Phenyl Silica Glass for Formation of Porous Dielectric Film
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fabricated 0.18-tim CMOS circuits using a-C:F ILDs and W plugs, and have reduced the periodic time (Tp) of ring oscillator circuits by reducing the wiring delay [2]. As feature sizes in integrated circuits approach 0.1 [tm, however, it is necessary to reduce the dielectric constant of the ILDs below 2, which means decreasing the density of the film. Porous silica films have been formed by drying liquid precursors after gelation [3]. Also polyimide nanoforms have been fabricated by block co-polymerization of thermally stable and thermally labile polymer blocks. The thermally labile blocks were then evaporated [4]. Both of them achieved the dielectric constant below 2 due to the reduction of the density. These reported films are spin-on materials deposited by a spinner and then thermally cured, whereas, we have been advocating the use of CVD to deposit low dielectric constant (low-k) polymers and have integrated chemical-vapor-deposited a-C:F films into CMOS circuits [2]. The CVD method uses a thermal excitation or glow discharge to activate species such as radicals from the original monomer. Polymer films are then deposited through various gas phases and surface reactions of these active species including ablation of the deposited films [5]. A layered structure that promotes adhesion can be easily fabricated by changing the source compounds and 49 Mat. Res. Soc. Symp. Proc. Vol. 565 © 1999 Materials Research Society
the characteristics of the films can be fine tuned by precisely controlling the deposition conditions. The fabrication difficulties when introducing low-k materials as ILD films can be overcome. In this study, we propose a new process for forming porous low-k films by using the CVD method. EXPERIMENT To decrease the density of ILD films, we make use of the techniques for fabricating xerogel film and polyimide nanoform. The density of xerogel film is reduced by evaporating of solvent after gelation of the film, and the density of a polyimide nanoform is reduced by evaporating the thermally labile blocks. This indicates that evaporating some components of a deposited film will decrease the density of the film formed by CVD.
Next we had to choose between a silica film or a polymer film such as a-C:F film to be a starting material. Silica films are hard enough to integrate and form ILDs, however, the dielectric constant of normal silica film is around 4 and the porosity of the film must be increased more than 0.5 to achieve a dielectric constant below 2. With a-C:F film, although it is not as hard as silica film, a dielectric constant of around 2.5 ensure low porosity to create a dielectric constant below 2. To determine which material is the most promising, we firstly selected a silica film and investigated the feasibility of using it to form porous film by CVD. Because the phenyl group forms a bulky structure in silica film, we deposited a phenyl silica glass and decreased the density by evaporating the phenyl groups from the film after the deposition. Phenyl silane is one promising source material, howe
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