UC-006

Particle removal efficiency evaluation of filters
in IPA
Tomoyuki Takakura and Shuichi Tsuzuki
Nihon Pall Ltd.
46, Kasuminosato, Ami-machi, Inashiki-gun, Ibaraki 300-0315 Japan
tomoyuki_takakura@ap.pall.com

Abstract – As part of the study to investigate filter performancein the actual conditions, particle removal efficiency evaluation
method of filters in isopropyl alcohol was developed. Threekinds of particles which exhibit each different charged state in
the chemical, and the sizes of which are around 10 nm, wereselected as test particles for the evaluation. Pall 10-nm-rated
surface-modified PTFE membrane filter was evaluated with theparticles. The result was compared with the removal efficiency
in deionized water. Also, the effect of adsorption on the removal
efficiency in the system was discussed.
INTRODUCTION
Isopropyl alcohol (IPA) is a chemical commonly used for thewafer drying process in the semiconductor industry. The
control level of contaminant in IPA is getting severer, as thefeature size of the semiconductor devices decreases. Filtration
plays an important role to reduce the contaminant. Particle sizeremoved by filter for IPA is required to be smaller inaccordance with the control level of the contaminant. Anotherimportant requirement of filters used in IPA is to minimize
extractables from the filters themselves. From this perspective,
polytetrafluoroethylene (PTFE) membrane filters are being
used in more cases, whereas high-density polyethylene (HDPE)
membrane filters had been most commonly used in IPA.
For filter removal rating, gold particle covered withmercaptosuccinic acid (Au MSA) is used as a test particle (i.e.
challenge particle) in deionized water (DIW); MSA minimizes
adsorption effect of gold particles to membranes [1]. It is theworst case condition for filter, since sieving effect almost
exclusively works for particle removal. In general, however,particle removal efficiency (PRE) of filters is affected by thechemical and the temperature employed. Therefore, to know
the PRE in the actual chemicals is of great importance for theappropriate filter usage. Based upon this idea, we havedeveloped PRE evaluation methods in H2SO4 [2, 3] and diluted
HCl [4, 5]. In this article, as part of these studies in the actual
chemicals, we report a PRE evaluation method in IPA.
EXPERIMENTAL
We evaluated Pall 10-nm-rated surface-modified PTFE (SMPTFE) membrane, which is one of filters used in the leadingedge semiconductor device manufacturing process.
Regarding test particle, Au MSA is not stably dispersed in IPA[6]. Alternatively, we chose the following three particles withthe size around the filter rating (10 nm) and each different

charged state because electrostatic interaction is a factor thataffects PRE: Platinum particle covered with Polyethylenimine(Tanaka Kikinzoku Kogyo, Pt PEI), Zirconia particle (Nissan
Chemical Industries, ZrO2), and gold particle covered with
Polyvinylpyrrolidone (Tanaka Kikinzoku Kogyo, Au PVP).
We evaluated size distribution and zeta potential of theseparticles in IPA by dynamic light scattering (DLS, Zetasizer
Nano ZS, Malvern).
Particle challenge tests of filters were performed with theseparticles. The test system described in Figure 1 was employed
as follows: Each particle was added to electronic grade IPA in
the PFA reservoir, and then the suspending solution (influent)
was filtered by each membrane at the flow rate of 5 ml/min; theeffluent was collected in a sampling bottle. Each Au, Pt, and Zr
concentration in the influent (=C0) and the effluent (=C) was
quantified with ICP-MS (7700s, Agilent) to calculate removal
efficiency (=[1-C/C0] × 100). For reference, tests with 10-nmsized Au MSA in DIW were performed. Also, 10-nm-rated
HDPE filter was tested with Pt PEI in IPA for data comparison.

Pressure
regulator
Compressed air

Pressuregauge
PFA reservoir
Influent

Test filter in 47 mm 
disk holder

Sampling bottle

Effluent

Figure 1. Schematic of particle challenge test system forparticle removal efficiency evaluation of filter membranes.
Constant flow rate (5 ml/min.) was implemented by adjusting
the pressure regulator.

50

As shown in Figure 2, the peak particle sizes of the threedifferent kinds of particles were around 10 nm in IPA with asmall variation in size; Pt PEI was slightly smaller than 10 nm,and ZrO2 was slightly larger than 10 nm. These results indicatethat the particles are not agglomerated in IPA, therefore can beused for PRE evaluation. The size distribution data of the Au
particle in DIW, which was used for PRE evaluation in DIW,
has a peak at 10 nm as reported in the previous study [1].

40

As shown in Figure 3, in IPA, Pt PEI was positively charged,
Au PVP was negatively charged, and ZrO2 was nearly neutral.

Zeta potential (mV)RESULTS AND DISCUSSION

302010

Au PVP0Pt PEI-10ZrO2-20-30-40

Volume (%)

Pt PEI in IPA-50

Figure 3. Zeta potential of the three kinds of particles in IPA
measured by DLS. These particles were used for the challenge
tests in IPA.

