1. Characterisation of technical surfaces at cryogenic temperature under electron bombardment.
Bernard HENRIST, CERN TE/VSC 5/6/2018

2. Overview Setup overview Measures and Procedures Conditioning curves
Gas coverage studies
Conclusions

3. Setup overview – Sample transfer
Advantages:
Fast access : Atmosphere to mbar in 10 minutes
Measurement of non baked sample in UHV environment
Better sensitivity (low background)
Sample rack in the storage chamber
Sample in the load lock
Loadlock and storage chamber

4. Setup overview – Manipulator
Fully motorized 4 axis manipulator: X,Y, Z and R
Standard Omicron flag type sample.
Sample floating for bias and current measurement.
Reproducibility of positions
Liquid helium cooling at 10K in 1h
Front cover to minimize thermal losses.
Second sample holder for accessory (not cooled)
Faraday cup

5. Setup overview – Gas injection
2 injection modes:
Injection through a diaphragm for flux measurement: Q=C∙∆𝑝 and pumping speed determination S= 𝑄 𝑝
Using a calibrated volume and injection through a retractable injector to build a ice layer on a cold surface 𝑛= ∆𝑝∙𝑘∙𝑇 𝑉

6. Setup overview – Beam visualization
Phosphor on secondary sample place
Dino-Lite Edge AM73115MTF 1X~70X 5.0MP Digital Microscope
Online size measurement capabilities

7. Measures and procedures - Beam current
Beam current must be known for SEY and ESD
Faraday cup on the gun
Faraday cup under the sample
Bias of the sample A
Electron Beam
Secondary
electrons
Bias
+45V Is Ib I2
𝐼 𝑠 = 𝐼 𝑏

8. Measures and procedures - SEY
Electron Beam
Secondary electrons
Bias
-45V Is Ib I2
Bias of -45V applied to the sample to repel the secondary electrons
Beam energy corrected of 45eV
Ib known -> SEY can be computed.
𝑆𝐸𝑌: 𝛿= 𝑒𝑚𝑖𝑡𝑡𝑒𝑑 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 = 𝐼 2 𝐼 𝑏 =1− 𝐼 𝑠 𝐼 𝑏

9. Measures and procedures - ESD
Desorption measurement of a gas layer:
Preparation of 500ML of gas:
Condensed gas surface: 2cm2
1ML=8∙ 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠/ 𝑐𝑚 2
500ML=8∙ 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠
V=0.75𝑙
∆𝑝= 𝑛∙𝑘∙𝑇 𝑉 =4.3∙ 10 −2 𝑚𝑏𝑎𝑟
Electron bombardment at 300eV:
𝜼 = 𝑛𝑏 𝑚𝑜𝑙𝑒𝑐. 𝑛𝑏 𝑒 − = 𝑄 [𝑚𝑜𝑙𝑒𝑐./𝑠] 𝐼 [ 𝑒 − /𝑠] [𝑚𝑜𝑙𝑒𝑐./ 𝑒 − ]
𝑄=∆𝑝∙𝑆

10. Conditioning - Copper Beam energy 300eV RT 10K Cu : OFE copper
5/2017 Prevac
Copper Flag type sample, as receive state
Beam energy 300eV RT
10K
Ref. LHC Project Report 433, EPAC 2000
– The secondary electron yield of technical materials and its variation with surface treatments.
V. Baglin et al.
Ref. Journal of Vacuum Science & Technology, 11/7/2012
– Secondary electron yield on cryogenic surfaces as a function of physisorbed gases.
A.Kuzucan et al.
SEY and conditionment results are confirmed in literature.
Same result of the conditioning at 10K

11. Conditioning - Copper DHP (Deoxidized High residual Phosphorous)
Beam energy 300eV
Cu-OFE RT
Cu-DHP RT
DHP copper give a lower SEY but quite no conditioning.

