Multi-level vertical memory device including inter-level channel connector

1. A semiconductor device, comprising:

a string of transistors stacked along a vertical direction above a substrate of the semiconductor device, the string comprising:

a first substring of transistors having a first channel structure that includes a first channel layer and a first gate dielectric structure that extend along the vertical direction;

a channel connector disposed above the first sub string; and

a second substring of transistors stacked above the channel connector, the second substring having a second channel structure that includes a second channel layer and a second gate dielectric structure that extend along the vertical direction, the channel connector electrically coupling the first channel layer and the second channel layer and being disposed below the second gate dielectric structure, wherein

the channel connector includes a first structure and a second structure,

the first structure includes a first recessed region that is overfilled by the second structure,

the second structure includes a second recessed region, and

the second channel layer extends into the second recessed region and contacts sidewalls of the second recessed region and a surface of the second recessed region is fully covered by a contact that is connected to the second channel layer, the contact electrically coupling the second channel layer and the second structure.

2. The semiconductor device according to claim 1 , wherein the second gate dielectric structure includes a tunnel insulating layer, a charge storage layer, and a blocking insulating layer that are sequentially formed over the second channel layer.

3. The semiconductor device according to claim 1 , wherein

the first structure is in contact with the first channel layer and has a first upper surface with the first recessed region; and

the second structure is in contact with the second channel layer and has a second upper surface with the second recessed region, the second gate dielectric structure being disposed on the second upper surface, the second upper surface being above the first upper surface.

4. The semiconductor device according to claim 1 , wherein

the second recessed region is formed in an upper surface of the channel connector; and

the second gate dielectric structure is disposed on the upper surface.

5. The semiconductor device according to claim 1 , wherein the second structure is in contact with the second channel structure, and the second structure is formed of an epitaxially grown material.

6. The semiconductor device according to claim 1 , wherein the channel connector includes polysilicon.

7. The semiconductor device according to claim 1 , wherein:

the second channel layer and the contact are made of an identical material.

8. The semiconductor device according to claim 1 , wherein

the first structure is separated from the second gate dielectric structure, and

the second structure is disposed adjacent to and below the second gate dielectric structure.

9. The semiconductor device according to claim 8 , wherein the first structure is adjacent to the first channel layer, the second structure is adjacent to the second channel layer, and a channel layer of the string includes the first channel layer and the second channel layer electrically coupled by the channel connector.

10. A semiconductor memory device, comprising:

a first deck of memory cells including a plurality of first substrings of memory cells over a substrate of the semiconductor memory device, the first substrings having respective first channel structures that include first channel layers and first gate dielectric structures that extend along a vertical direction above the substrate;

an inter-deck structure including a plurality of channel connectors, the channel connectors being disposed above respective first substrings of the plurality of first sub strings; and

a second deck of memory cells including a plurality of second substrings of memory cells, the second substrings being stacked above respective channel connectors of the plurality of channel connectors and having respective second channel structures that include second channel layers and second gate dielectric structures that extend along the vertical direction, the channel connectors electrically coupling the respective first channel layers and the second channel layers and being disposed below the respective second gate dielectric structures, wherein

the channel connectors include respective first structures and second structures,

the first structures include first recessed regions that are overfilled by the respective second structures,

the second structures include respective second recessed regions, and

the second channel layers extend into the respective second recessed regions and contact sidewalls of the respective second recessed regions and respective surfaces of the second recessed regions are fully covered by contacts that are connected to the respective second channel layers, the contacts electrically coupling the respective second channel layers and the respective second structures.

11. The semiconductor memory device according to claim 10 , wherein the second gate dielectric structures are adjacent to and disposed above the second structures, and are separated from the first structures via the second structures.

12. The semiconductor memory device according to claim 10 , wherein:

the first structures are in contact with the first channel layers and have first upper surfaces with the first recessed regions; and

the second structures are in contact with the second channel layers and have second upper surfaces with the second recessed regions.

13. The semiconductor memory device according to claim 10 , wherein:

the second channel layers and the contacts are made of an identical material.