Tailoring the Synthesis Environment for Sputter-deposited AlN-based Thin Films - Applications from Piezoelectric to Transparent Hard Coatings

Thin films and coatings are essential building blocks of many technologies we use every day. Physical vapor deposition (PVD) processes are commonly employed for deposition of functional thin films used in various fields such as electronics, data storage, medicine, decorative and protective coatings. Magnetron sputtering, a plasma-based PVD technique, is among the industrially most relevant techniques due to its scalability, high deposition rates and low cost. Furthermore, a wide range of process parameters are accessible, offering the possibility to adjust the functional material properties by varying the synthesis conditions. This thesis presents non-conventional process designs in a confocal magnetron sputtering setup, with the aim of depositing aluminum-based nitrides and oxynitrides with tailored functionality. An advanced magnetic field design and a modified gas inlet layout enable the control of the energetic environment at the substrate, as well as the chemical environment at the substrate and target, respectively. An additional magnetic field generated by a coil enables to control the low energy ion flux towards the substrate while keeping the ion energy and process parameters largely unaffected. The ion flux clearly influences the microstructure of the thin films. With an increasing ion flux, the morphology changes from an open to a closed grain boundary situation. This was consistent with a change of the residual stress from tensile to highly compressive (up to -4 GPa), while the crystalline orientation remained unaffected. The presented gas inlet design on the other hand is an inexpensive and robust modification of a reactive magnetron sputtering setup for deposition of oxynitrides. Using such a setup facilitates the control of the chemical environment and consequently the film stoichiometry during oxynitride deposition. The design principle is universally applicable, from lab-scale to industrial deposition systems, for reactive sputtering using two gases with vastly different reactivities. Ternary and quarternary materials, namely aluminum (silicon) oxynitrides, have been studied within this thesis. The addition of oxygen has a strong influence on the microstructure of the thin films. A model that illustrates the evolution of the microstructure with changing oxygen content is proposed. Going from zero atomic percentage to fully oxidized Al2O3, the morphology changes from a wurtzite solid solution into a fiber textured nanocomposite, then to a nanocomposite without uniaxial texture and finally to an amorphous solid solution. The evolution of the morphology is reflected in the hardness, elasticity, refractive index and residual stress of the thin films. The coatings were found to be suitable as protective coatings for displays, e.g. of mobile phones, or antireflective coatings. Additionally, the piezoelectric response of AlN coatings was investigated. To study the piezoelectric properties on a microscopic scale, piezoresponse force microscopy (PFM) was used. Because of the expected small response (displacements in the pico-meter range), resonance enhanced methods were applied. The microscopic analysis revealed that piezoelectric AlN thin films were deposited with a homogeneous polarization onto both electrode materials, aluminum and platinum, used within the scope of this thesis. Using the latter, a higher piezoelectric response was achieved, on the order of 3 pm/V, which is comparable to values found in literature. For an application in an AFM-setup, commercially available cantilevers were coated with a piezoelectric AlN thin film to provide an alternative way of actuation. The direct actuation of the cantilever, using the piezoelectric coating and a high bandwidth PI controller, was successfully integrated to measure in an intermittent contact mode. This setup allows for a faster actuation and could be used for high speed imaging in the future. Furthermore, the cantilever resonances were excited yielding a clean amplitude and phase signal.