A novel concept for reflective displays that uses two-dimensional photonic crystals with subwavelength gratings is introduced. A solar-powered reflective display with photonic crystal color filters was analyzed by a theoretical approach. We fabricated the photonic crystal color filters on a glass substrate by using low-cost nanoimprint lithography and multi-scan excimer laser annealing to produce RGB color filters through a single patterning process. The theoretical and experimental results show that the color filters have high reflectance and angular tolerance, which was qualitatively confirmed by chromaticity coordination analysis.
© 2010 OSA
Liquid crystal displays are primarily used indoors and employ a backlight unit (BLU) as the light source. The color filters in liquid crystal displays are required to have high transmittance for the specific wavelengths of the BLU. Recently, mobile display devices, such as digital cellular phones, portable multimedia players (PMP), and ultra-mobile personal computers (UMPC), have been both used indoors and outdoors due to their portability. These devices must have outdoor visibility despite the brightness of the ambient light [1, 2]. Ambient light reflecting off the surface of displays in bright environments may result in a decrease in the contrast, and the chromatic properties can be diminished due to the mixing of ambient light and the display panel light. To operate mobile display devices for long periods, the power consumption of the display panel should be reduced. Reflective displays, which use bright ambient light as the light source, have been considered as a potential candidate. The color quality of the reflective display can be enhanced if the color filter has a narrow bandwidth and a high transmittance . However, it is difficult for pigment-type color filters to have a narrow bandwidth without a significant loss in the transmittance [3, 4]. Alternative approaches for the color filter that use a subwavelength grating, based on guided-mode resonance (GMR), have been suggested [5–7]. For this approach, color is generated by the optical coupling between the incident light and the resonant modes that are determined by the modulation of the substrate refractive index. We previously developed a two-dimensional (2D) photonic crystal color filter for use in displays. The filter has high angular tolerance in the visible spectrum range, independent of the incident angle [6, 7]. However, multiple color filters could not be fabricated on the same substrate by a single patterning process because the pattern height required to generate each RGB color is different.
A solar-powered mobile device has been recently released for hours-long use in the outdoors . This mobile device has a full solar panel on its back, which can generate sufficient power to charge the phone. If the display device can use ambient light as the light source in a bright environment with a reflective photonic crystal color filter, its power consumption may be reduced. Moreover, if charging by solar energy is possible during the display device operation, we expect that the novel reflective display can increase the outdoor visibility and convenience of the device.
In this paper, we present a novel design for a reflective display that includes a light energy conversion device. The novel reflective display uses a GMR color filter. The reflected light expresses the desired colors, and the transmitted light is utilized as a light source for the light energy conversion device, located beneath the GMR color filter. Using the Rigorous Coupled-Waveguide Analysis (RCWA) method, we designed a 2D photonic crystal (PhC) color filter with the same pattern height for the RGB colors. The viability of the proposed reflective display was analytically investigated. We fabricated the 2D PhC color filter using low-cost, high-productivity nanoimprint lithography (NIL) and a multiline-scan excimer laser annealing (ELA) process. By using the NIL fabrication process for the color filter, three color filters, each corresponding to a different color, were fabricated on the same substrate by a single patterning process, unlike the conventional photolithography process.
2. Proposed novel reflective display
A conventional reflective display, shown in Fig. 1(a) , consists of a polarizer, a retardation film, color filters, a liquid crystal (LC) layer, and a reflective electrode . The diffusing reflective electrode integrates diffusion, reflection, and the electrode function. It can help minimize problems, such as image blurring, color mixing, or double images. This type display does not require a backlight. Thus, its power consumption and weight are greatly reduced. Because it is a reflective display, brighter ambient light can lead to a more vivid image. The proposed reflective display, depicted in Fig. 1(b), has a PhC color filter and does not need a dye or pigment to construct the RGB colors. It has a light energy conversion device located below the PhC color filter. The light energy conversion device can have the same structure as a general n-type or p-type silicon thin-film solar cell. For a conventional pigment-type color filter, when the incident light passes through a color filter that is selective for a specific wavelength range, the light for the corresponding wavelength is transmitted, and the remaining light may be absorbed into the color filter. If a PhC color filter, which is based on the GMR principle, is used in a reflective display device, a portion of the incident light on the color filter is reflected. This reflected light is used for color generation by the reflective display device, and the remainder of the incident light is transmitted through the PhC layer and the transparent substrate. The light transmitted through the transparent substrate may be used as an energy source by the solar cell, which may additionally produce a driving current.
