An Anodic Process for the Determination of Grain Boundary and Film Thickness Strengthening Effects in Aluminum Films on
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AN ANODIC PROCESS FOR THE DETERMINATION OF GRAIN BOUNDARY AND FILM THICKNESS STRENGTHENING EFFECTS IN ALUMINUM FILMS ON OXIDIZED SILICON. RAMNATH VENKATRAMAN AND JOHN C. BRAVMAN Dept. of Materials Science and Engineering, Stanford University CA 94305 ABSTRACT We have measured stress variations with temperature as a function of film thickness and a given grain size in pure Al and AI-0.5%Cu films on Si substrates. The variation in thickness for a given grain size is brought about by using the same film and the repeated controlled growth and dissolution of a barrier anodic oxide which can be grown uniformly on the film. Stress measurements were made as a function of temperature by measuring wafer curvature after successively removing each 0. lgtm of Al film. The components of strengthening due to the film thickness and the presence of grain boundaries were separated by assuming that the flow stress of the film is simply the sum of these two components. The observations are consistent with results obtained using laserreflowed films in an earlier work. The variations of these two components with temperature, and under tension and compression is discussed. INTRODUCTION Due to the effect of the rigid substrate to which it is bonded, the small grain size and other thin film effects, the mechanical behavior of aluminum thin films is very different from that of bulk samples. The variation of stress with thermal cycling in such films and their isothermal stress relaxation characteristics have been well documented [1-5]. They are expected to be greatly influenced by the thickness of the film, its grain size and the presence of alloying elements. When a thin film of aluminum is thermally cycled on a rigid substrate such as silicon, the imposed thermal strain is accomodated as elastic and plastic strains in the film. By measuring stress as a function of temperature one may obtain at a given temperature, the flow stress of the film, i.e. the stress at which the film would deform plastically at that temperature. However, it is important to understand the conditions under which the flow stress can be obtained from the stress-temperature curves. In a curve of stress vs. temperature, provided there are no volume changes, the slope can never exceed the value (E/(1-v)) Act where (E/(I-v)) is the biaxial modulus of the film, and Act is the difference in thermal expansion coefficients between the substrate and film. Only when the slope of the stress-temperature curve is significantly smaller than this elastic slope may the measured stress be called the flow stress. As will be explained later, thinner films have higher flow stresses and consequently require larger amounts of induced strain for them to flow plastically. As a result the final value of stress obtained on cooling to room temperature is likely to be less than the flow stress of the film at room temperature. In our previous work the effect of film thickness on the flow stress was investigated by using large-grained films obtained by laser-reflowing [6]. It was seen in these fil
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