A startup in California has developed a manufacturing technique that could substantially cut the cost of LED lightbulbs—a more energy-efficient type of lighting.
LEDs are conventionally made on a relatively costly substrate of silicon carbide or sapphire. Bridgelux has come up a new process takes advantage of existing fabrication machines used to make silicon computer chips, potentially cutting LED production costs by 75 percent, according to the company.
Despite their higher efficiencies and longer life, few homes and businesses use LED lighting—largely because of the initial cost. An LED chip makes up 30 to 60 percent of a commercial LED lightbulb. Electronic control circuits and heat management components take up the rest. So for a 60-watt equivalent bulb that costs $40, Bridgelux’s technology could bring the cost down by $9 to $18. Integrating the light chip with the electronics might further reduce costs.
LEDs made with the new technique produce 135 lumens for each watt of power. The U.S. Department of Energy’s Lighting Technology Roadmap calls for an efficiency of 150 lumens per watt by 2012. Some LED makers, such as Cree, in Durham, North Carolina, already sell LED lamps with efficiencies in that range. In contrast, incandescent bulbs emit around 15 lumens per watt, and fluorescent lightbulbs emit 50 to 100 lumens per watt.
Manufacturers typically make white LEDs by coating blue gallium nitride devices with yellow phosphors. The gallium nitride is grown on two- to four-inch sapphire or silicon carbide wafers. Cree builds its chips on silicon carbide wafers, “because we believe it produces superior LEDs,” says company spokesperson Michelle Murray.
Larger wafers mean more devices fabricated at once, which brings down cost. But large sapphire or silicon carbide wafers are more difficult, and expensive, to make. Companies such as Osram Opto Semiconductors in Germany are now moving to 15-centimeter sapphire wafers, most likely the largest size possible. Making 20-centimeter silicon wafers, on the other hand, is routine in the semiconductor chip-making industry. Bridgelux’s new silicon wafers were, in fact, made at an old silicon fabrication plant in Silicon Valley.
It is hard to grow gallium nitride on silicon, mainly because the materials expand and contract at very different rates, explains Colin Humphreys, a materials science researcher at Cambridge University. The process is carried out at temperatures around 1,000 °C, and, upon cooling, the gallium nitride cracks because it is under tension, Humphreys says. One way to solve the problem is to insert additional thin films around the gallium nitride to compress the material and balance out the tension produced during cooling. In fact, Humphreys and his colleagues have used this trick to make gallium nitride LEDs on silicon; their devices produce 70 lumens per watt. Bridgelux might be using a similar technique. “The result from Bridgelux is impressive,” Humphreys says. “It offers the promise of a large cost reduction without any reduction of efficiency.”
Other LED makers, including Osram, are also trying to make gallium nitride LEDs on silicon. Bridgelux expects to deliver its first commercial silicon-based LEDs in two to three years.