filters with 0.02-Ám pores are advantageous for reducing these defects,
small pore size alone does not eliminate them. In contrast, changes
in the surface chemistry of the membrane without further pore-size
reductions do result in the virtual elimination of defects in both
the resist and antireflective coating layers. While some performance
improvements depend on the filter surface and pore size only, other
improvements depend on the volume of photochemical processed by the
filter. In other words, once a prescribed volume of photoresist has
been processed, defect levels return to near-prefiltration
levels. This type of performance indicates that the filter surface
relies on an adsorptive mechanism to remove contamination. In such
cases, the membrane surface is active, enabling users to develop more-sophisticated
algorithms to estimate filter lifetime.
article discusses the nature of particulate and bubble defects in
193-nm photoresist processes and the interaction of the membrane surface
with the photochemicals. In addition, it presents the results from
laboratory and wafer-production tests performed by investigators at
Infineon Technologies. Finally, the article offers recommendations
for estimating filter lifetime.
Types and the Role of Filtration in Reducing Defects
fine linewidths require the use of new classes of photochemicals that
are exposed using argon fluoride (ArF) excimer lasers. Known as ArF
or 193-nm photochemicals, these resists rely on acid generators for
chemical amplification. ArF photoresists generate microbridging defects,
examples of which are shown in Figure 1.1 To achieve sharp
linewidths, 193-nm photochemicals also require the use of bottom antireflective
coatings (BARCs) and top antireflective coatings (TARCs). BARCs and
TARCs are acidic and have high concentrations of surfactant, resulting
in very low surface tension.
1: Examples of microbridging defects associated with 193-nm ArF
physical properties of photoresists and antireflective coatings have
prompted the semiconductor industry to modify its approach to the
purification of point-of-use chemicals. To reduce photochemical defect
levels, the industry is pursuing both an evolutionary approach (the
development of filters with finer pore sizes) and a revolutionary
approach (the modification of membrane surfaces).
first filters used in photochemical purification were based on nylon
or cellulosic microporous membranes. Initially, these membranes were
developed for the removal of microbiological organisms from pharmaceutical
fluids, but they were also able to reduce particle contamination in
photochemicals, which contain many solvent bases.
PTFE membranes were introduced to the semiconductor manufacturing
process because of their chemical compatibility. They were also used
in photochemical filtration with some success. While Teflon filters
were unaffected by the strong solvents used in photochemicals and
their membrane structures had comparatively low organic and ionic
extractables, the Teflon surface was much less wettable than the nylon
and cellulosic filters they replaced. For many photoresists, high
pressures are required to force the photochemical into the filters'
pore structure. In particular, the use of cyclohexanone or N-methyl
pyrrolidone–based photochemicals requires high pressure to wet
polyethylene (UPE) membranes were used quite successfully as photochemical
filters. The UPE membrane and high-density polyethylene (HDPE) filter
components were compatible with almost all photochemical solvents.
The UPE membrane wetted spontaneously with all solvent-based photochemicals,
eliminating the start-up difficulties of PTFE membranes. UPE membranes
reduced photoresist waste and resulted in a significant reduction
in wafer defects.2
UPE membranes wetted spontaneously with ArF photoresist and antireflective
coatings, there was empirical evidence that a membrane surface with
an even higher critical surface energy results in lower wafer defects.
A development program was initiated to understand the effects of that
membrane surface on defects.
Particle Retention. While 0.05-Ám membrane retention ratings
became the standard for deep-ultraviolet (DUV) resists, the finer
feature size of ArF photoresists required even finer filtration. Consequently,
membranes with 0.03- and 0.02-Ám retention ratings were developed.
membranes' retention was tested using mono-dispersed
0.034-Ám polystyrene latex beads and a modified Sematech test method.3
Retention was measured in terms of log reduction value (LRV), a very
sensitive method for detecting the slight passage of particles. The
LRV is a logarithm of the ratio of the number
of particles in the feed to the number of particles in the filtrate.
The maximum measurable LRV is controlled by the number of particles
in the feed and the background counts of the particle counter. In
the tests performed to determine the retention ratings of fine ArF
photoresist filters, background system particle counts were subtracted
from the filtrate values to improve sensitivity so that retention
performance could be compared at high LRVs.
0.05-Ám UPE membranes were able to retain a high percentage of 0.034-Ám
PSL beads, retention degraded as particle loading increased, limiting
the lifetime of filters in fluids containing high concentrations of
contaminating particles. In contrast, the 0.03- and 0.02-Ám rated
filters demonstrated better retention rates under very high particle-loading
conditions, as illustrated in Figure 2.
