Flat Field Forensics

TL;DR

This paper investigates subtle charge transport issues in flat field imaging, proposing modifications to transfer sequences and clocking methods to eliminate variance dips and structural traps, thereby improving image uniformity and sensor performance.

Contribution

It introduces new techniques to address charge spillage and structural traps in flat field sensors, enhancing uniformity and reducing artifacts.

Findings

Variance dips at high signal levels are eliminated by modified transfer sequences.

Increasing serial clock voltage reduces variance dips caused by traps.

Using three parallel phases reduces pixel area variation and improves uniformity.

Abstract

We present two subtle charge transport problems revealed by the statistics of flat fields. Mark Downing has presented photon transfer curves showing variance dips of order 25% at signal levels around 80% of blooming. These dips appear when substrate voltage is raised above zero, for - 0V to 8V parallel clock swing. We present a modified parallel transfer sequence that eliminates the dip, based on the hypothesis that it is caused by charge spillage from last line to the 2nd last line. We discuss an experiment to test whether the electrode map is incorrectly reported in the data sheet. A more subtle dip in the variance occurs at signals around 6000 e-. This is eliminated by increasing serial clock high by a few volts, suggesting the existence of a small structural trap at the parallel-serial interface. Tails above blooming stars are suppressed using an inverted clocking during readout and a positive clocking during exposure to maintain sharpness of the PTC. We show that integrating under three parallel phases, instead of the two recommended, reduces pixel area variations from 0.39% to 0.28%, while also eliminating striations observed along central columns in pixel area maps. We show that systematic line and column width errors at stitching boundaries (~15 nm) are now an order of magnitude less than the random pixel area variations.