by Kurt Werner, Phillip Tuma, Larry Zazzera, 3M
Executive overview
July 7, 2009 — Recent regulations adopted by the California Air Resources Board highlights the need for a strategy to reduce greenhouse gas emissions from microelectronic manufacturing facilities that use perfluorocarbons (PFCs) and perfluoropolyethers (PFPEs) heat transfer fluids. Publications on emissions of PFC gases and PFC and PFPE heat transfer fluids used in semiconductor manufacturing and physical property data on alternative heat transfer fluids are used to estimate potential emission reduction opportunities through adoption of the alternative fluids. The results indicate that greenhouse gas emissions from PFCs and PFPEs used in heat transfer applications can account for a significant fraction of a fab’s total PFC emissions and that these emissions can be reduced 58%-99% with the use of alternative dielectric heat transfer fluids. These findings are important because the use of alternative heat transfer liquids can be part of an industry strategy to further reduce PFC emissions.
Heat transfer fluids
Semiconductor manufacturing processes such as etch, plasma enhanced chemical vapor deposition (PECVD), ion implant, and test, require dielectric heat transfer liquids to maintain process or component temperatures. Both deionized-water-based liquids (DI water/glycol) PFC, and PFPE liquids are used in these applications. PFC and PFPE liquids are used because they are chemically inert and non-flammable; they have high dielectric strength and electrical resistivity; they evaporate cleanly and are practically non-toxic. As process temperature ranges continue to widen, the functional limits of deionized liquids are often inadequate. Various fluorinated fluids including PFCs and PFPEs are being used in an increasing percentage of applications not only because they posses the requisite safety, performance, maintenance and dielectric properties but because they can span a wide temperature range of -115°C to over 200°C (see Figure 1).
Operating temperature range of heat transfer fluids.
Reduction of PFC gas emissions
In 1995, the US semiconductor industry voluntarily committed to significantly reduce atmospheric emissions of specific PFC gases generated by plasma wafer etch and PECVD chamber cleaning processes, by signing a Memorandum of Understanding with the Environmental Protection Agency (EPA). In 1999, the global semiconductor industry agreed to reduce absolute PFC emissions to 10% below their respective baselines by 2010. In 2000, the US semiconductor industry negotiated a second voluntary agreement with the EPA. The PFC Reduction/Climate Partnership Memorandum of Understanding (MOU) supports the WSC agreement for a collective 10% reduction in emissions by 2010. By 2003 the US semiconductor industry had achieved the 10% reduction goal and they surpassed the goal in 2004. Emission reduction activities included: process optimization; abatement, capture and recycle; and the adoption of alternative plasma chemistries (1).
Emissions from PFC and PFPE heat transfer liquids
Given the growing use of PFC and PFPE heat transfer fluids tied to growing wafer demand, and the successful execution of emission reduction strategies for specific PFC gases, the percentage of PFC emissions attributable to PFC and PFPE heat transfer fluids is growing. PFC and PFPE fluids will evaporate when not contained, so emissions occur via equipment leaks, evaporation and spills. Though published atmospheric data for PFC liquids are somewhat sparse, they indicate that PFC liquids have atmospheric lifetimes and IR cross sections similar to the PFC gases listed in the MOU.
If the global warming potential (GWP) of a PFC liquid (or the vapor of that liquid) has not been measured, it is standard practice to substitute C6F14 data as a means of estimating the environmental properties of that material. Initial findings in a recent report prepared for the US EPA indicated that PFC emissions attributable to heat transfer fluids are ~20% of the total gaseous and liquid MMTCE emissions from a high-volume electronics manufacturing facility [2]. Some semiconductor manufacturers have recognized the growing contribution of PFC liquids and converted to alternatives where possible. For various reasons, equipment manufacturers have been hesitant to specify alternative fluid chemistries. As a result, opportunities for substantial emission reduction from the industry still exist.
Pending regulations
The California Global Warming Solutions Act of 2006 (specifically, the Health and Safety Code section 38560.5 [a]) stipulates that the California Air Resources Board identify and make public a list of discrete early action greenhouse gas emission reduction measures. On October 25, 2007, the Board approved the addition of the Semiconductor Perfluorocarbon Emissions Reduction Strategy as a discrete early action measure. This measure was adopted by the Board on February 26, 2009 and is enforceable by January 1, 2010. It requires semiconductor manufacturers in the state of California to certify the volume of heat transfer fluid purchased, its brand name, the amount put into each manufacturing tool, and whether that tool is new (being commissioned) or existing.
