Reduction of leakage current and program disturbs in flash memory devices
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 negative substrate bias voltage to reduce or eliminate leakage current that might otherwise conduct through the target memory cells. The negative substrate bias voltage also reduces the occurrence of program disturbs in cells adjacent to target cells by extending the depletion region deeper below the bit line that corresponds to the drain of the target device. The negative substrate 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 error 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 formed in a semiconductor substrate and corresponding to a bitline in the array, and a selectable drain/source formed in the semiconductor substrate and corresponding to a bitline in the array, the method comprising:
selecting a target cell in the array for programming;
applying a positive programming voltage to the wordline corresponding to the target cell;
applying a positive drain bias voltage to a first selectable bitline corresponding to the drain of the target cell;
grounding a second selectable bitline corresponding to the source of the target cell;
adjusting a negative substrate bias voltage in response to a write cycle status of the target cell and in response to the age of the nonvolatile memory device; and
controlling bitline leakage current with the negative substrate bias voltage at the semiconductor substrate of the target cell.
2. A method according to claim 1 , further comprising defining the negative substrate bias voltage in response to a bitline leakage tolerance.
3. A method according to claim 1 , further comprising defining the negative substrate bias voltage according to a program threshold voltage of the target cell.
4. A method according to claim 1 , further comprising applying the negative substrate bias voltage to the semiconductor substrate of the target cell.
5. A method according to claim 4 , wherein:
the bitlines of each cell have N-type conductivity;
the semiconductor substrate has P-type conductivity; and
applying the negative substrate bias voltage to the semiconductor substrate reduces bitline leakage current from the second selectable bitline to the semiconductor substrate.
6. A method according to claim 1 , further comprising reducing, with the negative substrate bias voltage, program disturb effects in an adjacent cell that shares the wordline corresponding to the target cell.
7. A method according to claim 6 , wherein reducing program disturb effects comprises extending a depletion region within the semiconductor substrate to reduce electron diffusion below the first selectable bitline.
8. A method according to claim 1 , wherein:
the programming voltage is between 7.0 volts and 10.0 volts;
the drain bias voltage is between 2.5 volts and 5.0 volts; and
the negative substrate bias voltage is between −0.5 volt and −5.0 volts.