3D structures in silicon control optical signals
Their optical properties are adjusted by structures of micrometre size.
Andreas Frölich from Karlsruhe Institute of Technology, explains, "Optical properties of materials can be influenced decisively by specific structurisation. 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. Typically, 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 micrometre. Professor Martin Wegener, of the Institute of Applied Physics and Institute of Nanotechnology of KIT and the coordinator of the DFG Centre for Functional Nanostructures (CFN), comments, "Our new SPRIE fabrication methods is based on established technologies, such as etching and innovative methods like self-organisation 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 micrometre-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 10µm, 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 centred around 1.5µm. With our etching method, we produce a corrugated structure in the micrometre 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.
- Julien Happich
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