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Materials Science and Engineering A 528 (2010) 698–705

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

oisture induced interface weakening in ACF package

i-Dong Sima, Chang-Kyu Chungb, Kyung-Wook Paikb, Soon-Bok Leea,∗

Department of Mechanical Engineering, KAIST, 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Republic of KoreaDepartment of Materials Science and Engineering, KAIST, 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

r t i c l e i n f o

rticle history:eceived 9 April 2010eceived in revised form5 September 2010ccepted 16 September 2010

eywords:nisotropic conductive film (ACF)

a b s t r a c t

One interesting degradation mechanism in flip–chip interconnections using anisotropic conductive film(ACF) is delamination between the adhesive and the sandwiched material. To understand the delamina-tion process, an attempt was made to quantitatively measure the interfacial fracture toughness of bothsilicon/ACF and flame retardant 4 (FR4)/ACF sandwiched specimens exposed to various humidity con-ditions using a 4-point bending test. For moisture absorption, specimens were stored inside a humiditychamber (85 ◦C, 85% RH) until they were fully saturated, and a pressure cooker test (PCT) was addition-ally performed to expose the specimen to harsher environmental conditions. Delamination occurrenceafter moisture absorption was checked by means of a scanning acoustic microscope (SAM). Based on

dhesionelaminationnvironmental degradationoisture absorption

the experimental results, we have determined the energy release rate of each interface under differ-ent humidity conditions and have located the weaker region where crack initiation would be likely tooccur under service conditions in interconnections using ACF. Evidence of crack propagation along theinterface was checked by cross section optical images and scanning electron microscopy (SEM). By com-paring the results of the specimens under different humidity conditions, interface weakening by moistureabsorption was observed and quantitatively analyzed. Interestingly, we found that ACF can show severedegradation of adhesion strength caused by moisture absorption without any delamination occurring.

. Introduction

Demand is increasing for integrated circuit (IC) packageshat are smaller and lighter with higher I/O and better elec-rical performance at lower cost. To satisfy these requirements,ip–chip interconnection using anisotropic conductive film (ACF)as become a popular option in electronic packaging due to its

ow bonding temperature, low cost capability (no flux and under-lling), fine pitch capability, and green process technology [1,2].

nterconnection technologies using ACFs are already major packag-ng methods for flat panel display modules [3]. Consequently, thelectrical stability and reliability of ACF is emerging as an impor-ant issue [4], and the mechanisms of degradation are an importantrea of research. One interesting degradation mechanism is delam-nation between the adhesive and the sandwiched material duringperation (Fig. 1(a)). Therefore, interface adhesion is becoming aore relevant parameter for the reliability of electronic packages.

A previous study on chip-on-board (COB) packages used scan-

ing acoustic microscopy and observed delamination beginningrom the edge of the silicon die. The results showed that thehip/ACF interface is relatively more prone to delamination than

∗ Corresponding author. Tel.: +82 42 350 3029; fax: +82 42 350 5028.E-mail address: sblee@kaist.ac.kr (S.-B. Lee).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.09.051

© 2010 Elsevier B.V. All rights reserved.

the substrate/ACF interface [5]. However, despite the significantrole of ACF properties in reliability assessment, the intrinsic adhe-sion properties of ACF are still not clear. Most of the research on theadhesion properties of ACF has been restricted to the full packagedstate [6]. As these studies include the effects of the metal, bumps,and terminals, the adhesion strength obtained may be affected bythe structural shape and components of the package. An importantissue is how to measure the adhesion properties of ACF, disregard-ing the effects of fillets, metal, bumps, and structure shape with arather simple and precise experiment.

