FFKM-Dichtungen für die Halbleiterfertigung: Plasmabeständigkeit, Vakuum und Kontaminationskontrolle

Seals are used throughout wafer-processing equipment, including:

• Process chamber lids
• Slit and gate valves
• Gas inlet systems
• Showerheads and gas manifolds
• Isolation valves
• Vacuum forelines
• Chamber doors
• Pump connections
• Wafer-transfer systems
• Chemical containers
• Flow controllers
• Filters and connectors
• Wet-processing valves and fittings

Although these components may represent a small portion of the equipment cost, their performance can affect:

• Vacuum stability
• Process-gas containment
• Chamber cleanliness
• Equipment uptime
• Preventive-maintenance intervals
• Wafer contamination
• Process repeatability
• Yield

DuPont identifies particulate contamination, outgassing and extractables caused by seal deterioration as important concerns in semiconductor production. Its semiconductor sealing guidance covers etching, deposition, ashing, oxidation, diffusion and wet processes.

A seal that continues to prevent visible leakage may still be unsuitable if its surface is eroding or releasing contamination into the process chamber.

FFKM has a highly fluorinated polymer structure that generally provides broader resistance to heat, chemicals and plasma environments than conventional elastomers such as FKM and silicone.

Semiconductor-specific FFKM compounds may be engineered to provide:

• Low plasma erosion
• Low particle generation
• Low trace-metal content
• Low outgassing
• Low extractables
• High-temperature stability
• Low compression set
• Broad wet-chemical resistance
• Reliable sealing under vacuum
• Controlled cleanroom manufacturing and packaging

Trelleborg describes semiconductor-grade FFKM materials as combining high purity, very low trace-metal content, plasma resistance, reduced particle generation and ultra-low outgassing in high-vacuum conditions.

These properties make FFKM an important option for both front-end wafer-processing equipment and the fluid-delivery systems that support it.

Semiconductor seal selection can be divided into three broad process groups:

1. Plasma and dry processes
2. Vacuum and thermal processes
3. Wet chemical processes

A compound that performs well in one group should not automatically be approved for another.

DuPont’s semiconductor material-selection system similarly separates semiconductor applications into plasma/gas, thermal and wet-process groups rather than using one general FFKM recommendation.

Plasma is used in semiconductor manufacturing for processes such as:

• Reactive ion etching
• Inductively coupled plasma etching
• Plasma-enhanced chemical vapor deposition
• Plasma-enhanced atomic layer deposition
• Resist stripping
• Ashing
• Chamber cleaning
• Surface treatment

Common plasma and process gases may include:

• Oxygen
• Nitrogen trifluoride
• Carbon tetrafluoride
• Chlorine
• Boron trichloride
• Sulfur hexafluoride
• Fluorocarbon mixtures
• Hydrogen-containing gases

These environments can attack elastomer surfaces and shorten seal life.

How plasma attacks an elastomer seal

Plasma attack may involve chemical or physical mechanisms.

In chemical attack, reactive radicals interact with the seal material and form volatile degradation products. In physical attack, energetic ions bombard and remove material from the seal surface.

The dominant mechanism depends on:

• Plasma reactor configuration
• Gas chemistry
• RF power
• Pressure
• Temperature
• Flow rate
• Ion energy
• Seal location
• Direct or remote exposure

DuPont’s plasma-seal research explains that reactive ion etch, inductively coupled plasma and downstream plasma systems can expose seals to very different conditions. It also distinguishes radical-driven chemical erosion from ion-driven physical sputtering.

This is why a statement such as “plasma-resistant FFKM” is not a complete specification.

Plasma chemistry matters

Different plasma chemistries can produce different degradation behavior.

For example:

• Oxygen plasma may oxidize and remove organic components.
• Fluorine plasma may attack particular fillers or polymer structures.
• Chlorine-containing plasma may create different compatibility demands.
• Remote plasma may produce mainly radical-driven chemical attack.
• Direct plasma exposure may include more energetic ion bombardment.

A compound that produces low weight loss in one plasma recipe may not provide the same performance in another.

For material evaluation, provide:

• Complete gas mixture
• Gas-flow ratios
• Chamber pressure
• Plasma power
• Exposure time
• Seal location
• Chamber temperature
• Cleaning chemistry
• Maintenance interval

Low erosion is important, but the seal must also degrade cleanly.

A compound may lose relatively little weight while still leaving particles or metallic residues in the chamber. Another compound may experience controlled erosion but produce mainly volatile products that can be removed through the vacuum system.

