An improved and optimized spectral spot-scanning system for visible focal plane array (FPA) sub-micron pixel photoresponse
testing is presented. This updated configuration includes: (1) additional diagnostic analysis tools which more
completely characterize the operation of the system; (2) a confocal microscope fitted into the optical system to aid in
more precise determination of spot focusing on the imager; (3) a post-acquisition transformation to imager pixel
response data to reduce overall data acquisition time. Wavelength-dependent pixel response data is presented to
demonstrate the repeatability of this setup as well as to quantify the impact of random and systematic experimental
errors.
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.
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