Design and optimization of one and two ports Two-Dimensional Mode Resonators for wide band RF applications

Novel Internet of Things (IoT) and 5G applications are driving the demand for faster wireless data exchange in a wide frequency range. Acoustic ultra-high-frequency (UHF) filters implementing piezoelectric resonators are among the most promising solutions to enable radio frequency (RF) front-end modules with large bandwidth and low insertions losses at the frequencies targeted by these new technologies. This thesis focuses on the design and optimization of one and two ports Aluminum Nitride based Two-Dimensional-Mode Resonators (2DMRs) for wide band applications. In the first part, a detailed analysis of the 2DMR state-of-the-art is reported and compared to other classes of acoustic resonators in terms of performance and scalability limitations. With the aim of improving the quality factor at resonance Q and the electromechanical coupling coefficient kt2 of 2DMRs preserving their ease of fabrication, four different studies have been investigated. The analyses have been carried on through 2D/3D FEM simulations with COMSOL Multiphysics and analytically described with mathematical models. Firstly, the minimization of anchor losses is demonstrated through the analysis of an equivalent electrical circuit where inactive acoustic waveguides have been analytically modelled as transmission lines. The introduction of “virtual fixed boundaries” at the edge of the active areas through lambda/4 acoustic transformers showed how the optimization problem can be split in the definition of two energy transfer functions governing the design of the bus and the anchors. 3D FEM simulations with different boundary conditions (Fixed Constraints and Perfectly Matched Layers) demonstrated excellent agreement with the analytical models. Secondly, a new class of resonators named Two-Dimensional Mode Multimodal Resonators (2DMR^M) have been introduced demonstrating a boosting in the classical 2DMRs kt2 thanks to a reactive coupling between the zero and the first order symmetric Lamb modes. A physical analysis of the multimodal resonators based on the dispersive relations of Lamb’s symmetric waves has been carried out and empirical design rules have been derived based on simulations with different materials and layers thicknesses. The frequency lithographic tunability of 2DMR^M has been computed and compared to the one of other MEMS resonators, demonstrating how this innovative configuration can overcome the limitations related to electrical loading and power handling common to many of other technologies. Moreover, as a way of reducing the residual stress in the piezoelectric film due to irregularities introduced by the bottom electrodes grating, an innovative 2DMR configuration including a continuous bottom electrode plate is discussed and performances of different electrodes materials are compared. Finally, several electrodes apodization shapes for spurious modes suppression have been simulated and compared obtaining unprecedent clean 2DMR^M admittance responses in 3D simulations without affecting the electromechanical coupling coefficient. To conclude, an innovative 2-ports device exploiting a frequency selective evanescent coupling of two 2DMRs with applications in RF power sensing has been engineered and analysed for the first time. To empirically demonstrate the results from simulations, an exhaustive experimental plan including 448 resonators has been outlined by parametrically drawing the lithographic masks with ad-hoc Python libraries.