A single layer of silicon atoms (black) binds to the moving silica tip of a scanning probe microscope, offering a precise way to remove the atoms. Image: Lei Chen/Southwest Jiaotong University.A precise, chemical-free method for etching nanoscale features on silicon wafers has been developed by a team comprising researchers from Penn State and Southwest Jiaotong University and Tsinghua University in China. The researchers report their method in a paper in Nature Communications.
In standard lithography, a photosensitive film is deposited on a silicon wafer and a mask is used to expose certain portions of this film, creating a pattern. Chemicals such as a potassium hydroxide solution are then used to etch this pattern into the silicon. Further steps are required to smooth out the roughened surface.
The team of researchers developed an entirely different, chemical- and mask-free, one-step method for creating such patterns. Their method involves lightly rubbing the rounded silica tip of an instrument called a scanning probe microscope across a silicon substrate – the material base typically used to make electronic devices. When exposed to the water vapor in air, the top layer of silicon forms bonds with the tip of the scanning probe, which causes a single layer of atoms to slide off as the probe moves across the silicon. Because the atoms below do not take part in the chemical reaction, they are completely undamaged.
"It's really quite a unique idea," said Seong Kim, professor of chemical engineering at Penn State. "It's a so-called tribochemical reaction. Unlike chemical reactions caused by heat, light or electric fields, which are all widely studied, mechanically-stimulated chemical reactions are less understood."
The removal mechanism is initiated when the silicon surface is exposed to air and the top layer of silicon atoms reacts with water molecules to form silicon-oxygen-hydrogen bonds. Next, the silica tip forms silicon-oxide-silicon bonds under the shear force produced as it moves across the surface, which facilitates the removal of silicon atoms from the topmost surface of the substrate.
Scientists working in nanofabrication who are trying to reduce the size of device features down to atomic-scale dimensions could find this technique useful, Kim believes. "Atomic layer etching can provide the depth resolution that people would like to get without the use of sacrificial layers and harsh chemicals," he said.
At the moment, this kind of patterning method is too slow for microfabrication, Kim acknowledged. However, researchers could use it to create a platform for testing electronic and microelectromechanical devices with features at the Angstrom or single-atom scale, far smaller than current devices. At least one company, IBM, has experimented with multiple probe arrays that could lead to large-scale patterning of devices.
"Our process could be combined with their process to scale up," Kim said. "This is the initial science part. Once we see the science, a lot of possibilities can be explored. For instance, we think this technique will work with other materials beyond silicon."
This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.