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

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.