Semiconductors require ever increasing purity in fluids that come in contact with the devices to reduce the defectivity during the manufacturing process. Defect control is extremely critical and continues to be one of biggest challenges in lithography processes for integrated device manufacturers (IDMs) as the critical dimension (CD) size shrinks [1]. Any defect could result in an unusable device, resulting in a financial loss for the IDMs. Particularly in 193nm lithography processes, there have been widespread occurrences of various defects in the coated films, and many factors could play a role in defect formation. For example, photoresist and BARC coating defects could be affected by the dispense process, cleanliness of the materials, and filtration process. Particle removal filters are used in almost every process step where a liquid comes in contact with a wafer. Implementation of polymer membrane-based microfiltration in the photochemical manufacturing process effectively improves the cleanliness of the materials. Furthermore, in today’s state-of-the art semiconductor fabs, an extra filtration step at the point of dispense on the coater module is adopted to further minimize coating defects by providing particle/bubble-free photochemical dispense on the wafer. With the continuous demands for defect reduction and high productivity, Entegris has developed a new, highly retentive 5nm rated asymmetric ultrahigh molecular weight polyethylene (UPE) filter to address these demands particularly in advanced lithography processes. The purpose of this application note is to provide data to show the performance of 5nm asymmetric UPE filters and its effectiveness of reducing the wafer defects for various lithographic processes.

1. AppLIcAtIon note
ImprovIng AdvAnced LIthogrAphy
process defectIvIty wIth A hIghLy
retentIve 5 nm AsymmetrIc Upe fILter
Authors: Aiwen Wu and Jennifer Braggin
Introduction Design of 5 nm Asymmetric
Semiconductors require ever increasing purity UPE Filter
in fluids that come in contact with the devices to
Various polymer materials could be used to manu­
reduce the defectivity during the manufacturing
facture particle filters for photochemical filtration,
process. Defect control is extremely critical and
such as UPE, PTFE and polyamide. Traditionally,
continues to be one of the biggest challenges in
all these material­based filters have symmetric
lithography processes for integrated device manu­
membranes, where pore size is constant across
facturers (IDMs) as the critical dimension (CD) the membrane thickness. In contrast, Entegris
size shrinks.1 Any defect could result in an unusable developed a new asymmetric UPE membrane using
device, resulting in a financial loss for the IDMs. its proprietary co­extrusion technology, where
Particularly in 193 nm lithography processes, pore size changes across the membrane thickness.
there have been widespread occurrences of various The upstream surface of the asymmetric membrane
defects in the coated films, and many factors has a larger pore size than on the downstream.
could play a role in defect formation. For example, This type of pore structure provides the improved
photoresist and BARC coating defects could be retention efficiency and capacity of the micro­
affected by the dispense process, cleanliness of porous membrane without sacrificing the flow.
the materials, and filtration process. This optimized asymmetric structure is specifi­
Particle removal filters are used in almost every cally designed to meet the unique requirements
process step where a liquid comes in contact with of advanced lithography processes below 65 nm
a wafer. Implementation of polymer membrane­ technology node, where pressure drop might be
based microfiltration in the photochemical a limiting factor in decreasing retention rating.
manufacturing process effectively improves the It allows for a wider window of operation on
cleanliness of the materials. Furthermore, in photochemical dispense pumps. Figure 1 shows
the scanning electron microscope (SEM) image of
today’s state­of­the­art semiconductor fabs, an
the cross­section of asymmetric UPE membrane.
extra filtration step at the point of dispense on
the coater module is adopted to further minimize
coating defects by providing particle/bubble­free
photochemical dispense on the wafer.
With the continuous demands for defect reduction
and high productivity, Entegris has developed a
new, highly retentive 5 nm rated asymmetric ultra­
high molecular weight polyethylene (UPE) filter
to address these demands particularly in advanced
lithography processes. The purpose of this applica­
tion note is to provide data to show the performance
of 5 nm asymmetric UPE filters and its effectiveness
of reducing the wafer defects for various litho­
graphic processes.
Open layer Intermediate layer Highly retentive layer
For increased flow High loading capacity Hard particle retention
Increased gel retention 5 nm removal
Increased impurity
residence time
Figure 1. Asymmetric membrane cross-section
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2. ImprovIng AdvAnced LIthogrAphy process defectIvIty
Improved Particle Retention well controlled. The experimental data shows
that using smaller 26 nm fluorescent particles to
Bubble point (BP) pressure is a common parameter challenge the filters results in more differentiation
to characterize microporous membranes and is among the filters. Figure 2 provides a correlation
defined as the minimum pressure required to begin between particle removal efficiency and bubble
displacing wetting liquid from the pores of a wetted point for Entegris UPE membranes. As membrane
membrane. This pressure is related to the largest pore size shrinks (BP increases), particle reten­
size pore in that membrane and is often determined tion increases.
by visual detection of “bubble” as applied differen­ Retention vs. Bubble Point
tial pressure is increased across the membrane.
