Rugate Optics: Chemically resistant porous silicon carbide spectral optics could have biosensing uses
Extremely fine porous structures can be generated in semiconductors, opening up new possibilities for novel sensors, optics, and electronics -- experiments in this area have already been done in silicon.
Extremely fine porous structures can be generated in semiconductors, opening up new possibilities for novel sensors, optics, and electronics—experiments in this area have already been done in silicon. Now, researchers at TU Wien (Austria) have come up with a way to fabricate porous silicon carbide (SiC) in a controlled manner.1Silicon carbide has some advantages over silicon, as its greater chemical resistance allows it to be used, for example, for biological applications without any additional coating required.
To demonstrate the potential of this new technology, a multilayer rugate (continuously varying refractive index) optical mirror that selectively reflects and transmits different colors of light was integrated into a SiC wafer by creating layers with a thickness of approximately 70 nm each, with different and continuously varying degrees of porosity, and thus with differing refractive indices (see figure).
A prototype porous silicon carbide (SiC) optical structure has a porosity (a) and thus a refractive index (b) that varies periodically with depth; individual 70-nm-thick porous layers in SiC are clearly visible in a scanning electron micrograph (c), and a prototype porous SiC optical structure designed to pass green and near-IR light and reflect red demonstrates the ability to create precise layers of different porosities and thus refractive indices (d). (Courtesy of TU Wien)
This could be very useful in sensor technology: for example, the refractive index of minute quantities of liquid could be measured using a porous semiconductor sensor, allowing a reliable distinction between different liquids.
Another attractive option from a technical and application-oriented perspective is to first make certain areas of the SiC wafer porous in a highly localized manner before depositing a new SiC layer over these porous areas, and then causing the latter to collapse in a controlled manner. This technique produces microstructures and nanostructures that can also play a key role in sensor technology.
"Until now, silicon has been used for this purpose, a material with which we already have a lot of experience," says Ulrich Schmid, one of the researchers. However, under harsh environmental conditions, such as in extreme heat or in alkaline solutions, structures made of silicon are attacked and rapidly destroyed.
Creating the color-selective mirror
The surface was cleaned, then partially covered with a thin layer of platinum. The SiC was then immersed in an etch solution and exposed to UV light to initiate oxidation, causing a thin porous layer—initially 1 μm thick—to form in the areas that were not coated with platinum. An electrical charge was also applied to precisely set the porosity and the thickness of the subsequent layers. Here, the first porous layer promoted the formation of the first pores when the electrical charge was applied. The resulting pores averaged about 350 nm2 in cross-sectional area.
"The porous structure spreads from the surface further and further into the interior of the material," explains Markus Leitgeb, another researcher. "By adjusting the electrical charge during this process, we can control what porosity we want to have at a given depth."
The resulting complex layered structure of SiC layers with higher and lower levels of porosity was separated from the bulk material by applying a high-voltage pulse. The thicknesses of the layers were selected so that the structure became an integrated, color-selective mirror/filter. The porosity as a function of depth was accurately determined in cross-sections via an image-processing technique.
1. M. Leitgeb et al., APL Mater., 5, 106106 (2017); https://doi.org/10.1063/1.5001876.