NAND flash memory string apparatus and methods of operation thereof

1. An NAND string comprising:

at least one memory cell;

a first selection transistor

a second selection transistor coupled between the at least one memory cell and the first selection transistor, wherein the second selection transistor has a pre-defined threshold voltage,

wherein the second selection transistor comprises:

a channel region between a source region and a drain region;

a gate region overlapping the channel region; and

a gate dielectric layer separating the gate region from the channel region, wherein the gate region comprises first to third layers, each of the first to third layers having conductive material, wherein the first layer is disposed in the second layer, contains an interpoly dielectric layer at a sidewall of the first layer and is in direct contact with the third layer on a top surface of the first layer, the second layer is in direct contact with the third layer.

2. The NAND string of claim 1 , further comprising:

a third selection transistor; and

a fourth selection transistor coupled between the third selection transistor and the at least one memory cell.

3. The NAND string of claim 2 , wherein a source of the first selection transistor is coupled to a source terminal of the NAND string.

4. The NAND string of claim 2 , wherein a drain of the third selection transistor is coupled to a drain terminal of the NAND string.

5. The NAND string of claim 2 , wherein the fourth selection transistor comprises:

a second channel region between a second source region and a second drain region;

a second gate region overlapping the second channel region; and

a second gate dielectric layer separating the second gate region from the second channel region, wherein the second gate region comprises first to third layers, each of the first to third layers of the second gate region having conductive material, wherein

the first layer of the second gate region is disposed in the second layer of the second gate region, contains an interpoly dielectric layer at a sidewall of the first layer of the second gate region and is in direct contact with the third layer of the second gate region on a top surface of the first layer of the second gate region, the second layer of the second gate region is in direct contact with the third layer of the second gate region.

6. A memory system comprising:

a controller unit;

an Input/Output unit interfaced with the controller unit to communicate data externally; and

a memory unit interfaced with the controller unit, the memory unit including a NAND string comprising:

at least one memory element;

a first selection transistor; and

a second selection transistor coupled between the at least one memory element and the first selection transistor, wherein the second selection transistor has a pre-defined threshold voltage, wherein the second selection transistor comprises:

a channel region between a source region and a drain region;

a gate region overlapping the channel region; and

a gate dielectric layer separating the gate region from the channel region,

wherein the gate region comprises first to third layers, each of the first to third layers having conductive material,

wherein the first layer is disposed in the second layer, contains an interpoly dielectric layer at a sidewall of the first layer and is in direct contact with the third layer on a top surface of the first layer, and the second layer is in direct contact with the third layer.

7. The memory system of claim 6 , wherein the NAND string further comprises:

a third selection transistor; and

a fourth selection transistor coupled between the third selection transistor and the at least one memory element.

8. The memory system of claim 7 , wherein a source of the first selection transistor is coupled to a source terminal of the NAND string and a drain of the third selection transistor is coupled to a drain terminal of the NAND string.

9. The memory system of claim 7 , wherein the fourth selection transistor comprises:

a second channel region between a second source region and a second drain region;

a second gate region overlapping the second channel region; and

a second gate dielectric layer separating the second gate region from the second channel region, wherein the second gate region comprises first to third layers, each of the first to third layers of the second gate region having conductive material,

wherein the first layer of the second gate region disposed in the second layer of the second gate region contains an interpoly dielectric layer at a sidewall of the first layer of the second gate region and is in direct contact with the third layer of the second gate region on a top surface of the first layer of the second gate region, the second layer of the second gate region is in direct contact with the third layer of the second gate region.

10. A memory device comprising:

a plurality of serially connected memory cells in a NAND string;

a first selection transistor on a source side of the NAND string;

a second selection transistor interposed between the plurality of serially connected memory cells and the first selection transistor on the source side of the NAND string; and

a third selection transistor on a drain side of the NAND string, wherein the second transistor has a pre-defined threshold voltage, wherein the second selection transistor comprises:

a channel region between a source region and a drain region;

a gate region overlapping the channel region; and

a gate dielectric layer separating the gate region from the channel region,

wherein the gate region comprises first to third layers, each of the first to third layers having conductive material,

wherein the first layer is disposed in the second layer, contains an interpoly dielectric layer at a sidewall of the first layer and is in direct contact with the third layer on a top surface of the first layer, the second layer is in direct contact with the third layer.