Complex interference multilayer systems typically implemented in high-performance optics consists of several layers of low and high refractive index materials. Low mechanical stress of the coatings is desired to avoid cracking and delamination of the film or a deformation of the substrate. It is known that the ion energies in plasma-assisted deposition can be employed to control material properties and thereby mechanical stress. In this study, we evaluate the influence of substrate biasing on mechanical stress and optical properties of alumina (Al2O3) coatings deposited by plasma enhanced atomic layer deposition (PEALD). Substrate biasing up to -300 V was applied during O2 plasma exposure in the second step of a two-step PEALD process. To distinguish the physical effect of ion bombardment from the physico-chemical effect, a substrate bias of -100 V was applied separately and only during Ar plasma exposure that constituted the third step of a three-step PEALD process. Al2O3 films were characterized using spectroscopic ellipsometry, spectrophotometry, xray photoelectron spectroscopy (XPS), x-ray diffractometry (XRD), x-ray reflectometry (XRR), Fourier transform infrared spectroscopy (FT-IR), wafer-curvature measurement and atomic force microscopy (AFM).