Infrared detector material comparison: vanadium oxide and amorphous silicon

Thanks to the rapid development of uncooled thermal detectors, convenient instruments utilizing IR detectors, such as the thermal imagers, are becoming more prevalent in commercial and industrial applications[1][2]. In modern thermal imagers, the IR detectors are usually focal plane array made up of uncooled microbolometers.

A microbolometers is a field effect transistor that’s constructed in such pattern: a thin film layer of IR absorbing thermistor elevated on top by electrodes, with a reflector underneath the thin film maximizing absorption. The structure is etched onto a silicon substrate that’s part of a readout integrated circuit(ROIC). The thin layer of thermistor should be kept at least 2 micrometers above the ROIC to minimize thermal signals introduced by the circuit itself[2]. Each one of these microbolometers is responsible for temperature measurement of one pixel on a thermal imager’s screen.

In the entire design of microbolometer, the performance of the IR absorbing material is of utmost importance. There are four major factors determining the quality of thermal detector materials: responsivity, response time, sensitivity, and resistance (Power consumption)[3]. Responsivity indicates the magnitude of the sensor's response induced by a radiometric flux of given intensity striking the sensor. Hence, it directly determines signal level going from the detector into the pre-amplifier. Response time of a detector is crucial because signal of interest may vary with time. The response speed of a detector is directly linked to its thermal imager’s maximum frame rate. Sensitivity specifies signal to noise ratio one should expect given an input flux. Resistance or power consumption contributes to how much thermal noise a circuit will likely generate given certain biased voltage. Sensitivity and resistance of microbolometers together determine whether minor details will be reliably distinguished from background noise on a thermal imager.

Currently, there are two mainstream IR absorbing materials on the market: Vanadium oxide(VOx) and amorphous silicon(A-Si).

Vanadium oxide (VOx) microbolometers were first developed in 1970s by Honeywell Corporate. VOx IR detectors generally have high temperature coefficient of resistance (the relative change of resistance per degree of temperature change) and excellent signal to noise ratio[4]. It is by far the most researched uncooled focal plane material. Therefore, Vox based thermal detectors can reliably deliver crisp and vibrant thermal images. However, to achieve higher responsivity, researchers introduced different composite of vanadium oxide (Ex VO2, V2O5)[1][4] onto microbolometer array, which also would also incur non-uniformity of signal response. Consequently, it is usually more technically demanding to achieve uniformity with VOx based thermal detectors.

Amorphous silicon(a-Si) microbolometers were developed by Texas instrument in 1970s in the attempt to compete with VOx based detectors. Thanks to a-Si’s intrinsic rigidity, the material can retain its integrity even when reduced to extremely thin layer structures. With its ultra-thin structure, the thermistor’s mass and the thermal conductance between the thermistor and the substrate are both very low[1]. These properties grant a-Si based microbolometers excellent response speed. In addition, due to the relative simplicity of a-Si microbolometer structure, the electrical array response is uniform and narrow, which is a great aid to noise-to-signal performance.

Of course, the performance of thermal imagers is not entirely dependent on material type of detectors, there are numerous factors to consider: craftmanship of the focal plane array, overall integration of the system, imaging algorithm, etc. As technology and manufacturing technique of thermal field effect transistor iterates, existing limits of microbolometers are likely to be overcome, and new types of detectors with greater performance will likely appear. But right now, having well-performed uncooled microbolometers definitely helps manufacture of good thermal imagers.

Fotric thermal cameras adopt their focal plane detectors from Lynred, one of world’s best IR detector manufacturers. Lynred is renowned for their expertise in producing high-quality a-Si based infrared detectors, and they are constantly making innovation in detector material and fabrication method for better and more affordable sensors. Fotric is also keen on pushing the performance limits of thermal imagers by keeping pace with most cutting-edge detectors and update our design and craftmanship.

[1] SOFRADIR-EC, Uncooled Infrared Imaging:Higher Performance, Lower Costs; 2017.

[2] Rogalski, Antoni. "History of infrared detectors." Opto-Electronics Review 20.3 (2012): 279-308.

[3] Boreman, Glenn D. "User's guide to IR detectors." Laser Metrology for Precision Measurement and Inspection in Industry. Vol. 4420. International Society for Optics and Photonics, 2001.

[4] Kohin, Margaret, and Neal R. Butler. "Performance limits of uncooled VOx microbolometer focal plane arrays." Infrared Technology and Applications XXX. Vol. 5406. International Society for Optics and Photonics, 2004.

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