A pixel-level automatic calibration circuit scheme for capacitive fingerprint sensor LSIs

We propose a pixel-level automatic calibration circuit scheme that initializes a capacitive fingerprint sensor LSI to eliminate the influence of the surface condition, which is degraded by dirt during long-time use. The scheme consists of an automatic calibration circuit for each pixel and a calibration control circuit for the pixel array. The calibration is executed by adjusting variable capacitance in each pixel to make the sensor signals of all pixels the same. The calibration control circuit selects the pixels in parallel, and calibrates all pixels in a short time. The scheme was applied to a fingerprint sensor LSI using the 0.5-μm CMOS process/sensor process, and clear fingerprint images were obtained even for a degraded surface condition. This confirms that the scheme is effective for capturing consistent clear images during long-time use.