Plasma Technology for Advanced Devices
Sidewall Passivation During Silicon Gate Etch
During silicon gate etching, anisotropy control is obtained by the formation of a passivation layer which forms on the gate sidewall and protects it from spontaneous etching reactions. The sidewall film thickness decreases along the gate sidewall showing that profile deformation is more likely to occur at the bottom of the gate profile. In the following, composition and thickness of the sidewall passivation layer will be investigated as a function as a function of the mask material, etch chemistry, and plasma operating conditions (slide 1).
XPS analyses performed on the gate sidewalls show that the passivation layer is a SiOxCly layer in a HBr/Cl2/O2 chemistry and a SiOxBry layer in a HBr/O2 chemistry (slide 2).
Slide 3 shows the influence of the mask material on the composition of the sidewall passivation layer. The overall differences are small, carbon from the resist mask seems not to play a major role in the passivation mechanism.
Comparing HBr/O2 and Cl2/O2 processes, the following observations were made: (1) The passivation film is a halogen rich oxide-like film, (2) The passivation film is thicker with HBr/O2 than with Cl2/O2, and (3) Chlorine in the gas mixture generates thin passivation layers (slide 4).
The passivation layer thickness is strongly dependent on the oxygen concentration in the gas phase (slide 5). Higher oxygen concentrations in the gas phase lead to thicker passivation layers.
XPS analysis after the silicon gate main etch and overetch show that during the overetch step, bromine is replaced by oxygen. Thus, the passivation layer is transformed into a more oxide like layer (silde 6).
Slide 7 summarizes the findings.