Negative tone development: gaining insight through physical simulation

A simple analysis of aerial image quality reveals that negative tone imaging is superior to positive tone for small dimension contacts and trenches. Negative Tone Development (NTD) of positive chemically amplified (de-protecting) photoresist is currently the favored method for realizing such images on the wafer. When experimental process windows are determined for NTD systems, it is apparent that the results far exceed the upper limit predicted using current physical modeling. Since real data transcends the capabilities of the current model to predict, some important physical process is clearly missing. In this work, we explore whether resist shrinkage during PEB can account for the observed discrepancies. Two very simple shrinkage models are developed and tested. Results show that shrinkage in the vertical direction explains some profile artifacts observed in actual NTD processes but has negligible impact on conventional positive tone processes. The horizontal shrinkage model reveals that this type of phenomenon can significantly increase the exposure latitude of a negative tone process but has marginal impact on positive tone exposure latitude only introducing a small CD offset. While horizontal shrinkage does enhance exposure latitude appreciably, the effect does not seem large enough on its own to account for the entire increase observed in the experimental results. Further work is ongoing to investigate other potential mechanisms for observed behavior.