Consistency and Cost are Key to Gas Management
John Baliga, Associate Editor -- Semiconductor International, 5/1/1999
Cost concerns have driven fabs to outsource gas management, chemical management and associated services to companies that used to just supply the materials and equipment. These services often include the installation of all equipment and piping, and even process tool hookup on demand. Given the demands made on these companies to ensure purity levels at point of use, they naturally have a great deal of influence in the design and fabrication of the gas handling system. They have a wealth of experience not only in supplying the gases, but in maintaining the equipment and ensuring delivery of the gas right to the tool that needs it at the required time and purity level (Fig. 1).
In many cases, these companies are also asked to take care of waste gas removal. If the gas management compa ny has expertise with wet chemicals, they are usually asked to handle those as well. Gas suppliers have been making alliances and acquisitions to be able to provide this service. Semiconductor companies generally prefer to go with one supplier for gas and chemical services, and many have long-term relationships with suppliers.
Suppliers are responding to the continuing need for a single gas and chemical contact by offering total gas management services for existing fabs regardless of which company's equipment the fab uses. For new installations, suppliers seem to be willing to do the same if customer demands make it necessary, though this can affect the level of quality assurance the supplier would be willing to provide.
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Fig. 1. Most new fabs have their gas service provider take care of gas management right through to the point of use. (Source: Air Products and Chemicals) |
More bulk delivery
One of the current trends in gas management is the move to more bulk delivery. Carrier gases like nitrogen have been delivered in bulk or even generated on site (Fig. 2), but as volume needs for other gases increase, it makes more sense to deliver those in bulk as well.
Marcelo Viera of Praxair (Danbury, Conn.) says that this is a major trend. He said, 'The advantages include fewer changeouts, which improves safety and reduces operating costs; higher, more consistent and reliable product purity; and significant cost of ownership benefits.'
One benefit is simplified logistics. Instead of keeping many cylinders of silane, for example, in the subfab and changing them on a daily or weekly basis, a tube trailer can be hooked up outside the fab, which can be left alone for months at a time (Fig. 3). Also, there is added safety in keeping the silane supply away from the fab.
Another advatage to bulk supply is improved contamination control. Less handling means fewer chances to generate contamination. After a changeout is done and any resulting contamination is handled, the gas supply will require relatively little attention from yield engineers for months.
In addition to silane, the etching gases C2F6 and NF3 are also starting to move to bulk delivery. Helium and HCl have also moved to bulk delivery in certain cases.
Increased partnering
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Fig. 2. Generating carrier gases on site is reliable and economical for large fab installations. (Source: Praxair) |
Most suppliers see no real problem meeting the industry roadmap targets for purity, even though that means less than one part per billion (ppb) purity in some cases. The real challenges, according to the suppliers, are about the supplier and the fab working together in the fab and at the supplier's plant.
Pascal Mauberger of Air Liquide (Paris, France) said, 'The biggest challenges in gas management are not technical. They are more what we could consider cultural.' He said that the total gas management (TGM) team has to become part of the fab team. 'The point is that we have to really integrate ourselves in their process. We have to measure ourselves against their indicators and ratios.' This kind of integration is easier to achieve between companies with long-term relationships.
Another way this trend manifests itself is in the quality requirements for specialty gases. In more cases, fabs are considering consistency as more important than absolute purity. Jerry Ermentrout of Air Products and Chemicals (Allentown, Pa.) said that fabs are getting more interested in looking at SPC data and knowing that the gas manufacturing process is under control than in looking at the purity at delivery. He said, 'Any variation in the incoming material can cause a problem, even if it's within the spec.'
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Fig. 3. Connecting 'tube trailers' at a gas pad is one of the many methods of bulk gas delivery. (Source: Praxair) |
One conflict that is becoming more frustrating in semiconductor manufacturing is the conflict between this consistency and process improvement. Ermentrout noted that these conflicts happen all the time, but that these fab customers 'have certain windows of opportunity for you to introduce new ideas.'
