Contamination Considerations for Perfluoroelastomer Seals used in Deposition Process
Reducing contamination from particles, metallic contaminants and outgassing caused by seal deterioration are major goals of semiconductor fabricators. Perfluoroelastomers (FFKMs) are used in deposition processes due to their extraordinary chemical resistance and thermal stability. Despite these qualities, FFKM performance can vary depending upon their chemical composition. Specially formulated products, such as DuPont™ Kalrez® 9100 are designed to help reduce the potential for contamination while maintaining sealing functionality in aggressive plasma environments.
Plasma is a powerful tool for etching, cleaning, deposition, etc. Fluorine-containing plasmas, e.g., NF3 and CF4, are used for deposition process chamber cleaning due to their high reactivity towards materials to be removed. Since all materials are consumed in plasma, seals need to withstand plasma attack, i.e., exhibit low weight loss (erosion) and leave minimal particles behind after being etched. Plasma attack can be chemical (seal exposed to radicals), physical (seal subjected to ion bombardment) or both. In most seal locations on wafer processing equipment, the plasma attack mechanism is mainly chemical. FFKMs exhibit better resistance to such environments versus other elastomeric materials.
Conventional FFKM products normally contain carbon black and/or mineral fillers. Newer products are either unfilled or polymeric filled. If the filler has high resistance to plasma (BaSO4, TiO2), it can “shield” the polymer to reduce weight loss, but have potential for particle generation by leaving particles behind once the polymer is etched.
Metallic contamination (barium, calcium) is also a concern in plasma processes. Plasma can break down materials to atomic or ionic species that can contaminate the deposited layer composition. Metallic contamination can produce negative effects at different levels of CMOS manufacturing. For instance, it can modify intrinsic properties of the film such as the dielectric constant, or negatively affect interface properties that are critical for integration. Unfilled and polymeric-filled products essentially contain only carbon, fluorine and oxygen, which can be etched to form volatiles, thereby significantly reducing the potential for contamination.
Precise control of deposition processing is critical as layers become thinner and approach the atomic level. A major concern is outgassing from sealing materials as it can interfere in the process by changing the composition and morphology of the deposited layer. It can also cause slow vacuum pump down to occur. Outgassing can originate from existing/adsorbed volatiles in seals or from small molecules resulting from seal degradation. The main components of outgassing at elevated temperatures are fluorine-containing molecules and/or fragments. The outgassing properties of FFKMs can be significantly different depending upon the crosslinking system employed (Figure 3).
DuPont™ Kalrez® 9100 perfluoroelastomer parts are polymeric filled. Since perfluoroelastomers and polymeric fillers degrade similarly in plasma to form volatile products, it offers reduced potential for particle and metallic contamination. It is also crosslinked using a proprietary curing system that provides excellent thermal stability while significantly reducing outgassing at elevated temperatures.