The proposed device was designed and analyzed using a self-made simulation tool based on the RCWA method. The 2D PhC color filter, shown in Fig. 2(a) , is comprised of a silicon layer with a high refractive index np that is formed on a glass substrate with a refractive index ns. The period, width, and height of the grating layer are L, d, and h, respectively. The period and width of the filters are: L = 390 nm and d = 160 nm for the red filter; L = 240 nm and d = 115 nm for the green filter; and L = 200 nm and d = 90 nm for the blue filter. The height of the grating layer was identical with h = 130 nm for each filter compared to our previous design  which has different pattern heights, and each color filter was fabricated on the same substrate by a single patterning process. For the refractive indices np and ns, the experimental results by Palik  were used to account for the dispersion characteristics, which change according to the visible region of the wavelength. The reflectance and transmittance for the PhC color filter at a reflective surface and a transmitting surface are illustrated in Figs. 2(b) – 2(d). As shown in Fig. 2(b), for incident light that includes all frequency components, only the light that is part of the red wavelength band is reflected by the PhC color filter, and the remaining light is transmitted through the PhC color filter. Accordingly, the reflected light with the red wavelength band may then be used for red color generation, and the non-reflected light can be used for charging the solar cell. The same mechanism applies to the green and blue PhC filters in Figs. 2(c) and 2(d). To precisely describe this mechanism, we determined the electric field distribution in the vicinity of the PhC slab for the green filter. The color filter in Fig. 2(c) has a maximum reflectance at a wavelength of 535 nm and has near zero reflectance at a wavelength of 650 nm. The optical reflectivity is significantly strengthened within a certain range of the spectral band, centered at a resonance wavelength. Figure 3 shows the electric field distribution in the vicinity of the slab for two wavelengths: (a) 535 nm and (b) 650 nm. When light with the resonance wavelength of 535 nm is incident on the green filter, the coupling of the incident wave induces a propagating mode that leads to the concentration of the electromagnetic field inside the slab, and most of the incident light is reflected at the boundary between the PhC slabs and the glass substrate (Fig. 3(a)). For incident light with a wavelength of 650 nm, most of the light is transmitted into the slabs and propagates through the glass substrate (Fig. 3(b)). Indeed, most of the light with the exception of a certain range of spectral band centered at the resonance wavelength, is transmitted and propagates into the PhC color filter without coupling and reflection.
The solar cell underneath the PhC filter absorbs the light that is transmitted through the transparent substrate and is incident to the solar cell. This light is not reflected by the red, green, and blue PhC units, and it reaches the solar cell to be converted into energy. Because the solar-powered mobile device has a solar panel on its back, it cannot generate power to charge the device during the operation of the display device. Our proposed device can generate power during the operation of the display device by using the transmitted light through the color filter. This can be accomplished because the display, including the color filter and the solar cell part, can be constructed in the same direction. Thus, we anticipate that the proposed display device can remarkably increase the outdoor convenience of the mobile display device. Ideally, a solar cell should absorb all useful photons. However, more than 30% of the incident light is reflected back from the surface of single-crystalline silicon solar cells. Antireflection coatings are widely used to improve the conversion efficiencies of silicon
solar cells. Recently, subwavelength-structured nipple arrays that mimic anti-reflective moth eyes were suggested . When the phase-matching condition between the incident light and the planar waveguide structure is satisfied, the PhC filter acts as a high-efficiency band-pass filter in which the diffracted fundamental wave is taken as the output. Hence, the light not used for color generation, which represents most of the incident light, can be converted into electrical charges by the solar cell. The PhC color filter can be used as a bi-functional device for color generation and increasing the efficiency of the solar cell through the anti-reflective coating.
