Research of room-temperature continuous-wave terahertz imaging array based on microbolometer

Abstract

We have designed a novel uncooled Terahertz (THz) imaging array based microbolometer. The Micro Electro Mechanical System (MEMS) technology is used to fabricate the imaging array which comprise bolometer layer, THz absorption layer, supporting layer (silicon nitride (SiNx)), resonant optical cavity and electrode. The vanadium oxide thin films is selected for the bolometer temperature sensing material because it has a high temperature coefficient of resistance (TCR) in the range 2%/K and 3%/K at room temperature and suitable square resistance 18.40kΩ/□, a small 1/f noise constant and can be integrated with signal read-out electronics in a cost efficient way. In order to provide a high absorption of the radiation in the bolometer membrane, a resonant optical cavity structure which has a reflection layer formed at the bottom of air gap is adopted for the targeted wavelength. The best air gap of the optical cavity structure is quarter of wavelength of interest, for example, 25μm for 3 THz radiation. The absorption mechanism is that the two reflected THz radiations coming from the absorption layer and reflection layer interfere destructively at THz absorption layer and free carriers in THz absorption layer absorb THz radiation, the absorbed THz radiation heats the imaging array so that the resistance values of bolometer change. The microbolometer need to be packaged in vacuum for best performance, so a cylindrical vacuum chamber which is sealed with polyethylene lamina for the THz radiation is fabricated. In order to maintain the vacuum performance of the chamber (conventional bolometers operate with vacuum levels <0.01mbar), the vacuum pump and molecular pump are adopted; furthermore the packaging technique of vacuum chamber is introduced in detail. Finally, because of its uncooled property of the microbolometer, it will have a low cost and be easy for fabrication of large array.