This means plasma performance should be evaluated using several measurements:

• Weight loss
• Surface cracking
• Particle count
• Particle size
• Particle composition
• Trace-metal release
• Seal-force retention
• Leakage after exposure

DuPont’s published research shows that filler selection can significantly affect particle and metallic-contamination behavior during plasma exposure.

Conventional FFKM compounds may contain:

• Carbon black
• Silica
• Mineral fillers
• Metallic-oxide fillers
• Polymeric fillers

These fillers can improve hardness, strength, thermal behavior or processability. However, they can also affect contamination performance.

Mineral and metallic fillers

Some mineral fillers resist plasma erosion better than the surrounding polymer.

As the polymer is removed, filler particles may remain exposed and eventually detach into the chamber. Depending on their composition, these particles may also introduce metallic or ionic contamination.

DuPont’s plasma research notes that plasma-resistant fillers such as certain oxides can shield the polymer but may increase particle-generation risk after the polymer around them has eroded.

Polymeric-filled and unfilled compounds

Polymeric-filled or unfilled FFKM compounds may reduce the risk of leaving hard inorganic particles behind.

Some formulations are designed so that the seal and filler degrade into volatile products more uniformly. This can reduce particulate and metallic contamination in certain plasma environments.

However, an unfilled material is not automatically best for every application. Fillers may be necessary for:

• Mechanical reinforcement
• Dimensional stability
• Wear resistance
• Extrusion resistance
• Dynamic seal performance

The compound must balance plasma cleanliness with the mechanical requirements of the seal location.

Particles released from a seal can come from:

• Plasma erosion
• Surface cracking
• Abrasion
• Dynamic valve movement
• Poor installation
• Excessive compression
• Sticking and tearing
• Chemical degradation
• Exposed fillers

Dynamic seals can be particularly important particle sources because their surfaces repeatedly move against hardware.

Common dynamic locations include:

• Slit valves
• Gate valves
• Isolation valves
• Moving chamber doors
• Reciprocating mechanisms

Greene Tweed reports that a semiconductor FFKM developed using dynamic testing generated substantially fewer particles than a benchmark material during a 60,000-cycle test. The result illustrates why static immersion or plasma tests alone may not predict performance in a moving valve.

Reducing particle generation

Particle-control measures may include:

• Selecting a low-particle semiconductor compound
• Matching the compound to the plasma chemistry
• Improving groove design
• Preventing seal rolling or twisting
• Reducing unnecessary friction
• Controlling surface finish
• Using an appropriate lubricant where permitted
• Selecting a purpose-designed valve seal
• Establishing preventive replacement intervals

DuPont has developed specialized bonded door and TriLobe seal designs for gate- and slit-valve applications where installation behavior, movement and particle control are important.

Trace metals can originate from:

• Compound fillers
• Pigments
• Processing equipment
• Mold-release agents
• Cleaning processes
• Packaging
• Seal degradation

Relevant elements may include:

• Aluminum
• Calcium
• Iron
• Magnesium
• Sodium
• Titanium
• Copper
• Zinc
• Other process-sensitive metals

Plasma can break down the seal surface and release material in atomic, ionic or particulate form. DuPont notes that conventional mineral-filled compounds may contain elements capable of contributing metallic contamination, while certain polymeric-filled or unfilled compounds contain fewer inorganic constituents.

When trace metals are critical, buyers should request actual analytical data for the proposed compound rather than relying on a general statement such as “high purity.”

Relevant test methods may include:

• ICP-MS
• Extractable-metal testing
• Surface analysis
• SEM/EDX particle analysis
• Ion chromatography
• Lot-specific contamination testing

Outgassing is the release of volatile compounds from a material under vacuum, heat or plasma exposure.

Possible sources include:

• Residual processing aids
• Cure by-products
• Absorbed moisture
• Low-molecular-weight ingredients
• Material-degradation products
• Cleaning residues

Outgassing can:

• Increase pump-down time
• Reduce vacuum stability
• Contaminate chamber surfaces
• Interfere with deposited films
• Increase background gases
• Shorten maintenance intervals

Semiconductor FFKM suppliers therefore develop and test compounds for low outgassing as well as chemical resistance.

Trelleborg states that its semiconductor FFKM range is designed for ultra-low outgassing in high-vacuum conditions, while Greene Tweed identifies low outgassing as a selection feature for specific dry- and wet-process compounds.

