Silicon to oxide etch selectivity is not the only parameter describing the thickness of the gate oxide after a gate etch process. For gate oxides with a thickness of less than 4 nm, the removal of the top oxide layer during the overetch step is accompanied by other processes which will be described below. Slide 1 shows the result of an ellipsometry study of the thickness of such a very thin gate oxide during the exposure to an HBr based overetch process with a nominal selectivity of 80:1. While one would expect the gate oxide thickness to decrease, the experiment shows actually an increase of the gate oxide thickness.
A more detailed analysis with spectroscopic ellipsometry (SE) and XPS shows that for the given etch process, the remaining gate oxide thickness was at least 3.7 nm even when the original gate oxide was much thinner. From this it appears that during the exposure of very thin gate oxides to a HBr / O2 based overetch recipe, a re-oxidation of the gate oxide occurs. Effectively, the gate oxide is grown during plasma exposure (slide 2).
Spectroscopic ellipsometry shows that a perturbed layer as thick as 8 nm is formed when the gate oxide is exposed to a hydrogen plasma. Hydrogen ions, atoms or molecules penetrate the thin gate oxide and damage the bulk silicon underneath the gate oxide layer (slide 3).
FWHM analysis of the XPS signal of the surface confirms that the bulk silicon is amorphized by the diffusion of hydrogen species (slide 4).
The consequences of the gate oxide re-oxidation during the overetch step are illustrated by the TEM cross sections in slide 5. After the wetclean, the bulk silicon is recessed by a significant amount which depends on the plasma conditions.
A more detailed study on the plasma conditions which favor a minimum gate oxide recess has been published by Vitale and Smith (J. Vac. Sci. Technol. B21 (2003) 2205). Some of their findings are summarized in slide 6.
The influence of the electric field on the diffusion of hydrogen and oxygen through the gate oxide is discussed in slide 7. In the absence of bias power applied to the wafer, hydrogen and oxygen induced effects in crystal silicon below the gate oxide are not observed. The damage to the c-Si bulk below the gate oxide can be explained as follows. Hydrogen ions can be directly implanted through the gate oxide into the bulk silicon. Oxygen ions implanted in SiO2 generate defects in the gate oxide which may favor oxygen diffusion through SiO2 and in the silicon wafer.
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