Pre-charge during programming for 3D memory using gate-induced drain leakage
In a programming operation of a 3D stacked non-volatile memory device, the channel of an inhibited NAND string is pre-charged by gate-induced drain leakage (GIDL) to achieve a high level of boosting which prevents program disturb in inhibited storage elements. In a program-verify iteration, prior to applying a program pulse, the drain-side select gate transistor is reverse biased to generate GIDL, causing the channel to be boosted to a pre-charge level such as 1.5V. Subsequently, when the program pulse is applied to a selected word line and pass voltages are applied to unselected word lines, the channel is boosted higher from the pre-charge level due to capacitive coupling. The pre-charge is effective even for a NAND string that is partially programmed because it does not rely on directly driving the channel from the bit line end.
1. A method for programming in a 3D stacked non-volatile memory device, comprising:
driving a voltage of a channel of an uninhibited NAND string, the uninhibited NAND string is selected for programming and comprises a selected non-volatile storage element which is selected for programming and a plurality of unselected non-volatile storage elements which are not selected for programming;
during the driving, floating a voltage of a channel of an inhibited NAND string by providing a drain-side select gate and a source-side select gate of the inhibited NAND string in a non-conductive state, the inhibited NAND string comprises a non-volatile storage element which is connected to the selected non-volatile storage element via a selected word line, and a plurality of non-volatile storage elements which are connected to the plurality of unselected non-volatile storage elements via unselected word lines;
during the floating, increasing the voltage of the channel of the inhibited NAND string to a pre-charge level using gate-induced drain leakage from the drain-side select gate and from the pre-charge level to a program inhibit level using capacitive coupling; and
while the voltage of the channel of the inhibited NAND string is at the program inhibit level, increasing a voltage of the selected word line to a programming level and maintaining the voltage of the selected word line at the programming level.
2. The method of claim 1 , wherein:
the voltage of the channel of the uninhibited NAND string is driven at a level which allows programming in the uninhibited NAND string.
3. The method of claim 1 , wherein:
the capacitive coupling is achieved by increasing a voltage on the unselected word lines from an initial level to a pass voltage level which is less than the programming level.
4. The method of claim 3 , wherein:
the voltage of the selected word line is increased from an initial level to the pass voltage level and then to the programming level.
5. The method of claim 1 , wherein:
the gate-induced drain leakage is achieved by providing a drain-to-gate voltage of the drain-side select gate at a level which is above a threshold level, the threshold level is associated with a generation of electron-holes pairs from gate-induced drain leakage.
6. The method of claim 5 , wherein:
the drain-to-gate voltage of the drain-side select gate is provided at the level which is above the threshold level by setting a voltage of a bit line connected to a drain of the drain-side select gate at a positive voltage and setting a voltage of a control gate of the drain-side select gate at 0 V or at a negative voltage.
7. The method of claim 5 , wherein:
during the increasing of the voltage of the channel of the inhibited NAND string from the pre-charge level to the program inhibit level using the capacitive coupling and during the maintaining of the voltage of the selected word line at the programming level, providing the drain-to-gate voltage of the drain-side select gate at a level which is below the threshold level.
8. The method of claim 7 , wherein:
the drain-to-gate voltage of the drain-side select gate is provided at the level which is below the threshold level by setting a voltage of a bit line connected to a drain of the drain-side select gate at a respective positive voltage and setting a voltage of a control gate of the drain-side select gate at a corresponding positive voltage.
9. The method of claim 8 , wherein:
the uninhibited NAND string comprises a drain-side select gate having a control gate;
a first bit line is connected to a drain of the drain-side select gate of the uninhibited NAND string;
a second bit line is connected to a drain of the drain-side select gate of the inhibited NAND string;
a control gate of the drain-side select gate of the inhibited NAND string is connected to the control gate of the drain-side select gate of the uninhibited NAND string; and
the driving of the voltage of the channel of the uninhibited NAND string comprises setting a voltage of the first bit line at a level which is below the corresponding positive voltage of the control gate of the drain-side select gate of the inhibited NAND string so that a control gate-to-drain voltage of the control gate of the drain-side select gate of the uninhibited NAND string is high enough to provide the drain-side select gate of the uninhibited NAND string in a conductive state.
10. The method of claim 1 , wherein:
the programming comprises a plurality of program-verify iterations;
each program-verify iteration comprises a program portion and a verify portion; and
the driving the voltage of the channel of the uninhibited NAND string and the floating of the voltage of the channel of the inhibited NAND string occur in the program portion of each program-verify iteration of the one or more program-verify iterations.