Suppression of Staebler-Wronski Effect Induced Electrical Crosstalk in a-Si:H-Based Image Sensors
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A10.6.1
Suppression of Staebler-Wronski Effect Induced Electrical Crosstalk in a-Si:H-Based Image Sensors Jeremy A. Theil Semiconductor Products Group, Agilent Technologies, 5301 Stevens Creek Blvd., MS 51L-GO, Santa Clara, CA, 95051, U.S.A.
ABSTRACT Hydrogenated amorphous silicon photodiodes have been considered for use in array-based image sensors. They promise to significantly reduce the size and cost of CMOS image sensors, while offering the promise of improved pixel sensitivity. However, Staebler-Wronski Effect (SWE) based electrical crosstalk degradation has been a major concern in their acceptance, due to degraded spatial contrast and color fidelity. Since the SWE is a fundamental mechanism of a Si:H, solutions to this issue must look to ways of mitigating the SWE on diode array performance rather than elimination of SWE. In order to study electrical crosstalk, a novel device structure that inhibits light from reaching portions of the a-Si:H/dielectric interface was designed and fabricated to directly measure interpixel leakage currents. Results from these structures indicate that edge leakage can be a significant contributing component to the measured signal. In addition, a CMOS-compatible structure to suppress electrical crosstalk was designed and fabricated. Results from these structures demonstrate suppression of crosstalk up to lateral electric fields of at least 2 x 104 V/cm. Such suppression is adequate for densely packed minimum-size pixel arrays. Aspects of the design and implementation of the structure will also be discussed.
INTRODUCTION Over the last ten years, there has been increasing interest in the use of a-Si:H in photodiode arrays that are monolithically integrated onto integrated circuits [1-3]. Such integration allows a combination of 1) reduced imaging pixel area, 2) reduced sensor cost, 3) lower photodiode leakage, and 4) improved pixel sensitivity. As pixel-level complexity (hence area) grows, the advantages become more apparent. However, one major issue that has hindered acceptance of these arrays for visible light applications are imaging artifacts induced by the Staebler-Wronski Effect. Exposure of a-Si:H imagers to focused sunlight (~3000 suns) produced dramatic changes in pixel behavior that were reversible upon heating. One of the most troublesome of these artifacts is the increase in interpixel crosstalk upon Staebler-Wronski based degradation. Sensors have been fabricated in which cross talk in non-degraded material is acceptable, however upon severe enough degradation, crosstalk becomes noticeable. While modifications to the a-Si:H including reducing the degree of hydrogenation have a positive effect, no materials based a-Si:H solution has succeeded in completely suppressing crosstalk. This paper describes the effect of an opaque light shielding structure that for the first time successfully suppresses Staebler-Wronski induced crosstalk by preventing localized degradation within the interpixel region.
EXPERIMENTAL Details of diode fabrication have been presented elsewhere,
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