High-Vacuum Technology Course Week 8

High-Vacuum Technology Course Week 8 Paul Nash HE Subject Leader (Engineering)

Vacuum Technology • Recap on last session • Progress on assignments to date

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

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

Conductance of Fittings • For fittings in series:

Conductance of Fittings • For fittings in parallel:

Conductance • Effect of an orifice on pumping speed: How much faster is pump B?

Conductance • Based on the equation we saw earlier: Pump A Pump B

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

System Design Considerations

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:

Gas and Vapour Load • Volume of System • Materials of construction and surface finish • Leakage into the system • Permeation and diffusion through walls and seals • Outgassing of process material • Back migration from pump

Gas and Vapour Load Leakage Outgassing Residual gas Process load (vapour, trapped volumes etc.) Back streaming To pumping system

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.)

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

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

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:

Outgassing • Total Gas Load • Pult = Ultimate pressure • Sp = Pump speed • q = Outgassing rate of material 1, 2 etc. • A = Area of material 1, 2 etc.

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)?

Outgassing • Examples: Now assume the same chamber is made of polished Stainless Steel What is the pressure after 4 hours – (assuming no leakage)?

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

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

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

Baking

High-Vacuum Technology Course Week 8