Theoretically it should be feasible to fab 350 nm without double-patterning by optimizing a simple immersion DLP/DMD i-line stepper.
I think ArF immersion with double-patterning should be able to do maskless 90 nm.
I'm not sure where it was, but I remember a seeing a project where someone made a rudimentary homebrew electron microscope by chemically etching the tungsten filament from a light bulb (to get the tip sharp enough) and attaching it to a piezo buzzer that was scored to separate it into four quadrants. The filament could be moved by applying various combinations of voltage to the piezo quadrants.
I didn't find the one I was thinking of (which I think was ca. 2002 and so maybe just vanished by now), but search results suggest that variations of this have been done by several people.
Decades ago computers used magnetic core memory. Those things operate on a macroscopic/classical physics level. You can make a core memory by hand if you buy the ferrous toroid first. But moreover, you mention 3d printers — it’s probably possible to manufacture the toroid on a sub-10k machine these days, be it a 3d printer or CNC machine. Some of these techniques generalize to multiple materials, meaning you could automate both the manufacturing of the toroid and the wires connecting them (and the assembly) and have an actual open-source, easily fabricated memory.
One thing not a lot of people know is that you can create clock-triggered combinatorial logic out of core memories just by routing the wires differently. So you’ve got your whole computation + volatile memory + non-volatile memory built on the same process using just two materials and at a macroscopic scale (think: millimeters). That sounds easier to bring up than silicon.
Yeah, macro-scale has its limitations (speed; power draw!), but it’s still enough to enable plenty of applications, and with room to scale it as the tech gets better.
The simplistic explanation for why this works is that electron beams can be easily focused using magnetic lenses into a beam that reaches the nano meter level.
These beams can then be deflected and controlled electronically which is what makes it possible to effectively make a cpu from a cad file.
Furthermore, It's very easy to see how the complexity of photolithography goes up exponentially as we scale down.
Therefore I believe it makes sense to abandon the concept of photolithography entirely if we want open souce silicon. I believe that this approach offers something similar to the sort of economics that enable 3D printers to become localized centers of automated manufacturing.
I should also mention that commercial E-beam machines are pretty expensive (something like 1-Mil) but that I dont think it would be that difficult to engineer one for a mere fraction of that price.