Refractive index measurements with improved accuracy for EUV/x-ray multilayer optics
Résumé
EUV/x-ray multilayer optics with complex structures and functionalities have become key components for various applications including photolithography, plasma physics, synchrotron, laser and ultrafast science, solar physics and astrophysics. Yet, the available refractive index values required to design and fabricate these optics, are often unreliable [1]. This is due to challenges inherent to the EUV/x-ray spectral region, such as the extreme sensitivity of materials to contamination and oxidation, the difficulty in fabricating appropriate thin film samples, the presence of near-edge absorption fine structure, and the presence of multiple reflections at the longer EUV wavelengths, which are complicating the measurements.
We are presenting a new methodology to measure the EUV refractive index and new sets of measurements for several important EUV/x-ray materials including Al, Cr, W, Pt [2-4].
We use combinations of transmittance and reflectance data in the spectral range 1.5 nm to 82.5 nm and demonstrate for the first time highly resolved fine structure in the regions of L, M, N and O absorption edges, in both the absorptive and dispersive portions of the refractive index, resulting in improvements of up to a factor of 3 compared to earlier values. The improved refractive index accuracy is validated by fitting experimental reflectance and transmittance data of multilayer coatings containing these materials. The development of new and improved measurement methodologies and the resulting new refractive index datasets will enable the accurate design, implementation and modeling of the in-band and out-of-band performance of the next generation of EUV/x-ray optics.