A THREE-DIMENSIONAL MEMORY DEVICE HAVING A BACKSIDE CONTACT VIA STRUCTURE WITH A LATERALLY BULGING PORTION AT A LEVEL OF SOURCE CONTACT LAYER

A lower source layer, a sacrificial source-level material layer, and an upper source layer are formed over a substrate. The lower source layer includes a recess trench in which a recessed surface of the lower source layer is vertically recessed relative to a topmost surface of the lower source layer. An alternating stack of insulating layers and spacer material layers is subsequently formed. Memory stack structures are formed through the alternating stack. A backside trench is formed through the alternating stack such that a bottom surface of the backside trench is formed within an area of the recess trench in a thickened portion of the sacrificial source-level material layer. The sacrificial source-level material layer is replaced with a source contact layer.