Radiation hard charge-coupled device operating at 77 ° K using nitride/oxide gate insulators

Radiation hard charge coupled devices have been fabricated which successfully continued to operate at cryogenic temperature (77°K) after having been exposed to ionizing radiation at this temperature. No thermal annealing was performed between the time of Co⁶⁰γ irradiation and the electrical measurements. These devices were p-type buried channel CCD´s with a coplanar polysilicon gate structure. The gate dielectric structure consisted of a thin oxide (70Å) overlayed by a thick nitride (750Å). The salient aspect of this effort was the development of a radiation hard CCD fabricated with a high temperature process compatable with high CCD yield. Generally, high temperature post gate processing steps are incompatable with radiation hard characteristics. It was determined that low temperature hysteresis and C-V plot distortions observed with high temperature post dielectric processing is due to the evolution of hydrogen from the dielectric. Annealing of the dielectric in hydrogen for 30 minutes at 900°C removed the deleterious effects. Threshold shifts of less than 1 volt were observed in the CCD read-out electrometer after irradiation with 10⁶rads (Si) at 77°K. Similarly, no measurable degradation of the CCD transfer efficiency (.9999) was observed with ionizing radiation at low temperature.