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For want of an o-ring, the mask was lost

O-ring seals are everywhere in a typical semiconductor fab. Any piece of vacuum equipment uses several of them to seal the openings where components of the process chamber fit together. Yet, as ubiquitous as they are, most process engineers don’t think about them very much.

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O-ring seals are everywhere in a typical semiconductor fab. Any piece of vacuum equipment uses several of them to seal the openings where components of the process chamber fit together. Yet, as ubiquitous as they are, most process engineers don’t think about them very much. They buy the seal specified by the equipment vendor, from the supplier with the most attractive price, and pretty much leave it at that.

Dalia Vernikovsky, CEO of Applied Seals North America, is trying to change that. As she explained in a conversation at Semicon West, o-rings were originally invented in 1937, for use in engines, not vacuum chambers. Most commonly used testing methods reflect that heritage, focusing on tensile tests and pulling. In the semiconductor industry, however, o-rings are more likely to encounter twisting and grinding as the parts on either side of the seal move, or fatigue as fittings are tightened into place and loosened again. Moreover, even a seal that appears to remain intact can leak filler particles and other contaminants into the process chamber.

Meanwhile, feature sizes are shrinking, and new materials and process gases are being used in vacuum chambers. Often, Vernikovsky said, neither process engineers nor seal designers fully understand how the process will interact with the seal. While the process temperature is less important than sometimes believed — the seal doesn’t reach plasma temperature — process gases can leech out components of the seal polymer. Similarly, many purchasers specify that seals must contain “no metals,” even though many metals are quite stable in many process environments. Silica, a common alternative, is non-reactive but tends to agglomerate into clumps. Metrology that can see killer defects for sub-32 nm devices has only recently become available, so the extent of the seal contamination issue is only starting to be understood.

For instance, in research with Sematech, ASNA tested the cleanest available seal material for 45 nm node production, a silica-filled elastomer — in an MOCVD system for EUV mask blank deposition. Defect levels were unacceptably high, with especially high levels of contamination from the elastomer materials. Mask defects as small as 7 nm can still print, but 7 nm is smaller than typical filler materials and the same order of magnitude as many commonly used elastomer molecules.

Based on these results, Sematech proposed a fundamental study on particle generation from sealing materials. Among other things, ASNA is working to develop nanocomposite filler materials with sub-40 nm particle sizes, and to replace the Viton used in many o-rings with a low-carbon alternative. Separately, the company is leading efforts to form a SEMI task force to develop seal standards that more accurately reflect the specialized needs of the semiconductor industry.