Fabrication of thermal-isolation structure for microheater elements applicable to fingerprint sensors

We are developing a fingerprint-sensor device that has one-dimensionally arrayed high-density microheater elements. To provide thermal isolation, we designed and fabricated two sets of insulation cavities for the heater elements, one to reduce the heat transferring from the heater elements to the substrate and the other one to reduce that of heat to the wiring. The insulation cavities in the first set are etched and laterally penetrate under the heater bridges, leaving a SiO2 diaphragm with the heater bridges on it. Those in the second set are etched lengthwise of each heater element, leaving a set of SiO2-wiring bridges over them. These SiO2-wiring bridges are used to perform metal-film wiring. The wiring (an electrical feed through) is formed using the lift-off method and running on the SiO2 film surface from the heater element to the bonding pad through the SiO2-wiring bridges; in this way, the amount of heat transferred to the wiring is reduced. The fabricated sensor device was made on a (1 0 0) silicon-on-insulator (SOI) wafer using micromachining technology. Each heater bridge was 5μm×17μm×50 μm with a pitch of 80 μm. Because of its small thermal capacity and effective thermal isolation, the sensor element is very sensitive. When a 0.4 V/20 μs pulse voltage was applied to the heater elements, their resistance reached a steady maximum value in about 4–5 μs, i.e. the temperature rise was saturated promptly.