Extreme Ultraviolet and Soft X-Ray Diffraction Efficiency of a Blazed Reflection Grating Fabricated by Thermally Activated Selective Topography Equilibration

Abstract

Future observatories utilizing reflection grating spectrometers for extreme ultraviolet (EUV) and soft x-ray (SXR) spectroscopy require high-fidelity gratings with both blazed groove facets and custom groove layouts that are often fanned or feature a slight curvature. While fabrication procedures centering on wet anisotropic etching in mono-crystalline silicon produce highly-efficient blazed gratings, the precision of a non-parallel groove layout is limited by the cubic structure of the silicon crystal. This motivates the pursuit of alternative techniques to grating manufacture, namely thermally activated selective topography equilibration (TASTE), which uses grayscale electron-beam lithography to pattern multi-leveled structures in resist followed by an optimized polymer thermal reflow to smooth the 3D patterns into continuous surface relief profiles. Using TASTE, a mold for a reflection grating with a periodicity of 400 nm and grooves resembling an asymmetric sawtooth was patterned in 130 nmthick poly(methyl methacrylate) resist on a silicon substrate over a 50 mm by 7.5 mm area. This structure was coated with 15 nm of gold by electron-beam physical vapor deposition using titanium as an adhesion layer and then tested for EUV and SXR diffraction efficiency at beamline 6.3.2 of the Advanced Light Source synchrotron facility. Results demonstrate a quasi-blaze response characteristic of a 27 • blaze angle with groove facets smooth to 1.5 nm RMS. Absolute peak order efficiency ranges from 75% to 25% while total relative efficiency measures 90% across the measured bandpass of 15.5 nm > λ > 1.55 nm.

X-ray verification of sol-gel resist shrinkage in substrate-conformal imprint lithography for a replicated blazed reflection grating

“…Previous test campaigns have demonstrated that reflection gratings operated in an extreme off-plane mount can be measured for soft x-ray diffraction efficiency using a beamline facility suitable for short-wavelength reflectometry [2,6,24,25]. The experiments described here took place at beamline 6.3.2 of the ALS, which provides a highly-coherent beam of radiation tunable over extreme UV and soft x-ray wavelengths that strikes a stage-mounted optic [19][20][21].…”

“…This layer was sputter-coated on the replica in an identical fashion to Miles, et al [2]: 5 nm of chromium was deposited for adhesion followed immediately by 15 of gold, without breaking vacuum. Because this thickness is several times larger than the 1/e penetration depth in gold at grazing-incidence angles, it is justified to treat this top film as a thick slab in this context [6,26].…”

“…1. Geometry for a reflection grating producing a conical diffraction pattern [2,6]. In an extreme off-plane mount, the incoming radiation is nearly parallel to the groove direction with 2 • while is free to match the blaze angle, , in a Littrow configuration with = = .…”

On obtaining high spectral resolution in extreme ultraviolet/soft X-ray monochromators operating off-plane diffraction in a divergent incident beam

“…Cash, 1991;McEntaffer, 2019). Recently, using state-of-the-art production technology to produce master structures of high quality for the grating replication process, very promising performance was experimentally verified at prototype structures by Miles et al (2018) and by McCoy et al (2020) for the soft X-ray range. The measurements concerned photon energies from 200 eV up to 1200 eV.…”

“…Unlike at a synchrotron, in space science one finds the sources at infinity, and then as shown by Cash (1983) the removal of the principal aberration in the diffraction process can be achieved in the grating structure by linearly varying the groove separation along the beam footprint in so-called radial groove gratings. The latest study by McCoy et al (2020) considers the production of such radial groove gratings. Instead Miles et al (2018) concentrated on the use of wet anisotropic etching into monocrystalline silicon.…”