The chemical origin of SEY at technical surfaces Rosanna Larciprete CNR-Istituto dei Sistemi Complessi, Roma, Italy and INFN-LFN, Frascati (RM), Italy ECLOUD'12 La Biodola, Isola d'Elba 7 June 2012
S econdary E lectron Y ield universal curve 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
Spread in the SEY data Ti Al Lin et al. SIA 2005, 37 895 SEY secondary electrons δ max =2.2 2.0 δ max =1.3 1.0 escape metal depth 0.0 rb. units) 0 100 200 300 400 Primary energy (eV) the effective SEY of the metal is strongly modified by the surface contamination
X-ray photoelectron spectroscopy θ emiss =0° C1s KE=hv-BE- φ hv=400 eV KE: kinetic energy FWHM = 250 meV φ : work function BE: binding energy 287 286 285 284 283 binding energy (eV) electron hv= 1253.6 eV analyzer FWHM = 0.95 eV 287 286 285 284 283 binding energy (eV)
XPS spectroscopy of technical samples hydrogen oxygen carbon metal C-C sp 3 C-H C1s C1s M C-O-C C-C sp 2 M-O C=O O-C=O 125 123 121 119 117 115 113 111 290 288 286 284 282 binding energy (eV) binding energy (eV)
SEY of technical samples �������� ���������� �������
SEY of technical samples incident beam secondary electrons bond dissociation M-O, C-OH, C-O, C=O, C bond rearrangement sp 2 sp 3 O 2 , H 2 O, H 2 CO, CO 2 electron beam desorption C C
SEY of technical samples incident beam secondary electrons dissociation of “environmental” molecules → reactions, film growth
co-laminated Cu for LHC beam screen e - beam 500 eV Cu3p SEY (arb. units) 2.0 O1s δ max =2.2 SEY SEY 1.0 C1s Cu2p 0.0 1000 800 600 400 200 0 0 100 200 300 400 Cu binding energy (eV) Primary energy (eV) 2.2 2.2 2.0 I=5 µ A 1.8 δ max 1.7 1.6 Q=1.2x10 -3 C/mm 2 1.4 1.25 1.2 58 59 60 61 62 63 y (mm) SEY decreases also outside the beam spot
co-laminated Cu for LHC beam screen 1.6 E=500 eV 1.5 I=5 µ A δ max 1.4 Q=1.2x10 -3 C/mm 2 1.3 56 58 60 62 64 66 y (mm) δ max =2.2 Cu 3p 3/2 C1s O1s 2.0 SEY (arb. units) C-C δ max =1.6 metal O-C oxide O-Cu C-H SEY δ max =1.25 1.0 C-O 0.0 940 936 932 928 292 290 288 286 284 282 0 100 200 300 400 538 536 534 532 530 528 binding energy (eV) binding energy (eV) binding energy (eV) Primary energy (eV) 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 δ max 1.4 1.3 56 58 60 62 64 66 y (mm) δ max =2.2 Cu 3p 3/2 C1s O1s 2.0 SEY (arb. units) C-C δ max =1.6 metal O-C oxide O-Cu C-H SEY δ max =1.25 1.0 C-O 0.0 940 936 932 928 292 290 288 286 284 282 0 100 200 300 400 538 536 534 532 530 528 binding energy (eV) binding energy (eV) binding energy (eV) Primary energy (eV) Ar + sputtering @ 2.2 KV + e - beam irradiation @ 500 eV, 10 µ A , 15 h, Q=3.6x10 -2 C/mm 2 δ max =1.3 δ δ δ 1.2 C1s O1s Cu 3p 3/2 δ max =1.2 δ δ δ 0.8 graphitic C SEY + sputtered Cu Ar 0.4 - beam irradiation after e 0.0 292 290 288 286 284 282 538 536 534 532 530 528 0 100 200 300 400 940 936 932 928 binding energy (eV) binding energy (eV) Ep (eV) binding energy (eV)
e - beam induced surface reactions O 2 Cu-O dissociation oxide reduction CO CO 2 reaction sp 2 sp 3 H 2 sp 3 → sp 2 conversion C-H dissociation C C CO O 2 C-O dissociation C film growth C O H Cu the contribution of all electron-induced surface reactions reduces δ max from 2.2 to 1.1
co-laminated Cu for LHC beam screen normal incidence 2.2 1.8 δ max Energy (EV) δ δ δ 1.4 10 20 50 200 500 -2 C/mm 2 @ 200 eV after 10 1.0 0 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 2 ) Dose (C/mm R. Cimino et al. submitted to PRL
co-laminated Cu for LHC beam screen C1s sp 3 2.0 C-H LHC δ max =2.2 as received 1.0 C-O O-C=O Intensity (arb. units) 0.0 sp 3 δ max =1.35 1.0 fully scrubbed sp 2 SEY (arb. units) 10 eV SEY 0.0 sp 2 1.0 fully scrubbed δ max =1.1 500 eV sp 3 0.0 1.0 δ max =1.05 HOPG 0.0 0 100 200 300 400 290 288 286 284 282 Primary energy (eV) Binding energy (eV) R. Cimino et al. submitted to PRL
