A method has been developed for mapping and analyzing the impact of each local region of the illumination distribution (i.e. "illumination source point"). The method makes directly visible the imaging impact/result for each one of those local regions. In this way, the entire available illumination region can then be broken down generally into regions of "good light" and "bad light" for each pattern under consideration. It is possible to then further subdivide the impact of each source point/local region into 'categories of impact' that can be each then independently evaluated (e.g. contrast/intensity modulation, Normalized Image Log Slope, CD range through a fixed focus region, CD change due to a given dose change, aberration sensitivity, etc.).
By applying the method and analyzing the full superset of all available source points for all mask patterns being considered, the optimization of the illumination distribution becomes a straightforward matter of finding the best combination of source points to deliver whatever is described as the target of the optimization.
An example of the application of the method will be provided and discussed, with highlights on key learning steps arising from this case study that might be applicable more generally.
By becoming familiar with the contributions of the various regions within the potential illumination region, one can begin to see both:
i. those regions which contribute only less than optimally to the imaging of any of the specified critical patterns, and:
ii. those regions which show positive contribution to the imaging (via analysis of NILS or CD/Focus, etc.)
Various methods to extract, analyze, and apply these results and knowledge, and turn them into a prescription for an optimized illuminator source design will be discussed.