P. Avila, B. Baloukas, O. Zabeida, J. Sapieha, L. Martinu, Polytechnique Montréal, Montreal, Quebec, Canada
The control of stresses is one of the key aspects in the development of high-performance optical thin films. Excessive stress development during film deposition can impact its optical and mechanical properties and lead to complete failure. Therefore, thoroughly understanding the mechanisms leading to stress generation and its dependency on processing parameters is fundamental. Recently, comprehensive models for stress generation have been developed for crystalline materials. However, little attention has been devoted to amorphous materials. Seeking to understand the particularities of stress generation in this class of materials, which is highly relevant for optical applications, we deposited model 500-nm-thick SiO₂ and amorphous Si films by RF magnetron sputtering under different conditions of working pressure and deposition rates. The stress development was assessed via in-situ substrate curvature measurements. The nature and the magnitude of the intrinsic stress were found to depend drastically on the gas pressure during the deposition. For instance, a decrease of 0.5 mTorr was sufficient to invert the nature of stress from tensile to compressive in the case of SiO₂ or increase the compressive stress more than two-fold, from -200 MPa to -500 MPa, for an a-Si film. The role of the deposition rate on stress development was also found to be pressure-dependent, promoting either tensile or compressive stresses with increasing deposition rate, depending on the average number of collisions of the arriving species. For hygroscopic materials such as SiO₂, extrinsic compressive stress originated from moisture absorption once the samples were exposed to air and its magnitude was also shown to depend on the processing conditions. The results show that the intrinsic stresses in amorphous films combine a tensile component arising from pore formation and a compressive component generated from energetic bombardment during film growth. Each of these components can thus be tuned by adjusting the deposition parameters, as demonstrated by a mapping of stress development for each material.