Semiconductor circuit board
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Seals for Semiconductor Manufacturing 

Seals for semiconductor manufacturing involve critical choices related to the material used. And this is not surprising, given the high stakes of semiconductor fabrication, where a single contamination event or seal failure can compromise an entire wafer batch, costing hundreds of thousands of dollars in yield loss.

While seals are small and often overlooked in semiconductor manufacturing, they are exposed to some of the most aggressive conditions imaginable in any industrial environment. However, selecting the right material means not only a reliable seal but also improved equipment uptime, yield, and process integrity.

The Hostile Environment of Semiconductor Manufacturing

There are several factors that contribute to the extremely hostile environment that semiconductor manufacturing seals must be able to withstand. Seals in semiconductor equipment are regularly exposed to acids, bases, solvents, amine-based strippers, and chlorinated gases, depending on the process step. 

There is also exposure to plasma: fluorine and oxygen plasmas, commonly used in dry etch and resist stripping, are among the most chemically reactive environments currently known. In addition, plasma exposure will rapidly degrade traditional elastomers that lack full fluorination.

Another challenge lies in extreme temperatures and vacuum pressures. Processes such as CVD (Chemical Vapor Deposition) and LPCVD nitride deposition both demand good thermal stability under sustained high temperatures. Also, many chambers operate under vacuum conditions where outgassing becomes a critical failure mode for the seal material used.

There are also UHP (Ultra-high purity) requirements. In short, any ionic contamination, particulate generation, or elevated TOC (Total Organic Carbon) introduced by a seal can corrupt the process chemistry or inadvertently dope the wafer. This has serious repercussions for semiconductor manufacturing processes and quality.

Seals for Semiconductors: Application-Specific Demands

No single material or seal geometry will work for every process step, so requirements vary significantly by application. 

CVD and LPCVD demand thermal stability and vacuum-compatible materials with minimal outgassing. On the other hand, dry and wet etch processes will require a material that is plasma-resistant and compatible with fluorine-based gases and corrosive media. 

CMP is especially interesting as it presents a dual challenge: exposure to an abrasive slurry and high-pH chemistry. This necessitates seals with strong abrasion and chemical resistance. Also consider track and lithography equipment that prioritizes solvent resistance, as photoresist developers and solvents readily degrade standard elastomers. Finally, resist stripping adds yet another layer of complexity, requiring seals that can withstand aggressive strip chemistries and sustained ozone exposure.

ProcessPrimary Sealing ChallengeKey Material Requirements
CVD / LPCVDHigh-temperature vacuum operationThermal stability, vacuum compatibility, minimal outgassing
Dry & Wet EtchHighly aggressive etch chemistriesPlasma resistance, compatibility with fluorine-based gases, and acid/base media
CMPAbrasive, alkaline slurry exposureAbrasion resistance, high-pH chemical resistance
Track & LithographyPhotoresist solvent and developer exposureBroad solvent resistance, dimensional stability
Resist StrippingOzone and aggressive strip chemistry exposureExceptional chemical resistance, ozone resistance

Material Solutions: Engineering Polymers With Superior Performance

The primary candidate materials for semiconductor manufacturing sealing solutions are polymers selected for their ability to meet the overlapping requirements just described. The engineering polymers discussed are well-adapted to the challenges related to semiconductor manufacturing environments. 

FFKM (Perfluoroelastomer)

FFKM is considered the top choice for O-rings in semiconductor manufacturing. It is able to deliver the sealing force and resilience of rubber, but with the chemical compatibility of PTFE. It offers a wide operating temperature, approximately -15°F to 620°F. In addition, there are various grades available for  UHP applications that have been optimized for minimal extractable ion content, low metallic contamination, and enhanced O₂/F₂ plasma resistance. FFKM is suitable for static and limited dynamic applications across CVD, CMP, etch, lithography, and stripping processes.

PTFE (Polytetrafluoroethylene)

PTFE is known for its extremely low coefficient of friction, self-lubrication, exceptional chemical compatibility, and high purity. It also performs extremely well in high-temperature, corrosive environments that degrade other material options. Note that PTFE is often the material of choice for highly effective spring-energized seals and reliable encapsulated O-ring jackets.

PEEK (Polyether Ether Ketone)

Like PTFE, PEEK offers good wear resistance and moderate friction properties. It also offers low outgassing, good plasma resistance, and excellent dimensional stability at elevated service temperatures (which can prove crucial). Because of these properties, PEEK is a strong candidate for labyrinth seals and spring-energized seal lips in dynamic semiconductor applications.

