Bottom profile degradation mechanism in high aspect ratio feature etching based on pattern transfer observation

The hypothetical mechanism of bottom profile degradation, such as distortion and twisting in high aspect ratio feature etching, was verified based on the pattern transfer observation of etched pattern. The authors mainly focused on trench pattern sample to make the investigation easier, that is, direct observation of the sidewall roughness, using an atomic force microscope, as well as analysis of the depth dependence of pattern deformation in high aspect ratio trench etching. Using Fourier transformation analysis for the trench sidewall roughness, it was found that lower spatial frequency component of the mask's sidewall roughness is amplified at the bottom region of the trench and that higher spatial frequency component of over 10 mu m(-1) disappears. However, the higher spatial frequency component is transferred directly to the upper sidewall of the trench. The observation of the pattern deformation profile, as a function of etch depth, revealed that the ratio of line width roughness to line edge roughness decreases linearly with increasing etch depth. This indicates that the deformation mode changes from roughness to wiggling as a function of etch depth. Also, the twisting depth, as determined by the cross section images, was found to be shallower when using lower wafer bias etching condition. Based upon these results, and previously proposed mechanisms, the authors conclude that the mask deformation should be considered as one of the root causes when bottom deformation occurs. This is due to the imbalance between ion flux and deposition amount created by mask asymmetric profile. In addition, the etching profile and etching direction should be affected by this phenomenon under polymer-rich process condition. In order to verify our assumed mechanism, the relationship between mask deformation degree, and bottom distortion and twisting amount was also evaluated by using different pitch hole patterns. As a result, the authors determined that the distortion is improved by 22% and the twisting is improved by 20% when the mask deformation is reduced by 50% with using a wider pitch pattern. (C) 2017 American Vacuum Society.