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!

Putting ions and electrons into cyclotron orbits would seem to be a
very good idea, since the electrons are much hotter than the ions when
the plasma is stuck. A lot of collisional interaction would help
bring things to thermal equilibrium. OTOH, given the difference in
masses and temps, the respective radii of cyclotron orbits at a given
field are very disparate.

Silicon monoxide is rather volatile. Reduction of inner silica wall
by the hydrogen plasma is your source. Cooling the wall might help -
but probably less than one would imagine.

Local erosion by energetic
plasma species doesn't much care about bulk temp. Can you run a
laminar flow gas annulus of straight argon at the wall with hydrogen
and methane addition at the core of the plasma column?

Putting ions and electrons into cyclotron orbits would seem to be a
very good idea, since the electrons are much hotter than the ions when
the plasma is stuck. A lot of collisional interaction would help
bring things to thermal equilibrium. OTOH, given the difference in
masses and temps, the respective radii of cyclotron orbits at a given
field are very disparate.

the plasma is already well into the thermal equilibrium area at the
pressures used in operation, so increasing the temp of ions and nuetral
species isnt what I need, but oviously this could be a plus if I decided to
try and deposit dimond at lower pressures..

Silicon monoxide is rather volatile. Reduction of inner silica wall
by the hydrogen plasma is your source. Cooling the wall might help -
but probably less than one would imagine.

the wall is already water cooled via a helical coil of copper bonded to the
glass reaction tube. the system is based upon an astex downstream reactive
species generator if you have any experience with such...
http://www.mksinst.com/cgi-bin/product.exe?pid=ASTeX-AX7610

Local erosion by energetic
plasma species doesn't much care about bulk temp. Can you run a
laminar flow gas annulus of straight argon at the wall with hydrogen
and methane addition at the core of the plasma column?

fluid dynamics has never been a strong point for me, but I'm going to guess
the answer would be no. the stalk that holds the sample only leaves about a
3 or 4mm clearance between it's outer endge and the id of the reaction tube.
while it might be possible to maintain a laminar flow in the upper portions
of the column, i assume that once the flow reaching the sample holder it
will become highly turbulant right at the single place when it needs to NOT
be turbulant... also, would I even be able to sustain a laminar flow in such
close proximity to a hot, dense plasma?

thanks for your input!

Let's SWAG some more. What dielectric resists reduction by a hydrogen
plasma? The lowest oxygen fugacity stuff around is yttria. Would it
be possible to lay down a micron or so CVD or sol/gel, or have a
sintered liner? (assuming it worked at all)

Let's SWAG some more. What dielectric resists reduction by a hydrogen
plasma? The lowest oxygen fugacity stuff around is yttria. Would it
be possible to lay down a micron or so CVD or sol/gel, or have a
sintered liner? (assuming it worked at all)

do you think that has any chance of working? were not tlaking about low
pressures here, but rather 50 to 100 torr, and plasma temps probably in the
10,000k ballpark.. ( I havent measured the reflected power, but there is a
whole lot of energy being dumped into 10 or 20 cm^3 worth of plamsa) I cant
see anything not eroding under that action of that kind of environment.
Thats why I was wanting to keep the plamsa away from the walls...

Zeta pinch. Pulsed is clever in various ways. I don't know about
continuous vs. all sorts of long term plasma flow instabilities.

We're here for public discussion. If nothing has solved the problem
then anything new and reasonable is worth debate. Discovery is not a
dirty word.
I agree entierly, thats why i posted here!

Has anybody used yttria in an argon/hydrogen/methane
plasma?
almost all the work i have seen, the device has used a fused sillica tube

Zirconia or thoria might also be interesting. All of them
are highly refractory, none of the three is easily reduced.

Has anybody used yttria in an argon/hydrogen/methane
plasma?
almost all the work i have seen, the device has used a fused sillica tube
(when the reaction chamber material type is specified)

Zirconia or thoria might also be interesting. All of them
are highly refractory, none of the three is easily reduced.

are we talking about the reactor walls being reduced? i had assumed that at
the temps we are talking about the main erosion mechanism is physical
ablation of the tube due to the high KE of the ions and or hot nuetral
species...

It is a question of
perisistent incorporated contamination, yes? Silicon conceivably fits
into the diamond lattice at very low loadings as silicon carbide.
Heavy elements will not fit and may be dynamically excluded or
scrubbed during deposition. CVD diamond requires immense energy
density at the deposition pedestal to achieve any kind of useful
kinetic net accumulation.
yup..

BTW, is the silicon functionally objectionable? If not, rewrite the
spec to require it
totally objectionable. it has a (comparitivly) serious impact on thermal

BTW, is the silicon functionally objectionable? If not, rewrite the
spec to require it
totally objectionable. it has a (comparitivly) serious impact on thermal
conductivity... seeing as how I'll be spending the better part of a megabuck
per kg to get the improved performance of isotopically enriched diamond, I
want to do anything to elliminate any potential source of less than ideal
thermal conductivity

C-12 diamond is a thermal miracle, but it must be absolutely clean of
C-13 or you get Umklapp phonon scattering and whatnot and a
disproportionately smaller miracle. Crass impurities are
unacceptable. Sigh. Try a heavy element oxide tube - sintered
zirconia, yttria, thoria.
now if only c-12 diamond had the kind of room temp thermal conductivity as

I've been diddling as a hobby with a condensed phase ambient pressure
diamond synthesis in "Devil Solvent." We have had impressive
chemistry then engineering problems with the nasty fused salt. It's
not near reduction to practice if it ever works at all - though it is
efficient in energy and carbon incorporation if it does work, and with
mammoth deposition rates on paper, carat/hr-cm^2. We're attempting
boron-doped Type IIb - phosphorescent orange-red after 254 nm UV
exposure. If it's in there we'll see it (working in the dark, as
usual).
a carat / hr / cm^2?! Now thats something I would like to see!... thats more

Has some wag suggested a deuterium atmosphere rather than protium?
It's a big isotope effect. Or an ultrasonically shaken platform to
create gas shocks at the interface. Good luck!
actually no. I did a quick google, and i didnt see anything that would

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!

(crossposted to sci.physics.fusion.. i'd post to physics.plasma, but it
seems dead in there...)

I've been diddling as a hobby with a condensed phase ambient pressure
diamond synthesis in "Devil Solvent." We have had impressive
chemistry then engineering problems with the nasty fused salt. It's
not near reduction to practice if it ever works at all - though it is
efficient in energy and carbon incorporation if it does work, and with
mammoth deposition rates on paper, carat/hr-cm^2. We're attempting
boron-doped Type IIb - phosphorescent orange-red after 254 nm UV
exposure. If it's in there we'll see it (working in the dark, as
usual).
a carat / hr / cm^2?! Now thats something I would like to see!... thats more
than a single order of magnitude higher than what any other machanism can
deposit, provided of course you could maintain such a rate over large areas.
I have seen some pretty wild numbers based upon the russian work with a
laserheated plasma torch method.. they could grow a mm thick film in single
shift workday (or close to it), but obviously the area for such deposition
very limited..

Has some wag suggested a deuterium atmosphere rather than protium?
It's a big isotope effect. Or an ultrasonically shaken platform to
create gas shocks at the interface. Good luck!
actually no. I did a quick google, and i didnt see anything that would
suggest increasing the thermal conductivity through such a mechanism... do
you have any refrences or recomendations where to look?

What is the substrate you are depositing the diamond on?

Bob

"
What is the substrate you are depositing the diamond on?

Bob

I'm depositing onto diamond, so the impurities arent comming from there, if
thats what you are thinking.