Membrane | Technology

he scale of semiconductor de-vices is continuously shrink-ing. This increases the need to

control both the size and number ofparticles present in the manufacturingprocess. Filtration is of great impor-tance for controlling the particle level.One filter selection criterion is the re-moval rating, which claims the size ofparticles to be removed by the filter.The filter rating method used for semi-conductor device manufacturing is typ-ically performed in deionized water(DIW) at ambient temperature [1]. Inthe actual manufacturing process, how-ever, the filters are used in a variety ofchemicals and at various temperatures.

In such cases, it is empirically knownthat removal efficiency of the filters dif-fers from that in the standard DIW andambient temperature test conditions.Thus, it is important to know the actualparticle removal efficiency (PRE) in theactual chemical.

In the semiconductor cleaningprocess, high temperature sulfuric acidis a commonly used chemical, and thereare some studies on PRE evaluation offilters in the chemical. Previous studiesevaluated particles in high temperaturesulfuric acid using liquid particle coun-ters (LPCs) with measurement of parti-cles in the > 40 and > 60 nm ranges [2,3]. However, the measurement ranges

of these LPCs are coarser than the filterrating used for the leading-edge semi-conductor processes (< 20 nm). A finermeasurement range would be preferred.In this study, an evaluation of PRE of fil-ters in high temperature sulfuric acidusing a LPC with sensitivity of 30 nmwas conducted.

EXPERIMENTALTest system

The chemical recirculation line de-scribed in Figure 1 was used as the testsystem. In this line, 96% sulfuric acid(electronic grade) was recirculated. Thefiltration temperature was set to 90 °C,because this is a typical condition inthe single wafer cleaning system. Forthe challenge solution, aluminananoparticles (Sigma-Aldrich*, < 50nm) were dispersed in DIW and addedto the chemical bath using a meteringpump. The particles in the line weremeasured by the LPC through the sam-pling lines placed at the upstream andthe downstream of the test filter. ARION** KS-19F, which has sensitivityof 30 nm, was used as the LPC. Twodifferent measurement ranges, > 30 nmand > 40 nm, were employed in thismeasurement. One LPC was utilizedfor both upstream and downstreammeasurements performing the chal-lenge test twice. The first was fordownstream and the second for up-stream. As depicted in Figure 1, the sul-furic acid was recirculated, but singlepass was substantively realized for thechallenge particle due to the cleanupfilter set at the downstream side of the

Changes in Particle Removal Efficiency of PTFE Membrane Filters in High Temperature Sulfuric Acid

By Anthony Shucosky, V.P. Marketing; Tomoyuki Takakura, Scientist II and Shuichi Tsuzuki, Principal Scientist, Pall Corporation

T

Figure 1. Test line for particle removal efficiency evaluation of filters in high tem-perature sulfuric acid.

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test filter. Finally, the particle removalefficiency of the test filter was calcu-lated by the following expression:

PRE = 100 × (Countup - Countdown) /Countup, … (1) where Countup is theparticle count monitored at the upstreamline of the test filter and Countdown is theone at the downstream line.

Evaluation of particle removal efficiencyAfter the initial preparation noted,

five different kinds of polytetrafluo-roethylene (PTFE) membrane filterswere evaluated in 90 °C, 96% sulfuricacid. The PRE was calculated for twodifferent measurement ranges, > 30 nmand > 40 nm. Table 1 summarizes theoverall condition of this evaluation.First, one each of Filter A ~ E was eval-uated at 10 L/min. For the Filter A, PREin room temperature (RT) deionizedwater (DIW) at 15 L/min was also eval-uated with the same procedure after theevaluation in 90 °C, 96% sulfuric acid.

RESULTS AND DISCUSSIONEvaluation of the particle counter

Figure 2 shows particle count data ofthe LPC for various concentrations of thechallenge solution. Both > 30 nm- and >40 nm-count are shown. In the lower par-ticle concentration range, the particlecount of the LPC linearly increases alongwith the actual concentration. In thehigher concentration, however, the parti-cle count gradually deviates from the lin-ear relation. This tendency is moresignificant for the finer range. Considering

this result, the particle concentration ofless than 0.2 ppb was adopted for all thetests in this study.

