High performance uncooled THz sensing structures based on antenna-coupled air-bridges

Antenna-coupled sensing elements, microbolometers as well as thermocouples, are potential solutions in focal plane arrays (FPA). The applications for these lightweight and low-cost sensors are specified in the field of infrared and millimeter wave imaging and sensing. Typically, these detectors can be operated uncooled in various frequency ranges. The purpose of this work is the fabrication of THz sensing structures with a low noise equivalent power (NEP) based on antenna-coupled thermocouples, which are arranged on air-bridges. For the first configuration level, a 2D thermoelectric array consisting of 2 x 4 pixels was investigated. Thereby, each pixel comprises 8 thermocouples, which are connected in series to increase the responsivity. A highly effective Bi0.87Sb0.13/Sb thermocouple materials combination was used. The thermoelectric structure was matched to the impedance (200 ) of the planar dipole antenna. To improve the responsivity of the sensor elements the substrate thickness was adapted to an operation frequency of 812 GHz and acts as an optical resonator in combination with a backside gold reflector and a tuned antenna geometry. The fabrication of the 30 µm long and 3 µm wide air-bridges was realized by surface micromachining with a polyimide sacrificial layer. Thermoelectric layers as well as antenna structure and diffusion layer were patterned almost exclusively by lift-off technique in order to obtain very small structure dimensions with high reliability. In first functional tests the NEP of so manufactured THz sensing structures with 8 thermocouples was determined to be 51 pW/Hz1/2 by an AC-DC-measurement at room temperature operation and under vacuum conditions.