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Electronic Imaging & Signal Processing

High-performance graphics arrays for uncooled IR imaging

Amorphous silicon microbolometers enable straightforward, high-volume production of highly sensitive, easy-to-use 17μm pixel-pitch uncooled IR focal plane arrays.
15 February 2012, SPIE Newsroom. DOI: 10.1117/2.1201202.004109

Detector arrays operating in the IR band are useful for a variety of military and commercial purposes, such as night vision and surveillance, firefighting, energy conservation, fever detection, and industrial monitoring. Cooled IR detector arrays are high performing but require costly mechanical coolers and expensive, electronics-grade semiconductor crystals. Uncooled IR sensor arrays are cheaper and have been developed for lightweight military applications since the 1980s, but over the past decade there has been a significant increase in production for commercial purposes. Commercial has now overtaken military production and is driving some of the most important developments in IR detector technologies. The challenge is to design detectors that are not only sensitive but also simple to produce in large volumes and that integrate easily into video cameras and other systems.

Microbolometers lead the market in uncooled thermal IR (8–14μm) detectors. Each microbolometer pixel is made up of a thin absorber film made of a material that changes resistivity with temperature. This is thermally isolated from a CMOS readout integrated circuit (ROIC) that signals the absorber's temperature (see Figure 1). In addition to the requirements mentioned previously, all pixels must show highly uniform behavior with temperature. Pixel electrical resistance uniformity depends mainly on the material property uniformity, and so the choice of thermometer semiconductor material is most important.

Amorphous silicon (a-Si) presents attractive electrical properties for microbolometer applications. It is easy to deposit uniformly on a 200mm CMOS substrate and is fully compatible with silicon microelectronics technology. Semiconductor electrical resistance is sensitive to temperature and to the activation energy, which is a property of the material. Alloys have the disadvantage that the activation energy will vary for different pixels; by contrast, each pixel of a-Si will have the same activation energy, and so an array of a-Si pixels will show a much higher spatial uniformity. This helps simplify the required algorithms and, therefore, the system electronics needed for detector operation.

Figure 1. Schematic of an uncooled thermal detector pixel.

Figure 2. Video graphics array (VGA) ceramic package (left). Extended graphics array (XGA) metallic package (right).

Figure 3. VGA output direct current (DC) level histogram at 293K.

Despite a-Si being easy to integrate onto CMOS, several technological improvements are key to successfully scaling the pixel pitch down from 25 to 17μm (or smaller). Advanced lithography in connection with a thinner a-Si absorber film clearly gives an edge to maintain, and even improve, thermal insulation when scaling down the pixel size. High operability and high manufacturing yield require a simple pixel microbridge structure, which also makes the sensor fast, with unusually small pixel time constants. The thermal time constant of a-Si 17μm pixels is less than the 10ms that is required for high video frame rate operation. By contrast, other technology gives a thermal time constant of 16–18ms.

Figure 4. Noise equivalent difference temperature (NEDT) histogram.

On the basis of this a-Si technology, we have designed video graphics array (VGA, 640×480 pixels) and extended graphics array (XGA, 1024×768 pixels) detectors with a pixel-pitch of 17μm. These two detectors can be packaged under vacuum without any focal plane temperature stabilization device (see Figure 2). The VGA sensors have a ceramics package; the XGA sensors use a metallic package that is easier to secure into the camera head for demanding applications such as driver vision enhancement for professional vehicles.

We measured the thermal variations of the IR sensor pixels due to the scene temperature deviations by a current-to-voltage conversion performed by a capacitive transimpedance amplifier in the CMOS readout circuit. We operated the array in rolling-shutter mode with a bidirectional scan in both row and column directions. We included a serial programmable interface in the ROIC design, which allowed device operation over a wide range of conditions. It also provided a large degree of flexibility in balancing scene dynamic range and sensitivity for different applications. We were able to achieve power consumption below 155mW.

Figure 5. VGA IR image.

Figure 6. XGA IR image.

We carried out electrical and electro-optical tests using an f/1 optical aperture and frame rate of 30Hz. We measured responsivity using two black bodies set at 293K and 308K, respectively.1 In the standard operational mode, we found a mean responsivity value of 12.1mV/K, hence offering a scene dynamic higher than 150K on a scene around room temperature (i.e., from −50° to +100°) without readout integrated circuit saturation. Figure 3 shows a typical output direct-current (DC) level uniformity histogram, the span of which represents only 11.5% of output dynamic. The sensitivity, expressed in noise equivalent difference temperature (NEDT), stands at 46mK—see Figure 4—at room temperature. The NEDT can be lowered to 38mK by increasing integration time at the expense of the dynamics, leading to a figure of merit defined as the product of NEDT by thermal time constant of <350mK.ms. Figures 5 and 6 illustrate the high image quality that can be achieved with 17μm-pixel devices.

In summary, we have developed the first a-Si XGA arrays with 17μm pixel-pitch and high uniform performance.2 We also demonstrated a high-performance VGA format sensor adapted to compact systems with a low thermal time constant (9ms), enabling frame rate operation of 50Hz. The 17μm sensor technology paves the way to high-volume applications. The next step is for us to take advantage of this a-Si technology to develop increasingly compact systems based on imminent developments such as even smaller pixel size (12μm), a new ‘silicon packaging’ technique and small array designs.

The author would like to thank the ULIS and Electronics and Information Technology Laboratory of the French Atomic Energy and Alternative Energies Commission (CEA LETI) teams who develop and produce uncooled IR microbolometer detectors.

Jean-Luc Tissot
Veurey-Voroize, France

From 1979 to 2002, Jean-Luc Tissot worked at the CEA LETI, France, to develop cooled and uncooled IR detector technologies. In 2002, he joined ULIS as technical director. Since 2006, he has been technical director and marketing division director at ULIS.

1. A. Durand, J. L. Tissot, P. Robert, S. Cortial, C. Roman, M. Vilain, O. Legras, VGA 17μm development for compact, low power systems, Proc. SPIE 8012, pp. 80121C, 2011. doi:10.1117/12.888095
2. C. Trouilleau, B. Fièque, S. Noblet, F. Giner, D. Pochic, A. Durand, P. Robert, S. Cortial, M. Vilain, J. L. Tissot, J. J. Yon, High-performance uncooled amorphous silicon TEC less XGA IRFPA with 17μm pixel-pitch, Proc. SPIE 7298, pp. 72980Q, 2009. doi:10.1117/12.819993