Fabrication of Antireflection-structured Film by Ultraviolet Nanoimprint Lithography and its Mold Lifetime Amelioration

Transcription

1 Fabrication of Antireflection-structured Film by Ultraviolet Nanoimprint Lithography and its Mold Lifetime Amelioration Nurhafizah Binti Abu Talip Student ID: Supervisor: Professor Jun Taniguchi Tokyo University of Science Faculty of Industrial Science and Technology Department of Applied Electronics January, 2016

2 ABSTRACT Antireflection (AR) nanostructures from biomimetic moth-eye structures which can eliminate the undesirable reflection and increase the light transmission on the film surface have various applications in micro-nano discipline and nanophotonics fields. The significant applications of AR nanostructures ranging from improving the visibility of flat panel displays (FPDs), enhancing the performance of solar cells, enhancing the optical data storage, ameliorating light extraction in LEDs, and increasing the performance of optical lenses. Bernhard first discovered the function of antireflection of moth-eye structures in the nanophotonics fields in Several methods are available for fabricating AR nanostructures, for instance, interface holographic lithography, electron-beam lithography, nanosphere lithography, anodic oxidation porous alumina and so on. Nevertheless, these methods involve expensive and sophisticated apparatus when it comes to large scale fabrication of AR film. Therefore, ultraviolet nanoimprint lithography or known as UV-NIL is a promising technique to fabricate AR films with excellent properties in large scale and with cost effective fabrication. The merits of using UV-NIL technique are its simple and room-temperature process, highthroughput, rapid and low-pressure process, and high-accuracy replication of pattern. One of the major obstacles for fabricating an AR film is to eliminate the reflection of interface that disrupts the optical performance of fabricated AR film. Interface reflection commonly occurs between the film substrate and the AR structures layer of fabricated AR film, which results in degradation of AR effect. Our laboratory i.e. Taniguchi et al. previously succeeded to fabricate a self-supporting AR-structured film which can reduce the reflection of interface to as low as 0.5% from UV-curable resin by UV-NIL. In this method, an intermediator film from film type of polyvinyl alcohol (PVA) material was employed to acquire a layer of self-supporting antireflection-structured film. However, the intermediator film that was used to support the replicated AR structure was from a low durability material. Due to the water-soluble synthetic polymer material of PVA film, a few regions from the replicated AR-structured layer were deteriorated and creases appeared during the I

3 dissolving process of PVA film. This phenomenon affects the performance of fabricated AR film. The improvement in AR film method fabrication is highly needed. The industry goal of AR film fabrication is to reduce the reflectivity as low as 0.1%. Therefore, to enhance the performance of fabricated AR film, the film type from polypropylene (PP) material is proposed to replace the intermediator film from PVA. This film gives better stability and better releasability during UV-NIL in concerning to get a single layer of selfsupporting film with an excellent AR property. As a result, we successfully improved the fabricated self-supporting AR-structured film by reducing the reflectivity to as low as 0.3% and allows 94 ± 0.5% of light transmission. The advantages of the fabricated AR film; it is flexible, disposable, simple process, and time-effectiveness. Then, an adhesive material is required in order to apply this replicated AR film to any surfaces. Nevertheless, reflection of interface still arises due to the difference in refractive index between the self-supporting AR-structured film and the adhesive material. This phenomenon exhibit high reflectivity which affects the optical performance of fabricated AR film. A study of film with AR property that capable to eliminate the interface reflection of the front and back surfaces is required. This type of AR film offers an excellent quasi-omnidirectional AR property on its film. Therefore, we proposed a double-sided self-supporting antireflectionstructured; hereafter, DSARS film that fabricated by UV-NIL, as a promising solution. In this method, the DSARS film was fabricated by sandwiching the film between two different material of molds i.e., from glass carbon (GC) as master mold and the replica film mold of the master mold. As a result, we successfully further improved the optical performance of fabricated DSARS film by eliminating the reflectivity as minimum as 0.1% at the wavelength of visible light. The fabricated DSARS film also presents excellent transmission of light, which presents ± 1.25% in the spectral range of wavelength. In this study, the determination of the commercial usability of fabricated DSARS film that specifically used in the surfaces of photovoltaic and LED was executed by demonstrating the application of fabricated DSARS film. The demonstration was executed by adhering the DSARS layer with adhesive material on top of the substrate of glass that had different refractive index. The merit of fabrication of DSARS film: it can be benefited in different shape of substrate. It also can effectively suppress the mismatch of the adhesive interface reflection that occurred during the multistacking of different refractive indices of film materials. II

4 Due to the high demand of the industry in mass fabrication of AR film, the improvement of UV-NIL technique is necessary. Based on the principle of UV-NIL, the presence of releasecoating-layer (RCL) on the AR mold is very crucial in order to impede the adhesion of resin during the mass fabrication of AR film. Nevertheless, one of the obstacles in mass fabrication of AR film is to extend the lifetime of RCL on the surface of AR mold. This issue arises due to the factors of chemical and mechanical that deteriorate the components of RCL from the surface gradually during the repetitive UV-NIL. According to the previous research reported by Takahashi et al., force that resulting from the complete filling of the resin into the mold of high aspect ratio will generate the strong release force (RF). The resulted strong RF will possibly shorten the lifetime of the mold. In addition, the strong RF also resulted from the large surface area of the complete filling of resin. Osari et al. also reported that the difference in filling pressure results in different resin filling behavior in UV-NIL mold. This phenomenon can affect the durability of RCL on the UV- NIL mold. They also claimed that the presence of capillary force in the RCL mold allows the management of resin filling. Taking into account the concept of partial-filling of polymer that used by Bogdanski et al., we considered that partial filling of resin will weaken the RF and reduce the aspect ratio of the fabricated AR film, eventually ameliorates the AR mold lifetime. Thus, by employing the technique of partially filling the UV-curable resin during UV-NIL, the amelioration of the lifetime of AR mold was proposed. We ameliorated the lifetime of the mold of ARS that was made by GC mold and coated with RCL by partially filling technique of UV-NIL. Then, we evaluated the amelioration of the lifetime of AR mold by evaluating the repeatability of the AR mold to fabricate the AR film with low reflectivity. For comparison, we also evaluated the complete-filling technique of UV-NIL. By utilizing the technique of partial-filling UV-NIL, as a result, 0.25 ± 0.15% reflectivity and 94.0 ± 0.50% transmittance at the spectral range of wavelengths of replicated AR films were successfully obtained in average up to the 150th imprint. While, by employing the complete-filling UV-NIL, low reflectivity of replicated AR film was only obtained up to the 50th imprint. Producing AR film using GC mold will allow only one sided fabrication due to the opacity of GC mold and GC is from brittle material. Besides, direct mass fabrication of DSARS film is not possible by using GC mold. It is necessary to reduce the dependency on GC mold and shift to the fabrication of AR film by using replica mold. It is also significant to produce the replica mold from III

5 the material that can contribute resilience and flexibility, translucent, excellent releasability, and environmental-friendly. Thus, replicating the ARS film from replica mold was proposed. The proposed replica mold is from release-agent-free-antireflection-structured (RAF-ARS) type mold. This replica mold is made from UV-curable resin that consists of fluorinated components. This replica mold also offers antisticking effect, antifouling effect, and tested durability. Nevertheless, extending the lifetime of fluorinated components in the replica molds of RAF-ARS becomes an issue in their mass fabrication. Therefore, by employing the same technique of partial-filling UV-NIL, we ameliorated the life-expectancy of RAF-ARS replica mold. For comparison, complete-filling technique was also evaluated. The investigation of the filling ratio effects on an RAF-ARS replica mold was also carried out. As a result, the lifetime of the RAF-ARS replica mold was successfully be prolonged up to the 100th imprint by employing the technique of partial-filling UV-NIL. In comparison, by employing the complete-filling UV-NIL technique, we can only fabricate up to the 75th imprint. IV

6 TABLE OF CONTENTS Abstract... I Table of contents... V Chapter Introduction Significance of Antireflection Structure Films in Device Applications Application of Single-sided AR Film Application of Double-sided AR Film Biomimetic antireflection structure Moth-eye structure background Antireflection moth-eye structure theory Fabrication method of biomimetic moth-eye antireflection structures Interference Holographic Lithography Electron-beam Lithography Nanosphere Lithography Anodic Oxidation Porous Alumina Nanoimprint Lithography Thermal Nanoimprint Lithography (T-NIL) Ultraviolet Nanoimprint Lithography (UV-NIL) V

7 1.4 UV-NIL Related Phenomenon Adhesion in UV-NIL Basic Principle of Capillary Force Shrinkage Phenomenon in UV-curable Resin Segregation Effect in Initiating Mold Releasing Material Properties of the UV-NIL Mold Properties of GC Structural Model of GC Motivation of Study Objective of Study Layout of the Thesis References Chapter Experimental Apparatus Ion Beam Irradiation Machining Apparatus ECR-type Ion Source Separation Grid Operating Room Scanning Electron Microscope (SEM) Operating Principle VI

8 2.2.2 Image Resolution Image yielded by Secondary Electron Image Enlargement and Scanning in SEM System Optical Column of Electron in SEM Operating Room of Sample UV-visible-near-infrared Spectrophotometer Excellent Evaluation Deep UV Technique Evaluation Spacious Sample Chamber High-resolution SEM Sample Coating System Resistively Heated Vacuum Evaporation System Mold-release Equipment Contact Angle Meter Lifetime Evaluation Machine References Chapter Fabrication of antireflection-structured films by ultraviolet nanoimprint lithography Fabrication of self-supporting antireflection-structured films by ultraviolet nanoimprint lithography Introduction VII

9 3.1.2 Theoretical calculation of fabricated AR film s reflectivity Experimental procedure Fabrication of AR mold Fabrication of self-supporting antireflection-structured film Evaluation of optical properties Result and discussion Conclusion Fabrication of double-sided self-supporting antireflection-structured (DSARS) film by ultraviolet nanoimprint lithography Introduction Experimental procedure Fabrication of molds Fabrication of DSARS film Evaluation of optical properties Result and discussion Fabrication of DSARS film and its optical properties Application of DSARS film on substrate with different refractive indices Conclusion References Chapter VIII

10 Mass fabrication of antireflection structure film by ameliorating the Lifetime of Antireflction structure mold by partial-filling ultraviolet nanoimprint lithography Lifetime amelioration of antireflection structure molds by means of partial-filling ultraviolet nanoimprint lithography Introduction Consideration of partially and completely filled pressure on the basis of theoretical calculation Experimental procedure Fabrication of ARS mold Repeatable UV-NIL Evaluation of ARS mold lifetime Result and discussion Fabricated ARS molds Lifetime amelioration result of ARS mold by partial-filling UV-NIL Conclusion Lifetime amelioration of release-agent-free antireflection-structured replica molds by means of partial-filling ultraviolet nanoimprint lithography Introduction Experimental procedure Fabrication of RAF-ARS replica molds Analysis of impacts of resin filling ratio on lifetime of RAF-ARS replica mold IX

11 Lifetime amelioration of RAF-ARS replica molds by partial-filling UV-NIL Results and discussion Fabricated RAF-ARS replica molds Analysis result of effects of resin filling ratio on lifetime of RAF-ARS replica mold Lifetime amelioration result of RAF-ARS replica molds by partial-filling UV- NIL Conclusion References Chapter Conclusion Summary of Achievement Future Work Key Publications Acknowledgement Curriculum Vitae List of Publications X

12 CHAPTER 1 INTRODUCTION In the advance of science and technology, nature has been an inspiration to humans for tackling challenges in engineering. Currently, biomimetic of nanostructures sparks fascinating interest in many application-based principles for instance, self-purification of lotus-leaf, antireflection (AR) of moth-eye, downsizing the water friction by using shark skin principle, basic coloring principle from butterfly wing, antisticking of gecko foot principle, and high-fluorescent of firefly principle [1 3]. Biomimetic moth-eye structure has been extensively explored in diverse application of nanophotonics industry for instance, flat panel display, lenses surfaces, photovoltaics, and light emitting diodes (LEDs). The presence of protuberance arrays in moth eyes has significant roles in suppressing the reflection and permitting the transmission of incident light on their surface, thus, presenting their AR properties. In 1967, Bernhard was first identified the AR property on motheye structure [4]. Then, Wilson and Hutley elucidated the application of moth-eye structure at the angle of gradual changes of effective gradient-index principle in terms of the difference of refractive index of multistacking materials [5]. While in 1973, the conical protuberance arrays of moth-eye structures were replicated onto the glass substrate by using photoresist coating technique. This technique were invented by Clapham and Hutley [6]. 1.1 Significance of Antireflection Structure Films in Device Applications Biomimetic AR structure films plays pivotal role in nanophotonics industry as mentioned above. Fabrication of biomimetic AR structure films offer flexibility, cost-effective, and satisfaction in handling technology. Followings are the related issues of biomimetic AR structure film in device applications that we are focused on. The application of AR film is divided into two parts i.e. application of single-sided AR film and application of double-sided AR film. 11

13 1.1.1 Application of Single-sided AR Film In the fields of photovoltaics [7 9], light-trapping technology become one of the important element in optimizing the absorption capability in thin-film solar cell. It is an urgency to develop an effective AR film on their protective cover to reduce the interface reflection losses and consequently increase their efficiencies. The reflection losses in the photovoltaics system is caused by the mismatch of the difference of refractive index between air interface and semiconductors materials. Fig illustrates the mechanism of the photovoltaic solar cell with AR layer and the propagation of the light through the photovoltaic system. Designing the optimal AR solution in photovoltaic is one of the most challenging because the whole range of incidence angles are importance as the sun moves from sunrise to sunset. The solar irradiation striking the surface of the earth contains a very broad spectrum of electromagnetic radiation which varies with time of day, relative position on the globe and weather conditions. It is a very unpredictable and dynamic system which pushes the limits of what current nanoengineering is able to achieve. Fig Schematic of the photovoltaic solar cell with AR layer in enhancing the efficiency of electrical conversion [10]. In the fields of LEDs [11 16], the performance of LED strongly depends on the performance of its internal quantum to extract light at a high intensity. If, the internal quantum performance can be improved, the light extraction can be reach almost 100%. Nevertheless, the real ability of LED 12

14 to extract the light is lower than expected due to the loss of internal reflection during the conversion of electricity. This phenomenon happened by reason of the massive mismatch difference of refractive indices among the lens materials and the air interface. As a result, fabricating antireflection-structured films with the purpose to enhance the performance of LED to extract the light become extremely essential in its application. Figure illustrates the example of the reduction of loss of internal reflection in the nanostructure-covered hemispherical LED lens. The figure shows that in the nanostructure-covered LED lens, the refractive index change gradually, resulting in the reduction of loss of incident light [16]. Fig Reduction of loss of internal reflection by the presences of antireflection property in nanostructure-covered hemispherical LED lens [16]. Over the past decade, the scientists have conducted various researches to improve the performance of light extraction in LEDs. Previously, a thin layer of quarter wavelength AR coating film was used as the conventional method. Nevertheless, the efficiency of light that is extracted is very low due to the extraction of light at a specific wavelength. A broadband and quasiomnidirectional application is extremely needed. Thus, it can be achieved by applying the appropriate design in stacking the AR coating [17, 18]. However, limitations in selecting the type 13

15 of material of the relevant refractive index and the capability of the thermal development in LED become challenges that need to be studied [19]. Therefore, many researchers focusing on the fabrication of antireflection structure based on the sub-wavelength theory that comprehensively have been studied in the fields of photonic crystals [20, 21], nanorod [22, 23], and biomimetic moth-eye [15, 17, 24]. Other than that, the manufacturing of flat panel displays (FPDs) [2, 25] is a dynamic and continuously evolving industry. FPDs for instances liquid crystal display panels or plasma display panels are expanding in resolution and size of length. As the screen size increases, the reflectivity from the surrounding lights also increases and consequently affects the visibility of the screen background. Thus, the best way to solve this problem is to fabricate the AR structures on the screen display. This technique can suppress the reflection that comes from the oblique incidence light Application of Double-sided AR Film It is well known that the solar cell is a device which capable in converting the sunlight into electricity. Various types of solar cell technologies are developed nowadays, for instances dyesensitized solar cell, crystalline silicon solar cell, and photovoltaic organic-based solar cell. However, the most famous technology among industries is based on crystalline silicon technology. This technology has been identified as a mature technology compared with other technologies in enhancing the performance of energy-conversion into electricity, in high capacity. Thus, a photovoltaic module has been established by combining certain amounts of solar cells together in one system. The solar cells are enclosed together by a backsheet at the bottom and a glass plate as a protective-cover at the top. Each cell is affixed by the polymer of encapsulant, as shown in Fig

16 Fig Schematic of a crystalline solar panel [26]. The presence of encapsulant polymer is very important in the construction of a photovoltaic system mechanism. It can provide the mechanical support, allow the UV absorption, provide protection from the environmental moisture, allow the isolation of electrical, and has a high resistance of temperature. Various types of materials can be used for encapsulation technique, but, the most popular material is from ethylene-vinyl-acetate (EVA) type [27, 28]. In the process of the photovoltaic construction, EVA sheet is arranged between the solar cells and the backsheet or between EVA sheet and glass. Then, EVA sheet will be heated at a certain temperature with a suitable period of time. Due to the dependency of EVA upon the degree of curing time, it is necessary to improve the technique of encapsulation. In encapsulation process, the existence of the double-side AR film that adhered between glass EVA sheet, EVA sheet cells, and EVA sheet backsheet plays an important role to reducing the mismatch of refractive index of every layers of different materials. This mismatch can affect the performance of crystalline solar panel. Thus, the application of double-sided AR film can possibly enhance the performance of crystalline solar panel fabrication. The illustration of application double-sided AR film in crystalline solar panel is shown in Fig

17 Fig Illustration of application of double-sided AR film in the encapsulation process of crystalline solar panel. 1.2 Biomimetic antireflection structure Moth-eye structure background C. G. Bernhard was discovered in 1967 that the moth eye can suppress the reflection of nearinfrared light from reflecting in their eyes to keep away from the predators [4]. The surface of moth-eye comprises an array of conical protuberances which normally has a size of 200nm in height and pitch. This conical protuberance is also known as corneal nipple [6]. The arrangement type of the conical-shaped of the moth-eye structure is almost the same as the shape of crystalline and look as pack of hexagonal. The pitch of the conical-shaped is in the range of nm. While the height of the conical-shaped is between 0 and 230 nm that known as papilionids and nymphalid, respectively. The presence of conical-shaped of the moth-eye structure on the lens creates an effective medium of gradient refractive index between the interface of air and the material of lens. Therefore, the reflectivity of the lens can be reduced during the propagation of the light [29]. The role of the hexagonal-type array of conical-shaped of moth-eye structure is to be as an antireflection layer or coating. This layer can suppress the reflection that comes into the eyes of 16

