Uncooled thermocouple air-bridge structure for a THz imaging system

We have developed an air-bridge structure supporting a thermocouple-based conducting strip, which acts as both a terminating resistance of an antenna and temperature sensor [1]. The fabrication of the 30 µm long and 3.5 µm wide air-bridge was accomplished by surface micromachining with a polyimide sacrificial layer. As the supporting layer, a 200 nm PE-CVD SixNy layer was patterned on the sacrificial layer. For passivation a 25 nm thin ALD-SiO2 with a low thermal conductance and a high conformal coverage was used. The influences of the geometry, the substrate thickness and the thermal properties of the freestanding air-bridges were investigated by computer simulations. Based on these results a compatible technology was developed to obtain high performance thermoelectric terahertz (THz) sensing structures. The responsivity of the thermocouple air-bridge structure was evaluated by electrical characterization. For these measurements the sensing structures were loaded by a well-defined AC bias, which simulated an absorbed incoming terahertz radiation and its conversion into heat resulting in a corresponding measurable DC signal. The NEP of the structure was measured to be 18 pW/Hz1/2 under vacuum conditions and 67 pW/Hz1/2 under atmospheric conditions, respectively. Key words: Surface micromachining, High detectivity, Antenna coupled, THz detector