Design Considerations for Process Chemical Distribution
Prior to designing a process chemical distribution system, users should ask questions about system design, capacity and purity requirements.
By Charles K. Gould and Todd A. Myers
Chemical delivery systems transfer high purity pressurized chemical through distribution tubing to process tools within a semiconductor fab cleanroom. Every prospective customer should know the tough questions to ask about system design, capacity and purity requirements.
In the mid-1980s, chemical delivery systems were introduced to simplify chemical supply in semiconductor fabs. With chemical distribution piping connected directly to the process tool, chemical is available on demand, and because the chemical is filtered prior to distribution, particle levels from a bulk distribution system are lower than the particle levels of bottled chemicals.1,2 Particle levels of prepackaged chemicals can increase during shipping, storage and transfer into process tanks or baths. Chemical delivery systems also eliminate many of the hazards associated with the handling and disposal of bottled chemicals.
Before the advent of delivery systems, process chemicals were stored and transported in bottles, totes or canisters which present hazards during transportation, use and disposal. Many of the bottles are glass and easily broken, potentially exposing the operator to toxic materials. Chemicals with a high vapor pressure will be released into the environment each time the bottle or container is opened. The chemicals may also be spilled or splashed when poured into the processing tanks or baths of the process tools. Finally, the bottles must be thoroughly rinsed prior to disposal or recycling. Here again, spilled chemicals or vapors can expose the operator to the hazardous chemical.
Semiconductor systems
All newly constructed semiconductor fabs utilize a bulk chemical delivery system and they are also gaining acceptance in other industries, such as the pharmaceutical and the medical device markets. Proven to reduce raw chemical and handling costs, control impurity levels and improve operator safety, the technology of chemical delivery systems has kept pace with the purity and capacity requirements of the semiconductor industry, with current state-of-the-art delivery systems supplying chemical with <1 particle/ml 0.2 µm in diameter and <500 ppt total metals added.
In the semiconductor industry, four types of chemicals are commonly delivered by a chemical delivery system: process cleaning chemicals, organic solvents, photolithography and specialty chemicals. (See Table 1). Many of the chemicals are toxic, hazardous or flammable. Proprietary chemicals introduced for new processes must be fully evaluated for special handling requirements before implementing the chemical into a delivery system. In addition, secondary containment of the chemical in the supply lines provides protection against potential leaks. For added safety, leak detectors are used throughout the distribution piping network allowing the system to automatically alert the system operator if a leak is detected. All delivery systems are designed with accessible emergency machine off (EMO) and emergency power off (EPO) switches to depressurize or power-off a system manually. Lastly, the problems associated with cleaning and disposing of containers is reduced by using larger returnable vessels, such as drums or reusable totes.
In addition to higher purity and improved safety, the use of a bulk delivery system also results in a lower overall cost per unit volume of chemical as well as less labor, less cleanroom space and reduced container disposal costs. These savings can more than offset capital costs associated with building, installing and maintaining a bulk delivery system. The payback period for chemical delivery systems installed in new facilities is typically less than 1.5 years.
System design
Today`s fabs have a wide range of volume and purity specifications for their process chemicals. In addition to chemical delivery systems, semiconductor fabs may require chemical blending, gas-to-chemical generation or waste reprocessing.
A chemical delivery system includes the means to pressurize chemical and control its distribution throughout the fab. Chemical is first withdrawn from supply containers and transferred to a day tank or pressure vessel via a vacuum source or a double diaphragm pump. Distribution through filters to the cleanroom process tools is accomplished with a pump or nitrogen pressurization. The choice of delivery method (i.e. pump type or nitrogen pressure) depends upon the chemical type, capacity and purity specifications required at the final point of use.
In a system designed for automatic delivery of chemical for limited points of use in a cleanroom or a piping chase, the required volume is typically low and a day tank is not needed. In this situation, a single pump transfers the chemical from supply containers and distributes it, on demand, directly to the process tool(s).
Most semiconductor applications require high delivery rates, as well as high purity chemical. The best delivery techniques for these applications utilize nitrogen pressurization and pulsation dampening along with submicron filtration to control the particulate contamination of the delivered chemical.
Because today`s fabs operate continuously, high purity chemical must be available on demand. Redundancy in essential components ensures that a failure does not interrupt chemical availability.
Compartmentalization for on-line maintenance, allows for isolation of system components for periodic maintenance or repair without interrupting operation. New chemical filters can be installed and qualified for both metal and particle levels before going on line without affecting the system operation. These systems may also be expanded to meet future needs for greater capacity. The bottom line in selecting a bulk chemical distribution system is performance. The particle and metal ion levels guaranteed at the point of use depend upon the specific system design, the chemical to be delivered and choice of chemical filters. Other performance criteria, such as mean time between failure and mean time to repair should also be evaluated to determine ultimate operating costs.
The need for system monitoring and on-line management often requires that the chemical delivery system interface with facility-wide safety and inventory control functions. System software can provide access to system status and real-time trending of chemical usage and on-line analytical measurements. An autopaging system for alarm notification and a chemical supply bar coding system are two common options which may be integrated into the monitoring system.
Particle contamination
Particulate contamination in process chemicals is controlled primarily by careful system design and submicron filtration. Since materials and components used in the delivery system ultimately affect chemical purity, they should be thoroughly tested in the lab for the release of particles and metal ions into process chemicals.
