Study on the crack mechanism of SiO2 anti-reflective layer prepared by sol-gel method

SiO2 anti-reflective (AR) layers have been prepared via sol-gel dip-coating technique and the mechanism study of crack formation has been then carried out. It is found that thermal expansion difference between substrate and layer, in combination with capillary tension caused by water vapor, was responsible for the production of considerable stress inside the sol-gel coating. Initial crack would be generated once this combined stress exceeded the self-relaxation and self-reconstruction capability of this sol-gel AR layer. It is also found that the morphology of most cracks exhibited to be a ridged edge accompanied with a collapse dent. This deformation was also assigned to the combination effect of capillary tension and expansion strain. In addition, a surface barrier was formed on the section of crack, owing to the incorporation between numerous hydroxyl groups and water vapor molecules. Such barrier would result in aggravation of initial cracks, in order to release the internal stress further.