Front-side illuminated CMOS spectral pixel response and modulation transfer function characterization: impact of pixel layout details and pixel depletion volume

Abstract

We have measured the pixel response and derived the spectral modulation transfer function (MTF) of a front-side illuminated complementary-metal-oxide-semiconductor (CMOS) focal plane array at wavelengths of 440, 544, 633, and 905 nanometers using a spot scanning technique. The spot scanning apparatus utilized a confocal microscope configuration with a spot diameter about 1.4 μm (wavelength dependent) to scan the CMOS imager 9-μm pixel pitch in 0.5-μm steps. The confocal microscope had a magnification of 200X, which enabled precise spot position verification as well as good characterization of the optical spot size. Two independent experimental techniques-a tilted knife-edge method and spot-scanning method-were used to derive the wavelength-dependent MTF data for the CMOS imager. The resultant MTFs from each technique were generally equivalent within the experimental errors of the two methods. Specific impacts of pixel circuitry layout and diffusion appear in the spectrally dependent MTFs derived from data acquired using each measurement technique.