MICROCONTAMINATION CONTROL | APPLICATION NOTE

The Intrinsic Role of Membrane
Morphology to Reduce Defectivity
in Advanced Photochemicals
Authors: T. Kohyama, Kozue Miura, Yasushi Ohyashiki, Aiwen Wu
ABSTRACT—Defect source reduction in leading-edge iArF resists is a criticalrequirement to improve device performance and overall yieldin lithography manufacturing processes. It is believed that somepolar polymers can aggregate and be responsible for single ormultiple micro-bridge defects. Further investigation into theformation of these defects is needed. We have previously presented the effective removal of gel-like polymers using nylon
media.1 However, as the industry is moving to smaller featuresizes, there is a need to further improve the defect removal
efficiency. In this paper, a filter, comprised of a novel membrane called Azora™ with unique morphology and high-flow
performance, is introduced. This new filter shows better onwafer and gel removal performance in an advanced ArF solution
than conventional nylon. In addition, it shows improved stability
during chemical storage, unlike traditional media such as nylonand UPE media. Results and possible retention mechanisms
are discussed.

INTRODUCTION—It becomes even more important as the feature size of semiconductor devices continues to decrease, attaining sub-10 nm
scales. Moreover, EUV lithography, new planarization processes,
and new resist chemistries introduce new challenges in defectcontrol. Filtration technology is an intrinsic part of the semiconductor manufacturing process to reduce defectivity, yet there
are only a few types of filtration media used in the industry.
Presently, several parameters are known to effectively assess
filtration performance, including flow rate, membrane surface
property, membrane structure, and surface properties.

For instance, it is well-known that an adsorption type filter
membrane such as nylon is effective at removing polar polymers.
On the other hand, particulate contaminants are often removed
via sieving retention like that of UPE membranes. The optimization and improvement of membrane surface properties canenhance filtration performance, however, filtration technology
innovation is still needed to meet the most recent defect targets.
In this paper, we present a membrane, Azora, with a morphologythat is distinctly different from the most commonly used membranes in semiconductor filtration. With its unique morphology,
the high-flow Azora membrane shows an excellent performancein on-wafer evaluations and gel removal capabilities. In addition,bottled ArF material filtered with Azora membrane shows
improved latent defects performance and particle size variability
when compared to UPE and nylon membranes.

EXPERIMENTAL—

Flow Rate Performance
Flow rate as a function of differential pressure was measured
comparing Azora and a typical asymmetric UPE membrane(comparable in pore size) in water. Tests were performed using
70 mm diameter 10-inch cartridge filter devices.

On-wafer Evaluation
ArF polymer solution was bottled after filtration with an
Optimizer-D™ filter device. The filtered solution was later dispensed on wafers at one week, one month, and two months
post-bottling intervals to measure coating defects and theassociated time dependencies. For bridge-type pattern defects,
the filtered solution was evaluated two weeks after bottling.

RESULTS—Filter flow rate performance is one of the most important factors responsible for improved productivity atbulk chemical suppliers, with higher flow rates (andassociated lower pressure drops) being more desirable for throughput. Our recent study revealed that ahigher pressure drop has a negative impact on defectperformance due to the fact that sufficiently highenough pressure makes gels deform and thereby
allows them to be squeezed through the membraneeven after once captured by the filter. Therefore, toaddress the design goals of higher flow and lower
pressure drop, Azora was developed to reduce pressure drop as much as possible through an increasein porosity as well as a reduction in membranethickness, both enabled by its novel morphology.
From Figure 1, it is clear that a 10” cartridge withthe Azora membrane has a 75% higher flow rate
when compared to an asymmetric UPE membraneof similar pore size.

Differential Pressure (kPa)

100

Interesting differences in time-dependent defectparticle size were observed between the different
types of filter membranes. Specifically, with UPE
and nylon membranes, on-wafer latent defect counts
from post-bottled solutions increased as a functionof storage time, whereas defect particle count forsolution filtered using Azora remained consistent,
independent of post-bottling storage time. Similarly,
for on-wafer microbridge defects, the same timedependent defect size phenomena was confirmed
for nylon but not observed for UPE (Figure 3). Theseobservations introduce the possibility that the Azora
membrane morphology may improve not only onwafer defectivity, but also minimize the variability
and latent defectivity of photoresist solutions postbottling well beyond that of currently available
asymmetric nylon and UPE membranes.

