Simulation of X-ray mask defect printability

Abstract

The printability of defects in X-ray masks was simulated in three dimensions using the CXrL toolset software developed at the University of Wisconsin and resist dissolution software developed in a collaboration between University of California at Berkeley and Motorola. Isolated defects on mask membranes and isolated defects on pellicle membranes mounted behind the mask membrane were modeled. Defects close to X-ray absorber features and absorber fabrication defects were also considered. Spheres and parallelepiped defect shapes composed of PMMA, ammonium sulfate, and stainless steel were modeled at exposure gaps in the range 10-50 μm. Attenuation of a variety of potential defect materials was calculated for the IBM Advanced Lithography Facility Hellos synchrotron source and beam-line X-ray spectrum. The dose-to-clear for 400 and 500 nm thickness APEX-E films was then used to predict what thickness of defect material would result in a printed defect. Image formation model predictions of defect printability in APEX-E resist were compared to attenuation calculations, indicating that defect shape and X-ray phase shift in the defect material has a profound impact on defect printability for materials that are not highly attenuating. Spheres printed more readily than parallelepipeds. Increasing the exposure gap reduced printability slightly. Experiments to determine the printability of organic spheres added to X-ray masks were compared to simulation to verify its accuracy. Based on modeling results, the minimum size of isolated defects on X-ray masks that printed are presented. The minimum size of defects that changed printed line-width were also discussed. Based on these results, defect inspection sensitivity, cleaning capability, and repair resolution for <175 nm line-width X-ray masks can be established.