Stray light reflected from the surface of imaging spectrometer components — in particular, the spectrometer slit — degrade the image quality. A technique has been developed for rapid, uniform, and cost-effective black silicon formation based on inductively coupled plasma (ICP) etching at cryogenic temperatures. Recent measurements show less than 1-percent total reflectance from 350–2,500 nm of doped black silicon formed in this way, making it an excellent option for texturing of component surfaces for reduction of stray light.
Oxygen combines with SF6 + Si etch byproducts to form a passivation layer atop the Si when the etch is performed at cryogenic temperatures. Excess flow of oxygen results in micromasking and the formation of black silicon. The process is repeatable and reliable, and provides control over etch depth and sidewall profile. Density of the needles can be controlled to some extent.
Regions to be textured can be patterned lithographically. Adhesion is not an issue as the nanotips are part of the underlying substrate. This is in contrast to surface growth/deposition techniques such as carbon nanotubes (CNTs).
The black Si surface is compatible with wet processing, including processing with solvents, the textured surface is completely inorganic, and it does not outgas.
In radiometry applications, optical absorbers are often constructed using “gold black” or CNTs. This black silicon technology is an improvement for these types of applications.
This work was done by Karl Y. Yee, Victor E. White, Pantazis Mouroulis, and Michael L. Eastwood of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47883
This Brief includes a Technical Support Package (TSP).
Cryo-Etched Black Silicon for Use as Optical Black
(reference NPO-47883) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in the development of Cryo-Etched Black Silicon (Black Si) for use as an optical black material. This technology is particularly relevant for applications in spectrometry and radiometry, where minimizing stray light is crucial for enhancing image quality and instrument performance.
The core of the document outlines a novel technique for creating black silicon through inductively coupled plasma (ICP) etching at cryogenic temperatures. This process results in a textured silicon surface that exhibits exceptionally low total reflectance, measured at less than 1% across a wavelength range of 350-2500 nm. Such low reflectance makes black silicon an excellent candidate for texturing slit surfaces in instruments like the Portable Remote Imaging Spectrometer (PRISM), which is designed to capture high-quality spectral data.
Key features of the black silicon formation process include its repeatability and reliability, the ability to pattern regions lithographically, and the fact that the nanotips formed during the etching process are integral to the underlying substrate, eliminating adhesion issues common with other surface growth techniques, such as carbon nanotubes (CNTs). Additionally, the black silicon surface is compatible with wet processing, making it versatile for various applications.
The document also outlines the project's milestones, including a three-month timeline for process development and a twelve-month timeline for delivering textured PRISM slits. The development of this technology is expected to advance the Technology Readiness Level (TRL) from 1 to 6, indicating its progression from concept to a stage suitable for operational use.
The document emphasizes the broader implications of this technology, suggesting that the advancements in black silicon formation could enhance the performance of all NASA spectrometers and radiometers, thereby improving scientific data collection and analysis. The contact information for further inquiries is provided, indicating that JPL is open to collaboration and partnerships in this innovative area.
Overall, this Technical Support Package highlights a significant technological advancement in optical materials, showcasing JPL's commitment to improving aerospace-related developments with potential applications beyond space exploration.
