Optimum tone for various feature types: positive versus negative

The continuing drive to reduce feature size is forcing resist processes to be tailored to specific levels, e.g. contact holes or isolated lines. Resist contrast, absorption, diffusion length and development characteristics are among the customized variables. For the most part, resist tone has not been among these variables, and the bulk of advanced lithography is done with positive tone resist processes. This paper will explore the optimum process tone for various feature types, and will include simple theoretical guidelines to help with this decision. Narrow resist lines are found to print best with a positive tone process while narrow trench geometries are found to print best with a negative tone process. Simple development bias models appear to accurately capture this behavior and are in agreement with full simulation.