Researchers at the University of Illinois have developed a chemical etching method for fabricating patterned arrays on gallium arsenide (GaAs) devices used in solar cells, lasers, light emitting diodes (LEDs), field effect transistors (FETs), capacitors and sensors.
Two etching methods are commonly used in semiconductor fabrication. Wet etching uses a chemical solution to erode the semiconductor material in all directions, while dry etching bombards the surface with a directed ion beam to carve out patterned structures. Dry etching is necessary for high aspect ratio nanostructures, – tiny features with a large ratio of height to width – which are essential for many advanced optoelectronic devices.
Unfortunately, dry etching can be difficult with semiconductor materials in the III-V group, which are more efficient than silicon in optoelectronic applications, because they are more prone to surface damage from the high-energy ion beam. To remedy this problem, Professor Xiuling Li and her group turned to metal-assisted chemical etching (MacEtch), a wet etching method they had previously developed for silicon. Unlike other wet methods, MacEtch works in a single direction, from the top down, and according to Li it is faster and less expensive than many dry etching methods. Her group reworked the MacEtch method to optimise the chemical solution and reaction conditions for the III-V semiconductor material gallium arsenide (GaAs).
The MacEtch process has two steps. First, a thin film of metal is patterned on the GaAs surface, and then the material with the metal pattern is immersed in the chemical solution. The metal catalyses the reaction, so only areas in contact with the metal are etched away, and high aspect ratio structures are formed as the metal sinks into the wafer. When etching is complete, the metal can be removed from the surface without damaging it.
Image: Xiuling Li, University of Illinois