Stress-free titanium-based thin films for inner ear microphones : The last missing part of a technology for totally implantable hearing aid implants

University essay from Linköpings universitet/Tunnfilmsfysik

Abstract: An implantable hearing aid device is being developed by a project group which is part of an EU initiative. This device contains a diaphragm consisting of a submicron thick freestanding titanium film, which should be free of internal stresses. Stress is the force exerted per unit cross-sectional area of the film and it can impair the functionality and performance of the device. The stress that evolves in a thin film during deposition at a substrate is compressive or/and tensile and affects the bending that occurs of the substrate due to the lateral force applied to the substrate by the stressed film. The goal of this diploma work was to contribute to the understanding of in situ stress evolution in a micron thick titanium film and thereby by tuning different physical parameters to obtain minimal residual stress in the films after growth. Titanium films were deposited on silicon sistrates using DC magnetron sputtering. The stress in the material varied, by tuning different physical parameters such as working pressure, power, distance between magnetron and the sample and substrate bias. For this thesis, firstly two different series were done; one where the changing parameter was the distance between the sample and the magnetron and one where it was the working pressure. Later a last series were done to see what effect the bias has on the stress. A multi-beam optical sensor system (MOS) was used to measure the stress in real-time during deposition. X-ray diffraction (XRD) was later used to make post-deposition stress measurements to verify the stress obtained from the MOS. However, the MOS shows the stress evolution in real time and XRD shows a ’final’ average value that can be compared with the stress obtained from MOS-data when the deposition is finished. The results showed that the stress goes from compressive to tensile as the working pressure and the distance between the magnetron and the sample increases. There are other factors, such as the temperature/heating in the main chamber, base pressure of the main chamber, cleaning of the sample and also where the argon gas is let in to the process chamber (in this project called the main chamber (MC)), that influences the results. This will in turn influence the repeatability of the data/measurements, since these effects can affect the process of nucleation and coalescence. The stress evolution can change if a bias is applied during the initial stage of the deposition process when the film has still not grown thick. This is could be due to the bias not having much of an impact on the stress evolution when the film is thicker and thereby more porous.

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