Jpn. J. Appl. Phys. Vol. rill tltlttll pp. 238 |—23tlr:
`Part 1. No. 4A. April Illltl
`Cc).‘![l01 The Japan Society offitpplicd Physics
`
`Thermal Stability of Liquid Crystal Alignment Layers
`
`Prepared by In—Sr'tu Ultra-Violet Exposure during lmidization of Polyimide
`
`Jae-Hoon KIM‘, Bharat R. ACHRKRYAI, Déna Mae AGRA' and Satycndra KIJMA It‘
`.‘)e'_imr'rnn=trt ct} i"’it_t-'.s't"c'.r. ffcriuittrn U.=rit*r'r.tv't_\«'. (?tum‘.’tnri, Kcmgm;-it-Do ."t’.li'l—?l'J.'.’. Korea
`'Dt‘_nut'mtL=tit nf."’h_it.r."¢'.r. Kent State lt'm'tic.I'.t'r't,1'. Kcttf. Ohio #4242. USA
`(Received Novcinbcr 2|], 2000: accepted for publication Docetnbcr 25, 2000)
`
`A novel method for liquid crystal alignment using in-.\v‘tt.« exposure to linearly polarized ultru—violct (LPUV) light during
`imidization oi‘ polyin-tide has been devised. The alignment layers prepared by this method exhibit higher thermal stability
`than the conventional method that crnploys l.l'-‘UV exposure ofter iniidization. Mulli-domain cells can be easily fabricated with
`the use uftt photo mask and mulli—stcp it:-silt: LPUV exposure during hard bake. With this metliod, it is also possible to gcticrzttc
`pretilt angle using two-step LFUV oxposttrc during irnidizulion.
`KEYWORDS:
`liquid crystals. alignment layer. UV exposure
`
`I.
`
`Introduction
`
`Tcchnologically, how to achieve liquid crystal alignment
`on substrates is crucial to have a reliable procedure that per-
`mits good control ofaligntncnt and yields high-quality align-
`ment ofliquid crystal (I.C") used in clcctro-optic dcviccs. Sur-
`face lrcntrrtcttts, such as, obliquely evaporated Sit),
`layers,
`Langmuir-Blodgctt Iilms, rubbed polymer films have been
`used to obtain liomogcncous alignment of Ills.” Among
`them, mechuniczztl rubbing ofpolyimidc (Pl) layers is the most"
`common alignment method used in mass production of LC.‘
`displays because of its simplicity and high thermal stability
`of the resultant alignment. The disadvantages of the rub-
`bing method are the generation of dust particles, electm-static
`charge, and physical damage which are dctritncntal to the fab-
`rication ofthin film transistor based devices.
`
`In recent years, photo—ulignmcnt has emerged as at promis-
`ing non-contact
`technique because of its simplicity and
`easy control of the alignment direction and anchoring en-
`ergy, so that tnulti-domain dcviccs, with improved view-
`ing angle characteristics.
`It has been demonstrated that
`polytvinyl}-l-tncdioxycinnamatc and poly(vinyl)cinn:1mutc
`films. when atnisotropically et'oss«|inked using linearly polar-
`ized ultra-violet (LPUV) light. can be very effective as align-
`ment laycr.3"' However, their performance dctcriotmcs with
`time.
`
`More recently. several research groups have reported align-
`ment ol‘ l.C's by Pl films exposed to the LPUV light.“"°’
`Fourier trztnsforrn infrared (FTIR} spectroscopy has shown
`that the UV irradiation anisotropically pltoto-dissociates pho-
`tosensitive chemicnl bonds in P1 including those in the imidc
`ring.”” This reduces the polarizability of P] molcculcsl"
`and changes the surface mot-pltology.“’ However, under this
`method the LPUV exposure is carried out after the irnidiza-
`tion oflhc film is complete. The Pla|ignmcntla_ve1's prepared
`by this method possess poor thorlnal and chemical stability.
