Microbolometer

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A microbolometer is a thermal sensor used to detect electromagnetic radiation. In addition to the detection of millimeter waves, UV and X-ray radiation , it is mainly used for the detection of medium and long-wave infrared radiation . As a two-dimensional infrared focal plane array (IRFPA), they represent the image sensor of thermal imaging cameras .

Microbolometers are mainly used as detectors in wavelength ranges above about 3 µm. In this area, uncooled photodiodes are technically difficult to implement due to the thermal excitation. Microbolometers can work at room temperature - that is, without complex cooling - but require temperature stabilization.

The microbolometer arrays u. a. in the fields of thermography , astronomy, surveillance, automotive, military and recently also smartphones.

Structure and functionality

Basic structure of a microbolometer

A microbolometer consists of a membrane made up of a sensor and absorber layer, which is suspended by two electrodes over a substrate in a vacuum and is therefore thermally insulated. Due to the absorption of the incident infrared radiation, the thermally insulated membrane heats up, which results in a change in the electrical resistance of the sensor layer. The resulting change in the measurement signal is detected with the aid of a readout circuit. A reflector is usually located below the membrane, as a result of which the radiation, which is partly transmitted, is reflected back and then absorbed by the absorber layer. The distance between membrane and reflector follows the λ / 4 condition.

Sensor material

The sensor layer is a core element of the microbolometer and has a decisive influence on the properties and performance of the microbolometer. There are several materials that are used for the sensor layer in microbolometers. The sensor layer and its material properties such. B. TCR , 1 / f noise and resistance, the responsiveness of the microbolometer is determined. Responsivity describes the ability to convert the incoming radiation into an electrical signal and thus the quality of the microbolometer.

The most commonly used sensor materials in microbolometers are amorphous silicon and vanadium oxide . Less common materials are Ti, YBaCuO, GeSiO, Poly-SiGe or BiLaSrMnO. Vanadium oxide is the original material system for microbolometers. It is with common CMOS - manufacturing processes compatible. There are several phases of VOx. VO2 has a low resistance but undergoes a metal-insulator phase change at 67 ° C and has a relatively low TCR. V2O5, on the other hand, has a high resistance and a high TCR. Currently, x ≈1.8 seems to be the most popular phase for microbolometer applications. Amorphous silicon (a-Si) is a newer technology than VOx. It can be integrated very well into the CMOS manufacturing process, is very stable, has a fast time constant and a long mean time before failure. In order to create the layer structure and the structuring, however, temperatures of up to 400 ° C are required.

Pixel pitch

Currently, IRFPAs with a pixel size (microbolometer) of 17 µm are state of the art in the commercial sector. In order to follow the trend of high-resolution IRFPAs and cameras, the new generation of IRFPAs consists of pixels with a size of 12 µm. To realize the ever smaller structures, ever higher demands are made on the microstructuring, z. B. at the Fraunhofer Institute for Microelectronic Circuits and Systems 12 µm bolometer u. a. realized via nanotube metal contacts.

Origin and list of manufacturers

Microbolometer technology was originally developed by Honeywell for the US Department of Defense beginning in the late 1970s. The US government released the technology in 1992. After the activation, Honeywell licensed its technology to several manufacturers. Microbolometers are subject to customs export controls. The following companies are involved in the development, production and sales of microbolometer arrays:

Thermal imaging camera with an array of 320 × 200 microbolometers

Individual evidence

  1. ↑ K.-M. Muckensturm, D. Weiler, F. Hochschulz, C. Busch, T. Geruschke: Measurement results of a 12 μm pixel size microbolometer array based on a novel thermally isolating structure using a 17 μm ROIC . In: Infrared Technology and Applications XLII . tape 9819 . International Society for Optics and Photonics, May 20, 2016, p. 98191N , doi : 10.1117 / 12.2223608 ( spiedigitallibrary.org [accessed May 25, 2018]).