NAND flash memory

1. A NAND flash memory comprising:

a NAND string in which a plurality of memory cells are connected in series to store information corresponding to an amount of charges held in a charge holding layer, the memory cells respectively comprising a semiconductor well on a surface of a semiconductor substrate, a first insulation film on the semiconductor well, the charge holding layer on the first insulation film, a second insulation film on the charge holding layer, and a control gate above the second insulation film; and

a control circuit configured to control voltages applied to the control gate and the semiconductor well, wherein

in a write operation, the control circuit applies a writing voltage between the control gate of a selected memory cell to be written and the semiconductor well, and

after the write operation and before performing a verification read operation of verifying whether data has been written into the selected memory cell, the control circuit performs a de-trapping operation, in which a first voltage of a same potential as that of the semiconductor well or a same polarity as that of the writing voltage is applied to the control gate of the selected memory cell and in which a second voltage of a same polarity as that of the writing voltage and larger than the first voltage as an absolute value is applied to a control gates of unselected memory cells not to be written.

2. The NAND flash memory of claim 1 , wherein the second voltage is substantially equal to a voltage applied to the control gates of the unselected memory cells, in the write operation.

3. A NAND flash memory comprising:

a NAND string in which a plurality of memory cells are connected in series to store information corresponding to an amount of charges held in a charge holding layer, the memory cells respectively comprising a semiconductor well on a surface of a semiconductor substrate, a first insulation film on the semiconductor well, the charge holding layer on the first insulation film, a second insulation film on the charge holding layer, and a control gate above the second insulation film; and

a control circuit configured to control voltages applied to the control gate and the semiconductor well, wherein

in a write operation, the control circuit applies a writing voltage between the control gate of a selected memory cell to be written and the semiconductor well, and

after the write operation and before performing a verification read operation of verifying whether data has been written into the selected memory cell, the control circuit performs a de-trapping operation, in which a first voltage of a same potential as that of the semiconductor well or a same polarity as that of the writing voltage is applied to the control gate of the selected memory cell and in which a second voltage of a same polarity as that of the writing voltage and larger than the first voltage as an absolute value is applied to a control gate of a first unselected memory cell which is not adjacent to the selected memory cell, and a third voltage of a same polarity as that of the writing voltage and smaller than the second voltage as an absolute value is applied to a control gate of a second unselected memory cell which is adjacent to the selected memory cell.

4. The NAND flash memory of claim 1 , wherein in the de-trapping operation, the control circuit applies the second voltage to the control gates of unselected memory cells near the selected memory cell, and applies a same potential as a potential of the semiconductor well to the control gates of the unselected memory cells separated from the selected memory cell.

5. The NAND flash memory of claim 2 , wherein in the de-trapping operation, the control circuit applies the second voltage or the third voltage to the control gate of unselected memory cells near the selected memory cell, and applies a same potential as a potential of the semiconductor well to the control gate of the unselected memory cells separated from the selected memory cell.

6. The NAND flash memory of claim 3 , wherein in the de-trapping operation, the control circuit applies the second voltage or the third voltage to the control gate of unselected memory cells near the selected memory cell, and applies a same potential as a potential of the semiconductor well to the control gate of the unselected memory cells separated from the selected memory cell.

7. The NAND flash memory of claim 1 , wherein in the de-trapping operation, the control circuit applies a higher voltage to the control gates of the unselected memory cells at a source side of the selected memory cell than a voltage of the control gates of the unselected memory cells at a drain side of the selected memory cell.

8. The NAND flash memory of claim 2 , wherein in the de-trapping operation, the control circuit applies a higher voltage to the control gates of the unselected memory cells at a source side of the selected memory cell than a voltage of the control gates of the unselected memory cells at a drain side of the selected memory cell.

9. The NAND flash memory of claim 3 , wherein in the de-trapping operation, the control circuit applies a higher voltage to the control gates of the unselected memory cells at a source side of the selected memory cell than a voltage of the control gates of the unselected memory cells at a drain side of the selected memory cell.

10. The NAND flash memory of claim 4 , wherein in the de-trapping operation, the control circuit applies a higher voltage to the control gates of the unselected memory cells at a source side of the selected memory cell than a voltage of the control gates of the unselected memory cells at a drain side of the selected memory cell.

11. The NAND flash memory of claim 1 , wherein in the de-trapping operation, the control circuit once applies the second voltage to the control gate of the selected memory cell, and thereafter applies the first voltage to the control gate of the selected memory cell.

12. The NAND flash memory of claim 2 , wherein in the de-trapping operation, the control circuit once applies the second voltage to the control gate of the selected memory cell, and thereafter applies the first voltage to the control gate of the selected memory cell.

13. The NAND flash memory of claim 3 , wherein in the de-trapping operation, the control circuit once applies the second voltage to the control gate of the selected memory cell, and thereafter applies the first voltage to the control gate of the selected memory cell.

14. The NAND flash memory of claim 4 , wherein in the de-trapping operation, the control circuit once applies the second voltage to the control gate of the selected memory cell, and thereafter applies the first voltage to the control gate of the selected memory cell.

15. The NAND flash memory of claim 7 , wherein in the de-trapping operation, the control circuit once applies the second voltage to the control gate of the selected memory cell, and thereafter applies the first voltage to the control gate of the selected memory cell.

16. The NAND flash memory of claim 3 , wherein in the de-trapping operation, the control circuit once applies the second voltage to the control gates of the second unselected memory cells, and thereafter applies the third voltage to the control gates of the second unselected memory cells.