Growth of plasma-polymerized thin films by PECVD method and ...

Surface & Coatings Technology 193 (2005) 142–146www.elsevier.com/locate/surfcoatGrowth of plasma-polymerized thin films by PECVD method andstudy on their surface and optical characteristicsI.-S. Bae a, *, S.-H. Cho a , S.-B. Lee a ,Y.Kim b , J.-H. Boo aa Department of Chemistry and Institute of Basic Science, Sungkyunkwan University, 300 ChunChun-Dong, Jangan, Suwon 440-746, Koreab Department of Mechanical Engineering, Sungkyunkwan University, 300 ChunChun-Dong, Jangan, Suwon 440-746, KoreaAvailable online 1 September 2004AbstractWe have deposited organic polymer thin films on glass and Si(100) substrates at room temperature using organic precursor by plasmaenhancedchemical vapor deposition (PECVD) method. Methylcyclohexane and ethylcyclohexane were utilized as organic precursors, andhydrogen and Ar were used as a bubbler and carrier gas. We especially compared the surface and optical properties of plasma-polymerizedorganic thin films with various RF power. AFM data showed that the plasma-polymerized films with smooth surface and sharp interfacecould be grown under various deposition conditions. The surface and optical properties of as-grown plasma-polymerized thin films wereanalyzed by contact angle measurement as well as FT-IR and UV–Visible spectrophotometer. As the plasma power was increased, the contactangle, refractive index, and main absorption peak of thin films were increased while the optical transmittance was decreased, signifying thatthe plasma-polymerized organic films have more low surface energy with increasing RF power.D 2004 Elsevier B.V. All rights reserved.Keywords: PECVD; Organic polymerization; Methylcyclohexane1. IntroductionThere has been an increase of interest in the use of glowdischarge for the polymerization of a number of organic andorganometallic compounds [1,2]. Plasma polymers are usedas dielectric and optical coating to inhibit corrosion. Theinvestigation of the optical properties of polymer films is ofparticular interest because of their use in optical devices [1].Among many CVD methods, plasma-enhanced chemicalvapor deposition (PECVD) process is very efficient methodto produce homogeneous organic thin films on large areasubstrates and offers good control over the film properties[3–7]. Plasma polymerization is known as a unique methodof organic thin film deposition. The films are usually highlycross-linked polymers and show chemically and physicallystable characteristics. However, the molecular structures ofthe films are different from starting monomers, because thepolymers are formed with highly decomposed moleculesunder ions and electron reactions with high energy. PACVD* Corresponding author. Tel.: +82 31 290 5972; fax: +82 31 290 7075.E-mail address: isbae0108@korea.com (I.-S. Bae).uses a glow discharge to create activated species such asradicals and ions from the original monomer, and thepolymer films are deposited through various gas phase andsurface reactions of these active species, including ablationof the deposited film. No water is generated during plasmapolymerization, and the influence of a solvent can thus beignored. Also, a layered structure that promotes adhesioncan be easily fabricated by changing the source compounds.In this study, we report our results on the growth ofplasma-polymerized organic thin films on glass and Si(100)substrates at room temperature using the methylcyclohexaneand ethylcyclohexane precursors by PECVD method. Thebody of this paper will focus on performing analysis ofoptical and surface properties of plasma-polymerizedorganic thin films.2. ExperimentPlasma polymerization was carried out in a vacuumchamber, which was made of stainless steel. Glass andSi(100) single crystal were used as substrates. After0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2004.07.022

