In the semiconductor fabrication process, hazardous and toxic gases are used in a number of manufacturing steps. While high-pressure cylinders are often used to store and deliver the gases today, they pose concerns. Use of high pressure cylinders for delivery of toxic gases can lead to accidental gas leaks and dangerous situations, which can shut down manufacturing facilities or, worse, be fatal. An alternative technology has existed for some time, and it has received attention for its inherent safety benefits. It is based on a physical property called adsorption, in which gas molecules bond to a surface. Given the right solid material, gas molecules will form a bond with its surface, effectively reducing cylinder pressure to below atmospheric pressure while maintaining the volume of gas stored. This virtually eliminates the possibility for a gas release—whether accidental or intentional. The case for using this approach to store and transport gas is so compelling that it was recently recognized by the United Nations for its safety and effectiveness in storing toxic gases.
The science behind the safety
Typically, in order for certain chemical compounds (molecules) to be accessible for use in manufacturing, they are compressed or liquefied inside their container under a charged pressure (high pressure). When materials are packaged in this manner they are defined as either compressed gases or liquefied compressed gases.
In order to compress or liquefy molecules in a container, a large amount of energy is applied to the molecules to “force” them into the container. The attendant energy contained is extremely hazardous as it provides a vehicle by which the gas can easily escape, and can result in the release of highly toxic, flammable, and corrosive chemicals to the workplace and/or environment.
For pressurized cylinders, a simple turn of the cylinder valve is all it takes to access the gas. Once opened, the pressure condition inside the cylinder forces the gas through the valve and out of the container. This can be catastrophic when handling lethal gases like arsine and phosphine. Due to this inherent hazard, the storage, use, and transportation of compressed gases are all highly regulated to protect life, property, and the environment.
Alternative solutions have been studied by scientists for many years. One such alternative is based on a naturally occurring physical attraction that was first studied in 1773. It was noted that vapor could be “taken up” inside of a charcoal structure, where its molecules would come to rest on the charcoal surface. Certain solids can form a weak bond with gases and effectively hold them (adsorption).
In 1994, Advanced Technology Materials Inc. (ATMI) introduced an adsorbent-based gas cylinder package (Figure 1) called SDS (Safe Delivery Source). It was designed and introduced to the semiconductor manufacturing market specifically to safely handle dangerous gases used in the ion implant process by way of weakly bonding (adsorbing) gas molecules inside of a nanoporous material. These adsorption forces hinder the mobility (kinetic energy) of the gas, thereby creating what’s called a subatmospheric condition and eliminating the potential risk for leaks. Put simply, if the cylinder were punctured, air would want to rush into the cylinder, versus gas wanting to come out.
The ion implant process is operated at a significant vacuum level (1X10-5 torr typical) far below atmospheric pressure. This strong vacuum pulls the gas molecules from the nanoporous material into the ion implant tool for use in the process.
The engineered nanoporous material inside the cylinder (Figure 2) is the key to its effective operation and safety. It starts with a proprietary polymer precursor material. The powdered polymer is first pressure-molded into rounded “pucks” to the exact dimensions necessary to fit inside the cylinders (Figure 3). They are then pyrolized inside custom-designed furnaces to produce high-density, non-graphitizing carbon that exhibits the desired molecular conformity. When completed, they will occupy up to or about 99% of the internal cylinder space. Yet, even with a cylinder full of carbon, the adsorbent properties allow the cylinder to hold more gas than a high pressure cylinder. In fact, the carbon-based system brings a combination of nanopores and density that provides over 500 football fields of adsorptive surface area inside a standard 18-in. cylinder.
Benefits to the community
Significant and independent testing and engineering analysis has been performed on the adsorbent-based gas cylinder technology. Independent engineering analysis shows the risk of a gas release from a subatmospheric absorbent-based gas storage technology to be 10,000 to 100,000 times less likely than with a high pressure gas storage cylinder. Any would-be gas release rate is 10,000 to 1 million times lower for a subatmospheric absorbent-based gas storage technology compared to a high pressure gas storage cylinder. And the quantity of gas released, should any occur, is miniscule compared to a high pressure gas storage cylinder.
Independent testing has also been performed in which subatmospheric absorbent-based gas storage technology and high pressure gas storage cylinders were exposed to controlled-temperature fires. Results showed that under the same fire conditions, the subatmospheric, absorbent-based gas storage technology would offer personnel egressing and first responders two to five times more evacuation and response time than a high pressure gas storage cylinder would before rupture occurs. In the event of a cylinder rupture, the blast pressure from an absorbent-based gas storage technology would be five to ten times less than that of a high pressure gas storage cylinder (Figure 4).
These results indicate significant safety benefits and indicate that adsorbent-based gas packaging technology not only protects the users but also everyone involved in the supply, shipment, handling, and even first responders and citizens in the communities through which these packages are transported and used.
Recognition by the U.N.