Evaluation of particle removal efficiencyFigure 3. The results of PREs in 90 °C,

96% sulfuric acid measured at > 30 nmand > 40 nm ranges. At > 40 nm range, thePREs were 95 ~ 99%; the difference

among each filter is not significant. Incontrast, at > 30 nm range, the PREs de-creased to 83 ~ 94%, and the differenceamong each filter is more significant.These results indicate that the finer meas-urement range (i.e., > 30 nm) significantlyimproves the resolution of the measure-ment. Additionally, PRE for the Filter A inRT DIW shown in Figure 4 was greater

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Table 1. Summary of the PRE evaluation conditions Figure 2. The relationship between the challenge concentrationand the particle count. The particle counter is RION KS-19F;> 30 nm and > 40 nm-count are shown.

Membrane | Technology

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than 99.5% for both measurement ranges.This result is reasonable because the re-moval rating of Filter A is 12 nm. But themethod is not adequate for accurate eval-uation of the 12 nm-rate filter in RT DIW

in light of resolution.In contrast, the PRE deteriorates in 90

°C, 96% sulfuric acid compared to thestandard condition for the filter rating. Thefine range (> 30 nm) of the LPC can eval-

uate the filter performance even thoughthe range is coarser than the filter rating.There are several possible causes for thisdeterioration. In liquid filtration system,PRE is affected by interactions among

Figure 4. The results of PRE evaluation for the Filter A in RTDIW using the LPC. This test was performed after the test in Fig-ure 3. PRE was calculated in two different measurement ranges(i.e., > 30 nm and > 40 nm). The filter showed > 99.5% in PREfor both measurement ranges. The flow rate was 15 L/min.

Figure 3. The results of PRE evaluation in 90 °C sulfuric acidusing the LPC. PREs were calculated in two different meas-urement ranges (i.e., > 30 nm and > 40 nm) for each filter.One each of Filter A ~ E was evaluated. The flow rate was 10L/min for all the filters in this figure.

chemical, particles, and the filter mem-brane. Temperature dependence of the ex-pansion coefficient of the PTFEmembrane may also affect the stability andstructure of the membrane morphology.Further investigations will explore thesepossibilities and mitigation of the reducedPRE in high temperature sulfuric acid.

CONCLUSIONPRE evaluation of several kinds of

PTFE membrane filters in 90 °C, 96%sulfuric acid using a LPC with sensitivityof 30 nm was performed. In the chemical,the PREs of five PTFE filters were in therange of 95 ~ 99% for > 40 nm range and83 ~ 94% for > 30 nm range at 10 L/min.Thus, it is indicated that the finer meas-urement range (i.e., > 30 nm) signifi-cantly improves the resolution of themeasurement and highlights the per-formance differences of each filter. Afterthe evaluation, PRE in standard test con-ditions of RT DIW was conducted for onefilter (Filter A, removal rating: 12 nm).The filter showed PRE of > 99.5% forboth > 30 and > 40 nm ranges. Based onthis, the PRE decrease in 90 °C, 96% sul-

furic acid was demonstrated. The effectsof different flow rates on retention and onbath clean up speed were also conducted.Result s can be found in the completestudy listed below [4].

For questions or comments contact:Tony Shucosky, VP Marketing, Chemical and Membrane Products Pall MicroelectronicsTel: 1-410-561-8538Email: tony_shucosky@pall.com

REFERENCES[1] T. Mizuno et al., IEEE transactions on semicon-ductor manufacturing Vol. 22, No. 4, 2009, pp. 452-461.[2] T. Nagafuchi et al., Solid State Phenomena Vols.145-146, 2009, pp. 69-72.[3] T. Takakura et al., Proceedings of The 60th JSAPSpring Meeting, 2013, 28P-G8-14.[4] T. Takakura, et al., Joint Symposium 2015 eMDCand ISSM, “Evaluation of particle removal effi-ciency of filters in high temperature sulfuric acidusing 30 nm liquid particle counter,” September2015.

*RION is a registered trademark of Rion Co., Ltd. **Sigma-Aldrich is a registered trademark ofSigma-Aldrich Co. LLC.

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