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

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

Setup overview – Sample transfer Advantages: Fast access : Atmosphere to 2.10-10 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

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

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

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

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

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

Measures and procedures - ESD Desorption measurement of a gas layer: Preparation of 500ML of gas: Condensed gas surface: 2cm2 1ML=8∙ 10 14 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠/ 𝑐𝑚 2 500ML=8∙ 10 17 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

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

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.

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

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

Gas coverage - Solid N2 phosphorescence 1 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

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 − ]

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

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

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.

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

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