Development of 3D WLCSP with Black Shielding for Optical Finger Print Sensor for the Application of Full Screen Smart Phone

As an important biometric technique, fingerprint verification is widely used for smartphone, IoT and payment due to the small size and high security. To meet full screen trend of smartphone, optical finger print sensor (FPS) using CIS received great interest and gradually replaced capacitive FPS. For such a device, a thin package size with better image performance is required. To meet these critical requirements, 3D wafer-level chip scale package (WLCSP) with via last TSV (through silicon via) was the best solution for CIS. In this paper, 3D WLCSP for FPS based on CIS with via last TSV interconnects was developed. For the optical FPS, the package size is 920×656μm with 580μm height, which includes 200μm thick glass, ~100μm thick optical layer, ~100μm thick CIS chip and BGAs of 80μm height. Electrical interconnections are implemented by via last TSVs, where big silicon trenches were formed on the backside of the two-row pads on the adjacent chips, then the small TSVs were etched for contacting with the pads of the backside. Dry etch was used to remove the oxide of the pads and PVD, electroplating and electroless plating were used to form redistribution layer (RDL).The light reflection and scattering at the sidewalls of the CIS chip and optical layer will affect the sensor performance greatly. Therefore, the coating of black materials on the sidewalls was developed to improve the performance of the optical FPS for high-end applications. The precut on dicing street was used to form the deep trench on CIS wafer and optical layer for black shielding. To form a void-free filling, the wetting property of the black glue is very important. Three different materials with required optical shielding properties were used. The process of dispensing of black glue into the trenches was developed. The droplet diameter of 50μm with a jetting speed of 70mm/s was used for black material filling. The results showed that one of the materials with 15μm thickness on the sidewalls with 99.9% optical shielding from 430-560nm light wavelength and voidfree filling was obtained which is good enough for the application. Reliability of the 3D WLCSP with black shielding was characterized by thermal cycling (TC) and highly accelerated stress test temperature storage (HAST) tests and the results show good package level reliability. The results indicate that the 3D WLCSP can provide a low cost and reliable solution for optical FPS.