Semiconductor
by Daniel Mays Daniel Mays No Comments

Meeting SEMI Standards for Seal Purity, Outgassing, and Chemical Resistance

Semiconductor fabrication facilities are among the most demanding environments for seal materials on the planet because a single contamination event can scrap a wafer batch worth hundreds of thousands of dollars. And every polymer, lubricant, and elastomeric material contains residual volatiles. SEMI standards deal with how to take these residual volatiles into account when designing components, such as seals, for wafer fabrication environments. The purpose of this blog post is to discuss what SEMI standards actually require and how engineers select bearings to meet them.

The SEMI Standards Landscape

SEMI standards serve as the common language of the global semiconductor supply chain. For example, SEMI standards define fitness for use by establishing minimum thresholds (e.g., outgassing limits, contamination levels, chemical compatibility requirements) that a component must meet to be considered safe for use in semiconductor manufacturing environments. These standards also protect the wafer from contamination that can destroy device yield and make suppliers accountable. 

By referencing a SEMI standard in a procurement specification, a fabrication engineer can require documented, third-party-verifiable compliance rather than relying on a supplier’s marketing claims. These standards also enable global interoperability: tools, chemicals, and components built to SEMI standards can move across fabrications and geographies without extensive re-qualification. This serves as a major economic benefit in a highly globalized industry.

SEMI F-Series (Facilities and Materials)

The SEMI F-series serves as the most relevant category for seals. Standards like SEMI F57 govern polymeric components and wetted surface materials used in UHP (ultra-high purity) gas distribution systems, covering surface finish requirements, material qualification, and cleanliness protocols. SEMI F2 addresses test methods for metallic contamination on wetted surfaces. These are the specs a seal or bearing supplier is most likely to be asked to demonstrate compliance with.

SEMI C-Series (Chemicals)

The SEMI C-Series of standards defines purity grades and acceptable contamination levels for process chemicals such as acids, solvents, and oxidizers. While these standards primarily govern the chemicals themselves, they are also critically important for material selection. If your seal is sitting in a bath governed by a C-series chemical specification, that specification indirectly defines the chemical resistance your seal must demonstrate. Engineers selecting materials for wetted components need to understand what chemical grade they are designing for.

SEMI S2 (Safety Guidelines for Semiconductor Manufacturing Equipment)

S2 is a broad equipment safety standard with direct implications for materials selection. It addresses chemical containment, materials of construction for chemical-wetted components, ventilation requirements, and labeling. It’s often the standard a tool OEM must satisfy as a condition of fab entry, making it a gateway document that touches bearing and seal design.

Purity: Metallic Contamination and Ionic Cleanliness

When it comes to cleanliness in fabrication environments, the core concern when it comes to contamination is the presence of trace metals (Fe, Na, K, Ca) as killer defects in semiconductor devices. Both dynamic seals are going to shed particles, and this includes wear debris, surface oxides, and manufacturing residues. Among the key material considerations are the use of PTFE bearings and seals, while other options include ceramic bearings (Si₃N₄, ZrO₂) as the go-to for ultra-pure applications and surface treatments (e.g., electropolishing, passivation, and their limits).

Outgassing: The Invisible Contamination Problem

Through outgassing, volatile organic compounds (VOCs) deposit on wafer surfaces and disrupt lithography, deposition, and etch processes. Two standards commonly apply: SEMI F10 and NASA ASTM E595, serving as the benchmarks engineers reference. And there are two key metrics that matter: ML (Total Mass Loss) and CVCM (Collected Volatile Condensable Materials). The general targets used by engineers are TML < 1.0% and CVCM < 0.1%. And remember that a material can pass TML and fail CVCM. Bearings and seals must comply with both. However, CVCM is the one that matters most because it captures what actually re-deposits on a cooler surface.

In seal assemblies, the vast majority of problems come from lubricants and greases (silicone-based greases are particularly problematic in vacuum) and adhesives, polymer cage materials, and elastomeric seals. This includes nylon and acetal bearing cages and standard elastomeric seals with curative that can volatize under heat and vacuum. To avoid these issues, industry best practice is to use …

  • PFPE-based dry film lubricants
  • PTFE lip seals (which are known for their high purity and self-lubrication)
  • Perfluoroelastomers (FFKM)

Bake-Out Procedures

As already discussed, every polymer, lubricant, and elastomeric material contains residual volatiles that may include trapped solvents from manufacturing, absorbed moisture, unreacted monomers, and plasticizer fractions. At room temperature, these substances release slowly over weeks or months. Elevated temperature, however, drives that same process in hours, forcing the material to shed its residual volatiles before it is ever installed. This process is known as bake-out and must be accomplished for a material to be safe to use in a high-purity environment.

Chemical Resistance: Surviving the Process Chemistry

The process chemistry involved with semiconductor manufacturers includes chemicals such as HF, H₂SO₄, HCl, H₂O₂, IPA, NMP, ozone, and aggressive plasma environments. And although a material may be listed as chemically resistant on a data sheet, that does not account for critical variables such as concentration, temperature, and exposure time. Swell, extraction, and stress cracking are the failure modes that tables consistently miss.

