1. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 1 Plasma Cleaning A new method of ultra-cleaning detector cryostats Sebastian Deiries, Armin Silber, Olaf Iwert, Evi Hummel, Jean Louis Lizon European Southern Observatory ESO (http://www.eso.org)http://www.eso.org

2. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 2 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

3. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 3 Conventional 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: mechanical mechanical cleaning, ultrasonic ultrasonic cleaning with special detergents, chemical chemical cleaning with isopropyl alcohol or acetone baking baking in vacuum oven

4. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 4 Cryostat components after conventional 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 introduce Plasma cleaning as a final ultra cleaning step:

5. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 5 Plasma cleaning process The surface is cleaned physically by ion bombardment and UV radiation and chemically by active oxygen. During plasma cleaning the surface is cleaned physically physically by ion bombardment (like sand-blasting) chemically chemically e.g. by active oxygen (oxidation and reduction). Application: Removal of very thin films of oil, oxides and greases

6. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 6 Realization of Plasma cleaning Existing vacuum oven, refurbished to a plasma oven

7. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 7

8. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 8 High voltage plasma controller LFG40 from company Diener Plasma control High voltage control Pressure regulation 0.1 – 1 mBar Voltage display Plasma regulation http://www.dienerelectronic.de

9. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 9 Plasma cleaning devices Modified vacuum oven, equipped with a high voltage anode Powerful dry vacuum pump: Alcatel ACP28 High voltage plasma controller LFG40

10. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 10 Use 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

11. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 11 Special arrangement: Plasma Cleaning inside every assembled cryostat Trans- former (Diener) High Voltage Anode mounted outside on the cryostat window Ground cable fixed at the case Controller Diener Electronic LFG40 Cryostat No disassembly necessary !

12. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 12 Plasma cleaning devices Successful 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!

13. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 13 OmegaCam cryostat during plasma cleaning

14. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 14 Video Video from Plasma ignition and cleaning

15. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 15 Safety 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

16. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 16 Plasma cleaning costs Procurement costs: Vacuum oven~8000 EUR Diener Electronic LFG40 Plasma Controller~5000 EUR Vacuum pump 7000 EUR Maintenance: Electrical costs per cleaning process0.20 EUR Costs of special gases~200 EUR per annum Maintenance>100 EUR per annum

17. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 17 Materials cleaned by plasma (Coated) Epoxy fiber glass Gold plated copper Aluminum Stainless steel PCBs

18. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 18 Limitations with plasma cleaning Plastic materials and PCBs should 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?

19. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 19 How to handle plasma cleaned parts A plasma cleaned cryostat part must be handled strictly under clean-room condition to avoid recontamination.

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

21. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 21 Test of cleaning result End vacuum test:End vacuum 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

22. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 22 Test of cleaning result: Many lines disappear or are reduced significantly Mass-spectrum test: Mass-spectrum test: Water CO O2 CO2

23. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 23 Conclusions Very effective cleaning of (hidden) contaminants as an addition to conventional cleaning Fast Cleaning method Successful cleaning of assembled cryostats (OmegaCam)

24. 20. June 2005SDW2005: Plasma Cleaning S. Deiries, ESO 24 Many Thanks for Listening