Effect of sidewall properties on the bottom microtrench during SiO2 etching in a CF4 plasma

Factors that affect microtrench formation during the etching of SiO2 in a CF4 plasma were investigated using an experimental set-up, which comprised a Faraday cage and step-shaped substrates consisting of a SiO2 bottom and slanted sidewalls of different materials. This set-up permitted the etched micropattern to be observed on a magnified scale under conditions that are similar to actual processes. SiO2 and fluorocarbon polymer were used as sidewall surface materials to investigate the effect of sidewall properties on the microtrench. The depth of the microtrench characteristically changed with the sidewall angle, reaching a maximum at 70° under the condition used in this study. This can be explained by the effect of two competitive factors: secondary etching of the bottom by ions reflected from the sidewall, which contributes to the bottom etch rate, and the shadowing effect of the sidewall, which decreases the etch rate. Secondary etching, as estimated from the difference in the bottom etch rates between two cases of an ordinary sidewall and a sidewall with an extremely rough surface, showed a maximum at a sidewall angle of 80°–82.5°, while the shadowing effect was increased monotonously with sidewall angle. The rates of secondary etching were higher for the polymer sidewall than for the SiO2 sidewall for sidewall angles lower than 70°, but opposite relative magnitudes were observed for angles higher than 70°. The result obtained at angles lower than 70° can be explained based on a model in which a fluorocarbon film formed on the sidewall, which was particularly thick for a polymer sidewall, acted as a source of etchant radicals, thus contributing to bottom etching. The opposite trend observed at angles higher than 70° can be attributed to the roughness of the sidewall surface, which was higher for the polymer sidewall, which affected the reflection of ions on the surface and, eventually, secondary etching.