SOURCE SIDE ASYMMETRICAL PRECHARGE PROGRAMMING SCHEME
First Claim
1. A method for programming a NAND flash string having a source line select device, memory cells and a string select device connected in series between a bitline and a source line, comprising:
- biasing the bitline to one of a first supply voltage level and a second supply voltage level;
asymmetrically precharging groupings of channels corresponding to the memory cells to different voltage levels from the source line for setting a selected memory cell channel to a program inhibit state independent of background data stored in unselected memory cells; and
programming the selected memory cell only when the bitline is biased to the second supply voltage level, the selected memory cell remaining in the program inhibit state when the bitline is biased to the first supply voltage level.
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Abstract
A method for programming NAND flash cells to minimize program stress while allowing for random page programming operations. The method includes asymmetrically precharging a NAND string from a positively biased source line while the bitline is decoupled from the NAND string, followed by the application of a programming voltage to the selected memory cell, and then followed by the application of bitline data. After asymmetrical precharging and application of the programming voltage, all the selected memory cells will be set to a program inhibit state as they will be decoupled from the other memory cells in their respective NAND strings, and their channels will be locally boosted to a voltage effective for inhibiting programming. A VSS biased bitline will discharge the locally boosted channel to VSS, thereby allowing programming of the selected memory cell to occur. A VDD biased bitline will have no effect on the precharged NAND string, thereby maintaining a program inhibited state of that selected memory cell.
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Citations
34 Claims
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1. A method for programming a NAND flash string having a source line select device, memory cells and a string select device connected in series between a bitline and a source line, comprising:
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biasing the bitline to one of a first supply voltage level and a second supply voltage level; asymmetrically precharging groupings of channels corresponding to the memory cells to different voltage levels from the source line for setting a selected memory cell channel to a program inhibit state independent of background data stored in unselected memory cells; and programming the selected memory cell only when the bitline is biased to the second supply voltage level, the selected memory cell remaining in the program inhibit state when the bitline is biased to the first supply voltage level. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
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20. A method for programming a NAND flash string having a source line select device, memory cells and a string select device connected in series between a bitline and a source line, comprising:
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biasing the bitline to one of a first supply voltage level and a second supply voltage level; precharging groupings of channels corresponding to the memory cells to different voltage levels from the source line for turning off a first memory cell adjacent to a selected memory cell; precharging the selected memory cell channel to a program inhibit state in response to an applied programming voltage; and
,driving the string select device to the first supply voltage level for coupling the bitline to the selected memory cell only when the bitline is biased to the second supply voltage level, the selected memory cell remaining in the program inhibit state when the bitline is biased to the first supply voltage level.
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21. A method for programming a NAND flash string having a source line select device, memory cells and a string select device connected in series between a bitline and a source line comprising:
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driving all wordlines to a first pass voltage for coupling a string precharge voltage provided by the source line to the memory cells, the string precharge voltage being greater than the first pass voltage; continuing driving all the wordlines except a first wordline corresponding to a first memory cell adjacent to a selected memory cell to a second pass voltage greater than the first pass voltage, the first memory cell being positioned between the selected memory cell and the string select device; driving a second wordline corresponding to a second memory cell adjacent to the selected memory cell to a first supply voltage for turning off the second memory cell; driving a third wordline corresponding to the selected memory cell to a programming voltage greater than the second pass voltage; and
,coupling the bitline to the selected memory cell. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28, 29)
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30. A flash memory device comprising:
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a driver for driving a source line select device, memory cells and a string select device connected in series between a bitline and a source line; and a controller for controlling the driver in a programming operation, the controller being configured to drive all wordlines of the memory cells to a first pass voltage for coupling a string precharge voltage provided by the source line to the memory cells, the string precharge voltage being greater than the first pass voltage, continue driving all the wordlines except a first wordline corresponding to a first memory cell adjacent to the a selected memory cell to second pass voltage greater than the first pass voltage, the first memory cell being positioned between the selected memory cell and the string select device, drive a second wordline corresponding to a second memory cell adjacent to the selected memory cell to a first supply voltage for turning off the second memory cell, drive a third wordline corresponding to the selected memory cell to a programming voltage greater than the second pass voltage, and couple the bitline to the selected memory cell. - View Dependent Claims (31, 32, 33, 34)
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Specification