Principles and design issues of bulk specialty-gas systems
05/01/2003
Overview
Over the past few years, various semiconductor manufacturers have ventured out to try bulk supply of specialty gases required for wafer processing. From this early adoption, along with crucial help from leading gas suppliers, have come the engineering disciplines needed to safely implement this more cost-effective and process consistent means of implementing bulk supply in larger fabs.
In recent years, gas suppliers have installed numerous bulk specialty-gas supply (BSGS) systems. Daily use of these systems has proven that their supply is a safe and reliable means of providing an entire facility's requirements, particularly 300mm fabs.
When considering the installation of a BSGS, one must carefully consider benefits and design issues. For example, it is important to properly review all thermodynamic design parameters; we have done this below with data from actual case studies and solutions to various BSGS supply demands.
Benefits of bulk supply
In general, a BSGS design must meet a user's requirements for enhanced cost, reliability, safety, and consistency of the delivered process gas.
BSGS schemes have the potential to provide significant savings through lower raw material pricing, reduced analytical costs, and reduced cylinder handling and change-out costs due to larger volume packages. Fewer cylinder changes mean that process excursions due to lot-to-lot product variation and potential system contamination are also minimized.
When container exchanges are minimized, there is less possibility of an inadvertent gas release. In addition, some users of BSGS systems have entered into supplier-customer site services agreements, where the gas supplier's expert technical staff performs container handling, eliminating the need for the customer to perform this activity.
Typically, a bulk-gas delivery system consists of a primary bulk supply container with redundancy provided by a backup container with automatic switchover. This method of supply allows continuous on-line process gas feed to wafer processing tools. Reliability is typically designed into BSGS systems through the duplication of the supply source manifold.
Issues of bulk supply
Several important issues must be addressed when transitioning from cylinders to a bulk supply source. For example, bulk supply has the potential for process interruption throughout a larger section of a fab if a BSGS unit fails. Failure of a typical single cylinder gas cabinet affects a maximum of eight process tools, but failure of a BSGS unit can affect up to 60 individual process tools. Appropriate system design engineering and redundancy is required to minimize this risk.
Due to the larger size of a BSGS, an appropriate larger footprint is required for a BSGS installation. Ton containers or even trailer bulk source containers will need to be stored on site. In many instances, this will require the appropriate system to be located outdoors.
Special handling equipment, including hand pallet jacks, fork trucks, and, in some cases, tractor-trailer rigs, may be required for handling bulk special gas source packages. The negative impact of mishandling a larger bulk container is greater than that of a smaller individual cylinder package.
Larger bulk packages also require increased abatement capabilities. The International Fire Code (2000 edition, section 3704.2.2.7.5) requires abatement of the complete release of a single vessel to a level of 1/2 the IDLH at the point of release within 40 min for gaseous bulk products or 240 min for liquefied bulk products. Larger capacity abatement systems will be required to comply with this regulation.
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Table 1 identifies products that are typically available as bulk specialty gases, with a comparison of the typical single cylinder and bulk volumes. On average, a bulk ton special gas supply source is equivalent to 10 standard gas cylinders.
Thermodynamics
Fundamental to most process gases are thermodynamic conditions such as the Joule-Thompson effect and associated potential for phase changes. A unique concern for liquefied process gases is the requirement to provide sufficient heat energy into them to sustain adequate vaporization and maintain a suitable vapor pressure to allow for transfer through the process delivery system.
Most of the energy power requirements are manifested during vaporization and are driven by the required process gas flow rates necessary to supply facility process tools. Liquefied special gases are stored in pressurized containers with a headspace of saturated gas above the liquid phase. The temperature of the liquid determines the pressure of the headspace. The relationship between the available pressure from a liquefied special gas and its storage location takes on importance when bulk containers are stored and used outside. Power, in the form of heat, is typically applied to individual cylinder systems through heater jackets to provide vaporization energy in higher flow systems. When heating systems are required, they need to be sized for total power levels, power densities (W/in2), and response time.
