The promising multi-bit/level programming operations for nano-scaled SONOS memory

•CHEI-P and PASHEI programs are compared in 90 nm SONOS devices.•CHEI-P program exhibits the superior reliability.•The accumulation charges in the nitride layer are investigated.•CHEI-P program exhibits the good characteristics at 4-bit 4-level states. In order to obtain a reliable multi-bit/level operation for nano-scaled polycrystalline silicon-oxide-nitride-oxide-silicon (SONOS) memory, two different localized charge-injection programming methods, the channel hot electron injection with a positive substrate bias (CHEI-P) and pulse agitated substrate hot electron injection (PASHEI), are operated in 90nm SONOS cells. It is found that the cells programmed by CHEI-P have the better endurance property than by PASHEI. The better endurance is due to the less accumulation of charges in the nitride layer, evidenced by surface potential profiling technique. CHEI-P program further exhibits the superior endurance and retention properties after 104 program/erase cycles in 4-bit/4-level operations. These results illustrate that CHEI-P program is a promising candidate for multi-bit/levels nano-sized SONOS memory.