Digital cameras have emerged as the dominant image capture technology. Among the most important trends in digital camera design is the development of CMOS image sensors. These sensors are being scaled with CMOS technology to enable increasing level of integration of capture and processing to reduce system power and cost.
Towards completing image sensor design integration, we have explored the possibility of introducing wavelength selectivity using only standard CMOS technology processing steps. Specifically, we have implemented light filters using metal patterns placed within each pixel to control the transmission of light to the photodetector. We refer to such pixel design as an integrated color pixel (ICP). ICPs may prove to be an alternative to current pixel designs that employ color filter arrays. Additionally, ICPs may be useful in a variety of image sensing applications, such as multi-spectral imaging.
We demonstrate for the first time a CMOS image sensor with ICPs based on sub-visible wavelength metal grids featuring visible wavelength selectivity in a standard 0.18um CMOS technology. We perform a comprehensive optical characterization of these CMOS ICPs, including transmittance measurements under various illumination conditions. We describe electromagnetic vector-field simulations, using a 2D finite-difference time domain (FDTD) method, of (a) the optical path between pixel surface and buried photodetector of a CMOS image sensor, (b) the effect of placing 1D metal patterns in this path. We then compare measured and predicted transmittances of 1D patterned metal layers. Finally, we predict the transmittance for more elaborate designs, that is, filters using more than one metal layer and filters in more advanced CMOS technologies.
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