Carpetlike random arrays of metal-coated silicon nanotips have been shown to be effective as antireflection surfaces. Now undergoing development for incorporation into Sun sensors that would provide guidance for robotic exploratory vehicles on Mars, nanotip carpets of this type could also have many uses on Earth as antireflection surfaces in instruments that handle or detect ultraviolet, visible, or infrared light.

Figure 1. A Random Array of Nanotips is formed on the flat substrate surface surrounding an aperture. The nanotips are coated with Cr/Au to enhance absorption of light. The Cr layer on the rear surface serves as a light attenuator.

In the original Sun-sensor application, what is required is an array of 50-μm-diameter apertures on what is otherwise an opaque, minimally reflective surface, as needed to implement a miniature multiple-pinhole camera. The process for fabrication of an antireflection nanotip carpet for this application (see Figure 1) includes, and goes somewhat beyond, the process described in "A New Process for Fabricating Random Silicon Nanotips" (NPO-40123), NASA Tech Briefs, Vol. 28, No. 1 (November 2004), page 62. In the first step, which is not part of the previously reported process, photolithography is performed to deposit etch masks to define the 50-μm apertures on a silicon substrate. In the second step, which is part of the previously reported process, the non-masked silicon area between the apertures is subjected to reactive ion etching (RIE) under a special combination of conditions that results in the growth of fluorine-based compounds in randomly distributed formations, known in the art as "polymer RIE grass," that have dimensions of the order of microns.

Figure 2. The Antireflection Nanotip Carpet surrounding the aperture absorbs most of the incident light. The reflectance of the carpet at a wavelength of 1 μm was found to be 0.06 percent of that of an aluminum mirror.

The polymer RIE grass formations serve as microscopic etch masks during the next step, in which deep reactive ion etching (DRIE) is performed. What remains after DRIE is the carpet of nanotips, which are high-aspect-ratio peaks, the tips of which have radii of the order of nanometers. Next, the nanotip array is evaporatively coated with Cr/Au to enhance the absorption of light (more specifically, infrared light in the Sun- sensor application). The photoresist etch masks protecting the apertures are then removed by dipping the substrate into acetone. Finally, for the Sun-sensor application, the back surface of the substrate is coated with a 57-nm-thick layer of Cr for attenuation of sunlight.

This work was done by Youngsam Bae, Sohrab Mobasser, Harish Manohara, and Choonsup Lee of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-42592, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Aluminum Automation Coatings & Adhesives Composites Fabrication Manufacturing & Prototyping Manufacturing processes Materials Metals Nanotechnology Polymers Robotics Sensors and actuators Sun and solar
This Brief includes a Technical Support Package (TSP).
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Nanotip Carpets as Antireflection Surfaces

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    Overview

    The document titled "Nanotip Carpets as Antireflection Surfaces" is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL), detailing a novel fabrication technique for antireflection surfaces using nanotip structures. This innovation is particularly relevant for micro sun sensors used in Mars rovers, addressing the critical issue of ghost images caused by multiple reflections.

    The fabrication process begins with photolithography to define an aperture array on a silicon substrate. This is followed by the creation of polymer "RIE-grass," which leads to the growth of fluorine-based compounds at randomly distributed sites, known as micro-etch masks. The substrate is then subjected to deep reactive ion etching (DRIE), which etches away material everywhere except at these sites, resulting in high aspect ratio structures with sharp tips measuring in the nanometer range.

    The document highlights the successful creation of nanotips with a radius of 100 nm and an aspect ratio of 200, achieved without the use of nanolithography. A layer of chromium/gold (Cr/Au) is evaporated onto the nano-tips to absorb infrared light, while the photoresist is removed using acetone. A 57-nm thick layer of chromium is then applied to the backside of the substrate to attenuate sunlight. The resulting nanotip substrate exhibits extremely low reflectance, approximately 0.06% at a target wavelength of 1 μm, significantly lower than that of traditional aluminum mirror surfaces.

    Figures included in the document provide visual representations of the fabrication process and the final product. Figure 2, for instance, shows the antireflection nano-tips substrate and the aperture array, demonstrating the effectiveness of the design in situations where dust may block one or more apertures.

    The document emphasizes the potential applications of this technology beyond Mars rovers, suggesting broader implications for various technological, scientific, and commercial fields. It serves as a resource for those interested in aerospace-related developments and highlights the importance of innovative partnerships in advancing research and technology.

    Overall, this Technical Support Package encapsulates a significant advancement in antireflection surface technology, showcasing how nanoscale engineering can solve complex challenges in space exploration and beyond.