The downward scaling of semiconductor circuits has put an increasing demand for contamination control in the lithography process. With the utilization of chemically amplified photoresists for enhanced photosensitivity, there have been widespread occurrences of various defects in the coated films. 193nm photoresist systems, typically consisting of a solvent system, a polymer system, photo acid generators (PAG), acid quenchers, additives, and surfactants, are particularly sensitive to particulate and bubble-induced defects. Microbridging defects in photoresist and cone defects in the anti-reflective coatings are particularly troublesome because they may form by contaminants smaller than the nominal size of the filter membrane pore and may only be evident in the subsequent process steps. Microbridging defects manifest themselves as resist remaining in unwanted places after exposure and development. Examples of these defects are shown in Figure 1. Microbridging was recognized as one of the critical patterning defects that were frequently observed in 193nm lithographic process in different formulations from different manufacturers. The problem becomes remarkable particularly in dense line/space L/S feature and should seriously damage the production yield. While the cause of micro-bridging defects has not been definitely identified, a hypothesis has been proposed which states that some compositional inhomogeneity potentially existing in the 193nm polymer was suspected for the root cause of micro-bridging defects. These “less-soluble” species exist in the polymer at a trace amount, which is supposed to agglomerate and eventually grow up into the size that would bridge the lines. The photoresist filtration is known to influence this defect density. With the continuous demands for defect reduction and high productivity, Entegris has developed a new and unique photochemical Point-Of-Use (POU) filter, Impact® Duo to address these demands particularly in advance lithography processes.

1. APPLICATION NOTE
A NEW DUAL-FUNCTIONALITY FILTER
FOR DEFECT REDUCTION OF ADVANCED
LITHOGRAPHY PROCESSES
Author: Aiwen Wu
Introduction
The downward scaling of semiconductor circuits
has put an increasing demand for contamination
control in the lithography process. With the
utilization of chemically amplified photoresists
for enhanced photosensitivity, there have been
widespread occurrences of various defects in the
coated films. 193 nm photoresist systems, typically
consisting of a solvent system, a polymer system,
photo acid generators (PAG), acid quenchers,
additives and surfactants, are particularly sensitive
to particulate and bubble-induced defects. Micro-
bridging defects in photoresist and cone defects
in the anti-reflective coatings are particularly
troublesome because they may form by contaminants
smaller than the nominal size of the filter membrane
pore and may only be evident in the subsequent
process steps.
Microbridging defects manifest themselves as resist
remaining in unwanted places after exposure and
development. Examples of these defects are shown
in Figure 1. Microbridging was recognized as one of
the critical patterning defects that were frequently
observed in 193 nm lithographic process in different
formulations from different manufacturers. The
problem becomes remarkable particularly in
dense line/space (L/S) feature and could seriously Figure 1. Microbridging type defects
damage the production yield. While the cause of
microbridging defects has not been definitely
identified, a hypothesis has been proposed which Polymer Materials Used
states that some compositional inhomogeneity
potentially existing in the 193 nm polymer was
for POU Photochemical
suspected for the root cause of microbridging Applications
defects. These “less-soluble” species exist in the
polymer at a trace amount, which is supposed to POU filtration acts as the last barrier to wafer
agglomerate and eventually grow up into the contamination from the process tool, fluid
size that would bridge the lines. The photoresist handling and system upsets. The ideal filter
filtration is known to influence this defect density. should have a combination of physical properties
that produces a structure capable to remove
With the continuous demands for defect reduction contamination from a photochemical. Some
and high productivity, Entegris has developed a key properties for a filter at the POU for
new and unique photochemical point-of-use (POU) photochemical filtration include:
filter, Impact® Duo, to address these demands • Filter membrane is wet spontaneously by
particularly in advanced lithography processes. photochemical
ENTEGRIS, INC. 1

2. A NEW DUAL-FUNCTIONALITY FILTER FOR DEFECT REDUCTION APPLICATION NOTE
UPE Membrane
Photoresist Solids
• Fast priming/bubble clearance
• Good retention
• High flow rate/low pressure drop
• Complete chemical compatibility with
photochemical
• High cleanliness/low extractables
1800 1500 1200 900
The most common materials used to manufacture
Wavenumbers (cm-1)
particle filters for POU photochemical filtration
Nylon Membrane
are ultra-high molecular weight polyethylene Photoresist Solids
(UPE) and polyamide. While both material
membranes wet spontaneously with all solvent-
based photochemicals and have been used as
photochemical filters, each membrane material
has its specific structure and associated properties.
