Study On The Measurement Of Thin Film’s Young’s Modulus Using LG/LD Surface Acoustic Waves

Films have been widely used in micro sensors, micro-mechanical systems and MEMS technology. Thin films’mechanical properties such as Young’s modulus, hardness, stiffness, and density of the material, directly affect the reliability of the components in a system, and among them, Young’s modulus is of great significance in the characterization of micro/nano materials, and hence, the development of measurement equipment capable to provide more accurate and reliable measurement of this property.Currently, there are several techniques which can measure the Young’s modulus, such as nanoindentation, Brillouin light scattering, acoustic microscopy and laser generated surface wave detection. Among them, the nanoindentation technique is a contact technology, which accuracy and lateral resolution are affected by the size of the probe; the Brillouin light scattering technique is a non-contact technology based on inelastic scattering of photons, and may cause energy absorbtion by the material affecting the property measurements. Acoustic microscopy is only suitable for testing thick films and its low ultrasonic frequency is not suitable for testing high frequency SAW’s. In comparison with these techniques, laser generated surface acoustic wave detection is a non-contact, non-destructive and relatively fast technique, which uses a pulse laser for generating surface acoustic waves, and it can be detected by piezoelectric or laser detection approaches, therefore, it has become one of the most promising methods in measuring the elastic properties of thin films in the micro and nano scale. However, the technique has still some drawbacks to further investigate, for example, the oblique incidence/oblique reflection of the probe beam of the laser ultrasonic detection system makes it difficult to be focused and the diffusion angle hard to be reduced, besides, it requires very sensitive equipment to acquire the weak SAW signals, and the signals are prone to get noise and interference from the laser beam reflections or mechanical vibrations.The present thesis is dedicated to the development of the equipment capable of generating and detecting surface acoustic waves in a layer-substrate thin film by an optical deflection laser beam detection method and use it to determine the film’s Young’s modulus. The feasibility of this technique is validated with an also developed piezoelectric transducer detection approach and by the nanoindentation technique. The main tasks developed and main results were the following: 1. The state-of-the-art of Young’s modulus measurement by different technologies was investigated; problems and opportinuities were summarized; the optical deflection detection for measuring Young’s modulus was described, and the research content was established.2. By studying the motion equation in solid media, the SAW propagation equation in a two-layered isotropic thin film/substrate model and the calculation of theoretical dispersion curves was verified, and an algorithm for processing the experimental surface waves and obtaining the dispersion curve was fully developed.3. A differential confocal detection system based on optical beam deflection was fully designed and built from the very beginning. By using a differential and a common beam path configuration, the interference was improved. A PVDF piezoelectric detection system was fully developed, for which the know-how technology to develop PVDF foil transducers was successfully gained, and different transducer designs were develop, among them, a four-quadrant piezoelectric sensor was patented.4. Based on experimental results, the bandwidth of the differential confocal detection system was broadened up to 300 MHz with a resolution of 0.05 GPa. In comparison with a commercial nanoindentation system, the relative error of the Young’s modulus was less than 1%, which confirmed that the differential confocal detection system is a feasible and accurate method to detect the Young’s modulus of thin films.5. A summary, conclusions, novelty, contributions to the research project, and future work are described.