Vacuum Fundamentals

High-Vacuum Technology Course

Week 8

Paul Nash

HE Subject Leader (Engineering)

Vacuum Fundamentals

• Recap on last session

• Progress on assignments to date

Vacuum Technology

Vacuum Fundamentals

Conductance

• Resistance to gas flow of the components has an influence on pumping speed and ultimate pressure obtainable

• Every component in the system has a volume to be pumped and in addition gives some resistance to gas flow

Vacuum Fundamentals

Conductance

• Components can include (in addition to the process chamber itself)– Valves– Gauge head fittings– Pipelines and fittings

• Each of these has a ‘Conductance’ and is generally in manufacturers data– This is the inverse of resistance

Vacuum Fundamentals

Conductance of Fittings

• For fittings in series:

..........1111

321 CCCCTOTAL

Vacuum Fundamentals

Conductance of Fittings

• For fittings in parallel:

..........321 CCCCTOTAL

Vacuum Fundamentals

Conductance

• Effect of an orifice on pumping speed:

How much faster is pump B?

Vacuum Fundamentals

Conductance

• Based on the equation we saw earlier: CS

CSS

P

Pc

121.116.11300

6.11300

lsSc

161.116.1130000

6.1130000

lsSc

Pump A

Pump B

Vacuum Fundamentals

Volume• The volume of the system is really the sum of

the parts – not just the chamber– Pipelines can have a significant impact– Some valves may have long flow paths– Extended tubulation should be avoided – remember

gauge head mounting?

Vacuum Fundamentals

• System Design• Gas and Vapour Load• Outgassing• Baking• Pump down time

Vacuum Technology

Vacuum Fundamentals

System Design Considerations

Vacuum Fundamentals

Gas and Vapour Load

• A significant factor in the selection of a pumping system

• The total gas and vapour load is affected by the following:

Vacuum Fundamentals

Gas and Vapour Load

1. Volume of System

2. Materials of construction and surface finish

3. Leakage into the system

4. Permeation and diffusion through walls and seals

5. Outgassing of process material

6. Back migration from pump

Vacuum Fundamentals

Gas and Vapour LoadLeakage Outgassing

Residual gas

To pumping system

Process load (vapour, trapped volumes etc.)

Back streaming

Vacuum Fundamentals

Gas and Vapour Load

• Vapour pressure of materials used must be much less than the required ultimate pressure

• Surfaces should produce minimum outgassing by being cleaned and preferably polished to reduce surface area

• Surfaces should not be affected by exposure to atmosphere (rust etc.)

Vacuum Fundamentals

Gas and Vapour Load

• Most common substance on surfaces is water vapour. Sytems under vacuum can be raised to atmosphere using an inert gas such as dry nitrogen. This helps subsequent pumpdown

• Baking will speed up the outgassing process – essential to achieve UHV

Vacuum Fundamentals

Outgassing

• The spontaneous evolution of gas from a material – becomes a significant factor as pressure reduces

• The outgassing rate is the quantity of gas given off per unit time by every unit of surface area in the system

• Usually expressed as mbar litres per second per cm2 mbar l s-1 cm-2

Vacuum Fundamentals

Outgassing

• Outgassing rate decreases with time and eventually reaches a constant value (typically after 4 hours)

• Polymers and elastomers outgas at rates 100s of times higher than most metals and glass

• Typical outgassing rates are shown on the next slide:

Vacuum Fundamentals

Outgassing

• Outgassing rate decreases with time and eventually reaches a constant value (typically after 4 hours)

• Polymers and elastomers outgas at rates 100s of times higher than most metals and glass

• Typical outgassing rates are shown on the next slide:

Vacuum Fundamentals

Vacuum Fundamentals

Outgassing

p

totult S

QP

..........2211 AqAqQtot • Total Gas Load

• Pult = Ultimate pressure

• Sp = Pump speed

• q = Outgassing rate of material 1, 2 etc.• A = Area of material 1, 2 etc.

Vacuum Fundamentals

Outgassing

• Examples:Process chamber of mild steel has surface area

50000 cm2

Seals made of Viton with surface are 250 cm2

Pump speed 280 ls-1

What is the pressure after 4 hours – (assuming no leakage)?

Vacuum Fundamentals

Outgassing

• Examples:Now assume the same chamber is made of

polished Stainless Steel

What is the pressure after 4 hours – (assuming no leakage)?

Vacuum Fundamentals

UHV System Design

• Eliminate elastomers, hydrocarbons and greases

• Avoid use of poor outgassing materials – eg: mild steel, porous materials, poor surface finishes

• Bake the system

Vacuum Fundamentals

UHV System Design

• Eliminate materials that cannot be baked

• Use clean techniques – gloves, clean atmosphere

• Stainless steel is popular– Low outgassing– Doesn’t readily corrode– Can be baked to 750oC

Vacuum Fundamentals

Baking

• Typical bakeout temperatures between 250oC and 450oC

• Maximum baking temperature may be limited by materials

• All components should be baked to similar temperatures – gases will transfer from hot areas to cooler areas

Vacuum Fundamentals

Baking

Vacuum Fundamentals

Baking