Laser Plasma and Laser-Matter Interactions Laboratory The effect of ionization on condensation in ablation plumes M. S. Tillack, D. Blair, S. S. Harilal Center for Energy Research and Mechanical and Aerospace Engineering Department Jacobs School of Engineering ARIES Town Meeting on Liquid Wall Chamber Dynamics Livermore, CA 5-6 May 2003

We are investigating late-stage laser ablation plume phenomena at UCSD 2.Modeling and experiments on homogeneous nucleation and growth of clusters 0 8 ns 1000 ns 1.Experimental studies of the expansion dynamics of plumes interpenetrating into ambient gases (with and without magnetic fields) 3.Spinodal decomposition and liquid droplet ejection

Lasers used in the UCSD Laser Plasma and Laser-Matter Interactions Laboratory Lambda Physik 420 mJ, 20 ns multi-gas excimer laser (248 nm with KrF) Spectra Physics 2-J, 8 ns Nd:YAG with harmonics 1064, 532, 355, 266 nm

Similarities and differences in ablation plume parameters * uncertainties in Ablator ionization

Theory

Classical theory of aerosol nucleation and growth Homogeneous Nucleation (Becker-Doring model) Condensation Growth Coagulation where the coagulation kernel is given by Transport and Rate of ChangeParticle Growth Rates ∂n/∂t = C  Z p s = p o exp[Q v /(kT b ) – Q v /(kT s )]

Dependence of homogeneous nucleation rate and critical radius on saturation ratio High saturation ratios result from rapid cooling due to plume expansion and heat transfer to background gas Extremely 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

Effect of ionization on cluster 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

Modeling

A 1-D multi-physics scoping tool was developed to help interpret plume condensation results  Laser absorption  Thermal response  Evaporation flux  Transient gasdynamics  Radiation transport  Condensation  Ionization/recombination I o e –  x, inverse bremsstrahlung cond., convection, heat of evaporation 2-fluid Navier-Stokes Stefan-Boltzmann model ion-modified Becker-Doring model high-n Saha, 3-body recombination Ablation plumes provide a highly dynamic, nonlinear, spatially inhomogeneous environment for condensation, where strong coupling of physics led us to a combined experimental and modeling approach.

Model prediction of expansion dynamics High ambient pressure prevents interpenetration (note, the 2-fluid model lacks single-particle effects) Target : Si Laser Intensity : 5x10 9 W cm -2 (peak of Gaussian) Ambient : 500 mTorr He

The plume front is accelerated to hypersonic velocities ~62 eV Thermal energy is converted into kinetic energy; collisions also appear to transfer energy from the bulk of the plume to the plume front

Model prediction of cluster birth and growth Spatial distribution of nucleation (*) and growth (o) rates at 500 ns Time-dependence of growth rate/birth rate Clusters are born at the contact surface and grow behind it Nucleation shuts down rapidly as the plume expands ss

Experiments

Experimental setup for studies of ablation plume dynamics Target : Al, Si Laser Intensity : 10 7 –5x10 9 W/cm 2 Ambient : Torr – 100 Torr air

Expansion of interpenetrating plumes depends strongly on the background pressure 0.01 Torr 1 Torr 0.1 Torr 10 Torr 100 Torr Free expansion (collisionless) Weakly collisional transition flow Collisional transition flow Fully collisional plume Confined plume

Example: plume behavior in weakly collisional transition regime (150 mTorr)

Strong interpenetration of the laser plasma and the ambient low density gas Plume splitting and sharpening observed 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 Plume behavior in weakly collisional transition regime (150 mTorr)

Plasma parameters are measured using spectroscopic techniques Electron Density: Measured using Stark broadening Initial ~ cm -3 Falls very rapidly within 200 ns Follows ~1/t – Adiabatic Temperature: Measured from line intensity ratios Initial ~8 eV falls very rapidly (Experiment Parameters: 5 GW cm -2, 150 mTorr air)

Besides spectroscopy, witness plates served as a primary diagnostic Start with single crystal Si HF acid dip to strip native oxide Spin, rinse, dry Controlled thermal oxide growth at 1350 K to ~1  m, 4 Å roughness Ta/Au sputter coat for SEM Locate witness plate near plume stagnation point Witness plate prior to exposure, showing a single defect in the native crystal structure Witness plate preparation technique:

Measurement of final condensate size 500 mTorr He 5x10 9 W/cm 2 5x10 8 W/cm 2

Cluster size distribution – comparison of theory & experiment note: the discrepancy at low irradiance is believed to be caused by anomolously high charge state induced by free electrons Good correlation between laser intensity and cluster size is observed. Is it due to increasing saturation ratio or charge state?

Saturation ratio and charge state derived from experimental measurements Maximum ionization state derived from spectroscopy, assuming LTE Saturation ratio derived from spectroscopy, assuming LTE Saturation ratio is inversely related to laser intensity!

Summary We have obtained a better understanding of the mechanisms which form particulate in laser plasma, through both modeling and experiments We have shown that ionization has a dominant effect on cluster formation in laser ablation plumes, even at low laser intensity The cluster sizes obtained are very small – of the order of 10 nm Model improvements are needed: 2-D, kinetic treatment,... In-situ particle measurements (scattering, cluster spectroscopy) would be very useful to further validate the mechanisms IFE relevance of experiments would be improved greatly with control of the background gas temperature Other applications include nanocluster formation, laser micromachining quality, thin film deposition by PLD