Young’s modulus of free-standing polycrystalline Al, Au, and W films with submicron thickness has been studied using a dynamic bulge-testing technique. For Au and Al films a clear frequency dependence of the modulus is observed at room temperature in the range 1×10−4–0.5 rad/s. The values of the moduli are considerably smaller than the corresponding values of bulk material. The modulus of W films measured under the same conditions does not depend on frequency and is equal to the bulk value. The origin of the behavior found in the Al and Au films is anelastic grain boundary sliding. As a consequence of the relatively small grain size of thin polycrystalline films this phenomenon is observable at room temperature in films with a relatively low melting point.
A newly designed bulge-testing apparatus for the mechanical testing of the tensile properties of free-standing thin films has been constructed and tested. With this instrument it is possible to measure the elastic modulus, tensile and compressive growth stress, and plain strain yield strength in thin films. The setup features a high strain resolution (4E-10), and a high stress resolution, e.g., >0.2 MPa for bulge heights larger than 10 μm. In our setup, thin films are stressed by a differential gas pressure across the film, while the deflection is measured by a scanning laser beam. The scanning laser beam measures the curvature of the bulge rather than the bulge height. This makes the setup insensitive to a possible initial nonflatness of the film. This also provides the possibility to measure the growth stress of films that were deposited under a compressive stress. We show the results of measurements of the plane strain modulus on thin tungsten films and on both flat and nonflat aluminum (alloy) film samples.
We developed a bulge-test setup which enables measurement of the elastic and plastic properties of free-standing thin film samples between room temperature and at least 300 °C. Mechanical stress is applied to the film by a differential gas pressure across the sample and the bulge height is measured by a scanning laser beam technique. To prevent sample oxidation the pressure cell containing the sample is mounted in a vacuum chamber. The correct operation of the setup is demonstrated by measurement of the thermal expansion of free-standing Al films. Creep experiments and tensile tests demonstrate measurement of the plastic deformation of these films at temperatures up to 200 °C.
We studied room-temperature transient creep in polycrystalline, free-standing Al films with a thickness between 220 and 550 nm using a high-resolution bulge test technique. A transient logarithmic creep strain is observed. The time and stress dependence of the creep strongly support the idea that dislocation glide, limited by forest dislocation cutting, is the prevailing rate limiting mechanism. This is in contradiction with the misfit dislocation model for thin-film strengthening but in agreement with recent work on plasticity in thin Ag and Cu films on a substrate. A comparison is made with data on bulk Al. Both the transient creep strain and the initial fast strain are at least three orders of magnitude smaller for the thin-film samples. We argue that the strain hardening coefficient is the key parameter distinguishing thin film from bulk creep.
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