Thermal characterization of micro heater arrays on a polyimide film substrate for fingerprint sensing applications

Abstract

A new type of micro fingerprint sensor was proposed and the sensing principle was demonstrated using micro heaters as the sensing elements. The arrayed sensing elements were micro resistors made of sputtered 200 nm thick platinum film on a 50 µm thick flexible polyimide film substrate. The temperature coefficient of resistance was 0.0029 (K−1) and showed good linearity. Because of the small thermal capacity and effective thermal isolation, the resistance of the heater showed high sensitivity and short response time to the applied power. By applying a pulsed voltage of 5.4 V at ambient air, the heater temperature could be increased from room temperature to 300 °C in 0.1 µs. When a fingerprint model (line and space: 100 µm; pattern height: 50 µm) made of silicone rubber was in contact with the sensor surface, contacting patterns of the fingerprint model were clearly recognized as the difference in resistance, i.e. in temperature. Micro through-holes with a diameter of 90 µm were wet etched vertically on the polyimide film substrate. The sensor array on the front surface and the wiring on the rear surface of the substrate were interconnected by depositing 1 µm thick copper film inside the through-holes using the electroless plating technique. The whole fabrication processes were simple and achieved in a low temperature range (<130 °C) on the non-silicon, low-cost and flexible substrate.