Fabrication of astronomical x-ray reflection gratings using thermally activated selective topography equilibration

Abstract

Thermally activated selective topography equilibration (TASTE) enables the creation of 3D structures in resist using grayscale electron-beam lithography followed by a thermal treatment to induce a selective polymer reflow. A blazed grating topography can be created by reflowing repeating staircase patterns in resist into wedge-like structures. Motivated by astronomical applications, such patterns with periodicities 840 nm and 400 nm have been fabricated in 130 nm-thick PMMA using TASTE to provide a base for X-ray reflection gratings. A path forward to integrate this alternative blazing technique into grating fabrication recipes is discussed.

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

“…GEBL processing for fabrication of the grating prototype surface relief mold is outlined in Figure 4: the staircase topography features two electron-exposed steps, a cleared area and an unexposed step, all of equal width consistent with a periodicity of d = 400 nm (i.e., w 0 = w 1 = w 2 = w 3 = 100 nm). Electron dosing for GEBL was performed according to the resist contrast curve provided by McCoy et al (2018), which is based on a room-temperature development recipe consisting of 2 minutes in a 1:1 mixture of methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA) followed by a 30-second rinse in IPA and a high-purity nitrogen blow-dry. This contrast curve is shown in Figure 5, where post-development PMMA thickness as measured by spectroscopic ellipsometry is plotted as a function of electron dose, D. These data were processed using the three-dimensional proximity effect correction (3DPEC) algorithm included in the Layout Beamer software package developed by GenISys GmbH 6 (Unal et al 2010) to generate a dose-corrected layout appropriate for achieving exposed staircase steps with h 1 ≈ 0.66h 0 and h 2 ≈ 0.33h 0 , where h 0 ≈ 130 nm is the spin-coat thickness.…”

“…Using the GEBL process outlined above, test patterns were exposed, developed and characterized by atomic force microscopy (AFM) to verify that the previously-reported staircase topography could be readily reproduced using the increased value for beam current enabled by a clock frequency upgrade to the EBPG5200. In an identical fashion to McCoy et al (2018), all AFM was carried out at the PSU Materials Characterization Laboratory 7 with a Bruker Icon instrument equipped with a SCANASYST-AIR tip over 2 µm in the direction of grating periodicity at 512 samples per line to yield a 3.9 nm pixel size using Bruker's PeakForce Tapping TM mode. A scan of the GEBL pattern exposed using an 8 nA beam current and a 400 µm aperture is shown in the top panel of Figure 6, where it is verified that the topography appears virtually indistinguishable from the previous result obtained using a 1 nA beam current and a 200 µm aperture.…”

“…An alternative to these methods of grating manufacture is thermally activated selective topography equilibration (TASTE), which combines grayscale electronbeam lithography (GEBL) and polymer thermal reflow to produce smooth, 3D surface relief profiles in poly(methyl methacrylate) (PMMA) or other thermoplastic resists such as ZEP520A and mr-PosEBR (Schleunitz et al 2014;Kirchner et al 2016;Pfirrmann et al 2016). Through optimization of TASTE, repeating staircase patterns in PMMA fabricated by GEBL can be equilibrated into wedgelike structures by selective thermal reflow to provide a template for a blazed grating with groove spacing on the order of hundreds of nanometers (McCoy et al 2018). With no dependence on the crystallographic structure of the substrate, TASTE has the potential for realizing reflection gratings that feature both a blazed surface topography and a non-parallel groove layout, thereby enabling high sensitivity and high λ/∆λ in an EUV/SXR spectrometer.…”

Limiting Spectral Resolution of a Reflection Grating Made via Electron-Beam Lithography

“…However, recent advances in EBL patterning have permitted the production of a VLS grating with a nominal ∼160 nm period over a 75 cm 2 area (Miles et al (2018)). EBL has also been used to write gratings with sculpted, triangular groove profiles directly using greyscale lithography and thermal reflow (McCoy et al (2018)). These direct-write blazed gratings have achieved high-diffraction efficiency in the X-ray when used in an echelle mounting (McCoy et al (2020); 2019)).…”

Single-digit 6-nm multilevel patterns by electron beam grayscale lithography

“…High-precision and high-resolution diffractive optics are important for large divergence optics, beam shape control and aerial image formation. Applications are, for example, data transmission and barcode scanners [2], optical tweezers [17], optical lithography in the UV [18] and EUV range [19], hybrid optics [2,[20][21][22], nanofluidics [14], hierarchical surfaces [23], astronomical gratings [24] and spectrometers [25]. Grayscale electron beam lithography has been applied across scales from sub-10-μm [26] down to sub-100-nm feature height and width before [24] ( Figure 1).…”