Costs, audits and long-term planning
Cost issues keep entering into the equation for fab managers. Actually, the material cost of the gases is small compared to handling and managing them. As fab managers look for more places to reduce costs, their evaluation of gas and chemical services should start with an accurate evaluation of where their costs lie. This includes the value of avoiding a large loss (see 'Continuous Gas Monitoring Reduces Losses').
Chris Elsmore of BOC Edwards noted, 'We've done a number of audits last year at customer facilities where we've looked to offer savings by combining gas, chemical and vacuum services, and the most difficult part is getting to the customer's root cost of managing the product themselves. By that, I mean they may have 10 people, each with 10% of their job managing chemicals, and that cost is not included in the calculation when they look at outsourcing. Once the root cost is identified, the savings of outsourcing are tangible and significant.'
After dealing with the large ticket items like the process tools, details of gas and chemical systems can easily receive less attention than they need. Small up front costs are attractive, but they can lead to very large total costs over time (see 'Costs and Gas Monitoring').
Recovery and recycling
Gas handling companies have been asked to handle the waste effluent as well as supply the process gas. This may seem to be another one of many tasks to place on the gas company, but it is also an opportunity for recovery and recycling.
Typically, the waste effluent is treated by running it through a scrubber, possibly having been burned first, and then released into the air. However, with some processing, the original gas can instead be filtered out of the effluent and used as a feedstock for the gas provider to manufacture process quality gas.
The idea of recycling a gas in the fab has been tossed around in recent years, but it is not yet a practical reality. On site purification of gases and chemicals is not widespread, nor does it appear that it will be any time soon.
Consistency
Semiconductor companies with multiple fab facilities strive for a very high degree of consistency between the facilities and in each fab over time. Chris Elsmore of BOC Edwards said, 'When we work with a customer in Japan, America and the UK, they definitely want the same systems and procedures throughout every facility based on BOPs (best operating practices).'
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Fig. 4. Consistent practices are a must, especially for companies with fabs in multiple countries. (Source: BOC Edwards) |
Jerry Ermentrout of Air Products believes that providing services at multiple facilities gives them a way to develop their best practices. 'We collect data at all the sites and do a lot of sharing of best practices across the sites. So, our learnings exceed even those of let's say the large semiconductor companies, because very few of them operate the number of fabs that we do.'
Another advantage to the approach is when an issue arises in one facility, the solution to that issue can be implemented in all the other facilities (Fig. 4). Elsmore continued, 'We've found in the past that if the customer has an issue on a site in Japan, procedures must be in place to immediately communicate this, as the next morning you can bet you're bottom dollar they'll knock on your door in the UK asking what we are doing about it over here to make sure it doesn't happen.'
Conflicts can arise, however, with this approach. One is the balancing act between the vendor's best practices and confidentiality agreements with their customers. Elsmore says, 'Anything we do with a customer is always confidential, so our BOPs draw experience from our install base while simultaneously maintaining customer confidentiality.'
Conclusion
More gases are being delivered to fabs in bulk, and this
trend offers a number of advantages to a fab beyond the cost savings. As gas
management providers develop and offer more services, fab managers should
accurately evaluate all the costs associated with managing their gas systems for
the life of the fab before evaluating proposals for outsourcing. Although
opportunities always arise to improve the processes for handling these gases,
maintaining consistency is the key to successful gas management. 
