Design Considerations for Large Format FIR Array Detectors

Efficient long wavelength broadband and spectral imaging on SOFIA and in future FIR/sub-mm missions will require large two-dimensional detector arrays. While monolithic near infrared arrays of up to 2048 x 2048 pixels are available, observations at wavelengths beyond 40 microns are still limited to mosaics of 32 x 32 or less pixels. We describe how to combine state-of-the-art FIR/sub-mm photoconductor technology and the elaborate industrial production techniques of the near and mid-infrared to produce larger, more reliable and eventually easier to make long wavelength arrays. This approach includes monolithic photoconductor array configurations optimized for quantum efficiency and dark current. For electrical connection to a two-dimensional readout chip indium bump bonds similar to those in shorter wavelengths large-format arrays are used. The readout is based on the successful cryo-CMOS technology developed for SIRTF and SOFIA/AIRES. Differences in thermal contraction between detector and readout materials are addressed using techniques developed for large HgCdTe arrays. Initially, we plan on extrinsic Germanium photoconductor arrays of up to 128 x 128 pixels for wavelengths out to 130 microns. Longer wavelengths can be covered as blocked-impurity band Germanium, GaAs or other new detectors become available. Larger arrays may become feasible once the design concepts have been proven.