Ultra-low Reflectivity Black Silicon Pupil Masks
Manufacturing
Ultra-low Reflectivity Black Silicon Pupil Masks (GSC-TOPS-370)
Enhancing coronagraph performance with cryogenically etched black silicon
Overview
NASA’s direct imaging missions for exoplanet discovery rely on coronagraph masks to reduce light scattering and diffraction. These masks play a crucial role in improving contrast and suppressing unwanted reflections, enabling the detection of faint celestial objects. However, traditional masks have limitations in absorption and surface uniformity, leading to residual reflections that hinder optimal imaging performance.
This NASA innovation is a novel process utilizing cryogenic etching of black silicon (BSi) to fabricate highly absorptive pupil masks for coronagraphs. By employing black silicon structures, the technology achieves extremely high light absorption and significantly reduces specular reflectance, thereby enhancing the effectiveness of coronagraphic imaging.
The Technology
Fabrication of NASA's pupil mask begins with the preparation of a silicon wafer, which serves as the foundation for the black silicon structure. The wafer undergoes ion beam figuring (IBF), a non-contact technique that precisely removes surface irregularities at the nanometer scale. This process ensures that the silicon surface is diffraction-limited, eliminating errors that could degrade optical performance. Once the wafer is polished to the required precision, it is then processed lithographically to define the mask pattern, creating reflective and absorptive regions essential for controlling light propagation.
To achieve the desired high absorption characteristics, the lithographically patterned wafer undergoes cryogenic etching, a sophisticated process that transforms the silicon surface into a highly textured, black silicon structure. This method utilizes a controlled plasma environment with sulfur hexafluoride (SF6) and oxygen to etch the surface at cryogenic temperatures. The process is carefully optimized by adjusting parameters such as gas flow rates, chamber pressure, ion density, and etch duration, leading to the formation of high-aspect-ratio nanostructures on the silicon substrate. These structures, resembling a dense “forest” of silicon nanospikes, trap and diffuse incoming light, drastically reducing specular reflection. The resulting surface exhibits an ultra-low reflectivity that is orders of magnitude lower than conventional polished silicon.
By leveraging NASA’s cutting-edge fabrication technique, the newly developed black silicon pupil mask offers a powerful solution for high-contrast astronomical imaging. Its ability to minimize scattered light and enhance optical contrast makes it an ideal component for space telescopes tasked with directly imaging exoplanets as well as other applications requiring ultra low reflectivity systems.


Benefits
- Ultra-high light absorption: Cryogenic black silicon achieves extremely low reflectivity, improving light suppression in coronagraphs.
- Precision fabrication: Ion beam figuring ensures diffraction-limited surface quality, optimizing optical performance.
- Scalability & adaptability: The process can be applied to various optical components for different applications.
- Improved image contrast: Enhances direct exoplanet imaging capabilities by reducing background noise.
- Robust & stable: Fabricated black silicon surfaces exhibit long-term stability under space conditions.
Applications
- Space telescopes & astronomy (to enhance coronagraph performance in missions such as High-Contrast Imager for Complex Aperture Telescope [HiCAT] and future exoplanet direct imaging instruments)
- High-contrast imaging systems
- Advanced optical sensing
- Photodetectors
- Solar cells
- Near-infrared avalanche photodiodes
Technology Details
Manufacturing
GSC-TOPS-370
GSC-18878-1
“Highly Absorptive Pupil Mask Fabricated with Black Silicon,” Ron Shiri et al, 9/9/2019, https://ntrs.nasa.gov/citations/20210013534
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