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Continuous Gas Monitoring Reduces Losses Variations in gas quality can severely affect fab yields and the bottom line, as the following scenario illustrates. Something is wrong with the metalization. Monitor wafers indicate that the sputter-deposited metal layers are thinner than they should be. It was fine the week before. All the deposition parameters - power, time, pressure - are as they should be. Eventually it is discovered that someone either turned on or neglected to turn off a critical valve after a gas supply was changed. The films, instead of being sputter-deposited by pure argon, were sputtered in an argon/hydrogen mixture. Two thousand wafers are affected, some near the end of the process, each valued at $1,000 or more. In many fabs, the major bulk gases, nitrogen, hydrogen, oxygen, argon and helium, are supplied by a gas distribution system linked to bulk storage tanks. Unfortunately, the possibility of an event such as just described is all too real, and it can be costly. Such a loss can be prevented or significantly reduced by continuously monitoring the gas distribution system with quadrupole mass spectrometers (QMSs) set up to display simple red/green indicators signifying go-no-go conditions. Continuous monitoring can provide early detection of cross contamination. It can also provide continuous purity certification at every delivery point, on-line leak detection and the needed information for trouble shooting. Modern software can control multiple independently operating sensors and simultaneously display simple red/green enunciators on a PC screen. A red alarm indicator can also give basic information on the nature of the contaminant gas and be quickly switched to a full-spectrum display. Data history can be displayed to indicate the exact time that the problem occurred and the history of the degree and nature of the contamination. Rapid sampling can be done at gas line pressures by pressure reduction to the level that a QMS closed source ionizer can operate, typically <10 mTorr. Although atmospheric pressure ionization mass spectrometers (APIMS) can be used, they are more expensive than a closed ion source QMS system. Miniature high-pressure QMAs that can operate at over 20 mTorr would be less costly, but would also be less sensitive and would still require a pressure reduction system. Since the primary objective of in-line continuous monitoring is to catch catastrophes, extreme ppb sensitivity is not required. A typical QMA sampling a gas at 30 psig can detect <10 ppm of contaminants (~10-9 Torr minimum detectable partial pressure in a sample reduced to 3 mTorr). This is sufficient for most leak detection and trouble-shooting as well as detecting percentage-level contamination problems. In a typical five-line installation, each gas line has a dedicated QMS. Each instrument has its own operational 'recipe' set for the particular gas and contaminants, with upper alarm limits for peaks that represent the other four gases. Multi-peak, multi-event matrix alarm setups can be used to immediately identify and display possible alarm causes such as residual air or air leaks, hydrocarbon contamination or cross-contamination by another pure gas. The system can be connected to a factory automation system and via a LAN to a PC at an engineer's desk. Since the
pressure of the gas being monitored is known, calibration can simply be
the periodic adjustment of the QMA electron multiplier voltage to bring a
principal gas peak (e.g., mass 14 or 28 for nitrogen, mass 2 for hydrogen)
up to a standard calibration set point. This procedure can be automated
with a software recipe.
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Costs and Gas Monitoring The primary criterion in gas detector selection continues to be the reliability of the monitoring systems. Not only must they alert personnel to any emergency situation, but they must also avoid false alarms. False alarms raise questions about the credibility of the detection system, and they can cost anywhere from $80,000 to $500,000 per event. If the detection system 'falls asleep' without warning, a situation much worse than an expensive exercise may occur. All of the reliability and cost issues of the detection technologies must be considered up front. The two main technologies for detecting toxic and corrosive gases are colormetric tapes and electrochemical cells. Tape systems tend to cost more up front than electrochemical cell systems, but there are reliability and long-term cost advantages to consider. Tape systems have historically been less susceptible to false alarms. Cross sensitivity issues can cause a false alarm in some electrochemical systems. Some electrochemical systems, if poorly shielded, are susceptible to a false alarm from RFI events such as keying a walkie talkie. Also, electrochemical systems have had a failure mode in which they 'fall asleep' without warning. Precautions have been taken to ensure that these problems do not occur, and they add cost to the systems. For the record, Zellweger Analytics offers both of these detector technologies. Other costs include consumables use and calibration costs. For these, a look at the long-term costs is necessary, because they can be significantly different for the two technologies over the course of five to 10 years. For combustible gas detection, the majority of companies
use catalytic bead systems to monitor primarily hydrogen but also
solvents. Tapes and beads may seem to be 'low-tech' methods, requiring
visual monitoring. They also can be the lowest cost and most reliable
methods to employ in a fab. |
