Nanostructuring of PDMS surfaces: Dependence on casting solvents

M.-E. Vlachopoulou a,b, A. Tserepi a,*, K. Beltsios b, G. Boulousis a, E. Gogolides a

a Institute of Microelectronics-NCSR Demokritos, Aghia Paraskevi 15310, Greeceb Materials Science and Engineering Department, University of Ioannina, Ioannina 45110, Greece

Available online 4 February 2007

Abstract

In this work, the processing properties of poly-dimethylsiloxane (PDMS) are investigated as factors a!ecting its surface nanostruc-turing by means of plasma etching. When PDMS films are exposed to SF6 plasmas under conditions ensuring anisotropic etching, col-umn-like structures appear on the PDMS surface. The prime aim of this work is to show that the PDMS surface topography can be tunedby the proper choice of the casting solvent used for the deposition of the PDMS films. In addition, we have prompted the e!ect of shorttime chain relaxation of the spun film structure before the introduction of crosslinks through thermal curing. By proper choice of solventquality and other processing parameters it is found possible to achieve as much as 30% controlled variation of rms roughness and 50%variation of column spacing for a given set of representative plasma conditions.! 2007 Elsevier B.V. All rights reserved.

Keywords: PDMS; SF6 plasma; Surface topography; Columnar structures; Chain packing

1. Introduction

Poly-dimethylsiloxane (PDMS) is a common type of sil-icone rubber with appealing properties as a structuralmaterial in microfluidics. Its main advantage is the easeof patterning by means of soft lithography [1] for the fab-rication of soft stamps, molds, and microfluidic devices.Especially for the latter, the surface topography of PDMSis a crucial parameter, as it determines the surface wettingproperties [2].

In addition, nanostructuring of polymeric surfaces toperform patterning on the nanoscale from non-litho-graphic methods is of potential technological importance,where cheap, large-scale devices are required. Our methodto achieve nanostructuring on PDMS is based on plasmatreatment of PDMS surfaces under conditions appropriatefor selective etching of certain components of the polymerwith simultaneous creation of etching resistant compo-nents. If in addition anisotropic etching is performed, wehave shown [3] that column-like structures are created on

the PDMS surface, with new potential applications in thefield of microfluidics and analytical devices based on thehigh surface to volume ratio of such nanostructured micro-channel walls; for such applications, it is crucial to maxi-mize our capacity to tune the structures in consideration.Therefore, in this work, we explore the dependence ofPDMS surface topography on the processing parameters,emphasizing the e!ect of the casting solvent used for thepreparation of the PDMS samples.

We show that PDMS samples, cast and spin-coated withsolutions of the PDMS prepolymer in di!erent solvents,and exposed to the same anisotropic etching conditionsin SF6 plasmas, exhibit di!erent etching rates, roughnessheight as well as di!erent densities of the column-like struc-tures. Therefore, casting solvents a!ect PDMS surfacenanostructuring, presumably because they lead to di!erentpacking arrangements of polymeric chains in the bulkPDMS; hence the casting solvent constitutes a processingparameter which can be used to tune, within certain range,the PDMS surface nanostructuring.

The possibility of a!ecting the chain arrangements ofPDMS layers through proper choice of casting solventshas been demonstrated before in the case of Langmuir–Blodgett (LB) films [4]; yet the present is a di!erent case

0167-9317/$ - see front matter ! 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.mee.2007.01.169

* Corresponding author. Tel.: +30 210 650 3264; fax: +30 210 651 1723.E-mail address: atserepi@imel.demokritos.gr (A. Tserepi).

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Microelectronic Engineering 84 (2007) 1476–1479

as the casting process involves spinning (i.e. centrifugalforces) and, also, the cast material is essentially threedimensional, while a monolayer compressed up to 5–10times was explored in the LB case.

2. Experiment

2.1. Materials

A commercial PDMS material (Sylgard 184, supplied byDow Corning) was used. Thin films of PDMS were pre-pared by spin-coating a mixture of base and its curingagent, dissolved into di!erent solvents, at a mixing ratioof 10:1 on Si substrates, to form PDMS films of 6 lm thick-ness after thermal crosslinking of the material. Thermalcrosslinking followed either immediately after spin-coatingor five days later (during this time, the samples were storedat 5 "C or !20 "C in order to retard thermal crosslinkingand to allow for a relaxation of the polymeric chainstowards equilibrium configurations). The film thicknesswas measured afterwards by multi-wavelength spectro-scopic ellipsometry.

2.2. Plasma treatment

The prepared PDMS samples were treated in SF6 plas-mas generated in an inductively-coupled plasma (ICP)reactor (MET, from Alcatel), described in detail elsewhere[2,5]. The etching plasma was produced at conditions(1900 W, !100 V, 10 mTorr SF6) ensuring anisotropicetching of the exposed PDMS surface. The vertical etchrate was measured in-situ and in real-time by laserinterferometry.

