Nanostructure Fabrication Using Laser Interference Lithography

Nanostructure Fabrication Using Laser Interference Lithography ANTONIO JOU XIE, Class of 2012, Major: Electrical Engineering Mentor: SANG-WOO SEO, Professor, Electrical Engineering ABSTRACT: The constructive and deconstructive property of light is used in this experiment to create nano-grating patterns on Silicon Nitride samples coated with positive photoresist S1805. The set up for this experiment involves the use of Lloyd s Mirror Interferometer to achieve the interference patterns. Single and double exposure of samples led to uniform and continuous patterns that were 1-dimensional and 2-dimensional. These patterns were created successfully with grating periodicity ranging from 700nm to 300nm. This technique can be further applied to different types of photoresist to create such patterns. The value of these patterns lies in its uses and application in several nano-devices. KEYWORDS: photonic crystals, grating, nanostructures I. INTRODUCTION Over the past few decades, the field of nanotechnology has been an area of enormous interest for research. By fabricating structures in the nanoscale, 10 to 100 times the size of individual atoms, materials behave very differently from their corresponding bulk properties [1]. This ability gives way to new fields and allows exciting applications. Periodic nanostructures are a category of nanostructures that have a geometry in which the material properties repeat in space over a certain period. They have broad applications in photonic and electronic devices as well as in the field of biotechnology [1]. Artificial structuring of a material in a nanoscopic scale creates unique electrical and optical properties that can be strongly modified from its bulk properties. For example, photonic crystals are periodic nanostructures that are designed to control the motion of photons, similarly to the way that periodicity of a semiconductor crystal affects the motion of electrons. Periodic dielectric or metallic nanostructures affect the propagation of electromagnetic waves to form photonic bandgaps for which the propagation of electromagnetic waves is forbidden in all spatial directions. This unique optical phenomenon led to many fundamental discoveries and novel device applications. Some examples are inhibition of spontaneous emission [2], antireflection structures [3, 4], surface plasmonic resonance [5], and low-loss waveguiding structure [6]. The sustainable growth in this area is closely related to low cost, reliable fabrication of nanostructure in various sizes and dimensions. The main focus of this project is to develop nanoscale gratings that are the first step in the fabrication of such photonic crystals. II. EXPERIMENTAL METHODS The gratings are created by exposing positive photoresist, S1805, to ultraviolet light through an interferometer. This causes the portion of the photoresist that was exposed to light, to become soluble to the photoresist developer, MIF319. This effectively creates a grating pattern that consists of ridges and valleys formed by the S1805 material. A. Lloyd s Mirror Interferometer Lloyd s mirror interferometer configuration is used to create the gratings. The interferometer, shown in Figure 1, is composed of a mirror positioned perpendicular to a sample and a sample Figure 1. Lloyd s Mirror Interferometer holder. An expanded beam illuminates both the mirror and the sample. Part of the light is reflected on the mirror, creating a virtual light source that is effectively superimposed on the portion of the beam that is directly illuminating the sample. This superposition of the reflected and the expanded light wave on the sample creates a phase difference between the two waves. Waves that are in phase will exhibit constructive interference while those out of phase will undergo destructive interference and thus create a grating pattern on the sample. The periodicity of such interference is varied by changing the angle of exposure. This is accomplished by rotating the sample holder relative to the incident laser beam. www.gsoejsr.org VOLUME 5, AUGUST 2012 25

STUDENT AUTHOR: ANTONIO JOU XIE Antonio Jou Xie is currently pursuing his undergraduate degree in Electrical Engineering with secondary fields of specialization in mathematics and physics at The City College of New York. Antonio has always had a fascination with the inner workings of the world - from what makes people tick to how every single thing works. Prior to enrollment at City College, he was part of a pilot engineering program in his high school that introduced students to engineering. This program included classes ranging from mechatronics to management and planning. During this time, Antonio participated in various robotic competitions, including the FIRST LEGO competitions. It was all of these activities, along with encouraging teachers and mentors, that propelled him to enter the world of Engineering. As an undergraduate student, he worked in the CUNYSAT-1 space program under the tutelage of Professor Charles Watkins. The program, which involved a collaborative effort from different CUNY campuses and Cornell University, was aimed at launching a picosatellite into low-earth-orbit to study ionospheric disturbances using GPS. He also worked in CITYSAT, a student-independent project that would fabricate custom-built picosatellites for various science missions. Antonio