A CMOS Micromachined Capacitive Sensor Array for Fingerprint Detection

Biometric techniques, such as fingerprint detection, have been widely applied to identify a specific user for security and safety reasons. This work presents an integrated capacitive sensor array for fingerprint detection based on the difference of pressures induced by the ridges and valleys of a finger tip. The 8 × 32 sensing membranes are released by wet etch of the complementary metal oxide semiconductor (CMOS) metal layers and no additional thin-film deposition is required to form the capacitive electrodes. Each membrane contains the CMOS dielectric and metal layers with a total thickness of 3.375 μm. The size of each sensing pixel is 65 × 65 μm³, equivalent to an imaging resolution of 390 dpi. The sensing capacitance and capacitive sensitivity are 12 fF and 1.41 fF/MPa, respectively. The measured spring constant of the membrane is close to the simulated value of 1400 N/m. Part of the fingerprint image is successfully produced by the CMOS sensor array.