Reducing Wafer Contamination
Kalrez® FFKM Seals Help Minimize Wafer Process Contamination
Perfluoroelastomers (FFKMs) are used as seals on waferprocessing equipment due to their extraordinary resistance tochemicals, including reactive plasmas and extreme heat as highas 327°C. FFKM performance can varydepending upon the chemical composition. Specially formulatedKalrez® products may be required to help reducewafer contamination.
Weight loss / Particle generation
Fabricators have found that plasma is a verypowerful tool for etching, CVD and stripping because all materialsare consumed. FFKM seals are used in theseprocesses because of their exceptional resistance to aggressivemedia. Prolonged exposure to plasmas can degradeseal surfaces resulting in particulate contamination evenbefore sealing functionality is lost. The ideal seal, therefore, would resist surface degradation whilemaintaining sealing functionality.FFKM seals are designed to withstand chemical attack and provide extended seal life. They exhibit low weight lossand particle generation, thus improving wafer yield,increasing process reliability and reducing frequency of equipmentmaintenance.Figure 1 illustrates the relative particlegeneration in plasma for four FFKM compounds.
High heat and temperature spikes can “cook” elastomeric sealscausing 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 processenvironment. Thermal processes like oxidation anddiffusion need seals that resist not only theprocess chemicals, but also the extreme temperatures required.Reliable, in-service temperature ratings for sealing materials arebest defined by long-term (672 hour) testing for seal force retention.Figure 2 shows the extremely low outgassing properties ofa typical FFKM compound from room temperature up to 400°C.
Metallic, ionic and TOC extractables
To transform raw semiconducting materials into useful devices,aggressive acids, solvents (including amines), and bases are used. These chemicals can attack elastomericseals causing them to swell and degrade or leach undesirable metallicand ionic extractables. This maycause wafer contamination and subsequently integrated circuit functionality.
FFKM seals are used in wafer cleaning, wet etching, and photolithography. Specially-designed FFKM compoundsfeature low levels of metallic, ionic and TOC extractables aswell as excellent chemical resistance. Figure 3 shows the extractables performance of severalFFKM compounds after 1 month in UPDI water, piranha and SC1.