Surface and Coatings Technology 180 – 181 (2004) 244–249
A structural study of organo-silicon polymeric thin films deposited by remote microwave plasma enhanced chemical vapour deposition a ´ A. Barrancoa,b,*, J. Cotrinoa,c, F. Yuberoa, T. Girardeaud, S. Cameliod, A.R. Gonzalez-Elipe a
´ ´ Instituto de Ciencia de Materiales de Sevilla CSIC-Universidad de Sevilla and Dpt. Q. Inorganica. cy Americo Vespucio syn. 41092 Seville, Spain b Swiss Federal Laboratories for Materials Testing and Research (EMPA). Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland c ´ ´ ´ Dpto de Fısica Atomica, Molecular y Nuclear. Facultad de Fısica, Universidad de Sevilla. Avda. Reina Mercedes syn. Seville, Spain d ´ Laboratoire de Metallurgie-Physique Bat SP2M 86962 Futuroscope Chasseneuil Cedex, France
Abstract SiOxCyHz thin films with different SiyC and SiyO ratios have been prepared by plasma enhanced chemical vapour deposition, using (CH3)3SiCl as precursor and oxygen as plasma gas. Thin films with compositions ranging from SiO2 :H to SiO2 C4.7 H7 were prepared by varying the oxygen to precursor ratio. The stoichiometry and structure of the films were determined by Fouriertransform infrared spectroscopy, Rutherford backscattering spectrometry, electron recoil detection analysis, X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy at the Si K edge. It has been shown that in all the films there are bonding structures of the type Si–C andyor Si–O–C, besides a basic Si–O–Si skeleton. The preservation of this Si–O–Si structure for a so wide range of C content in the film is a key feature of the synthesised films. The optical properties of the films were also investigated by spectroscopic ellipsometry. It has been found that the refractive index increases with the carbon content in the films. The films were transparent in the visible although they presented a high absorption in the ultraviolet region that increases with the C content. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Plasma polymerization; Organo-silicon polymers; Optical properties; XANES; Si K edge; Thin films
1. Introduction The study of silicone-like plasma polymeric thin films (also designed as organo-silicon plasma polymers, SiOxCyHz or SiCOH thin films), made of Si, O, C and H, have attracted much attention because of their use for different applications w1–3x. Thus, these films have been recently employed as low dielectric constant films in microelectronics w4x, gas filtering membranes w5x optical films w6,7x, protective coatings w8,9x or biocompatible films w10x. For each of these specific applications it is critical a precise control of the microstructure and composition of the films. Consequently, a great effort has been devoted to develop specific experimental protocols enabling the tailored synthesis of organo-silicon layers. SiOxCyHz polymeric films can be deposited from organosilicon precursors in oxygen plasmas. Either inor*Corresponding author. Fax: q34-95-446-0665. E-mail address:
[email protected] (A. Barranco).
ganic silica films or polymeric organosilicon deposits can be obtained by changing the percentage of oxygen in the discharge w1–3x. A point of particular interest is the type of local structure surrounding silicon and the way the different organic functional groups are bonded among them and with silicon. In a previous paper we have published preliminary results of the synthesis of SiOCH polymers by remote plasma enhanced chemical vapour deposition (PECVD) using clorotrimethylsilane (ClTMS) as precursor and oxygen as plasma gas w11x. In those experiments no deposition was observed with tetramethylsilane (TMS) and oxygen. The present manuscript is devoted to the chemical and structural analysis of these films by means of a series of surface and bulk techniques. Besides classical methods such as Fourier-transform infrared (FT-IR) or X-ray photoelectron spectroscopy (XPS), we report the use of X-ray absorption near edge spectroscopy (XANES) to analyse the environment around the Si atoms in the films.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.031
A. Barranco et al. / Surface and Coatings Technology 180 – 181 (2004) 244–249
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Finally, the optical properties of the organo-silicon layers are also analysed and parameters like refractive index and absorption coefficient are correlated with their composition and structure. 2. Experimental In this paper we study silicone-like plasma polymers deposited by PECVD in a remote microwave reactor. The experimental conditions for the preparation of the films and a complete description of the reactor have been recently published w11,12x. SiOxCyHz thin films with different compositions were prepared by changing the ratio between the flows of oxygen and silicon precursor (R) during preparation. The films were deposited on Si(1 0 0) wafers at a total pressure of 5=10y3 Torr and had an average thickness of ;500 nm. FT-IR spectra were collected in transmission at normal geometry in a Nicolet 510 spectrometer. The composition of the SiOxCyHz layers was determined by Rutherford backscattering spectroscopy (RBS) and electron recoil detection analysis (ERDA). The measurements were performed in the Ion accelerator Aramis (Orsay, France) by using 1.5 and 3 MeV a particles, respectively. RBS and ERDA spectra were simulated using the RUMP code. The RBS spectra were recorded in channelling geometry. XPS spectra were acquired with an ESCALAB 210 spectrometer from VG operated at constant pass energy with a value of 20 eV. The Mg Ka radiation was used as excitation source. Atomic ratios and surface concentrations were quantitatively determined from the area of the Si 2p, C 1s and O 1s peaks after a Shirley-type background subtraction and correction by the reported sensitivity factors of the peaks w13x. XANES spectra at the Si K edge were recorded in the SA32 beamline at the Super-Aco synchrotron (LURE, Orsay). An InSb(1 1 1) double crystal monochromator was used for selection of photon energy (resolution of 0.75 at 2000 eV). The spectra were recorded by measuring the drain current through the sample (total electron yield detection). Spectroscopic ellipsometry experiments were performed using a SOPRA commercial system. The measurements were made in a wavelength range comprised between 0.21 and 1.2 mm at different angles of incidence (65, 70, 758). These are the most sensitive angles, since they are close to the Brewster angle of the silicon substrate. The analysis procedure consists of a regression using the Cauchy law to simulate the refractive index n(l) and to extract the absorption coefficient k(l) of the layers.
