a-Si:H photodiode technology for advanced CMOS active pixel sensor imagers

Hydrogenated amorphous silicon (a-Si:H) holds the promise of realizing three-dimensional semiconductor integrated circuits by placing the photodiode above the pixel control circuitry rather than in-plane with it. This has the obvious advantages of enabling large die-size reduction and higher light collection efficiency compared to standard crystalline silicon arrays. We have developed a photodiode array technology that is fully compatible with 0.35 μm CMOS process flows to produce image sensors arrays with 10-bit dynamic range that are 30% smaller than comparable standard crystalline silicon photodiodes. These sensors have 50% higher sensitivity, and two times lower dark current when compared to bulk silicon sensors of the same design. The various methods of interconnection of the diode to the array and their advantages will be presented. Diode leakage currents as low as 30 pA/cm2 have been measured. The effect of doped layer thickness and concentration on quantum efficiency (as high as 80% around 560 nm), and the effect of a-Si:H defect concentration on diode performance will be discussed.