Flash memory programming and verification with reduced leakage current
A flash memory system configured in accordance with an example embodiment of the invention employs a virtual ground array architecture. During programming operations, target memory cells are biased with a positive source bias voltage to reduce or eliminate leakage current that might otherwise conduct through the target memory cells. A positive source bias voltage may also be applied to target memory cells during verification operations (program verify, soft program verify, erase verify) to reduce or eliminate leakage current that might otherwise introduce errors in the verification operations.
1. A method of programming a nonvolatile memory device having an array of cells arranged in a virtual ground architecture, each cell including a gate corresponding to a wordline in the array, a selectable source/drain corresponding to a bitline in the array, and a selectable drain/source corresponding to a bitline in the array, the method comprising:
selecting a target cell in the array for programming;
applying a programming voltage to the wordline corresponding to the target cell;
applying a drain bias voltage to a first selectable bitline corresponding to the drain of the target cell; and
establishing a positive source bias voltage at a second selectable bitline corresponding to the source of the target cell, the positive source bias voltage being adjusted in response to a write cycle status of the array.
2. A method according to claim 1 , wherein establishing the positive source bias voltage comprises applying the positive source bias voltage to the second selectable bitline.
3. A method according to claim 1 , wherein establishing the positive source bias voltage comprises passively generating the positive source bias voltage in response to current conducting through the second selectable bitline.
4. A method according to claim 3 , wherein passively generating the positive source bias voltage comprises conducting the current through a passive resistance coupled between the second selectable bitline and ground.
5. A method according to claim 1 , wherein:
the programming voltage is between 8 volts and 11 volts;
the drain bias voltage is between 3.5 volts and 5.0 volts; and
the positive source bias voltage is between 0.3 volt and 1.0 volt.
6. A method according to claim 1 , further comprising defining the drain bias voltage and the positive source bias voltage according to a program threshold voltage of the target cell.
7. A method according to claim 1 , wherein:
the bitlines of each cell are formed in a semiconductor substrate; and
the method further comprises grounding the semiconductor substrate.
8. A method according to claim 7 , wherein:
the bitlines of each cell have N-type conductivity;
the semiconductor substrate has P-type conductivity; and
establishing the positive source bias voltage at the second selectable bitline reduces bitline leakage current from the second selectable bitline to the first selectable bitline.
9. A method of programming a nonvolatile memory device having an array of cells arranged in a virtual ground architecture, each cell including a gate corresponding to a wordline in the array, a selectable source/drain corresponding to a bitline in the array, and a selectable drain/source corresponding to a bitline in the array, the method comprising:
selecting a target cell in the array for programming;
applying a programming voltage to the wordline corresponding to the target cell;
applying a drain bias voltage to a first selectable bitline corresponding to the drain of the target cell;
adjusting a positive source bias voltage in response to a write cycle status of the array; and
controlling bitline leakage current with the positive source bias voltage at a second selectable bitline corresponding to the source of the target cell.
10. A method according to claim 9 , further comprising defining the positive source bias voltage in response to a bitline leakage tolerance.
11. A method according to claim 9 , further comprising adjusting the drain bias voltage in response to the positive source bias voltage.
12. A method of programming a nonvolatile memory device having an array of cells arranged in a virtual ground architecture, each cell including a gate corresponding to a wordline in the array, a selectable source/drain corresponding to a bitline in the array, and a selectable drain/source corresponding to a bitline in the array, the method comprising:
selecting a target cell in the array for programming;
applying a programming voltage to the wordline corresponding to the target cell;
applying a drain bias voltage to a first selectable bitline corresponding to the drain of the target cell; and
establishing a positive source bias voltage at a second selectable bitline corresponding to the source of the target cell, the positive source bias voltage being adjusted in response to the age of the nonvolatile memory device.