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 semiconductor memory device comprising:
first and second bit lines;
first and second word lines;
first and second select transistors;
a source line;
a plurality of memory cells including first, second and third memory cells, each memory cell capable of storing n bits of data (n is a natural number equal to or larger than two), the first memory cell connected to the first word line, coupled to the first bit line and coupled to the source line via the first select transistor, the second memory cell being adjacent to the first memory cell along the first word line, connected to the first word line, coupled to the second bit line and coupled to the source line via the second select transistor, the third memory cell connected to the second word line, coupled to the first bit line and coupled to the source line via the first select transistor and the first memory cell, a source of the third memory cell connected to a drain of the first memory cell; and
a write circuit configured to write a first bit of data into the first memory cell, write a second bit of data into the second memory cell and write a third bit of data into the third memory cell, before writing a fourth bit of data into the first memory cell without erasing the first bit of data in the first memory cell, wherein:
the write circuit is configured to write the first bit of data into the first memory cell by making a threshold voltage of the first memory cell higher than a first verify potential, according to a first logical state of the first bit of data;
the write circuit is configured to write the fourth bit of data into the first memory cell by making the threshold voltage of the first memory cell, which has been made higher than the first verify potential, higher than a second verify potential, according to a first logical state of the fourth bit of data;
the write circuit is configured to write the fourth bit of data into the first memory cell by making the threshold voltage of the first memory cell, which has been made higher than the first verify potential, higher than a third verify potential, according to a second logical state of the fourth bit of data; and
the third verify potential is higher than the second verify potential.
2. The semiconductor memory device according to claim 1 , wherein the first logical state of the first bit of data is “0”.
3. The semiconductor memory device according to claim 1 , wherein:
the first logical state of the fourth bit of data is “0”; and
the second logical state of the fourth bit of data is “1”.
4. The semiconductor memory device according to claim 1 , wherein the semiconductor memory device is a NAND-type flash memory.
5. The semiconductor memory device according to claim 1 , wherein:
the write circuit is configured to write the first bit of data into the first memory cell before writing the second bit of data into the second memory cell; and
the write circuit is configured to write the second bit of data into the second memory cell before writing the third bit of data into the third memory cell.
6. The semiconductor memory device according to claim 1 , the semiconductor memory device further comprising a third word line, wherein:
the memory cells further include fourth and fifth memory cells;
the fourth memory cell is adjacent to the third memory cell along the second word line, connected to the second word line, coupled to the second bit line and coupled to the source line via the second select transistor and the second memory cell;
the fifth memory cell is connected to the third word line, coupled to the first bit line and coupled to the source line via the first select transistor and the first and third memory cells; and
the write circuit is configured to write a fifth bit of data into the fourth memory cell and write a sixth bit of data into the fifth memory cell before writing a seventh bit of data into the third memory cell without erasing the third bit of data in the third memory cell.
7. The semiconductor memory device according to claim 6 , wherein the write circuit is configured to write the fourth bit of data into the first memory cell, write the seventh bit of data into the third memory cell and write an eighth bit of data into the second memory cell, before writing a ninth bit of data into the first memory cell.
8. The semiconductor memory device according to claim 7 , wherein:
the write circuit is configured to write the ninth bit of data into the first memory cell by making the threshold voltage of the first memory cell, which has been made higher than the second verify potential, higher than a fourth verify potential, according to a first logical state of the ninth bit of data;
the write circuit is configured to write the ninth bit of data into the first memory cell by making the threshold voltage of the first memory cell, which has been made higher than the second verify potential, higher than a fifth verify potential, according to a second logical state of the ninth bit of data;
the write circuit is configured to write the ninth bit of data into the first memory cell by making the threshold voltage of the first memory cell, which has been made higher than the third verify potential, higher than a sixth verify potential, according to a first logical state of the ninth bit of data; and
the write circuit is configured to write the ninth bit of data into the first memory cell by making the threshold voltage of the first memory cell, which has been made higher than the third verify potential, higher than a seventh verify potential, according to a second logical state of the ninth bit of data.
9. The semiconductor memory device according to claim 1 , wherein:
the write circuit includes a plurality of data storage circuits configured to store data to be written in the memory cells; and
the first and second bit lines are coupled to the same data storage circuit.
10. The semiconductor memory device according to claim 1 , wherein the write circuit is configured to write the first bit of data into the first memory cell by maintaining the threshold voltage of the first memory cell at a first voltage lower than the first verify potential according to a second logical state of the first bit of data.
11. The semiconductor memory device according to claim 1 , the semiconductor memory device further comprising a plurality of flag cells, wherein
the write circuit is configured to write flag information indicating that the fourth bit of data has been written into the first memory cell, into one of the flag cells selected concurrently with the first memory cell, when the write circuit writes the fourth bit of data into the first memory cell.