The disclosed invention is generally directed to infrared radiation detectors, and is more particularly directed to an infrared radiation detector structure which includes integral feedback capacitance for use with a capacitive-feedback transimpedance readout amplifier.
Infrared detectors are utilized to detect radiation having wavelengths in the infrared spectral region. Generally, infrared detectors are semiconductor devices which provide electrical signals as outputs in response to incident infrared radiation. For example, infrared detectors may be made of silicon doped with appropriate impurities such as gallium or arsenic.
Infrared detectors are generally fabricated in the form of integrated circuit detector assemblies wherein a detector assembly includes a semiconductor radiation detecting layer having a plurality of implanted detector contacts arranged in a grid or array, for example. The implanted detector contacts define the pixels of the detector and function to collect free charge carriers that result from incident radiation received by the portions of the radiation detecting layer adjacent the respective implanted contacts.
Integrated circuit detector assemblies are often incorporated in hybridized detector/readout assemblies. Such hybridized assemblies include a generally planar integrated circuit detector assembly and a generally planar integrated circuit readout amplifier assembly. The detector assembly and the readout amplifier assembly may be mechanically secured to each other in layered fashion by cold welds formed with indium bumps which conductively connect the implanted detector contacts to contacts formed in the readout amplifier assembly.
For some applications, cold-welded indium bumps are not structurally sufficient to withstand anticipated mechanical stresses during use. To provide additional mechanical strength, wicking epoxy is used to supplement the cold welds. The viscosity of the epoxy is chosen to be low enough to allow the epoxy to squeeze between and around the cold-welded bumps. However, the epoxy layers are not always uniform, in that air gaps and other discontinuities may form between the cold-welded bumps. This leads to nonuniform detector response problems in the finished device.
A particular type of amplifier utilized in readout amplifier assemblies is known as a capacitive feedback transimpedance amplifier which includes a high gain operational amplifier and feedback capacitance. The capacitive feedback transimpedance amplifier integrates the detector current over a predetermined integration period, at the end of which the amplifier output is sampled.
In known hybridized detector/readout assemblies, both the operational amplifier and the feedback capacitors are formed in the readout assembly. The feedback capacitance for each of the operational amplifiers is provided by respective parallel plate capacitors disposed in the readout assembly. Particularly, each feedback capacitor comprises two parallel plates formed within the readout array, separated by a dielectric material. The first plate is conductively coupled by means of an indium bump, for example, to a planar implanted contact in the detector array.
The implanted detector contact, which is connected to the first capacitor plate, generally forms a larger area than the second capacitor plate in the readout array. The difference in area creates unwanted E-field lines that terminate in the substrate of the readout array. The result is parasitic capacitance to ground which limits the signal-to-noise ratio and gain of the system.
Further considerations with the foregoing structure include limitations on packing density for the readout array since some of the components for providing the feedback capacitance are formed therein. Further, the foregoing structure requires precise hybridization of the detector assembly and the readout assembly since responsivity variations may result from nonuniformities in the spacing between the detector assembly and the readout assembly, voids in any wicking epoxy utilized, misalignment between the detector assembly and the readout array, as well as other variations resulting from hybridization. Such hybridization nonuniformities result in nonuniformities in gain as between the respective transimpedance amplifiers in a given hybridized assembly, which in turn may result in a degraded signal-to-noise ratio since smaller, nonoptimum biasing must be applied to the detectors.