2: Retention rates of 0.05-, 0.03-, and 0.02-µm UPE membranes
for 0.034-µm PSL beads.
Bubbles. In photochemical purification applications, the
elimination of bubbles is as important as particle removal. Bubbles
in photoresist or antireflective coating layers result in wafer defects.
Bubbles form in liquids when the solubility of dissolved gases decreases.
In addition, pressure fluctuations, such as those created during fluid
pumping, can cause bubbles to form. Three bubble-formation mechanisms
have been proposed in the literature: homogeneous nucleation, heterogeneous
nucleation, and cavitation.4
Homogeneous nucleation results in the formation of microbubbles
everywhere in a liquid when gas molecules form clusters and grow to
a defined size. This phenomenon occurs when supersaturated dissolved
gas in a liquid suddenly becomes insoluble—for example, when
pressure is reduced.
Heterogeneous nucleation is defined as bubble growth on hydrophobic
surfaces. Hydrophobic surfaces or particles act as catalysts for bubble
formation when gas solubility in a liquid is reduced.
Cavitation is characterized by bubble formation at nucleation sites
caused by a sudden pressure drop of a moving fluid.
rare occurrence, homogeneous nucleation is not a likely mechanism
for bubble formation in TARCs. In contrast, both heterogeneous nucleation
and cavitation are the likely mechanisms for bubble formation in photochemicals.
Both the surface energy and pressure drop of a membrane surface may
play a role in microbubble formation, and the membrane's pore size
plays a role in microbubble removal. Filter manufacturers must perform
a balancing act by developing membrane surfaces that both remove bubbles
from photochemical fluid and retard bubble formation downstream of
Filter Surface Properties. The optimal filter has a combination
of physical properties that enables it to remove contamination from
Its membrane is wetted spontaneously by the chemicals.
It has high capillary forces to completely wet surfaces.
It eliminates vapor from voids.
It has a small pore size so
that particles and bubbles can be removed.
Because of its low-pressure-drop characteristic, it can have a small
membrane area and reduce outgassing downstream of
Photochemical compatible, it does not have extractables and has a
thermal-induced phase-separation UPE membranes meet most of these
requirements, photochemicals with high surface tension (>35 dyn/cm)
do not wet the membrane surface spontaneously. Hence, a modified UPE
surface has been developed that accommodates chemicals with the surface
tension of water (72 dyn/cm) and higher. The modified UPE surface
has the following properties:
A base membrane that is inert to photochemicals.
A smooth structure with few crevices.
A smooth surface with a cross-linked polymer matrix that increases
the membrane's surface energy and maintains the bulk property of the
3 presents a schematic diagram of the modified UPE membrane.
addition to the size-exclusion and wetting attributes of the membrane
surface, the membrane may exhibit adsorptive effects. The surface
removes trace contaminants as a result of hydrophobic binding or charge
effects. Depending on the nature of the surface, these effects can
be significant. While the membrane surface can be well characterized,
its interactions with complicated photochemical chemistries can be
difficult to predict. Nevertheless, these interactions can be measured
empirically. Once they are recognized, a hypothesis describing their
function can be developed.
3: Schematic diagram of the modified UPE membrane.
to optimize filter design commenced when process engineers at Infineon
observed that unacceptably high defect levels resulted from using
0.05-Ám filtration at the point of use to purify an ArF photoresist.
While reducing the pore size of the UPE membrane to 0.02 Ám reduced
wafer defect levels, those levels remained high. Figure 4 shows that
defect levels increased over time when 0.05- and 0.02-Ám filters were
used in the lithography process.
4: Wafer defect levels resulting from the use of 0.05- and 0.02-µm
address this problem, investigators from Infineon and Mykrolis (Billerica,
MA) began to consider the use of nylon. Although they knew that the
bulk properties of nylon limit the material's chemical compatibility,
empirical evidence indicated that filters with a nylon surface can
reduce wafer defect levels. Investigators hypothesized that a nylon
surface has higher surface free energy than UPE alone—in other
words, it is more hydrophilic or wets with high-surface-tension fluids.
In addition, they observed what appeared to be an adsorptive interaction
between the membrane surface and the resist (high-molecular-weight
contaminants in the resist are adsorbed by the amide surface).
Mykrolis membrane scientists developed a filter with a nylonlike surface
membrane that maintains the bulk chemical compatibility of UPE. Figure
5 compares Fourier transform infrared data from four filters that
were used to filter ArF photoresist. Initially, none of the membranes
had an adsorption peak at 1720 cm–1. While filter modification
1 (a nonsieving UPE filter with a surface similar to nylon) adsorbed
some contaminant material from the photoresist in the nylon-adsorption
area, the base UPE membrane did not adsorb material. Since filter
modification 2 had more amide functionality per mass than nylon, resulting
in more contaminant adsorption, that modification became the basis
for Mykrolis's PCM filter. The filter was tested at Infineon.
