A color filter based on a subwavelength patterned grating in poly silicon was proposed and realized on a quartz substrate. It was produced by utilizing the laser interference lithography technique to feature wide effective area compared to the costly e-beam lithography. An oxide layer was introduced on top of the silicon grating layer as a mask to facilitate the silicon-etching and to enhance the filtering selectivity as well. The structural parameters for the device include the grating pitch and height of 450 nm and 100 nm respectively, the silicon stripe width of 150 nm, and the oxide thickness of 200 nm. The fabricated device offered a spectral response suitable for a blue color filter, exhibiting the center wavelength of ~460 nm, the bandwidth ~90 nm and the peak transmission 40%. The positional dependence of its performance was examined to find the effective area of 3×3 mm2, where the variation in the relative transmission efficiency and in the center wavelength was less than 10% and 2 nm respectively. Finally, the influence of the angle of the incident beam upon the transfer characteristics of the device was investigated to reveal that the rate of change in the relative transmission was equivalent to about 1.5%/degree.
©2008 Optical Society of America
A color filter attracted abundant attention as a key element for such diverse applications as complementary metal-oxide-semiconductor (CMOS) image sensors, liquid crystal display devices, light emitting diodes, etc [1–3]. Previously it was mainly implemented by resorting to spin-cast dye films. Recently a bandpass color filter was realized incorporating a one-dimensional (1D) subwavelength grating in single-crystalline silicon or a 2D grating in a metal film via the e-beam lithography [4, 5]. This type of thin-film filters may feature prominent merits in terms of the outstanding compatibility with the prevalent CMOS process, the flexible integration with other optical/electronic components, and the cost effectiveness. But the adoption of the costly e-beam lithography causes an unavoidable drawback that the operation area of the device was practically limited to as small as about 100×100 µm2. Meanwhile, there was a recent report that a nano-scale periodic grating pattern can be created over substantially large area with the help of the laser interference lithography (LIL) [6, 7]. And, regarding the formation of a thin film in two different types of silicon, the PVD (physical vapor deposition) was mainly exploited for the single-crystalline silicon and the CVD (chemical vapor deposition) for the poly silicon. The poly-silicon film is known to offer better uniformity in film thickness, better film quality and higher flexibility for multi-layer structures compared to the single-crystalline silicon film .
In this paper we aimed at demonstrating a visible color filter by taking advantage of a subwavelength 1D grating in poly silicon. The LIL technique was employed to implement the grating, thereby enabling the extension of the effective area of the filter without further expense unlike the e-beam lithography. An oxide layer was introduced on top of the silicon grating to serve as an etching mask and to enhance the filtering selectivity. Finally, the impact of the angle of the incident beam upon the device performance as well as the positional dependence of it was studied as well.
2. Device design and simulation results
In this work we made an attempt to accomplish a blue-color filter operating in the visible range by incorporating a 1D subwavelength patterned grating in poly silicon. Figure 1 depicts the schematic and cross-sectional configuration of the device. A silicon grating with the pitch of less than the optical wavelength is first formed on a quartz substrate, and an oxide (SiO2) layer is placed on top of the silicon grating alone. When an input white light impinges on the device, a particular colored light could be acquired at the output. As illustrated in Fig. 1(a) and 1(b), the structural parameters related to the device are denoted as: Λ for the grating pitch, W for the width of the silicon stripe, Hp for the silicon grating height, and Hs for the oxide layer thickness. And the refractive indices for the silicon and oxide films are represented as np and ns respectively.
The device depicted in Fig. 1 is considered to consist of three planar layers in the vertical z-direction: the quartz substrate as a lower cladding, the silicon grating as a core, and the air combined with the oxide layer as an upper cladding. The core layer corresponding to the silicon grating is known to have an equivalent refractive index between the refractive index of the silicon and that of the surrounding medium, which is larger than that of the lower and upper cladding layers . As a result, the three-layer structure may function as a planar waveguide supporting guided modes in the lateral x-direction. When a light is launched normally toward the color filter as shown in Fig. 1, various diffracted waves are generated by means of the silicon grating. Among those diffracted waves, the fundamental order propagates in the same direction as the incoming light while other waves of higher orders in different directions. It is well accepted that a strong guided-mode resonance (GMR) between the incoming propagating light and the guided modes of the aforementioned three-layer planar waveguide structure can be brought about as long as a phase matching among them is satisfied [9, 10]. It is known that for a TE-polarized light with the electric field aligned in the direction of the grating, a subwavelength silicon waveguide grating may generate a transmission peak that corresponds to a resonance minimum in the reflection induced by the GMR effect under a certain condition [4, 11, 12]. Hence it plays a role as a bandpass filter with the diffracted fundamental wave taken as its output, where the optical transmission is significantly strengthened within a certain range of spectral band centered at a resonance wavelength but it is remarkably diminished otherwise.
We designed and analyzed the proposed device by relying on a commercially available tool based on the finite-difference time-domain method, OptiFDTD (Optiwave, Canada), taking account of the dispersion characteristics of the poly silicon material . The quartz substrate was assumed to possess a constant refractive index of 1.5 with negligible loss. And the oxide layer thickness was fixed at 200 nm, playing the role of a protecting mask during the etching of the underlying silicon film. Consequently, the structural parameters for the designed filter were determined to be: Λ=450 nm, W=150 nm, Hp=100 nm and Hs=200 nm. Its theoretical transfer characteristics are described in Fig. 2 for the case with and without the oxide layer. It is proven that the inclusion of an oxide layer tends to elevate the selectivity of the spectral transmission by suppressing undesired sub-peaks outside of the passband. As for the calculated device performance, the center wavelength was 460 nm in the blue color band and the peak transmission was 40% or so. A calculated reflection spectrum for the proposed waveguide-grating filter is also given in Fig. 2, indicating that a resonance minimum occurs near that transmission peak at 460 nm as addressed above.
