Barry E. Burke

Toward fast, low-noise, low-power digital CCDs for Lynx and other high-energy astrophysics missions

Future X-ray missions such as Lynx require large-format imaging detectors with performance at least as good as the best current-generation devices but with much higher readout rates. We are investigating a Digital CCD detector architecture, under development at MIT Lincoln Laboratory, for use in such missions. This architecture features a CMOS-compatible detector integrated with parallel CMOS signal processing chains. Fast, low-noise amplifiers and highly parallel signal processing provide the high frame-rates required. CMOS-compatibility of the CCD provides low-power charge transfer and signal processing. We report on the performance of CMOS-compatible test CCDs read at rates up to 5 Mpix s−1 (50 times faster than Chandra ACIS CCDs), with transfer clock swings as low as ±1.5 V (power/area < 10% of that of ACIS CCDs). We measure read noise below 6 electrons RMS at 2.5 MHz and X-ray spectral resolution better than 150 eV FWHM at 5.9 keV for single-pixel events. We discuss expected detector radiation tolerance at these relatively high transfer rates. We point out that the high pixel ’aspect ratio’ (depletion-depth : pixel size ≈ 9 : 1) of our test devices is similar to that expected for Lynx detectors, and illustrate some of the implications of this geometry for X-ray performance and noise requirements.

Broadband (200–1000 NM) Back-Illuminated CCD Imagers

Improved and stable blue/UV quantum efficiency has been demonstrated on 2K×4K imagers using molecular-beam epitaxy to create a thin doped layer on the back surface. Quantum efficiency data on thick (40–50 μm) imagers with single and dual-layer anti-reflection coatings is presented that demonstrates high and broadband response. Measurements of the optical point-spread response show the devices to be fully depleted with good response across a broad spectrum, but interesting features appear in the near-IR as a result of deeply penetrating light being scattered off the surface structure of the CCD.

Oxide-bonded molecular-beam epitaxial backside passivation process for large-format CCDs

We describe recent advances in backside passivation of large-format charge-coupled devices (CCDs) fabricated on 200- mm diameter wafers. These CCDs utilize direct oxide bonding and molecular-beam epitaxial (MBE) growth to enable high quantum efficiency in the ultraviolet (UV) and soft X-ray bands. In particular, the development of low-temperature MBE growth techniques and oxide bonding processes, which can withstand MBE processing, are described. Several highperformance large-format CCD designs were successfully back-illuminated using the presented process and excellent quantum efficiency (QE) and dark current are measured on these devices. Reflection-limited QE is measured from 200 nm to 800 nm, and dark current of less than 1e- /pixel/sec is measured at 40°C for a 9.5 μm pixel.

Development of germanium charge-coupled devices

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.