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BER1990

Mechanical Measurements Many expérimental techniques hâve been used to study the mechanical properties of metallic superlattices. Thèse techniques can be divided into two catégories, depending on whether the technique requires the film to be removed from the substrate or not: (1) techniques that require removal from the substrate include bulge test, 5 vibrating reed, 6 vibrating membrane, 7 and continuous ultrasonic wave measurements; 8 (2) techniques that do not require removal from the substrate include Brillouin scattering, 9 pico-reflectance, 10 surface acoustic wave, 11 and nano-indenter. 12 The obvious question arises — does removal from the substrate alter the mechanical properties of the films? We believe that most results to date imply that removal from the substrate does not alter substantially the mechanical properties of the films. However, it should be recognized that many of the measurements on self-supporting films are subject to expérimental artifacts caused by the removal from the substrate. Tables I and II list the various expérimental techniques used to study the mechanical properties of metallic superlattices, the elastic properties that they can détermine, and possible expérimental difficulties. Examining Tables I a n d II clearly shows that the complète characterization of the mechanical properties of thin films requires the application of a wide battery of tests, on the same samples, together with simultaneous structural characterization. Since no single laboratory can apply ail thèse techniques, it is désirable to exchange samples between the various research laboratories. Structural Properties The number of Systems that hâve been studied is very large and includes com-

binations of éléments, compounds, and alloys. Although corrélations between physical properties and the binary thermodynamic phase diagram of the constituents hâve been pointed out, 1 we believe that this only affects the mechanical properties through changes in the structure, which are known to cause drastic effects on their transport properties. 13 Moreover, small changes in the structure of thin films are theoretically expected to hâve drastic effects on the mechanical properties. For instance, an early theoretical model 14 has shown that small changes (about a few percent) in the lattice parameters may drastically affect the elastic properties. It is therefore important to perform précise, detailed structural studies 15 " 16 to ascertain whether the origin of the elastic anomalies is correlated and/or caused by structural or electronic effects. X-ray diffraction has been one of the most successful, précise techniques for determining the structure of thin films and superlattices. Although x-ray diffraction has been used for many years for précise, quantitative déterminations of the structure of bulk materials, to date its application to superlattices has only been semiquantitative. The reason is that many of the properties of interest, such as magnitude and type of interfacial roughness, interdiffusion, and lattice