Investigation of Si and Ge etching mechanisms in radiofrequency CF2-O2plasma based on surface reactivities

Reactive ion etching (RIE) of silicon and germanium in CF 4 -O 2 radio frequency plasmas has been investigated by means of optical emission spectroscopy, mass spectrometry and quasi in situ X-ray photoelectron spectroscopy. Above 10% O 2 , germanium etching is selective with respect to silicon. In the gas phase, two etching products are detected: SiF 4 and GeF 4 . Their concentration is shown to be correlated with the amount of atomic fluorine and oxygen as measured by actinometry. In agreement with the plasma characterization, XPS analysis reveals that the Si etch rate appears to be controlled primarily by the thickness of the SiO x F y superficial layer whereas the formation of GeO x F y does not inhibit Ge etching. A simple model is used to express the surface reactivity in function of the experimental data. Application of this model to experimental CF 4 -O 2 plasma etching suggests that surface composition controls silicon etching, whereas germanium etching depends mainly on the fluorine flux on the surface.