Fingerprint imager based on a-Si:H active-matrix photo-diode arrays

We present a thin-format optical fingerprint image sensor based on hydrogenated amorphous silicon (a-Si:H) n-i-p photo-diode technology. The fingerprint sensor has resolution of 403 dpi. Optical Monte Carlo simulations and experimental data obtained for an edge-light illuminated sensor structure shows that the fingerprint image is formed by total internal reflection (TIR) of photons in finger skin valleys. Conversely, for a back-light illuminated sensor structure the image is formed by reflection of photons in finger skin ridges. Finally, a fingerprint sensor prototype has been built to demonstrate these two types of sensor technology.