Sub-angstrom critical dimension metrology with EUV scatterometry (Invited Paper)

Critical dimensions (CDs) on EUV photomasks are rapidly shrinking to print higher resolution features, enhance contrast with sub-resolution assist features, and control the stitching black border with sub-resolution gratings. Reducing mask CDs to 10s of nm necessitates single-nm CD process control and CD metrology with single-angstrom accuracy or better. Furthermore, the ideal probe for an EUV photomask is EUV radiation, due to its short wavelength and sensitivity to any and all optical effects that could potentially impact the EUV aerial image. In this work we present actinic CD metrology using EUV scatterometry performed on the EUV Tech ENK tool, capable of delivering sub-nm or even sub-angstrom CD accuracy. Furthermore, we explore extending the technique to measure more complicated dimensions such as the sidewall angle (SWA) of the absorber. We present experimental demonstrations on the ENK tool of CD and SWA metrology on EUV photomasks.