1. Field of the Invention
This invention is in the field of apparatus which are responsive to photons of light to produce an electrical output response. More particularly, the present invention relates to a detector for providing an electrical response to infrared light. A detector array apparatus includes a multitude of such detectors, each of which is responsive to infrared light to produce an individual electrical signal. An optical system is generally used in connection with the array device to focus light from a distant scene to form an image. This image is directed upon the detector array, and the multitude of electrical signals provided by the detectors of the array then contain image information which was carried in the light falling upon the array. In the present case, the detectors and the array device are responsive to light in the infrared spectral band to produce an electrical signal carrying image information. Such infrared light is invisible to the human eye. Consequently, the light received by the detector array may be from a scene which is not visible to the natural vision of a human observer. However, the electrical signal from the detector array may be processed for display on a device, such as a video monitor, which provides a visible image for viewing by a human user of the device. Such a viewing process is generally referred to as thermal imaging, and the apparatus may be referred to as a thermal imager. A method of making the apparatus is also disclosed.
2. Related Technology
Conventional infrared detectors used for thermal imaging generally utilize materials such as Indium Antimonide (InSb), Mercury Cadmium Telluride (HCT), and Platinum Silicide (PtSi), which change their electrical characteristics in response to photons of light in the infrared spectral band. Because of this change in an electrical characteristic of some materials in response to infrared light, photoconductive materials can be used in detectors allowing current to flow and providing an electrical signal in response to light in the infrared spectral band. Such sensitivity to infrared light also allows these, and other, materials to be used in arrays of such detectors, which arrays will provide an electrical signal carrying image information from the light focused on the array. That is, the detectors, or pixels, of the detector array each receive light from a portion, or pixel, of a scene. The detector pixels each provide a corresponding portion of an electrical signal, which in analog or in digital form, carries the image information from the scene.
However, InSb, HCT, and PtSi sensors must be cooled to cryogenic temperatures (in the range of 77.degree. K) in order to provide adequate sensitivity to infrared light. The substrate materials upon which the detectors of InSb and HCT material are fabricated are generally expensive. Additionally, the fabrication of some of these detectors requires use of epitaxial crystal growth, and of MOCVD (metal-oxide chemical vapor deposition) reactors. Thus, the fabrication and packaging of these conventional detectors and detector arrays is comparatively expensive and has a comparatively low yield of usable detectors and arrays.
A less expensive photoconductive material which has been used in detectors and in linear detector arrays is lead selenide (PbSe). This material is less expensive to use because it is not single-crystalline in nature and can be applied, delineated, electroded, and passivated with minimal processing, and with a high yield of usable detector arrays. This processing does not require the use of MOCVD reactors. Additionally, the substrate upon which the photoconductive PbSe material is carried can be relatively inexpensive quartz material.
Such conventional detectors and detector arrays generally employ the photoconductive material in the form of discreet tiles (or areas in plan view of the detectors) of the photoconductive material disposed on a surface of the substrate. With a linear detector array, the separations between the tiles simply prevent cross talk between adjacent detectors under dynamic conditions. The same is true of a two-dimensional array. Both linear and two-dimensional arrays incorporate discontinuities in the photoconductive material so that discreet detector elements are defined in both a physical and an electrical sense. These detector areas of photoconductive material are generally thin in comparison to their dimensions in the plane of the substrate surface upon which they are carried so that they might be visualized as discreet separated tiles upon a floor. These discreet tiles of the photoconductive material generally have electrodes or electrical leads connected to opposite sides of the photoconductive material so that in response to the receipt of infrared light photons and a resulting change in the conductivity of the material, a changed current level will flow between these electrodes or electrical leads. This change in the electrical current level is used to provide the electrical output signal from the detector or detector array.
However, conventional photoconductive detectors and detector arrays, including those which used PbSe material, have suffered from a lower than desired signal-to-noise (SN) ratio. This low SN ratio is believed to be caused in part by the edges of the detector tiles, to which no electrical connection is made, interrupting equipotential lines in the photoconductive material. That is, an "edge" of such a detector tile might be viewed as a physical boundary or plane which transects or cuts equipotential lines of the photoconductive material. At such an edge of the detector, the current flow perpendicular to the equipotential lines along the edge of the detector is non-uniform and not proportional to any input signal, and thereby constitutes a source of noise.
Photoconductive detectors are generally compared by using a parameter known as the D* (d-star) detectivity figure of merit. This D* parameter includes a factor for SN ratio, as well as the bandwidth and active area of the detector. Consequently, all conventional detectors suffer a reduction in their performance rating because of the edge noise created by the discreet photoconductive detector tiles of the detectors.
Another disadvantage of conventional detectors which have discreet tiles of photoconductive material is that the necessary separations between the detector tiles uses a significant portion of the available area on the substrate. Consequently, only a fractional portion of the substrate surface is actually used by the active detector tiles. This comparatively smaller area of the substrate, which is the active area of the detector because it carries the tiles of photoconductive material responsive to the incident infrared light, means that conventional detectors also have a decreased fill factor, which is defined as the ratio of the active detection area to the total available area of the pixel.