Semiconductor memory device which stores plural data in a cell
A memory cell array is configured to have a plurality of memory cells arranged in a matrix, each of the memory cells being connected to a word line and a bit line and being capable of storing n values (n is a natural number equal to or larger than 3). A control circuit controls the potentials of the word line and bit line according to input data and writes data into a memory cell. The control circuit writes data into the memory cell to a k-valued threshold voltage (k<=n) in a write operation, precharges the bit line once, and then changes the potential of the word line an i number of times to verify whether the memory cell has reached an i-valued (i<=k) threshold voltage.
1. A method for controlling a semiconductor memory device comprising a memory cell array configured to have a plurality of memory cells arranged in a matrix, each of the plurality of memory cells being connected to a word line and a bit line and being capable of storing n values (n is a natural number equal to or larger than 3), the method comprising:
controlling potentials of the word line and the bit line according to input data to write data into a memory cell;
writing data into the memory cell to an a1-valued (a1≦n) threshold voltage in a first write operation, to an a2-valued (a2≦n) threshold voltage in a second write operation, and to an ak-valued (ak≦n) threshold voltage in a k-th write operation (k is a natural number equal to or larger than 2: k≦n), in the first to k-th write operations; and
performing write operations by repeating program and verify operations while raising a program voltage in increments of ΔVpgm,
wherein ΔVpgm in the first to k-th write operations fulfill the following expression:
ΔVpgm in the first write operation>ΔVpgm in the second write operation> . . . >ΔVpgm in the kth write operation.
2. The method according to claim 1 , wherein threshold voltages of the memory cell sequentially increase in the first to k-th write operations.
3. The method according to claim 1 , wherein the verify operation includes verifying a threshold voltage of the memory cell by applying a verify voltage to the word line.
4. The method according to claim 1 , wherein the memory cells constitute a NAND flash memory.