Desorption and Sputtering on Solid Surfaces by Low-energy Multicharged Ions K. Motohashi Department of Biomedical Engineering, Toyo University motohashi@toyonet.toyo.ac.jp 1. Background Sputtering and desorption on solid surfaces induced by slow multicharged ions have been attracting attention because of 1. high sputtering yield and its strong charge state (or potential energy) dependence as well as weak kinetic energy dependence, and following 2. nanostructure formation in every ion impact.
1-1. Sputtering and desorption OES SIMS Quartz And also in here Bullets go in here Species Microbalance H* H + O + Ti + Si + (UO x ) + y Total Substrate Si(1)/H 2 Si(1) 1 1 H 1) Si(111) 1 1 H 2) Projectiles Xe q+ 6 q 22 Xe q+ 4 q 12 I q+ 17 q 53 SAM( MUD A and DDT) Ar q+ 4 q 1 TiO 2 (1) Si(111) 1 1 H 2) U LiF 3) NaCl 4) I q+ 25 q 51 I q+ 17 q 53 Au q+ 36 q 69 Xe q+ 17 q 52 SiO 2 3) Yield ~ q 2.8 ±.3 ~ q 5 ~ q 3.4 ~ q 5 ~ q 1.4 Authors Deiwiks (26) 1) Kuroki (23) 2) Tona (26) Flores (29) Tona (28) Tona (26) 3) Sporn (1997) 4) Varga (1997) 1-2. Nanostructure formation Bullets go in here Mica HOPG And also in here LiF KBr Si TiO CaF 2 2 shape Blister 1) or Hillock 2-5) Hillock 7) Pit 8) Crater 9) Caldera 1) Hilock 2) E P dep. of size V EP d q h q V E P (E P >14 kev) V E P Authors 1) Shneider (1993) 2) Parks (1998) 3) Minniti (21) 4) Hayderer (21) 5) Nakamura (25) 6) Tona (27) 7) El-Said (27) 8) Heller (28) 9) Tona (27) 1) Tona (28)
1-3. Nanostructuring and Sputtering Contribution of Potential Energy E P Contribution of Kinetic Energy E K Nanostructuring Sputtering Volume ~ E P (Diameter ~ q, Height ~ q) Coulomb Explosion Hydrogen Atoms/Molecules/Ions Yield ~ q n (3 n 5) Pair-wise Potential Sputtering Heavy Atoms/Molecules/Ions Yield ~ q m (1 m 2) Defect Mediated Potential Sputtering Independence or weak dependence of E K Coulomb Explosion Inelastic Thermal Spike Hydrogen Atoms/Molecules/Ions Negative dependence of E K Heavy Atoms/Molecules/Ions Weak dependence of E K Kinetically Assisted Potential Sputtering 2. Motivation Synergy between E P and E K? Roles of neutrals? Kinematics or dynamics Spectroscopic data
3. Objective To measure velocity or kinetic energy of particles participating in the collisions in order to investigate the kinematics of sputtering, desorption, and nanostructuring induced by slow multicharged ion collisions with solid surfaces. 4. Experimental Methods 4.1 TOF-SIMS studies 4.1.1 3D-momentum spectroscopy of secondary ions 4.1.2 Simultaneous TOF-SIMS/LEIS analysis 4.2 OES study
4.1. TOF-SIMS study Secondary ions mass spectrometry (SIMS) measured in coincidence with scattered atoms or ions in off-normal incidence Coincidence SIMS Stop Start Secondary ions Scattered atoms /ions Multicharged ions eē- 4-1. Experimental setups of SIMS Secondary ions STOP TOF-SIMS with velocity-map imaging z Momentum distribution of y desorbing or sputtering ions x A q+ TOF-SIMS with 2D LEIS imaging Scattered neutral atoms START z y Secondary ions STOP x Kinetic energy distribution of scattered ions correlating with secondary ion emission Secondary electrons START Scattered ions START
4.2. OES study Optical emission spectroscopy (OES) of sputtered neutral atoms in normal incidence E V Z V Excited Atom Bulk Monochromator Doppler Broadening v λ = c 2λ Intensity decay as a function of Z = z I I exp v τ 6. Results and discussion 6.1. TOF-SIMS with velocity map imaging