Fig. 1. Evolution of atomic percentages in the SiOxCyHz thin films as a function of the parameter R. (Top) Percentages determined by RBSyERDA analysis for the bulk of the films. (Bottom) percentages determined by XPS analysis for the surface of the films.
3. Results 3.1. Composition of thin films Fig. 1 shows the evolution as a function of R of the bulk atomic percentages of Si, O, C and H, determined by channelling-RBSyERDA. From this figure it is possible to realise that the relative amounts of C and H in the films increase with the flow of oxygen. In general, for R;1 the films had a silica composition that was maintained for R)1 although the percentage of H in the film decreases. Organo-silicon films were obtained for R-1, the relative amount of C and H increasing as R decreases. It is also worth noting that the films deposited with a R value as low as 0.01 still have a SiyO ratio close to 2. No deposition was observed in plasma containing the pure precursor only w11x. In none case chlorine was detected in the bulk of the films. This suggests that this element dissociates from ClTMS after activation by the plasma and that it is removed from the deposition chamber w11x. The XPS analysis of the surface of the films shows that their composition is stable with time (i.e. from several minutes to months). The fact that the surface and bulk compositions, reported, respectively, in Fig. 1a, b, follow similar trend points to a constant in-depth stoichiometry profile. A fitting analysis of the Si 2p photoemission spectra for samples prepared with different R ratios is presented in Fig. 2. The full width at half maximum (FWHM) of the Si 2p peaks in this figure varies from ;2.40 for Rs0.01 to ;1.85 for Rs1, remaining constant for
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Fig. 2. Fitted Si 2p photoemission spectra for a series of SiOxCyHz thin films prepared with the indicated values of R.
CH3–C). Other bands, not presented in the figure, were one at 1720 cmy1 (low intense C_ O stretching band only visible in the films prepared with very low R ratios) and some others detected in the region 2500– 3500 cmy1. A first one centered at ;2900 cmy1 corresponds to the stretching of CHn, while a small and broad band at ;3500 cmy1 is attributed to O–H bonds. It is interesting that no Si–H band, commonly detected at 2120 cmy1 in other organo-silicon thin films w3,4,19x, is observed in these layers. The films deposited at R) 1 present FT-IR bands typical of inorganic silica films w16,17x. Their spectra are characterised by a very intense Si–O–Si stretching band at ;1060 cmy1, plus the additional bands corresponding to the Si–O–Si bending mode at 800 cmy1 and the Si–OH band at 940 cmy1. The shoulder on the high wave number side of the Si–O–Si stretching band in SiO2 films (;1200 cmy1) has been attributed to a longitudinal–transversal optical splitting that can be observed only in films that contain SiO4 tetrahedral units w16,17,20x. From the spectra in Fig. 3 it appears that the intensity of the organic bands decreases as R increases. However, it is interesting that even for R ratios as low as 5=10y3 the Si–O–Si band is still visible in the spectrum, thus suggesting that a Si–O–Si bonding structure is preserved in all the films independently of their actual composition.
