alt Hacker NewsYou could make in a garage some decent analog integrated circuits, e.g. audio amplifiers or operational amplifiers or even radio-frequency circuits for not too high frequency ranges.
However you cannot make useful digital circuits. For digital circuits, the best that you can do is to be content to only design them and buy an FPGA for implementing them, instead of attempting to manufacture a custom IC.
With the kind of digital circuits that you could make in a garage, the most complex thing that you could do would be something like a very big table or wall digital clock, made not with a single IC like today, but with a few dozen ICs.
Anything more complex than that would need far too many ICs.
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What are the factors you expect to limit the integration scale in a garage fab?
Not true. You are confusing "digital" with "microprocessor". You wouldn't be able to do any single-chip microprocessor, of course, but something like 74181 is very doable at this scale, and building a 1970s-era computer out of a few dozen of these is something enthusiasts still do. The main problem isn't logic, it's memory - memory needs density (thin film magnetics anyone?).
Then, of course, if by "useful" you mean "commercially viable", it is indeed not going to be competitive against either TSMC or your local 500nm foundry ever.