Abstract:
Fabrication of far infrared photoconductor arrays, especially for low background astronomy, is particularly challenging due to arrays&#39; relatively large pixel size, susceptibility to stray radiation, and the requirement for low bias levels. A hybrid-far infrared photoconductor array as presented, provides a system and method for the development of large-format far IR arrays. The hybrid-far infrared array is provided with a blocking layer, situated in between a detecting layer and a readout layer, which allows detection of far infrared signals without complications based on readout glow. In particular, the readout glow, detector heating, and thermal mismatch between the readout and the detecting layer are addressed by careful selection of the materials for the blocking layer. The blocking layer is provided with an array of conductive vias passing through the bulk of the blocking layer in order to efficiently transmit electrical signals between the detecting layer and the readout layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/546,700, filed Feb. 20, 2004, which is incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention pertains, in general, to sensing far-infrared radiation, and in particular, to a photoconductor array for sensing radiation in the far infrared region of the electromagnetic spectrum. 
   2. Description of the Background Art 
   In general, sensing devices are designed to detect radiant energy found in certain regions of the electromagnetic spectrum. Development of a sensing device that detects all wavelengths of radiation from the electromagnetic spectrum can be quite daunting due to the size and complexity of such a device. Therefore, sensing device manufactures have produced devices that are sensitive to certain discrete regions of radiant energy, including far ultraviolet, near ultraviolet, visible, near infrared, and far infrared, within the electromagnetic spectrum. Manufacturing of sensing devices for each of these regions varies depending on the energy of light that is contained within each region. In some cases, like the detection of far infrared light, a sensor includes a one-dimensional line of light sensitive pixels or a two dimensional array of pixels comprised of stacked bars of light sensitive material. Once the incoming light has been detected, readout electronics process electric signals generated by the incident radiation interacting with the detecting material. The challenge of efficiently transferring information (in the form of electronic signals) from the detecting medium to the readout medium has confounded sensing device manufacturers for decades. 
   For conventional, far-infrared sensing devices, the interface between the detecting array and readout electronics is carried out by wire bonds connecting each detection pixel to the corresponding unit cell within the readout electronics. The wire bonds can be manufactured from an electrically conducting material like copper. However, the extreme size of the detecting and readout media, and, subsequently, the large number of pixels and unit cells to be connected, render wire bonding an inefficient means for interfacing the detecting array with the readout electronics. 
   Several other challenges also plague the far infrared sensing device community, including the inherent temperature increase of the sensing media, the inherent radiant glow from the readout multiplexer, and a possible thermal mismatch between the sensing material and the readout material. For near infrared sensing devices, an increase in detector temperature and a radiant glow from the readout medium is of little concern due to the high energy of the incoming radiation to be detected and the detectors operating temperature. Any incoming near infrared signal is significantly greater than the background signal generated by detector temperature and readout glow. 
   However, for far infrared detection, where incoming radiation is substantially less energetic than near infrared radiation and the detectors need to operate in much lower temperatures, readout glow and detector heating become problematic. Any stray light reaching the detecting surface due to glow from the readout multiplexer will compromise detector performance. In addition, the heat generated by the readout affects the performance of the detector array by raising its temperature. 
   This, in turn, causes an increase in excess dark current, thus resulting in the degradation of detector performance. Also, the effects of readout glow are especially pronounced in the low temperature environment necessary for far infrared detection. 
   In the case that the detecting media and readout media are constructed from different materials, a thermal mismatch is created. The detecting and readout materials contract and/or expand at different rates when exposed to the low temperatures required for far infrared sensing. When this happens, the overall yield of a detecting device can be significantly decreased. 
   Another difficulty that arises when detecting radiant energy of any particular wavelength relates to the presence of optical crosstalk between adjacent pixels on the surface of a detecting array. For a conventional detector array, typically a single wafer of some thickness is pixelized on one surface of the array by depositing metallic pads in a desired pattern. These metalized pads are the connections to the readout inputs. In the conventional design of linear arrays made of single bars of detector material, the pixelization is commonly accomplished by cutting grooves into the bar mainly in an effort to eliminate optical crosstalk. However, this approach to eliminating optical crosstalk is not conducive for implementation on large surface area arrays, like those needed for far infrared detection, due to the increase in processing costs for cutting grooves over such a large area. 
   Furthermore, another significant challenge facing far infrared sensor manufacturers involves maintaining a stable operating voltage for the detecting media during the time that the detecting media waits for its signal to be integrated by the readout media. When the overall voltage (or bias) of the detecting media becomes unstable, or debiased, a significant limitation in detector efficiency is encountered. 
   What is needed is a sensing device that will efficiently detect far infrared radiation that does not suffer from deficiencies introduced by a) detector heating b), readout glow, c) thermal mismatching, d) detector debiasing, and e) optical crosstalk. 
