<title>Practical topography design for alternating phase-shifting mask</title>

Abstract

Topographical structures for a dual-trench type alternating phase-shifting mask whose shifters were made of perpendicular trenches with different depth alternately, have been successfully designed using direct Maxwell's equation solver. The structures could reduce the difference of the adjacent peak intensities of the grouped line image on the wafer due to light scattering effects at sidewalls of the trenches. Detailed design of the structures was performed in accordance with the concept of 'effective transmission' and 'effective phase error'. It was clear that the former could be controlled by shallow trench depth, and the latter, which was defined as the phase difference between 'effective phase difference' and 180 degrees, could be reduced by controlling the difference in depth between deep and shallow trenches. For 0.175micrometers lines and spaces, the optimum shallow and deep trench depths corresponded to approximately 270 degrees and 447 degrees in phase, respectively. After the optimization, the depth of focus obtained by exposure-defocus tree was about 0.9 times as large as that obtained for an ideal alternating PSM having rectangle-shaped distribution of complex transmission (Kirchhoff's assumption).