Supporting Materials: PI, PCTFE, and PAI (Torlon)

There are some additional supporting materials, namely:

  • Polyimide (PI): known for its low outgassing, excellent cryogenic performance, and high purity
  • PCTFE: offers improved strength and hardness over PTFE while retaining excellent chemical inertness
  • PAI (Torlon): ideal where high pressure, cryogenic temperatures, and corrosive media are all part of the operating environment; known for excellent wear resistance and self-lubrication

Choosing the Right Material

ProcessRecommended MaterialsRationale
CVD / LPCVDFFKM, PEEK, PI (Polyimide)High thermal stability, low outgassing, vacuum-compatible; FFKM grades available for water vapor and ammonia processes
Dry & Wet EtchFFKM, PTFE, PCTFESuperior plasma resistance and broad chemical inertness, including fluorine-based gas compatibility; minimal particle generation
CMP (Chemical Mechanical Polishing)FFKM, PAI (Torlon), PEEKAbrasion-resistant with strong resistance to high-pH slurry chemistry; PAI offers excellent wear resistance and self-lubrication
Track & LithographyFFKM, PTFE, PCTFEBroad solvent resistance; chemically inert to photoresist solvents and developers that attack standard elastomers
Resist StrippingFFKM, PTFEOutstanding ozone resistance and broad chemical compatibility; FFKM specifically rated for ozone and aggressive strip process chemistry

Material selection for semiconductor sealing applications must be matched carefully to the demands of each individual process. For CVD and LPCVD, FFKM, PEEK, and polyimide (PI) are the leading candidates, as all three offer the thermal stability and low outgassing required for sustained high-temperature vacuum operation. Specialized FFKM grades are also available that are formulated specifically for processes involving water vapor and ammonia.

In dry and wet etch environments, FFKM, PTFE, and PCTFE are the materials of choice, owing to their broad chemical inertness and resistance to fluorine-based plasmas and corrosive etch chemistries. Minimal particle generation is a critical secondary requirement in these applications, and all three materials perform well in this regard. For CMP, the combination of abrasive slurry and high-pH chemistry narrows the field to FFKM, PAI (Torlon), and PEEK, with PAI offering a particular advantage due to its exceptional wear resistance and self-lubricating properties.

Track and lithography equipment demands seals that can withstand the photoresist solvents and developers that rapidly degrade conventional elastomers, making FFKM, PTFE, and PCTFE the preferred options given their broad solvent resistance and dimensional stability. Resist stripping represents perhaps the most chemically aggressive environment of all, with ozone exposure compounding an already demanding chemical profile. Here, FFKM and PTFE are the primary recommendations, with select FFKM grades formulated specifically for ozone-intensive strip process chemistry.

Conclusion

In semiconductor manufacturing applications, the seal material selection has an incredible impact on process reliability, contamination control, and total cost of ownership. And there is no universal, one-size-fits-all solution to choosing a seal material. Engineers must look for the optimal combination of material properties to be matched to each process step’s specific chemical, thermal, and purity demands and there are engineering polymers available that are ideal for different applications.

 Advanced EMC is your partner when it comes to semiconductor sealing solutions. With over 100 years of combined experience, Advanced EMC has access to UHP-grade FFKM along with a full range of engineered polymer seal type. The Advanced EMC Sealing Solutions Team is ready to identify and supply the right sealing solution for any semiconductor application, whether standard or custom-designed. Contact Advanced EMC today for a consultation or to request a quote.

FEP Encapsulated O-Rings | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Encapsulated O-Rings for Chemical Service

Encapsulated O-rings occupy a narrow but critical niche in chemical service sealing. These O-rings are usually specified when elastomeric solutions lack chemical compatibility against aggressive chemicals, but solid PTFE O-rings lack elasticity. Industries such as chemical processing, semiconductor manufacturing, pharmaceutical systems, and specialty fluid handling all involve problematic combinations of aggressive media, temperature variation, and intermittent pressure. These applications often require an encapsulated O-ring.

This blog post explains why encapsulated O-rings work so well for chemical service, and then continues with a discussion of jack material tradeoffs, why a system-level view of O-rings is important, and then continues with a discussion of permeability, compression set, and the complications involved in vacuum conditions.

Why Encapsulated O-Rings Are Commonly Specified for Chemical Service

Fluoropolymers such as FEP, PFA, and PTFE providea very broad chemical resistance across media such as acids, bases, solvents, and oxidizers. However, when used alone, these materials have low modulus recovery and are extremely susceptible to issues like creep and stress relaxation. Encapsulated O-rings, on the other hand, bridge this gap by pairing a chemically inert fluoropolymer jacket with an internal energizing core that supplies sealing force.

The result is not a chemically inert O-ring, but rather a hybrid sealing system. The resulting system, when correctly engineered, can help strike that delicate balance between chemical compatibility, consistent contact stress, and dimensional stability.

