Stress mitigation in Mo/Si multilayers for EUV lithography

Although Mo/Si multilayers are now widely used in EUV lithography development programs, multilayer induced substrate stress continues to be a major issue. Standard stress values of -350 to -450 MPa, reported for Mo/Si systems produced by magnetron sputtering, induced an intolerable deformation of the surface figure of EUV optical components. Stress in e-beam deposited Mo/Si multilayers has not been reported before. At FOM Rijnhuizen, an extensive stress mitigation program has been carried out on multilayers produced by e-beam deposition and medium energy ion polishing. The stress in our standard, high reflectance Mo/Si multilayers is less than -200 MPa. Although e-beam deposition apparently halves the typical stress values obtained by sputter deposition, it is still above the allowable limit for the first lithographic system, the so-called Alpha Tool. To further reduce stress, the influence of the Mo fraction, the number of periods and the multilayer period or d-spacing has been investigated. Varying the Molybdenum fraction in e-beam deposited multilayers results in a similar dependency as reported for magnetron sputtered coatings, though at strongly reduced absolute values. Furthermore, variation of the d-spacing has a small influence on stress. The number of periods however, has no influence on the stress value in the range from 20 to 50 periods. Applying stress mitigation techniques based on adjustment of the Mo fraction, a high reflectance of above 69% at near normal incidence at 13.5 nm has been obtained for multilayers with a stress value of only -33 MPa. This has been achieved by using Mo and Si only. This stress value is sufficiently low to enable the first generations of EUVL optics.