Components used to construct chemical delivery systems must also be made of materials which are compatible with the chemicals to be delivered. Complete compatibility is essential. For example, small amounts of corrosion can result in a large release of particles and significant metallic contamination. Note that the addition of 1 mg/liter (l ppb) of material with a density of 1.0 g/cc can form 1,900,000 particles/ml with a 0.1-µm diameter.3
Fluoropolymer tubing and components are typically used for acidic and caustic chemicals because of their high chemical compatibility and relative inertness. For some applications, especially organic solvents, stainless steel tubing and components may be required.
Because on-line technology for the removal of metal ions is limited, when possible, system components are constructed from fluoropolymers, primarily perfluoroalkoxy (PFA) and polytetrafluoroethylene (PTFE). These components must also be manufactured with strict procedures to limit contamination during molding, machining, packaging or shipping.
The filters used for the delivery of semiconductor chemicals are typically a pleated, flouropolymer membrane, with pore size ranging from 0.05 to 0.1 microns. Proper selection of system filters must take into account pore size, retention characteristics, particle shedding properties and chemical compatibility.
The particle levels in chemical dispensed by a chemical delivery system may be affected by changes in flow rate through the filters. Hydraulic shocks caused by stopping or changing the rate of chemical flow may also dislodge particles from the filters.
System qualification and continuous monitoring
Newly installed chemical delivery systems are “qualified” before use to assure that they meet specified particle levels and do not add metals to process chemicals. Typically, a system is flushed with chemical until the metal levels in the chemical at the point of use are equivalent to the incoming metal levels. In another method, process chemical is recirculated through the system and samples are withdrawn periodically for metal ion analysis. From the change in metal ion concentration with time, the time at which a system meets specifications can be calculated. To ensure that particle and metal ion contamination remains within specification, most fabs will also have a continuous monitoring program.
Perhaps the largest source of error in measuring metallic extractables from chemical delivery systems is related to system holdup volume and variability of incoming chemical. Metal ion extraction is usually measured by determining the difference between the contamination levels at the process tools and the levels in the incoming chemical. A change in the concentration of the incoming chemical causes a change in this difference that can be erroneously interpreted as metal ion extraction.6
Bulk chemical delivery is only one aspect of total chemical management. Chemical delivery systems can be easily combined with chemical supply generation or blending systems. Also, as the recycling of spent chemical becomes economically attractive, chemical reclamation may be recycled back into the chemical supply loop.
Dilute process chemicals can be made safely and inexpensively by on-site blending of fab water and concentrated chemical. Positive photoresist developers, including those with surfactant, can be prepared in blending modules with precise chemical measurement and assay techniques. On-site blending offers superior chemical purity in part because the water available at semiconductor fabs is usually of better quality than that available at most chemical manufacturing facilities. As the concentrated chemical is diluted with fab water, the contaminants are also diluted. The result is high-purity dilute chemical at reduced cost. An added advantage is that the fab manager has control of purity and concentration accuracy. Chemicals such as ammonium hydroxide, hydrochloric acid and hydrofluoric acid can be generated onsite by mixing clean anhydrous gases with fab ultrapure water.
Gas-to-chemical generation and blending are recognized as cost effective alternatives for high purity chemical supply. As fabs move to total chemical management, on-site chemical generation will increase. Waste collection and reprocessing are areas of rapid technology development, driven by the reduced expense of purchased chemicals, lowered disposal costs and environmental regulation. Several fabs, for example, have already installed cost effective sulfuric acid recycling systems.n
References
1. Gruver R., R. Silverman and J. Kehley. “Correlation of Particulates in Process Liquids and Wafer Contamination,” Proceedings of the 36th Annual Technical Meeting, Institute of Environmental Sciences, pp. 312-316, 1990.
2. Pate, K. “Examining the Design, Capabilities, and Benefits of Bulk Chemical Delivery Systems,” Microcontamination, 9 (10), pp. 25-30, 1991.
3. Grant, D.C. and S. Clifford. “Advantages of Central Chemical Delivery Systems,” in CleanRooms `93 Conference Proceedings, 1993.
4. Grant, D.C., L. Clark, T. Lemke, N. Powell, “Qualification of Chemical Delivery Systems for Elemental Extraction Using the Dynamic Extraction Technique,” to be presented at the 13th International Symposium on Contamination Control, The Hague, The Netherlands, Sept. 16-20, 1996.
5. Lemke, T. and D.C. Grant, “Long Term Performance of ChemFill Chemical Delivery Systems Supplying High Purity Chemicals in a Semiconductor Fab,” FSI Technical Report.
6. Grant, D.C., S. Van Dyke, and D. Wilkes, “Issues involved in qualifying chemical delivery systems for metallic extractables,” Proceedings of the Institute of Environmental Sciences, 1993.
Charles K. Gould is a marketing specialist for chemical distribution products within the Chemical Management Division of FSI International, Inc. (Chaska, MN). He has a B.S. in chemical engineering from the University of Minnesota.
Todd A. Myers is the senior product sales specialist for FSI International`s Chemical Management Division. He has more than seven years experience in the semiconductor industry. He received a BS in electrical engineering from the University of Minnesota.
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Figure 1. A typical chemical dispense module uses off-line and on-line pumps to get chemical to the cleanroom process tools.
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The ChemFill Model 1000 is a chemical delivery module manufactured by FSI International. The chemical management system meets requirements of sub-0.5-micron linewidth fabs and can be upgraded to meet the requirements projected beyond 0.18-micron linewidths.