Species-resolved imaging and gated photon counting Spectroscopy of laser ablation plume dynamics During krf- and arf-las
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ABSTRACT Gated photon counting spectroscopy and species-resolved ICCD photography have been applied to study the weak plasma luminescence which occurs following the propagation of the initial ablation plume in vacuum and during the 'rebound' of the plume with a substrate during pulsed laser deposition of amorphous diamond. These time- and spatially-resolved spectroscopic techniques were required in order to investigate notable differences between amorphous diamondlike carbon films formed by pulsed laser deposition from ArF (193 nm) and KrF (248 nm) irradiation of pyrolytic graphite in vacuum. Three principal regions of plume emission have been characterized: (1) a bright luminescent ball (v -3-5 cm/gs) displaying nearly entirely C+ emission which appears to result from laser interaction with the initial ejecta, (2) a spherical ball of emission (v -1 cm/ts) displaying neutral carbon atomic emission lines and, at early times, jets of excited C2 , and (3) a well-defined region of broadband emission (v - 0.3 cmrgs) near the target surface first containing emission bands from C2 , then weak, continuum emission thought to result from C3 and higher clusters and/or blackbody emission from hot clusters or nanoparticles. For both lasers, the measurements reveal an explosive interaction within the plume which results in a variety of new gas dynamic observations in vacuum:. These include (a) generation of instabilities or jets, (b) confinement of a residual part of the plume near the pellet surface, (c) cluster formation in the collisional, confined regions of the plume, and (d) reflection of the confined region backward to splash and redeposit on the pellet surface. Evidence for gas-phase formation of these clusters in vacuum is indicated from the dynamic evolution of the same cluster bands observed during the collision of the plume with the substrate surface during film growth. Addition of background gases strongly enhances the third (cluster) component, in accordance with plume-splitting phenomena. The combination of sensitive imaging and photon-counting diagnostic techniques permit an
understanding of the importance of gas dynamic effects, such as clustering, on the time-of-flight distributions of species arriving during the deposition of thin films in both vacuum and background gases. INTRODUCTION Recently, a new amorphous form of diamond has been grown in thin film form by filtered vacuum arc 1 and pulsed laser deposition 2 -' 2 techniques. This new allotrope of pure carbon is distinguished from the diamond-like1 hydrogenated carbons2 (a-C:H) by the term tetrahedrally coordinated amorphous carbon (ta-C) or amorphic diamond. -5 In its purest form, the disordered tetrahedral carbon is almost entirely sp3 bonded and has a density, hardness, and electrical resistivity near that of natural diamond. Films with the highest sp 3.'sp 2 ratios exhibit very high compressive stresses, which limits maximum thicknesses to -1 l.tm. Indeed the diamond-like bonding is thought to arise from the compressive stress generated by the shallow
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