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 Molecular 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 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