Control of physical and microstructural properties in molybdenum by direct current magnetron sputtering deposition producing bilayer thin film

Highlights: • Reduce molybdenum (Mo) surface roughness, R_a by depositing top Mo layer of smaller R_a. • Similar sputtering power for top and bottom layer yields good crystals in Mo bilayer. • Strong association between tensile stress and compressive strain in Mo mono/bilayer. - Abstract: Molybdenum (Mo) thin films are widely used in microelectromechanical systems (MEMS) applications. Mo bilayer deposition by direct current (DC) magnetron sputtering has been proposed in order to attain the desired smooth surface, small in-plane tensile stress and high degree crystallisation of Mo thin film for the fabrication of MEMS actuators and electrodes. The influences of sputtering time (10 min–40 min) and sputtering DC power (100 W–250 W) on the physical and microstructural properties of single layer Mo thin film have been evaluated. The optimised sputtering conditions for bottom and top layer of Mo bilayer have been determined and the individual influence of each layer on the resulting Mo bilayer has been discussed. Our studies reveal that the deposition of top Mo layer with small surface roughness but high in-plane tensile stress has reduced the high surface roughness of bottom Mo layer and simultaneously retained the small in-plane tensile stress. Reducing the sputtering time and/or using similar sputtering power for bottom and top layers improved the crystallinity of Mo bilayer along the preferred 〈110〉 direction. Mo bilayer thin films of total thickness 19 nm, surface roughness <2.5 nm and in-plane tensile stress <500 MPa have been attained.