1. Molecular Dynamics Simulations of Ion Irradiation of a Surface under an Electric Field
S. Parviainen, F. Djurabekova

2. Ion irradiation during breakdown
Breakdown: plasma above surface
Ions from the plasma are accelerated towards the surface
Irradiation
Plasma sheath, strong electric field
Irradiation in general is well studied, but what about irradiation under breakdown conditions?
Plasma sputtering invented in 1852, but still little data?

3. What I will tell you
Sputtering yield (=sputtered atoms/incident ions) is modified by:
The presence of an electric field
Surface features (e.g. protrusions)
Also modifies local flux
Relevance for breakdown research: plasma is maintained by sputtered atoms
Input for plasma simulations
So far these have not included the modified yields

4. The modeled system Flat surface, flat surface w/ protrusion
Cu+ ions on Cu surface perpendicular to surface
DC - constant electric field
Room temperature
“Low” flux (No multiple hits in same spot)
Hybrid Molecular Dynamics - Electrodynamics
See Flyura's talk for details

5. Plasma-surface interaction
Plasma is formed + +
Field electron emission
Sputtering
And so on ...
Ion bombardment

6. Energy of incident ions
~8 keV
(H. Timkó)

7. Sputtering yield (Cu on Cu)
Matsunami et al. semi-empirical
N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, K. Morita, R. Shimizu, and H. Tawara, in Energy Dependence of the Yields of Ion-Induced Sputtering of Monatomic Solids, IPPJ-AM-32 (Institute of Plasma Physics, Nagoya University, Japan, 1983).

8. Simulation E
14GV/m, 2keV
Color indicates kinetic energy

9. Sputtering yield vs. field

10. 14GV/m, 2keV
0.4e
Color indicates charge

11. Sputtering or evaporation?
Normalized sputtering rate (1/ps)

12. Sputtering or evaporation?

13. Modified sputtering yield

14. Surface features Field emitters are present on surface
e.g. Protrusions may appear on an initially flat surface in the presence of a strong electric field
How do these affect sputtering?
A. Pohjonen et al., J. Appl. Phys. 114, (2013)
E=13GV/m
(void)

15. Insert picuture here

16. Surface features
Field enhancement isoline
h=5nm, r=1nm

17. Local fluence
“Extreme” case with very low energy ion. Illustrates how there are many more ions hitting at the protrusion than around. Tip radius =~1nm and height = ~5nm

18. Local fluence
Another way to look at it. Flux density is much increased around the tip (approximate radius 1nm) but goes to normal fairly rapidly with distance.

19. Effect of tip size
With increasingly large protrusions the field distortion reaches further.
r=1nm, h=5nm
r=2nm, h=10nm

20. Effect of tip size Stronger electric field can grab faster ions
Larger: h=10nm, d=4nm
Smaller:h=5nm,d=2nm
Stronger electric field can grab faster ions
Increasing the initial energy of the ions prevent the ions from being deflected towards the protrusion. Should try to fit a function to data, but at least does not seem to be directly 1/x. Will check still.

21. Sputtering yield on protrusion

22. For the future Energy of incident ions much lower than thought
See Kyrre's talk tomorrow
~10-100eV
New simulations in this range
First results by tomorrow?
Study other geometries

23. Conclusions
Sputtering yield in enhanced significantly when an electric field is applied
Increase several hundred percent at high fields
This should be considered in e.g. plasma calculations
“Sputtering” yield a bit misleading: actually from field evaporation
Sputtering yield is much higher from protrusions than flat surface
The local flux is also increased locally around protrusions
i.e. probability of hitting a protrusion (if they exist) is increased