"Low temperature deposition and characterisation of high quality nanochrystalline diamond films for the fabrication of highly sensitive pressure sensing membranes"
Both hydrogen-terminated and boron doped diamond conductive membranes show piezoresistive behavior and can be used as pressure sensors. In order to make sensitive pressure sensing membranes, it is necessary to identify all parameters that have an influence on the final pressure sensitivity of the membrane. To this end, nanocrystalline diamond membranes were fabricated on glass and their pressure sensing properties were related to their size, dopant concentration and internal stress levels. In this work a guideline for the optimisation of B-doped nanocrystalline membranes, and other materials based on the same design, is presented. The sensitivity of pressure sensing membranes is found to increase with decreasing doping level and increasing membrane size. In addition, there is an influence of the underlying substrate, i.e. B:NCD membranes contain a certain stress level after deposition because of the mismatch in thermal expansion coefficients between the diamond layer and the substrate. In addition, diamond deposition at lower temperatures leads to lower stress levels. Therefore, the second part of this thesis focused on low temperature growth of nanocrystalline diamond layers with novel linear antenna technology. High quality diamond layers were grown at temperatures between 300°C and 400°C, but show a particular morphology, i.e. co-existence of plates and octahedral diamond grains. In contrast to previous reports claiming the need of high temperatures (T > 1000 °C), in this work low temperatures (320 °C ≤ T ≤410 °C) were sufficient to deposit diamond plate structures. A model is proposed that accounts for the initial development of these plates full of stacking faults and the growth of this morphology at low temperatures. The model is based on the presence of foreign atoms, which can enhance the growth of diamond plates in various ways; the impurities induce the formation of stacking faults, they block a certain crystal facet from further growth, or they ensure enhanced etching of a crystal facet.