Surface roughness studies of deep plasma etching in crystalline silicon

A study of the parameters influencing the roughness of plasma etched surfaces of monocrystalline silicon was conducted, using a commercial reactive ion etcher (RIE) and a Gaseous Electronic Conference (GEC) Plasma Standard Cell as the etching tools. Relatively deep (6 micrometer) etching was done using pure SF₆ and mixtures of SF₆ with additions of either oxygen or hydrogen. Other parameters studied included chamber pressure and rf power. Roughness was determined qualitatively by the use of a Scanning Electron Microscope (SEM). An Atomic Force Microscope (AFM) was used on selected samples to obtain quantitative roughness measurements. A wide range of surface roughness was observed. Both pure SF₆ and SF₆ plus 7.5% H₂ mere capable of producing very smooth bottom and wall surfaces. Increasing H₂ to 17.5% caused the sidewalls to become rough while maintaining bottom smoothness. Oxygen additions caused all surfaces to be rough. With most gas compositions, surfaces could be made smoother by decreasing power and increasing pressure. Silicon etch rates generally increased with power, but showed a maximum near a pressure of 400 mT, decreasing at both higher and lower pressures. Although similar trends were observed with both etch tools, further comparisons will require a more detailed investigation