Chemical Sputtering. von Kohlenstoff durch Wasserstoff. W. Jacob

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1 Chemical Sputtering von Kohlenstoff durch Wasserstoff W. Jacob Centre for Interdisciplinary Plasma Science Max-Planck-Institut für Plasmaphysik, Garching Content: Definitions: Chemical erosion, physical sputtering, chemical sputtering Physical sputtering Chemical erosion Chemical sputtering The MAJESTIX experiment: Ion- interaction: Chemical Sputtering Summary

2 Plasma-surface interaction A large variety of species impinges on the surface Classes of species: stable neutrals (mostly working gas) neutral radicals ions General assumption non reactive reactive, sticking at surface What is the sticking coefficient? stick, enhance sticking of radicals modify deposited material All these species show mutual interactions! Radical/radical interaction Ion/radical interaction ion/c3 ion/ little is known, example: C 3 synergism ion-induced stitching postulated in literature ion-induced sticking ion-induced etching (reactive ion etching, RIE) chemical sputtering

3 Chemical vs. physical sputtering LRQVRUQUJWLFQXWUDOV QRQUDFWLY UDFWLYLRQV +DWRPV Physical sputtering threshold energy energy dependence (TRIM.SP) isotope effect (kinematic factor) no significant T dependence all species (incl. inert gases) Chemical sputtering ions + neutrals energy dependence T dependence very low threshold energy isotope effect ion-to-neutral ratio dep. high erosion yield Chemical erosion thermally activated (no threshold energy) no isotope effect requires chemically reactive species

4 Chemical vs. physical sputtering Chemical sputtering: l strong variation with surface T l eroded species = molecules involving atoms of target and projectiles l no (?) or very low threshold energy l highly selective (depends sensitively on target projectile combination) l activation or inhibition possible Physical sputtering: l T independent (more or less, for C measurable effect at high T) l eroded species = atoms or small clusters of target material l high (>10 - >100 ev) threshold energy (M 1, M 2, E SB ) l depends on energy transfer only (T max = 4 M 1 M 2 /(M 1 + M 2 ) 2 ) Energy distribution of emitted species: E mean kt surface E mean some ev

5 Physical sputtering l well understood (for the most part) l key parameter is the surface binding energy E SB (= 7.4 ev for carbon) l depends on energy l depends on projectile mass l only weakly T dependent l threshold energy depends on target/projectile combination l depends on angle of incidence l depends on substrate material l depends on target roughness Open questions / problems: energy transfer: T max = 4 M 1 M 2 /(M 1 + M 2 ) 2 isotope effect l molecular ions atomic ions (at low and high energy) (most data are measured using 2 + or 3 + ions!) l surface roughness (dynamical development during process)

6 Ion-solid interaction Schematic representation of C impinging on a-c: Relevant processes: sputtering implantation backscattering displacement Elastic energy loss: due to collisions with the nuclei (= nuclear energy loss) W. Jacob, Thin Solid Films 326 (1998) Inelastic energy loss: due to energy transfer to the electrons, continuos along the path, no change of direction

7 Chemical erosion -atom induced chemical erosion k C 3 dehydrogenation sp 3 k D 2 Thermally-activated release of C 3 sp x k hydrogenation k T> 600K TDS T>600K k 2 C x y erosion sp x C 3 k x T> 400K Precursor for chemical erosion is a C 3 group adjacent to a dangling bond site both are produced by interaction with atomic hydrogen sp 2 C 3

8 Chemical erosion E.Vietzke et al., Surf. Coat. Technol. 47 (1991)156 Total Erosion Yield [C/ o or C/D o ] 1.E-01 1.E-02 1.E-03 more soft D o on a-c: hard o on graphite 1000 X 1.E-01 1.E-02 1.E-03 Three orders of magnitude difference in total erosion. Í reactivity of the surface depends critically on the surface structure. 1.E-04 1.E-04 o on diamond films 1.E Temperature [K] 1.E-05

9 Sputtering with reactive ions: physics meets chemistry and D bombardment of carbon Chemical erosion, ion-induced chemical erosion, ion-enhanced chemical erosion, ion-induced etching, reactive ion etching, chemically enhanced physical sputtering, chemical sputtering,... A simple picture: Chemical reactions take place at the end of range, when isotopes are thermalized. Molecules are formed locally, then they diffuse to the surface and desorb. Í temperature dependence of process

10 Chemical erosion: T dependence and influence of doping -atom induced chemical erosion 0.05 boron-doped graphite (15 at%) 50 ev D k C 3 sp 3 dehydrogenation k D 2 Chemical erosion yield (a. u.) experimental data pyrol. graph. USB 15 (B) model E rel = 1.8 ev E rel = 1.2 ev Temperature (K) 200 ev D sp x release k hydrogenation k T> 600K TDS T>600K sp 2 2 C x y sp x C 3 Maximum in erosion yield = competition between release (T act = 1.8 ev) and C 3 release (T act = 1.6 ev). Doping reduces T act for release interruption of chemical erosion cycle k erosion k x T> 400K C 3 C 3 release C. García-Rosales, J. Roth, J. Nucl. Mater (1992) 573.

