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Novel etch method produces 3D microstructures in silicon

01 Nov 2012

Promising new methods for optical processing for telecoms

In modern, high-speed telecommunications, light carries digital information over hundreds or thousands of kilometers within seconds. Switching, modulating and multiplexing devices based on adapted optical materials control the light signals.

Now researchers from Karlsruhe Institute of Technology, Germany, the Université Catholique de Louvain, Belgium, and Humboldt University, Berlin, Germany, have developed a novel method to produce photonic crystals to manipulate such optical signals. Their optical properties are adjusted by structures of micrometer size. The method is rapid, cheap, and simple and partly uses the self-organization principle. The work has been published in the journal Advanced Functional Materials.

Andreas Frölich from Karlsruhe Institute of Technology, explains, “Optical properties of materials can be influenced decisively by specific structurization. Silicon is used in components, such as filters or deflectors, for telecommunications. So far, however, all of these components have been flat, that is two-dimensional. Entirely novel concepts might be feasible using three-dimensional components. Typcially, the cost of structuring the silicon in this way is high.”

The desired functional structure has to be very regular in all three spatial directions and details usually measure just one micrometer. Professor Martin Wegener, of the Institute of Applied Physics and Institute of Nanotechnology of KIT and the coordinator of the DFG Center for Functional Nanostructures (CFN), comments, “Our new SPRIE fabrication methods is based on established technologies, such as etching and innovative methods like self-organization and combines them in a creative manner.”

This method known as SPRIE (Sequential Passivation and Reactive Ion Etching) is applied to structure the silicon on large areas in a simple and three-dimensional manner. First, a solution with micrometer-sized spheres of polystyrene is applied to the silicon’s surface. After drying, these spheres automatically form a dense monolayer over the silicon. Upon metal coating and the removal of the spheres, a honeycomb etching mask remains on the silicon surface.

2D template: 3D structures

“This etching mask is our two-dimensional template for the construction of the three-dimensional structure,” says Frölich. “The free areas are removed by etching with a reactive plasma gas. An electric field is then applied to make the gas particles etch into the depth only or homogeneously in all directions. In addition, we can specifically passivate the walls of the hole, which means that it is protected from further etching by a polymer layer.”

Repeated etching and passivation make the holes of the etching mask grow to the desired functional depth. At up to 10 micrometers, hole depth typically exceeds its width by a factor of more than 10. The process steps and the electric field are adjusted precisely to control the structure of the walls. Instead of a simple hole with vertical smooth walls, every etching step produces a spherical depression with a curved surface. This curvature is the basis for the regular repeating structures of novel waveguides.

Frölich adds, “Optical telecommunications are usually carried on wavelengths centered around 1.5 µm. With our etching method, we produce a corrugated structure in the micrometer range along the wall.” The nearby field and deep, structured holes together act like a regular crystal that refracts [infrared light] in the desired manner.
The SPRIE method can produce a three-dimensional photonic crystal within a few minutes, as it is based on conventional industrial processes. In principle, a three-dimensional structure can be generated in silicon with a user-selectable mask.

The research partners say that this approach opens up new possibilities for meeting the requirements for advanced optical components in telecommunications. 
Different designs of photonic crystals are available: some are applied as waveguides with very small curvature radii and small losses; others as extremely small-band optical filters and multiplexers. In few decades, computers working with light instead of electricity might be feasible.

About the Author

Matthew Peach is a contributing editor to optics.org

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