The chemical origin of SEY at technical surfaces Rosanna - - PowerPoint PPT Presentation

Rosanna Larciprete

CNR-Istituto dei Sistemi Complessi, Roma, Italy and INFN-LFN, Frascati (RM), Italy

The chemical origin of SEY at technical surfaces

ECLOUD'12 La Biodola, Isola d'Elba 7 June 2012

universal curve

Secondary Electron Yield

secondary electron emission three-step process:

• production of SEs at a depth z
• transport of the SE toward the surface
• emission of SE across the surface barrier

the material parameters influencing SEY are: penetration depth of the primary electrons, stopping power, escape depth of the secondary electrons, work function - Z number

Lin et al. SIA 2005, 37 895

Spread in the SEY data

metal secondary electrons

escape depth

Al

2.0 1.0 0.0

• rb. units)

400 300 200 100 Primary energy (eV)

δmax=2.2 δmax=1.3

Ti the effective SEY of the metal is strongly modified by the surface contamination

SEY

electron analyzer

X-ray photoelectron spectroscopy

KE: kinetic energy BE: binding energy

φ: work function KE=hv-BE-φ

287 286 285 284 283 binding energy (eV)

C1s

FWHM = 250 meV

θemiss=0°

hv=400 eV

287 286 285 284 283 binding energy (eV)

hv=1253.6 eV FWHM = 0.95 eV

290 288 286 284 282

carbon hydrogen

• xygen

metal

binding energy (eV) binding energy (eV)

M M-O C1s C=O C1s C-C sp3 C-H C-C sp2 C-O-C O-C=O

XPS spectroscopy of technical samples

125 123 121 119 117 115 113 111

SEY of technical samples

SEY of technical samples

incident beam

electron beam desorption O2, H2O, H2 CO, CO2

sp3 sp2

C C secondary electrons

bond dissociation M-O, C-OH, C-O, C=O, C bond rearrangement

SEY of technical samples

incident beam secondary electrons dissociation of “environmental” molecules → reactions, film growth

1000 800 600 400 200 binding energy (eV)

400 300 200 100 Primary energy (eV) 2.0 1.0 0.0 SEY (arb. units)

δmax=2.2

e- beam 500 eV

2.2 2.0 1.8 1.6 1.4 1.2

δmax

63 62 61 60 59 58 y (mm)

I=5µA Q=1.2x10-3 C/mm2

co-laminated Cu for LHC beam screen

C1s O1s Cu3p Cu2p

2.2 1.25 1.7

SEY decreases also outside the beam spot

Cu

SEY SEY

C1s O1s

C-O C-C C-H O-C O-Cu

Cu 3p3/2

400 300 200 100 Primary energy (eV) 2.0 1.0 0.0 SEY (arb. units)

δmax=2.2 δmax=1.25 δmax=1.6

the beam spot but also the surrounding area is modified in the beam spot the quantity of surface C increases → graphitic film growth

co-laminated Cu for LHC beam screen

1.6 1.5 1.4 1.3

δmax

66 64 62 60 58 56 y (mm) 292 290 288 286 284 282 binding energy (eV)

940 936 932 928 binding energy (eV)

538 536 534 532 530 528 binding energy (eV)

• xide

metal

E=500 eV I=5µA Q=1.2x10-3 C/mm2

SEY

SEY

1.6 1.5 1.4 1.3

δmax

66 64 62 60 58 56 y (mm)

C1s O1s

C-O C-C C-H O-C O-Cu

Cu 3p3/2

400 300 200 100 Primary energy (eV) 2.0 1.0 0.0 SEY (arb. units)

δmax=2.2 δmax=1.25 δmax=1.6

538 536 534 532 530 528 binding energy (eV) 292 290 288 286 284 282 binding energy (eV)

Ar+ sputtering @ 2.2 KV + e-beam irradiation @ 500 eV, 10µA, 15 h, Q=3.6x10-2 C/mm2 Cu 3p3/2 C1s O1s

co-laminated Cu for LHC beam screen

292 290 288 286 284 282 binding energy (eV)

