NAND string pre-charge during programming by injecting holes via substrate
Apparatuses and techniques are described for pre-charging NAND string channels in a pre-charge phase of a program operation. In one aspect, a hole-type pre-charge process is used at the source end of a NAND string, where a bottom of the NAND string is connected to a p-well of a substrate. By applying a positive voltage to the p-well and a lower voltage, such as 0 V or a negative voltage, to the source-side select gate transistors and the memory cells, the holes from the p-well are injected into the channel In another approach, the hole-type pre-charge process and an electron-type pre-charge process are used sequentially in separate time periods. In another approach, the hole-type pre-charge process is used at the source end of a NAND string while the electron-type pre-charge process is used at the drain end of the NAND string.
1. An apparatus, comprising;
a NAND string extending vertically in a stack of spaced apart conductive layers, the NAND string comprising a source end and a drain end, the NAND string comprising a plurality of memory cells, a source-side select gate transistor at the source end, and a channel, and the spaced apart conductive layers comprise a plurality of word lines connected to the plurality of memory cells and a source-side select gate control line connected to the source-side select gate transistor;
a substrate comprising a p-well in contact with the source end of the NAND string, an n-type contact in the p-well and a p+ contact in the p-well; and
a control circuit, the control circuit is configured to pre-charge the channel in a program operation, wherein to pre-charge the channel, the control circuit is configured to inject holes into the channel at the source end of the NAND string, the injecting of the holes into the channel at the source end of the NAND string comprises concurrently biasing the source-side select gate control line with a respective voltage of no more than 0 V, the n-type contact with a respective positive voltage and the p+ contact with a respective positive voltage.
2. The apparatus of claim 1 , wherein:
the respective voltage of the source-side select gate control line is a negative voltage.
3. The apparatus of claim 1 , wherein:
to inject the holes into the channel at the source end of the NAND string, the control circuit is configured to bias a source-side subset of the plurality of word lines at the source end of the NAND string with a respective voltage of no more than 0 V concurrent with the biasing of the source-side select gate control line with the respective voltage of no more than 0 V, the n-type contact with the respective positive voltage and the p+ contact with the respective positive voltage.
4. The apparatus of claim 3 , wherein:
the respective voltage of the source-side subset of the plurality of word lines is a negative voltage.
5. The apparatus of claim 1 , wherein:
to pre-charge the channel, before the injecting of the holes into the channel at the source end of the NAND string, the control circuit is configured to extract electrons from the channel at the source end of the NAND string, the extracting of the electrons from the channel at the source end of the NAND string comprises concurrently biasing a source-side subset of the plurality of word lines at the source end of the NAND string with a respective positive voltage, the source-side select gate control line with a respective positive voltage, the n-type contact with a respective positive voltage and the p+ contact with a respective positive voltage.
6. The apparatus of claim 5 , wherein:
the program operation comprises a top to bottom word line programming order in the stack.
7. The apparatus of claim 1 , wherein:
the NAND string comprises a drain-side select gate transistor at the drain end;
the spaced apart conductive layers comprise a drain-side select gate control line connected to the drain-side select gate transistor; and
to pre-charge the channel, during the injecting of the holes into the channel at the source end of the NAND string, the control circuit is configured to extract electrons from the channel at the drain end of the NAND string, the extracting electrons from the channel at the drain end of the NAND string comprises concurrently biasing a drain-side subset of the plurality of word lines at the drain end of the NAND string with a respective positive voltage, the drain-side select gate control line with a respective positive voltage and the drain end with a respective positive voltage.
8. The apparatus of claim 7 , wherein:
the extracting of the electrons from the channel at the drain end of the NAND string comprises biasing a drain-side subset of the plurality of word lines at the drain end of the NAND string with a respective positive voltage.
9. The apparatus of claim 8 , wherein:
the injecting of the holes into the channel at the source end of the NAND string comprises biasing a source-side subset of the plurality of word lines at the source end of the NAND string with a respective voltage of no more than 0 V, concurrent with the biasing of the drain-side subset of the plurality of word lines at the drain end of the NAND string with the respective positive voltage.
10. The apparatus of claim 9 , wherein:
the source-side subset of the plurality of word lines comprises successive word lines ranging from a source-side word line of the plurality of word lines to a source-side adjacent word line of a selected word line among the plurality of word lines; and
the drain-side subset of the plurality of word lines comprises successive word lines ranging from a drain-side word line to the selected word line.
11. The apparatus of claim 9 , wherein:
the respective voltage of the source-side subset of the plurality of word lines is a negative voltage.
12. A method, comprising:
pre-charging channels of NAND strings in a block in a pre-charge phase of a program loop of a program operation, the channel of each NAND string extends between a source-side select gate transistor at a source end of the NAND string and a drain-side select gate transistor at a drain end of the NAND string, the source ends of the NAND strings are connected to a substrate, the NAND strings are connected to a plurality of word lines, and the pre-charging of the channels comprises injecting holes from the substrate into the channels at the source ends of the NAND strings; and
after the pre-charging of the channels, applying a program pulse to a selected word line of the plurality of word lines.
13. The method of claim 12 , wherein:
a plurality of word lines are connected to memory cells of the NAND strings;
a control line is connected to the source-side select gate transistor; and
the injecting of the holes from the substrate into the channels at the source ends of the NAND strings comprises applying a positive voltage to the substrate and a voltage of no more than 0 V to the control line and to a subset of the plurality of word lines which is adjacent to the source-side select gate transistors.
14. The method of claim 12 , wherein:
the pre-charging of the channels comprises extracting electrons from the channels into the substrate at the source ends of the NAND strings, separately in time from the injecting holes from the substrate into the channels at the source ends of the NAND strings.
15. The method of claim 12 , wherein:
the drain ends of the NAND strings are connected to respective bit lines; and
the pre-charging of the channels comprises extracting electrons from the channels into the bit lines at the drain ends of the NAND strings concurrently with the injecting of the holes from the substrate into the channels at the source ends of the NAND strings.
16. An apparatus, comprising;
a substrate;
a block of memory cells arranged on the substrate in NAND strings, each NAND string comprising a source-side select gate transistor at a source end of the NAND string, a drain-side select gate transistor at a drain end of the NAND string, memory cells between the source end and the drain end, and a channel extending from the source end to the drain end, wherein the source ends of the NAND strings are connected to the substrate, the NAND strings extend upward from the substrate, and the memory cells are connected to a plurality of word lines; and
a control circuit, the control circuit is configured to pre-charge the channels before applying a program voltage to a selected word line of the plurality of word lines in a program operation, wherein to pre-charge the channels, the control circuit is configured to apply respective voltage signals to the substrate, the source-side select gate transistors and the plurality of word lines for a first time period, the respective voltage signals initially have respective positive voltages, after which, in a second time period, the respective voltage signals for the source-side select gate transistors and the plurality of word lines are reduced while the respective voltage signal for the substrate remains at its respective positive level.
17. The apparatus of claim 16 , wherein:
a level to which the respective voltage signals for the source-side select gate transistors and the plurality of word lines are reduced is no more than 0 V.
18. The apparatus of claim 16 , wherein:
a level to which the respective voltage signals for the source-side select gate transistors and the plurality of word lines are reduced is a negative voltage.
19. The apparatus of claim 16 , wherein:
the second time period is a function of a position of the selected word line in the plurality of word lines.
20. The apparatus of claim 16 , wherein:
a ratio of the second time period to the first time period is a function of a position of the selected word line in the plurality of word lines.