Adaptive scheme for incremental step pulse programming of flash memory
According to an adaptive programming method for flash memories, the cell programming speed is detected by programming a number of cells using a fixed trial program voltage. A starting program voltage V start is then adjusted based on the detected cell programming speed. The ISPP is then carried out using the adjusted value for V start .
1. A method of programming a flash memory, the method comprising:
performing a programing/verifying operation on a plurality of memory cells using a trial program voltage to determine a cell programming speed;
modifying a starting program voltage based on the determined cell programming speed; and
performing a sequence of programing/verifying operations to program a plurality of selected memory cells to a target program state, wherein in the first of the sequence of programing/verifying operations, a program voltage equal to the modified starting program voltage is used, and the program voltage is incrementally increased from the modified starting program voltage in subsequent programing/verifying operations until the plurality of selected memory cells are programmed to the target state.
2. The method of claim 1 wherein the trial program voltage is a constant voltage over the life of the flash memory.
3. The method of claim 1 wherein the starting program voltage is set to the trial program voltage less a delta voltage, the delta voltage representing a difference between the uptail voltage of the trial program voltage distribution and an uptail voltage of a threshold voltage distribution corresponding to one of a plurality of cell states.
4. The method of claim 1 wherein both the trial program voltage and the uptail voltage of the threshold voltage distribution are constant voltages.
5. The method of claim 1 where the trial program voltage is selected so that an uptail of the trial program voltage distribution is greater than an uptail of a first threshold voltage distribution corresponding to lowest program state in a plurality of distinct program states, and is smaller than an uptail of the last threshold voltage distribution corresponding to the highest program state in the plurality of distinct program states.
6. A method of programming a flash memory, the method comprising:
programming a plurality of memory cells by applying a trial program voltage to the plurality of memory cells;
performing multiple program verify operations to determine an uptail voltage of the trial program voltage distribution for the programmed plurality of memory cells; and
programing a plurality of memory cells using a starting program voltage, wherein the starting program voltage is set to the trial program voltage less a delta voltage, the delta voltage representing a difference between the uptail voltage of the trial program voltage distribution and an uptail voltage of a threshold voltage distribution corresponding to one of a plurality of cell states.
7. The method of claim 6 wherein both the trial program voltage and the uptail voltage of the threshold voltage distribution are constant voltages.
8. The method of claim 6 where the trial program voltage is selected so that an uptail of the trial program voltage distribution is greater than an uptail of a first threshold voltage distribution corresponding to lowest program state in a plurality of distinct program states, and is smaller than an uptail of the last threshold voltage distribution corresponding to the highest program state in the plurality of distinct program states.
9. The method of claim 6 wherein performing multiple program verify operations includes performing a binary search to determine the uptail voltage of the trial program voltage distribution.
10. A method of programming a flash memory, the method comprising:
programming a plurality of memory cells by applying a trial program voltage to the plurality of memory cells;
determining an uptail voltage of the trial program voltage distribution for the programmed plurality of memory cells;
obtaining a voltage difference between the uptail of the trial program voltage distribution and an uptail voltage of a threshold voltage distribution corresponding to one of a plurality of cell states;
reducing the trial program voltage by an amount equal to the obtained voltage difference;
setting a value of a starting program voltage to the reduced trial program voltage; and
performing a sequence of programing/verifying operations to program a plurality of selected memory cells to a target program state, wherein in the first of the sequence of programing/verifying operations, a program voltage equal to the starting program voltage is used.
11. The method of claim 10 wherein both the trial program voltage and the uptail voltage of the threshold voltage distribution are constant voltages.
12. The method of claim 10 where the trial program voltage is selected so that an uptail of the trial program voltage distribution is greater than an uptail of a first threshold voltage distribution corresponding to lowest program state in a plurality of distinct program states, and is smaller than an uptail of the last threshold voltage distribution corresponding to the highest program state in the plurality of distinct program states.
13. The method of claim 10 wherein the uptail voltage of the trial program voltage distribution is determine using a binary search.