Stainless steel samples from RICH@BNL E=500 eV C1s 2.0 sp 3 C-H δ max =2.2 as received 1.0 O-C=O 0.0 sp 3 1.0 δ max =1.3 SEY (arb. units) sp 2 C-H Intensity (arb. units) SEY 0.0 1.0 δ max =1.1 fully scrubbed C-O x 0.0 1.0 δ max =1.05 HOPG 0.0 0 100 200 300 400 290 288 286 284 282 Primary energy (eV) Binding energy (eV)
Al samples from Petra III C1s O1s Al2s Al2p as received Intensity (arb. units) 3.0 1.2x10 -1 C/mm 2 @ 500 eV 2.7 Ta SEY (arb. units) N1s 2.0 1.8 SEY Ar + sputtering 1.3 1.0 0.0 0 100 200 300 400 Primary Energy (eV) 600 500 400 300 200 100 0 Binding energy (eV) D. Grosso et al. submitted to PR-ST
Al samples from Petra III Al 2p + sputtering Ar 78 76 74 72 70 Binding energy (eV) 72.5 eV Al metallic the minimal partial pressure of H 2 O contained in the residual gas is sufficient 73.4 eV Al bonded to chemisorbed O to hinder the achievement of a stable, 73.9 eV tetrahedral Al 2 O 3 clean Al surface. 75.1 eV octahedral Al 2 O 3 After prolonged ion bombardment there are still Al atoms bonded to O even in a Al 2 O 3 phases D. Grosso et al. submitted to PR-ST
Al samples from Petra III C1s O1s 3.5 3.0 Al2s Al2p 2.7 SEY (arb. units) as received 2.0 1.8 Intensity (arb. units) SEY 1.2x10 -1 C/mm 2 @ 500 eV Ta N1s 1.3 1.0 Ar + sputtering Al2s Al2p e - beam 0.0 0 100 200 300 400 500 eV 2.9x10 -2 C/mm 2 Primary Energy (eV) dissociation of residual gas molecules 1.4 C/mm 2 as H 2 O and CO induced at the metal surface by the e - beam determines a 600 500 400 300 200 100 0 rapid oxidation of the irradiated area, as Binding energy (eV) well as, although to a lesser extent, of the surrounding region D. Grosso et al. submitted to PR-ST
Al samples from Petra III Al 2p Al2p e- beam irradiation 2 ) Q (C/mm 1.4 dramatic enhancement exclusively of the most oxidized Al 2 O 3 phase 2 ) Q (C/mm -2 2.9x10 C1 C3 C2 C4 Al 2p 72.5 eV Al metallic + sputtering Ar 73.4 eV Al bonded to chemisorbed O 73.9 eV tetrahedral Al 2 O 3 75.1 eV octahedral Al 2 O 3 78 76 74 72 70 Binding energy (eV) D. Grosso et al. submitted to PR-ST
Al samples from Petra III 1.5 4 - 500 eV O1s area, C4 area (arb. units) e as 1.0 3 - 500 eV received e δ max - 500 eV e 0.5 2 + 2 KeV Ar 0.0 1 time the SEY variation follows the oxygen content of the Al surface D. Grosso et al. submitted to PR-ST
e - beam induced surface reactions O 2 oxide reduction Al-O dissociation CO 2 CO reaction H 2 H 2 O 2Cu 2 O +O 2 =4CuO 0 oxidation dissociation C O H -200 4Cu+ 2O 2 —> 2 Cu 2 O Gibbs free energy (kJ) Al -400 2Fe+ O 2 —> 2 FeO SEY is determined by the rates of Al oxidation and reduction -600 reactions involving C play a minor role -800 4Al + 3O 2 —> 2Al 2 O 3 H 2 -1000 sp 3 → sp 2 conversion C-H dissociation 400 800 1200 1600 2000 temperature (K) CO O 2 C-O dissociation C film growth
C films on polycrystalline Cu a-C films C film thickness 2-3 nm magnetron sputtering @ RT p(Ar)= 10 -2 mbar ∆ t = 2min δ δ max =1.3 δ δ C1s 1.2 δ δ δ max =1.17 δ 0.8 O1s Cu3s SEY Cu3p C/poly-Cu 0.4 Cu substrate C film poly-Cu 0.0 500 400 300 200 100 0 0 100 200 300 400 Binding Energy (eV) Ep (eV)
C films on polycristalline Cu δ 1.17 hv=40.8 eV C1s valence band 2p- σ hv=1253.6 eV FWHM (eV) 1.10 1.8 1.0 1.4 0.95 2p- π 1.3 0.6 E F 0.9 RT 460 ° C 0.2 RT 700 ° C 460 ° C HOPG 700 ° C 288 286 284 282 0 100 200 300 400 12 8 4 0 Binding energy (eV) Binding energy (eV) Ep (eV) 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 δ max values. This determines the high variation of the experimental values. For Cu samples electron conditioning at 500 eV reduces the SEY and lowers δ max 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 δ max 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 δ max depends on the sp 3 /sp 2 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.
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