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Selective Etching of Silicon in Preference to Germanium and Si<sub>0.5</sub>Ge<sub>0.5</sub>
17
Citations
15
References
2017
Year
The selective etching characteristics of silicon, germanium, and Si<sub>0.5</sub>Ge<sub>0.5</sub> subjected to a downstream H<sub>2</sub>/CF<sub>4</sub>/Ar plasma have been studied using a pair of in situ quartz crystal microbalances (QCMs) and X-ray photoelectron spectroscopy (XPS). At 50 °C and 760 mTorr, Si can be etched in preference to Ge and Si<sub>0.5</sub>Ge<sub>0.5</sub>, with an essentially infinite Si/Ge etch-rate ratio (ERR), whereas for Si/Si<sub>0.5</sub>Ge<sub>0.5</sub>, the ERR is infinite at 22 °C and 760 mTorr. XPS data showed that the selectivity is due to the differential suppression of etching by a ∼2 ML thick C<sub>x</sub>H<sub>y</sub>F<sub>z</sub> layer formed by the H<sub>2</sub>/CF<sub>4</sub>/Ar plasma on Si, Ge, and Si<sub>0.5</sub>Ge<sub>0.5</sub>. The data are consistent with the less exothermic reaction of fluorine radicals with Ge or Si<sub>0.5</sub>Ge<sub>0.5</sub> being strongly suppressed by the C<sub>x</sub>H<sub>y</sub>F<sub>z</sub> layer, whereas, on Si, the C<sub>x</sub>H<sub>y</sub>F<sub>z</sub> layer is not sufficient to completely suppress etching. Replacing H<sub>2</sub> with D<sub>2</sub> in the feed gas resulted in an inverse kinetic isotope effect (IKIE) where the Si and Si<sub>0.5</sub>Ge<sub>0.5</sub> etch rates were increased by ∼30 times with retention of significant etch selectivity. The use of D<sub>2</sub>/CF<sub>4</sub>/Ar instead of H<sub>2</sub>/CF<sub>4</sub>/Ar resulted in less total carbon deposition on Si and Si<sub>0.5</sub>Ge<sub>0.5</sub> and gave less Ge enrichment of Si<sub>0.5</sub>Ge<sub>0.5</sub>. These results are consistent with the selectivity being due to the differential suppression of etching by an angstrom-scale carbon layer.
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