Studies on the spin coating process of silica films

Silica xerogel films with good uniformity and various thicknesses were successfully prepared by a two-step acid-base catalyst procedure and spin coating. After the addition of ammonia catalyst, the sol changes gradually from a Newtonian fluid to a shear-thinning one due to the progress of polymerization reactions. An empirical equation using the relative ratio of reaction time to gel time can give successful prediction of sol viscosity. Our systematic study of the coating process indicates the existence of a process window in terms of t/tgel and spin speed can be defined for obtaining uniform and defect-free films. It is difficult to obtain films with good quality when the viscosity of the sol is either too high or too low. Another parameter that also exhibits strong influence on film thickness is the concentration of TEOS in the starting solution. Finally, the film thickness was found to correlate well with sol viscosity and spin speed as h=A η0.45 ϖ−0.62.