QWIP - not only a night vision
Development and production of QWIP focal plane arrays at Acreo
bernhard.hirschauer@acreo.se
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 role in the future development and production of QWIPs.
From vision to research and development
Traditional cooled long wavelength infrared sensors (> 3 µm) 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 arrays 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-80s 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 Dr 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%. 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 Technologies 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:
- Development of a 640 x 480 pixel detector.
- Further independent development of our ROIC.
- Development of QWIPs operating at medium wavelength (3 - 5 µm) as well as simultaneous two colour detection (8 - 12 µm and 3 - 5 µm)
- Development of both advanced growth process on metal organic vapour phase epitaxy (MOVPE) and GaAs-process and control.
Dare to touch it: 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 320x240 pixel detectors for FLIR Systems on a weekly delivery plan. This detector type is mainly used in the FLIR System ThermaCAMTM SC3000 but also for FLIRs new triple-QWIP system and SaabTech Electronics BIRC. 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 20 mK (@ f/2 optics, 300 K background) and operability higher than 99,95%. 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.
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% in the world during the next 5 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. Especially the already strong airborne sector for both civil and military applications, aside of the traditional night vision, is expected to grow with about 20% per year. ACREO is one of the three companies who supply QWIP detectors for IR-cameras in volumes. ACREOs partner FLIR Systems is with 26% market share by far the leader on the infrared imaging equipment market. 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.