What are Infrared Detectors?
An infrared detector is simply a transducer of radiant energy, converting radiant energy in the infrared into a measurable form. Infrared detectors can be used for a variety of applications in the military, scientific, industrial, medical, security and automotive arenas. Since infrared radiation does not rely on visible light, it offers the possibility of seeing in the dark or through obscured conditions, by detecting the infrared energy emitted by objects. The detected energy is translated into imagery showing the energy differences between objects, thus allowing an otherwise obscured scene to be seen.
Under infrared light, the world reveals features not apparent under regular visible light. People and animals are easily seen in total darkness, weaknesses are revealed in structures, components close to failure glow brighter, visibility is improved in adverse condition such as smoke or fog.
Infrared Detector Formats
Infrared detectors are available as single element detectors in circular, rectangular, cruciform, and other geometries for reticle systems, as linear arrays, and as 2D focal plane arrays
Single element infrared detectors are normally frontside illuminated and wire bonded devices. Linear and 2D arrays may be fabricated with a variety of device and signal output architectures.
First generation linear arrays were usually frontside illuminated, with the detector signal output connected by wire bonding to each element in the array. The signal from each element was then brought out of the vacuum package and connected to an individual room temperature preamplifier prior to interfacing with the imaging system display. Gain adjustments were usually made in the preamplifier circuitry. This approach limited first generation linear arrays to less than two hundred elements.
Second generation arrays, both linear and 2D, are frequently backside illuminated through a transparent substrate. Several alternative focal plane architectures are illustrated in the graph below.
Monolithic detector arrays (c) have integrated detector and readout functions. Generally, in these arrays, the command and control signal processing electronics are adjacent to the detector array, rather than underneath. In this case, the signal processing circuits may be connected to the detector by wire bonds. In the monolithic configuration it is not necessary for the signal processing circuits to be on the same substrate as the detector/readout (as shown in the figure) or at the same temperature as the detector. Monolithic PtSi detector arrays can be made with signal processing incorporated on the periphery of the detector/readout chip through the use of silicon-based detector technology.
Z technology, as illustrated in figure (d), provides extended signal processing real estate for each pixel in the readout chip by extending the structure in the orthogonal direction. In the approach illustrated, stacked, thinned readout chips are glued together, and the detector array is connected to the edge of this signal processing stack with indium.
Finally, a “Loophole” approach, as illustrated in figure (e), relies on thinning the detector material after adhesively bonding it to the silicon readout. Detector elements are connected to the underlying readout with vias, which are etched through the detector material to contact pads on the readout and metallized.
Infrared detector types
||Vanadium Oxide (V2o5)
||Lithium Tantalite (LiTa)
||GaAs / AlGaAs
Many of these infrared detector materials are based on compound semiconductors made of III-V elements such as indium, gallium, arsenic, antimony, or on the II-VI elements mercury, cadmium and telluride, or on the IV-VI elements lead, sulfur and selenide. They can be combined into binary compounds such as GaAs, InSb, PbS and PbSe or into ternaries such as InGaAs or HgCdTe.