Emissions of a greenhouse gas that has 17,000 times the planet-warming capacity of carbon dioxide are at least four times higher than had been previously estimated. Nitrogen trifluoride (NF3) is used mainly by the semiconductor industry to clean the chambers in which silicon chips are made. The industry had in the past estimated that most of the gas was expended during the cleaning process and only about 2 percent escaped into the air. But the first-ever measurements of nitrogen trifluoride levels in the atmosphere, published recently in the journal Geophysical Research Letters show that emissions could be as high as 16 percent.
The results might not have immediate repercussions—nitrogen trifluoride currently adds 0.04 percent of the global warming effect created by carbon dioxide emitted from sources such as coal-burning power plants and cars. More and more gas will be needed, however, as flat-panel LCD televisions become standard in American living rooms and the fledgling thin-film solar cell industry takes off; nitrogen trifluoride is used as a cleaning agent in the manufacture of both.
The production of the gas is nearly doubling every year, says Michael Prather, atmospheric chemist at University of California, Irvine, who had predicted earlier this year that emissions would likely exceed the industry’s claim that only 2 percent of the gas is released into the atmosphere.
Despite its potential consequences, the gas is not regulated and electronics companies are not required to keep a record of how much they use or emit. “Nobody really knows how much [nitrogen trifluoride] is used…we don’t know how much is being produced and also don’t [know if the emissions rate] is correct,” says Ray Weiss, a geochemist at the Scripps Institution of Oceanography at the University of California, San Diego, who led the new work.
Emissions numbers conflict depending on whom you ask. The semiconductor industry started to use nitrogen trifluoride as a greener alternative for cleaning chipmaking equipment in the late 1980s. Making integrated circuits involves depositing layers of materials such as semiconductors and metals on a silicon wafer. These materials also stick to chamber walls. So after each layer is deposited, nitrogen fluoride is pumped into the chamber and is split to release highly reactive fluorine atoms that clean the walls. Air Products and Chemicals, Inc., based in Allentown, Pa., which manufactures a third of the world’s nitrogen trifluoride, claims that most of the gas is utilized during the process and what is leftover is trapped and destroyed in a special waste system.
But conditions in a silicon chip–fabrication facility are anyone’s guess due to the lack of regulation or regulatory oversight, Prather says. The waste systems might be designed to destroy 97 percent of the gas, but that is under perfect conditions. “Most [semiconductor manufacturers] don’t achieve that because they’re hurrying in production,” he says. The gas tanks themselves might leak or be mishandled during transport and disposal. Besides, manufacturers might not even be using control measures. “There is a whole chain of events, so I don’t think two to three percent [emission] is real.”
Weiss’s study lends proof to Prather’s worries. The concentration of nitrogen trifluoride in the atmosphere is about 0.5 part per trillion, making it very hard to measure. Weiss had to distill, heat and pass the air samples over adsorbents to remove gases such as carbon dioxide and krypton that could foul the extremely sensitive detector. He found that about 563 metric tons of nitrogen trifluoride was emitted in 2006. From his measurements, he calculates that the emissions have already increased to 620 metric tons in 2008, which is about 16 percent of the 4,000 metric tons that Prather estimates will be produced and used this year.
The emissions will escalate as nitrogen trifluoride’s use increases. Although a chipmaking chamber is about the size of a refrigerator, those used to make LCD panels are the size of a van, says Steve Pilgrim, global marketing manager at Munich, Germany–based The Linde Group, a nitrogen trifluoride producer. Meanwhile, thousands of megawatts worth of thin-film solar cells are in the manufacturing pipeline. “For every megawatt of solar panel capacity, you’ll need a ton of NF3 for cleaning the equipment,” Pilgrim says.
Air Products claims that worldwide production of nitrogen trifluoride has reached 7,300 metric tons. The company is now building a 500-metric ton plant that will take the company’s capacity to about 2,400 tons next year.
Some companies are solving the problem by adopting alternatives to nitrogen trifluoride. Toshiba Matsushita Display, Samsung and LG have installed systems that generate fluorine on-site at some of their LCD and semiconductor facilities. The system, made by Linde, splits hydrogen fluoride into fluorine. That takes less energy than splitting nitrogen fluoride and there is no global warming risk, Pilgrim says. However, the system does need upfront costs that smaller LCD manufacturers might not want to bear. Any accidental release of fluorine could also be an issue: Fluorine is a toxic and corrosive gas and, at high concentrations, can retard plant growth and damage teeth and bones.
Prather says we should now be following nitrogen trifluoride concentrations in the atmosphere closely. There needs to be pressure on the electronics industry to report emissions, he says. A good start would be including nitrogen trifluoride in the list of greenhouse gases being regulated by the Kyoto Protocol, which aims to reduce discharges of carbon dioxide and six other greenhouse gases by assigning emission limits to countries that have ratified it. “The real issue is we’re missing international reporting,” he says. “We should start reporting it immediately and measuring it, and then we’ll find out how important it is.”