cleaning for vacuum
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Cleaning for Vacuum
1
19 May 2003
Review HW 5
Review for Final Exam
Cleaning for Vacuum
Final Exam Venue
Friday, 23rd May from 5:30 to 7:30 PM
Exam will be two hours in Length
Exam is 40% of the grade
It is possible to do poorly on the exam and still get an “A” in this class.
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Things to bring
It is recommended that you collate your class notes into a 3-ring binder.
First pages should be the Formulae
Calculator
Pencil and eraser
Guidelines for Review of the Final
Understand the use of the equations
Understand how to read and plot logarithmic data
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Requirements of Vacuum Parts Cleaning for wetted surfaces
Parts should EconomicallySupport a low base pressure
Favor a shortest reasonable pump down time
Introduce a low contaminant level to the system
Be easily inspected
Be easily cleaned
Supporting a low Base Pressure
Have a Low outgassing rate
Support a bake out if required
Subsystems of parts are already leak-checked or qualified
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Favoring a short Pump down time
Minimal real or virtual leaks (trapped volumes)
Surfaces have a low ability to absorb or desorb gases
Introducing allow contaminant level to the system
Particles can migrate, interfering with process, base pressure or seal surfaces
Some vacuum components generate particles in vacuum (unlubricated sliding metal surfaces for example)
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Being easily inspected & Cleaned
Dirt, cleaning spots, or oils cannot easily hide on shiny surfaces in under bright light conditions. This makes for easier quality assurance.
Cleaning costs are usually not a high percentage of total fabrication costs for small components, but can be for big ones.
Surfaces easy to visually QA include
Machined Surfaces
Electropolished surfaces
Ball Polished Surfaces
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Economics of Cleaning
The amount of effort put in to cleaning should be economically influenced by the process goals
Freeze drying apples
Decorative coatings
Micro electronics
Surface science
increasing cleanlinessrequired
Examples of contaminants
Films
native oxides, Aluminum, Stainless, etc
oils from machined, handled parts}
Salt (sodium chloride) from sweat glands
Coatings that can be contaminants or create trapped volumes, such as electroplated metals, paint, varnish, plastic, Cadmium, Lead
Sulfur bearing machining oils that are subsurface imbedded
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Particle Contaminants that dominate our world
Soot, organic Fibers, metal flakes, residual organic matter
General techniques
Films:
In general are removed by wet chemical techniques; mechanical methods may alter the part
Particles:
Removed by kinetic energy, such as wiping, blowing, high pressure spray, ultrasonics,
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Cleaning Methods
Machining, Sand blasting
Vapor degreasing
Ultrasonic Bath
Water Based Detergents
Chemical polishing
Electro-polishing
High press. spray
Chemical methods
Vacuum bake-out
De-ionized H20 Rinse
General order of cleaning steps
Mechanical cleaning
Degreasing
Detergent cleaning
High Pressure Spray
Chemical cleaning (pickling)
Electrolytic polishing
Vacuum bake-out
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Mechanical cleaning
Machining
Sand blasting
Bead blasting
Wire brushing
Notes on machining
Particles may become imbedded in the machined part!
Cutting fluids will be on the surface of the part and in any open pores. Avoid cutting fluids with high vapor pressure components such as Sulfur, Lithium.
Use sharp tools to avoid entrapping cutting fluids.
Surface finish WILL affect outgassing rate
(Torr-liters/sec/cm2 )
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Sand and bead blasting
° Surface area of the part will significantly increase!
° Part will be more difficult to degrease in the future
° Abrasive particles may become imbedded in the part
Soap & detergent
detergents
soaps
phosphatessilicates
glycerines
salts of fatty acids
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Detergents
Synthetic materials that have a cleansing action and act as surfactants
Anionic: sodium alkylbenzene sulfonate
Non-ionic: ethoxylated long chain alcohols
Cationic: ammonium chlorides
Amphoteric: alkyl imidazoline
ref: Metals Handbook, 9th ed. Volume 5, pp.3-28
Soaps
Oil, grease, fat + alkali (NaOH) => sodium oleate
Na(C17H33O2)
Na+ C17H33O2
−
sodium hydrocarbon
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Emulsion cleaning
Removing heavy soils from surfaces using
organic solvents dispersed in water, and an
emulsifying agentParticles of the contaminant are separated from
the surface of the part, & held in suspension
ref: Cleaning and Pickling for Electroplating, American Electroplater's Society
Solvent cleaning
Dissolution of contaminant by a liquid.
Solvents:
trichloroethylene
methylene chloride
benzene
ethanol
acetone
New ES&H guidelines may prohibit the use of the most efficient solvents!
Identify the contaminant, use the correct solvent!
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EPA approved solvents:
Ethyl alcoholIsopropyl alcohol
TurpentineAcetone
Benzene
Carbon tet.
