Three-dimensional memory device including a dummy word line with tapered corner and method of making the same
A memory device includes a first alternating stack of first insulating layers and first electrically conductive layers located over a substrate, a memory opening vertically extending through the first alternating stack and having a tapered sidewall surface at a level of one of the first electrically conductive layers, and a memory opening fill structure located in the memory opening and comprising a vertical stack of memory elements and a vertical semiconductor channel. One of the first electrically conductive layers includes a taper-containing electrically conductive layer that is located at a level of the lateral protrusion of the memory opening and has a contoured sidewall having a tapered sidewall segment that is parallel to the tapered sidewall surface of the lateral protrusion.
1. A method of forming a memory device, comprising:
forming a first alternating stack of first insulating layers and first sacrificial material layers located over a substrate, wherein one of the first sacrificial material layers comprises a composite sacrificial material layer including a primary sacrificial material sublayer including a first sacrificial material and a secondary sacrificial material sublayer including a second sacrificial material that is different from the first sacrificial material;
forming a first-tier memory opening through the first alternating stack;
performing an isotropic recess etch process that isotropically etches the second sacrificial material at a higher average etch rate than the first sacrificial material, wherein a recessed sidewall of the composite sacrificial material layer comprises a tapered recessed surface segment; and
forming a memory opening fill structure within a volume including the first-tier memory opening, wherein the memory opening fill structure comprises a vertical stack of memory elements and a vertical semiconductor channel, and comprises a lateral protrusion having a tapered sidewall surface that is parallel to the tapered recessed surface segment.
2. The method of claim 1 , further comprising replacing the first sacrificial material layers with first electrically conductive layers, wherein one of the first electrically conductive layers comprises a taper-containing electrically conductive layer that is formed in a volume from which the composite sacrificial material layer is removed.
3. The method of claim 2 , wherein the taper-containing electrically conductive layer comprises a dummy word line having a contoured sidewall having a tapered sidewall segment that is parallel to the tapered sidewall surface of the lateral protrusion.
4. The method of claim 1 , wherein the recessed sidewall of the composite sacrificial material layer comprises a cylindrical vertical surface segment that is adjoined to the tapered recessed surface segment.
5. The method of claim 1 , wherein:
the first sacrificial material comprises a first silicon nitride material; and
the second sacrificial material comprises a second silicon nitride material having a higher average etch rate during the isotropic recess etch process than an etch rate of the first sacrificial material during the isotropic recess etch process.
6. The method of claim 1 , further comprising:
forming a second alternating stack of second insulating layers and second sacrificial material layers over the first alternating stack;
forming a second-tier memory opening through the second alternating stack over a volume of the first-tier memory opening;
forming an inter-tier memory opening including volumes of the second-tier memory opening and the first-tier memory opening, wherein the memory opening fill structure is formed within the inter-tier memory opening; and
replacing the first sacrificial material layers and the second sacrificial material layers with first electrically conductive layers and second electrically conductive layers, respectively.