Seal of approval

Whenever a piece of equipment fails in a fab – or has to be taken out of service for maintenance – the costs can be enormous in terms of lost production. David Holt of Perlast reports on a new seal that promises to improve the reliability of a vital component of many deposition tools.

Plasma enhanced processing in etching, physical vapour deposition and chemical vapour deposition tools is particularly damaging to sealing materials. High temperatures and highly reactive process gasses combined with fluorine-based cleaning gases can rapidly degrade seal performance. As a consequence, high levels of maintenance are needed if process contamination from impurities released from the degrading seal is to be avoided.

For sometime, the seal industry has been researching the development of new elastomeric materials that are sufficiently pure to prevent contamination as the seal degrades but still retain their mechanical properties. Such sealing materials hold out the prospect of reduced machine downtime for seal replacement, and thus increased semiconductor manufacturing equipment productivity.

Seals used in semiconductor manufacture must have excellent chemical resistance to the highly reactive free radicals and fluorine-based CIF3 and NF3 chemistries, exceptional purity and robust mechanical properties. Until recently, the best a semiconductor equipment manufacturer could hope for was two out of three of these. Highly filled perfluoroelastomers (FFKM) offer the highest resistance and mechanical properties but lack the purity needed.

Standard FFKM compounds with conventional filler systems based on inorganic compounds are rich in metal oxides that are released by the seal as it degrades, leading to process contamination. However FFKM without fillers do not have the required mechanical properties.

Some attempts to reduce contamination and improve the mechanical properties involved compounding perfluoroelastomers with special silica (SiO2-based fillers). However, for semiconductor applications, the results have proved to be inadequate.

In another approach, powdered PTFE was used as a filler. This achieved the required purity and chemical resistance but at the price of poorer mechanical properties due to the lack of interfacial bonding between the PTFE powder and the perfluoro-elastomer matrix.

New perfluoro-elastomer materials are now being developed that take a different approach to achieving the balance between exceptional purity and mechanical performance. These fluorine-resistant perfluoroelastomers use a novel nanofiller system without any metallic or carbon-based elements to produce an exceptionally pure elastomer that virtually eliminates any risk of particulation and process contamination.

Pure perfluoroelastomers
The pure FFKM compound uses crystalline nanofiller of perfluoropolymer in the form of nanoparticles dispersed in the FFKM matrix. The nano-dimensions of the crystalline domains allow the seals to be highly transparent, ie the particle size of the fillers is significantly smaller than the wavelength of visible light.

High mechanical gain can be achieved with such reinforcement. This is essentially due to the enormous increase in interfacial area between the nanoparticles and the elastomeric matrix.

In order to develop a polymer that matches the needs of dry and wet semiconductor applications, the FFKM manufacturing conditions have also been optimised. Fine tuning the curing and post-curing conditions and optimisation of the curing system – in terms of cross-linking agent/peroxide ratio – yields a FKKM elastomer with the best combination of purity, temperature and chemical resistance properties.

In order to highlight the differences and the improvements due to the nanoreinforcement, in Table 1 the mechanical and sealing properties of nanofilled perfluoro-elastomer are compared with a perfluoroelastomer (filled with carbon black N990) and with a perfluoroelastomer without filler.

By comparing nanofilled with the perfluoroelastomer without filler (1st and 3rd columns of Table 1 respectively), it shows that nanofilled has far higher values of tensile and modulus, with a similar elongation at break. These results have also been observed after press moulding and post curing.

Besides, as can be noted by comparing the first and the second columns in Table 1, the mechanical properties of nanofilled (in particular tensile and elongation at break) are very close to those of the typical perfluoroelastomer (1st column,). This experimental finding clearly indicates that the fine dispersion of the polymeric filler in the FFKM matrix counterbalances the low interaction energy between the two polymers, thus obtaining a compound whose mechanical properties are close to those obtained if carbon-based filler is used.

With respect to compression set performance (Table 1), the nanofilled perfluoroelastomer presents the best values as compared with the perfluoroelastomer with carbon-based fillers or without fillers. It can be noted that the nanofilled perfluoroelastomer shows even better compression set values than standard formulated perfluoroelastomer after 70 hours at 200°C (35% vs 43%). Moreover, after 70 hours at 230°C the nanofilled perfluoroelastomer compression set value is still close to that of the standard perfluoroelastomer.

Purity comparison
In tests to simulate the heat ageing breakdown of nanofilled perfluoroelastomer and a typical perfluoroelastomer, the results for energy-dispersive X-ray analysis (Figures 1 and 2) show the impurities released by the filled perfluoroelastomer compared with the lack of impurities from the nanofilled perfluoroelastomer. In summary, the nanofilled perfluoroelastomer is a new translucent, high purity perfluoroelastomer with good mechanical properties and compression together with excellent resistance to the fluorinated gases typically used in semiconductor manu-facture processes.

Moreover, the develop-ment of high purity nanofilled perfluoro-elastomer seals that retain their physical integrity for longer during manufac-ture can lead to significant cost of ownership savings. Trials of the nanofilled perfluoroelastomer seal in dry-clean systems have so far lead to 25% higher capital productivity and 25% lower costs of ownership.

Author details
David Holt is interna-tional business manager for Perlast, a range of perfluoroelastomers used in the semiconductor and chemicals industry. He joined Perlast in 1998 and is a member of the Chartered Institute of Marketing and the Institute of Incorporated Engineers.

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Table 1