An additional issue to consider is heat transfer into the liquid and gas phases of the source gas. In many cases, sufficient energy needed to vaporize the liquid can be derived from heat conducted from ambient air through the source gas container walls into the liquid itself. This transfer is sensitive to ambient air temperatures as well as the velocity of the air circulating over the surface of the container. Heat transfer to the contained gas is also dependent on the nature of the container — including the container footprint, surface coatings, obstructions in the form of straps, tie-downs or handling cradles — as well as frost or ice buildup.
Heat transfer relies on a temperature differential. The critical issue with ambient air heating is the ability to find a balance between the temperature differential, the heat flux, and the heat of vaporization that is stable over the duration of the source gas use. When this balance is not stable, the temperature will continue to decrease. This temperature decrease will be accelerated by an increasing heat of vaporization, a decreasing reserve of liquefied gas, a smaller effective surface area through which to transfer the heat, and a potential for formation of frost and ice on the container.
Based on a thermal conductivity advantage of liquefied gas over vaporized gas (~15:1 for HCl), the primary path for transfer of heat energy into the liquefied gas product is through the container wall surface from the bottom of the container up to the liquid level.
 Figure 1. Liquid level surface area and thermocouple (TC) locations in bulk HCl ton container. |
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Figure 1 illustrates the surface area available for heat energy to pass directly through the source container walls into the liquid product. This area drops dramatically as the liquid level in the container decreases. As the heat transfer area decreases with product consumption, the power density must increase to maintain a constant or slightly increasing power for vaporization.
When we looked at actual thermocouple signal results obtained from analysis of an HCl system providing a steady gas flow rate of ~300slpm (Fig. 2), we found that the thermocouple readouts served as a good indicator of container liquid level.
In this example using HCl, the heat transfer is based entirely on ambient airflow. Initially, the airflow was close to zero. After ~1.5 hrs, several fans were used to increase airflow across the container surface to ~250 ft/min. This changed the slope of the temperature curve, but still did not provide sufficient heat transfer to reach temperature equilibrium. Contributing to the inability to reach equilibrium was the diminishing product level and resulting loss of effective heat transfer area across the ton container walls.
The use of controlled airflow across cylinders is inherent in the design of cylinder gas cabinets. The heat transfer enabled by this airflow is often sufficient to avoid the need for powered heating systems.
Liquefaction issues
Many semiconductor gases used in BSGS systems are stored in and dispensed from a liquid state. This complicates thermodynamic issues, making correctly designing a gas delivery system even more challenging.
One of the greatest concerns occurs within liquefied gas after vaporization. The resulting saturated vapor can easily revert into the liquid phase with small decreases in the energy level (i.e., temperature) of the gas. The amount of gas that re-liquefies depends on the magnitude of the change in the product energy level.
It is a common practice to wrap gas distribution lines with heat tracing tape combined with insulation to prevent liquefaction. Some primary rules that should be reviewed for applying system heat tracing on BSGS systems in liquefied gases service, and located outdoors, are:
- Use a temperature set point that is at least 5°C higher than the maximum container source heater set point (if source heater is used) or the highest daytime ambient temperature.
- Heat trace all components along the pipeline from the source container at least to the first pressure regulation point. Heat tracing beyond the regulation point may not be required depending on the magnitude of the pressure drop and the gas involved.
- Be sure to provide adequate heat tape and insulation where pipelines are secured with thermally conducting support brackets. These can remove heat from the pipeline and lower the system temperature.
- Thoroughly wrap components, such as valves, filters, and transducers, because they can be heat loss points.
- Heat trace the system flow path through cabinets and valve manifold boxes because there are many components in the gas flow path and there is typically an airflow that encourages heat extraction from exposed surfaces of components. The manifold panel backing plate acts as a large heat sink in this situation.
 Figure 2. Bulk HCl ton container gas transfer temperature profile. |
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 Figure 3. Heating prior to regulation in a bulk HCl supply system. |
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Heat tracing requirements are sometimes worsened by the requirement to transport a gas over a long distance from an outdoor storage area to a building and into a temperature-controlled fab area. An alternative that is sometimes used is a two-stage regulation scheme in which the first regulation stage is located near the storage container. The pressure is reduced sufficiently within the first stage of regulation to avoid sensitivity to liquefaction. This engineered solution may minimize liquefaction while still achieving the advantages of minimizing pressure drop along the long pipeline. In such cases, the final regulation stage is accomplished near the point of use. Depending on the gas and the conditions, the need for heat tracing between regulation stages may be eliminated.
