Three-dimensional memory device containing structurally reinforced pedestal channel portions and method of making the same

1. A method of forming a three-dimensional memory device, comprising:

forming a source select level silicon nitride layer including a first silicon nitride material over a substrate;

forming an alternating stack of insulating layers and sacrificial silicon nitride layers including a second silicon nitride material over the source select level silicon nitride layer;

forming a memory opening through the alternating stack and the source select level silicon nitride layer;

laterally recessing a sidewall of the source select level silicon nitride layer farther than sidewalls of the sacrificial silicon nitride layers employing an isotropic etch process that etches the first silicon nitride material at a greater etch rate than the second silicon nitride material;

forming a pedestal channel portion at a bottom region of the memory opening, wherein the pedestal channel portion contacts a remaining portion of the source select level silicon nitride layer;

forming a memory stack structure on the pedestal channel portion in the memory opening, wherein the memory stack structure comprises a memory film and a vertical semiconductor channel located inside the memory film;

forming a tubular dielectric spacer by converting a cylindrical surface portion of the pedestal channel portion into a semiconductor oxide portion by an oxidation process; and

replacing the source select level silicon nitride layer and the sacrificial silicon nitride layers with a source select level electrically conductive layer and word line electrically conductive layers, respectively.

2. The method of claim 1 , further comprising:

forming a backside trench through the alternating stack and the source select level sacrificial material layer; and

forming backside recesses by etching the source select level silicon nitride layer and the sacrificial silicon nitride layers selective to the pedestal channel portion and the memory film,

wherein the tubular dielectric spacer is formed after formation of the backside recesses and prior to formation of the source select level electrically conductive layer and the word line electrically conductive layers.

3. The method of claim 1 , wherein:

the source select level silicon nitride layer is recessed further from a central axis of the memory opening than the sacrificial silicon nitride layers to form a lateral recess extending laterally from the memory opening to the source select level silicon nitride layer;

forming the pedestal channel portion comprises growing the pedestal channel portion in the memory opening such that an initial laterally protruding annular portion of the pedestal channel portion is grown in the lateral recess during the step of growing the pedestal channel portion and such that the initial laterally protruding annular portion contacts the remaining portion of the source select level silicon nitride layer in the lateral recess; and

the pedestal channel portion comprises a laterally protruding annular portion that laterally protrudes farther outward than sidewalls of the insulating layers at a periphery of the memory opening after formation of the tubular dielectric spacer.

4. The method of claim 1 , wherein:

the second silicon nitride material differs in at least one of structure or composition from the first silicon nitride material;

the first silicon nitride material has a greater etch rate in hydrofluoric acid than the second silicon nitride material; and

the isotropic etch process is a hydrofluoric acid wet etch process.

5. The method of claim 4 , wherein the sacrificial silicon nitride layers are formed by a deposition process selected from a plasma enhanced chemical vapor deposition process and a low pressure chemical vapor deposition process.

6. The method of claim 4 , wherein:

the source select level silicon nitride layer is formed by a first plasma enhanced chemical vapor deposition process; and

the sacrificial silicon nitride layers are formed by a deposition process selected from a low pressure chemical vapor deposition process and a second plasma enhanced chemical vapor deposition process that provides a higher deposition rate than the first plasma enhanced chemical vapor deposition process.

7. The method of claim 1 , wherein a ratio of an etch rate of the first silicon nitride material in the isotropic etch process to an etch rate of the second silicon nitride material in the isotropic etch process is greater than 1.5.

8. The method of claim 1 , wherein the source select level silicon nitride layer is recessed further from a central axis of the memory opening than the sacrificial silicon nitride layers to form a lateral recess extending laterally from the memory opening to the source select level silicon nitride layer.

9. The method of claim 1 , wherein:

the three-dimensional memory device comprises a monolithic three-dimensional NAND memory device;

the word line electrically conductive layers comprise, or are 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 word line electrically conductive layers comprise a plurality of control gate electrodes having a strip shape extending substantially parallel to the top surface of the substrate, the plurality of 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; and

the array of monolithic three-dimensional NAND strings comprises:

a plurality of semiconductor channels, wherein at least one end portion of each of the plurality of semiconductor channels extends substantially perpendicular to a top surface of the substrate, and

a plurality of charge storage elements, each charge storage element located adjacent to a respective one of the plurality of semiconductor channels.

