Gases & Instrumentation - January/February 2010

of fit-up and workmanship. The sample weld is then cut open (sectioned) to show the entire ID weld bead and analyzed at the job site. The sample weld is checked against the weld criteria in SEMI F81 for penetration, bead concavity, bead variation and oxidation. Once the primary standard sample has been deemed acceptable, the primary sample becomes the on-site work sample against which other weld coupons are evaluated.

Sample welds are made periodically at the beginning and end of a shift, when weld parameters are changed, when the material heat number4 is changed, for a change in tube size or wall thickness, a change in ambient temperature of +/- 20 degrees F, or when the power source or welding machine, weld head, electrode, etc. is changed. The ID purge source used for couponing must be the same as that used for production welds.

Production welding is permitted only after the sample weld has been accepted. The sample weld may be a production weld only when it is possible to examine the ID with a borescope or sight tube. The welding procedure details the conditions under which welds may be repaired or when they must be cut out.

Each weld has a unique number and is cross referenced to a specific drawing. Traceability is assured by a weld log which must be maintained for all welds, including coupon welds.

Operator training

Operator training is essential. Welding operators must not only be able to operate the welding equipment, but must understand and be able to carry out welding procedures as described in SEMI F78. Training is provided by the welding equipment manufacturers, and training classes in orbital welding and cleanroom protocol are offered for apprentices by the United Association of Journeymen and Apprenticeships in the plumbing and pipefitting industry of the United States and Canada5.

Materials

AISI Type 316L stainless steel has been the material of choice for semi-conductor process gas lines. Other corrosion resistant alloys, such as hastelloy C- 22, have been

used to a lesser extent with corrosive gases. Refinements in steel production technology have lead to materials relatively free of inclusions and other contaminants , and when welded, have exceptionally smooth weld beads.

The Effect of Sulfur on Welding

One of the initial problems in welding Type 316L was the wide range in sulfur content in the AISI (American Institute of Steel and Iron) specification that specified a maximum sulfur content of 0.030 wt.%, but no minimum.

Sulfur has a dynamic effect on weld pool characteristics. The direction of fluid flow is based on surface tension, (i.e. the Maran-goni effect) which has a positive correlation with temperature when sulfur is present

Autogenous orbital
tube welding is
completely automatic

and a negative correlation with temperature at low concentrations.

When the sulphur concentration is at the mid- to high-range for the AISI specification, the direction of fluid flow in the weld pool is from the edges of the weld pool inward creating a weld with deep penetration. At the low end of the sulfur specification, typically less than 0.005 wt.%, fluid flow in the weld pool is reversed so that, in this case, the fluid flow is from the center of the weld pool outwards creating a wide, shallow weld bead. Other elements such as oxygen and selenium have similar, but less noticeable effects.

Since sulphur combines with manganese to form inclusions, the surface finish of the higher sulfur heats is unacceptably rough. The semiconductor industry elected to specify sulphur concentrations at the lower end of the range. SEMI F206 specifies an upper limit of sulfur of 0.012 wt.% for general purpose tubing and 0.010 wt.% for high purity and ultra- high purity tubing.

Sulfur mismatch, which occurs when
one heat of material2 is significantly higher
in sulfur than the other component being
joined, can result in the weld pool shifting
towards the low sulfur heat. This is less
likely to occur when both components
are in the lower sulfur range, but welding
of very low sulfur materials requires sup-
port of the weld puddle during welding
by pressurization to achieve an acceptable
weld bead profile. This must be done in a
controlled manner to achieve consistent
results as specified by SEMI F78.

Conclusions

Orbital GTA welding has been of fundamental importance in the development of semiconductor fabrication technology. There have been advances in the welding technology, improvements in welding equipment, increases in gas purity, and developments in materials science that have all contributed to better, cleaner, more repeatable welds. This would not have been possible without orbital welding.

The SEMI Standards, particularly SEMI F78 Practice, reflects a tremendous amount of technical expertise by end users, installing contractors, welding operators, weld equipment manufacturers, and quality assurance personnel. Their combined experience has gone into the development of practical welding specifications for this technologically demanding industry. G&I

References

1. SEMI F78-0304 Practice for Gas Tungsten Arc (GTA) Welding of Fluid Distribution Systems in Semiconductor Manufacturing Applications

2.SEMI F81-1103 Specification for Visual Inspection and Acceptance that details weld acceptance criteria.

3. Thorium, cerium and lanthanum are types of tungsten differentiated by the dopant that is added to the pure tungsten to make it more conductive and thus facilitate arc strike.

4. Different melts of steel, even of the same type of steel, are differentiated by a “Heat Number”. This is traceable back to the steel mill and provides a list of the base metal, alloying elements and some trace elements in that specific melt of steel. The percentage of alloying and trace elements affect the weldability of each heat of material to some extent.