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

A three-dimensional memory device comprising:

• a vertically alternating stack of electrically conductive layers and insulating layers located over a source semiconductor layer over a substrate;
• an array of memory stack structures that extend through the vertically alternating stack and into an upper portion of the source semiconductor layer, each memory stack structure including a semiconductor channel and a memory film laterally surrounding the semiconductor channel, wherein a lower portion of each memory stack structure has a bulging portion that has a greater lateral dimension than an overlying portion of the respective memory stack structure that adjoins a top end of the bulging portion;
• and
• at least one source strap structure contacting a respective subset of the semiconductor channels of the memory stack structures at a level of the bulging portion of each memory stack structure, wherein the at least one source strap structure comprises source strap rails, the source strap rails laterally extend along a first horizontal direction, and the memory film of each memory stack structure includes a lateral opening through which a respective source strap rail extends to provide physical contact between a respective semiconductor channel and the respective source strap rail;
• and
• an insulating wall structure that vertically extends through the vertically alternating stack and laterally extends along a second horizontal direction that is different from the first horizontal direction and straddles each of the source strap rails.