Chemical effects during silicon dioxide chemical mechanical planarization

Chemical mechanical planarization or chemical mechanical polishing (CMP) is an essential process step in the manufacturing of integrated circuits in the semiconductor industry. Silicon dioxide CMP is used in shallow trench isolation, planarization of interlevel dielectrics, and manufacturing of micro-electromechanical systems (MEMS). As the name implies, the polishing mechanism has both chemical and mechanical components. The most scientifically based models of the process have focused almost exclusively on mechanical aspects of the mechanism, such as abrasive wear and pad-wafer contact mechanics. The chemical aspects of the mechanism have been largely ignored in the literature or have been dealt with in only a very cursory manner. One reason for this is the lack of fundamental data, such as reaction equilibrium constants and diffusion coefficients for the silicon dioxide-water system. The goal of this work is to begin to fill that gap.; The reaction of water with silicon dioxide at low temperatures relative to CMP (20°C to 80°C) was examined and equilibrium constants were determined. Negligible silanol (SiOH) formation was observed in silicon dioxide samples exposed to pure water. The addition of potassium ions to the water reduced the activity of the water and consequently enhanced silanol formation. Increasing pH in aqueous potassium hydroxide solutions was found to further enhance silanol formation. Additionally, the diffusion behavior of water in silicon dioxide in this temperature regime was measured and modeled. Based on a Fickean concentration profile and the diffusion coefficients for water in silicon dioxide measured in this work, it is expected that water penetrates less than 5 nm into a silicon dioxide film during CMP. The presence of potassium ions in the water was found to enhance silanol formation. No apparent effect of pH on diffusional flux of water into silicon dioxide was observed. An apparent time dependence of the diffusion coefficient of water in silicon dioxide was observed. This was explained as the result of diffusion-induced stress. Such stresses can be significant at the silicon dioxide surface and are expected to contribute to the CMP removal mechanism.