Optimization of piezoelectric thin film sputtering

Piezoelectric thin films play a crucial role in the modern communication world, as they are the core of radio frequency (RF) filters used in virtually all mobile devices. These components ensure the transmission and reception of signals at specific frequencies. Surface Acoustic Wave resonators (SAW) and Bulk Acoustic Wave (BAW) resonators are the current standard for piezoelectric-based RF filters, with Aluminium Nitride (AlN) being one of the most commonly used material. As mobile network technology advances, RF filters must work at higher frequencies with larger bandwidths, and the devices’ designs and materials must be developed too. In this context, this thesis aims to optimize the sputtering deposition of piezoelectric thin films. The experimental work has been carried out in the Center of MicroNanotechnology (CMi) at the École Polytechnique Fédérale de Lausanne (EPFL) under the direction of the Advanced NEMS Laboratory led by Prof. Guillermo Villanueva. The optimization process focused on tuning the deposition parameters of a newly available sputtering cluster tool, the Alliance Concept CT200. The research was organized keeping in mind the final application of the thin films. The optimization process hence included the development of a quality bottom electrode in Platinum, the deposition of AlN thin films, and the characterization of their properties. To study the piezoelectric properties of the deposited films, simple microdevices were realized. A brief investigation of the potentiality of doping the AlN thin films with Sc via co-sputtering was also carried out. Crystallographic measurements indicated the production of poly-crystalline AlN thin films with a good degree of orientation, proving the efficacy of the tool to deposit piezoelectric thin films on the bottom electrodes. Composition and thickness of the thin films were proven consistent over several depositions by measurements, while film crystallinity results were less conclusive, as they were influenced by tool contamination. The realization of Free-Standing Bulk Acoustic Resonators (FBAR) was successful, and the devices exhibited a good effective coupling coefficient, proving that the AlN thin films deposited were compatible with the use for RF microdevices. The composition analysis of the Sc-doped materials proved the feasibility of depositing thin film with a controlled doping content.