Cryo-Etched Black Silicon for Use as Optical Black

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

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