Welding techniques for delivering UHP gases to process tools

To produce a quality weld, three parameters should be controlled — impurity levels in the stainless steel, total welding time and welding gas quality.

By P. Cruickshank, D. Ostoja, S. Krishnan and M. Saleem

Minimizing contamination in process gas delivery systems transporting specialty gases to process tools is required as semiconductor processing enters the new era of 300 mm wafer size. In addition to improving the quality of stainless steel tubes used in transporting process gases, welding techniques play an important role in achieving this goal.

Semiconductor processing requires the use of ultra high purity (UHP) gases with impurity levels in the range of ~1 ppb for specialty and ~100 ppb for bulk gases. Delivering UHP gases without contamination to the process tools poses challenges for designing and manufacturing gas delivery systems. Semiconductor-grade electropolished (EP) stainless steel surfaces used in tubing and components in gas delivery systems are satisfactory to maintain the high purity levels of process gases up to the process tool. However, this passivated EP layer is destroyed in the weld bead and in the area — known as the heat-affected zone (HAZ) — adjacent to the weld bead. Also, Manganese (Mn) outgasses from the bulk stainless steel during welding and deposits in the HAZ. The Mn deposits in the HAZ have a high surface area and readily absorb moisture from the gas distribution system. Thus, the welded area is a source of significant contamination in the process gas delivery system due to possible contamination of UHP gases and the processing environment. Depending on the process gas being delivered, weld bead contamination can range from gas phase contamination to particle generation.

Welding parameters

Non-pulsed or continuous welding is used in semiconductor gas delivery applications since it produces a weld bead with less surface area and a smaller HAZ. In order to produce a good quality weld suitable for a UHP gas delivery system, three parameters should be controlled: (1) impurity levels in the stainless steel, (2) total welding time, and 3) welding gas quality.

(1) The impurity levels are defined and controlled through specifications for single and double-melt stainless steel materials. Because single-melt stainless steel consists of higher levels of impurities than double-melt stainless steel, the welding currents required to produce a weld are lower for this material than for double-melt stainless steel due to the formation of eutectic (i.e., lower melting) mixtures.

(2) The total welding time determines the heat input into the weld. This is a function of weld current and weld speed. Conventional orbital welding utilizes speeds of ~5 rpm in an uncontrolled ambient while ultraclean welding uses speeds of ~10 rpm in a Class 100 cleanroom. In order to maintain a fixed heat input into the weld, an increase in weld speed has to be accompanied by an increase in weld current supplied to the electrode used for welding.

(3) The use of appropriate weld gases and backsealing gases is also important to achieve an optimum weld. For example, H2 is used as part of the arc gas in order to obtain high temperatures at the electrode and at the tubing surface to be welded. In addition, H2 acts as a reducing agent for bulk and trace metal oxides formed due to high temperatures reached during welding. In the absence of H2, higher weld currents will be required to achieve the same quality of the weld bead. Additionally, the use of high purity gases is critical to prevent oxidation or undesirable side reactions from occuring in the weld area. Thus, moisture (H2O), oxygen, and hydrocarbon impurities in the weld and back seal gases have to be strictly controlled in the ppb concentration levels. The backsealing gas acts as a medium to prevent the adsorption of outgassed materials during the welding process onto the inner surfaces of the tubing. After completion of the welding process, it is used as a purge gas to remove contaminants from the welded area.

Weld bead quality

Conventional single-melt stainless steel has a weld bead surface roughness of 140-200 &#181in (microinches) (See Figure 1). In contrast, the weld bead of double-melt stainless steel exhibits surface roughness of ~70 &#181in (See Figure 2) [1,2]. The conventional weld site is capable of excessive moisture entrapment due to greater surface roughness. In addition to the quality of stainless steel, other factors that affect the quality of the weld include the welding speed, gases, and ambient. During the welding process, elements like sulfur and Mn, which have relatively high vapor pressure at welding temperatures, outgas from the weld area and re-deposit in the HAZ following a crude vapor deposition mechanism [3]. This is particularly detrimental in applications involving corrosive gas delivery where the contaminated HAZ can exhibit decreased corrosion-resistance to reactive gases, and can result in particle generation. The ultraclean welding methods use welding speeds of ~10 rpm in the presence of UHP nitrogen purge gas (<1 ppb H2O, O2) in a Class 100 cleanroom to reduce the amount of Mn vaporized and redeposited downstream of the weld bead.

The quality of weld beads also depends on alignment between parts to be welded, presence of particles or chemical impurities on the surfaces of the coupons to be welded, and purity of welding gases.

Ultraclean, high speed welding will be required for special welding applications such as welding of chromium passivated (CrP) [3] and fluorine passivated components.

Conclusions

Delivering UHP gases to process tools in a semiconductor wafer fab without microcontamination poses a significant challenge in the design and manufacture of gas delivery systems. Improvements in welding technology have been made to accomplish this goal. Conventional welding techniques result in destruction of the chromium-rich electropolished layer of stainless steel surfaces and in the deposition of Mn in the HAZ of weld sites. In applications involving corrosive gas delivery, Mn deposits catalyze corrosion resulting in particle generation. Ultraclean, high speed welding is useful in minimizing the destruction of the chromium-enriched electropolished layer and in reducing Mn outgassing in the HAZ. Thus, the use of high purity gases and high speed welding in a controlled ambient is critical for the manufacture of weldments or gas panels used in semiconductor process gas delivery.

References

1. S. Krishnan, S. Grube, O. Laparra, and A. Tudhope, “Site Specific Corrosion in Gas Delivery Tubing Exposed to Semiconductor Grade HCl”, Supplement to CleanRooms, October 1995, pp. S11-S15.

2. S. Krishnan, A. Tudhope, O. Laparra, and J. Grob, “Ultraclean Gas Delivery Systems: A Case Study”, Semiconductor International, April 1995.

3. T. Ohmi, “Corrosion-free Cr2O3 Passivated Gas Tubing System for Specialty Gases”, Supplement to CleanRooms, October 1995, pp. S18-S22.

Dr. Sowmya Krishnan is manager of technology and applications development at Ultra Clean Technology (Menlo Park, CA), a manufacturer of ultra high purity gas delivery systems. Mohamed Saleem is a research associate in the technology development group. Phillip Cruickshank is a lead welder in the manufacturing group. Dave Ostoja is a welder in the manufacturing group.