Characterization of polyimid-multi-layer thin films combining laser ultrasonic measurements and numerical evaluations

Modern microelectronic devices frequently require the use of thin films and multi-layer systems [1]. Especially the application of advanced material models to simulate e.g. reliability issues for such components relies on the accurate determination of the layers' elastic properties [2]. Polyimides (PI) show many beneficial mechanical and chemical properties for applications in the field of microelectronics, e.g. in the realization of MEMS packages [3]. In the work reported here, the combination of a laser ultrasonic measurement and a numerically solved theoretical model [4] is presented in order to determine the elastic properties of a multi-layer system. The multi-layer system consists of a polyimide layer of 11 μm thickness and an 800 nm silicon nitride film on a (100) silicon substrate. The theoretical model uses a partial wave ansatz and a global matrix method in order to determine the frequency dependent phase velocity [5]. For the measurement of the frequency dependent phase velocity, nanosecond laser pulses were focused to a line shape onto the sample surface. This line excitation generates plane broadband surface acoustic waves (SAWs). The phase velocity depends on the frequency as a consequence of the different sound velocities of substrate and layers. The low frequency SAWs propagate mainly in the substrate, whereas higher frequency waves propagate mostly in the thin layers. An optical beam deflection method was applied to detect the SAWs. The Young's Modulus and the Poisson ratio of the polyimide layer were derived by fitting the theoretical curve to the experiment. The presented method provides the possibility to measure contactless on wafer level. It represents a valuable tool regarding the non-destructive evaluation of elastic properties of thin films in multi-layered systems. The Young's modulus and the Poisson ratio can serve as essential input for various advanced measurement techniques and simulations [2].