New techniques toward the nanofabrication of custom, blazed UV gratings to enable next-generation spectroscopy

The report from the most recent decadal survey in astronomy and astrophysics, Pathways to Discovery in Astronomy and Astrophysics for the 2020s (ASTRO2020), had two top recommendations for the future of NASA astrophysics: the Habitable Worlds Observatory (HWO) as the next flagship mission, and the Great Observatories Maturation Program (GOMAP) to develop the technology and mission architecture necessary to realize a successful HWO. As part of GOMAP, several key technologies must be further developed in order to enable the high-performance instruments for next-generation observatories. One such technology, as outlined in NASA's Astrophysics Strategic Technology Gap (ASTG) program and discussed in the instrument design for the LUVOIR Ultraviolet Multi-Object Spectrograph (LUMOS), is high-performance UV diffraction gratings. Historically, UV gratings used in astronomical instruments have been fabricated through either mechanical ruing or holography. Both techniques suffer from inherent fabrication limitations that make it difficult to achieve the high-efficiency blazed gratings that will be necessary to enable a high-resolution UV spectrograph on the HWO. Further, each process offers limited customizability in terms of variable line spacing and aberration control in the groove pattern, resulting in little flexibility in grating design and performance for UV spectrographs. In response to NASA's ASTG program, there has been substantial evolution in recent years in UV gratings fabricated via electron-beam lithography (EBL). EBL and associated nanofabrication techniques allow for customizable groove layouts, densities, and facet angles, all of which are defined prior to fabrication. The state of the art in blazed EBL gratings for UV applications relies on KOH etching to produce sharp, angled facets. While capable of high diffraction efficiency due to the atomic-layer etching achievable with KOH exposure, gratings fabricated in this way are only applicable for groove layouts with parallel grooves or a very small amount of groove curvature due to the reliance on the plane structure of monocrystalline silicon. To realize the performance needed for complex, aberration-correcting grove profiles such as those needed for the HWO, alternative techniques must be developed to enable blazed groove facets that are independent of the substrate structure. In this study we will investigate two such techniques, EBL paired with ion-beam etching (EBL+IBE) to shape the groove facets, and thermally-activated selective topography equilibration (TASTE). Both approaches introduce new processes to replace KOH etching and achieve high diffraction efficiency while retaining the high spectral resolution achievable with EBL exposures. We take a process-oriented approach to develop repeatable fabrication processes for EBL+IBE and TASTE. Our development will be motivated by three specific grating requirements from the ASTG program and LUMOS and will include process optimization and small-scale grating demonstrations. The conclusion of the program will include several large-scale samples that are performance tested, advancing the TRL of these approaches to TRL5. Our study will produce proven techniques for blazing EBL gratings over a wide range of blaze angles and groove densities, paving the way for fully customizable grating designs for the HWO and other future UV observatories.

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