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 hard-

sphere 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 titanium-containing 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 measurementCapacitance 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 measurementThermal 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