Fabrication and Diffraction Efficiency of a Large-format, Replicated X-Ray Reflection Grating

Abstract

We present the methodology used to fabricate an X-ray reflection grating and describe a technique for grating replication. Further, we present the experimental procedure and results of a study to measure the diffraction efficiency of a replicated X-ray reflection grating in an extreme off-plane geometry. The blazed grating demonstrates a total diffraction efficiency of ∼60% from 0.34 to 1.2 keV at a grazing angle of ∼1.°5, with single-order efficiency ranging from ∼35% to 65% for energies within the blaze envelope. The diffraction efficiency of the grating measured relative to the reflectivity of the metal coating averages ∼90% above 0.34 keV. Data collected as a function of beam position on the grating indicate a relative variation in total efficiency of <1% rms across the grating surface.

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

“…Following the methodology outlined in Section 2.1 and detailed by Miles et al (2018), the grating prototype was tested for EUV and SXR diffraction efficiency at beamline 6.3.2 of the ALS. Figure 11 shows the gold-coated grating prototype installed inside the beamline test chamber in an extreme off-plane mount, where the dispersion direction, x, is roughly parallel with the direction of horizontal stage motion for the photodiode detector, which is seen masked with a 0.5 mm-wide vertical slit.…”

“…An especially important feature of the TASTE process is its ability to define a sawtooth-like topography over a layout defined by electron-beam lithography while also avoiding the dependences on crystallographic structure that exist in processes that use anisotropic wet etching to provide a grating blaze (Franke et al 1997;Chang 2003;McEntaffer et al 2013;Miles et al 2018). This is particularly advantageous for realizing fanned, curved or other variable-linespace groove layouts that are required for achieving high spectral resolving power, λ/∆λ, while also having blazed groove facets that enable high spectral sensitivity.…”

“…At a distance L away from the point of incidence on the grating, diffracted orders are each separated by a distance λL/d along the direction of grating periodicity (i.e., the dispersion direction), where d is the groove spacing. Figure can be determined experimentally following the test procedures outlined by Miles et al (2018), where the photodiode detector mounted on vertical goniometric and horizontal linear staging at a distance L ≈ 235 mm away from the point of incidence on the grating is used to measure I n (λ) for each propagating order and I inc (λ). The grating prototype described in this paper was designed specifically for taking diffraction efficiency measurements at this beamline in a grazing-incidence, extreme off-plane mount where the incident radiation is nearly parallel to the groove direction and propagating orders are confined to the surface of a cone with a small opening angle (Cash 1991).…”

“…where, as illustrated in the left panel of Figure 1, γ is the half-opening angle of the cone, α is the azimuthal incidence angle and β is the azimuthal diffracted angle of the n th diffracted order. The testing methodology adopted from Miles et al (2018) relies on the radius of the diffracted arc, given by being smaller than the 10 mm by 10 mm collecting area of the photodiode detector used at the beamline. With propagating orders being dispersed a distance from 0 th order along the cross-groove direction given by…”

“…The state-of-the-art for blazed gratings that perform with high diffraction efficiency at EUV and SXR wavelengths are those fabricated by wet anisotropic etching in mono-crystalline silicon, where typically either interference lithography (Franke et al 1997;Chang 2003) or electron-beam lithography (Voronov et al 2011;Miles et al 2018) is used to define a groove layout in resist before the pattern is transferred into the underlying silicon crystal structure to produce atomically-smooth sawtooth facets. However, interference lithography faces severe limitations in its ability to pattern non-parallel layouts and even with the direct-write capabilities of electron-beam lithography, the cubic structure of mono-crystalline silicon prevents the formation of fanned or curved grooves with smooth and continuous triangular facets.…”