Pattern Formation in PMMA Film Induced by Electric Field

Transcription

1 Pattern Formation in PMMA Film Induced by Electric Field OLEKSIY LYUTAKOV, 1 IVAN HÜTTEL, 1 VÁCLAV PRAJZLER, 2 VÍTĚZSLAV JEŘÁBEK, 2 ALEXANDER JANČÁREK, 3 VLADIMÍR HNATOWICZ, 4 VÁCLAV ŠVORČÍK 1 1 Department of Solid State Engineering, Institute of Chemical Technology, Prague, Czech Republic 2 Faculty of Electrical Engineering, Department of Microelectronics, Czech Technical University in Prague, Prague, Czech Republic 3 Faculty of Nuclear Sciences and Physical Engineering, Department of Physical Electronics, Czech Technical University in Prague, Prague, Czech Republic 4 Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rez, Czech Republic Received 14 November 2008; revised 10 March 2009; accepted 26 March 2009 DOI: /polb Published online in Wiley InterScience ( ABSTRACT: Micro-sized patterns were created on thin poly(methyl methacrylate) (PMMA) films by the effect of external field, perpendicular to the film surface. The PMMA film, prepared by spin-coating onto Si wafer, was heated to the fluid temperature (275 C) and a linear pattern was created by the effect of electric field produced by a strip electrode. In another experiment, a round pattern was created as a result of local laser heating of the PMMA film under homogenous electrical field. The created patterns were analyzed by optical microscopy and profile meter. The dependence of the form and size of the created patterns on the intensity of the electric field, exposure time, and initial film thickness was examined. Wave guiding property of a linear pattern, produced by the above technique, was examined in a simple experiment. VC 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: , 2009 Keywords: optics; spin coating; thin films INTRODUCTION Polymers, with their satisfactory light-guiding parameters and transparency at all important communication wavelengths, can be used for fabrication of optical components such as waveguides. Polymer films can be spin-coated onto most optical materials and they can easily be integrated in various structures. Large thermo-optic coefficient and large photosensitivity are other Correspondence to: V. Švorčík ( vaclav.svorcik@ vscht.cz), Vol. 47, (2009) VC 2009 Wiley Periodicals, Inc. useful properties of polymers which could be utilized in construction of optical devices. There are several well established methods of patterning of polymer surface (e.g., nanoimprint lithography and soft lithography). One relatively new, promising technique for polymer patterning is a process based on hydrodynamic instabilities of liquid polymer surface induced by external electric field. The effect of electric field on the stability of fluid surfaces has been an area of extensive experimental and theoretical research from 60 ties. More recently, there has been a renewed interest in the area due to possible use of these phenomena for creation of well-controlled patterns on thin polymer films subjected to external electric field. A lot of experimental work has been done by Schaffer 1131

