Incorporating more gas control within cylinders
11/01/2003
Overview
Delivery of gases from cylinders, especially specialty gases, has always been a part of wafer fabrication. Process and safety demands, however, have dictated recent changes to conventional cylinder gas delivery. This has led to the integration of flow, pressure, purification, and other functions into the cylinder itself.
The primary functions of a compressed-gas cylinder valve are to contain the gas inside the cylinder (isolation) and to allow controlled release of product to the delivery system. The valve outlet serves as the mechanical seal with the delivery system and is designed to prevent inadvertent connection of incompatible gases. Conventionally, gas-control functions, such as flow control, pressure regulation, purification, and weight measurement must be accomplished using downstream equipment. Recently, we have integrated several of these delivery-system functions into a family of cylinder valves generically called "compact controllers." These systems provide operational safety benefits and also reduce a gas user's cost-of-ownership.
Automatic shutoff
An automatic shutoff (ASO) valve is generally required in delivery systems for gases with toxicity, flammability, or other hazards. Because it is desirable for an ASO to be located close to its gas cylinder to enhance system safety, wafer fabs frequently use a pneumatic cylinder valve, which embodies the principle of integrating gas-control function into the cylinder package. When combined with a restrictive flow orifice (RFO), these pneumatically actuated cylinder valves can significantly mitigate risks associated with most plausible leakage scenarios. These normally closed valves interface with local sensors that detect low ventilation, excess gas flow, toxic gas release, or fire to release the opening force and interrupt the flow of hazardous gas.
 Figure 1. A schematic of MegaBIP operation. |
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Purification
By employing built-in purification (BIP), a consistent high-purity gas can be withdrawn from the cylinder independent of the cylinder pressure or vapor-liquid partitioning. BIP is achieved using an appropriately chosen purification medium contained in a purifier assembly, which is situated within the cylinder [1]. The purifier need not be designed as a pressure vessel because it is contained within the cylinder. A separate filling path into the cylinder that bypasses the purifier is necessary to ensure its efficiency (Fig. 1).
We use a residual pressure valve to prevent the cylinder from being completely depressurized; this eliminates the risk of trace impurities bleeding off of the purifier media. In addition, it protects the integrity of the purifier and avoids cross-contamination of the cylinder from backflow of other gases.
To take advantage of BIP, it is necessary to provide integral filtration to capture any particles that might come from the purification bed. By using an all-welded high-efficiency filter (0.003µm nominal retention size) in the BIP assembly, a strictly particle-free gas stream can be guaranteed. Even when a full purifier bed is not required to remove molecular contaminants, this built-in filtration capability may still be valuable, particularly when dealing with gases prone to particle formation, such as diborane and silane.
Pressure regulation
To safely and accurately control gas flow, the high pressure within the cylinder is typically lowered to a consistent value using a pressure regulator. Even in low-vapor-pressure liquefied gases, an absolute pressure regulator is frequently used to ensure a consistent delivery pressure and prevent condensation of liquid in delivery lines. When the regulator is downstream of the outlet connection (Fig. 2a), trapped atmospheric contamination must be removed with vacuum-pressure purging that requires purge isolation valves both upstream and downstream of the regulator.
By incorporating a high-integrity pressure regulator directly into the valve, we have simplified reduced pressure delivery (RPD, Fig. 2b). Similar to BIP, the integral regulator requires a separate gas path to be used by the supplier that is locked from access by the end user.
 Figure 2. A typical purge panel required for a) conventional pneumatically actuated cylinder and b) reduced-pressure delivery (RPD) cylinder. |
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 Figure 3. The compact controller components. |
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By slightly modifying the regulator, it is possible to set the delivery pressure at either a subatmospheric pressure (e.g., 10psia or -4.7psig) or superatmospheric pressure (e.g., 30psig). A regulator in which the delivery pressure is adjustable only by the supplier is preferred to minimize the possibility of human error. The choice of preset pressure depends on the user's flow and pressure requirements.
Contents gauge
For nonliquefied compressed gases, the best way to determine the contents remaining inside of a cylinder is to monitor internal pressure. When a pressure regulator is integrated into the cylinder valve, it is necessary to make this pressure measurement upstream of the regulator. The pressure transmitter can either be in constant communication with the cylinder pressure (an active gauge) or isolated from the cylinder by a valve (a passive gauge). The latter configuration is preferred for hazardous or high-purity gases, because it mitigates potential leakage and permits repair or replacement of the pressure sensor by the gas supplier without risking contamination of the cylinder.
Other useful functions that can be integrated onto a specialty-gas cylinder valve are under development, including liquid-level measurements, variable pressure control, and flow control (Fig. 3).