Another important factor in assessing the reliability of ACF pack-ages is the environmental effect. In fact, shear failure in thermalcycling testing and delamination failure in moisture absorptiontesting are the two major reliability problems with chip-on-boardpackages using ACF flip–chip interconnection [7,8]. Both failureconditions can be analyzed as a bending mode due to coefficientof thermal expansion (CTE) or coefficient of moisture expansion(CME) mismatch between the IC chip and the substrate (Fig. 1(b)).Therefore, it is critical to quantitatively evaluate interface delam-ination in the electronic structure which has been exposed to

high temperature or has absorbed moisture. In this study, wefocus on the influence of moisture absorption on the adhesionproperty of ACF. Moisture penetration into the flip–chip pack-age is known to cause critical effects on the package reliability.Moisture induced swelling and popcorn cracking are well known

G.-D. Sim et al. / Materials Science and Engineering A 528 (2010) 698–705 699

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ig. 1. (a) Delamination along the bimaterial interface, and (b) bending mode at lowemperature.

henomena that may cause degradation of the package perfor-ance, and interface delamination could occur during operation

9]. Therefore, precise information about moisture-induced inter-ace weakening is very important. As polymeric materials andnorganic materials have different values of CME, previous studiesave analyzed CME-mismatch-induced stress and strain problemsuch as CTE-mismatch problems [10]. These results are very helpfuln evaluating moisture-induced failure of plastic integrated circuitIC) packages. However, in this type of package structure, the inter-ace weakening effect directly caused by the properties of the ACFtself is hard to abstract. Therefore, experimental analysis of the

oisture absorption effect on the intrinsic adhesion properties ofCF may provide important insights into the degradation of ACFackages.

Of the various adhesion test methods which can be utilized toeasure adhesion force or energy, delamination of a sandwiched

pecimen has been noted as the most useful for the initial investi-ation of fracture energy [11]. Moreover, other traditional methodshat are frequently used in the electronic packaging field are usually

Fig. 3. Schematic illustration of t

Fig. 2. Schematic illustration of flip chip interconnection using ACF.

qualitative and dependent on residual stresses, whereas four-pointbending test results are quantitative and reproducible [12]. Anotherimportant advantage of this method is the fact that after specimenfabrication and during debonding the film is still bonded to one sideof the substrate. Therefore, residual stress stored in the film is notrelieved and does not affect the energy release rate. Finally, becauseused two separate specimens for study, namely, silicon/ACF andFR4/ACF, strain induced by CTE or CME mismatch was not expected.Consequently, these advantages support our experiment methodto investigate the intrinsic adhesion properties of ACF with varioussubstrates and humidity conditions.

In this study, the adhesion strength of ACF was produced andevaluated precisely and was evaluated quantitatively for both sil-icon/ACF and FR4/ACF structures by applying a novel four-pointbending test to flip–chip interconnection. Furthermore, the mecha-nism of moisture induced interface weakening will be elucidated bycomparison of the results from differently humidified specimens.

2. Experimental methods

2.1. Materials preparation

Anisotropic conductive film (ACF) is an adhesive betweenelectronic components which consist of an epoxy-based poly-mer matrix and conductive fillers for electrical conduction andmechanical interconnection. A schematic diagram of a flip–chip

he ACF bonding procedure.

7 and Engineering A 528 (2010) 698–705

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00 G.-D. Sim et al. / Materials Science

nterconnection using ACF is shown in Fig. 2. The adhesive resinsed for the ACFs was an epoxy-based formulation which washosen due to its good adhesion to various substrates, high glassransition temperature, and favorable melt viscosity required forhe thermo-compression bonding process. Five micron-diameter

etal-coated polymer balls were used as conductive particles inhe ACF resin, and the thickness of the ACF was 35 �m. Silicon andR4 were the materials used as the substrate. In COB packages, Siepresents integrated circuits, while FR4 represents printed circuitoard. The upper and lower substrates were 30 mm long, 3 mmide, and 1 mm thick for both Si and FR4. Especially for the upper

nd lower Si substrate, the polished sides were placed to face theCF in order to mimic real flip–chip assemblies and to measurerecise adhesion strengths.

For the specimen fabrication, a thermo-compression bondingrocedure was adopted [13]. A schematic illustration of the bond-

ng process is shown in Fig. 3. First, the lower part was pre-bondedith ACF at 80 ◦C for 3 s. Next, the upper part and the lower partere aligned over the ACF bonding plate. Finally, the whole struc-

ure was bonded at 190 ◦C and 50 N for 20 s. As the upper and lowerubstrates were identical for both specimens, shear strain inducedy CTE mismatch was not observed. Therefore, shear stress inducedracks at the edge can be disregarded, and the fabrication processoes not affect the loading phase angle. Also, we can assume thathe adhesion of the ACF is homogenous over the whole length.nlike the whole flip–chip package shown in Fig. 2, we excluded

he metal bump and fillet to remove their effects on adhesion.