Chloroform
Methanol
MEK
TCE
Methylene choride
Toluene
Xylene
Hazardous wastes:
ref: 40 CFR Sections 261.31, 261.33(f)
Saponification
Alkalai in the cleaning agent combines with oils to form water soluble soap that is washed away
oil film
Na+water solublesoap
part
ref: Cleaning and Pickling for Electroplating, American Electroplater's Society
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Dispersion
Solid particles are broken up into smaller fragments by the action of surfactants and active ingredients in the cleaner.
ref: Cleaning and Pickling for Electroplating, American Electroplater's Society
Ultra-sonic cleaning
High frequency energy used to create pressure waves in the cleaning fluid to cavitate contaminants off the part
° May use a wide range of solvents.
° Generally used on small parts.
° Re-contamination is possible.
° Most effective on small particles (< 100µm diam.)
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Megahertz Transducers require line of sight to the surface
Basics of Ultrasonic Cleaning
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Hydraulic Microcavities form when local pressure drops below ~18 Torr. Collapse is
catastrophic
High, sweeping frequencies are required for small particle cleaning
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The fluid becomes an acoustic structure with standing waves.
Vapor degreasing
Hot vapor is used to dissolve oils from a part suspended above the boiling solvent.
° Proper match of solvent to contaminant is essential.
° Part must be in vapor for sufficient time.
° Not an effective technique for removal of small particles.
° Solvents: TCE, perchlor, Freon TF&TE.
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Chemical cleaning
° Surface may be roughened by grain boundary etching
° Sharp edges of parts may become rounded
° Surface of part will be contaminated with cleaning solution (Cl, Na, P, F, S, etc.)
° Post treatment required (neutralizing, rinsing)
Part is immersed in an active chemical (strong acid or base) to remove surface films
Electrolytic cleaning
The part, immersed in an electrolyte, is made an electrode in an electrolytic cell (opposite of electroplating)
° Metal removal will be highest at sharp corners and high spots
° Near surface will absorb hydrogen (post-treatment degassing required)
° Fluids: aluminum- Electro-glo 100
stainless steel: Electro-glo 300
° Voltage: 5-10 VDC, 0.5 to 1.0 A/sq in
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High pressure spray
Impact of high velocity liquid spray removes particulates
° Used on optical components
° 50 to 8000 PSI pressures used
° Cleanroom operation
Vacuum bake-out (thermal desorption)
Goal is to remove any adsorbed or absorbed gases remaining in the part following previous cleaning procedures
Consider the part when selecting the bake-out temperature & duration. (Grain growth, diffusion, decomposition, creep, delamination)
Monitor the process with RGA
Literature references : Journal of Vacuum Science & Technology 6, 13
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Photo desorption
Contaminants on surfaces inside a vacuum
vessel may be removed by high intensity
ultraviolet radiation° Photodesorption works well to remove water
vapor from surfaces inside the vessel
Other In-situ cleaning
° Electron bombardment- electrons emitted by a tungsten filament are accelerate to part by applied electric field
° Ion bombardment- ionized gases are electrostatically attracted to biased part (glow discharge cleaning)
° Beam may be directed at specific areas
° Reactive gas (oxygen) may be used
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Easy Mistakes
Wrapping parts in ordinary kitchen aluminum foil after cleaning. Use instead special aluminum foil designed for vacuum system packaging.
Leaving parts on a shelf in a non-clean area.
Cleaning parts in an HVAC system shared with a machine shop.
Typical 1980’s cleaning procedure for UHV stainless steel:
° Vapor degrease in trichloroethylene
° Detergent clean (Oakite) with scrubbing
° Hot water rinse. (DI water)
° Acid clean (33% HNO3 + 33% HF + 33% H2 O room temp.)
° DI water rinse
° Dry with air or gas which is free of oil & water
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UHV Gate Valve at Hanford Ligo
Note Stamping here
Two Extreme Cases
DCS Exhaust at LAM
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Cleanroom at Lam Research Corporation
Two pairs of booties
two pairs of gloves
Drink water before you enter (for smokers)
Optional for class 100: Face Mask and battery powered HEPA systems.
Great hangout for allergy sufferers.
No Grease permitted in the LBL/ALS Tunnel. Why?
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Typical Fabrication and Assembly procedures
Use of Poly bags between manufacture and assembly
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O-rings are post-cured in heated chambers to ensure low outgassing
Special handling fixture manages UHV assemblies in preparation for welding
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Fabrication Processes are tailored to vacuum needs
Machining Operations Welding
Environmentally Friendly Cleaning Procedures
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Three Step Hot DI rinse with Ultrasonic power
UHV RF Gate Valves for SLAC B Factory
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Modern Clean rooms for Assembly
tools for Managing Defects: Clean rooms and Minienvironments
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Moving air properly in cleanrooms can be expensive
Proper Lighting is important in assembly areas
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Vacuum Grease
Proper management of this substance is essential in cleanroomassembly.
Ball Polishing
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RGA can be the final word on cleanliness
Sample pumped at room temperature for 16 hours, and then sampled with an RGA in a prequalified, baked system.
A reasonable UHV SpecThe resulting highest peak above atomic mass 44 is < 1/1000th the peak of Water (Mass 18).
Other tools are also available
Electron Spectroscopy for Chemical Analysis
Auger Electron Spectroscopy.
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