While pixel size have shrunk considerably over the last decade (from more than 10μm to less than 2μm), designing efficient colour filters that can scale down with the pixel size has remained a challenge. In a paper published in the Nano Letters under the title "Visible Wavelength Color Filters Using Dielectric Subwavelength Gratings for Backside-Illuminated CMOS Image Sensor Technologies", the researchers highlight the limits of dye-doped polymers often used for RGB colour filters in digital colour imaging and discard plasmonic colour filters for their relatively low absolute efficiency (in the 40 to 50% range), to replace them with seemingly simple dielectric subwavelength gratings.
The researchers created several colour filter designs (an RGB Bayer pattern on top of a silicon photodiode) at various pixel sizes ranging from 5μm to 1μm. Made of polycrystalline silicon (poly-Si) on top of a SiO2 spacer right over the Si photodiode layer, the micro gratings were demonstrated to exhibit a highly efficient transmission while being nearly insensitive to the light angular incidence.
Experimenting with different hole patterns, they realized an optimum RGB colour filter structure consisting of an 80nm-thick poly-Si slab with air holes, placed on a 115nm thick SiO2 spacer layer sitting on backside-illumination (BSI) Si photodiodes. The red part was consisted of 90nm-diameter holes in a hexagonal lattice with a 250nm periode, the green was designed with a period of 180nm with 140nm-diameter holes in a square lattice, and the blue design had 240nm-diameter holes in a hexagonal lattice with a period of 270nm.