142 Fault Tolerant Active Pixel Sensors for Large Area Digital Imaging Systems

Digital imaging detectors are growing larger in silicon area and pixel count, which increases fabrication time defects, reducing yield, hence increasing costs and limiting area. In harsh environments, like high radiation conditions, what used to work might fail with time. Fault tolerant Active Pixel Sensors have been created by splitting the photodiode and readout transistors into two parallel operating halves with only a small area cost. These offer standard operation normally, but produce a recoverable image of half illumination sensitivity for single defects. The single-defect case can be compensated by a multiplication of two, whereas the double-defect case is much less likely but can be corrected via software. This paper presents the experimental and simulation results obtained from the fault-tolerant APS ' fabricated in CMOS 0.18-micron technology, disregarding the effects of interpolation. Test results suggest that after compensation, the percentage differences between the normally operating fault tolerant APS and the single-defect optically stuck-high and stuck-low cases are 0.5 % and 1.5 % respectively, which falls within experimental errors. Combining these fault tolerant APS ' with a software interpolation technique results in a system where initial simulations show the production of almost defect free images under error conditions with hundreds of dead pixels