Author:

Keywords:

thin film, characterization, photoacoustics, anisotropy

Abstract:

As devices get smaller, a challenge is presented to those who determine the physical properties of the materials used within these devices in a fast and reliable way. This thesis attempts to answer, in part, to that challenge, with a focus on the elastic and, to a lesser extent, thermal properties of thin functional filmsthat are deposited on single crystal substrates. The elastic anisotropy introduced by the single crystal substrate has a major effect on the surface acoustic wave (SAW) velocity and its dispersion. The aim of this work is to extract the elastic properties of functional coatings, and exploit the anisotropy of the substrate, through the fitting of the SAW velocity dispersion. This approach brings an addition to earlier work at ATF (KU Leuven), the Applied optics group (University of Nottingham) and the Keith Nelson research group (MIT).Experimentally the SAW velocity dispersion is measured through two all optical methods. Impulsive stimulated thermal scattering (ISTS, also known as heterodyne diffraction or transient grating) and grating induced laser beam deflection (GILBD) both excite and detect SAWs, with a chosen wavelength, on reflective surfaces. Several films were investigated, each presenting the applicability of the presented approach toother situations. Nanocrystalline diamond (NCD) attempts to bring the extraordinary properties of diamond to applications. In this thesis the elastic and thermal properties were determined, and were found to be in line with literature values. It also reveals that this technique is able to characterize a rough isotropic intermediate layer (a Cr top coating was deposited to enhance reflectivity). Sintered porous silicon is typically used as a sacrificial layer to transfer a functional layer from a bulk Si substrate to a glass substrate for use in photovoltaic cells. c44 and c'=(c11-c12)/2 were determined for samples with different porosities and were found to be within the boundaries set by a finite element model. Additionally it was observed that the microscopic anisotropy of the Si matrix translates to a macroscopic anisotropy in the SAW velocity dispersion. This study shows the applicability of the technique on soft (porous) anisotropic coatings on anisotropic substrates. MnO2 is a typical cathode material in batteries and is studied here as a thin film (500 nm). The resulting value for the Young's modulus and porosity were confirmed by independent techniques. The presented method is thus able to simultaneously determine both the Young's modulus and the porosity of an isotropic coating with sub-micron thickness, with a error estimate that is smaller than the other presented methods.Ni2MnGa is a magnetic shape memory alloy with great potential for the development of microsensors and microactuators. The presented approach allowed to determine a value for c44 and c' of the cubic superstructure of the twinned martensitic phase of the material. Additionally the martensite to austenitephase transition couldbe monitored using SAWs and the effective thermal diffusivity at a temperature higher than the Curie temperature (hence making the M-A transition invisible to magnetic measurements) and for a very small sample with a thin (500 nm) coating (hence making the transition very difficult to detect using calorimetric methods). Bi2Te3, finally, is a thermoelectric material of which the figure of merit, the ratio of the electric conductivity to thermal conductivity, expresses its ability to convert thermal energy into electrical energy (and vice versa). In this study the thermal diffusivity was determined of two films. The extracted values were a factor 2 to 5 lower than those found in literature, which was explained by the granular morphology of the BiTe film. The different case studies illustrate the feasible of the presented approach to extract elastic properties from coatings that are mounted on single crystals. The anisotropy that the single crystal substrate introduces is a factor that has to be accounted for, and which was exploited to extract more elastic information from the coating. For coatings with cubic anisotropy, or a cubic superstructure, it was shown that it is possible to reliably estimate a value for c44 and c'. For isotropic coatings it was shown, through a simulation feasibility study, that it is possible to extract the Young's modulus, the thickness, the density and Poisson's ratio. The latter is a parameter that is difficult to measure and is often given an ad hoc value. A technique that is sensitive to it is therefore valuable. However, the reliable extraction of Poisson's ratio was not confirmed by the experimental results on MnO2. Future investigation on a custom made sample with good optical quality and known properties should allow to cross-validate the feasibility study. Future research should attempt to characterize films with a thickness smaller than 50 nm, as in this is the current working range in the microelectronic industry. Additionally, in this thickness regime the properties of the films change and so interesting physics (confinement effects) can be investigated.