Opto-numerical analysis of AIN piezoelectric thin film operating as an actuation layer in MEMS cantilevers

摘要

The goal of this Ph.D. thesis is the analysis and the determination of the properties of aluminium nitride (AlN) piezoelectric thin film operating as an actuation layer within bimorph microcantilevers and the evaluation of their micromechanical performances. The research described in this thesis was accomplished with the use of a hybrid methodology combining the optical interferometry, the techniques of nanoindentation, the numerical simulations as well as the analytical considerations meaning the association in which the interactions between the physical and mathematical models of the studied objects were investigated. This research was performed within the framework of the “co-tutelle” Ph.D. thesis between l'Université de Franche-Comté and the Warsaw University of Technology. In France, this work was accomplished in the Département d'Optique P.M. Duffieux of the Institute FEMTO-ST. In Poland, the work was caried out by the Institute of Micromechanics and Photonics. The present manuscript proposes the development of hybrid opto-numerical methodology to study the MEMS structures. This methodology allows a complex characterisation of samples by identifying and explaining the main sources of the discrepancies between their physical and mathematical models. This hybrid approach was applied to study the silicon microcantilevers actuated by a piezoelectric transducer containing a thin film of AlN sandwiched between two metal electrodes. The properties of AlN thin films as well as micromechanical behaviours and life time of the tested samples were determined. The physical model of these multilayer elements was created with a help of full field and non-contact Twyman-Green interferometry, offering high-resolution data. Thus, the data concerning static as well as dynamic performances, information on geometry as well as material and mechanical properties of the investigated microobjects were obtained. To complete this data, some reliability tests of the studied samples were realised. The mathematical models combining the analytical calculations and the Finite Element Method (FEM) calculations (ANSYS software) were performed taking into account the multilayer structure of mentioned elements. They allowed to understand better the functionality of the real microdevices with special emphasis on the identification of their mechanical failures. Moreover, the research focused also on some in-use problems dealing with both the ageing and fatigue accelerated life tests and the operational stability of the samples. Resulting data allowed a better knowledge about the most likely failure modes affecting the operation of the microobjects and the factors that invoke these failures. They led to determine the operational conditions and helped in analysis and evaluation of the AlN-driven microcantilever technology in order to improve their manufacturing process. The proposed hybrid methodology has extended the metrological capability of the laser interferometry techniques. It has allowed to develop the methods serving to characterise microstructures, determine their reliability and support their technology in realisation of the reliable devices with required parameters. Implementation of the mentioned methodology within the framework of this Ph.D. thesis also allowed to obtain knowledge about the properties of the AlN thin films and the performances of the piezoelectric microcantilevers leading to increase a number of their applications with improve their quality and controllability.