The reflective response characteristics for each RGB color with the 2D PhC filter are shown in Fig. 4 . The maximum reflectances of 85.4%, 75.9%, and 61.9% were obtained for the red, green, and blue filters, respectively. The 2D PhC filter with a large refractive index ns produces nearly dispersionless modes in the range of the specific wavelength [6,7], and this results in resonance reflection peaks for the incident light, independent of the incident angle. However, the reflectance intensities of the transverse electric (TE) and transverse magnetic (TM) waves have different features. As shown in Figs. 4(a) – 4(c), although the incident angle of the TE wave was 20°, almost no change in the maximum reflectance and central wavelengths occurred. The reflectance of the TM wave was influenced by the incident angle. Most of the light incident on the red filter was transmitted into the grating layer with little reflectance. In a study on the perception of color, one of the first mathematically defined color spaces was the CIE XYZ color space (also known as the CIE 1931 color space), created by the Commission Internationale de l’Éclairage in 1931. Figure 4(d) shows the RGB space of the PhC color filter depicted on the CIE 1931 color space chromaticity diagram. On the CIE 1931 color diagram, the simulated reflective spectra are located at two color coordinates (x, y), which are (0.5875, 0.3313), (0.2135, 0.5640), and (0.1298, 0.1348) for the red, green, and blue filters, respectively. Compared to the National Television System Committee (NTSC) standard, shown by the dashed line in Fig. 4(d), the color reproducibility of the PhC color filter, which is calculated from the area ratio of the color diagram between the NTSC and the PhC color filters, is 56.9%. The color reproducibility of paper, which is a typical reflective display, is approximately 20%. This value shows that the PhC color filter may have a remarkable color performance.
3. Fabrication of PhC color filters
The 2D PhC color filter can be fabricated using the NIL and the line-scan type of ELA process, presented in Fig. 5 . Although only the PhC color filter process is introduced in this paper, the proposed process is compatible with the solar cell fabrication process for the suggested solar-powered reflective display. The master for the NIL process can be fabricated on a silicon wafer, using the conventional e-beam lithography (EBL) process. We created a master through EBL, using a JBX-9300FS system (JEOL Ltd.) with a negative CAR-type resist of NEB-22S68 (Sumitomo Ltd.). Inductively-coupled plasma-reactive ion etching (ICP-RIE) was used to obtain an RGB color filter with the 2D PhC patterns through the NIL process. The Si master used in the NIL process was etched by first applying an SiO2 (300 Å) hard mask to etch the Si master with an RF1 bias power of 80 W, an RF2 source power of 480 W, and an operating chamber pressure of 7 mTorr. The gases used in this step were C4F8 (10 sccm), CHF3 (20 sccm), O2 (5 sccm), and Ar (45 sccm). The etching time and etching rate were 14 sec and ~33 Å sec−1, respectively. Second, each pattern on the hard mask was transferred onto the Si substrate with an RF1 bias power of 80 W, RF2 source power of 540 W, and an operating chamber pressure of 4 mTorr. The gases used in this step were HBr (80 sccm), Cl2 (18 sccm), and O2 (5 sccm). The etching time and etching rate were 31 sec and 73.6 Å sec−1, respectively. Figure 6 shows an optical microscope image and SEM micrographs for each pattern profile and a cross-sectional view of the 228-nm-high pillars after etching. The pitch of the color filter array is 150 μm × 50 μm, and the subpixel size for each color filter is 142 μm × 42 μm. The pattern size for each RGB color filter is different, and different colors were observed with the optical microscope. The critical dimension tolerance was less than ± 2%, and the line edge roughness was smaller than 5 nm. The pattern pitch for each color filter did not change during the etching process. The fabricated Si master had a good uniformity of 93% over the entire pillar pattern, and the pillar profile cross section was more than 88°.
