Low dielectric constant porous spin-on glass for microelectronic applications.

摘要

This research work focused on the development, characterization, and optimization of the properties of silsesquioxane-based, low dielectric constant porous materials for use as interlevel dielectrics in integrated circuits. A commercially available spin-on glass (methylsilsesquioxane, MSQ) was modified by the introduction of porosity. The porosity reduced the effective dielectric constant of the MSQ by the incorporation of air. The porous methylsilsesquioxane films were created by making polymer blends with trimethoxysilyl norbornene (TMSNB) and triethoxysilyl norbornene (TESNB), where the polymer served as a sacrificial “place-holder”. Upon exposure to elevated temperatures (∼425°C), the polymers decomposed within the MSQ matrix to form nanometer-size voids in the films. Different pore microstructures were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM), depending on the functional groups on the polymeric sacrificial material used. TEM cross-sectional micrographs showed pores of nearly spherical geometry with 3–10 nm diameters distributed within the TMSNB:MSQ films after heat treatment at 425°C for 1.5 hours. In the case of the TESNB:MSQ blends, only large pore cavities were observed at the surface of the films. The differences in microstructure were correlated to variations in the chemical reactivity between the sacrificial polymer and the MSQ in the mixtures. Solid-state nuclear magnetic resonance and Fourier-transform infrared spectroscopies were used to probe the chemical bonding between the mixture components. Indications of chemical bonding were observed in the TMSNB:MSQ blends, and also in the TESNB:MSQ mixtures, but only upon the addition of an acid catalyst to the system.{09}The absence of chemical bonding was assumed responsible for the complete phase-separation induced in the TESNB:MSQ blends without the added acid catalyst. The electrical, optical, and mechanical properties of the porous films were evaluated as a function of type and molecular weight of the sacrificial polymer used to prepare the samples, and the polymer loading in the films.{09}The dielectric constant, index of refraction, elastic modulus and hardness reduced with increasing porosity in the films (polymer loading). Significant improvements were observed in the fracture toughness, or the resistance to crack propagation in the films, with TESNB polymer loading as compared with the non-porous MSQ films.