QWIP IR Detector

Detector technology for high performance imaging in the long wavelength infrared range. Detection of low energy photons requires a semiconductor material with a very small bandgap. These materials are difficult to produce with a stable quality. By stacking nanometer thin layers of 2 different semiconductor material artificial potential-wells can be created. Acreo developed this so called quantum-well technology based on GaAs. This is a well developed technology used for many different applications thus assuring a reliable high yield production.

To allow the incoming radiation to be absorbed in the quantum well structure the electric field has to be perpendicular to the QW layers. Acreo solved this by developing a new patented design with gratings formed on top of the detector, reflecting the normal incident radiation with an angle. Read-out of large pixel arrays is achieved by flip-chip bonding a CMOS circuit to the GaAs detector array. A hybridization technology using dense arrays of indium bumps has been developed, and is now a basis also for the fabrication of other types of detectors combining advanced materials with CMOS technology.

Application areas

High performance IR cameras can be divided into:

• Space
• Defence & Security
• Search & Rescue
• Industrial & Commercial applications

These have different requirements regarding resolution and sensitivity. For some applications small lightweight IR systems are needed, while others have their critical parameter in the lifetime of the system.

Commercial development

The technology was spun off from Acreo in 2007 by starting the company IRnova. All development and production of QWIP FPA (focal plane arrays) is taken place in the Electrum Laboratory. IRnova develop and supply high quality, high performance infrared detectors and related components to infrared module, camera and system manufacturers all over the world. This includes the whole IR camera modules including dewar cooler, A/D converters, preamplifiers, software and proxy electronics.

From research to production and spin-off

QWIP Technology

Introduction and Basic Principles

QWIP's belong to the category of so called photon detectors; the absorption of an infrared photon results directly in some specific quantum event, such as the photoelectric emission of electrons from a surface, or electronic interband transitions in semiconductor materials. Therefore, the output of photon detectors is governed by the rate of absorption of photons and not directly by the photon energy. They require cooling down to cryogenic temperatures in order to get rid of excessive dark current, but in return their general performance is high. QWIP's are most often used as photoconductive detectors. In this type of detectors photo-generated charge carriers increase the conductivity of the device material.
In QWIP's the quantum wells (QW) are formed by layers of different materials with different bandgap. This gives rise to potential-wells for charge carriers in the conduction band as well as in the valence band.

When these layers are sufficiently thin, the energy levels will show confinement; the continuous energy levels found in bulk material become discrete energy levels in these layers. In the following part of the discussion we will only consider the conduction band. By choosing the right barrier and well materials and layer thicknesses exactly two energy levels will form in the well. With bias voltage applied the higher level aligns to the edge of the conduction band. This structure can then act as a photon detector: when a photon with the right energy arrives, it will excite an electron from the ground state to the higher state. This electron can freely participate in the charge transport in the conduction band. This will result in a photocurrent in the detector.

However not only photons can excite an electron, phonons can as well. Phonons are quantized vibration modes of the atomic lattice and are generally generated by the temperature of the lattice. For all temperatures above absolute zero, electrons will be excited and contribute to the so called dark current. For a detector responding in the range 8-9.5 µm, cooling to temperatures near 70 K is necessary in order to reduce dark currents to a sufficiently low level compared to the photon generated current.

By an appropriate choice of material and design of the quantum wells, the energy levels can be tailored to absorb radiation in the infrared region from 3 to 20 µm. An excellent material combination in this respect is the aluminium gallium arsenide/gallium arsenide (A1GaAs/GaAs) material system, with gallium arsenide (GaAs) being the substrate material. The advantage of using GaAs is its mature processing technology.

Due to quantum mechanical selection rules normal incident radiation is not absorbed and only incident radiation with a component of the electric field perpendicular to the QW layers will be absorbed. IRnova holds an exclusive license of a patent for a method to accomplish just that. A grating is formed on top of the detector to reflect the normal incident radiation with an angle and thus allow the incoming radiation to be absorbed.

Fabrication

A large detector arrays can readily be fabricated based on the principles outlined before. Such detector arrays consist of a QWIP chip indium bump flip-chip bonded to a silicon CMOS readout integrated circuit (ROIC).The manufacturing process can be summarized by the following steps:

y

The first step is to grow the QWIP structure by MOVPE (Metal Organic Vapour Phase Epitaxy) starting with a semi-insulating GaAs wafer. A typical QWIP structure consists of 50 quantum wells, each of width 5.0 nm surrounded på AlGaAs layers (x = 0.28) of width 35 nm. On either side of the QW structure is a contact layer consisting of highly n-doped GaAs.
The next step is to lithographically define and etch a two dimensional grating into the uppermost part of the mesa. Then detector mesas are fabricated by etching down to the lower contact layer. Finally metal contacts are made and a layer of gold deposited over the grating. The latter acts as a reflector for the radiation.

