Effect of the Amount of Water for Hydrolysis on Cracking and Stress Evolution in Alkoxide-Derived Sol-Gel Silica Coating Films

抄録

Silica gel films were deposited on single-crystal Si substrates using starting solutions of mole ratios, Si(OC₂H₅)₄ : H₂O : HNO₃ C₂H₅OH=1 : x : 0.01 : 4, where x=2, 4 and 10. Cracking of the gel films was observed in situ on heating using a near infrared image furnace equipped with an optical microscope. The gel films were found to be cracked at lower temperatures when the H₂O/TEOS ratios in solutions were larger. In-plane stress in gel films was also measured in situ on heating with a thin film stress measurement apparatus equipped with an electric furnace. Tensile stress generated on heating, and higher stress evolved at larger H₂O/TEOS ratios, which was thought to allow cracking at larger H₂O/TEOS ratios. However, higher rates of increase in stress at larger H₂O/TEOS ratios were seen only up to 100°C, while the slope of the stress-temperature relation was similar over that temperature irrespective of the H₂O/TEOS ratios. It was thought to be the higher capillary pressure due to the high surface tension of H₂O that gave rise to the higher rates of increase in stress at larger H₂O/TEOS ratios observed below 100°C. In other words, higher surface tension of H₂O is the most possible cause of the lower cracking temperatures at larger H₂O/TEOS ratios. The shape of the cracks tended to shift from scale-like to worm-like, and then to linear shape as the cracking onset temperature increases. In other words, the cracks propagate curled at lower temperature and straight at higher temperatures.