A Four-Bit-Per-Cell Program Method with Substrate-Bias Assisted Hot Electron Injection for Charge Trap Flash Memory Devices

We propose a four-bit-per-cell program method using a two-step sequence with substrate-bias assisted hot electron (SAHE) injection into the charge trap flash memory devices in order to overcome the limitations of conventional four-bit program methods, which use channel hot electron (CHE) injection. With this proposed method, a localized charge injection near the junction edge with an acceptable read margin was clearly observed, along with a threshold voltage difference of 1 V between the forward and the reverse read. In addition, a multi-level storage was easily obtained using a drain voltage step of 1 V at each level of the three programmed states, along with a fast program time of 1 μs. Finally, by using charge pumping methods, we directly observed the detailed information on the spatial distribution of the local threshold voltage in each level of the four states, for each physical bit, as a function of the program voltage.