The equation for bubble point shows an inversely Retention
BP
Membrane Bubble Point
proportional relationship between the pore
Membrane Retention
diameter and the bubble point value. 2
UPE UPE UPE UPE UPE UPE
5 nm 10 nm 10 nm 20 nm 30 nm 50 nm
Asym- Asym-
metric metric
Figure 2. Correlation between 26 nm PSL bead retention for
UPE membranes
While 5 nm asymmetric UPE membrane greatly
improves particle retention performance compared
to 10 nm rating UPE, it doesn’t sacrifice the flow
Filters with 20 nm and 10 nm retention ratings have performance. With pore size greatly shrinking and
become the standard for 193 nm photoresist filtra­ thickness increasing, the pressure drop of 5 nm
tion. These filters were tested for particle retention asymmetric UPE membrane does not significantly
with monodispersed 33 nm polystyrene latex (PSL) increase. Figure 3 presents the flow comparison of
beads and optical particle counters (OPC) using Entegris point­of­use (POU) photochemical filter
a modified SEMATECH® test method.3 However, Impact® 2 V2.
one of the issues with using an OPC to measure PSL 25
particles down to 33 nm in size is that it is difficult
to differentiate very tight filters from one another. 20
Recently a new method has been developed by
Pressure Drop (PSI)
Entegris to measure particle removal efficiencies 15 Impact 2 V2 5 nm asymmetric
for sub­30 nm pore size rated filters using fluo­
rescence spectroscopy. In this method, a filter is 10
challenged with fluorescent particles, which are Impact 2 V2 20 nm
then counted by measuring fluorescent signals of
5
upstream and downstream solutions. Retention can Impact 2 V2 10 nm
be calculated by comparing the fluorescent signals
0
of the two solutions. There are several advantages 0 5 10 15 20
to the fluorescence method compared to traditional Flow Rate for 1 cp Fluid (cc/sec)
OPC techniques, including high sensitivity, easy Figure 3. Flow performance comparison of point-of-use filters
spectral measurement and ability to detect fluores­
cent PSL particles whose size has been relatively
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3. ImprovIng AdvAnced LIthogrAphy process defectIvIty
Wafer Defect Reduction of of­use filtration methods on microbridging
defectivity.6 This study focused on the ability of
5 nm Asymmetric UPE Filter point­of­use filtration on the lithography track to
reduce the level of microbridging defectivity in a
The decrease of critical line widths is requiring 45 nm line/space pattern created through immer­
the use of tighter filtration for critical photochem­ sion lithography. A design of experiment method­
icals. Filters rated at 10 nm and 20 nm filtration ology was used to determine the effect of filter
have been implemented in both the photochemical retention rating, filtration rate and controlled
manufacturing process and in the point­of­use filtration pressure on microbridging defectivity.
spin­coating process in track systems, and have
been shown to be effective in reducing defects.4, 5 In the experiment, wafers were coated and
However, the implementation of finer filtration exposed on the Sokudo™ RF 3Si/ASML™
raises concerns as high differential pressure across XT:1900Gi cluster. Entegris’ IntelliGen® Mini
the filter causes possible outgassing of the photo­ dispense system was used with Entegris’ Impact
chemical and finer pore size approaches the size Mini point­of­use UPE filters with various pore
of large molecular weight polymers in the photo­ sizes. IMEC’s Defect 45 mask was used for the expo­
chemical. These conditions can cause polymer sures. The Defect 45 mask is designed to print nine
shearing, micro bubble formation and gel particle uniform sub­dies of 45 nm lines with 90 nm pitch.
formation. Filter materials with finer pore size may The resist stack for this experiment consisted of 95
have affinity with certain species in the photo­ nm BARC and 105 nm TOK TARF Pi6 001ME resist
chemical formulations, causing photochemical which intentionally contaminated high levels of
changes to occur before it reaches the wafer. To microbridging defects. A post­soak step was added
address all these concerns and demonstrate the after the exposure, and development was done
benefits of highly retentive 5 nm asymmetric UPE with the slit­scan nozzle. In a particular test, an
filter, a series of evaluations have been conducted intentionally contaminated resist with 10 times
under actual production conditions at a number of the microbridging levels of the process of record
different customer sites. (POR) resist chemistry was used to determine the
effect of filtration parameters on defect density.
