Standfirst: Increased used of perfluoropolyether (PFPE) fluids in coolant systems for both 200mm and 300mm etching tools has suffered from leakage problems for this expensive material. John Baxter of Swagelok describes the process by which the site of the leakage was traced and his companys solution consisting of replacing O-rings with ball-valves and silicone seals

Perfluoropolyether fluids are gaining acceptance in a variety of semiconductor applications, particularly as a coolant in heat transfer systems where fluid must be circulated between a process tool and a chiller. The increased use of perfluoropolyether (PFPE) fluids in coolant systems for both 200mm and 300mm etching tools is largely attributable to its properties, which allow for maximum heat transfer from the etching tool while providing resistance to the high voltages and magnetic fields associated with the process. The dielectric properties of PFPE coolants do not break down, thus eliminating the need for expensive resistivity monitors and filtration systems that often have been required for other coolants, such as ethylene glycol, glycol/dionised water mixtures and Freon. PFPE fluids are known for being clean and inert and having the ability to withstand a wide range of temperatures.
Although Freon at one time was a coolant of choice, it gradually is being supplanted by PFPE fluids in part because of concerns about monitoring for and detecting leaks in systems, as Freon evaporates upon contact with the atmosphere, making it difficult to visually detect system leaks.

Despite the overriding benefits of PFPE fluids, the material has come under growing scrutiny recently by equipment manufacturers and end users, some of whom have encountered seal problems within their heat transfer systems. PFPE fluids are expensive, costing in the neighbourhood of $130/litre. Yet some fabs reportedly have opted to replace from four to eight litres per day of PFPE fluid that leak from systems rather than incur the even more expensive proposition of halting production to replace seals and fix leaks. Multiplying the cost for the volume lost per day by the number of days the tool is in operation gives the yearly loss from leakages. In some circumstances, the cost of the replacement coolant material alone can reach $500,000 within the course of a year. This leakage problem can be exacerbated if too much PFPE fluid escapes, causing the tool to shut down and requiring it to be thoroughly cleaned, brought to the proper working temperature, and qualified for production. Consequently, tool downtime can pose an even more expensive problem, where the total cost per shutdown is equal to the number of hours it takes to recommission a tool multiplied by the hourly rate of the tool.

Team solution

Last year, sealing problems prompted a semiconductor original equipment manufacturer (OEM) and Solvay Solexis, a supplier of thermal fluids and other chemicals, to team with Swagelok to develop a solution that would serve as a sealing method for an etch tool. Specifically, Swagelok developed a leak-tight valve assembly centred on a ball valve design that replaced the plug valve previously used.
Swagelok was contacted after the OEM determined that PFPE fluid was leaking from the etch tools coolant lines at several fabs throughout the world. The severity and frequency of the leaks appeared to be related not only to the system fluid, but also operating temperatures.
PFPE fluids typically flow through flexible hoses that connect a chiller to a valve manifold box in the chase and ultimately link to the etch chamber. In one case, a fab cooled two metal etch chambers from 85ºC to 25ºC and then shut off the coolant lines for maintenance. A coolant leak appeared to come from the plug valves.
Upon examining valves returned from the field, the OEM ascertained that the failure appeared to result from compression set in the plug valve O-rings. Compression set is the relaxation and deformation of an elastomer and leads to a reduction in the ability of the O-ring to maintain a seal.

O-ring material problems

Historically, plug valves have been the most common valve design in coolant systems. To create a seal, plug valves rely on elastomer O-rings that are inside the valve body and are lubricated to reduce seal friction and wear during valve actuation. Extensive tests undertaken by Swagelok found that the elastomer seals in the plug valves were the primary source of the leakage. The dynamic seals in a plug valve rely on the ability of the O-ring to remain elastic and dimensionally stable. In the case of this particular OEM, the elastomer O-rings used in the leaking plug valves were made of EPDM (ethylene-propylene-diene monomer). PFPE can act as a weak solvent for some materials, capable of leaching out the plasticisers used to help make elastomers like EPDM supple, as well as removing lubricants that minimise the friction and wear of dynamic seals. This effect, in conjunction with the temperature cycling, contributed to the failure of the elastomer.

In developing a solution, Swagelok focused on targeting the compatibility and functionality of a complete family of products with PFPE fluids, specifically those manufactured by Solvay Solexis, over a range of service temperatures. Solvay Solexis played an important role in the process by working closely with Swagelok in the research and testing.
The Swagelok independent distributor network also assisted in gaining insight into the problem and the inherent challenges. Specifically, Sunnyvale Valve and Fitting in California worked with the tool manufacturer. Portland Valve and Fitting in Oregon worked with an end user who experienced leaks. Finally, Penn Valve and Fitting in Pennsylvania worked with Solvay Solexis. Their knowledge and presence in the field played an integral role in arriving at a quick and effective solution.
The underlying objective was to satisfy OEM concerns about using PFPE fluids in current and future equipment. Swagelok also sought to develop a solution for the industry as a means of effectively containing these fluids.

Success with ball valves and silicone

Recognising that the dynamic seal created by the plug valves was a contributing leakage factor, Swagelok theorised that its 60 series ball valve would better serve the application. The ball valve was considered because, unlike a plug valve, it does not contain dynamic elastomer seals. Rather than relying on lubricants and O-rings, the ball valve features a spring-loaded ball seal. The springs flex and ensure there is constant pressure on the ball seal at all times, regardless of the temperature or type of fluid flowing through the system. Static O-ring seals are used to prevent leakage through the valve body flanges.
After assessing the leakage problems, Swageloks engineers and researchers turned their attention to the material in the flange seal O-rings, opting to test silicone against the more traditional fluorocarbon FKM (trademarked as Viton) elastomer. Silicone was selected because it has the ability to withstand a wide range of temperatures while continuing to maintain its sealing capabilities.
Swagelok completed a comprehensive series of low-temperature and high-temperature tests of the 60 series ball valve with silicone flange seals to determine the seal performance during thermal cycling. Swagelok tube fittings, VCR face seal fittings, quick connects and hoses were also tested. Test temperatures ranged from -60ºC to 180ºC and all components were pressurised with PFPE fluid (100psi~7bar). High-temperature thermal cycles extended over a period of 32 days, 26 of which were spent at 180ºC.
The findings showed that the ball valves with the silicone flange seal, as well as the fittings and quick connects had no measurable external leakage. In the case of the 60 series ball valve, the static silicone seal appeared to be unaffected by the PFPE fluid and performed reliably over the entire temperature range.

Adapted for field use

As a result of the testing, Swagelok developed a solution by customising the 60 series ball valve with an additional purge port, flow sensor and end ports/flanges for a semiconductor OEM. A product assembly consisting of the 60 series ball valve, VCR and tube fittings, quick connects and hoses also was developed. The supply and return loop assembly was placed into an enclosure and was beta tested. Product test reports were also compiled to document the performance of this product under the conditions mentioned and are available for review. The first 22 units have gone into operation and have performed without leakage.

The combined effort has resulted in better tool utilisation and minimised losses with respect to PFPE fluid containment. The sole source solution can be implemented in 200mm and 300mm tool coolant systems, either for new tool production or in retrofits to existing systems. This cost-saving design will be expanded to other tools using PFPE thermal fluids and made available for field replacement in existing systems.

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