Volume (%)

ZrO2 in IPA

Volume (%)

Au PVP in IPA

110Size (nm)100Figure 2. Particle size distribution in IPA measured by DLS.
These particles were used for the challenge tests in IPA. The
three particles’ sizes were around 10 nm with a small variationin size; Pt PEI was slightly smaller than 10 nm, and ZrO2 wasslightly larger than 10 nm.

The size distribution of ZrO2 remained unchanged for 24 hours
after making the suspension, though it agglomerated after 1week presumably due to the neutral charge; the ZrO2 was usedwithin 5 hours after preparation. The result that zeta potentialof ZrO2 shows neutral value in IPA is similar to that of silica
and silicon particles [7] which are presumed to be ones of actual
contaminants during wafer cleaning process.
Figure 4 shows PREs of 10-nm-rated SM-PTFE membranes
challenged with three kinds of particles in IPA. PREs in IPA
showed sufficiently high values for all the three kinds ofparticles, though we need to note each particle has somedifference in size. Overall, since the SM-PTFE membrane
removed three kinds of particles with each different chargedstates (i.e. negative, positive, and neutral) effectively, it can beconcluded that this filter is capable of removing various
particles that may exist in IPA.

In DIW, with Au MSA

90

100

8090

Particle removal efficiency (%)

Particle removal efficiency (%)

100

7060

50403020In IPA, with Pt PEI80706050403020100100Au PVP
Pt PEIParticle used for challenge tests
ZrO2
ZrO2

Figure 4. Challenge test results of Pall 10-nm-rated SM-PTFE
membranes with three kinds of particles in IPA. Each test wasrepeated twice.
Figure 5 shows PRE comparison between 10-nm-rated SMPTFE and HDPE membranes in DIW and in IPA; significant
difference in PRE between DIW and IPA is shown. In DIW,
both membranes showed PREs of > 95% for 10-nm-sized Au
particle. In IPA, SM-PTFE membranes showed PREs of > 95%,
whereas HDPE membranes were around 50%. Since the size ofPt PEI is smaller than 10 nm as shown in Figure 2, the resultsof HDPE membranes are reasonable if we consider only sieving
effect. Thus, the result that PREs of SM-PTFE with Pt PEI in
IPA were greater than that in DIW indicates adsorbing effectworked for the particle removal as well as sieving effect.

10 nm

10 nm

10 nm

10 nm

SM-PTFE

HDPE

SM-PTFE

HDPE

Membrane filters tested
Figure 5. Challenge test results of Pall 10-nm-rated SM-PTFEand HDPE membranes in DIW (light gray) and IPA (dark gray).
The challenge particles used were Au MSA in DIW and Pt PEIin IPA. The data in DIW are average of several tests with error
bars. The same data in Figure 4 are shown for the results of SMPTFE in IPA. Note that the size of Pt PEI is smaller than 10 nm
as shown in Figure 2.
CONCLUSION
A PRE evaluation method of filters in IPA was developed. The
evaluation was performed for Pall 10-nm-rated SM-PTFE
membrane filters with three kinds of particles, the sizes ofwhich are around 10 nm. The SM-PTFE filters showed PREsof > 95% with all the three kinds of particles. Since the SMPTFE membrane removed three kinds of particles with eachdifferent charged states (i.e. negative, positive, and neutral)effectively, it can be concluded that this filter is capable ofremoving various particles that may exist in IPA. Comparison
data between PREs of SM-PTFE and HDPE, and PREs in DIW
and in IPA indicate that the SM-PTFE filter removed particles
in IPA by adsorbing effect as well as sieving effect. Further
investigations will explore the detailed mechanism ofadsorption in this system.
REFERENCES[1] T. Mizuno, et al., "A Novel Filter Rating Method Using
Less Than 30-nm Gold Nanoparticle and Protective
Ligand," IEEE transactions on semiconductor
manufacturing, Vol. 22, No. 4, pp. 452-461, 2009.
[2] T. Takakura and S. Tsuzuki, "Particle removal efficiency
evaluation of filters in 150 C sulfuric acid," presented at
the 75th JSAP Autumn Meeting, 17P-A14-6, 2014.
[3] T. Takakura and S. Tsuzuki, “Evaluation of particle
removal efficiency of filters in high temperature sulfuric

acid using 30 nm liquid particle counter,” in Proceedingsof Joint eMDC & 2015 ISSM, 2015.
[4] M. Hashimoto and S. Tsuzuki, "Removal efficiency withactual chemical for wafer cleaning process," presented at
the 60th JSAP Spring Meeting, 28P-G8-14, 2013.
[5] T. Takakura and S. Tsuzuki, "Particle removal efficiency
evaluation of filters in 70 C diluted hydrochloric acid,"presented at the 63rd JSAP Spring Meeting, 19p-S223-10,
2016.
[6] T. Mizuno, et al., "Dispersion stability of standard
nanoparticles for filter performance evaluation," presented
at the 57th JSAP Spring Meeting, 20P-C-7, 2010.
[7] Jin-Goo Park, et al., "Interfacial and Electrokinetic
Characterization of IPA Solutions Related toSemiconductor Wafer Drying and Cleaning," J.
Electrochem. Soc., vol. 153, pp G811-G814, 2006.