In Situ Laser Ablation Plasma Diagnostics in the Film Growth Regime - Effects of Ambient Background Gases
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IN SITU LASER ABLATION PLASMA DIAGNOSTICS IN THE FILM GROWTH REGIME - EFFECTS OF AMBIENT BACKGROUND GASES D.B. GEOHEGAN Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN. ABSTRACT The propagation of the laser-induced plasma formed by KrF irradiation of Y I Ba 2Cu 3O 7 has been characterized in background pressures of oxygen and argon typically used for thin film growth. The ion current transmitted through the background gases was recorded along the normal to the irradiated pellet as a function of distance in order to measure the decreasing velocity and magnitude of the expanding plasma current due to collisional slowing and attenuation of the laser plume. The integrated ion charge delivered to a substrate at low pressures can be described by elastic scattering giving a general integral cross sections of 0e[O21 = 3.2 x 10-16 cm 2 and oe1Arl = 2.7 x 10-16 cm 2 . At higher pressures, inelastic scattering leads to increased recombination and reactive conversion of ions indicated by increased fluorescence of all the species, which becomes dominated by fluorescence of YO and BaO. Spatially resolved fluorescence measurements indicate that the luminous boundary to the plasma follows a weak shock front which coincides with the ion flux propagation. The ion transmission is found to drop exponentially with distance and background pressure, in agreement with a simple scattering model which yields general scattering cross sections for ionargon 0 i-Ar = 2.1 x 10-16 cm 2 and ion-oxygen 0Ti-02 = 2.3 x 10.16 cm 2 interactions in background pressures up to 300 mTorr. The general features of the plume deceleration are described in terms of a drag force model. INTRODUCTION The expansion of the laser produced plasma from a multicomponent target in vacuum can be regarded as an assemblage of supersonic free expansion beams, with individual stream velocity and kinetic energy distributions determined from the surface layer temperature. However, experimental measurements reveal hyperthermal beams of atoms and ions with similar center-of-mass velocities as well as fluorescence far downstream which indicates the persistence of collisions even in the "collisionless" regime. Collisions in a dense Knudsen layer in the early stages of expansion were used by Kelly and Dreyfus I l I to explain the observed common centerof-mass velocity. Fluorescence from excited neutrals and ions, the most easily observed diagnostic of plume transport, results principally from plasma kinetics due to the ions and electrons in the plume, and does not represent the full picture of plume transport such as the non-emitting ground state species 121. Ion probes and absorption spectroscopy allow one to observe components of the non-emitting species in the plume: ions which have not recombined or converted and neutrals which have relaxed to their ground states 13 1. In situ processing of 1:2:3 superconductor films requires relatively low pressures (typically 10-300 mTorr) of background oxygen in the chamber during deposition. Under these conditions, the plu
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