Technology and reliability of sub-micron 1T-flash EEPROM

As device dimensions scale into the submicron regime, stability and reliability effects become increasingly important. As charges are cycled in and out of the isolated floating gate through the tunnel oxide, the effects of oxide wear-out, charge trapping and defects affects the reliability of Flash Electrically-Erasable Programmable Read-Only Memory (EEPROM) devices.; In this research study submicron One-Transistor Flash (1T-Flash) devices were fabricated using advanced LOCOS isolation and I-line lithography. Device parametrics were extracted to characterize the tunnel oxide and Oxide-Nitride-Oxide inter-poly dielectrics. The results indicate that the tunnel/ONO oxides for fast and normal bits are comparable. The 1T-Flash EEPROM device reliability was studied and quantified with discrete devices and a large array of bits. Memory threshold voltages have been characterized for all bits and the overerased or fast bits identified. Results indicates that fast bit threshold voltages can vary from −0.2V to 0.5V which is 1.7V to 1 V lower than that of a normal bit. The characterization showed that a fast and normal bit had similar data retention characteristics, and hence the fast bit was not inherently defective. Experiments were performed to the assess the reliability of fast bits. It was demonstrated that floating gate polysilicon grain size plays a significant role in fast bit generation. Finally, analytical model for programming of fast bit threshold voltage including floating gate polysilicon area and field enhancement effects was developed for 1T-Flash EEPROM.