Stress and microstructure evolution during initial growth of Pt on amorphous substrates
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An understanding of the relationship between stress and the corresponding microstructure at various stages of thin film growth might allow prediction and control of both microstructure and film stress during thin film deposition. In the present study, a combination of in situ curvature measurement and ex situ microstructural characterization was used to make correlations between stress and microstructure for the growth of Pt on SiO2. Plan view transmission electron micrographs of Pt films with average thicknesses ranging from 3 to 35 Å show the evolution of microstructure from isolated islands to a coalesced film, in agreement with models for stress behavior during the early stages of film growth. Quantitative measurements of grain size, island size, and areal fraction covered are extracted from these micrographs and, in conjunction with an island coalescence model, used to calculate the magnitude of the tensile stresses generated during coalescence. The predicted curvature is shown to compare favorably with the measured stresses.
I. INTRODUCTION
Controlling stresses in deposited thin films has become one of the more important challenges of microelectronic technology. It is well known that there are relationships between the film stress and the corresponding microstructure at various stages of growth. Better understanding of how film microstructure and stress evolve may lead to better control of final stress states and microstructures in deposited thin films. Film stress, obtained from substrate curvature, is measured during growth of sputter-deposited Pt on amorphous substrates and is observed to be slightly compressive at thicknesses less than about 5 Å. This is followed by a change to tension in thicker films, leading to a tensile maximum at about 33 Å, after which the stress becomes compressive again. Such stress behavior is commonly observed during the deposition of highmobility metal films by evaporation, although the maximum tensile stress is typically observed at larger thicknesses in these studies.1 The initial compressive behavior has been associated with capillarity and residual elastic strain effects during the growth of isolated crystallites.2 Evidence suggests that the tensile stresses are caused by the spontaneous snapping together of individual islands, and the tensile stress maximum then marks the completion of film coalescence.3 In this paper we establish that crystallite coalescence is responsible for the development of tensile stresses, primarily through the use of plan view transmission electron 2540
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J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 19 Mar 2015
microscopy (TEM) observations of sputtered Pt films on SiO2 substrates. We also present a simple quantitative model of the tensile stresses that develop through crystallite coalescence, taking account of island size and areal coverage of the substrate. We use a model developed by Hutchinson and his co-workers4 for describing curvature changes associated with cracking in residually stressed films to
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