has participated in the Kaylie Prize for Entrepreneurship competition, in which he was part of a team aiming to build a natural alarm clock integrated into a lightweight-sleeping mask. Although this project did not attract much attention, his second project involving a portable non-invasive blood glucose meter made him and his colleagues finalists in the competition. For his capstone design project, Antonio was part of the Advanced Laser Fluorometry group and worked to detect chlorophyll concentrations in water based on the fluorescence of chlorophyll present in algae. This proved more efficient than direct chlorophyll measurements. Antonio has worked as a research assistant in the Nuclear Magnetic Resonance Laboratory, conducting simulations of electron spin interactions under the Physics Department. Now he is currently working as a research assistant under Professor Sang-Woo Seo, further developing the periodic nanostructure research project described in this paper. In addition, Antonio has been deeply involved in the CCNY community. He has served for Tau Beta Pi, the engineering honor society; as well as Eta Kappa Nu, the electrical engineering honor society. He is currently the president of The City College student branch of the Institute of Electrical and Electronics Engineers (IEEE) and has worked with the Career Center to bring industry recruiters on campus. He has also served as one of the prime organizers of annual events such as the Electrical Engineering Mixer. DR. SANG-WOO SEO received his B.S. degree in Electrical Engineering from Ajou University, Korea, an M.S. degree from Kwangju Institute of Science and Technology, Korea, and a Ph.D. degree in electrical engineering from the Georgia Institute of Technology, Atlanta, in 1997, 1999, and 2003, respectively. From 2003 to 2005, he worked as a research engineer at Georgia Institute of Technology and Duke University to develop thin film photonic devices and integration method for optical sensors and optical interconnections. In 2005, he worked as a research scientist at the University of California, Davis to develop InP based chip-scale optical code-division multiplex access and arbitrary waveform generator systems. Dr. Seo joined the Department of Electrical Engineering Professor Seo s Team (from left to right): Jing Xiao, Saad Memon, at The City College of New York in 2007. He has established Dr. Sang-Woo Seo, Antonio Jou, and Fuchuan Song. the Advanced Photonic Integrated System laboratory. The laboratory is a state-of-the-art photonic design, nano/micro fabrication, and measurement facility. Research in his lab covers broadly optical materials, devices, and system integrations. His current research focus is on the development of scalable heterogeneous integration method to combine optical active/passive devices with other technological functional systems (such as Silicon circuits, Radio-frequency (RF) systems, micromechanical systems, or microfluidic devices). To achieve this goal, his group is developing thin optical device structure and its scalable integration method. Active optical devices (such as lasers, photodetectors) are fabricated from their preferred substrate. Using a post-integration process, they are separated from their bulky substrate and selectively integrated on any host substrate. Based on this approach, his group is combining optical functional devices with other technological functional systems for various applications in optical sensors and communications. Some of his current research interests include three-dimensional optoelectronic device integration, integrated microfluidic/photonic sensors, Integrated THz systems, and optical micromechanical systems. 26 JOURNAL OF STUDENT RESEARCH www.gsoejsr.org

His laboratory is also used for various undergraduate/graduate student research and outreach programs. Throughout those activities, students can experience small-scale micro/nano device fabrications and create innovative materials and devices in his laboratory. Students at all levels from undergraduate to graduate are welcome to discuss their interests on nano/micro materials, devices, and system research. Figure 2. Experiment Setup. The formula for the desired grating period is given by Λ = λ/(2sinθ) where Λ is the desired grating period, λ is the wavelength of the laser, and θ is the angle of the incident light beam as seen in Figure 1. The experiment set up consists of the interferometer in conjunction with an ultraviolet 30mW 405nm laser, pinhole lens, objective lens, shutter, and convex lens. A diagram of the set up is shown in Figure 2. B. Sample Preparation Samples were prepared using Silicon nitride (Si3N4) coated Silicon wafers with a Miller index of <100>. Wafers were cleaved along their crystalline orientation into smaller square samples according to the dimensions of the sample holder and cleaned using a 3-step cleaning process with acetone, methanol, and isopropyl alcohol. The samples were dehydrated at 95 C for at least 60 seconds and allowed to cool to room temperature before spinning. A coat of bis(trimethylsilyl)amine (HMDS) was used to facilitate the adhesion of the photoresist. HMDS was added on top of the samples and was spread at 500 rpm for 15 seconds and spun at 2500 rpm for 30 seconds. S1805 was added subsequently on top of the HMDS coating, spread at 500 rpm for 15 seconds and spun at 5000 rpm for 30 seconds. This created a uniform layer of