EUV exposure experiment using programmed multilayer defects for refining printability simulation

Availability of defect-free masks is one of the most critical issues for enabling EUV lithography. Among others, multilayer phase defects embedded in EUV blanks are primary concern because multilayer defects as low as 2nm are expected to be printable. Therefore, thorough understanding of defect printability through experiments is anticipated to derive critical sizes of multilayer defects. A test mask with programmed multilayer defects has been fabricated for this purpose with various sizes of defects as well as various protrusion sizes of defects from the absorber lines. As the first step toward rigorous printability prediction that can comprehend any arbitrary-shaped multilayer defects, line defects parallel to absorber lines have been selected for printability experiments using micro exposure tool (MET) in Lawrence Berkeley National Lab. for direct comparison with 2D simulation. The simulation of electro-magnetic field with multilayer defects has been carried out using finite-element-method with triangular cells that well match the needs for incorporation of minute change in multilayer profile as observed in a TEM photograph. The experiments have proved that there is systematic correlation between the sizes of defect protrusion from beneath the absorber line with printed line CD. The defect with protrusion size of 33nm on mask, however, did not show any evidence of defects while aerial image simulation indicated there should be detectable difference in print results. Root cause investigation for this gap indicated that the multilayer bump affected the absorber width and profile above and hence generated the gap between experiments and simulation that assumed identical absorbers. Simulation with exact absorber shape input well matched experimental results.