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Subwavelength Color Filter
Introduction
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.
Color Filter Array
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.
Types of Color Filters
These images demonstrate other types of color filters.
Features
- Pigment/Dye filters are well known and widely used, but small pixels are more susceptible to cross talk and slowly fade with UV exposure.
- Plasmonic filters are very sensitive, which has both pros and cons. These are potentially easier to fabricate, and do not degrade, but have low transmittance.
- Dielectric filters can be easy to fabricate, do not degrade, and have high transmittance compared to plasmonic filters.
Plasmonic 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.
Dielectric Color Filters
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.
Improved Blue Configuration
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:
- Scan over Period_custom and Factor and measure transmission at ~450nm
- Calculate spectra at optimal point Period_custom = 150nm, Factor = 0.9
This retains angular insensitivity but has higher cross talk. Cross talk can be reduced if a slightly higher ‘blue’ wavelength is used (about 480nm).
Studying 'Finite Pixel
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:
- Peak transmission is lower, most likely due to edge effects and finite size of pixels.
- Blue peak is lower and has high cross talk, blue pixel should be optimized for finite size,but not infinite size.
Conclusion
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.
Video Presentation
Watch the video presentation "Design of Sub-wavelength Color Filters for CMOS Image Sensors."
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