Multilayer interconnect or multiple layers of transistors?
Many chips as in flash (and recently also DRAM), but stacking logic is fairly new.
Exactly! Stacked logic is the key to the fabled smart matter. Heat dissipation is going to be an issue, although I think IBM had some ideas on microfluidics. Also, there's been a ton of work on dilution fridges for quantum chips, and some startups are doing cryo logic, these two could complement
Multiple layers of transistors - at least for RAM (HBM). Multiple logic layers are coming really soon.