Combination of Acoustic Methods and the Indentation Technique for the Measurement of Film Properties

New techniques are continuously being developed to produce films and thin films, whose properties typically depend on the preparation process, and can be significantly different from those of the material in bulk form. The characterization of thin layers remains an open issue. A precise knowledge of the mechanical properties is crucial in several cases, and is of general interest. A full mechanical characterization includes the determination of both the elastic properties, which characterize the reversible deformations, and the properties which characterize the reversible behaviour. In most cases the elastic behaviour can be completely characterized by the elastic moduli, or equivalently by the components of the elastic tensor. It is well known that also in the simplest case, the homogeneous isotropic continuum, the elastic stiffness cannot be characterized by a single parameter, but needs two independent parameters; in the case of anisotropic solids the number of independent parameters further increases. The inelastic behaviour is typically more complex. Among the methods to perform the mechanical characterization, a specific class exploits vibrations of acoustic nature as a probe of the material behaviour. These methods are non destructive, and involve only elastic strains; therefore, they are intrinsically unable to give indications about any inelastic behaviour. On the other hand, due to the complete absence of inelastic strains, the relationship between the raw measurement results and the stiffness parameters can be more straightforward, and less subjected to uncertainties or to spurious effects, possibly allowing better accuracies. The mechanical characterization of supported films typically requires specific methods. The most widespread technique is indentation, for which a specific standard exists, and which induces both elastic and inelastic strains: It supplies significant information about irreversible deformation, but the extraction of the information concerning the elastic behaviour is non trivial, and typically leads to a single parameter, usually referred to as 'indentation modulus'. If a reasonable assumption about the value of Poisson???s ratio is available, a value of Young modulus can be derived, which obviously depends on the reliability of the adopted assumption. In the case of films, since the nano and micro-structure can be different from that of bulk samples, a well grounded assumption about the value of Poisson???s ratio might be not available. It is also well known that, when supported films are measured, care must be exercised to avoid the influence of the substrate properties. Methods which exploit acoustic vibrations have been developed also for supported films. Acoustic properties depend on stiffness and inertia; therefore, as it happens for bulky samples, acoustic methods require a value of mass density, independently measured. However in acoustic methods the intrinsic absence of inelastic strains makes the derivation of the stiffness parameters less subjected to spurious effects, and less dependent on specific modelling assumptions. Among the techniques based on acoustic excitations, the so called laser ultrasonics techniques rely on impulsive, therefore broadband, excitation, while quantitative acoustic microscopy relies on monochromatic excitation. In the detection of vibrational excitations, substantial advantages are offered by light, a contact-less and inertia-less probe; such advantages are particularly relevant in the measurement of films and small structures. They are exploited by Brillouin spectroscopy, which relies on Brillouin scattering: the inelastic scattering of light by acoustic excitations. Brillouin spectrometry relies on spontaneous thermal excitation, which has a small amplitude, but has the broadest band, allowing access to the GHz and multi GHz band. For all these methods, the outcome is the measurement of the propagation velocity of one or more acoustic modes. If sufficient information is gathered, a full elastic characterization can be achieved by purely vibrational means, if an independent value of the mass density is available. However, a complete elastic characterization by only acoustic means is not always achievable. The results of acoustic methods and of indentation can therefore be combined, with the purpose to obtain a complete elastic characterization, not achievable by each of the techniques alone. This can be particularly useful in the case of new materials or of films of unconventiona structures, for which a reliable assumption about the value of Poisson???s ratio, needed by indentation, is not available. And the combination of techniques anyhow offers a useful cross-check among techniques based on completely different principles. This chapter is devoted to this combination of indentation with acoustic techniques, namely quantitative acoustic microscopy and Brillouin spectroscopy.