Three-dimensional memory device with double-sided stepped surfaces and method of making thereof

A method of forming a semiconductor structure, comprising:

• forming an alternating stack of insulating layers and spacer material layers over a substrate comprising a semiconductor material layer, the alternating stack comprising a first layer stack of first insulating layers and first spacer material layers and a second layer stack of second insulating layers and second spacer material layers that is more distal from the substrate than the first layer stack is from the substrate, and the first spacer material layers and the second spacer material layers are formed as, or are subsequently replaced with, first electrically conductive layers and second electrically conductive layers, respectively;
• forming distal stepped surfaces by patterning the second layer stack, wherein lateral extents of the second spacer material layers decrease with a vertical distance from the substrate upon formation of the distal stepped surfaces;
• forming memory stack structures through the alternating stack, wherein each of the memory stack structures comprises a vertical semiconductor channel and a memory film;
• forming an opening through the semiconductor material layer;
• forming proximal stepped surfaces by patterning a portion of the first layer stack located within an area of the opening through the semiconductor material layer employing a plurality of masked anisotropic etch processes that etches unmasked regions of the portion of the first layer stack along a direction that points from the substrate toward the alternating stack;
• and
• forming an upright-stepped dielectric material portion on the proximal stepped surfaces of the first layer stack.