Case 1: Reducing Microbridging Defects for Figure 4 shows the best results for combination of
193 nm Top-coatless Immersion Photoresist filtration rate and pressure applied to the filtrate
Microbridging was recognized as one of the critical for 5 nm asymmetric UPE and 20 nm symmetric
patterning defects that were frequently observed in UPE filters. Further analysis of the experimental
193 nm lithographic process in different formula­ data suggests that filter pore size is statistically
tions from different manufacturers. The problem significant for reducing microbridging defects. The
becomes remarkable particularly in dense line/ data also indicates that different filter designs will
space patterns and seriously damages the pro­ require different, optimized filtration rates and
duction yield. Entegris has engaged with the applied pressures to further reduce microbridging
Interuniversity Microelectronics Center (IMEC) defect density.
in Leuven, Belgium, to study the effects of point­
Normalized Defect Data Defective Chemistry Sample
120%
No filter (Medium rate/ 20 nm (Low rate/ 5 nm (High rate/
Medium pressure) High pressure) High pressure)
100%
80%
60%
40%
20%
0
Total Irregular Embedded Burst Bubble Line
Microbridging Bridging Contaminant Defect Nodule
Figure 4. Results of 5 nm asymmetric UPE evaluation under best conditions at IMEC
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4. ImprovIng AdvAnced LIthogrAphy process defectIvIty
Case 2: Reducing Overall Coating Defects for Case 3: Reducing Residue-type Defects for
193 nm BARC Process 193 nm Top-coatless Immersion Photoresist
The implementation of BARC processes in 193 nm A study had been started to reduce residue­type
dry and immersion lithography has been accom­ defects for the latest 193 nm top­coatless immersion
panied by defect reduction challenges on fine resist. To reduce this residue­type defect, 10 nm
patterns. In a joint study with AZ Electronic asymmetric and 5 nm asymmetric UPE filters were
Materials, the efficiency and performance of compared. The tested resist was well controlled for
Entegris’ Impact 2 UPE filters with various pore elimination of microbridging­type defect. The focus
sizes in reducing BARC process related defects was on eliminating residue­type defects. Table 1
was examined. AZ® ArF­1C5D BARC coating was is the brief summary of testing condition. The eval­
employed in this study. The testing was accom­ uation results in Figure 6 showed that the 5 nm
plished using an Entegris IntelliGen Mini dispense asymmetric UPE filter reduced the residue­type
pump integrated in the coating module of a defects by over 70% compared to the 10 nm
TEL® CLEAN TRACK ACT 8. BARC was coated on asymmetric UPE filter.
an unprimed silicon wafer. The dispense recipe and
coat recipe were kept constant as the filter pore tAbLe 1. brIef sUmmAry of testIng condItIon
size changed. Spin speed was adjusted such that Substrate 200 mm bare Si with BARC
37 nm BARC film thickness was achieved. The
softbake condition was 200°C for 60 seconds. Top coat None
Three ACI wafers were coated and the wafers were Mask 75 nm L/S
measured for defects. A KLA­Tencor® 2360 High­ Exposure Dry ArF scanner
Resolution Imaging Inspection System was used
with a 0.20 µm pixel size under bright field imaging Dispense pump Single stage
mode to count defects on the coated wafers. Then Inspection KLA 2360
the defect review was carried out using an Applied
Materials® SEMVision™ cX defect review tool. 100
10 nm UPE
Figure 5 shows the total defect results for the 5 nm UPE
Normalized Defect Count (%)
80
tested filters. In the first run, 3 point­of­use
UPE filters with various pore sizes were tested.
60
Compared to a 20 nm rating filter, smaller pore size
filters significantly improved the BARC defectivity.
40
Furthermore, 5 nm asymmetric filter performed
the best, with the lowest number of total defects.
20
Then a confirmation run was conducted using
20 nm and 5 nm asymmetric filters to confirm the
findings in the first round of testing. The defect 0
Fall on Residue
results were very similar to the first round of Figure 6. Effect of filter pore size on residue-type defect
testing as shown in Figure 6. The 5 nm asymmetric reduction in ArF immersion resist
filter showed much lower defect counts than the
20 nm filter.
First run
Confirmation run
Total Defects
Impact 2 V2 20 nm Impact 2 V2 10 nm Impact 2 V2 5 nm
symmetric symmetric asymmetric
Figure 5. Total defectivity comparison by filter
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5. ImprovIng AdvAnced LIthogrAphy process defectIvIty
Filter Priming 0.2 µm. This OPC was installed on the outlet line
of the dispense system, monitoring the entire
The most noticeable attribute of photochemical downstream of the testing filters. The effluent was
POU filters is the ability to purge air and contami­ recycled to the reservoir. The filters were primed
nants introduced during system maintenance or with the solvent ethyl lactate and the dispense
filter changeout. Customers continue to have issues recipe was continually performed until particle
with filter priming and associated tool downtime. counts leveled off. Since each new testing filter
As the finer feature size of advanced lithography was installed after the particle counts reached very
processes requires even finer filtration for photo­ low background with a filter in place, the particle
chemicals, there is more concern on the finer levels shown by the counter indicated the level
filter priming speed. The unique design of 5 nm of microbubbles in the dispense line during the
asymmetric UPE membrane ensures a high level of testing. While optical particle counters are not
retention to reduce advance process defects while designed to count bubbles, the results can be used
also maintaining the ability of fast priming during in a semi­quantitative manner to see differences
filter changeout. in filter performance.
