1. Laser Plasma and Laser-Matter Interactions Laboratory Laser ablation plume dynamics and particulate generation M. S. Tillack Mechanical and Aerospace Engineering Department ECE Departmental Seminar 31 October 2003

2. Laser-produced plasmas have numerous applications in science and industry.  Micromachining  Thin film deposition  Cluster production  Nanotube production  Surface modification  Surface cleaning  Elemental analysis  X-ray laser  Photolithography  EUV lithography  Medicine  Inertial fusion energy

3. Contaminated surfaceAfter cleaning Polyimide laser ink-jet printer head (courtesy of HP) However, several problems limit their use. e.g., workpiece & equipment contamination are important problems in micromachining http://www.oxfordlasers.com

4. Understanding and control of target emissions is needed in order to develop inertial fusion energy. Target explosions yield ~50 MJ of 0.1-10 MeV ions

5. Laser-produced plasma is a potential light source for EUV lithography at 13.4 nm. Achieving higher efficiency and lower contamination are key issues for EUV light sources

6.  Absorption, reflection  Heat transfer  Thermodynamics (phase change)  Plasma breakdown  Shock waves (gas)  Stress waves (solid)  Laser-plasma interactions  Gas dynamic expansion  Atomic & molecular processes Understanding the dynamics, emission character- istics and mechanisms of particulate formation will enable greater use of lasers in industry. A wide range of physics is involved in laser-matter interactions:

7. This seminar surveys several phenomena, with emphasis on our research at UCSD. 4.Spinodal decomposition 3.Homogeneous nucleation and growth of clusters in the presence of ions 1.Ambient gases and 2.Magnetic fields Expansion dynamics of plumes interpenetrating into

8. 1.Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases 0.01Torr 1Torr 0.1Torr 10Torr 100Torr S. S. Harilal, C. V. Bindhu, M. S. Tillack, F. Najmabadi and A. C. Gaeris, "Internal Structure and Expansion Dynamics of Laser Ablation Plumes into Ambient Gases," J. Applied Physics 93, 5 (2003) 2380-2388. S. S. Harilal, C. V. Bindhu, M. S. Tillack, F. Najmabadi and A. C. Gaeris, "Plume Splitting and Sharpening in Laser-produced Aluminum Plasma," Journal of Physics D: Applied Physics 35 (2002) 2935-2938.

9. Experimental setup for time-resolved studies of ablation plume dynamics Target : Al, Si Laser Intensity : 10 7 –5x10 9 W/cm 2 Ambient : 10 -8 – 100 Torr air Spectra Physics 2-J, 8 ns Nd:YAG with harmonics 1064, 532, 355, 266 nm

10. Below 10 mTorr the plume expands freely Laser intensity = 5 GWcm -2, Intensification time = 2 ns Each image is obtained from a single laser pulse Plume edge maintains a constant velocity (~ 10 7 cm/s) P = 10 -6 TorrP = 10 -2 Torr

11. The plume splits and sharpens at 150 mTorr Strong interpenetration of the laser plasma and the ambient low density gas Observed plume splitting and sharpening. This pressure range falls in the region of transition from collisionless to collisional interaction of the plume species with the gas Enhanced emission from all species

12. Instabilities appear at 1.3 Torr  Plume decelerates  Instability appears  Intensity peaks in slower component

13. Above 10 Torr the plume remains confined P = 10 Torr airP = 100 Torr air

14. Summary of plume dynamics vs. pressure Fitting Models:  Free expansion: R ~ t  Shock Model: R ~ (E o /  o ) 1/5 t 2/5  Drag Model: R = R o (1–e -bt )  Best fit at 150 mTorr R ~ t 0.445

15. 2.Magnetic confinement of laser-produced plasma M. S. Tillack, S. S. Harilal, F. Najmabadi and J. O'Shay, "Magnetic Confinement of an Expanding Laser-Produced Plasma," Inertial Fusion Science and Applications 2003, Monterey CA, Sept. 2003. S. S. Harilal, M. S. Tillack, B. O'Shay, C. V. Bindhu, F. Najmabadi, "Confinement and dynamics of laser-produced plasma expanding across a transverse magnetic field," Phys Rev. E (accepted).

