Metal-Assisted Chemical Etching
- Tuesday, 27 December 2011
University of Illinois researchers have developed a method to chemically etch patterned arrays in the semiconductor gallium arsenide - used in solar cells, lasers, light-emitting diodes (LEDs), field effect transistors (FETs), capacitors, and sensors.
A semiconductor’s physical properties can vary depending on its structure, so semiconductor wafers are etched into structures that tune their electrical and optical properties and connectivity before they are assembled into chips. Semiconductors are commonly etched with two techniques - wet etching uses a chemical solution to erode the semiconductor in all directions, while dry etching uses a directed beam of ions to bombard the surface, carving out a directed pattern. Such patterns are required for high-aspect-ratio nanostructures, or tiny shapes that have a large ratio of height to width. High-aspect-ratio structures are essential to many high-end optoelectronic device applications.
While silicon is the most pervasive material in semiconductor devices, materials in the III-V group are more efficient in optoelectronic applications, such as solar cells or lasers. Unfortunately, these materials can be difficult to dry etch, as the high-energy ion blasts damage the semiconductor’s surface. III-V semiconductors are especially susceptible to damage.
To address this problem, electrical and computer engineering professor Xiuling Li and her group turned to metal-assisted chemical etching (MacEtch), a wet-etching approach they had previously developed for silicon. Unlike other wet methods, MacEtch works in one direction, from the top down. It is faster and less expensive than many dry etch techniques, according to Li. Her group revisited the MacEtch technique, optimizing the chemical solution and reaction conditions for the III-V semiconductor gallium arsenide (GaAs).
The process has two steps. First, a thin film of metal is patterned on the GaAs surface. Then, the semiconductor with the metal pattern is immersed in the MacEtch chemical solution. The metal catalyzes the reaction so that only the areas touching metal are etched away, and high-aspect-ratio structures are formed as the metal sinks into the wafer. When the etching is done, the metal can be cleaned from the surface without damaging it.
To create metal film patterns on the GaAs surface, Li’s team used a patterning technique pioneered by John Rogers, a professor of materials science and engineering at the university. Their research teams joined forces to optimize the method, called soft lithography, for chemical compatibility while protecting the GaAs surface. Soft lithography is applied to the whole semiconductor wafer, as opposed to small segments, creating patterns over large areas – without expensive optical equipment.
“The combination of soft lithography and MacEtch make the perfect combination to produce large-area, high-aspect-ratio III-V nanostructures in a low-cost fashion,” said Li.