18 the moths. This layer also creates the perfect of broadband antireflection property [30]. The conical-shaped structure that formed in the eyes of the moths is also responsible in increasing the transmission of the light during the night or dark situation and consequently increases the sensitivity of the visual light. The subwavelength conical-shaped structure in moths forms an effective medium of gradient-refractive index. Thus, it can produce the impedance matching of the optical in the gradient-index profile [1, 5, 31]. The antireflection nanostructured that biomimetic from the moths has successfully created a broadband antireflection effect on the surface of any substrate. This broadband effect of the antireflection provides a broad range of illumination from various angles and can be benefited in improving the conventional method of antireflection coating. It is well-known that the antireflection coating has limitations in suppressing the reflection of light at oblique incidence. Thus, by fabricating an antireflection structure on the surface of substrate can perfectly suppress the reflection at oblique incidence. The antireflection structure that is fabricated on the surface of substrate is in accordance with the height and the pitch of the subwavelength. The conical-tapered shape of the antireflection structure plays a role to change the refractive index gradually between two different mediums that are air and substrate mediums. This technique can prevent the phenomenon of large difference in refractive index which results reflection. In the visible light of condition, the size of the pitch of the antireflection structure has to be less than 150 nm and the height has to be in the range of nm [6, 25] Antireflection moth-eye structure theory The most important element in fabricating the structure of antireflection is to determine the type of appropriate materials used and to create the relevant topography on the substrate. The subwavelength of the antireflection structure is innovated by observing the natural pattern of the eyes of moths. This structure can be identified as an alternative technique for fabricating the antireflection effect on the surface of substrate based on the relevant theories and the experiment results. The surface of antireflection structures properties can be described from two categories of theories i.e. gradient refractive index theory (GRIN) and effective medium theory (EMT). The light that incidence on the roughness surface of nanostructures will be reflected and diffused. The roughness surface of the substrate is constructed by sub-wavelength nanostructures with a tapered profile. This structure is capable to prevent the reflection from various angles. It 17

19 can also increase the transmission dramatically. This phenomenon is depicted in Fig Figure (a) show the reflected of light from non-patterned surface substrate, where the difference of refractive index (RI) from two different interfaces will cause an ineffective medium. This can be described from the graph that gives sharp changes of gradient-index of film. While, by grating the surface with moth-eye structures [Fig (b)], the shape of the moth-eye structures eliminated the difference of RI between ambient air and the substrate which creates an effective medium to reduce the reflection, consequently gives the gradual changes of gradientindex of film. Fig Effective medium theory of AR film. In the instance of the profile of continuous surface-relief or commonly known as the sinusoidal profile, antireflection effect in thin-film system resulted from the graded index technique. The graded antireflection structure height is determined by the length of the subwavelength that can give an effective effect of antireflection properties on the substrate. If the appropriate height has been determined, the size pitch that resulted is also small enough to form the effect of antireflection on the substrate. Thus, the biomimetic moth-eye structures that will be fabricated in this work is assumed as the sinusoidal profile surface. 18

20 Fig Effective medium for a sinusoidal surface profile [32]. In other words, the EMT theory can explain the synergy between the light and the diversity of the formation of the subwavelength structure on the surface of substrate. This diversity of the structure is limited to homogeneous form in order to ensure that it can present the constant single optical properties effectiveness. The EMT theory also depends on the ordinary of weighted spatial of the characteristic of optical profile in a specific order. While, for the profile of sinusoidal surface condition, the effective medium of the film is created by the film that has gradient property. Thus, the illustration of the gradient-refractive index films are shown in Fig In the surface of sinusoidal profile, the refractive index increments through the thickness of the film during the propagation of the light. This phenomenon creates a refractive index difference in the film. This simple equation can be described as ( ). The best technique to optimize the design of the antireflection grating is through a simple calculation based on technique of rigorous coupled-wave analysis (RCWA). The technique of RCWA is first proposed by Moharam in 1981 [33]. This technique known as a simple technique that used the Maxwell s equations. The Maxwell s equations can accurately analyze the diffraction of the electromagnetic wave in a periodic structure of subwavelength. Based on the expectation of the simulation results of RCWA, the reflectivity can be reduced effectively with the increment of aspect ratio h/p, where h and p represents height and pitch of the subwavelength structure of antireflection layer, respectively. In this thesis, we hybridize the antireflection structure theory with the multilayer antireflection equation for the theoretical calculation of the reflection i.e. by assuming that the refractive index of the fabricated moth-eye structure can be segmented into some layer of effective refractive index for the purpose of eliminating the reflection from the incident light. The assumption made is based 19

21 on the theory of the sinusoidal surface profile. RI will increase along the thickness of the sinusoidal profile. The equation of single and double layer of AR films is shown in eq and For eq ,, and represent RI indices of air, 1st layer of effective medium of AR, and the substrate, respectively. While for eq ,, and represent RI indices of air, 1st layer of effective medium of AR, 2nd layer of effective medium of AR, and the substrate, respectively. ( ) ( ) 1.3 Fabrication method of biomimetic moth-eye antireflection structures AR property of moth-eye structures has attracted the attention of researcher in the micro nano science and engineering fields. Nevertheless, in the 1960s and 1970s, fabrication of moth-eye structure of sub-wavelength dimensions for visible applications was difficult. Since the development of technology in fabrication techniques have ameliorated, various technique of replication of moth-eye structure have been proposed. Here, five most widely used fabrication techniques are reviewed. A brief explanation of each technique is given along with a detailed example. A few successful examples of fabrication and optical results for each of method are presented Interference Holographic Lithography Bernhard was the first pioneer to fabricate AR moth-eye structure by designing the structures that can only be performed in the range spectral of microwave [4]. The model was manufactured from a mixture of beeswax and paraffin for ensuring that the refractive index was close to chitin. After that, Clapham and Hutley [6] further researched and was first invented the fabrication of moth-eye structure that performed in the visible of wavelength by using interference holography 20

22 lithography. In the technique of Clapham and Hutley, they used the interference fringes at the intersection of two collimated krypton laser beams with wavelength 315nm at an angle of 120 o with regard to form sinusoidal protuberances in photoresist on a glass substrate. The technique is shown in Fig (a). Fig Illustration of (a) two-beam interference holography with a beam splitter [34] and (b) with Lloyd s mirror configuration [35]. Here, the photoresist had a refractive index that identical to glass material. The variation of the intensity of the sinusoidal beams formed the sinusoidal grooves in the photoresist. The formation of sinusoidal grooves is shown in Fig This technique is applicable only for flat surfaces. The refractive index of the fabricated structure was 1.6 and the spacing between protuberances was approximately 210nm. Then, configured with Lloyd s mirror as shown in Fig (b), the 21

23 glass was rotated 90 o between two exposures of laser beams. This resulted in less than 0.5% of reflectivity in the range of nm wavelength at normal incidence. Fig Illustration of the formation of the sinusoidal groove in the photoresist. The spacing between the protuberances depends on the spacing between the fringes produced by the intersecting laser beam as shown in the following equation: ( ) Here, is the spacing between protuberances, is the half angle of interfering laser beams, and is the wavelength. With the possible maximum value of angle between the two laser beams, the minimum value of d was calculated as 0.6λ which corresponds to 120 o angle between the laser beams. Based on the calculation, in the case of the krypton laser beam, 210nm was the minimum spacing that can be fabricated. While the depth of the grooves h was difficult to control. The depth varies depends on the exposure time and the response of the photoresist to the exposure. Beside, photoresist had an extra sensitivity at shorter wavelengths. Thus, there is a limitation between the ratio of which makes h and d strictly dependent on the changes of each other. This become the limitation of this technique, particularly in fabricating the higher protuberances due to ameliorate the anti-reflectivity performance of the moth-eye structures. 22

24 Afterwards, Wilson and Hutley [5] used 458nm wavelength of an argon ion laser which formed an identical structure with a minimum spacing of 275nm. The fabricated moth-eye structures had a different periodicity and depth. This resulted in reflectivity values less than 3%. They also introduced the idea of using a metal replica to transfer the pattern into photoresist. The demerit of this technique for certain cases, the separation of the master and the replica was difficult, consequently caused deterioration to the master. Based on above reviewed technique, they showed that the performance of the fabricated AR moth-eye structure greatly potential in replacing the commercial multilayer of AR layers. However, due to the cost constraint for mass fabrication and the nature of the photoresist, this technique inadequate for many application. The example of the applications such as fabrication of AR coating with high performance of optical components, mass fabrication of plastic components, solar cell, and LEDs. Thus, amelioration in fabrication techniques and material properties is highly required. For further amelioration, this technique was used to transfer the moth-eye pattern to substrates made of other materials like glass [36] and silicon [37]. First, the substrate needs to be coated with a certain thickness of photoresist. The resist is exposed with the two intersecting coherent UV lasers to transfer the pattern of the optical standing wave on to the resist in one dimension. The substrate is rotated 90 o in order for the pattern to be transferred in the second dimension. Then, the pattern is replicated onto the silicon by the reactive ion etching method. Finally the remaining resist is removed by immersing the substrate in acids. Lalanne and Morris [38] used the same technique to create a moth-eye pattern on silicon working in the visible wavelength. However, they used an intermediate layer of SiO2 between the substrate and resist. This layer ensures that the grooves can be etched deep enough into the photoresist. As a result, this technique is capable in forming structures with a pitch (periodicity) of 200nm to 4μm and a minimum feature size of 100nm. Nevertheless, the minimum feature size of techniques like Electron Beam Lithography and Nanoimprint Lithography is even smaller. The other demerit of this technique is that it is limited in the design of the protuberances and arrays. The height of the grooves and the periodicity depend on each other, thus, the height is limited by 23

25 the thickness of the resist and the etching ratio of the substrate to resist. However, by using metal replicas or embossing, it is possible to fabricate large areas of the structure at low cost. Therefore, for fabrication structures that has minimum size of pattern for example 100nm and a lattice geometry, it can be made by interfering beams i.e. square or hexagonal structure, holographic lithography is recommended Electron-beam Lithography One of the method that used to fabricate the biomimetic moth-eye structure is by employing the electron-beam lithography technique. Figure shows the basic step of this technique. First, a layer of resist is coated onto the substrate as shown in Fig (b). Two types of resist will normally use from positive and negative type resist. For the negative resist, the removal occurred at the non-exposed resist area. Whereas positive resist, the removal occurred at the exposed resist area. As shown in Fig (d), the development of the tapered resist is occurred after the removal from the exposure area and the chemical treatment. Then, the pattern is replicated into the substrate by means of etching process. The type of the etching process play a pivotal role in shaping the replicated pattern [Fig (e)]. After the etching process, the residing resist is removed using acid wash [Fig (f)]. Fig Fabrication process of AR structures by E-beam lithography [39]. Several of silicon moth-eye structures have been fabricated by means of E-beam lithography. Kanamori used an e-beam lithography technique along with SF6 Fast Atom Beam (FAB) etching 24

26 technique [39]. They consumed the anisotropic nature of the FAB etching technique to form deep structures. A 400nm positive resist was coated on a 200μm thick silicon wafer. A 1.2mm 1.2mm area was exposed with the E-beam machine for about 10h to write the pattern into the resist. After developing the resist, the wafer was etched with SF6 FAB etching and consequently replicating the pattern to the targeted silicon substrate. The residing resist is removed by immersing the wafer into an equal parts solution of H2SO4 and H2O2. As a result, based on the scanning electronic microscope (SEM) image, the fabricated protuberances had sharp conical shapes with a pitch of 150nm and height of 350nm. The fabricated structure showed a large reduction of reflectivity with less than 3% within the visible wavelength compared to flat silicon. Besides, at a wavelength of 632.8nm, the reflectivity of s-polarized light at angles of incidence from 5 80 o of the fabricated structure decreased from that of the silicon substrate. The p-polarized reflected light is also lower than silicon, but only up to an incidence angle of 65 o. The minimum reflectivity was 1.65% at 40 o. They claimed that the minimum reflectivity seems to be caused by an effect similar to that of the Brewster angle. Employing different etching processes, removing the resist by means of different etching processes or a lift off process, or adding more layers as an etching mask to the process makes it possible to fabricate moth-eye structures in different materials. Toyota et al. [40] used E-beam lithography along with reactive ion etching and a chrome disk array as an etching mask to fabricate moth-eye structures in fused silica. SEM image result showed that their fabricated structure had conical protuberances with a pitch of 250nm and height of 750nm. The reflectivity result that they obtained was below than 0.5% of reflectivity in the visible light wavelength of nm at normal incidence and 3% for the angle of incidence range of 5 55 at 632.8nm E-beam lithography provides a smaller minimum feature size compared to interference holography lithography. Nevertheless, the writing process is time consuming for even the small areas. Thus this technique is sophisticated and expensive in terms of time and equipment Nanosphere Lithography One of the approaches to reduce the cost of the moth-eye structure fabrication is to utilize the nanosphere as masks to replicate the pattern directly to the targeted substrate. In this technique, the nanospheres is self-assembly arranged on top of the targeted substrate. Here, a number of 25

27 spheres on a flat surface form a hexagonal structure. This is a bottom-up technique which does not involve the expensive pattern writing of E-beam lithography. The setup equipment is much cheaper compared to previous techniques. Figure illustrates the basic fabrication of the nanosphere lithography [41]. First, a monolayer of nanospheres is deposited on the substrate as illustrated in Fig (a). The size of the nanosphere is customized according to the desired feature size. Next, the etching process is executed to form the hexagonal pattern of protuberances in the targeted substrate as shown in Fig (b) and (c). Finally, additional etching step is need to smooth the shape of the protuberances [Fig (d)]. Fig Illustration of basic AR fabrication of nanosphere lithography [41]. Sun et al. [24] took the initiative to fabricate AR moth-eye structure in the silicon by depositing a monolayer silica colloidal crystals with non-close-packed as a mask for etching use. The 360nm silica sphere of non-close-packed is arranged on the silicon by spin-coating technique. Then, a SF6 dry etch was performed to etch the underlying silicon through the nanosphere mask. The etching time determines the height of the fabricated protuberances. Finally, the residing silica particles are removed by a hydrofluoric acid wash. Based on the SEM result, the fabricated protuberances had 26

28 a height of 800nm and a base radius of 210nm. While the optical performance of the fabricated moth-eye showed less than 2.5% of reflectivity at the spectral range of nm. Then, Chuang et al. [41] used the same technique to fabricate the silicon AR moth-eye structures. They succeeded to reduce the fabricated protuberance in the silicon i.e. the base diameter of 90nm and the height of 150nm. The reflectivity was less than 5% for incidence angles from 5 70 o at 632.8nm wavelength. Min et al. [42] also used the same technique to fabricate GaSb moth-eye structures for the application of thermophotovoltaic. The fabricated structures were etched for 2.5min and showed a reduction of reflectivity from 35% of the GaSb flat surface to just 2% of the etched surface. Therefore, the advantage of this technique is simple, rapid technique and low cost compared to pattern writing techniques such as E-beam lithography. In addition, nanospheres have been used to fabricate self-assembly protuberances such as holes, cones, rings and nanotubes. However, in order to fabricate samples in a large scale, a bulk of spheres are needed which makes the technique expensive. This technique only capable for fabricating a shape of hexagonal structure Anodic Oxidation Porous Alumina Recently, the use of the membrane form porous anodic alumina (PAA) with hexagonal order of nanopores as a template of fabrication tool has been the focus of researchers due to the diversity of its function. PAA membranes are mostly applied in many areas for instances nanowires, nanoporous membranes, nanostructure antireflection, and nanotubes. Figure shows the two basic steps of fabrication process of anodization PAA [43]. First, the formation of alumina membrane is carried out by using a sterile aluminum sheet in order to go through the process of anodically oxidized. Next, the formed membrane is removed by using acid solution. Commonly used acid solution is from oxalic acid, phosphoric acid, or sulfuric acid liquid. The acid solution used in the second time process of anodic oxidation responsible to build pattern structures on the targeted substrate [Fig (b)]. After the second process of the anodic oxidation, the nanopores in the PAA membrane is well-organized and formed [Fig (c)]. Then, the final film is released from the barrier layer promptly by the pulse voltage exceed than 5V. The success at this stage will ensure the establishment of the free-standing of the PAA 27

29 membrane [Fig (d)] and will prevent the layer of alumina to cover the aluminum substrate [Fig (e)]. Fig Fabrication procedure of two-step anodization of porous anodic alumina [43]. Yanagishita et al. [44] has taken the initiative to fabricate the polymer film of antireflection structure that consisting an array of the hole structure by using UV-NIL method. The holestructured UV-NIL template that used to fabricate the film of AR was from a metal mold that made by PAA technique. The arrangement of the hole arrays that formed on the substrate were resulted from the long period of the first process of the anodization. The perfect arrangement of the hole arrays on the surface of the film substrate were resulted from the second process of the anodization after the removal step of the oxide layer. The first process of the anodization was conducted in the solution of 0.3M oxalic acid at the temperature of 17 o C under the condition of 40V for 3 hours. The formation of the holes in the tapered shape of PAA were resulting from the process of the saturation that combining with the anodization process and consequently widening the pore of holes. While, the treatment of the pore-widening was conducted for 30s in the second process of the anodization by using the solution of 5wt% of phosphoric acid. In the final step, a fabricated 28

