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 cm<sup>2</sup> ) 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.
Although HgCdTe imagers are a well-established technology, photodetectors fabricated using the same process still yield a large variation in their performance characteristics, largely stemming from hard-to-control pecu- liarities at the interface between the surface passivation and the active region of each photodiode. This work investigates the dark current characteristics of long-wave IR (cutoff wavelength of 10um) Hg<sub>0.774</sub>Cd<sub>0.226</sub>Te mesa photodiodes, which have been passivated with a CdTe film. We use a 2-D model of a p-on-n device structure to study how interface states and Cadmium diffusion at the passivation interface can influence the photodiode dark current.
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. Fe<sup>55</sup> and Cd<sup>109</sup> 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.
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.