How would manufacturing a hollow triangular prism, or a cube, be accomplished more cheaply than assembling planar components? When people discuss the possibility of 3D semiconductor fabrication, they're usually talking about additive manufacturing of multiple layers of transistors atop a single silicon wafer substrate. That's only going to be economical if most of the volume deposited is used for transistors rather than needing to be removed later. Large hollow shapes would be difficult and wasteful to manufacture that way; transistors aren't as easy to 3d-print as plastic filament.
I can't imagine it myself, though a massively parallel thing that didn't need to worry about heat might work. IDK is that what an optical type chip is supposed to somehow do?
I know a little more about how semiconductors work; the heat parts are related to the waste energy from the transistors change between conducting and not. Since the semi-conduction parts need the conduction paths opened or closed by changing charge levels in the narrow parts. I can barely imagine some sort of interference based optical chip might not generate heat at the logic combination part, but something some-where's gotta flip/flop and that's probably where the heat will exist.
The other option we've heard about for decades involves better conduction of the heat out of the chip. If that's the case thermally conductive layers could sandwich between stacked 2D layers. Maybe that'll finally be economically viable for high density logic. As far as RAM stacking seems to work well enough so far.
That would still be starting from the planar wafers we're manufacturing today, but the question was about making 3D structures directly without just assembling 2D chips as we have them currently.
