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Sofradir Builds A New Factory To Ramp HgCdTe Chip Production

For decades, infrared detector development has drawn on many different types of compound semiconductor material. Philippe Tribolet explains why Sofradir has selected HgCdTe for its future plans. These include building a new $12 million fab to cut chip manufacturing costs, which should ultimately lower detector prices and drive up sales of these high-specification imagers.

Infrared imaging systems have several advantages over their optical equivalents. They can operate at night, because image contrast is solely provided by temperature differences, and they can work in poor weather conditions by detecting radiation in particular wavelength bands. Image quality is also less affected by reflections from sunlight and emission from bright light sources, such as car headlights, particularly when the detection band in the 8–12 μm range is selected.

As a result, these semiconductor-based imagers are suitable for a wide variety of everyday uses. However, cost has limited their deployment to military and space applications, where they are predominantly used for surveillance and targeting of hot objects, such as rockets and vehicle motors.

The military s use of these detectors began in the 1970s with first-generation cameras featuring cooled, small linear arrays of photodetectors coupled to a complex two-dimensional scanning system. By the 1980s second-generation versions, which included a read-out integrated circuit for signal pre-processing and multiplexing on the focal plane, had been built in research labs. These were then commercialized in the early 1990s.

The progression to second-generation detectors opened the way for high-resolution scanned arrays featuring time delay and integration, and high-resolution two-dimensional arrays with signal processing on the focal plane. Cooled "staring" two-dimensional arrays operating in the medium- and long-wavelength infrared bands were also developed, which have been dubbed "2.5-generation" infrared detectors. These imagers, which were launched commercially in the late 1990s, have arrays with typically 320 × 256 or 640 × 512 pixels, and are suitable for applications such as missile detection. The improved resolution and performance of all these second-generation detectors have doubled the effective range for target acquisition, and enabled identification of enemy vehicles at night through smoke and dust at distances of 6 km and more.

These various generations of infrared imagers have been fabricated with a variety of technologies, and include detector arrays made from HgCdTe, InSb, InGaAs and GaAs quantum wells (see figure 1 for an overview). Each of these detectors has particular merits, and selection is application specific.

The performance of these detectors can be evaluated by their ability to distinguish a small target from an infrared background with a similar temperature. Military, security and surveillance applications use this as a test, and rate the imagers performance in terms of detection or identification range, and ability to deal with adverse weather.

The highest class of detectors can image objects at distances of 10 km or more, depending on atmospheric conditions and altitude, and are suitable for many different applications. These include missile guidance and tracking, missile warning, providing tank and airborne vision enhancement, carrying out reconnaissance and climate observations, agricultural monitoring, pollution analysis, and astronomical observations. Comparable detectors with an operating range of 6–10 km can also be used for scientific applications, including analysis of relatively weak signals for gas spectroscopy.

The detectors that operate over these ranges require cooling, and are predominantly based on HgCdTe and InSb chips. HgCdTe has several advantages (see "The strengths of HgCdTe"), including a sensitivity over a spectral range stretching from visible wavelengths to 18 μm, a high quantum efficiency coupled to a high signal-to-noise ratio, and a relatively high operating temperature that reduces the cost of the associated cooler. LPE has been used for many years to produce these chips, but single- and dual-band chips covering short-wave (1–3 μm) and medium-wave (3–5 μm) infrared bands can now be produced in large volumes and at low costs using MBE.

By comparison, InSb is only sensitive in the medium-wave infrared band (3–5 μm). Technological limitations also hinder improvements in operating temperature and very small pixel sizes, making this material unsuitable for the most demanding applications. Alternative technologies include quantum-well infrared photo-detectors (QWIPs), which can be used for long-range detection (8–12 μm) but have slow frame rates and require lower operating temperatures, and type II superlattices, which are still under development.

Another class of detectors are those with a 2–6 km detection range. These can serve civilian needs such as fire surveillance; security applications such as police surveillance and tracking, border surveillance, and airborne landing vision enhancement; and also provide imaging in military small armored vehicles and certain types of unmanned airborne vehicles. Cooling the detector is mandatory in all these applications. In some cases the selection of a very high-performance detector can actually cut the overall system cost, as this can reduce the size of the optics, simplify the signal processing and ease reliability constraints.

The candidates for providing infrared detection over these distances are similar to those for the longer ranges. However, QWIPs operating in the infrared long-wavelength band can offer good value for money if their limited efficiency and relatively high dark current can be tolerated. InGaAs cameras operating at shorter wavelengths are also competitive, but they can suffer from read-out circuit noise when the input signal is low, and they cannot detect beyond 1.9 μm.