   SUMMARY OF THE INVENTION 
   A hybrid photoconductor array enables the detection of far infrared radiation without significant interference from the problems described above. An embodiment of the photoconductor array places a blocking layer in between the detecting layer and the readout layer, significantly reducing signal contamination generated by detector heating and readout glow. The blocking layer advantageously provides an electrical connection between the detecting layer and the readout layer. An embodiment provides the blocking layer with conductive vias for directing electrical signals from the detecting layer to the readout layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is diagram of a far infrared detector in accordance with an embodiment of the present invention. 
       FIG. 2  is a detailed view of a far infrared detector in accordance with an embodiment of the present invention. 
       FIG. 3  is a detailed view of a far infrared detector in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a far infrared (IR) photoconductor array  100 , according to one embodiment of the present invention. Photoconductor array  100  is designed such that a blocking layer  110  is sandwiched between a detecting layer  105  and a readout layer  115 . The design of photoconductor array  100  provides superior protection against readout glow by having the blocking layer  110  restrict the glow from reaching the detecting layer  105 . 
   Detecting layer  105  converts radiant energy, particularly far infrared radiation, into electrical signals containing information about the incoming radiation that are processed by readout layer  115  and subsequently analyzed by a computer-based micro-processing device. In addition to blocking the glow from the readout layer  115  from reaching detecting layer  105 , the blocking layer  110  provides an effective electrical conduit between detecting layer  105  and readout layer  115  for transferring electrical signals. Readout layer  115  specifically converts electrical current (or photocurrent) from the detecting layer  105  into a usable voltage that can be processed as needed. 
   A wide range of materials are available for construction of detecting layer  105 , including GaAs, InAs, InSb, Ge:Ga, CdTe, Ge:Sb, and HgTe. GaAs offers the potential of extending the long wavelength response of detecting layer  105  to 300 μm, which is advantageous for sensing a variety of phenomena that emit radiation in the far infrared. InAs and InSb have the added capability of extending the long wavelength response of detecting layer  105  even further, to 886 μm and 1771 μm respectively. 
   In an embodiment, the blocking layer  110  is constructed from alumina (Al 2 O 3 ). Alumina, as a material, is partially opaque in the far IR and sub-millimeter region and can further be formulated in manufacturing to provide a superior optical block. In another embodiment, blocking layer  110  is constructed from any particular thermally conductive, electrically insulating material that is generally opaque to radiation from the far infrared region of the electromagnetic spectrum. Opaque materials, like alumina, absorb radiation within the far IR, thus significantly reducing effects of readout glow. 
   An embodiment of readout layer  115 , is a capacitive transimpedance amplifier (CTIA). CTIA readouts employ an integration-reset scheme to convert the detecting layer  105  photocurrent to a voltage that can subsequently be processed as needed. In one embodiment, the front end of the CTIA is a high gain amplifier, typically a pair of p-type MOSFETs in the cascade configuration, with a capacitor in the feedback loop. To achieve high open-loop gain, one embodiment uses a pair of n-type MOSFETs, also in the cascade configuration, as a load for this amplifier. CMOS technology allows fabrication of both p-type and n-type MOSFETs on the same chip, which provide amplifiers with high open-loop gains and superior performance for integrating low-level signals. A multi-gain circuit provides operation under a broad range of infrared flux levels. This can be accomplished by using multiple feedback capacitors enabled as needed to provide different well capacities and, therefore, different gain settings. During the integration time, when the reset switches are open, the photocurrent from the detecting layer  105  accumulates as an integrated charge on the feedback capacitor. By virtue of the negative feedback, the capacitor also pins the detecting layer  105  input node to a constant voltage, thereby keeping the bias of detecting layer  105  constant, regardless of the integrated signal. 
     FIG. 2  is a cutaway view showing an embodiment of photoconductor array  100 , according to an embodiment of the present invention, where far infrared (IR) radiation  204  is incident upon a top surface  206  of the detecting layer  105 . The top surface  206 , opposite to an array of detection pads  208 , serves as a transparent contact that receives the incident far IR radiation  204  and also serves as a common bias pad  202 . The array of detection pads  208  is positioned on the bottom surface of the detecting layer  105  and serves as a contact point for transmitting the photocurrent generated by detecting layer  105  to readout layer  115 . Due to reflection of the far infrared radiation  204  at the detection pads  208 , the effective optical depth of the detecting array is, in essence, twice the thickness of the detecting layer  105 . 
