Design and fabrication of two-dimensional photonic crystal color filters using nanoimprint and excimer laser annealing

다국어 초록 (Multilingual Abstract)

The color quality of typical displays can be enhanced if the color filter has a narrow bandwidth and a high transmittance. However, it is difficult for conven-tional pigment-type color filters to have a narrow bandwidth without a significant loss in the transmittance. On the other hand, many creatures in nature, such as Morpho butterflies and peacocks, display unique brilliant colors, known as structural colors, which result from the interaction of light with periodic nanostructures on their surfaces. Under inspiration of these nature’s structural colors, new approaches for the color filter that use a subwavelength grating, based on guided-mode resonance (GMR), have been suggested. But for practical display applications, a high angular tolerance in the visible spectrum range, independent of incident angle, is essential; until now, no procedure has satisfied that need. In addition, no study exists on the material properties of the grating necessary for a high-efficiency color filter and the high productive fabrication method for photonic crystal (PhC) color filters has not been suggested.
In this thesis, we aim to develop high angular tolerant PhC color filters including their theoretical analysis, fabrication and characterization, and present high fidelity nanofabrication technologies which are applicable to realize a variety of nanodevices. Furthermore, a novel concept for reflective displays that uses the proposed PhC color filters is introduced and the feasibility study on a large-sized patterning process is discussed in detail.
On the basis of the rigorous coupled-wave analysis (RCWA) method, two-dimensional (2D) PhC color filters are designed and their optical performances are analytically evaluated and it is shown that the proposed color filters have high reflectance and angular tolerance, where the dispersion characteristics of gratings are carefully considered because it has an important effect on the color filter efficiency. We also show that high angular tolerant characteristics originate from the high refractive index contrast of the grating structures for the photonic bandgap analysis. The proposed color filters are first realized using the 2D patterning by lift-off e-beam lithography (EBL) after the crystallization of an amorphous silicon (a-Si) layer by solid phase crystallization (SPC). Even though the fabricated color filters excepting the blue color filter have high reflectance and angular tolerance similarly to the simulated results, the applied process results in low efficiency of the blue color filter and an undesired sidelobe peak and a wavelength shift due to some reduction in the effective height by diffusion during the SPC process. In order to diminish these side effects, we suggest a different approach that can achieve single crystal-like Si on an amorphous substrate with a remarkably short process time at a low temperature. It is accomplished by the nanopatterning of an a-Si film using low-cost, high-productivity nanoimprint lithography (NIL) followed by the excimer laser annealing (ELA) process for the crystallization of the patterned a-Si layer. By isolating the a-Si film into 2D patterns before the crystallization process, we found that single crystal-like Si patterns with a crystallinity of 96% can be obtained with only low-energy multi-shot irradiation, which result in high-efficiency PhC color filters while alleviating the side effects of the former method. In addition, for the first time the close relationship between silicon crystallinity and PhC color filter reflectance was examined in detail.
A novel concept for reflective displays that uses 2D PhCs with subwavelength gratings is introduced. A solar-powered reflective display with PhC color filters is analyzed by a theoretical approach. We could make sure that the 2D PhC color filter can be fabricated on the same substrate by a single patterning process with the same pattern height, unlike the conventional photolithography process. Finally, the novel fabrication process to realize the large-sized PhC color filters over 8-inch-diagonal size is presented and its feasibility study is performed. The highly crystallized silicon patterning scheme presented here may be very attractive for a variety of devices requiring high carrier mobility and/or high optical efficiency. In addition, it is certainly expected that the proposed large-size pattering method will be one of the most hopeful technologies for providing the large-sized flexible stamp with nano-sized patterns in the roll printing process as well as effectively applying the proposed color filters to large-sized display applications.