The final class of detectors are those used for imaging objects at distances between tens of meters and 2 km. They can also serve civilian applications, such as building inspection, industrial process control, industrial site surveillance, and automotive driving enhancement, and other uses will emerge as the cost of these detectors fall. Uncooled thermal detectors, such as micro-bolometers, which include imagers based on amorphous silicon that is fully compatible with CMOS silicon technologies, offer the best value for money and are best suited to these tasks.

The French legacy

Many of these different types of chip-based detectors have been commercialized in France, a country with a very strong history of infrared detector development. Currently over 500 people are employed for the research, advancement, and manufacture of these devices in the Grenoble area, which makes this region a major global player in this technology.

The research has been led by the national research and development center CEA-Leti, which started developing infrared detectors in the 1970s. This knowledge has been transferred to Sofradir, which is headquartered in Chatenay-Malabry and has production facilities in Veurey-Voroize. The research from CEA-Leti has been used in our production of second- and 2.5-generation detectors, which we have been manufacturing for over 13 years, and the amorphous silicon micro-bolometers that we have been building since 2003 through our subsidiary, ULIS. QWIPs developed at Thales Research and Technologies (TRT), France, are also being mass-produced in cooperation with us, and InSb and InGaAs technologies are being developed within France.

Today, we are manufacturing thousands of cooled infrared detectors, alongside tens of thousand uncooled detectors through ULIS. In addition, we are currently building a new €9 million ($12.1 million) MBE fab for HgCdTe chip production, which will scale up our production from 2 inch to 4 inch material. When installation is completed in summer 2008, the increases in manufacturing volumes and efficiencies will reduce our production costs and also enable us to make larger chips. In addition, the lower costs should have an effect on the price of the longer-range detectors that will employ our chips, and should drive an increase in sales of this type of imager. One-third of the cost of an infrared detector is associated with the focal-plane array, so lower chip manufacturing costs can have a big impact on the price of the overall system.

Third-generation detectors

Our new fab will be used to mass-produce third-generation detectors that have been developed in-house. These devices provide multicolor operation, have large focal-plane arrays with homogenous active layers, can resolve more image detail and operate more effectively in poor weather.

The chips will be produced by MBE, a technique capable of growing the multiple layers required by multicolor detectors on a wide variety of large-diameter substrates. This replaces our current growth technique used for production, LPE, which is carried out on lattice-matched CdZnTe substrates. These substrates are only available in small sizes, so many fabs are now developing silicon or GaAs platforms that offer low cost and a compatibility with the thermo-mechanical characteristics of the read-out circuit. In France, however, germanium has been the preferred alternative, because the lower stability of its oxides makes its surface easier to prepare, both ex situ and in situ, prior to MBE growth.

The MBE process on germanium (211) was demonstrated several years ago at CEA-Leti, and is suitable for making larger arrays. This substrate is already available in 4 inch and larger versions, which can be used to make the high-quality HgCdTe films needed for two-dimensional short-wavelength and medium-wavelength infrared arrays. We plan to start mass-producing both of these types of arrays during summer 2008, using two MBE reactors with a 1 × 4 inch and a 3 × 4 inch capacity. The switch to a larger wafer size will give us an advantage over InSb and QWIP manufacturers, who are still using 3 inch substrates for mass-production (see figure 2). In addition, it will allow us to affordably fabricate 1280 × 1024 pixel infrared focal-plane arrays with a 15 μm pixel pitch that will provide a greater image identification capability than detectors with fewer pixels.

We are also continuing to improve our third-generation detectors, so that manufacturing costs can be cut and selling prices reduced. In particular, we are focusing on the development of smaller pixel pitches and larger formats, and improving the performance of our avalanche photodiodes (APDs) and multicolor detectors. Our HgCdTe APDs are a unique design based on electron-impact ionization, and deliver exceptional detection. These electron initiated APDs deliver a thousandfold multiplication gain at an inverse bias of only 10 V. The high gain at low bias, combined with a low noise factor, makes these APDs particularly well-suited for integration in the latest FPAs. These are ideal for active laser-based imaging, but they can also serve many passive imaging applications.

Our continual development of HgCdTe for a wide variety of focal-plane arrays will lead to fabrication of a series of very high performance detectors. These devices, which will outperform detectors fabricated with competing technologies, will benefit from the use of electron initiated APDs. With this technology we can meet our customers needs for greater performance, lowered system cost or fewer constraints.

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