30 metal mold was electrochemically deposited by using an acidic solution. The fabricated metal mold that comprising a tapered holes was resulted from the alumina template that submerged in the solution of the 30wt% of NaOH after the electrodeposition process of the nickel. Thus, as a result, they successfully fabricated the AR film that can prevent the reflection to as low as 1% reflectivity. Then, they expand the potential of AR fabrication with the same technique to the different type of substrate. They fabricated the AR structures on the hemispherical lenses, i.e. the technique is allows fabrication of small structure on a curved shape [45]. Therefore, the advantage of this technique is simple, suitable for fine pattern fabrication structure, and applicable in large scale fabrication. However, this technique has limitation where the reduction of reflection on the fabricated surface is poor. This technique is also a time consuming process resulting in the high maintenance cost Nanoimprint Lithography Fabricating sample products on a large scale by using e-beam lithography very timeconsuming and costly. Nanoimprint lithography (NIL) [39] becomes an alternative technique in improving the technique of e-beam lithography. NIL is known as the simple and inexpensive technique. By simply using a template or mold, the replication of the pattern onto the substrate can be done. By applying the appropriate release-coating-layer on top of the mold, the lifetime of the mold can be extended and will allow the repetitive replication of the pattern in a large scale [46]. There are two type of nanoimprint lithography i.e. thermal nanoimprint lithography and ultraviolet nanoimprint lithography Thermal Nanoimprint Lithography (T-NIL) The fabrication technique start by making the mold. The mold is fabricated by means of e- beam lithography for features smaller than 200nm or optical Interference Lithography (Holographic Lithography) for bigger features. Then it is pressed onto a heated substrate, which the correct amount of pressure and cooling the substrate, the mold is removed. The pattern on the polymer is then replicated onto the substrate by using the dry etching technique. By using the chemical methods, the residual polymer resist is removed. The basic process of T-NIL is illustrated 29

31 in Fig The minimum feature size in this technique is determined by the features on the mold. However, a minimum feature size of 10nm has been achieved using nanoimprint lithography with e-beam lithography to make the mold. Since the mold can be used many times the time needed to make the mold by e-beam lithography is not that much of a concern. Fig Illustration of basic process of thermal nanoimprint lithography (T-NIL). In T-NIL, viscous state is reached by heating thermoplastic materials above their glass transition temperature Tg. The flow behavior of a thermoplastic material depends highly on the temperature. At the microscopic scale, the thermoplastic material could be imagined as a networker of linear, entangled polymer chains. Well below Tg, the material behaves almost elastic because the chains cannot be displaced permanently by the applied force. When the temperature increases, the polymer chains can move entanglement points. This elongation is reversible and the material still behaves elastic. But to partially reconfigure entanglements and to enable polymer chains to move against each other, even higher temperatures than Tg are required. The respective temperature is termed terminal flow temperature Tf. 30

32 In an attempt to fabricate an antireflection structure layers, Yu et al. [47] fabricated silicon antireflection moth-eye structures by means of trilayer nanoimprint lithography and a lift off on silicon. In their process, a nickel mask of pyramids is made by means of trilayer photoresist nanoimprinting lithography and lift off. Reactive ion etching (CHF3/O2) was then used to etch the underlying substrate through the mask to give a conical moth-eye array with 200nm period and 250nm groove depth. This moth-eye structure exhibited a reflection of less than 1% at normal incidence within the visible regime ( nm). Also, at a wavelength of 632.8nm their structure showed a reflection of less than 3% for angles of incidence less than 65 o. Boden et al. [48] also fabricated silicon moth-eye structures by means of nanoimprint. They employed dry etching processes to produce protuberances similar to moth-eye structures on 1cm 1cm samples. Reflection was as low as 10% for incident light in the visible regime. Using the same technique, moth-eye structures have also been fabricated on glass [49, 50] and GaAs [51]. T-NIL has the advantage of large scale fabrication along with small feature size as small as e- beam lithography. However, like e-beam lithography, the control over the shape of protuberances is still very poor, and remains dependent upon the etching process Ultraviolet Nanoimprint Lithography (UV-NIL) The advance of the fine patterning technology in the next generation lithography fields requires an expensive process in order to fabricate the nanostructure in mass fabrication. UV-NIL has demonstrated a great success in the fabrication of the nanostructure in a high-resolution, highaccuracy of replication, cost effectiveness, and high-throughput. The low viscous imprint materials allow UV-NIL to be conducted at room temperature and much lower pressures than T-NIL. This increases the mold lifetime [52] and decreases the process time for the same mold-layout. The viscosity of the imprint material can be reduced by using low molecular weight materials like monomers or oligomers. For example, typical acrylate monomers can have viscosities down to 1 mpa s. The imprint material can be spun-on or selectively dispensed. The mold is lowered into the liquid imprint material, which starts to fill the mold cavities. To preserve the mold pattern, the imprint material must be solidified. This could be achieved with curable moieties and a UV-flood-exposure. Mostly UV-curable formulations are used. Among 31

33 them, free radical curing is most often used besides anionic or cationic initiator systems. After curing, the mold is separated from the imprint. UV-NIL requires a transparent mold or substrate to enable the UV-light to reach the imprint material. Mostly silicon substrates are used and thus a transparent mold is required. Figure shows the basic process of UV-NIL. The polymer flow in UV-NIL is less complex compared to T-NIL. However, depending on the mold layout, special measures against bubble trapping might be taken in account. Mixed pattern sizes are easier to replicate with UV-NIL due to the less complex polymer flow. Chemical functionalities can be easily incorporated in monomer formulations. Hence, UV-NIL is highly suited for customized polymer materials, which is often need for functional systems. A further advantageous for functional materials to preserve the particular functionalities of chemical additives or other fillers. Thus in this thesis, we are focusing on using the UV-NIL technique in fabricating the AR film. Fig Illustration of basic process of ultraviolet nanoimprint lithography (UV-NIL). 1.4 UV-NIL Related Phenomenon Adhesion in UV-NIL The following section is based on [53] and summarize the theories that related to the adhesion in UV-NIL. Adhesion is describes as the interaction between different surfaces. Adhesion is measure of the extent of the mechanical load that can be transferred from one surface to another 32

34 surface. Adhesion can be mediated by a respective material that known as adhesive. In UV-NIL, the adhesive material must comply with both the substrate not to the mold. Therefore, it requires a certain amount of work to separate these surfaces after they have been in contact with each other. There are several intermolecular forces and mechanisms become a source of adhesion. They can be grouped into physical, chemical, and mechanical interactions. In UV-NIL, physical interactions always exist between mold and resin. The interaction is known as van-der-waals interactions that comprises from a sub-group of electrostatic forces. Van der Waals forces can be divided into several groups i.e. dispersion force, dipole-induced dipole force, and dipole-dipole force. The synergy of the dipole-dipole arise when the partial electric deviate within a molecule as a result of different electronegativity values in chemical elements. For instance in the molecule CF3-CH3, the electrons deviated toward the fluorine that lead to permanent dipole molecule [53]. Chemical interactions that possibly involved in UV-NIL can be a covalent bonds which shared electron pairs between bonding partners. The bond strength depend on the bonding partners. An important group of covalent bonds are acid-based interactions of an electron donor providing electrons to an electron acceptor. While, the mechanical adhesion interactions is based on interlocking, friction, and closely related to surface roughness [54] Basic Principle of Capillary Force The capillary force that acting in the liquid of thermodynamics is resulted from the existence of adhesion and the surface tension. The liquid adhesion force that acting on the capillary walls result in the formation of the meniscus that form on the repulsion force upward. Meanwhile, the surface tension contributes to strengthening the attractiveness of the liquid to the surface of the capillary. It is essential to describe the meniscus that formed by the attraction of the capillary force with two radius of the curvature that act as the sphere-like shape of liquid. Cross-section of a small part of the curved surface is shown in Fig (a). R1 and R2 shows the radius of the curves that acting within the capillary. 33

35 Fig (a) Illustration of the mechanical equilibrium in the curved surface of the liquid (b) Meniscus in a capillary when R1 R2; (c) Capillary condition when R1=R2. The movement of the liquid outward from the initial position to a certain position in the capillary tube will produce a small change of area of. From the small change of area, the work W can be derived. In addition, the difference pressure = P1-P2 of the entire surface also can be obtained. P1 and P2 indicate the internal pressure and the external pressure, respectively. These both pressures are acting on the xy area and through the distance of dz. 34

36 ( ) ( ) With regard to the equilateral triangle condition, the following equation is obtained; ( ) ( ) If the surface of the liquid is in the condition of equilibrium mechanical, two types of work that resulting from R1 and R2 must be equivalent to each other. Then, the eq can be replaced by dx and dy; ( ) In the case of the liquid that circularly that meet the wall of the cylindrical capillary at certain angle as shown in Figs (b) and (c), the meniscus that act as a spherical-like shape can be assumed as /cos with condition. The capillary force can be derived as following. 35

37 ( ) Shrinkage Phenomenon in UV-curable Resin During UV-NIL technique, the photoinitiator in the resin will absorb the UV irradiation at the relevant intensity of wavelength. The reaction of the free radical polymerization is performed by the existence of free radicals. Then, the double bond of carbon of the acrylic group in monomer will be activated. This reaction creates an interactions and consequently form the groups of acetate [55]. The covalent of carbon-carbon interact each other after the interaction of van deer Waals between the individual monomers. The polymerization shrinkage in the resin occurs during the UV irradiation can be evaluated by the technique of Hudson et al. [56]. R. F. Brady et al. [57] have reported that the difference in shrinkage can be attributed to the polymerization style during the polymer curing system. During the shrinkage, the additional polymerization is taken by the radical polymerization of acrylates. Then, the polymerization of cationic of epoxides and oxetanes will take the polymerization of ring-opening. However, exclude the polymerization of the cationic of vinyl ether, it will falls directly into additional polymerization. The basic structure of the molecular of cationic curable resin and the degree of the reactivity is illustrated in Fig Fig Basic structure of molecular of the cationic curable resins and the degree of the reactivity. 36

38 Fig Mechanism of curing shrinkage during polymerization. (a) The additional polymerization of an acrylate; (b) the polymerization of the ring-opening of cycloaliphatic epoxides [58]. The mechanism of shrinkage phenomenon is shown in Fig Figure (a) shows the distance changes between 3.41 Å of van deer Waals and 1.54 Å of covalent in the polymerization process of monomers. Meanwhile, Fig (b) shows the shortening of the distance between monomers during the polymerization of ring-opening. This phenomenon also known as additional polymerization. Nevertheless, the polymerization of ring-opening united to form a covalent bond of C-O in epoxy. This phenomenon can present the bond of ether between the monomers and yielding the distance between two atoms and the van der Waals. Therefore, the amount of the shrinkage of the polymerization of open-ring will be smaller than the additional polymerization. In addition, the volume of UV-curable resin that consists of acrylate monomers is known to be shrunk between 3~16% [59,60]. Consequently causes the mismatch from the actual mold. 37

39 1.4.4 Segregation Effect in Initiating Mold Releasing Reducing the surface energy between UV-curable resin (polymer) and mold interface is essential for reducing the adhesion issue in releasing mold during UV-NIL. Therefore, surface treatment of the mold using release agent treatments has been proposed. The best-known technique is a fluorinated anti-sticking layer coating that uses a silane coupling agent on the mold surface [61 63]. Anti-sticking layer coating also known as release coating layer (RCL). The fluorine compound that has groups of silanol groups in the chain of molecular chain will make the connection firmed with hydroxyl groups that present in the mold surface by silane-coupling reaction. However, due to its poor durability, mass production become its challenge. To enhance the durability, the segregation agent that is basically from fluorine-based or siliconbased additive compound in UV-curable resin has been reviewed [64 66]. This additive is similar to an internal release agent that has a segregation effect. The phenomenon of segregation occurs due to the existence of the thermodynamic driving force that is driven by the difference of the energy state in the resin. The difference state of energy weakening the surface of interface energy based on the compound of additive [64,67,68]. The additive component tends to segregate to the interface of the layer of resin during the heat treatment or UV treatment. This phenomenon is also known as the migration effect. The segregation effect is illustrated in Fig Segregation is always related to a Brownian motion which can be associated to other phenomenon which enhance or limit the particle movements [69]. Particles diffusion in a liquid state environment is well defined by the Fick s first and second laws. The tools that used to analyze the effects of the segregation on the surface of the cured resin is like the measurement of the contact angle and X-ray photoelectron spectroscopy (XPS). 38

40 Fig Conceptual schematic of the segregation phenomenon Material Properties of the UV-NIL Mold Glassy carbon (GC) currently has been drawing attention as a potential mold material in fabricating UV-NIL mold. Due to the physical properties of GC, it has a promising outlook in the next generation of lithography technology. Thus, in our study we proposed GC as one of the master molds. The following is the detail elucidation of the GC in terms of its properties and model of structure Properties of GC GC is a turbostratic form of carbon which is produced by carbonizing a polymer under carefully controlled conditions of temperature in the range o C and pressure. The resulting carbon are hard, resistance to high temperature heat treatment and cannot easily be graphitized i.e. non-graphitizable carbon by usual heat treatment. Good electrical conductivity although the conductivity is less than that of graphite. It has a low density of 1.5 g/cm 3 and is reported to have a very low gas permeability [70]. GC has been tested its levels of oxidation with oxygen, water vapor, and carbon dioxide. Results of the test showed that the level of the oxidation of GC was lower than any other carbon. GC also has a high resistance to the reaction of acid. GC is very firm and not affected by any treatment that meted out on it. Under the normal circumstances, graphite is form in powder that made from a mixture of concentrated sulfuric and nitric acid at room temperature [70]. 39

41 Structural Model of GC Fig Schematic of Franklin s structural model of (a) a soft carbon from graphitizable carbon and (b) a hard carbon from non-graphitizable [71]. Figure shows the difference of crystallite growth in carbons between a graphitizing and non-graphitizing carbons. This theory is based on the paper reported by Franklin in 1951 [71]. In this theory, the basic units of the small graphitic crystallites are comprising several layers of string. The strings intersect with each other. By using this theory, the assumption should be made that the crystallite growth rate depends on the movement of the whole or parts of the individual atoms. Crystal growth rate also depends on the orientation of the individual structural units and the intersection between the strings. In the condition of the graphitizing carbon, the structural units are parallel to each other as illustrated in Fig (a). The bond between the adjacent units are considered to be weak. Meanwhile, the condition of non-graphitizing, the individual structural is in random orientation as illustrated in Fig (b). The vigorous bond between the units can prevent the movement of the layers of string. 40

42 Then, Jenkins and Kawamura in 1972 [72] have reported that the non-graphitizing carbon that heat treated at certain temperature will be formed a ribbon-shaped structure as illustrated in Fig The structure of this model is described on the assumption that the molecules of the polymer precursor material retains after the carbonization. The graphitic carbon structure is visible as the fibrils that are curved, rounded, and tangled. Fig Ribbon-like structure of heat treated non-graphitizing glassy carbon. Lc denotes as height of crystallite and La denotes as diameter of crystallite [72]. Therefore, it can be concluded that GC is a shapeless or amorphous, non-graphitizing, and isotropic. The structure of GC is ribbon-like structured shape that twisted, tangled, and intersected. The intersected carbon ribbons create internal voids, consequently contribute to GC porosity structures and low density value. Due to the porous structure of GC, it can easily be formed by selective etching between void areas of GC surface in structuring nanoscale pattern on its surface. 1.5 Motivation of Study Our laboratory succeeded in fabricating AR structure on glassy carbon (GC) substrate with less than 0.1% reflectivity by simple technique of oxygen ion beam etching [25]. In this technique, we acknowledged that oxygen ion beam energy of 500eV formed the finest pitch of conical AR structures, and that an irradiation time of more than 24min could fabricate conical AR structures with heights more than 250nm..This property is very useful in improving the visibility in FPD. In 41

43 this application, large area of AR structures are necessary. This method simply involves irradiation of oxygen ion beam to GC surface, so it is easy to implement and the processing is rapid. In addition, prevention of oblique angle reflection is indicated up to 30mm square. Therefore, this method is suitable for FPD application. However, GC is an opaque material. Thus, it would be required to transfer printing of AR structures on to materials that exhibit transparency such as polymer film. For practical use, we successfully replicated AR film from GC mold with 0.5% reflectivity by using UV-curable resin [73]. The replicated AR structured from self-supporting film with AR properties. The merit of self-supporting AR film is to eliminate the interface reflection that generated between two material substrate i.e. AR-structured layer and film substrate (support film). Figure show the fabricated result of self-supporting AR film fabricated by Taniguchi et al. [73]. Fig (a) SEM images of GC mold and replicated self-supporting AR film. (b) Reflectivities of GC mold and self-supporting AR film. (c) Comparison of the appearance of self-supporting AR film i.e. without support film (poly (ethylene terephthalate) (PET) film) and with support PET film [73]. However, the improvement in method fabrication of AR film from self-supporting AR film is needed in order to enhance the performance of AR film. 42

44 There are several challenges that we have to encounter during the fabrication of AR structure films. The challenges are as follows: Eliminating the difference refractive index between two materials by fabricating the AR structure on film. Fabricating AR film with reflectivity to as low as 0.1% and can transmit 99.9% of the light. Fabricating AR film that excellent quasi-omnidirectional AR properties. Fabricating AR film with an effective method and cost. Mass fabrication of AR film. 1.6 Objective of Study This thesis presents the fabrication of AR film that has great potential in eliminating the interface reflection and transmitting the light at the visible wavelength. In this study, several method in fabricating an excellent performance of AR film is proposed. The fabrication method that used for this work is based on the ultraviolet nanoimprint lithography technique that offer simplicity and low cost. This work has pursued the following objectives: To fabricate the self-supporting antireflection-structured film with the method that can eliminate the interface reflection at the visible light wavelength. Improve the previous method. The target of this study is to reduce the reflectivity to as low as 0.1% and increase the transmittance close to 99.9%. To study the optical properties of the fabricated AR structure film. To study the amelioration method of fabricating an excellent performance of AR film. To ameliorate the lifetime of mold for mass fabrication of AR film 1.7 Layout of the Thesis The chapters of this thesis are as follows: 43

45 CHAPTER 1 provides the introduction of the antireflection (AR) structures that consist of application of AR structures, related theory of AR, and fabrication methods. Then, the phenomena that related to the fabricating AR structure during ultraviolet nanoimprint lithography process is summarized in this chapter. CHAPTER 2 introduce the experimental apparatus that used in this study. Briefly explains the mechanism of the apparatus and the features of the apparatus. CHAPTER 3 presents the study of fabrication method of self-supporting AR nanostructure film by ultraviolet nanoimprint lithography (UV-NIL). In this chapter, the experiment is divided into two parts. The experimental procedure and the discussion of result is presented. CHAPTER 4 presents the mass fabrication of antireflection-structured film by ameliorating the lifetime of AR structure mold using partial-filling UV-NIL technique. In this chapter, the technique of the partial-filling UV-NIL is developed. The experiment is divided into two parts. The amelioration of the release-coated AR structure mold and the release-agent-free AR structure mold is presented. Here, the evaluation of the lifetime of the AR mold to fabricate AR film in mass fabrication is investigated. CHAPTER 5 provides the main conclusions drawn from this work. Suggestion for further studies and future works are also presented. 44