The material compatibility hierarchy for seals is:

  • FFKM (Kalrez®, Perlast®): broadest chemical resistance, low extractables
  • PTFE: excellent passive resistance but poor dynamic sealing performance
  • EPDM / FKM (Viton®): acceptable in some applications, disqualified in others (HF is a hard no for FKM)

Conclusion

SEMI compliance is not just a checklist, but rather a systems-level engineering discipline. Material selection, outgassing qualification, and chemical resistance validation must work together to result in a fully SEMI-compliant seal. Engineers who treat these as a system rather than separate line items build better products. If you are looking for a SEMI-compliant sealing solution, contact the engineers at Advanced EMC today.

by Daniel Mays Daniel Mays No Comments

Understanding Leakage Rates for Metal Seals

Leakage rates on seal datasheets are often overlooked until a cryogenic vacuum loses hold of its vacuum or a clean-room system fails certification. In fact, the leakage rate of seals is one of the most easily misunderstood seal specifications. This is partly because the units for leakage are not very intuitive and partly because engineers may not realize that zero leakage is a technically meaningless goal. 

This blog post examines what the seal leakage rate for metal seals measures, how it is tested, and why correctly interpreting it gives engineers an excellent tool for system design and accurate seal selection.

What Does Leakage Rate Measure?

The leakage rate quantifies the mass flow of gas passing through (or past) a seal per unit time. The units are expressed as a pressure-volume product per second (Pa·m³/s). The reason that 

Pa·m³/s, not ml/min or cc/s, is used by engineers; this is simple: including pressure makes it more physically meaningful for compressible fluids (all gases and vapors are considered compressible fluids). 

Here is an analogy: 1×10⁻¹⁰ Pa·m³/s is roughly equivalent to one cubic centimeter of gas at atmospheric pressure leaking every ~10,000 seconds, a leakage rate that is essentially imperceptible in most industrial settings.

There is a distinct difference between leak rate and permeation. Permeation is the process of molecular diffusion through the seal material itself, which is only relevant for polymers and elastomers. Metal seals leak through the interface gap, while polymers and elastomers can potentially link through both the gap and by permeation.

How Leakage Rate is Measured

The primary approach to measuring the leakage rate of a metal seal is helium mass spectrometry, in which a mass spectrometer “sniffs” for the presence of helium. Note that helium is used because of its inertness, small atomic size, and high detector sensitivity. In fact, it leaks about 2.7 times faster than nitrogen due to its molecular weight. If a heavier gas is involved, the test results will have to be corrected. Leakage rate tests are also usually performed at ambient temperatures. Still, real-world temperatures may involve expansion and contraction that will impact contract stress and need to be accounted for in the final design.

There are two standard test configurations used: the bombing method and the vacuum method. 

In the bombing/pressurization method, the seal assembly is pressurized with helium while the mass spectrometer detector measures (sniffs) the downstream side. The vacuum method is the opposite: the seal assembly is placed under vacuum, helium is applied upstream, and the spectrometer measures the breakthrough.

Several standards apply, including 

  • ASTM F2132 (metallic gasket testing)
  • ISO 3530 / MS-LNCE (aerospace leak test standards)
  • SEMI standards for semiconductor process equipment

The Leakage Rate Spectrum

Let’s look at the leakage rates for some of the most common types of seals. As you can see, metal seals outperform in terms of leakage rates. 

How does this compare to the thresholds in real-world applications? Here is an approximate visualization assuming well-prepared, precision-machined seals with properly prepared joints.

What Drives Leakage at the Seal Interface?

Five common issues lead to leakage at the interface of a metal seal: contact stress, surface finish, hardness differential, bolt load and seating stress, and thermal cycling. 

Contact stress

No machined surface is perfectly flat: microscopically, such surfaces are full of jagged peaks and valleys. A high contact stress physically crushes the seal into these imperfections. Whereas rubber seals just stretch into the gaps, metal seals require a great deal of pressure to permanently yield (i.e., plastically deform) to create a custom-fit, leak-proof barrier.

Surface finish (Ra)

The Roughness Average (Ra) of the flange measures the depth of microscopic scratches on it. If the surface is too rough, the scratches will act as tiny leak tunnels under the seal. Even a highly compressed, coated metal O-ring cannot fill deep canyons, which is why tight leak specifications demand a finely polished finish (usually Ra ≤ 0.8 µm) to minimize gaps.

Hardness differential

The seal must always be softer than the mating flange. These requirements are so that it deforms instead of gouging the expensive hardware. High-performance metal seals often use a stiff, spring-like core (e.g., Inconel) wrapped in a softer, ductile coating (e.g.,  silver or PTFE) that smears into the harder stainless steel flange without causing damage.

Bolt load and seating stress

In a metal seal, bolts act like stiff springs that clamp the flanges together and crush the seal. If a joint is under-torqued or the load is not evenly distributed, portions of the seal will simply not reach the required pressure to deform properly. This leaves microscopic gaps, making improper bolting a leading cause of field failures for metal seals.

Thermal cycling effects

As temperatures fluctuate, different metals expand and contract at different rates. This causes the bolts to slowly lose their clamping force over time in a process known as stress relaxation. To prevent leaks due to thermal expansion and contraction, engineers use spring-energized seals. These specialized metal seals act like mechanical batteries, automatically expanding outward to maintain unbroken contact even if the flanges slightly pull apart.

Leakage Rate Check List for Metal Seals

Finally, here is a checklist to help engineers as they start specifying a metal seal for a design. For more information on how leakage rates are calculated, check out this article from Pumps & Systems: Calculate the Leakage Rate of a Mechanical Seal.

Conclusion

Remember that 1×10⁻¹⁰ Pa·m³/s isn’t just a number on a datasheet. It represents a specific, measurable physical limit with direct consequences for system performance and safety. And achieving good system performance requires the right seal profile, coating, and groove preparation to work together. To learn more about metal seals for leak-sensitive applications, we encourage you to contact the sealing experts at Advanced EMC.