.2. Environment conditions

.2.1. Moisture absorptionCured sandwiched specimens were stored in the humidity

hamber (Fig. 4) at 85 ◦C and 85% relative humidity (RH) until theyere fully saturated. The saturation state was checked by measur-

ng the weight of the specimen from time to time as shown in Fig. 5.nlike the moisture diffusion of the FR4 specimen, which is Fick-

an, the moisture diffusion of the Si specimen has a stepped weight

ncrement. This is because Si itself does not absorb moisture. That is,he weight increase inside the Si specimen totally occurs in the ACF.s the weight increment was smaller than the measurement reso-

ution, a clear Fickian curve was not possible in this case. However,s our object was to check the saturation time for both specimens,

Fig. 5. Weight measurement du

Fig. 4. Humidity chamber for moisture absorption.

this test result seems adequate. Based on our result, the Si and FR4specimens stored longer than 300 h were subject to the four-pointbending test.

2.2.2. Pressure cooker test (PCT)In order to investigate the moisture-related reliability of ACF

flip–chip assemblies, a PCT was also performed. The PCT was con-ducted at 121 ◦C and 100% relative humidity (RH) at 2 atm. Thespecimens were exposed to these environment conditions for 99 h.

By comparing the moisture absorbed specimen and the PCT spec-imen, we gained further understanding about the degradationmechanism induced by moisture absorption.

ring moisture absorption.

G.-D. Sim et al. / Materials Science and Engineering A 528 (2010) 698–705 701

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Table 1Specification of the materials used in this study.

Thickness (mm) Young’s modulus (GPa) Poisson’s ratio

ACF 0.035 1.2 0.35Si 1 130 0.27FR4 1 17.2 0.15

Table 2Test conditions for experiment.

Dimension (mm)

ig. 6. Four-point bending setup of the sandwiched specimen: (a) illustration andb) during real experiment.

.3. Experiment procedure

.3.1. Four-point bending testThe four-point bending adhesion test was carried out using a

tandard sandwiched structure, as shown in Fig. 6. For crack initia-ion, a notch was cut into both the Si and FR4 samples. To minimizehe effect of the glass fiber existing inside the FR4 substrate, which

Fig. 7. Load–displacement curve for various displacement rate condi

Length 30Width 3Pin spacing 5.5

cause fluctuations in the load–displacement curve, a notch depth of930 �m was chosen. The experiment was performed in a displace-ment control mode with an optimized displacement rate for eachspecimen. Further explanation will be given in the next section.

By applying beam theory, the strain energy release rate can becalculated as in [14] as

Gc = 21(1 − �2)Mc2

4Eb2h3= 21(1 − �2)Pc

2l2

16Eb2h3(1)

where Mc = Pcl/2 represents the critical bending moment, Pc is theplateau load, l is the spacing between the inner and outer loadinglines, b is the sample width, h is the specimen height, and E and � areYoung’s modulus and Poisson’s ratio of the substrate, respectively.Properties of the materials analyzed in our experiment and the test

conditions are shown in Table 1 [15,16] and Table 2, respectively.The equation is based on a homogeneous solution which is reliablewhen the thickness of the layer is a hundredth or even a thousandthof the length scale of the overall geometry [17].

tions: (a) 0.2 �m/s, (b) 0.3 �m/s (c) 0.4 �m/s and (d) 0.5 �m/s.

702 G.-D. Sim et al. / Materials Science and Engineering A 528 (2010) 698–705

Table 3Optimization of the displacement rate.

0.1 �m/s 0.2 �m/s 0.3 �m/s 0.4 �m/s 0.5 �m/s

Experiment Time X X O O O

X

2

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3

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beyond the position of failure. From this evidence, we conclude that

TI

Reliable Test results O O O O X

: unacceptable, O: acceptable.

