spin-coated ultrathin multilayers and their ... · korea-australia rheology journal march 2003 vol....

Korea-Australia Rheology JournalVol. 15, No. 1, March 2003 pp. 1-7

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Spin-coated ultrathin multilayers and their micropatterningusing microfluidic channels

Hongseok Jang, Sangcheol Kim, Jinhan Cho and Kookheon Char*School of Chemical Engineering, Seoul National UniversitySan 56-1 Shilim-dong, Kwanak-gu, Seoul 151-744, Korea

(Received July 6, 2002)

Abstract

A new method is introduced to build up organic/organic multilayer films composed of cationic poly(ally-lamine hydrochloride) (PAH) and negatively charged poly (sodium 4-styrenesulfonate) (PSS) using the spinning process. The adsorption process is governed by both the viscous force induced by fast solveelimination and the electrostatic interaction between oppositely charged species. On the other hand, the cetrifugal and air shear forces applied by the spinning process significantly enhances desorption of weaklbound polyelectrolyte chains and also induce the planarization of the adsorbed polyelectrolyte layer. Thfilm thickness per bilayer adsorbed by the conventional dipping process and the spinning process was founto be about 4Å and 24Å, respectively. The surface of the multilayer films prepared with the spinning pro-cess is quite homogeneous and smooth. Also, a new approach to create multilayer ultrathin films with welldefined micropatterns in a short process time is introduced. To achieve such micropatterns with high lineresolution in organic multilayer films, microfluidic channels were combined with the convective self-assem-bly process employing both hydrogen bonding and electrostatic intermolecular interactions. The channelwere initially filled with polymer solution by capillary pressure and the residual solution was then removedby the spinning process.

Keywords: spin self-assembly method, multilayer, electrostatic attraction, hydrogen bonding, micropattern,microfluidic channel, convective self-assembly

1. Introduction

Fabrication of multilayer thin films have attracted muchinterest because they are of great use in thin film devicesbased on electrochemical processes such as sensors, inte-grated optics, rectifiers, or light-emitting devices (LED)(He et al., 1999; Ho et al., 1998; 2000; Hong et al., 1998;Onitsuka et al., 1996; Zhang et al., 1999). For the appli-cations of organic or organic/inorganic hybrid multilayerthin films to high performance devices, the internal struc-ture of the films should be on the degree of high molecularorder and the ability to pattern with at least micrometer-scale feature on the films are also required. Since the ioniclayer-by-layer self-assembly (SA) technique was intro-duced for the fabrication of polyelectrolyte multilayer(Decher, 1997), this self-assembling technique has beenextended to conducting polymer composites (Kotov et al.,1995), nonlinear optical dyes (Laschewasky et al., 1997;Lenahan et al., 1998), and the assembly of nanoparticles or

biomolecular systems (Lvov et al., 1995). This layer-by-layer deposition by dipping method is principally based the self-diffusion of adsorbing polymer and the rearrangment on the surface and thus various experimental facshould be taken into account in order to obtain the unifosurface coverage of a polymer layer adsorbing onto a sstrate: the adsorption time, the pH, the concentrationpolyelectrolyte and the amount of added ionic salt (Choetal., 2000; Mendelson et al., 2000). After the adsorption ofa polyelectrolyte layer, thorough washing using a flow opure solvent is required because the weakly adsorbed pelectrolyte chains significantly increase the surface rouness of the multilayer films.

Up until now, several methods of multilayer patterninhave been reported, including template-directed selecdeposition with microcontact printing, electric field directed deposition, and water-based subtractive patterningthese methods, however, chemically patterned templashould be prepared on the substrates because the adtion process is based on the self-diffusion and rearranment due to the interactions between adsorbing molecuand the patterned templates.

