A parametric study of single-crystal silicon roughness induced by an SF6 plasma has been carried out by means of atomic force microscopy. An helicon source (also called resonant inductive plasma etcher) has been used to study the relation between plasma parameters and subsequent surface damage. The surface damage has been examined in terms of height roughness analysis and in terms of spatial (lateral) extent of the surface roughness. The central result is that roughness scales with the ratio of the ion flux over the reactive neutral flux (J+/JF), showing the combined role of both ionic and neutral species. At low ion flux, the neutrals smooth the surface, while at higher ion flux, they propagate the ion-induced defects, allowing the roughness to be enhanced. Influences of other parameters such as exposure duration, ion energy, or substrate temperature have also been quantified. It is shown that the roughness growth is well described by an empirical law: rms∝(1/√E)(J+/JF)ηtβ, with η≊0.45 and β≊1 (rms is the root mean square of the roughness). In other respects, we analyze the data with a Fourier transform analysis. The main advantage is to minimize noise and to separate the magnitude of the roughness, the lateral correlation length on which the roughness is growing, and the behavior of short and long range roughness. The results are identical to the rms analysis, especially, the above scaling law. The time evolution of the lateral correlation length follows a scaling law (which is not accessible by means of the rms) leading to a fractal dimension of 2.67. Also is observed a variation of the short range roughness as a function of the substrate bias voltage. Consequence for further scaling down of integrated circuits is called to mind.

The influence of the surface temperature on tungsten etching in a SF6 plasma diffusing from a helicon source has been studied in detail. The surface temperature dependence of the etching kinetics has been analyzed. The influence of the other parameters such as oxygen content of the plasma and ion energy have also been considered. The etching mechanism depends on the temperature range: the etching is partly spontaneous at higher temperatures (T≳−20 °C), becomes a chemical sputtering type at intermediate temperature (−20 °C<T<−40 °C), and a chemically enhanced physical sputtering at lower temperature (T<−40 °C). Surface analyses by x-ray photoelectron spectroscopy provide an understanding of how the surface chemistry is modified with temperature.

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.

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Articles you may be interested inEffects of etch chemistry on SF6based tungsten etching by electron cyclotron resonance reactive ion etching Tungsten etching in pulsed plasmas has been investigated in a helicon plasma source reactor. The time dependence of the fluorine atom concentration has been measured using the time-resolved actinometry technique and related to the etch rate. According to our observations, it appears that fluorine adsorption on the tungsten substrate surface continues in the post-discharge period until the surface saturates. Moreover, it appears that two etching regimes exist. For short discharge off periods, the etching is limited by the fluorine adsorption ability of the surface, whereas for long periods, the etching is limited by the desorption rate of the etch products. Experiments were performed at different substrate temperatures and plasma gas pressures. An empirical model has been developed, in good agreement with the experimental data.