Increasing mechanical strength of mesoporous silica thin films by addition of tetrapropylammonium hydroxide and refluxing processes

The incorporation of mesopores into silica films is an effective way to reduce the dielectric constant. However, the pores reduce the film mechanical strength. This study investigates two steps for preparing coating solution. One was the reflux of the silica colloid at 70 °C. The other was the addition of TPAOH (tetrapropylammonium hydroxide) into the colloid. The reflux step can increase the mechanical strength, reduce the flat band voltage and reduce the leakage current of the films. Nevertheless, the low-k value (k represents dielectric constant) increases as the porosity of the film falls. Adding a slight amount of TPAOH before the reflux process can recover both the porosity and the low k value, while maintaining the high mechanical strength and the low flat band voltage. Results of this study demonstrate that two more steps (the addition of TPAOH and the reflux) in the preparation of the coating solution can increase the film hardness and elastic modulus from 0.8 to 1.4 GPa and from 5.8 to 9.9 GPa respectively, while maintaining the low-k value close to 2.05.