Semiconductor memory device for storing multivalued data
View Patent ↗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 5 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 10 the first memory cells in the bit line direction.
1. A method of operating the non-volatile semiconductor memory, the non-volatile semiconductor memory including a bit line coupled to a memory cell, a word line coupled to the memory cell, a first transistor coupled between the bit line and a sense node, a first register coupled to the sense node, a second register coupled to the first register, and a second transistor coupled between the sense node and a source of a power voltage, the memory cell being capable of storing n bits of data (n is a natural number equal to or larger than two), the method comprising:
receiving a first bit of data from an outside of the non-volatile semiconductor memory;
making a threshold voltage of the memory cell higher than a first level if the first bit of data is a first value;
receiving a second bit of data from the outside of the non-volatile semiconductor memory;
making the threshold voltage of the memory cell, that has been made higher than the first level, higher than a second level, if the second bit of data is a second value, the second level being higher than the first level; and
making the threshold voltage of the memory cell, that has been made higher than the first level, higher than a third level, if the second bit of data is a third value, the third level being higher than the second level, and the third value differing from the second value,
wherein the step of making the threshold voltage of the memory cell higher than the first level comprises:
storing data of a first logic level in the first register, the first logic level corresponding to the first value of the first bit of data;
applying a first program voltage to the memory cell to alter the threshold voltage of the memory cell;
determining whether the threshold voltage of the memory cell is lower than the first level; and
applying a second program voltage to the memory cell to alter the threshold voltage of the memory cell if it has been determined that the threshold voltage of the memory cell is lower than the first level, the second program voltage being higher than the first program voltage by a first amount, and
wherein the step of making the threshold voltage of the memory cell higher than the second level comprises:
storing data of the first logic level in the first register;
storing data of a second logic level in the second register, the second logic level corresponding to the second value of the second bit of data;
applying a third program voltage to the memory cell to alter the threshold voltage of the memory cell;
determining whether the threshold voltage of the memory cell is lower than the third level; and
applying a fourth program voltage to the memory cell to alter the threshold voltage of the memory cell if it has been determined that the threshold voltage of the memory cell is lower than the third level, the fourth program voltage being higher than the third program voltage by a second amount.
2. The method according to claim 1 , wherein:
the step of making the threshold voltage of the memory cell higher than the first level further comprises storing data of a third logic level in the first register if it has been determined that the threshold voltage of the memory cell has reached the first level, the third logic level differing from the first logic level; and
the step of making the threshold voltage of the memory cell higher than the second level further comprises storing data of the third logic level in the first register if it has been determined that the threshold voltage of the memory cell has reached the second level.
3. The method according to claim 2 , wherein the first logic level is a low level and the third logic level is a high level.
4. The method according to claim 2 , wherein the step of making the threshold voltage of the memory cell higher than the third level comprises:
storing data of the first logic level in the first register; and
storing data of a fourth logic level in the second register, the fourth logic level corresponding to the third value of the second bit of data, the fourth logic level differing from the second logic level.
5. The method according to claim 2 , wherein the step of determining whether the threshold voltage of the memory cell is lower than the first level comprises:
electrically connecting the bit line to the source of the power voltage through the first and second transistors by applying a first control voltage to the gate of the first transistor;
applying a read voltage to the word line, after applying the first control voltage to the gate of the first transistor; and
electrically connecting the bit line to the sense node by applying a second control voltage to the gate of the first transistor so that a voltage on the bit line is reflected to a voltage on the sense node, after applying the read voltage to the word line.
6. The method according to claim 5 , wherein the data of the third logic level is stored in the first register according to the voltage on the sense node, after applying the read voltage to the word line.
7. The method according to claim 5 , wherein the first control voltage causes the first transistor to charge the bit line to a voltage lower than the power voltage.
8. The method according to claim 5 , wherein the first control voltage causes the first transistor to charge the bit line to a voltage lower than a voltage on the sense node.
9. The method according to claim 5 , wherein:
the first transistor is an N-channel transistor; and
the first control voltage is higher than the second control voltage.
10. The method according to claim 1 , wherein the second amount is smaller than the first amount.
11. The method according to claim 1 , wherein the non-volatile semiconductor memory is a NAND type flash memory.
12. The method according to claim 1 , wherein the first value is “0”.
13. The method according to claim 1 , wherein the second value is “0” and the third value is “1”.
14. The method according to claim 1 , the method further comprising maintaining the threshold voltage of the memory cell if the first bit of data is a fourth value, the fourth value differing from the first value.
15. The method according to claim 14 , wherein the fourth value is “1”.
16. The method according to claim 1 , wherein:
the step of making the threshold voltage of the memory cell higher than the second level further comprises determining whether the threshold voltage of the memory cell is lower than a fourth level, before applying the third program voltage, the fourth level being lower than the first level.
17. The method according to claim 1 , wherein the step of making the threshold voltage of the memory cell higher than the second level further comprises transferring the data of the second logic level in the second register to the first register.
18. The method according to claim 1 , wherein, in the step of applying the first program voltage to the memory cell, the first register storing the data of the first logic level provides a ground potential to the bit line through the first transistor.