Diffusion Pump inconsistencies

[[email protected]]

Hi,

I'm reading university textbook from a professor who has made LED and
diode fabricating equipment for his students. He uses a rotary vane
pump fed to diffusion pump. He says the rotary vane gets it down to
0.1 torr, which is common for those pumps. I've read various websites
saying the lowest pressure a diffusion pump will work, but no
agreement, all kinds of different answers, inconsistent, from 1e-2 to
1e-3 torr. Well, my rotary vane pump goes down to 0.075 torr, a bit
lower than the professors, but no where near 1e-2 torr. Before buying
a cheap diffusion pump on ebay (on a budget here), how can I know it
will work with my vane pump at 0.075 torr? Maybe diffusion pumps have
a heck of a time at 0.1 torr, but will still work a bit. Perhaps they
take awhile to get going at 0.1 torr. No idea.

Thanks for the help,
Anon

 

[[email protected]]

Hi,

I'm reading university textbook from a professor who has made LED and
diode fabricating equipment for his students. He uses a rotary vane
pump fed to diffusion pump. He says the rotary vane gets it down to
0.1 torr, which is common for those pumps. I've read various websites
saying the lowest pressure a diffusion pump will work, but no
agreement, all kinds of different answers, inconsistent, from 1e-2 to
1e-3 torr. Well, my rotary vane pump goes down to 0.075 torr, a bit
lower than the professors, but no where near 1e-2 torr. Before buying
a cheap diffusion pump on ebay (on a budget here), how can I know it
will work with my vane pump at 0.075 torr? Maybe diffusion pumps have
a heck of a time at 0.1 torr, but will still work a bit. Perhaps they
take awhile to get going at 0.1 torr. No idea.

As far as I know, backing a diffusion pump with a rotary backing pump
is absolutely standard. That how I pumped down my vacuum line when I
was doing my Ph.D. in physical chemistry back in the 1960's - just
like everybody else who was working with vacuum lines - and that's
what everybody did where I was doing my post-doc a few years later.

When I was working on electron microscopes at Cambridge Instruments in
the 1980's, their standard set-up relied on a turbomolecular pump
backed by the same sort of rotary vacuum pump. A turbomolecular pump
cost a bit more than a diffusion pump but needs less cooling.

Where the electron microscopes were coupled up to big specimen
chambers, the turbomolecular pumps were bit too slow, and we used
cryopumps instead - still back by rotary pumps (albeit tolerably big
ones).

I've heard it claimed that you can back a diffusion pump with a water
pump, which only gets you down to about 15 torr, so I'd say that you
should go ahead and buy your diffusion pump.

 

[Rich Grise]

I'm reading university textbook from a professor who has made LED and
diode fabricating equipment for his students. He uses a rotary vane pump
fed to diffusion pump. He says the rotary vane gets it down to 0.1 torr,
which is common for those pumps. I've read various websites saying the
lowest pressure a diffusion pump will work, but no agreement, all kinds of
different answers, inconsistent, from 1e-2 to 1e-3 torr. Well, my rotary
vane pump goes down to 0.075 torr, a bit lower than the professors, but no
where near 1e-2 torr. Before buying a cheap diffusion pump on ebay (on a
budget here), how can I know it will work with my vane pump at 0.075 torr?
Maybe diffusion pumps have a heck of a time at 0.1 torr, but will still
work a bit. Perhaps they take awhile to get going at 0.1 torr. No idea.

If you want a clean vacuum, DO NOT USE an oil diffusion pump. The whole
chamber fills up with pump oil. Look into sorption pumps, ion pumps,
getter pumps, cryopumps; and, well, if your vane pump is also known as a
turbo pump, you've got that covered.

Good Luck!
Rich

 

[[email protected]]

As far as I know, backing a diffusion pump with a rotary backing pump
is absolutely standard. That how I pumped down my vacuum line when I
was doing my Ph.D. in physical chemistry back in the 1960's - just
like everybody else who was working with vacuum lines - and that's
what everybody did where I was doing my post-doc a few years later.

When I was working on electron microscopes at Cambridge Instruments in
the 1980's, their standard set-up relied on a turbomolecular pump
backed by the same sort of rotary vacuum pump. A turbomolecular pump
cost a bit more than a diffusion pump but needs less cooling.

Where the electron microscopes were coupled up to big specimen
chambers, the turbomolecular pumps were bit too slow, and we used
cryopumps instead - still back by rotary pumps (albeit tolerably big
ones).

I've heard it claimed that you can back a diffusion pump with a water
pump, which only gets you down to about 15 torr, so I'd say that you
should go ahead and buy your diffusion pump.

Thanks! I can beat 15 torr, so no problemo! I'll buy it this week,
but first Rich Grise said it fills chamber up with oil. I'll wait to
see what he says.