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50 CHAPTER 2 EXPERIMENTAL APPARATUS In this chapter, we introduce the experiment apparatus that we used to conduct the experiment in chapter 3 and chapter 4. The ion beam irradiation machining apparatus was used for fabricating antireflection structure (ARS) mold. Scanning electron microscope (SEM) was used for studying the morphological surface of molds and replicated ARS films. UV-visible-near-infrared spectrophotometer was used to measure the optical properties of the fabricated mold and replicated ARS. High-resolution SEM sample coating system was used to deposit a thin film layer on the sample substrate for creating an electrical conductivity, so that the high quality of SEM image can be obtained. Resistively heated vacuum evaporation system is used to create a thin films on the fabricated mold. Mold-release equipment is used to release the adhered two substrate in contact during UV-NIL. This equipment also provide the measurement of release force or adhesion force. Contact angle meter is used to measure the surface energy of the fabricated sample (mold and replica). Parallel-plate-type UV-NIL machine is used to execute the repetitive UV-NIL uniformly. The details elucidation in terms of mechanism and properties as following. 2.1 Ion Beam Irradiation Machining Apparatus The machine of ion beam irradiation was performed by an ion shower type equipment from ELIONIX Inc. EIS-210R. This machine is designed with three main parts that is electroncyclotron-resonance (ECR)-type ion source, separation grid, and operating room or chamber [1, 2]. The ion beam in this machine can be generated widely which is around 30nm in diameter with 0.3 3keV of ion energy. The electrical insulator of ion beam machine can be produced by neutralizing the ion charge with electron injection method. Electrical insulation is necessary to generate a repulsive force against the incident of ions. Thus, the ion neutralizer is needed to allow 49

51 the thermions emissions in this machine. The thermions emissions are resulted from the heat treated of the tungsten wire that saturated with the ion beam. Figure illustrates the type of ion shower system that equipped by the ECR-type ion source. Fig The mechanism of the ECR-type ion source system ECR-type Ion Source Ions in the ECR machine that generated from magnetic restricted plasma is heat treated by microwave [3-5]. Plasma is classified as a gas of quasi-neutral that can generate the collective action during the electromagnetic fields exposure. In addition, plasma is capable in producing the 50

52 collective movement reaction within a long distance through the interaction of Coulomb. A clear difference between plasma and neutral gas is determined by the amount of charges that independently moving and the ability to produce an excellent conductor. Actually, a phase transformation from gas to plasma that composing of ions, molecules, or electrons cannot be clearly defined. Plasma also has characteristics of liquids that is not easy to be compressed. The admittance of the neutral gas and microwaves into the plasma room will lead to the ionization process. Normally, at this point, a few of free electrons will be circled in the plasma room to perform the helical-shaped orbits. When there is a rapid change of the external electric field (Ẽext) induced by the microwaves, free electrons can obtained the energy easily. This condition occurs when the frequency of gyration (ωce) is equivalent to the frequency of the use of the electromagnetic waves (ωrf). (2.1.1) From eq , B is the magnetic field. The system of the free electron movement that formed an orbital shape is the system of electron-cyclotron-resonance. The resonance layer position in the plasma room is dependent on the strength of magnetic field and the frequency that is generated by the microwaves. Ionization process occurs when the electron plasma is heated and activated for each contravention. The activated plasma will generate the free electrons shower step by step until balancing of the electrons happened. The balance of the electrons in the plasma room is very important in order to get the neutral ion charges Separation Grid The separation grids are assembled and aligned between the plasma source room and the operating room with the high vacuum condition of 0.01Pa. The grids are made up by an array of holes sheets that parallel to each other. The release of the ions from the plasma room to the releasetargeted place is aided by the high voltage that directing the electrons to comply with the lines of magnetic field. The ions release process in the electric field occur rapidly due to the discrepancy between the ion source and the lines of beam. 51

53 2.1.3 Operating Room From the separation grid the ion flux is led to the operating room where the ions freely drift to the work piece holder. The work piece holder is placed on an aluminum work table which can rotate at the speed of 0.67rpm. The stage can be tilted in a range of 0 o to ±90 o i.e. it can be uptilted as well as down-tilted. 2.2 Scanning Electron Microscope (SEM) Operating Principle The topography of the antireflection structure film can be observed by using the scanning electron microscope (SEM) from ELIONIX ERA-8800FE [6]. SEM used the magnification system of the sample surface on the monitor of CRT. The fine electron beam is used to scan the sample surface and the image will be transmitted to the monitor screen. The brightness of the image pixel is displayed on the CRT that yielded from the amount of the secondary electron intensity. The scanning electron microscope will play the role to read the obtained of the secondary electron image. The amount of the vertical lines in the scanned image is approximately Due to the 1/1000 of the time needed to identify the lines of the secondary electron intensity, the monitor scree is segmented into pixels. The image in the digital form can be displayed, analyzed, and saved as a result of the brightness of digitization of each image pixel. Figure shows the mechanism of a SEM. 52

54 Fig The mechanism of a scanning electron microscope Image Resolution High resolution image of SEM will be generated by depending on the size of the scanning of electron beam lines. The 5,000 of magnification image will fill the 100mm 2 area of CRT monitor. Thus, the size of the observation area is 20 m that gained from the calculation of 100 5,000. An assumption is made to prepare the 20nm 2 of area for every pixel by segmenting the 20 m area to 1000 of division in both horizontal and vertical axis. 53

55 One of the major factor of the generation of the high resolution images is depends on the successful of the generated finest of electron beam lines. Each pixel on a CRT monitor will be 0.1mm 2 of area when it is segmented into 1000 in both horizontal and vertical axis. The result of the pixel will be shown in smooth and clear in the resolution of human eye is due to the range of resolution human eye is in mm Image yielded by Secondary Electron The acquisition image from the secondary image is developed by the low energy of electron that comes from the bombardment process of the targeted surface of sample. The possessed secondary electrons will be detected by the detector or sensor which has a positive voltage. The positive voltage will serve to agitate the scintillator to transmit the light. The estimated value of the positive voltage is 10kV. The transmitted light will be processed by the photoelectron multiplier to develop the signal of electric. The developed signal of electric will detect the secondary electrons and will control the electrode to be emitted in a CRT monitor. This phenomenon will also generate the intensity of the brightness in the monitor CRT. The light development mechanism is depicted in Fig Fig The development of the image of secondary electron in the SEM system. High energy will result from the backscattered electron beam that coming from the surface of sample. The same energy value as shown as from incident electron. The backscattered electrons will travel all over the direction of the surface of sample. This condition will cause some of the electron will not be detected because of it sheltered from the detector. Then, the resulted image is 54

56 clear and smooth because there is no interruption from the backscattered electrons during the generation image. The generated image seems to be detected directly by an electron probe. Meanwhile, the generation of the images of secondary electrons is used the same principle as in the reflected electron images. However, the contrast of the generated image is different to each other. The potential of electrical is employed to expedite the rate of the secondary electron detection sensitivity, so that, the resulting image is in high resolution. The secondary electron detector will investigate the radiation exhaled from various angle of directions. The contrast image actually generated by the exposure intensity of the electrons instead of the reflected beam images. Figure illustrates the detection mechanism of the image from the secondary image. Fig Mechanism of the detection of the images from the secondary electrons Image Enlargement and Scanning in SEM System The position between electron beam radiation on the surface of the sample and the emission result at the CRT monitor must be appropriate to each other. This situation is very crucial to ensure that the synchronization occurs between electrons during the scanning of images. Besides, the enlargement or magnification of image also is determined by the ratio of the size of detected electron beam scanning. 55

57 2.2.5 Optical Column of Electron in SEM The optical column of electron in the SEM system has a function to manufacture and transmit the electron beam fibers that will be used in the scanning process of images. Normally the electrons are developed by heating up the filament of tungsten. The filament of tungsten used is from the type of LaB6 filament or ZrO/W filament. Then, the resulted electrons will be concentrated and will be segmented into three levels of electromagnetic lenses. The process of concentration of electron beam will generate focus in the generated images Operating Room of Sample The sample room or also known as chamber is built up with two main parts that are sample stage and the detector device of the secondary electron. The secondary electrons generated as a result of a fine electron beam that emitted into the optical column of the electrons during the scanning process of the sample surface. The generated electrons are assembled and detected by a collector device of secondary electron. These generated electrons will be processed and transformed into the electrical signal and will be sent to the monitor of screen. Fig Operating station of SEM. 56

58 2.3 UV-visible-near-infrared Spectrophotometer The evaluation of optical properties of the sample surface of film was done by utilizing Shimadzu SolidSpec-3700 [7]. It is developed to have a very high sensitivity measurement. This device is equipped by a measurement of deep UV and the spacious sample chamber for operating the evaluation process. The spectrophotometer can handle the application of optical, semiconductor, and flat panel display Excellent Evaluation The evaluation of the optical properties such as reflectance and transmittance required high precision of measurement method. This SolidSpec-3700 Spectrophotometer is designed to have three types of sensors that can detect the spectrum from ultraviolet region up to infrared region. When the evaluation is approaching the infrared region, the sensitivity of the device is enhanced by the use of sensors from the types of InGaAs and PbS. The ability of this device to detect the spectrum from the range of ultraviolet to infrared resulting the high accuracy and high sensitivity of the evaluation value of the optical properties of targeted sample. Compared with the conventional spectrophotometer, the improvement in feature is done by inserting sensor from the type of InGaAs. This sensor capable in providing the highly sensitive, excellent, and precise measurements Deep UV Technique Evaluation The use of ultraviolet laser such as ArF excimer laser can improve the measurement of optical properties in the region of deep ultraviolet. This SolidSpec-3700 allows measurement in the range of nm and integrated with direct detection unit to allow measurement of the range nm. Thus, measurements that obtained more sensitive and precise. The acquisition of the deep UV is due to the ability of the nitrogen in the sample chamber removes the oxygen molecules that interfered. The molecules oxygen in the atmosphere very easy to absorb the ultraviolet light below 190nm. The sensitivity of the measurement can be obtained if the cleaning process of molecules oxygen by nitrogen can be done very well. 57

59 A sufficient amount of light is needed to perform high precision measurements due to achieve the deep UV region. PMT sensor from the type of fused silica is used as a window material and the resin is integrated to act as coating material inside the sample chamber. These two materials is very essential in ensuring the absorption of deep ultraviolet light. In the deep UV condition, the gained spectrum has low noise and the evaluation will be successfully obtained Spacious Sample Chamber The advantage of using SolidSpec-3700 is it has a spacious sample chamber which allows the measurement of any size of sample. The size of the interior chamber is 900mm of width, 700mm of diameter, and 350nm of height. Thus, the maximum size of sample can be measured is around 700mm of width, 560nm of diameter, and 40mm of height. 2.4 High-resolution SEM Sample Coating System High-resolution SEM sample coating system was utilized to coat a thin layer of film on the surface of sample substrate for creating an electrical conductivity. Creating an electrical conductivity is very crucial for the use of SEM observation to avoid the distortion on the image resolution. In this work, we used high-resolution SEM sample coating system from ELIONIX Inc. ESC-101 [8]. This machine is function as a result of the combination between the ion source from magnetron and the ion source from the anode hollow. This device is used to coat the platinum on the surface of the fabricated AR film before observing the pattern using SEM. The type of coating that provided in this machine is from material of platinum or tungsten. This machine is capable to coat the surface of substrate with very small particles that has diameter size around 1nm or less. The thickness of the coating result is in the range of nm. This machine provide a quick coating technique with 3 to 5 minutes after the vacuum condition reached below 1Pa. This quick coating techniques can reduce the damage to the sample surface due to the minimal heat that is used. The heat that used is under 50 o C. This coating technique can coat the entire surface of the sample even to the narrowest profile and the contamination that resulted from the coating also can be reduced. 58

60 2.5 Resistively Heated Vacuum Evaporation System Vacuum evaporation is a method used to create thin films by the condensation and deposition of vaporized atoms and molecules directly from an evaporation source (materials such as compounds or metals) onto a compatible work piece surface by heating the evaporation source in a high vacuum environment. Evaporation sources to heat the evaporation source material are offered in boat, filament, and crucible types. The evaporation source is depending on the type and the shape of intended film deposition materials such as Ag, Al, Au, Cu, Dy, Ni, Pt, Ti Alq3, NPB, LiF, etc. In this study, we used VPC-260F, ULVAC KIKO Inc. [9] for resistively heated vacuum evaporation system. In this system, the evaporation source is designed in boat type. VPC-260F is designed as a compact installation space and easy maintenance. The vacuum chamber can be opened without stopping the main pump. Main pump (oil diffusion pump 200L/sec) is protected by an automatic leak valve in case of a power failure. The ultimate pressure to operate this system is Pa and the evacuation time is Pa/20min. Fig Principle of resistively heated vacuum evaporation system. 59

61 2.6 Mold-release Equipment Mold-release equipment (Mitsui Electric) [10] is used to release the adhered two substrate in contact during UV-NIL. This apparatus is complemented by a mold platform on the bottom plate, a sample platform at the top plat, a push-up-down jig, and a load cell. This apparatus works to contribute a motion of peel-release in one direction during the measurement of the force of release. Normally, when we separate the two plates that are adhered together, the vertical release motion techniques will be used. This technique will cause damage to the pattern on every plate during the release process. Creating a peeling motion technique to this apparatus can reduce the damage of pattern on the plate during the release process from the mold. Although, the aspect ratio of the pattern on the mold is excessively high. The replicated pattern of film that in contact with the mold will place on the mold-release plate by using adhesive tape. Then, the top of the plate that has the adhesive tape will move down close to the bottom plate where the film and mold intact together assisted by the push-up-down jig. After some force is applied to the intact film and mold. The push-up-down jig will move the top plate upwards to release the film from the mold in parallel direction. Fig Mold-release equipment The maximum force that resulted from the release process is called as release force. This release force is measured by the load cell that provided from this apparatus. The angle of peeling 60

62 motion is 12 o and takes 3.5-s of time release to perform the process of release. The speed of the push-up-down jig is 0.086m/s and the speed that provided during the release process is 0.41m/s. The technique of peel-release-motion allows the separation of the film and mold in vertical direction at the same time. This technique can decrease the deterioration of pattern on the film or mold. 2.7 Contact Angle Meter Contact angle meter from DropMaster DM-701 [11] was used to measure the energy surface of the sample. DropMaster DM-701 is produced by Kyowa Interface Science and is used for the purpose of the measurement of contact angle (CA). The measurement technique of CA is used as an indicator to understand the principle of wettability of surface. This technique has been universally used in industry to measure the wettability of the sample surface. This model of contact angle meter is adjustable, easy to manage, and compact. This model is very famous among the researchers to study the related field of surface. This apparatus is automatically controlled by the computer and integrated with the liquid dispenser. The maximum size of the sample surface area that can be measured is mm 2. The computing system in this apparatus will handle the movement of the liquid dispenser onto the surface of the sample automatically with a single command to perform the measurement process. Multiple points of water dropped that automatically commanded by the computer system will calculate the wettability of the sample surface based on the result of the CA. The degree of wettability is shown in Fig Fig Degree of wettability. 61

63 Normally, when a liquid is dropped onto the surface of the treated sample, this will disrupt the condition of its surface tension [Fig ]. Then, the following equation is established; (2.7.1) The eq is known as the Young Equation in the principle of wettability. The tangent that resulting from water droplets on the surface of the sample formed an angle in the surface of the sample. The result of this reaction is called as CA. Figure illustrates the young equation of surface tension. Fig Surface tension according to Young Equation. The half angle method or known as θ/2 Method is applied to measure the CA of the sample surface. In calculating the CA, the equation of is manipulated. From this equation, r indicates the radius of the water droplet, while h indicates the height of the water droplet. CA is obtained from a combination of straight lines that connecting the end of the left and right ends, and the peak of the water droplet that applied to the surface. The water droplets that dropped onto the sample surface can be considered as part of the imaginary sphere. Figure illustrates the droplet of water on the sample surface. From the geometric theorem, is equal to Because of the shape of the water droplet can be considered as the parts of an imaginary sphere in this method, the gravitational effect can be neglected during the measurement of CA. This method also allows the measurement of the small quantity of water droplet. It also can be analyzed rapidly and the calculation can be made easily. 62

64 Fig Water droplet on the substrate and can be assumed as part of a sphere. 2.8 Lifetime Evaluation Machine The machine from parallel-plate type UV-NIL that is assembled by Mitsui Co. Ltd is utilized to evaluate the lifetime of the mold. This machine is capable to imprint repeatedly the patterns on the mold to the targeted film. This machine contributes to the pressing and releasing techniques uniformly. This machine also facilitate the removal of air bubbles during the imprint process. The mechanism of the repeatable UV-NIL machine is illustrated in Fig In this evaluation, the position of mold is adjusted to be parallel to the load cell. The high intensity of UV source is installed in the load cell. The mold is affixed with the adhesive tape onto the sample stage. At this condition, the distance between the mold and targeted film is 0.75 ± 0.25mm. The resin is dropped onto the mold by the dispenser that received the command from the computer system that we setup before executing the process. Then, the jig was then pushed upward by the sample stage, and the load cell pressed the film substrate onto the mold with the desired pressure. After the UV exposure, the jig pulled the stage downward to separate the mold and film at a release speed of 200μm/s. 63

65 Fig Schematic of parallel-plate-type UV-NIL machine. 64

66 REFERENCES [1] Miyamoto, J. Taniguchi, S. Kiyohara, New Diamond and frontier Carbon Technol. 10, (2000) 63. [2] ELIONIX Inc. EIS-210ER, [3] R. Geller, Electron Cyclotron Resonance Ion Source and ECR Plasmas, Institute of Physics Publishing, London, ISBN (1996). [4] R. Geller, Proceeding of the 15 th International workshop on ECR ion sources, Jyvaskyla, 1 (2002). [5] J. Arianer and R. Geller, Annu. Rev. Nucl. Part. Sci. 31, (1981) 19. [6] Scanning Electron Microscope ELIONIX ERA-8800FE, [7] Shimadzu SolidSpec-3700, [8] ELIONIX Inc. ESC-101, [9] ULVAC KIKO Inc. VPC-260F, [10] J. Taniguchi, Y. Kamiya, T. Ohsaki, and N. Sakai, Microelectron. Eng. 87, (2010) 859. [11] Kyowa Interface Science DM-701, 65