1,000,000

Nylon

80UPE60UPE

40

On-wafer Defect @ ≥35 nm

Flow Rate Performance

Nylon

100,000
10,000

UPE

1,000
Azora

100Azora

1020

InitialAzora

1 week1 month 2 months

Figure 2. Coating defects inspection.

00

12345

Flow Rate (L/min) @ Water, 25°C (77°F)

200

On-wafer Evaluation
On-wafer evaluations are very useful to ultimately
judge if filtration works to improve defectivity, as it isdifficult to observe and confirm correlations between
lab scale testing results and yield data at advancednodes. In this study, metrology inspections for bothpost-coating and pattern defects were conducted.

Nylon

UPE

On-wafer Pattern Defect(Bridge Type)

Figure 1. Comparative flow rate performance.

160

UPE120
Nylon

80
Azora

400Azora

Initial2 weeks

Figure 3. Patterned defects inspection.

The filter with Azora membrane produces different
data on coating and pattern defects comparedto those of UPE and nylon. The Azora filter had
the fewest coating defect counts and least timedependent variability compared to both nylon andUPE membrane filters that produced peak defectcounts after one week of storage that somewhat
decreased over time, but remained two orders ofmagnitude higher than that of Azora (Figure 2).
2

DISCUSSION—Membrane Morphology

Liquid flow

Membrane morphology is a key factor to determine
filter performance, as it governs the manner in which
particles/contaminants in a chemical will interact withthe membrane. Figure 4 shows a schematic of three
membrane architectures. The symmetric and asymmetric architectures are commonly encountered inboth UPE and nylon membranes and both thesearchitectures have some advantages/disadvantages.
In recent times, as the filter pore sizes have becometighter, asymmetric membranes have become morecommon to compensate for the loss of flow which
has resulted in the tightest layers becoming thinner.
In contrast, the Azora membrane has a tortuous,complex structure which is very different from existing
membranes. As shown in the results section, thisunique morphology overcomes the flow loss issue
at tighter pore sizes while being able to impart a new
functionality to the membrane. Further investigation isrequired to understand the mechanism of reductionof latent defects/defects on wafer in ArF photoresist
solutions – this will ultimately be useful in further
optimization of the membrane.

Tortuous

Asymmetric
Filter material

Symmetric
CavityFigure 4. Schematic of membrane architectures.

Current Hypothesis on Mechanism of DefectReduction
Our current results suggest that the unique morphology of the high-flow Azora membrane helps inreduction of gel particles subsequently leading toreduction in both coating defects and microbridging
defects. Though the exact mechanism needs further
investigation, we hypothesize that when gel particles
are in a continuously mixed bulk phase, such as ina solution at a bulk chemical manufacturer prior tofiltration, there is negligible interaction (i.e., aggregation) between them, leading to relatively large interstitial spaces (Figure 5). This dispersion of gel particles
creates a unique challenge for typical UPE and nylon
membranes where there is possibly a limited interaction due to the symmetric/asymmetric morphology.
Conversely, the tortuous path and the high flow of theAzora membrane increases the chance of interactions

between (a) multiple gel particles in the microscopic
porous spaces leading to aggregation and capture,and (b) the gel particles and membranes surface
leading to their capture via Van der Waals forces. In
our future experiments, we plan to further investigate
this potential capture mechanism of gel particles
allowing for novel filtration concepts and technology.

Close proximity

Contact

Big gel particle

Figure 5. Hypothesis regarding gel aggregation.

CONCLUSIONS—

Current portfolio of filtration membranes including
UPE and nylon have demonstrated satisfactory
performance in reduction of gel particles from ArF
polymer solutions. However, to successfully enable
the high-volume manufacturing of semiconductor
processes in the sub-10 nm nodes, we need a fundamental innovation in filter membrane technology toachieve the ever-reducing defect targets. In this study,
a filter using the high-flow Azora membrane showeda unique functionality in reduction of gel particles insolution, attributed to its morphology, leading to
on-wafer microbridge reduction and reduction inlatent defects, especially in aged resist. The results
provide a potentially multifaceted improvement tothe bulk chemical manufacturers (from improved
flow, improved chemical batch specifications, andincreased shelf-life), as well as integrated device
manufacturers (from reduced pressure drop andimproved on-wafer performance).

REFERENCES—1. K ohyama, T, “The filter adsorption mechanism inphotoresist materials” Proc. SPIE 8325 (2012).

This paper was originally presented at a 2018 SPIE Advanced
Lithography poster session.
3

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