- Introduction and Basic Principles
- Fabrication
- Publications on QWIPs

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 (See Figure 1). This gives rise to potential-wells for charge carriers in the conduction band as well as in the valence band.

Figure 1. Band diagram of a quantum well.

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 (Figure 2).

Figure 2. Quantum well with applied bias.

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.

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:

1. Epitaxial growth of QWIP structure
2. Processing of the QWIP array
3. Fabrication of ROIC (based on silicon CMOS)
4. Processing of indium bumps
5. Hybridization -Flip-chip bonding
6. Mounting and wire bonding

Figure 3. Production steps of a QWIP array.

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. Figure 4 shows the result after GaAs processing.

Figure 4. Detector mesas with grating and metal contacts.

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.
Figure 5 shows a cross-section through a detector pixel.

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.

Figure 5. Scheme of detector pixel in cross-section.

Figure 6. QWIP array mounted on a substrate.

The hybridized QWIP array mounted onto a ceramic substrate and wire bonded is shown in Figure 6.

Publications on QWIPs