Cloud native EDA tools & pre-optimized hardware platforms
Tools Used: FullWAVE FDTD, DiffractMOD RCWA, MOST
Cameras are becoming smaller in order to meet the need for increased resolution and smaller form factors, such as demand for compact devices. This miniaturization requires smaller pixels and a redesign of traditional color filters. Many color filters are based on traditional absorptive dyes which are susceptible to cross talk as pixel size decreases and fade over time.
In this application note, we explore two alternative color filter types: plasmonic based color filters built from thin metal films, and Dielectric color filters built from dielectric materials.
Cameras utilize color filter arrays to accurately detect color.
There are many possible color filter arrangements, the Bayer mosaic is shown below. This color filter uses four sensors per pixel: 1 blue, 1 red, and 2 green, the resulting color image is found via post processing.
These images demonstrate other types of color filters.
In this example, all gratings are hexagonal nano hole arrays in
a 150nm thick Al plate in a SiO 2 background. Here we show individually tuned transmissive gratings for Red, Green, and Blue.
The results calculated by FullWAVE FDTD agree well with the reference, with the exception of an additional resonance at shorter wavelengths which contributes to cross talk. We can note that the reference did not give exact geometry and resonances are sensitive to geometry.
In this example, all gratings are air holes in a 80nm poly Si slab on a 115nm SiO 2 spacer and the transmissive gratings are individually tuned for Red, Green, and Blue.
The results calculated by DiffractMOD agree very well with the reference.
The angular sensitivity results also agree with the reference.
The Red and Green configurations have about 75 percent transmission, while Blue only has about 60 percent. This could be due to the fact that poly Si is more absorptive at smaller wavelengths, and smaller wavelengths require smaller structures, which introduces possible fabrication difficulties. We can use MOST to explore the parameter space to see if we can improve the performance of the Blue configuration.
In MOST, we take the following steps to improve blue configuration:
This retains angular insensitivity but has higher cross talk. Cross talk can be reduced if a slightly higher ‘blue’ wavelength is used (about 480nm).
So far, we have studied infinite structures but in reality, pixel size is finite. We can use FullWAVE FDTD to study a single pixel using the new ‘optimized’ blue sub pixel. With a sub pixel pitch of 1.05 µm, we'll measure transmission through each subpixel. We use periodic boundary conditions to effectively consider an infinite 2x2 array of RGB subpixels.
The normalized results are similar to ideal infinite simulation, with some expected differences:
The miniaturization of cameras requires smaller pixels and redesigned color filters. Color filters based on traditional absorptive dyes are susceptible to cross talk as pixel size decreases and can fade over time. Plasmonic color filters are promising but suffer from high loss, and dielectric color filters are a good alternative that can fit into existing processes.
Watch the video presentation "Design of Sub-wavelength Color Filters for CMOS Image Sensors."
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