Bulking up specialty gas delivery
09/01/2000
by Sheila Galatowitsch
 |
The pressure is on: Gas system vendors prepare to deliver reliable, pure and safe bulk specialty gases to 300-mm pilot line facilities
In two decades of supplying gases to semiconductor fabs worldwide, major gas suppliers have built a wide variety of delivery systems, pipeline networks and standalone on-site supply systems. For bulk gas systems, semiconductor manufacturers may opt for an air separation unit to deliver nitrogen and oxygen, or they may receive as much as 40,000 pounds of liquid nitrogen, oxygen, argon or hydrogen transported via tractor trailer to storage tanks at fab sites. Specialty gases, however, are typically supplied in cylinders, which hold about 50 pounds of product.
But for new 300-mm pilot line facilities expected to become operational over the next year, specialty gases are bulking up. Production of 300-mm wafers will require even more gas consumption than production of 200 mm wafers—as much as 200 percent more. And higher production levels may mean more tools tied into a gas supply system.
That's why semiconductor manufacturers are replacing traditional low-volume cylinder delivery systems with new bulk gas systems, and gas system suppliers like Praxair Inc. (Danbury, CT), Air Products and Chemicals Inc. (Allentown, PA), BOC Edwards (Wilmington, MA) and Air Liquide Electronics (Dallas) are gearing up to deliver reliable, pure and safe specialty gases in bulk. Some bulk systems used in various processes, including plasma etching, chemical vapor deposition and chamber cleaning, will contain from 1000 to 8000 pounds of specialty gases, such as nitrous oxide and halocarbons with ammonia flowing at rates up to 530 pounds.
More practical solution
The higher flow rates required for the larger wafers simply make bulk supply more practical, says David Coveney, global business manager for semiconductor process gases at Praxair. "A cylinder may only have 50 pounds of gas in it, whereas bulk supply would have 10 times that amount." The new wafer technology also requires larger chambers to form the wafer layers—another reason bulk gas delivery makes more sense.
 An Air Products bulk specialty gas system installation at a major semiconductor manufacturing facility. |
Bulk specialty gas is altogether more efficient for both vendor and manufacturer. Apart from the significant cost advantages inherent in larger-volume systems, the bulk approach will save on labor costs associated with analyzing and replenishing materials and maintaining delivery systems. It will make the logistics of transport and distribution more efficient. And because it is delivered in such large volumes, bulk gas will provide a more consistent, reliable supply of gas.
Bulk specialty gas delivery will also improve personnel safety. With operators changing out cylinders every couple of months, rather than every few days or weeks, it minimizes exposure to hazardous gases. "Every time you avoid making a change-out, you reduce potential risk to personnel," says Coveney.
 Air Products' purification system removes contaminants that can cause process variability. |
Perhaps as important, bulk specialty gas systems will provide a higher level of contamination-free manufacturing, says Joseph Stockunas, worldwide marketing manager in Air Products' electronics division.
Fewer cylinder change-outs mean fewer opportunities to introduce contamination into the gas delivery system. And because bulk supply methods draw from a larger reservoir of the same material, fewer qualifications are required compared to low-volume cylinder delivery.
The greater consistency of the material in bulk systems will also help reduce the risk of contamination for the thinner layers on the 300-mm wafers. In fact, BOC Edwards has found that the consistency of process gases is "every bit as important to our customers as the absolute gas purity," says Chris Caso, global product manager for electronic materials. For example, even though a particular gas may be within specification, there still could be significant variability within that specification.
"For increasingly complex semiconductor processes this is no longer acceptable, since these variations—although within the absolute specification—can still cause repeatability problems within the process," says Caso. For these reasons, BOC Edwards performs statistical quality control (SQC) analysis of its manufacturing processes, with the goal of eliminating the variability. SQC takes into account all aspects of the manufacturing process, from cylinder preparation to purification, filling and final analysis.
Designing proper liquefied systems
The changeover to bulk, however, often overlooks the physical changes that accompany the switch, says Paul Espitalier-Noel, BOC Edwards' general manager for systems engineering. Because many specialty gases are stored under pressure in liquid form, they are often placed in bulk liquefied gas containers and located outside or in remote buildings, either for safety purposes, economical use of building space or to accommodate container handling. This outdoor placement exposes the gases to temperature extremes they didn't see in the cylinder cabinet. While the airflow in a cylinder cabinet is regulated to meet specs and codes, there is no guarantee of any airflow outside, says Espitalier-Noel.
 Air Products' MegaBIP purification system. |
Such key thermodynamic issues must be taken into account in a good design of a bulk specialty gas system. To better predict system performance and optimize designs, BOC Edwards has developed modeling tools and simulations that track the entire process from the gas source to point-of-use. "Detailed design modeling is a trend that will only intensify in 300-mm plants," says Espitalier-Noel.
