Gases/Chemicals: Gas and chemicals

01/01/2000

Mitch Sisemore, National Semiconductor, Arlington, Texas

Special Section - Technology Outlook

As we enter 2000, it is important to look at the future of gas and chemical technologies from a clear perspective of our accomplishments and present status. The challenges can be viewed as problems, but all indications point to success in solving them, taking them out of the realm of impossibility and into reality.

Component lifetimes in corrosive service

Probably the biggest challenge facing gas systems technology is effectively managing corrosive gas delivery, especially in such high-use, high-flow applications as epitaxial silicon processing (which uses large volumes of HCl, corrosive silicon sources, and dopants). Component failures or gas leaks can be disastrous. The potential impacts to the process, cost of repair, cost of downtime, and safety concerns make managing corrosive gas service a critical topic.

Some benefits can be gained with highly corrosion-resistant materials such as Hastelloy. In addition, where justifiable, changing to a bulk delivery system can reduce failure rates by reducing the number of cylinder changes and thereby reducing the exposure to atmosphere. The risk is still substantial, however. Additional improvements need to be made. Some gains will be possible with new, even more corrosion-resistant materials such as low-manganese stainless steel, as well as improved component designs like surface mount components.

An emerging technology that is gaining interest is in the area of high-pressure corrosive purifiers. They can operate in the cylinder pressure ranges of corrosive gases (e.g., >600psi for HCl). Whether as a separate component or integrated into the gas cylinder, a high-pressure purifier will remove moisture and oxygen from a process gas prior to its entrance into the gas panel. By removing the essential ingredients of corrosion (i.e., moisture and oxygen), the purifier may significantly reduce the corrosion of gas panel components. Reducing corrosion extends component lifetimes, improves process reliability by reducing metal contamination and particles, reduces safety risk for component leaks, and decreases overall maintenance costs of components in gas service. Within five years, purifiers of this type will be available for use in the field and will aid significantly in simplifying the maintenance of gas cabinets and improving component reliability.

Autoconnection

When dealing with hazardous gases, safety is the most important consideration. Under normal operating conditions, the risk for a hazardous exposure is substantial any time a gas line is opened, whether during a cylinder change or other maintenance activity. During a cylinder change, one can be exposed in two main ways:

• Residual gas remaining in the pigtail — because of a poor purge or faulty cylinder valve — can be released when an "empty" cylinder is disconnected.
• A leak can be present and cause a gas release when the new cylinder is opened.

A potential safety improvement would be a gas cabinet-cylinder combination with an autoconnection capability. The connection could be similar to docking a spacecraft and would not require the direct contact of a person to either tighten the connection or open the cylinder. With the gas cylinder safely contained in an exhausted gas cabinet, personnel would be separated from hazardous exposure risks. The empty cylinder would disconnect automatically and could be capped and removed safely. The full cylinder could be installed, uncapped, and placed in position safely. Autoconnection could then complete the cylinder change.

As a person who has completed cylinder changes and is responsible for developing safe procedures for others to perform cylinder changes, I know there is room for improvement. Technology already exists for pneumatic cylinder valves. Standardizing an interface and developing the autoconnection feature is not an insurmountable problem. The next step would be to determine if there is consensus regarding the need and then evaluate potential solutions.

Modular gas system components

While the initial cost of modular gas-system-handling components (i.e., surface mount technology) is greater than that of traditional linear components, the industry expects the overall cost of stocking, assembly, and maintenance to be lower [1]. Surface mount allows standard components to be stocked that can be easily and quickly configured onto a base-block without welding. This ease of configuration allows much greater flexibility, while at the same time reducing components that need to be stocked. Maintenance is simplified because any failing component can be replaced quickly and easily — a kind of plug-in replacement. In addition, surface mount components possess a much smaller footprint compared to standard gas components, which makes them extremely attractive for applications in the gas boxes of implant and CVD tools.