11 Chemical sputtering of carbon: D, Í C 1 D + Í C 1 + Í C Total erosion yield (C/D) T RT T max Total erosion yield (C/) T max TRIM.SP Deuterium impact energy (kev) T RT TRIM.SP ydrogen impact energy (kev) M. Balden, J. Roth, J. Nucl. Mater. 280, (2000) New weight-loss measurements of the chemical erosion yields of carbon materials under hydrogen ion bombardment

12 Particle-beam experiments Chemical sputtering

13 Measuring erosion yields &+ILOP TXDQWLILGLRQEDP TXDQWLILG K\GURJQ EDP VXEVWUDW URVLRQ\LOG PDVXUGURVLRQUDWLQURGGFDUERQDWRPVSUFP V LPSLQJLQJLRQIOX[SUFP V

14 Experimental set-up UV experiment with 2 radical beam sources and one ion beam source $U N9 + :LQ )LOWU 1 &+ G FOUDWLRQ UISODVPD + $U 9 &+ OOLSVRPWU\LQIUDUG VXEVWUDW VXEVWUDW SUSDUDWLRQFKDPEU 8+9FKDPEU W. Jacob, Ch. opf, A. von Keudell, M. Meier, and T. Schwarz-Selinger: Particle-beam Experiment to Study eterogeneous Surface Reactions Relevant to Plasma-assisted Thin Film Growth and Etching, Review of Scientific Instruments 74, (2003).

15 Ion synergism Y (eroded C atoms per ion) Ion flux density = 3.5 * cm -2 s Chemical Sputtering -1 flux density = 1.4 * 1015 cm-2 s-1 Y(Ar + / ), Y(Ar + ) TRIM.SP, E sb = 0.1 ev TRIM.SP, E sb = 4.5 ev Γ() ion energy (ev) Chemical erosion due to atomic alone T = 340 K erosion rate Γ (cm -2 s -1 ) Physical sputtering threshold: ~ 60 ev Christian opf, PhD Thesis Ch. opf, A. von Keudell, and W. Jacob, Chemical Sputtering of ydrocarbon Films by Low-energy Ar + Ions and Atom Impact, Nuclear Fusion 42, L27 (2002). Ch. opf, A. von Keudell, and W. Jacob, Chemical Sputtering of ydrocarbon Films, J. Appl. Phys. 94, 2373 (2003). Erosion of a-c: layers comparison of simple physical sputtering (red symbols) due to Ar ions with erosion due to simultaneous interaction of and Ar + (blue symbols). enhanced erosion above 200 ev for simultaneous interaction erosion below threshold for physical sputtering (threshold energy for physical sputtering 60 ev erosion at 20 ev >> pure chemical erosion chemical sputtering separation of chemical and kinematical effects due to use of Ar + and neutral / ion ratio 400

16 Chemical sputtering of silicon -:&REXUQ+):LQWUV-$SSO3K\V +):LQWUV-:&REXUQ6XUIDF6FLQF5SRUWV

17 Chemical sputtering: Definition.F. Winters, J.W. Coburn, Surface Science Reports 14 (1992) 162: For the propose of this paper Chemical sputtering is defined as a process whereby ion bombardment causes or allows a chemical reaction to occur which produces a particle that is weakly bound to the surface and hence easily desorbed in the gas phase. The key process leading to desorption is the chemical reaction. The collision cascade physics... is operative in inducing the chemical reaction.

18 Chemical sputtering mechanism ions break C C bonds passivates broken bonds (1) and (2) formation of volatile hydrocarbons below the surface diffusion of C compounds to the surface and desorption

19 Energy dependence Y ( ions ) ybb ( x) p pass ( x) dx bond breaking due to ion impact Y ( ions ) passivation by atomic = a displacement events per depth interval calculated by TRIM.SP a is a fit parameter y dp ( x) e ( x / λ ) exponential decay, maximum range about 2 nm, known from plasma experiments dx y dp C (x) [nm -1 ] y dp (x) p Pass (x) ion energy (ev) depth x [nm] E dp C = 5 ev, λ = 0.4 nm p Pass (x)

20 Energy dependence Ar + / 2+ / data Y(Ar + / ) Y C dp model Y( + 2 / ) data 10-3 Y C dp model ( + 2 = 2+ ) model ( + 2 ) Ar + energy (ev) + 2 energy (ev) a = 0.4 a = 0.4 j = cm -2 s -1, j Ar+ = cm -2 s -1, R = j / j Ar 400

21 Energy dependence: Modeling results 10 chemical sputtering yield (per ion) N + 2 Ar + Ne + e energy (ev) a = 0.4 R 400

22 Energy dependence: Modeling results 0.8 chemical sputtering yield e + T + D a = 0.4 R 400 ion energy (ev)

23 Growth competition: Deposition chem. sputtering Ausbeute / 2 = Growth ion-induced sticking R = j / j Ausbeute chemical sputtering here: constant contribution from sticking - flux ratio dep. contribution from chemical sputtering

24 Summary Physical sputtering: for the most part well understood well modeled by TRIM.SP (binary collision approximation) energy, projectile mass, angle, roughness Chemical erosion: for the most part well understood thermally activated process can be influenced by doping Chemical sputtering: increase of yield and lowering of threshold mechanistic model for chemical sputtering flux ratio dependence (rate equation model): high fluxes required energy dependence: bond breaking passivation predictions for other ions, e.g., D, T, e, N 2,.. Growth of a-c: is always a competition between deposition and erosion (chemical sputtering).

25 The end Collaborators: Christian opf Achim von Keudell Matthias Meier Thomas Schwarz-Selinger