`lividcntly, many researches are attetnpting to develop ncw
`non-cotttact alignment method fbrproducing stable alignment
`layer.
`In order to improve the thermal stability of prepared
`alignment layer, wc proposed new non-contact UV alignment
`method which employs LPUV exposure during the imidiza—
`tion ot'Pl. 1 "
`
`In this paper, we report detail experimental results about
`the thermal stability of the alignment layers prepared by the
`new method. The. alignment layers prepared by this mctltod
`has much higher thermal stability than of the alignment layers
`prepared by the conventional [JV method for PI Iilms. And
`also, we report the pretilt angle generation using two—stcp
`t'n-sits.» LPLJV exposure during hard bake.
`
`2. Experimental
`
`We tested the in-Sim method using several Pls from var-
`ious chemical companies. The main Pls used in this Study
`are the Pl S1-L610, 7311, and 7511 (Nissan Chemical Co.).
`And the ncmatic LC E48 (British Drug House} is used to
`make LC cells. P] lilms were prepared by hcnt curing of
`precursor polyamic acid (FAA) solutions which was synthe-
`sized from the rcuction bctwccn l:Cl.l'i.l.CEtl“l‘)0KyllC diaultydridc
`and diarnincs. Cilass substrates were spin coated with it solu-
`tion ofPAA (unimidizcd Pl) in N-methyl-2-pyrrolidinonc at
`3000 rpm for 30 s. The films were then soil baked at ltJt}“‘C‘
`for It) min to evaporate the solvent. The iinidization temper-
`atures of PAA films are depending on the Pls used. We t‘ol-
`lowcd the recommendation temperature from the companies.
`In case of S}-L610. it is hard baked at 250°C‘ for 1 It. During
`the hard bake, the precursor FAA lilm undergoes imidizattion
`forming P1.
`In the conventional method, the spin coatcd Pl
`lllm followed by thermal imidimtion is exposed to l.P1.lV at
`room to-mperaturc.‘”'? 9’ In our study, we exposed LPUV for
`30 min during thcrinal imidization (or hard bake). To distin-
`guish it from the conventional method. we are calling it an
`t'u—sr'm UV exposure method.
`Figure I shows schematically the experimental setup. A
`collimated beam from a Xe lamp was polarized using an
`Oriel UV shcct polarizer. The intensity of the polarized UV
`light was approximately 6mW2‘cm2 at the film's sttrli-icc. The
`PIIPAA coated substrate was placed perpendicular to the po-
`larized UV beam with the polymer side toward the lamp. The
`hot stage can bc rotating for oblique LPUV exposure for pic-
`Lilt angle generation.
`We measured the optical anisotropy induced by LPUV dur-
`ing exposure as shown in Fig. I. We used 21 pltoto-elastic
`modulator (PE.M9U, Hinds Instruments) with a fused silica
`head and a Ho-Ne laser for optical phase retardation men-
`
`' Eunnil address: jhoont_tt;'|tn||y1n.ac.kr
`
`138.!
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`23 8.‘!
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`Jpn. J. Appl. Phys. Vol. 40{2Utll 1 Pt. I. No. 4A
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`J.—|l. KIM or al.
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`'13cutI--
`"9
`E"Iu_/
`C1
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`‘£5
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`ES:
`
`1)
`Ed
`
`D
`
`90
`
`180
`
`270
`
`360
`
`Angle (deg)
`Fig. 2. Optical phase retardation as a fuIu:Jli-on ofthe rotation angle of the
`sample for (at AL lflfil (bl SI-I 1132. t1nd(c)JALS 2l4R Pl films. The
`LPUV is exposed 30 min during ilnidizutinn.
`
`(103rad.)
`Retardation
`
`0
`
`20
`
`40
`
`on
`
`Time (min)
`Fig. 3. Cotripsrrison ofoplicol phase retztrtlation ol‘SE olfl l'| as :1 fimctitni
`of the LPUV CJSPOSUTC time for la} in-tritrt method and {b) conventional
`method. The itnidimlion temperature was 2Stl"['.