*Corresponding author: [email protected]© 2003 by The Korean Society of Rheology

Korea-Australia Rheology Journal March 2003 Vol. 15, No. 1 1

Hongseok Jang, Sangcheol Kim, Jinhan Cho and Kookheon Char

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In this work, we describe the new spin self-assembly(SA) method as an alternative for producing well-orga-nized multilayer films in short process time. Through theanalyses by UV-Vis spectroscopy and AFM, the multilayerthin films fabricated by the spin SA method were com-pared with those prepared by the conventional dippingmethod. We have also investigated the micropatterning ofmultilayers based on the hydrogen bonding and electro-static intermolecular interaction. These patterned multilay-ers are obtained by using microfluidic channels composedof poly (dimethylsiloxane) (PDMS) and hydrophilic sub-strate.

2. Experimental

2.1. MaterialsPoly(allylamine hydrochloride) (PAH, Mn=50,000~65,000),

poly(diallyldimethyl ammonium chloride) (PDAC, Mw=100,000~200,000), poly(sodium 4-styrenesulfonate) (PSS,MW =70,000), Poly(4-vinylpyridine) (PVP, Mw=60,000) andpoly(acrylic acid) (PAA, Mv~450,000) were purchasedfrom Aldrich and used without further purification. 16-mercaptohexadecanoic acid was obtained from Aldrich andused in absolute ethanol solution. Poly(dimethylsiloxane)(PDMS) and curing agent were purchased from Dow Corn-ing and used as recommended from the literature.

2.2. Multilayer preparationQuartz substrates for the deposition of polyelectrolytes

were initially cleaned by ultrasonification in a hot mixtureof H2SO4/H2O2 (7/3) for 3 hours. They were then heated ina mixture of H2O/H2O2/NH3 (5/1/1) at 80

oC for 1 hour, andthen subsequently dried by N2 gas purging. After this pro-cedure, the substrates were negatively charged and used forthe polyelectrolyte deposition starting with the cationicPAH. In the conventional layer-by-layer SA method, thesubstrates were immersed alternately in the cationic solu-tion and in the anionic solution for 20 min for each dep-osition. After each deposition step, the surface of the self-assembled film was washed by dipping into the deionizedwater for 2 min and then blown dry with a stream of nitro-gen. In the spin SA method, the deposition steps were car-ried out as follows: a few drops of polyelectrolyte solutionwere placed on the substrate and then the substrate wasrotated at a fixed rotating speed (typically, 4000 rpm) for 8to 15 seconds. After the deposition of each polyelectrolytelayer, the substrates were thoroughly rinsed twice with aplenty of deionized water. The spinning time and the speedfor the washing step were also identical to those for thelayer deposition.

2.3. Micropatterning of multilayersA silicon wafer was initially coated with a 1000Å buffer

layer of silicon oxide and followed by deposition of a 100

Å adhesive layer of titanium. A 1000Å film of gold wasthen sputtered on the titanium surface. The COOH-tminated hydrophilic substrate was prepared by immersthe gold film into a 5 mM 16-mercaptohexadecanoic acin absolute ethanol for 30 min. The deposition steps wcarried out as follows: polymer solution was first alloweto fill the microfluidic channels, which were formed by thcontact between a PDMS mold and a hydrophilic substrby capillary pressure. The channel filling with a polymsolution also enables the polymer to adsorb onto the face and the removal of residual solution is then carried by the spinning process (typically, 6000 rpm for 2 min

2.4. CharacterizationThe characteristics of multilayer films were analyzed

UV-vis spectrometer, atomic force microscopy (AFM). Aellipsometer was also employed for the thickness msurement.

3. Results and discussion

Figure 1 shows the difference in UV-vis absorbance multilayer films prepared by both spin SA and dipping Smethods as a function of number of bilayers. Due to contribution of the adsorbed PSS chains, the absorbaoccurs at 225 nm in the UV region. The aqueous poelectrolyte concentrations of PAH and PSS used in the different deposition methods were all identical to be mM (monomer unit basis). The dipping process was pformed for an adsorption time of 20 min per layer in ordto allow enough time for the saturated adsorption. In sSA process, however, the amount of polymers adsorbeeach deposition step was found to be much larger. It shobe noted at this point that the multilayer film fabricated the spin SA method is adsorbed onto only one side o

Fig. 1.Absorbance at 225 nm of PAH/PSS multilayers preparby (�) spin SA method and (�) dipping SA method.

