Exactly what happens at the interface between the exposed and unexposed photoresist has become an important issue for the design of 32-nanometer processes. Most of the exposed areas of the photoresist swell slightly and dissolve away when washed with the developer. However this swelling can induce the polymer formulation to separate (like oil and water) and alter the unexposed portions of the resist at the edges of the pattern, roughening the edge. For a 32-nanometer feature, manufacturers want to hold this roughness to at most about two or three nanometers.
Industry models of the process have assumed a fairly simple relationship in which edge roughness in the exposed “latent” image in the photoresist transfers directly to the developed pattern, but the NIST measurements reveal a much more complicated process. By substituting deuterium-based heavy water in the chemistry, the NIST team was able to use neutrons to observe the entire process at a nanometer scale. They found that at the edges of exposed areas the photoresist components interact to allow the developer to penetrate several nanometers into the unexposed resist. This interface region swells up and remains swollen during the rinsing process, collapsing when the surface is dried. The magnitude of the swelling is significantly larger than the molecules in the resist, and the end effect can limit the ability of the photoresist to achieve the needed edge resolution. On the plus side, say the researchers, their measurements give new insight into how the resist chemistry could be modified to control the swelling to optimal levels. ###
The research, funded by SEMATECH, is part of a NIST-industry effort to better understand the complex chemistry of photoresists in order to meet the needs of next-generation photolithography.
* V.M. Prabhu, B.D. Vogt, S. Kang , A. Rao , E.K. Lin and S.K. Satija. Direct measurement of the spatial extent of the in situ developed latent image by neutron reflectivity. Journal of Vacuum Science and Technology B, 25(6), 2514-2520 (2007).
Contact: Michael Baum firstname.lastname@example.org 301-975-2763 National Institute of Standards and Technology (NIST)
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