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October 08, 2012

Building 3D Microstructures from a 2D Template

Karlsruhe Institute of Technology - A new etching method produces three-dimensional microstructures in silicon for the processing of light signals in telecommunications.

In modern telecommunications, light carries digital information over kilometers within seconds. Adapted optical materials con-trol the light signals. In the AFM journal, researchers from Ber-lin, Louvain, and from Karlsruhe Institute of Technology present a method to produce photonic crystals. Their optical properties are adjusted by structures of micrometer size. The method is rapid, cheap, and simple and partly uses the self-organization principle.

“Optical properties of materials can be influenced decisively by spe-cific structurization,” explains Andreas Frölich from Karlsruhe Institute of Technology. Silicon is used in components, e.g. filters or deflectors, for telecommunications. So far, however, all these com-ponents have been flat, i.e. two-dimensional. Entirely novel concepts might be feasible using three-dimensional components. The ex-penditure required to structure the silicon is very high. The structure has to be very regular in all three spatial directions and details are to have the size of about one micrometer, which corresponds to one hundredth of the thickness of a hair.

“Our new SPRIE fabrication methods uses established technologies, such as etching and innovative methods like self-organization and combines them in a very creative manner,” says Martin Wegener, Professor of the Institute of Applied Physics and Institute of Nano-technology of KIT and coordinator of the DFG Center for Functional Nanostructures (CFN). The SPRIE method is applied to structure 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 surface. After drying, these spheres automati-cally form in a dense monolayer on the silicon. Upon metal coating and the removal of the spheres, a honeycomb etching mask remains on the silicon surface.


Deep below the silicon surface, the SPRIE method produces regular structures in the micrometer range that refract light. (Photo: KIT/CFN)

Advanced Functional Materials journal - Direct Transcription of Two-Dimensional Colloidal Crystal Arrays into Three-Dimensional Photonic Crystals



“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 applied to make the gas particles etch into the depth only or ho-mogeneously 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 makes the holes of the etching mask grow into the depth. With up to 10 micrometers, their depth exceeds their 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. “Optical telecommunication takes place at a wavelength of 1.5 µm. With our etching method, we produce a corrugated structure in the micrometer range along the wall.” The field at closely adjacent and very deep, structured holes acts like a regular crystal that refracts in the desired manner.

The SPRIE (Sequential Passivation and Reactive Ion Etching) 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 using a freely choosable mask. This opens up new possibilities for meeting the requirements made on optical components in telecomunications.

Different designs of photonic crystals are available. Some are applied as waveguides with very small curvature radii and small losses or as extremely small-band optical filters and multiplexers. In few decades, computers working with light instead of electricity might be feasible. Apart from KIT, the Belgian Université catholique de Louvain and Humboldt University, Berlin, were involved in the development.


ABSTRACT - A simple protocol for the fabrication of three-dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one-step processing protocol relying on sequential passivation and reactive ion etching reactions using C4F8 and SF6 plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two-dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop-bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large-scale fabrication of photonic structures.

7 pages of supplemental material

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