`
`The dependence of optical retardation to the UV expo-
`sure time for the in—.t'i'ru method is shown along with that of
`conventional film for comparison in Fig. 3.
`In order to re-
`move thickness effect, we measured the optical retardation in
`the same sample: we covered one half of‘ the sample by a
`photo-mask during imidization and we measured optical re-
`tardation in the second half exposed to LPUV during imidiza—
`tion. After then, we cooled down the temperature and exposed
`LPUV in the first half of the sample with measuring optical
`retardation. The optical anisotropy gradually increases with
`the exposure time and becomes saturated in l h in both 1neth~
`ods. And it is clear that the magnitude of retardation for the
`sample prepared by r'n—.sim method is larger than of the sample
`prepared by conventional method.
`
`
`
`I. The schernatic diagram of C7t]JI.‘.I'll]1|.‘fllfll setup; (I) UV source. (2)
`Fig.
`UV pnlarizer, {3} UV transparent window, [-4) FAA layer, (5) Glass sub-
`strate. and to} Hot stage, (7) lle~Ne laser. (8) Folurizers. (9) PEM. ( ID)
`Detector. fl 1} Lnck—in amplilier, {IE} Computer. The sample hot Stage is
`Inountcd such that it can be rotate for oblique exposure to generate finite
`prelllt.
`
`surcments. The photo-elastic modulator (PEM) was placed
`between two crossed polarizcrs with its optic axis at 45” to
`the axes ofpolarizcr and analyzer. The LC cell prepared with
`photo-alignment layers was placed between PEM and the an-
`alyzer. The signal from the photo detector was fed to a lock~in
`amplifier {l:'.G&Ci Princeton Applied Research. Model 5210)
`for measuring the ac signal and a digital multimeter for the dc
`signal. The lock—in amplifier was tuned to the 50 kHz refer-
`ence signal from PEM. The laser beam was incident normal
`to the sample ccll‘s surface. The signal was monitored while
`rotating the sample with respect to the surface normal. The
`sensitivity of tltis method enables us to measure the phase re-
`tardation with a precision of :l:l).0l"’_
`
`3. Results and Discussion
`
`The optical anisotropy was as a function of the angle of re-
`tation for various PI films (ALI 051 , SE1 132, and .lALS2l 4R}
`prepared by n:—sn'tt method (see Fig. 2]. Though the triag-
`nitude is difierent for different PI lilms, it
`is very clear that
`the optical anisotropy is induced by LPUV exposure dur-
`ing imidization. We note that the magnitude of the optical
`anisotropy is depending on the hard baking temperature, UV
`intensity, exposure time, thickness of films, and so on. With
`comparing rubbed sample, the magnitude of the retardation
`is comparable but has the opposite sign to that of the rubbed
`lilrn. This suggests that the polymer chains are aligned per-
`pendicularly to the direction of polarization of LPUV. From
`FTIR study, we find that,
`the orientation of P1 molecules
`change after LPU V exposure and appears to be primarily
`due to preferential degradation of PI molecules parallel to
`the electric lield of Ll’UV.' " Reorientation of PI chains due
`to breaking of imide bonds may be contributing to the opti-
`cal anisotropy and alignment of’ LC molecules. The broken
`bonds reduce the polarizability of the Pl molecules.
`In con-
`trast to the previous report that the LC alignment is mainly
`achieved via the ittteraetions of LC rnolec-ulcs with the polar
`functional group in P1 produced by LPUV," our results show
`that anisotropic irreversible depolymerization is primarily rc-
`sponsiblc for LC alignment on LPUV exposed Pl films during
`the imidization.
