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Completed TRL 2 (started at 2, targeting 3)
New fabrication techniques are enabling both thinner and more precise features for reflective optics in space telescopes. While this may increase the collecting area and is beneficial from a mass and cost reduction standpoint, intrinsic stresses due to processing and coating reflective thin films are amplified on thinner substrates, resulting in mirror deformation. These stresses have negative impacts on all types of optics, though this proposal will primarily explore the effects of intrinsic stress on X-ray reflection gratings. Many of NASA’s future science and astronomy objectives, including characterizing black holes, galaxy clusters, and supernova remnants, will be made possible by improving the X-ray spectroscopy instrumentation and high-efficiency and high-resolution X-ray optics have been rated as Tier 1 Technology Gaps by the NASA Physics of the Cosmos (PCOS) program. To achieve the rigorous requirements of future missions, such as Lynx, an off-plane X-ray grating has been designed to be blazed in profile and radially-ruled to achieve both high diffraction efficiencies and spectral resolving power, respectively. However, evaluation of gratings fabricated under current conditions show intrinsic thin film stresses that cause degradation in X-ray grating performance, particularly in spectral resolving power. This proposal will implement a three-phase approach to understand and modify X-ray grating features and critical dimensions (CDs)by understanding thin film stresses and using stress-compensation techniques as an engineering tool to mitigate current fabrication and design challenges. Though excellent work has been done to optimize the X-ray grating fabrication process, the intrinsic stresses across X-ray gratings are still not well understood. Phase one of this study will focus on identifying stress contributions from individual film layers and fabrication steps and understanding how these stresses interact with each other to induce a global stress profile. Phase two will then focus on compensating for stresses through both passive (process-based) and active (post-processing) techniques. New fabrication methods will first be evaluated to identify stress-minimizing fabrication processes and conditions. Active stress compensation techniques will then be investigated by patterning the backside of the wafer using grayscale electron-beam lithography (GEBL) to locally relieve stresses. This will theoretically give high-precision control of intrinsic stress across a grating. Finally, phase three of this study will explore actively using techniques developed in phase two to direct intrinsic stresses as an engineering tool to modify grating features and critical dimensions. While fabrication of blazed groove facets or radially-ruled gratings have been successfully demonstrated, blazed radially-ruled gratings are currently approximated, thus limiting spectral resolution. Feature modification using directed stresses will, in theory, allow for grating pattern manipulation and will hopefully enable better blazed radially-ruled X-ray reflective gratings than current methods allow, thus increasing overall spectral resolution.
Feature modification using directed stresses will, in theory, allow for grating pattern manipulation and will hopefully enable better blazed radially-ruled X-ray reflective gratings than current methods allow, thus increasing overall spectral resolution.
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