Silicon microbolometer technology
The silicon microbolometer is fabricated using microengineering technology - also known as MicroSystemsTechnology (MST), and MicroElectroMechanicalSystems (MEMS). The modern microbolometer was first developed at the Defence Science and Technology Organisation (DSTO), Australia, during the late 1970s and first patented in 1980. The inventor is Kevin Liddiard, who is Proprietor/Owner of Electro-optic Sensor Design.
EOSD draws on this core technology to design microbolometer infrared focal plane arrays (FPA).
EOSD has long experience in the fabrication of microbolometer technology using amorphous silicon and other silicon alloys.
An historical comment: contrary to Wikipedia, US development of microbolometer technology started in the 1980s.
Mosaic pixel microbolometer technology
Conventional optical detectors have a pixel size defined by the radiation absorbing area of the detector. Focal plane detector arrays have a pixel size defined by the inter-detector spacing. The ratio of pixel area to absorbing area is the fill factor, which should be as large as possible, maximum unity. Detected signal are read out in sequence from the individual detectors in the array.
With the mosaic pixel focal plane array (MP-FPA) concept each imaging pixel comprises a number of functional detectors electrically interconnected, in the case of resistance microbolometers as a resistor network. The microbolometers may be interconnected in parallel, series or series-parallel to meet the requirements for specific applications. The interconnections are made at the pixel, so that signals are read out from the composite mosaic pixel, not the individual microbolometers.
Left: 2x2 parallel MP-FPA
Right: 2x2 series-parallel MP-FPA
Left: connection within pixel
Right: Connection at substrate
Mosaic pixel technology offers the ability to modify the performance of FPA to meet the requirements for different applications. For a given microbolometer design a mosaic pixel can be manufactured to change resistance, thermal conductance and thermal capacitance, and reduce noise to achieve the desired sensitivity and speed of response.
Parallel connected MP-FPAs are particularly suited for large pixels used in short range detection, where a high performance can be achieved at a fast speed of response with a mechanically robust design and high production yield.
The technology can also be employed to advantage in series-parallel connection, for example enhanced performance for industrial thermal imagers.
Passive infrared (PIR) security sensor technology
Mosaic pixel focal plane array (MP-FPA) technology can offer substantially enhanced performance compared to current pyroelectric PIR sensors; in particular, longer detection range, low false alarm rate and ability to detect slow temperatures changes as may indicate developing fire or equipment failure.
A prototype MP-FPA (pictured) has been developed for concept demonstration, using amorphous silicon technology.
K.C.Liddiard, “Novel concepts for low-cost IR security sensors”, Proc. SPIE, 5783, p.693 (2005)
K.C.Liddiard, “PIR security sensors – developing the next generation”, Proc. SPIE, 6542, p.1Q (2007)
Liddiard, K.C, “New design enhancements for microbolometer PIR security sensors”, Proc. SPIE 7854, 01-10 (2010)
Enhanced performance industrial thermography
Thermal imaging for preventative maintenance and other diagnostic applications such as building inspection is a rapidly growing market with many companies competing for market share. These cameras typically display a calibrated thermogram of the field of view. They must be cheap, rugged and easy to operate.
Low cost thermography cameras invariably employ microbolometer focal plane arrays (FPA) for IR sensing. Current products typically have a sensitivity (NETD) of 100mK. This is too high to meet new applications identified by EOSD, which require an NETD of <50mK. There is strong competition from many companies, so there is a need for product advantage.
Mosaic pixel technology can be employed for enhanced performance in low cost thermal imaging, with the ability to achieve temperature sensitivity of <25mK, comparable to that of high cost cryogenically cooled photodetectors.
Low cost IR sensors for non-contact temperature measurement
Uncooled infrared sensors with a sensitivity of ≤10mK and fast speed of response can be fabricated with mosaic pixel microbolometer technology. This performance is achieved for applications where high optical resolution is not needed. A cheap expendable sensor containing a radio frequency identification tag (RFID) and GPS chip could give early alert to emergency services.
Potential applications include:
People monitoring and counting
Industrial process monitoring
Forest and domestic fire detection
Pedestrian safety sensors
Greenhouse gas detection
Solar irradiance monitoring
K.C.Liddiard, “Further applications for mosaic pixel FPA technology”, Proc. SPIE 8012, pp. 3O1- 11 (2011)
K.C.Liddiard, "Application of mosaic pixel microbolometer technology to very high performance, low cost thermography and pedestrian detection”. Proc. SPIE, Vol. 8704, (2013)
IR Sensors and the Internet of Things
Here we consider the future role of IR sensors for the Internet of Things (IoT), the networked interconnection of everyday objects. There are said to be 50,000 billion ‘things’ on earth, and every human is surrounded by thousands of objects. The concept of IoT is that if objects were equipped with radio frequency identification tags (RFID), they could be identified and inventoried by computer. Thus the position, nature and status of an object could be determined and if necessary appropriate action taken.
This is not a new idea, but is now gaining significant credibility worldwide, and viewed by many as the nature of things to come. The IoT concept applies to many sensor types. We can envisage a small IR sensor roughly the size of a mobile telephone with internal GPS and RFID. The application might be industrial or domestic security, schoolyard security, home or forest fire detection. Many other applications can be identified. An alarm is transmitted to a land base, aircraft or satellite, and then onward transmitted to emergency services, personal mobile phones and computers.
K.Liddiard, “Further applications for mosaic pixel FPA technology”, Proc. SPIE 8012, pp. 301- 11 (2011)
The Hammersmith Group, “The Internet of things: Networked objects and smart devices”, research report (2010)
Contact EOSD for further enquiries on the above technology and applications