Apparent relaxation of in-plane residual stress of sol–gel-derived crystalline and glass oxide thin films

Abstract

Crystalline or glass oxide thin films with thickness of ca. 50–210 nm and porosity of ca. 5–17% were prepared on Si(100) wafers by the sol–gel method. The films were left standing at room temperature in dry and humid atmosphere as well as in water for varying periods of time. We measured the in-plane residual stress of the films as a function of time during storing at room temperature by measuring the radius of curvature of the substrate surface. The anatase, silica, and ceria films had tensile in-plane residual stress when fired at 1000 °C, 600 °C, and 800 °C, respectively. Such tensile stress was found to decrease with time, finally becoming constant, at room temperature, where the rate and extent of stress reduction were larger in the order, “in water” > “in humid atmosphere” > “in dry atmosphere.” On the other hand, the silica films had compressive in-plane residual stress when fired at 1000 °C. Such compressive stress was stable, not decreasing with time, at room temperature. The ceria film, after showing a decrease in tensile stress in humid atmosphere, exhibited an increase in stress when heated at 300 °C. When stored again at room temperature in humid atmosphere, the film showed a decrease in stress with time over again. Then we excluded the structural relaxation as the origin of the tensile stress reduction at room temperature, considering the stability of the compressive stress and the reversibility of the tensile stress. The ceria films showed a decrease in tensile stress even when stored in acetone and n-heptane vapors, suggesting that the film expansion resulting from the vapor-molecule adsorption may be the origin of the decrease in tensile stress. Yttria-stabilized zirconia films fired at 1000 °C also had tensile stress. The stress, however, was constant with time at room temperature, the reason for which is unknown, and further studies should be made.