Collisional Effects of Background Gases on Pulsed Laser Deposition Plasma Beams
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of key processing parameters with distance, namel): kinetic energy, density, and temperature. Reported here are combined diagnostic measurements of silicon and yttrium ablation plume penetration through inert background gases during a key transitional regime in which the ion flux is split into distinct fast and slowed components. This apparently general phenomenon occurs over a limited range of distances at ambient pressures, including those sometimes used for PLD (as reported for YBCO ablation into 02).1,2,6,7 This "plume-splitting" is significant because a 'fast' component of ions can arrive at the ion probe (or substrate) with little or no delay compared to propagation in vacuum (e.g., Y+ kinetic energies in this study were up to 250 eV, and Si+ up to 215 eV). Kools recently employed a Monte Carlo simulation involving elastic collisions to prove this possibility. 8 However, at longer distances this 'fast' component is completely attenuated, and a single, slowed distribution of ions is observed. This 'fast' component is easily overlooked in imaging studies because enhanced plume luminescence occurs in the slowed distribution. However, the transmitted flux is readily observed with fast ion probes which provide convenient measurements of the magnitude and time-of-flight of the ion current (flux) in plasma plumes propagating through background gases. 1 ,2,5-7, 9 Figure 1 serves to review the ion probe measurements and illustrate the splitting effect. Figure l(a) shows the ion flux arriving at d = 5 cm along the normal to a silicon target following 2.8 J/cm 2 KrF-laser irradiation in vacuum and various helium ambient pressures. As in the case of YBCO penetrating through 02,1,2,6,7 the ion flux in these Si/He and Si/Ar studies was attenuated exponentially with background pressure and/or distance in general agreement with the simple scattering model (effective cross sections in both cases -1 x 10-16 cm 2 ) utilized for YBCO. 1 ,2,6,7 At long distances and long time delays, ion probe and fast imaging measurements 21 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
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Figure 1: Ion probe current waveforms measured d = 5 cm along the normal from a silicon target following 2.8 J/cm 2 KrF-laser irradiation in vacuum and background helium and argon: (a) Slowing and attenuation vs. helium ambient pressure (unscaled), (b) Comparison of fast and slow ion flux arrival at d = 5 cm through helium (200 mTorr) and argon (80 mTorr) with that in vacuum. In addition to the slowed material, a fast component of ions arrives in both cases with little or no delay compared to the propagation in vacuum. respectively record a decelerating, stable shock-structure which propagates in accordance with dr
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