The principle particle retention mechanism of
microporous membranes is based on sieving or 1800 1500 1200 900
screening, that is, particles larger than the pore Wavenumbers (cm-1)
size do not pass through the filter. The decrease Figure 2. FTIR spectra of used filters in 193 nm photoresist
of critical linewidths is requiring the use of tighter
filtration for photochemicals. 10 nm and 20 nm
nylon and UPE filter surfaces after filtering a
filtration has been implemented in both the
193 nm photoresist. None of the membranes
photoresist manufacturing process and in the
initially had peaks evident at 1790 to 1720 cm-1.
POU spin-coating process in track systems, and
The nylon membrane shows the adsorption of some
has been shown to be effective to reduce defects.
material from the photoresist in the area while the
While finer pore size filters are advantageous for
UPE membrane shows no adsorption at all.
reducing wafer defects, small pore alone will not
eliminate some process-specific defects such as It was hypothesized that the adsorption effect
microbridging. In addition, the implementation of of polyamide membrane was due to the polar
finer filtration raises concerns as high differential functional groups of polyamide polymer. Figure 3
pressure across the filter causes outgassing of the shows the chemical structure of polyamide and
photoresist and finer pore size approaches the polyethylene polymers and the polar functional
size of large molecular weight polymers in the groups of polyamide polymer. This nonsieving
photoresist. retention capability of polyamide membrane would
be instrumental in reducing the wafer defect level
In addition to the size exclusion, there may be
without further reducing the membrane pore size.
some adsorption effects of the membrane. Trace
contaminants, even smaller than membrane pore
in size, are attracted and captured by the membrane
surface due to hydrophobic binding or charge
effect. This is referred as nonsieving retention.
While the membrane surface can be well Polar Functional Groups
characterized, the interactions with complicated
photochemical chemistry can be difficult to predict.
Empirical evidence indicated that there appeared
to be an adsorptive interaction of the polyamide
membrane surface with the photoresist, in which UPE
“less-soluble”, polar, defect-causing high molecular
Polyamide
weight polymers in the resist were adsorbed by the
membrane. Figure 2 shows the FTIR spectra of
Figure 3. Chemical structure of polyamide and polyethylene
2 ENTEGRIS, INC.

3. A NEW DUAL-FUNCTIONALITY FILTER FOR DEFECT REDUCTION APPLICATION NOTE
While the surface of polyamide is beneficial for On the other hand, ultra-high molecular weight
particle removal, there are some concerns about polyethylene membranes offer outstanding
polyamide membrane. chemical resistance to organic solvents used in
photoresist and bottom anti-reflective coatings
Lifetime concerns (BARC) and have found considerable success as
photoresist and BARC filters in the lithography
Improvement in defectivity due to nonsieving
processes. The UPE membrane and high-density
mechanism is likely volume dependent. Once a
polyethylene (HDPE) filter components are
prescribed volume of photoresist has been processed,
compatible with almost all photochemical solvents.
defects would break through as functional groups
Highly sieving UPE filters down to 10 nm have
become saturated. It is difficult to predict the
been demonstrated to be effective to reduce
breakthrough point, since this point is affected by
defects on the wafers.
many external factors.
Side effects concerns Impact Duo Filter Design
The adsorption of polar functional groups of The Impact Duo filter is specifically designed
polyamide membrane is non-selective. The polar to meet the unique requirements of advanced
functional groups may adsorb the defect-causing lithography processes. It uses Entegris’ dual-
impurities while at the same time they may also functionality technology, which is the combination
adsorb the additives which are important to of a polyamide layer and a tight UPE membrane.
photoresist properties, such as PAGs, acid quenchers Figure 4 is a schematic representation of membrane
etc. The adsorption of these important additives layout in the filter. The filter is constructed with
may induce some CD variations. the pleated dual layer membranes and with HDPE
supports, components and housing.
Compatibility concerns
Polyamide is a highly crystalline form with a Support Polyamide UPE 0.03 µm or 0.01 µm
high degree of hydrogen bonding. It tends to be
decomposed in solvents with high polarity. Moisture,
chemical attack, photolysis and oxidation are all
routes to attack polyamide. The degradation of Upstream Downstream
polyamide filter membrane can be detected in the
process in a number of ways. The most common
effect is loss of particle performance. As a result,
Impact Duo (4 layers)
small particles can be released, known as particle
shedding, or the pore size can increase, releasing
Figure 4. Schematic of membrane layout of the “Duo” filter
retained particles back into the process stream.
