THREE-DIMENSIONAL MEMORY DEVICE INCLUDING DISCRETE CHARGE STORAGE ELEMENTS AND METHODS OF MAKING THEREOF
A method of forming a memory device includes forming an insulating layer and a composite sacrificial material layer having a vertical compositional change that is stepwise or gradual such that a bottommost portion and a topmost portion of the composite sacrificial material layer a different etch rate in an isotropic etchant than the middle portion, forming a memory opening, laterally recessing the composite sacrificial material layers selective to the insulating layers around the memory opening by introducing the isotropic etchant into the memory opening to form lateral recesses in the composite sacrificial material layers, forming a memory opening fill structure within the memory opening, where the memory opening fill structure includes a vertical stack of memory elements that are formed in the lateral recesses, a dielectric material liner, and a vertical semiconductor channel, and replacing the composite sacrificial material layers with electrically conductive layers.
1 . A method of forming a memory device, comprising:
forming a vertical repetition of a unit layer stack, wherein the unit layer stack comprises, from bottom to top, an insulating layer and a composite sacrificial material layer having a vertical compositional change that is stepwise or gradual such that a bottommost portion and a topmost portion of the composite sacrificial material layer have a first etch rate in an isotropic etchant, and a middle portion of the composite sacrificial material layer has a second etch rate in the isotropic etchant that is different than the first etch rate;
forming a memory opening having a straight sidewall through the vertical repetition;
laterally recessing the composite sacrificial material layers selective to the insulating layers around the memory opening by introducing the isotropic etchant into the memory opening to form lateral recesses in the composite sacrificial material layers;
forming a memory opening fill structure within the memory opening, wherein the memory opening fill structure comprises a vertical stack of memory elements that are formed in the lateral recesses, a dielectric material liner, and a vertical semiconductor channel; and
replacing the composite sacrificial material layers with electrically conductive layers.
2 . The method of claim 1 , wherein the first etch rate is greater than the second etch rate.
3 . The method of claim 2 , wherein the lateral recesses have a straight vertical outer sidewall, and the memory elements have a straight outer vertical sidewall facing the electrically conductive layers.
4 . The method of claim 3 , wherein the vertical stack of memory elements comprises a vertical stack of discrete memory elements.
5 . The method of claim 4 , wherein the discrete memory elements have a straight inner sidewall facing the vertical semiconductor channel.
6 . The method of claim 5 , wherein discrete memory elements comprise discrete silicon nitride charge storage elements.
7 . The method of claim 3 , further comprising a vertically extending connecting portion which vertically connects the memory elements to each other.
8 . The method of claim 7 , wherein memory elements comprise silicon nitride charge storage elements and the vertically extending connecting portion comprises a vertically extending silicon nitride layer.
9 . The method of claim 3 , wherein the electrically conductive layers have a straight inner sidewall facing the straight outer vertical sidewall of the memory elements.
10 . The method of claim 9 , wherein the memory opening fill structure further comprises a blocking dielectric layer in contact with an entirety of the outer sidewalls of the vertical stack of memory elements.
11 . The method of claim 10 , wherein the blocking dielectric layer has straight outer vertical sidewalls located in the lateral recesses and facing the straight inner sidewalls of the electrically conductive layers.
12 . The memory device of claim 1 , wherein:
the memory elements comprise charge storage elements;
the dielectric material liner comprises a tunneling dielectric layer; and
the memory device comprises a vertical NAND string.
13 . The method of claim 1 , wherein:
the middle portion of each of the composite sacrificial material layers comprises a first silicon nitride material; and
the topmost portion and the bottommost portion of each of the composite sacrificial material layers comprises a second silicon nitride material having at least one of a different porosity or different silicon-to-nitrogen ratio that the first silicon nitride material of the middle portion.
14 . The method of claim 13 , wherein each of the composite sacrificial material layers has a vertical compositional profile that changes stepwise along a vertical direction.
15 . The method of claim 14 , wherein the bottommost portion comprises a first silicon nitride layer, the middle portion comprises a second silicon nitride layer, and the topmost portion comprises a third silicon nitride layer.
16 . The method of claim 13 , wherein each of the composite sacrificial material layers comprises a respective compositionally-graded silicon nitride layer in which a porosity or silicon-to-nitrogen ratio gradually changes from a bottommost surface thereof to the middle portion thereof, and from the middle portion thereof to a topmost surface thereof.
17 . The method of claim 13 , wherein the isotropic etchant comprises a wet etch solution comprising hot phosphoric acid.
18 . The method of claim 13 , wherein the isotropic etchant comprises a wet etch solution including a mixture of hydrofluoric acid and ethylene glycol.
19 . The method of claim 13 , wherein the isotropic etchant comprises a wet etch solution including mixture of hydrofluoric acid and nitric acid.
20 . The method of claim 1 , wherein the memory opening fill structure further comprises a drain region located in contact with an upper portion of the vertical semiconductor channel.