12. Conditioning - LESS Beam energy 300eV RT 10K
LESS : Laser-Engineered Surface Structures on DHP copper (type C)
Copper Flag type sample (Ferrovac) clean at CERN (LHC procedure)
LESS treatment 15/3/2018 Dundee.
Parameters: “Coldex like”.
Linearly polarized 10-ps laser beam wavelength of 532 nm 200 kHz.
Diameter ~ 12 µm.
Average laser pulse energy of 5 µJ, energy fluence 4.2 J cm-2
Beam intensity of ~ 0.4 TW cm-2 .
Raster scanned 10 mm s-1 (LH) scanning. 240 pulses per spot.
distance between consecutive lines 24 µm.
Treatment under laminar N2 flow.
Beam energy 300eV RT
10K
Ref. IPAC 2016
– Low secondary electron yield of laser treated surfaces
of copper, aluminium and stainless steel.
R. Valizadeh et al.
Initial SEY results is lower than literature probably due to the DHP copper.
No difference between RT and 10K

13. Conditioning - aCC Beam energy 300eV RT 10K
aCC : Amorphous carbon coating on DHP copper
Copper Flag type sample (Ferrovac)
clean at CERN (LHC procedure)
Carbon coating: 17/3/2018 CERN
500nmTi + 50nmC
Beam energy 300eV RT
10K
Ref. Vacuum 98 (2013) 29-36
– Carbon coating with low secondary electron yield.
P.Costa Pinto et al.
SEY and conditioning results at room temperature confirmed in literature.
No difference between RT and 10K

14. Gas coverage - Solid N2 phosphorescence1 2 3 4 1 2 3 4
Green spot. Similitude in the spectrometry of aurora.
Stable pressure during drilling of a hole.
Once the hole appear, the desorption start to decrease.
Ref. Nature May 17, 1924
– The Auroral Spectrum and the Upper Atmosphere.
L. Vegard

15. Gas coverage – ESD of 500ML N2 on LESS at 10K at 300eV
N2 pic28 variation:
∆𝑝 𝑁 2 =1.4∙ 10 −10 [𝑚𝑏𝑎𝑟]
𝑆 𝑁 2 10𝐾 =3700 [𝑙/𝑠]
𝑄 𝑁 2 =5.3∙ 10 −7 [𝑚𝑏𝑎𝑟∙𝑙/𝑠]
𝐼 𝑏 =1.16∙ 10 −6 [𝐴]
𝜼 𝑵 𝟐 = 𝑛𝑏 𝑚𝑜𝑙𝑒𝑐. 𝑁 2 𝑛𝑏 𝑒 − =𝟏.𝟖 [𝑚𝑜𝑙𝑒𝑐. 𝑁 2 / 𝑒 − ]

16. Gas coverage - SEY Is I2 Ib 𝑆𝐸𝑌: 𝛿= 𝐼 2 𝐼 𝑏 =1− 𝐼 𝑠 𝐼 𝑏 Bias -45V A
𝑆𝐸𝑌: 𝛿= 𝐼 2 𝐼 𝑏 =1− 𝐼 𝑠 𝐼 𝑏 A
Electron Beam
Secondary electrons
Bias
-45V Is Ib I2
Charging effect

17. Gas coverage – ESD of 500ML N2 on aCC at 10K at 300eV
ESD seems to be not dependent
to the substrate
𝜼 𝑵 𝟐 =𝟏.𝟗 [𝑚𝑜𝑙𝑒𝑐. 𝑁 2 / 𝑒 − ]
Charging effect

18. Conclusion
The setup is designed to measure ESD and SEY at cryogenic temperature.
SEY of aCC and LESS at cryogenic temperature remain bellow 1.
The conditioning behavior are the same at room temperature and at cryogenic temperature.
A thick layer of nitrogen at 10K has a ESD of about 1.8 molecule per electron not linked to the substrate (aCC or LESS).
The DHP copper conditioned differently than OFE copper.
Charging effect is a limitation for the SEY measurement with this procedure.

19. Thank you for your attention
Acknowledgments:
Michal Haubner – Technical student
Remi Dupuis – Doctoral student
Vincent Baglin – Section leader
Antoine Benoit – Informatics engineer

23. Gas coverage - SEY 𝑆𝐸𝑌: 𝛿= 𝐼 2 𝐼 𝑏 =1− 𝐼 𝑠 𝐼 𝑏 Charging effect
𝑆𝐸𝑌: 𝛿= 𝐼 2 𝐼 𝑏 =1− 𝐼 𝑠 𝐼 𝑏
Ref. Journal of Vacuum Science & Technology, 11/7/2012
– Secondary electron yield on cryogenic surfaces as a function of physisorbed gases
Charging effect