Anon

 

[[email protected]]

If you want a clean vacuum, DO NOT USE an oil diffusion pump. The whole
chamber fills up with pump oil. Look into sorption pumps, ion pumps,
getter pumps, cryopumps; and, well, if your vane pump is also known as a
turbo pump, you've got that covered.

Good Luck!
Rich

Ohhh, I wish my budget was for a turbomolecular! I don't like ion /
gutters because they're one timers, need to replace the material,
right? This is for a garage project, so would ion pump be afordable
to buy the materail to absorbe the gases every run. Maybe I should
learn more about ion pumps.

As far as oil in the camber, I know what you mean, but oil's a lot
better than oxygen as far as deposition. I think silicon molecules
love to stick to oxygen molecules more than oil. So long as the
pressure gets to at least 1e-4 torr.

I'm not going to use a liquid nitrogen cryotrap to prevent
backstreaming because it's a garage project, but I read that
polyphenyl ethers make a cryotrap unnecessary. I thought that maybe
placing a lengthy metal tubing coil cooled a bit would trap enough
backstreaming oil. You know, similar to what hotrodders use to cool
the gas prior to carburator. In hotrodding, the gas flows through the
metal coil, and the coil is inside a canister with ice. My only
concers is how much metal surface is allowed in a vacuum because atoms
are continually escaping the surface in a vacuum. You know, if there's
too much surface then the vacuum pumps can't keep up with it.

Thanks,
Anon

 

[Carl Ijames]

Diff pumps will start to pump in the 1 torr range, enough to get themselves
down under 1e-3 torr where they are happy. Just make sure you don't have
any leaks that will keep it in the transition region any longer than
necessary since this is when you get the majority of backstreaming. Use
pentaphenyl ether (Santovac 5) oil, it is very good about not creeping along
surfaces so the backstreaming is usually limited to line of sight to the
pump inlet. Hang the pump from an elbow off the side of your chamber and
that does just about as good as a refrigerated baffle, or fabricate a metal
disc to sit over the pump inlet spaced an inch or so above the bottom of the
chamber so only the bottom of the disc has line of sight into the pump
inlet. You also need a trap (zeolite, copper wool, something) between the
rough pump and the diff pump to stop oil from the rough pump from migrating
into the diff pump where they will promptly pollute the chamber.

Try to get a copy of O'Hanlon's book A User's Guide to Vacuum Technology, it
is very readable and will fill in lots of gaps in how to set up and operate
a high vacuum system.

-----
Regards,
Carl Ijames

If you want a clean vacuum, DO NOT USE an oil diffusion pump. The whole
chamber fills up with pump oil. Look into sorption pumps, ion pumps,
getter pumps, cryopumps; and, well, if your vane pump is also known as a
turbo pump, you've got that covered.

Good Luck!
Rich

Ohhh, I wish my budget was for a turbomolecular! I don't like ion /
gutters because they're one timers, need to replace the material,
right? This is for a garage project, so would ion pump be afordable
to buy the materail to absorbe the gases every run. Maybe I should
learn more about ion pumps.

As far as oil in the camber, I know what you mean, but oil's a lot
better than oxygen as far as deposition. I think silicon molecules
love to stick to oxygen molecules more than oil. So long as the
pressure gets to at least 1e-4 torr.

I'm not going to use a liquid nitrogen cryotrap to prevent
backstreaming because it's a garage project, but I read that
polyphenyl ethers make a cryotrap unnecessary. I thought that maybe
placing a lengthy metal tubing coil cooled a bit would trap enough
backstreaming oil. You know, similar to what hotrodders use to cool
the gas prior to carburator. In hotrodding, the gas flows through the
metal coil, and the coil is inside a canister with ice. My only
concers is how much metal surface is allowed in a vacuum because atoms
are continually escaping the surface in a vacuum. You know, if there's
too much surface then the vacuum pumps can't keep up with it.

Thanks,
Anon

 

[Jamie]

Hi,

I'm reading university textbook from a professor who has made LED and
diode fabricating equipment for his students. He uses a rotary vane
pump fed to diffusion pump. He says the rotary vane gets it down to
0.1 torr, which is common for those pumps. I've read various websites
saying the lowest pressure a diffusion pump will work, but no
agreement, all kinds of different answers, inconsistent, from 1e-2 to
1e-3 torr. Well, my rotary vane pump goes down to 0.075 torr, a bit
lower than the professors, but no where near 1e-2 torr. Before buying
a cheap diffusion pump on ebay (on a budget here), how can I know it
will work with my vane pump at 0.075 torr? Maybe diffusion pumps have
a heck of a time at 0.1 torr, but will still work a bit. Perhaps they
take awhile to get going at 0.1 torr. No idea.

