20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Plasma CleaningA new method of ultra-cleaning

detector cryostats

Sebastian Deiries, Armin Silber, Olaf Iwert,

Evi Hummel, Jean Louis LizonEuropean Southern Observatory ESO (http://

www.eso.org)

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Introduction

• Why cleaning of CCD cryostats?• Contamination is more a problem with optical than IR

detectors blue wavelengths• Conventional cleaning (mechanical, with detergent,

isopropyl-alcohol, acetone & baking) insufficient?• Plasma cleaning used in industry for ultra cleaning• It is a simple method with easy installation:

First results after a few days!• It is rather cheap and has very low cleaning costs• It gives very high efficient cleaning results• The risks are low

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Conventional CleaningConventional Cleaning

(See also “Ultra-clean CCD Cryostat – CCD contamination can be kept under control” from S. Deiries, SDW2002 )

Cryostat part in ultrasonic bath

Baking of cryostat parts in the vacuum oven depending from the

material up to 160 o C

Chemicals and detergents for ultrasonic and chemical cleaning Precautions and

careful handling of the chemicals

Before plasma cleaning can be applied, the piece or cryostat should roughly be clean. Normally we apply our standard cleaning processes:

• mechanicalmechanical cleaning, • ultrasonic ultrasonic cleaning with special detergents, • chemical chemical cleaning with isopropyl alcohol or acetone• bakingbaking in vacuum oven

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Cryostat components after Cryostat components after conventional cleaningconventional cleaning

Because cryostat looks clean after conventional cleaning: but there are still (organic) contaminations on the surface.

My colleague Armin Silber had the idea to introducePlasma cleaning as a final ultra cleaning step:

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Plasma Plasma cleaningcleaning process process

During plasma cleaning the surface is cleaned• physicallyphysically by ion bombardment (like sand-blasting)• chemicallychemically e.g. by active oxygen (oxidation and reduction).

Application: Removal of very thin films of oil, oxides and greases

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Realization of Plasma cleaningRealization of Plasma cleaning

Existing vacuum oven, refurbished to a plasma oven

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20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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High voltage High voltage plasma controllerplasma controller

LFG40 from company DienerLFG40 from company Diener

Plasma control

High voltage control

Pressureregulation

0.1 – 1 mBarVoltage display

Plasma regulation

http://www.dienerelectronic.dehttp://www.dienerelectronic.de

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Plasma cleaning devicesPlasma cleaning devices

Modified vacuum oven, equipped with a high voltage anode

Powerful dry vacuum pump:Alcatel ACP28

High voltage plasma controller LFG40

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Use of different gasesUse of different gases

• Oxygen - Best for cleaning of aluminium and stainless steel

• Ambient air - Sufficient for normal cleaning

• Argon - Cleaning without chemical reaction

• Hydrogen - Best for cleaning of gold, silver and copper

• Nitrogen - Similar as Argon

• Flour-containing gases - Chemical cleaning

• Mixtures of the gases mentioned above

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Special arrangement: Special arrangement: Plasma Cleaning inside everyPlasma Cleaning inside every

assembled cryostatassembled cryostat

Trans-former

(Diener)

High Voltage Anodemounted outside on the cryostat window

Ground cable fixed at the case

Controller Diener

Electronic LFG40

CryostatNo

disassembly necessary !

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Plasma cleaning devicesPlasma cleaning devicesSuccessful plasma cleaning inside the assembled OmegaCam cryostat

After some minutes the large volume (approx. 200 l) of the cryostat was filled by a violet glowing plasma. A normal cleaning process takes not longer than 10 minutes!

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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OmegaCam cryostat during plasma cleaningOmegaCam cryostat during plasma cleaning

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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VideoVideofrom Plasma ignitionand cleaning

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Safety measuresSafety measures

• Exhaust gasesExhaust gases should not go to the working laboratory (small toxic risk)

• UV radiationUV radiation is absorbed by oven window

• Electrical safetyElectrical safety

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Plasma cleaning costsPlasma cleaning costs

Procurement costs:Vacuum oven ~8000 EURDiener Electronic LFG40 Plasma Controller ~5000 EURVacuum pump 7000 EUR

Maintenance:Electrical costs per cleaning process 0.20 EURCosts of special gases ~200 EUR per annumMaintenance >100 EUR per annum

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Materials cleaned by plasmaMaterials cleaned by plasma

(Coated) Epoxy fiber glass

Gold plated copper

AluminumStainlesssteel

PCBs

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Limitations with plasma Limitations with plasma cleaningcleaning

• Plastic materials and PCBsshould not be plasma-treated longer than 10 minutes; risk of becoming black

• Damage of CCD detectors?Test ongoing; which of the manufacturer present at the SDW2005 is interested and would supply a sample CCD?

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How to handle plasma How to handle plasma cleaned partscleaned parts

A plasma cleaned cryostat part must be handled strictly under clean-room condition to avoid recontamination.

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Test of cleaning resultTest of cleaning result

• Water drop method:Water drop method:

A sample, which is contaminated shows a round water drop on its surface. After the plasma cleaning there is less adhesion, therefore no water drop on the surface possible: the water is flowing away.

contaminatedPlasma cleaned

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Test of cleaning resultTest of cleaning result

• End End vacuumvacuum test: test:

Typical vacuum end pressure values of ESO standard cryostats (after 1h pumping time):

Before plasma cleaning: 1 x 10E-3mBar

After plasma cleaning: 5x 10E-5mBar

20. June 2005 SDW2005: Plasma Cleaning S. Deiries, ESO

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Test of cleaning result:Test of cleaning result:

Many lines disappear or are reduced significantly

• Mass-spectrum test:Mass-spectrum test:

Water

COO2 CO2

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ConclusionsConclusions

• Very effective cleaning of (hidden) contaminants as an addition to conventional cleaning

• Fast Cleaning method

• Successful cleaning of assembled cryostats (OmegaCam)

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Many Thanks Many Thanks for Listeningfor Listening