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MEMS devices absolutely blew my mind the first time I read about them. Thinking: "yeah, this just shouldn't exist". Like, how can you make something with moving

MEMS devices absolutely blew my mind the first time I read about them. Thinking: "yeah, this just shouldn't exist". Like, how can you make something with moving parts out of something that normally doesn't move (polycrystalline silicon), and is very fragile at that, all the while doing it at a scale that you require similar processes as 'normal' semiconductors. You can make physical oscillators like the peeps do in this article and have surrounding circuitry to drive and measure them, levers and actuators to mount actual tools onto: probes to manipulate single cells, mirrors to control light (looking at you, DLP) and so many other things. Super interesting!

nwiswell 2y ago
> out of something that normally doesn't move (polycrystalline silicon) More commonly, bulk micromachined MEMS devices are monocrystalline. > and is very fragile at that Normalizing for density, monocrystalline silicon is stronger than steel!
- theideaofcoffee 2y ago
  Whoops, yep, thanks for the correction! What's a Greek prefix between friends (save for crystal properties).
- rcxdude 2y ago
  >Normalizing for density, monocrystalline silicon is stronger than steel! So's glass, and silicon's a lot like glass (very brittle)
  - nwiswell 2y ago
    Glass has a tensile strength more than an order of magnitude below mono-Si. The elastic modulus for soda-lime is within a factor of 2, but that's not usually what is meant by "strength", rather "stiffness". You're right that fracture toughness is comparable, but that's not usually as relevant in a MEMS context: for one thing there's less potential for unanticipated collisions, but also crack propagation is significantly inhibited at microscopic length scales.
amelius 2y ago
I once heard a story about a MEMS device. It was about a tiny motor that could do a million revolutions per second. However, as it turned out it would break after 1 second.
- rcxdude 2y ago
  Yeah, this is a common issue. There was a lot of hype initially about basically just taking large-scale mechanical designs and shrinking them. The issue is that anything where you have moving parts in contact, they wear extremely quickly because things tend to stick together at that scale (combined with extremely fast speeds involved in scaling things down) All the MEMS that are actually used in practice avoid this: they are effectively entirely flexural structures, nothing rubs against anything else, they just flex in place. This kind of structure can last almost indefinitely (especially since they're usually made of a single crystal, making the usual creep/cracking wear mechanisms mostly a non-issue)

nwiswell 2y ago

> out of something that normally doesn't move (polycrystalline silicon) More commonly, bulk micromachined MEMS devices are monocrystalline. > and is very fragile at that Normalizing for density, monocrystalline silicon is stronger than steel!

theideaofcoffee 2y ago
Whoops, yep, thanks for the correction! What's a Greek prefix between friends (save for crystal properties).
rcxdude 2y ago
>Normalizing for density, monocrystalline silicon is stronger than steel! So's glass, and silicon's a lot like glass (very brittle)
- nwiswell 2y ago
  Glass has a tensile strength more than an order of magnitude below mono-Si. The elastic modulus for soda-lime is within a factor of 2, but that's not usually what is meant by "strength", rather "stiffness". You're right that fracture toughness is comparable, but that's not usually as relevant in a MEMS context: for one thing there's less potential for unanticipated collisions, but also crack propagation is significantly inhibited at microscopic length scales.

amelius 2y ago

I once heard a story about a MEMS device. It was about a tiny motor that could do a million revolutions per second. However, as it turned out it would break after 1 second.

rcxdude 2y ago
Yeah, this is a common issue. There was a lot of hype initially about basically just taking large-scale mechanical designs and shrinking them. The issue is that anything where you have moving parts in contact, they wear extremely quickly because things tend to stick together at that scale (combined with extremely fast speeds involved in scaling things down) All the MEMS that are actually used in practice avoid this: they are effectively entirely flexural structures, nothing rubs against anything else, they just flex in place. This kind of structure can last almost indefinitely (especially since they're usually made of a single crystal, making the usual creep/cracking wear mechanisms mostly a non-issue)