Outgassing data must be compared carefully

Published outgassing results may use different:

• Temperatures
• Vacuum levels
• Exposure times
• Sample sizes
• Analytical methods
• Preconditioning procedures

Two compounds cannot be compared reliably unless the test conditions are similar.

For a critical chamber, request:

• Total mass loss
• Collected volatile condensable material
• Residual gas analysis
• Specific detected species
• Test temperature
• Vacuum level
• Bake-out history

Outgassing and permeation are related but different.

• Outgassing comes from substances released by the seal itself.
• Permeation is the movement of external gas through the elastomer.

Under high vacuum, excessive gas permeation can contribute to:

• Increased gas load
• Longer pump-down time
• Difficulty reaching base pressure
• Background contamination
• Process instability

Seal geometry, cross-section and exposed surface area influence permeation as well as the compound itself.

For high-vacuum applications, evaluate:

• Target vacuum level
• Acceptable leak rate
• Process gas
• Seal cross-section
• Seal temperature
• Pressure differential
• Exposure area
• Pumping capacity
• Required pump-down time

DuPont has highlighted low-permeation FFKM seals for high-temperature and high-vacuum semiconductor use, illustrating that vacuum performance may require a dedicated material or design rather than a general-purpose O-ring.

A seal can be chemically resistant and still fail because it loses elastic sealing force.

Compression set becomes especially important in:

• Heated process chambers
• Diffusion systems
• CVD and ALD equipment
• Vacuum flanges
• Chamber lids
• Equipment with long maintenance cycles

Possible symptoms include:

• Failure to reach vacuum
• Leakage after chamber cooldown
• Longer pump-down time
• Leakage after maintenance
• Permanent flattening
• Reduced contact pressure

A semiconductor FFKM compound should be reviewed for:

• Compression set
• Seal-force retention
• Maximum continuous temperature
• Thermal cycling
• Vacuum performance
• Chemical exposure at temperature

The highest published temperature rating is not always the best selection. A compound optimized for extreme heat may have different plasma, outgassing or purity characteristics from a grade designed for deposition or etching.

FFKM is also used in wet wafer-processing and chemical-delivery systems.

Typical fluids include:

• Ultrapure water
• Hydrofluoric acid
• Sulfuric acid
• Nitric acid
• Phosphoric acid
• Hydrogen peroxide
• Ammonium hydroxide
• Piranha solutions
• SC-1 and SC-2 cleaning solutions
• Photoresist strippers
• NMP
• Hydroxylamine
• IPA
• DMSO
• Copper-plating solutions

DuPont’s wet-process selection guide lists wafer preparation, wet etching, stripping and copper plating as semiconductor applications for specialized high-purity FFKM seals.

Wet-process selection criteria

For wet processes, prioritize:

• Chemical compatibility
• Low extractables
• Low ionic contamination
• Low trace-metal content
• Resistance to swelling
• Mechanical-property retention
• Cleanroom processing and packaging

Greene Tweed reports low total organic carbon, low leachable trace metals and low ionic contamination for semiconductor wet-process seal materials.

A seal compound selected for dry plasma should not automatically be used in a wet bench. The wet-process chemistry and contamination specifications must be reviewed separately.

A high-purity compound can be contaminated after molding if it is handled, cleaned or packaged incorrectly.

Possible contamination sources include:

• Dust
• Skin contact
• Cutting debris
• Mold-release residue
• Cleaning chemicals
• Packaging materials
• Uncontrolled storage
• Mixed production equipment

For critical semiconductor seals, buyers may need to specify:

• Cleanroom manufacturing class
• Cleaning method
• Ultrapure-water rinsing
• Particle inspection
• Double bagging
• Lot identification
• Date code
• Certificate of conformance
• Trace-metal limits
• Packaging material

Trelleborg states that selected semiconductor FFKM seals are manufactured and packaged in controlled cleanroom conditions, emphasizing that process cleanliness extends beyond the compound formulation itself.

Static seal locations

Examples include:

• Chamber lids
• Gas-manifold connections
• Vacuum flanges
• Sensor housings
• Chemical fittings

Key requirements include:

• Compression-set resistance
• Low outgassing
• Chemical compatibility
• Vacuum retention
• Thermal stability

Dynamic seal locations

Examples include:

• Slit valves
• Gate valves
• Isolation valves
• Moving chamber components

Dynamic locations also require:

• Wear resistance
• Low friction
• Tear resistance
• Particle control
• Resistance to sticking
• Stable installation geometry

Do not select a dynamic seal using only static plasma-erosion or chemical-immersion data.

