Understanding the litho-impact of phase due to 3D mask effects when using off-axis illumination

Aberration sensitive structures, such as two-bar structures, in general suffer from phase effects. The topography of the mask leads to a phase distribution across the diffraction-orders, and hence the image of a two-bar is potentially also distorted due to the topography, leading to full wafer-CDU being larger than required. The phase across the diffraction orders is dependent on the angle of incidence of the imaging light. For ArF it was found before that optimizing the phase distribution leads to a better process-window. It is of interest to investigate the origin and impact of the phase distribution for EUV imaging, since in this case the phase differences depend even stronger on angle. By using analysis from a double-diffraction model and simulations it is shown how illumination setting and stack properties influence the image. Specifically it is shown that two-bar features have better overlapping process window for thinner absorbers whereas spaces through pitch require a thicker absorber.