Nanosphere Lithography Derec Ciafre 1, Lingyun Miao 2, and Keita Oka 1 1 Institute of Optics / 2 ECE Dept. University of Rochester Abstract Nanosphere Lithography is quickly emerging as an efficient, low cost, material specific process for producing features on the sub wavelength scale. In this paper we will first explore and refine the methods behind creating uniform patterns of nanostructures and try to develop a new process to reduce the feature sizes previously achieved. Introduction Submicron fabrication techniques have been extensively explored in recent years as today s technologies require smaller and more efficient structures. For instance, surface-enhanced Raman spectroscopy is sensitive to the surface which it works on. The surfaces are often prepared with a distribution of metal nanoparticles. The shape and the size of such particles strongly affect the factor of enhancements of Raman scattering because they change absorption and scattering rates on the surface [1]. Another application of nanostructures is in solar cells. The most recent technology requires depositing semiconductor material onto a transparent substrate to make the solar cells 100 times thinner than a silicon wafer. Since the solar cell is so thin and delicate, it is essential that the semiconductor material is printed on the substrate with the right size and shape for it to function efficiently [2]. Nanostructure fabrication processes which have been used over the past few years include electron-beam lithography (EBL) and x-ray lithography (XRL). EBL can achieve a feature size as small as 1-2nm, but is not suitable for mass-volume fabrication in today s industry because it is not a parallel process. On the other hand, XRL is able to produce a much higher volume but cannot compete with the resolution limits of EBL. Despite these promising processes, nanostructure fabrication using these lithography techniques is fast approaching its capability limits [3]. Nanosphere lithography (NSL), the subject of this project, is an inexpensive, material specific and high-output nanostructure fabrication process which can systematically produce a 2-D array of periodic structures. It makes use of placing nanospheres in a tightly packed pattern on a substrate in order to create a mask for pattern transfer. By removing the spheres after thin film deposition or etching, the remaining 2-D array on the substrate has triangular shaped nanostructures in a hexagonal pattern, often called a Fischer pattern [4]. Our efforts in this lab explored ways to efficiently create a uniform single layer mask over a large area and to reduce feature size. Using the nanostructures prepared by NSL, we also explored fluorescence quenching using confocal microscopy. Experimental Procedure and Results Part I. Single Layer Nanosphere Mask Nanosphere Lithography can be performed using several techniques, each with different variables that will affect the overall size, shape, and uniformity of the nanostructures. In order to minimize the feature size, a mask more complicated than a single layer of nanospheres is required. As the mask increases in complexity, the parameters that need to be adjusted in order to create a uniform pattern also increase. Previous groups who attempted to create nanostructures using a double layer mask ran into
difficulties. Due to this fact, it was believed that understanding the method for creating a uniform single layer must first be explored in order to create a more complex mask. To create a single layer of nanospheres, cover slips were initially cleaned with a strong acid and then rinsed with deionized water. Half the cover slips were treated with 3-Aminopropyltriethoxysilane (APTES) to alter the surface charge of the cover slips for better adhesion. Polystyrene spheres with a 1um diameter were then diluted into five different concentrations ranging from 10 7 to 10 10 particles/ml. The solutions were drop-coated onto the cover slips, which had been dried with nitrogen. Each concentration was dropped onto two slips, one treated and the other untreated with APTES. The samples were covered and allowed to dry overnight before being viewed with the optical microscope. Samples that had the largest region of a uniform hexagonal close pack were coated in a thermal evaporator with thin films of approximately 10 nm of titanium followed by 100 nm of gold. Using scotch tape, the spheres were removed from the cover slips. While complete removal of the spheres was desired, some were left attached to the cover slips by this method. The samples were viewed by scanning electron microscopy (SEM) and Fischer Patterns were observed over large areas. The concentration which resulted in the largest single layer uniform close pack was 5x10 9 particles/ml on a cover slip not treated with APTES, which can be seen in Figure 1a. Samples with lower concentrations displayed sparse areas of tightly packed spheres, while higher concentrations produced multilayers. The resulting Fischer Pattern after the removal of the spheres can be seen in Figure 1c. The APTES treated cover slips did not significantly enhance the uniformity of patterns, which could be explained as below. First, this could be due to the limited sample size of our experiments. Second, given that the spheres were drop-coated onto the cover slips, if the surface charge condition was not optimized, the spontaneous spreading of the spheres could be hindered hence the uniformity of the patterns was weakened. (a) (b) (c) Figure 1: (a) SEM image of large uniform single layer, (b) SEM image of hexagonal close pack and (c) resulting Fischer Pattern (5x10 9 particles/ml of 1um spheres)
