Memory block erasing in a flash memory device
View Patent ↗The erase and verify method performs an erase operation and an erase verify read operation. If the erase verify read operation fails because unerased memory cells have been found, a normal memory read operation is performed in order to determine which memory cells are still programmed. A selective erase operation is then performed on the memory cells such that only the rows that comprise unerased memory cells undergo additional erase operations.
1. A method for erasing a flash memory device comprising a plurality of memory blocks, each memory block having a plurality of memory cells organized in columns and rows, the method comprising:
performing an erase operation on a first memory block;
determining if any of the plurality of memory cells of the first memory block is unerased by performing an erase verification operation that comprises biasing all of the rows of the first memory block substantially simultaneously with an erase verify read potential; and
performing a selective erase operation only on rows comprising an unerased memory cell in response to a normal memory read operation that determines which memory cells of the first memory block remain unerased wherein the normal memory read operation comprises biasing only a selected row of the first memory block with a normal read potential and biasing the remaining rows of the first memory block at a V PASS voltage that is greater than the normal read potential.
2. The method of claim 1 wherein the selective erase operation comprises biasing, at a predetermined potential, only rows that comprise an unerased memory cell.
3. The method of claim 2 wherein the predetermined potential is ground potential.
4. The method of claim 1 wherein the normal memory read operation comprises individually biasing in turn, at the normal read potential, each of the rows of the first memory block while select columns of the first memory block are biased at a precharge voltage, unselected columns are biased at ground potential, and unselected rows are biased at the V PASS voltage.
5. The method of claim 4 wherein the normal read potential is ground potential and the precharge voltage is in a range of 0V to V CC .
6. The method of claim 2 and further including allowing to float all remaining rows and all columns of the memory block during the selective erase operation.
7. A flash memory device comprising:
a memory array having a plurality of memory blocks that are each comprised of a plurality of memory cells arranged in rows and columns; and
control circuitry that is adapted to execute a memory erase and verify method comprising performing an erase operation on a first memory block of the plurality of memory blocks, determining if any of the plurality of memory cells of the first memory block is unerased by performing an erase verification operation that comprises biasing all of the rows of the first memory block substantially simultaneously with an erase verify read potential, and performing a selective erase operation only on rows comprising unerased memory cells in response to a normal memory read operation that determines which memory cells remain unerased wherein the normal memory read operation comprises biasing only a selected row of the first memory block with a normal read potential and biasing the remaining rows of the first memory block at a V PASS voltage that is greater than the normal read potential.
8. The flash memory device of claim 7 wherein the memory array is arranged in a NAND architecture.
9. The flash memory device of claim 7 wherein the memory array is arranged in a NOR architecture.
10. A method for erasing a flash memory device comprising a plurality of memory blocks, each memory block having a plurality of memory cells organized in columns and rows, the method comprising:
performing an erase operation on a first memory block;
determining if any of the plurality of memory cells of the first memory block is unerased by performing an erase verification operation; and
performing a selective erase operation only on rows comprising unerased memory cells in response to a normal memory read operation that determines which memory cells of the first memory block remain unerased wherein the normal memory read operation biases rows differently than the erase verification operation.