2.3. Surface characterization

The topographically modified PDMS surfaces werecharacterized by atomic force microscopy (AFM). ANanoscope III Digital Instruments AFM (in the tappingmode) was used for surface topographical characterization,after plasma treatment. The average roughness (rms) aswell as the average distance between the nanocolumnswas then calculated from the AFM images.

3. Results and discussion

The formation of periodic column-like structures onPDMS is shown in the AFM image of Fig. 1. A randomarray of columns with top diameter in the 100-nm rangeat an average spacing of a few hundred nm’s is shown,for a PDMS sample treated for 2 min in the SF6 plasma.For this sample, the PDMS prepolymer was dissolved inMIBK.

We next used a variety of solvents (toluene, benzene,methyl isobutyl ketone (MIBK), methyl ethyl ketone(MEK), xylene, hexane, and n-methyl-2-pyrrolidone(NMP)), for the preparation of PDMS solutions for spin-

coating. The ‘quality’ of these solvents with respect toPDMS solutions was initially determined through swellingexperiments. For these experiments, PDMS blocks(10 · 10 · 1 mm) were prepared, sunk and left in the sol-vents until equilibrium swelling was achieved. The swellingof PDMS in each solvent was estimated by measuring theweight of PDMS before and after its residence in the sol-vent. The results of the swelling experiments expressed interms of the final to initial PDMS volume ratio are shownin Fig. 2, for PDMS samples prepared at two crosslink den-sities, the regular one (base to curing agent mixing ratio of10:1), and a higher crosslink density (mixing ratio of 10:3).The higher the PDMS swelling, the better the quality of thecasting solvent for PDMS, and, thus, according to Fig. 2,the six solvents investigated were put in a ‘quality’ order;toluene constitutes the ‘best’ solvent, while MEK andNMP are the ‘worst’ ones, among those explored.

For plasma processing purposes, the PDMS prepolymerwas dissolved in the aforementioned solvents and theresulting solutions were spin-coated on Si wafers. PDMSfilms were formed after thermal crosslinking of the material

Fig. 1. AFM images (top and 3-D view) of the surface topography ofPDMS dissolved in MIBK casting solvent.

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that followed immediately after spin-coating (‘regular’samples) or after 5 days refrigeration (‘refrigerated’ sam-ples at T = 5 "C and !20"C). The prepared PDMS filmswere subsequently exposed to SF6 etching plasmas, andthe measured etching rates are shown in Fig. 3 in orderof increasing solvent quality. The plot shows a relativelymonotonic dependence of the PDMS etch rate on the sol-vent quality for regular and films refrigerated at 5 "C, withfilm thickness kept fixed at 6 lm. This result reveals that, atthese conditions, the quality of solvent, which apparentlyleaves an imprint in the packing arrangement of polymericchains, a!ects the etch rate of PDMS. As regards to therelaxation allowed during the refrigeration period, thee!ect on etching rate is rather small at T = 5 "C. Hence,the degree of cooling at T = 5 "C was insu"cient for thee!ective quench of crosslinking reactions inhibiting di!u-sional rearrangement of the chains. Stronger refrigeration(T = !20 "C) led to stronger quench of crosslinking reac-tions and allowed chain relaxation su"cient for major filmrestructuring. In this case, a tendency is observed for theetch rate to converge to a single, independent of casting sol-vent, value.

The plasma-modified PDMS surfaces were examined byAFM and information about the surface topography andthe column height (rms value), in specific, is shown inFig. 4. The plot shows the dependence of rms roughnesson the solvent quality, which is qualitatively opposite tothe dependence of etch rate on the solvent quality, but stilla monotonic one (see Fig. 3).

At this point it is important to note that the quality ofthe solvent also a!ects the film thickness for a given spinrate. Hence one can either aim at a given film thicknessby varying the spin rate (as in the case of Figs. 3,4) orcan keep the spin rate fixed and obtain a thickness thatdepends on the casting solvent. The dependence of theetched structure in the two cases is somewhat di!erent.Fig. 5 shows the e!ect of the casting solvent on the etchrate when the spin rate is kept fixed. A comparison ofFig. 3 and Fig. 5 shows that the dependence of etch rateon solvent quality is, roughly, the same in terms of trend,yet di!erent in detail. In general, such di!erences can beattributed to the contribution of parameters beyond sol-vent quality; such parameters are the spin rate of the solu-tion, the evaporation rate of the solvent etc.

