Ultra-high efficiency multilayer blazed gratings through deposition kinetic control

Abstract

Diffraction efficiency of multilayer coated blazed gratings (MBG) strongly depends on the perfection of the saw-toothshaped layers in the overall composite structure. Growth of multilayers on saw-tooth substrates should be carefully optimized in order to reduce groove profile distortion and at the same time to avoid significant roughening of multilayer interfaces. In this work we report on a new way to optimize growth of sputter-deposited Mo/Si multilayers on saw-tooth substrates through variation of the sputtering gas pressure. A new record for diffraction efficiency of 44% was achieved for a optimized MBG with groove density of 5250 lines/mm at the wavelength of 13.1 nm. © 2010 Optical Society of America OSIS codes: 050.1950, 120.6660, 340.7480, 230.4170, 310.1860 High efficiency diffraction gratings with high groove density are of great importance for a wide range of extreme ultraviolet (EUV) and soft x-ray applications including EUV lithography [1], astrophysics and solar science [2], and high-resolution Resonance Inelastic X-ray Scattering [3]. Multilayer-coated Blazed Gratings (MBG) can offer vastly improved performance over conventional single layer-coated gratings provided that the grating fabrication process can ensure coherent addition of each layer in the multilayer stack. The main technological challenges are fabrication of nano-period saw-tooth substrates with a perfect groove profile and an atomically smooth surface of blazed facets, and deposition of a multilayer (ML) on the highly corrugated surface of the substrates. Recent progress in fabrication of saw-tooth gratings with anisotropic etching of silicon single crystals [4,5] provided the necessary quality of the substrates [6]. The main remaining problem however is coherent replication of this structure from the substrate throughout the whole multilayer stack.Simulation of the diffraction efficiency shows that smoothening of the saw-tooth substrate by a ML deposited on it is a main limiting factor for MBG performance. Traditional deposition techniques optimized for growth of the MLs on flat substrates aim to provide interfaces of a ML stack as smooth as possible via promotion of surface relaxation processes. However, this approach does not work well for the highly corrugated surface of saw-tooth substrates, because a triangular shaped groove suffers greatly from excessive diffusion mobility of deposited atoms, caused in particular by bombardment of growing surface with energetic particles. The mobility provides effective smoothening of random roughness of interfaces, but in the same manner affects the high-frequency harmonics of a Fourier spectrum of a saw-tooth surface, causing degradation of the initial groove profile in the course the ML deposition [7,8]. As a result of this, a MBG loses its blazing ability and diffraction efficiency can be substantially reduced compared to the theoretical maximum.To avoid groove degradation one should reduce the adatom mobility. However, too low a surface mobility would result in rough interfaces and incre...

Multilayer-coated blazed grating with variable line spacing and a variable blaze angle

“…how fast the efficiency decays with deviation from the ideal blazing condition. Our experimental experience in MBG fabrication [1][2][3] indicates that a modest deviation from (1) is not harmful and in many cases can be tolerated. Another question raised by Underwood et al, [7] is how refraction of x-rays in multilayer media affects the blazing condition.…”

“…Recent progress in the technology of multilayercoated blazed gratings (MBG) [1][2][3] opens important opportunities for the development of a new generation of spectrometers for EUV and soft x-rays [4,5]. The gratings have superior efficiency characteristics over lamellar gratings due to their blazing ability, which is provided by the slanted facets of the sawtooth grooves.…”

Nonlinear continuum growth model of multiscale reliefs as applied to rigorous analysis of multilayer short-wave scattering intensity. I. Gratings

“…A comparison of calculated with measured reflection coefficients of a multilayer mirror (witness) revealed that the parameters À Zr = 0.4 and Á = 10.43 nm, incorporated in a model of multilayer Al/Zr grating coating, correlate well with the growth values and that the average interface roughness can be defined by ' 0.9 nm ( 0 ' 0.4 nm, 0 = 10 nm) (Voronov, Gawlitza et al, 2012;Voronov, Anderson et al, 2012). The data obtained in this comparison argue for $90% TE-polarized incident radiation (intensity).…”

“…Diffraction gratings on blazed-profile Si substrates are fabricated by interference or electron lithography and selective etching in KOH of Si plates cut at an angle ' (Voronov et al, 2010;Voronov, Anderson, Cambie, Cabrini et al, 2011;Voronov, Gawlitza et al, 2012;Voronov, Anderson et al, 2012). While fabrication of gratings of a large size with d 200 nm for the short-wave range shows considerable promise, realization of the available design potential requires that the relevant technology produces a grating profile close to the ideal triangular geometry and coatings made up of tens or hundreds of atomic layers with subatomically smooth boundaries.…”

Spectral tailoring of nanoscale EUV and soft x-ray multilayer optics

“…[116][117][118] Voronov et al have made significant development in high groove density (small period) MBGs. [119][120][121][122][123] A record of 52% diffraction efficiency at k ¼ 13.4 nm was achieved using the 2nd order of a Mo/Si coated MBG with a groove density of 2525 lines/mm ( Figure 9). 121 A 10 000 line/mm MBG with 13.2% efficiency at normal incidence was demonstrated at k ¼ 19.2 nm.…”