In-plane stress development in sol–gel-derived titania and silica thin films on Si(100) substrates

Abstract

In order to study (i) how the in-plane stress develops in alkoxide-derived oxide gel films on heating and (ii) how the residual stress changes with firing temperature, we prepared titania and silica gel films on Si(100) wafers from alkoxide solutions. The intrinsic stress was previously measured in situ during heating up to 500 °C for the gel films, where the stress was calculated from the radius of curvature of the substrate bending. Because the commercialized apparatus that measures the substrate bending has an upper limitation of temperature at 500 °C, we devised a method for estimating the intrinsic stress over 500 °C in the present work. The gel films were heated from 100 °C up to 900 or 1000 °C successively and cumulatively. Then we estimated the intrinsic stress over 500 °C by monitoring the changes in stress in situ during heating and cooling for the cumulatively fired films. Combining the estimated data over 500 °C with those measured below 500 °C, we found that the intrinsic stress increases and then decreases around to zero with temperature for both titania and silica gel films, which we recognized as a general trend in alkoxide-derived oxide thin films in conjunction with our previous work on yttria stabilized zirconia (YSZ) films. Regarding the residual stress, the titania and silica films exhibited different behaviors. The titania film showed an increase followed by a slight decrease, and finally a significant increase in residual tensile stress with increasing firing temperature. The silica films also exhibited an increase and a decrease in residual tensile stress with increasing firing temperature, but the stress continued to decrease and turned into compressive one at ca 800 °C. The last decrease in residual tensile stress that turned into compressive one resulted from the development of compressive thermal stress during cooling, which is due to the smaller thermal expansion coefficient of silica than silicon. It was thus clarified that the difference in thermal expansion coefficient between the film and the substrate becomes prominent in residual stress when the films are fired at high temperatures like 800 °C. Thus we could successfully obtain a general view on how the residual stress changes with firing temperature as well as on how the in-plane stress develops during heating thanks to the information on the stress development in the sol–gel-derived titania and silica films that was added to the information on the YSZ films.

Highlights

• In-plane stress development was studied on alkoxide-derived titania and silica thin films on Si(100) wafers by measuring the substrate bending using a commercialized apparatus.

• We devised a method to estimate the changes in intrinsic stress during heating over 500 °C, which is the upper limit of temperature available for the apparatus.

• Employing the estimated values we found that in-plane tensile stress develops during heating, followed by a decrease down around to zero with increasing temperature for alkoxide-derived oxide thin films.

• The residual stress was demonstrated to be tensile at lower firing temperatures, followed by a decrease with increasing firing temperature, and finally to increase or decrease with turning into compressive one, depending on the difference in thermal expansion coefficient between the film and the substrate.