Mechanical behavior of thin Cu films studied by a four-point bending technique
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Mechanical behavior of thin Cu films studied by a four-point bending technique Volker Weihnacht and Winfried Brückner Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, D-01069 Dresden/ Germany, [email protected]
ABSTRACT Four-point bending experiments in combination with thermal cycling of thin films on substrates were performed in a dedicated apparatus. Strains up to ±0.8% could be imposed into Cu films of 0.2, 0.5, and 1.0 µm thickness on Si substrates by bending the substrates at various temperatures in high vacuum. After relief of the bending, the residual stress was measured by the wafer-curvature method. At temperatures below 250°C, the yield behavior is asymmetric in tension and compression. The amount of plastic strain introduced by external bending increases with film thickness, but the absolute values of the introduced plastic strains are very low throughout. At higher temperatures, there is no clear thickness dependence and no asymmetry in tension and compression. The results are discussed in connection with the formation of misfit dislocations during plastic deformation of thin films. INTRODUCTION Thin metallic films play an important role in many modern technologies due to their electrical, magnetical and optical properties. The ongoing miniaturization of the used structures leads to stability problems due to cracking, hillock and void formation, and local plastic deformation. Therefore, the mechanical properties of thin metallic films have been intensively studied and modeled. For mechanical testing of thin films some methods from bulk testing have been adapted to the thin film geometry, e.g. micro-tensile testing [1] and nanoindentation measurement [2]. Also new methods were developed, such as bulge testing [3], micro-beam bending [4], tensile testing of films on flexible or plastic substrates [5], and stress measurement during thermal cycling [6]. The most common technique is thermal cycling of films on Si substrates. It gives a good insight into the plastic behavior under thermal stresses, but is hampered by the fact that temperature and film stress can not be varied independently. It turned out that thin-film plasticity is different from bulk plasticity for the same material. In particular, the difference shows up in very high film strengths. This could not be explained by microstructural reasons, because also pure single-crystalline films are significantly stronger than the corresponding bulk material [7]. Further peculiarities of mechanical thin-film behavior are effective strengthening during cooling and a Bauschinger effect with low compressive stresses during reheating in the thermal cycling experiment [8]. In this work, in addition to the thermal cycling behavior, information was obtained by imposing external stresses into Cu films on Si samples by four-point bending. This technique enabled us to measure the isothermal stress-strain behavior at various temperatures. The results are compared with stress measurement during thermal cycling between room temp
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