MICROBOLOMETERS KORAY POLAT 500612007 2013-SPRING

Outline Theory of operation Applications Consequences Referances

Microbolometer Thermal sensor convert radiation to heat and measure temperature difference for IR sensing. And this type of sensors are called with the name BOLOMETER A microbolometer is a specific type of bolometer used as a detector in a thermal camera. The resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.

Infrared radiation with wavelengths between 7 Infrared radiation with wavelengths between 7.5-14 μm strikes the detector material Unwanted light waves filtered Remaining waves hit the sensor Temperature increases This changes the electrical resistance

Theory of Operation A microbolometer is an uncooled thermal sensor. Unlike expensive cooling methods including stirling cycle coolers and liquid nitrogen coolers bolometers not need to any cooling method This sensors also gain 10 minutes unlike the other thermal resolution sensors.

The diagram of microbolometer is in figure below; Each company that manufactures microbolometers has their own unique procedure for producing them and they even use a variety of different absorbing materials 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.

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 below, 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

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 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 17 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.

Detecting material properties The devices responsivity is a main factor , how well the device will work . Responsivity is the ability of the device to convert the incoming radiation into an electrical signal. Detector material properties effect this value hence 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. his is related to the material's temperature coefficient of resistance (TCR) specifically its negative temperature coefficient. Industry currently manufactures microbolometers that contain materials with TCRs near -2%. Although many materials exist that have far higher TCRs, there are several other factors that need to be taken into consideration when producing optimized microbolometers.

1/f noise 1/f noise, like other noises, causes a disturbance that affects the signal and that may distort the information carried by the signal. Changes in temperature across the absorbing material are determined by changes in the bias current or voltage flowing through the detecting material. If the noise is large then small changes that occur may not be seen clearly and the device is useless a detector material that has a minimum amount of 1/f noise allows for a clearer signal to be maintained between IR detection and the output that is displayed.

Resistance Using a material that has low room temperature resistance is also important. resistance across the detecting material mean less power will need to be used. Also, there is a relationship between resistance and noise, the higher the resistance the higher the noise. Thus, for easier detection and to satisfy the low noise requirement, resistance should be low.

Detecting materials The two most commonly used IR radiation detecting materials in microbolometers are amorphous silicon and vanadium oxide. Amorphous Si (a-Si) works well mainly because it can easily be integrated into the CMOS fabrication process. To create the layered structure and patterning, the CMOS fabrication process can be used but it requires temperatures to stay below 200˚C on average. A problem with some potential materials is that to create the desirable properties their deposition temperatures may be too high although this is not a problem for a-Si thin films. a-Si also possesses reasonable values for TCR, 1/f noise and resistance when the deposition parameters are optimized.

Vanadium oxide thin films may also be integrated into the CMOS fabrication process although not as easily as a-Si for temperature reasons. VO2 has low resistance but undergoes a metal-insulator phase change near 67 °C and also has a lower value of TCR. On the other hand, V2O5 exhibits high resistance and also high TCR. Many phases of VOx exist although it seems that x≈1.8 has become the most popular for microbolometer applications.

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 thin film transistor (TFT), which is a special kind of field effect transistor. Main change in these devices would be the addition of a gate electrode. Although the main concepts of the devices are similar, using this design allows for the advantages of the TFT to be utilized.

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 MTBF. Less expensive compared to cameras based on cooled detectors.

Disadvantages Less sensitive than cooled thermal and photon detector imagers. Cannot be used for multispectral or high-speed infrared applications. Have not been able to match the resolution of cooled semiconductor based approaches. Higher noise than cooled semiconductor based approaches.

Performance limits The sensitivity is partly limited by the thermal conductance of the pixel., the speed of response is limited by the thermal heat capacity divided by the thermal conductance. Reducing the heat capacity increases the speed but also increases statistical mechanical thermal temperature fluctuations (noise). Increasing the thermal conductance raises the speed, but decreases sensitivity.

Applications The application areas of the uncooled detectors can be summarized as: Military Applications : Simple surveillance This sensor type is a general bolometer technology and the night vision gets for security and surveillance. This not also use for military but also civil security systems.

Rifle sights In military, for rifles there are night vision camera aparatus. This thermal sensing is very important and developing by the technology.

Advanced threat warning For the national security bolometers are used for monitoring dangerous threat, especially for border monitoring.

Unattended ground sensors  Unattended ground sensors are small ground-based sensors that collect intelligence through seismic, acoustic, Radiological Nuclear and Electro-Optic means. These sensors are networked devices that provide an early warning system to supplement a platoon size element and are capable of remote operation.

Long range scouts In military application current years provide to long scout range. Scanning long range is important for countries border lines against enemies.

Missile seeker

Civilian Applications Night vision enhancers for drivers To decrease the accidents which are result with death The automobile technology choose bolometers for thermal sensing to decrase accidents

Satellite instruments Monitoring world and give information about the thermal changes bolometers choose for satallite systems

Fire fighting Bolometers especially choose by the fire fighting . By a sensing camera in a fire action fighters monitoring the different thermal values and maybe save people or an animal.

Medical sensing Many applications of bolometers are thermal based and seem like the others for example in military nigh vision rifle sights and in civil life for security systems work with same procedure. Howevever , in medical applications thermal sensing ( bolometer ) is very important for modern medical systems. Skin cancer detection : Dental use:

Consequences Microbolometers are thermal sensing devices. By the advantages of bolometers and by the help of technology using areas are increasing. The use of bolometers are played an important role on industry and army. The advent of uncooled microbolometers is set to change how to diseases are detected and monitored. Over the next decade increased reserch in the terahertz spectrum will lead to more breakthroughs that will change the field of Biomems.

Referances Uncooled Thermal Imaging Arrays, Systems, and Applications, Paul W Kruse Weiguo Liu, Bin Jiang, and Weiguang Zhu Microelectronics Center, School of Electronic and Electrical Engineering, Nanyang Technological University, Singapore 639798  C. HANSON, H. BERATAN and S. MCKENNEY, Proc. SPIEInt. Soc. Opt. Eng., 1735 Infrared Detector  R. W. WHATMORE, Ferroelectrics  S. NOMURO and S. SAWADA, J. Phys. Soc. Japan Micro electro mechanical system research and application center METU