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 NAND Flash memory comprising:
a first well sector having a first NAND cell string for selectively receiving an erase voltage during an erase operation;
a second well sector having a second NAND cell string for selectively receiving the erase voltage during the erase operation;
a bitline electrically connected to the first NAND cell string and the second NAND cell string; and,
a page buffer electrically connected to the bitline.
2. The NAND Flash memory of claim 1 , wherein the first well sector includes a third NAND cell string electrically connected to a second bitline, and the second well sector includes a fourth NAND cell string electrically connected to the second bitline, the second bitline being electrically connected to the page buffer.
3. The NAND Flash memory of claim 2 , wherein the first NAND cell string and the third NAND cell string are part of one memory block, and the second NAND cell string and the fourth NAND cell string are part of another memory block.
4. The NAND Flash memory of claim 1 , wherein the first well sector includes a third NAND cell string electrically connected to the bitline, and the second well sector includes a fourth NAND cell string electrically connected to the bitline.
5. The NAND Flash memory of claim 4 , wherein the first NAND cell string is part of a first memory block, the third NAND cell string is part of a second memory block, the second NAND cell string is part of a third memory block, and the fourth NAND cell string is part of a fourth memory block.
6. The NAND Flash memory of claim 5 , further including a block decoder for selecting one of the first memory block, the second memory block, the third memory block and the fourth memory block for erasure, in response to a block address.
7. The NAND Flash memory of claim 6 , further including
a charge pump for providing an erase voltage, and
a selector for coupling the erase voltage to one of the first well sector and the second well sector in response to the block address.
8. The NAND Flash memory of claim 1 , wherein the bitline includes a first bitline segment electrically connected to the first NAND cell string and a second bitline segment electrically connected to the second NAND cell string through an isolation device.
9. The NAND Flash memory of claim 8 , wherein the isolation device is located between the first well sector and the second well sector.
10. The NAND Flash memory of claim 8 , wherein the isolation device has a gate terminal biased to a voltage greater than a supply voltage VDD during a program operation, a read operation and the erase operation.
11. The NAND Flash memory of claim 8 , wherein the isolation device is electrically non-conductive in the erase operation for isolating the first bitline segment from the second bitline segment when one of the first NAND cell string and the second NAND cell string is selected for erasure.
12. The NAND Flash memory of claim 8 , wherein the isolation device is turned off in response to a control signal.
13. The NAND Flash memory of claim 8 , wherein the isolation device is turned off in response to a well sector selection signal.
14. The NAND Flash memory of claim 8 , further including a bitline segment decoder for enabling the isolation device in response to a well sector selection signal during a read operation, the bitline segment decoder disabling the isolation device in response to an erase control signal during an erase operation.
15. The NAND Flash memory of claim 14 , wherein the bitline segment decoder includes an isolation device driver for receiving the erase control signal and the well sector selection signal, the isolation device driver providing an isolation drive signal for controlling the isolation device when the well sector selection signal is at an active logic level.
16. The NAND Flash memory of claim 15 , wherein the isolation device driver includes an override circuit for driving the well sector selection signal to the active logic level in response to another well sector selection signal at the active logic level.
17. A NAND Flash memory comprising:
at least two well sectors each including at least one memory block of NAND cell strings, the at least one memory block in each of the at least two well sectors being electrically connected to corresponding bitline segments; and
isolation devices coupled between the bitline segments corresponding to the at least two well sectors.
18. The NAND Flash memory of claim 17 , wherein the isolation devices have gate terminals biased to a predetermined voltage.
19. The NAND Flash memory of claim 18 , wherein the predetermined voltage is greater than a supply voltage VDD.
20. The NAND Flash memory of claim 17 , wherein the isolation devices are turned off during an erase operation.
21. The NAND Flash memory of claim 17 , wherein the isolation devices are selectively turned off during a read operation.
22. The NAND Flash memory of claim 21 , wherein the isolation devices between a selected well sector including a selected memory block and a page buffer are turned on during a read operation.
23. The NAND Flash memory of claim 17 , wherein each of the at least two well sectors includes two memory blocks.
24. The NAND Flash memory of claim 17 , further including a selector for selectively passing an erase voltage to one of the at least two well sectors.
25. The NAND Flash memory of claim 24 , wherein the selector couples the erase voltage to one of the at least two well sectors in response to a portion of a block address, the block address being decoded to select one memory block for erasure.
26. The NAND Flash memory of claim 17 , further including a page buffer electrically connected to the bitline segments corresponding to one of the at least two well sectors.
27. A method for erasing a selected memory block in a NAND Flash device, comprising:
selecting a memory block in a first well sector, the first well sector including at least two memory blocks electrically coupled to at least two memory blocks in a second well sector by a logical bitline;
biasing the memory block formed in the first well sector for erasure;
biasing an unselected memory block formed in the first well sector for inhibiting erasure;
applying an erase voltage to the first well sector; and,
inhibiting application of the erase voltage to the second well sector.
28. The method of claim 27 , wherein the logical bitline includes bitline segments corresponding to the first well sector and the second well sector, the method further including decoupling the bitline segments corresponding to the first well sector and the second well sector from each other before applying the erase voltage to the first well sector.
29. The method of claim 27 , wherein the logical bitline includes bitline segments corresponding to the first well sector and the second well sector, the method further including decoupling the bitline segments corresponding to the first well sector and the second well sector from each other with an isolation device when a bitline voltage of the first well sector is at least a predetermined bias voltage applied to a gate terminal of the isolation device.