Microelectronic devices include a stack structure of insulative structures vertically alternating with conductive structures and arranged in tiers forming opposing staircase structures. A polysilicon fill material substantially fills an opening (e.g., a high-aspect-ratio opening) between the opposing staircase structures. The polysilicon fill material may have non-compressive stress such that the stack structure may be partitioned into blocks without the blocks bending and without contacts—formed in at least one of the polysilicon fill material and the stack structure—deforming, misaligning, or forming electrical shorts with neighboring contacts.