Pressure
1 atmosphere ~ 1 bar ~ 760 mm Hg ~ 760 torr ~ 100,000 Pa
Ion gauges read in mbar i.e. 1x10-10 mbar = 1x10-13 atm.
Sometimes ion gauges read in torr but ours are set to mbar
Lower Pressure
1x10-4 mbar
Rough Vacuum
1x10-8 mbar
High Vacuum
Ultra High Vacuum
Pressure
Pressure Range
Pressure/ mbar
Low Vacuum
103-100
Medium Vacuum
100-10-3
High Vacuum
10-3-10-7
Ultra High Vacuum
10-7-10-12
Extreme Ultra High Vacuum
Pressure
(mbar)
Molecular
density at 298 K
Molecular mean free
path λ N2 at 295 K
Monolayer
time
10-12
2.5 x 10-4 cm-3
3.4 x 104 km
44 days
10-15
25 cm-3
3.4 x 107 km
120 years
10-19
Interstellar space
0.25m-3
2.3 x 103 AU
1.2 x 106 years
Redhead, P. A. Review Literature And Arts Of The Americas 213–226 (1958).
Three main types of pumps
Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow
gases to flow in from the chamber, seal off the cavity, and exhaust it to the
atmosphere.
Momentum transfer pumps, also called molecular pumps, use high speed jets of
dense fluid or high speed rotating blades to knock gas molecules out of the chamber.
Entrapment pumps capture gases in a solid or adsorbed state. This includes
cryopumps, getters, and ion pumps.
Pumping Ranges
people.rit.edu/vwlsps/LabTech/Pumps.pdf
Pumping Ranges
people.rit.edu/vwlsps/LabTech/Pumps.pdf
Viscous vs. Molecular Flow Regimes
The gas in a vacuum system can be in a viscous state, in a molecular state or in a
state which is intermediate between these two.
When a system is brought from the atmospheric pressure to "high vacuum", the gas
in the system goes through all these states. The mean free path of the gas
molecules is very small at atmospheric pressure so that the flow of the gas is limited
by its viscosity.
At low pressures where the mean free path of the molecules is similar to the
dimensions of the vacuum enclosure, the flow of the gas is governed by viscosity as
well as by molecular phenomena; this is the intermediate flow.
At very low pressures where the mean free path is much larger than the dimensions
of the vacuum enclosure, the flow is molecular.
Viscous > 10-4
Molecular < 10-6
Rotary Pump
http://www.quorumtech.com/Products/RV5PUMP.jpg
Rotary Pump
Check oil level once a month
Ensure the pump is able to cool
Mechanical pumps can introduce noise in STM
Turbomolecular Pump
http://www.varianinc.com
http://www.pfeiffer-vacuum.com
Turbomolecular Pump
Turbo pumps utilize a stack of turbine blades which rotate at very high speed
(of order 50,000 rpm) to move gas from the inlet port to the exhaust port.
Turbo pumps are very effective at low pressures (<10 torr), essentially in the
molecular flow regime in which gas densities are so low that the molecules
collide with chamber walls far more often than with each other.
As a result, turbo pumps can achieve chamber base pressures of 10-9 torr or
below.
However, the high packing of fan blades and the high rotation speed of the
turbo pump make it ineffective at higher pressures, where fluid (viscous) flow
dominates.
Powering a turbo pump alone at atmospheric pressure will barely cause the
blades to rotate. THEREFORE TURBOS ARE BACKED BY ROTARY PUMPS
Stalling of turbo pump
Ion Pumps
1 Permanent magnets
2 Pump envelope
3 Titanium cathodes
4 Anode cell array
5 Positive high voltage lead
http://www.thermionics.com/ip_too.htm
Ion Pumps
Sputter ion pumps operate by ionizing gas within a magnetically confined cold
cathode discharge.
The events that combine to enable pumping of gases under vacuum are:
Entrapment of electrons in orbit by a magnetic field.
Ionization of gas by collision with electrons.
Sputtering of titanium by ion bombardment.
Titanium gettering of active gases.
Pumping of heavy noble gases by ion burial.
Diffusion of hydrogen and helium into titanium.
Dissociation of complex molecules into simple ones for pumping ease, e.g., CH4
breaks down into C and 2H2. Hydrogen is pumped separately. Carbon is no
longer part of the residual gas and resides in solid form.
3 -7 kV range. 5 kV
No moving parts or oil: no maintenance or vibration
http://www.thermionics.com/ip_too.htm
Titanium Sublimation Pumps (TSPs)
Resistively heat Ti metal
Thin layer of Ti on chamber walls
This reacts with O2 etc and pumps chamber
N.B. Sample areas must be shielded!
Titanium Sublimation Pumps (TSPs)
Can’t pump un-reactive gases: Noble gases, Argon
TSP are not flashed while at cryogenic temperatures.
This leads to: cryogen boil off due to high temperature
outgassing from the TSP. This will then adhere to any cold
surface, including the sample.
http://www.aiv.it/?q=content/gettering-and-ion-pumping
Cyropumping
N2(L)
He(L)
Overtime the surface area becomes saturated and pumping speed drops to zero
Systems needs to be recharged by warming and pumping outgas by other means
During He(L) fills H2 and CO are outgassed
Dawson, J. & Haygood, J. Cryopumping. Cryogenics (1965)
Useful Websites
Folder in the general user folder on the N drive
http://www.pfeiffer-vacuum.com/know-how
http://www.aip.org/avsguide/refguide/workingpress.html
http://www.duniway.com/images/pdf/pg/ion-pump-op-troubleshoot.pdf
Ion pump working
http://www.gammavacuum.com/operation.asp
Bake out and outgassing
Even once all this is in place, you still cannot achieve UHV. The problem is
that a lot of gas molecules are adsorbed to the walls of your vacuum chamber.
Once you lower the pressure, these molecules are sometimes desorbed and
pumped away but this is a very slow process. The way to accelerate this
(exponentially!) is to increase the temperature. Therefore, one has to perform
a so-called bakeout of the system in which the entire UHV system is heated to
a high temperature, typically higher than 100 degrees Celsius. Heating the
system up to this temperature can take a long time, especially if you have
thermally well-insulated parts inside which are only heated by radiation. The
same is true for cooling it down again, so the whole procedure can take a
more than a day.
When cooling down, it is a good idea to operate any filaments (and devices
containing filaments, such as ion gauges, sputter guns, titanium sublimation
pumps, residual gas analyzers and so on) at a higher temperature than you
usually use under normal operation, in order to avoid massive desorption of
the adsorbed rest gas later. This procedure is called outgassing.
Useful Websites
http://www.aip.org/avsguide/refguide/workingpress.html