Amorphous molybdenum-based thin films for surface micromachining: Dissertation

Microelectromechanical systems (MEMS) are often based on silicon technology. This thesis studies two molybdenum-based thin films, amorphous Mo-N and Mo-Si-N deposited by reactive sputtering, for an alternative material choice for surface micromachining. Bulk amorphous metals stand out from other engineering materials because of their high elasticity, which would be an interesting feature for the structural layer of MEMS devices. Since elemental metal films are practically always polycrystalline, molybdenum was amorphised first by nitrogen. The resulting Mo-N films were characterised for their deposition and etch rates, composition, resistivity and residual stress. Because the amorphisation was incomplete, silicon was added.The Mo-Si-N films were characterised for their deposition and etch rates, composition, resistivity, residual stress, microstructure, surface roughness, elastic modulus, hardness, elastic recovery, coefficient of thermal expansion, temperature coefficient of resistance and complex refractive index. It was found the resistivity of these amorphous Mo-Si-N films is 1...2 mOcm, and their residual stress can be tuned to low tensile values (around 100 MPa) by the sputtering pressure.The thermal stability of Mo-N and Mo-Si-N films was studied in particular. The first signs of oxidation were observed at 350°C, and structural changes even below. The unsealing surface oxidation can be prevented by a thin protective silicon cap on top of the films. The residual stress of the Mo-Si-N films sputtered from separate Mo and Si targets depends on the post-deposition annealing temperature, while the Mo-Si-N films sputtered from a Mo5Si3 compound target are more resistant to annealing-induced structural changes.By the end of this study, surface micromachined MEMS devices with Mo-Si-N films as their structural layer were demonstrated. The capacitive RF MEMS devices operated at frequencies up to 110 GHz, and were fully functional after the actuation of 50 million cycles between the up- and down-states of their MEMS bridges. Mo-Si-N films were also applied to thin film absorbers designed for the visible and near-infrared wavelengths (350...2000 nm). The absorption was measured to be higher than 93 % over the whole spectrum of interest.In conclusion, amorphous Mo-N and Mo-Si-N films are suitable for several kinds of MEMS devices on condition that they are not exposed to increased temperatures in an oxidising atmosphere without a protective silicon cap. Their integration with conventional MEMS processes is convenient, as they can be deposited by sputtering and patterned with common dry and wet etch chemistries. The demonstrated MEMS fabrication process was CMOS compatible. The low process temperature enables the use of a polymeric sacrificial layer and provides an opportunity for the monolithic integration of MEMS and CMOS.