photoresist material with a thickness of about 6000 ångström. The samples were baked for 60 seconds at 95 C and were allowed to cool before exposure. C. Sample Exposure Due to the light-sensitive nature of the photoresist, necessary precautions were taken to prevent any damage to the samples by (1) ambient light. In addition, the ultraviolet laser used was powered on five minutes before the sample exposure to prevent any laser power fluctuations during exposure due to initial temperature changes. The samples were placed in the sample holder, as seen in Figure 1, perpendicular to the mirror and the angle was adjusted accordingly to yield the desired grating periodicity. In our laboratory setup, the angle indicator placed under the sample holder does not directly show the incident angle of the laser beam. However, this angle indicator can be used to calculate the incident angle of the beam and is given by the following formula: θ incident = θ indicator 45 The shutter was removed and the sample was exposed for a predetermined amount of time. Once the samples were exposed, the shutter was closed to prevent overexposure of the photoresist. Additionally, to produce two-dimensional patterns, samples underwent a double exposure procedure. These samples were exposed in one orientation for a predetermined amount of time, rotated 90 with the shutter on, and exposed in the new orientation for a predetermined amount of time. D. Sample Development The exposed samples were dipped in a solution of MIF319, a photoresist developer for S1805, for a set amount of time until the exposed samples were developed appropriately. This process involved developing the samples with diluted or undiluted developer, depending on the developing behavior of the exposed samples. The diluted solutions were diluted with deionized water with specific ratios of developer to water. E. Sample Observation Samples that were exposed and developed correctly diffracted ambient light, a phenomena visible to the naked eye. The developed samples were then examined under a highresolution microscope. Images of each sample and information such as the grating periodicity were collected as well. Other characteristics such as the uniformity of the photoresist and continuity of the interference patterns were also recorded. In addition, since the grating periodicities were in the nanoscale, a scanning electron microscope (SEM) had to be (2) www.gsoejsr.org VOLUME 5, AUGUST 2012 27

used to more precisely measure the grating periodicity and the photoresist thickness after exposure and development. III. RESULTS Figure 3 shows images of the patterns obtained using different incident angles during the sample exposure procedure. As predicted in Equation (1), the grating period decreased with an increase of the incident angle. The samples shown were exposed to incident angles of 18, 25, and 35 from top to bottom, respectively. Samples were developed at higher incident angles than those shown, up to 42, however, due to resolution limitations, the gratings in these samples could not be seen by the optical microscopes. Samples exhibited a minuscule amount of detail at best. Note that these images were taken with the same microscopic zoom. Thus, each grating image is to scale with the other and comparable. A. Sample Exposure and Development Times Samples exposed to incident angles of 18, 25, and 35 each had an exposure time of 60 seconds. The samples were developed using undiluted MIF319 developer for 15 seconds, 10 seconds, and 7 seconds, respectively. Samples that were exposed at 42 had an exposure time of 50 seconds and a short developing time of 3 to 5 seconds before pattern degradation. B. Sample Exposure Angle vs. Grating Period In Figure 4, the gratings were further analyzed under a SEM, providing much greater detail and information. The periodicities of the gratings were obtained. At an incident angle of 18, the grating period was about 700nm. The grating period decreased as predicted--an increase of the incident angle to 42 yielded a grating period of 310nm. In addition, Figure 4 shows that the photoresist layers are not fully dissolved for the lower two samples, i.e., the lower most regions do not reach the substrate. At 35, the S1805 is partially dissolved. At a higher angle of 42, only the peaks are prominent in the grating. Another detail shown in Figure 4 is that the pattern edges for the 25 and 35 incident angle cases are not smooth or straight as the edges seen in the first case. Figure 5 was obtained by plotting the experimental results from the grating period measurements done through the SEM against their respective incident angles. The data was plotted against the theoretical value given in Equation (1). As seen from the figure, there is a large deviation from the theoretical values at lower angles. At higher angles, the grating periods are closer to their theoretical values. Figure 5. Desired Grating Period versus Incident Angle. In Figure 6, the obtained results at different exposure angles are listed with their respective experimental and theoretical values. Angle Theoretical Grating Period Experimental Grating Period 18 655.3 nm 700.2 nm 25 479.2 nm 642.5 nm 35 353.1 nm 433.4 nm 42 302.6 nm 313.3 nm Figure 6. Tabulated results for the exposed angles. (Left) Figure 3. Developed grating with incident angle of 18, 25, 35 (from top to bottom). (Right) Figure 4. Developed grating with incident angle of 18, 25, 35, 42 (from top to bottom). C. Double Exposure Pattern The pattern shown in Figure 7 shows a double exposed sample obtained with an incident angle of 25. The pattern was exposed for 50 seconds during its first exposure and 60 seconds during its second exposure. Developing time was 10 seconds using an undiluted MIF319 developer. Unlike the one-dimensional 28 JOURNAL OF STUDENT RESEARCH www.gsoejsr.org

