Low Dielectric Constant Functionalized Silica Gels
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LOW DIELECTRIC CONSTANT FUNCTIONALIZED SILICA GELS Brian P. Gorman, Rosa A. Orozco-Teran, Jodi A. Roepsch, Dennis W. Mueller and Richard F. Reidy* Department of Materials Science, University of North Texas, Denton, TX. ABSTRACT Scaling requirements of the semiconductor industry demand interlayer dielectric materials with low dielectric constants, chemical compatibility with Cu, thermal and mechanical stability, and the capability of integration into planned device structures. We have developed a novel low-k silica gel processing technique capable of both bulk and spin-on film architectures. Derived from fluorinated silica, this gel exhibits very low dielectric constants (1.18 in bulk and 2.1 in thin film measured using AC impedance methods). Structural determination from FTIR illustrates a fluorinated silica structure with shortened Si-O bonds, however, the fluorine is lost after annealing at elevated temperatures. Microstructural analysis by TEM indicate a highly unusual morphology with highly linked features and pore sizes in the 30nm range, coinciding with nitrogen adsorption results. Mechanical properties of the thin films, as studied by nanoindentation methods, shows that the films have extraordinarily high elastic moduli (12GPa) and hardness (1GPa). INTRODUCTION Fluorinated-silica thin films have incurred interest as interlayer dielectrics due to their compatibility with large-scale integration and low dielectric constants (1,2). Low measured dielectric constants have enticed many studies into the cause for this behavior (2-4). In the literature, methods of fluorine doping of silica most often employ plasmaenhanced chemical vapor deposition (PECVD) of silica with a fluorine source (e.g., CF4, SiF4) or PECVD of triethoxyfluorsilane (TEFS)(2,3,4). The causes for this change in dielectric properties are still somewhat disputed. Efforts by Demkov and Coffer indicate fluorine ditorts Si-O rings and increases silica tetrahedral framework formation, respectively.(4,5) These ring distortions may limit lattice polarizability and, consequently, decrease the dielectric constant. FTIR spectroscopy studies have shown up-shifting of Si-O stretching modes and decreases in the Si-O stretch peak area to be correlated with decreases in the dielectric constant and increasing fluorine content (3,6). These decreases in the dielectric constant were found to be due not only to decreasing ionic polarization, but mainly resulted from a decrease in orientational polarization attributed to Si-OH bonding, which also decreased as a result of fluorine doping. In addition to low dielectric constants, the mechanical properties of low density silica films are critical to their implementation as interlevel dielectrics. Several studies have examined the relationship between dielectric constant and mechanical properties.(7,8,9) In mesoporous films, the elastic modulus and hardness have been shown to be highly dependent upon the degree of Si-O networking (measured as a function of annealing temperature), which is unfortunately inversely related
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