Optimized programming of multilevel flash EEPROMs

The trade-off between speed and dispersion of programmed threshold voltages is investigated in 0.25 μm flash memory technology. It is shown that ramped gate programming provides tighter distributions of programmed threshold voltages than its conventional box-waveform counterpart, allowing a larger number of bits per second to be written. In particular, at low programming speed, ramped gate programming is shown to allow four level schemes without program and verify operations, with a program bandwidth potentially approaching 30 Mbits/s in the conventional one-bit-per-cell scheme (and correspondingly higher values in the multi-level case). Instead, sixteen level schemes without program and verify do not seem practically feasible