2 Korea-Australia Rheology Journal

Spin-coated ultrathin multilayers and their micropatterning using microfluidic channels

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quartz wafer while the film prepared by the dipping SAmethod is adsorbed onto both sides of a quartz wafer. Thefilm thickness per bilayer adsorbed by the dipping processand the spinning process was found to be about 4Å and 24Å, respectively, as determined from the ellipsometric mea-surement.

We also found that the spin SA method has a significanteffect on the surface roughness of prepared multilayerfilms presumably due to the air shear force during the spin-ning process (Chou et al., 2000). In order to investigate thiseffect in detail, we measured the surface roughness of theself-assembled films using atomic force microscopy(AFM) in tapping mode as shown in figure 2. Although theflattened adsorption of a polyelectrolyte onto a substratewas obtained from a low ionic strength of the solution, thesurface roughness of the dipping SA film composed of(PAH/PSS)30 multilayer was measured to be about 8Å. Onthe other hand, the (PAH/PSS)30 multilayer film preparedby the spin SA method shows a surface roughness of about3 Å. These results are quite reproducible and clearly dem-onstrate that the air shear force driven by the spinning pro-cess significantly enhances the surface planarization of themultilayer films. In addition, the surface roughness ofabout 3 Å and the bilayer thickness of 24 Å for the spin-ning process compared with 8Å and 4Å for the dippingprocess provides an indirect evidence that the internalstructure of the spin SA films is highly ordered. This alsopoints to the fact that the film prepared with the spin pro-cess contains rather distinct interfaces between respectivelayers in contrast to the film obtained by the dippingmethod although no Bragg peaks in the X-ray reflectivityspectra of the PAH/PSS multilayer films to distinguish

such internal structure were observed due to the small etron density difference between PAH and PSS.

The significant difference in the adsorbed amount and surface roughness between the dipping and the spinnmethod is caused by the different adsorption mechaniThe conventional SA method by the dipping processprincipally based on the self-diffusion in which chargepolyelectrolyte chains are adsorbed onto an oppositcharged surface and then rearranged owing to the etrostatic interaction. As the presence of water moleculethe assemblies screens the electrostatic attraction, theface coverage of polyelectrolyte chains is restricted andweakly adsorbed chains are produced. In the spin SA pcess, however, the adsorption and rearrangement of pelectrolyte chains on the surface and the eliminationweakly bound chains from the substrate are almost simtaneously achieved by high spinning speed for a short tiFast elimination rate of water by the spinning process snificantly increases the concentration of polyelectrolysolution during the short deposition time. It also improvthe electrostatic attraction between oppositely chargpolymers. As a result, the adsorption of polyelectrolychains is more enhanced and thicker layers are obtaalthough the thin layer is favoured by the centrifugal forand air shear force. The surface planarization of the mtilayer films is also achieved because the weakly adsorchains are easily eliminated by the air shear for(Lawrence, 1988; Meyerhofer, 1978; Ohara et al., 1989).

To understand quantitatively the formation of multilayein spinning process, the effects of polyelectrolyte conctration and spinning speed were investigated. Figure 3shows that the absorbance increases linearly with the n

Fig. 2.Tapping mode AFM images (Digital Instruments, Nanoscope IIIa) of (a) a (PAH/PSS)30 film prepared with the dipping SA and(b) a (PAH/PSS)30 film fabricated with the spin SA.



