Effect of Scratch Stress on the Surface Hardness and Inner Structures of a Capacitive Fingerprint Sensor LSI

Abstract

The authors performed a scratch test in which a weighted needle applies scratch stress to the surface of a capacitive fingerprint sensor large scale integration (LSI), which has a grounded wall (GND wall) structure where each sensor plate is surrounded by a lattice-like wall. The scratch stress degrades not only the sensor's surface but also the metal interlayer. Increasing the thickness of the surface passivation film and that of the interlayer reduce the degradation of the surface and inner structures. To confirm the influence of thick passivation film on the electrostatic discharge (ESD) hardness and the intensity of sensing of the fingerprint sensor LSI, the authors performed an air discharge test and fingerprint identification. The tests show that a thick passivation film and a thick interlayer are effective in preventing scratch stress with ESD hardness and the intensity of sensing of the fingerprint sensor LSI.