R01 when it has the value typical of stoichiometric silica w14x. The fact that the FWHM increases as R decreases indicates that the environment around the silicon become more heterogeneous. The Si 2p spectra in Fig. 2 have been fitted with four bands corresponding to SiO4 (103.4 eV), SiO3C (102.6 eV), SiO2C2 (101.7 eV) and SiO3C (100.7 eV) local environments. The differences in binding energies among the components are close to 1 eV per oxidation state and have been taken from Refs. w5,14x. As R increases, the contribution of the SiO4 peak increases, being interesting to notice that for a R ratio as low as 0.01 the fractions of SiO4 (;32%) and SiO3C (;58%) species are dominant. 3.2. Film structure and FT-IR analysis Fig. 3 shows the FT-IR spectra of a series of films prepared with different values of R. The following vibrational bands corresponding to specific groups can be recognised w2–4,14–19x: 885 and 800 cmy1 (Si–(CH3)n rocking), 1000 and 1100 cmy1 (Si–O–Si stretching at ;1060 cmy1 and Si–O–R stretching at ;1020 cmy1), 1260 cmy1 (Si–CH3 deformation), 1380 cmy1 (C–H symmetric bending in –CH3 andyor methyl symmetric bending in CH3–C), 1410 (CH2 symmetrical scissoring in Si–CH2) 1460 cmy1 (C–H asymmetric bending in –CH3 andyor methyl asymmetric binding in
Fig. 3. FT-IR spectra for a series of SiOx Cy Hz thin films prepared with the indicated values of R.
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that the spectra in Fig. 4 are due to Si atoms bonded to either O (peak A) or C atoms (peak B). Meanwhile, the absence of additional structures between peaks A and C indicates the amorphous nature of the films w23x. 3.4. Optical properties The refractive index and extinction coefficient of the polymeric films as a function of R have been plotted in Fig. 5. The n values for the films prepared at Rs1.5 are slightly lower than those of thermal silicon oxide (1.456 at 632.8 nm). The n values increase with R to reach a value of 1.590 at 632.8 nm for Rs0.01. The curves of the extinction coefficient show that the films are transparent for wavelengths larger than 500 nm. Below this energy, the extinction coefficients in the ultraviolet region increase as R decreases. The films prepared with R)1 do not absorb light in all the analysed energy ranges as observed in stoichiometric silica layers. 4. Discussion
3.3. Film structure and Si K-edge XANES spectra
Fig. 1 indicates that the composition of the films is rather uniform from the surface to the bulk. The differences in atomic percents found between bulk and surface are always below 20% and may be due to a contamination of the surface of the films with spurious carbon. The OySi ratios are always in the interval 1.9–2.2, even
Fig. 4 shows the Si K-edge XANES spectra of selected films as a function of the value of R. These spectra can be considered as the p-projection of the Si 2p levels in the conduction band of the organo-silicon layers. The shape of the spectrum of the sample prepared with Rs1.5 is identical to the spectrum of stoichiometric amorphous silicon dioxide w21,22x. This spectrum shows a sharp and intense absorption peak (white line) at 1847 eV (marked as A in Fig. 4) that has been assigned to a localized quasi-atomic Si 1s™t2(Si 3px,y,z) electronic transition w22,23x. Peak A shifts to lower energies when R decreases. Its minimum value was 1846.3 eV for Rs 0.01. A second resonance (marked as B in Fig. 4) at approximately 1844.8 eV was detected for R-1. This energy position is similar to that of the main peak characterising the spectrum of SiC thin films w23–25x. The intensity of peak B increases as R decreases, until it reaches an intensity similar to that of peak A for Rs 0.01. Another interesting feature in these spectra is the broad band located approximately 17 eV above peak A (marked as C in Fig. 4) that shifts to lower energies as R decreases. Spectra with a C structure similar to that in Fig. 4 have been described in terms of multi-scattering effects within the first andyor second co-ordination shells of Si atoms w23,24x. Concerning the SiOxCyHz thin films studied here, these previous studies support
Fig. 5. (Top) Refractive index, n(l) and absorption coefficient, k(l) curves determined by ellipsometry for a series of SiOxCy Hz thin films prepared with the indicated values of R.
Fig. 4. Si K-edge XANES spectra for a series of SiOxCyHz thin films prepared with the indicated values of R.
