Analysis of Thermally Induced Distortion of Optics for Soft X-ray Projection Lithography

The development of lens systems for projection x-ray lithography presents significant challenges associated with the fabrication and testing of ultra precise optical surfaces. Once assembled, these projection lenses must further be dimensionally stable to tolerances determined by the wavelength of the soft x-rays used for illumination, typically between 100Å and 300Å. Lens systems capable of printing over large areas will contain a number of mirrors with reflectivities in the range of 60±10%. For these systems, the first element will be subjected to a significant incident x-ray flux, of which ~40% will be absorbed. This absorbed power causes heating which in turn will cause a distortion of the optical surface. The intent of this study has been to examine the magnitude of these distortions under a variety of conditions.