Bulb-shaped memory stack structures for direct source contact in three-dimensional memory device

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

• forming a source semiconductor layer over a substrate;
• forming a line trench through the source conductive layer;
• forming a etch stop semiconductor rail within the line trench;
• forming a laterally alternating stack of dielectric rails and sacrificial semiconductor rails over the source conductive layer and the etch stop semiconductor rail;
• forming a vertically alternating stack of insulating layers and spacer material layers over the laterally alternating stack, wherein the spacer material layers are formed as, or are subsequently replaced with, electrically conductive layers;
• forming memory stack structures through the vertically alternating stack and the laterally alternating stack, each memory stack structure including a semiconductor channel and a memory film laterally surrounding the semiconductor channel;
• forming a backside trench through the vertically alternating stack and the laterally alternating stack by an anisotropic etch process that employs the etch stop semiconductor rail as an etch stop structure;
• forming source cavities by removing the sacrificial semiconductor rails and portions of each memory film adjacent to the sacrificial semiconductor rails;
• and
• forming source strap rails in the source cavities and directly on sidewalls of the semiconductor channels.