1. Vacuum science

2. Outline Kinetic theory of gases Performance measures Vacuum pumps
Positive displacement
Momentum transfer
Entrainment
Pressure measurement

3. Kinetic theory of gases
Maxwell speed distribution: Average magnitude of velocity: Root mean square velocity:

4. 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:

5. 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

6. 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

7. Vacuum pump pressure ranges

8. 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

9. Positive displacement
A simple piston pump

10. Positive displacement
Rotary vane pump
Roots blower pump

11. Momentum transfer
Diffusion pump
Turbomolecular pump

12. 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

13. 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

14. 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

15. 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