2 1132 LYUTAKOV ET AL. and coworkers 1 4 and by Chou et al. 5,6 Theoretically, the electric field induced instabilities and pattern formation in thin liquid films was studied, e.g., by Sharma and coworkers. 7,8 Here, we describe results of our recent experiments on electric field assisted patterning of thin PMAA films. The main goal of the work was creation of a linear wave-guiding structure and examination of its parameters in dependence on experimental conditions. In another experiment, creation of spatially limited pattern induced by local laser heating of polymer film under homogenous electric field was demonstrated. EXPERIMENTAL The present experiments were performed on poly (methyl methacrylate) (PMMA) of optical purity supplied by Goodfellow. The glass transition and fluid temperatures (T g ¼ 112 C, T f ¼ 275 C) of the polymer were determined by the standard calorimetric method using DSC 2920 technique. The molecular weights of polymer (M w ¼ 1,459,000, M n ¼ 708,000) were measured with typical uncertainty of 5% by gel permeatic chromatography (GPC) technique. PMMA films, ,000 nm thick, were spin-coated (1500 rpm) from wt % solution of PMMA in chloroform onto a silicon wafer [crystallographic orientation (100), resistance X cm, refractile index n ¼ 3.505]. 9,10 Figure 1(A) shows a schematic of our experimental setup. The PMMA film on the bottom was exposed to electric field created between upper strip electrode (50 lm wide and several lm long) deposited on a glass plate and Si backing. Strip electrode was prepared onto glass substrate using photolithography method. The distance from the upper electrode to the PMMA film was about 10 lm and the field intensities do not exceed a breakdown limit of about V/m. The whole assembly under electric field was heated in an oven to PMMA fluid temperature of 275 C 5 C. After exposure, the sample was allowed to cool down spontaneously to room temperature under the continued effect of the electric field. Figure 1(B) shows a schematic of another experiment on the effect of local heating of PMMA film under homogenous electric field. The PMMA film on Si substrate was situated in the device upper part. The local heating was accomplished by irradiation with the light from 10 W laser Synrad 48-1S emitting at 10.6 lm wavelength. At this Figure 1. Schematic representation of the experimental setups: (a) PMMA film spin coated onto Si substrate (bottom) is viewed by glass mask with metallic strip electrode. Whole assembly is heated on the temperature 275 C 5 C and electrical field is applied between conductive Si substrate and metallic strip on the mask. (b) PMMA film spin coated onto Si/ SiO 2 substrate is viewed by flat metallic electrode. Homogenous electric field is applied to the full area of the polymer and simultaneously one part of the film is heated by laser radiation coming from above. wavelength, the light is not absorbed in the Si substrate and only PMMA film is heated. The laser was RF pumped with modulation frequency of 5 khz and the average power of the laser beam depends linearly on duty cycle variation. The laser beam was focused by a spherical ZnSe lens to a spot with a diameter of 40 lm. Homogenous electric field was created between Si substrate and bottom metal electrode. The shape and properties of the resulting structures created on the PMMA layer were studied using optical microscope Olympus. Profiles of the structures were measured by profile meter Hommel 1000 and Talystep (standard deviation 10%). The wave-guiding properties of the prepared linear structures were studied with 630 nm laser light delivered by optical fiber (PMMA has negligible internal absorption of 630 nm light). Optical loss was determined using a dispersive method and a CMOS camera. The loss was calculated from the change in intensity of dispersed light as measured along the structure. RESULTS AND DISCUSSION Application of local electric field produced by the strip electrode to liquid PMMA film results in creation of a linear structure, the evolution of which as a function of the exposure time, is illustrated by microscope images in Figure 2. It is seen from

3 PATTERN FORMATION 1133 Figure 2. Microscopic images of PMMA structures prepared under the electric field of V/m (at 275 C 5 C) taken at various exposure times (min). the interference patterns that the structure growth starts at random spots which later join each other and homogenous and continuous structure-channel is created after about 60 min exposure. For non-absorbing materials, the interference is generally a function of the film thickness and the refraction index, the later being responsible for just few percent of the whole effect. In the present case, rich interference pattern with many interference lines reflects mostly radial variation of the structure thickness. Typical cross section profile of the linear structure created by exposing PMMA film to the electric field of V/m is shown in the inset of Figure 3. The interaction with local electric field results in a redistribution of PMMA mass. The resulting profile consists of Gaussian shaped central part where the PMMA mass was pulled above initial film surface and depleted wings. The height of the profile central part as a function of the applied electric field is shown in Figure 3. The intensity of the electric field was increased up to the electric breakdown occurring at V/m. As could be expected, the height is a rapidly increasing function of the intensity of the electric field. The final height of the structure results from local equilibrium between electrostatic force and surface tension of liquid PMMA. Under the same experimental conditions, the elongation of the structure should increase with increasing electric field intensity up to the moment when an equilibrium between electrostatic force and Figure 3. Dependence of the height of the linear structures prepared under temperature 275 C 5 C on an applied electric field. The insert shows the typical profile of a channel prepared at the electric field V/m.

