Scaling up multispectral color filters with binary lithography and reflow (BLR)

Abstract

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. Herein, we introduce an approach to produce at least 16 distinct filters based on a single low-resolution lithographic step with minimum feature size of 0.6 μm. Distinct from grayscale lithography, we employ standard binary lithography but achieve height variations in polymeric resist through a post-development reflow process. The resulting transparent polymeric films were incorporated in Fabry-Perot cavity structures with cavity thickness ranging from 90 to 230 nm to produce transmittance across the visible spectrum. This binary lithography and reflow (BLR) process demonstrates control of the dielectric layer thickness down to ∼15 nm. This new process provides a cost-effective alternative to traditional techniques in fabricating microscopic transmission filters, and other applications where precise thickness variation across the substrate is required.

Keywords: binary lithography and reflow; electron beam lithography; multispectral filters; structural colors.

Figure 1:

Schematic illustrating the process flow of fabricating transmission color filters.

Figure 2:

Optical micrographs (OM) and scanning electron microscopy (SEM) images of three types of patterns. The (a) brightfield and (b) darkfield OM images of the 16 pixels of P1 (square holes), P2 (lines) and P3 (meshes). (c) SEM images of P1 pixel 1, P2 pixel 1 and P3 pixel 8.

Figure 3:

The optical microscopy (OM) images of P1, P2 and P3 printed on Si substrate, after the reflow-process was carried out: (a) reflow at 180 °C – 30 s, (b) reflow at 180 °C – 2 min and, (c) reflow at 250 °C – 2 min, respectively. The white arrows indicate the range of pixels that forms flat surfaces during the reflow process.

Figure 4:

Brightfield OM of the three patterns in each step of spectral filter fabrication and corresponding transmission spectra. (a) OM of the pattern-1, pattern-2, and pattern-3 printed on the PMMA/Ag/Glass structure, images are taken after the patterns are developed. (b) Brightfield OM of the same sample after the reflow process, and (c) after Ag coating on the reflowed PMMA/Ag/Glass structure. (d) Transmission OM of the fabricated filters by Ag/PMMA/Ag/Glass. The measured transmission spectra of the filters fabricated by (e) P1, (f) P2, (g) P3. The arrow in (e), (f) and (g) indicate the trend of the spectra with variation of nos. of pixels in three patterns.