Trends in Gas Management and Use

		At a Glance
	Gas suppliers are working to deliver higher levels of purity with an acceptable cost-of-ownership and provide solutions for new gases, such as ClF3, while still meeting stringent environmental, safety and health concerns.

Dozens of different gases are used in the 350 or so individual process steps required to manufacture an integrated circuit in the 0.25 µm (250 nm) regime. These are traditionally defined as either "bulk" gases -- oxygen, nitrogen, helium and argon -- or "process" or "specialty" gases, which includes everything else.

Key trends

Beyond the need for a reliable, uninterrupted supply of a wide variety of gases, semiconductor manufacturers continue to push gas suppliers to deliver higher levels of purity, measured not only in terms of particle levels but by trace levels of impurities, especially metallic impurities such as iron. At the same time, there is a desire to reduce the cost-of-ownership of gases, which has resulted in a significant trend toward the delivery of heavily used gases, such as silane, in bulk containers.

The semiconductor industry is also beginning to use a variety of fundamentally new materials, including copper interconnects, new diffusion barriers (i.e., WN) and insulators with a low dielectric constant. Work also continues to widen the process window and improve the performance of existing etch and CVD processes, as well as to find a new solution for chamber cleaning. These trends -- especially in the area of CVD, etch and chamber cleaning -- create strong interest in new types of gases. One notable gas that is finding increased use in the United States for chamber cleaning is ClF₃, which has been used for some time in Japan. "Chlorine trifluoride, which dissociates without the need for plasma, can be a very effective cleaning tool," said Noel Leeson, vice president of electronics for BOC Edwards (Murray Hill, N.J.). "And it can enhance productivity, because you only have to have about a fifth the number of wet cleans." Nitric oxide (NO) is another gas finding increased use, mainly for gate oxide nitridation where it replaces N₂O.

1. One of semiconductor manufacturers' most basic requirements is for a reliable, uninterrupted supply of high-purity gases, delivered in a safe and environmentally responsible way. (Source: BOC Edwards)

The increased emphasis on using gases responsibly to keep workers' safety and health intact while also minimizing the impact to the environment is also having a significant impact on how and what gases are used in semiconductor manufacturing. Many gases are highly reactive and can be corrosive, flammable, pyrophoric, toxic and/or carcinogenic. This impacts not only handling protocol during events such as cylinder change-outs, but the piping (double-wall tubing may be required), monitoring, gas cabinets and a variety of other issues. Environmental concerns dictate which materials are used and also how the gaseous byproducts of process reactions are treated. Perfluorocarbons (PFCs) in particular have been targeted as an environmental threat, and major efforts, mostly voluntary, are under way to curb their release into the atmosphere.

 

Finally, there is a strong push to combine new analytical techniques with an understanding of how gases interact to help gain knowledge of what is going on inside processes. Increasingly, tools such as residual gas analyzers and other in situ sensors are used to analyze gas reactions during a process and then use that information to characterize, track and generally improve the process.

How gases are used

2. From the gas cabinet, gases are piped through the fab to the point of use.

Gases can be made available to the fab in one of three ways: they are generated on-site; they can be delivered in bulk tube trailers or 1000 lb tankers; or they are delivered in more familiar 100 lb cylinders.

Bulk gases such as oxygen, nitrogen, helium and argon are usually generated on-site with what is basically a miniature air separation plant. These are used in such large quantities that this proves to be quite cost-effective. More recently, technology has also been developed to generate arsine and phosphine on-site, near the process tool. This is driven by safety concerns, since it enables highly toxic arsine and phosphine gases to be produced from a relatively benign source, close to where the gases are to be used.

Cost and safety issues have also led to the increased use of bulk delivery of some gases. Here, trailers with a number of long cylinder (tube trailers) or larger tanker trucks are parked outside the fab, and the gas is piped in. This trend initially started with silane, which is pyrophoric (meaning it burns upon exposure to air), but has now encompassed other types of gases used in high volumes. "That's quite a strong trend, and we see that coming into more and more materials, such as the halocarbons and NO, where there's some good economics in going to a bulk distribution system," Leeson explained.

In addition to the cost savings, such an approach is also safer, as Mark McClear, worldwide marketing director of Praxair, Electronic Gases, Services and Systems (Austin, Texas), noted: "A tube trailer of silane eliminates the change-out of 300-400 cylinders per year, and every one of those change-outs is the opportunity for a safety event. You only change your tube trailer once a year." Reducing the number of change-outs not only helps improve contamination control and reduce worker exposure, but it also reduces the risk of error. "The greatest opportunity for danger in any of these toxic or hazardous or pyrophoric gases is the human element," said Air Liquide's (Walnut Creek, Calif.) Les Polgar.

The third way of supplying gas, which is actually the way most gases are supplied, is in a gas cylinder. Gas suppliers go to great lengths to treat the interiors of these cylinders to minimize contamination, to the point of including a polished coating of nickel for reactive process gases. In the fab, these cylinders are kept in a secure storage area or "bunker" and when needed, installed in a gas cabinet (Fig. 1). A gas cabinet typically holds one or two cylinders and has special venting in case of leaks, a scale to monitor how much of the gas has been used and a complex gas panel used for pressure and flow control, venting and purging. They may also include fire sensors and/or toxic gas monitors as appropriate.

From the gas cabinet, gases are piped through the fab to the point of use: the process tool (Fig. 2). The installation of this piping, usually made of stainless steel, is a science unto itself, especially the fittings, which must be carefully welded to minimize contamination. Various analysis, including particle measurements, is often done at various points in the distribution process (Fig. 3).

3. Gas suppliers are being asked to provide sophisticated gas analysis and monitoring capabilities. (Source: BOC Edwards)

At the process tool, gases are controlled by another complex gas panel, which often includes valves, mass flow controllers, gas filters and pressure regulators and transducers. The gases are precisely metered into the process chamber, where they react with each other and with the wafer. Unused gases and process byproducts are swept downstream by a vacuum pump (or series of pumps) and subsequently trapped, treated so that they are safe to release to the atmosphere or recycled.

The cost/purity tradeoff

The gas industry has been asked to supply gases with higher and higher levels of purity -- usually measured in the number of "nines" -- and they have delivered. Bulk gases are now available in almost unbelievably pure levels, with seven nines purity (99.99999%) not uncommon. Process gases are generally not as pure but, worst case, are usually in the four nines (99.99%) range. The reactivity of process gases creates problems not only in maintaining high purity, in that they can react with the cylinder and materials in the distribution to create particles, but they can also present some difficult particle measurement challenges. Beyond particle contamination, which Leeson said is now being controlled in the 0.1 µm range, there is a strong focus on oxygen and moisture content, as well as the presence of trace elemental impurities, especially metals.

The 1997 Roadmap for Semiconductors specifically calls out allowable levels of H₂O, O₂, CO₂ and CH₄ in bulk gases (<100 ppt for the 0.15 µm (150 nm) generation and beyond) and allowable particle levels (<0.1 particles/liter larger than the critical size, the critical size being defined as one-half the design rule). For specialty gases, two critical-size particles are allowed at the point of use per liter of gas. O₂ and H₂O levels are also called out for corrosive gases.

The unanswered question here is whether this trend to high-purity gases is really worth the extra cost, especially when you consider that the gases are often dumped into process chambers, where particles are typically measured in the ppm range. "We often find that the high-purity piping systems that we design, build and hook up are at far better levels than are often seen elsewhere in the process," Polgar noted. "Somewhere, someone has to ask the question: 'How much extra does it cost to really deliver this?' Very few organizations have evaluated the costs of this added protection," he added.