Cleaning for Vacuum

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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.