Application Considerations for Perfluoroelastomer Seals Used on Wafer Processing Equipment
Wafer processing involves many aggressive chemicals and operates in severe environments. The trend towards larger wafers, smaller feature sizes and decreasing thickness of deposited layers has placed increased emphasis on the need to minimize or eliminate unwanted sources of process contamination. Since purity is critical to high wafer yield, reducing contamination from particulates, outgassing and extractables caused by seal deterioration are major goals of semiconductor fabricators.
Perfluoroelastomers (FFKMs) are widely used as seals on wafer processing equipment due to their extraordinary resistance to chemicals, including reactive plasmas and extreme heat as high as 327°C. Despite these qualities, FFKM performance can vary widely depending upon the chemical composition. Specially-formulated FFKM compounds may be required to help reduce contamination.
Semiconductor fabricators have found that plasma is a very powerful tool for etching, CVD and stripping because all materials are consumed. Seals made from FFKM are used in these processes because of their exceptional resistance to aggressive media. Despite this, prolonged exposure to plasmas can degrade seal surfaces resulting in particulate contamination even before sealing functionality is lost. The ideal seal for plasma applications, therefore, would resist surface degradation while maintaining sealing functionality.
FFKM seals are designed to withstand chemical attack and provide extended seal life in plasma. They exhibit low weight loss (etch rate) and particle generation, thus improving wafer yield, increasing process reliability and reducing frequency of equipment maintenance.
Fluorinated plasmas, oxygen and a mixture of oxygen and fluorine plasma are common chemicals used in semiconductor wafer processing. Figure 1 illustrates the relative particle generation in plasma for four FFKM compounds.
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High heat and temperature spikes can "cook" elastomeric seals beyond recognition, causing them to become hard and brittle. When this occurs, their crosslink structure, the key to elasticity, becomes irreversibly damaged making effective sealing impossible. Elastomers can also degrade under high temperatures, causing outgassing to occur, thereby contaminating the process environment. Thermal processes like RTP, LPCVD, oxidation, diffusion, lamp annealing, etc., need seals that resist not only the process chemicals, but also the extreme temperatures required. Reliable, in-service temperature ratings for sealing materials are best defi ned by long-term (672 hour) testing for seal force retention. Figure 2 shows the extremely low outgassing properties of a typical FFKM compound from room temperature up to 400°C.
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To transform raw semiconducting materials into useful devices, aggressive acids, solvents (including amines), and bases are used to clean, rinse, etch or strip unwanted materials and contaminants from the wafer surface. These chemicals can attack elastomeric seals causing them to swell and degrade or leach undesirable metallic and ionic extractables that affect integrated circuit functionality. FFKM seals are used in wafer cleaning, wet etching, photolithography and copper plating applications. Specially-designed FFKM compounds feature low levels of metallic, ionic and TOC extractables as well as excellent chemical resistance and superior contamination performance. Figure 3 shows the extractables performance of several FFKM compounds after 1 month in UPDI water, piranha and SC1.
For further information about FFKM seals in wafer processing environments, refer to www.dupontelastomers.com/semiarticle1/.
