Effects of sintering on dielectric constants of mesoporous silica

Abstract

The presence of –OH groups increases the dielectric constant of mesoporous silica films making them less suitable as interlayer dielectrics. To reduce the concentration of –OH groups a sintering process was used. The films were prepared by a spin-on, binary solvent technique. The film porosity was controlled by changing the amount of glycol, a low-vapor-pressure pore former. The dielectric constants of these mesoporous silica films were measured electrically over a wide porosity range before and after sintering to investigate the effects of porosity and annealing. At a porosity of ∼50%, the dielectric constant agrees with Hrubesh’s correlation, Kgel=1+7.1(η−1)=1+1.48ρgel, a correlation determined using bulk aerogels, where K is the dielectric constant, and η and ρgel are the refractive index and density of the mesoporous material, respectively. Above and below 50% porosity, the data show substantial deviations from this correlation. At porosities above 60%, the measured dielectric constant is lower than that predicted by Hrubesh’s correlation. At porosities <40%, the dielectric constant is much higher, even higher than thermal SiO2. There is no significant difference in the measured dielectric constant between as-prepared and sintered films above 60% porosity with surface modification by trimethylchlorosilane (TMCS), but starting with porosities ranging from 30% to 55%, there is a significant improvement in the dielectric constants of sintered mesoporous films. The annealing process improves the dielectric property of the mesoporous film because it eliminates –OH groups by a condensation reaction and heals the structure by reattachment at the Si–O–Si bonds that the surface modifier cannot reach at porosities below 40%. The surface chemistry and thermal stability of the films was investigated using thermogravimetric analysis and Fourier-transform infrared spectroscopy. Ellipsometry and field-emission scanning electron microscopy were used to observe the film morphology and determine the porosity and the thickness of the films before and after sintering. The internal surface area of the prepared mesoporous film was measured by ellipsometric porosimetry (IMEC) before and after the sintering to calculate the concentration of –OH groups that would exist in the film at equilibrium. This calculated value was then compared with actual values for OH concentration based on dielectric measurement results.