Fig. 6 shows the dependence of column spacing, k, oncasting solvent quality for PDMS samples processed atthe same conditions as in Fig. 5, i.e. same spin rate and dif-ferent resulting thickness. It is immediately obvious thatthe relatively monotonous variation of the structural out-come as a function of the solvent quality is lost. Yet, acomparison of Figs. 5 and 6 suggests a di!erent correlationbased on the finer features of the two graphs. For five outof the six solvents employed, the ‘waviness’ of the twographs is, qualitatively, of the same type; the waviness inFig. 6 is stronger and any sense of monotonous variationof the structural outcome with the solvent quality is lost.The substantial similarity in the trends observed in Figs.5 and 6 can be understood if one considers the physicalmeaning of k. As we have demonstrated in previous work[3], the columns created as a result of anisotropic and selec-

1,0

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i (fin

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MIBK MEK NMP HEXANE BENZENE XYLENE TOLUENE

10:310:1Base/curing agent ratio (Degree of crosslinking)

Fig. 2. PDMS swelling in various solvents for two di!erent crosslinkingdensities. The standard PDMS crosslinking density corresponds to 10:1base: curing agent mixing ratio.

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regular samples "refrigerated samples" at 5 °

°

C "refrigerated samples" at -20 C

etch

rate

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mek hexane mibkxylene

benzenetoluene

Fig. 3. Dependence of the PDMS etching rate on the casting solventquality (increasing order). Fixed film thickness (6.5 ± 0.3 lm), di!erentspin rate.

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toluenebenzenemibkxylene

hexanemek

Fig. 4. Dependence of the column height (rms value) on casting solventquality (increasing order), for SF6 plasma-treated PDMS surfaces. Fixedfilm thickness (6.5 ± 0.3 lm), di!erent spin rate.

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tive plasma etching represent etch resistant polymeric com-ponents, or, in other words, elements retarding the fastetching of the surface. The closer together these elementsare, or the smaller k is, the smaller will be the etch rate,due to the reduction of the cross-section of the surface withthe plasma active etch species. Therefore, the similarity inthe dependence of etch rate and k on the casting solventquality is justified.

All variations described herein are the outcome of ourexploration focusing on the contribution of casting solvent.It is known from polymer solution theory [6] that as solu-tions become more concentrated the chains tend to followGaussian statistics at all scales regardless of the quality ofsolvent; nevertheless at small scales less entangled, self-avoiding configurations are preserved in the case of goodsolvents. When the solvent fully evaporates, no chain con-figuration trace of the solvent quality will be left, provided

that the melt is allowed to undergo a full equilibration.When the equilibration is not full (as it happens for spunPDMS films of practical importance) traces of the originalconfigurations found in solution are left in the solvent-freepolymer layer and the structural features of the etchedstructures exhibit di!erentiation.

What is clearly demonstrated is that the fine structure ofsurface topography can be tuned within a certain range(50% variation of column spacing k and 30% variation ofrms roughness) by proper choice of solvent. However, thecomplexity of the interfering processing parameters allowsfor somewhat di!erent trends for di!erent choices ofremaining process parameters. This description is consis-tent with the assumption that, upon etching, di!erent chainarrangements at small scales lead to fragments, possibly ofSiOxFy composition, having di!erent capacity to lead ulti-mately to the building of non-volatile etch resistant areas.It is also possible that preserved di!erences of film struc-ture extend to the medium size range and also contributeto di!erent behaviours during plasma etching.

4. Conclusion

In this work, we demonstrate, for the first time, that pro-cessing properties of PDMS, and in specific, the chainpacking arrangement can a!ect the topography of PDMSunder plasma-based nanostructuring of its surface. Thedetailed e!ect of additional properties remains to be exam-ined in the future, due to the technological importance oftuning at will surface nanostructuring and the scientific sig-nificance of illuminating the basic mechanisms responsiblefor that.

Acknowledgment

The authors wish to thank Dr. V. Constantoudis, Insti-tute of Microelectronics, for the statistical analysis of AFMimages.

References

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Valamontes, J. Appl. Phys. 98 (2005) 11. Art no. 113502.[3] A. Tserepi, M.-E. Vlachopoulou, E. Gogolides, Nanotechnology 17

(2006) 3977.[4] K. Beltsios, E. Soterakou, G. Tsangaris, N. Kanellopoulos, J. Nanosci.

Nanotechnol. 1 (2001) 1.[5] A. Tserepi, G. Cordoyiannis, G.P. Patsis, V. Constantoudis, E.S.

Valamontes, E. Gogolides, D. Eon, M.C. Peignon, Ch. Cardinaud, G.Turban, J. Vac. Sci. Techol. B 21 (2003) 174.

[6] M. Daoud, G. Jannink, J. Phys. 65 (1976) 973.

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mek hexane mibkxylene

benzene toluene

Fig. 5. Dependence of the etch rate of PDMS samples on casting solventquality (order of increasing quality). Same spin rate, di!erent filmthickness (3.3–7.7 lm).

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mek hexane mibkxylene

benzene toluene

Fig. 6. Dependence of the column spacing for PDMS samples on castingsolvent quality (order of increasing quality). Same spin rate, di!erent filmthickness (3.3–7.7 lm).

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