Failure analysis should examine the seal together with chamber history, plasma recipe, cleaning cycle and hardware condition.

Useful techniques may include:

• Optical inspection
• Weight-loss measurement
• Hardness testing
• Dimensional measurement
• SEM/EDX
• Surface spectroscopy
• Particle counting
• Leak testing
• Compression-set evaluation

Use the following selection process.

1. Identify the process segment

Specify whether the seal is used in:

• Etch
• Deposition
• Ashing or stripping
• Thermal processing
• Ion implantation
• Wet cleaning
• Chemical distribution
• Vacuum transfer

2. Define the complete chemistry

Provide:

• Process gases
• Plasma gases
• Cleaning gases
• Wet chemicals
• Concentrations
• Gas-flow ratios
• Reaction by-products

3. Define plasma conditions

Include:

• Direct or remote plasma
• Reactor type
• RF power
• Pressure
• Temperature
• Exposure duration
• Seal location

4. Define temperature and vacuum

Provide:

• Normal temperature
• Peak temperature
• Bake-out temperature
• Base pressure
• Operating pressure
• Acceptable leak rate
• Pump-down requirement

5. Define contamination limits

Specify requirements for:

• Particle generation
• Trace metals
• Outgassing
• Extractables
• Ionic contamination
• Total organic carbon

6. Define seal movement

Identify:

• Static or dynamic use
• Valve cycle rate
• Speed
• Stroke
• Surface finish
• Lubrication restrictions

7. Review the seal design

Provide:

• O-ring dimensions
• Groove width and depth
• Seal squeeze
• Stretch
• Extrusion clearance
• Hardware tolerances
• Drawing or physical sample

8. Validate the actual compound

For critical applications, consider:

• Plasma erosion testing
• Particle testing
• Outgassing analysis
• Trace-metal analysis
• Chemical immersion
• Thermal cycling
• Dynamic cycle testing
• Chamber-level qualification

The final specification should identify a named compound and approved manufacturing process rather than simply stating “semiconductor-grade FFKM.”

Semiconductor FFKM seals can have a high initial purchase price, but seal cost should be evaluated against:

• Chamber downtime
• Preventive-maintenance labor
• Lost wafers
• Chamber cleaning
• Requalification
• Yield loss
• Spare-parts inventory
• Valve or hardware damage

DuPont’s lifecycle-cost guidance recommends comparing seal price with mean time between repair, unscheduled downtime and the operational costs associated with replacement.

A more expensive compound may provide lower total cost when it:

• Reduces particle generation
• Extends chamber-cleaning intervals
• Improves valve cycle life
• Reduces vacuum failures
• Prevents contamination
• Supports longer maintenance intervals

The best compound is not necessarily the one with the highest temperature rating or purchase price. It is the least costly qualified material that meets the process, purity and reliability requirements.

Selecting one FFKM for every semiconductor process

Etching, deposition, wet processing and thermal equipment create different sealing conditions.

Evaluating only plasma weight loss

A compound may have low erosion but still generate particles or trace-metal contamination.

Ignoring filler chemistry

Filler type can influence particle and metallic-residue behavior.

Using a dry-process compound in a wet application

Plasma resistance does not confirm compatibility with acids, bases, solvents or strippers.

Ignoring dynamic particle generation

A slit-valve seal should be evaluated under representative movement and cycle conditions.

Comparing outgassing values from different test methods

Temperature, pressure, duration and sample preparation must be comparable.

Specifying only “high-purity FFKM”

The exact compound, cleaning process, packaging and contamination limits should be defined.

Ignoring the groove and hardware

Even the correct compound can fail because of excessive squeeze, poor surface finish, twisting or insufficient groove volume.

FFKM seals help semiconductor equipment operate in environments involving plasma, reactive gases, high vacuum, extreme temperatures and aggressive wet chemicals.

However, successful semiconductor sealing requires more than broad chemical resistance.

The selected compound must provide the right balance of:

• Plasma resistance
• Low erosion
• Low particle generation
• Low trace-metal content
• Low outgassing
• Low extractables
• Vacuum stability
• Compression-set resistance
• Wet-chemical compatibility
• Mechanical performance
• Cleanroom processing

Different etch, deposition, thermal and wet processes may require different FFKM formulations.

For critical applications, compound selection should be based on the complete plasma recipe, process chemistry, temperature, vacuum level, seal movement and contamination requirements. Qualification testing should reproduce the actual process as closely as practical.