Cadmium Mercury Telluride Detectors Research Articles

Characterization of mid-wavelength quantum infrared cameras using the photon transfer technique

Noise plays a fundamental role in connection with theoretical limits in the field of inverse thermal problems, since the solution accuracy for such inverse problems is decisively influenced by noise-induced errors in temperature measurements. There are eight standard assumptions proposed by Beck and Arnold (1977), Beck (1979), Beck et al. (1985) and Özisik and Orlande (1999) for the statistical description of measurement errors (noise) which should be fulfilled in order to adequately solve an inverse heat conduction problem. Therefore, it is of utmost importance for thermography measurements that the real infrared camera properties (and especially the noise performance) are well characterized and their influence on the measurement results is known and considered. Within the scope of this work three different mid-wavelength infrared cameras (based on indium antimonide and cadmium mercury telluride detectors) from various manufacturers were characterized with respect to the above mentioned prerequisites by means of the so called photon transfer technique. One of the results show that the tested IR quantum detectors are mostly photon-noise limited and therefore subject to a pronounced Poisson noise characteristic, i.e. the noise signal level (signal variance) depends on the signal intensity. This in turn violates one of the basic prerequisites of most inverse heat conduction approaches, which are relying on an additive noise model with constant variance. In addition, the camera gain constants for two camera models were determined using the photon transfer technique. They are 433 photons/DN or 281 photons/DN and are a measure of the sensitivity of the camera, i.e. the number of incident photons that result in a measurable signal change in the thermographic image.

Technological and Electrical Performances of Ultrafine-Pitch Flip-Chip Assembly Based on Room-Temperature Vertical Interconnection

Flip-chip is a high-density and highly reliable interconnection technology, which is widely used for the fabrication of infrared complementary metal-oxide-semiconductor imagers. In order to increase the format and the resolution of staring arrays to 2000 × 2000 pixels or even larger complexities, while keeping substrate dimensions and device cost low, there is a general trend in reducing pixel size. But this results in a very challenging process control for ultrafine-pitch and high-count flip-chip bonding technology. A new room-temperature insertion technology has been proposed and developed to alleviate most of these issues. It overcomes the planarity and thermal mismatch issues of this heterogeneous assembly made of cadmium mercury telluride detectors hybridized on silicon read-out circuits while reducing the bonding thermocompression forces and the hybridization temperature. The technological preparation of the assembled devices is described in this paper. A process characterization, using a modified nanoindenter with instrumentation for single tip insertion, has been achieved to evaluate insertion mechanical parameters. This paper gives also the first measured interconnection yield and vertical access resistance of 2000 × 2000 hybridized arrays of 10 μm pitch, fabricated using the proposed insertion flip-chip technique. Connection results are discussed, and a comparative study proves that parallelism during insertion is a key parameter. When providing a perfect parallelism control with advanced new flip-chip bonders, the microtip insertion technique is scalable to complexities of over 4 million connections at very fine pitch. Moreover, this new low-temperature insertion concept could possibly be applied to 3-D interstrata interconnections for electronic devices. Indeed, it avoids the issues caused by cumulative high-temperature cycles, which are typical of more conventional processes.

Recent developments in cadmium mercury telluride infrared detectors

A II–VI compound, cadmium mercury telluride, has dominated recent advances in the detection of infrared radiation. Although the main application is in thermal imaging, other applications include instrumentation and guidance. In this paper we describe the history of the development of cadmium mercury telluride as a detector material, with emphasis on the importance of material parameters and the role of the material scientist. High purity material, free from defects and of good crystal quality, is needed in order to ensure good minority carrier lifetime. Because of the segregation of impurities during solidification, material produced by the Bridgman technique offers considerable advantages over material produced by cast recrystallize techniques. Detectors based on a principle established by C.T. Elliott of the RSRE, Malvern, have led to families of thermal imagers that produce near perfect imagery. These detectors incorporate signal processing in the element by providing the time delay and integration functions that are normally performed off the focal plane in conventional serial scanned systems. A particular requirement of these SPRITE detectors is for long minority carrier lifetime. This has called for improvements in both material and fabrication technology which have led to an advanced technology that has benefited all aspects of the manufacture of cadmium mercury telluride detectors. Near background limited performance will be described in both the 8–14 and the 3–5 μm atmospheric transmission bands. There is, however, more image blurring in the 3–5 μm band than occurs in the 8–14 μm band. The direction of present work is towards detectors combining cadmium mercury telluride elements with advanced integrated circuits to provide more complex signal processing in the focal plane. This has been driven by the need to improve the sensitivity of thermal imaging systems where the scan rates are too low to allow useful time delay and integration in SPRITE detectors and by future needs for simplified missile guidance without mechanical scanning. Although the approaches have included extrinsic silicon, IR sensitive Schottky barriers and silicon hybridised with a variety of IR sensitive materials, the main thrust of present work in Europe and the United States is towards cadmium mercury telluride-silicon hybrids and intrinsic cadmium mercury telluride CCD focal planes. In the United Kingdom, the emphasis has been on two dimensional staring arrays of cadmium mercury telluride photodiodes combined with silicon substrates that incorporate either CCD circuits or MOSFET switch addressing. The switched approach has led to a novel line scanned array (LSA) that has enabled the first demonstrations of staring array imagery in the 8–14 μm band. Recent work will be described both on LSA and on CCD focal planes.