Thermal and infrared performance of a free-standing microthermocouple

This paper discusses the thermal and optical characteristics of a free-standing microthermocouple with a view to its application as an infrared detector. This device is a radiation thermocouple in which the hot junction is thermally isolated from the substrate by making it free-standing. When radiation is absorbed by the device, the free-standing structure heats while the much larger cold junction, which is in physical and thermal contact with the substrate, remains at ambient temperature. Copper and constantan have been used as the thermocouple materials. Electrical and optical measurements have been performed for free-standing wire lengths of 20, 40, and 60 μm and have been theoretically modeled. Thermal analysis shows that convective heat loss is significant and from the measurements a convection coefficient of 20 000 W m^-2 K^-1, for the free-standing wires, has been deduced. The time constants of these devices were of the order of a few microseconds. The device output has been shown to depend on the polarization of the input infrared radiation. The output was largest when the free-standing wires were parallel to the polarization of the incident radiation.