To pattern the 2D gratings with the NIL process, a silicon-on-glass (SOG) wafer with a thickness of 130 nm was required. The amorphous silicon (a-Si) SOG (a-SOG) was formed from a 30-nm-thick hard-mask layer (SiO2), a 130-nm-thick a-Si layer, and 100-nm-thick barrier layer (SiO2) on a 525-μm-thick glass substrate with the low-temperature plasma- enhanced chemical vapor deposition (PECVD) process. The barrier layer minimized the process errors (e.g., under-trenching) by acting as an etch-stop layer and also prevented impurities from penetrating into the a-Si layer during the crystallization process. The hard-mask layer on the a-Si layer was applied to facilitate the etching of the a-Si layer and to enhance the filtering selectivity. We fabricated a soft-polymer stamp using the nanoimprint process to preserve the relatively costly Si master and to easily create a number of stamps. The soft stamp is advantageous because it can apply a more uniform pressure than a hard stamp, like the Si master, and this results in uniform patterning and a large area of extensity for display applications. To make the soft mold, we used urethane methacrylate (FLK MD 700, Solvay Solexis) and included 4% w/w photoinitiator (Darocur 1173, Ciba) as the polymer resin. The soft mold did not require an additional anti-adhesion treatment on its surface. The 2D grating patterns were formed on the a-SOG wafer by the NIL process, using the fabricated soft mold and a UV-curable resin. A mixture of a fluorinated oligomer with perfluoroalkyl chains and an acrylate monomer with carbon double bonds was used as the UV-curable resin for imprinting. The NIL was processed by our in-house system. The NIL parameters, including vacuum pressure, pressing force, delay time, and UV-exposure time, were optimized, and high-fidelity NIL conditions were established. The pattern from the soft stamp was well transferred, preserving the square shape of the Si master. The residual thickness was less than 5 nm. After creating the 2D grating patterns using the NIL process, the oxide layer was selectively etched using the ICP-RIE process with the NIL pattern serving as a mask. The a-Si layer was also selectively etched using the ICP-RIE process with conditions similar to the Si master etching process. Figure 7 shows the etched pattern resulting from the NIL process. As shown in the optical microscope image in Fig. 7(a), the reflected colors for each filter do not show the desired RGB colors because the crystallization process used to increase the crystallinity of the amorphous silicon patterns had not been applied. The critical dimension changes for the design parameters of the red, green, and blue filters were 5%, 7%, and 13%, respectively. It is thought that these correspond to the different etching characteristics according to each pattern size. Design modifications for the Si master were required because of the critical dimension changes for each color filter. The pattern pitch for each color filter does not change after the NIL and etching processes. The uniformity for the entire pillar patterns is over 90%, and the pillar profile cross section is approximately 90°. The images in Figs. 7(b) – (d) show that the squareness of the pattern is well maintained after the etching process.
Silicon crystallinity is important for the enhancement of the color filter reflectance. Although the 2D patterns made by the NIL and ICP-RIE processes are similar to the desired parameters, they are composed of amorphous silicon. The internal absorption is relatively large for amorphous silicon. It has been shown that the maximum reflectance is significantly reduced, and there is large shift in the central wavelength . Moreover in our previous study , it was analytically and experimentally proved that the color filter reflectance was remarkably enhanced and closely approached the simulated result after crystallization of the patterned a-Si layer. We thus used the line-scan type of the ELA process to correspond to the large size of the proposed device. In the ELA process, the pattern size is important because the probability of crystal nucleation is proportional to the volume of the material to be crystallized. Single-nucleus crystallization can be realized in a small volume, hence irradiation at low laser energy, through which material quasi-melting can occur, may be sufficient. At a low laser energy, it is expected that the proper degree of multi-scan irradiation is effective for enhancing the extent of crystallization. After the ELA irradiation, the reflectance was measured by a UV visible spectrometer (Model UV-2450, Simadzu), and a nanostructural analysis was carried out using transmission electron microscopy (TEM). Figure 8 shows the reflectances and bright-field TEM micrographs of the green PhC filter at several ELA conditions. We first applied a two-scan irradiation at 85 mJ cm−2 using a XeCl excimer laser (308 nm) with an overlap of 97.5% to the etched sample without the dehydrogenation process. As shown in Figs. 8(a) and 8(b), the reflectance was lowcompared to the simulated result, and a markedly deformed shape is shown in the TEM image. The relatively large grain and pore defects are observable in the upper region of the pattern, and the small grain is at the bottom of the pattern. The pore defects result from the presence of impurities, such as hydrogen. Thus, the dehydrogenation process is required in the PhC color filter process when fabricating an a-SOG substrate before the ELA process. Hydrogen may also result in the early triggering of explosive crystallization . High energy laser intensity with excessive overlap was added to the hydrogen impurities, and it was expected that pattern deformation occurred in its upper region. To improve these problems, the dehydrogenation process and a decreased overlap of 50% were applied. As shown in Figs. 8(c) and 8(d), the pore defects were solved with these conditions. By increasing the scan number and the laser energy instead of decreasing the overlap, a large-grained polycrystal Si pattern was acquired without large-scale changes in the shape. We could obtain a high reflectance, similar to the simulated result. The grain size increased with increasing energy density, and the crystallization appearance affected the optical performance of the PhC color filter.
