Backside-Illuminated Lateral PIN Photodiode for CMOS Image Sensor on SOS Substrate

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In this paper, we propose a method to design charge-sensing elements for CMOS image sensor pixels on a silicon-on-sapphire (SOS) substrate. To address the low quantum efficiency problem due to very thin active film used, a backside illuminated lateral PIN photodiode on an SOS substrate is proposed and developed. It has the advantages of higher photo response with a PIN structure and improved optical transmission with a backside illumination through a transparent sapphire substrate. An active pixel sensor (APS) based on the PIN and backside illumination has been implemented in a commercially available SOS CMOS process. Acceptable sensitivity in optical conversion from the APS can be achieved, even with the ultrathin silicon film. The APS is demonstrated to function at 1.2 V, giving a dynamic range of 51 dB.