67 CHAPTER 3 FABRICATION OF ANTIREFLECTION-STRUCTURED FILMS BY ULTRAVIOLET NANOIMPRINT LITHOGRAPHY Novel approaches in fabricating antireflection (AR) structure [1 9] is essential in the advancement of the nanophotonics field for instance. AR structure serves to eliminate undesirable reflections and escalate the performance of light transmission. Among the most critical applications offered by AR structure is escalating the visual acuity of flat panel display (FPD) [10,11], enhancing the effectiveness of light absorption by solar cells [12 19], increasing the light output of light-emitting diode [20 22], improving the accuracy of data storage, and boosting the visibility of the optical lenses. Encouraged by the biomimetic moth-eye structure [23 25], its needlelike-shaped structure that bulging and tapering is believed to alter the refractive index (RI) difference between two different material types of interface gradually. This phenomenon yields an antireflection property [23 26]. Several approaches are available for fabricating AR structure, for instance, electron-beam lithography [27 29], interface holographic lithography [30 33], photolithography [34], nanosphere lithography [35], colloidal self-assembly [5,6], self-masking etching [36], and so on. Nevertheless, these methods involve expensive and sophisticated apparatus when it comes to mass fabrication. We have discussed briefly in chapter 1. Previously, we have successfully reported a simple fabrication method of AR structure by employing the replication method of mold [11]. We also investigated and analyzed the mold release properties during ultraviolet nanoimprint lithography (UV-NIL) [37]. Our main objective is to fabricate an AR film with excellent performance that can reduce the reflectivity to as low as 0.1% and can transmit almost 100% of light. The major challenge for a successful AR film technology is to eliminate the interface reflection that disrupts the performance and optical properties of the film. In this chapter, by the polymer replication method utilizing UV NIL [38], 66

68 the fabrication of single-sided self-supporting antireflection-structured (SSARS) and double-sided self-supporting antireflection-structured (DSARS) films are discussed in sec. 3.1 and 3.2, respectively. This chapter is structured in two parts. First, we developed and fabricated the self-supporting antireflection-structured film that can eliminate the interface reflection in sec In this work, we targeted to obtain an AR film that has reflectivity less than 0.3% and almost 99% of light transmission. An adhesive material is required in order to apply this replicated AR film to any surfaces. Nevertheless, reflection of interface still arises due to the difference in refractive index between the replicated AR film and the adhesive material. This phenomenon exhibits high reflectivity which affects the optical performance of fabricated AR film. Therefore, a study of film with AR property that is capable to eliminate the interface reflection of the front and back surfaces is required. In the second part, we ameliorated the replication method of AR film in regards to maximizing the performance of AR film by developing the DSARS film in sec This film can eliminate the interface reflection for both side of the films, consequently, lowered the reflectivity to as low as 0.1%. 67

69 3.1 Fabrication of self-supporting antireflection-structured film by ultraviolet nanoimprint lithography Introduction Material from glassy carbon (GC) type was employed as a master mold to fabricate AR films. The etching technique of oxygen ion beam irradiation was applied to self-assemble AR structure on top of the GC mold. GC is a shapeless or amorphous, non-graphitizing, and isotropic. The structure of GC is ribbon-like structured shape that twisted, tangled, and intersected. The intersected carbon ribbons create internal voids. Due to the porous structure of GC, it is easily formed by oxygen ion beam irradiation by selective etching between void area of GC surface [11]. Details of GC are is explained in chapter 1 at sec As a result, we were successfully fabricated AR-structured mold which has reflectivity of less than 0.1%. GC is a non-transparency material, for the practical use, fabrication of AR structure from transparent material is highly required. Thus, the replication of AR structure on to a material of plastic-like by employing UV-NIL technique was studied. We also employed the release-coating technique on top of GC mold in order to avoid the adhesion of resin during UV-NIL. As a result, the AR structure was successfully replicated onto a transparent film of poly (ethylene terephthalate) (PET) [39]. As mentioned in chapter 1, UV-NIL is one of the promising and competent technique for AR structure replication on numerous type of film substrate [40]. The merits of using UV-NIL technique are its simple and room-temperature process, high-throughput, rapid and low-pressure process, and high-accuracy replication of pattern [41]. However, among the obstacles in the fabrication of AR film is to eliminate the interface reflection that disrupts the optical performance of fabricated AR film. Interface reflection frequently arises between the film substrate and the AR structures layer of fabricated AR film, which results in deterioration of AR effect. For the sake of eliminating the high reflectivity on the replicated AR film, the fabrication of self-supporting antireflection structure (SSARS) film becomes possible approach. Previously, we succeeded to fabricate a self-supporting AR-structured film which can reduce the reflection of interface to as low as 0.5% from UV-curable resin by UV- 68

70 NIL. In this method, an intermediator film from film type of polyvinyl alcohol (PVA) material was employed to acquire a layer of self-supporting antireflection-structured film [39]. Nevertheless, the intermediator film that was used to support the replicated AR structure was from a low durability material. Due to the water-soluble synthetic polymer material of PVA film, a few regions from the replicated AR-structured layer were deteriorated and creases appeared during the dissolving process of PVA film. This phenomenon affects the quality of imprinting and the performance of fabricated AR film. The film type from polypropylene (PP) material is suggested to substitute the intermediator film from PVA, in order to enhance the performance of fabricated AR film. This film can give better stability and better releasability during UV-NIL in concerning to get a single layer of selfsupporting film with an excellent AR property. In addition, the characteristic of this film, it is humid-resistant material, weightless, flexible, UV penetrable material, and cost effectiveness. Thus, in this section, we introduced a new approach in fabricating a self-supporting antireflection-structured film by employing the PP film as the intermediator film. We also analyzed the optical properties of the fabricated mold and replicated AR film Theoretical calculation of fabricated AR film s reflectivity In this section, the film reflectivity of self-supporting AR-structured is calculated theoretically before the experimental procedure is executed in the next section. This calculation is very crucial to show that the capability of the self-supporting AR-structured film to eliminate reflection from light of incident. Based on the sub-topic in chapter 1, the moth-eye structures is classified as a surface of sinusoidal contour which can create an effective medium in gradient refractive index (RI). In the theory of the sinusoidal surface profile, the increasing RI along the thickness of the surface profile caused the gradual changes of the graph of the gradient-index film of the AR film. Thus, to calculate the reflection value of self-supporting AR film, the hybridization of theory of gradient-index medium and the multilayer AR theory is made. The eq of double layer AR film is used to show the increase of the RI in the fabricated self-supporting AR film. In this study, the material of the AR film from the UV-curable resin. The type of UV-curable resin that we employed is PAK-01-CL from Toyo Gosei [42] and has RI, The RI of AR structure 69

71 i.e. is assumed at the range of and the RI of air is The illustration of the gradual changes of the gradient-index medium of the AR film is shown in Fig Fig Gradual change of gradient-index medium of self-supporting AR film. By using the eq in chapter 1, the AR structure of self-supporting AR-structured film i.e. conical in shape is segmented into two parts. This assumption is to show the increase of the RI in the AR film during the propagation of the light through the AR film substrate. The height of the AR structures, or referred to as thickness d is assumed as 200nm and the pitch is assumed as 150nm. These values are based on the theory of Clapham i.e. pitch should be less than 150nm and the height of the AR structure should be in the length of nm [27]. The segmentation is divided into two effective RI that are equal in thickness i.e. 100 nm 100nm for every layer. The value of RI for every part of the segment is calculated based on the volume ratio of the conical shape of AR structures. The illustration of the assumption of the value of RI is based on the volume ratio of the conical shape of AR is shown in Fig Fig Illustration of AR structure segmented conical shape s volume ratio calculation. 70

72 After the cone is segmented into two parts equally, we assumed that the radius r of segment a and b are 37.5 and 75.0nm, respectively. By using the equation of volume of cone i.e., the volume of segment a is nm and the volume of segment b is nm. From these values, we simplified that the volume ratio between segment a and b for RI is 1:10. Thus, the value of RI for every segment in the range of is made based on the volume ratio of every segment. This assumption is to show the increasing of the RI during the propagation of light through AR structures. Then, based on the calculation from the volume ratio of the conical shape of AR structure i.e. shape of moth-eye structure, we assumed the 1st RI of effective medium of segmentation is 1.05 and the 2nd RI of effective medium of segmentation is The illustration of segmentation of AR structures into two layers in the self-supporting AR-structured film is shown in Fig Based on these assumptions, the reflectivity of the self-supporting AR-structured film is calculated. Fig Illustration of segmented AR structures of self-supporting AR-structured film in creating the gradient-index medium. The calculation of the self-supporting AR-structured film based on the double layer AR equation as following; 71

73 Based on the theoretical calculation, the reflectivity of the self-supporting AR-structured film can be reduced to as low as 0.02%. This value is to show that, the capability of the AR film to suppress the reflectivity almost near to zero percentage. This excellent performance of AR film can ameliorate the performance of the AR film in any electronic devices Experimental procedure Fabrication of AR mold The outline of the ARS mold fabrication method is illustrated in Fig Initially, the AR structure was self-assembled on top of the mirror-finished glassy carbon from Tokai Carbon Co., Ltd. by etching technique of oxygen ion beam irradiation [Fig (a)] [37,43]. The abbreviation of glassy carbon is known as GC. We used an EIS-210ER (ELIONIX Co.) ion beam apparatus equipped with an electron cyclotron resonance (ECR)-type ion source. Ion beam irradiation was carried out at a microwave power of 100W, ion-beam acceleration voltage of 300V, an oxygen gas flow rate of 4.0sccm, and for an irradiation time of 60 min at room temperature. This apparatus was sucked out and maintained at a background pressure of less than Pa, whereas at the time of oxygen gas introduction it was kept at a pressure of less than Pa. The gap between the sample of GC and the source of ion beam was 170mm. The angle of incident of ion beam was perpendicular to the surface. With this position, the uniformity of the fabrication of AR structure over a region up to 30mm square was possible. The current density of the ion beam was measured by Faraday cup at a distance of 62mm from the source of ion beam. The mechanism of the ECR system as discussed in sec. 2.1 in chapter 2. Afterwards, a 30 nm thickness of chromium layer was deposited on top of GC mold by utilizing the heated system of vacuum evaporation from VPC-260F, ULVAC KIKO Inc. The reaction of oxygen to the chromium layer from the effects of air exposure, convert it to Cr2O3 [44]. The existence of this oxide layer is very crucial in enhancing the performance of the treatment of GC with Optool DSX 1.0wt% of fluorinated silane coupling release agent from Daikin Co. The 72

74 deposited GC mold with Cr was coated by dipping it in release agent as shown in Fig (b). The dipping conditions were as follows: 24-h of dipping time, 100 o C of baking temperature, and 3-min of baking time [41,45]. Fig Outline of ARS mold fabrication. (a) Fabrication of ARS on top of GC by etching process; and (b) Release coating Fabrication of self-supporting antireflection-structured film To acquire the replica film from self-supporting antireflection-structured, the release treatment and UV-NIL process were performed. This replicated film is made up from a single layer of polymer that has AR structure without any supported substrate. Firstly, GC mold was covered by the UV-curable resin of PAK-01-CL from Toyo Gosei [42]. Then, the 200μm thickness of PP film was pressed against the PAK-01-CL and eventually exposed to the UV (λ= 365nm) with 25mW/cm 2 of UV power for 8-s, as illustrated in Fig (a). The force that applied for spreading the resin was only from the weight of the PP film. The replicated PAK-01-CL was peeled 73

75 off faithfully from the PP film with tweezers after the hardening process of PAK-01-CL, as illustrated in Fig (b). Lastly, the replicated AR structure in contact with PP film was released from the GC mold. The force desired during the release action was evaluated in favor of assessing the durability of the PP film during release process. The adhesive force between the interface of PAK-01-CL and PP film was also measured. Nevertheless, this experiment resulted in severe degradation of replicated AR structure. Fig (c) illustrates the forces that measured by release force measurement tool [41]. Fig Outline of self-supporting antireflection-structured film fabrication steps. (a) UV- NIL technique; (b) peel-release of replicated AR-structured layer from PP film; and (d) technique of release of PAK-01-CL supplement with the measurement of release and adhesive forces. 74

76 Evaluation of optical properties The morphology of the replica AR film i.e. self-supporting antireflection-structured film and the GC mold was analyzed by scanning electron microscopy (SEM, ERA-8800FE, ELIONIX Co.) from the angle of top and 75 o tilted views. The reflectivity at the incident light angle of 5 o were also measured by a UV-visible-near-infrared spectrophotometer (SolidSpec-3700, Shimadzu) in the spectrum region of nm for both GC mold and replicated AR film Result and discussion As delineated in Fig , the release force [Fig (a)] is described as the desired force desired to release the PAK-01-CL in contact with PP film from GC mold. While, the adhesive force [Fig (b)] is described as the force between the Pak-01-CL and the PP film. Fig Measurement of release and adhesive forces during durability test of PP film. Figure presents the relation between adhesive force and release force of replicated AR film in contact with GC mold during release of UV-NIL. The graphs shows apparently that the adhesive force is greater than the release force during the UV-NIL process. From the calculation, the average of adhesive force μ was 63.1N, and its standard deviation was to be ± 3.5N. From the graph also, we acknowledged that the release force increment with the increment number of the imprint time. The increment force is due to the mechanical eradication of the release coating during UV-NIL. This phenomenon has been reported by Tada et al. in Ref. [46]. In addition, we found that a strong anchor effect [47] that yielded between the GC mold and AR-structured PP film 75

77 contribute to the increment of the release force. Nevertheless, the release of PP film in contact with AR-structured layer from GC mold without severe damage was proved. This experiment also proved that the stability of PP film as an intermediator film that employed in UV-NIL process results in excellent imprint quality compared to previous research with PVA film. The motion of peel-release of PAK-01-CL from the GC mold was successfully executed. Fig Relationship between release and adhesive forces that act during the UV-NIL release process by replicated AR-structured film from the actual mold i.e. GC mold. Figure shows the top and 75 o tilted views of the fabricated GC mold and the replicated AR structure from self-supporting type film. The white dots identify in the top view of the GC mold conform to the structures of black hole of replicated AR-structured film in the image of top view. The diameter of conical-shaped structure of replicated AR was 60 ± 1nm. This value relatively similar as the hole structures i.e. black region on the GC mold that gives value of 68 ± 0.5nm. These values proves the excellent replication of the AR pattern on the GC mold by UV- NIL. It is also clearly be seen from Fig that the replicated height of 264 ± 1nm was succeeded to be partially imprinted from the height of 421 ± 5nm of actual mold. We speculated 76

78 that the reduction of the height of replicated AR-structured film from the actual mold was influenced by the shrinkage effect [48,49] which resulted from the disclosure of UV. Furthermore, the presence of capillary force [50] from coated GC mold with release agent inhibits the complete filling of high viscosity of PAK-01-CL [42] into the AR pattern on the GC mold. Even though the replicated AR pattern height was smaller than the actual size, the height that emulated was sufficient to exhibit the excellent property of AR. Based on the theory of Clapham [27], concerning to fabricate the AR structure on any substrate, the size of the AR structure must be less than 150nm and the height suggested is between the length of nm. Fig Top and 75 o tilted views of SEM images. (a) GC mold surface; (b) replicated selfsupporting antireflection-structured film. Figure shows the optical properties of the replicated AR-structured film and the GC mold in the wavelength of visible light. From the optical properties graph, the reflectivities values of replicated AR-structured film and the GC mold were 0.3 ± 0.05 and 0.1 ± 0.05% in the spectral 77

79 region of nm, respectively. While the 94 ± 0.5% value of the light that can transmitted through the film in the spectral region of nm was also given by the graph. With the supported PET film that we reported previously [39], the relative reflectance that we obtained from the replicated AR-structured film was relatively high that was 10 14% of reflectivity. Yet, by employing our newly suggested method, we can successfully reduce the reflectivity to as minimum as 0.3% by PP film compared to the previous method that only can reduce the reflectivity to as low as 0.5% by PVA film. Fig Optical properties of the replicated AR-structured from self-supporting type film and the GC mold in the wavelength of visible light. The faithful fabrication of self-supporting AR-structured film was obtained due to the merits of PP film. By employing the PP film as the intermediator film, the peel-releasing of PAK-01-CL without damage can easily be made. Besides, the aim of the industry is to minimize the reflectivity of AR film to as low as 0.1% or less. Even though our approach in this research seems to be close to the targeted aim, further study on the effective fabrication of AR film is highly required. 78

80 3.1.5 Conclusion We have conducted the fabrication of replicated AR-structured from self-supporting type film by UV-NIL technique, which polypropylene (PP) film was employed as an intermediator film. Self-supporting antireflection-structured film is very important in reducing the high reflectivity that caused by the reflection of interface that resulted from two different refractive indices of PAK- 01-CL and PP film. Furthermore, in our recommended technique, the deterioration on the replicated AR-structured from self-supporting type film can be minimized due to the greater adhesive force between the PP film and the PAK-01-CL. This phenomenon assists the UV-NIL process without remaining any UV-curable resin from PAK-01-CL on top of the GC mold during the peel-release of UV-NIL process. As a result, the fabricated self-supporting antireflectionstructured film successfully reduced the reflectivity to as minimum as 0.3% in the wavelength of visible light. Thu, we have demonstrated a cost-effectiveness technique in fabricating ARstructured films from self-supporting type with high resolution and outstanding performance of optical properties. 79

81 3.2 Fabrication of double-sided self-supporting antireflection-structured (DSARS) film by ultraviolet nanoimprint lithography Introduction In sec. 3.1, we have successfully fabricated a single-sided self-supporting antireflectionstructured film or named as SSARS film by the polymer replication technique by UV-NIL [38]. This fabricated AR film can efficiently reduce the interface reflection. The advantages of this method is incomplex procedure, high-throughput, time- and cost-effectiveness. From this method, the reflectivity of replicated AR-structured film only can be reduced to 0.3% [38]. However, our target is to minimize the reflectivity to be as low as 0.1% and enhance the transmittance in the wavelength of visible light. Low reflectivity of replicated AR film is highly demanded to optimize the optical performance of AR film. In this section, we proposed a film that can suppress the reflection from the front and back surfaces. It also can confer excellent quasi-omnidirectional AR properties [51 53] on films. The mechanism of interface reflection suppression is illustrated in Fig Figure 3.2.1(a) illustrates the suppression of reflection in double-sided self-supporting antireflection-structured (DSARS) film, while in Fig (b) delineates the suppression of reflection in SSARS film for comparison. Fig Outline of the suppression of interface reflection on the (a) DSARS and (b) SSARS films. 80

