Thermoelectric radiation sensors for the space mission BepiColombo to Mercury

Abstract

We present a newly designed thermoelectric detector chip of high detectivity for the space mission BepiColombo to Mercury. The sensor is part of the MERTIS radiometer, which enables radiometric measurements in the spectral range from 7-40 micron to study the thermo-physical properties of the planet's surface material. In collaboration with the DLR Institute of Planetary Research, the Institute of Photonic Technology has developed a sensor array with a specific detectivity <i>D*</i> of 1.3 x 10⁹ Jones in vacuum environment and 2 x 15 individual readable channels. In addition, it has an optical slit in the middle, which serves as the entrance slit of a spectrometer downstream. The sensor area is coated with an absorbing layer, in this case silver black having an absorption coefficient of nearly 100 percent in a wavelength range from 0.4 up to 20 micron. To minimize the thermal cross talk between the individual pixels, each pixel is separated by a 50 micron slit in the self-supporting silicon nitride membrane. For good mechanical stability of the pixels, the pixel membrane is tensioned by 10 micron bridges like braces. The sensor is electrically contacted with a star-flex PCB by direct wire bonding and both are mounted on milled aluminum housing. At the Institute of Photonic Technology (IPHT), high detectivity radiation sensors are developed and based on the thermoelectric principle. The thermoelectric materials used are the highly effective combination of n-bismuth(87%)- antimony(13%) / p-antimony. The sensors are designed, in the main, as miniaturized multi-junction thermocouples and made by state of the art thin film technologies allowing for achievable and reproducible detectivities <i>D*</i> in the range of 10⁸ up to 2 x 10⁹ Jones.