A NOVEL MONOS NONVOLATILE MEMORY DEVICE ENSURING 10-YEAR DATA RETENTION AFTER 10(7) ERASE WRITE CYCLES

This work describes a highly-reliable nonvolatile memory device suitable for high-density Electrically Erasable. and Programmable Read Only Memories (EEPROM's). The Metal-Nitride-Oxide-Semiconductor (MNOS) structure is currently one of the most reliable devices for EEPROM's, giving 10(5)-cycle endurance with 10-year data retention. For higher endurance with long-term retention, we propose a new Metal-Oxide-Nitride-Oxide-Semiconductor (MONOS) structure whose top oxide is fabricated by Chemical Vapor Deposition (CVD) on the nitride. This CVD is oxide is densified by pyrogenic annealing and has stoichiometric SiO2 characteristics. Its potential barrier, which prevents stored charges from decaying through the top oxide to the gate, thus becomes sharper than that of the thermally grown top oxide used in the conventional MONOS structure. For comparison between the proposed MONOS, conventional MONOS, and MNOS structures, we have made three devices on the same process line. The 16.7-nm nitride thickness in combination with a top oxide thickness of 4.0 nm results in a gate capacitance equivalent to that of the conventional MNOS structure with a 23.5-nm nitride thickness. Moreover, an asymmetric erase/write programming voltage has been adopted to the MONOS device operation by considering both erased-state degradation and written-state retention. At 85-degrees-C, the proposed MONOS device has 10(7)-cycle endurance with 10-year data retention. This endurance is far superior not only to the 10(5) cycles of a MNOS device but also to the 10(6) cycles of the conventional MONOS device.