Nanofabrication of a variable-line-spaced, blazed master grating for the off-plane grating rocket experiment pathfinder mission

Development of variable-line-spaced, blazed diffraction gratings is needed to achieve the spectral resolving power and effective area performance of next-generation x-ray and UV spectrographs in the coming decade(s). The nanofabrication development efforts in producing the master grating for the flight of the suborbital rocket payload, the Pathfinder Off-plane Grating Rocket Experiment (OGRE-P), will be discussed here. The OGRE-P master grating is radially grooved to match the converging x-ray beam from the Wolter-I type optic, with an average groove period of ≈315 nm in order to achieve high resolving power (R⪆1500). The replica gratings made from the master will be blazed at ≈ 28° to increase diffraction efficiency in key science line orders, which is crucial for a suborbital rocket flight due to the short (≈150 second) observation time. The master grating will be replicated by SCIL Nanoimprint Solutions using substrate conformal imprint lithography in order to produce the ≈150 flight gratings used in the OGRE-P spectrograph. Nanofabrication processing of the master grating was performed at Penn State University's Materials Research Institute Nanofabrication Lab using a RAITH 5200 EBPG+ electron-beam lithography (EBL) tool for thermally activated selective topographical equilibration (TASTE) to create a blazed groove profile utilizing greyscale lithography. Efforts in optimizing the greyscale EBL process to produce a high-fidelity radial groove pattern over the ≈ 5000mm² grating area as well as the blazed groove profile will be discussed. The OGRE-P gratings will serve as an important step towards achieving the demanding performance requirements of future orbital spectrographs.