Surface finish requirements for soft x-ray mirrors.

Area of Science:

• Materials Science
• Optics
• Metrology

Background:

• X-ray multilayer mirrors are critical for various applications requiring high reflectance.
• Surface finish directly impacts mirror performance, especially at normal incidence.
• Characterizing nanoscale surface features is essential for optimizing mirror fabrication.

Purpose of the Study:

• To correlate direct surface-finish metrology with soft x-ray reflectance measurements.
• To establish critical surface roughness parameters for high-reflectance x-ray mirrors.
• To evaluate the suitability of different metrology techniques for x-ray mirror characterization.

Main Methods:

• Comparison of scanning-probe metrology with optical and stylus profilometry.
• Analysis of surface-finish Fourier spectra of highly polished x-ray mirrors.
• Measurement of normal-incidence, soft x-ray reflectance.

Main Results:

• High reflectance requires root-mean-square (rms) surface roughness below ~1 Å.
• This roughness criterion applies to spatial frequencies from 1 to 100 µm⁻¹ (1 µm to 10 nm wavelengths).
• Surface-finish Fourier spectra of these mirrors follow an inverse power law.

Conclusions:

• Scanning-probe metrology is essential for characterizing the relevant spatial frequencies.
• Bandwidth-limited roughness measurements from lower-spatial-frequency instruments are uncorrelated with soft x-ray reflectance.
• Inaccurate metrology can lead to erroneous conclusions about x-ray mirror performance.