Laboratory experiments were conducted to examine The result of filter priming testing is shown in
the priming performance of Impact 2 V2 5 nm Figure 7. The priming speed of Impact 2 V2 5 nm
asymmetric UPE filter and compare to a standard asymmetric UPE filters is slightly better or equal
Impact 2 V2 10 nm symmetric UPE filter on an to a standard Impact 2 V2 10 nm symmetric UPE
Entegris two­stage­technology IntelliGen Mini filter. This result shows that when the membrane
dispense pump. A recirculating chemical test structure is optimized correctly, the 5 nm asym­
stand was assembled using a chemical reservoir, metric filter can maintain the fast priming speed.
a dispense pump, a filter manifold, a test filter As a result, the chemical waste and dispense­point
and an OPC. The OPC is a PMS LiQuilaz® SO2, downtime are reduced.
capable of detecting and sizing particles down to
Bubble Cleanup Testing on IntelliGen Mini Pump with Ethyl Lactate
(Dispense Volume = 5 mL; Dispense Rate = 1 mL/sec; Filtration Rate = 1 mL/sec
14
12 Impact 2 V2 10 nm symmetric UPE
Impact 2 V2 5 nm asymmetric UPE #1
Impact 2 V2 5 nm aymmetric UPE #2
10
Particles/mL ≥0.2 µm
8
6
4
2
0
0 50 100 150 200 250 300 350 400 450
Dispense Cycle
Figure 7. Bubble flushup of Impact 2 V2 5 nm asymmetric UPE filter
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6. ImprovIng AdvAnced LIthogrAphy process defectIvIty
Conclusion References
Asymmetric UPE technology is a new applica­ 1. Tamada, M. Sanada, “Mechanism Study of
tion­specific filter family designed to optimize Defect Improvement by Short Develop Time
performance in critical photochemical filtration Process,” Proc. SPIE, Vol. 5753, 996 –1007, 2005.
processes requiring a combination of high flow 2. Porter, Mark C., “Handbook of Industrial
and extreme retention. The 5 nm asymmetric UPE Membrane Technology.”
filter has been shown to provide superior retention 3. Lee, J.K. et al., “Latex Sphere Retention by
that can lead to reduction on microbridging defects, Microporous Membranes in Liquid Filtration,”
residue defects and overall defects in advanced Journal of the IES, January/February 1993,
lithography processes. There is no evidence that 26 – 36.
finer filtration down to a 5 nm rating changes the
integrity of advance photolithography chemistries 4. Amari, M.; Wu, A.; Yang, H. J.; Chen, L.;
resulting in increased defectivity. To enable Bowling T. and Watt, M., “Effect of Filter
further reduction in chip line widths with mini­ Surface Chemistry and Morphology on
mized defects, the finer filtration is required 193 nm Lithography Applications,”
at both photochemical manufacturing and SEMICON Korea, 2005.
point­of­use locations. 5. Wu, A. and Chow, W., “Defect Reduction in
Advanced Lithography Processes Using a New
Dual Functionality Filter,” International
Symposium on Semiconductor Manufacturing,
Tokyo, Japan, October 27– 29, 2008.
6. Braggin, J.; Gronheid, R.; Cheng S.; Van Den
Heuvel, D.; Bernard, S.; Foubert, P. and
Rosslee, C., “Analysis of the Effect of Point­of­
Use Filtration on Microbridging Defectivity,”
SPIE 2009.
Entegris®, Impact® and IntelliGen® are registered trademarks of Entegris, Inc.
SEMATECH® is a registered trademark of Sematech, Inc.
Sokudo™ is a trademark of Sokudo, USA, L.L.C.
ASML™ is a trademark of ASML Holding N.V.
AZ® is a registered trademark of AZ Electronic Materials.
TEL® is a registered trademark of Tokyo Electron Kabushiki Kaisha.
KLA­Tencor® is a registered trademark of KLA­Tencor Corporation.
SEMVision™ is a trademark of Applied Materials, Inc.
LiQuilaz® is a registered trademark of Particle Measuring Systems.
entegrIs, Inc.
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