In astronomy, charge-coupled devices (CCDs) record the light captured by the major telescopes of the world. CCDs provide greater sensitivity than photographic film, which was commonly used in the past. But film typically sacrifices sensitivity to the red and infrared wavelengths to obtain the required sensitivity to blue light.
Unlike CCDs found in consumer equipment, astronomical CCDs rely on backlighting rather than frontlighting. That's because frontlighting circuitry blocks blue light. Also, the thickness of the silicon substrate in frontlit CCDs must shrink to just 20 µm. This ensures that enough blue-wavelength photon-generated charges successfully traverse the chip to reach the CCD's charge-collecting potential wells. The resulting device, however, is less sensitive to reds than blues. It's also fragile, leading to low production yields and high costs.
A novel CCD devised by Steve Holland, an electrical engineer in the Physics Division's Microsystems Lab at Lawrence Berkeley Laboratories, Berkeley, Calif., overcomes the problems of thin-chip CCDs. The new 300-µm thick, backlit CCD doesn't require thinning, yet it is still sufficiently sensitive to blue light. Two features make this possible.Added Features Propel New CCD One is the use of very pure n-type silicon in which electrically active dopants account for just one part per 100 billion. The techniques required to obtain this level of purity were first developed in high-energy physics particle-detector experiments. The pure silicon produced photodetectors with low dark current.
Fully depleted silicon is the second characteristic. Holland explains, "By layering a thin, transparent window onto the back of the n-type silicon substrate—a window that also acts as an electrode—we can apply a bias voltage between the window and the positively doped channel layer under the front circuitry." The bias voltage removes charge carriers in the path from the back to the front of the chip. Therefore, when photons of blue light strike the back of the chip, they produce electrons that can safely cross the chip without experiencing recombination.
With its thick-chip construction, charge carriers associated with red light follow a mostly straight path to the CCD's potential wells and suffer little sideways diffusion or "fringes" produced by reflection. As a result, the CCD's response to red and infrared light is much greater than its thin predecessors, giving astronomers a better tool for studying red-shifted objects.
The light-sensitive surface on the back of the chip is unobstructed by CCD circuitry, which is located on the front side. Consequently, the CCDs can be placed side by side to form large arrays. For example, a CCD with 4 million pixels was built for the University of California's Lick Observatory, located near San Jose.
Measurements taken on the Lick Observatory's sensor confirm the design's enhanced red-light performance. This particular device is said to exhibit greater quantum efficiency (the ratio of photons converted to electric charge) in the near-infrared region than any other astronomical CCD in use. Some even larger CCD imagers are in the works, such as a CCD with 8 million pixels for spectroscopy at the Keck Telescope in Hawaii, and one with almost a billion pixels for use in the SuperNova/Acceleration Probe (SNAP) satellite. The new CCD's potential benefits aren't limited to astronomy.
For more information, point your browser to http://cdc.lbl.gov/. Or contact Paul Preuss at (510) 486-6249; [email protected].