Reducing emissions from PFC heat transfer liquids
There are two different approaches for reducing emissions resulting from lost PFC heat transfer fluids. The first is to identify and eliminate leaks and evaporative losses. The second approach is the adoption of an alternative liquid with a lower GWP. A critical on-site review of a heat transfer system installation can often reveal causes of fluid loss. Causes of liquid leaks include inappropriate or damaged connections, and inappropriate valves or pumps. An often overlooked mechanism is evaporative loss resulting from temperature cycles. An understanding of this mechanism can, in some instances, reduce losses dramatically. With a nominal investment of time, eliminating leaks and reducing evaporative losses will immediately reduce PFC emissions and decrease the costs of replacing lost liquid. However, the effectiveness varies widely and on average, losses are reduced no more than 20%.
Alternative liquids
The most effective strategy for reducing emissions resulting from lost PFC liquids is replacement of these liquids with low-GWP alternatives. While a variety of alternatives can satisfy the temperature requirements of semiconductor applications, many suffer limitations that make their use impractical. For example, petroleum-based products, silicone oils, and citrus-based oils tend to be flammable or unstable. Chlorofluorocarbons are ozone depleting and other chlorinated materials like dichloroethylene, trichloroethylene and methylene chloride are flammable, unstable, or highly regulated due to their toxicity.
Commercially available segregated hydrofluoroether (HFE) and hydrofluoropolyether (HFPE) liquids have GWPs that are 1%-48% those of common PFC liquids. These PFC and PFPE alternatives have already been used successfully in semiconductor manufacturing applications, most notably in automatic test equipment. These alternative fluids are used because they posses many of the desirable safety, dielectric, compatibility, and stability properties of PFCs and PFPEs.
Physical and environmental properties of heat transfer liquids
Like PFCs and PFPEs, the alternative fluids are non-ozone depleting. Because they are not photolyzed in the lower atmosphere, these materials do not contribute to atmospheric smog formation. In the troposphere, reactive hydroxyl radicals quickly break down these molecules, resulting in short atmospheric lifetimes and GWPs that are 1%-50% that of C6F14. The table below contains physical and environmental properties of the most popular PFC liquids along with HFE and HFPE alternatives. Numerous other HFE and HFPE fluids are available.
Physical and environmental properties of various heat transfer liquids.
Table 1 also lists various properties of heat transfer liquids. GWP is the global warming potential for a 100 year time horizon relative to carbon dioxide as published by or calculated with methods used by the Intergovernmental Panel on Climate Change (IPCC) [3]. Relative GWP is the GWP of the listed material divided by 9300, the GWP of C6F14. Boiling Pt. is the boiling point in degrees Celsius and defines the upper operating temperature range. Low Temp is the lowest practical operating temperature based on viscosity or freeze point considerations.
Conclusion
A fab’s PFC heat transfer fluid emissions are a significant and growing portion of its total PFC emissions. Use of commercially available alternative fluids can reduce global warming emissions attributable to PFC liquids by 58% to over 99%. These alternative heat transfer fluids possess a balance of properties that makes them well suited for semiconductor heat transfer applications.
Acknowledgments
Galden HT Fluids and H-Galden ZT Fluids are registered trademarks of Solvay Seolexis.
Fluorinert FC Fluids and Novec HFE Fluids are registered trademarks of 3M.
References
1. L. Beu, “Reduction of PFC Emissions: 2005 State of the Technology Report,” International Sematech Manufacturing Initiative (ISMI), Tech. Transfer # 05104693A-ENG, Dec. 2, 2005.
2. C. Shepherd Burton, “Uses and Air Emissions of Liquid PFC Heat Transfer Fluids from the Electronics Sector,” EPA Document # EPA-430-R-06-901.
3. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; eds.: S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, H.L. Miller; Cambridge U. Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp, 2007.
Biographies
Kurt Werner received his BS in biology from St. John’s U., (Collegeville, MN), an MS from the U. of Minnesota and is Board Certified in General Toxicology by the American Board of Toxicology. He is the Environmental Affairs Manager in 3M’s Electronics Markets Materials Division, 3M Center, 251-2A-08, St. Paul, MN 55144 USA.
Phil Tuma received a BA degree from the U. of St. Thomas, a BSME from the U. of Minnesota, and an MSME from Arizona State U. He is an advanced application development specialist in 3M’s Electronics Markets Materials Division.
Larry Zazzera received a BA degree in chemistry from the U. of Delaware and a PhD in materials chemistry from the U. of Minnesota. He is a technical manager in 3M’s Electronics Markets Materials Division.