Ion beam sputtered Ge–Si–O amorphous thin films for microbolometer infrared detectors and their application in earth sensors

Abstract

Ge–Si–O thin films are prepared by ion beam sputtering technique with argon (Ar) alone and argon and oxygen as sputtering species, using sputtering targets of different compositions of Ge and SiO2. The deposited thin films are amorphous in nature and have chemical compositions close to that of the target. The study of electrical properties has shown that the activation energy and hence the thermistor constant (β) and electrical resistivity (ρ) are sensitive to oxygen flow rate, and they are the least for thin films prepared with Ar alone as the sputtering species. Different thermal isolation structures (TIS), consisting of silicon nitride (Si3N4) membrane of different thicknesses, Ge–Si–O thin film and, chromium coating on the rear side of the membrane, are prepared by bulk micro-machining technique, whose thermal conductance (Gth) properties are evaluated from the experimentally determined current–voltage (I–V) characteristics. Gth shows non-linear dependence with respect to raise in temperature of thin film thermistor due to Joule heating. The infrared micro-bolometer detectors, fabricated using one of the TIS structures have shown responsivity (ℜ) close to 115V/W at a bias voltage of 1.5V and chopping frequency of 10Hz, thermal time constant (τ) of 2.5ms and noise voltage of 255nV/Hz1/2 against the corresponding thermal properties of Gth and thermal capacitance Cth equal to 9.0×10−5W/K and 1.95×10−7J/K respectively. The detectors are found to have uniform spectral response in the infrared region from 2 to 20μm, and NEDT in the range from 108 to 574mK when used with an F/1 optical system. The detector, in an infrared earth sensor system, is tested before an extended black body which simulates the earth disc in the laboratory and the results are discussed.