Cleaning for Ultra- High Vacuum (UHV)

Cleaning for Ultra-
High Vacuum (UHV)
Keith Middleman
Vacuum Science Group
ASTeC
CCLRC Daresbury Laboratory

Outline of Presentation
• UHV Limiting Factors
• Why Do We Need To Clean for Accelerators
• Methods to Reduce Outgassing
• SRS Cleaning Procedure
• Methods of Testing
• Search for New Cleaning Solvents

Limiting Factors for achieving UHV
• Material Choice
• Gas Sources
• Contaminant: prevents vacuum system reaching required base pressure or
introduces unwanted species into the residual gas.
• Outgassing: The outgassing rate is the time-dependent rate at which gases
and vapours are released under vacuum. This limits the ultimate pressure
achievable and can introduce contamination into the vacuum system.

Why Clean for Accelerator Vacuum Systems
• UHV Total Pressures Required ~ 1x10 -9 mbar
• Maintain Satisfactory Lifetime Stored Electron Beam
• Electron Scatter ∝ Atomic Number 2
• Low presence of high mass species
• Hydrocarbons < 0.1% Pump Lubricants < 0.01%
• Stimulated desorption
• Electron or Ion induced - Major gas source
• Desorption Yield (Number of molecules per incident photon)
• Maintain Clean In-Vacuum Surfaces
• Prevent Particle Target Poisoning
• Maintain Efficient Optical Properties for EM Radiation Transport

Methods to Reduce Outgassing
• Bakeout - If the pressure is not low enough we can
reduce the thermal outgassing rates by reducing the
surface coverage N or the temperature T.
• Beam Cleaning
• Passivation
Oxide films
Nitride coatings (TiN, BN)
Active films (NEG coatings)

Methods to Reduce Outgassing
• Ultrasonic Cleaning - widely used
• Glow discharge
• Electropolishing

SRS Cleaning Procedure
CLEANING PROCEDURE
EX-SITU BAKEOUT
• Hot water jet with detergent
• Surface stripping with alumina
beads
• Ultrasonic wash in hot Triklone
• Vapour Wash in hot Triklone
• Rinse with de-min water
• Immerse in hot alkaline bath of P3-
Almeco 36 at 60°C
• Rinse in de-min water
• Dry in warm, dust free air, bag and
seal.
• Leak check (< 10 -10
• Bake to 250°C for 24 hours
• Record RGA scan
mbar l s -1 )
Effective pre-treatment
ensures outgassing rates
are reduced to < 10-11 mbar
l s -1 cm -2

Cleaning Facilities
Water Jet and Detergent Phase

Cleaning Facilities
Vapour wash

Testing - Is it ‘Clean’
• Surface Analysis Techniques (Detailed Surface Composition)
• AES - Auger Electron Spectroscopy
• SIMS - Secondary Ion Mass Spectrometry
• Scanning Electron Micrograph
• Phenomenological Testing (Surface Vacuum Performance)
• Measure Total Outgassing Rates - Total Pressure Gauge
• Measure Partial Pressures - RGA
• Stimulated Desorption (Friction, Electron or Ion Induced, Thermal)

Replacement of 1,1,2-Trichloroethylene
• Why Change - Reclassification of 1,1,2-Trichloroethylene (Trike TM )
• What is important to us - Thermal outgassing and ESD
• Throughput method of measuring outgassing rates
Q
=
P1
− P2
A
⋅C
• Comparative Tests - existing procedure proven for 20 years

Outgassing Measurement Facility

Thermal Outgassing Results
Experiment Information
Thermal Outgassing Rate
Q (mbar l s -1 cm -2 )
Blank Run
No Contamination Cleaned with Trike
Full Contamination. Cleaned with Trike
Full Contamination. Cleaned with Lenium
Full Contamination. Cleaned with Leksol
Full Contamination. Cleaned with Novec HFE
Full Contamination. Cleaned with IPA
Full Contamination. Cleaned with Micro 90
Full Contamination. Cleaned with Lancerclean
9.3E-14
7.9E-13
9.3E-13
2.8E-12
9.3E-14
7.5E-13
7.5E-13
3.7E-13
4.6E-12

RGA Data - Thermal Outgassing
Acceptable UHV Scan
Hydrocarbon < 0.1%
Pump lubricants < 0.01%
Unacceptable UHV Scan
Hydrocarbon ~ 25%
Pump lubricants ~ 5%

RGA Data - Electron Stimulated Desorption
Desorption Yield ~ 10 -3 molecules / e -
Desorption Yield ~ 4 molecules / e -

Summary
• Accelerators (storage rings) require UHV
• Need to minimise thermal outgassing and
stimulated desorption
• Requires a detailed cleaning recipe
• Phenomenological Testing