I.-S. Bae et al. / Surface & Coatings Technology 193 (2005) 142–146 143cleaning the substrates using acetone, isopropyl alcohol anddistilled water, the pre-cleaned substrates were in situ pretreatedwith Ar plasma to make an oxygen-free surface and/or a buffer layer for enhancing film adhesion. The generaldeposition pressure and temperature were 2–410 1 torrand room temperature, respectively. The substrates wereheated using indirect heating method and the temperature ofthe chamber wall was detected by Chromel–Alumelthermocouple. The typical conditions of PECVD processapplied in this study for film deposition are 30–100 W of RFpower and 20 sccm of Ar carrier gas, and 20 sccm of H 2bubbler gas. Methylcyclohexane and ethylcyclohexane wereused as organic precursors. Due to high vapor pressure ofthe precursors themselves, there was no need to heat thesource during deposition. The surface properties of asgrownplasma-polymerized thin films were analyzed bycontact angle measurements and AFM analysis. The opticalproperties of as-grown plasma-polymerized organic thinfilms were also ex situ characterized with FT-IR, UV–Visspectroscopy, and ellipsometry.3. Results and discussionFig. 1a shows FT-IR spectra of plasma-polymerizedmethylcyclohexane and ethylcyclohexane thin films. InFig. 1a, we can see two strong absorption peaks at about2910 and 1400 cm 1 . The absorption peak at 2910 cm 1 canbe assigned to the aliphatic CUH stretching mode by methylgroup. And the polymers produce CUH bending modesaround 1400 cm 1 . The small absorption peaks around1400–1600 cm 1 may attribute to CUC and CMC stretchingvibration. We could also see the increase of absorption peakintensities whenever the polymerization was carried out athigh RF power. Fig. 1b and c show UV–Vis transmittancespectra of the plasma-polymerized thin films that weredeposited at room temperature under the condition ofAr:H 2 =1:1 and different RF powers. From both figures, wecould see a change of optical transmittance in the range 300–350 nm with a sharp slope and a high value of constanttransmission from 350 to 2500 nm. Higher transmittancethan 80% could be obtained from the methylcyclohexanefilms (see Fig. 1c) grown at RF power of below 100 W.However, the optical transmittance was gradually decreasedwith increasing RF powers due a degree of CUC orCMCbonding in the film layers. In the case of the ethylcyclohexanefilms, on the other hand, the optical transmittance wasrelatively lower than that of the methylcyclohexane films.From these results, we could make the conclusion that withincreasing RF power and number of carbon contents in thefunctional group precursor, the optical transmittance oforganic thin films decreases. This suggests that withincreasing RF power, high degree cross-linking density ofthe electrons could be overlapped, resulting in formation ofmore stable polymers. The increase of absorption peakintensity with increasing RF power can be explained withFig. 1. (a) FT-IR spectra of plasma-polymerized methylcyclohexane andethylcyclohexane thin films. UV–visible transmittance spectra obtainedfrom (b) ethylcyclohexane and (c) methylcyclohexane thin films grown onglass at various RF powers.either the increase of carbon contents or the scatteredreflection caused by plasma bombardment. This indicatesthat more high-quality films could be deposited using highRF power. Thus, we obtained better crystalline polymer thinfilms from the condition of 100 W RF power.Fig. 2a shows the variation of the refractive indexobtained by ellipsometry measurements for the plasma-

144I.-S. Bae et al. / Surface & Coatings Technology 193 (2005) 142–146Fig. 2. Variations of refractive index and deposition rate as a function of RFpower. (a) Refractive index and (b) deposition rate.polymerized organic thin films as a function of plasmapower. As the plasma power was increased, the refractiveindex of both methylcyclohexane and ethylcyclohexanefilms were also increased. However, the refractive index ofmethylcyclohexane films has always a higher value thanthose of ethylcyclohexane. This means that the polymer-likemethylcyclohexane thin films have more dense structure andthe RF power is the main effect of enhancing the refractiveindex. This is the same trend as FT-IR and UV–Visible data.At this moment, we don’t know in detail why themethylcyclohexane polymer film has high optical refractiveindex and transmittance. The increase of working temperaturecan also contribute to the increase in the opticalrefractive index of plasma-polymerized organic thin films.Fig. 2b shows the variation of deposition rate with RFpowers. From this figure, we could see that the depositionrates of the films are linearly increased with RF power, andobtain the maximum deposition rates (98 and 90 nm/min)when the methylcyclohexane and ethylcyclohexane filmswere respectively grown at 100 W of RF power. This isquite a higher value compared with that grown by theconventional method. Increasing the deposition rate withincreasing RF power can be simply explained in terms of theincrease of plasma density with enough available energywhich could be used for plasma polymerization ofmethylcyclohexane and ethylcyclohexane precursors. Basedon this assumption, we can understand why the methylcyclohexanefilm has more a dense structure and higherdeposition rate, in comparison with the ethylcyclohexanefilm.Fig. 3 shows three-dimensional AFM images of theplasma-polymerized thin films deposited at RT and variousFig. 3. The three-dimensional AFM images of (a,b) methylcyclohexane and (c,d) ethylcyclohexane thin films grown on Si(100) with different RF powers.

