Non-volatile memory with countermeasures for select gate disturb during program pre-charge
Program disturb is a condition that includes the unintended programming while performing a programming process for memory cells, where the program disturb can affect both memory cells and select gates in a NAND structure. During a pre-charge phase of a programming operation, a drain side select gate may be biased to a higher voltage than an adjacent word line, resulting in a disturb of the select gate due to hot-electron injection. This can raise the threshold voltage of the select gate, causing error in reading the NAND string or even making it inaccessible. To help avoid this problem, during a program pre-charge, the voltage applied to the select gate is raised in a sequence of steps, rather than driving the select gate directly to its final pre-charge voltage level.
1. An apparatus, comprising:
a select gate control line connected to a first select gate;
a first word line connected to a first memory cell;
a second word line connected to a second memory cell, the second memory cell connected in series with the first memory cell and the first select gate and located adjacent to the first select gate; and
a pre-charge circuit configured to pre-charge a channel coupled to the first select gate, the first memory cell and the second memory cell by concurrently:
biasing the first word line to ground;
driving the second word line at a non-selected pre-charge level above ground; and
driving the select gate control line with a voltage waveform of a plurality of steps increasing from ground to a voltage level higher than the non-selected pre-charge level; and
a programming circuit configured to apply a program pulse to the first word line subsequent to pre-charge the channel.
2. The apparatus of claim 1 , further comprising:
a first bit line, the first memory cell and second memory cell connected to the first bit line through the first select gate, wherein the pre-charge circuit is further configured to bias the first bit line at ground while driving the select gate control line with the voltage waveform.
3. The apparatus of claim 2 , further comprising:
a second bit line;
a second select gate connected to the select gate control line;
a third memory cell connected to the first word line; and
a fourth memory cell connected to the second word line, the third and fourth memory cells connected to the second bit line through the second select gate, wherein the pre-charge circuit is further configured to bias the second bit line at a program inhibit voltage greater than ground while driving the select gate control line with the voltage waveform.
4. The apparatus of claim 1 , wherein the plurality of steps includes a first step having a voltage level higher than the non-selected pre-charge level and a subsequent second step having a voltage level higher than the first step.
5. The apparatus of claim 1 , wherein the plurality of steps includes a sequence of three or more steps of a common size increasing at a constant rate.
6. The apparatus of claim 1 , wherein the plurality of steps includes a first sequence of a plurality of steps of a first size increasing at a first rate to an intermediate value, followed by a plurality of steps of a second size increasing at a second rate from the intermediate value to a final value.
7. The apparatus of claim 1 , wherein the first select gate is a drain side select gate.
8. The apparatus of claim 1 , wherein the second word line is a dummy word line.
9. An apparatus, comprising:
a NAND string, including a selected memory cell and a non-selected memory cell, the non-selected memory cell located on the NAND string adjacent to a select gate; and
a control circuit connected to the NAND string and configured to program the selected memory cell by a series of pulse operations, each pulse operation including applying a pulse to the selected memory cell and, prior to the applying the pulse, pre-charging the NAND string by applying an increasing sequence of a plurality of voltage levels to the select gate while applying a non-selected voltage level to the non-selected memory cell, one or more of the voltage levels of the increasing sequence of voltage levels being greater than non-selected voltage level.
10. The apparatus of claim 9 , wherein the increasing sequence of voltage levels includes a first step having a voltage level higher than the non-selected voltage level and a subsequent second step having a voltage level higher than the first step.
11. The apparatus of claim 9 , wherein the increasing sequence of voltage levels includes a sequence of three or more steps of a common size increasing at a constant rate.
12. The apparatus of claim 9 , wherein the increasing sequence of voltage levels includes a first sequence of a plurality of steps of a first size increasing at a first rate to an intermediate value, followed by a plurality of steps of a second size increasing at a second rate from the intermediate value to a final value.
13. The apparatus of claim 9 , wherein apparatus comprises a memory array of a monolithic three-dimensional semiconductor memory device in which the memory cells are arranged in multiple physical levels above a silicon substrate and comprise a charge storage medium.
14. A method, comprising:
pre-charging a first NAND string, comprising:
driving a control line connected to a first select gate with a staircase waveform of a sequence of voltage levels sequentially increasing to a select gate pre-charge voltage; and
while driving the first select gate with the staircase waveform, biasing a first word line connected to a non-selected memory cell with a bypass voltage, where the non-selected memory cell is adjacent to the first select gate and is between the first select gate and a selected memory cell on the first NAND string, and the bypass voltage is lower than the select gate pre-charge voltage; and
subsequent to pre-charging the first NAND string, applying a programming pulse to a second word line connected the selected memory cell.
15. The method of claim 14 , wherein the first NAND string is connected to a first bit line through the first select gate, and pre-charging the first NAND string further comprises:
biasing the first bit line at ground.
16. The method of claim 15 , wherein programming the selected memory cell further comprises:
concurrent with pre-charging the first NAND string, pre-charging a second NAND string having a first memory cell connected to the first word line, a second memory sell connected to the second word line, and a second select gate connected to the control line, wherein the second NAND string is connected to a second bit line through the second select gate and pre-charging the second NAND string includes:
biasing the second bit line at program inhibit voltage.
17. The method of claim 14 , wherein the staircase waveform includes a first step having a voltage level higher than the bypass voltage and lower than the select gate pre-charge voltage, and a subsequent second step having a voltage level at the select gate pre-charge voltage.
18. The method of claim 14 , wherein the staircase waveform includes a sequence of three or more steps of a common size increasing at a constant rate.
19. The method of claim 14 , wherein the staircase waveform includes a first sequence of a plurality of steps of a first size increasing at a first rate to an intermediate value, followed by a plurality of steps of a second size increasing at a second rate from the intermediate value to the select gate pre-charge voltage.
20. An apparatus, comprising:
an array of non-volatile memory cells including a plurality of NAND strings, each NAND string including a select gate through which the NAND string is connected to a corresponding bit line, a first memory cell adjacent to the select gate and a second memory cell, wherein each of the select gates is connect to a select line, each of the first memory cells is connected to a first word line, and each of the second memory cells is connected to a second word line;
means for programming the second memory cell on selected ones of the NAND strings; and
means for pre-charging the plurality of NAND strings, including both the selected NAND strings and non-selected ones of the NAND strings, for the programming, the pre-charging including biasing bit lines corresponding to the selected NAND strings to a program enable value, biasing bit lines corresponding to non-selected NAND strings to a program inhibit value, biasing the first word line to turn on the first memory cells, and biasing the select line with a rising waveform having a plurality of steps increasing to a value sufficient to turn on the select gates of the non-selected NAND strings.