Germanium charge-coupled devices for hard x-ray astronomy

A germanium charge-coupled device (CCD) offers all of the advantages of silicon CCDs – notably excellent uniformity, high energy resolution, and noiseless on-chip charge summation – but covers an even broader spectral range, extending into the hard X-ray band. MIT Lincoln Laboratory has been developing germanium CCDs for applications in astrophysics, with the goal of realizing megapixel-class arrays with read noise less than a few electrons, sensitivity to both soft and hard X-rays, and high energy resolution. We recently realized our first small pixel arrays and have since been working to increase both the format and performance of these devices. In this article, we discuss performance improvements, identification of yield-limiting process steps in fabrication of frontside-illuminated devices, fabrication and analysis of our first backside-illuminated detectors, and design of prototypes which includes a 512 × 512 pixel frame-transfer CCD with 24 µm pixel pitch.