The mechanical properties of the filter can also be
The “Duo” filter provides superior sieving and
compromised, resulting in a weakened membrane
nonsieving (adsorptive/purification) retention.
and defects in the membrane structure, which
The technology is targeted at specific 193 nm
could allow unfiltered chemicals to come in
photoresist and BARC applications where the
contact with the wafer.
customer is experiencing defects caused by
The lifetime of POU photochemical filters is impurities not removed by standard sieving
generally in the 3- to 12-month range because the technology. The upstream polyamide layer uses
particle loading is low and the tool downtime costs an asymmetric membrane structure, where the
involved with filter changeout are quite high. In downstream surface of the membrane is very
the POU application, the filter must have exemplary thin and has more retentive pore structure. The
chemical compatibility because it is in contact remaining membrane thickness has a more open
with the chemical for up to one year. Even if the pore structure, providing strength and some depth
filter is idle, chemical attack can occur, with the filtration. This kind of composite pore structure
by-products of degradation being released into the provides the retention capacity of the microporous
process on startup. membrane without the sacrifice of the flow. Figure 5
shows the scanning electron microscope (SEM)
of the cross-section of the symmetric UPE and
asymmetric polyamide membranes used in the
Impact Duo filter.
ENTEGRIS, INC. 3

4. A NEW DUAL-FUNCTIONALITY FILTER FOR DEFECT REDUCTION APPLICATION NOTE
In addition, the polar functional groups of The Impact Duo’s superior sieving retention is
polyamide membranes may remove difficult accomplished mostly by the UPE membrane
contaminants from specific photoresists due to used as the downstream membrane layer. Sieving
an adsorptive property, where a single retention retention of 30 nm and below is effective at
technology might not succeed. However, by its reducing gels, hard/soft particles and some
nature, the capacity of an adsorptive removal molecular contaminants that can lead to defects.
mechanism is limited. The number of sites on the The defect-causing substances adsorbed in the
surface and the amount of contamination in the first nonsieving layer may nucleate and become
photoresist control the lifetime of the filter. By larger gels. These large gels may penetrate
using an asymmetric pore structure, the thickness through the nonsieving layer. The sieving UPE
of the polyamide membrane can be increased, layer downstream of the nonsieving layer can
resulting in higher numbers of adsorptive sites on protect the processes from these gels or other
the surface, longer residence time of the photoresist variables, such as compatibility induced by the
in the tortuous membrane pore structure and, first layer and extend the filter lifetime after active
therefore, enhanced nonsieving retention sites of the first layer are saturated. In addition,
efficiency and filter lifetime. this filter design mimics photoresist manufacturing
process (PMP) of most advance chemicals, where
nonsieving technology is used as a pre-filter and a
highly retentive sieving UPE membrane is used as
a final filter. By using the combination of a thick,
asymmetric polyamide layer and a thin, tight UPE
membrane, the “Duo” technology provides superior
sieving and nonsieving particle retention, flow and
chemical compatibility.
Improving Filter
Performance
Particle Retention
The finer feature size of advanced lithography
processes requires even finer filtration for
Asymmetric polyamide membrane
photoresists. The filters with 30 nm, 20 nm, even
10 nm retention ratings have become the standard
for ArF resists. These filters were tested for
particle retention with monodispersed 0.034 µm
polystyrene latex (PSL) beads using a modified
SEMATECH test method1. In the testing, the filters
were continually challenged with 0.034 µm PSL
beads and filter retention values were measured as a
function of particle loading. The retention capability
of the filters is reported as log reduction value
(LRV), a very sensitive method for detecting slight
passage of particles. LRV is defined as the logarithm
of the ratio of the number of particles in the feed
to the number of particles in the filtrate.
Symmetric UPE membrane
Figure 5. Membrane cross-section
4 ENTEGRIS, INC.

5. A NEW DUAL-FUNCTIONALITY FILTER FOR DEFECT REDUCTION APPLICATION NOTE
Figure 6 shows the particle retention comparison METAL EXTRACTABLES RESULTS FOR IMPACT DUO FILTERS
of the newly developed Impact Duo 30 nm and Filter Impact Duo Impact Duo Competitor’s Nylon
10 nm filters, and a 50 nm polyamide single-layer
Pore Size 10 nm 30 nm 40 nm
filter. The Impact Duo filters show improved
retention compared to a 50 nm polyamide single- Metal µg/device µg/device µg/device
layer filter. Both the 30 nm and 10 nm rated Na 0.40 0.26 2.80
Impact Duo filters are extremely efficient in Mg 0.92 0.96 0.79
removing PSL particles from the process fluid, Al 0.43 0.46 0.96
even with very high particle loading. The Impact K 0.14 0.07 0.62
Duo 10 nm filter maintains superior particle Ca 0.08 0.08 2.48
removal performance throughout the whole
Ti 0.01 0.04 0.55
particle challenge period. This represents an
Cr 0.00 0.00 0.11
extended lifetime in fluids containing high
concentrations of contaminating particles. Mn 0.00 0.00 0.18
Fe 0.05 0.11 0.04
30 nm LRV vs. Particles/Filter Ni 0.02 0.01 2.35
Cu 0.05 0.03 0.55
50 nm polyamide
Impact Duo 30 nm Zn 0.09 0.04 0.65
Impact Duo 10 nm
Pb 0.00 0.00 0.24
10.00
Total 2.19 2.07 12.32
Table 1. Metal extractables results for Impact Duo filters
Retention (LRV)
1.00 The organic extractables of the Impact Duo filters
was measured as non-volatile residue (NVR). The
filters were soaked with a solvent for a period of
time and the solvent drained into a clean beaker.