3. The device fabrication and experimental results
The procedure used for producing the proposed color filter is briefly described in Fig. 3: On a 4” quartz substrate a poly-silicon film of 100 nm thickness was deposited with the LPCVD (low pressure chemical vapor deposition), and an oxide (SiO2) layer of 200 nm thickness was subsequently formed with the PECVD (plasma enhanced chemical vapor deposition). A Cr layer was introduced on top of the oxide layer to reinforce adhesion prior to spin coating the photoresist. A 1D grating pattern was generated in the photoresist layer by employing the Lloyds-mirror type LIL setup, where a He-Cd laser at 352 nm was used. The grating pattern was formed through the interference between the two beams impinging upon the device plane at an angle-one coming directly from the light source and the other traversing a reflecting mirror. The detailed parameters used for the grating recording include: the source laser power of 50 mW, the exposure time of 300 sec, and the power density of 0.1mW/cm2. And the Cr and oxide layers were selectively etched through the RIE (reactive ion etching). After the photoresist and Cr layers were removed, the poly-silicon film was appropriately etched to complete the grating pattern in it with the ICP (inductively coupled plasma), with the oxide film serving as a mask.
The scanning electron micrograph of the prepared grating involved in our filter is illustrated in Fig. 4, revealing that the grating pitch Λ=446 nm, the silicon line width W=156 nm, the grating thickness Hp=96 nm, and the oxide layer thickness Hs=198 nm. These results offer a good agreement with those of the design. For the purpose of evaluating the fabricated filter, it was first mounted onto a translation stage, then a TE-polarized light beam from a halogen lamp (Model LS-1, Ocean Optics), which was gained by inserting a Glan-Thompson polarizer in between the light source and the device, was launched normally to it. The output from the device was detected with a spectrum analyzer (Model USB 4000-VIS-NIR, Ocean Optics). Here the diameter of the light beam on the grating sample was about 3 mm. Its measured spectral response is plotted in Fig. 5. As anticipated, a bandpass filtering in the blue-color band was achieved with the center wavelength at 470 nm and the 3-dB bandwidth of ~90 nm. Its peak transmission efficiency was as high as 40%. And, from the theoretical reflection spectrum of the filter shown in Fig. 2, it is deduced that about 15% of the input beam power was reflected back and the remaining 45% of it could be dominantly accounted for by the absorption in the poly silicon layer  as well as the scattering by the practical grating patterns with rough sidewalls and surfaces. The device design needs to be further optimized to remarkably enhance the transmission efficiency .
As shown in the Fig. 5, overall, the measured transfer curve appears to be in decent agreement with the calculated one. In order to prove the performance of the filter, its output was then captured with a camera with an input white light illuminated. As known from the inset included in Fig. 5, we successfully got a clear blue-color image.
In view of practical applications of the color filter, it is required to note that the source light is mostly likely to be incident upon the device obliquely with an angle due to its intrinsic divergence rather than normally. Therefore the effect of the angle of the incident beam on the device performance deserves investigation. The relative peak transmission as a function of the angle of incidence θ was specifically examined both experimentally and theoretically as shown in Fig. 6. A maximum transmission was attained for the case of the normal incidence with θ=0° as expected, and as θ was changed from 0° up to 16° the relative transmission declined approximately linearly by 24% in measurement and 18% in theory. Accordingly, the measured and calculated rate of change in the relative transmission was estimated to be 1.5%/degree and 1.0%/degree respectively.
Next we paid attention to the positional dependence of the transfer characteristics of the device over an effective area of 3×3 mm2 on the device as shown in Fig. 7. A total of nine positions (P0 ~ P8) were chosen to observe their spectral response. On the whole, a fairly consistent response was attained within the effective region with the variation in the relative transmission and in the center wavelength of about 10% and 2 nm respectively. This non-uniformity in the device performance is believed to be primarily attributed to the fluctuation in local structural parameters associated with the silicon grating, which is to be mitigated by improving the LIL process to extend the effective operation area of the device . It was also confirmed numerically that other color filters different from the blue one addressed in this paper will be readily made possible by adjusting the device parameters like the period and height of the silicon grating. Furthermore, the nanoimprint technique based on a stamp made out of the LIL can be introduced to enable the mass production of the nano-structured device such as color filters at low cost .
In summary, we have presented a blue color filter utilizing a subwavelength patterned grating in poly silicon. It was designed by using the FDTD method and implemented by counting on the LIL, featuring large effective area and low-cost production. The positional dependence of its transfer characteristics and the influence of the angle of the incident beam on them were taken into consideration.
This research work was supported by Center for Nanoscale Mechatronics & Manufacturing of the 21st Frontier Project supported by Korean Ministry of Science & Technology and by the nano IP/SoC promotion group of Seoul R&BD Program. The authors would like to thank Dr. J. K. Yang at the Korea Advanced Institute of Science and Technology (KAIST) and Prof. I. K. Hwang at Chonnam University for their help.
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