Plasma environments—used in etching, deposition, cleaning,
and ashing—are among the most aggressive sealing contexts in semiconductor
fabrication. Between energetic ions, reactive radicals, UV radiation, and
thermal cycling, seals are pushed far beyond what many standard elastomers can
endure.
For fabs, mis-choosing a seal material leads to compromised
yields, frequent maintenance, larger particle loads, and unscheduled downtime.
In contrast, perfluoroelastomer (FFKM) O-rings offer distinct advantages in
plasma environments. Below is a technical breakdown of how & why.
1. Plasma Erosion: FKM vs FFKM
·
A paper from Tohoku University (“Study on
CF₄/O₂ plasma resistance of O-ring elastomer materials”) compared FKM and FFKM
exposed to CF₄/O₂ plasma using a microwave-excited surface-wave plasma source. It
found that both FKM and FFKM are eroded by low-energy ion bombardment (< 10
eV), but FFKM remains substantially more stable under neutral radical attack compared to FKM.
·
In the same study, they showed that increasing
the ion kinetic energy (via RF bias) worsens erosion in both materials, but the
rate of erosion in FFKM increases much more slowly than in FKM as conditions
become harsher.
2. Thermal + Oxidative Ageing Under Plasma-Like
Conditions
-
A
study from BAM/DLR compared the lifetime of FKM and FFKM O-rings under thermo-oxidative
ageing at 200-300 °C (air atmosphere) which, though not full plasma,
approximates harsh oxidative stresses some seals see in plasma pre- and
post-process cycles. Results: FKM O-rings had a lifetime of ~75 days at 200 °C before failing, while FFKM lasted ~135 days under similar
ageing.
-
This
suggests that under continuous high-temperature exposure (common in plasma
tool purge, clean, or bake cycles), FFKM resists oxidative damage much
longer than FKM.
3. Material Properties That Matter in Plasma Environments
Based on available literature and supplier data, here are
key properties where FFKM outperforms:
|
Property
|
FKM Limitation in Plasma Environments
|
How FFKM Excels
|
|
Radical / Ion Attack / Erosion
|
FKM loses mass faster; radical attack causes surface
pitting and cracking.
|
FFKM shows significantly lower erosion and better radical
stability.
|
|
Temperature & Oxidation Resistance
|
Prolonged exposure > 200-230 °C accelerates degradation
in FKM via chain scission, dehydrofluorination.
|
FFKM maintains integrity at higher continuous
temperatures, resists thermo‐oxidative ageing.
|
|
Particle Generation / Purity
|
Seal breakdown in FKM releases debris, accelerates yield
issues.
|
FFKM, when properly compounded (low fillers, low
ionic/metal contaminants), produces far fewer particles. Parker FF302 is an
example.
|
|
Compression Set & Mechanical Longevity
|
FKM shows higher permanent deformation after exposure
(temperature + plasma), reducing sealing force.
|
FFKM retains more shape and sealing force through cycling
and plasma exposure. Supplier guides highlight this.
|
4. Case in Real Tools: Field Data & ROI
-
DuPont’s “Extending Lifetime of Critical Seals in HDPCVD Processes” case:
Older FFKM compounds in isolation valves exposed to NF₃ plasma had to be
replaced every ~5,000 wafer cycles. But with newer FFKM (e.g., Kalrez®
9100), seal life doubled or even tripled in the same application,
meaning fewer interventions, fewer chamber vented/reattached parts, and
reduced downtime.
-
In
one field use mentioned by Parker, the FF302 perfluoroelastomer showed no
erosion in places where competitive materials lost ~20% of seal volume
under aggressive HDP-CVD or etch tool plasma roles involving SiH₄
deposition + NF₃ / O₂ plasma cleans.
5. Practical Guidelines for Selecting FFKM Seals for
Plasma Environments
To get the benefit of FFKM in plasma, here are engineering
& selection best practices:
-
Material
Grade: Use FFKM grades designed specifically for low erosion in
plasma; often these are low-fill, low metallic/ionic contamination, high
cross-link density.
-
Temperature
Rating: Make sure continuous and peak operating temps (including
purge/clean cycles) are within the material rating (many FFKM grades are
good to ~300-320 °C).
-
Compression/Gland
Design: Overcompression or undercompression both cost you—overcompress
leads to faster mechanical fatigue, undercompress leads to leakage. Proper
groove design and retention are crucial.
-
Cycle
Between Exposure & Cooling: Repeated plasma exposure + cooling
cycles accelerate fatigue. Where possible, minimize thermal cycling or
design for it.
-
Purity
& Contamination Control: Ensure seals have low extractables, low
ionic content, and very low particle shedding. Suppliers sometimes offer
“high-purity” or “semiconductor grade” FFKM. For example, Parker’s
FF370-75 is formulated for high-purity plasma environments.
For semiconductor fabs pushing for higher tool uptime, lower
particle counts, and fewer maintenance interventions, FFKM O-rings offer a
clear technical evidence base of improved performance in plasma
environments over FKM:
-
Significantly
lower erosion in CF₄/O₂ or similar plasma gases.
-
Better
thermo-oxidative ageing resistance (e.g. ~135 days vs ~75 days at 200 °C
for BAM aging tests).
-
Real
tool data showing doubling or tripling of seal lifetimes in demanding
HV-CVD / HDP-CVD / isolation valve positions.
While FFKM comes at a premium, the data shows that for
plasma environments, the return on investment (ROI) is strong,
especially when downtime, contamination, and yield losses are factored in.