The ROIC is based on direct injection and is manufactured in a standard CMOS process. The output signal has a serial analogue format. Amplification, A/D conversion and pixel correction is done off chip.

In the hybridisation process indium bumps are processed onto the chips, after which the QWIP and ROIC chips are aligned and bonded in a flip-chip bonder. The GaAs substrate is finally thinned down by a combination of lapping and chemical etching.

1 Dielectric reflector
2 QWIP structure
3 Indiumbump
4 Readoutcircuit.

The arrows show the incident radiation, together with the multiple passes of the radiation diffracted by the grating.

Publications

1. J. Y. Andersson and L. Lundqvist, "Near-unity quantum efficiency of AlGaAs/GaAs quantum well infrared detectors using a waveguide with a doubly periodic grating coupler", Appl.Phys. Lett 59 (1991) p. 857-859.
2. J. Y. Andersson and L. Lundqvist, "Grating coupled quantumwell infrared detectors: Theory and performance", J. Appl. Phys. 71 (1992) p. 3600-3610.
3. L. Lundqvist, J. Y. Andersson, Z. F. Paska, J. Borglind, and D. Haga, "Efficiency ofgrating coupled AlGaAs/GaAs quantum well infrareddetectors", Appl. Phys. Lett.,63 (1993) p.3361-3363.
4. J. Y. Andersson and L. Lundqvist, "Grating coupled quantum well detectors", in Long Wavelength Infrared Detectors, Vol. 1, Ed.M. Razeghi, part of the series Optoelectronic Properties of Semiconductors and Superlattices, p. 207-270, Series Editor M. O. Manasreh, Gordon and Breach Science Publishers, ISBN2-88449-208-9.
5. J. Y. Andersson, "Dark current mechanisms and conditions of background radiation limitation of n-doped AlGaAs/GaAs quantum-well infrared detectors", J. Appl. Phys. 78, pp. 6298-6304, 1993.
6. J. Y. Andersson, J. Alverbro, J. Borglind, P. Helander, H. Martijn, and M. Östlund,"320x240 pixels quantum well infrared photodetector (QWIP) array for thermal imaging: fabrication and evaluation", Proceeding of the SPIE, Infrared Technology and Applications XXIII, Vol. 3061,pp. 740-748, 1997.
7. H. Martijn, U. Halldin, P. Helander, J. Alverbro, J.Y. Andersson, "Finding the optimal readout integrated circuit for high-resolution quantum well infrared photodetectors", Proceedings of the SPIE - The International Society for Optical Engineering, v 3698, 1999, p 789-98.
8. P. Helander, J.Y. Andersson, J. Alverbro, J. Borglind, Z. Fakoor-Biniaz, Y. Eriksson, U. Halldin, H. Martijn, P.J. Tolf, M. Ostlund, "A 320×240 pixels quantum well infrared photodetector array for thermal imaging", Physica Scripta Volume T, v T79, 1999, p 138-42.
9. H. Martijn, U. Halldin, P. Helander, J.Y. Andersson, "A 640 by 480 pixels readout circuit for IR imaging", Analog Integrated Circuits and Signal Processing, v 22, n 1, Jan. 2000, p 71-9.
10. B. Hirschauer, J. Alverbro, J Anderssonm J. Borglind, A. Bustamente, Z. Fakoor-Biniaz, U. Halldin, P. Helander., Y. Lindberg-Eriksson, H. Malm, H. Martijn, C. Nordahl, O. Oberg, "Development and production of QWIP focal plane arrays at ACREO", Infrared Physics & Technology, v 42, n 3-5, June-Oct. 2001, p 329-32.
11. H. Martijn, A. Gromov, S. Smuk, H. Malm, C. Asplund, J. Borglind, S. Becanovic, J. Alverbro, U. Halldin, B. Hirschauer, "Far-IR linear detector array for DARWIN", Infrared Physics & Technology, v 47, n 1-2, Oct. 2005, 106-14.
12. S. Smuk, A. Gromov, J. Alverbro, P. Merken, T. Souverijns, D. Haga, H. Malm, C. Asplund, J. Borglind, S. Becanovic, P. Tinghag, H. Martijn, B. Hirschauer, "Optimisation of QWIP detectors for space applications", Proceedings of SPIE - The International Society for Optical Engineering, v 5978, Sensors, Systems, and Next-Generation Satellites IX, 2005, p 59781B.
13. A. Gromov, C. Asplund, S. Smuk, H. Martijn, "Optimisation of QWIP performance for high temperature and low background applications", Proceedings of SPIE - The International Society for Optical Engineering, v 6395, Electro-Optical and Infrared Systems: Technology and Applications III, 2006, p 639502.