Bolometer array and read-out electronics

ExpertiseImaging

Imaging

RISE Acreo develops new imaging devices for non-visible wavelengths. We use advanced semiconductor materials and nanotechnology fabrication methods and integrate with CMOS read-out circuits to functional arrays.

The work is organised around our excellence centre IMAGIC, where we have a close collaboration with industrial and resarch partners.

We specialise on:

Low-cost IR imaging

Highly sensitive CMOS compatible thermistor material for night vision
Watch the video on possible application:

Night vision system increasing safety on the road.

High performance IR imaging

High-performance IR cameras are needed for applications in defence and security, space technology, search and rescue or other industrial applications. IMAGIC developes semiconductor materials and technologies for new infrared photon detectors based on quantum-dot and superlattice detectors.

UV sensing technology

A new generation of UV detectors based on wide-bandgap semiconductor materials (silicon carbide, gallium nitride, and zinc oxide) is under development. These have the potential to provide very sensitive, compact and low cost solutions for UV detection and imaging.

X-ray imaging

X-ray detector development for medical and industrial applications.

Terahertz imaging

Terahertz (THz) is a relative newly exploited wavelength region in the electromagnetic spectrum. The technology has recently been adapted for security applications (airport body scanning cameras) and advanced medical diagnostics, TDS (Time domain spectroscopy). IMAGIC developes SiGe Quantum Well microbolometers technology for THz imaging.

3D imaging

Range information för accurate image segmentation and object recognition for security and entertainment.

Packaging

Integration of imaging and optoelectronic devices.

Project References

Microbolometer for nigth vision

People

Stéphane Junique
Senior Scientist
+46 (0)70 772 77 48
stephane.junique [at] ri.se

Duncan Platt
Group Manager
+46 704 559964
duncan.platt [at] ri.se

Björn Samel
Department Manager
+46 (0)70 475 00 81
bjorn.samel [at] ri.se

Publications
Low-Cost IR Imaging

Also referred to as un-cooled IR imaging

Imaging in the far infrared region has traditionally been the privilege of user who can accept bulky, heavy and power hungry equipment with a high price tag. However, with its possibility of generating images in complete darkness or make contactless temperature images of equipment in operation, the number of applications are virtually unlimited if it weren’t for the cost. In contrast to the cooled systems mentioned above, un-cooled infrared sensors offer an alternative for users where performance requirements have to be balanced against price, weight, size and power consumption. Applications which fall into this category are, e.g., consumer night vision systems for automotive or marine use, IR cameras for craftsmen working with insulation of buildings or for preventive maintenance of industrial machinery. Portability, battery life and cost are essential for wide spread use in these cases.

Un-cooled IR imaging offer these possibilities by basing the infrared sensor on a different working principle than cooled sensors. The most common un-cooled image sensor is based on the bolometer principle. Here, an infrared image is formed on a focal plane array of heat absorbing and temperature sensitive resistors. The absorbed IR energy result in a temperature change of the bolometer, and the temperature change can be measured by an integrated temperature sensitive resistor – a thermistor. On the market today, there are two dominating technologies at use which differ mainly in the material chosen for the thermistor: Vanadium Oxide (VOx) and amorphous Silicon (a-Si). Due to the close connection to the military industry, availability of sensors based on these technologies is still limited and surrounded by export license regulations preventing wide spread adoption.

Acreo brings to the un-cooled scene a new material for the thermistor and a slightly different approach to bolometer design. Based on our experience with engineered materials, we have developed a thermistor using alternating layers of Si and SiGe. The thermistor material is highly sensitivity to small temperature changes and generates very little internal noise  - both properties of vital importance for the production of high performance IR imagers. Further more, the thermistor material is fully compatible with CMOS processing equipment and procedures lowering the threshold for commercial implementation.

Within IMAGIC, Acreo is working on further improving the performance of the Si/SiGe thermistor material. Issues that are being addressed not only include fundamental material investigations, but also look at important manufacturing aspects such as sensor material uniformity and repeatability. Autoliv and Sensonor are the primary partners within IMAGIC for this work.

High Performance IR Imaging

Cooled thermal IR imaging

Called this way since the detector element needs cooling in order to produce a good signal, cooled IR is part of the technology used to develop high performance IR imaging detectors.

The physical principle behind this technique involves absorption of an IR photon which excites an electron and thus produces a change in the measured electrical conduction or voltage. Examples of this is the quantum well infrared photodetector (QWIP) which makes use of intersubband transitions in quantum wells or InSb and MCT (Mercury Cadmium Telluride) that are bulk material detectors.

More stringent requirements for thermal IR cameras such as higher operating temperature create a demand for detectors that use more advanced materials. Therefore within IMAGIC, two candidates are being developed. These are QDIPs (quantum dot infrared photodetectors) and antimonide superlattice detectors.

Quantum dots (QD) are small islands of a semiconducting material surrounded by another semiconducting material with a larger bandgap, thus creating a potential well where electrons and holes are trapped. The detection energy / wavelength can be tuned by adjusting the size of the quantum dot or the depth of the potential well. For further tuning possibilities, the dots can be inserted in a quantum well, a so-called Dots-in-a-Well structure (DWELL). The detection wavelength is then partly determined by the dot and partly by the well, and the wavelength window can be more easily achieved.

Antimonide superlattice detectors consist of alternating thin layers of InAs/GaInSb (indium arsenide/gallium indium antimonide). This new material combines the performance of bulk detectors with the easy processing of III/V materials as QWIP. With its possibility to reduce dark current and raise operation temperature and at the same time maintain high detectivity and short integration times. Antimonide superlattices will significantly enhance the performance of detectors and as a consequence lead to a cheaper camera system.

The main advantage of both materials over the QWIP is the reduction achieved in the dark current. A lower dark current permits an increased operating temperature, which reduces the cooling requirements of the detector and hence the cost of the camera.

QD infrared photodetectors (QDIP) single pixel components have been fabricated at Acreo. A dot density of ~1000 dots/µm2 has been achieved, which means that a 25µm pixel with ten layers of quantum dots consists of 6.25 million quantum dots! The peak responsivity has been measured as 60 mA/W at an applied bias of 2.25V, and the peak wavelength is situated at 8.3µm. The dark current at a temperature of 77K has been measured to be lower than the dark current of a conventional QWIP at 65K, demonstrating that the expected reduction in dark current can be achieved.

Antimonide superlattice development started at a later date than the QDIP development, and material development is currently under way. It is aimed to have a device demonstrator ready in 2 years time.

Read more about QWIP: www.ir-nova.se/.