Dependences of Young’s modulus of porous silica low dielectric constant films on skeletal structure and porosity

Abstract

Porous silica films were prepared by spin coating the mixtures of acidic silica sol and nonionic surfactant template. The (a) porosity and (b) skeletal structure of the films were varied by adjusting the (a) template concentration and the (b) annealing temperature, respectively. Fourier transform infrared spectroscopic ellipsometry was employed to evaluate the skeletal silica structure of the films. The analysis was focused on the midinfrared (1000–1300cm−1) spectral structure which is assigned as the asymmetric stretching vibration mode of Si–O–Si bonds [Kamitsos et al., Phys. Rev. B 48, 12499 (1993)]. The spectral structure depended on both porosity and chemical bonding structure. Bruggemann’s effective medium theory was employed to obtain the spectrum of “skeletal” silica from that of “porous” silica. The skeletal silica structure was then discussed in terms of the peak positions of the transverse optical (ωTO) and longitudinal optical (ωLO) vibration modes of Si–O–Si network. It was shown that the Young’s elastic modulus of skeletal silica correlates well with ωLO2∕ωTO2. We have obtained good correlations between ωLO2∕ωTO2 of skeletal silica and elastic modulus E for two series of porous silica films with around 55% and 40% porosity. The experimental results show that the structural change in silica skeleton strongly affects the mechanical properties of porous silica low-k films.