Experimental demonstration of dark field illumination using contact hole features

Abstract

The use of off-axis illumination in present lithography processes has enabled patterning to push below k1<0.40. This continual drive toward printing smaller feature sizes at the 193nm wavelength has led to the investigation and successful implementation of a number of new resolution enhancement technique (RET) approaches. In this work, the application of dark field imaging as a RET for low-k1 contact hole features was examined using both simulation and experiment on a standard lithography tool set. Here dark field imaging is defined as imaging where a portion of the illumination source is placed outside of the projection optics. This approach is similar to standard off-axis illumination approaches using the familiar two-beam imaging but at greater incident angle where the zeroth diffraction order is not captured and image modulation is created by higher diffraction orders. This approach creates a number of “unexpected” effects and the authors show these for contact hole patterns using ASML Twinscan lithography tools operationally modified to operate in dark field mode. They demonstrate the following new dark field effects for contact holes: reduced and negative mask error enhancement factor, inverted develop critical dimension as a function of pitch, and image tone reversal at phase edges. Additionally, experimentally relevant items such as stray light and dark field exposure energy requirements are discussed.