Figure 1: Schematic illustrating the process flow of fabricating transmission color filters.
1. Pre-reading
Spectral sensors are essential components in digital cameras and mobile phones, utilizing color filters fabricated above photodiodes to exhibit color sensitivity. Traditionally, these color filters are composed of organic dye filters, but they suffer from drawbacks such as lack of durability under high temperatures, high fabrication costs, and impracticality for multispectral imaging where more than three primary colors are needed. Alternatives to organic dye filters include diffraction gratings, plasmonic structures, thin lossy-dielectric coatings, and high refractive index dielectric metasurfaces. Additionally, nanostructuring of silicon has been proposed to combine color filters and photodiodes into a single element. Despite these innovations, none of these alternatives currently meet industry standards for multispectral filters due to limitations in their optical properties, such as low transmission values or limited spectral range.
A promising candidate for multispectral transmission and reflective filters is the Fabry–Perot (F–P) cavity structure, which consists of an intermediate dielectric layer sandwiched between two metallic reflectors. The Ag/SiO2/Ag structures are particularly notable for their polarization-independent transmittance with narrowband full width half maximum (FWHM). However, achieving varied thicknesses of the intermediate dielectric layers to enable multispectral filters on a single chip remains a challenge. Addressing this, Prof. Joel Yang’s group from the Singapore University of Technology and Design, in collaboration with LiteOn Semiconductor, has proposed the Binary Lithography and Reflow (BLR) process. This method controls the intermediate thickness of the polymeric dielectric film in the F-P cavity structure, enabling a broad range of thicknesses (15–200 nm) with narrow steps (~6 nm) between pixels. Utilizing standard binary lithography followed by thermal reflow, this technique achieves sharp transitions of less than 2 μm at pixel borders, applicable to micron-scale lithography processes. The BLR method, which focuses on producing regions with flat and uniform thickness rather than complex 2.5D structures, shows significant potential for fabricating both transmission and reflective spectral filters.
2. Background
Efforts to increase the number of filters are driven by the demand for miniaturized spectrometers and multispectral imaging. However, processes that rely on sequential fabrication of each filter are cost ineffective. The BLR process presented here enables a single lithographic step to control the thickness of F–P cavities, suited for scalable manufacturing of multiple high-efficiency transmission color filters. The Ag/PMMA/Ag structure was considered as F–P cavity structure and the optical filters span visible spectrum. Three designs were considered for the patterning on the PMMA/Ag, followed by reflow process to provide systematically controlled cavity steps. As the exposure dose of EBL was constant, the pattern-design and fill factor determine the thickness of the cavity. Remarkably, the resulting thickness spanning ∼8–95 % of the original PMMA thickness could be achieved. Accurate thickness control with increments of ∼6 nm was achieved by varying the pitch of the binary pattern. Though we have shown simple rectilinear patterns exposed using EBL in PMMA, this work can be extended to more sophisticated patterns for better reflow behavior, and patterning using other lithographic methods, e.g. projection lithography or nanoimprint lithography, with other suitable polymers or resins.
3. Innovative research
Figure 1 shows a process flow of the proposed method to fabricate transmission color filters. Details of the fabrication process are in the experimental section. Briefly, a 24-nm thick Ag film is coated on a glass substrate forming the bottom metallic layer of the F–P cavity. A 210 nm-thick positive-tone PMMA film is spin coated as the dielectric layer of the MIM structure. A 3 by 16 array of 50 × 50 μm2 squares is patterned by electron beam lithography with a chosen infill pattern of varying density. Patterns consist of squares, lines, or meshes. After development, square holes, trenches or square posts are formed in PMMA. The sample was heated on a hot plate under three different conditions: 180 °C for 30 s, 180 °C for 2 min and 250 °C for 30 s to induce reflow of the patterned PMMA to soften and merge into films of varying thickness depending on the initial filling factors. We later observed that 180 °C for 2 min is close to optimal. In the entire patterning process, the exposure dose was kept constant. Thus, the thickness of the final dielectric film, d, is controlled by the design of the pattern in binary lithography. The Ag film is finally deposited to complete the F–P cavity structure.
