Fundamentals of topographic substrate leveling

The leveling of 100–500-μm-wide, 1-μm-deep isolated holes and trenches on a silicon substrate by 1–3-μm-thick silicone oil films was observed by measuring film thickness changes at the centers of the features using a noncontact, interferometric technique. The dependence of the leveling time t on feature width w, film viscosity η, and the initial film thickness h0 was investigated and compared to theoretical predictions. Experimental data were obtained for various values of w, η, and h0. Except when the film thickness was about 1 μm, the data for each different type of geometry fell on a single curve when the degree of leveling or planarization was plotted against T≡tγh30/ηw4 where γ is the surface tension of the film. At the same value of T, the degree of planarization of isolated holes was about twice that of isolated trenches. The planarization versus T curves should apply to all Newtonian liquids and may be used to predict the degree of planarization that will be achieved at specified leveling times by materials having specified values of η, h0, and γ. Thus, the curves may be useful in selecting planarizing materials for semiconductor fabrication steps that require substrate topography leveling. Simulations of the leveling process based on capillarity-driven flow were performed and agreed well with the experimental data. The simulations predicted some interesting and unexpected behavior that was observed experimentally. At short times the planarization became worse before improving at longer times and bumps appeared in the film profiles that apparently were a first step in minimizing surface energy. Such simulations may prove to be useful in predicting the difficulty of planarizing various types of topographic geometries and in designing ‘‘dummy topography’’ that will make them easier to planarize.