Electrical and microstructural characterization of molybdenum tungsten electrodes using a combinatorial thin film sputtering technique

A combinatorial rf magnetron sputter deposition technique was employed to investigate the electrical characteristics and microstructural properties of molybdenum tungsten (MoW) high temperature electrodes as a function of the binary composition. In addition to the composition, the effect of substrate bias and temperature was investigated. The electrical resistivity of MoW samples deposited at room temperature with zero bias followed the typical Nordheim’s rule as a function of composition. The resistivity increases with tungsten fraction and is a maximum around 0.5 atomic fraction of tungsten. A metastable β-W phase was identified and the relative amount of the β-W phase scales with the resistivity. Samples deposited at higher temperature (250°C) also followed Nordheim’s rule as a function of composition, however, it did not contain the metastable β-W phase and consequently had a lower resistivity. The resistivity of samples deposited with substrate bias is uniformly lower and obeyed the rule of mixtures as a function of composition. The molybdenum-rich compositions had a lower resistivity, contrary to expectations based on bulk resistivity values, and is attributed to high electron-dislocation scattering cross sections in tungsten versus molybdenum. The metastable β-W phase was not observed in the biased films even when deposited at room temperature. High resolution scanning electron microscopy revealed a more dense structure for the biased films, which is correlated to the significantly lower film resistivity.