Determination of Thin Film Elastic Modulus by Magnetostrictive Sensor and Finite Element Simulation

This paper reports on the measurement of elastic modulus of thin film materials by a magnetostrictive sensor. Thin film materials of Cu, Cr, Al, Au, Sn, In, Au-rich lead free solder (AuSn) and SiC were sputter deposited onto well defined strips of Metglas sensor. The elastic modulus of the thin film materials was determined by measuring the resonant frequency of the sensors before and after film deposition and using the frequency shift. Thin film material’s elastic modulus of Cu and Au was also determined by finite element simulation to verify the experimental results. The as sputtered films were examined by X-ray Diffraction (XRD) and Scanning Election Microscope (SEM) to characterize their microstructures. Elastic modulus of Au and Cu films was experimentally determined with the value of 75.9GPa and 139.2 GPa, respectively. The simulated results show about 2.5% to 4% higher than the experimental ones. This method represents a potentially new, non-destructive method to determine critical material properties of as deposited materials. INTRODUCTION: Thin film materials have been widely applied in microelectronics and microelectromechanical systems (MEMS) for interconnection and packaging. Such as Al, Cu and Au films are dominant materials in the electric circuit interconnection. Solder materials have played an equal important role for packaging and assembly microelectronic and MEMS devices. Indium and AuSn (80/20 wt %) thin film solders, in particular, are often applied in the assembly of optical fiber and components. A Au-rich, AuSn eutectic solder for example, is the ideal choice for passive assembly photonics devices [1], not only because of its excellent creep and fatigue resistance and self alignment, but also the facts that AuSn solder can be reflowed without using the flux process[2], since flux can easily contaminate the photonics facet. Moreover, AuSn and Indium solders are often used for hermetical packaging for optical device and MEMS[3]. In addition, Pb containing solder materials are limited in use in the European Community Japan and other countries due to environmental contamination issues from disposed electronic products. High temperature, lead free AuSn (80/20 wt %) eutectic solder then becomes one of the most important alternatives for high temperature power device packaging and assembly. The mechanical properties of film materials employed in interconnection and packaging are critical to successful function of the device, and they may differ from their bulk counterparts, since the volume of materials applied in those applications is much less than the volume of bulk materials used for conventional testing. To assess the mechanical property of thin film materials, many available techniques [4-6] require additional fabrication steps to construct the test structures. These steps are not only very cost ineffective, but also may alter the properties of the film material. . Nanoindentation is simple approach but it may be influenced by the substrate and the depth of indentation self. All of the techniques employed for measuring materials in bulk form are not applicable for the thin film case[7]. We report on our recent investigation of measuring the elastic modulus of thin film materials via magnetostrictive sensors. Magnetostrictive behavior exists in soft magnetic materials when the applied external magnetic field changes, the magnetostrictive materials will be subjected to compression and tension in longitudinal direction. The oscillation occurs when the applied filed has the same frequency as the material vibrating in its resonant frequency. The 1st resonant frequency of a free standing beam vibrating in longitudinal mode is given by the following equation [8]. ρ E L f ′ = 2 1 0 (1) Where L, ρ and E’ are the length, density and effective elastic modulus in isotropic elasticity of sensor material, respectively. If we take into account the changes of both elastic modulus and mass of the sensor due to a solid thin film rigidly and continuously deposited onto the beam surface, the effective resonant frequency and the shift will be expressed in Equations (2) and (3)