Microstructural Development of Dispersion Strengthened Cu Thin Films
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ABSTRACT The internal oxidation method is applied for the first time to produce a fine dispersion of second phase particles in thin films. A processing route is presented which includes ultra-high vacuum magnetron sputtering of about 1 pm thick alloy films onto Si substrates followed by insitu annealing and oxidation. Two different Cu-base alloys are examined, Cu-Y and Cu-Al, in which the extent of miscibilitiy differs significantly. This has considerable influence on the grain growth behavior. Nanoindention and wafer-curvature experiments show a drastic improvement of both room-temperature and high-temperature strength. Phenomena well known from bulk oxide-dispersion strengthened (ODS) alloys are found to appear in the thin films as well: Results on abnormal grain growth and the formation of creep voids are presented and discussed in terms of particle effects. INTRODUCTION Both the microstructural development and the mechanical properties of thin metallic films firmly attached to a stiff substrate give rise to manifold questions; e. g. the stresses supported by thin films are found to be significantly higher than the strength of corresponding bulk materials. The dimensional constraint (film-thickness effect), grain size, texture and dislocation density have been found to determine the stress evolution in a thin film during a thermal cycle, e.g. [ 1,2]. Grain growth upon annealing of a thin film usually stagnates when the grain diameter reaches the range of the film thickness [3]. However, as in bulk materials, abnormal grain growth can occur under certain circumstances yielding grain sizes significantly larger than the film thickness. In this work the effect of a fine dispersion of stable second phase particles within columnar grains on the thin film yield strength at temperatures up to 600 'C is studied systematically. Ceramic particles with a very low solubility product have the potential to withstand ripening. An ODS-type creep-resistant film material with high thermal and electrical conductivity may also find application in micro-electro-mechanical systems. Evaporated metal films with a dispersed ceramic phase were investigated by different groups mainly in the 70s (see e.g. [4]). These films were much thicker (up to 1 mm) than those presented in this paper and did not have a columnar grain structure. Oberle et al. [5] report in a more recent work on electro-codepositon of films with ceramic particles which were added to the electrolyte. These films, however, also had bulk-like microstructures. In the present work, a different processing route, internal oxidation (10) of a thin sputtered alloy films, was chosen to produce ceramic particles within the metal matrix. A noble matrix metal, in our case Cu, is co-sputtered with about I at.% of an element with a high oxygen affinity, Y or Al. Upon annealing with a controlled oxygen partial pressure, preferential oxidation of the less nobel metal within the noble metal matrix takes place. The technique of 10 is excellently reviewed in the article by Meijering [6]. 359
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