.3.2. Optimization of displacement rate for humidified specimensFor specimens stored at room temperature and room humid-

ty conditions, the experiment was performed in a displacementontrol mode with a rate of 0.1 �m/s. However, it was necessaryo change the displacement rate for humidified specimens becauserevious researchers have pointed out that experiments should beone before moisture spreads out in laboratory humidity condi-ion [18]. As an example, a 30-min standard has been introduced.n our case, the Si specimen coped well with the time limit with aisplacement rate of 0.1 �m/s. However, as the elastic modulus ofR4 is 10 times less than that of Si, the experiment time exceededhe standard in the case of FR4. Therefore, we decided to increasehe displacement rate for the FR4 specimen to match up with thexperiment time limit.

To optimize the displacement rate, less experiment timeith reliable, reproducible test results were considered. The

oad–displacement curves for various rate conditions are shownn Fig. 7. Especially for few test results at 0.5 �m/s, it was hard tobserve a load plateau region. Suitability of the rate for the moisturebsorbed specimens is summarized in Table 3. Between 0.3 �m/snd 0.4 �m/s, we chose 0.3 �m/s as the optimized displacementate for the FR4 specimen.

. Results and discussion

.1. Crack initiation and propagation

As the bending moment increases, a crack initiates from theotched tensile side surface and propagates vertically to the

nterface. When the crack meets the interface, if the interface isufficiently weak, the crack deflects into the interface and propa-ates along the interface. Fig. 8 shows a crack propagated betweenhe ACF and the substrate taken by scanning electron microscopySEM). The crack initiated from the notch and propagated towardshe interface. At the moment crack approached the interface,ebonding, rather than penetration, occurred. By observing the

oad–displacement curve, the plateau load is determined, and fromq. (1), the energy release rate is calculated.

The plateau loads for both the silicon and FR4 specimensere chosen as shown in Fig. 9. At the beginning of the

oad–displacement curve, load increases drawing an ambiguousurve. This is due to the roller on the upper part of the tester whichnables the pin to adjust its position at the initial state. After adjust-

ent, the first elastic loading region appears which is shown more

learly for the FR4 specimen due to its larger span. When the loadeaches a certain value, debonding initiates, and the load drops untilteady crack growth occurs at the plateau load.

able 4nterface fracture energy and stress intensity factors for various environment conditions.

Si/ACF interface

Room 85 ◦C/85%

Interface fracture energy G (J/m2) 2.16 1.26KI (MPa m1/2) 0.42 0.319KII (MPa m1/2) 0.36 0.276Relative strength (%) 100 58.33

Fig. 8. Crack path shown by cross sectional view with scanning electron microscopy(SEM) for (a) Si specimen and (b) FR4 specimen. In case of silicon specimen, brittlefracture occurred as the crack propagated toward the loading pin.

During the experiment, distinguishing features were observedfor each substrate. First, for the Si specimen (Fig. 9(a)), load dropwas hardly seen after the first elastic loading region. This couldlead to doubt about the chosen plateau load. Therefore, we verifiedthe plateau load by performing additional experiments. When ourpresumed plateau load region ended and the second elastic load-ing region started, we stopped the experiment and looked throughthe cross section of the specimen through an optical microscope(Fig. 10). In the figure, crack initiation at the notch and propagationalong the interface were both observed. This is supported by thepreviously mentioned SEM image (Fig. 8(a)) in which the Si fail-ure was observed. Although the Si substrate failed due to the highload at the second elastic loading, the crack was observed to extend

the presumed plateau load is reasonable. The missing load drop isexplained by the brittleness of Si and the deep notch. Also, for theFR4 specimen, continuous load fluctuation was observed before itreached the plateau load. This is due to the glass fiber comprising

FR4/ACF interface

PCT 99 h Room 85 ◦C/85% PCT 99 h

1.25 414.7 350.68 314.710.318 2.04 1.88 1.780.275 1.77 1.63 1.54

57.87 100 84.56 75.89

G.-D. Sim et al. / Materials Science and E

Fig. 9. Load–displacement curve during the four-point bending test of the (a) Sispecimen and (b) FR4 specimen.

