Development of polycrystalline and amorphous silicon carbide thin films for membrane-based MEMS devices

This dissertation presents the systematic development of polycrystalline 3C-SiC (poly-SiC) and amorphous hydrogenated SiC (a-SiC:H) thin films for membrane-based MEMS applications. Poly-SiC membranes were fabricated from low stress, high conductivity, (111)-oriented polycrystalline 3C-SiC thin films deposited from dichlorosilane, acetylene and ammonia at 900°C by LPCVD. Using nano-indentation and bulge test techniques, the Young's modulus and Poisson's ratio for such films were found to be 350 GPa and 0.122, respectively. Burst testing reveals an average burst strength of 1000 MPa and an ultimate strain of 0.29%. Membrane-based capacitive pressure sensors were fabricated from these films using a vacuum-based wafer bonding technique. The device exhibited repeatable sensing performance up to 500°C, limited by packaging. FEA simulation suggests that thermal expansion-induced stress in the poly-SiC diaphragm is the major contributor to a shift in sensor performance.; Amorphous hydrogenated SiC membranes were fabricated from electrically insulating films deposited by PECVD at 350°C using trimethylsilane as the precursor. The as-deposited films exhibited compressive residual stresses that could be converted to tensile stresses by thermal annealing at 450°C. The average values of Young's modulus and Poisson's ratio of annealed films were found to be around 80 GPa and 0.23, respectively. These films have a burst strength between 700 to 800 MPa and an ultimate strain between 0.79% and 0.90%, depending on the annealing time. Compositional analyses indicate that annealing at 450°C not only causes hydrogen effusion but also the formation of additional Si-C chemical bonds. The as-deposited films have superb surface morphology making them well suited for wafer bonding. A a-SiC:H/Si direct bonding process with a maximum temperature of 450°C was developed. Fabrication of membrane-based MEMS structures, such as nano-gap channels and micron-deep, vacuum-sealed reservoirs capped with a-SiC:H membranes were demonstrated. The lower limit of the bonding strength of the bonding interface was estimated to be mJ/m2.; The findings of this research suggest that poly-SiC thin films deposited by LPCVD using dichlorosilane and acetylene and a-SiC:H thin films deposited by PECVD using trimethylsilane are well-suited for membrane-based MEMS applications.