Magnetically enhanced reactive ion etching of silicon and silicon dioxide

The influence of magnetic fields from 0 to 10 mT applied perpendicular to the substrate surface in SF6(CHF3 plasmas was studied for the etching of monocrystalline silicon and silicon dioxide using resist as masking material. A saturation in the silicon etchrates was observed above 6 mT, weakly depending on the pressures and RF powers. Actinometric optical emission spectroscopy yielded a linear etch rate dependence on the atomic fluorine density. This phenomenon together with the fairly anisotropic profiles can be explained with a simple modelling approach. The etching of silicon dioxide was investigated with different cathode arrangements for gaining optimum etch homogeneity/etch rate conditions. An overall variation of the oxide etch rates with changing magnetic fields was attributed to effective ion current densities, which were calculated from a well established model for SiO2 etching and includes a "damage yield" that depends on the ion energy, i.e. approximately on the DC-bias at the substrate electrode. Selectivities Si : SiO2 were found to increase with increasing magnetic fields from 3 : 1 at O mT to 9 : 1 at the upper field limit. At magnetic fields above 6 mT they could be expressed as inversely proportional to the damage yield.