Fig. 10. Crack initiation and propagation shown by optical microscope for un-failedSi/ACF specimen.

ngineering A 528 (2010) 698–705 703

the FR4, which can be seen in Fig. 8(b). For the crack initiated at thenotch to reach the interface, it penetrates through the glass fibers,and this may affect the load–displacement curve. However, thisdid not affect the plateau load for the FR4 specimen, and it showedreliable and reproducible results.

3.2. Interfacial fracture toughness

The energy release rate from (1) originates from the superposi-tion of pure mode I and mode II. The energy release rate for eachcase of pure mode I and mode II are calculated as in [19] as

GI = 3(1 − �2)Pc2l2

4Eb2h3(2)

GII = 9(1 − �2)Pc2l2

16Eb2h3(3)

The relation between energy release rate G and the stress inten-sity factor K in a plain strain condition is known as Irwin’s universalrelation in linear elastic fracture mechanics [20], which is given as

G = (1 − �2)K2

E(4)

The total stress intensity factor Ki can be calculated, takingaccount of the effect of mode I and mode II, as

Ki =√KI

2 + KII2 (5)

The critical energy release rate calculated from Eq. (1) can beapplied to represent the adhesion strength of ACF at differentinterfaces. Energy release rates and corresponding stress intensityfactors are shown in Table 4. Experimental results showed that theFR4/ACF interface is a hundred times tougher than the Si/ACF inter-face. This result agrees well with the experimental result for the fullpackage structure [5]. This is because ACF and FR4 are both epoxy-based materials, and chemical bonding may occur between thesematerials during thermo-compression bonding.

However, as energy release rate G is a function of the phaseangle , these results represents the fracture toughness under sucha condition. The phase angle for the four-point bending specimenis given as

= tan−1(KII

KI

)≈ 41◦ (6)

The local phase angle is known as

= ∞ +ω + ε ln(l

h

)(7)

where ω is an angle that depends on the elastic properties of thefilms and the substrates, and ε is the bimaterial constant defined asin [21] as

ε = 12�

ln

[(3 − 4�1)/�1 + 1/�2

(3 − 4�2)/�2 + 1/�1

](8)

However, as the elastic mismatch is not too large for most cases, itis common to specify the phase angle as the remote phase angle.In electronic packaging, due to mismatches of CTE or CME, warp-ing occurs during operation of the electronic structure [22], whichshows a mixed mode loading condition. Therefore, the mixed modecondition handled in our experiment by bending is reasonably com-parable to a real service load.

As a fracture criterion, delamination of each interface occurswhen the energy release rate at the service load exceeds their inter-face toughness Gc:

G( ) ≥ Gc( ) (9)

704 G.-D. Sim et al. / Materials Science and Engineering A 528 (2010) 698–705

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ig. 11. SAM images before moisture absorption for (a) Si and (b) FR4 specimen.

.3. Moisture absorption effect

Two major reliability problems should be discussed in consid-ring the reliability of moisturized specimens. The first problem ishe adhesion strength weakening caused by the hygroswelling ofCF. The second problem is the vaporization effect and delamina-

ion caused by vapor pressure. Our current research focuses on theffect of moisture on the intrinsic adhesion properties of ACF.

Most of the currently produced ACFs generally consist of ahermosetting epoxy with minor thermoplastic polymers. Ther-

oplastic polymers are known to have porous chains. Moisturean work its way between these polymer chains [23]; therefore,welling may occur inside the polymer resin. As shown in Table 4,he adhesion property of ACF seemed to weaken by hygroswellinguring moisture absorption. This effect was remarkable for thei specimen. Compared to the Si specimen, the FR4/ACF interfacedhesion strength was observed to be much more tolerable.

In the previous section, we observed that the interface fractureoughness of the FR4/ACF interface is much greater than that of thei/ACF interface because ACF and FR4 are both epoxy-based mate-ials. Similarly, in the moisture absorbed specimen, hygroswellingtress in the FR4 package was not strong enough to intercept cross-inked chains [24] formed between ACF and FR4. On the other hand,he Si/ACF interface was observed to be prone to hygroswelling.ur experimental results suggest that moisture uptake can bettributed to the degradation of the intrinsic adhesion propertiesf the polymer resin inside ACF.

