A Physics-based Model for Negative Tone Development Materials

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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. One of the challenges for these materials is prediction of cross-section shape. Cross-section shape is often critical in leading-edge lithography processes where resist thinning or top loss can lead to pattern failure during the etch process.<br>There are two important effects that make prediction of cross-section shape more difficult for NTD materials. First, NTD materials typically do not have the develop contrast of positive tone develop (PTD) systems. NTD often has a larger minimum (unexposed) develop rate, and a smaller maximum (fully exposed) develop rate. Second, photoresist typically shrinks after post-exposure bake in regions where de-protection is high. For PTD, these regions dissolve, and the features are formed by the protected areas which do not show shrinkage. The opposite is true for NTD, where the regions with the largest amount of shrinkage form the features on the wafer.<br>We demonstrate a photoresist model that incorporates resist shrinkage following the elasticity theory described by Flory-volume loss due to de-protection leads to a stress in the photoresist, and the material deforms in a way that minimizes the free energy of the system. We show that this model, combined with accurate dissolution rate measurements, can accurately predict cross-section shape for isolated and dense trenches formed with an example NTD material.

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