Tungsten etching mechanisms in low-pressure SF6 plasma

A detailed study of tungsten low-pressure etching in a helicon source reactor was performed. In correlation with surface analyses (transmission electronic microscope, Rutherford backscattering, nuclear reaction analysis), a complete parametric study of the plasma and etching parameters versus the macroscopic parameters [gas pressure, radio frequency (rf) power, substrate bias voltage) has been carried out. Using a model developed by Hoffman and Heinrich for silicon etching (Proceedings of the 9th ISPC, Pugnochiuso, Italy, 1989, p. 1003), and taking into account the experimental results, it has been shown that tungsten etching mechanisms can be expressed by the formation and destruction of a low-density reactive top layer. Sputtering of this layer under argon-ion bombardment has been studied by optical emission spectroscopy. Consequently, the tungsten etch rate can be expressed as an analytical function of the macroscopic parameters such as gas pressure, rf power, and substrate bias voltage.