940 936 932 928 binding energy (eV)

538 536 534 532 530 528 binding energy (eV)

• xide

metal graphitic C

940 936 932 928 binding energy (eV)

1.2 0.8 0.4 0.0 400 300 200 100 Ep (eV)

Ar

+ sputtered Cu

after e

• beam irradiation

δ δ δ δmax=1.3 δ δ δ δmax=1.2

SEY

C-O C O

Cu

H dissociation C film growth O2 reaction CO2 CO CO Cu-O dissociation O2 C-H dissociation H2 sp3→sp2 conversion

e- beam induced surface reactions

the contribution of all electron-induced surface reactions reduces δmax from 2.2 to 1.1

sp3 sp2

C C

• xide reduction

2.2 1.8 1.4 1.0

δ δ δ δmax

10

• 7

10

• 6

10

• 5

10

• 4

10

• 3

10

• 2

Dose (C/mm

2)

normal incidence

10 20 50 200 500

Energy (EV)

after 10

• 2C/mm

2 @ 200 eV

co-laminated Cu for LHC beam screen

• R. Cimino et al. submitted to PRL

290 288 286 284 282 Binding energy (eV)

C1s

Intensity (arb. units)

HOPG as received

LHC

fully scrubbed 500 eV fully scrubbed 10 eV

2.0 1.0 0.0 SEY (arb. units) 400 300 200 100 Primary energy (eV) 1.0 0.0

1.0 0.0

1.0 0.0

δmax=2.2 δmax=1.35 δmax=1.1 δmax=1.05

co-laminated Cu for LHC beam screen

sp2 sp3 sp3 sp2 sp3 C-H C-O O-C=O

• R. Cimino et al. submitted to PRL

SEY

Stainless steel samples from RICH@BNL

290 288 286 284 282 Binding energy (eV)

C1s

Intensity (arb. units)

HOPG as received fully scrubbed

1.0 0.0

400 300 200 100 Primary energy (eV) 2.0 1.0 0.0 SEY (arb. units) 1.0 0.0 1.0 0.0

δmax=2.2 δmax=1.3 δmax=1.1 δmax=1.05

sp3 C-H O-C=O sp2 sp3 C-H C-Ox

E=500 eV

SEY

3.0 2.0 1.0 0.0 SEY (arb. units) 400 300 200 100 Primary Energy (eV)

2.7 1.8 1.3 Intensity (arb. units) 600 500 400 300 200 100 Binding energy (eV)

O1s C1s N1s Ta Al2s Al2p Ar+ sputtering as received 1.2x10-1C/mm2 @ 500 eV

Al samples from Petra III

• D. Grosso et al. submitted to PR-ST

SEY

78 76 74 72 70 Binding energy (eV) Ar

+ sputtering

Al 2p

72.5 eV Al metallic 73.4 eV Al bonded to chemisorbed O 73.9 eV tetrahedral Al2O3 75.1 eV octahedral Al2O3

the minimal partial pressure of H2O contained in the residual gas is sufficient to hinder the achievement of a stable, clean Al surface. After prolonged ion bombardment there are still Al atoms bonded to O even in a Al2O3 phases

Al samples from Petra III

• D. Grosso et al. submitted to PR-ST

Intensity (arb. units) 600 500 400 300 200 100 Binding energy (eV)

O1s C1s N1s Ta Al2s Al2p Al2s Al2p

3.0 2.0 1.0 0.0 SEY (arb. units) 400 300 200 100 Primary Energy (eV)

3.5 2.7 1.8 1.3 Ar+ sputtering as received 1.2x10-1C/mm2 @ 500 eV 2.9x10-2C/mm2 1.4 C/mm2

e- beam 500 eV

dissociation of residual gas molecules as H2O and CO induced at the metal surface by the e- beam determines a rapid oxidation of the irradiated area, as well as, although to a lesser extent, of the surrounding region

Al samples from Petra III

• D. Grosso et al. submitted to PR-ST

SEY

78 76 74 72 70 Binding energy (eV)

Al2p

Ar

+ sputtering

e- beam irradiation Q (C/mm

2)

