Parylene supported 20um*20um uncooled thermoelectric infrared detector with high fill factor

Presented is a novel design for an uncooled surface-micromachined thermoelectric (TE) infrared (IR) detector. The detector features a P-doped polysilicon/Nichrome (Cr20-Ni80) thermocouple, which is embedded into a thin layer of Parylene-N to provide structural support. The low thermal conductivity (~0.1W/m.K), chemical resistance, and ease of deposition/patterning of Parylene-N make it an excellent choice of material for use in MEMS thermal detectors. This detector also features an umbrella-like IR absorber composed of a three layer stack of NiCr/SiN/NiCr to optimize IR absorption. The total device area is 20 um * 20 um per pixel with an absorber area of ~19 um * 19 um resulting in a fill factor of 90%. At room temperature, a DC responsivity of ~170V/W with a rise time of less than 8 ms is measured from the fabricated devices in vacuum when viewing a 500K blackbody without any concentrating optics. The dominant source of noise in thermoelectric IR detectors is typically Johnson noise when the detectors are operating in an open circuit condition. The fabricated detectors have resistances about 85KOhm which results in Johnson noise of about 38nV/Hz^0.5. The D* is calculated to be 9 * 106 cm*Hz0.5/ W. Preliminary finite element analysis indicates that the thermal conduction from the hot junction to the substrate through the TE wires is dominant ( GTE >> Gparylene) considering the fabricated dimensions of the parylene film and the TE wires. Thus, by further reducing the size of the TE wires, GTE can be decreased and hence, responsivity can be improved while the parylene film sustains the structural integrity of the cell.

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