Better chips need purer gases
BY HANK HOGAN
Recent announcements from Pall Microelectronics, a part of Pall Corp. (East Hills, N.Y.; www.pall.com), and Mykrolis Corp. (Billerica, Mass.; www.mykrolis.com) may portend a time when gas filters and purifiers are the same device, and when cleanroom air itself is purified, as well as filtered, to control contamination.
Cleaning up carbon monoxide
In late September, Pall Microelectronics announced it had developed a technology based on its inorganic purification media, AresKleen, to purify carbon monoxide (CO) down to a sub-parts per billion (ppb) level. Carbon monoxide is widely used today in the semiconductor industry to improve plasma etches in advanced processes. In the future, the gas could help produce carbon nanotubes.
The problem is that carbon monoxide isn't pure. It contains trace amounts of oxygen, moisture, carbon dioxide and metallic compounds known as metal carbonyls. The carbonyls can be deposited on a wafer's surface during an etch, leading to a change in device electrical characteristics and performance.
One source of contamination is the gas itself. The minute amount of water present in carbon monoxide reacts with the cylinder and the tubing used for transport.
Mykrolis Corp.'s Aeronex Infinity systems will be used to purify hydrogen, nitrogen and ammonia used in the manufacture of high-brightness LEDs in Taiwan.
Bottle vendors get around this problem by eliminating steel- and nickel-wetted parts in favor of an aluminum cylinder, a brass valve, and a copper-based pressure relief device. "It is possible to provide high-purity CO in a bottle," says Ralph Richardson, director of business development of the electronics division of the gas supplier Air Products and Chemicals Inc. (Allentown, Pa.; www.airproducts.com).
Richardson acknowledges that while there are issues with carbonyl generation at high pressure, with proper generation, a careful choice of containment materials, and bulk material purification, it is possible to deliver high-purity carbon monoxide without a point-of-use purifier. "It is common practice for some users to include a point-of-use purifier in the gas cabinet for insurance but it's not necessary," he remarks.
Pall Microelectronics' Vice President of Scientific and Laboratory Services, Barry Gotlinsky, however, favors the contamination-control insurance policy of point-of-use purifiers because of chemistry issues. Because of the pressure of the bottled gas, he says, "these impurities form readily. The moisture essentially catalyzes the formation of these nickel and iron carbonyls."
That's why, Gotlinsky believes, purifying carbon monoxide as close to the point of use as possible is so important.
Looking forward, Gotlinsky sees the use of combination purifiers and filters to remove particulates and molecular contamination. The key, he notes, is that the combined device must fit in the same space as present day filters and must not demand too high a gas flow. Such filter purifier combinations are already in development.
Stepping into the light
A mid-September announcement from Mykrolis that it was shipping gas purification systems to be used in light emitting diode (LED) production is another example of increased gas purification awareness. Sent to Taiwanese manufacturers, the Mykrolis Aeronex Infinity systems will be used to purify hydrogen, nitrogen and ammonia gas for the manufacture of high-brightness LEDs.
"The market for these devices has been very robust, growing at 45 to 50 percent per year since 1995," says Robert Steele, director of optoelectronics at the market research firm Strategies Unlimited (Mountain View, Calif.; www.strategies-u.com). "Recent market numbers are $1.8 billion in 2002 and $2.7 billion in 2003. We are projecting $3.6 billion in 2004."
High-brightness LEDs, which are based on compound semiconductors, are produced via epitaxy using high-purity gas precursors, such as trimethyl gallium, trimethyl indium, ammonia and phosphine.
For volume production, however, LED manufacturers need an ongoing gas supply. The purity level required depends on the particular gas. Several specifications call for contamination levels of less than one part per billion (1 ppb), but that level of purity won't be needed in standard CMOS semiconductor technology for a few years, according to Dan Alvarez Jr., technology director of the Mykrolis gas microcontamination business unit.
While 1 ppb levels could be achieved by point-of-use purifiers, Alvarez argues that single-system purifiers will eventually have to be swapped out, leading to downtime during the changeover. In contrast, Alvarez claims the dual-bed nature of the Mykrolis' systems avoids the problem of having to swap out the purifier. "One bed can be in regeneration mode while the other is operating; then, they switch back and forth," explains Alvarez.
Every breath you take
One area of gas purification affects every semiconductor cleanroom using state-of-the-art processes. Cutting-edge lithographic tools using 193-nanometer (nm) lasers, because of the wavelength of the light and the resulting energy of the beam, are forcing manufacturers to consider airborne molecular contaminants that had previously been ignored.
"There's now a closer look at lighter, low molecular weight organics, especially those that contain things that can end up on the lens and be difficult to clean off," says Dave Ruede, group manager for filtration products at chemical filter maker Extraction Systems Inc. (Franklin, Mass.; www.extraction.com), which specializes in cleaning up and monitoring the air and gases used in lithographic tools and process.
Among the potential problems, Ruede lists silicon, boron, and phosphorus containing compounds. By themselves, these molecules would not adhere to tool optics but when blasted by a 193-nm beam, the compounds can be converted to sand and other substances that are difficult to remove.
Advanced semiconductor processes could also introduce problems from the use of novel materials, such as low-k and high-k dielectrics. While the exact impact on gas purification needs is unknown, many industry watchers expect that the purity levels will have to go up. Although the introduction of novel materials to advanced semiconductor fabrication can both improve performance and enhance yield, there's also a contamination control consideration. As pointed out by Pall's Gotlinsky, "You do improve the process, but you've got the process uniformity to such a point that now new contaminants come out that you haven't seen before." Clearly, says Gotlinsky, part of the solution is to improve the purity of the materials going in.
It's likely that greater care will have to be taken to segregate air flows within tools. There also could be a need to improve monitoring of the air in the cleanroom itself. What some see happening is that gas purification won't be just for the materials going into tools and process chambers, but for the air in the cleanroom itself—possibly leading to the introduction of units that take the place of today's HEPA filters. These combination products would filter particles while removing chemical contaminants.
It's an evolution that some see as inevitable. "At some point, we think it will be necessary to have bulk air purification," says Mykrolis' Alvarez.
A fluorine alternative
WILMINGTON, Mass. — Fluorine gas is favored for use in next-generation semiconductor chemical vapor deposition (CVD) chamber cleaning because of its effectiveness and because it does not directly contribute to PFC emissions. But expense, plus handling and safety concerns when supplied via high-pressure cylinders, have hindered its sweeping acceptance.
To counter concerns, companies such as BOC Edwards (www.bocedwards.com) have developed on-site fluorine generation technology designed to offer a less expensive and safer solution.
Recently, Hynix Semiconductor (Ichon, South Korea) became the sixth manufacturer to install BOC Edwards' Generation F fluorine-generation system for use in its 300-mm wafer processes. The company's on-site technology has also been installed for use in LCD manufacturing.
Rather than supplying extremely volatile fluorine via high-pressure cylinders, the Generation F systems provide low-pressure packaging directly to the semiconductor tool via hydrogen fluoride (HF) feedstock at the manufacturing facility. The system includes cell, fluorine purification and compression, a storage buffer, and controls within a ventilated enclosure.
The F400 module, rated at 400 liters per hour, is designed to deliver up to 16 kilograms of pure fluorine per day to the process tool.