A new method to study cyclic deformation of thin films in tension and compression
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A new method to study cyclic deformation of thin films in tension and compression M. Hommel, O. Kraft, and E. Arzt Max-Planck-Institut f¨ur Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany (Received 27 August 1998; accepted 22 February 1999)
In this paper, a new method to study cyclic plastic deformation in thin metal films is presented. Cu films were deposited onto compliant substrates allowing the film to be subjected to tensile and compressive stresses on loading and unloading of the film/substrate composite. The film stress was measured in situ by x-ray diffraction. First results lend to characteristic stress-strain hysteresis curves, indicative of fatigue processes in small dimensions. Metal thin films are widely used in microelectronic or micromechanical devices. For the design and reliability of such devices, the understanding of the mechanical behavior of the films used is important. In particular, plasticity in metal thin films may limit their functionality or provide a desirable mechanism of stress relaxation. Due to the small dimensions, however, investigating thin film mechanical behavior can be very difficult. In the past, several methods were used to investigate plastic deformation processes, e.g., micro-tensile tests of freestanding films1 or of films on substrates,2 bulge testing,3 and microbeam deflection.4 It is also common to study the stress evolution in thin films on silicon substrates when they are subjected to thermal cycles.5–8 Due to the mismatch in thermal expansion coefficient between, for instance, a metal film and the silicon substrate, compressive stresses in the film develop on heating and tensile stresses on cooling. The measured stress-temperature hystereses are commonly used to characterize the plastic deformation mechanisms in the films. Unfortunately, this method does not allow for varying stress and temperature independently. Further, the total strain to which the film can be subjected is limited by the difference in the thermal expansion and the imposed temperature change. In this paper, we introduce a new tensile-testing method to study plastic deformation in metallic thin films during cyclic loading. In our experiments, the film is deposited onto a compliant substrate which is mechanically strained by a tensile-tester. The compliant substrate behaves fully elastic up to strains of about 10%; the thin film, however, is plastically deformed. For the stress measurements, the micro-tensile machine is mounted in an x-ray diffractometer9 which is used to measure the film stress independently, as in earlier studies.2 The new element here is that, because of the compliance of the substrate, effects of cyclic loading and, eventually, of fatigue in thin films can be studied. Copper films were deposited by magnetron sputtering under HV conditions on 125 mm thick polyimide substrates (Kapton , DuPont). The sample had a rectangular shape with a length of 60 mm and a width of 15 mm. To improve the adhesion of the copper film to the s
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