Integrated circuits including memory arrays having memory cell strings and methods including methods for forming memory arrays including memory cell strings

An integrated circuit including a memory array having strings of memory cells includes laterally spaced memory blocks individually including a first vertical stack having interleaved insulation levels and conductive levels. A string of memory cells includes a string of channel material extending through the insulating layer and the conductive layer. The conductive levels individually include horizontally elongated conductive lines. A second vertical stack is located alongside the first vertical stack. The second vertical stack includes an upper portion and a lower portion. The upper portion includes vertically interleaved first and second insulating stages of different compositions relative to each other. The lower portion includes an upper polysilicon-containing layer, a lower polysilicon-containing layer, an interposer layer vertically interposed between the upper and lower polysilicon-containing layers. An upper intermediate layer is vertically between the upper polysilicon-containing layer and the interpo