Elastic Properties of Thin Films
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two temperature regions. Below approximately 10 K and above a characteristic temperature that depends on the measuring frequency, the internal friction of amorphous solids is constant, with a value that typically lies between 10-3 and 10-4. This region is known as the plateau region. Below this characteristic temperature, the internal friction begins to decrease and eventually falls off as T 3 . This should again be contrasted with the internal friction of very pure single crystals which can be as low as 10-10 throughout this temperature region. 2 Even in crystalline material which is not single crystal, the anelastic processes which contribute to the damping, such as dislocation motion, diffusion, grain boundary motion and point defect relaxation are often frozen out at temperatures above I K. 3 This results in an internal friction that is much lower than is typically found in an amorphous solid. We propose that the anomalous low temperature thermal and elastic properties of amorphous solids can serve as a fingerprint of the disordered state. In particular we propose that these anomalous properties can be used to quantify the disorder present in solids. Because of this, we have developed a technique for measuring the internal friction and shear modulus of thin films. This technique should be capable of quantifying the disorder present in these films. This would be particularly useful in areas such as studies of the removal of damage to crystalline films caused by ion implantation, studies of the crystalline perfection of thin films produced by molecular beam epitaxy, and studies of the elastic properties of amorphous solids which can only be produced in thin film form. In what follows, a description of the technique we have developed for measuring the elastic properties of thin films is presented. As an illustration of the sensitivity of the technique, results from a study of the internal friction of thin Si0 2 films are discussed. 567 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
EXPERIMENTAL TECHNIQUE The experimental technique we have developed for measuring the elastic properties of thin films is very similar to a technique developed by Berry et al. which utilized a vibrating reed. 4 The idea is to utilize an oscillator, which has well characterized elastic properties, as a substrate on which the thin film of interest can be applied. The resulting changes in the elastic properties of the oscillator caused by application of the thin film can then be used to determine the elastic properties of the thin film itself. In order to gain a better understanding of this technique, consider the effect the application of a thin film has on the internal friction of an oscillator. The internal friction is defined in the usual manner as Q-1 =AE
2•rt•(1)
27rE
where AE is the energy lost per cycle by the oscillator and E is the total energy stored in the oscillator. When the oscillator is composed of a thin film and substrate, AE can be broken into two parts, the energy lost in the substrate, AEsub,
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