82 To determine the commercial usability of the replicated DSARS film, particularly in photovoltaic and LED surfaces, we also demonstrated the application of the replicated DSARS film to the glass substrate by applying an adhesive material. Therefore, a new method of fabricating the DSARS film by UV NIL is presented. Then, the optical properties of the replicated DSARS film are evaluated Experimental procedure Fabrication of molds For the direct fabrication of the DSARS film, a mold from glassy carbon (GC) and a mold from a transparent polyester (PES) film (Toyobo Cosmoshine A4300) that free from release agent were prepared in sequence. The GC mold is defined as the mold that imprints the needlelike AR nanostructures on back side of the DSARS film, and the film mold is defined as the mold that imprints the needlelike AR nanostructures on front side of the DSARS film. A summary of the molds fabrication method is shown in Fig Figure (a) illustrates the fabrication of the GC mold. First, the needlelike AR nanostructure was fabricated on top of the mirror-finished GC (Tokai Carbon) by ion-beamreactive etching [37,43]. We used an EIS-210ER (Elionix) ion-beam apparatus equipped with an electron cyclotron resonance (ECR)-type ion source. An ion beam was irradiated at a microwave power of 100W, ion-beam acceleration voltage of 300V, and an oxygen gas flow rate of 3.0sccm for an irradiation time of 60min at room temperature. To fabricate a robust needlelike AR nanostructure array on the GC mold, the mold was treated by chromium deposition and fluorine coating; the procedure is described in detail elsewhere in sec

83 Fig Summary of molds fabrication method. (a) Fabrication of needlelike AR nanostructures on top of the GC mold by oxygen-ion-beam etching finished by Cr deposition and release treatment; (b) Fabrication of RAF-ARS mold from X433-3; and (c) Heat treatment of X433-3 mold (RAF-ARS mold). Next, the fabrication of the release-agent-free antireflection-structured (RAF-ARS) mold from UV-curable resin (Autex PARQIT OEX-028-X433-3; hereafter, X433-3) is shown in Fig (b). First, X433-3 was dispensed on top of the GC mold [the same mold as that shown in Fig (a)]. Then, the GC mold was covered with the PES film and illuminated with UV 82

84 (365nm) to obtain X433-3 mold. X433-3 is the amelioration of X433T [54], which is composed of cationic polymerized UV-curable resin and epoxy-modified fluorine resin. The viscosity of X433-3 was 70 mpa-s and it had a pencil hardness of 5H (JIS K5400) after UV curing. The sturdiness of X433-3 after the UV curing is useful for the repetitive UV NIL. The UV conditions for X433-3 were as follows: a PES film area of 5cm 2, filling time of 1 min, UV power of 25mW, and illumination time 60-s, i.e., the UV dose was 1500mJ/cm 2. After the solidification of the X433-3 PES film, it was peeled off and heat-treated at 80 C for 30min [Fig (c)]. X433-3 contains a fluorine polymer and this fluorinated component segregates at the cured resin surface after heat treatment [54] resulting in a release-agent-free layer to facilitate the release process. The detail explanation of the segregation phenomenon is discussed in chapter 4. Furthermore, preparing the transparent mold that can allow UV penetration is very crucial in the fabrication of the DSARS film. The contact angles of the GC mold and X433-3 mold were also examined using a contact angle instrument (Kyowa Interface Science DM-701). This measurement is very crucial to the examination of the antisticking properties of the molds surfaces Fabrication of DSARS film The fabrication of the DSARS film by UV NIL is shown in Fig The DSARS film is a UV-curable resin polymer, where the front and back surfaces were replicated with needlelike AR nanostructures by UV-NIL. The objective of the fabrication of the DSARS film is to produce a film with the quasi-omnidirectional antireflective property on both sides. This property can produce an effective medium between air and any substrate interface [55]. Thus, the performance of the AR film can be optimized. In this work, first, a gap of 100μm was formed on top of the GC mold for controlling the thickness of the DSARS film during UV NIL. Then, the PAK-01-CL was dispensed onto the GC mold. Next, the film mold (thickness 100μm) was pressed against the PAK-01-CL and then exposed to UV (365nm). UV conditions for the PAK-01-CL were as follows: filling time, 1min; applied force, from the 50g weight of the glass substrate; UV power, 25mW; and illumination time, 30-s; i.e., a UV dose of 750mJ/cm 2. After the solidification of PAK-01-CL, the X433-3 mold together with the intact replicated ARS (replicated needlelike AR nanostructures on PAK-01-CL) 83

85 was released from the GC mold. Finally, the replicated PAK-01-CL that covered the top of the X433-3 mold was carefully peeled off using tweezers. Fig Fabrication of DSARS film Evaluation of optical properties The morphologies of the GC mold, X433-3 mold, and DSARS film (replica pattern) were observed by scanning electron microscopy (SEM; Elionix ERA-8800FE) from the top view and 75 tilted view. The optical properties such as reflectivity and transmittance of the DSARS film were measured using a UV-visible-near-infrared spectrophotometer (SolidSpec-3700, Shimadzu) the spectral range of nm Result and discussion Fabrication of DSARS film and its optical properties Figure shows the top view and the 75 tilted view of SEM images of both the GC mold and film mold. From the SEM images, the white dots in the top-view film mold correspond to the hole structure of the top-view GC mold. According to the morphology of the GC and X

86 molds, it can be distinctly seen that the pitch, diameter, and height of the needlelike AR nanostructure on the replica mold (X433-3 mold) were different from those of the actual mold (GC mold). This difference was due to the filling and shrinkage effects during UV-NIL [38]. Fig SEM images of top view and 75 o tilted view of the imprint molds. (a) GC mold; and (b) X433-3 mold. However, despite this difference, both of the imprint molds were relevant to the fabrication of the DSARS film. The contact angles of the GC and film molds were 165 and 114, respectively. This indicates that both imprint molds were superhydrophobic, and therefore, have excellent antisticking surface properties required for obtaining the faithful fabrication of the DSARS film. A low-surface-energy release layer on the mold helps improve the imprint quality and the peeling of the DSARS film during UV-NIL [56]. To demonstrate the AR properties on both sides of molds, reflectivities were measured. Fig shows the reflectivity of the GC and X433-3 molds in the visible light wavelength range. The reflectivity of the GC and film molds were ± and 0.3 ± 0.025%, respectively in the spectral range of nm. 85

87 Fig Reflectivity of the GC and X433-3 molds. Figure shows the surface morphology of the needlelike AR nanostructure on both sides of the DSARS film from the top view and the 75 tilted view. Theoretically, in the case of visible light, the pitch of the needlelike AR nanostructures needs to be less than 150nm, whereas the height should be only nm [27] to conform to the prescribed dimensions of needlelike nanostructures with effective AR [23]. From the SEM images it can clearly be seen that, the needlelike AR nanostructures were successfully replicated at the front [Fig (a)] and back [Fig (b)] surfaces of PAK-01-CL. Figure (c) shows the appearance of the DSARS film. The thickness of the DSARS film was 96.5 ± 0.5μm. The replicated area also appeared to be uniform and considerably free of defects. From calculation, the pitch, diameter, and height of the needlelike AR nanostructures on the front surface of the DSARS film were 76 ± 0.5, 67 ± 1, and 219 ± 1nm, whereas, those of the nanostructures on the back surface of the DSARS film were 72 ± 2, 62 ± 1, and 237 ± 3nm, respectively. 86

88 Fig SEM images of the top view and 75 o tilted view of the DSARS film. (a) SEM image of the front surface of the DSARS film; (b) SEM image of the back surface of the DSARS film; (c) Appearance of the DSARS film. Figure and presents the amelioration of the DSARS film compared with SSARS film (the fabrication of the SSARS film was well-described in our previous report [38]). The details are summarized in Table

89 Table Reflectivity and transmittance of DSARS, SSARS, and unpatterned PAK-01-CL (reference) films. Fig Comparison of reflectivity between DSARS, SSARS, and unpatterned PAK-01-CL (reference) films. 88

90 Fig Comparison of transmittance between DSARS, SSARS, and unpatterned PAK-01-CL (reference) films. Figure shows the comparison of reflectivity between the DSARS, SSARS, and unpatterned self-supporting PAK-01-CL (reference) films. Here, the DSARS film shows a minimum reflectivity of 0.10 ± 0.05% as compared with 0.35 ± 0.10% for the SSARS film and 3.30 ± 0.50% for the unpatterned self-supporting PAK-01-CL. The transmittance study was supplemented by a needlelike AR nanostructure transmittance measurement, as shown in Fig From the graph, it is evident that the transmittance of the unpatterned self-supporting PAK- 01-CL film increased from ± 1.25 to ± 1.25%, a 20.9% increase due to the needlelike ARS on both sides of the PAK-01-CL film. In the case of the SSARS film, transmittance increased by 16.2%, with a maximum transmittance of ± 0.85%. Based on the Lambert Beer law [57,58], we assumed that light was not 100% transmitted during propagation inside the DSARS film because the light was absorbed or scattered out of the beam owing to the thickness of the DSARS film. Furthermore, during the UV curing, the volume 89

91 of PAK-01-CL is known to decrease owing to the free radical polymerization reaction between the acrylate groups in monomers (or oligomers) [49,59 61]. From the graph, it is apparent that the DSARS film offers better suppression of the reflection and transmittance than the SSARS film, owing to the quasi-omnidirectional AR property observed in the DSARS film. In the next section, the evaluation of the applications of the DSARS film with different refractive indices will be explained. This additional experiment was essential to ascertain the impact of the optical properties of the DSARS film when multistacked with another substrate above it Application of DSARS film on substrate with different refractive indices In this evaluation, the fabricated DSARS film was placed on top of the micro-glass slide (thickness 1.5mm; Matsunami Glass) with the UV-curable resin (Autex PARQIT OEX- 004-S1; hereafter, 004-S1; UV dose = mJ/cm2) as the adhesive material. The refractive indices of 004-S1 and micro-glass slide were 1.59 and 1.51, respectively. The purpose of using different refractive indices was to distinguish the performance of the DSARS film from that of the SSARS film we previously fabricated [38]. Then, the reflectivity and transmittance of the multistacked DSARS were measured. We also fabricated the SSARS and unpatterned PAK-01-CL films on the glass substrates for comparison. From the illustration in Fig , we surmise that the DSARS film can better suppress the adhesive interface reflection than the SSARS film. 90

92 Fig Illustration of the performance of DSARS, SSARS, and unpatterned PAK-01-CL films when applied on the glass with the adhesive OEX-004-S1. (a) Unpatterned self-supporting PAK-01-CL in contact with micro-glass slide; (b) SSARS film in contact with micro-glass slide; (c) DSARS film in contact with micro-glass slide. The idea of suppressing interface reflection was proved by measuring the reflectivity and transmittance of each type of film, as shown in Fig and The reflectivity and transmittance are summarized in Table From the comparison graph, it is evident that the DSARS film shows excellent performance. It has only 0.95 ± 0.15% reflectivity and transmits ± 1.10% visible light wavelength compared with the other film. 91

93 Table Reflectivity and transmittance of the optical properties of DSARS film, SSARS film, and unpatterned PAK-01-CL (reference) on a glass with the adhesive OEX-004-S1. Fig Comparison of reflectivity between the DSARS, SSARS, and unpatterned PAK-01- CL films. 92

94 Fig Comparison of transmittance between the DSARS, SSARS, and unpatterned PAK- 01-CL films. From this evaluation, we acknowledged that the refractive-index difference can also cause reflections at the interface between two or more materials. These reflections can lead to significant transmission lost in devices with multilayer stacks. Thus, the fabrication of the DSARS film is a possible suggestion to overcome this obstacle Conclusion We have successfully fabricated AR-structured from double-sided self-supporting antireflection-structured (DSARS) type film by employing UV-NIL technique. The DSARS film is a self-supporting UV-curable resin without an interface and has AR properties on both sides of the surfaces. Utilizing our recommended method, we successfully fabricated the DSARS film with 0.10 ± 0.05% reflectivity in the wavelength of visible light. Furthermore, the DSARS film displays excellent transmission of light with ± 1.25% transmittance at the visible light wavelengths. Finally, we examined the applicability of the fabricated DSARS film by multistacking it with an 93

95 adhesive material and a glass substrate with different refractive indices. Consequently, the concept idea that the fabricated DSARS film can enhance the reduction of the interface reflection was validated. We believed that the DSARS film can improve the efficiency of the solar cells, enhancing the light extraction of LED, for example. Thus, we have demonstrated a simple, timeefficient, and low-cost method of fabricating DSARS films with high resolution and excellent performance. 94

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99 CHAPTER 4 MASS FABRICATION OF ANTIREFLECTION STRUCTURE FILM BY AMELIORATING THE LIFETIME OF ANTIREFLCTION STRUCTURE MOLD BY PARTIAL-FILLING ULTRAVIOLET NANOIMPRINT LITHOGRAPHY There is a great demand of massively fabricated antireflection structure (ARS) by ultraviolet nanoimprint lithography (UV-NIL) [1] to support the fast development in the flexible electronics field. Based on the UV-NIL principle, one of the important elements is the release agent, which is used to inhibit the adhesion of resin on top of ARS mold during replication process. However, extending the lifetime of release agent on ARS mold surfaces is the bottleneck in the mass fabrication process. This is caused by both of the chemical and mechanical factors that contribute to the degradation of the release agent. Tada et al. claimed that that the release agent is mechanically removed during the recurring UV-NIL [2]. Furthermore, Truffier-Boutry et al. also stated that the strikes of radicals on the release agent reduced its density. The radicals are by product of UV curable resin during ultraviolet light exposure of UV-NIL [3]. In this chapter, we found that the solution for ameliorating the ARS mold is by partially filling the UV-curable resin during UV-NIL. This chapter is structured in two parts. First, we ameliorate the lifetime of the AR mold from GC mold that is coated with release agent i.e. release coating layer (RCL) in sec. 4.1 by partial-filling UV-NIL. In this section, we assessed the lifetime of GC mold in the partial-filling UV-NIL. For comparison purpose, the complete-filling UV-NIL was also performed. Producing ARS film using GC mold will allow only one sided fabrication due to the opacity of GC mold. In addition, the fabricated ARS from GC material is brittle. Therefore, to reduce the dependency on GC mold, replicated ARS films from replica mold is highly required. It is also 98

100 necessary to fabricate ARS molds that are free from RCL and from a substance that will offer antisticking effect, antifouling effect, and a tested sturdiness. Thus, the second part, we ameliorate the release-agent-free antireflection-structured (RAF-ARS) replica mold with the same method i.e. partial-filling UV-NIL in sec In this study, we evaluated the replicated ARS films that obtained from the RAF-ARS during repetitive partial-filling UV-NIL. Again, for comparison, complete-filling UV-NIL was also executed. 99

101 4.1 Lifetime amelioration of antireflection structure molds by means of partial-filling ultraviolet nanoimprint lithography Introduction Ultraviolet nanoimprint lithography (UV-NIL) [1] has the potential to be used effectively in the mass fabrication of antireflection-structured (ARS) films. ARS films play an essential role in improving the performance of electronic devices such as displays, LEDs, and solar cells. However, prolonging the lifetime of ARS molds is a challenge for the mass fabrication. Even if a release coating layer (RCL) is applied to the ARS mold, there are still unresolved problems such as the resin filing failure, resin adhesion, and imprint flaws. These problems arise due to the deterioration of the RCL by mechanical [2] and chemical [3] reasons as we mentioned previously. Based on our previous investigation [4], the complete filling of the resin for a high-aspect-ratio mold results in a strong release force (RF). As a result, the RCL rapidly deteriorates and consequently shortens the lifespan of the ARS molds. We also discussed the connection between the resin filling behavior and the vitality of the RCL at different filling pressure in Ref. [5]. In view of the concept of partial filling in Ref. [6 9], we assumed from the results in Ref. [4,5] that the RF and the aspect ratio of the replicated ARS film can be reduced by partially filling the resin during UV-NIL. Thus, prolongs the lifetime of the ARS mold. In this study, the idea of partial-filling UV-NIL is associated with the presence of capillary force (Pc) that acts on the substrate owing to the difference of surface energy and the formation of fine nanostructures [10,11] that affects the filling behavior. The needlelike ARS shape in our study is similar to the shape of the capillary, thus, the equation of Pc that we used in elucidating the phenomenon of resin fillings as following [12]; (4.1.1) 100

102 From the equation, γ is the surface tension of the resin, θ is the contact angle (CA) in the capillary of ARS mold and a is the pitch of the needlelike AR structures on the ARS mold. The diagram of resin filling behavior of ARS mold is elucidated in Fig Fig Phenomenon of resin filling behaviors in UV-NIL: (a) Positive value of capillary force that attracts resin filling in hydrophilic ARS mold indicates as Pc; (b) Negative value of Pc that against the resin filling in hydrophobic ARS mold indicates as Pʹc. In normal cases, with an uncoated hydrophilic ARS mold in Fig (a), the positive value of and the narrow pitches of the needlelike ARS structures generates a positive value of Pc for dragging the resin downward. In contrast, an ARS molds with an RCL, i.e., hydrophobic molds [Fig (b)], is negative and gives a negative value of Pc for pushing the resin upward. This negative value of Pc has been confirmed at nano-scale feature size mold with RCL in Ref. [5,13 15]. The presence of RCL and ARS on molds reduces the surface energy and exacerbates the capillarity that complicates the fillings. Thus, a sufficient filling pressure equivalent to Pc is required to assist the resin filling process. This phenomenon gives merit to the development of the partial-filling UV-NIL technique [Fig (a)]. In addition, the shrinkage effect [16,17] also contributes in shaping the fine pattern of replicated AR film. 101

103 Fig (a) Partial filling with filling pressure P1 i.e. P1 < Pʹc and its replicated result. (b) Complete filling with filling pressure P2 i.e. P2 Pʹc > P1 and its replicated result. Therefore, we established a method to ameliorate the lifetime of the ARS mold through partialfilling UV-NIL [Fig (a)]; complete-filling UV-NIL [Fig (b)] was also performed for comparison. Evaluation on the release properties, i.e. the RF, CA and optical properties of the fabricated partially and completely filled ARS films was performed to determine the lifetime of the ARS molds. 102

