Sc-doped AlN: high performance piezoelectric MEMS resonators and high-polarization ferroelectric

The present Master's Thesis regards the impact of Scandium doping on Aluminum Nitride (AlN) thin films. Two very interesting applications of Sc-doped AlN (ScAlN) are treated in depth, namely the improvement in MEMS piezoelectric resonator performances and the study of its ferroelectric properties. While the first study poses itself as an optimization of something already present, the second one represents a step towards the opening of new research lines. The present work is divided into three main Chapters. The first one is the introduction of the Thesis work, describing the state of the art in the field of MEMS resonators. Moreover, physical descriptions of the phenomena of piezoelectricity and ferroelectricity, the ones on which the studied technologies rely, are given. Finally, the electrical description of piezoelectric MEMS resonators, together with the working mechanisms of a specific topology of filters which uses them as building blocks, are given. The second chapter introduces a relatively new technology of piezoelectric MEMS resonators called Cross-Sectional Lame' Mode Resonator (CLMR) and explains why it poses itself as an improvement to the current state of the art. A systematic optimization study is performed on AlN CLMRs, concerning both the geometrical structure and the electrode materials. Moreover, the impact of Scandium doping of AlN thin films is quantified as regards the performances of CLMRs, obtaining noteworthy improvements compared to the case of pure AlN. In addition, two experimental plans have been designed and clearly described in order to verify the theoretical and simulation analysis, one on AlN and the other on 30% doped ScAlN. The third chapter regards ScAlN ferroelectricity. Firstly, an explanation of the appearance of this phenomenon is given, together with the applications that such a discovery is able to open in the field of RF MEMS. After that, descriptions of the employed thin films sputtering and characterization tools are given. With those, experimental results obtained on ScAlN ferroelectric capacitors fabricated in-house are shown, with interesting considerations on how the film properties vary according to different deposition and measurement parameters. Finally, for the first time, a Finite Element Model (FEM) utilizing the commercial software COMSOL Multiphysics able to describe the behaviour of ferroelectric materials is developed and clearly explained. Starting from an empiric mathematical model, the hysteresis and butterfly curves are obtained from COMSOL with a desirable level of agreement, at least for this early stage of development. Moreover, the model is embedded in the simulation of a reconfigurable resonator, opening the possibility of simulating this kind of MEMS devices.