THREE-DIMENSIONAL MEMORY DEVICE CONTAINING TOP SOURCE CONTACT TO DOPED SEMICONDUCTOR SOURCE TIPS AND METHODS FOR FORMING THE SAME
An alternating stack of insulating layers and spacer material layers is formed over a substrate. The spacer material layers are formed as, or are subsequently replaced with, electrically conductive layers. A memory opening is formed through the alternating stack. A memory material layer, a semiconductor source structure, a vertical semiconductor channel, a dielectric core, and a drain region are formed in the memory opening. Dopants in the semiconductor source structure are activated after formation of the drain region. Subsequently, the substrate and a bottom portion of the memory film are removed and a metallic source layer is formed on the semiconductor source structure.
1 . A memory device, comprising:
an alternating stack of insulating layers and electrically conductive layers located over a semiconductor material layer;
a memory opening vertically extending through the alternating stack and through the semiconductor material layer; and
a memory opening fill structure located in the memory opening and comprising a vertical semiconductor channel that vertically extends through each electrically conductive layer in the alternating stack, a semiconductor source structure adjoined to a first end of the vertical semiconductor channel and laterally surrounded by the semiconductor material layer, and a drain region contacting a second end of the vertical semiconductor channel,
wherein the semiconductor source structure comprises a pillar semiconductor source portion having a first width along a horizontal direction and a tubular semiconductor source portion having an inner cavity and an outer cylindrical surface which has a second width along the horizontal direction, the second width being greater than the first width.
2 . The memory device of claim 1 , wherein the semiconductor source structure further comprises a conical semiconductor source portion connecting the pillar semiconductor source portion and the tubular semiconductor source portion.
3 . The memory device of claim 2 , wherein:
the pillar semiconductor source portion comprises a cylindrical sidewall surface; and
the conical semiconductor source portion comprises a lateral conical surface that connects a top periphery of the cylindrical sidewall surface of the pillar semiconductor source portion to a bottom periphery of the outer cylindrical surface of the tubular semiconductor source portion.
4 . The memory device of claim 2 , wherein the vertical semiconductor channel comprises:
a first tubular channel portion having a first cylindrical outer sidewall that has the second width along the horizontal direction; and
a second tubular channel portion having a second cylindrical outer sidewall that has a third width along the horizontal direction, the third width being greater than the second width.
5 . The memory device of claim 4 , wherein the vertical semiconductor channel further comprises a connecting tapered tubular channel portion that connects the first tubular channel portion and the second tubular channel portion, and having a variable lateral dimension that increases with a vertical distance from a horizontal plane including a bottom surface of the semiconductor material layer.
6 . The memory device of claim 4 , wherein a bottom periphery of the first cylindrical outer sidewall of the first tubular channel portion coincides with a top periphery of the outer cylindrical surface of the tubular semiconductor source portion.
7 . The memory device of claim 4 , wherein the memory opening fill structure further comprises a dielectric core that is laterally surrounded by the vertical semiconductor channel.
8 . The memory device of claim 7 , wherein the dielectric core comprises:
a first cylindrical dielectric core portion that is laterally surrounded by the first tubular channel portion and the tubular semiconductor source portion and has a first lateral dimension; and
a second cylindrical dielectric core portion that is laterally surrounded by the second tubular channel portion and has a second lateral dimension that is greater than the first lateral dimension.
9 . The memory device of claim 7 , wherein:
the semiconductor source structure comprises atoms of an electrical dopant therein; and
the dielectric core has a homogeneous material composition throughout and is free of the electrical dopant.
10 . The memory device of claim 1 , wherein the memory opening fill structure comprises a memory film that laterally surrounds the vertical semiconductor channel and the semiconductor source structure.
11 . The memory device of claim 1 , further comprising an annular dielectric semiconductor oxide spacer comprising an outer cylindrical sidewall that contacts an upper cylindrical sidewall surface segment of an opening in the semiconductor material layer.
12 . The memory device of claim 11 , further comprising a metallic source layer that comprises a horizontally-extending portion that contacts a bottom surface of the semiconductor material layer and an upward-protruding tubular portion that contacts a lower cylindrical surface segment of the opening in the semiconductor material layer.
13 . The memory device of claim 12 , wherein:
the metallic source layer further comprises a downward-protruding portion that underlies the semiconductor source structure;
the upward-protruding tubular portion of the metallic source layer contacts an entirety of a cylindrical sidewall surface of the pillar semiconductor source portion.
14 . The memory device of claim 1 , wherein:
the vertical semiconductor channel comprises p-type polysilicon;
the semiconductor material layer comprises intrinsic or lightly doped polysilicon layer; and
the semiconductor source structure comprises heavily doped n-type polysilicon having a higher doping concentration than the semiconductor material layer.
15 . A method of forming a device structure, comprising:
forming a semiconductor material layer and an alternating stack of insulating layers and spacer material layers over a semiconductor substrate, wherein the spacer material layers are formed as, or are subsequently replaced with, electrically conductive layers;
forming a memory opening vertically extending through the alternating stack, the semiconductor material layer, and an upper portion of the semiconductor substrate;
oxidizing surface portions of the semiconductor material layer and the semiconductor substrate around a bottom portion of the memory opening to form a semiconductor oxide spacer structure including a first cylindrical portion having a first thickness at a level of the semiconductor substrate and a second cylindrical portion having a second thickness at a level of the semiconductor material layer, wherein the first thickness is greater than the second thickness;
forming a memory opening fill structure comprising a memory film, a semiconductor source structure, a vertical semiconductor channel, a dielectric core, and a drain region in a remaining volume of the memory opening;
removing the semiconductor substrate, a bottom portion of the semiconductor oxide spacer structure, and a bottom portion of the memory film; and
forming a metallic source layer on the semiconductor source structure.
16 . The method of claim 15 , wherein:
the semiconductor source structure is formed after formation of the memory film within a first void of the memory opening that is laterally surrounded by the first cylindrical portion of the semiconductor oxide spacer structure; and
a top surface of the semiconductor source structure is formed at or below a horizontal plane including a bottom surface of the semiconductor material layer.
17 . The method of claim 15 , wherein:
the semiconductor source structure is formed by conformally depositing a heavily-doped semiconductor material layer over the memory film and by isotropically recessing the doped semiconductor material layer; and
the vertical semiconductor channel is formed by depositing a semiconductor channel material layer 60 L that is intrinsic or includes dopants at an atomic concentration less than 3×10 16 /cm 3 .
18 . The method of claim 15 , further comprising crystallizing the semiconductor source structure prior to removing the semiconductor substrate.
19 . The method of claim 15 , wherein:
a top portion of the semiconductor oxide spacer structure that remains after removal of the bottom portion of the semiconductor oxide spacer structure comprises an annular dielectric semiconductor oxide spacer;
an outer cylindrical sidewall of the annular dielectric semiconductor oxide spacer contacts an upper cylindrical surface segment of an opening in the semiconductor material layer; and
an inner cylindrical sidewall of the annular dielectric semiconductor oxide spacer contacts a cylindrical surface segment of the memory film.
20 . The method of claim 15 , wherein:
the metallic source layer is formed directly on a lower cylindrical surface segment of an opening in the semiconductor material layer;
the vertical semiconductor channel comprises p-type polysilicon;
the semiconductor material layer comprises intrinsic or lightly doped polysilicon layer; and
the semiconductor source structure comprises heavily doped n-type polysilicon having a higher doping concentration than the semiconductor material layer.