As a saturated gas flows from the supply container to the first point of regulation, there is usually little change in the state of the gas. The distribution piping acts as an extension of the supply container. The saturated gas may begin to liquefy if conditions along the pipeline support such a change of state.
The enthalpy chart in Fig. 3 illustrates the need to preheat the product prior to regulation to avoid transition to the two-phase region that generates some product liquefaction and can cause accelerated corrosion in a corrosive gas BSGS. The example shown is HCl in a vessel at 85°F. The gas is regulated to 100psig without preheating. This results in the product entering a two-phase flow regime. Additionally, ~2% of the product reverts to the liquid phase. The temperature at the regulator outlet is approx. -38°F, which will crystallize any moisture in the gas stream. This may result in regulator failure.
In the same system, a regulation pre-heater can be used to transition away from the two-phase liquefaction region and maintain a regulator outlet temperature, which is above freezing.
 Figure 4. Ambient system temperature cycling. |
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A liquefaction source seldom accounted for is the natural diurnal cycle of temperature and its impact on the outside storage container and associated system piping. Storage containers account for large thermal masses. The associated stainless steel distribution piping constitutes a much lower thermal mass and will respond to changes in ambient temperatures much more rapidly than the storage container and its contents.
Temperature cycling will follow a daily cyclical pattern. The net effect is that when ambient temperatures rise, the piping and storage vessel track this change with increasing lags during the morning and afternoon hours. During the daytime warm-up, the saturated gas leaving the vessel will typically encounter warmer piping and will be protected from liquefaction. During the evening, the thermal race reverses, and the vessel is the slowest to cool down with decreasing ambient temperatures.
This creates a negative temperature gradient, which may lead to liquefaction in system piping and components prior to regulation. Figure 4 illustrates actual thermal cycle data acquired from a ton container ("tonner") of N2O over four days. This is another situation where design analysis is used to determine if heat tracing and insulation may be required.
Why go bulk?
There are several clear benefits to using bulk gas supply for fabs:
- cost savings from reductions in the supplier's time and labor in the filling, in analytical operations, and in delivery trips to a fab;
- reduction in site labor and time associated with change-outs for replenishment (typically a factor of ~10:1);
- product consistency inherent in the lack of cylinder lot to cylinder lot variations that may affect the quality of wafer processing or IC yield;
- savings in total dedicated floor space by replacing multiple gas cabinets with a single BSGS system with source containers located outside; and
- decreased initial capital equipment cost as a single BSGS can be utilized in place of multiple individual gas cabinet systems.
The benefits of BSGS systems come into clearer focus with the increasing experience available in the industry. Individual case-by-case assessment of the operational and environmental conditions is required for such systems to ensure performance under all expected conditions.
A "more exact assessment" is best accomplished using good field verifiable thermodynamic and fluid dynamic computer simulation tools. BOC Edwards simulation and analysis tools perform reviews of newly proposed systems as well as providing engineered solutions of existing systems to assure trouble-free operations of BSGS systems. It is expected that the industry will move toward this requirement as a design standard as these tools become more generally available to system suppliers.
Dave Ruppert, Walter Preller, Michael Marmaro, BOC Edwards, Wilmington, Massachusetts
Dave Ruppert received his masters in EE from Stevens Institute of Technology. He is manager of systems engineering for the Electronic Materials Group at BOC Edwards, Murray Hill, NJ; ph 908/771-1732, fax 908/508-3931, e-mail [email protected].
Walter Preller has more than 30 years of experience in the operation, design, and installation of special gas systems. He is a senior systems engineer in the BOC Edwards Electronic Materials Group.
Michael Marmaro received his BS from the US Military Academy and MBA from The U. of Portland. He is global product manager for special gas equipment at BOC Edwards.