Thin absorber EUV photomask based on mixed Ni and TaN material

Abstract

Lithographic patterning at the 7 and 5 nm nodes will likely require EUV (λ=13.5 nm) lithography for many of the critical levels. All optical elements in an EUV scanner are reflective which requires the EUV photomask to be illuminated at an angle to its normal. Current scanners have an incidence of 6 degree, but future designs will be >6 degrees for high-NA systems. Non-telecentricity has been shown to cause H-V bias due to shadowing, pattern shift through focus, and image contrast lost due to apodization by the reflective mask coating. A thinner EUV absorber can dramatically reduce these issues. Ni offers better EUV absorption than Ta-based materials, which hold promise as a thinner absorber candidate. Unfortunately, the challenge of etching Ni has prevented its adoption into manufacturing. We propose a new absorber material that infuses Ni nanoparticles into the TaN host medium, allowing for the use of established Ta etching chemistry. A thinner is absorber is created due to the enhanced absorption properties of the Ni-Ta nano-composite material. Finite integral method and effective medium theory-based transfer matrix method have been independently developed to analyze the performance of the nano-composite absorption layer. We show that inserting 15% volume fraction Ni nanoparticles into 40-nm of TaN absorber material can reduce the reflection below 2% over the EUV range. Numerical simulations confirm that the reduced reflectivity is due to the increased absorption of Ni, while scattering only contributes to approximately 0.2% of the reduction in reflectivity.