Black border, mask 3D effects: covering challenges of EUV mask architecture for 22nm node and beyond

Photomask is at the heart of a lithographic scanner’s optical path. It cannot be left non-optimized from the imaging point of view. In this work we provide new insights on two critical aspects of EUV mask architecture: optimization of absorber for 16 nm half-pitch imaging and a systematic approach to black border EUV and DUV reflectance specifications. Good 16 nm imaging is demonstrated on ASML NXE:3300 EUV scanner. Currently a relatively high dose resist is used for imaging and the dose reduction is desired. Optimization (reduction) of absorber height and mask CD bias can allow for up to 30% dose reduction without essential contrast loss. Disadvantages of absorber height reduction are ~7 nm increase of best focus range through pitch and tighter absorber height mean to target and uniformity requirements. A disadvantage of a smaller reticle CD (down to 14 nm 1x) is manufacturing process uniformity over the reticle. A systematic approach of black border reflections impact on imaging is established. The image border is a pattern free dark area surrounding the image field and preventing exposure of the image field neighborhood on wafer. Currently accepted design of the black border on EUV reticle is an image border where the absorber and multilayer stack are etched down to the substrate and EUV reflectance is reduced to <0.05%. DUV reflectance of such a black border is about 5%. It is shown that a tighter DUV reflectance specification <1.5% is required driven by the impact of DUV reflections from the black border on imaging. NXE:3300 and NXE:3100 experimental imaging results are shown. The need of low DUV wavelength reflectance metrology (in the range 100-300 nm) is demonstrated using an estimated NXE scanner out-of-band DUV spectrum. Promising results of low DUV reflectance of the black border are shown.