Radiation Hardening of CCD Imagers Using "Defect Control"

CCDS SUBJECT TO PROTON IRRADIATION ARE DEGRADED BY THE DEVELOPMENT OF CRYSTALLINE DEFECTS WHICH IMPACT QUALITY. SINCE THE CONCENTRATION OF ACTIVE DEFECTS IS PROPORTIONAL TO THE NIEL OF PROTONS IN SILICON, IT HAS BEEN ASSUMED THAT THE AMOUNT OF DAMAGE CANNOT BE CONTROLLED. HOWEVER, EXAMINATION SHOWS THAT CCD DEGRADATION IS CAUSED BY VACANCIES COMBINING WITH DOPANT ATOMS OR DEFECTS TO CAUSE VACANCY COMPLEXES SUCH AS E-CENTERS AND/OR DIVACANCIES, AND THEIR CONTRATIONS CAN DEPEND ON CRYSTALLINE ORIENTATION (THRU DAMAGE STRAGGLING), DOPANT LEVELS, ETC. FURTHERMORE, SINCE THE E-CENTER IS AN ELECTRON TRAP, WILL CONTROL THE HAVIOR OF N-TYPE CCDs, WHILE THE DIVACANCY, BEING A HOLE TRAP, WILL CONTROL THE BEHAVIOR OF P-TYPE CCDs. THUS, P- AND N- CCDs WILL DIFFER IN HARDNESS. UNDER THIS PROGRAM, THE EQUATIONS FOR THE MASS BALANCE OF E-CENTERS AND DIVACANCIES WILL BE ANALYZED, AND A MODEL DEVELOPED RELATING CONCENTRATIONS OF EACH DEFECT TYPE TO NIEL, CRYSTALLINE ORIENTATION, ETC. BASED ON THIS MODEL, CCD DESIGNS WILL BE DEVELOPED WHICH IMPROVE HARDNESS BY SELECTING THE SPECIFIC CARRIER TYPE DEVICE AND ORIENTATION WHICH UNDERGOES THE LEAST DEGRADATION IN PERFORMANCE. PHASE II WILL FABRICATE PROTOTYPE HARDENED CCDs BASED ON THESE DESIGNS.