NAND flash memory having multiple cell substrates
View Patent ↗A NAND flash memory bank having a plurality of bitlines of a memory array connected to a page buffer, where NAND cell strings connected to the same bitline are formed in at least two well sectors. At least one well sector can be selectively coupled to an erase voltage during an erase operation, such that unselected well sectors are inhibited from receiving the erase voltage. When the area of the well sectors decrease, a corresponding decrease in the capacitance of each well sector results. Accordingly, higher speed erasing of the NAND flash memory cells relative to a single well memory bank is obtained when the charge pump circuit drive capacity remains unchanged. Alternately, a constant erase speed corresponding to a single well memory bank is obtained by matching a well segment having a specific area to a charge pump with reduced drive capacity. A reduced drive capacity charge pump will occupy less semiconductor chip area, thereby reducing cost.
1. A method for selectively providing an erase voltage to a NAND flash device having a first well sector, a second well sector and at least one memory block formed within each of the first and second well sectors, the method comprising:
selecting one of the at least two memory blocks for erasure of NAND cell strings in response to a block address;
selecting the first well sector in response to at least a portion of the block address; and,
applying the erase voltage to the first well sector.
2. The method of claim 1 , wherein selecting one of the at least two memory blocks includes biasing wordlines of the one of the at least two memory blocks to a voltage level for erasing memory cells when the erase voltage is applied to the selected one of the at least two well sectors.
3. The method of claim 1 , wherein selecting the first well sector includes inhibiting application of the erase voltage to the second well sector in response to the at least a portion of the block address.
4. The method of claim 1 , wherein one of the at least two memory blocks is connected to a first bitline segment and the other of the at least two memory blocks is connected to a second bitline segment, the method further comprising decoupling the first bitline segment from the second bitline segment before applying the erase voltage to the first well sector.
5. The method of claim 4 , wherein decoupling includes turning off an isolation device connected between the first bitline segment and the second bitline segment.
6. The method of claim 5 , wherein decoupling further includes applying VSS to a gate terminal of the isolation device to turn off the isolation device.
7. The method of claim 5 , wherein decoupling further includes applying a predetermined bias voltage to a gate terminal of the isolation device to turn off the isolation device, once the first bitline segment reaches the predetermined bias voltage.
8. A method for controlling a NAND flash device having at least two well sectors, each having at least one memory block, comprising:
turning off an isolation device connected between a first bitline segment connected to a first memory block in a first well sector and to a second bitline segment connected to a second memory block in a second well sector in a memory operation.
9. The method of claim 8 , wherein the memory operation is a read operation, and includes precharging the first bitline segment and the second bitline segment.
10. The method of claim 9 , wherein precharging includes turning on the isolation device for connecting the first bitline segment to the second bitline segment.
11. The method of claim 10 , wherein turning on includes selecting an addressed isolation device for turning on in response to a portion of a block address used for selecting a memory block for the read operation.
12. The method of claim 11 , wherein the first bitline segment and the second bitline segment are part of a logical bitline having a page buffer connected at one end of the logical bitline and a precharge circuit connected to another end of the logical bitline, turning on further includes turning on all isolation devices between the addressed isolation device and the page buffer.
13. The method of claim 12 , wherein turning off includes turning off isolation devices between the addressed isolation device and the precharge circuit.
14. The method of claim 8 , wherein the a memory operation
includes an erase operation, further including
selecting the first memory block for erasure in response to a block address;
selecting the first well sector in response to at least a portion of the block address; and, applying the erase voltage to the first well sector.
15. The method of claim 14 , wherein selecting the first well sector includes inhibiting application of the erase voltage to the second well sector in response to the at least a portion of the block address.
16. The method of claim 8 , wherein the first bitline segment and the second bitline segment are part of a logical bitline having a page buffer connected at one end of the logical bitline and a precharge circuit connected to another end of the logical bitline, turning off includes turning off all isolation transistors between the precharge circuit and the page buffer in the erase operation.
17. The method of claim 8 , wherein the first bitline segment and the second bitline segment are part of a logical bitline having a page buffer connected at one end of the logical bitline and a precharge circuit connected to another end of the logical bitline, turning off includes turning off a pair of isolation transistors for isolating a bitline segment from other bitline segments in response to at the at least a portion of the block address in the erase operation.