[urthor]

https://3dfabric.tsmc.com/english/dedicatedFoundry/technolog...

What you're describing is called 3D stacking, and it works.

It's just extraordinarily complex to resolve the intra-die latency issues, and many others, when going vertical. Hence expensive.

[petra]

3D is already being used for ram/flash IC's. That's possible because most memory cells aren't being used in a memory chip - so heat density is reasonable.

3D is also used in AMD's 3D cache.

But 3D logic on logic is more complicated because of heat issues. AFAIK there's no workable technical solution for that yet.