Add Epistar to a long list of LED manufacturers that are researching the possibility of building LEDs on silicon (Si) wafers rather than the sapphire or silicon carbide wafers that are widely used today. Epistar and Azzurro Semiconductors announced that the former had transferred its existing gallium nitride (GaN) LED structure to an Si wafer in just 16 weeks.
Si wafers offer many potential benefits, including lower-cost unprocessed wafers enabled in part by the broad IC industry. Moreover, a move to silicon wafers would allow LED makers to utilize the automated in depreciated IC fabrication lines once the epitaxial layers are grown on the Si substrate, thereby cutting costs in the backend of the manufacturing process. Cost reductions could accelerate the adoption of solid-state lighting (SSL)
Buffer layer and templates
A major issue with GaN-on-Si manufacturing, however, is a crystal lattice mismatch between the materials that can lead to bowing and cracking of wafers at the high temperatures used in the epitaxial process. Azzurro says that they have solved the problem with a buffer layer between the GaN and Si materials. The company is also producing template wafers that include the Si substrate, the buffer layer, and a highly-doped GaN layer. The LED maker then grows the additional GaN layers that are specific to their LED structure and architecture on the template wafers.
The Epistar project utilized 150-mm (6-in) Azzurro template wafers. And while Epistar has not announced plans for production of GaN-on-Si LEDs, the research project is notable because of the fast transition of the Epistar's GaN LED structure to the new substrate.
"We are very excited about the outcome of this joined exercise which has exceeded all expectations regarding speed and cost of migration," said Epistar Chairman Lee Biing-Jye. "The success helps us to utilize GaN‐on‐Si which is a game changer for the industry ‐ and Epistar as a leading innovator is part of it."
Azzurro also recently received a large grant that it is using to pursue 200-mm (8-in) GaN-on-Si wafer technology. The larger wafers could further reduce costs by packing more LED die per wafer.
Other wafer developments
The field is starting to look crowed for Si-based LED manufacturing. Bridgelux and Toshiba have partnered on Si technology developed by the former. Toshiba recently said that it would begin production of Si-based LEDs on a 200-mm line this month.
Osram Opto Semiconductors announced R&D results from its Si-based research back in January. Moreover, Osram had worked with Azzurro at one time as well as having developed internal Si R&D. Also in January, Lattice Power and Plessey Semiconductors said that they would begin production of GaN-on-Si LEDs this year.
Si , however, isn't the only substrate that may impact the industry. Startup Soraa is touting its GaN-on-GaN technology that would provide a perfect lattice match, thereby supporting higher current density and delivering brighter LEDs. But GaN wafers are much more expensive for now. Still, Soraa recently announced that Japanese lighting manufacturer Ushio will begin selling an MR-16 retrofit lamp using Soraa LEDs in November.
