Study of SEY Degradation of Amorphous Carbon Coatings Nenad Bundaleski Centro de Física e Investigaçaõ Tecnológica Universidade Nova de Lisboa, Caparica, Portugal

Initial idea Why does the SEY of a-carbon coatings increase in air? The idea: to expose the samples to different gases and try to identify which air constituent is responsible for ageing… Stainless steel substrate aC coating (<1µm) Glass vessel

Experimental setup Electron Gun Energy Analyzer Ion Gun X-Ray Source The experiments were performed in the multipurpose surface analysis apparatus . Base pressure in low 10-10 mbar region. XPS, AES, SIMS, LEIS, SEY, WF. Sample holder allows heating/cooling of the sample. Ion Gun SEY – e-gun and Faraday cup X-Ray Source Sample Entry lock And, we have TOF-SIMS…

Initial idea CERN, we have a problem: increase of SEY was observed for any sample, ‘even’ the reference sample which was in 1.10-2 mbar! And then… pbase ~ 2.10-6 mbar Ageing in high vacuum!?

Fast ageing under HV conditions We bake the HV system 24 h, 106° C, p ~ 2.10-5 mbar with the sample inside… SEY grows in HV much faster than in air Particularly intensive aging during baking

Identifying the cause of SEY increase RGA analysis: H2O H2 CO mass 43…oil from the rotary pump? There is no evidence of fast ageing.

Identifying the cause of SEY increase Exposure to vapour of mineral oils (saturated hydrocarbons): 5 min at 5x10-2 mbar Hydrocarbons are responsible for the fast ageing in HV!

Identifying the cause of SEY increase Even 1.10-9 mbar is not a safe environment if… achieved with oil diffusion pumps

XPS analysis Composition analysis Oxygen is the main impurity. SEY increases with the concentration of impurities

XPS analysis ≈0.2 eV C 1s Peak shift due to ageing is too small to draw reliable conclusions Photoelectron intensity at higher binding energies decreases with ageing

XPS analysis O 1s Shape of the O 1s peak changes with ageing

SEE spectra Energy spectra of secondary electrons – another fingerprint of ageing!

Annealing in air – another surprise The sample annealed in air ages much slower in HV

Annealing in air – another surprise The sample annealed in air ages faster in air?

Annealing in air – another surprise Initially bad sample may be recovered by prolonged annealing in air

TOF-SIMS analysis – first results SEY increases with the hydrogen content in the coating Annealing in air triggers surface segregation of hydrogen

Conclusions Hydrocarbons are responsible for fast ageing in vacuum Quality of ‘vacuum’ is more important than the total pressure O 1s peak in XPS and SEE spectra are fingerprints for ageing SEY increases with hydrogen contents SEY properties can be tailored by sample annealing in air. Could hydrocarbons be also responsible for ageing in air? If yes, why the ageing is slower? Their partial pressures are much beyond 10-6 mbar…

Surface Science and Vacuum Technology Research Line prof. Orlando Teodoro prof. Ana Silva prof. Augusto Moutinho Surface science laboratory Electron Spectroscopies Dr. Nenad Bundaleski Joao Trigueiro Sara Candeias Bobbie-Jean Shaw* TOF-SIMS Dr. Aleksandar Tolstoguzov Amjad Gouman Aderito Santos

Experimental setup for SEY measurements SEY (d) : number of secondary electrons emitted from a surface per incident primary electron.