Micromachined planar arrays of detectors that could operate either as photodiodes or as bolometers have been proposed for use in detecting photons in applications in which broad spectral responses are required. The availability of either of the two modes would enable operation throughout the spectrum, from x rays through infrared. Potential markets for these devices could lie in the automotive industry (infrared-image detectors for night vision), the consumer electronics industry (infrared detectors for security systems), the semiconductor industry (process-monitoring equipment), and medical electronics (x-ray detectors).
In general, bolometers enable the detection of infrared radiation, without the need for cooling. The most common and most sensitive microbolometers now in use are based on thermistors made of amorphous semiconductors; the change in current through a thermally isolated microthermistor is indicative of the amount of absorbed radiation. Amorphous semiconductors are attractive as microthermistor materials because they are compatible with surface micromachining techniques that can be used to impart high degrees of thermal isolation. The disadvantages of amorphous semiconductors are low coefficients of thermal resistance and high levels of low-frequency ("1/f") noise.
In a device of the proposed type, the detectors would be single-crystal junction diodes within a micromachined structure that would provide a high degree of thermal isolation to the diodes. The diodes could be operated with forward bias to obtain the bolometric mode - usually best for detecting infrared radiation. Alternatively, the diodes could be operated with reverse bias to make them respond similarly to common photodiodes.
With respect to the functionalities of photodiodes and microthermistors, the single-crystal semiconductor materials that would be used in the proposed devices offer two important advantages over amorphous and polycrystalline semiconductors; namely, lower leakage currents and higher coefficients of thermal resistance. Exceptional thermal isolation and low thermal mass of detectors could be obtained by fabricating an array of detectors on one chip and its readout circuitry on another chip, then mating the two chips by use of hybridization techniques. For example, the detector array could be fabricated on a silicon-on-insulator chip by use of standard patterning and doping techniques, while the readout circuitry could be fabricated by standard complementary metal oxide/semiconductor processes. In the fabrication of the detector array, the thermal isolation of the detectors would be maximized and their thermal mass minimized by using surface micromachining techniques to remove most of the detector-supporting layer. The detector and readout chips could then be joined to each other by a standard technique of bonding via indium bumps on electrical-contact pads.
The sensitivity of a device of this type in the bolometric mode could be adjusted by varying the forward bias. By appropriate weighting of image data acquired in both bias modes, it should be possible to separate an image into visible and infrared parts. Depending on the selected bias, operating temperature, and spectrum of interest, a device of the proposed type may offer performance rivalling that of a quantum-well infrared photodetector (QWIP), but with a capability of operation over a wavelength range much broader than that of a QWIP. The performances of these devices may even approach theoretical limits for both thermal infrared sensors and photodiodes.
This work was done by Mark Wadsworth, Marc Foote, and Robert Beye of Caltech for NASA's Jet Propulsion Laboratory.
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