Compaction scheme in NVM
First Claim
1. A method of erasing a nonvolatile memory (NVM) that includes a plurality of NVM cells, each cell having a gate, a source, a drain, and a channel between the source and the drain, the method comprising:
- (a) applying an erase pulse to the NVM cell;
(b) determining whether erasure was effective; and
(c) if erasure is determined to be effective, compacting threshold voltages of NVM cells.
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Abstract
A method of erasing a semiconductor nonvolatile memory (NVM) so as to compact the distribution of cell erased threshold voltages within a restricted range around a target erased threshold voltage. Erase pulses are applied to NVM cells until a determination is made by, for example, sensing total column source current that adequate erasure has been realized. An optional soft program signal may be applied subsequent to each erase pulse in order to impede over-erasure. Once erasure has been verified, the distribution of erased threshold voltages is compacted by sustaining, for a predetermined length of time, the simultaneous application of a gate voltage that is equal to the target erased threshold voltage and a highly positive drain voltage.
87 Citations
27 Claims
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1. A method of erasing a nonvolatile memory (NVM) that includes a plurality of NVM cells, each cell having a gate, a source, a drain, and a channel between the source and the drain, the method comprising:
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(a) applying an erase pulse to the NVM cell;
(b) determining whether erasure was effective; and
(c) if erasure is determined to be effective, compacting threshold voltages of NVM cells. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. An array of nonvolatile memory (NVM) cells arranged in a plurality of columns and plurality of rows, each memory cell comprising:
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body of semiconductor material having a surface;
a drain formed in the body beneath the surface;
a source formed in the body beneath the surface, the drain and the source defining a channel therebetween;
a gate dielectric disposed on the surface over at least a portion of the channel, the gate dielectric comprising an ONO stack;
a gate conductor overlying the gate dielectric;
a first bit line coupled to the drain of each NVM cell in a first column of the plurality of columns;
a second bit line coupled to the drain of each NVM cell in a second column of the plurality of columns;
a first word line coupled to the gate conductor of each memory cell in a first row of the plurality of rows;
a second word line coupled to the gate conductor of each memory cell in a second row of the plurality of rows;
a source line coupled to the source of each NVM cell in the first row and to the source of each NVM cell in the second row;
a first voltage source coupled to both the first word line and to the second word line, the first voltage source providing first voltage approximately equal to a target erased threshold voltage; and
a second voltage source coupled to the first bit line, the second voltage source providing a second voltage that is greater than the first voltage
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13. A method of erasing a nonvolatile memory (NVM) cell having a gate, a source, and a drain, the method comprising:
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applying an erase signal to the NVM cell;
applying a signal to the NVM cell so as to impede over-erasure of the cell;
determining whether the NVM cell is effectively erased; and
applying a compaction signal to the NVM cell. - View Dependent Claims (12, 14, 15, 16, 17, 18, 19, 20)
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21. In a semiconductor nonvolatile memory (NVM) that comprises a plurality of NVM cells, each all having a drain, a source, a channel between the gate and the source, a gate dielectric in the form of an ONO stack disposed over at least a portion of the channel, and a gate conductor overlying the gate dielectric, a method of erasing the NVM by successively performing the steps:
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(a) applying an erase pulse to an NVM cell;
(b) soft programming the NVM cell;
(c) determining whether the NVM cell is erased;
(d) repeating steps (a), (b) and (c); and
(e) applying to the gate conductor, for a predetermined length of time, a target erased threshold voltage. - View Dependent Claims (22, 23, 24, 25, 26, 27)
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Specification