Instability of residual stress of crystalline and glass oxide thin films prepared by solgel method

Abstract

Previously, we demonstrated that solgel-derived glass and ceramic thin films show a decrease in in-plane residual tensile stress during storing in humid atmosphere at room temperature. To clarify the mechanism, we prepared SiO2 glass films on Si(100) wafers via spin-coating followed by firing and obtained in-plane stress by measuring the substrate curvature. The SiO2 film fired at 600 °C showed a decrease in tensile stress during storing but showed recovery when heated afterward at 300 °C. We detected an increase in the infrared O-H absorption peak area as well as in the film thickness during storing, the latter of which was detected by in situ ellipsometry for the SiO2 film fired at 600 °C. On the other hand, when fired at 880 and 885 °C, the SiO2 films gained “compressive” residual stress, which slightly increased with time during storing. Based on these observations the decrease in tensile stress during storing has been confirmed to be caused basically by the film expansion induced by water adsorption. The SiO2 film fired at 600 °C showed recovery in stress on heating at 300 °C, which was incomplete in contrast to the CeO2 film that showed complete recovery as reported previously. Such recovery in stress was accompanied by decrease and turning back of the O-H absorption peak are to original values, the latter of which was incomplete in the SiO2 film but complete in the CeO2 film. These results on stress recovery and IR absorption suggest that besides water “adsorption,” water “absorption” cannot be ruled out for the SiO2 film as the partial cause of the decrease in “tensile” stress during storing.