The effect of ion bombardment on the microstructure and properties of molybdenum films

Abstract

Typically, evaporated films of refractory metals are fine grained and porous with lower than bulk values of reflectance. In the present work, ion bombardment during deposition was used in an attempt to obtain nore dense, bulk-like films of molybdenum. Films were produced by both electron beam evaporation with concurrent bombardment using argon ions from an ion gun and by steered-arc evaporation using a substrate bias voltage to enhance the ion bombardment. Transmission electron microscopy examination showed that the molybdenum films produces by electron beam evaporation became less porous and had larger grain size with increasing ion bombardment. However, the maximum grain size, about 25 nm, was significantly smaller than the grain size, about 200 nm, found in films deposited by arc evaporation with substrate bias. In addition, the films produced by arc evaporation had an equiaxial grain structure which is normally found only in films deposited at high substrate temperature. Molybdenum films with less porosity and larger grains were found to have higher values of reflectance and lower values of resistivity as might be expected on the basis of an electron mean free path argument. Film stress was also found to be a function of ion bombardment and could be correlated with the microstructure. Hence, arc evaporation with a substrate bias appears to offer advantages over ion-assisted electron beam evaporation for producing films with a dense, large grain, equiaxial microstructure and near-bulk values of reflectance.