Vacuum science
Outline
• Kinetic theory of gases
• Performance measures
• Vacuum pumps
– Positive displacement
– Momentum transfer
– Entrainment
• Pressure measurement
Kinetic theory of gases
Maxwell speed distribution:
Average magnitude of velocity:
Root mean square velocity:
Mean free path
• The average distance a gas molecule travels before
colliding with another gas molecule or the container
walls
• Let
be the particle diameter.
• Then
will be the cross-sectional area for hardsphere collisions, and the mean free path:
Gas flow
• Viscous flow
– The gas behaves like a coherent coherent, collective
medium
– Collisions with each other are more frequent than with the
walls
– Occurs at pressures greater than 1 mtorr
• Molecular flow
– Collisions with walls are more frequent than with each
other
– Occurs at pressures less than 1 mtorr
– Mean free path is larger than the chamber length
Performance measures
• Pumping speed (S) is the volumetric flow rate
of a pump at its inlet, volume per unit of time
• Throughput Q
– Q ̴ mass flow rate at const. T
– torr-L/s
– Q = PS
Vacuum pump pressure ranges
Vacuum pumps
• Positive displacement (rough vacuum)
– Mechanical
– 3 steps: capture a volume of gas, compression of the captured
volume, and gas expulsion
• Momentum transfer (high vacuum)
– Knock gas molecules out of the chamber
• High speed jets of dense fluid
• High speed rotating blades
• Entrainment (to and including ultrahigh vacuum)
– Capture gases in a solid or adsorbed state
Positive displacement
• A simple piston pump
Positive displacement
Rotary vane pump
Roots blower pump
Momentum transfer
Diffusion pump
Turbomolecular pump
Entrainment
Ultrahigh vacuum
• Titanium sublimation pump
– Chemisorption or gettering of active gases
– Deposit a thin film of highly reactive metal on the
inside surface of the pump by heating a titaniumcontaining filament
• Sputter-ion pump
– Chemisorption and burying
• On impact the accelerated ions will typically bury themselves
in the cathode as well as sputter some of the cathode
• Also pumps noble gases by burying
Pressure measurement
Capacitance manometer
• Mechanical
• Depends on a pressure difference between
the chamber to be measured and a reference
volume
• Detects the movement of a thin metal
diaphragm
• For rough vacuum - difficult to measure below
1 mtorr
Pressure measurement
Thermal conductivity
• Measures the thermal conductivity of the gas
• Operate by passing a current through a wire
and measure its tempreature
• For rough vacuum - above 1 mtorr
Pressure measurement
Ionization
• For high and ultrahigh vacuum
• An electron stream ionize the gas in the gauge which
again are collected by an electrical field
• This ion current is a function of the chamber
pressure
• Hot cathode (filament)
torr down to
torr
• Cold cathode (plasma)
torr down to
torr