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ber of bilayers for all the polymer concentrations employedin this study. The slope indicates the deposited amount ofpolyelectrolytes after a cycle of the spin self-assembly ofboth PAH and PSS. This amount is initially dependent onthe concentration of polymer solution, then levels off above16 mM. As shown in figure 3(b), the film thickness perbilayer increases from 5Å to 40Å and shows the samedependence on the solution concentration. This asymptoticbehavior may be due to the saturation of polyelectrolyteson the surface. Although the increase of polymer concen-tration tends to produce a thicker adsorbed polymer layerdue to the adsorption driving forces such as viscous andelectrostatic forces, the excess adsorbed layer with weakbinding sites is readily eliminated by the desorption drivingforces such as centrifugal and air shear forces. This obser-vation suggests that it is possible to precisely control theadsorbed amount of polyelectrolytes in multilayer films bysimply changing a wide range of polymer concentration inspite of the strong repulsion among polyelectrolyte chainswith the same charge.

According to the theoretical models and experimenresults based on the spin coating of single-layer films, film thickness (H) is dependent on the spinning speed (Ω)and the initial solution concentration (Ci) as follows:

H ~ Ω −α Ciβ (1)

When the thickness varies from micrometer to millimter, the value of α is 0.5 and β has the value between 1.5and 2.47. As mentioned above, the absorbance (A) is early proportional to the film thickness. Thus, by substuting A for H, the equation (1) is examined for the spSA multilayer thin films. At a fixed solution concentrationthe absorbance per bilayer decreases linearly with the speed as shown in figure 4(a) and the exponent of α isfound to be 0.34. The polyelectrolyte concentrations used only below 16 mM, above which the surface is fusaturated and is not affected by the concentration as cfirmed in figure 3. Figure 4(b) represents a log-log plot

Fig. 3. (a) The increase of absorbance of (PAH/PSS)n as a func-tion of bilayer number. (b) The effect of mole concen-tration of polyelectrolytes on the adsorbed amount (� )and the thickness per PAH/PSS bilayer (�) prepared byspin SA method.

Fig. 4.The effect of spinning speed (a) and mole concentratof polyelectrolytes (b) ranging from 1 mM to 10 mM onthe adsorbed amount of PAH/PSS bilayers prepared the spin SA method. Each point in the figure represethe average absorbance per bilayer for up to 20 bilaye



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the average absorbance per bilayer as a function of moleconcentration. For the spinning speeds ranging from 2000to 6000 rpm, the absorbance increases with the polyelec-trolyte concentration with a scaling exponent of 0.74. Thedifference in the power exponents with respect to both thespinning speed and the initial solution concentration maybe caused by the characteristics of the spin SA method. Inthe spin self-assembled multilayer thin films, the weaklybound chains are thoroughly eliminated by the centrifugalforce and air shear force during the washing step. As aresult, the stable and ultrathin layer is obtained at each dep-osition step. It means that the next adsorption becomesstrongly dependent on the interaction between a depositedsublayer (or a substrate) and a coating overlayer. Comparedwith a thick single layer, the adsorption and desorptionforces, which are determined by only the spinning speedand mole concentration, become less important.

The dimension of each channel is 10 µm wide, 1.2 µmhigh, and 5 mm long. When a polymer solution fills themicrofluidic channel with a rectangular cross section, thetime τ taken to travel the channel with a length L is givenby the following equation:

∆P = γ[{cosθsubstrate+ cosθPDMS}/ a + 2cosθPDMS/b] (2)

Cg= {ab/(a + b)} 2/8 (3)

τ = ηL2/(2Cg∆P) (4)

where γ is the surface tension of liquid, θ is the contactangle of liquid, a is the height and b is the width of chan-nel, η is the viscosity of a polymer solution, ∆P is the pres-sure drop across the channel and Cg is the geometric factorof the capillary channel. Generally, we note that the chan-

nel is filled in short time as far as the solvent preferentiawets the channel surface. For the case of N, N-dimethyl-formamide (DMF) as a solvent, the time taken for the coplete filling was observed to be within 3s, which is agreement with the time estimated by assuming ηDMF =0.794×10−3 kg/ms, ∆PDMF ≈ 48 kPa, and Cg ≈ 1.4×10−13 m2.The multilayers in the microfluidic channels are obtainby the convective SA method through the adsorption arearrangement of polymer chains onto a substrate anddesorption of weakly bound chains in the presence oshear field. In the spin SA process without mold confinment, there are several driving forces such as centrifu

Fig. 5.Schematic of micropatterning of multilayer films usingthe convective self-assembly process.