104 4.1.2 Consideration of partially and completely filled pressure on the basis of theoretical calculation In this section, we investigated the filling pressure based on the theoretical calculation that was experimentally acquired by Osari et al. in 2010 [13] in favor of initiating the technique of partialand complete-filling UV-NIL. This calculation is very crucial to predict the range of standard value of UV-NIL filling pressure before the experimental procedure is performed. In order to determine the value of filling pressure UV-NIL, we have to calculate the value of Pc of ARS mold. The major challenge in calculating the Pc is to measure the CA inside the capillary of needlelike ARS mold due to the fine-pattern of nanoscale. Based on the completely filled replicated mid-air structure glass result as shown in Fig [13,14], with 500 of aperture size a, the result of the Pc is 0.88MPa. While, the value of CA in the channel is calculated from scanning electron microscope image and the obtained [14]. Fig The schematic model of mid-air structured mold with release agent, (referred Fig. 1 in Ref. [13]). Then, the value of will be used as the constant value of CA in the capillary of needlelike ARS mold [Fig ]. Here, the assumption that the value of = (the schematic of completely filled resin, as shown in Fig (b)) is made. Then, the pitch size of the 103

105 needlelike ARS is the same as the aperture size of mid-air structure i.e. a = 500 and the surface tension of the PAK-01-CL that we will use in the next section is By inserting these values in the eq , the value of as following; Fig The schematic model of a needlelike ARS capillary. Based on the calculation above, the value of Pc obtained is negative, and that shows the Pc is pushing the resin upward during filling UV-NIL. By assuming the complete filling pressure is equal to the Pc, the maximum value of completely filling pressure is 0.07MPa ( ). From this value, we assumed that the maximum value of partial filling pressure will be half of, that gives is 0.035MPa. The possible range of partial-filling UV-NIL pressure is 0.018MPa 0.035MPa. We conclude that, the possible range of filling pressure is within 0.018MPa 0.07MPa. Thus, based on this value assumption, the ARS mold life time was extended through partial-filling UV-NIL execution. 104

106 4.1.3 Experimental procedure Fabrication of ARS mold We prepared two mirror-finished glassy carbon (GC; Tokai Carbon Co., Ltd.) substrates, 15mm 15mm in size as the master molds for the purpose of partial- and complete-filling cases; named as GC A and GC B molds, respectively. The mold fabrication procedure is shown in Fig Needlelike ARSs were fabricated on top of the GC substrate by oxygen ion-beam-reactive dry etching [Fig (a)] [18]. An EIS-210ER (Elionix, Inc.) ion-beam apparatus equipped with an electron-cyclotron-resonance-type ion source was used. In this work, the ARS was selfassembled on the GCs at low acceleration voltage of 300V for 60min of etching condition; details are given in [19]. Fabricated molds at low acceleration voltage can possibly generate small RF during UV-NIL owing to the formation of the narrow pitches and effective heights on the molds. Fig (a) Fabrication of ARS on top of GC by etching process. (b) Fluorine coating to construct a robust ARS mold. To form a robust ARS, the fabricated molds were finished by a 30nm chromium deposition and then treated by 1wt% Optool DSX (Daikin Co.) as the RCL [Fig (b)]. The coating conditions were as follows: a 24-h dipping time, 100- C baking temperature, and 3-min baking time [4,20]. To obtain the same initial condition of both molds, GCs were simultaneously etched and treated with release agent in the same condition and time. 105

107 Repeatable UV-NIL A parallel-plate-type UV-NIL machine (Mitsui Co. Ltd.) was employed to repeatedly imprint the ARS onto a polyester (PES) film (thickness = 100μm; Toyobo Cosmoshine A4300). This machine provides uniform pressing and releasing. Also, it assists in the dissolution of air bubbles. In comparing the ARS mold lifetime trend, the filling pressures to initiate both partial and complete filling of the resin were investigated from the theoretical postulation of the filling pressure in sec i.e MPa 0.07MPa ; as a result, the applied pressures were 0.02 and 0.05MPa, respectively. The UV-NIL conditions were a 15-s filling time, 620-mJ/cm 2 UV dose, and a 5-s irradiation time; the UV-curable resin was PAK-01-CL (62.4 mpa-s viscosity at 25 o C and 30.6mN/m of surface tension; Toyo Gosei Co., Ltd. [21]). Fig Repeated fabrication of ARS film by parallel-plate-type UV-NIL machine. Figure briefly summarizes the repeated UV-NIL mechanism. The ARS mold was aligned to be parallel with the load cell equipped with the high-intensity UV source (λ = 365nm) and was secured on the sample stage using adhesive tape. In this case, the gap between the PES 106

108 film and ARS mold was 0.75 ± 0.25mm. The PAK-01-CL was distributed onto the PES film by the resin dispenser and subsequently moved to be above the center of the ARS mold. The jig was then pushed upward by the sample stage, and the load cell pressed the PES film onto the ARS mold with the desired pressure. After the UV exposure, the jig pulled the stage downward to separate the mold and film at a release speed of 200μm/s Evaluation of ARS mold lifetime The CA and RF during release of the ARS mold after release were measured by utilizing an RF measurement apparatus [4] and a CA instrument (DM-701, Kyowa Interface Science Inc.), respectively. The RF measurement apparatus provides peeling motion in one direction to permit the authentic values of the RF. The distilled water droplet was utilized for CA. The RF and CA values are to indicate the robustness of RCL on the ARS molds. The RF and CA were measured every 10th imprint of the UV-NIL process until the 50th and then at every 25th imprint. Repetitive partial- and complete-filling UV-NIL were stopped when the resin adhered to the ARS mold. The reflectivity at a 5 incident light angle and the transmittance of the partial- and completefilled ARS films were evaluated using a UV visible near-infrared spectrophotometer (SolidSpec- 3700, Shimadzu) over a spectral range of nm. The study of optical properties is used to determine the lifetime of the ARS molds. The morphologies of the ARS mold (before and after UV-NIL), replicated partial-filled ARS, and complete-filled ARS films (PAK-01-CL) were also observed utilizing a scanning electron microscope (SEM; ERA-8800FE, Elionix Co.); both top view and 75 tilted view images were obtained Result and discussion Fabricated ARS molds Figure shows top and 75 tilted views of the ARS molds as master molds. The aspect ratios, reflectivities and surface areas of needlelike ARS of GC A and GC B were 7.75 and 7.94 which are relatively high (>1), 0.18 ± 0.08 and 0.15 ± 0.07% at visible wavelengths, and and m 2, respectively. The detailed calculation of surface area is elucidated in Ref. [22]. The application of the RCL altered the surface from hydrophilic (CA = 6 ± 2 ) to superhydrophobic (CA > 152 ± 1 ) for both molds. These features shows that the ARS molds 107

109 conforms to the prescribed definition of an effective ARS [23] and has an outstanding anti-sticking property for repetitive UV-NIL. Fig SEM image of the master molds: (a) partial-filling use that denotes as GC A mold and (b) complete-filling use that denotes as GC B mold Lifetime amelioration result of ARS mold by partial-filling UV-NIL Figure presents the reliance of the RF and CA on the number of imprints in the instance of partial- and complete-filling UV-NIL. From the graph, it is apparent that the RF escalated and the CA declined as the number imprints escalated for both filling cases, and the partial-filling RF values were smaller than the complete-filling values up to the lifetime of the ARS mold. In the instance of partial and complete filling, the RF escalated from 63.8 to 217N/cm 2 until the 350th imprint and from 79.7 to 165N/cm 2 until the 100th imprint, respectively. The CA of the ARS mold after partial and complete filling declined from 153 to 29.3 until the 350th imprint and from 153 to 92.4 until the 100th imprint, respectively. We consider that the release agent (1wt% Optool 108

110 DSX) on the ARS mold was imbibed to the PAK-01-CL during UV-NIL, leading to the increase in the RF and decrease in the CA. We also assumed that the tendency of rapid ripped off of RCL from the ARS mold is higher in complete-filling UV-NIL compared to partial-filling UV-NIL due to the strong RF. These results indicate that partial-filling UV-NIL extends the lifetime of the ARS mold compared to complete-filling UV-NIL. Fig RF and CA as a function of the number of imprints in the case of partial- and complete-filling UV-NIL. Figure and presents selected SEM images of partial-filled ARSs (150th, 250th, and 350th imprints) and complete-filled ARSs (50th, 75th, and 100th imprints) of PAK-01-CL. The selection criterion of SEM images is based on the difference in quality of replicated ARS for both partial- and complete-filling UV-NIL. Details of the morphologies are summed up in Table

111 From the SEM images, it can be seen clearly that deficiencies happened at the 250th and 75th imprints in the case of partial- and complete-filling UV-NIL, respectively. These deficiencies corresponded to deficiencies on the ARS mold. The heights of the replicated PAK-01-CL features reduced, while the pitches and diameters of the features expanded as the number of imprints escalated. These resulted in lower aspect ratios. This type of analysis is indispensable for detecting defects in the replicated PAK-01-CL and precisely evaluating the lifetime of the ARS mold. From the morphological study (Fig , Fig , and Fig ), we also conclude that partialfilling UV-NIL reduced the anchor effect [24] during release of replicated PAK-01-CLs from the high-density surface area of ARS molds. In the instance of complete-filling UV-NIL, the replicated surface area of complete-filled of PAK-01-CLs was higher compared to partial-filling UV-NIL. The existence of anchor effect during release also disrupted the durability of RCL on the ARS mold. Table Morphologies of partial- and complete-filled ARS films. 110

112 Fig SEM images of partially filled of replicated ARS film by PAK-01-CL. 111

113 Fig SEM images of completely filled of replicated ARS film by PAK-01-CL. 112

114 The optical properties, i.e., the reflectivity and transmittance, of selected partial-filled ARSs (150th, 250th, and 350th imprints) and complete-filled ARSs (50th, 75th, and 100th imprints) of PAK-01-CL over the spectral range of nm are shown in Fig and Fig , respectively. Fig Reflectivity at 5 incident light angle of partially and completely filled ARS films at visible wavelengths. 113

115 Fig Transmittance of partially and completely filled ARS films at visible wavelengths. We found that the transmittance decreased and reflectivity increased the as the number of imprints escalated. This presents the declining trend in optical properties of replicated partial- and complete-filled ARS of PAK-01-CL as the patterns in ARS molds deteriorated. The graph also shows that partial-filling UV-NIL produces ARS films that sustain an outstanding performance until the 150th imprint, i.e., films with a reflectivity of 0.25 ± 0.15% and a transmittance of 94.0 ± 0.50% at visible wavelengths. Though the deficiency of ARS of PAK-01-CL was observed at the 250th imprint for partial-filled UV-NIL, it still displayed good optical properties (reflectivity; 0.5%, and transmittance; 93.5% at the spectral range of 300 nm 750nm). In comparison, ARS films with good optical properties are acquired up until the 50th imprint with complete-filling UV-NIL. Furthermore, the decrease on the aspect ratio of replicated PAK- 01-CL due to the degradation of ARS mold, opposed the prescribed effective AR property (pitch; p < 150nm, and height; 200nm < h < 300nm) [23], thereby leading to the poor performance of the optical properties of replicated PAK-01-CLs. 114

116 Figure presents the relation between of the surface area and the error area ratio of replicated partially- and completely filled ARS films on the number of imprints in the case of partial- and complete-filling UV-NIL, respectively. From the graph, it is apparent that the surface area of replicated ARS films decreased and the error area ratio of replicated ARS films escalated as the number imprints escalated for both filling cases. This graph corresponds to the degradation of the replicated partially- and completely-filled ARS films, which also indicated the lifetime of the ARS mold. This result also presents the changes of the features of the replicated ARS shape owing to the changes of the condition of the ARS mold. As we can see, the replicated error area ratio of the partially filled ARS film gradually increased up to 41% of error until 350th of imprints. While in the case of the completely filled ARS film, the graph shows the rapidly increased about 41% of the error area ratio at the 100th of imprints. Thus, this trend of graph shows that, partialfilling UV-NIL technique can expand the lifetime of the ARS mold and prolongs its lifetime. Fig Surface area and error area ratio as function of the number of imprints in the case of partial- and complete-filling UV-NIL. 115

117 Figure shows the SEM image of ARS mold after the 350th imprint of partial-filling UV-NIL. From the top view of SEM image, we can see the adhesion of resin in the interstices of the ARS mold. Whereas from the 75 tilted view, the damaged area on the surface of ARS mold and the condition of the needlelike structures of ARS can clearly be observed. The reflectivity of ARS mold after repetitive of partial-filling UV-NIL was 1.50 ± 0.75 % in the spectral range of nm. Fig SEM image of the ARS mold after the 350th imprint of partial-filling UV-NIL. We performed this experiment twice, and in both cases, we perceived the same trend of experiment results. We target to achieve 1000 of ARS film imprints per mold by means of partialfilling UV-NIL. Fabricated ARS from GC material is brittle and RCL has a limitation. It becomes a pressing need to fabricate ARS molds that free from RCL and from a material that will offer antisticking effect, antifouling effect, and a tested sturdiness. Our future work will be focused towards this goal. Thus, in sec. 5.2, the amelioration of the replica ARS mold is discussed Conclusion We have achieved lifetime amelioration of ARS molds by means of partial-filling UV-NIL. We successfully fabricated ARS films with exceptional performance until the 150th imprint, i.e., reflectivity of 0.25 ± 0.15% and transmittance of 94.0 ± 0.50% at visible wavelengths, compared to complete-filling UV-NIL, i.e., until the 50th imprint. We examined the resin filling behavior that affects the lifetime of the ARS mold and found that the durability of the RCL plays an 116

118 important role in maintaining the lifetime of the ARS mold during repetitive UV-NIL. However, different resin fillings give rise to different RFs. A strong RF will lead to quick degradation of the RCL and consequently affect the mold lifetime. Thus, we have established an effective technique for the mass fabrication of ARS films that offers simplicity and cost-effectiveness. We also believe that our proposed technique has a great advantage in improving the roll-to-roll nanoimprint lithography (R2R UV-NIL) technique. 117

119 4.2 Lifetime amelioration of release-agent-free antireflection-structured replica molds by means of partial-filling ultraviolet nanoimprint lithography Introduction The fabrication of films that have properties of AR on both sides of surface or known as double-sided self-supporting antireflection-structured (DSARS) films [25 28] has been discussed in detail at sec. 3.2 in chapter 3. This type of film gains great interest of researchers owing to their ability to ameliorate the performance of electronic devices, for instance, photovoltaic devices [29,30] organic light-emitting diodes [31,32], X-ray imaging devices [33,34], and displays. These films offer an outstanding quasi-omnidirectional properties of AR [35 37] in the mechanism of electronic devices. Ultraviolet nanoimprint lithography (UV-NIL) [1,38] is considered to be a practical method for the mass fabrication of DSARS films by deploying cost-effective methods, for example, roll-to-roll (R2R) and roll-to-plate (R2P) nanoimprint lithography (NIL) [39 42]. In order to cut down the intricacy of the fabrication process of DSARS films, a tougher UVcurable fluorinated film-based material replica mold or known as release-agent-free replica mold is indispensable for their direct fabrication. This type of replica mold has a lot of benefits, for example it can contribute to resilience and flexibility, translucent, excellent releasability, and environmental-friendly. A fluorinated component that exists in fabricated antireflection-structured (ARS) replica molds is vital in order to obstruct the adherence of UV-curable resin on the mold during the UV-NIL process [43 47]. We succeeded in managing the fabrication of release-agent-free antireflection-structured (RAF-ARS) replica mold in our previous studies [48]. The RAF-ARS replica mold that we fabricated contains the fluorinated component that was made from an antifouling-effect UVcurable resin. Nevertheless, extending the life-expectancy of the RAF-ARS replica mold becomes one of the obstacles when fabricating AR film in mass fabrication. This issue arises due to the factors of chemical and mechanical that deteriorate the fluorinated components from the surface gradually during the repetitive UV-NIL [2,3]. 118

120 According to the previous research reported by Takahashi et al. [4] that we have discussed in sec. 4.1, force that resulting from the complete filling of the resin into the mold of high aspect ratio will generate the strong release force (RF). The resulted strong RF will possibly shorten the lifetime or life-expectancy of the mold. In addition, the strong RF also resulted from the large surface area of the complete filling of resin. Osari et al. [5] also reported that the difference in filling pressure results in different resin filling behavior in UV-NIL mold. This phenomenon can affect the durability of fluorinated components from the mold. Taking into account the concept of partial-filling of polymer that utilized by Bogdanski et al. [6 9,49], we considered from the results experiment in Ref. [4,5] that partial filling of resin will weaken the RF and reduce the aspect ratio of the fabricated AR film, eventually ameliorates the AR mold lifetime. The motivation behind the breakthrough of the partial-filling UV-NIL technique is associated with existence of capillary force (Pc) that resulted from the alteration character of the substrate surface. Pc is believed to be the outcome of the differences in the surface energy and the construction of fine nanopatterns [10,11,15] that dominates the type of filling behavior [5,13,14,50]. The behavior of fillings resin is depicted in Fig The eq. (4.1.1) i.e. is used for Pc in elucidating the phenomenon of resin filling in the replica mold. Here, a is the pitch of the needlelike AR structures on the replica mold, is the surface tension of the resin, and is the contact angle (CA) in the capillary. 119

121 Fig Analogy of resin filling behaviors in UV-NIL. (a) A positive capillary force indicated as Pc, promotes resin filling in a hydrophilic replica mold. (b) A negative value of Pc, indicated as Pʹc, obstructs resin filling in a hydrophobic replica mold. Without fluorinated components or named as a hydrophilic replica mold in Fig (a), a positive value of and a narrow pitch of the needlelike AR nanostructures produce a positive value of Pc, dragging the resin downward. While, in the instance of the replica mold with fluorinated components or named as a hydrophobic RAF-ARS replica mold [Fig (b)], is negative and a negative value of Pc is produced, boosting the resin upward. The existence of fluorinated components in the replica mold weakens the surface tension and aggravates the capillarity. Therefore, an adequate filling pressure identical to Pc is essential to boost the resin filling process. This phenomenon addresses the urgency for development of a partial-filling UV- NIL technique [Fig (a)]. In addition, the effect of shrinkage [16,17] also plays an important role in molding the finest pattern of replicated result during partial-filling UV-NIL process. 120

