Aspect ratio dependence of silicon trench oxidation in downstream of microwave oxygen plasma

Abstract

Silicon trench oxidation is studied in microwave oxygen plasma downstream under the conditions of radio frequency bias as well as DC bias applications. Particularly, an aspect ratio dependence of trench oxidation is precisely examined within its range from 1 to 5. It is shown that the oxidation film obtained at the trench bottom is drastically reduced at higher aspect ratios even in the bias condition in which the oxidation at the trench top shows a maximum. It is also demonstrated that the ratio of film thickness at the bottom to the top of the trench is consistent with oxygen radical fluxes coming onto the silicon dioxide film surface, indicating that the oxidation is ion-assisted but is radical flux-limited. This is because the oxidant species responsible for oxidation is O− which is formed on the surface by attachment with radicals or molecules and diffused in the silicon dioxide film. It is emphasized that the negative ions O− in plasma are also important in enhancing the oxidation by bombardment with their kinetic energy, while the positive ions are for neutralization of the charge, particularly at the bottom of trench.