In a message dated 5/6/98 3:32:57 AM Central Daylight Time, pcmeas@hotmail.com
writes:
<< Arcs do not depend on the existence of a gas. Do you believe you could
never get an arc in outer space, for instance? Please refer, if you
will, to your sophomore physics book. In a vacuum, with adequate
potential between two conductors, the electromagnetic forces ultimately
cause particles to traverse the vacuum gap from one conductor to
another. Thus, in a vacuum, and arc causes material to transfer. This
is the physics behind sputtering, which, by the way, does not produce
rough particulate material deposition, but actually produces very
uniform film deposition, which is why sputtering is used to coat disk
drive surfaces, and put metal films on glass for LCD computer screens.
>>
Arlen,
I think what you are describing is not sputtering, but cathodic arc deposition
(Catarc). It's been a while, but as I recall it works like this:
An arc is struck between two electrodes by burning away a thin film of metal
coating an insulator between the two electrodes. These metal atoms for a
moment form a conductor in the vacuum which allows a high current/low voltage
arc to be established between the electrodes. When the high current arc is
established, large quantities of metal are ejected from the target (by a
mechanism which I don't remember) which then coat everything in sight
including the ignitor. You cannot use this technique with a target material
unless it is a conductor. This technique is popular for coating big stuff
like automobile bumpers, etc.
Sputtering is produced through a mechanical action. An atom is accelerated
into a target material. Some of the target material is ejected as a result of
the transfer of kinetic energy. Typically an argon gas atmosphere of about 10
mT is used. The argon is ionized and accelerated using a magnet/DC potential
when the target is a conductor. RF energy, typically 13.58 MHz at several kW,
is used when the target is a nonconductor. Sputtering has the advantage that
you can coat chicken fat on a substrate if you care to. Sputtering is
typically used to coat small substrates which high precision, such as
semiconductor wafers. At no time is there an arc between the target and an
electrode. Both Catarc and sputtering generally result in lower substrate
temperatures than an evaporative process.
Sputtering can occur in a vacuum tube when the gas is ionized by the electron
beam and accelerated towards the cathode. I would expect the rate of cathode
contamination and subsequent emission loss is increased by excessive gas in
the tube. No gas in tube -> no sputtering.
While I have everyone's attention,
I enjoy this reflector for the technical information and discussions. I would
hope that we can all keep our discussion and debates at a professional level,
concentrating on the subject material and not on the author. This reflects
best on all parties involved.
73's
Chris, KF0FX
KF0FX@aol.com
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