It is even more important to have good engineering and design modeling for low-vapor pressure gases, adds Caso. "The margins for error are much smaller for low-vapor pressure gases because they operate over a reduced pressure range and are highly sensitive to the temperatures they encounter. The impact of a poor design, which doesn't take into account the thermodynamic properties of the gas, can include low pressure and low or no flow at the process tool." BOC Edwards' extensive gas properties database and modeling software have allowed it to develop a cost-effective standard bulk design for both low-pressure vapor and liquefied compressed gases.
 Praxair's Kingman, AZ, facility for semiconductor process gases. |
Properly heating liquefied gases in bulk quantity poses another challenging problem. "It becomes a double-edged sword," says Hwa-Chi Wang, manager of worldwide electronic research and development at Air Liquide Electronics. "Inadequate heating results in insufficient vaporization of the liquids and delivery of a vapor-liquid mixture to the process chamber. This has been identified as a major problem to process control and corrosion," says Wang. "On the other hand, over-heating can cause re-condensation of the vapor in the downstream distribution network, which again forms a vapor-liquid mixture."
To meet these challenges, Air Liquide has developed a distribution system that delivers liquefied specialty gases in bulk quantities. The heating is controlled by programmable logic control and is based on the temperature of the liquid instead of the outer wall of the container. "This is a critical aspect," says Wang. "There can be a huge difference between the two temperatures because of the enormous thermal mass of the container."
Besides the consumption flow rate and duration, Wang reiterates that optimum design of the bulk specialty systems also depends on the geographic location and local regulations. For example, if a bulk system needs to be located outdoors at below-zero ambient temperatures, heating solutions would become very expensive, if not impossible. Air Liquide has developed a bulk specialty gas system, called the BEPS-L, that more efficiently withdraws and heats liquid from the container. Says Wang: "In BEPS-L, you only heat the amount of the liquid to be delivered to the process chamber, which is a tiny fraction of the entire quantity in the container. This concept reduces significantly the size of the heating system and thereby the cost."
Improving gas system purity
Beyond preparing to supply specialty gases in bulk containers to 300-mm pilot plants, these vendors are also finding ways to improve the purity and analytical capabilities of gas delivery systems, especially because higher flow rates have been known to carry more particles through the systems.
 Praxair's Kingman, AZ, facility for semiconductor process gases. |
In addition, rapid vaporization with droplet formation can entrain more of the liquid-borne impurities into the gas phase.
Air Products has developed a gas system technology, called MegaBIP, that features a built-in purifier inside the gas cylinder, replacing point-of-use purifiers typically attached to the cylinder. The purifier has several valve features that maintain integrity, prevent any contaminants from being backfilled into the purifier and ensure that gas flows only one way through the purifier bed. The technology, which will be used in both 200-mm and 300-mm fabs, also protects against impurities that are created when cylinder contents are depleted and gas pressure decreases.
Praxair has developed new analytical tools that check the purity of bulk gases. The company's portable argon analyzer detects multiple impurities such as moisture, nitrogen and methane at part-per-billion levels. Its start-up, purge and operation are automated, and it has an on-board expert system to validate data and instrument diagnostics. Another new tool, the mobile analytical test cart, is a customized cabinet that maintains analytical equipment and data while in transit with an integrated uninterruptible power supply. It measures multiple inert or reactive base gases in traditional bulk gas systems.
 The cylinder shipping yard at Praxair's Kingman, AZ, facility. |
Caso says that BOC Edwards fills and analyzes bulk specialty gas containers using the same high-quality preparation techniques as cylinders. The process minimizes impurities introduced during the initial preparation, which will desorb over time creating a continuous source of impurities into the semiconductor manufacturer's process and negatively impact product performance. "The goal is to provide our customers with the same specification gas in bulk form as they currently receive in cylinders, eliminating process re-qualification costs upon transition," he says.
Because gas distribution in a fab requires such specialized knowledge, more manufacturers are outsourcing either all or part of the gas system management to suppliers. All of these suppliers say that their outsourcing business is booming. BOC Edwards has 700 experts working in fabs worldwide. Air Products has more than 450 on-site operations technicians maintaining gas management outsourcing contracts with 69 fabs worldwide.
Fab owners "would rather focus on their core competency making computer chips," than hassle with the complexities of gas delivery, says Coveney, who reports that Praxair's fab customers are requesting more help with installation of the bulk gas supply system and integration all the way up to the tools.
In addition, most suppliers have created multi-disciplinary teams that can determine how a process change will impact gas and chemical delivery and other parts of the manufacturing process. Such teams help speed up response time, which will be ever more critical in 300-mm fabs. For gas system suppliers, look's like the pressure's on—to stay.