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Semiconductor etching gets easier

Photonics Spectra
Mar 2012
Ashley N. Paddock, ashley.paddock@photonics.com

CHAMPAIGN, Ill. – A method that chemically etches patterned arrays in gallium arsenide will make high-end optoelectronic devices easier to manufacture.

Developed by a team led by Xiuling Li of the University of Illinois, the technique will enable faster, less expensive gallium arsenide-based devices such as solar cells, lasers, LEDs, field effect transistors, capacitors and sensors.

The physical properties of a semiconductor can vary depending on its structure, so a semiconductor wafer must be etched to tune its electro-optical properties and connectivity before it is assembled into chips. Wet etching uses a chemical solution to erode the semiconductor in all directions, and dry etching uses a directed beam of ions to bombard the surface, carving out a directed pattern.


Scanning electron microscope image of nanopillars etched in gallium arsenide via metal-assisted chemical etching. Images courtesy of Xiuling Li.


Although silicon is the most common material in semiconductor devices, materials in the III-V group are more efficient in optoelectronic applications, such as solar cells or lasers. Unfortunately, all of these materials can be difficult to dry-etch because high-energy ion blasts can damage the semiconductor’s surface. III-V semiconductors are especially susceptible to damage.

To address this problem, the researchers used metal-assisted chemical etching (MacEtch), a wet-etching approach they had 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, Li said. Her group optimized the MacEtch chemical solution and reaction conditions for the III-V semiconductor gallium arsenide (GaAs).


Metal-assisted chemical etching uses two steps. First, a thin layer of gold is patterned on top of a semiconductor wafer using soft lithography (left). The gold catalyzes a chemical reaction that etches the semiconductor from the top down, creating 3-D structures for optoelectronic applications (right).


The process involves patterning a thin film of metal onto the GaAs surface, then immersing the metal pattern into the Mac-Etch 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.

Realization of high-aspect-ratio III-V nanostructure arrays by wet etching can transform the fabrication of semiconductor lasers where surface grating is fabricated by dry etching, which is expensive and causes surface damage,” Li said.

Li’s group used a patterning technique developed by John Rogers, a professor of materials science and engineering at the university. The two research teams collaborated 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.

By using soft lithography and MacEtch together, the teams produced large-area, high-aspect-ratio III-V nanostructures at a minimal cost, Li said. The technique was published in Nano Letters (doi: 10.1021/ nl202708d).

Next, the researchers hope to further optimize conditions for GaAs etching and to establish parameters for MacEtch of other III-V semiconductors. They hope to demonstrate device fabrication, including distributed Bragg reflector lasers and photonic crystals.


GLOSSARY
optoelectronics
A sub-field of photonics that pertains to an electronic device that responds to optical power, emits or modifies optical radiation, or utilizes optical radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer. Electro-optic often is used erroneously as a synonym.
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