Comparison of positive tone versus negative tone resist pattern collapse behavior

In this work, e-beam lithography patterns have been specifically designed and fabricated which provide the opportunity to probe the collapse behavior of both positive and negative tone systems. The pattern layout includes adjacent parallel line structures that both vary in the line size and also in the distance by which they are separated by the space between them. This type of structure allows for the control and modulation of the capillary forces, and ultimately the stresses, experienced by the photoresist line pairs during the final rinse and drying steps of the development process. Using such structures, it is possible to determine the critical stress, i.e., the maximum stress experienced by the photoresist lines before collapse, as a function of a variety of parameters including: material type, substrate preparation conditions, resist film thickness, and resist feature width. In this article, such a modular approach has been used to compare the pattern collapse behavior of a prototypical positive tone resist formulated using a protected hydroxystyrene-based copolymer and a prototypical negative tone epoxide-based molecular photoresist (4-EP). It was found that the critical stress at the point of pattern collapse decreased both as the thickness and the feature width of the resist lines decreased, though this trend was observed to a much lesser extent in the negative tone 4-EP material. It is observed that the negative tone resist, whose imaging mechanism involves cross-linking, shows far superior pattern collapse performance as compared to the positive tone deprotection based resist and is in general able to achieve significantly higher aspect ratio patterning at equivalent feature linewidths.