Development of ultra-high efficiency multilayer-coated blazed diffraction gratings for EUV and soft x-rays applications

 

We report on recent progress achieved at the Advanced Light Source (ALS) on development, fabrication, and characterization of ultra-high efficiency multilayer-coated blazed gratings (MBG) suitable for EUV and soft x-ray applications. As diffraction elements, MBGs have several important advantages compared with sinusoidal or laminar gratings. First, with blazed gratings most of the incoming radiation m ay potentially be diffracted into one specific, desired, diffracted order. Second, use of high efficiency multilayer coatings potentially allows extending the advantages of normal incidence grating spectrometer designs to EUV and soft x-ray wavelengths. In order to realize this potential, the MBG must have a nano-period structure with an ideal saw-tooth groove profile, coherently replicated over the entire structure and ensured to have an extremely low micro-roughness of the substrate’s and multilayer’s interfacial surfaces.

Using e-beam and interference lithography techniques followed by wet anisotropic etching of silicon [1], we fabricated saw-tooth substrates with groove densities up to 10,000 lines/mm having near-atomically plane groove facets. The multilayers are deposited on the substrates by magnetron sputtering in Argon. With cross-sectional transmission-electron microscopy (TEM), we show that careful optimization of the deposition parameters [2] allows us to avoid significant smoothing of the groove profile and increase of the surface roughness. This observation is in excellent agreement with the results of diffraction measurements. An absolute efficiency of 44% was achieved for the first blazed order of a 5250 lines/mm MBG (consisting of 30 Mo/Si bilayers) at a wavelength of 13.1 nm. Simulations of diffraction efficiency of soft x-ray MBGs, based on the integral method for solving Maxwell’s equations for a periodic structure [3], show that the efficiency of an optimized grating relative to the reflectance of the multilayer can approach 100%.

We also discuss a route to achieving high diffraction efficiency in high order at wavelengths throughout the soft x-ray energy range with revolutionary applications in synchrotron science.

The authors are grateful to R. M. Feshchenko, V. V. Kondratenko, Y. P. Pershin, A. G. Ponomarenko, and A. V. Vinogradov for very useful discussions. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, Material Science Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 at Lawrence Berkeley National Laboratory.

 

1. D. L. Voronov, E. H. Anderson, R. Cambie, S. Dhuey, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H. A. Padmore, Nucl. Instr. and Meth. (2011) A649(1), 156.

2. D. L. Voronov, E. H. Anderson, E. M. Gullikson, F. Salmassi, T. Warwick, V. V. Yashchuk, and H.A. Padmore, Opt. Lett. (2012) (in press).

3. PCGrate® software (http://iigrate.com/about/pcgrates/sxseries).

© 2012 Ioffe Physical -Technical Institute of the Russian Academy of Sciences.