Safety solutions for high-pressure gas cylinders
07/01/2001
Ray Dubois, ATMI Materials Lifecycle Solutions, Phoenix, Arizona
James Mayer, ATMI Materials Lifecycle Solutions, Austin, Texas
overview
Since the early 1960s, the use of high-pressure, specialty gases in semiconductor manufacturing has significantly contributed to the rapid evolution of integrated circuit technology. Compressed gas cylinders typically contain pressures up to 2500 psig. The potential energy controlled by these cylinders is a function of fill pressure and gas reactivity. This combination has led to specialty gas cylinders often being referred to as "a sleeping giant."
The care and handling of high-pressure cylinder specialty gases have always been a challenge. In the 1960s it was quite common to find gas cylinders standing alone in fab areas with exposed regulators and piping. A leak could cause significant harm to facilities and/or personnel. The industry as a whole realized that this posed unacceptable risk (Fig. 1). Efforts were undertaken to contain gas cylinders and provide control systems that could isolate and exhaust escaping hazardous gases.
 Figure 1. The potential damage from energy restrained by a high-pressure cylinder is illustrated in the ruptured cylinder. |
High-pressure cylinder packages are constructed of aluminum, stainless steel, and a variety of steel alloys, and are manufactured without seams or welded joints through a spinning or piercing process of the bulk metal [1]. Specifications for the manufacture, applicability, and maintenance of compressed gas cylinders are governed domestically by the US Dept. of Transportation and supported by agencies such as the American Society of Mechanical Engineers (ASME), American Society for Testing And Materials (ASTM), and the Compressed Gas Association to ensure adequate safety standards within many industries [2].
Silsco was one of the first US companies to provide a complete gas containment system — the forerunner to the modern gas cabinet [3]. The design laid the foundation for gas distribution cabinets that provide multiple cylinders, purging manifolds, automatic switchover, fire suppression, and process connections.
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These systems quickly became the industry standard for safer handling, containment and distribution of specialty gases. Operator error, however, could still result in serious accidents if encountered during cylinder changeout. To help reduce these errors, manually operated manifolds were replaced with air-operated valves and electronic controllers. These systems allowed for automatically sequenced valve operation, and this led to automated purge and cylinder change protocols, essentially giving operators a "go, no-go" condition to change cylinders safely.
Today, automated gas-handling systems are widespread in the industry and have become even more sophisticated. Communication with process tools, gas detection systems, seismic monitors, fire detection and even Internet connectivity has evolved [4]. Improved gas-distribution system design had been a significant step toward safer high-pressure cylinder operation.
Advent of flow/pressure control
While safer gas delivery systems were deployed, the high-pressure connection between the cylinder valve outlet and the gas panel remained a risk. To reduce the impact of a system failure, cylinder valves needed the capability for remote operation. It took several years before reliable fail-safe pneumatic operators were adopted and integrated for use with the more hazardous specialty gases [5, 6].
During this valve development period, the restrictive flow orifice (RFO) was introduced to the specialty gas market [7]. Already in use for bulk flammable gases, the installation of RFOs into the cylinder valve outlet dramatically reduced the rate of accidental release possible from any given cylinder. This solution has become a standard for many hazardous production materials (HPMs), such as silane, arsine, and phosphine. For many HPMs, most local code authorities now require the use of RFOs, ranging in size from 0.006 to 0.040 in., with the 0.010-in. orifice being most common. This does nothing to reduce high-pressure risk, however, and further development is still required at the cylinder valve connection point (Fig. 2).
High pressure risk continues
Through the 1980s, several significant attempts were made to integrate pressure control into the high-pressure gas cylinder package. One device, an integrated cylinder valve
egulator combination, provided remote actuation and pressure reduction, but proved too complex and unreliable for broad use and was discontinued [8].
Another device was designed for installation under the cylinder valve, deep in the neck threads of the cylinder. This design utilized a ceramic pin that would break and close the flow path if a cylinder fell over, thereby shearing off the cylinder valve. This product was not broadly adopted, as most semiconductor quality cylinders utilize stainless steel valves that are unlikely to shear in a fall.
Other technologies have also been engineered to eliminate the high-pressure hazard. For example, on-site gas generation has typically been viewed as a potentially safe alternative to the distribution of high-pressure gases such as arsine and fluorine. These systems generate materials at low pressure and high purity. Unfortunately, high costs, inconvenience of operation, and a narrow range of possible applications have limited their use. The result has been a greater focus on developing high-integrity, high-purity cylinder and valve packages.
The weak link for gas packages
Twenty years ago, an effort to improve the leak integrity and safety of the cylinder valve outlet connection resulted in the creation of the DISS (diameter indexed safety system) connections. These were developed specifically for the semiconductor industry. New types of cylinder valve connections were also implemented — valve outlet and cylinder to valve. Historically, cylinder to valve connections have been mechanical pipe threads, subject to leakage. The treads can act as cavities, trapping impurities that can be deposited and released. To address this issue, a metal gasket flange system was developed to allow threadless mating of the valve to the cylinder [5]. The main barrier to widespread adoption has been the cost of this special cylinder design and the size of the fleet required to load this package into the market place. Safety concerns will dictate how quickly this package becomes adopted.