10. A method of forming a three-dimensional memory device, comprising:

forming a source select level silicon nitride layer including a first silicon nitride material over a substrate;

forming an alternating stack of insulating layers and sacrificial silicon nitride layers including a second silicon nitride material over the source select level silicon nitride layer;

forming a memory opening through the alternating stack and the source select level silicon nitride layer;

laterally recessing a sidewall of the source select level silicon nitride layer farther than sidewalls of the sacrificial silicon nitride layers employing an isotropic etch process that etches the first silicon nitride material at a greater etch rate than the second silicon nitride material;

forming a pedestal channel portion at a bottom region of the memory opening, wherein the pedestal channel portion contacts a remaining portion of the source select level silicon nitride layer;

forming a memory stack structure on the pedestal channel portion in the memory opening, wherein the memory stack structure comprises a memory film and a vertical semiconductor channel located inside the memory film; and

replacing the source select level silicon nitride layer and the sacrificial silicon nitride layers with a source select level electrically conductive layer and word line electrically conductive layers, respectively;

wherein:

the second silicon nitride material differs in at least one of structure or composition from the first silicon nitride material;

the first silicon nitride material has a greater etch rate in hydrofluoric acid than the second silicon nitride material;

the isotropic etch process is a hydrofluoric acid wet etch process;

the source select level silicon nitride layer is formed by a first plasma enhanced chemical vapor deposition process; and

the sacrificial silicon nitride layers are formed by a deposition process selected from a low pressure chemical vapor deposition process and a second plasma enhanced chemical vapor deposition process that provides a higher deposition rate than the first plasma enhanced chemical vapor deposition process.

11. A method of forming a three-dimensional memory device, comprising:

forming a source select level silicon nitride layer including a first silicon nitride material over a substrate;

forming an alternating stack of insulating layers and sacrificial silicon nitride layers including a second silicon nitride material over the source select level silicon nitride layer;

forming a memory opening through the alternating stack and the source select level silicon nitride layer;

laterally recessing a sidewall of the source select level silicon nitride layer farther than sidewalls of the sacrificial silicon nitride layers employing an isotropic etch process that etches the first silicon nitride material at a greater etch rate than the second silicon nitride material;

forming a pedestal channel portion at a bottom region of the memory opening, wherein the pedestal channel portion contacts a remaining portion of the source select level silicon nitride layer;

forming a memory stack structure on the pedestal channel portion in the memory opening, wherein the memory stack structure comprises a memory film and a vertical semiconductor channel located inside the memory film; and

replacing the source select level silicon nitride layer and the sacrificial silicon nitride layers with a source select level electrically conductive layer and word line electrically conductive layers, respectively;

wherein a ratio of an etch rate of the first silicon nitride material in the isotropic etch process to an etch rate of the second silicon nitride material in the isotropic etch process is greater than 1.5.

12. A method of forming a three-dimensional memory device, comprising:

forming a source select level silicon nitride layer including a first silicon nitride material over a substrate;

forming an alternating stack of insulating layers and sacrificial silicon nitride layers including a second silicon nitride material over the source select level silicon nitride layer;

forming a memory opening through the alternating stack and the source select level silicon nitride layer;

laterally recessing a sidewall of the source select level silicon nitride layer farther than sidewalls of the sacrificial silicon nitride layers employing an isotropic etch process that etches the first silicon nitride material at a greater etch rate than the second silicon nitride material;

forming a pedestal channel portion at a bottom region of the memory opening, wherein the pedestal channel portion contacts a remaining portion of the source select level silicon nitride layer;

forming a memory stack structure on the pedestal channel portion in the memory opening, wherein the memory stack structure comprises a memory film and a vertical semiconductor channel located inside the memory film; and

replacing the source select level silicon nitride layer and the sacrificial silicon nitride layers with a source select level electrically conductive layer and word line electrically conductive layers, respectively;

wherein:

the three-dimensional memory device comprises a monolithic three-dimensional NAND memory device;

the word line electrically conductive layers comprise, or are 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 word line electrically conductive layers comprise a plurality of control gate electrodes having a strip shape extending substantially parallel to the top surface of the substrate, the plurality of 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; and

the array of monolithic three-dimensional NAND strings comprises:

a plurality of semiconductor channels, wherein at least one end portion of each of the plurality of semiconductor channels extends substantially perpendicular to a top surface of the substrate, and

a plurality of charge storage elements, each charge storage element located adjacent to a respective one of the plurality of semiconductor channels.