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The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for film growth by pulsed-laser deposition of materials since the magnitude and kinetic energy of the species arriving at the deposition substrate are key processing parameters. Dynamical calculations of plume propagation in vacuum and in background gas have been performed using particle-in-cell hydrodynamics, continuum gas dynamics, and scattering models. Results from these calculations are presented and compared with experimental observations. In this process, a pulsed laser, usually of the excimer variety such as KrF (krypton fluoride) and ArF (argon fluoride) with pulse lengths of tens of nanoseconds, is used to ablate material gently at fluences of a few J/cm2. The resulting neutrayweakly ionized plasma plume is allowed to expand, in vacuum or in background gas, before depositing on a substrate of compatible material at a suitable distance away from the target.While laser ablation is reportedly simple conceptually and experimentally,* the physics ingredients that come into play are quite complicated3 given that they involve lasersolid interactions at the target, plasma formation off the target, vapor/plasma plume transport toward the deposition substrate with its associated hydrodynamics and atomic physics, as well as plume-solid interactions at the deposition substrate.We have been pursuing a global physics and computational modeling approach to laser ablation that relies on thermal models to describe laser-solid interactions for neutral plume formation; on kinetic breakdown models of plasma formation in the vapor plume; on variety of hydrodynamics, gas dynamics, and collisional or scattering transport models for the neutrayweakly ionized plasma plume; as well as on molecular dynamics methods to treat plume-substrate interactions. The many facets of this modeling effort have been summarized el~ewhere.~ Here we concentrate on the dynamics of plume propagation and on comparisons with experimental results.Experimental observations have shown marked differences between plume expansion in vacuum and in the presence of a higher pressure background gas. These observations are common to a wide range of laser-ablated materials including silicon, carbon, yttrium, and high-temperature superconducting compounds such as YBCO (yttrium-barium-copper oxide). Ablation in high-pressure ambient gases results in shock 2 waves and expansion fronts propagating through the background gases.5 Time-of-flight measurements also show two components in the ion probe signals: an energetic component that propagates at vacuum speed and another that is more or less significantly slowed down depending on the pressure of the background gas.6 These differences are important since the magnitude and kinetic energy of the species arriving at the deposition substrate are key processing parameters.We have applied our transport models to study plume expansion in near vacuum and .-in a higher pressure background gas. Results from some of these models w...

A dynamic source effect that accelerates the expansion of laser-ablated material in the direction perpendicular to the target is demonstrated. A self-similar theory shows that the maximum expansion velocity is proportional to c s ͞a, where 1 2 a is the slope of the velocity profile and c s is the sound speed. Numerical hydrodynamic modeling is in good agreement with the theory. A dynamic partial ionization effect is also studied. With these effects, a is reduced and the maximum expansion velocity is significantly increased over that found from conventional models.

Various physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed-laser deposition of materials. These include thermal models of laser-solid target interactions that initiate the vapor plume, plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms, hydrodynamic and collisional descriptions of plume transport, and molecular dynamics models of the interaction of plume particles with the deposition substrate.

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