Sub-atmospheric gas cylinder packaging
Through the early 1990s, safe specialty gas handling focused on reducing the storage and delivery pressure of specialty gas cylinders. Investigation of low-pressure and sub-atmospheric (SA) delivery technologies offered the potential to deliver high-purity gases without the drawbacks of conventional delivery systems [9-11] (Fig. 3).
 Figure 3. A timeline shows the evolution of sub-atmospheric delivery options. |
Compared to a compressed gas, most available sub-atmospheric gas sources are based on the physical adsorption of toxic gases onto a specially prepared, micro- porous media [9-13]. The reversible adsorption dramatically lowers the potential energy, allowing the gas species to remain on the adsorbent at sub-atmospheric pressures (<760torr) and normal temperatures throughout the lifecycle of the cylinder. The adsorbed gas can be extracted from the adsorbent media and delivered to the tool with the application of heat or vacuum source. A typical sub-atmospheric gas source contains four components: a standard DOT-approved cylinder; a tied diaphragm cylinder valve, manual or air actuated; an internal containment filter; and a highly purified adsorbent media [9-11]. The reduction of gas phase impurities in the final package provides consistent delivered gas purity that exceeds 99.9996% [14].
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In 1994, the first sub-atmospheric gas sources became commercially available. Since then, more than 25,000 cylinders have been shipped [15]. Sub-atmospheric dopant gases have been successfully used in ion implantation [10], CVD processes including HDP-CVD [9, 16], doped polysilicon [11], and MOVPE [17]. Typical capacities and applications for SA gas sources are shown in Table 1.
Safety enhancements
Sub-atmospheric gas source packages are used to deliver toxic gas precursors used in many semiconductor processes. The binding of the gas on the adsorbent prevents accidental gas release concentrations in excess of the threshold limit values (TLV) of common dopant gases [18].
To determine the magnitude of release reductions to high-pressure cylinders of similar volume, full cylinder release concentrations were measured for 2.2 liter Arsine [18] and 49 liter SAGE PH3 [19] cylinders (Fig. 4). The worst-case release for a room-temperature SA source produced gas concentrations <1.4 ppmv with average releases <130 ppbv [19]. These concentrations are on the order of 60,000 times less than expected from a similarly sized high-pressure cylinder fitted with a RFO and containing an equivalent quantity of high-pressure phosphine.
Gas source integration
Since adsorption coefficients vary depending on gas species and adsorbent, SA gas sources deliver only 100% pure gas. The elimination of balance gas has led to dramatically increased cylinder capacity compared to high-pressure gas mixtures (Table 2). The impact may be significant reductions in downtime and required process re-qualifications as a result of cylinder changes. In many cases, higher-capacity gas sources can improve overall equipment effectiveness (OEE) and help optimize throughput and cylinder-to-cylinder process repeatability [20].
SA gas sources
The application of sub-atmospheric gas sources to semiconductor processing does not come without limitations. Clearly, atmospheric pressure processes lack the vacuum system necessary to extract the gas from SA cylinders.
For processes operating at reduced pressure, adsorbent cooling during the endothermic gas desorption process limits maximum flow capability. However, larger cylinders (>16 liter internal volume) have demonstrated >1.0 slm flows maintained throughout the lifetime of the SA cylinder. Although flows from sub-atmospheric sources are significantly less than the capabilities of a high-pressure bottle, they do confirm that delivery to multiple CVD chambers is possible and that this technology is a viable source for CVD.
To feed processes operating at higher base pressures (100-650 torr) and to overcome the effects of desorption-induced cooling, improvements in heat transfer and a method to boost gas extraction rates have been developed [17]. Also, a recent DOT exemption has allowed the preparation of SA gas source cylinders in welded vessels up to 450L in volume [21]. These large-capacity gas sources are more easily adapted to overcome thermal management issues and thus improve delivery rates and capacity for SA gas sources.
Integrated pressure control — VAC technology
One of the most significant barriers to the proliferation of SA gas sources appears to be the development of adsorbents for new gas species. Development can take years because of the complexity of testing for new gas species compatibility and purity. In order to facilitate the continued growth of safer gas delivery systems, alternative technologies are needed to bridge the gap between high-pressure and SA storage and delivery. Several technologies are under development by a number of suppliers, though few are available.
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Figure 4. Simulated valve failure depicts a) worst case PH3 concentrations; b) average release concentrations at full (700torr), half full (375torr), and near empty (78torr) pressures; and c) release concentrations for a 2.2L arsine cylinder.
