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 7.5-14 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 aSi 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 ElectroOptic 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:
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.
•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