Adaptive erase scheme for a memory device

1 . A method of erasing a plurality of memory cells in a memory device, comprising the steps of:

preparing a memory device that includes a plurality of memory blocks, the memory blocks including respective first and second sub-blocks that can be programmed and erased independently of one another, the first and second sub-blocks including an array of memory cells arranged in a plurality of word lines; and

erasing the memory cells of a selected memory block of the plurality of memory blocks in at least one erase loop, the at least one erase loop including

simultaneously applying an erase voltage to a plurality of channels in the selected memory block, applying a first bias voltage to the plurality of word lines of the first sub-block, and applying a second bias voltage to the plurality of word lines of the second sub-block,

wherein the first bias voltage is different than the second bias voltage so that the memory cells of the first and second sub-blocks are erased at different speeds,

wherein the first sub-block is a lower sub-block on a source side of the memory block and the second sub-block is an upper sub-block on a drain side of the memory block,

wherein the at least one erase loop includes a plurality of erase loops, and wherein between erase loops

the first bias voltage that is applied to the word lines of the lower sub-block is adjusted by a first voltage step size, and

the second bias voltage that is applied to the word lines of the upper sub-block is adjusted by a second voltage step size,

wherein the first voltage step size is greater than the second voltage step size.

2 . The method as set forth in claim 1 , wherein the lower sub-block includes a plurality of lower memory holes and wherein the upper sub-block includes a plurality of upper memory holes.

3 . The method as set forth in claim 2 , wherein the at least one erase loop further includes the steps of:

performing an erase-verify operation on only the memory cells of the upper sub-block; and

performing an erase-verify operation on only the memory cells of the lower sub-block.

4 . The method as set forth in claim 3 , wherein in response to the memory cells of the lower sub-block passing the erase-verify operation, the method further includes the steps of, in at least one additional erase loop:

simultaneously applying an erase voltage to a plurality of channels in the selected memory block, applying an erase-inhibit voltage to the plurality of word lines of the lower sub-block, and applying the second bias voltage to the plurality of word lines of the upper sub-block.

5 . The method as set forth in claim 4 , wherein the at least one additional erase loop further including the steps of performing the erase-verify operation on only the memory cells of the upper sub-block; and

skipping the erase-verify operation for the memory cells of the lower sub-block.

6 . A memory device, comprising:

a plurality of memory blocks, the memory blocks including respective first and second sub-blocks that can be programmed and erased independently of one another, and the first and second sub-blocks including an array of memory cells arranged in a plurality of word lines; and

control circuitry, the control circuitry being configured to erase the memory cells of a selected memory block of the plurality of memory blocks in at least one erase loop, in the at least one erase loop, the control circuitry being configured to;

simultaneously apply an erase voltage to a plurality of channels in the selected memory block, apply a first bias voltage to the plurality of word lines of the first sub-block, and apply a second bias voltage to the plurality of word lines of the second sub-block,

wherein the first bias voltage is different than the second bias voltage so that the memory cells of the first and second sub-blocks are erased at different speeds,

wherein the first sub-block is a lower sub-block on a source side of the memory block and the second sub-block is an upper sub-block on a drain side of the memory block,

wherein the at least one erase loop includes a plurality of erase loops, and wherein between erase loops, the control circuitry is configured to

adjust the first bias voltage that is applied to the word lines of the lower sub-block by a first voltage step size, and

adjust the second bias voltage that is applied to the word lines of the upper sub-block by a second voltage step size,

wherein the first voltage step size is greater than the second voltage step size.

7 . The memory device as set forth in claim 6 , wherein the lower sub-block includes a plurality of lower memory holes and wherein the upper sub-block includes a plurality of upper memory holes.

8 . The memory device as set forth in claim 7 , wherein during the at least one erase loop the control circuitry is further configured to:

perform an erase-verify operation on only the memory cells of the upper sub-block; and

perform an erase-verify operation on only the memory cells of the lower sub-block.

9 . The memory device as set forth in claim 8 , wherein in response to the memory cells of the lower sub-block passing the erase-verify operation, in at least one additional erase loop, the control circuitry is further configured to:

simultaneously apply an erase voltage to a plurality of channels in the selected sub-memory block, apply an erase-inhibit voltage to the plurality of word lines of the lower sub-block, and apply the second bias voltage to the plurality of word lines of the upper sub-block.

10 . The memory device as set forth in claim 9 , wherein during the at least one additional erase loop, the control circuitry is further configured to:

perform the erase-verify operation on only the memory cells of the upper sub-block; and

skip the erase-verify operation for the memory cells of the lower sub-block.

11 . An apparatus, comprising:

a plurality of memory blocks, the memory blocks including respective first and second sub-blocks that can be programmed and erased independently of one another, the first and second sub-blocks including an array of memory cells arranged in a plurality of word lines, and the memory blocks each including a plurality of upper memory holes and a plurality of lower memory holes; and

an erasing means for erasing the memory cells of a selected memory block of the plurality of memory blocks in at least one erase loop, in the at least one erase loop, the erasing means being configured to;

simultaneously apply an erase voltage to a plurality of channels in the selected memory block, apply a first bias voltage to the plurality of word lines that are aligned with the plurality of lower memory holes, and apply a second bias voltage to the plurality of word lines that are aligned with the upper memory holes,

wherein the first bias voltage is different than the second bias voltage so that the memory cells of the plurality of word lines that are aligned with the upper memory holes are erased at a different speed than the memory cells of the word lines that are aligned with the lower memory holes,

wherein the at least one erase loop includes a plurality of erase loops, and wherein between erase loops, the erasing means is configured to

adjust the first bias voltage that is applied to the word lines that are aligned with the lower memory holes by a first voltage step size, and

adjust the second bias voltage that is applied to the word lines that are aligned with the upper memory holes by a second voltage step size,

wherein the first voltage step size is greater than the second voltage step size.

12 . The apparatus as set forth in claim 11 , wherein the erasing means is further configured to:

perform an erase-verify operation on only the memory cells of the word lines that are aligned with the upper memory holes; and

perform an erase-verify operation on only the memory cells of the word lines that are aligned with the lower memory holes.