Thanks for the help,
Anon

Our pumps go down to 2e-7 torr, one of ours will hit -8 now and then
but I suspect an inconsistency with the ion tube.

http://webpages.charter.net/jamie_5"

 

[Jamie]

If you want a clean vacuum, DO NOT USE an oil diffusion pump. The whole
chamber fills up with pump oil. Look into sorption pumps, ion pumps,
getter pumps, cryopumps; and, well, if your vane pump is also known as a
turbo pump, you've got that covered.

Good Luck!
Rich

we operate our 2 pole turbo's up to 1000 hz.



http://webpages.charter.net/jamie_5"

 

[[email protected]]

If you want a clean vacuum, DO NOT USE an oil diffusion pump. The whole
chamber fills up with pump oil. Look into sorption pumps, ion pumps,
getter pumps, cryopumps; and, well, if your vane pump is also known as a
turbo pump, you've got that covered.

We always put a liquid nitrogen cooled trap between the diffusion pump
and the rest of the line; I had two, more to protect the oil in the
backing pump from the bromine in the vacuum line than to protect the
vacuum line from the oil in the diffusion pump.

Electron microscopes mostly make do without this - in any event the
specimen chamber of an electron microscope contains mechanical moving
parts which have to be lubricated so there's always oil vapour around.

Liquid nitrogen costs about the same as milk per unit volume and it's
rather easier to get it delivered to your house or garage; you need a
big vacuum flask that will take about a week's supply but they aren't
all that expensive - a quick Google suggests something around $500.

 

[[email protected]]

Hi,

I'm reading university textbook from a professor who has made LED and
diode fabricating equipment for his students. He uses a rotary vane
pump fed to diffusion pump. He says the rotary vane gets it down to
0.1 torr, which is common for those pumps. I've read various websites
saying the lowest pressure a diffusion pump will work, but no
agreement, all kinds of different answers, inconsistent, from 1e-2 to
1e-3 torr. Well, my rotary vane pump goes down to 0.075 torr, a bit
lower than the professors, but no where near 1e-2 torr. Before buying
a cheap diffusion pump on ebay (on a budget here), how can I know it
will work with my vane pump at 0.075 torr? Maybe diffusion pumps have
a heck of a time at 0.1 torr, but will still work a bit. Perhaps they
take awhile to get going at 0.1 torr. No idea.

Thanks for the help,
Anon

Lots of good suggestions above. Someone said you can back a Diff pump
with 15 Torr. This might OK but backing and starting the diff pump
are two different things. Your rotary vane pump that gets down to 0.1
Torr will be fine. (You need some valves so that once the rotary pump
has got the system down to 0.1 Torr it can be switched over and used
as the backing pump on the diff pump.) Other wise you need two pumps.
With only one mechanical pump you can get the system to work but you
have to do some careful valving.

You might look into the text "Building Scientific Apparatus" by Moore
Davis and Coplan. I would definitely use a nitrogen trap with the
system. You might also look into cryo pumps. These can't pump things
like neon, hydrogen and helium, but if you can flush your system with
nitrogen then they are very clean pumps.

Good luck,
George Herold

 

[Rich Grise]

Ohhh, I wish my budget was for a turbomolecular! I don't like ion /
gutters because they're one timers, need to replace the material, right?
This is for a garage project, so would ion pump be afordable to buy the
materail to absorbe the gases every run. Maybe I should learn more about
ion pumps.

AFAIK, the getter pumps and ion pumps got purged pretty well when they
baked out the bell jar.

As far as oil in the camber, I know what you mean, but oil's a lot better
than oxygen as far as deposition. I think silicon molecules love to stick
to oxygen molecules more than oil. So long as the pressure gets to at
least 1e-4 torr.

Unfortunately, I don't remember what vacuum pressure they were going for,
but with the cryopump it was good enough for molecular beam epitaxy.

I'm not going to use a liquid nitrogen cryotrap to prevent backstreaming
because it's a garage project, but I read that polyphenyl ethers make a
cryotrap unnecessary.

Well, actually, I wasn't talking "cryotrap" - I'm talkng about a pair of
cannisters about 6" diameter and about 12-15" long, full of Zeolite.
They'd hang dewars around them, fill the dewars with LN2, and when the
LN2 stopped boiling, they'd open the valve. You could hear the air being
sucked out ot the chamber, and the LN2 would start boiling again. When it
got down to some level, they'd close the valve and start using the other
pumps. When they'd let the chamber come up to air (to change experiments or
hatever), all the air would come out of the sorption pumps, and they'd
bake them out.

It seemed to work; the company was so rich that they built a full-on gym
and a couple raquetball courts. Every three months they had a Profit
Sharing party, and they'd bring a keg of beer into the lunchroom. ;-)

Unfortunately, my wonderful boss quit one day presumably for more money,
and his replacement was such a moron, I arranged to get myself fired
(so I could collect unemployment), then found an even better job (well,
not as exciting, but an infinitely better boss.) ;-)

Cheers!
Rich