Semiconductor memory device for storing multivalued data
Data storage circuits are connected to the bit lines in a one-to-one correspondence. A write circuit writes the data on a first page into a plurality of first memory cells selected simultaneously by a word line. Thereafter, the write circuit writes the data on a second page into the plurality of first memory cell. Then, the write circuit writes the data on the first and second pages into second memory cells adjoining the first memory cells in the bit line direction.
1. A method of programming a nonvolatile memory cell capable of storing n bits of data (n is a natural number equal to or larger than two), the method comprising:
applying a first write voltage to the memory cell so as to alter a threshold voltage of the memory cell, when a first bit of data is a first value;
determining whether the threshold voltage of the memory cell has reached a first verify level, after applying the first write voltage;
increasing the first write voltage by a first amount if it is determined that the threshold voltage of the memory cell has not reached the first verify level;
applying the increased first write voltage to the memory cell so as to alter the threshold voltage of the memory cell;
applying a second write voltage to the memory cell so as to alter the threshold voltage of the memory cell after determining the memory cell has reached the first verify level;
determining whether the threshold voltage of the memory cell has reached a second verify level, after applying the second write voltage, if a second bit of data is a second value;
determining whether the threshold voltage of the memory cell has reached a third verify level, after applying the second write voltage, if the second bit of data is a third value, the third value differing from the second value;
increasing the second write voltage by a second amount if it is determined that the threshold voltage of the memory cell has not reached the second verify level; and
applying the increased second write voltage to the memory cell so as to alter the threshold voltage of the memory cell.
2. The method according to claim 1 , wherein the first amount is larger than the second amount.
3. The method according to claim 1 , wherein the first value is “0”.
4. The method according to claim 1 , wherein the second value is “0” and the third value is “1”.
5. The method according to claim 4 , wherein the third verify voltage is higher than the second verify voltage.
6. The method according to claim 1 , the method further comprising: maintaining the threshold voltage of the memory cell when the first bit of data is a fourth value, the fourth value differing from the first value.
7. The method according to claim 6 , wherein the fourth value is “1”.
8. A method of programming a plurality of nonvolatile memory cells including first and second memory cells, the first memory cell being adjacent to the second memory cell, each memory cell capable of storing n bits of data (n is a natural number equal to or larger than two), the method comprising:
applying a first write voltage to the first memory cell so as to alter a threshold voltage of the first memory cell, when a first bit of data to be stored in the first memory cell is a first value;
determining whether the threshold voltage of the first memory cell has reached a first verify level, after applying the first write voltage;
increasing the first write voltage by a first amount if it is determined that the threshold voltage of the first memory cell has not reached the first verify level;
applying the increased first write voltage to the first memory cell so as to alter the threshold voltage of the first memory cell;
applying a second write voltage to the second memory cell so as to alter a threshold voltage of the second memory cell, when a second bit of data to be stored in the second memory cell is a second value;
determining whether the threshold voltage of the second memory cell has reached the first verify level, after applying the second write voltage;
increasing the second write voltage by the first amount if it is determined that the threshold voltage of the second memory cell has not reached the first verify level;
applying the increased second write voltage to the second memory cell so as to alter the threshold voltage of the second memory cell;
applying a third write voltage to the first memory cell so as to alter the threshold voltage of the first memory cell that is determined to have reached the first verify level, after programming the first bit of data into the first memory cell and programming the second bit of data into the second memory cell;
determining whether the threshold voltage of the first memory cell has reached a second verify level, after applying the third write voltage, when a third bit of data to be stored in the first memory cell is a third value;
determining whether the threshold voltage of the first memory cell has reached a third verify level, after applying the third write voltage, when the third bit of data to be stored in the first memory cell is a fourth value, the fourth value differing from the third value;
increasing the third write voltage by a second amount if it is determined that the threshold voltage of the first memory cell has not reached the second verify level; and
applying the increased third write voltage to the first memory cell so as to alter the threshold voltage of the first memory cell.
9. The method according to claim 8 , wherein the first amount is larger than the second amount.
10. The method according to claim 8 , wherein the first and second values are “0”s.
11. The method according to claim 8 , wherein the third value is “0” and the fourth value is “1”.
12. The semiconductor memory device according to claim 11 , wherein the third verify voltage is higher than the second verify voltage.
13. The method according to claim 8 , the method further comprising:
maintaining the threshold voltage of the first memory cell when the first bit of data is a fifth value, the fifth value differing from the first value; and
maintaining the threshold voltage of the second memory cell when the second bit of data is a sixth value, the sixth value differing from the second value.
14. The method according to claim 13 , wherein the fifth and sixth values are “1”s.
15. The method according to claim 8 , wherein the first memory cell is adjacent to the second memory cell in the direction of a word line.
16. The method according to claim 8 , wherein the first memory cell is adjacent to the second memory cell in the direction of a bit line.