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This thesis covers interdisciplinary research, using vibrations in the ultrasonic frequency range to analyse the behaviour of suspended structures made from Ge, 3CSiC, and SiN. These structures can be used in micro-electromechanical Systems (MEMS) for applications such as pressure sensors. A laser interferometer was used to measure the vibration resonance patterns, along with a self-developed system which can do two-dimensional (2D) scanning of the samples at reduced pressures, and over a range of temperatures, in order to probe the behaviour of the materials. This work presents the feasibility of this non-contact and non-destructive method for material characterisation of suspended structures through the information obtained from the deflection and the structures resonant frequencies. Vibration testing is typically done in the linear regime, and linear vibrations of the structures are initially investigated with the 2D scanning system. Larger amplitude vibrations lead to non-linear behaviour. The non-linear dynamics experimental results show the amplitude-frequency response curve has a position dependence for where the laser probe is on the sample, and is related to the strength of excitation force. A critical displacement of the oscillation amplitude is identified for the samples in these experimental measurements. The experimental results from the non-linear dynamic measurements are used to consider the applicability of several theoretical studies from the literature to this type of sample. The laser probe was shown to have a small thermal effect on the measurements. The linear and non-linear 2D scan studies used low laser power to minimise heating. A calibration measurement was then done on the thermal effects which were induced by the laser probe for different detection laser powers. The dynamic thermal field generated by the moving laser during a 2D mapping process causes different thermal gradients across the sample when the laser is placed at different positions on the suspended thin film. The dynamic thermal field could be used as a novel tool for material characterisation following further investigation. Finally, the onset of damage followed by self-healing behaviour was observed on a Ge thin film with sub-micron and nano meter cracks. The damage was analysed by comparing the percentage change of the residual stress between theoretical predictions from considering the thermal expansion of the thin film and substrate materials, with experimental measurements done under a varying temperature. Additionally, the quality-factors of the resonance were also monitored during the measurement and showed the onset of damage. This work when taken as a whole offers a new novel analysis technique for suspended structures used in MEMS applications, with the potential to characterise the material properties, and to inspect them for quality assessment or damage detection. The method could be made to be fully non-contact through using activation using e.g. air-coupled transducers, leading to a potential on-line analysis technique. |
