Evaluation of lithographic performance of extreme ultra violet mask using coherent scattering microscope

Abstract

The authors report the application of a coherent scattering microscope (CSM) equipped with a femtosecond laser based extreme ultra violet (EUV) source to analyze aerial images for an EUV lithography mask. The CSM employs an EUV beam with a 13.5 nm wavelength and an incident angle of 6 degrees for the wafer scanner emulation. The CSM consists of a condenser optics that delivers the EUV beam to the mask and detector that measures the field spectrum diffracted by the mask pattern. Using a pair of spherical and plane EUV mirrors, coherent EUV light is relayed to the mask and the light diffracted by the mask pattern is measured by a 1 in. size charge-coupled device detector. The integration time of the EUV beam in the detector is controlled by a high speed shutter. This device experimentally records the coherent diffraction pattern from the EUV mask and reconstructs its aerial image using a phase retrieval algorithm. These features made it possible to investigate the projected image properties, such as depth-of-focus, exposure latitude, mask error enhancement factor, horizontal-vertical critical dimension (H-V CD) bias, normalized image log slope (NILS), and so on. The device also provides a mask CD from a rigorous coupled-wave analysis algorithm modeling based on the Maxwell equation through measured diffraction patterns. NILS according to half-pitch design node was measured by CSM and simulated using a Solid-EUV simulator. Illumination conditions of NA and sigma (sigma) values are 0.25 and 0.8, respectively, which are the same as those of the EUV exposure tool. The NILS measured by CSM and that simulated using the Solid-EUV have good consistency.