`
`Page 2 of 6
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`Jpn. J. Appl. Phys. Vol. 40 (ZUUIJ Pt. I, No. 4A
`,_\,__jT..---.. .. _
`._
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`J.-H. KIM {*1 ml.
`
`2383
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`Q3
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`-
`
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`
`~ ..
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`_ ,,-£.'n,._».3
`
`is.
`
`(a)
`(b)
`Fig. 4. Polarizing microscopy texture of at homogeneously aligned t:t:l| prepared by conventional UV method, {a} before and {bi alter
`thermal amicaling at llJl'.|° C for l2 II. The loss alignment in (bl: shows thcnnal instability of this .'1|ignn1t:nt method.
`
`
`
`
`
`(a)
`
`(b)
`(C)
`Polarizing microscopy texture for a hotnogcneously aligned ncrnatic Cell prepared by the r'n—.rr'.ru UV exposure method. (:1) before
`Fig. 5.
`and (lo) after thermal annealing at 100°C for llh, and (C) after thermal annealing at 150°C [2 h. Evidently, the aligltrncnt is more
`stable ctnnpztrcd to that ofthe conventional method shown in Fig. 4.
`
`We believe that there are several factors responsible for the
`cnhancctncnt in optical retardation for the fr:-.s'it:: method. In
`conventional method, the LPUV dissociates bonds in poly-
`mer chains after polymerization {imidizationl had completed.
`As a result, smaller chains (segments) are left in the direc-
`tion of polarization while the orthogonal direction is popu-
`lated by longer chains.” The smaller segments are not able to
`relax and perhaps reorient as the UV exposure is conducted
`at room temperature. Consequently, there is significant strain
`energy stored in these films which is released at higher tem-
`peratures during thermal annealing. This relaxation process
`renders the polymer chain orientation more random which in
`turn loses the ability to align liquid crystal molecules. On the
`other hand,
`in the in-nit: method, the depolymcrization by
`LPUV and polyrncrization by thermal reaction occur simulta-
`neously. Thercforc, the imidization rate is anisotropic. More-
`over, since we expose LPUV at high temperature in the in-sift:
`method, the mobility of polymer chains is higher. Small poly-
`mer chains that reorient and become perpendicular to the di-
`rection of polarization are likely to undergo imidization and
`thus increase the number and length of the chains in that di-
`rection. Thus, we can expect that the resulting alignment
`Iilms are not only free of strain energy and hence more sta-
`ble than the conventionally prepared films.
`The LC texture in a cell prepared with the alignment layer
`using the conventional method is initially uniform as shown in
`
`Fig. 4(a). However after thermal annealing at 100°C‘ for 12 h,
`it shows sehlicren tcxturc indicating that the LC‘ molecules
`have lost their alignment [Fig -‘-l(b)'|. The optical texture of
`the cell prepared by the in-.sit‘ri method is also uniform over
`the whole area as shown in Fig. 5(a). After thermal anncaling
`under similar conditions, no degradation of alignment is ob-
`served [Fig 5{b)]. At elevated temperature (l50°C for 1211},
`some loss of alignment is observed as shown in Fig. 5(c].
`From the result, it is clear that thc in—.t‘irtt method produces
`more thcnnally stablc alignment layers than conventional
`method.
`It is believed that this method holds the promise of
`producing even more stable alignment layers, when all pa-
`rameters, such as temperatures of soft» and hard—bake, inten-
`sity of UV, duration, and the time of UV exposure, have been
`optimized.
`To optimize the bake temperature, for SE'.r'3l l and 751 I,
`we measured optical anisotropy as a function of baking tem-
`perature as shown in Fig. 6. Both Pls show higher retardation
`at 200°C than other temperatures. So, we fixed imidization
`temperature at 200°C for further investigation.