Today, the greatest concern about surface mount gas components is the need to define a single standard; there are more than seven surface mount standards available or under consideration [1]. With a single standard, the benefits of the technology will be realized and these can then be passed on to customers. Without a standard, cost reductions and inventory reductions will not be possible and long-term use of the technology is likely to be minimal.

Improved reliability via redundancy

When using bulk chemical or gas supply, more and more of a fab's production capability rests on one system. This means that no downtime is acceptable. Preventive maintenance (PM) is not allowed to interfere and breakdowns can mean unemployment for the poor soul deemed responsible.

Redundancy is critical to providing an uninterrupted chemical or gas supply to a fab. With redundant systems, PMs can be accomplished by temporarily switching to a backup system while maintenance activity is completed. In the event of an unexpected failure, the chemical system can automatically switch to the backup, so the fab is never impacted and no one even realizes the primary system is no longer operational. The additional expense of a redundant system is simple to justify when weighed against shutting down an entire fab for even a few minutes.

This represents a changing attitude for process chemicals and specialty gases. One-of-a-kind tools may exist in multiple areas within a fab. Fab tools are often so expensive and cleanroom space at such a premium that it is difficult to justify redundancy. However, every fab has some degree of redundancy within its electrical system or within a facility gas such as process nitrogen. Bulk chemical systems are just as critical; for example, systems such as hydrogen peroxide feed most of the wet tools in a fab. Spiking occurs so often that even a moment of downtime can shut down several tools in the fab.

It should be understood that no matter how hard one tries, certain points will always exist as potential single-point failures. It is crucial to identify these points and develop and implement plans for as quick a recovery as possible.

Total site management

Some fabs define responsibilities so that the engineer in charge of the fab process tool is accountable for support equipment that often includes gas cabinets, valve manifold boxes, or bulk chemical systems. But gas and chemical management is a core competency all its own. Many sites are moving toward varying degrees of outsourcing the onsite management of gases and chemicals. This is done to allow a fab to concentrate on its core competency, "making wafers," and to allow experts in handling and managing gases and chemicals to work within their competency.

The downside to outsourcing is that the moment you pay someone else to perform a job you could do, you are leaving at least the opportunity for additional profit untapped.

Bulk supplies

Fabs are increasingly employing bulk delivery systems for their gas and chemical needs. Bulk supplies provide several benefits to a semiconductor fab, the most obvious being reduced chemical cost through buying in larger quantities. Also, safety is improved due to reduced exposure to chemical changes. Bulk supplies can enhance process control and improve yields by reducing the variability of chemicals used in the process. Risk of contamination due to "mispours" or connecting incorrect drums is greatly improved by transitioning to a bulk supply.

The key disadvantages to a bulk system are initial capital expense and increased risk of a truly catastrophic failure. Justifying the capital to install a bulk chemical system is straightforward. Chemical cost savings are simple to calculate and the costs of installation are fairly clear and can easily be input into a cost-of-ownership model for justification. To reduce the risk of catastrophe, the careful design and maintenance of the system are essential. To maximize reliability, an appropriate level of redundancy should be used in the design of a bulk system.

Into 2000 . . .

Clearly, there are both problems and interesting opportunities in the gases and chemicals arena for semiconductor manufacturing. Improved design and management methods will make redundant systems and total site management systems the norm. Improved equipment such as surface mount components will increase the flexibility and ease of maintenance for gas components. Advances in purifier technology will improve reliability and lifetime of gas systems. A potential safety improvement — autoconnecting cylinders — may or may not find its way into the marketplace.

Reference

1. J. Snow, S. Tison, W. Plante, "Evolving Gas Flow, Measurement, and Control Technologies," Solid State Technology, Vol. 42, No. 10, p. 51, October 1999.

Mitch Sisemore received his BS in mechanical engineering from Texas A&M University. He is the gas and chemical services equipment engineer at National Semiconductor, 1111 W. Bardin Rd., Arlington, Texas 76017; ph 817/468-6748, fax 817/557-7644, e-mail [email protected].