Performance and quality analysis of Mo–Si multilayers formed by ion-beam and magnetron sputtering for extreme ultraviolet lithography

Mo–Si multilayer structures were grown by ion-beam and magnetron sputtering to make high-performance mask blanks for practical use in extreme-ultraviolet (EUV) lithography. For ion-beam sputtering, the effect of using Ar or Xe as the sputtering gas, and the impact of the acceleration voltage of Ar or Xe ions on the EUV reflectivity of multilayers were evaluated. In the wavelength range of 12.5–14.5 nm, the peak EUV reflectivity was 60–63% for 40 Mo–Si bilayers grown by ion-beam sputtering, and 62–65% for those grown by magnetron sputtering. Transmission electron microscopy images of Mo–Si multilayers revealed interface layers between the Mo and Si layers. They were found to be composed of a mixture of Mo and Si and to be formed during sputtering. They had a thickness of 1.5–2.0 nm for the deposition sequence Mo-on-Si, and 0.5–1.0 nm for Si-on-Mo. In addition, they were 20–30% thicker for ion-beam sputtering than for magnetron sputtering. Calculations of the EUV reflectivity spectrum for 40 Mo–Si bilayers indicate that interface layer thickness plays a crucial role in determining the EUV performance of multilayers. Finally, an ion-implantation model was found to provide a better explanation of the mechanism of interface layer formation than a thermal-interdiffusion model.