Above:
A starshade mask on a thin silicon wafer recently fabricated at MDL.
Black Silicon
Blacker Than Black
Karl Yee - Victor White
How do you create the darkest black substance possible? Black silicon is the answer, utilizing the surface texturing of single crystal silicon. This process creates a surface with a dense forest of dark, needle-like structures. With this modification the material becomes highly absorbing of visible and infrared radiation—it is the darkest material that can be manufactured. This new technology is being utilized by the MDL to provide the maximum reduction of stray light in optical instrumentation.
MDL’s microfabricated black materials greatly reduce stray light in optical instruments. Candidate choices include gold black, carbon nanotubes, and black silicon. Black silicon is chosen for its robustness, lower reflectivity, and wider bandwidth. Since the proof of concept in 2008, ultraprecise optical spectrometer slits with black silicon built into them have flown on multiple instruments (e.g., HyTES, AVIRIS, UCIS, HyspIRI, MaRS2, PRISM, NEON, and SWIS), and the technology is considered standard for JPL’s imaging spectrometers. A recent application of this material is in the masks of the WFIRST shaped-pupil coronagraph, the goal of which is to image and characterize exoplanets. The suppression of the glare from the parent star, while simultaneously imaging the orbiting planet, requires typical contrast levels of a billion to one—a problem that has been described as being analogous to imaging a firefly sitting on a searchlight from over 1000 miles away.
To accomplish this, shaped-pupil masks with extremely black micron-scale features in a highly reflective background were designed and developed. Black silicon, optimized for a very low specular reflectivity of just ~10–7 in the wavelength band from 0.4 to 1 micron, accomplished these goals. This instrument complements other techniques for searching for exoplanets and will also enable space-based spectroscopic analysis of light from these planets. MDL is also using electron-beam lithography and deep reactive ion etching of thin silicon wafers to produce small, laboratory-scale starshade masks for experiments at Princeton University. These experiments are validating the concept of flying a large starshade, tens of meters in diameter, in front of a space telescope, to block starlight and enable the detection of the faint reflected light from Earth-like planets orbiting the star. Measurements of the light-suppression effectiveness of these beautiful submicron, precision flower-like apertures support the expectations for the performance of future full-scale starshades.
Dr. Yee is a senior technologist in the Nano and Micro Systems (NaMS) group, with expertise in MEMS design, fabrication, and packaging processes. His primary area of research is in resonant vibratory inertial sensors. He has been the principal investigator on several programs for NASA, DARPA, and the Army.
Victor White is a process engineer in the Nano and Micro Systems (NaMS) group at JPL’s Microdevices Laboratory (MDL). Victor specializes in using MDL’s fabrication tools to shape silicon-based materials into a host of flight-enabling structures. Most of his current work involves applications of black silicon, which is among the blackest engineered materials on the planet, into ultraprecise optical components for flight applications.
A black silicon mask to suppress starlight to reveal light from an orbiting exoplanet that is up to a billion times fainter.
+ Larger image
Development of black Si technology to enhance the performance of imaging spectrometers for Earth science applications.
+ Larger image