5: Adsorption of material from ArF photoresist on different membrane
to field testing, the chemical compatibility of the PCM membrane was
compared with that of nylon to ensure that its physical properties
did not degrade. UPE membrane samples with a PCM surface and nylon
membranes were soaked in photoresist for one month. Then the strength
and elongation of the membranes were evaluated using an instrument
from Instron (Norwood, MA). Samples of the membrane were pulled at
a controlled rate while force and elongation were measured. The samples'
strength and elongation before and after the one-month soaking period
are illustrated in Figures 6 and 7, respectively. While the nylon
material's strength declined over that period and its elongation decreased
by 90%, the UPE membrane's strength increased slightly and its elongation
fell by only 50%.
6: Tensile strength of membranes before and after exposure to
7: Elongation of membranes before and after exposure to DUV resist.
determine the effects of filter designs on wafer defect levels, tests
were performed involving several filter samples with different surface
types and pore sizes. The objective of the tests was to determine
if pore size alone or surface alone could reduce defect levels. First,
wafers were processed in an ACT 8 coater/developer i-line track tool
from Tokyo Electron (Tokyo). Standard pump parameters and 193-nm acrylate-based
photoresist were used. Finally, the wafers were inspected using a
KLA 2351 from KLA-Tencor (San Jose). The filters tested are listed
in Table I.
Impact LHVD (Mykrolis)
UPE (tight pore size)
and 0.02-µm double-layer UPE (improved gel particle
UPE with modification 1 (check for adsorption)
UPE with modification 1 (check for adsorption and tight
nylon/0.02-µm UPE (check for adsorption and tight
UPE with modification 2 (PCM)
(check for adsorption)
I: Filters tested to determine the effects of surface type and
pore size on wafer defect levels.
8 summarizes the results of the tests. While both tighter membranes
and improved surfaces reduced defect levels, the combination of a
tighter membrane and the maximum amide surface concentration (Filter
F) resulted in the lowest defect levels. Figure 9 shows the defect
performance of a production PCM filter over an extended period of
8: Production wafer defect levels resulting from the use of different
9: Defect performance of filter F (PCM filter) in a production
setting over an extended period of time.
of Top Antireflective Coatings
are the major source of defects in TARCs. Since TARCs have high levels
of surfactant, resulting in low surface tension, they have a high
propensity for outgassing. To determine bubble levels, investigators
conducted laboratory experiments using a Mykrolis IntelliGen 2 dispense
system and AZ Aquatar TARC from AZ Electronic Materials (Somerville,
NJ). A LiQuilaz SO2 optical particle counter from Particle Measuring
Systems (Boulder, CO) was installed on the dispense line. While optical
particle counters are not designed to count bubbles, particle results
can be used in a semiquantitative manner to determine differences
in filter performance. All membranes tested were of the flat-sheet
pleated variety, with the exception of a 0.1-Ám hollow-fiber UPE membrane.
The filters were primed with TARC and the dispense recipe was performed
continually until particle counts leveled off.
results showing several different filters' retention efficiencies
are shown in Figure 10. (Retention efficiency has the greatest effect
on particle counts.) The lowest particle count (i.e., the best particle-retention
efficiency) was achieved using a PCM 0.02-Ám hydrophilic filter. Figure
11 shows the results of a flow-stoppage test. The use of the PCM filter
resulted in the lowest particle counts when flow was reestablished.
That filter resulted in a smaller particle-count spike than Mykrolis's
LHVD 0.05-Ám hydrophobic membrane and minimized bubble formation and
10: Laboratory tests showing particle levels resulting from the
use of different filters. Particle levels indicate the level of
microbubbles in the dispense line.
11: Laboratory tests comparing wafer particle performance (bubble
level) of a 0.02-µm hydrophilic filter and a 0.05-µm
hydrophobic filter after flow had been stopped for 2 hours.
the laboratory tests, 0.02-Ám Impact Plus PCM filters were evaluated
at Infineon. The filters were installed in a Mykrolis Two-Stage Technology
dispense system using AZ Aquatar TARC. The TARC-coated wafers were
analyzed for defects using a KLA-Tencor AIT 2 patterned-wafer inspection
tool. The tests revealed that the filter's pore size, retention efficiency,
and surface energy had a dramatic effect on wafer-level defects. The
PCM filter resulted in 57% fewer defects than a 0.04-Ám nylon membrane,
85% fewer defects than a 0.1-Ám nylon membrane, and 88% fewer defects
than a 0.1-Ám hollow-fiber UPE membrane.
