Carrier transport properties and reliability issues in flash EEPROM cells.

摘要

Flash memory has become the mainstream non-volatile memory. The NOR-type stacked-gate flash cells use channel hot electron injection and Fowler-Nordheim (F-N) tunneling for the programming operation and the erasing operation, respectively. As the feature sizes of the flash cells continue to scale down, other carrier transport mechanisms, such as Drain-Avalanche Hot Carrier (DAHC) injection, Substrate-Current-Induced Hot Electron (SCIHE) injection and oxide direct tunneling, become more important. With hot carrier injection and oxide tunneling as the carrier transport mechanisms, the flash cells are subjected to various device reliability issues, such as the wide cell threshold voltage distribution due to the over-erasure phenomena and device degradation due to hot hole injection.; This work is an attempt to investigate the carrier transport properties and reliability issues within the flash cells. To study the characteristics of ultra-low hot carrier injection, a floating gate extraction technique based on a lumped circuit model has been developed. To accurately model the oxide tunneling mechanism, the quantization effects have been taken into account by developing a simulator, MIS2D, and then evaluating the "effective" dielectric thickness and deeper band bending within Si substrates due to the quantization phenomena. The threshold voltage shifts due to quantization effects are also simulated using MIS2D. F-N tunneling and oxide direct tunneling have been simulated and then verified by electrical characterization with quantization effect correction. DAHC injection and SCIHE injection have been modeled and compared with experimental data. Both the DAHC injection and the SCIHE injection mechanisms have been applied to the convergence schemes, which are used to converge the wide cell threshold voltage distribution. The principle of the convergence operations has been analyzed and two design guidelines have been established. Compared to the DAHC scheme, the newly proposed SCIHE scheme can significantly improve the overall performance and long term reliability of the convergence operations and can be further optimized for block convergence.