Modeling effects of common molecular contaminants on the Euclid infrared detectors

Cleanliness specifications for infrared detector arrays are usually so stringent that effects are neglibile. However, the specifications determine only the level of particulates and areal density of molecular layer on the surface, but the chemical composition of these contaminants are not specified. Here, we use a model to assess the impact on system quantum efficiency from possible contaminants that could accidentally transfer or cryopump to the detector during instrument or spacecraft testing and on orbit operation. Contaminant layers thin enough to meet typical specifications, < 0.5 mu gram/cm(2), have a negligible effect on the net quantum efficiency of the detector, provided that the contaminant does not react with the detector surface, Performance impacts from these contaminant plating onto the surface become important for thicknesses 5 - 50 mu gram/cm(2). Importantly, detectable change in the "ripple" of the anti reflection coating occurs at these coverages and can enhance the system quantum efficiency. This is a factor 10 less coverage for which loss from molecular absorption lines is important. Thus, should contamination be suspected during instrument test or flight, detailed modelling of the layer on the detector and response to very well known calibrations sources would be useful to determine the impact on detector performance.