Hillock formation and thermal stresses in thin Au films on Si substrates
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Hillock formation and thermal stresses in thin Au films on Si substrates Linda Sauter, T. John Balk1, Gerhard Dehm2, Julie A. Nucci and Eduard Arzt Max Planck Institute for Metals Research and Institut für Metallkunde, University of Stuttgart, 70569 Stuttgart, Germany 1 Now at: University of Kentucky, Department of Chemical and Materials Engineering, Lexington, KY 40506, USA 2 Now at: Erich Schmid Institut für Materialwissenschaft, Österreichische Akademie der Wissenschaften, and Department für Materialphysik, Montanuniversität Leoben, 8700 Leoben, Austria ABSTRACT The wafer curvature technique was used to analyze stresses in fine-grained, 50 nm to 2 µm thick Au films on silicon substrates between room temperature and 500 °C. The microstructural evolution was analyzed by scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In situ heating experiments inside a scanning electron microscope provided a comparison between the morphological development and the stress-temperature behavior of the film. Hillock formation was observed, but it can only partially account for the stress relaxation measured by the wafer curvature technique. INTRODUCTION Thin metal films on rigid substrates often sustain high stresses during thermal cycling and annealing. Compressive stresses at elevated temperatures can enhance grain growth, grain boundary diffusion, or hillock formation. Au films are typically deposited onto metallic adhesion layers including V, Ti, Cu, Ni, Sn, In, and Cr [1-3]. These elements diffuse into the Au film, segregate to grain boundaries and/or the surface, and may form a surface oxide. The microstructural evolution, especially hillock and void formation, has been found to depend crucially upon the behavior of these interlayers [2]. Hillock growth was also reported in Au films with an interlayer of immiscible Mo [4]. The lateral diameter of these hillocks was predicted as a function of isothermal annealing time, in reasonable agreement with the experimental data. Zhang et al. [5] found that a 40 nm alumina passivation layer on Au/Si bilayer beams with a 20 nm Cr interlayer prevented grain coarsening and hillock formation. Thus, since the stress and morphological evolution in thin Au films is complicated by interaction with the surrounding layers, a clear picture of the origins and consequences of hillocks in Au films is not yet available. In the present study we deposited Au films directly onto silicon nitride-coated silicon wafers. A thin metallic interlayer was not necessary, as the Au adhered to the silicon nitride throughout the temperature cycling. This avoided the complications imposed by an interlayer and allowed more accurate film stress measurements over a broad film thickness range (50 nm to 2 µm). Hillock formation and stress evolution in these films were studied systematically.
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EXPERIMENTAL Nine different Au film thicknesses between 50 nm and 2 µm were DC magnetron sputtered at room temperature under ultrahigh vacuum conditions
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