Controlling hazards
A gas cylinder with an integral pressure-reducing regulator — an integrated cylinder valve combining a vacuum regulator isolated both upstream and downstream with diaphragm valves (Fig. 4) — can eliminate the chance of a significant gas release of hazardous specialty gases if the fixed delivery pressure is set below atmospheric pressure. In this case, no gas will flow, even with the isolation valves open, until negative gauge pressure is applied to the cylinder valve outlet.
The pressure regulator used in this system cannot be adjusted by the end user and ensures that the delivery pressure is always below the prevailing atmospheric pressure during transportation, storage, and use. The isolation valve upstream of the regulator allows the mechanism to be thoroughly evacuated before and after use to remove traces of process gas, while the low-pressure isolation valve downstream can be used to prevent ingress of atmospheric contamination and provides the added security of double isolation from the hazardous material.
Because an implanter's ion sources typically float at high voltage relative to ground, the gas lines leading to the source must also be electrically isolated from ground and contained within the implanter chassis. Because these hazardous gases are used in cleanrooms and in areas not designated for hazardous production materials (HPM), the added safety afforded by such a system may be warranted. Building and fire codes may limit the total quantity of gas stored outside the HPM room and thus set an upper limit for fill volume in the vacuum delivery-system (VDS) cylinder. Nevertheless, a VDS cylinder will contain more usable gas than a dilute compressed gas mixture or gas adsorbed at low pressure on a sorbent medium. This increased product quantity reduces cylinder changeout frequency, thus enhancing safety, quality, and potential extended ion-implanter uptime.
Reduced pressure delivery
An RPD system is recommended for superatmospheric wafer fab processes (e.g., atmospheric pressure CVD) to allow flow control of mass flow controllers. An RPD valve is very similar to VDS except the pressure regulator is optimized for superatmospheric pressure regulation (Fig. 5). Gases considered for RPD applications include silane, methylsilane, and trimethylsilane for CVD and diffusion, and arsine and phosphine for MOCVD. There is also an opportunity to provide hydride mixtures with this valve functionality.
In the configuration shown in Fig. 5, the RPD device is fitted with a normally closed ASO cylinder valve upstream of the pressure regulator to improve the intrinsic safety of the system. The detachable pneumatic actuator remains at the user's location and can be safely and easily fitted to the valve via a quick connect. In transit, a lock-down cap provides additional closing force on the valve. The RPD valve can also include a built-in pressure transducer that measures cylinder contents whenever the ASO is opened.
 Figure 4. The VDS subatmospheric delivery system designed for ion implanters. |
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The outlet of the RPD valve is fitted with a restrictive flow orifice (RFO), the size of which can be selected by the user and installed by the gas supplier. The RFO size is chosen to passively mitigate the worst-case release from the cylinder in the case of a leak while still allowing adequate gas-flow rates from the cylinder to meet the maximum requirement of the process tools. In conventional high-pressure compressed-gas cylinders, the tradeoff between the worst-case release rate and the process requirement is complicated by the fact that the pressure delivered from the cylinder drops steadily as the gas is withdrawn. Hence, the worst-case release rate is dependent upon maximum cylinder pressure in the full cylinder, whereas maximum guaranteed flow to the process is limited by the lowest pressure achieved before the cylinder must be replaced.
 Figure 5. An RPD device showing a detachable pneumatic actuator. |
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Using a pressure regulator integral to the gas-cylinder package produces a constant pressure upstream of the RFO, so maximum flow capacity does not change during the useful life of the cylinder. Using an RFO in the gas-distribution system as opposed to one installed in the body of a cylinder valve does not provide the same benefit because it cannot mitigate the most likely scenario for gas releases, relating to the cylinder not being properly connected to the distribution panel.
Benefits
The industry recognizes the need for the highest level of risk mitigation when using silane (Fig. 6). Therefore, a conventional cylinder typically has an RFO installed to control the release rate below a maximum rate determined by a facility risk assessment and the available engineering controls. RFO size is typically 0.010 in. dia.; smaller openings might be prone to blockage.
When the conventional cylinder is filled to its maximum pressure, the resulting worst-case release rate of silane through the RFO may be too great. The gas supplier can put less gas in the cylinder to reach an acceptable release rate; however, this approach adds cost and potentially reduces consistency as a result of more frequent cylinder changes. As the silane is taken from the cylinder, the delivery pressure steadily falls, and with it, the maximum flow through the RFO drops proportionally. At some point, the cylinder is no longer capable of supplying the gas at the desired delivery pressure at a rate sufficient for process demand. The cylinder must be taken offline at this point, wasting valuable gas.
