Soft-x-ray CCD imagers for AXAF
October 1, 1997
We describe the key features and performance data of a 1024 x 1026-pixel frame-transfer imager for use as a soft-x-ray detector on the NASA X-ray observatory Advanced X-ray Astrophysics Facility (AXAF). The four-port device features a floating-diffusion output circuit with a responsivity of 20/spl mu/V/e/sup -/ and noise of about 2 e/sup -/ at a 100-kHz data rate. Techniques for achieving the low sense-node capacitance of 5 fF are described. The CCD is fabricated on high-resistivity p-type silicon for deep depletion and includes narrow potential troughs for transfer inefficiencies of around 10/sup -7/ (ten to the negative 7). To achieve good sensitivity at energies below 1 keV, we have developed a back-illumination process that features low recombination losses at the back surface and has produced efficiencies of about 0.7 at 277 eV (carbon K/spl alpha/).