ROIC glow reduction in very low flux short wave infra-red focal plane arrays for astronomy

CEA and Lynred develop very large focal plane arrays (FPA) in the short wave infrared range (SWIR) with ultra-low dark current for space and astronomy applications. The structure of such arrays is based on a HgCdTe sensitive layer flipchipped onto a Si ROIC. This ROIC is based on a source follower per detector input and output stage giving access to very high gains for very low flux (below 1ph/s) and very low noise (11.5e-) measurements. However, during previous characterisations, these FPAs appeared particularly sensitive to electro-luminescence emitted by ROIC source follower output stage transistors in the saturation regime. Indeed, the emitted photons are in the sensitive wavelength range of the HgCdTe layer (2.1 μm cut-off). They are then collected by the photodiodes thus degrading the measured dark current. This phenomenon, called ROIC glow, is the limiting mechanism of dark current at low temperature for such arrays. We describe here a solution to reduce to ground level this ROIC glow. MOSFET drain to source voltage is a major parameter for limiting glow and our results show good agreement with hot electrons light emission models. Transfer function characterisation of the ROIC was also performed to highlight the limits of the proposed procedure, which is that the source follower MOSFETs must stay in the saturated regime. Measurements carried out on different characterisation benches and several detectors at CEA-LETI and CEA-DAP show dark currents below 0.03e/s/pixel after glow mitigation.