This invention relates to infrared imagers, and, more particularly, to imagers in the form of focal plane arrays of detectors of two types with different infrared spectral sensitivities.
Infrared imagers in the form of infrared focal plane arrays have numerous applications such as space object identification, missile guidance, and thermal imagers. There are many different technologies for fabricating two dimensional infrared focal plane arrays and many ways of extracting the detection information. The most common approach is to use a monolithic array of photodiode fabricated on a narrow band gap semiconductor and connect this to silicon integrated multiplexing circuitry. Alloys of mercury, cadmium and tellurium are frequently used to fabricate the narrow band gap semiconductor because of the ability to select the band gap by merely specifying the proportions of mercury to cadmium. Indeed, HgTe is a semimetal and CdTe has a band gap of about 1.5 eV, and alloys of the two show a continuous variation in band gap.
For example, Hg.sub..8 Cd.sub..2 Te is commonly called 14 micron HgCdTe because it has a band gap approximately equal to the energy of a photon having a wavelength of 14 microns. See, generally, Wolfe and Zissis, Eds. The Infrared Handbook, ch. 11 (1978). Various methods of fabrication of alloys of mercury, cadmium and tellurium are known; for example, Castro, U.S. Pat. No. 4,374,678 discloses a laser-based selective interdiffusion process.
The integrated multiplexing circuitry is commonly a charge coupled device with the frontal currents generated in the photodiode array being injected into the CCD during a stare time, followed by the accumulated charge being clocked out of the CCD between stare time intervals. Alternatively, the photocurrent generated in the photo-diode array can be sampled and amplified by a low impedance current integrator. See, generally, Wolfe and Zissis, Eds., The Infrared Handbook, ch. 12 (1978).
However, the performance of such infrared detectors is limited because they are not capable of infrared spectral discrimination; that is, they only see one color. Detection of two or more colors, that is, two or more infrared spectral regions, will allow easier analysis of the target being viewed because the spectral signature (emissivity versus wavelength) can be determined. This will allow discrimination between targets that would appear identical for a one color detector.
Previous two color detectors were basically brute force type adaptations of one color detectors. For example, merely inserting a filter in front of a long wavelength detector (small bandgap semiconductor used to fabricate the photodiodes) will simulate a short wavelength detector; and alternately applying and removing the filter will give a sequence of two color detections. This filter modification method has the obvious problems associated with moving the filter around, reliability of filter mounting method due to the proximity to and low temperature of the detector, and also the problem of a decrease in sensitivity of the detector due to the lower efficiency of a small bandgap semiconductor being limited to detection of only higher energy photons.
Another two color approach is to simply bond, in an alternating fashion, small detectors of two different types to a common carrier. A problem with this common carrier approach is the physical size of the detectors being bonded onto the carrier; for example, if the individual detectors were on the order of a few millimeters in length and width, then the array formed on the common carrier would be like a checkerboard with each of the squares a few millimeters on a side. In addition to the obvious lack of resolution, any time delay due to scanning would make detection of moving targets unreliable.
Monolithic structures with photodiodes of two or more infrared color sensitivity have been reported. Barrowcliff and Wood, Integrated Multicolor Detector Structure, Meeting of the Iris Specialty Group on Infrared Detectors (Jun. 12-13, 1979) formed photodiodes of InAs.sub..87 Sb.sub..13 and Ga.sub..97 In.sub..03 Sb on GaSb substrates by selective etching and liquid phase epitaxial growth, followed by polishing and ion implantation of Be. Similarly, Chu, Bouley and Black, Performance of Multispectral Response Lead Salt Infrared Detectors, 409 Proceedings of the SPIE--the International Society for Optical Engineering 89 (Apr. 5-6, 1983) reported epitaxial growth of multilayers of p-type PbS PbS.sub..5 Se.sub..5 and PbSe and formed Pb metal Schottky contacts. These metallic lead-lead salt diodes show multispectral response in the infrared. A further problem, however, with all of the foregoing two color detectors and focal plane arrays is the complexity of two sets of connections and circuitry for extracting the two color information. Moreover, this previous art is restricted to wavelength less than 6 microns which excludes the useful atmospheric window between 8 and 12 micron wavelength.