To check whether delamination occurs during hygroswelling,canning acoustic microscopy (SAM) images were taken for bothpecimens before (Fig. 11) and after moisture absorption (Fig. 12).eflection mode was applied to the silicon specimens, which showelamination in bright images. On the other hand, as FR4 consistf glass fiber, diffraction occurred for the FR4 sandwiched speci-en. To solve this problem, transmission mode was applied to the

pecimen, where the black area indicates the occurrence of delam-nation. Due to the glass fiber effect, we were not able to get clearmages for the FR4 specimens; however, compared with the origi-al state without any moisture absorption, the FR4 images showedo significant difference. Through discussion, we concluded thatelamination did not occur after moisture absorption for both thei and FR4 specimens.

Consequently, the SAM images indicate that swelling is not theain factor for delamination. Therefore, we may conclude that

oisture absorption induced hygroswelling, without delamination

ccurrence, contributes to the degradation of the adhesion strengthf ACF.

ig. 12. SAM images after moisture absorption for (a) Si and (b) FR4 specimen.

Fig. 13. SAM images after PCT for (a) Si and (b) FR4 specimen.

3.4. Harsh environment effect

A PCT test was additionally performed with the saturated spec-imens to expose them to harsher environment conditions. Ourexpectation before the experiment was to observe popcorn crack-ing at the Si/ACF or FR4/ACF interface due to high temperatureinside the chamber. However, unlike the full-package failure causedby delamination at the interfaces during the PCT [25], no delami-nation occurred for our type of sample (Fig. 13). This difference isdue to the specific geometry of our specimens in which the ACFis sandwiched by a material which shows negligible CTE or CMEmismatch-induced stress in the full package state. Therefore, wecan conclude that delamination under the harsh environment ismainly caused by shear stress rather than direct vaporization. Onemore interesting result is the degradation of the FR4/ACF adhe-sion strength during the PCT. Compared to the Si/ACF interface,which was already severely damaged after moisture absorption, theFR4/ACF interface showed weakening of adhesion strength duringthe PCT. We believe this is caused by high temperature and highpressure, which force moisture penetration and interferes with thebonding between the FR4 and ACF materials.

Further discussions may be required to understand the pre-cise mechanism. However, based on our experiment results, weconclude that moisture absorption during service load of an ACFpackage may cause a severe degradation in adhesion strength evenwhen delamination is not coevolved.

4. Conclusion

A novel experimental method using a four-point bending testwas introduced and verified to quantitatively evaluate the interfacetoughness of flip chip interconnections using ACF. Si and FR4, whichare important component materials in electronic packaging, wereselected as the substrate for sandwiched specimens. By observingthe load–displacement curve, the plateau load was determined, andthe interfacial toughness of each specimen was calculated quan-titatively based on linear elastic fracture mechanics. Experimentalresults showed that Si/ACF interface is much weaker than FR4/ACF;therefore, it is the crucial region where delamination might occurduring the service load. The intrinsic adhesion value of ACF wasmeasured under various humidity conditions to analyze interfaceweakening due to moisture absorption. Displacement rates wereoptimized to obtain reliable and reproducible results for both Siand FR4 specimens. Cross sectional images were taken to check forevidence of crack propagation through the interface. Additionally,we took SAM images to observe whether delamination occurredinside the sandwiched specimens.

Based on our experiment results, we concluded that stress fromCTE or CME mismatch would be the main cause for delaminationrather than popcorn cracking from vaporization or hygroswellingfrom moisture absorption. However, more interestingly, we foundthat moisture absorption itself without any delamination could

degrade the intrinsic adhesion property of ACF material and induceinterface weakening. These results provide useful insights into thefailure mechanism of the full package state structure and can serveas failure criteria for FEM analysis.

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G.-D. Sim et al. / Materials Science

cknowledgements

This research was supported by the National Research Founda-ion of Korea (NRF) grant funded by the Korea government (MEST)No. 2010-0026884), and by a grant(2010K000197) from Center foranoscale Mechatronics & Manufacturing, one of the 21st Centuryrontier Research Programs, which are supported by Ministry ofducation, Science and Technology, KOREA.

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