Method of making a multilevel memory stack structure using a cavity containing a sacrificial fill material

A monolithic three-dimensional memory device comprising:

• a lower stack structure comprising a first alternating stack including first insulating layers and first electrically conductive layers and located over a substrate;
• an upper stack structure comprising a second alternating stack including second insulating layers and second electrically conductive layers and located over the lower stack structure;
• and
• a plurality of memory stack structures extending through the lower stack structure and the upper stack structure;
• wherein:
• each of the memory stack structures is located in a memory opening and comprises a lower blocking dielectric physically contacting a sidewall of a lower portion of the memory opening and continuously extending from a bottommost first insulating layer in the first alternating stack to an interface between the lower stack structure and the upper stack structure, and an upper blocking dielectric contacting a sidewall of an upper portion of the memory opening and continuously extending from the interface between the lower stack structure and the upper stack structure to a topmost layer in the upper stack structure and not in physical contact with the lower blocking dielectric;
• the sidewall of the lower portion of the memory opening is adjoined to the sidewall of the upper portion of the memory opening by a horizontal bottom surface of the upper stack structure;
• each of the plurality of memory stack structures comprises a memory film including a memory material layer and a tunneling dielectric layer that extends through the first alternating stack and the second alternating stack as a single continuous material layer;
• and
• each memory material layer contacts the horizontal bottom surface of the upper stack structure.