Extremely fine porous structures with tiny holes — resembling a kind of sponge at the nano level — can be generated in semiconductors. A method was developed for the controlled manufacture of porous silicon carbide (SiC), which has significant advantages over silicon. It possesses greater chemical resistance and can be used for biological applications, for example, without any additional coating required.
To demonstrate the potential of the new technology, a mirror that selectively reflects different colors of light was integrated into a SiC wafer by creating thin layers with a thickness of approximately 70 nm each, and with different degrees of porosity. The process makes a porous structure with nano holes from a solid piece of a semiconductor material. The porous structure influences the manner in which light waves are affected by the material. Controlling the porosity means also controlling the optical refractive index of the material. This can be very useful in sensor technology; for example, the refractive index of tiny quantities of liquid can be measured using a porous semiconductor sensor, thus 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.
In all these techniques, it is crucial that the appropriate starting material is selected. Until now, silicon has been used for this purpose; however, silicon has significant drawbacks. Under harsh environmental conditions such as extreme heat or in alkaline solutions, structures made of silicon are attacked and rapidly destroyed. Therefore, sensors made of silicon are often not suitable for biological or electrochemical applications. For this reason, a similar method was attempted with the semiconductor SiC, which is biocompatible and considerably more robust from a chemical perspective.
First, the surface is cleaned, and then partially covered with a thin layer of platinum. The SiC is then immersed in an etching solution and exposed to UV light in order to initiate the oxidation processes. This causes a thin, porous layer — initially 1 μm thick — to form in these areas that are not coated with platinum. An electrical charge is then applied in order to precisely set the porosity and thickness of the subsequent layers. The first porous layer promotes the formation of the first pores when the electrical charge is applied.
The porous structure spreads from the surface further into the interior of the material. By adjusting the electrical charge during this process, the porosity can be controlled at a given depth. In this way, it is possible to produce a complex layered structure of SiC layers with higher and lower levels of porosity that is finally separated from the bulk material by applying a high-voltage pulse. The thickness of the individual layers can be selected such that the layered structure reflects certain light wavelengths particularly well, or allows certain light wavelengths to pass through (see figure), resulting in an integrated, color-selective mirror.