Three-dimensional memory device including composite backside metal fill structures and methods for forming the same

A three dimensional memory device, comprising:

• an alternating stack of insulating layers and electrically conductive layers;
• memory openings vertically extending through the alternating stack;
• memory opening fill structures located in the memory openings and comprising a respective vertical semiconductor channel and a respective vertical stack of memory elements;
• and
• a backside trench fill structure laterally extending along a first horizontal direction, wherein:
• the backside trench fill structure comprises a backside trench insulating spacer and a backside contact via structure that is laterally surrounded by the backside trench insulating spacer;
• and
• the backside contact via structure comprises a first metallic nitride liner in contact with the backside trench insulating spacer, a tapered metallic nitride liner having a tapered vertical profile such that a lateral thickness of the tapered metallic nitride liner decreases with a vertical distance from a horizontal plane including a topmost surface of the backside contact via structure and has a vertical extent that is less than a vertical extent of the first metallic nitride liner, and at least one core fill conductive material portion that is laterally surrounded by the first metallic nitride liner.