Method for compacting the erased threshold voltage distribution of flash memory devices during writing operations
View Patent ↗A method for operating a flash memory device. The memory device includes a matrix of memory cells each one having a programmable threshold voltage defining a value stored in the memory cell. The method includes the steps of erasing a block of memory cells, and compacting the threshold voltages of the memory cells of the block within a predefined compacting range, wherein the step of compacting includes: selecting at least one first memory cell of the block for writing a target value; restoring the threshold voltage of a subset of the memory cells of the block to the compacting range, the subset including the at least one first memory cell and/or at least one second memory cell of the block being adjacent to the at least one first memory cell; and at least partially writing the target value into the at least one first memory cell.
1. A method for operating a flash memory device including a matrix of memory cells each one having a programmable threshold voltage defining a value stored in the memory cell, wherein the method includes the steps of:
erasing a block of memory cells, and
compacting the threshold voltages of the memory cells of the block within a predefined compacting range,
wherein the step of compacting includes:
selecting at least one first memory cell of the block for writing a target value;
restoring the threshold voltage of a subset of the memory cells of the block to the compacting range, the subset comprising the at least one first memory cell and/or at least one second memory cell of the block being adjacent to the at least one first memory cell; and
at least partially writing the target value into the at least one first memory cell.
2. The method according to claim 1 , wherein the subset comprises the at least one first memory cell, and the at least partially writing comprises partially writing, the step of compacting further including:
the selecting, restoring and partially writing of at least one third memory cell of the block being adjacent to the at least one first memory cell, and
completing the writing of the target value into the at least one first memory cell.
3. The method according to claim 2 , wherein the value stored in each memory cell is defined by a level of the threshold voltage selected among an erased level and a plurality of programmed levels, each erased memory cell having the threshold voltage at the erased level, wherein the step of partially writing includes:
leaving the at least one first memory cell unaffected or programming the at least one first memory cell to bring the threshold voltage to a first programmed level adjacent to the erased level according to the target value, and wherein the step of completing the writing includes:
leaving the at least one first memory cell unaffected or programming the at least one first memory cell to bring the threshold voltage to another programmed level different from the first programmed level according to the target value.
4. The method according to claim 3 , wherein the at least one first memory cell comprises a page of first memory cells, the steps of restoring and partially writing the first memory cells including:
applying a series of increasing first pulses to all the first memory cells, the application of the first pulses being individually disabled for each first memory cell not to be programmed once restored,
applying a series of further increasing second pulses to each first memory cell still to be restored or to be programmed, the application of the second pulses being individually disabled for each first memory cell not to be programmed once restored and for each first memory cell to be programmed once programmed, and
applying a series of still further increasing third pulses to each first memory cell still to be programmed, the application of the third pulses being individually disabled for each first memory cell to be programmed once programmed.
5. The method according to claim 3 , wherein the value stored in each memory cell is represented by a plurality of bits, the erased level and the first programmed level being associated with different conditions of one of the bits and a same condition of the remaining bits.
6. The method according to claim 1 , wherein the subset comprises the at least one second memory cell.
7. The method according to claim 6 , wherein the memory cells of the block are written in succession according to a predetermined writing sequence, the step of compacting further including, before selecting a starting first memory cell of the sequence:
the restoring of the starting first memory cell and at least one third memory cell of the block being adjacent to the starting first memory cell different from the corresponding at least one second memory cell.
8. The method according to claim 7 , wherein the at least one second memory cell comprises a single second memory cell.
9. The method according to claim 8 , wherein the matrix is arranged in a plurality of rows and columns with the columns grouped in pairs each one of an even column and an odd column associated with a common page buffer, wherein the memory cells are ordered in the writing sequence from each memory cell in the even column to the memory cell in the same row and in the odd column, and from each memory cell in the odd column to the memory cell in the even column and in the next row, and wherein the second memory cell corresponding to each first memory cell in the even column comprises the memory cell in the odd column and in the next row, and the second memory cell corresponding to each first memory cell in the odd column comprises the memory cell in the even column and in the second next row.
10. A software program for performing the method of claim 1 , wherein when the program is executed in a micro-controller of a flash memory device.
11. A flash memory device including means for performing the steps of the method according to claim 1 .