Joint Symposium 2017 – eMDC & ISSM
IDJo in t

Sym p o siu m201 7

Particle removal efficiency evaluation of filters in high temperature IPA
Tomoyuki Takakura, Shusaku Daikoku, and Shuichi Tsuzuki
tomoyuki_takakura@ap.pall.com / shusaku_daikoku@ap.pall.com / shuichi_tsuzuki@ap.pall.com
Nihon Pall Ltd.
46, Kasuminosato, Ami-machi, Inashiki-gun, Ibaraki 300-0315 Japan
Phone: +81-80-1274-0578 Fax: +81-29-889-1957
Keywords: filtration, isopropyl alcohol (IPA), particle removal efficiency (PRE)varies with each user’s condition. However, since theboiling point of IPA is 82 C, temperature of 70 °C orlower is adopted in most cases.

INTRODUCTION
The need of isopropyl alcohol (IPA) for wafer drying has
been increasing in the semiconductor industry, because thestructure of semiconductor devices is getting more
complicated and IPA drying is more preferable to avoid
pattern disruption of complicated device structure. Also, as
the feature size of the semiconductor devices continuouslydecreasing, the cleanliness level of IPA needs to be further
improved. Filtration is an indispensable technology tocontrol the cleanliness, and more and more filters with finer
removal rating are now being used to achieve the requiredcleanliness level.

For filter’s PRE evaluation, we need to choose test particle
to be challenged to test filters. In the previous PRE
evaluation in RT IPA [6], we used the following three kindsof test particles: Platinum particle covered with
polyethylenimine (Pt PEI), zirconia particle (ZrO2), andgold particle covered with polyvinylpyrrolidone (Au PVP).
These particles have each different charged state; Pt PEI is
positively charged, Au PVP is negatively charged, andZrO2 is nearly neutral. Jin-Goo Park et al. [7] conductedzeta potential measurement of silica and silicon particles inwater and water/IPA mixture, and reported that zeta
potential of the particles approached neutral values as theconcentration of IPA reached 50 volume%. In view of thesefacts, we assumed that ZrO2, silica, and silicon particles
have similar charged stated in 100% IPA. Since silica andsilicon particles are presumed to be ones of actual
contaminants during wafer cleaning process, we decided tofocus on the test using ZrO2 that shows similar behavior
with silica and silicon particles.

Filter’s removal rating is determined by particle removal
efficiency (PRE) under a standard condition, which istypically room temperature (RT) water [1]. In general,
however, the actual PRE varies depending on the chemicaland temperature to be used. Therefore, understanding thePRE in the actual chemicals is important as well as
selecting an appropriate removal rating for effective usageof filters. For this reason, we have developed PRE
evaluation methods in several chemicals which arecommonly used in the semiconductor manufacturing
processes [2~6]. Regarding PRE in IPA, we evaluated Pall
10-nm-rated surface-modified polytetrafluoroethylene(SM-PTFE) membrane filter using three kinds of test
particles with their size around 10 nm at RT, and found thatthe filter shows sufficiently high PRE in the chemical. On
the other hand, IPA is often used at elevated temperaturewhen used in the actual processes. Therefore, in this study,
we developed a method that can evaluate PRE in hightemperature IPA in order to investigate the influence oftemperature on filter’s PRE. The test method was applied toevaluate two kinds of Pall SM-PTFE filters with eachdifferent removal rating, and the difference in actual PRE
between the filters in RT and high temperature IPA wasdiscussed.

Previously, the particle size distribution in IPA wasmeasured by means of dynamic light scattering (DLS,Zetasizer* Nano ZS, Malvern*) at RT. In this time, theparticle needs to be dispersed in 70 C IPA as well. In orderto confirm the fact, the ZrO2 added to IPA (the suspendingsolution) in a PFA bottle was heated up to 70 C and kept
for 20 minutes; then, the solution was cooled down to RT
and measured by DLS. We have assumed that if the particle
is dispersed at RT after heating, it was dispersed during theheating as well. Another solution without heating was also
prepared and measured for comparison. Although themeasurement tool (DLS) is capable of measuring theparticle size at 70 C, we adopted the procedure describedabove, because measurement at high temperature needs toconsider the temperature dependence of other parameterssuch as refractive index of the ZrO2 and IPA, which makes
direct data comparison between at RT and at hightemperature somewhat difficult.

EXPERIMENTAL
Particle size distribution measurement in IPA (beforeand after heating)Particle challenge test of filters in RT and 70 C IPA

The heating temperature of IPA was decided to be 70 C. In
the actual manufacturing process, the temperature of IPA-In order to evaluate PRE in RT and 70 C IPA, particle
challenge tests of filters were performed using the test1-Testcondition
No.
12
3system shown in Figure 1. First, the test particle (ZrO2) wasadded to electronic grade IPA to make suspending solution
and it was poured in the reservoir (i.e. at the upstream ofthe disk holder). For the test in 70 C IPA, the reservoir wasplaced in the water bath at 70 C for 10 minutes to stabilize
the designated temperature. For the test in RT IPA, thesame filtration system was employed except for not using
the water bath. Next, the suspending solution (influent) wasfiltered by each filter at the flow rate of 5 ml/min., and theeffluent was collected in a sampling bottle. Finally, thezirconium (Zr) concentration in the influent (= C0) and theeffluent (= C) was quantified with ICP-MS (7700s, Agilent)to calculate PRE (= [1-C/C0] × 100).

