Matsushita Completes Ultra High Purity Nitrogen System Reducing Contaminants to PPB Levels
Matsushita Completes Ultra High Purity Nitrogen System Reducing Contaminants to PPB Levels
Pilchuck Mechanical, Inc. installed an ultra high purity (UHP) nitrogen system at Matsushita Semiconductor of America's (MASCA) Puyallup, Washington facility. The nitrogen distribution system is capable of delivering nitrogen of remarkably high purity at approximately 120 points of use from a piping system comprised of over fourteen hundred linear feet of tubing with contaminants measured in the low parts per billion (PPB) range (see Table 1). Achieving this level of purity requires an excellent purification system which was provided by a Nippon Sanso Pegasus purifier and ultra high purity gas filter (Figure 1.), but it also requires an extremely clean process gas piping system to avoid contamination of the gas as it travels from the filter to the points of use. The construction of this nitrogen system, which exceeded even the exceptionally high standards it was designed to achieve, demanded not only clean materials, but required extraordinary care in fabrication.
The installation of the UHP nitrogen system was part of a facilities upgrade in preparation for the manufacture of a new product line which will be produced on six-inch wafers. In addition to the nitrogen system, Pilchuck also completed the construction of an UHP oxygen system and an UHP deionized water system which provides UP Dl water where needed with a total organic carbon (TOC) content of less than 1.5 PPB and particle counts of less-than/equal-to 1.0 for particles of 0.05 microns and larger in size. They also installed piping for hazardous process gases which required double containment of the process gas lines. Construction was constrained by the need to keep the facility in full production throughout the construction period with only minor interference with operations while maintaining a class 1 working environment.
Figure 1. Ultra high purity filters used for nitrogen system.
This wafer fabrication facility was originally built by National Semiconductor before being bought by Matsushita. The wafer fab and all related additions were designed by the Architect/Engineering firm of Lockwood Green of Dallas, Texas. MASCA was determined to eliminate as many unnecessary expenses as possible on their facilities upgrade and still be able to construct a piping system that would meet or exceed the specified standards. This goal was met by creating a team comprised of MASCA, Air Products and Pilchuck Mechanical, Inc. The UHP piping systems were designed for optimum cleanliness with no deadlegs which would create areas of stagnation. Engineering drawings that were not considered essential were not done in the interest of saving money and time. The facility owners did not skimp in areas they felt necessary to achieve their UHP goals.
Pilchuck was able to save some money for the customer since they are not simply a mechanical contractor, but are a general contractor as well. This means that they can bid on a job as a general contractor and do not have to have a separate mechanical contractor. Pilchuck acting as a general contractor managed equipment installation or "fitup". Typically fitup costs about 10% of the process equipment purchase price. MASCA's goal was to limit equipment fitup costs to 5-8%. Pilchuck Mechanical helped MASCA reach their goal.
Orbital welding, which has long been the standard method of joining tubing for semiconductor process gas lines because of the smooth inner weld bead that is characteristic of this joining process, was considered to be essential. The smooth inner surface provided by this technology is mandated because a rough surface would provide an entrapment site for particulates that could be released and cause irreparable damage to submicron linewidth products, or could cause turbulence in the gas flow that would affect sensitive semiconductor processes. MASCA owns two and Pilchuck Mechanical, Inc. owns three Model 107 orbital tube welding power supplies manufactured in Pacoima, California by Arc Machines, Inc. During the fitup, Pilchuck acquired a Model 207 microprocessor-based power supply also made by AMI. These power supplies are used for fusion welding of tubing or thin-walled pipe in applications where the addition of filler metal to the weld is not required. These power supplies control weld parameters including weld current, pulsation, level times, travel speed (RPM), etc. to within ± 1% accuracy. Weld programs or schedules for each size of tubing to be welded are stored in the memory of the Model 207 for convenient recall when ready to weld.
Figure 2. Preparing to weld assembly. Welding power supply is shown at right.
The welds are completed inside the weld heads which form an enclosed chamber that fills with inert gas, usually argon, to protect the molten metal from oxidation. An arc is struck between a tungsten electrode inserted in the weld head rotor and the tube, and the electrode rotates around the joint to complete the weld.
Arc Machines, Inc. Model 9AF-1500 weld heads, which are used for welding sizes ranging from 1/4 inch OD to 1-1/2 inches OD, were used to weld tube-to-tube, tube-to-Veriflow drop valves, for welding stub ends onto tubing, and to weld purge ports and all VCR glands. The number of persons actually performing the high purity welds was limited to one. This was done in an effort to more closely monitor and control the overall quality of the total system installation.
The welding technician was an experienced welder who had learned to operate the Model 107 power supply three years earlier at MASCA. He is an apprentice in Local 82 in Tacoma, Washington, of the United Association of Journeymen and Apprentices of the Plumbing and Pipefitting Industries of the United States and Canada.
Materials of Construction
Seamless 316L stainless steel tubing electropolished on the interior surface to provide a 10 RA maximum surface finish was used for the UHP piping systems. Some vent tubes were bent with a tube bender, but for the high purity systems, fittings to provide 45 degree and 90 degree bends that had been electropolished after bending were purchased to avoid altering of the electropolished surface finish as a result of bending. Type 316L stainless steel is typically used for high purity piping systems because it is relatively easy to weld and fabricate and contains about 2% molybdenum to provide additional corrosion resistance compared to 304 stainless steel. The "L" grade contains less carbon (0.03%) than the standard grade of 316 which contains a maximum 0.08%. This means that there is less loss of corrosion resistance as a result of carbide precipitation during welding. The combination of orbital welding technology and "L" grades of stainless steel have virtually eliminated carbide precipitation which was frequently a problem when manual welding techniques were used.
Clean Fabrication Area
Welding with orbital weld head.
Welding and fabrication of UHP piping systems are typically done in a cleanroom trailer located adjacent to the jobsite. Completed assemblies are capped and bagged to prevent contamination of these parts while in transit to the site of installation. The caps and bags are then removed just prior to making the field welds. At the Matsushita site, the expense of a cleanroom trailer was eliminated. Instead, a clean area 8' x 30' was established for precutting tubing near the site where the assemblies were to be installed. The area was separated from the shop area by static-proof Visqueen curtains, and HEPA (High Efficiency Particulate Air) filters were installed in the ceiling to purify the air within the enclosure. A sticky floor mat that was easily maintainable in a clean condition was used to control particulates at the floor level.
A clean bench was used as a work area. Welding and fitting technicians wore booties and cleanroom gloves while working in this area and full cleanroom dress when welding in the cleanroom. The gloves are worn to prevent fingerprint oils from getting onto the metal. This would be a carbon source that could affect the corrosion resistance of the welds or might result in the evolution of gases from the interior surface of the piping system in service.
Any tubing or other object to be moved between this clean preparation area and the cleanroom was bagged and the tubing was recleaned by blowing a wad through the inside with a nitrogen or argon purge.
Figure 4. Orbital welds on outer containment of double contained gas line.
Generally Matsushita Puyallup believes that the interior of the tubing and valves needs the highest levels of cleaning and protection since it is this surface that sees the process gases. As a result, emphasis is placed on keeping these areas free of contamination through positive purging with ultrapure gases. Successful results with orbital welding require consistently good tubing end preparation. This was accomplished with a Tri Tool Model 572 Severmaster end-preparation tool that cuts and makes a machined end suitable for orbital welding. The tube ends must be squarely cut so that they touch all around the weld joint with no gap. There must be no chamfer or bevel on the end which would cause a reduction or irregularity of the tube wall thickness. The Severmaster makes a finished preparation on one of the cut ends, but the other end must be refaced or simply have the burr removed with a stainless steel knife. The tool is used for sizes ranging between 1/2 to 1-1/2 inches OD. An excellent weld joint preparation is important for achieving weld consistency even when ATW or ABW fittings designed for orbital welding are used.
After the cutting and prepping operation, the parts were cleaned with Freon TA (CFC 113-90% + Acetone 10%) and double-bagged for delivery to the cleanroom.
Purging, especially of the weld interior, is critical for achieving a high level of purity in a piping system. If no purge at all were used on the weld interior during the weld, the resulting weld area would be black and crusty from oxidation. This condition is called "sugared" as it resembles burnt carbonized sugar. If a purge is used, but the duration is not sufficient to replace the atmosphere with highly purified argon, or if the argon is contaminated with atmospheric oxygen or moisture, a weld with varying amounts of blue, gray, or brownish discoloration will result. This discoloration is referred to as "heat tint". Heat tint is undesirable and has been associated with an increased rate of corrosion. There is also the fear that the products of oxidation, especially of the type which can easily be wiped off, will result in contamination of the gases that are transported through the piping system. Achieving a weld interior with no sign of discoloration resulting from oxidation requires special care. For this installation, special cryogenic dewars of liquid argon certified for UHP applications to be 99.999% pure were used for purging the weld head and interior of the tubing during welding.
Figure 5. Welding technician making an in-place weld on piping in the cleanroom.
As a precaution to prevent contamination of the welding gas, a welded stainless steel purging system was connected between the flowmeter and the welding power supply and a 0.02 µ Millipore filter was placed on the Model 107 at the outlet to the weld head.
For the ID purge, one concern would be the build-up of pressure which would cause concavity of the inner weld bead, or in an extreme case, could blow the molten metal onto the tungsten electrode. It takes pressure of only about 1/2 inch of water on the weld ID to cause a measurable displacement of the weld bead. This means that, although the weld joint must be purged, there has to be an outlet from which the purge gas can escape to the atmosphere. If the distance between the weld joint and the exit orifice is short, there is the possibility that air can be sucked back into the tube and contaminate the ID purge gas sufficiently to produce noticeable oxidation.
To prevent this, a 5-foot long spiral-shaped stainless steel exit tube was welded to the fitting used to contain the purge gas in the tube. (See figure 6.) A dial reading of 0.0 ppm on a portable oxygen analyzer, Teledyne Model 311D, measuring in the 5ppb range, was used as a criterion to assure that the oxygen level inside the tube was sufficiently low to prevent discoloration prior to starting the weld sequence. An argon purge was maintained on the piping system for 24 hours a day to prevent atmospheric contamination. The flow rate was turned down at night and up to about 30 SCFH during the day for welding. The flow rate was adjusted depending on the tubing OD to prevent excessive ID pressurization.
Weld quality had to meet the strict standards of the facilities owners. All welding personnel made test coupons (qualification welds) to establish weld criteria at the beginning of the construction project. The welds had to be fully penetrated around the entire weld perimeter since a lack-of-fusion defect would provide an entrapment site that could result in contamination of the system. This site would also be vulnerable to crevice corrosion which could contribute contaminants in service. The welds were expected to be smooth and flat (not concave) on the outside and to have a weld bead of uniform width on the inside but with little or no convexity of the inner weld bead. No concavity of the inner weld bead was permitted. No visible signs of oxidation of the weld interior were allowed. The owner's representatives were closely involved with the development of the weld procedures and gradually gained confidence that they were getting the level of quality that would meet their specifications.
During the construction phase of the project, daily test coupons were made and inspected before production welding could begin to make certain that the production welds were of the same quality as the qualification welds. Additional test coupons were made for any type of change in the process, such as the addition of an extension cable to the weld head, change in argon dewar, change in tube size, or material heat number. All test coupons were dated, identified by welder, and stored for future reference.
Validation of the Ultra High Purity Nitrogen System
Before a piping system can be put into service, it must be validated to assure that it meets all the criteria it was designed to meet. In the case of the UHP nitrogen system, it had to be shown that it could produce and deliver nitrogen gas containing no more than the specified levels of contaminants. Since the specified levels of these contaminants were so low, very complex and expensive apparatus was required to make the measurements. Furthermore, the measuring equipment requires precise calibration procedures in a controlled atmosphere to assure accuracy. Rather than attempt to make these measurements on site, when construction of the piping system was completed, MASCA collected nitrogen that had passed through the UHP filter in a gas cylinder at 100 PSIG and sent the gas sample to Air Products in Allentown, Pennsylvania for analysis. Prior to collecting the test sample, a nitrogen purge was allowed to flow through the sample gas cylinder for a week to assure that the cylinder was clean enough so that it would not contaminate the sample.
Figure 6. ID purge arrangement for orbital welding.
Gas samples were taken from several points of use and sent to Air Products for testing. The results of that analysis are shown in Table 1.
Prefabricated welded assemblies were carried to the cleanroom for field installation into the UHP piping system. A Model 107 power supply was stationed on the "dirty" side of the cleanroom (service chase) just outside the Class 1 area. This machine was frequently cleaned with isopropyl alcohol to avoid dust or particulate contamination. Several hundred orbital welds required to complete the piping system in the Class 1 areas were made using extension cables. With extension cables, the weld heads could be positioned in the Class 1 areas without the need to relocate the particle generating power supply.
Figure 7. Piping installed on “dirty” side of cleanroom.
A nitrogen purge was used to maintain a continuous positive pressure inside all tubing until final system connections were made. This was done to minimize or eliminate the exposure of the interior tube walls to outside conditions.
Hazardous Gas Piping
The hazardous gas piping in the Matsushita plant was done in accordance with Article 80 of the Uniform Fire Code. The code specifies that consideration must be given to the prevention of the escape of hazardous (toxic, pyrophoric, and corrosive) gases to the atmosphere or work environment. In compliance with this code doubly-contained piping systems were installed by Pilchuck at MASCA. This code was originated in Santa Clara, California, and has received nationwide attention. Process gas lines for transport of silane, phosphene, chlorine, diborane, and nitrogen trifluoride are generally 1/4 inch diameter tubing with a surrounding outer containment tube. A spring or other type of spacer is placed between the process tube and the containment which is 1/2 inch tube. The primary tubing is welded orbitally and subjected to a helium leak test at 10 (to-the-minus-7) torr before the secondary containment can be welded. A short section of 5/8 inch diameter sleeve is placed over the 1/2 OD containment tube and welded to the 1/2 tubing at both ends of the sleeve. (Figure 4.) In order to get consistently good welds, the sleeve must have a uniformly good fit to the tube.
The double containment piping system works as follows: the internal 1/4 inch diameter transport line is pressurized to about 30 psi. The area between the outer containment pipe and the transport line is pressurized to about 60 psi. This area is monitored and if the pressure drops the system is automatically shut down and both local and remote alarms are activated.
All hazardous gas cylinders are confined to gas cabinets located in specially constructed gas buildings and each cylinder has its own monitor and alarm system. Similarly, each piece of equipment in the cleanroom which uses a hazardous gas has a separate detector and alarm system. Gas bottles containing hazardous gases are constantly monitored to detect weight changes indicating that the cylinder needs to be changed.
Pilchuck Mechanical, Inc. completed the installation of the UHP nitrogen system and was able to meet Matsushita's specified purity goals on schedule and within budget. This required partnering between the contractor and the user to establish which procedures were actually necessary and which could be eliminated without compromising the required quality standards. Increasing demands for finer linewidths in semiconductor products are driving the demand for greater purity in process gas piping systems. This, in turn, is driving up construction costs for installing piping systems of the required levels of purity. If future costs are to be contained, it will become increasingly important to know which fabrication practices can be eliminated without imparing quality and which are of vital importance.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
The author would like to acknowledge the collaboration and technical assistance of Facilities Manager, Bob Frisbie, and Supervisor, Ted Vance, at Matsushita, and Arch E. Van Belle, President, as well as Tom Wilburn and Matt Pettibone of Pilchuck Mechanical, Inc. Photos were provided by Pilchuck.