Stress-free ultrahard NEMS cantilevers fabricated from atomically-smooth nickel-molybdenum nanocomposite thin films.

摘要

The objective of this study was to design a microstructure that was metallic, atomically smooth, strong, stable at elevated temperatures and stress free. We picked nickel-molybdenum (Ni-Mo) and performed a systematic study of microstructure and properties of 1 mum thick films as a function of composition by using room temperature co-sputtering. This investigation resulted in an optimum film composition of Ni-44atomic%Mo, with a unique nanocomposite microstructure of Mo-rich nanocrystallites randomly and densely dispersed in a Ni-rich amorphous matrix. This stress-free film showed a very high nanoindentation hardness of 11 GPa and an order of magnitude reduction in surface roughness (0.8 nm) compared to those of pure Ni (4.5 GPa and 10 nm) and pure Mo (7.5 GPa and 8 nm), while maintaining the resistivity in metallic range. The alloy is also stable at high temperatures of the order of 400°C. As a further step, we conducted a detailed study on the prediction of the composition range of amorphous formation according to the different existing models. Among these models, we found that the prediction range of Miedema's thermodynamic model of Ni-38at.%Mo to Ni-68at.%Mo was in a good agreement with the amorphous range obtained from our experimental data. As a proof-of-principle, we synthesized freestanding single-anchored cantilevers from Ni-44at.%Mo film that were 50 nm thick, 400 nm wide and ranged in length from 1-6 mum. The synthesized cantilevers have the potential to be used as sensors in atomic force microscopy, petroleum industry, microfluidics and biomedical chips, and generally wherever operations at elevated temperature and good conductivity are required.