`In Table I, we compare the thermal stability at ltJU°(.‘ For
`various alignment methods using SE 731i and 751]. The UV
`intensity and exposure time are 400 W and 60min, respec-
`tively. Among the method, the rubbing produces t|1c most
`stable alignment layers as expected. However, for both Pls,
`the thermal stability of in-sfrti sample is better titan that of
`
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`2.134
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`Jpn. J. Appl. Phys. \«"ol.4tl (IUD!) PI.
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`l,l"vlo. -"IA
`
`5
`
`J.—H. KIM t.-'l'r..'tl.
`
`15' LPUV
`
`2nd LPUV
`
`Eat) Retardation
`(l0'3rad.)
`
`J)-
`
`PU
`
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`1
`170
`
`180
`
`190
`
`200
`
`21D
`
`220
`
`230
`
`Temperature (“(3)
`t‘t. Optical phase rt.-tnrdation as a function cl’ imidizalion temperature
`Fig.
`For la) SE 7'3! I and (b) SE '.-‘SI 1. The Ll\-" intensity and exposure time
`were 400 W and 31} min.
`
`Table I.
`T5 I
`l .
`
`T|tet'm£Il stability Fordiflercnl alignment methods for SE T311 and
`
`-
`Annealing [til
`Pl
`Method
`—T
`{J
`24
`48
`'32
`96
`
`
`SE 73!!
`
`SE Till
`
`Rubbing
`Convtsntionttl
`.’n—.S'i‘:u
`
`Rubbing
`Conventional
`irt~5iI!t
`
`O
`O
`O
`
`O
`O
`O
`
`Q
`O
`O
`
`O
`O
`O
`
`O
`O
`O
`
`O
`x
`
`O
`x
`C)
`
`O
`>'
`X
`
`Q
`x
`x
`
`x
`x
`x
`
`0: Alignment tu.-ittttrt: is uniform.
`x: Disclinalion line or l'|1lC]"0-i.lUl1‘H‘,l:lIl appears.
`
`'l‘.1bIe ll.
`
`Tl'lI.‘T‘I'II.1.l stability For varying UV exposure titnc I'o." SL-' 73] l.
`
`ms
`
`132
`
`UV cxpm-um
`“ms (mint
`
`an
`5“
`
`49
`
`{l
`
`O
`Cl
`
`O
`
`24
`
`C)
`C3
`
`O
`
`Annealing timc th}
`43
`‘E2
`9t:
`
`Q
`O
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`O
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`Q
`O
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`0
`
`x
`>‘
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`Cl
`
`x
`X
`
`O
`
`O i o
`' 0
`30
`0: Alignment texture is uni fonn.
`x‘. Dtsclination line or micro-domain appenns.
`
`O
`
`O _ O
`
`x
`X
`
`><
`
`x
`
`conventional method. And SE':'3l 1 shows better thcnnal sta-
`
`bility than SE'2’5l 1. It is probably due to the di1Tercntchemi-
`cal structure ofboth Pls.
`
`Now, we studied the influence of UV exposure time on
`thermal stability in .='n—.s-.-‘tn method for SE 7311. The UV
`intensity was 400 W. and annealing temperature was l0(J"C.
`The results are summarized in Table II. The samples with
`LPUV exposure for 30min and 40 min show tltc most stable
`
`Page 4 of 6
`
`
`
`(b)
`st.‘I—ttp for tnulli-doI1r.tin
`(:1) Schctnatic drawing ol‘ I.‘}(|:IE!l‘lIl1L‘]‘1l.i1l
`Fig. 3'.
`lb) Appearaltce of a hotncgencously aligtted tnttlti—dontain cell
`cells.
`between crossed polarizcrs. The polzirization direction ol‘ lit-st exposed
`LPUV coincides with the axis of one of the crossed poIari?.crs. The po-
`latization direction was at 45” to the polarizcr uxcs as sltown in the lit-,-
`ure. D:trk(bright) regions niarked as |([[) rcprcscnt nncttwnj LFUV expu-
`su1'e(5.'}.