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for the SiOxCyHz films prepared with a R value as low as 0.01. This result is interesting because the high oxygen content is accompanied by a high C content (e.g. the CySi ratio reaches a value as high as 4.7 for Rs0.01). Moreover, with respect to the precursor molecule, the films prepared at low R values are defective in H but not in C. These findings differ from the general trend found by the plasma polymerization of organosilicon molecules w3x. Thus, although the OySi ratios obtained here at low R values are slightly lower than those found in organo-silicon thin films deposited from TEOSyO2 plasmas w19,26x, they are higher than those reported for HDMSOyO2 plasmas w26,7x or even for ClTMSyO2 and TMSyO2 in a surfatron plasma reactor w14,27x or TMSyO2 in a parallel plate RF reactor w4x. On the other hand, the CySi ratios at low R values are higher than in films deposited from pure TMS plasmas w3,4x, TEOSyO2 and HDMSOyO2 plasmas in RF reactors w19,26x. Hence, the observed dependence of atomic percentages on R seems to be a particular feature of both the employ of a ClTMS precursor and the use of a downstream microwave ECR plasma. Another peculiarity of our SiOxCyHz thin films concerns the structure of the first shell of atoms around Si. The analyses by XPS, FT-IR and XANES confirm the existence of a Si–O–Si bonding structure in all the films and that the contribution of Si–O–C and Si–C bonding structures increases as R decreases. Interestingly, the Si–O–Si structure is preserved even for Rs0.005 when the corresponding FT-IR band is still noticeable (Fig. 3). Note that the intensity of the FT-IR band corresponding to Si–CH3, although increasing for low R values, is always relatively small. The XANES and XPS spectra also support that a well defined Si–O–Si network co-exists with a SiO4yxCx tetrahedral co-ordination around Si. Thus, a progressive bonding of Si with carbon groups (i.e. Si–C–Si andyor Si–O–C bonding structures) is congruent with the fitting analysis of the XPS spectra. On the other hand, a welldefined white line (peak A in Fig. 4) is clearly observed in the XANES spectra of all the films. A shift in its position reflects a change in the electronegativity of the atoms andyor groups of atoms forming the tetrahedral environment of Si w23–25x. We attribute the observed shift when R decreases to the partial substitution of oxygen by carbon atoms in the first shell around Si. At the same time, it is possible to observe an increase in the intensity of peak B, attributed to Si–C resonances. A structure at ;1845 eV has been also observed in SiOx (x(2) thin films w22x and assigned to Si3q species bonded to one silicon and three oxygen atoms w22x. This attribution to Si–C resonances is confirmed by the fact that the analysis of the XANES and XPS spectra discards the presence of Si–Si bonds in the films w22x. According to Fig. 5, the refractive index of the films decreases with the oxygen concentration in the gas feed
to a value close to that of thermal silica for the layers deposited with R)1. The n values obtained for C rich thin films are higher that those reported in literature for TEOS–O2 RF discharges w19x and pure HSMSO discharges w28x and similar to those reported for HDMSO– O2 mixtures in a planar RF reactor w7x. On the other hand, the k values in the ultraviolet region of the spectra (cf. Fig. 5) are very high in comparison with previous literature results w6,7,19,28x. As suggested by Zajickova et al. w7x organo-silicon plasma polymers transparent in the visible and absorbing in this region of the spectra could be used for the protection of commercial polymer substrates against ultraviolet radiation. According to Martinu w6x, for similar compactness and porosity, the increase with R of n and k, this latter in the UV region, can be related to the progressive incorporation of carbon into the layers. Although the FT-IR analysis of the samples (cf. Fig. 3) reveals the existence of –CH3 groups, the fact that the HyC ratios in Fig. 1 are lower than 3 (e.g. 1.5 for Rs0.01) discards that all the carbon can be accounted for by the alkyl groups. Therefore, in agreement with other literature results on the optical properties of amorphous carbon films w6,29x, we assume that such a carbon excess, in the form of highly cross-linked C atoms serving as bridge between silicon atoms andyor as small clusters, may be responsible of the high n and k values. Acknowledgments ´ We thank the Ministerio de Ciencia y Tecnologıa (MAT2001-2820) and the EU COST 527 network for financial support. We want to acknowledge the contribution of C. Clerc (Aramis, Orsay) for her assistance in the RBS analysis. A.B. acknowledges a post-doctoral ´ Ciencia y fellowship from Ministerio de Educacion Deporte (Spain). References w1x H. Yasuda, Plasma Polymerization, Academic Press, 1985. w2x R. d’Agostino, P. Favia, F. Fracassi (Eds.), Plasma Processing of Polymers, Kluwer Academic, 1996, and references therein. w3x A.M. Wrobel, ´ M.R. Whertheimer, Plasma polymerized organosilicones and organometallics, in: R. d’Agostino (Ed.), Plasma Deposition Treatment and Etching of Polymers, Academic Press, 1990, p. 163. w4x A. Grill, V. Patel, J. Appl. Phys. 85 (1999) 3314. w5x S. Roualdes, A. Van der Lee, R. Berjoan, J. Sanchez, ´ J. Durand, AIChE J. 7 (1999) 1575. w6x L. Martinu, D. Poitras, J. Vac. Sci. Technol. A 18 (2000) 2619. w7x L. Zajickova, V. Bursickova, V. Perina, et al., Surf. Coat. Technol. 142 (2001) 449. w8x C. Vautrin-Ul, C. Boisse-Laporte, N. Benissad, A. Chausse, P. Leprince, R. Messina, Prog. Org. Coat. 38 (2000) 9. w9x A. Quede, C. Jama, P. Supiot, et al., Surf. Coat. Technol. 151– 152 (2002) 424.
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