Circuit and method for erasing flash memory array
AssigneeAMIC Technology Corporation
InventorsKou-Su Chen, David K. Y. Liu
Life of this patent
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Abstract
A circuit and method for achieving compressed distributions of erased cell threshold voltages in an EEPROM array is disclosed. The invention, when used to condition flash memory cell arrays, results in increased endurance of such arrays, and eliminates the need for pre-programming operations before a bulk erase can take place. By eliminating the need to pre-program the memory array before each erasure, the process provides a signicant improvement for low power applications, because battery life is extended and write cycle time is enhanced.
Claims38
A method of correcting over-erased memory cells in a flash memory cell array, including the steps of:
- (a) detecting the existence of any over-erased cells, such over-erased cells having a threshold voltage below a threshold voltage Vt.sub.emin, where Vt.sub.emin is a minimum target threshold value for a cell in an erased state;
- (b) generating a threshold adjustment voltage signal, which adjustment signal is not applied to the entire array, to adjust threshold voltages of any over-erased cells, so that threshold voltages for any of such cells are set to an approximate threshold voltage Vt.sub.sp, where Vt.sub.sp has a value greater than or equal to Vt.sub.emin and less than Vt.sub.emax, where Vt.sub.emax is a maximum target threshold voltage value for a cell in an erased state.
A method of controlling a distribution of threshold voltages of memory cells in a flash memory cell array, including the steps of:
- (a) detecting any over-erased cells;
- (b) if any over-erased cells are detected, adjusting only such over-erased cells by applying a first adjustment signal to modify the over-erased cell threshold voltages to be approximately equal to Vt.sub.sp, where Vt.sub.emin <=Vt.sub.sp <=Vt.sub.emax, and where Vt.sub.emin and Vt.sub.emax represent minimum and maximum target threshold voltage values for a cell in an erased state, respectively;
- and
- (c) detecting any under-erased cells;
- and
- (d) if any under-erased cells are detected, generating an erase pulse that is applied to the entire array;
- (e) applying a second and third adjustment signal to compress said threshold voltages to a range between Vt.sub.emin and Vt.sub.emax by adjusting threshold voltage values of both over-erased and under-erased cells.
A method of correcting threshold voltage values for over-erased memory cells in a flash memory cell array, including the steps of:
- (a) detecting the existence of any over-erased cells, said over-erased cells having a threshold voltage below a threshold voltage Vt.sub.emin, where Vt.sub.emin is a minimum target threshold value for a cell in an erased state;
- (b) adjusting said threshold voltages of said over-erased cells by generating a threshold adjustment voltage signal, said adjustment voltage signal being configured and applied to such over-erased cells in order to set threshold voltages for over-erased cells to a value between Vt.sub.emin and Vt.sub.emax, where Vt.sub.emax is a maximum target threshold value for a cell in an erased state;
- and
- wherein said adjusting step is performed without applying said threshold adjustment voltage signal to all of such cells in such array.
A method of erasing memory cells in a flash memory cell array, including the steps of:
- (a) applying an erase pulse to the array during an erase operation;
- and
- (b) adjusting threshold voltages of such cells after such erase operation with a threshold adjustment signal, so that such erased cells have a threshold voltage above a threshold voltage Vt.sub.emin, where Vt.sub.emin is a minimum target threshold value for a cell in an erased state, and such cells further have a threshold voltage value below Vt.sub.emax, where Vt.sub.emax is a maximum target threshold voltage value for a cell in an erased state;
- and
- wherein such array is erased without first pre-programming such cells to a program threshold voltage value Vt.sub.pmin, where Vt.sub.pmin is a minimum program threshold value for such array, and Vt.sub.pmin >Vt.sub.emax.
A method of controlling voltage threshold distributions of memory cells in a flash memory cell array, including the steps of:
- (a) detecting any over-erased cells;
- and
- (b) correcting any such over-erased cells;
- and
- (c) detecting any under-erased cells;
- and
- (d) applying an erase pulse to the array during an erase operation, such erase pulse being applied without first pre-programming such cells to a program threshold voltage value Vt.sub.pmin, where Vt.sub.pmin is a minimum program threshold value for such array, and Vt.sub.pmin >Vt.sub.emax where Vt.sub.emax is a maximum target threshold voltage value for a cell in an erased state.
A method of converging voltage thresholds of memory cells in a flash memory cell array, including the steps of:
- (a) detecting the existence of any over-erased cells, such over-erased cells having a threshold voltage below a threshold voltage Vt.sub.emin, where Vt.sub.emin is a minimum target threshold value for a cell in an erased state;
- (b) detecting the existence of any under-erased cells, such under-erased cells having a threshold voltage above a threshold voltage Vt.sub.emax, where Vt.sub.emax is a maximum target threshold value for a cell in an erased state;
- (c) generating a first type of threshold adjustment voltage signal to adjust threshold voltages of any over-erased cells;
- (d) generating a second type of threshold adjustment voltage signal to adjust threshold voltages of any under-erased cells;
- wherein threshold voltages for such cells converge during steps (a) to (d) to a value between Vt.sub.emin and Vt.sub.emax.
A method of converging voltage thresholds of memory cells in a flash memory cell array, including the steps of:
- (a) erasing the memory cells using an erase pulse;
- (b) detecting the existence of any over-erased cells from such erase pulse, such over-erased cells having a threshold voltage below a threshold voltage Vt.sub.emin, where Vt.sub.emin is a minimum target threshold value for a cell in an erased state;
- (c) applying a soft program signal to such array when any over-erased cells are detected;
- (d) detecting the existence of any under-erased cells, such under-erased cells having a threshold voltage above a threshold voltage Vt.sub.emax, where Vt.sub.emax is a maximum target threshold value for a cell in an erased state;
- (e) applying a soft erase signal to such array when any under-erased cells are detected;
- wherein threshold voltages for such cells are adjusted with said soft program and soft erase signals to converge to a value between Vt.sub.emin and Vt.sub.emax.
Dependencies are inferred from the claim text.