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In Tim Scott's October 2, 2023 video "The largest telescope that will ever be built*" (note the asterisk, link below) there is a discussion of the "re-silvering" of the four main mirrors of the Very Large Telescope (VLT) every 2-3 years when despite regular $\require{mhchem}\ce{CO_2}$ cleaning they accumulate too much dust/dirt:

They atomise aluminium to create the mirror layer. And when I say atomise, I don't mean like perfume sprayer atomiser. I mean literally, they use plasma to make individual atoms of aluminium float around in a vacuum, and then they just let it softly rain down onto the mirror until they have a coating about 1,000 atoms thick.

One common way to deposit thin aluminum films is thermal evaporation - a ceramic alumina ($\require{mhchem}\ce{Al_2O_3}$) crucible containing aluminum metal is heated to a point where the vapor pressure of aluminum results in a flux of atomic aluminum vapor that is emitted at thermal velocity and generally then stick to to whichever surface they hit.

But in this case, at least according to Scott, a plasma deposition process is used.

Question: Why is plasma used to deposit aluminum on the VLT's four large mirrors?

I've in fact never heard of plasma deposition of aluminum. I've done a lot of Plasma-enhanced chemical vapor deposition for dielectrics (e.g. $\ce{Si_3N_4}$, $\ce{SiO_2}$) and PECVD diamond-like carbon or DLC is quite popular, but plasma deposition of layers of pure metal is new to me.

So any details about the mechanism will be appreciated!


cued at 11:07, just before the block quote begins:


Somewhat related:

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    $\begingroup$ Sputtering with argon is a good guess. Standard semiconductor processing step. $\endgroup$
    – Jon Custer
    Oct 13, 2023 at 23:59
  • $\begingroup$ @JonCuster Ah, (e.g. argon) plasma sputtering of aluminum, not aluminum plasma, ya I think you've nailed it. I'll bet googling something like "sputtering VLT" will get you the supporting source for an answer. $\endgroup$
    – uhoh
    Oct 14, 2023 at 0:07

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As shown in an ESO image, the mirrors are coated in a giant plasma sputter system, where ions produced in a (presumably argon) plasma are accelerated and strike the pure aluminum, freeing neutral aluminum atoms which then reach the miror surface and (usually) stick.

It has a diameter of more than 9 meters and a volume of 122,000 liters, pumped with 8 large cryopumps. During deposition the mirror is spun at a stately 5 revolutions per hour under the sputter gun array.

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