Microbolometer
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A microbolometer is a specific type of
Theory of construction
A microbolometer is an uncooled thermal sensor. High resolution thermal sensors require exotic and expensive cooling methods including stirling cycle coolers and liquid nitrogen coolers. These methods of cooling high resolution thermal imagers are expensive to operate and unwieldy to move. Also, high resolution thermal imagers require a cool down time in excess of 10 minutes before being usable.
A microbolometer consists of an array of pixels, each pixel being made up of several layers. The cross-sectional diagram shown in Figure 1 provides a generalized view of the pixel. Each company that manufactures microbolometers has their own unique procedure for producing them and they even use a variety of different IR absorbing materials. In this example the bottom layer consists of a silicon substrate and a readout integrated circuit (ROIC). Electrical contacts are deposited and then selectively etched away. A reflector, for example, a titanium mirror, is created beneath the IR absorbing material. Since some light is able to pass through the absorbing layer, the reflector redirects this light back up to ensure the greatest possible absorption, hence allowing a stronger signal to be produced. Next, a sacrificial layer is deposited so that later in the process a gap can be created to thermally isolate the IR absorbing material from the ROIC. A layer of absorbing material is then deposited and selectively etched so that the final contacts can be created. To create the final bridge like structure shown in Figure 1, the sacrificial layer is removed so that the absorbing material is suspended approximately 2 μm above the readout circuit. Because microbolometers do not undergo any cooling, the absorbing material must be thermally isolated from the bottom ROIC and the bridge like structure allows for this to occur. After the array of pixels is created the microbolometer is encapsulated under a vacuum to increase the longevity of the device. In some cases the entire fabrication process is done without breaking vacuum.
The microbolometer array is commonly found in two sizes, 320×240 pixels or less expensive 160×120 pixels. Current technology has led to the production of devices with 640×480 or 1024x768 pixels.[citation needed] There has also been a decrease in the individual pixel dimensions. The pixel size was typically 45 μm in older devices and has been decreased to 12 μm in current devices. As the pixel size is decreased and the number of pixels per unit area is increased proportionally, an image with higher resolution is created, but with a higher NETD (Noise Equivalent Temperature Difference (differential)) due to smaller pixels being less sensitive to IR radiation.
Detecting material properties
There is a wide variety of materials that are used for the detector element in microbolometers. A main factor in dictating how well the device will work is the device's responsivity. Responsivity is the ability of the device to convert the incoming radiation into an electrical signal. Detector material properties influence this value and thus several main material properties should be investigated: TCR, 1/f Noise, and Resistance.
Temperature coefficient of resistance (TCR)
The material used in the detector must demonstrate large changes in resistance as a result of minute changes in temperature. As the material is heated, due to the incoming infrared radiation, the resistance of the material decreases. This is related to the material's
1/f noise
Resistance
Using a material that has low room temperature resistance is important for two reasons. First, lower resistance across the detecting material means less power will need to be used. Second, higher resistances comes with higher Johnson–Nyquist noise.
Detecting materials
The two most commonly used IR radiation detecting materials in microbolometers are
Amorphous Si (a-Si) thin films can easily be integrated into the CMOS fabrication process using low deposition temperatures, is highly stable, has a fast time constant, and has a long mean time before failure. To create the layered structure and patterning using the CMOS fabrication process requires temperatures to stay below 200˚C on average. a-Si also possesses excellent values for TCR, 1/f noise and resistance when deposition parameters are optimized.
The market share of VOx is much higher than any other technology. VOx market share is 70% where as Amorphous Silicon is 13%. Also, VOx technology based thermal cameras are being used in Defence Sector due to its sensitivity, image stability and reliability.
The use of infrared optical antennae together with small-size microbolometer materials can enhance its detection efficiency.[1][2]
Active vs passive microbolometers
Most microbolometers contain a temperature sensitive resistor which makes them a passive electronic device. In 1994 one company, Electro-Optic Sensor Design (EOSD), began looking into producing microbolometers that used a
Advantages
- They are small and lightweight. For applications requiring relatively short ranges, the physical dimensions of the camera are even smaller. This property enables, for example, the mounting of uncooled microbolometer thermal imagers on helmets.
- Provide real video output immediately after power on.
- Low power consumption relative to cooled detector thermal imagers.
- Very long mean time between failures.
- Less expensive compared to cameras based on cooled detectors.
Disadvantages
- Less sensitive (due to higher noise) than cooled thermal and photon detector imagers, and as a result have not been able to match the resolution of cooled semiconductor based approaches.
Performance limits
The sensitivity is partly limited by the
Origins
Microbolometer technology was originally developed by
Manufacturers of microbolometer arrays
- Xenics[3]
- BAE Systems
- DRS Technologies
- Teledyne FLIR Systems
- Teledyne Dalsa[4]
- Fraunhofer IMS
- GUIDEIR[5]
- Honeywell (Manufactured for Infrared Solutions)
- Institut National d'Optique (INO)
- L-3 Communications Infrared Products
- InfraredVision Technology Corporation (affiliated with L-3)
- Mikrosens Electronics Inc.[6]
- NEC
- Opgal Optronics
- Qioptiq
- Raytheon
- SemiConductor Devices[7]
- Seek Thermal
- Sofradiret ULIS)
References
- PMID 31434970.
- .
- ^ "Xenics | Infrared Solutions". Xenics. Retrieved 2022-10-29.
- ^ "Infrared Detectors | Teledyne DALSA". www.teledynedalsa.com. Retrieved 2022-10-29.
- ^ "CB360-Guide Sensmart". www.guideir.com. Retrieved 2022-10-29.
- ^ "MikroSens | Low Cost Thermal Imaging". www.mikrosens.com.tr. Archived from the original on 2022-11-30. Retrieved 2022-10-29.
- ^ "Micro-Bolometers | Core Technologies | Technologies | SemiConductor Devices". www.scd.co.il. Retrieved 2018-08-10.
- Notes
- Wang, Hongchen; Xinjian Yi; Jianjun Lai & Yi Li (31 January 2005). "Fabricating Microbolometer Array on Unplanar Readout Integrated Circuit". International Journal of Infrared and Millimeter Waves. 26 (5): 751–762. S2CID 110889363.
- LETI. "Microbolometers". Archived from the original on 2015-04-13. Retrieved 2007-12-03.
- Deb, K.K; Ionescu, A.C.; Li, C. (August 2000). "Protein-based thin films: A new high-TCR material". Sensors. 17 (8). Peterborough, NH: Advanstar Communications: 52–55. Archived from the original on 2008-04-28. Retrieved 2007-12-03.
- Kumar, R.T. Rajendra; B. Karunagarana; D. Mangalaraja; Sa.K. Narayandassa; et al. (18 March 2003). "Room temperature deposited vanadium oxide thin films for uncooled infrared detectors". Materials Research Bulletin. 38 (7): 1235–1240. .
- Liddiard, Kevin C. (2004). "The active microbolometer: a new concept in infrared detection". In Abbott, Derek; Eshraghian, Kamran; Musca, Charles A; Pavlidis, Dimitris; Weste, Neil (eds.). Proceedings of SPIE: Microelectronics: Design, Technology, and Packaging. Vol. 5274. Bellingham, WA: SPIE. pp. 227–238. S2CID 108830862.