With an RPD system, the delivery pressure remains constant through most of the useful life of the cylinder. Since the delivery pressure (100psig in the example) is significantly less than the internal pressure of the cylinder, a somewhat larger RFO can be used, reducing the risk of clogging. More significant, the larger RFO and the fact that the flow capacity of the system does not steadily fall as the product is withdrawn make it possible to use a greater fraction of the gas within the cylinder. The RPD system also allows the cylinder to be filled to its maximum pressure without increasing the flow through the RFO, since the RPD valve will maintain constant flow through the RFO as long as the cylinder pressure is above the RPD control pressure. Therefore, by increasing fill pressure and by depleting the cylinder to a lower pressure, a user may use 25% more product from each cylinder, thereby reducing the frequency of cylinder changes.
Overall advantages
The obvious advantage of integrated gas-cylinder capabilities is eliminating the need to purchase and maintain standalone pressure regulators and purifiers. A regulated cylinder valve also eliminates the need for some system valves. Other advantages include more complete usage of a cylinder's gas and faster system cleanup during cylinder changes. The integrated pressure transducer can take the place of the pressure measurement device usually associated with the gas panel.
 Figure 6. A comparison of delivery capabilities of a conventional silane cylinder to those of an RPD cylinder. |
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While high-integrity valves, purifiers, and regulators are typically very reliable, they do need occasional replacement or repair. The use of these integrated functions on a cylinder shifts this responsibility from the gas user to the gas supplier. Because a pressure regulator should never be exposed to atmospheric contamination in normal operation (a separate path is used by the gas supplier to fill the cylinders), it is expected that the maintenance requirement for the built-in regulators should be lower than that for regulators on gas panels, which may be exposed to atmospheric contamination during every cylinder change.
Depending on local safety and building codes, it is likely that the added safety benefits of VDS cylinders will permit them to deliver hazardous gases without the need to purchase additional safety equipment or special mass flow controllers. VDS cylinders may allow users to increase threshold quantity limits for storage and use of highly hazardous materials (e.g., pyrophorics or dopants) at their facilities. The use of an RPD package fitted with an appropriately sized RFO installed downstream of the regulator and low-pressure shutoff valve may also safely achieve higher flow rates from each cylinder. An individual cylinder may supply more process tools; the RPD device may allow greater fill densities inside the cylinder. End users avoid the cost of installing added HPM space or gas-delivery panels to meet a given gas requirement.
Operational savings
Because VDS or RPD devices allow greater cylinder fill densities compared with the alternatives (such as dilute gas mixtures or adsorbent-filled vessels in the case of VDS, or lower fill pressures limited by RFO sizing in the case of RPD), there can be savings based on fewer cylinder changes. Prudent operational practice requires trained technicians with appropriate protective equipment to conduct hazardous cylinder changes along with the associated purging steps before and after the changes. Less frequent cylinder changes save on labor and potentially reduce process equipment downtime. Similarly, these integral pressure-regulated gas-supply packages eliminate downtime caused by routine regulator maintenance and replacement.
Safety and quality
Reduced delivery pressure can lessen the consequences of an unintentional gas release. Furthermore, the lower delivery pressure improves the reliability of components in the delivery system, making failures less likely and the consequences of a component failure less severe [2]. Surveys indicate that at least 20%, and perhaps as many as 65%, of all incidents involving gas containers are related to container-change operations [3, 4]. The ability to contain a larger quantity of gas than a conventional cylinder decreases the required frequency of container changes and results in improved safety.
To ensure that these safety and reliability benefits are fully realized, we have subjected VDS and RPD devices to extensive testing. Representative samples of each valve design were tested and proven to meet all of the pertinent regulatory requirements. Extensive regulator, manual valve, and pneumatic actuator cycling tests were also completed with no premature failures detected.
For example, the RPD valve's regulator characteristics remained unchanged after exercising the regulator for >100,000 cycles by interrupting a 2slpm nitrogen flow for 30 sec every minute. The integrated valves performed as expected when tested with typical reactive specialty gases, such as boron trifluoride, phosphine, and silane.
James J. Hart, John Irven, Russel E. Parise, Ronald M. Pearlstein,
James VanOmmeren, Air Products and Chemicals Inc., Allentown, Pennsylvania
Acknowledgments
Additional authors include Neil A. Downie and Mark Clune. MegaBIP is a registered trademark of Air Products and Chemicals Inc.
References
- B.L. Hertzler, et al., Solid State Technology, 43, 11, pp. 73–80, November 2000.
- K. Olander, SESHA Journal 2002, 15 (3/4), 9.
- T. Roigelstad, Technology Transfer 94062405A-ENG, International Sematech, Austin, TX, 1994.
- C.A. Fields, Intel Corp., Santa Clara, CA, 1993.
For more information, contact Ronald M. Pearlstein at Air Products and Chemicals Inc., 7201 Hamilton Blvd., Allentown, PA 18195; ph 610/481-8594, fax 610/481-5361, e-mail [email protected].