16. The plasma “blows a high beta bubble”, which rapidly stalls as the plasma cools Other effects include enhancement of soft x-ray emission, plasma instabilities, ion acceleration Magnetic fields can slow down or divert an expanding high-beta plasma and thermalize directed ion energy RbRb

17. 3 cm Experiments were performed with NdFeB permanent magnets 1.5 cm

18. Stagnation length is determined from R-t plot free expansion v=6x10 6 cm/s 5 GW/cm 2

19. The average electron temperature rises in the vicinity of stagnation

20. Expansion in an axial magnetic field exhibits tip sharpening 50 GW/cm 2 fast ions: ~10 7 cm/s plume front: 4x10 6 cm/s

21. Homogeneous nucleation and growth from the vapor phase – Supersaturated vapor, ion or impurity seeded Phase decomposition from the liquid phase – Thermally driven phase explosion – Pressure driven fracture Hydrodynamic droplet formation Several mechanisms can create aerosol from short-pulse lasers.

22. 3.Modeling and experiments on homogeneous nucleation and growth of clusters Target Contact Surface Shock Condensed Particulates M. S. Tillack, D. Blair and S. S. Harilal, "The effect of ionization on cluster formation in laser ablation plumes,” Nanotechnology (accepted, Oct. 2003).

23. Homogeneous nucleation: Supersaturation drives rapid condensation. High saturation ratios result from rapid cooling due to plume expansion and heat transfer to the background gas Very high nucleation rate and small critical radius result Reduction in S due to condensation shuts down HNR quickly; competition between homogeneous and heterogeneous condensation determines final size and density distribution Si, n=10 20 cm –3, T=2000 K J h = Z  C*  =k 

24. Homogeneous nucleation: Ions enhance the nucleation rate. Ion jacketing produces seed sites Dielectric constant of vapor reduces free energy Si, n=10 20 cm –3, T=2000 K, Z eff =0.01

25. A 1-D multi-physics code was developed to account for the highly dynamic plume behavior.  Laser absorption  Thermal response  Evaporation flux  Transient gasdynamics  Radiation transport  Condensation  Ionization/recombination absorption I o e – x, w/plasma shielding cond., convection, heat of condensation 2-fluid Navier-Stokes simple Stephan-Boltzmann model modified Becker-Doring model modified Saha, 3-body recombination Target : Si Laser Intensity : 10 7 –10 9 W cm -2 Ambient : 500 mTorr He

26. 500 mTorr He 5x10 8 W/cm 2 5x10 9 W/cm 2 5x10 8 W/cm 2 5x10 7 W/cm 2 In addition, plume spectroscopy and post- test examination of witness plates was used.

27. As laser intensity increases, ionization increases but saturation ratio decreases. Maximum charge state at 50 ns, 1 mm from Al target, as derived from spectroscopy and assuming LTE. Saturation ratio at 1 mm, derived from spectroscopy and assuming LTE.

28. Smaller particle sizes confirm the dominant influence of ionization. Comparison of experiments and modeling of mean cluster size vs. laser intensity. Model prediction of mean cluster size with the effect of ionization “turned off”

29. 4.Modeling of explosive phase change (spinodal decomposition)  G < 0 G v   G/  v   < 0

30. A rapid rise in temperature or drop in pressure will drive materials into a metastable state. Heating faster than the homogeneous vapor nucleation rate can superheat a liquid beyond equilibrium. The metastable liquid has an excess of free energy (   G/  v   < 0 ), so it decomposes rapidly into liquid and vapor phases. Compression Tension P(v) U(v) U coh P th dp/dv=0 spinodal! p = -dU/dv

31. As T/T tc  spinodal, Becker-Döhring theory predicts an avalanche-like explosive growth of the nucleation rate (by 20-30 orders of magnitude) The nucleation rate grows exponentially as the material approaches its critical temperature.

32. This has been observed as a threshold phenomenon for modest laser fluences V. Craciun, et al., “Laser-induced explosive boiling during nanosecond laser ablation of silicon,” Applied Surface Science 186 (2002) 288-292. N. M. Bulgakova and A. V. Bulgakov, “Pulsed laser ablation of solids: transition from normal vaporization to phase explosion,” Appl. Phys. A 199-208 (2001).

33. Example result: depth of material removed by x-rays in liquid Flibe wall at R=6.5 Cohesion energy (total evaporation energy) 2.5 Evap. region 10.4 2-phase region Sensible energy (energy to reach saturation) 0.9 T critical 4.1 Explosive boiling region

34. Acknowledgements Expansion dynamics – S. S. Harial, C. V. Bindhu Magnetic diversion – S. S. Harial, Beau O’Shay, Farrokh Najmabadi Cluster condensation – Dustin Blair Phase decomposition – Brian Christensen, Rene Raffray, Mofreh Zaghloul Funding – US Dept. of Energy Office of Science, Defense Programs – Hewlett Packard, General Atomics