For years, optoelectronics manufacturers have focused on ways to replace expensive gallium arsenide (GaAs) substrate materials with silicon (Si) wafers, which are economical, easy to manufacture, and available in diameters as large as 300 mm. Now, a unique alliance between Motorola Inc. (Schaumburg, IL) and IQE Plc (Cardiff, UK) has culminated in the world's first demonstration 8-in. and 12-in. GaAs-on-Si wafers. In essence, the new technology is the first to successfully combine the best properties of workhorse silicon technology with the speed and optical capabilities of high-performance compound semiconductors.
Ravi Droopad displays a 12-in. GaAs-on-silicon wafer. (Motorola)
The materials technology involves the molecular beam epitaxial growth of strontium titanate (STO) on silicon wafersa combination popular with developers of advanced DRAMs. The key to the process is that STO is well matched not only to the crystalline properties of silicon but also to those of GaAs, making it possible to grow GaAs structures on top of the coated silicon wafer. The work was initially carried out at the Phoenix, AZ, labs of Motorola and later in the Bethlehem, PA, production facility of IQE. Subsequently, some of these wafers were shipped to IQE's metal-oxide chemical vapor deposition (MOCVD) facility (Cardiff, Wales, UK). Here, engineers deposited multiple layers of III-V semiconductors on the substrates, fabricating light-emitting devices and other optoelectronic devices.
STO makes the Motorola-IQE approach unique. Previous attempts at mating GaAs and silicon involved intermediate layers such as those made from germanium or III-V semiconductor materials, including indium phosphide (InP), silicon germanium, and gallium arsenide. These approaches yielded encouraging, but far from perfect, results: Devices made from these combinations were functional, but performance and/or lifetime was inferior to devices made of GaAs on GaAs.
As GaAs crystal quality and wafer pricing improved, the need for alternative substrates became less pressing. Still, the economics of III-V device processing have seldom been out of the spotlight. Today, device makers are more intent than ever to reduce manufacturing costs and improve their products for a competitive marketplace.
"[STO-on-Si] opens the door to significantly less-expensive optical communications and other devices by potentially eliminating the current cost barriers holding back many advanced applications," says Chris Meadows, marketing manager at IQE.
Analysts don't expect the technology to hurt III-V substrate markets in the short-term. Discrete die size is fairly constant for opto devices. Current fabs are fitted for 2-in. wafers and will take time to tool-up for bigger wafers. Nevertheless, in due course, price pressure will force this change.
Strategy Analytics (Luton, UK) sees the capability of being able to produce GaAs-on-silicon as a breakthrough for the semiconductor industry. "It may represent the next large and crucial evolutionary step that offers GaAs technology the opportunity to become a truly pervasive technology, with much more widespread utilization of GaAs's uniquely superior properties in everyday applications," says Stephen Entwistle, director of the GaAs and high-speed circuits service. "In the longer term, technologies such as Si BiCMOS and SiGe could become obsolete."
The potential of the technology was recently demonstrated with the fabrication of several light-emitting devices. The group has also constructed working mobile-phone components. Although the recent results have been based on GaAs-on-Si, the method is not restricted to this combination. Motorola Labs is working on developing the optimum intermediate layer for other important opto materials such as InP. "One of our next goals is to complete the task of growing indium phosphide on silicon," says Jim Prendergast, vice president and director of Motorola Labs' physical sciences research lab. "This technology should support chip clock speeds of more than 70 GHz and long-wavelength lasers that are critical to fiber-optic communications." Having secured a number of patents on this technology, Motorola intends to share it via licensing arrangements.