device geometries shrink, particulate contamination below 0.05 Ám
must be controlled. Both a filter's pore size and surface properties
must be optimized to purify ArF photochemicals. The use of filter
membranes with a fine (0.02-Ám) pore structure reduces defects on
the wafer surface. In addition, the membrane must be wettable in the
photochemical to prevent the formation of gas bubbles and make filter
start-up as easy as possible.
many ArF resists, a surface with amide functionality may remove contamination
because of its adsorptive nature. However, an adsorptive removal mechanism
has a finite capacity. The number of sites on the wafer surface and
the amount of contamination in the chemical control filter lifetime.
In addition, flow rate, or residence time, can play a minor role in
contamination removal. Membranes are particularly good adsorptive
surfaces because they are in intimate contact with the chemical. While
the removal kinetics of membranes are more favorable than those of
resin beads, their capacity is lower than resins that are designed
to remove particles using adsorption.
resist contamination levels are generally an uncontrolled variable,
it is difficult to predict the lifetime of filters that use adsorption
as a purification technique. Contamination levels follow a breakthrough
curve, where defect levels start to increase gradually before significant
degradation occurs. The higher the flow rate, the more apparent the
degradation, since higher flow rates reduce the residence time. The
only good monitor of resist contamination is wafer defect levels.
When wafer defects increase, the filter should be replaced.
filter surfaces can be used to develop new photoresists. By testing
each component of the resist formulation with the active filter, contamination
sources can be segregated and controlled. The easiest point in the
production cycle to remove contamination is during the resist manufacturing
process, when sophisticated analytical techniques can be used. The
active filter surface then becomes an insurance policy in the fab
rather than the last line of defense, resulting in longer filter lifetime
and a lower cost of ownership.
article is an edited version of a presentation given at Semicon Korea,
February 2–4, 2005, in Seoul. The authors would like to thank
Infineon Technologies for providing the manufacturing-based data appearing
in the article and for providing Figure 1. They also thank Joseph
Zahka, Saksatha Ly, and Bipin Parekh for their technical contributions
and for preparing the membranes evaluated in this study.
PB Sahoo et al., "Progress in Deep-UV Photoresists,"
Bulletin of Material Science 25, no. 6 (2002): 553–556.
L Mouche et al., "Photoresist Filtration Performance of
UPE and PTFE Filters," in Proceedings of the IES Annual Technical
Meeting (Mount Prospect, IL: Institute of Environmental Sciences,
J-K Lee, BYH Liu, and KL Rubow, "Latex Sphere Retention
by Microporous Membranes in Liquid Filtration," Journal of
the IES 36, no. 1 (1993): 26–36.
J Duffner, "Defect Reduction in Top Antireflective Coating,"
Applications Note AN1019ENUS (Billerica, MA: Mykrolis, 2004).
Amari is manager of applications technology development at
Mykrolis in Tokyo. He received a BS in industrial chemistry from Ibaraki
University. (Amari can be reached at +81 3 54429713 or email@example.com.)
Wu, PhD, is an applications engineer at Mykrolis in Billerica,
MA. Since joining the company in 2003, he has focused on wet etch
and clean filtration, electrochemical-plating filtration, and photochemical
filtration products. He received a PhD in chemical engineering from
the University of New Hampshire in Durham. (Wu can be reached at 978/436-6820
Jun Yang is an Asia applications manager for Mykrolis. He
has been with the company since 1996, working in application development
for microelectronics, chemical manufacturing, and flat-panel manufacturing
processes. Before joining Mykrolis, Yang was section chief of the
process integration division of Hyundai Electronics. He received BS
and MS degrees in materials sciences and engineering from Hanyang
University, in Seoul. (Yang can be reached at +82 31 7385331 or firstname.lastname@example.org.)
Chen, PhD, is a lithographic process engineer at Infineon
Technologies in Richmond, VA. In 1998 she received a PhD in chemical
engineering from the University of Rochester in Rochester, NY. (Chen
can be reached at 804/952-8098 or Linda.email@example.com.)
Bowling is a lithography equipment engineer at Infineon.
Active in the semiconductor industry for 15 years, he received a degree
in mechanical engineering in 1989 from the West Virginia Institute
of Technology in Montgomery. (Bowling can be reached at 804/952-7607
Watt is a senior staff process engineer in the lithography
department at Infineon. He received a degree in mechanical engineering
from the University of Glasgow in Scotland in 1983. (Watt can be reached
at 804/952-6131 or firstname.lastname@example.org.)