Self-limiting multi-level programming states

摘要

A method of programming flash EEPROM devices that provides self-limiting multi-level programming states. Each cell in the flash EEPROM device can be programmed to have one of multiple threshold voltages. Each cell to be programmed has a programming voltage applied to the gate, a programming voltage applied to the drain and bias voltage applied to either the source (Vs) or to the substrate (Vsub) or both. The bias voltages Vs or Vsub are determined during a precharacterization procedure and each desired threshold voltage has a corresponding bias voltage Vs or Vsub that provides the desired threshold voltage during the programming procedure. The bias voltages Vs or Vsub are selected to provide self-limiting programming by the effective vertical field Ev=Vg -Vt-(either Vs or Vsub), where Vt increases during programming until the programming stops. The lateral field El=Vd-(either Vs and/or Vsub) is adjusted during programming to keep the lateral field El equal to Vd.