Effects of Radiation Damage on Scientific Charge Coupled Devices

The charge coupled devices (CCDs) operate in the charge domain. The electrical signals that propagate through the device are small bundles of charge carriers, either electrons or holes. These charge packets are created, stored, and moved around inside the device to perform the operations required. The fundamental unit of a CCD is a metal–insulator–semiconductor (MIS) capacitor on which the charge packets are stored. If two of these capacitors are placed close enough together, the charge can be transferred from one to the other by manipulating the voltages on their gates. Charge transfer between capacitors is the key operation performed by a CCD and is the origin of the term “charge coupled device.” In many scientific imaging applications, it is necessary to subject the detector to harmful radiative environments. Such applications include almost any space mission, x-ray crystallography, certain forms of medical imaging, and energetic particle detection. Radiation deposits its energy in silicon in various ways, some of which can result in permanent damage. At its most benign, the radiation energy may simply be transferred to mechanical vibration of the silicon atoms and be manifested as heat. Two of the more harmful effects are of most concern in electronic devices: the first is ionization and the second is atomic displacement.