SONOS Nonvolatile Shadow RAMs for Space Applications

Several versions of shadow random access memories have been commercialized. These memories consist of an SRAM shadowed by a nonvolatile memory on a single chip of silicon. Both floating gate and silicon nitride devices have been used for the nonvolatile shadow memory storage elements. Because radiation hardened SRAMs have been proven for space applications and silicon nitride devices have a high tolerance to radiation exposure, shadow RAMs based on the silicon nitride technology can be combined with radiation hardened CMOS to provide rugged nonvolatile semiconductor memories for space applications. These memories have high endurance, fast write times and low power performance because these parameters are determined by the SRAM portion of the memory. The nonvolatile shadow memory is automatically written only on power down. Therefore the endurance limitation of silicon nitride to approximately 10(exp 6) cycles applies only to the number of power down cycles. Nonvolatile retention times of greater than ten years can be obtained at 140 deg C.