Figure 7. Double Exposure Pattern with incident angle of 25. patterns shown in Figure 3, the two-dimensional pattern consists of individual circles throughout the sample structure. IV. DISCUSSION/CONCLUSIONS The results show that the technique formulated above was successful at developing nanoscale interference patterns or gratings for the positive photoresist S1805. These results can lead to potential applications, such as the development and manufacturing of photonic crystals and other nanodevices. A. Grating Depth Nevertheless, there were some discrepancies in the results that required further investigation. At lower incident angles, the regions where the photoresist was exposed were completely dissolved and reached the substrate, whereas at higher incident angles, excess photoresist material was left in between the peaks. The periods of such gratings, however, were effectively reduced to smaller grating periods with higher incident angles. The presence of excess photoresist can be attributed to the fact that at such a small scale, the photoresist is very unstable during the development process. Developing the material for even a few more seconds could render the grating unusable since the grating structure could be damaged or the entire photoresist coating could be removed. Further experimentation should be done using a diluted developer to develop the pattern at a slower and more stable pace. B. Secondary Interference The grating edges in some cases did not show a smooth and straight edge but rather a jagged one. This is the result of a secondary interference pattern that undergoes inside the photoresist. The incident beam penetrates the photoresist and it is reflected back by the substrate, which results in the irregular edges seen in Figure 4 for the 25 and 35 incident beam cases. To resolve this issue, an anti-reflective material can be coated on the substrate prior to the addition of the HDMS and the photoresist to prevent the incident beam from reflecting by the substrate back into the photoresist. C. Double Exposure Pattern In the double exposed samples, exposing both orientations for the same amount of time led to patterns that were developed faster in one orientation than in the other. The results showed that the first exposure developed faster than the second exposure. Thus, exposure times were adjusted accordingly. The image shown in Figure 7 demonstrates the result of such adjustment in exposure time. Further experiment can be conducted to increase the exposure incident angle and thus decrease the twodimensional pattern periodicity. D. Experimental Deviations Figures 5 and 6 show that there was a deviation in our experiment from the theoretical values obtained using Equation (1). This was caused by the setup of our experiment. Note that in Figure 2 the laser beam goes through a convex lens before being exposed to the sample. The convex lens is used to focus the 25mm diameter laser beam into a 10mm diameter laser beam. This shortens the exposure time due to the higher power intensity that reaches the samples, but also causes the laser beam to hit the mirror and sample at a slightly different angle than it would if it were a parallel beam. The result is higher grating periods at the given angles, however, this deviation decreases as incident angles increase. E. Future Work Similar experiments will be repeated for the S1805 photoresist in order to find optimal parameters for higher incident angles for the one-dimensional and two-dimensional case. These experiments will include but not be limited to variation of the initial baking temperature, exposure time, and developing time. Further experimentation needs to be conducted using a diluted MIF319 developer at different concentrations as well. In addition, different types of photoresist can be employed using this technique, such as negative photoresist. This might contribute to the development of different periodic patterns. Ultimately, the findings from this experiment could contribute to the fabrication of various nanodevices that make use of these periodic nanostructures. REFERENCES [1] J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, Second ed.: Princeton University Press, 2008. [2] E. Yablonovitch, Inhibited Spontaneous Emission in Solid-State Physics and Electronics, Physical Review Letters, vol. 58, pp. 2059-2062, 1987. [3] Y. Zhao, J. Wang, and G. Mao, Colloidal subwavelength nanostructures for antireflection optical coatings, Opt. Lett., vol. 30, pp. 1885-1887, 2005. [4] S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, and L. C. Chen, Anti-reflecting and photonic nanostructures, Materials Science and Engineering: R: Reports, vol. 69, pp. 1-35. [5] W. L. Barnes, A. Dereux, and T. W. Ebbesen, Surface plasmon subwavelength optics, Nature, vol. 424, pp. 824-830, 2003. [6] S. J. McNab, N. Moll, and Y. A. Vlasov, Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides, Opt. Express, vol. 11, pp. 2927-2939, 2003. www.gsoejsr.org VOLUME 5, AUGUST 2012 29