Search

A germanium charge-coupled device (CCD) offers the advantages of a silicon CCD for X-ray detection – excellent uniformity, low read noise, high energy resolution, and noiseless on-chip charge summation – while covering an even broader spectral range. Notably, a germanium CCD offers the potential for broadband X-ray sensitivity with similar or even superior energy resolution than silicon, albeit requiring lower operating temperatures (≤ 150K) to achieve sufficiently low dark noise due to the lower band gap of this material. The recent demonstration of high-quality gate dielectrics on germanium with low surface-state density and low gate leakage is foundational for realization of high-quality imaging devices on this material. Building on this advancement, MIT Lincoln Laboratory has been developing germanium CCDs for several years, with design, fabrication, and characterization of kpixel-class front-illuminated devices discussed recently. In this article, we describe plans to scale these small arrays to megapixel-class imaging devices with performance suitable for scientific applications. Specifically, we discuss our efforts to increase charge-transfer efficiency, reduce dark current, improve fabrication yield, and fabricate backside-illuminated devices with excellent sensitivity.

Silicon charge-coupled devices (CCDs) are commonly utilized for scientific imaging in wavebands spanning the near infrared to soft X-ray. These devices offer numerous advantages including large format, excellent uniformity, low read noise, noiseless on-chip charge summation, and high energy resolution in the soft X-ray band. By building CCDs on bulk germanium, we can realize all of these advantages while covering an even broader spectral range, notably including the short-wave infrared (SWIR) and hard X-ray bands. Since germanium is available in wafer diameters up to 200 mm and can be processed in the same tools used to build silicon CCDs, large-format (>10 MPixel, >10 cm2 ) germanium imaging devices with narrow pixel pitch can be fabricated. Furthermore, devices fabricated on germanium have recently demonstrated the combination of low surface state density and high carrier lifetime required to achieve low dark current in a CCD. At MIT Lincoln Laboratory, we have been developing germanium imaging devices with the goal of fabricating large-format CCDs with SWIR or broadband X-ray sensitivity, and we recently realized our first front-illuminated CCDs built on bulk germanium. In this article, we describe design and fabrication of these arrays, analysis of read noise and dark current on these devices, and efforts to scale to larger device formats.

The Transiting Exoplanet Survey Satellite (TESS) is an Explorer-class mission dedicated to finding planets around bright, nearby stars so that more detailed follow-up studies can be done. TESS is due to launch in 2017 and careful characterization of the detectors will need to be completed on ground before then to ensure that the cameras will be within their photometric requirement of 60ppm/hr. TESS will fly MITLincoln Laboratories CCID-80s as the main scientific detector for its four cameras. They are 100μm deep depletion devices which have low dark current noise levels and can operate at low light levels at room temperature. They also each have a frame store region, which reduces smearing during readout and allows for near continuous integration. This paper describes the hardware and methodology that were developed for testing and characterizing individual CCID-80s. A dark system with no stimuli was used to measure the dark current. Fe55 and Cd109 X-ray sources were used to establish gain at low signal levels and its temperature dependence. An LED system that generates a programmable series of pulses was used in conjunction with an integrating sphere to measure pixel response non-uniformity (PRNU) and gain at higher signal levels. The same LED system was used with a pinhole system to evaluate the linearity and charge conservation capability of the CCID-80s.

Germanium exhibits high sensitivity to short-wave infrared (SWIR) and X-ray radiation, making it an interesting candidate for imaging applications in these bands. Recent advances in germanium processing allow for high-quality charge-coupled devices (CCDs) to be realized in this material. In this article, we discuss our evaluation of germanium as an absorber material for CCDs via fabrication and analysis of discrete devices such as diodes, metal-insulator-semiconductor capacitors, and buried-channel metal-oxide-semiconductor field-effect transistors (MOSFETs). We then describe fabrication of our first imaging device on germanium, a 32 × 1 × 8.1 μm linear shift register. Based on this work, we find that germanium is a promising material for CCDs imaging in the SWIR and X-ray bands.

We report on two recently developed charge-coupled devices (CCDs) for adaptive optics wavefront sensing, both designed to provide exceptional sensitivity (low noise and high quantum efficiency) in high-frame-rate low-latency readout applications. The first imager, the CCID75, is a back-illuminated 16-port 160×160-pixel CCD that has been demonstrated to operate at frame rates above 1,300 fps with noise of < 3 e-. We will describe the architecture of this CCD that enables this level of performance, present and discuss characterization data, and review additional design features that enable unique operating modes for adaptive optics wavefront sensing. We will also present an architectural overview and initial characterization data of a recently designed variation on the CCID75 architecture, the CCID82, which incorporates an electronic shutter to support adaptive optics using Rayleigh beacons.

Until quite recently, practically any major tragic event in our country—whether a natural calamity, a railroad accident, or a mine explosion—was surrounded by a dense curtain of secrecy. There is hardly any need to demonstrate that the dearth of information and the half-truths have left their mark. But this is not the only issue. The consequence of treating any issue as "closed" is that specialists competent in the matter are unable to act; they have no way to study the causes and the preconditions of a disaster and to analyze the problems to which it gives rise.

Hierarchy is nowadays defined as a universal structural principle of any organizational system—biological, technical, or social. With regard to social systems, this principle is discernible at all levels, from small groups to society as a whole. Indeed, mankind does not seem to know any other mode of association than that of a pyramid. However, the inevitability of this principle in social organizations means that the problems to which it gives rise are also inevitable. These problems derive from the characteristics of social relations that are typical of a hierarchy, namely, relations of subordination, dependence, and inequality. In an organization, one worker is consciously given the preeminent right to take decisions concerning another, and, moreover, he also receives the means for monitoring the job behavior of the other. Such workers form a clear minority which, however, decides for the majority. Herein lies one of the most important characteristics of intra-organizational relations, and the objectiv...