Fig. 6. AFM topography images of micropatterns of (PVPPAA)5 multilayers prepared with: (a) DMF, (b) ethanoand (c) ethanol/H2O (9/1, v/v). (d) (PDAC/PSS)5 mul-tilayer in water (Height scale is 50 nm).



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force, viscous force, and air shear force. Because theremoval of the polymer solution in the microfluidic chan-nels is restricted by the confined geometry, these forces aredependent on the position of the PDMS mold on a sub-strate. Typically, in present work, the mold is located nearthe rim of the SAM-coated silicon wafer and the channeldirection is aligned in parallel with the radial direction.

By using the microfluidic channels, the patterning ofmultilayer films with alternating PVP and PAA layers wasperformed. As shown in figure 6, the micropatterns of spin-coating ultrathin multilayers are significantly dependent onsolvent used.

Micropattern with clear edge boundaries and small ridgeswas developed by using DMF as a solvent. For ethanol,high ridge is observed at the edge of the patterned lines,while the ridge was remarkably reduced by mixing ethanolwith water. In previous works, the ridge formation phe-nomena were also observed in chemically patterned filmsand were attributed to the retraction of a thin film of poly-mer from the resist alkanethiolate. However, in the case ofthe microfluidic channels employed in present work, theridge formation is due to the wetting of a solvent on thePDMS walls. It is consistent with the fact that the contactangle of ethanol on the PDMS is 31o, much lower than thatof DMF (i.e., 63o).

We also achieved the micropatterning of (PDAC/PSS)5multilayer, which is based on the electrostatic intermo-lecular interaction. Figure 6(d) shows the AFM image of(PDAC/PSS)5 multilayer in aqueous solution. The featureheight of the micropatterned multilayer is lower than thatof (PVP/PAA)5, but no ridge is observed. As mentionedabove, the ridge formation at the edge boundary is sup-pressed by the poor wettability on the PDMS mold (i.e.,the contact angle is about 108o).

4. Conclusions

In summary, we have demonstrated for the first time thatthe spin SA process utilizing the centrifugal force, viscousforce, air shear force and electrostatic interactions causesthe adsorption, the rearrangement of polymer chains onto asubstrate and the desorption of weakly bound chains in avery short time of approximately 10 seconds. This newultrathin film-forming process yields a highly orderedinternal structure far superior to the structure obtained bythe conventional dipping SA process although it is muchsimpler and faster. It also allows us to precisely control andpredict the bilayer thickness as well as the surface rough-ness. The patterned multilayer films employing the hydro-gen bonding and the electrostatic attraction can also befabricated by the convective SA method combined withmicrofluidic channels. In the light of the wide applicationrange of the ultrathin multilayer films, the spin SA processdeveloped in the present study opens up new possibilities

for highly efficient electronic/photonic devices based omultilayer structures.

Acknowledgement

This work was financially supported by the NationResearch Laboratory Program (Grant M1-0104-00-01and funded in part by the Ministry of Education througthe Brain Korea 21 Program at Seoul National Univers

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Spin-coated ultrathin multilayers and their micropatterning using microfluidic channelsHongseok Jang, Sangcheol Kim, Jinhan Cho and Kookheon Char*School of Chemical Engineering, Seoul National University San 56-1 Shilim-dong, Kwanak-gu, Seoul ...(Received July 6, 2002)

AbstractA new method is introduced to build up organic/organic multilayer films composed of cationic poly...Keywords�:�spin self-assembly method, multilayer, electrostatic attraction, hydrogen bonding, mic...

spin-coated ultrathin multilayers and their ... · korea-australia rheology journal march 2003 vol....

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