Methods of forming a semiconductor device structure including a stair step structure

A method of forming a semiconductor device structure, comprising:

• forming a non-conductive stack structure comprising non-conductive tiers over a substrate, each of the non-conductive tiers independently comprising a silicon oxide structure vertically overlying a silicon nitride structure;
• forming a masking structure directly vertically adjacent a portion of a substantially planar uppermost surface of the non-conductive stack structure defined by the silicon oxide structure of an uppermost tier of the non-conductive stack structure, the masking structure exhibiting a substantially planar upper surface and having a different material composition than the silicon oxide structure and the silicon nitride structure of each of the non-conductive tiers of the non-conductive stack structure;
• forming a non-conformal photoresist over exposed surfaces of the masking structure and the non-conductive stack structure;
• subjecting the non-conductive stack structure to multiple material removal processes using the masking structure and portions of the non-conformal photoresist as masks to form a stair step structure comprising at least three steps in the non-conductive stack structure, a vertically lowermost step of the at least three steps of the stair step structure positioned directly laterally adjacent the masking structure, and portions of the silicon oxide structure of the uppermost tier of the non-conductive stack structure defining an upper surface of a vertically uppermost step of the at least three steps of the stair step structure substantially coplanar with a lower vertical boundary of the masking structure;
• and
• replacing at least a portion of the silicon nitride structure of each of the non-conductive tiers with a conductive material to form a conductive stack structure comprising partially conductive tiers over the substrate.