Neutron Damage Mechanisms in Charge Transfer Devices

A study of neutron damage mechanisms in charge transfer devices has been performed with emphasis placed on investigation of dark current increases. MOS capacitors were used to determine damage coefficients that are applicable to the radiation response of CCDs. Measurements of dark current density in CCDs were made following neutron bombardment. A unique value for generation-lifetime damage coefficient was determined (Kg = 7.0 × 106 n-sec/cm2) using MOS capacitors on both n- and p-type silicon substrates and this value was then used to calculate the expected change in dark current density with neutron fluence in a charge transfer device. The calculated value is in good agreement with the present experimental value of 4×l0-11 nanoamps per neutron. A qualitative explanation is given to account for the nearly two-orders-of-magnitude difference between Kg and recombination-lifetime damage coefficients which is based in part on the nature of neutron damage in a depletion region. An explanation is also given to account for the more than six-orders-of-magnitude difference between Kg and storage-time damage coefficient. It is demonstrated that a moderate reduction in the operating temperature of a charge transfer device should result in substantial radiation tolerance in terms of neutron-induced increases in dark current.