Semiconductor memory device and method of erasing data therein
A semiconductor memory device includes a memory cell array of NAND cell units. The NAND cell unit includes a plurality of electrically erasable programmable nonvolatile memory cells connected serially, and a first and a second selection transistor provided to connect both ends of the memory cells to a bit line and a source line, respectively. The semiconductor memory device also includes dummy cells inserted in the NAND cell unit adjacent to the first and second selection transistors, respectively. The dummy cells in the NAND cell unit are erased simultaneously with the memory cells under a weaker erase potential condition than that for the memory cells and set in a higher threshold distribution than an erased state of the memory cells.
1. A semiconductor memory device having a memory cell array configured by arranging a plurality of NAND cell units, said NAND cell unit comprises:
a plurality of electrically erasable programmable nonvolatile memory cells connected serially;
a first and a second selection transistor provided to connect both ends of said memory cells to a bit line and a source line, respectively; and
dummy cells inserted in said NAND cell unit adjacent to said first and second selection transistors, respectively,
wherein said memory cells and said dummy cells have respective control gates which are connected to regular word lines and dummy word lines, respectively, and said first and second selection transistors have respective gates which are connected to first and second selection gate lines, respectively, wherein
a plurality of said NAND cell units sharing said regular word lines, dummy word lines and selection gate lines configures an erase unit, and wherein
said memory cells and dummy cells in said erase unit are erased simultaneously by applying an erase voltage to a well region including said memory cell array formed therein, applying a first voltage of just or near 0V to said regular word lines in said erase unit, and applying a second voltage higher than said first voltage to said dummy word lines in said erase unit.
2. The semiconductor memory device according to claim 1 , wherein
said memory cells and dummy cells in said erase unit are subjected to an erase verifying operation in which a first erase verify voltage is applied to said regular word lines and a second erase verify voltage higher than said first erase verify voltage is applied to said dummy word lines.
3. The semiconductor memory device according to claim 1 , wherein
after erasing all NAND cell units in said erase unit, writing is executed to said dummy cells in said erase unit for setting a higher threshold distribution than said erased state of said memory cells.
4. The semiconductor memory device according to claim 3 , wherein
after said writing, soft programming is executed for dissolving an over-erased cell in said erase unit.
5. A semiconductor memory device having a memory cell array configured by arranging a plurality of NAND cell units, said NAND cell unit comprises:
a plurality of electrically erasable programmable nonvolatile memory cells connected serially;
a first and a second selection transistor provided to connect both ends of said memory cells to a bit line and a source line, respectively; and
dummy cells inserted in said NAND cell unit adjacent to said first and second selection transistors, respectively, wherein
said memory cells and said dummy cells have respective control gates which are connected to regular word lines and dummy word lines, respectively, and said first and second selection transistors have respective gates which are connected to first and second selection gate lines, respectively, wherein
a plurality of said NAND cell units sharing said regular word lines, dummy word lines and selection gate lines configures an erase unit, and wherein
said memory cells and dummy cells in said erase unit are erased simultaneously and subjected to an erase verifying operation in which a first erase verify voltage is applied to said regular word lines and a second erase verify voltage higher than said first erase verify voltage is applied to said dummy word lines.
6. The semiconductor memory device according to claim 5 , wherein
after erasing all NAND cell units in said erase unit, writing is executed to said dummy cells in said erase unit for setting a higher threshold distribution than said erased state of said memory cells.
7. The semiconductor memory device according to claim 6 , wherein
after said writing, soft programming is executed for dissolving an over-erased cell in said erase unit.
8. A semiconductor memory device having a memory cell array configured by arranging a plurality of NAND cell units, said NAND cell unit comprises:
a plurality of electrically erasable programmable nonvolatile memory cells connected serially;
a first and a second selection transistor provided to connect both ends of said memory cells to a bit line and a source line, respectively; and
dummy cells inserted in said NAND cell unit adjacent to said first and second selection transistors, respectively, wherein
said memory cells and said dummy cells have respective control gates which are connected to regular word lines and dummy word lines, respectively, and said first and second selection transistors have respective gates which are connected to first and second selection gate lines, respectively, wherein
a plurality of said NAND cell units sharing said regular word lines, dummy word lines and selection gate lines configures an erase unit, wherein
said memory cells and dummy cells in said erase unit are erased simultaneously and subjected to soft programming after erasing for dissolving an over-erased cell in said erase unit, and wherein
said soft programming operation includes a soft programming verifying operation in which a first soft program verify voltage is applied to said regular word lines and a second soft program verify voltage higher than said first soft program verify voltage is applied to said dummy word lines.
9. The semiconductor memory device according to claim 8 , wherein
after erasing all NAND cell units in said erase unit, writing is executed to said dummy cells in said erase unit for setting a higher threshold distribution than said erased state of said memory cells.
10. The semiconductor memory device according to claim 9 , wherein
after said writing, soft programming is executed for dissolving an over-erased cell in said erase unit.