Effects of heating rate on stress evolution in alkoxide-derived silica gel-coating films

Silica gel films were deposited by spin coating on single-crystal Si wafers using an acid-catalyzed Si(OC2H5)(4) solution as a coating solution. The gel films were heated at various rates, where in situ stress measurement was conducted. In-plane tensile stress developed during the course of heating, and was found to be larger at lower heating rates at temperatures up to 350 degrees C. The larger stress was thought to cause cracking at lower temperatures, which was previously observed at lower heating rates in in situ observation. The larger stress at lower heating rates was basically ascribed to the larger degrees of densification, which was revealed in the larger extent of reduction in thickness as well as in Si-OH/Si-O-Si and O-H/Si-O-Si infrared absorption band area ratios at lower heating rates. The difference in stress at different heating rates appeared to originate mainly in the difference observed at low temperatures below 130 degrees C, suggesting that the heating rate particularly affects the densification that occurs via solvent evaporation. The increment in stress was reduced over 400 degrees C when the heating rate was low, which was thought to result from the higher degree of densification already achieved below 400 degrees C as well as the structural relaxation occurring at such high temperatures.