6.1.1. Proton desorption from GaN surfaces 3D-momentum image of protons emitted from a GaN(1) surface interacting with Ar 8+ at glancing angle (.5 ) H + Ar 8+ GaN(1) Ar Ar 8+ beam K. Motohashi et al., NIMB 232 (25) 254. 6-1-2. Angular dependence of protons +9 Ar X Z H + Ar q+ α α = tan E k = Al surface Y 1( Py / Pz ) 2 2 2 ( P + P + P )/( 2m) x Simulation (SIMION) y z This analyzer can detect all protons which have the kinetic energy less than 2eV. -9 K. Motohashi and S. Tsurubuchi, NIMB 264 (27) 15
6. Results and discussion 6.2. TOF-SIMS with 2D-LEIS imaging 6-2-2. Velocity dispersion with E B filter z GaN(1) Ar 6+ (15. kev) Wien Filter (EB filter) 1 mm B Charge state separator v E CS E Ar (6-m)+ E CS (V/cm) 67 133 2 2 333 Ar v + v y v - v x 2D-LEIS image Ar + Ar Ar + Ar + 1 2 3 4 5 6 7 8
6-2-3. Energy calibration Ar 6+ (15.2 kev) 1 5 z (mm) Ar 6+ (15. kev) [ 1-2 ] z/ E = 5.2 mm/kev E = ±.3 kev 2 1-1 -5 Normalized intensity (arb. units) 6-2-4. Result of LEIS Ar 6+ 2 (15 kev) GaN (1) 2 2 E f ( cosθ + A sin θ ) N/E 5 + 4 3 2 1 :Ar + :Ar 6+ (Primary) N (2 nd layer) k = E i = Ga (1 st layer) ( 1+ A) 2 13 14 15 16 E (kev)
6-2-5. Result of SIMS measured in coincidence with scattered ions Cou unts 4 3 2 1 H + C 3 H 3 + + (b) + C 2 H 2 C 4 H 4 T Ga + 1 N + T 2 (<< T 1 ) GaN + 1 2 3 4 5 6 7 z (m mm) 2 1-1 -2 (a) 1 5 9 13 17 21 25 29 33 37 4 1 2 3 4 5 6 7 t 12 (1-6 s) 2 1 Ar +.66 Ar.23-1 Ar 2+.11-2 (c) 4 8 Counts Proton emission followed by elastic scattering between Ar and Ga or N +15.5 T 1 = 4.65 µs +15. T 1 (Proton) z T 1 (Proton) z E (kev) +14.5 +14. Ga N T 2 = 1.87 µs T 2 = 1.95 µs T 2 = 1.87µs (14.6 kev) T 2 = 1.95µs (13.9 kev) +13.5 +13. 1.5 2 2.5 3 3.5 t 12 (sec) Ar Ga N H [ 1-6 ]
6. Results and discussion 6.3. OES 6.3.1 <v > of Al * (Previous study) <v > ( 1 4 m/ /s ) 5 4 3 2 1 q+ v Ar v q+ Ar q+ (45 kev) :Al :Al 2 O 3 2 4 6 8 q v v Al * Al and Al 2 O 3 surfaces There was no significant change of <v > among different charge states as well as between metal and insulator surfaces. K. Motohashi et al., Phys. Scr. T73 (1997) 329.
6-3-2. <v > of Ti * (in progress) I / I 1.2 1..8.6.4.2 -.2 (a) -2 2 4 6 Z (mm) Ti * (3d 2 4s4p x 3 G) τ = 1.6 ns <v > = (7±3) 1 4 m/s I / I 1 1 Z I = I k exp (b) 1 1-1 τ v k k v 1-2 1 2 3 4 5 6 Z (mm) Measurement of charge-state dependence of <v >isnow on going. Summary TOF-SIMS coincidence measurements on scattered atoms/ions and secondary ions, and OES measurements of sputtered neutral atoms were conducted to investigate the dynamics of desorption and sputtering due to electron capture by slow multicharged ions. Anisotropic distributions in the proton emissions for glancing-incidence collisions were observed. It was found in the simultaneous TOF-SIMS and LEIS measurement that potential sputtering of protons induced by electron capture is followed by elastic scattering between projectile ions and surface atoms in the first and second layers. The mean velocity, parallel to the surface, of sputtered substrate atoms were almost independent on charge state of incident ions. The measurements of mean velocity, perpendicular to the surface, of sputtered substrate atoms are now in progress.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (B) (193616) and Reimei Research project of Japan Atomic Energy Agency (JAEA). I would like to thank Prof. S. Tsurubuchi, Prof. Y. Yamazaki, Dr.Y.Kanai,Dr.M.Flores,Dr.Y.Saitoh,andDr.S.Kitazawa for fruitful discussion and assistance. Thank you for your attention.