1.4 2.9x10

• 2

Q (C/mm

2)

C1 C2 C3 C4

Al 2p

72.5 eV Al metallic 73.4 eV Al bonded to chemisorbed O 73.9 eV tetrahedral Al2O3 75.1 eV octahedral Al2O3

Al 2p dramatic enhancement exclusively

• f the most oxidized Al2O3 phase

Al samples from Petra III

• D. Grosso et al. submitted to PR-ST

Al samples from Petra III

1.5 1.0 0.5 0.0 O1s area, C4 area (arb. units)

4 3 2 1

δmax

time as received e

• 500 eV

Ar

+ 2 KeV

e

• 500 eV

e

• 500 eV

the SEY variation follows the oxygen content of the Al surface

• D. Grosso et al. submitted to PR-ST

O2 C O

Al

H Al-O dissociation reaction CO2 CO

• xide reduction

H2O dissociation H2

• xidation

e- beam induced surface reactions

SEY is determined by the rates of Al oxidation and reduction

C-O C-H dissociation H2 dissociation C film growth O2 CO sp3→sp2 conversion

reactions involving C play a minor role

• 1000
• 800
• 600
• 400
• 200

Gibbs free energy (kJ) 2000 1600 1200 800 400 4Al + 3O2 —> 2Al2O3 2Fe+ O2 —> 2 FeO 4Cu+ 2O2 —> 2 Cu2O 2Cu2O +O2=4CuO temperature (K)

C films on polycrystalline Cu

500 400 300 200 100 Binding Energy (eV) C1s O1s

Cu3p Cu3s poly-Cu C/poly-Cu

C film thickness 2-3 nm

1.2 0.8 0.4 0.0 400 300 200 100 Ep (eV)

Cu substrate C film

δ δ δ δmax=1.3 δ δ δ δmax=1.17

a-C films magnetron sputtering @ RT p(Ar)= 10-2 mbar ∆t = 2min

SEY

12 8 4 Binding energy (eV)

valence band

hv=40.8 eV RT 460 ° C 700 ° C

EF

C films on polycristalline Cu

288 286 284 282 Binding energy (eV)

C1s

hv=1253.6 eV FWHM (eV)

1.8 0.9 1.4 1.3

RT 460 ° C 700 ° C HOPG

1.0 0.6 0.2 400 300 200 100 Ep (eV)

1.17 1.10 0.95 δ

2p-π 2p-σ

the graphitization of the C films corresponds to a lower SEY

Conclusions

The SEY of technical samples is strongly affected by the chemical composition of the surface as the presence and the nature of contaminating adsorbates can heavily modify the effectve δmax values. This determines the high variation of the experimental values. For Cu samples electron conditioning at 500 eV reduces the SEY and lowers δmax from 2.2 to 1.1 . Both direct beam and secondary electrons have a role in the chemical reactions which decrease the SEY. Similar results were found for stainless steel samples. On the contrary for Al samples electron conditioning at 500 eV does not succeed in lowering δmax below 1.8 (1.5). In this case the composition of the residual gas in the UHV chamber is extremely importantin limiting the e- beam induced oxidation. For ultrathin C films deposited by magnetron sputtering on copper δmax depends on the sp3/sp2 ratio. The knowledge of the chemical state of a “technical” surface can elucidate the origin of the measured SEY curves and in general provide profitable information for the e-cloud mitigation.

Acknowledgements

Roberto Cimino, INFN-LNF, Frascati (RM), Italy Davide Remo Grosso, INFN-LNF, Frascati (RM), Italy Mario Commisso, INFN-LNF, Frascati (RM), Italy Theo Demma, INFN-LNF, Frascati (RM), Italy Roberto Flammini, CNR-IMIP, Montelibretti (RM), Italy INFN-LNF, Frascati (RM), Italy Reiner Wanzenberg, Desy, Hamburg, Germany Vincent Baglin, CERN, Geneva, Switzerland Wolfram Fischer, Brookhaven Nat. Lab., USA Antonio Di Trolio, CNR-ISC, Roma, Italy

Thanks for your attention