A Fast-Response Thermopile Photodetector Fabricated by a Photolithography Technique Combined with Laser-Beam Machining

Abstract

A micrometer-scale thermopile formed on Si substrate using a photolithography technique combined with laser-beam machining is described. Initially, the surface of an 11-mm-square and 0.3-mm-thick Si substrate was oxidized to grow a SiO₂ thin-film. A 300-μm-long, 100-μm-wide and 0.5-μm-thick SiO₂ air bridge was then formed on the Si substrate using a photolithography technique. Next, copper and constantan were evaporated in layers on the middle of the air bridge. This evaporated thin film functioned as a type-T thermocouple. Then, in order to improve the sensitivity of the thermocouple, three of the type-T thin-film thermocouples were shaped by cutting the evaporated film into three regions using a laser-beam machining technique. Finally, the thermopile was completed by connecting the three thermocouples in series, locating their hot junctions on the center of the air bridge and cold junctions on the pad areas used as electrodes. Because the heat capacity and the heat dissipation of the hot junction areas differ from those of the cold junction area, a temperature difference occurs between the hot and cold junctions when the thermopile receives radiation. This temperature difference generates a thermo-electromotive force between the cold and hot junctions, which is a measure of the intensity of the incident radiation. Due to a small heat capacity combined with a relatively large radiation reception area, the time-constant of this type of thermopile for light-illumination was found to be less than 1 ms, which was more than ten times faster than that of conventional thermopiles presently on the market.