Fast and accurate programming method for multi-level NAND EEPROMs

For the replacement of conventional hard disks by NAND EEPROMs, a very high density and a high programming speed are required. An increased density can be achieved by using multi-level memory cells. With the new method, using staircase programming pulses combined with a bit-by-bit verify, a very narrow threshold voltage distribution of 0.7 V, necessary for 4-level or 2-bit operation, and a high programming speed of 300 /spl mu/s/page or 590 ns/byte can be obtained.