A 216 × 216 Global-Shutter CMOS Image Sensor With Embedded Analog Memory and Automatic Exposure Control for Under-Display Optical Fingerprint Sensing Applications

A 216 × 216 under-display optical fingerprint CMOS image sensor (CIS) is proposed and was prototyped using 0.11-μm CIS technology. Two analog storage nodes that act as a ping-pong buffer are embedded in each pixel of the global-shutter CIS, rendering a digital buffer unnecessary and increasing the operating frame rate. A processor with automatic exposure (AE) and black-level correction is embedded in the CIS to shorten the data transmission time. The AE and dynamic range enhancement functions are initiated to increase the success rate of fingerprint recognition. The full well capacity and sensitivity of the global-shutter CIS are 9 ke- and 21 V/(lux · s), respectively. A prototype of the fingerprint sensor consumed only 26 mW with a 3.3-V supply voltage. An imaging lens and the CIS were combined to form the prototype of a compact camera module (CCM). Each pixel size is 6.8 × 6.8 μm2, equivalent to a panel resolution of 747 dots per inch with an optical ratio of 5. The sensor chip size is 2.23×2.39 mm2, and the sensing area is 1.469×1.469 mm2, occupying 40.5% of the chip. The prototype CCM was successfully embedded in a mobile phone, and the performance of the CIS was validated in the temperature range of -20°C to 50°C. Even when the interrupt interval for the serial peripheral interface communication was 300 ms, the CIS as a slave device could capture the fingerprint images clearly.