Surface finish requirements for soft x-ray mirrors.

We have examined the correlations between direct surface-finish metrology techniques and normalincidence, soft x-ray reflectance measurements of highly polished x-ray multilayer mirrors. We find that, to maintain high reflectance, the rms surface roughness of these mirrors must be less than ~ 1 Å over the range of spatial frequencies extending approximately from 1 to 100 µm(-1)1 (i.e., spatial wavelengths from 1 µm to 10 nm). This range of spatial frequencies is accessible directly only through scanning-probe metrology. Because the surface-finish Fourier spectrum of such highly polished mirrors is described approximately by an inverse power law (unlike a conventional surface), bandwidth-limited rms roughness values measured with instruments that are sensitive to only lower spatial frequencies (i.e., optical or stylus profileres) are generally uncorrelated with the soft x-ray reflectance and can lead to erroneous conclusions regarding the expected performance of substrates for x-ray mirrors.

[1]  David L. Windt,et al.  Soft x-ray reflectometry of multilayer coatings using a laser-plasma source , 1992, Optics & Photonics.

[2]  Daniel G. Stearns,et al.  Stochastic model for thin film growth and erosion , 1993 .

[3]  J. Bennett,et al.  Relation between the angular dependence of scattering and the statistical properties of optical surfaces , 1979 .

[4]  J. B. Kortright,et al.  Substrate Effects on X-Ray Specular Reflectance and Non-Specular Scattering from X-Ray Multilayers, , 1992 .

[5]  Gregory Peter Kochanski,et al.  Scanning probe metrology , 1992, Advanced Lithography.

[6]  E. Church.  Fractal surface finish. , 1988, Applied optics.

[7]  Daniel G. Stearns,et al.  X‐ray scattering from interfacial roughness in multilayer structures , 1992 .

[8]  P. Hall.  Introduction to Surface Roughness and Scattering , 1989 .

[9]  J. E. Griffith,et al.  A ROCKING BEAM ELECTROSTATIC BALANCE FOR THE MEASUREMENT OF SMALL FORCES , 1991 .

[10]  M. Stedman.  Limits of topographic measurement by the scanning tunnelling and atomic force microscopes , 1988 .

[11]  Richard H. Stulen,et al.  Developing a soft X-ray projection lithography tool , 1991, AT&T Technical Journal.

[12]  D. E. Savage,et al.  Determination of roughness correlations in multilayer films for x‐ray mirrors , 1991 .

[13]  M. J. Vasile,et al.  Scanning probe metrology , 1992 .

[14]  M. J. Vasile,et al.  Scanning probe tips formed by focused ion beams , 1991 .

[15]  David L. Windt,et al.  Interface imperfections in metal/Si multilayers , 1992 .

[16]  J. E. Griffith,et al.  Dimensional metrology with scanning probe microscopes , 1993 .