The solvent in the beaker was then evaporated
0.10 and the mass of the residue was weighed with an
1E+11 1E+12 1E+13
analytical balance. As seen in Table 2, the Impact
Number of 0.034 µm PSL Challenge Particles
Duo filters have very low organic extractables
Figure 6. Particle retention of Impact Duo filters using levels.
PSL beads
ORGANIC EXTRACTABLES RESULTS MEASURED BY NVR
Cleanliness/Extractables Filter NVR (mg/device)
Competitor Nylon 20 nm 8.6
Impact Duo filters are much cleaner and have
much lower metal extractables and organic Competitor Nylon 40 nm 8.2
extractables than other commercially available Impact 2 UPE 3.0
photochemical POU filters. The metal extractables Impact Duo 1.3
of the Impact Duo filters was determined after Table 2. Organic extractables results for Impact Duo filters as
placing the filters in a known volume of an measured by NVR
extracting fluid over a period of time. The filters
were drained of the fluid and the extracting fluid
was analyzed for key metal elements using ICP-MS The Impact Duo filters use ultra-clean raw
technique. The metal concentrations measured in materials and are cleaned using Entegris’ most
ppb (parts-per-billion) were multiplied by the advanced proprietary cleaning technology for
extract volume and the total reported as µg/device. ultra-low organic extractables. Careful handling,
Table 1 provides a summary of the metal extractables manufacturing and flushing techniques are also
results for Impact Duo filters and a competitor’s responsible for the Impact Duo filters’ extractables
nylon filter. The metal extractables results show performance.
the Impact Duo filters are much cleaner as
indicated by the total metals extracted from the
filters compared to a competitor’s nylon filter.
ENTEGRIS, INC. 5

6. A NEW DUAL-FUNCTIONALITY FILTER FOR DEFECT REDUCTION APPLICATION NOTE
Filter Priming
Bubble Flushup of Impact Duo Filters
The most noticeable attribute of photochemical on a Two-stage Dispense Pump
POU filters is the ability to purge air and
contaminants introduced during system Impact Plus 10 nm UPE single-layer
Impact Duo 10 nm
maintenance or filter changeout. Many fabs are
Impact Duo 30 nm
reluctant to perform filter change prior to failure
due to chemical consumption and tool downtime. 1000
>= 0.15 µm Particle Counts (pts/ml)
With constant throughput fab processes, lengthy
fabrication tool downtime means corporate
profitability is reduced. The Impact Duo filter’s
@RION KL-27
100
unique design ensures a high level of retention
to reduce advanced process defects, while also
maintaining the ability of fast priming during 10
filter changeout.
Laboratory experiments were conducted to examine
the priming performance of Impact Duo filters 1
-25 0 25 50 75 100 125 150
and compare it to a standard Impact Plus UPE
Operation Cycle
single-layer filter on an Entegris IntelliGen® Mini
dispense pump or a single-stage dispense pump. A Figure 7. Bubble flush-up of Impact Duo filters on a two-
recirculating chemical test stand was assembled stage dispense pump
using a chemical reservoir, a dispense pump, a
filter manifold, a test filter and an optical particle
Bubble Flushup of Impact Duo Filters
counter (OPC). The OPC is a Rion KL-27, capable on a Single-stage Dispense Pump
of detecting and sizing particles 10 nm to 50 nm.
