LED Firms Mull New Wafer Sizes And Materials

Silicon, gallium nitride and sapphire are all being tested against price, stability and performance.

popularity

By Mark LaPedus
Seeking to reduce the cost of solid-state lighting and related applications, LED manufacturers are taking a page from the IC industry: They are looking at larger wafer sizes and new materials in the fab.

Today, the state-of-the-art LED fab is a 150mm (6-inch) facility, but a large percentage of these plants are still using 50mm (2-inch) substrates. The vast majority of LED suppliers use substrates based on sapphire technology. The exception to the rule is Cree, which uses silicon carbide (SiC) substrates.

In what could drive down manufacturing costs, Bridgelux, Lattice Power, Osram, Philips Lumileds, Toshiba and others are exploring or beginning to ramp up LEDs based on a lower cost substrate material: silicon. Toshiba, for one, is ramping up a gallium nitride (GaN) on silicon process using 200mm wafers.

In addition, other vendors are starting to ramp up LEDs based on separate GaN-on-GaN or semi non-polar GaN substrates using 50mm or smaller wafer sizes. GaN promises to generate more lumens per area, but the substrate costs are still expensive.

GaN-on-silicon processes on 200mm substrates has a 1- to 2-cent per square mm die cost advantage over traditional sapphire in LED production, said Jy Bhardwaj, vice president of research and development at Philips Lumileds, one of the world’s largest LED makers. “It isn’t like silicon will take over everything,” Bhardwaj said. “Silicon is indeed under development as a cost play, feasible only when the performance disadvantages have been overcome. Silicon must also enter at 200mm without any increased processing costs.”

Sapphire and SiC, the incumbent technologies, will continue to make advances amid a push towards silicon. “The sapphire industry is not going to stand still. The sapphire world is also investing in 200mm,” he said.

Compared to semiconductor plants, LED fab costs are reasonable. A 50mm (2-inch) LED fab runs from $50 million to $60 million, said Christian Dieseldorff, an analyst with SEMI. In comparison, a big LED fab runs from $120 million to $150 million. “Most LED fabs run a mix of wafer sizes,” Dieseldorff said. “There is not really a pure 6-inch dedicated LED fab out there, because 6-inch sapphire is very expensive.”

It’s unlikely that manufacturers would build a dedicated 200mm LED fab. Today, global LED demand can be entirely met by a single 200mm line with 50,000 wafer starts per month, according to Philips Lumileds. So, don’t expect 300mm LED fabs in the near term.

Sea of change
Still, there is a sea of change taking place in LEDs. After years of sizzling growth, the LED market is currently suffering from a capacity glut and sluggish demand. According to SEMI, total LED fab capital equipment spending is projected to hit $1.917 billion in 2012, down from $2.52 billion in 2011. In 2013, LED fab capital spending is projected to be $1.96 billion.

One of the reasons for the glut is that the Chinese government handed out subsidies of up to 50% to buy the key LED equipment, metal organic chemical vapor deposition (MOCVD) tools, to LED makers in China. China recently ended most of the MOCVD subsidies, but the move caused excess capacity in the worldwide market.

LED demand is also mixed. The LED market is maturing for TVs and mobile products. The next big thing is solid-state lighting, which could save $120 billion in energy costs over the next 20 years, according to the U.S. Department of Energy.

There is still a glaring cost delta between LED lamps and the lowly incandescent light bulb. The cost of a 60-watt LED replacement bulb is roughly $25 to $40, compared to a mere 50 cents for today’s incandescent bulb, said Robert Steele, a consultant for Strategies Unlimited, a research firm. “LEDs are five times more efficient, but the price gap for a LED replacement bulb still remains too high for consumers,” he said.

China makes the cheapest LED bulbs at about $14 to $24 per unit. High-end LED makers are looking to reduce the cost of the package, which in turn could help bring the price of an LED bulb to around $11.06 by 2020, according to Lux Research.

LED suppliers also are focusing on the substrate to reduce costs. In a simple LED manufacturing flow, the substrate goes through an MOCVD or epitaxy process, followed by etch and packaging. The MOCVD tools represent 50% or more of LED fab capital expenditures.

Hydride vapor phase epitaxy (HVPE) is an alternative approach that is said to speed up the flow for thick GaN layers in LEDs. Using HVPE, LED makers can reduce the cost of GaN template wafers, said Chantal Arena, vice president and general manager of Soitec Phoenix Labs, a subsidiary of Soitec. “Our strategy is to use production-proven silicon epitaxy equipment features and add our gallium source and delivery system to create a high productivity HVPE equipment,” Arena said.

New LED substrate materials
At present, there are five main LED substrate types. With its patented SiC substrate technology, Cree announced a 150mm LED fab in 2010. At that time, Philips Lumileds became one of the first sapphire LED vendors to move to 150mm substrates.

Silicon and GaN are the new kids on the block. In June, China’s Lattice Power announced what the company claimed were the world’s first GaN-on-silicon LEDs in production. Lattice Power’s LEDs are made on 50mm wafers.

Then, in July, Toshiba said it will shortly start mass production of white LEDs based on a GaN-on-silicon technology on a 200mm line in Japan. Toshiba and its partner, Bridgelux, have been collaborating on the development of GaN-on-silicon technology.

“The opportunity for silicon is not on 6-inch, because 6-inch costs for sapphire are really coming down. The opportunity for silicon is going to be on 8-inch,” said Philips Lumileds’ Bhardwaj. “But that opportunity is only there for a short period of time, because sapphire will be available on 8-inch and it will come down in cost. So the question is, can silicon deliver in the next one or two years? The performance has to be at parity or very near it. Today, the best case is 5% below the best in class sapphire. The worst case is 10% to 15% below.”

Like sapphire and SiC, GaN-on-silicon suffers from material stress due to a combination of lattice and thermal expansion mismatch. “Therefore, you’ve got to create this interface matching and growth nucleation layers. The greater the mismatch, the higher the number of dislocation defects and it is critical minimize the threading dislocation density (TDD),” he said.

Lattice Power addresses the problem using patterned substrates that isolate the stress caused by cracking. It utilizes an AlGaN/AlGaN multilayer buffer that manages internal strain. With this approach, Lattice Power claims a manufacturing yield of more than 95%.

On the GaN front, startup Soraa is developing LED lamps based on a GaN-on-GaN technology. Unlike sapphire, SiC and silicon, GaN is latticed matched, which lowers the TDD. Others are developing semi non-polar GaN. With this technology, the substrates are free of stacking faults, thereby reducing TDD by 10,000 times, according to Kyma, a supplier of substrate products.

“There is a lot of interest in GaN,” said Strategies Unlimited’s Steele. “You can drive more current through the LED, but it’s very expensive. The challenge is to get substrate costs down.”

There could be room for all substrate technologies, especially in the fledging solid-state lighting market. Lighting only represents 10% to 15% of the overall LED market and there is still room for growth. “LEDs are still rapidly growing in lighting,” Steele said.