One alternative technology, vacuum actuated cylinder (VAC), couples a high-pressure cylinder and gas with a specially designed cylinder valve assembly [22]. The valve is fitted with an internally integrated pressure control device that limits the cylinder valve outlet pressure to a pre-determined set point. The set point chosen for the integrated pressure control device defines the nature of the package. A VAC cylinder will not release gas until the process line pressure is less than or equal to the set point pressure of the pressure control device. With an RFO installed in the delivery port of the valve, a maximum release rate is dictated by the RFO size and the set point pressure of the VAC cylinder. The resulting release flows are expected to be significantly reduced when compared to equivalent high-pressure bottles. For a high-pressure cylinder, the release flow is defined by the size of the RFO and the cylinder outlet pressure.
In positive pressure delivery applications, the VAC cylinder can be optimized by selecting the appropriate outlet set point pressure and RFO sizing. This flexibility can lower the risks normally associated with specialty gas installations. Delivery characteristics inherent to three gas sources — SDS, SAGE, and VAC — to traditional high-pressure technology are compared in Table 3.
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As the materials used in semiconductor processes continue to diversify, improvements in gas handling will facilitate the introduction of new, better, specialty gases that will help improve yields, OEE, and device performance.
References
- Private communications with Harsco, Camp Hill, PA.
- US Dept. of Transportation, Code of Federal Regulations. Many supporting documents are available through www.dot.gov.
- Private communication with Silsco, Dallas, TX.
- "The Sudden Surge and Interest in Wafer Fab e-Diagnostics," P. Burggraaf, Solid State Tech. (on-line), March 2001.
- Handbook of Compressed Gases, Compressed Gas Association, 4th edition; available at www.cganet.com.
- Ceodeux Inc. (PA) first successfully commercialized the air-actuated high-pressure cylinder valve actuator.
- EWAL Manufacturing and Superior Valve Inc. were early manufacturers of RFOs.
- Union Carbide Corp., Linde Division and Veriflo Corp., Linde Valve Brochure, circa 1986.
- M. Donnatucci, et al., "Sub-Atmospheric Pressure Gas Delivery System for CVD," Semi's Workshop on Gas Distribution Systems, Semicon West, July 1998.
- T. Romig, et al., "Advances in Ion Implanter Productivity & Safety," Solid State Tech., No. 12, pp. 69-74, 1996.
- R. Dubois, et al., "Safe Alternative To High Pressure Gases," Proceedings of the Semiconductor Safety Association, Phoenix Chapter, Emerging Technologies Workshop, Tempe, AZ, Sept. 25/26, 2000.
- US Patent No. 5,518,528, "Storage and Delivery System for Gaseous Hydride, Halide, and Organometallic Group V Compounds," May 21, 1996.
- M. Donnatucci, R. Frye, J. Mcmannus, L. Wang, ATMI internal technical reports 1994-2001. Contact the author of this paper for additional details.
- Gas purities can vary from gas species to gas species. Specification information and analytical details for the various SAGE gas sources are available from ATMI. Contact the author of this paper for additional details.
- SDS is a registered trademark of Matheson Tri-Gas and ATMI Inc. SAGE is a trademark of ATMI Inc.
- D. Armburst, et al., "High Density Plasma CVD Phosphosilicate Glass Films for Advanced CMOS Technology," Proc. '99 DUMIC, 106-115.
- R. Dubois, R. Faller, B. Cheung, Proceedings of the Semiconductor Safety Association Annual Meeting, New Orleans, LA, April 11-13, 2001.
- Roy F. Weston Inc. Environmental Technology Laboratory, Toxic Gas Release Study and Report commissioned by ATMI, May 1997.
- ATMI internal technical reports 1997-2001. Contact the author of this paper for additional details.
- J.V. McManus, et al., "A New Era in Gas Handling Safety: Sub-Atmospheric Pressure Gas Sources," Semiconductor Fabtech, 7th edition.
- DOT exemption # E12221 allows the loading of a wide variety of gases into absorbent based sub-atmospheric packages with operating pressure >75 psig and up to 450 liters.
- W.K. Olander, L. Wang, M. Donatucci, R. Frye, "Reducing the HPM Risk: Pressure-actuated Gas Delivery," Semiconductor Fabtech, 12th edition.
Ray Dubois received his BSc from Saint Mary's University in Halifax, Nova Scotia, in 1989, and his PhD in inorganic chemistry from the University of Alabama in 1994. He joined ATMI in 1996 as chemical manufacturing manager for the Materials Division. In 1998, he was named product development manager for ATMI Gas Operations, 617 River Oaks Parkway, San Jose, CA 95134; ph 602/659-2594, email [email protected].
Jim Mayer has spent more than 25 years working with specialty gases supplied to the semiconductor industry. His breadth of experience includes plant construction, operations, sales and marketing, and gas analysis while working for major specialty gas suppliers. He is the product manager for implant source materials at ATMI.