   In an embodiment, the array of detection pads  208  is ion implanted on the bottom surface of the detecting layer  105  through a shadowmasking process in order to create a pixelised array. Through the shadowmasking process, an area of high electrical conductivity is defined by depositing an inverted mask where an electrical contact is to be made. Utilizing a shadowmasking approach in detector design significantly simplifies the array construction, especially for the large format arrays. In an embodiment, detection pads are constructed from gold, aluminum, copper, or any particular conducting material, to provide sufficient electrical conductivity between the detecting layer  105  and the blocking layer  110 . 
   In an embodiment, the blocking layer  110  is provided with a first array of conductive pads  216  on the top surface of the blocking layer  110  and a second array of conductive pads  218  on the bottom surface of the blocking layer  100 . The first array of conductive pads  216  is aligned with the array of detection pads  208  and the second array of conductive pads  218  is aligned with an array of readout pads  220  on a top surface of the readout layer  115 . In an embodiment, readout pads are constructed from gold, aluminum, copper, or any particular conducting material. 
   In order to provide a more efficient processing of far infrared signals between the detecting layer  105  and the readout layer  115 , in one embodiment, a first  210  and second  212  array of bump bonds and an array of conductive vias  214  are manufactured with blocking layer  110 , as shown in  FIG. 2 . The first array of bump bonds  210  also serves as a heat sink into the blocking layer  110  and the array of conductive vias  214 , in order to further dissipate heat generated by the detecting layer  105  at the array of detection pads  208 . The second array of bump bonds  212  also serves as a heat sink into the blocking layer  110  and the array of conductive vias  214 , in order to further dissipate heat generated by the readout layer  115 . Furthermore, the conductive vias  214  provide an electrical connection between the first  216  and second  218  array of conductive pads, thus allowing detector photocurrent to pass through the blocking layer, while restricting the amount of readout glow from being transmitted to detecting layer  105 . In one embodiment, the array of conductive vias  214  are constructed from aluminum or any particularly conductive material. 
   In one embodiment, the first  210  and second  212  array of bump bonds are constructed from indium. In another embodiment, the first  210  and second  212  array of bump bonds are constructed from any particular conductive medium, like a conductive epoxy. Bump bonding (as opposed to wire bonding) is a beneficial means for providing an electrical connection between the detecting layer  105  and readout layer  115  because the overall size of the far IR photoconductor array  100  is reduced as compared to conventional detecting arrays. 
   In an embodiment, the blocking layer  110  is selected based on its thermal conductivity and expansion coefficient in order to: provide proper heat sinking of the detecting layer so that the detecting layer  105  reaches an optimum cryogenic operating temperature; and to dampen the inherent effects of thermal mismatching between the detecting layer  105  and readout layer  115 . By selecting blocking layer  110  with an expansion coefficient substantially equal to the expansion coefficient of the detecting layer  105  and readout layer  115 , the blocking layer  110  effectively alleviates undue stress on the first  210  and second  212  array of bump bonds that might occur while the detecting layer  105  and readout layer  115  are cooling, and therefore contracting, at different rates. According to an embodiment, the detecting layer  105  is constructed from germanium (Ge) and the readout layer  115  is constructed from silicon (Si). In this case, alumina is an adequate selection for the blocking layer because the expansion coefficient of alumina is in between that of silicon and germanium. 
   According to an embodiment of the present invention, a blocking pad is imbedded within the blocking layer  110 , substantially near the array of conductive vias  214 , in order to further reduce transmission of background radiation due to readout glow. According to an embodiment, the blocking pad is constructed from a conducting materials such as aluminum, copper, or gold. According to another embodiment, a plurality of blocking pads  302 , as shown in  FIG. 3 , are positioned such that any stray radiation from the readout layer is forced to travel through a meandering path, and thus further reducing the effects of readout glow on the detecting layer. Blocking pads  302  are arranged substantially parallel to the first  216  and second  218  array of conductive pads and extend substantially beyond one end of a conductive pad from the first array of conductive pads  216  and a conductive pad from the second array of conductive pads  218 . 
   In order to effectively transfer electrical signals from the detecting layer  105 , through the blocking layer  110 , to the readout layer  115 , the array of readout pads  220 , is positioned on the top surface of the readout layer  115 , and aligned with the array of detection pads  208 . The array of readout pads  220  serves as a contact point for receiving photocurrent from detecting layer  105  and transforming the photocurrent into a usable voltage by readout layer  115 . Proper alignment of the conductive arrays with the corresponding detection pads is important for maintaining an effective electrical connection between the detecting layer  105  and the readout layer  115 . Also, precise alignment of these elements insures that electrical signals received by the readout layer  115  are limited only to those produced by the detecting layer  105  and not by an outside source. Furthermore, selecting the blocking layer  110  such that the thermal conductivity of the blocking layer is adequately high allows for adequate heat sinking of the detecting layer  105  during cryogenic cooling. 
   While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention.