Highly Successful Semiconductor Cleanroom Fabrication Operation in UK Features Orbital Welding

Highly Successful Semiconductor Cleanroom Fabrication Operation in UK Features Orbital Welding


Cambridge Fluid Systems Builds Assemblies for Worldwide Semiconductor Market


Figure 1.
Orbital Welding operator in cleanroom attire prepares to weld an assembly using the Model 9-500 weld head. Note use of gloves. Height gauges are used as supports for the weld assembly.

Fabrication for the rapidly moving semiconductor industry is a never-ending challenge. As the requirements for smaller and smaller line widths on devices have gone from micron to sub-micron to tenths of microns, and microchips get more and more complex with ever-expanding wafer size and complexity, the specifications for fabrication of components become increasingly exacting. Cambridge Fluid Systems (CFS) of Cambridge, UK manufactures gas control products which include manifolds and assemblies for handling process gases used in semiconductor manufacturing. CFS have upgraded their facilities and improved their procedures to comply with requests from their clients to meet the stricter standards. They have also improved their productivity since the semiconductor industry is expected to continue expanding well into the next century.

Orbital welding is a prerequisite for the joining of tubing and fittings for semiconductor process gas systems. A very smooth inner weld bead is essential to prevent entrapment of minute particulates that might be released into the process stream. Even sub-micron particulates can ruin a costly device, while rough spots on the tubing ID may lead to particle shedding through the piping system and affect the precision of gas delivery for key process steps. Either condition would have a detrimental effect on product yield. CFS is very proficient in orbital welding. They have many years experience with microprocessor-based orbital welding power supplies manufactured by Arc Machines, Inc., of Pacoima, California USA. The power supply has a counter that records the number of welds the machine has made - up to 99,000 welds. Several of CFS's machines have been "around the clock". CFS are presently purchasing more Arc Machines' Model 207 power supplies and weld heads to keep up with the increased demand for their products.


Figure 2.
Cambridge Fluid Systems specializes in fabrication of assemblies used in semiconductor gas distribution systems. Fabrication is done in a cleanroom using state-of-the-art orbital welding technology.

CFS have enlarged their cleanroom. The expansion has made it possible for them to compete more effectively in the semiconductor market by enabling them to increase their production while upgrading their procedures at the same time. With the exception of the vacuum system, CFS has made their own cleanroom which adds 1,000 square feet of usable space. Improvements such as the cleanroom windows that allow workers to see out and visitors to see in are contributing to better working conditions. The new main area is Class 100 with a Class 1,000 service area. A separate Class 1,000 pipe-prep area has been added which perhaps will eventually meet Class 100 specifications. The old Class 100 areas were located under laminar flow hoods outside the main body of the cleanroom, but eventually all the Class 100 space will be included in one large Class 100 shop which will improve efficiency.

CFS is continually improving upon their fabrication facility with purchases of sophisticated equipment designed to be compatible with current semiconductor technology. These improvements include recently installed equipment using reverse osmosis (RO) to produce their own highly-purified 18M? deionized water for cleaning components and finished assemblies. They have also installed "ppb" level moisture and oxygen analysers in addition to "state of the art" helium leak detectors and advanced gas purification systems. They are gradually upgrading their materials and over the past 8 years have increased the use of electropolished 316L stainless steel tubing over chromatography grade SC 1 tubing which is simply cleanroom packed and prepared.

Gas purity

There have been dramatic improvements in the purity level of gases used in the industry during the past o years. "Five nines" or 99,999% pure used to be the norm for high-purity welding. With the purifiers available today, gas of 7 or 8 nines purity is fairly common CFS has made the investment to purify their argon gas used for welding to the ppb level of purity. The orbital weld heads are of the enclosed type and form a chamber filled With inert argon gas which protects the stainless steel weld pool from oxidation which would occur at welding temperatures in the presence of atmospheric oxygen. Similarly, purified argon is supplied to the inside (ID) of the tubing during welding to protect the inner weld bead from oxidation. If the argon used for purging contains more than 1-2 ppm oxygen or moisture, a light brownish or bluish discoloration occurs on the weld. If this should form an aerosol and be released into the gas stream it would contaminate the process, so most semiconductor specifications require that there be no visible sign of oxidation on the weld ID. The bluish discoloration that occurs has been associated with oxides that vaporize from the molten weld pool and precipitate in bands on either side of the weld. Oxidation, or heat tint, as well as manganese deposits have been associated with corrosion in the heat-affected-zone (HAZ) of welds in process gas lines transporting corrosive gases such as HC1 or HBr.

Even with gas of ppb quality, successful purging of the weld joint can only be accomplished if the gas reaches the weld joint without absorbing atmospheric oxygen or moisture through the hoses delivering the gas or from elsewhere in the purge system such as diaphragm leaks in the regulator or from around the fittings. CFS uses stainless steel and PTFE hoses with Utra-Torr® fittings, which can be tightened and loosened repeatedly, to connect the gas hose to the part being welded. Hoses should be impermeable and non-particulating. Welded stainless steel fits this description but is impractical so most people use plastic hose of some type, of which some are better than others, but none are perfect. For effective purging of the weld OD the flow rates of gas to the weld head must be appropriate to the size of weld head being used. CFS has developed weld schedules which include purge and shield gas flow rates appropriate to specific tube sizes.

Pressure is created when the exit orifice is smaller than the tube delivering the gas. The right amount of pressure on the tube ID helps to achieve a flat smooth inner weld bead which is so important to semiconductor processing, but care must be taken not to over pressurise the tube. To control oxidation, CFS maintains a continuous ID purge. For site work they say it can take up to one or two days to get an acceptable test weld because of the time required to purge the lines. Field work can make purging difficult since welding to a 36m long 1/4" OD tube will add enough pressure to blow out a weld. They do not weld with a check valve in the line as this would create too much pressure, consequently the gas supply used for purging must be separate from any process gas line.

Washing


Figure 3.
New washing equipment in the cleanroom is used for washing components prior to welding and for washing assemblies before shipping, CFS makes their own 18M water for this purpose.

Tubing for assemblies arrives at CFS precleaned, bagged, and ready for use. Tubing is re-cleaned before use to remove any contaminants that might have been picked up. Contamination control includes the use of nitrile rubber gloves for handling tubing and components. The QC Manager feels that if you don't get it dirty, you won't have to clean it.

Ultrasonic solvent-based cleaning procedures have been replaced with the new hot DI water baths. An aqueous ion-free detergent solution at 60°C is followed by 2 DI washes with 18M water. Parts are then dried with a hot nitrogen drier for 10 minutes. Finished assemblies are also washed and dried before shipping.

Welding equipment, maintenance and productivity

Arc Machines' Model 9-500 weld heads with tube clamp assemblies (TCAs) for 1/4, 3/8, and 1/2 inch diameter tubing are in use constantly for CFS's high-production high-purity operation. They can do up to 20 welds per hour if they don't have to change tube sizing. With two 9-hour shifts each day and with each welding operator averaging 135 welds a shift, they complete about 270 welds per day per machine.


Figure 4.
Arc Machines' Model 207 orbital welding power supply located beneath workbench and Model 9-500 weld head with assembly ready for welding. Weld sequence is initiated via remote operator pendant in holder at left. Note use of height gauges to support weld assemblies.

CFS has implemented a regular maintenance program to assure that the weld heads will continue to operate properly with this type of use. TCA's are routinely cleaned every other day with Scotch-Brite to remove all traces of accumulated contaminants. Similarly, each weld head is dismantled on a weekly basis and traces of contamination inside the head and on the gears is removed to minimize the possibility of arcing problems. Tungsten electrodes are machined to length with a consistent tip geometry. They are specified to be 2% Ceriated tungsten which give a more stable arc and superior arc strike reliability than thoriated tungstens. As a precaution, the tungstens are replaced periodically rather than waiting for them to fail. CFS experiences very little downtime with their weld heads as a result of their systematic approach to maintenance.

Welding Procedures

For any type of production welding, but especially for high-purity or UHP welding, it is most important to have a written welding procedure in-place. CFS has their In-House Welding STANDARD OPERATION PROCEDURES posted in the cleanroom. The SOP's include the situations which require the making of test welds. Test welds are made at the start and end of every day, with a change in tube size, after each 15 welds or after a change of electrodes. Welds may be inspected on the ID with a borescope, but when it is not possible to inspect each weld, acceptable test welds indicate that the procedure is working correctly, and the repeatability of the orbital welding process provides some assurance that the production of acceptable welds will continue. SOP's also include required purge times and gas flow rates for each tubing size and specify the use of a restrictor on 3/8 and 1/2 inch lines to a 1/4 inch outlet.

A Weld Procedure Qualification (WPQ) by Lloyd's Register of Industrial Services is in effect certifying that test samples have successfully passed bend and tensile tests that prove the ductility and strength of the weldments. Weld qualification testing was done by Incon. The weld standards are in compliance with German DIN norms and meet the requirements for BS 4870 and newer EN 287 and EN 288 standards.


Figure 5.
Completed welds are inspected routinely with a borescope. Test coupons are made on a regular schedule and the samples cut open and examined visually.

CFS has sample welds mounted on a board in the cleanroom that show examples of acceptable welds and welds with rejectable defects including incomplete penetration of the weld joint, discoloration from oxidation (none allowed), misalignment, and OD concavity. All welds must be completely penetrated on the ID. The weld bead should be of uniform width around the entire circumference of the weld joint. There is no restriction on the maximum width of the weld bead. Weld beads on materials very low in sulfur tend to be much wider than those with sulfur content over 0.008% by weight. Arc deflection severe enough to cause a lack of weld bead penetration may occur when welding materials from two different heats when one is very low in sulfur and the other is 0.010% or greater. CFS has had no problems with sulfur mismatches although some of their valves and components are of VIM/VAR material which is typically very low in sulfur.

Tubes and fittings must be well-aligned for proper weld joint configuration, and since the weld head alone was not intended to provide full support of the components being welded, CFS uses heavy-weight engineering height gages to hold components in position. A precision fit-up is required for orbital fusion butt welding and CFS uses a portable end-preparation tool to get good square butt-weld preparations which are essential for uniform weld quality. During the facing operation the tubing is purged with nitrogen to prevent contamination of the ID. The consistency of orbital welding results in uniform shrinkage of components during welding so that the amount of shrinkage can be factored-in during the design phase. Allowances for shrinkage per weld are 0.15 mm for 1/4 inch OD tubing, 0.30 mm for 3/8 inch tubing and 0.50 mm for 1/2 inch diameter.

All welds are completed in a single pass. This is efficient from a production standpoint and results in lower heat input and a smaller heat-affected-zone (HAZ) than with welds of more than one pass. A weld program is developed for each tubing size and stored in the microprocessor-based power supply where they can easily be recalled to make a weld without effect on other stored programs. A pulsed-arc GTAW (gas tungsten arc welding) process is used with the primary welding current proportional to the wall thickness, i.e., about 35 amperes for 0.035" wall material, and 49 amperes for 0.049" wall tubing. A tungsten electrode installed in the rotor of the weld head orbits the weld joint to complete the weld. Tungsten RPM is based on electrode travel speed of about 5-6 IPM which is about 3.2 RPM for a 1/2 inch diameter tube and about 8 RPM for a 1/4 inch diameter tube. Pulse times have an effect on weld bead appearance and can be as short as 0.01 sec but are typically set closer to 0.1 sec.

Well-trained orbital welding personnel are an essential element of a successful cleanroom fabrication operation. Welding operators were trained at the Arc Machines UK office in Derby how to set up and operate the equipment, write weld programs for each size tube and to program the welding power supply. Welding personnel must be able to evaluate the welds and make adjustments if necessary. The slight variability in trace elements from heat-to-heat of stainless steel affects the welding properties such that slight adjustments of welding amperage may be required. In addition to training offered by the orbital welding equipment manufacturer, personnel are also qualified to BS4870 Part IV.

Testing of Finished Assemblies

Figure 6.
Helium leak-testing of an orbitally welded gas control panel in the cleanroom at Cambridge Fluid Systems. Leak detection level is 10-11 atm/cc/sec.

Accurate leak-testing of gas panels and other assemblies is essential for use in critical semiconductor processes where maintaining gas purity to the point of use is mandatory. Furthermore, with process gases that are highly toxic or pyrophoric, containment is just as important as the prevention of contamination. CFS has leak-detection units, UL 4OOs, which detect leaks to 2 X 10-11 atm/cc/sec and UL 100 units which detect leaks to 1 X 10-10 atm/cc/sec. Helium is used for detection of leaks as it has a very small molecule that passes through minute spaces when sufficient vacuum is applied to the other side.

CFS use four methods for leak testing with helium:

1) The first method requires the unit under test (UUT) to be evacuated by a helium leak tester with 100% He sprayed around all welded and de-mountable joints on the outside and checked for He leaking to the inside;

2) The UUT is then filled with Electronics grade (99,9995% pure) 5% He in 95% N2 systematically so that valve seat leakage can be determined at working pressure;

3) When the UUT is fully pressurized (1.2 x working pressure) a He sniffer probe is used to check all welded and de-mountable joints for leaks;

4) The final test uses a vacuum chamber which was made to CFS's own design. The UUT is placed pressurised into the chamber and a leak tester is used to sample the overall system leak rate within the chamber. The final vacuum chamber test has been very useful as leaks are found in valve bodies and weld fittings at a failure rate of 1 per 1,000. In addition to leak testing, assemblies are tested to ensure that they do not contribute particulate matter to more than 10 particles of 0.1 µ size per arketing/Expansion of Operations

Sales Director Steven Hales says that CFS has a major share of the market for gas control units for the UK semiconductor fabrication industry. In 1995/1996 CFS's core business was £7 million, most of which was based on orbital welding. CFS is working to maintain their market share and are making another 1,800 sq ft cleanroom in Scotland close to the NEC facility in Scotland's Silicon Glen where more wafer fabs will be built in the next few years. This will place CFS in a position to offer more on-site support. CFS are also expanding into the Far East with good prospects for sales of gas control systems.

CFS' orbital welding work is not limited to cleanroom fabrication - they also do field installations in the semiconductor industry as well as in several other industries. They have used Arc Machines' orbital welding to install instrumentation lines for refineries and even done orbital welding of large-diameter schedule pipe for the nuclear industry. By effective systematic use of orbital welding technology, CFS has established themselves as a leader in the demanding semiconductor industry and positioned themselves to take advantage of other opportunities in the global marketplace.


By Barbara K. Henon, Ph.D., Arc Machines, Inc.

Cajon® and Utra-Torr® are the registered trademark of the Swagelok Corporation.

Scotch-Brite® is the registered trademark of the 3M Company.cubic ft and less than a 10 ppb contribution of H2O or O2 to the gas stream passing through them.

Design Considerations


Figure 7.
CFS has modified the TCA of the Model 9-500 weld head to accept a large configuration.

enior Production Supervisor, Chris Mitchell, is one of the CFS engineering group. In designing the manifolds and panels they consider the dimensions of the orbital weld heads. However, to accommodate special configurations the heads may require slight modifications. For example, to weld the Cajon® fitting SS-4-VCR-4TB2 to a microfit elbow in the Model 9-500 weld head CFS cut away part of the weld head TCA to accommodate the nut. In very tight situations, the nut on the fitting was modified slightly to achieve the fit. Since the system in which the assembly was installed had low pressure requirements, this was acceptable. CFS's Managing Director, Soon Lim Lee, says that they could use some slight modifications of the weld heads to ease some clearance situations, but that they are generally very satisfied with the performance and the output of the orbital welding equipment at the present time.

Marketing/Expansion of Operations

Sales Director Steven Hales says that CFS has a major share of the market for gas control units for the UK semiconductor fabrication industry. In 1995/1996 CFS's core business was £7 million, most of which was based on orbital welding. CFS is working to maintain their market share and are making another 1,800 sq ft cleanroom in Scotland close to the NEC facility in Scotland's Silicon Glen where more wafer fabs will be built in the next few years. This will place CFS in a position to offer more on-site support. CFS are also expanding into the Far East with good prospects for sales of gas control systems.

CFS' orbital welding work is not limited to cleanroom fabrication - they also do field installations in the semiconductor industry as well as in several other industries. They have used Arc Machines' orbital welding to install instrumentation lines for refineries and even done orbital welding of large-diameter schedule pipe for the nuclear industry. By effective systematic use of orbital welding technology, CFS has established themselves as a leader in the demanding semiconductor industry and positioned themselves to take advantage of other opportunities in the global marketplace.


By Barbara K. Henon, Ph.D., Arc Machines, Inc.

Cajon® and Utra-Torr® are the registered trademark of the Swagelok Corporation.

Scotch-Brite® is the registered trademark of the 3M Company.