The central conceit of these kinds of thought experiments is this:
A lot of them are just that, conceptual: you could hang out with da Vinci, language barrier aside, and have interesting discussions on a variety of subjects. He would surely learn a lot, and you would surely stand to learn many things as well.
But as it turns out, semiconductor manufacture actually is
hard. It's multidisciplinary, involving many branches of chemistry and physics (with a bit of psychology thrown in to manage the inevitable meat-based support), on scales and precision unprecedented until then.
To say that semiconductor production could've been advanced, even by say ten years, is probably a very aggressive forecast!
In the late 19th century, say: machining was well matured, with fair precision, say, less than a tenth of a mm being a regular thing. That's nice, but still a broad side of a barn compared to the micron precision demanded by semiconductors as we know them. You simply can't have a machinist turning handles on a (large-reduction-ratio pantograph) Etch-a-Sketch and expect it to come out consistently.
At the same time (turn of the last century, say), some semiconductor theory and observation was just beginning to develop, but the theoretical tools were unprepared to handle it (physicists quickly learned that condensed-matter physics is hard, too!). Observations were inconsistent, spooky even. One would just as well write off another's results as "impossible to reproduce", or worse things. Likewise, those working with such materials might've been disinclined to publish due to possibly being labeled as a loon?
The materials were partly to blame, but they didn't have a good way of knowing that. Analytical chemistry couldn't detect parts per billion, except in some rare cases. Studies would've been easier if the materials were pure and homogeneous from the start, but again, there wasn't much understanding of impurities, and dislocations and other defects.
And it took advances in polymer chemistry and photochemistry to make the step-and-repeat method possible, as we know it today. At that time, photography was a fairly ordinary thing, but it tended to be grainy, and color photography for example was only in laboratories. The best plastics at the time were mostly natural: rubber, lacquer, asphalt, and bakelite came a bit later (early 20th century). It wasn't until much later that organic photochemistry became much better understood, and photographic resists became possible.
These just to name a few!
Incidentally, I don't think they would've had too much trouble cooking up the other process handling equipment, things like electric heaters, plasma generators and vacuum chambers. Hard vacuum was studied at the time, though I don't know how useful it was in terms of engineering or other practice (ahem, quack medicine using x-rays aside). Not that such equipment would've been accurate enough, either: you need a feedback control system to operate these things, which would've been quite the challenge before electronic controls came along. (You can make a hydraulic or mechanical control system, but it's quite noisy (due to turbulence or rotating and sliding parts), and inevitably slow.)
Most of all, with so many highly-refined (for the time) technologies committed to one single goal, what good is it? Who's buying these chips? What are they using them for?
The driving force through the 50s and 60s was military technology, willing to pay high prices for top tech. Consumer applications came as well, but only with the crudest of parts (germanium BJTs), and sparingly at that (unless they were faulty, in which case you might have a "six transistor" (or more) pocket radio
).
You can't change an economy overnight. It takes decades upon decades of rollout to get everyone invested into new technologies, and driving their further advance. Nay, it's better to think of semiconductors as one spoke, among many, in the enormous feedback wheel that has pushed technology forward over the last century.
On a related subject: what, then, would be most practical to send back in history?
Probably, vacuum tubes would be achievable within ones' [remaining] lifetime, given suitable patronage.
Going back to da Vinci: suppose you brought with you, in your head, the designs for electrical power generation, transmission and transformation; designs for vacuum pumps and handling equipment; and the chemical and metallurgical knowledge to win the required materials from ores. While all this would be most peculiar to da Vinci, let's say you win his confidence and you work together to create these fantastical creations.
You could start by grading and stockpiling ores, refining them to the various metals needed (iron and copper being relatively easy; nickel less so, and harder still for tungsten, molybdenum and others). This already requires a lot of support apparatus, as only a few of these can be processed in the traditional method (as you'd find in a handy copy of, not De Re Metallica, but perhaps the parent texts it would be compiled from). You would have to do much of the analytical work of, say, Lavoisier, but three centuries earlier. For example, tungsten and molybdenum might be hydrogen-reduced (since, you can't make a fire hot enough to melt them, and you wouldn't want to anyway, because both readily form carbides). This requires oil of vitriol (sulfuric acid), say.
In the end, what would you create? You need something to show your patron. You might create a telegraph, and show how it can be deployed over great distances to communicate between towns, or how an army might deploy wires as it marches, so it can be commanded instantly. (Which, I suspect, is a very modern conception of warfare, and without the rapid supply lines we take for granted today, rapid communication probably wouldn't be all that tactically useful after all.)
Tim
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