QWIP – not only for night vision: development and production of QWIP focal plane arrays at ACREO

Citation

(2001), "QWIP – not only for night vision: development and production of QWIP focal plane arrays at ACREO", Sensor Review, Vol. 21 No. 3. https://doi.org/10.1108/sr.2001.08721caf.001

QWIP – not only for night vision: development and production of QWIP focal plane arrays at ACREO

QWIP – not only for night vision: development and production of QWIP focal plane arrays at ACREO

Keyword: Thermal imaging

Many years of research and development of Quantum Well Infrared Photodetectors (QWIPs) made this new infrared-detector technology ready for the market. ACREO will play a significant roll in the future development and production of QWIPs.

From vision to research and development

Traditional cooled long wavelength infrared sensors (> 3µm) with high sensitivity use material systems like HgCdTe, InSb, PtSi and doped Si. These detector types are based on an interband process where an electron is excited from the valence band to the conduction band by the absorption of a photon. QWIPs based on GaAs-material system make use of intersubband transitions in quantum wells. Each one of the traditional materials has fabrication limitations, while QWIPs have the advantages of high yield and therefore low cost, suitable for large array and dual wavelengths. The lower responsivity of this detector type is more than well compensated by its high signal to noise ratio and uniformity.

The quantum well array (QWA) technology was pioneered in the mid-1980s by Lucent Technologies Bell Laboratories. ACREO has been involved in QWIP research and development (R&D) since 1986. During the first years a small group led by Jan Andersson was dedicated to research on QWIP structures and means of coupling radiation into the quantum-well structure. The latter was an essential task due to the peculiar absorption characteristics of intersubband transitions in quantum wells. One of the research results is a two-dimensional optical grating coupler to create an optimal electric field pattern for highest possible absorption and responsivity. By careful design of the profile and dimensions of the grating, suitable grating modes can be enhanced and thus the infrared absorption. The absorption quantum efficiency becomes typically 50 per cent. Besides providing efficiency, the structure is also comparatively easy to process. ACREO holds a patent for this grating coupler.

Since 1992 FLIR Systems (earlier AGEMA Infrared Systems AB), FMV, FOA and NUTEK have sponsored an R&D project with the goal to develop QWIP detectors and start up production. A complete QWIP is pixelated and consists of a GaAs detector which is hybridised on top of a Si-based read-out integrated circuit (ROIC). Soon it became clear that an adapted ROIC design and the hybridisation of the focal plane array (FPA) are key issues in order to achieve the highest possible performance and operability of the complete detector device for volume production. Expanded measures where taken in 1996.

In 1997 the industrial interest increased further and also CelsiusTech and Saab Dynamics now merged to SAAB AB started to sponsor the R&D project. Because of its success during the last years it has now been expanded and scheduled until 2003. The major goals are:

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  development of a 640 x 480 pixel detector;

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  further independent development of our ROIC;

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  development of QWIPs operating at medium wavelength (3-5 µm) as well as simultaneous two colour detection (8-12 µm and 3-5 µm); and

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  development of both advanced growth process on metal organic vapour phase epitaxy (MOVPE) and GaAs-process and control.

From research and development to production

As a result of intense R&D a process qualification could be achieved which allowed steady and reproducible results. A pre-production line was established during 1997 to 1999 in close co-operation with FLIR Systems in Danderyd, Sweden. At the beginning of the year 2000 ACREO started to produce 320 x 240 pixel detectors for FLIR Systems on a weekly delivery plan (Plate 1). This detector type is mainly used in the FLIR System ThermaCAM™ SC3000 but also for FLIR's new triple-QWIP system and SAAB's Birch. All process steps except the ROIC production are done in ACREOs own laboratories, which gives high control over quality and availability of the product. The close co-operation between R&D and production steadily improved the performance of the detectors. A quick feedback results in improved quality and processing. Detectors which leave the production line now have a noise equivalent temperature difference (NEDT) better than 20mK (@ f/2 optics, 300 K background) and operability higher than 99.95 per cent. The high performance of the detectors produced by ACREO triggered an increasing demand by the customer. In order to meet those needs further production projects are planned for the very near future.

Plate 1 The image of a 320 x 240 QWIP as it was developee by ACREO and is produced for FLIR Systems

Visions for IR vision from ACREO

According to the newest report of Maxtech International, a compound annual growth rate (CAGR) for cooled infrared imaging equipment is expected to be 14 per cent in the world during the next five years. Applications like surveillance and long distance sight are traditionally dominating but there is a growing market for predictive maintenance, process control and quality control, driver's vision enhancement, law enforcement, medical applications or fire fighting. In particular, the already strong airborne sector for both civil and military applications, aside from the traditional night vision, is expected to grow with about 20 per cent per year. ACREO is one of the three companies which supply QWIP detectors for IR-cameras in volumes. ACREO's partner, FLIR Systems, is, with 26 per cent market share, by far the leader on the infrared imaging equipment market (Plate 2). Considering these facts, the coming years will see a prosperous development of QWIP-related activities which is already visible in rising budget and funding as well as increased personal resources at ACREO.

Plate 2 An infra-red image of a PC card

For more information, please contact: Dr Bernhard Hirschauer, ACREO AB-Imaging, Electrum 236, 164 40 Kista, Sweden. Tel: +46 8 632 77 83; Fax: +46 8 750 54 30.