Microstructural Evolution and Stress Relaxation in Sputtered Tungsten Films
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F. M. Ross, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720; R. R. Kola, R. Hull and J. C. Bean, AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974. ABSTRACT We have investigated the relationship between microstructure and stress in very thin sputtered W films. We discuss features of the microstructure, in particular the presence of voids in compressively stressed films, in terms of the evolution of the structure from a metastable P3-phase. By developing a novel specimen geometry for the transmission electron microscope (TEM), we present dynamic observations of the O-W---ct-W transformation. INTRODUCTION Sputtered tungsten thin films find many applications as absorbers for x-ray masks and in VLSI metallisation. However, these films are usually deposited with a high intrinsic stress, and this can result in voiding in metallisation; it is also disadvantageous in x-ray masks where distortion of the pattern must be minimised. Stress is also known to alter the kinetics of relaxation of a metastable substrate on which the film is deposited [1]. It is therefore important to control both the initial stress state of the film and any changes in stress during subsequent device processing. It is now well established that the primary parameter controlling stress in films sputtered at room temperature is the pressure of the working gas during deposition [2-5]. Small changes in the pressure can alter the stress state from highly compressive to highly tensile, in both very thin (tens of nanometers) and thicker (micron) W films. The mechanism for this effect is thought to be a competition between scattering in the plasma, which decreases the directionality of the incident ions, and the enhancement of surface mobility by ion bombardment [3]. This model predicts a strong correlation between microstructure and stress state. W films with compressive andctensile stresses are, in fact, known to have distinctive microstructures, in terms of grain size, voids, argon concentration and lattice parameter [6-9], although the quantitative relationship between these parameters and the stress is still not fully understood, particularly for the case of very thin films. In this paper we describe an investigation into the microstructure and stress in sputtered W films. In a previous study [1], we investigated the effect of film stress on the relaxation of metastable GeSi/Si substrates. We found that the sign of the stress in the W film influenced both the onset of relaxation and the ultimate density of misfit dislocations formed at the GeSi/Si interfaces on heating. This result suggested that a suitably stressed metal overlayer may be useful in diminishing the driving force for the formation of dislocations (which usually originate at the edges of patterned mesas) during the processing of strained layer systems. In the present investigation we have considered the relationship between stress and microstructure in these W overlayers. We have characterised the differences in microstructure between compressive and tens
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