This OPC was installed on the outlet line of the Impact Plus 10 nm UPE single-layer
Impact Duo 10 nm
dispense system, monitoring the entire downstream
Impact Duo 30 nm
of the testing filters. The effluent was recycled to
the reservoir. The filters were primed with the 1000
>= 0.15 µm Particle Counts (pts/ml)
solvent propylene glycol monomethyl ether acetate
(PGMEA) and the dispense recipe was continually
performed until particle counts leveled off. Since
@RION KL-27
100
each new testing filter was installed after the
particle counts reached very low background with
a filter in place, the particle levels shown by the 10
counter indicated the level of microbubbles in the
dispense line during the testing. While optical
particle counters are not designed to count bubbles,
1
the results can be used in a semi-quantitative -25 0 25 50 75 100 125 150
manner to see differences in filter performance. Operation Cycle
The results of filter priming testing are showed Figure 8. Bubble flush-up of Impact Duo filters on a single-
in Figures 7 and 8. The priming speed of Impact stage dispense pump
Duo filters is slightly better or equal to a standard
Impact Plus 10 nm UPE single-layer filter on both a
two-stage dispense pump and a single-stage dispense On-wafer Performance
pump. These results show that when the device
design and membrane structure are combined The benefits of Impact Duo filters have been
correctly, Impact Duo filters can maintain the fast demonstrated under actual production conditions
priming speed. As a result, the chemical waste and at a number of different semiconductor
dispense-point downtime are reduced. manufacturing sites. One customer in Asia had
been facing bottom bridge defect issue for latest
6 ENTEGRIS, INC.

7. A NEW DUAL-FUNCTIONALITY FILTER FOR DEFECT REDUCTION APPLICATION NOTE
ArF resist. The customer evaluated several filters Particle for Duo Filter Evaluation
to reduce this defect issue. Figure 9 summarizes Pct_duo Control limit
the results of the evaluation over a period of time.
15
While tighter filtration for BARC reduced the
defect level, Impact Duo technology resulted in 10
almost “zero” bottom bridge defect. 5
Effect of Impact Duo Filter on Bridge 0
Defect Reduction in ArF Resist
-5
ArF Resist: UPE 50 nm ArF Resist: UPE 50 nm
ArF BARC: UPE 50 nm ArF BARC: UPE 10 nm -10
1 3 5 7 9 11 13 15 17 19 21 23 25 27
Number of Bottom Bridge Defects
12 Weeks
Particle Pre-pump Change
2 Weeks (Nylon 40 nm)
ArF Resist: Duo 10 nm
ArF BARC: UPE 10 nm PC0001_C Control limit
15
8 Weeks
10
5
1 2 3 4 5 6 7 8 9 10 11 12
0
Figure 9. Effect of Impact Duo filter on bridge defect
reduction in ArF resist -5
Impact Duo filters were also evaluated at an Asian -10 1 3 5 7 9 11 13 15 17 19 21 23 25 27
semiconductor foundry. The 30 nm Impact Duo
Figure 10. Comparison of wafer defects as measured by
filter was installed in a RDS-01 dispense pump in surface scanning
a TEL® Act 8 track with a DUV resist. The resist-
coated wafers were analyzed for defects with a
KLA-Tencor® AIT. As shown in Figure 10, particle semiconductor manufacturing conditions, to
counts on wafer were variable when a nylon 40 nm provide superior sieving and nonsieving (adsorptive/
single-layer filter was installed as the standard purification) retention that can lead to defect
filter. However, for the Impact Duo filter, particle reduction in advanced lithography processes and
levels on wafer were not only reduced, but also superior chemical compatibility and lifetime,
were more stable. especially for certain resists where bridging defect
or cone defect is a problem. Entegris’ proprietary
These tests demonstrated that Impact Duo filters cleaning technology ensures the cleanliness of
were effective at reducing bridge defects and Impact Duo filters. The fast priming speed due to
provided lower particle concentrations on wafer optimized device design and membrane structure
than comparable pore size single-layer filters for effectively reduces tool downtime and chemical
the testing photoresists. consumption required for filter changeout.
Conclusion References
Impact Duo, a new photochemical point-of-use 1.Lee, J.K. et al, “Latex Sphere Retention by
filter based on dual-functionality membrane Microporous Membranes in Liquid Filtration,”
technology has been shown, under laboratory and Journal of the IES, January/February 1993, 26-36.
TEL® is a registered trademark of Tokyo Electron, Inc.
KLA-Tencor® is a registered trademark of KLA-Tencor, Inc.
Entegris®, Impact® and IntelliGen® are registered trademarks of Entegris, Inc.
Rion KL-27 is a trademark of Rion Company, LTD.
Sematech® is a registered trademark of Sematech, Inc.
ENTEGRIS, INC.
Corporate Headquarters / 3500 Lyman Boulevard / Chaska, Minnesota 55318 USA
Customer Service Tel. 952-556-4188 / Customer Service Fax 952-556-8022
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