From: sfaber@cbnewse.cb.att.com (steven.r.faber)
Newsgroups: rec.pyrotechnics
Subject: Re: Vacuum deposition of aluminium
Message-ID: <1992Mar25.142351.27811@cbnewse.cb.att.com>
Date: 25 Mar 92 14:23:51 GMT

From article <1992Mar24.140307.22983@demon.co.uk>, by dingbat@cix.compulink.co.uk (Codesmiths):
> In-Reply-To:  Ian Stirling <printf@cix.compulink.co.uk>
>
>> I'm trying to vacuum deposit aluminium on microscope cover slips
>
> The process you're after is called vacuum sputtering. What you have
> here sounds more like vacuum spluttering !
>
> All of the following is from memory, as it's several years since I
> did this stuff. If you have a university library nearby, then you
> could check the vacuum / optical manufacture section in there, it's a
> standard process.
>
> You need a much lower pressure. The mechanism should involve atoms of
> aluminium being transferred indivually. The low pressure is needed so
> that the mean free path (without colliding with an air molecule) is
> at least as large as the separation between source & target - it's
> not just about avoiding oxidation. With small samples, life is much
> easier as the vacuum needed (thus the mean free path length) is
> related to the size of the workpiece. Large sputter chambers need
> oil diffussion pumps or similar, but a mechanical roughing pump will
> do for small chambers. (The usual suggestions involving surplus
> pumps, or fridge compressors.)

Yes, pressures around 10^-3 torr (mm Hg) come to mind.

Sputtering involves passing a electrical discharge through usually
Argon gas, which is accelerated with a high voltage potential to
the target material, usually a fairly refractory metal.
The positive Argon ions then knock off the target metal and cause
it to be deposited in a thin layer on the thing you want coated.

Al melts at a fairly low temp. so you don't need to sputter it,
just put some in a tungsten boat-filament heated with a high current
source in the vacuum chamber and it evaporates nicely and coats your
mirror.  You still need the fancy equipment to do it that way.

Steve