Fabrication of Aspherical Mirrors for Extreme Ultra-Violet Lithography (EUVL) Using Deposition Techniques

Abstract

Mirrors for the imaging optics of extreme ultra-violet lithography (EUVL) systems require highly precise figure processing. We have examined a method of fabricating the aspherical surfaces by deposition. A film of graded thickness is deposited on a spherical substrate using an RF-plasma-enhanced magnetron sputtering system with a deposition mask. The distribution in thickness of the deposited film is close to the designed value, and the method was shown to be feasible for figuring aspherical surfaces. The deposition films for the method are required to have little surface roughness and low residual stress. We have compared the roughnesses and residual stresses in Mo single layer films and Mo/Si multilayer films used in mirrors for soft X-rays. Both values for Mo/Si multilayers were about one order of magnitude smaller than those for Mo single layer films. The roughness of Mo/Si multilayer films almost satisfies the required value for EUVL optics; however, further study is necessary to diminish the residual stress.