From: Mike Rosing <rosing@neurophys.wisc.edu>
Newsgroups: sci.physics.plasma
Subject: Re: Need help with a microwave plasma deposition problem.
Organization: University of Wisconsin, Madison
References: <bfsjhb$f486$1@saturn.cs.uml.edu>


bob wrote:
> Hello,
> I have a bit of a problem with a NIRIM style cvd diamond deposition system
> and am looking for advice. Before I go any further please let me state that
> I'm a laser jock and DONT have any formal experience with this hardware.
> It's a project my company picked up on the cheap, and were just in the
> 'fooling around' stages...
>
> For the sake of reference, a NIRIM cvd reactor is essentially little more
> than a quartz tube that has been 'stabed' through a waveguide connected to a
> 2.45 gig microwave soucre and tuner. the tube is pumped down to 10 to 100
> torr and process gasses are maintained by mass flow controllers. The MW
> radiation sustains an arc and allows for the deposition of diamond while at
> the same time free hydrogen etches any graphite phase carbon that may be
> deposited onto a heated substrate holder in the plasma column.
>
> Now for my problem. I have noticed silicon inpurities in diamond films
> produced by this reactor. As I understand it these types of reactors are
> known to have impurity problems due to etching of the reactor wall tube by
> the MW plasma. In an effort to reduce this problem, why cant the tube be
> centered in a solinoid that squeezes the plasma into the central portions of
> the process tube (if etching of the process tube isnt stopped, it should at
> least be lessened I would think). I checked into the literture, and the only
> such arraingments I could find related to ECR style systems where very low,
> large area dischages (in other reactor designs, NOT an NIRIM reactor) lead
> to a non-isothermal plasma and the magnetic feild only serves to aid in
> energy transfer from hot electrons to the heavier gas species.
>
> So.. What am i missing here? Is there some reason I'm overlooking that would
> inhibit me from making use of such a magnetic feild to reduce this
> contamination issue?
>
> Thanks in advance for any help!

I guess I'm a bit confused, is there silicon on the walls of the reactor?
One thing you might try is changing the operating mode of the MW. Set it up
so you have several wave lengths across with standing wave nodes. Put the
thing to be etched/plated in the middle where you have high field strength
and use the mag field to prevent wall ions from getting into the central
section. Note that they will neutralize, travel accros the field, and
then get reionized where you don't want them, so for long run times this
won't work to well. Another simple thing to try is just to increase the
pressure of the stuff you want to be there and reduce the amount of impurity.
Again, not so great but better than nothing.

An expensive solution is to change the reactor so there are no impurities to
get into the system in the first place. but you knew that :-)

Patience, persistence, truth,
Dr. mike




--
Mike Rosing
www.beastrider.com BeastRider, LLC
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