A photonic crystal (PC) is a periodic nanostructure comprising an arrangement of alternating high- and low-dielectric-constant materials. This structure affects the flow of photons in a way similar to how crystal periodicity in semiconductors affects electron flow. For a photonic crystal to work, the periodic structure must be about half the resonance wavelength. For visible light this is between 400 and 700nm, making the fabrication of PCs both cumbersome and complex. Although nanofabrication techniques such as focused-ion-beam milling and electron-beam lithography have improved significantly over the last decade, high fabrication costs has severely hindered the commercial realization of PC-related optoelectronic devices. Consequently, finding an economic method of fabricating PCs with high precision is an important area of PC research.
A number of approaches have been proposed, including nanoimprint1 and UV-interference lithography.2 Nanoimprint is the most mature method, and fabrication machines are commercially available. However, expensive nanofabricating techniques are still required to produce the hard mold for imprinting. Correctly fabricating periodic patterns using UV interference lithography involves expensive UV lasers and complex optical systems. At the moment, mask-based photolithography is still the preferred method for most electronics industries. Nanosphere lithography3 and photolithography using nanosphere arrays as phase masks4, 5 have already been reported. These techniques can be used to make 2D or 3D periodic nanostructures at low cost. We have spent several years developing this approach and have recently modified it to form PCs at a selected area.
The nanospheres we used in this study were polystyrene spheres with diameters of 500 and 1000nm. To begin, we spin-coated a thin layer of photoresist on top of the substrate. Nanospheres subsequently formed a single-layered hexagonal close-packed array by convective self-assembly. Figure 1(a) shows a scanning electron microscope (SEM) image of the resulting structure. We used a commercial mask aligner during the UV exposure process. The polystyrene nanospheres function as nanoscale spherical lenses, focusing the incident UV light into a cylindrical-shaped pattern. We call our method nanospherical-lens lithography (NLL). Figure 1(b) shows the result of a simulation that confirms the collimating function of a single nanosphere using 3D finite-difference time-domain analysis. The exposed photoresist film can be developed into photoresist hole arrays. Metals or dielectric materials can be evaporated onto the samples to form nanodisk arrays after the photoresist lift-off process.
Figure 1. (a) A single-layer array of polystyrene nanospheres atop a photoresist (PR) film. (b) Simulated photon energy distribution of the incident UV light.
SEM images of the hole arrays and the resulting nanodisk array are shown in Figure 2(a) and (b), respectively. The perfectly round-shaped holes and nanodisks show the quality of fabrication using our approach. In addition, the NLL photoresist lift-off process allows us to deposit thick film, which in turn produces thick nanodisks. Figure 2(c) and (d) shows bird's-eye-view SEM images of 100nm-thick nanodisks with vertical and smooth sidewalls. These nanodisks could not be easily fabricated using a state-of-the-art focused-ion-beam instrument.
Figure 2. Scanning electron microscope (SEM) images of (a) photoresist nanoholes and (b) 20nm-thick silver nanodisk array. (c) and (d) Bird's-eye-view SEM images of thick (100nm) silver nanodisk arrays.
We have modified NLL to make nanodisk and nanohole arrays at a preselected location. Figure 3 illustrates this concept. By adding a regular photomask on top of the nanospheres, we were able to form hexagon-shaped PC patterns: see Figure 4(a). The clear region of the photomask is outlined with a white broken line. Figure 4(b) shows an enlarged image of the PCs. The nanodisk arrays are formed only inside this outline.
Figure 3. Schematic illustration of the selected-area nanospherical-lens lithography process.
Figure 4. (a) SEM image of a silver nanodisk array constrained within a hexagonal pattern. White broken lines have been added to show the outline of the hexagon. (b) Enlarged SEM image showing the edge.
In summary, we have demonstrated the fabrication of nanodisk/nanohole arrays using NLL. Tiny polystyrene nanospheres are used as nanoscale lenses to collimate the incident UV light, exposing the underlying photoresist film. We can make near-perfect round-disk or hole arrays. By adding a photomask during exposure, we can limit the formation of nanostructures to a selected area. Up to now, we have been able to fabricate PCs within micron-sized patterns. We hope to optimize the experimental conditions so we can shrink the size of the pattern to build PC devices. We have also observed diffraction effects of UV light near the edge of the metal pattern on photomasks, and intend to perform more comprehensive studies to understand these effects. Our final goal is to be able to fabricate several useful PC structures to show the potential of this method.
We acknowledge financial support from the National Science Council, Taiwan (grants NSC 99-2112-M-006-015-MY2 and NSC 100-2627-B-006-014-). We are also grateful for collaboration from the Center of Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan, and the National Center for High-Performance Computing of Taiwan.
Yun-Chorng Chang, Jyun-Sen Huang, Hsin-Chan Chung
Department of Photonics
Advanced Optoelectronic Technology Center
National Cheng Kung University
Yun-Chorng Chang is an associate professor in the Department of Photonics. His research interests span nanophotonics, semiconductors, and biophotonics.
1. S. W. Lee, K. S. Lee, J. Ahn, J. J. Lee, M. G. Kim, Y. B. Shin, Highly sensitive biosensing using arrays of plasmonic Au nanodisks realized by nanoimprint lithography, ACS Nano
5(2), p. 897–904, 2011. doi:10.1021/Nn102041m
2. N. Feth, C. Enkrich, M. Wegener, S. Linden, Large-area magnetic metamaterials via compact interference lithography, Opt. Exp. 15(2), p. 501–507, 2007.
3. J. C. Hulteen, R. P. Van Duyn, Nanosphere lithography—a materials general fabrication process for periodic particle array surfaces, J. Vac. Sci. Technol. A 13(3), p. 1553–1558, 1995.
4. C. H. Hou, S. Z. Tseng, C. H. Chan, T. J. Chen, H. T. Chien, F. L. Hsiao, H. K. Chiu, C. C. Lee, Y. L. Tsai, C. C. Chen, Output power enhancement of light-emitting diodes via two-dimensional hole arrays generated by a monolayer of microspheres, Appl. Phys. Lett.
95(13), p. 133105, 2009. doi:10.1063/1.3238360
5. C. H. Chang, L. Tian, W. R. Hesse, H. Gao, H. J. Choi, J. G. Kim, M. Siddiqui, G. Barbastathis, From two-dimensional colloidal self-assembly to three-dimensional nanolithography, Nano Lett.
11(6), p. 2533–2537, 2011. doi:10.1021/nl2011824