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157-nm attenuated phase-shift mask materials with irradiation stability

The next suite of optical lithography tools beyond 193nm will use 157nm irradiation to illuminate the mask pattern onto a semiconductor wafer. As the illumination wavelength decreases, the number of materials that can be used to create attenuated phase shift masks decreases dramatically. Especially the number of materials that maintain constant transmission after prolonged irradiation. The Ta-based and Cr-based materials have been recognized as two such sets of materials that remain optically unchanged due to prolonged VUV irradiation. Optical characterization of these materials by spectroscopic ellipsometry has been used to simulate several material systems to achieve proper transmission and phase shift while simultaneously improving the inspection contrast of the patterned mask. Both simulation and experimental results will be presented for Ta-based and/or Cr-based material systems that maintain relatively constant transmission for more than 50 million pulses under 157nm irradiation.