Focal Planes for the MIRI

The MIRI will use three identical 1024x1024 pixel infrared arrays, under development at Raytheon Vision Systems. The detectors for these devices are Si:As IBC, that is, arsenic doped silicon impurity band conduction devices. These detectors have a layer of silicon moderately heavily doped with arsenic (concentration of order 1016 cm-3) that absorbs the infrared photons. Thermally generated charge carriers are blocked by another, thinner layer that is high purity and hence has no impurity band to carry thermally excited charge carriers across it.

The grid of detectors is assembled onto a grid of readouts by indium bump bonding. A tiny indium dot is evaporated onto each detector output and onto the mating amplifier input, the two grids are carefully aligned, and a controlled force applied. If all these steps have been done correctly, the indium bumps all cold weld to create the million plus solder connections needed by the array. The completed device is mounted in a chip carrier where fine wires can bring signals in and out to control the readout and to carry signals to the electronics (figure from G. Rieke, "The Detection of Light," Cambridge University Press).

The MIRI focal planes have direct heritage to those developed for SIRTF by the IRAC and IRS instrument teams.They provide broad spectral response (good quantum efficiency from 5 to 27 microns), low noise and dark current, and good performance in the space environment.

For the MIRI, the focal planes will be mounted in enclosures that provide shielding from cosmic rays and thermal isolation from the optical bench, so the arrays can be heat sunk solidly to the solid hydrogen in the MIRI cryostat.	The assembled focal plane module provides a robust and simple interface to the optical bench of the instrument.