As digital imaging arrays increase in size and resolution, defect correction could lower costs and improve yields. A fault tolerant active pixel sensor (APS) has been designed that will operate in the presence of a single point defect. The photosensitive area of the pixel is split in half and both halves operate in parallel. The output of each half is combined using a common row select transistor. The common pixel defects are optically stuck high (bright pixel) and optically stuck low (dark pixel). Simulations showed that a non-defective pixel would function normally and if one pixel half was defective, the other half would operate normally with half the sensitivity of a non-defective pixel. Fault tolerant photodiode and photogate APS’ were designed and fabricated in CMOS 0.18-micron technology. Half stuck high and half stuck low defects were induced on the fault tolerant pixels and the sensitivity ratio of non-defective to half defective pixels was measured (ideally 2). The experimental ratios ranged from 1.89 (stuck high) and 2.02 (stuck low) for the photodiode APS to 1.73 (stuck low) and 1.77 (stuck high) for the photogate APS. Non-defective fault tolerant pixels have also shown a 2x increase in sensitivity over normal APS’.
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.