`
`alignment capabilities. Longer UV exposure than 40 min de-
`creasc the thczrnal stability. This may be due to the fact that
`the prolonged UV exposure eventually dissociates even the
`bonds oriented perpcndiculay to the polarization direction.
`Since. in the fit-sitt: method. LPUV exposure takes place
`during imidization ofP[, the processing time are significantly
`reduced compared to the conventional process.
`lvloreover,
`multi-domain cells can be easily fabricated with multi-step
`LPUV exposure using a photo mask during the hard lJ£]l(t:
`[l7ig. 7(a)]. In our study, whole area ofthc substrate was ex-
`posed to normally incident LFUV for the first 20 min ofhard
`baking. During the next 20 min ofhard baking. one halfoftltc
`substrate was covered by 3 photo-mask and the second half
`was exposed normally to LPUV with polarization direction
`rotated by 45° with respect to the lirst exposure. Figure ?'('b_}
`shows the microscopic texture of the sample.
`In one region
`(marked as I). which is exposed to LPUV only once, the po-
`larization direction ofthc LPUV exposure coincides with the
`axis of one of the cross polarizers and lTlll1lI‘l'lLll11 transmit-
`tance is obtained. as expected. The other region (marked as
`ll), with two LPUV exposures with polari:-cation direction to-
`tated throttgh 45° during the second exposure. appears bright.
`in applicational aspects, the generation of prctilt angle is
`one of the most important factors to eliminate oricntational
`defects which significantly deteriorate the pcrfonnattcc of LC
`displays. We tested the prctilt angle generation using in-.tim
`method by the same method using in conventionztl method.
`i.e.
`two—step UV exposure method. Whole area of the sub«
`strate was exposed to nonnally incident LPUV for the lirst
`
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`Jpn. J. Appl. Phys. \a'oI.4tJ (2001) Pt. I, No.4A
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`2385
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`LPUV
`
`i
`
`LPUV
`
`(:1) Scltcmatic drawing of experimental sctmp for generating pretilt angle. (b);-’\lignn1ent textures between crossed polarizers for
`Fig. 8.
`single LPUV exposure with different applied voltages. (C) Alignment textures between Crossed poiarizers for double LPUV exposure
`with different applied voltages. Applied voltage was I V (left) and 3 V {right} in (b) and (c).
`
`20min of hard baking. During the next 10 min of hard bak-
`ing, LPUV with polarization direction rotated by 90” with re-
`spect to the first exposure is exposed obliquely [Fig. 8(a)].
`Figures 8(1)) and 8(c) are textures of the samples for single
`LPUV exposure with normal direction to the substrates and
`double LPUV exposure as described above, respectively, with
`different applied voltages. In low voltage regime, both sam-
`
`In higher voltage, however, it
`ples Show unifoml alignment.
`appears disclination lines in single exposed sample due to re-
`verse tilt.
`It means that the prctilt angle for this sample is
`nearly zero. In double exposed sample, it still shows uniform
`alignment which means finite pretilt angle is generated on the
`surface. Using the crystal rotation method, we found that the
`preti It angle is about 3°.
`
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`4. Concluding Remarks
`
`In conclusion, we have demonstrated It nevcl method for
`LC alignment using LPUV exposure during imidization of
`pelyimide. The results show that samples prepared by this
`method have better thennal stability and require less process-
`ing time. And also. we can easily make multi~dernair1 align.-
`mcnt layers and generate pretilt angles using mu]ti—step UV
`exposure. We note that this method may also be applicable
`to other plm-t0polyine:' films during evaporation ofthe Se-lvenl
`and to solutions efa cross-linkable resin and a curing agent.
`
`Acknowledgements
`
`This work was supported in part by NSF Science and
`Technology Center ALCOM grant DMR-89-20147, and by
`the Hallyrn Academy of Sciences, Hallym University, Korea,
`200 I .
`
`ll
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`2)
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`4)
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`6)
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`8:
`9)
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`10)
`1|)
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