I.-S. Bae et al. / Surface & Coatings Technology 193 (2005) 142–146 145RF powers. All AFM images showed quite smooth surfaceswith no cracks. In the case of the methylcyclohexane film,for example, the rms roughness decreases from 7.375 to0.257 nm with increasing RF power to 100 W (see Fig. 3aand b). Moreover, the rms roughness of the ethylcyclohexanefilm is also decreased from 7.925 to 0.444 nm withincreasing RF power. In addition, we could clearly see thecluster on the film surface grown. From Fig. 3a and b, wecan see that the clusters of the films have 1 Am size at 30Wof RF power. But it was not entirely the cluster from thesurface of 100 W. This could be explained by the differentdissociation rates of precursor or sputtering effect. In otherwords, when the RF power increases, the element ofprecursors goes well disjointing from air or surface. Sothe polymer is formed by the size of the disjointed element.When the RF power increases, the element size becomessmall because of the plasma bombardment. We confirmedthat the RF power mainly determines the roughness ofsurface.Fig. 4 shows the variation of contact angles of thepolymerized thin films. Contact angle was measured froma drop of liquid resting on a solid surface. The drop ofliquid forming an angle may be considered as resting inequilibrium by balancing the three forces involved. Theangle within the liquid phase is known as the contact angleor wetting. In generally, a hydrophilic surface will have acontact angle less than ~708, and a hydrophobic surface ischaracterized by a contact angle of 70–908 or larger. FromFig. 4, the values of contact angles for ethylcyclohexanefilms are also increased from 72.98 to 79.38 withincreasing RF power to 100 W, while those for methylcyclohexanefilms are also increased from 83.08 to 89.38,respectively. This indicates that the plasma-polymerizedmethylcyclohexane films will relatively have more highsurface energy than that of ethylcyclohexane thin films,and both films could have hydrophobic surface and lowsurface energy with increasing RF power. However, moredetailed experiments are still needed to confine thisphenomenon.4. ConclusionsWe have deposited organic polymer thin films on Si(100)and glass substrates at room temperature using organicmonomer precursors by plasma-enhanced chemical vapordeposition (PECVD) method. Radiofrequency with 13.56MHz was applied for the ignition of the plasma, and theirRF power were changed in the range from 30 to 100 Wunder gas ratio of Ar:H 2 =1:1. Methylcyclohexane andethylcyclohexane were utilized as organic precursors, andH 2 and Ar were used as a bubbler and carrier gas,respectively. The as-grown organic thin films also showedquite high optical transmittance up to 80%. FT-IR and UV–Visible results show that the as-grown films have someoriented structures. And AFM data show quite smooth anddense surface morphology with increasing RF power. Theoptical refractive index of methylcyclohexane polymer filmsshow more higher value than that of ethylcyclohexane filmswhile the contact angles of methylcyclohexane films haverelatively lower values than that of ethylcyclohexane films,indicating more high surface energy for the methylcyclohexanefilms. The maximum growth rate of this study wasfound to be 98 nm/min.Fig. 4. The contact angle of the (a,b) methylcyclohexane and (c,d) ethylcyclohexane thin films.

146I.-S. Bae et al. / Surface & Coatings Technology 193 (2005) 142–146AcknowledgmentsThis work was supported in part by the BK21 project ofthe Ministry of Education, Korea and by the Center forAdvanced Plasma Surface Technology at the SungkyunkwanUniversity.[2] N. Dilsiz, G. Akovali, Polymer 37 (2) (1996) 333.[3] D. Jung, H. Pang, J.-H. Park, Y.W. Park, Y. Son, Jpn. J. Appl. Phys. 38(1999) L84.[4] J. Joo, Y.C. Quan, D. Jung, J. Mater. Res. 15 (1) (2000) 228.[5] N.V. Bhat, D.S. Wavhal, J. Appl. Polym. Sci. 70 (1998) 203.[6] S. Morita, J. Tamano, S. Hattori, M. Ieda, J. Appl. Phys. 51 (7) (1980)3983.[7] Y.C. Quan, J. Joo, D. Jung, Jpn. J. Appl. Phys. 38 (1999) 1356.References[1] A.B.M. Shah Jalal, S. Ahmed, A.H. Bhuiyan, M. Ibrahim, Thin SolidFilms 288 (1996) 108.