A Swipe-type Fingerprint Sensor using Fixed-Displacement Shuttering Scheme

Biometric sensors for wearable devices have a dilemma between the small physical size to fit within the small device and the large sensing area to obtain enough information for reliable recognition. A swipe-type fingerprint sensor is a good candidate that satisfies both of small size and large sensing area. This paper proposes a compact swipe-type fingerprint sensor using a fixed-displacement shuttering scheme to avoid the swiping-speed dependent image distortion. The shutter for a line image sensor is adaptively controlled by the displacement measure unit, which are consist of two lines of pixel array, Soft-XOR circuits and digital control circuits. The displacement of a finger is detected by calculating the pattern matching of two line images, which are separated for fixed distance. The proposed system utilizes optical pixels for high-speed image capturing. The prototype sensor chip was fabricated with $0.25~\mu \text{m}$ standard CMOS process. The sensor chip occupies only 4mm $\times 0.9$ mm and provides 625 dpi up to 25cm/s of swiping speed. Experimental results obtained from the sensor demonstrate that the proposed shuttering method reduced minutiae distribution by factor of 3 over the conventional fixed-time interval shuttering.