This invention relates in general to infrared detectors and, more particularly, to infrared detectors in which the thermally sensitive material is amorphous silicon. 
Over the years, various types of infrared detectors have been developed.  Many include a substrate having thereon a focal plane array, the focal plane array including a plurality of detector elements that each correspond to a respective pixel.  The substrate contains an integrated circuit which is electrically coupled to the detector elements, and which is commonly known as a read out integrated circuit (ROIC). 
Each detector element includes a membrane which is suspended at a location spaced above the top surface of the substrate, in order to facilitate thermal isolation.  The membrane includes a thermally sensitive material, such as vanadium oxide (VOx).  The membrane also includes two electrodes, which are each coupled to the thermally sensitive material, and which are also coupled to the ROIC in the substrate.  As the temperature of the thermally sensitive material varies, the resistance of the thermally sensitive material also varies, and the ROIC in the substrate can determine the amount of thermal energy which has been received at a detector element by sensing the resistance of that detector element.  Existing ROIC circuits have been developed to be compatible with input impedance requirements of the associated detector elements. 
One common type of ROIC circuit is known as a pulse-biased mode current skimming ROIC, and is frequently used with vanadium oxide detector elements.  In this type of ROIC, the bias voltage to the electrodes in each membrane is not continuous.  Instead, the bias voltage is kept off until the ROIC is ready to read the detector element.  The bias voltage is then turned on, the current which then flows through the thermally sensitive material in the detector element is integrated, the result of the integration is read, and the bias voltage is turned off.  This type of pulsed-mode operation limits heating within the detector element caused by the current flow from the bias voltage.  The input impedance of this type of ROIC is typically in the range of about 10 Kxcexa9 to 1000 Kxcexa9.  A different type of known ROIC circuit is a direct current bias switched capacitor integrating amplifier per unit cell ROIC.  In this approach, the bias voltage to each detector element is kept on continuously, and the output of the detector element is periodically switched to an integrating amplifier.  The input impedance of this type of ROIC is typically in the range of about 10 Mxcexa9 to 100 Mxcexa9. 
Although infrared detector devices of this known type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.  For example, some disadvantages include limitations relating to the thermal mass in the membrane, the difficulty in balancing stress between layers in the membrane, high noise levels, and a relatively low temperature coefficient of resistance (TCR), where TCR is a relatively standard measure of sensitivity for purposes of measuring thermal radiation. 
An approach to resolving some of the existing problems has been based on the use of amorphous silicon as the thermally sensitive material.  Amorphous silicon has the advantage that a relatively high TCR value and thus a high level of sensitivity can be obtained by providing a low level of doping, or even by leaving the amorphous silicon undoped.  However, when amorphous silicon is undoped or has a low level of doping, it also has a very high resistance.  As a result, problems have been encountered in attempting to use amorphous silicon to implement detector elements which have a high TCR and thus a high level of sensitivity, but which also have an impedance that is matched to the input impedance requirements of existing ROIC designs.  This is because the effective resistance of the detector element was linked to the doping level that also provided the high level of sensitivity.  For a desirably high level of sensitivity, the resistivity of an amorphous silicon detector tended to be about three orders of magnitude higher than would be appropriate for impedance matching with a pre-existing ROIC.  Finally, prior to the present invention, it was impractical to use a relatively low doped amorphous silicon detector element with a pre-existing ROIC circuit designed for a lower impedance detector element (1 Kxcexa9 to 1000 Kxcexa9), such as a pulsed-bias mode ROIC, because the impedance requirements could not be matched in a suitable manner. 
A related consideration is that there are applications in which it is desirable to operate an infrared detector at a higher frame rate than the standard video frame rate of 30 frames per second.  But where the detector elements have a high resistance, it is difficult to operate them at a high rate of speed because a very high current would be required to read information rapidly out of a detector element with a high resistance, and the requisite high current would present problems in terms of power consumption, damage to the integrated circuit device, and so forth. 
One preexisting detector element based on amorphous silicon has a membrane which is about 2300 xc3x85 thick.  The membrane contains an amorphous silicon layer, and has two spaced electrodes which each contact the amorphous silicon layer adjacent a respective end thereof.  The amorphous silicon layer and the electrodes are sandwiched between two insulating layers.  One of the insulating layers has embedded therein a relatively large absorber layer made from a material which absorbs infrared radiation.  The electrodes are provided outwardly from the absorber layer on opposite sides thereof.  Consequently, there is a relatively significant spacing between the electrodes.  This distance between the electrodes is needed for optimum operation of the absorber layer.  However, it also means that, when the amorphous silicon layer is undoped or lightly doped in order to obtain a high TCR, the effective resistance created between the electrodes by the amorphous silicon layer is so high that the impedance of the detector element can not realistically be matched to the impedance requirements of existing ROIC designs.  While it would theoretically be possible to change existing ROIC designs in order to obtain an ROIC with an input impedance matched to the impedance of the detector elements, there is a strong incentive to avoid the expense and problems involved is with attempting to change an existing ROIC design. 
From the foregoing, it may be appreciated that a need has arisen for an infrared detector, and a method of making it, where the infrared detector provides a high level of sensitivity while reducing problems of the type discussed above, in a manner so that an effective resistance for a detector element is substantially independent of the temperature coefficient of resistance and the level of doping.  According to a first form of the present invention, a method and apparatus are provided to address this need, and involve the provision of an amorphous silicon portion which has a selected temperature coefficient of resistance, and the fabrication of first and second electrodes which are at spaced locations on the amorphous silicon portion and which are electrically coupled to the amorphous silicon portion.  The electrodes and the amorphous silicon portion are structurally configured so as to provide between the electrodes through the amorphous silicon portion at a given temperature a resistance selected independently of the temperature coefficient of resistance. 
It will also be recognized that a need has arisen for an infrared detector, and a method of making it, in which the infrared radiation is efficiently absorbed.  According to further form of the present invention, a method and apparatus are provided to meet this need, and involve: providing a thermally sensitive portion having a resistance which varies with temperature; and fabricating a thermal absorber portion which absorbs infrared radiation, which is in thermal communication with the thermally sensitive portion, and which is made of an alloy that includes titanium and aluminum.