We observed that the net effect of the etch process is to improve LCDU, although the final LCDU is both stack- and resist-dependent. Different resists demonstrate the same LCDU improvement, so that the LCDU after etch will depend on the initial resist performance. Using a stack different from the one used to set up the etch process can undermine the LCDU improvement.
The impact of the various etch steps is investigated in order to identify the physical mechanisms responsible for the LCDU improvement through etch. Both top-down and cross section Scanning Electron Microscopy (SEM) are used. The step-by-step analysis of the etch process showed that the main LCDU improvement is achieved during oxide etch, while the other process steps are either ineffective or detrimental in terms of LCDU. The main cause of the LCDU improvement is then attributed to the polymerization of the CH surface happening during the oxide etch.
Finally, the LCDU improvement caused by the etch process is investigated as a function of the initial LCDU after litho in a relatively broad range (2-15nm). The ratio between LCDU after litho over LCDU after etch is investigated as a function of the initial LCDU after litho for two different resists. The results indicate that the impact of etch on LCDU is characterized by a single curve, specific to the etch process in use and independent of the resist type. In addition, we observe that the percentage LCDU improvement is constant above a certain threshold, in agreement with the throughpitch results.