Historically, due to the danger from potential leaks when transporting toxic gases, regulating agencies have classified their shipment according to four main categories through the “Condition of Transport for Gases.” The conditions are compressed, liquefied (high or low pressure), refrigerated liquefied, and dissolved gases. These categories limited the mode of transport to customers, resulting in slower transport times and complicated transport logistics. These considerations are especially important in an environment where semiconductor manufacturers face continual pressure to reduce operating costs, energy costs, and capital costs.
Due to the long history and proven safety performance of adsorptive technologies in the storage of toxic gases, the United Nations’ Sub-Committee of Experts on the Transportation of Dangerous Goods (SCETDG) acted to amend the “U.N. Model of Regulations for the Transportation of Dangerous Goods” (Model of Regulations) to create the adsorbed gases condition of transport classification. The 18th revised edition of the regulations also added 17 new proper shipping names and U.N. numbers to allow expanded transportation options for adsorbed gas materials and packages.
As the U.N. sub-committee took up the issue and completed its assessment, its members unanimously voted in favor of creating an adsorbed gases transport condition classification. The new class is defined by the U.N. as “a gas when packaged for transport is adsorbed onto a solid porous material resulting in an internal receptacle pressure of less than 101.3 kPa at 20 deg. C, and < 300 kPa at 50 deg. C.”
The assessment is consistent with the National Fire Protection Agency (NFPA) 318 and International Code Council (ICC) Model Codes covering subatmospheric gas sources. In fact, the improved safety of adsorbent-based gas packaging has previously been recognized by a variety of national and international organizations including NFPA, ICC, the U.S. Department of Transportation (USDOT), Factory Mutual, and the International Organization for Standardization (ISO). Further, the NFPA Code 318 2012, titled “The Standard for the Protection of Semiconductor Fabrication Facilities,” recommends that subatmospheric packages be used instead of compressed gas packages wherever practical.
Moving forward, USDOT and other agencies around the world are expected to adopt the new U.N. provisions (possibly over the next two years). The goal of the Model of Regulations is to harmonize rules governing the worldwide transportation of dangerous goods in order to maximize public safety. These U.N. regulations are widely accepted and form the basis for most national and international shipping requirements. The U.N. maintains a list of dangerous goods of commercial interest assigning proper shipping names and numbers, specifying general packing requirements, testing, marking, labeling, placarding, and transport.
What this means for manufacturers
The U.N. reclassification could ultimately allow for adsorbed toxic gases to be transported as international airfreight versus the current requirement of shipping toxic gases by land and water only. Transportation time for gas products shipped as adsorbed gases could also be significantly reduced, decreasing costs and system down time for end users that rely on these specialty gases for the production of semiconductors. Additionally, this reclassification of adsorbed gas technology will allow more options for transporting materials, onsite storage limits, cylinder sizes, as well as how and even where adsorbed gas packages are used. All of these factors combine to enable improved efficiency and speed in the supply chain, which in turn enables end-users to reduce their local inventories, supply chain management expenses, and overall cost of ownership.
Now that the safety benefits of this technology have driven the assignment of new U.N. numbers and descriptions—all consistent with the properties of an adsorbed gas—this allows emergency response teams and first responders to more accurately assess the magnitude of potential gas release, which will lead to better critical evacuation and isolation strategies. The adsorbent-based cylinder technology and gas delivery system has greatly reduced the number of accidental gas releases in the semiconductor industry. Because of its safety record, adsorbed gas systems have also found favor with insurance companies, many of which are offering reduced premiums for using adsorbent-based gas cylinders.
Conclusion
Storage and transportation of hazardous materials has long represented an uncomfortable and challenging situation for semiconductor manufacturers and global transportation authorities—especially when it comes to toxic gases packaged in high pressure cylinders. Adsorbent-based gas packaging technology has tamed dangerous and highly toxic gases and reduced the risks and the worry of potentially disastrous situations that come with using high pressure cylinders to store and deliver these gases. Moreover, the adsorption of the gas by the carbon pores offers a solution that is cost effective and in line with supply chain and facility management needs. It enables speed, efficiency, and cost reduction in the supply chain, reduces overall storage requirements, and enables usage efficiency. Most significantly, adsorbent-based gas storage technology increases the levels of safety achieved in supply, storage, and use of toxic gases.
This provides significant peace of mind for suppliers, shippers, handlers, and users. The validation of this type of technology from the U.N. allows for more freedom in transportation options, including the possibility for transportation via air freight. Transportation times could be significantly reduced thus decreasing costs and reducing the potential for system down time for end users. It can be comforting to know that physics and natural laws can combine with technology to form cleaner, greener, safer, and more efficient solutions for industry.
Danny Elzer has 24 years of experience in semiconductor manufacturing and engineering, with specific expertise in the ion implant process and gas packaging and handling. He has been a leader in the marketing and sales efforts for ATMI’s SDS and VAC gas packaging systems for the last seven years. Safe Delivery Source and SDS are registered trademarks of Advanced Technology Materials Inc., in the U.S., other countries, or both.
This article appeared in the November/December 2013 issue of Controlled Environments.
