Surface roughness generated by plasma etching processes of silicon

The authors used atomic force microscopy to analyze the roughness generated on c-Si (100) surfaces when etched in high-density plasmas over a wide range of conditions (pressure, rf power) using SF6, CF4, Cl-2, and HBr chemistries. The authors demonstrate unambiguously that high-density plasmas do not generate roughness during silicon etching; but on the contrary, they tend to smooth the existing surface roughness if already present. This is evidenced by analyzing the time evolution of the shape of self-organized silicon nanopillars (patterned on the Si wafer by using diblock copolymers as an etch mask). The 20-nm-high, 20-nm-wide pillars separated by 10 nm are rapidly smoothed by exposure to Cl-2 and SF6 plasmas, thus restoring a flat silicon surface. In high-density plasmas, the local etch rate is generally limited by the availability of reactive radicals. In these conditions, the smoothing mechanism is due to the fact that the hills of a rough surface receive a higher flux of etchant radicals than the valleys. Finally, the authors show that the roughening of silicon surfaces in F-based plasma, often reported in the literature, is only due to the micromasking of silicon by AlFx particles originating from the sputtering of the (Al2O3) reactor walls. A high percentage of Al is indeed detected on the surface after etching in F-based plasmas. However, when the chamber walls are intentionally coated by a carbon layer prior to the silicon etching process, the F-based plasmas behave like the other etching chemistries investigated: they rapidly smooth any existing roughness. (c) 2008 American Vacuum Society.