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

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-