Plasma Technology for Advanced Devices
The Selfclean Concept in Silicon Gate Etching
For the traditional HBr/Cl2/O2 silicon gate etch chemistry, high concentrations of etch products are present in the gas phase. These by-products can dissociate and form non volatile species. This leads to re-deposition of siliconoxyhalogenides on the feature sidewalls and in the presence of oxygen in the plasma to oxydation to form SiOx on the sidewalls. Direct line of sight re-deposition from the bottom of the features is also possible. The same mechanisms holds to explain the formation of SiOx on the chamber walls. The chamber wall will be covered with SiOx deposits (slide 1).
When CF4 is added, the formation of silicon oxyhalogenides is suppressed and silicon fluorides or oxyfluorides are formed. The SiOx based passivation film is replaced by a thin carbon rich layer (slide 2).
XPS studies reveal that the SiOxCly based passivation film is transformed in a CFxCly based passivation film when CF4 is added to the standard HBr/Cl2/O2 chemistry (slide 3).
In the steady state etching regime, a thin halogen rich fluorocarbon film is formed on the silicon surface at the bottom of the features (slide 4).