Memory system and programming method thereof

1. A method of operating a nonvolatile memory device in which a plurality of strings are connected to a bit line, each string including at least one string selection transistor, a plurality of memory cells, and at least one ground selection transistor formed in a direction perpendicular to a substrate, the nonvolatile memory device configured to select a pre-pulse mode and a non pre-pulse mode, the method comprising:

determining whether an operation mode of the nonvolatile memory device is the pre-pulse mode or the non pre-pulse mode, wherein a result of the determining is different between a read operation and a program verification operation;

when the operation mode is the pre-pulse mode, applying a pre-pulse having a particular level to a string selection line connected to a gate of a string selection transistor of at least one unselected string of the plurality of strings for a particular time period, and then performing the read operation or the program verification operation for at least one memory cell of the plurality of memory cells; and

when the operation mode is the non pre-pulse mode, performing the read operation or the program verification operation for the at least one memory cell of the plurality of memory cells without applying the pre-pulse to the at least one string selection line.

2. The method of claim 1 , wherein the operation mode is the pre-pulse mode in the read operation.

3. The method of claim 1 , wherein the operation mode is the non pre-pulse mode in the program verification operation.

4. The method of claim 1 , wherein the operation mode is the non pre-pulse mode in the program verification operation when an off cell is not detected at a previous program verification operation.

5. The method of claim 4 , wherein the operation mode is the pre-pulse mode in the program verification operation when the off cell is detected at the previous program verification operation.

6. The method of claim 1 , wherein the operation mode is the non pre-pulse mode in the program verification operation when a number of program loops does not exceed a predetermined value.

7. The method of claim 6 , wherein the operation mode is the pre-pulse mode in the program verification in operation when the number of program loops exceeds the predetermined value.

8. The method of claim 1 , wherein information indicating whether the operation mode is the pre-pulse mode or non pre-pulse mode is provided from an external device.

9. The method of claim 1 , wherein the particular time period for which the pre-pulse is applied is varied according to a target state of the at least one memory cell.

10. The method of claim 9 , wherein the time period is selected to be longer when the target state of the at least one memory cell has a higher level, and is selected to be shorter when the target state of the at least one memory cell has a lower level.

11. A storage device comprising:

at least one nonvolatile memory device in which a plurality of strings are connected to a bit line, each string including at least one string selection transistor, a plurality of memory cells, and at least one ground selection transistor formed in a direction perpendicular to a substrate, the nonvolatile memory device configured to select a pre-pulse mode and a non pre-pulse mode; and

a memory controller configured to control the at least one nonvolatile memory device,

wherein the at least one nonvolatile memory device determines whether an operation mode of the nonvolatile memory device is the pre-pulse mode or the non pre-pulse mode,

wherein a result of the determining is different between a read operation and a program verification operation, and

wherein a pre-pulse having a particular level is applied to a string selection line connected to a gate of at least one string selection transistor of at least one unselected string for a particular time period in the pre-pulse mode.

12. The storage device of claim 11 , wherein each of the plurality of strings is connected between the bit line and a common source line and is formed in a direction perpendicular to the substrate.

13. The storage device of claim 11 , wherein when the operation mode is the pre-pulse mode, the at least one nonvolatile memory device applies the pre-pulse to the string selection line, and then performs the read operation or the program verification operation for at least one memory cell of the plurality of memory cells.

14. The storage device of claim 13 , wherein when the operation mode is the non pre-pulse mode, the at least one nonvolatile memory device performs the read operation or the program verification operation for the at least one memory cell of the plurality of memory cells without applying the pre-pulse to the at least one string selection line.

15. The storage device of claim 11 , wherein the at least one string selection transistor is turned on by the pre-pulse.

16. The storage device of claim 11 , wherein the operation mode is the pre-pulse mode in the program verification operation when an off cell is detected at the previous program verification operation.

17. The storage device of claim 11 , wherein the operation mode is the pre-pulse mode in the program verification operation when the number of program loops exceeds the predetermined value.

18. A nonvolatile memory device comprising:

a memory cell array including a plurality of memory blocks, each memory block including a plurality of strings connected between a bit line and a common source line, each string including at least one string selection transistor, a plurality of memory cells, and at least one ground selection transistor formed in a direction perpendicular to a substrate; and

a control logic configured to control the at least one nonvolatile memory device and to select a pre-pulse mode and a non pre-pulse mode, wherein the control logic determines whether an operation mode of the nonvolatile memory device is the pre-pulse mode or the non pre-pulse mode,

wherein when the operation mode is the pre-pulse mode, a pre-pulse having a particular level is applied to a string selection line connected to a gate of a string selection transistor of at least one unselected string of the plurality of strings for a particular time period, and then the read operation or the program verification operation for at least one memory cell of the plurality of memory cells is performed, and

wherein when the operation mode is the non pre-pulse mode, the read operation or the program verification operation for the at least one memory cell of the plurality of memory cells without applying the pre-pulse to the at least one string selection line is performed.

19. The nonvolatile memory device of claim 18 , wherein the operation mode is the pre-pulse mode in the read operation.

20. The nonvolatile memory device of claim 18 , wherein the operation mode is the non pre-pulse mode in the program verification operation.