Arrays of elevationally-extending strings of memory cells and methods used in forming an array of elevationally-extending strings of memory cells

A method used in forming an array of elevationally-extending strings of memory cells, comprising:

• forming upper and lower stacks individually comprising vertically-alternating insulative tiers and wordline tiers;
• forming lower channel openings in the lower stack;
• forming upper channel openings into the upper stack to individual of the lower channel openings to form interconnected channel openings individually comprising one of individual of the lower channel openings and one of individual of the upper channel openings;
• at least one of the upper and lower stacks comprising an intra-stack etch-stop tier between a top tier and a bottom tier of the respective upper or lower stack, the intra-stack etch-stop tier being of different composition from those of the insulative tiers and the wordline tiers of the respective upper or lower stack;
• the forming of at least one of all of the lower channel openings and all of the upper channel openings comprising:
• etching into the insulative tiers and the wordline tiers that are above the intra-stack etch-stop tier to the intra-stack etch-stop tier to form the respective lower channel openings or the upper channel openings to have individual bases comprising the intra-stack etch-stop tier;
• penetrating through the intra-stack etch-stop tier to extend individual of the respective lower channel openings or the upper channel openings there-through;
• and
• after extending the individual channel openings through the intra-stack etch-stop tier, etching into and through the insulative tiers and the wordline tiers that are below the intra-stack etch-stop tier to extend the respective individual upper or lower channel openings deeper into the respective upper or lower stack below the intra-stack etch-stop tier;
• and
• forming transistor channel material in the individual upper and lower channel openings elevationally along the intra-stack etch-stop tier and along the insulative tiers and the wordline tiers that are above and below the intra-stack etch-stop tier.