Sputtered AlN on SiO₂ for monolithic integrated piezoelectric photonics

Integrated photonics started to surpass their traditional applications in tele- and datacommunication towards a variety of new domains including health care, artificial intelligence, and quantum applications. With this diversification, also the need for general-purpose solutions - similar to field programmable gate arrays (FPGAs) - grows, to enable for fast prototyping and lower the time and costs required for the development of new integrated photonic devices. A waveguide mesh consisting of tunable phase shifters and power splitters could serve this need. These active devices are required to provide broadband functionality with low power consumption, crosstalk, and optical losses on a small footprint with a highly scalable approach. Physical alteration of waveguide structures via micro-electro-mechanical systems (MEMS) is expected to fulfill these requirements. While electrostatic MEMS have been integrated into silicon (Si) photonic platforms, piezoelectric MEMS overcome the encountered challenge of pull-in and provide bidirectional movement of waveguides. Aluminum nitride (AlN) is an emerging material platform for integrated photonics providing also piezoelectric properties. Within this thesis the feasibility of integrating such piezoelectric MEMS for tuning of integrated photonic components within the same AlN material platform is explored. To achieve this, several challenges are addressed, starting with the sputter-deposition of AlN on silicon dioxide (SiO2). The impact of sputtering parameters on material characteristics including optical properties, crystal quality, surface roughness and thin film stress is investigated. A path towards low stress thin films suitable for the targeted free-standing MEMS and integrated photonic components is presented. Next, a platform for passive integrated photonic components is developed targeting a wavelength of 1.55 µm. The development of the process flow focuses on the critical AlN etching process, for which a correlation of the thin film stress and grain size with the etch rate is found. The characterization of directional couplers, optical ring resonators (ORRs), and straight waveguides for different AlN-on-SiO2 thin films reveals a tendency of increased propagation loss for smaller grain sizes. In a last step the integration of the MEMS components on the developed material platform is tackled. A directional coupler with in-plane piezoelectric bending actuators for gap tuning is designed, fabricated, and characterized. Open challenges encountered during the fabrication process are discussed and possible solutions are provided. A first proof-of-concept for monolithic integrated piezoelectric MEMS for active integrated photonics is demonstrated. Overall, this work provides a comprehensive study of required steps towards the monolithic integration of piezoelectric MEMS on the emerging AlN-on-SiO2 integrated photonics platform. It contributes relevant correlations between material characteristics, etching