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Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Vacuum Systems

Lecture 6

G.J. Mankey

gmankey@mint.ua.edu

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Pumping Speed and Throughput


The mass flow or throughput of a pump is given by the
equation Q = SP where S is the pumping speed and P is
the pressure. For a conductive element Q = C(P
1
-
P
2
)


For elements of a system connected in series, we must
add the conductance of these elements as in an
electrical circuit: 1/C = 1/C
1

+ 1/C
2

+ 1/C
3

+…


Conductance depends on pressure in the low to
medium vacuum regions, and is independent of
pressure in high to ultrahigh vacuum regions.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Calculations of Conductance


In the molecular flow region, the conductance of a long straight
circular tube is C = 12 d
3
/z liters/sec where d is the diameter(cm)
and z is the length(cm).


For an orifice C = 12 A liters/sec where A is the area in square
centimeters.


These equations should be used to estimate the effect of
connecting pumps, hoses, etc. to a system to insure the pumps are
properly utilized.


The effective pumping speed of a system is then given by the
equation 1/S
eff

= 1/S + 1/C where S is the pumping speed of the
pump and C is the conductance of the associated flanges and
fittings.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Standard Flanges


Conflat flanges use viton or copper gaskets with a knife
edge for high to ultrahigh vacuum applications.


Care must be taken not to damage the flange knife edge.


Standard sizes are mini (¾" ID), 2 ¾" (1 ½" ID), 4 ½" (2
¾" ID), 6" (4" ID), 8" (6" ID) and 10" (8" ID).


Medium vacuum applications use ISO and ASA flanges,
low vacuum uses KF quik flanges.

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Differential Pumping


The amount of gas Q is equated:

SP
2

= Q = C(P
1



P
2
)


This trick can be used to maintain
a constant pressure difference
between two vessels.

P
1

P
2

C

S

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Rotary Vane Pump


An oil seal between a phenolic vane and a steel
cylinder is used to scavenge gas from the
vacuum region and exhaust it to the
atmosphere.


This pump works from atmosphere to about 0.1
mTorr.


Precautions must be taken at low pressures to
avoid oil backstreaming into the vacuum vessel.


It is also used as a backing pump for
compression pumps like a diffusion pump or
turbomolecular pump.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Oil Diffusion Pump


Oil vapor forced through jets in the
stack transfer momentum to gas
molecules and force them down
through the pump and out the exhaust
(must be backed).


The pump is characterized by a
compression ratio and an ultimate
pressure.


Economical (no moving parts).


If used with a cryogenic trap, UHV
can be routinely achieved.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Turbomolecular Pump


Turbine blades rotating at high speed
transfer momentum to gas molecules to
force them out the exhaust (must be
backed).


The pump is characterized by a
compression ratio and ultimate pressure.


Expensive (>$10k).


UHV can be readily achieved (better if
used in combination with a titanium
sublimation pump).

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Gas Compression Ratio


Since the pump works by
momentum transfer, the
compression ratio depends
on the atomic mass.


The thermal velocity of light
gas is much greater, so the
molecules are pumped less
efficiently.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Mass Spectrum of Turbo System


The gas composition
reflects the difference in
compression ration of
light gases and the
composition dependent
outgassing rates of
stainless steel.


Usually hydrogen is the
main constituent of a well
-
baked system.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Titanium Sublimation Pump


High current (50 A) is passed through a
titanium impregnated molybdenum filament to
sublimate a fresh coating onto the cryoshroud
walls.


The film is highly reactive to H, CO and O and
catalytically converts H
2

and CO to CH
4

which
is more readily pumped by a turbo pump.


Cooling the cryoshroud with liquid nitrogen
goes the extra mile to get into the low 10
-
10

mbar range.


Pumping speed depends on gas, activated area
and wall temperature (can be quite high, i.e.
limited by inlet flange size).

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Ion Pump


A high voltage combined with a magnetic
field causes electrons to travel in a helical
path with an energy sufficient to ionize
gas atoms.


The ions are accelerated so they strike a
Ti plate and become buried in the plate.


Can be started below 10
-
6

mbar.


Pumping speed is gas dependent and
drops off below 10
-
9

mbar.


Buries the gas in the plate, so no backing
pump is required.


A little less expensive than turbo pumps.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Ion Pump Types


Diode pump: center Ti
electrode is biased positively to
accelerate ions toward pump
wall.


Triode Pump: Intermediate Ti
electrode is biased negatively to
accelerate ions toward pump
wall.

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Cryopump


A He refrigerator is used to
cool a large
-
area surface
where gas is condensed.


The gas absorption depends
on the bonding mechanism
to the cryopanels.


After prolonged use, the
pump must be
“regenerated.”

Ref: Inficon Instrumentation Catalog (2000
-
2001)

Center for Materials for Information Technology

an NSF Materials Science and Engineering Center

Cryo Pump Speed for Various Gases


Pumping speed depends on type of gas and area of
selected cryopanel.

Ref: Inficon Instrumentation Catalog (2000
-
2001)