Encapsulated O-Rings: Jacket + Core System

Encapsulated O-rings are composite assemblies: the fluoropolymer jacket provides chemical isolation and surface characteristics, while the core is responsible for contact stress, recovery, and dimensional tolerance.

The jacket limits the core’s ability to energize the seal, which is the primary factor that makes these O-rings extremely effective. The core compensates for factors such as jacket creep, surface irregularities, and pressure fluctuations. 

Failure to account for the interaction between the core and the jack leads to seals that are chemically compatible but mechanically unstable, however. A system-level view like this is essential when selecting encapsulated O-rings for chemical service, particularly in long-duration or safety-critical applications.

Jacket Material Tradeoffs: FEP, PFA, and PTFE

The jacket material serves as the main barrier between the process media and the internal core. In addition, it exerts a dominant influence on seal stiffness, installation force, and recovery behavior.

FEP is widely regarded as the industry standard for encapsulated O-rings. This material offers a combination of excellent chemical resistance, relatively low stiffness, and good flexibility at moderate temperatures. These characteristics of FEP make it an excellent choice for general chemical service where sealing force is limited and good dimensional compliance is necessary.

PFA  is another option that provides similar chemical resistance to FEP but also provides better thermal stability and resistance to fatigue cracking at elevated temperatures. FEP has a higher melt strength and reduced permeability, both of which make it attractive for hot chemical service or applications with repeated thermal cycling. However, these benefits come at the cost of increased stiffness compared to FEP.

PTFE jackets offer the highest temperature capability and the lowest permeability among the three materials discussed. These jackets also exhibit the greatest stiffness and lowest compliance. PTFE-encapsulated O-rings usually require significantly higher gland squeeze to achieve the necessary sealing force. These factors can be problematic in low-pressure systems or designs with limited hardware stiffness.

Also, keep in mind that as jacket stiffness and thickness increase, chemical impermeability and vacuum resistance improve. At the same time, the ability of the core to energize the seal is going to be significantly reduced. Jacket hardness, wall thickness, and gland geometry must be accounted for together to avoid under-energized or over-compressed seals.

Permeability

Fluoropolymers are chemically resistant but not impermeable. Small molecules can diffuse through them over time because of concentration gradients, temperature differentials, and pressure differentials.This is problematic in applications involving solvents, gases, or sustained exposure at elevated temperatures. Among the jacket materials discussed, PTFE generally exhibits the lowest permeation rates, followed by PFA, with FEP being the most permeable.

Permeation can lead to core swelling, blistering during depressurization, or distortion of the jacket. In some cases, these effects are merely cosmetic. In others, they compromise the geometry of the seal and contact stress, leading to leakage or mechanical failure. Fortunately, there exist design strategies to manage permeation. These strategies include selecting lower-permeability jackets, increasing wall thickness, limiting temperature exposure, or transitioning to spring-energized cores that are less sensitive to swelling.

Compression Set 

The core is responsible for maintaining the sealing force as the jacket creeps and relaxes. Elastomeric cores (e.g., silicone or fluoroelastomer) are often used because of their resilience and ease of manufacture. There is another option: spring-energized cores that replace elastomer recovery with mechanical force. The use of metal springs leads to a highly consistent load across wide temperature ranges and eliminates the problem of compression set. Spring-energized O-rings are effective in high-temperature chemical service, long dwell applications, and systems that experience frequent pressure cycling.

Vacuum Conditions

Encapsulated O-rings face some unique challenges in vacuum service. Outgassing from elastomeric cores can contaminate vacuum environments. Permeation through the jacket, usually driven by high pressure differentials, can lead to leaks. Then, at low differential pressures, stiff fluoropolymer jackets may collapse or lose conformity to the gland walls. 

Proper gland geometry is essential. 

Excessive squeezing will increase both creep and cold flow. Insufficient squeeze, though, results in a loss of contact stress under vacuum. Encapsulated O-rings can perform well in moderate vacuum environments, but high and ultra-high vacuum applications often require alternative sealing technologies or design modifications such as spring-energized PTFE seals.

Conclusion

Encapsulated O-rings are not drop-in replacements for elastomer O-rings. They are complex sealing systems whose performance depends on considering the interaction between jacket material, permeability, core behavior, and the operating environment. 

 By engaging seal specialists early in the design process, it becomes much easier to align material selection, gland geometry, and operating conditions for a successful O-ring seal.

For applications involving aggressive chemicals, thermal cycling, or vacuum exposure, Advanced EMC is an expert in evaluating encapsulated O-ring designs and navigating the tradeoffs inherent in chemical service sealing. Contact us today!