Design and implementation of pixel-based adjustable ESD protection circuits for capacitive fingerprint biometric sensors

The trade-off design between ESD endurability and polymer thickness on sensor plate for capacitive fingerprint is emerging in these years since the image quality of fingerprint is vital for biometrics authentication. In this paper, we propose a pixel-based ESD protection circuitry in parallel to GDPMOS and GGNMOS and an adjustable body terminal in proposed pixel-based ESD circuit to meet and exceed air discharge ±15 kV without thicker polymer. With the proposed pixel-based adjustable ESD protection circuit, the maximum air discharge achieved by (18 kV, −17 kV), and 10 out of 10 samples passed +15 kV, and 10 out of 10 samples passed −15 kV. Compared to conventional ground wall fingerprint sensor, the maximum air discharge was stopped by (15 kV, −15 kV), and just merely five out 10 samples passed 15 kV, seven out of 10 samples passed −15 kV. Besides the excellent ESD endurability, the proposed pixel-based ESD protection circuit achieves air-discharge 15.5 kV and grayscale count 230 at merely 50 μm thickness of polymer. Compared to the conventionally used ground wall sensor types, it passes only 13 kV and a grayscale count of 210 at a polymer thickness of 50 μm, but fails 15 KV Level 4 of IEC-61000-4-2. Even with polymer thickness increased to 250 μm, ground wall sensor types barely passed 15 kV, and the grayscale count dropped to 120, which is far from PIV image standard.