Metallic thin films for NEMS/MEMS: from fundamental behaviour to microstructural design and fabrication

Abstract

The main focus of this thesis is the study of thin metal films in microelectromechanical/nanoelectromechanical systems (MEMS/NEMS), ranging from application to fundamental behaviour. The use of metallic structural components is desirable since they are electrically conductive, optically reflective and ductile. However, polycrystalline metallic thin films typically exhibit low strength and hardness, high surface roughness and significant incremental stress, making them unusable for NEMS/MEMS. By co-sputtering Ni-Mo thin films we are able to tailor the microstructure and surface morphology such that these limitations are overcome. As such, uncurled NEMS cantilevers possessing enhanced hardness, metallic conductivity and sub-nanometer roughness are fabricated with resonant frequencies in the MHz regime, and quality factors ranging from 200-900. Following this, the use and design of all-metal atomic force microscope (AFM) probes is investigated. This is motivated by the growing number of AFM applications which make use of metal-coated probes, and as a result of the metallization suffers from stress-induced cantilever bending, thermal expansion mismatch, increased tip radius and limited device lifetime due to coating wear. To this end, monostructural all-metal AFM probes having 1 µm thickness, lengths of 100-400 µm, and tip radii ranging from 10 to 40 nm are fabricated. This is accomplished through microstructural design of Cu-Hf thin films, where an optimal combination of resistivity (96 µΩcm), hardness (5.2 GPa), ductility and incremental stress. Lastly, in many MEMS/NEMS applications the unique properties of nonmetallic components are required, but a metallization layer is still needed. As metallization layers become increasingly thinner, film stability can become problematic, due to the phenomenon of solid-state dewetting. The fundamental mechanisms of solid-state dewetting are investigated in Ni thin films on The main focus of this thesis is the study of thin metal films in microelectromechanical/nanoelectromechanical systems (MEMS/NEMS), ranging from application to fundamental behaviour. The use of metallic structural components is desirable since they are electrically conductive, optically reflective and ductile. However, polycrystalline metallic thin films typically exhibit low strength and hardness, high surface roughness and significant incremental stress, making them unusable for NEMS/MEMS. By co-sputtering Ni-Mo thin films we are able to tailor the microstructure and surface morphology such that these limitations are overcome. As such, uncurled NEMS cantilevers possessing enhanced hardness, metallic conductivity and sub-nanometer roughness are fabricated with resonant frequencies in the MHz regime, and quality factors ranging from 200-900. Following this, the use and design of all-metal atomic force microscope (AFM) probes is investigated. This is motivated by the growing number of AFM applications which make use of metal-coated probes, and as a result of the metallization suffers from stress-induced cantilever bending, thermal expansion mismatch, increased tip radius and limited device lifetime due to coating wear. To this end, monostructural all-metal AFM probes having 1 µm thickness, lengths of 100-400 µm, and tip radii ranging from 10 to 40 nm are fabricated. This is accomplished through microstructural design of Cu-Hf thin films, where an optimal combination of resistivity (96 µΩcm), hardness (5.2 GPa), ductility and incremental stress. Lastly, in many MEMS/NEMS applications the unique properties of nonmetallic components are required, but a metallization layer is still needed. As metallization layers become increasingly thinner, film stability can become problematic, due to the phenomenon of solid-state dewetting. The fundamental mechanisms of solid-state dewetting are investigated in Ni thin films on SiO2. This phenomenon is monitored in situ using time resolved differential reflectometry (TRDR) and ex situ using AFM. It is found that Ni dewetting on SiO2 occurs through the sequential processes of grain growth, grain boundary grooving, hole growth and particle coarsening. Kinetic analysis of the TRDR data revealed two rate-limiting processes, with activation energies of 0.31±0.04 and 0.59±0.06 eV. It is hypothesized that these kinetic pathways correspond to Ni grain growth and surface mass self-diffusion on the Ni(111) planes, respectively.