Three designs consisting of square holes, lines, and mesh patterns corresponding to pattern-1(P1), pattern-2 (P2) and pattern-3 (P3) were studied. The optical microscopy (OM) images of these three types of patterns are presented in Figure 2 (a). The pixel-to-pixel distance was 20 μm for all three designs. In P1, we patterned squares with a constant nominal width, x = 0.6 μm, such that pattern density was varied by varying the distance between these squares. With PMMA as a positive resist, these exposed square patterns translate into holes after development. Similarly, both P2 and P3 consist of lines also with width x = 0.6 μm. In P2, lines were formed in only one direction while in P3, lines in both x and y directions were patterned, resulting in isolated PMMA islands. Figure 2(b) shows SEM images of three representative pixels for unit cells of P1, P2 and P3.
The OM image of pixels after the reflow process is shown in Figure 2(c)–(e) where three reflow conditions: 180 °C for 30 s, 180 °C for 2 min and 250 °C for 30 s are presented. We observe different reflow behaviors in P1, P2 and P3. For instance, P1 reflowed earlier than P2 and P3, with all regions fully forming continuous films after 30 s at 180 °C, as indicated by the white arrows. The rate of reflow was higher for higher pattern density or lower fill factor. In contrast, the lower density patterns in P2 required a longer heating duration of 2 min to form a continuous film, with the sparsest patterns never forming films unless heated to 250 °C. P3 exhibited the lowest tendency to form continuous films. However, as it had the lowest filling fraction of PMMA patterns, it achieves the thinnest films of the three patterns. The films increase in thickness from left to right. At 250 °C - 30 s, all the pixels are formed, due to the fluid nature of PMMA at this temperature but exhibit a thickness gradient at the interpixel boundary.
Figure 2: Optical microscopy (OM) and scanning electron microscopy (SEM) images of three types of patterns printed on Si substrate. The (a) brightfield of the 16 pixels of P1 (square holes), P2 (lines) and P3 (meshes). (b) SEM images of P1 pixel 1, P2 pixel 1 and P3 pixel 8. The OM images of these patterns after the reflow-process was carried out: (c) reflow at 180 ◦C - 30 s, (d) reflow at 180 ◦C - 2 min and, (e) reflow at 250 ◦C - 2 min, respectively.
Figure 3 shows brightfield OM images of the filters at various stages, along with transmission color images, and measured transmission spectra. Figure 3(a) presents OM images of P1, P2 and P3 printed on PMMA (210 nm)/Ag/Glass. Figure 3(b) exhibits the optical micrographs after the reflow process of the same structures shown in Figure 3(a). Figure 3(c) shows OM images after the Ag top layer is coated. The series of colors generated for each pixel provides visual evidence of the systematic step creation of PMMA by reflow process. The transmission color images are shown in Figure 3(d). The transmission spectra of the highlighting color pixels span the visible spectrum from 450 to 750 nm wavelengths at ∼50 nm steps as shown in Figure 3(e)–(g).
Figure 3: Brightfield OM of the three patterns printed on the (a) PMMA/Ag/Glass, (b) after the reflow process. The (c) reflection and (d) transmission color after Ag coating on the reflowed PMMA/Ag/Glass structure. The measured transmission spectra of the filters fabricated by (e) P1, (f) P2, (g) P3.
4. Applications and perspectives
The present study introduces a novel method for controlling PMMA resist’s step height through binary lithography and reflow (BLR). Unlike the other conventional approaches such as grayscale, which lacks repeatability and is impractical for industrial applications due to its sensitive dose requirement, BLR offers a one-time exposure and reflow process to achieve step heights, reducing fabrication steps and increasing process latitude for industry application.
These research results are published online with the title “Scaling up multispectral color filters with binary lithography and reflow (BLR)” on Nanophotonics.
The authors of this article are Md Abdur Rahman, Soroosh Daqiqeh Rezaei, Deepshikha Arora, Hao Wang, Tomohiro Mori, Ser Chern Chia, John You En Chan, Parvathi Nair Suseela Nair, Siam Uddin, Cheng-Feng Pan, Wang Zhang, Hongtao Wang, Zheng Ruitao, Lim Sin Heng and Joel K. W. Yang. Prof. Joel K. W. Yang’s research group is affiliated to Engineering Product Development pillar, Singapore University of Technology and Design. Zheng Ruitao and Lim Sin Heng are from LiteOn Semiconductor.
