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The 128x128 Si:As Array

This array was developed under the leadership of the IRS team at Cornell, with processing by Boeing (formerly Rockwell, now DRS). The Si:As arrays are "direct hybrids", similar to the whole suite of infrared arrays developed over the last 15 years for use between 1 and 30 microns. They are made by fabricating both a grid of infrared detectors in a suitably optimized material, and also a silicon readout containing a grid of amplifiers. Indium bumps are evaporated on matching electrodes on these two grids. The detector array and silicon readouts are bonded by aligning the bumps and applying pressure; the mating bumps cold weld. When a bias is applied across the detectors, currents generated when photons are absorbed are conducted across the welded indium bumps into the input transistors of the readout amplifiers. The outputs of the amplifiers are multiplexed electronically in the readout and brought to bonding pads at the side of the device. Conventional wire bonding connects these points to pads in a chip carrier, and a similar set of wires carries service lines and clocks from the chip carrier contacts into the readout. See the figure below, from G. H. Rieke, "Detection of Light", Cambridge University Press -- no library is complete without this book!

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Here is the flight unit. The detector array itself is the gray square in the middle. The large aluminum housing is to protect it from damage by the low energy protons in the solar wind (more aluminum in the instrument protects the exposed upper direction). The array is mounted on a fiberglass stalk to isolate it from the SIRTF helium bath, and a heater raises it to an optimum temperature near 5.5K. Surrounding it is a circuit board with electrical components and traces that eventually connect the inputs and outputs to the connector in front. Instst5.jpg (94666 bytes)

The devices MIPS will be using are described in more detail by Van Cleve et al., "Evaluation of Si:As and Si:Sb Blocked Impurity Detectors for SIRTF and WIRE", Proc. SPIE, 2553, pages 502 - 513 (1995).

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