4Temperature

1010
5RT70 CRT

570 °C

ChemicalIPA

RESULTS AND DISCUSSION
Particle size distribution measurement in IPA (beforeand after heating)
As shown in Figure 2, the ZrO2 particle used for thechallenge test showed almost no change in the size
distribution even after heating up to 70 C in IPA, and thus
the particle in 70 C IPA was presumed to be dispersed
same as in RT IPA. These data ensure that we can directlycompare the PRE at RT and 70 C in IPA.

Test filters were Pall 10-nm-rated and 5-nm-rated
SM-PTFE membranes, both of which are filters used in theleading-edge
semiconductor
devicemanufacturing
processes. Table 1 summarizes the overall conditions of thisevaluation. Test was repeated five times. Note that the test
condition No.1 in the table 1 is the same condition as in theprevious study [6].
Challenge solution(Particle in IPA)

Filter rating(nm)

Before heating
After heatingRegulator

20Compressed air

Samplingbottle

Volume (%)

Heater47 mm  diskholder
integrated withreservoir

15105Water bathElectronic balance

0Figure 1. Schematic of particle challenge test system for
PRE evaluation of filters in RT and 70 C IPA. Test filters
were 47 mm  disk format. The water bath was only used
for the tests at 70 C to keep the elevated temperature.
Constant flow rate (5 ml/min.) was implemented byadjusting the pressure regulator.

110100Size (nm)Figure 2. Particle size distribution (in volume) of ZrO2 inIPA. This particle was used for the filter’s challenge test inIPA. The dotted black line indicates the distribution before
heating, and the red line indicates the distribution after
heating up to 70 C.

Table 1. Conditions of the particle challenge tests of theSM-PTFE filters.

Particle challenge test of filters in RT and 70 C IPA
Figure 3 shows results of the challenge tests in IPA. In RT
IPA, both of the 10-nm-rated and 5-nm-rated filters showedsufficiently high PRE (> 99%). In contrast, PRE of the10-nm-rated filter slightly deteriorated at 70 C comparedto that at RT, whereas the 5-nm-rated filter maintained highefficiency at 70 C. These results indicate the filter removal
rating determined based on the test in RT water is reflectedon the PRE in 70 C IPA.

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REFERENCES

In the past study, we have reported that PRE of a 12nm-rated SM-PTFE filter decreased in 90 C 96% sulfuricacid (H2SO4) [3], whereas PRE of a 10 nm-rated SM-PTFE
didn’t decrease in 70 C diluted (pH1) hydrochloric acid(HCl) [5]. Considering these results including the current
study, filters’ PRE tends to deteriorate in high temperaturechemicals, though the degree of decrease depends on thechemical and temperature employed.

[1] T. Mizuno, et al., "A Novel Filter Rating Method
Using Less Than 30-nm Gold Nanoparticle andProtective
Ligand,"IEEEtransactions
onsemiconductor 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.

10 nm-rated SM-PTFE filter
5 nm-rated SM-PTFE filter[3] T. Takakura and S. Tsuzuki, “Evaluation of particle
removal efficiency of filters in high temperature
sulfuric acid using 30 nm liquid particle counter,” inProceedings of Joint eMDC & 2015 ISSM, 2015.

100%
Particle Removal Efficiency (%)

90%
80%

[4] M. Hashimoto and S. Tsuzuki, "Removal efficiency
with actual chemical for wafer cleaning process,"
presented at the 60th JSAP Spring Meeting,
28P-G8-14, 2013.

70%
60%
50%

[5] T. Takakura and S. Tsuzuki, "Particle removal
efficiency evaluation of filters in 70 C dilutedhydrochloric acid," presented at the 63rd JSAP Spring
Meeting, 19p-S223-10, 2016.

40%
30%
20%

[6] T. Takakura and S. Tsuzuki, “Particle removal
efficiency evaluation of filters in IPA,” in Proceedingsof 2016 ISSM, 2016.

10%
0%
RT IPA[7]

70 70
CCIPA

Figure 3. Results of the particle (ZrO2) challenge tests in
RT and 70 C IPA. PRE of each test condition is shown.
For each condition, tests were repeated five times and theaverage is shown with the error bar (standard deviation).

* Malvern and Zetasizer are trademarks of Malvern
Instruments Ltd.

CONCLUSION
A PRE evaluation method of filters in high temperature IPA
was developed. The evaluation was performed for Pall
10-nm-rated and 5-nm-rated SM-PTFE membrane filterswith ZrO2 particles, the size of which is around 10 nm bothin RT and 70 C IPA. In RT IPA, both of the 10-nm-rated
and 5-nm-rated filters showed sufficiently high PRE (>99%). In contrast, PRE of the 10-nm-rated filter slightly
deteriorated at 70 C compared to that at RT, whereas the5-nm-rated filter maintained high efficiency at 70 C. Theseresults indicate the filter removal rating determined basedon the test in RT water is reflected on the PRE in 70 C IPA.
Considering the PRE evaluation results in the past [3, 5]
and the current study, filters’ PRE tends to deteriorate inhigh temperature chemicals, though the degree of decreasedepends on the chemical and temperature employed.

-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.

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