Three-dimensional memory device containing multi-threshold-voltage drain select gates and method of making the same
A memory device contains a stack of insulating layers and electrically conductive word line layers, at least one first drain select gate electrode located over the stack and extending through a first drain select transistor and a second drain select transistor, at least one second drain select gate electrode located between the first drain select electrode and the stack, and extending through a third drain select transistor and a fourth drain select transistor. The first drain select transistor and the third drain select transistor are located in a first NAND memory string. The second drain select transistor and the fourth drain select transistor are located in a second NAND memory string different from the first NAND memory string. The first drain select transistor has a higher threshold voltage than the second drain select transistor. The third drain select transistor has a lower threshold voltage than the fourth drain select transistor.
1. A three-dimensional memory device comprising:
an alternating stack of insulating layers and electrically conductive layers located over a substrate; and
a plurality of rows of memory stack structures extending through the alternating stack, wherein each of the memory stack structures comprises charge storage elements located at levels of the electrically conductive layers and laterally surrounding a respective vertical semiconductor channel, wherein:
at least one of the electrically conductive layers comprises a first select gate electrode for multiple rows of memory stack structures within the plurality of rows;
at least another of the electrically conductive layers comprises a second select gate electrode for the multiple rows of memory stack structures;
a first row of memory stack structures among the multiple rows of memory stack structures has a higher threshold voltage for the first select gate electrode than a second row of memory stack structures among the multiple rows of memory stack structures has for the first select gate electrode; and
the first row of memory stack structures has a lower threshold voltage for the second select gate electrode than the second row of memory stack structures has for the second select gate electrode.
2. The three-dimensional memory device of claim 1 , wherein:
the first select gate electrode is located above the second select gate electrode;
portions of the vertical semiconductor channels at a level of the first select gate electrode within the first row of memory stack structures have a greater dopant concentration of electrical dopants than portions of the vertical semiconductor channels at the level of the first select gate electrode within the second row of memory stack structures; and
portions of the vertical semiconductor channels at a level of the second select gate electrode within the second row of memory stack structures have a greater dopant concentration of electrical dopants than portions of the vertical semiconductor channels at the level of the second select gate electrode within the first row of memory stack structures.
3. The three-dimensional memory device of claim 2 , wherein:
the electrical dopants in the vertical semiconductor channels are p-type electrical dopants; and
a top end of each of the vertical semiconductor channels adjoins an n-doped drain region.
4. The three-dimensional memory device of claim 1 , wherein:
portions of the vertical semiconductor channels at a level of the first select gate electrode within the first row of memory stack structures have a greater dopant concentration of electrical dopants than portions of the vertical semiconductor channels at the level of the first select gate electrode within the second row of memory stack structures; and
charge storage elements laterally surrounding portions of the vertical semiconductor channels at a level of the second select gate electrode within the second row of memory stack structures have more stored electrical charges per charge storage element than charge storage elements laterally surrounding portions of the vertical semiconductor channels at the level of the second select gate electrode within the first row of memory stack structures.
5. The three-dimensional memory device of claim 1 , further comprising a first insulating material portion and a second insulating material portion vertically extending from a topmost one of the electrically conductive layers to a bottommost one of the electrically conductive layers, wherein each insulating layer in the alternating stack laterally extends from the first insulating material portion to the second insulating material portion.
6. The three-dimensional memory device of claim 5 , wherein each of the at least one of the electrically conductive layers comprising the first select gate electrode and each of the at least another of the electrically conductive layers comprising the second select gate electrode laterally extends from the first insulating material portion to the second insulating material portion.
7. The three-dimensional memory device of claim 5 , further comprising a dielectric isolation structure embedded in an upper portion of the alternating stack,
wherein:
the at least one of the electrically conductive layers overlie the at least another of the electrically conductive layers;
the dielectric isolation structure laterally divides each of the at least one of the electrically conductive layers into the first select gate electrode and an additional first select gate electrode;
each of the first select gate electrode and the additional first select gate electrode has a lateral extent that is less than a lateral distance between the first insulating material portion and the second insulating material portion;
each of the at least another of the electrically conductive layers laterally extends from the first insulating material portion to the second insulating material portion;
a third row of memory stack structures and a fourth row of memory stack structures among the memory stack structures extend through the additional first select gate electrode and the second select gate electrode;
the third row of memory stack structures has a higher threshold voltage for the additional first select gate electrode than the fourth row of memory stack structures has for the additional first select gate electrode; and
the third row of memory stack structures has a lower threshold voltage for the second select gate electrode than the fourth row of memory stack structures has for the second select gate electrode.
8. The three-dimensional memory device of claim 1 , wherein:
a third row of memory stack structures among the multiple rows of memory stack structures has a lower threshold voltage for the first select gate electrode than the second row of memory stack structures has for the first select gate electrode; and
the third row of memory stack structures has a higher threshold voltage for the second select gate electrode than the second row of memory stack structures has for the second select gate electrode.
9. The three-dimensional memory device of claim 1 , wherein:
at least one additional one of the electrically conductive layers comprise a third select gate electrode for the multiple rows of memory stack structures; and
the first row of memory stack structures has a higher threshold voltage for the third select gate electrode than the second row of memory stack structures has for the third select gate electrode.
10. The three-dimensional memory device of claim 1 , further comprising a plurality of bit lines extending over the memory stack structures, wherein:
the first row of memory stack structures is located in a first memory string;
the second row of memory stack structures is located in a second memory string;
a first bit line of the plurality of bit lines is electrically connected by first drain contact via structure to a first memory stack structure in the first memory string and is electrically connected by a second drain contact via structure to a second memory stack structure in the second memory string;
a portion of the electrically conductive layers located under the first and the second select gate electrodes comprise word lines; and
the word lines contact the first memory stack structure and the second memory stack structure without being cut by a trench or a dielectric isolation structure.
11. The three-dimensional memory device of claim 1 , wherein:
the three-dimensional memory device comprises a monolithic three-dimensional NAND memory device;
a subset of the electrically conductive layers located below the drain select gate electrodes comprises, or is electrically connected to, a respective word line of the monolithic three-dimensional NAND memory device;
the substrate comprises a silicon substrate;
the monolithic three-dimensional NAND memory device comprises an array of monolithic three-dimensional NAND strings over the silicon substrate;
at least one memory cell in a first device level of the array of monolithic three-dimensional NAND strings is located over another memory cell in a second device level of the array of monolithic three-dimensional NAND strings;
the silicon substrate contains an integrated circuit comprising a driver circuit for the memory device located thereon;
the control gate electrodes have a strip shape extending substantially parallel to the top surface of the substrate, and the control gate electrodes comprise at least a first control gate electrode located in the first device level and a second control gate electrode located in the second device level.