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

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

• forming an alternating stack of insulating layers and spacer material layers over a substrate;
• forming memory openings through the alternating stack;
• forming memory opening fill structures in the memory openings, wherein the memory opening fill structures comprise a respective vertical semiconductor channel and a respective vertical stack of memory elements;
• forming a backside trench through the alternating stack;
• forming a backside trench insulating spacer on a sidewall of the backside trench;
• and
• forming a backside contact via structure in the backside trench over the backside trench insulating spacer, wherein the backside contact via structure comprises:
• a first metallic nitride liner comprising a nitride of a first metal;
• a first metal core fill conductive material portion consisting essentially of a second metal;
• and
• a second metallic nitride liner comprising a nitride of the second metal located between an inner sidewall of the first metallic nitride liner and an outer sidewall of the first metal core fill conductive material portion.