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:
an internal voltage generator configured to provide an erase voltage; and
a plurality of memory planes coupled to the internal voltage generator, each memory plane comprising:
a plurality of memory blocks, each memory block having a plurality of NAND cell strings and a plurality of wordlines, each wordline coupled to corresponding gates of the NAND cell strings;
a plurality of well sectors, each well sector having one or more memory blocks, the erase voltage being applied to a selected well sector during a block erase operation;
a plurality of bitlines, each bitline coupled to a corresponding NAND cell string at each memory block; and
a page buffer coupled to the plurality of memory blocks via the plurality of bitlines.
2. The NAND flash memory device of claim 1 further comprising a selector configured to receive a block address and to selectively provide the erase voltage to the corresponding memory block according to the block address.
3. The NAND flash memory device of claim 2 , wherein each of the plurality of well sectors comprises a n-type isolation well and a p-type well.
4. The NAND flash memory device of claim 3 , wherein the p-type well is formed within the n-type isolation well.
5. The NAND flash memory device of claim 3 , wherein the erase voltage is applied to the selected well sector, and a ground voltage is applied to an unselected well sector.
6. The NAND flash memory device of claim 5 , wherein the ground voltage is applied to the wordlines of a selected memory block of the selected well sector while the wordlines of unselected memory blocks of the selected well sector are self-boosted to a voltage level less than the erase voltage.
7. The NAND flash memory device of claim 1 , wherein each bitline is divided into a plurality of bitline segments by an isolation transistor located between the well sectors.
8. The NAND Flash memory of claim 7 , wherein the isolation transistor has a gate terminal biased to a voltage greater than a supply voltage VDD during a program operation, a read operation, and the block erase operation, and
wherein the isolation transistor is electrically non-conductive to isolate the bitline segments during the block erase operation.