122 Fig (a) Partial filling with filling pressure P1, i.e., P1 < Pʹc, and the result of replication. (b) Complete filling with filling pressure P2, i.e., P2 Pʹc > P1, and the result of replication. Thus, we developed a technique to ameliorate the lifetime of RAF-ARS replica molds by partial-filling UV-NIL in this study. Complete-filling UV-NIL [Fig (d)] was also conducted for comparison. The release and optical properties of replica molds and replicated ARS films are evaluated to analyze the lifetime of RAF-ARS replica molds Experimental procedure Fabrication of RAF-ARS replica molds Figure summarizes the fabrication technique of RAF-ARS replica molds. First, we fabricated the master mold. The needlelike AR structures were fabricated on a mirror-finished glassy carbon (GC; Tokai Carbon) substrate of size 15 15mm 2 by oxygen ion-beam-reactive dry 121

123 etching technique, as shown in (a). An EIS-210ER (Elionix) ion-beam machine fitted with an electron-cyclotron-resonance-type ion source was employed. The profiles of the AR structures were managed and manipulated at 300V of 60-min for creating a low acceleration voltage. Then, deposition of chromium and release coating were finished for mold fabrication; the step of the fabrication process of GC mold is explained in detail somewhere [18,22,26]. In the instance of the fabrication of RAF-ARS replica molds by UV-NIL, a UV-curable resin from Autex PARQIT OEX-028-X433-3 resin or named as X433-3 was applied. Initially, the needlelike AR structures from fabricated GC mold was replicated onto a 25 25mm 2 polyester (PES) film (thickness = 100μm; Toyobo Cosmoshine A4300) coated with X433-3 by utilizing the UV-NIL technique, as illustrated in Fig (b). The UV-NIL conditions for X433-3 were a filling pressure of 0.1 MPa, filling time of 15-s, UV dose of 620mJ/cm 2, and UV irradiation time of 60-s. Then, the UV-cured X433-3 that is in contact with PES film was peeled off. The fabricated X433-3 mold was then conducted to proceed with the heat treatment at 85 o C for 30-min in order to finish the process. Three pieces of RAF-ARS replica molds from the same GC mold were prepared for the intention of experimental analysis which indicates as the X433-3 A, X433-3 B, and X433-3 C molds. 122

124 Fig (a) Fabrication of GC mold as a master mold. (b) Fabrication of RAF-ARS replica mold from X433-3 resin. X433-3 is cultivated utilizing a cationic UV-cured epoxy system and builds a hard cured resin that is suitable for repeated UV-NIL. X433-3 consists of a blend of cationically polymerizable UV-curable and epoxy-modified fluorinated segregation agent resins. The cationically polymerizable UV-curable resin is blended with several alicyclic epoxy resins as a basic recipe. A typical alicyclic epoxy resin is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, whose structure is shown in Fig The fluorinated components of X433-3 segregate at the surface boundary of the cured resin after UV curing and heat treatment, producing an antifouling effect at the resin surface, as delineated in Fig , thereby allowing it to act as an RAF-ARS replica mold. This segregation phenomenon arises due to the existence of a thermodynamic driving force that is driven by the energy state in the X433-3 resin, consequently weakening the surface interface energy through its 123

125 fluorinated additive [51 53]. This phenomenon is also named as the migration effect. The viscosity of X433-3 is 70mPa-s and it has a pencil hardness of 5H (JIS K5400) after UV and heat treatment [26,48,54]. Fig Segregation of fluorinated components at the surface boundary of RAF-ARS replica mold Analysis of impacts of resin filling ratio on lifetime of RAF-ARS replica mold Before evaluating the improvement of the lifetime of the RAF-ARS replica mold by repeated UV-NIL technique that is greatly time-consuming, an experimental analysis of the impact ratio of the filling of resin on the life-expectancy of mold was carried out. The RAF-ARS replica mold from X433-3 A mold was employed for this analysis. The filling pressure values of 0.007, 0.02, 0.03, 0.04, 0.05, and 0.06MPa were occupied. A filling time of 15-s, UV dose of 620mJ/cm 2, UV irradiation time of 5-s, and the UV-curable resin from PAK-01-CL type (viscosity of 62.4mPa-s and surface tension of 30.6 mn/m at 25 o C ; Toyo Gosei [21]) were employed to meet the appropriate UV-NIL conditions. Then, the selection of an adequate value of filling pressure that can further improve the life-expectancy of the replica mold was conducted at this step. Also, the 124

126 selected filling pressure values have to be integrated with the definition of an effective AR [23] on the replicated AR films Lifetime amelioration of RAF-ARS replica molds by partial-filling UV-NIL Repetition of partial-filling UV-NIL technique was implemented by employing a parallelplate-type UV-NIL machine that is assembled by Mitsui Co. Ltd. This machine is employed to evaluate the life-expectancy of the RAF-ARS replica mold that has been fabricated. Completefilling UV-NIL was also carried out for comparison. The X433-3 B and C molds were utilized in this evaluation for the purpose of partial- and complete-filling UV-NIL technique, respectively. This machine has the advantage of creating the homogeneous pressing and releasing conditions during UV-NIL process. Also, it can easily assist to get rid of the air bubbles to ensure the faithful replication occurs properly. The mechanism of the repetition of UV-NIL process briefly analogized in Fig Fig Repeated fabrication of ARS films using parallel-plate-type UV-NIL machine. The structures of AR on the X433-3 replica mold were repetitively imprinted onto a PES film. The replica mold of X433-3 was adjusted to be parallel to a load cell that fitted with a high intensity 125

127 of UV source (λ = 365nm). The replica mold was locked on the sample holder by utilizing adhesive tape. The conditions of UV-NIL that employed were identical with the conditions of UV-NIL in the experiment in sec PAK-01-CL was dispensed onto the PES film by a resin dispenser, pressed to the X433-3 mold by the load cell with the desired filling pressure, and then released after UV curing. The occupied filling pressure value was selected based on the result from the analysis in sec The repetitive partial- and complete-filling UV-NIL process were ended when the PAK-01-CL were severely adhered on top of the surface of the replica mold of X433-3 B and X433-3 C molds, respectively. Lifetime assessment of the X433-3 B and X433-3 C molds were evaluated by measuring the value of RF during the release and the value of CA after the release of the replica molds of X The RF value was calculated by RF measurement special apparatus [4] and the value of CA was measured by CA equipment that fabricated from DM-701, Kyowa Interface Science Inc. The RF measurement apparatus can create a motion of peeling in one direction to permit precise values of the RF. A water droplet from distilled water was utilized for the CA assessment. The RF and CA values were recorded at every 10th imprint in the UV-NIL process until the 50th imprint and then at every 25th imprint. The RF and CA values were exploited to validate the existence of fluorinated components on the surfaces of the X433-3 molds during repetitive UV-NIL. The lifetime of the X433-3 molds was also analyzed by the performance of the replicated partially and completely filled ARS films (replicated ARS PAK-01-CLs). The optical properties, i.e., the reflectivity at a 5 incident light angle and the transmittance of the replicated ARS PAK- 01-CLs, were assessed by employing a UV visible near-infrared spectrophotometer (SolidSpec- 3700, Shimadzu) over a spectral range of nm. Morphological studies of the X433-3 molds (before and after repetitive UV-NIL), and replicated PAK-01-CLs were also executed by employing a scanning electron microscope (SEM; ERA-8800FE, Elionix Co.); both top-view and 75 -tilted-view images were acquired. 126

128 4.2.3 Results and discussion Fabricated RAF-ARS replica molds Figure presents top and 75 -tilted views of the fabricated X433-3 molds, and Fig delineates their optical properties. The morphological properties of the AR structures in the GC mold that was employed as the master mold and the X433-3 molds are summed up in Table Fig SEM images of fabricated RAF-ARS replica molds (X433-3 A, X433-3 B, and X433-3 C). 127

129 Fig Optical properties of fabricated RAF-ARS replica molds. Table Morphological properties of AR structures in master and RAF-ARS replica molds. From the SEM images and the graph, it can apparently be examined that the initial conditions of the X433-3 A, X433-3 B, and X433-3 C molds were almost the same. The optical properties of the X433-3 molds also fulfilled the prescribed definition of an effective AR structure [23]. The CA values of the X433-3 A, X433-3 B, and X433-3 C molds were 141, 142, and 141 o, respectively. 128

130 These values denoted that the segregation of fluorinated components that convey the antifouling effect at the surface of the replica molds arisen, resulting in an outstanding hydrophobic surface, permitting the UV-NIL process to be accomplished. In other words, these features ensured the performance of the X433-3 molds as RAF-ARS replica molds and their aptness for further evaluation of lifetime Analysis result of effects of resin filling ratio on lifetime of RAF-ARS replica mold Figure presents the filling ratio and surface area as a function of filling pressure for the replicated ARS PAK-01-CLs. Based on the SEM images, the surface areas of the replicated ARS PAK-01-CLs were calculated from the formula, where is the radius of the AR structure and is its height [22]. Figure presents the dependence of the reflectivity of the replicated ARS PAK-01-CLs on the filling ratio at different filling pressures. The characteristics of the replicated ARS PAK-01-CLs are summed up in Table From Fig , it can apparently be observed that the filling ratio and surface area of the replicated ARS PAK-01-CLs increment with the increment of filling pressure. From this result, we believed that when the surface area of resin adhesion (PAK-01-CL) expanded, the adhesion force also elevated. This phenomenon is known as the anchor effect [22,24,55,56]. The increment in the strength of the anchor effect on nanoscale AR structures will drive to a strong RF during UV-NIL. The strong RF gradually interrupts the condition of fluorinated components at the surface of the RAF-ARS replica mold, disturbing its lifetime. From the graph, we understand that by the partial filling of PAK-01-CL at 0.02MPa filling pressure, the lifetime of the RAF-ARS replica mold can efficiently be improved. The result also revealed that in the case of 50.5% partial filling of PAK-01-CL in the replica mold, the surface area of the replicated ARS was successfully minimized. The reflectivity exhibited by the replicated ARS PAK-01-CL also satisfies the prescribed effective AR property (pitch p < 150nm and height 200nm < h < 300 m) [23]. Therefore, for further lifetime assessment, filling pressures of 0.02 and 0.05MPa were selected to create partial- and complete-filling UV- NIL, respectively. In the next section, the lifetime amelioration of RAF-ARS replica molds is discussed. 129

131 Fig Relationship between filling pressure, filling ratio, and surface area of replicated ARS PAK-01-CL. 130

132 Fig Dependence of optical properties of replicated ARS PAK-01-CL on filling pressure. Table Dependence of morphological properties of replicated AR structures on filling pressure. 131

133 Lifetime amelioration result of RAF-ARS replica molds by partial-filling UV-NIL Figure presents the dependence of the RF and CA on the number of imprints in the instances of partial- and complete-filling UV-NIL. The existence of the fluorinated components at the surface of the X433-3 molds was determined by the values of RF and CA. The graph presents that the RF escalated and the CA declined as the number of imprints escalated for both types of filling. Fig RF and CA as functions of the number of imprints in the case of partial- and complete-filling UV-NIL. In partial and complete filling, the RF escalated from an initial value of 84.2N/cm 2 to 221N/cm 2 after the 200th imprint and from an initial value of 123N/cm 2 to 303N/cm 2 after the 150th imprint, respectively. The CA of the X433-3 molds after partial and complete filling declined from 141 to 58.9 until the 200th imprint and from 140 to 59.8 after the 150th imprint, respectively. Partial and complete filling UV-NIL could no longer be executed after the 200th and 150th imprints when the PAK-01-CL critically adhered on top of the X433-3 B and C molds, respectively. We consider 132

134 that the fluorinated components on the X433-3 molds were imbibed on the PAK-01-CL during repetitive UV-NIL [4], weakening their hydrophobicity, i.e., expanding the surface energy and anchor effect of both X433-3 replica molds. This mainly accounted for the increment in the RF and the reduction in the CA values. In comparison, the partial-filling RF values were smaller than the complete-filling values up to the lifetime of the X433-3 molds. Thus, it is translucent that partial-filling UV-NIL ameliorates the lifetime of the ARS replica mold compared with completefilling UV-NIL. Figure presents selected SEM images of partially filled ARSs (100th, 150th, and 200th imprints) and Fig presents selected SEM images of completely filled ARSs (75th, 100th, and 150th imprints) of PAK-01-CL. The selection criterion of the SEM images is based on the difference in quality of the replicated ARS PAK-01-CLs between partial- and complete-filling UV-NIL. From the SEM images, it can be seen clearly that defects happened at the 150th and 100th imprints in partial- and complete-filling UV-NIL, respectively. These deficiencies corresponded to deficiencies on the X433-3 molds due to the increment of RF values. The morphological properties of the replicated ARS PAK-01-CLs are summed up in Table From the table, it can be seen that the heights of the replicated PAK-01-CLs declined while the pitches and diameters of the features escalated as the number of imprints escalated for both types of filling. We believe that the AR structures of the X433-3 molds were shrunk by the polymerization [16,17] during repetitive UV-NIL for both types of filling, which affected the replicated ARS PAK-01-CLs. Such analyses are crucial for identifying deficiencies in replicated PAK-01-CLs and accurately assessing the lifetime of X433-3 molds. 133

135 Fig SEM images of partially filled of replicated ARS film by PAK-01-CL. 134

136 Fig SEM images of completely filled of replicated ARS film by PAK-01-CL. 135

137 Table Morphological properties of AR structures in partially and completely filled ARS films. The optical properties, i.e., the reflectivity and transmittance, of selected partially filled ARSs (100th, 150th, and 200th imprints) and completely filled ARSs (75th, 100th, and 150th imprints) of PAK-01-CL over the spectral range of nm are shown in Figs and , respectively. The reflectivity escalated and the transmittance declined as the number of imprints escalated. This presents the degradation in the optical properties of the replicated partially and completely filled AR structures of PAK-01-CLs as the patterns in the X433-3 molds degraded. The graph also presents that partial-filling UV-NIL produces ARS films that provide an outstanding performance until the 100th imprint, i.e., films with a reflectivity of 0.30 ± 0.15% and a transmittance of 93.0 ± 0.50% at visible wavelengths. Even though a deficiency in the ARS of PAK-01-CL was discovered at the 150th imprint for partial-filled UV-NIL, it still offered good optical properties (reflectivity 0.40 ± 0.25% and transmittance 92.5 ± 1.50% at visible wavelengths). In comparison, ARS films with good optical properties were obtained up until the 75th imprint for complete-filling UV-NIL. In addition, the reduction in the aspect ratio of the replicated PAK-01-CLs due to the degradation of the X433-3 molds drove to the non-satisfaction of the prescribed effective AR property (pitch p < 150nm and height 200nm < h < 300nm) [23], thereby driving to poor optical properties of the replicated PAK- 01-CLs. 136

138 Fig Reflectivity at 5 incident light angle of partially and completely filled ARS films at visible wavelengths. 137

139 Fig Transmittance of partially and completely filled ARS films at visible wavelengths. Figure presents the relationship of the surface area and the error area ratio of replicated partially- and completely filled ARS films with the number of imprints in the case of partial- and complete-filling UV-NIL, respectively. From the graph, it can clearly be seen that the surface area of replicated ARS films decreased and the error area ratio of replicated ARS films increased as the number imprints increased for both filling cases. This graph corresponds to the degradation on the RAF-ARS mold as the number of imprints increased during the replication of the ARS films. The changes of the surface area during the imprints process proved the changes of the features of the replicated results of the ARS films. From the graph, the result of error area ratio is added to show the error that occurred during the imprints process for both filling cases. In the case of partialfilling UV-NIL, the maximum error that occurred is up to 57% at the 200th imprint. While in the case of complete-filling UV-NIL, the graph shows 52% of error at the 150th imprint. From the result, we concluded that the partial-filling UV-NIL technique can expand the lifetime of the RAF- ARS mold and prolongs its lifetime. 138

140 Fig Surface area and error area ratio as function of the number of imprints in the case of partial- and complete-filling UV-NIL. Figure presents SEM images of the X433-3 B mold after the 200th imprint in the instance of partial-filling UV-NIL. From both the top-view and the 75 -tilted-view SEM images, we can observe the adhesion of PAK-01-CL in the interstices of the AR structures of the X433-3 B mold. This proves that the deterioration of the fluorinated components drives to a strong adhesion force between the PAK-01-CL and the replica mold. We also believe that the shrinkage effect acting on the X433-3 B mold disrupts the position of the fluorinated components and affects the lifetime of the X433-3 B mold. After repeated partial-filling UV-NIL, the height, diameter, and pitch of the AR structures in the X433-3 B were 372 ± 5, 85 ± 2, and 98 ± 3nm, respectively, meaning that the X433-3 B mold shrunk by 8 9% as a result of polymerization. Also, the reflectivity of the replica mold after the 200th imprint is relatively high, i.e., 1.20 ± 0.25% in the spectral range of nm. 139

141 Fig SEM images of the ARS replica mold, i.e., X433-3 B mold, after the 200th imprint in the case of partial-filling UV-NIL. We conducted this experiment twice, and determined the same trend in the experimental results for both types of filling. Our objective is to obtain 1000 ARS film imprints per RAF-ARS replica mold by partial-filling UV-NIL and maintain their low reflectivities. Nevertheless, the release properties of X433-3 are insufficient for practical purposes. To further improve the lifetime of RAF-ARS replica molds for repetitive UV-NIL, a tougher resin with better release properties than X433-3 is extremely needed. Our future work will be centered towards accomplishing this objective Conclusion The lifetime amelioration of RAF-ARS replica molds by partial-filling UV-NIL was carried out. As indicated by the outstanding AR properties of replicated ARS films, we successfully extended the lifetime of RAF-ARS replica molds fabricated from X433-3 resin by partial filling to the 100th imprint, compared with the 75th imprint for complete-filling UV-NIL. We also analyzed the effects of the resin filling ratio on the lifetime of the RAF-ARS replica mold and figured out that a different surface area of replicated ARS films leads to a different RF value. A strong RF resulting from a strong adhesion force drives to the deterioration of the fluorinated components, consequently disturbing the lifetime of the RAF-ARS replica mold. Thus, we have demonstrated an effective technique for the mass fabrication of ARS films that is also simple and cost-effective. This technique is expected to be highly advantageous in improving the roll-to-roll nanoimprint lithography (R2R UV-NIL) technique. 140