Experimental Characterization and Synthesis of Nanotwinned Ni-Mo-W Alloys
Johns Hopkins University
Microelectromechanical systems (MEMS) have transformed consumer and industrial products through the integration of mechanical and electrical components within a single package. MEMS are ubiquitous in society, found predominantly in consumer electronics and automotive industries, providing interconnectivity across a wide variety of devices and everyday objects. To date, the materials selection for the structural element of many MEMS devices has been limited to a relatively small subset of materials, with silicon being the dominant choice. Employing MEMS sensors and switches in extreme environments will need advanced materials with a synergistic balance of properties, e.g. high strength, density, electrical conductivity, dimensional stability, and microscale manufacturability, but MEMS materials with this suite of properties are not readily available. Metallic systems are especially attractive for these applications due to their high density, strength and electrical conductivity. For this reason, metal MEMS materials are the motivation and focus for this dissertation. The synthesis of nanotwinned nickel-molybdenum-tungsten (Ni-Mo-W) alloys resulted in thin films with a very favorable suite of properties. Combinatorial techniques were employed to deposit a compositional spread of Ni85MoxW15-x, alloys and to investigate their physical and mechanical properties as a function of alloy chemistry. The addition of Mo and W was shown to significantly decrease the coefficient of thermal expansion (CTE) and provide a route for tailoring the CTE and its temperature dependence with compositional control. The measured CTE values for Ni-Mo-W matched that of commercial glass substrates currently employed in MEMS devices, broadening the spectrum of materials with the requisite dimensional stability for use in layered structures. Microscale mechanical testing was used to measure the in-plane tensile properties; a linear-elastic response with fracture strengths ranging from 2-3 GPa was uncovered. The ultrahigh tensile strengths are attributed to the presence of highly-aligned nanotwins and their effectiveness as obstacles to dislocation motion. In situ micropillar experiments demonstrated compressive strengths of 3-4 GPa and extremely localized plasticity, both of which are strongly orientation dependent. The nanoscale twins underpinning this mechanical behavior do not impede motion of electrons, and nanotwinned Ni-Mo-W thin films were found to posses the electrical conductivity of bulk Ni alloys. Taken as a whole, this study highlights the balance of physical, thermal and mechanical properties for Ni-Mo-W, driven by nanoscale twin formation. Deposition of Ni-Mo-W films displayed a wide processing window for the formation of the requisite nanotwinned microstructure and attendant properties (CTE, strength, ductility and electrical resistivity). Microcantilever beams were designed and fabricated using traditional integrated circuit processing to translate thin film properties into prototype MEMS device structures. Laser interferometry was used to certify the dimensional stability of the cantilever beams as-fabricated and after thermal exposure at elevated temperatures associated with wafer bonding. Micromachined cantilever beams showed excellent dimensional stability with beam deflection profiles on the order of tens of nanometers, elucidating a path beyond outstanding material properties to actual device structures for next generation metal MEMS devices.
nanotwinned metals, thin films, MEMS