We fabricated the red PhC filter, which has the large-sized pattern, with the same conditions for fabricating the green PhC filter. Figure 9 shows the reflectance and TEM results of the red PhC filter. The measured reflectance differed significantly from the simulated result. This can be explained by the TEM images in Figs. 9(b) and 9(c). At low energy there is no change in the crystallization within the pattern, and at high energy there is some change in the crystallization at the top of the pattern, which has a small grain size. This result proves that the crystallization characteristics depend on the volume of the pattern. To obtain the optimal condition for the ELA process, we increased the laser energy and scan number. For the scan number, the reflectance had a slight change over two scans. Thus, we fixed the scan number at two. For a laser energy of 130 mJ cm−2, the pattern did not have a notable change in size. Figure 10 shows the optical performances of the RGB color filters and the TEM results after the optimized ELA condition were applied. For the blue and green filters, the reflectances are similar to the simulated results. Figure 10(c) shows that there was no change in the size even if the higher laser energy condition was applied to the blue color filter. Thus, the green color filter will have the same crystallization appearance. Although there is some improvement when compared to Fig. 9(a), the measured reflectance for the red color filter has the same difference with the simulation. In Fig. 10(b), the interior grain size in the pattern is small, and the higher laser energy may be required to increase its grain size. If the laser energy were increased more, the reflectance of the red color filter could be improved. It is expected that there are some changes in the shape and decreases in the reflectance for the blue and green color filter. To maintain the pattern shape during ELA, the deposition of an additional capping layer after etching may be performed.
We measured the reflectance of the fabricated color filter while changing the incident angle from 0° to 45° using a bi-directional scattering distribution (BRDF: RT-300, J&C Tech) system. To qualitatively analyze the color characteristics according to the incident angle, we drew the chromaticity diagram with the above experimental results. Figure 11 shows that color shifts in RGB are not large even if the incident angle changes. The color changes are much more gradual, and almost no change in the color could be observed up to 45° of the incident angle. The color reproducibility of the fabricated PhC color filter was approximately 10% less than the simulated result. This difference results from the critical dimension difference and the inefficient crystallization process. Nevertheless, the 2D PhC crystal color filter and the fabrication process are promising technologies to realize the proposed novel reflective display.
A novel reflective display that can be charged by solar energy while operating is introduced. Its operating mechanism was analytically explained with the RCWA method. The proposed display would use a 2D PhC color filter to embody the RGB colors, and it would have a high reflectance and angular tolerance. 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. Its optical performances are theoretically proved.
The 2D PhC color filter was fabricated using low-cost nanoimprint lithography and a crystallization process using the line-scan type of ELA, which is required for large-sized displays. In the nanoimprint lithography, a soft mold replicated from a high-fidelity Si master formed by the EBL and ICP-RIE processes was applied to create 2D grating patterns on an a-SOG wafer. All of the patterns were well transferred with a near-zero residual and a pattern uniformity over 90% for the whole area. After etching the a-Si layer using the 2D nanoimprint patterns as an etch mask, multi-scan ELA successfully enabled single-nucleus crystallization. The close relationship between the silicon crystallinity and the PhC color filter reflectance was examined in detail with a UV visible spectrometer and TEM analysis.
The 2D PhC color filter has a high reflectance, over 60%, and a high angular tolerance. The high angular tolerance was qualitatively proved with the CIE chromaticity diagram, which provides human color perception information. The novel reflective display with the 2D PhC color filter could be a promising candidate for mobile displays.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0000769).
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