Abstract

In the research on the thermomechanical stability of ZrO₂ evaporated films with and without ion-assisted deposition (IAD), we performed interferometry in a vacuum oven to study the stress in the films as a function of temperature. In thermal cycling, all the plots of stress versus temperature for IAD and non-IAD films exhibit hysteresis. To understand the hysteresis, we studied micro-structure and water effects. The results show that the likely mechanisms are water desorption, recrystallization, and phase transformation, and we believe that a combination of all three occurred. In our presentation, we will emphasize the effect of water adsorption and desorption. In the research, we defined thermomechanical instability as the width of the hysteresis and environmental mechanical instability as the stress change from vacuum to air. These parameters have proven to be good at describing thermomechanical and environmental mechanical stability, respectively. Our results also show that high bombardment (increasing oxygen ion bombardment tends to give more tensile stress) and high deposition temperature (increasing deposition temperature tends to give less tensile stress) result in more stable films, both thermomechanically and optically. It is well known that thermal stress is due to a thermal expansion coefficient mismatch between substrate and film. However, if thermal-expansion coefficients are to be derived from thermal stress, then great care must be taken to eliminate the water effect; otherwise, the results will be totally wrong. Furthermore, it is almost impossible to obtain a correct thermal-expansion coefficient from postdeposition thermal stress measurements because at the temperature and after the time required to eliminate the water, the film under measurement will already have experienced a crystal structure transition. Therefore, in situ thermal stress studies are needed.

© 1990 Optical Society of America