> not being a chip engineer, i wonder while 3D chip layer stacking isn't in the wide use. Short version: The process to make a 2D layer of transistors require

> not being a chip engineer, i wonder while 3D chip layer stacking isn't in the wide use. Short version: The process to make a 2D layer of transistors requires something like 20-30 masking steps, and many of those steps require significant heating. Adding a second plane of 2D transistors on top subjects all of the transistors underneath to more heating, so it is hard to build that structure on top without making the transistors underneath nonfunctional.