While examining the cover slips, some unexpected variations were discovered, which can be seen in Figure 2. A contamination in the solution due to alternate sphere sizes is believed to have impacted the close pack and affected the resulting Fischer Pattern. It is hypothesized that if a sphere smaller than 1 um should fall into the voids between the spheres, a pattern such as the one seen in Figure 2b will result. (a) (b) Figure 2: SEM image of variations in the Fischer Pattern (5x10 9 particles/ml of 1um spheres) Part II. Reduction of Fischer pattern feature size Using the parameters discovered in Part I for a large area of closely packed spheres, a Fischer Pattern using spheres 350nm in diameter was attempted. Since the 350nm spheres are much smaller compared to the 1um spheres, the void between the spheres should also become much smaller, resulting in a reduction of the nanostructure size. Applying the same procedure as in Part I, we were successful to create a large uniform Fischer pattern with reduced feature size (Figure 3). Figure 3: SEM image of Fischer Pattern with reduced feature size (particles/ml of 350nm spheres) Based on our observations of the single layer mask of 1um spheres (Figure 2), we believe that by applying spheres of two different diameters with the correct ratio would also reduce feature size. A single layer of 1um spheres was produced using the procedure and parameters from Part I and allowed to dry overnight. The slips were then drop-coated with the 350 nm spheres at five different concentrations ranging from 10 9 to 10 12 particles/ml. It is believed that the 350 nm spheres would rest in the area of lowest potential energy, creating the desired reduced features (Figure 4). After allowing the cover slips to
dry overnight, they were once again coated with a thin film of titanium and a thicker film of gold. The spheres were then removed with the same method as in Part I. The samples were then viewed by SEM. Figure 4: Desired packing and resulting nanostructures from 1 um and 350 nm spheres The results from the 1um and 350 nm sphere combination varied between the different samples and within each concentration. While they yielded a reduction of feature size, the results were not repeatable or consistent over a large area. As can be seen in Figure 5a, the smaller spheres did not fill the voids between the larger spheres as expected. In some instances the spheres grouped together forming a separate single layer or overfilling the voids. These results can mainly be attributed to the fact that the correct concentration has not yet been discovered. (a) (b) Figure 5: (a) SEM image of double layer mask of 1um and 350 nm nanospheres (b) and resulting Fischer Pattern Part III. Optical Experiments utilizing quenching In order to optically characterize the Fischer patterns produced by NSL, we used a phenomenon called quenching. Quenching refers to any process which decreases the fluorescence intensity of a given substance [5]. First, samples were created using NSL as stated in Part I. A thin film of Au (10nm) was deposited using sputtering. The spherical particles were then removed by sticky tape. We spin-coated a layer of Nile Blue dye molecules on the stripped substrate. Nile Blue absorbs at 630nm and radiates at 660nm. If a laser beam with wavelength around 630nm is focused on the substrate, Nile Blue molecules will be excited and give rise to fluorescence (Figure 6). On the other hand, close to metal the excited electrons relax through non-radiative energy channels, and therefore the fluorescence intensity will be
reduced in that area. If we scan the laser beam across the substrate and measure the fluorescence intensity at each point, we can observe quenching and reconstruct the Fischer patterns. Figure 6: Schematic showing where fluorescence will be seen. The experiment for quenching was setup as follows (Figure 7a). A laser beam emitting at 637nm was reflected by a dichroic mirror and focused by an objective onto the sample, which is located on the scan stage. Nile Blue molecules on the sample were excited by the focused laser beam and the areas without the metal emitted fluorescence at 660nm. The fluorescence passed through the dichroic mirror and then through a long pass optical filter, which has a cut-off wavelength of 650nm (Figure 7b). As a result, only the emitted fluorescence photons reach the photo detector. The photo detector converts the optical signal into an electrical signal, which is sent to a computer for data analysis and image reconstruction. (a) (b) Figure 7: (a) Schematic of the experimental setup and (b) the optical filter characteristics.
Our quenching experiments reproduced metal Fischer patterns observed by SEM. As can be seen in Figure 8b, fluorescence was observed in regions where there is no metal. There was no fluorescence coming from regions near the metal structures. The image in Figure 8b shows conclusive evidence for fluorescence quenching. (a) (b) Figure 8: (a) SEM image of metal Fischer patterns and (b) the optical scanning image showing Nile Blue fluorescence quenching near the metal structures. Conclusion Throughout this project, we were able to make significant steps towards discovering the different dimensions of nanosphere lithography. In Part I of the project, we explored the formation of a uniform, closely packed, single-layer nanosphere lattice and found that a concentration of 5x10 9 particles/ml on a cover slip not coated with APTES yielded the best results. The objective of Part II was to reduce the feature size of the nanostructures. We successfully created Fischer Patterns with 350nm spheres, thereby reducing the size of our nanostructures. While we were unsuccessful in creating repeatable results with the 350nm and 1um combination, we saw great potential in the process we attempted. After experimenting with different parameters, such as concentration and coating of the cover slips, we see the method introduced in this project to be a candidate for reducing feature size even further with repeatable results. Part III of this project involved optically characterizing the Fischer patterns by coating the substrate with a fluorescent dye. After observing the samples under the confocal microscope, we successfully observed fluorescence quenching. Overall, our team discovered an efficient way for creating single-layer nanosphere structures and found a potentially new way for reducing feature size. With further research, we believe a controlled, repeatable and high-output 2-D array of periodic nanostructures is achievable. Acknowledgement We would like to thank Professor Novotny for all his help throughout the class. We would also like to thank Ryan Beams and Palash Bharadwaj for the many hours they spent with us working in the lab making equipment available and explaining the theory behind the project. Without their help, we would not have been able to make progress in the nanosphere lithography project. References
[1] J. J. Mock, M. Barbic, D. R. Smith, D. A. Schultz and S. Schultz. J. of Chem. Phys. 116:15 2002. [2] http://www.nanosolar.com/technology.htm [3] J. C. Hulteen and R. P. Van Duyne. J. Vac. Sci. Technol. A. 13(3) 1995. [4] U.C. Fischer and H.P. Zingsheim. J. Vac. Sci. Tech., 19, 1981. [5] http://en.wikipedia.org/wiki/quenching_(fluorescence)