4 1134 LYUTAKOV ET AL. Figure 4. Dependence of the height of the linear structures on the initial thickness of PMMA film. The structures were prepared at the temperature of 275 C 5 C and electric field intensity V/m. surface tension is achieved. In contrast to some earlier experimental studies, 3 a mechanical contact between elongated PMMA film and upper electrode was not achieved in the present experimental arrangement. Repeated experiments performed under the same conditions show good reproducibility of the procedure; the parameters of the created structures (width and height) vary by less than 15%. The longitudinal parameters of the created linear structures were controlled by profile meter and the height and width of the structure were found to vary within 10% along the whole structure length. Within these limits, the produced structures can be considered as homogenous and continuous. Another parameter, which could affect the parameters of the created structure is the initial thickness of the PMMA film and in turn the amount of material which can, in principle, be incorporated into the structure. The height of the structure produced at the electric field intensity of V/m was examined as a function of the initial film thickness and the result is shown in Figure 4. For initial film thicknesses, from 0.5 to 7.0 lm, the height of the prepared structure is a linearly increasing function of the layer initial thickness. For thicker layers, saturation takes place and the resulting structure height does not depend on the initial film thickness. Obviously, the total amount of the PMMA material which couldbeincorporatedintothestructureisstrongly limited in the present experimental arrangement. Figure 5. Binding and guiding of laser radiation (wavelength 630 nm) in curvilinear channel waveguide (a). The laser radiation was supplied into the waveguide structure using face to face coupling method. The arc was prepared at the electric field V/m and temperature of 275 C 5 Cby application of mask with curvilinear strip electrode (b). The height and width of the structure, measured by profile meter, were 7 and 75 lm, respectively. [Color figure can be viewed in the online issue, which is available at To examine light-guiding properties of the PMMA linear structures created by the present procedure, a curvilinear structure [refer microscopic image in Fig. 5(b)] was prepared on the PMMA film, 7 lm thick spin-coated on Si wafer with a SiO 2 layer (cladding). The wave-guiding properties of the structure were examined by feeding it with 630 nm laser light (refer experimental details). The optical loss was estimated from the intensity drop of dispersed light along Figure 6. Microscopy image of the structure created on the PMMA film by local heating with laser light under electric field of V/m (a). Height profile of PMMA structure prepared with this method as measured by profile meter across the structure center (b). [Color figure can be viewed in the online issue, which is available at

5 PATTERN FORMATION 1135 the structure length [refer Fig. 5(a)]. Observed optical loss about 0.85 db/cm is acceptable for potential usage in integrated optics. The stability of the structure and its properties were checked in an experiment when the system was kept at 100 C for 100 h and no measurable changes in the structure shape and properties were observed. The last experiment should demonstrate that spatially limited structures can be prepared by local melting of PMMA film subjected to homogenous electric field. The experimental details were described above. The result obtained on PMMA film 0.5 lm thick laser heated under the electric field intensity of V/m is illustrated by microscopy image shown in Figure 6(a). As can be seen from the interference pattern, the created structure copies the laser light spot about 40 lm in diameter. In Figure 6(b), the height profile of the structure measured across its center is shown. Contrary to the linear structures produced before no side wings depleted of PMMA are observed in this case. This may indicate that by this procedure, a hollow, bubble-like structure is produced. CONCLUSIONS Linear structures were created by the effect of locally limited electric field in thin PMMA films heated to fluid temperature. The form and parameters of the structures were examined as a function of the exposure time, electric field intensity, and the initial thickness of the film. It was shown that with increasing exposure time, the structure evolves from comprising discrete islands to continuous and homogenous one at longer exposures. With increasing intensity of electric field, the height of the structure increases but no mechanical touch between the structure and the opposite electrode was observed in the present experimental arrangement. As a function of the initial film thickness, the resulting height of the structure increases with the increasing thickness until saturation is reached for the initial thickness of about 7 lm. The present results confirm the experimental data obtained earlier under different experimental conditions. For the first time, it was demonstrated that a spatially limited structure can be created by local laser heating of the PMMA layer under homogenous electric field. Curvilinear structure was created on Si/SiO 2 substrate and its light-guiding property was demonstrated for the first time. This work was supported by GA CR under the project 102/06/0424, Ministry of Education of the CR under Research Programs , /7, and LC06041, and GAAS CR under the projects KAN and A REFERENCES AND NOTES 1. Schaffer, E.; Thurn-Albrecht, T.; Russel, T.; Steiner, U. Nature 2000, 403, Schaffer, E.; Thurn-Albrecht, T.; Russel, T.; Steiner, U. Eur Phys Lett 2001, 53, Morariu, M.; Voicu, N.; Schaffer, E.; Lin, Z.; Russel, T.; Steiner, U. Nat Mater 2003, 2, Schaffer, E.; Harkema S.; Roerdink, M.; Blossey, R.; Steiner, U. Macromolecules 2003, 36, Chou, S. Y.; Zhuang, L. J Vac Sci Technol B 1999, 17, Chou, S.; Zhuang, L.; Guo, L. Appl Phys Lett 1999, 75, Shankar, V.; Sharma, A. J Colloid Interface Sci 2004, 274, Verma, R.; Sharma, A.; Kargupta, K.; Bhaumik, J. Langmuir 2005, 21, Švorčík, V.; Lyutakov, O.; Huttel, I. J Mater Sci Mater Electron 2008, 19, Lyutakov, O.; Švorčík, V.; Huttel, I.; Siegel, J.; Kasálková, N.; Slepička, P. J Mater Sci Mater Electron 2008, 19,