Proceedings Article | 13 September 1994
Proc. SPIE. 2209, Space Optics 1994: Earth Observation and Astronomy
KEYWORDS: Infrared sensors, Sensors, Spectroscopy, Multiplexing, Infrared spectroscopy, Diodes, Charge-coupled devices, Mars, Planets, Space operations
The MARS 94 spacecraft is devoted to a mission to the planet Mars. Planned to be launched in October 1994, it shall reach the planet in September 1995. The lifetime is one Mars year. A second spacecraft will follow two years later. This paper presents the characteristics of the IR detection modules for the IR OMEGA spectrometer which consists of InSb multiplexed detectors. The purpose of this Meudon Observatory instrument is to observe mineralogy, water, ice and atmosphere in the IR spectrum. SAT has developed the Identification Model, Development Model and Fight Models of detectors, each fitted with 2 arrays and IR filters in a dedicated space housing equipped with a cold flex line and a connector. An array is composed of 128 multiplexed InSb photovoltaic detectors surrounded by a specific IR filter. Each filter is divided into 2 zones. For the IR Short Wavelength filter, one zone operates from 1 to 1.8 micrometers and a second one from 1.8 to 2.7 micrometers ; for the IR Long Wavelength filter, the 2 zones are 2.7 to 4.7 and 4.7 to 5.3 micrometers . The housing is designed in collaboration with the Observation de Paris-Meudon. The InSb photodiodes of sensitive areas 90 X 120 micrometers , and pitch of 120 micrometers , operate at 77 K, with the following characteristics: by means of a scanning operation, the sensitive areas are measured at half maximum of height and the diameters are: 120 +/- 1 et 92 +/- 0,7 mum. The signal is illustrated by the I(V) curve: the main parameters are: I<SUB>sc</SUB> equals 10 - 100 pA and R<SUB>0</SUB> equals 10 - 25 G(Omega) . The quantum efficiency is (eta) equals 60% - 65% and the peak responsivity 2,5 - 2.6 A/W. The detectivity of non multiplexed pixels is: D* at (lambda) pic (FOV, 1800 Hz, 1 Hz) equals 2 X 10<SUP>11</SUP> cm Hz<SUP>1/2</SUP> W<SUP>-1</SUP>. Measurements are made under low flux corresponding to the conditions of the OMEGA experience, the planet Mars being a cold body of about 270 K. The 1/f noise is very low: fc equals 1 - 2 Hz at reverse bias voltage of Vp equals -200 mV. The cross-talk under photon flux between adjacent diodes is < -40 dB. The multiplexed readout is obtained with a Si CCD from Thomson developed for the SPOT IV Mid IR channel. This CCD normally operates at room temperature and is space - qualified for this Earth observation project. It has not been modified for our 77 K conditions, only the drive electronics, bias conditions and phase clocking have been adapted to the 128 detectors. This 2 phase - CCD operates in a specific mode: discharge of a capacitance with a subtraction of electrons corresponding to the optical signal. The integration time is T<SUB>i</SUB> equals 5 ms for a charge of Q<SUB>max</SUB> equals 0.5 pC with I equals 100 pA. The performances of multiplexed InSb diodes at 77 K are given by the mean total noise of 2800 electrons including a 2300 noise contribution due to the CCD. Environmental tests have been performed and results are reported including mechanical tests (vibrations, shocks), thermal cycling, electrical tests (electrostatic discharge), and irradiations under 3 and 6 krad.
