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Atmospheric pressure chemical vapour deposition of polycarbosilane films via UV laser-induced polymerization of ethynyltrimethylsilane
Surface and Coatings Technology 149 (2002) 129–134
Atmospheric pressure chemical vapour deposition of polycarbosilane films via UV laser-induced polymerization of ethynyltrimethylsilane d ˇ J. Polaa,*, Z. Bastlb, A. Ouchic, J. Subrt , H. Moritae a
Academy of Sciences of the Czech Republic, Institute of Chemical Process Fundamentals, Laser Chemistry Group, Rozvojova Str., 16502 Prague, Czech Republic b J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague, Czech Republic c National Institute of Materials and Chemical Research, Tsukuba, Ibaraki, 305–8565, Japan d ˇ ˇ Czech Republic Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Rez, e Graduate School of Science and Technology, Chiba University, Chiba 263, Japan Received 6 November 2000; accepted in revised form 13 August 2001
Abstract ArF laser-induced polymerization of gaseous ethynyltrimethylsilane at atmospheric pressure of He represents a convenient way of efficient chemical vapour deposition of polycarbosilane films. The films are produced at ambient temperature of metals, quartz and glass, and are adhesive to these substrates, which makes this process promising for fabrication of protective coatings on thermally unstable surfaces. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Polycarbosilane films; Laser; Chemical vapour deposition; Ethynyltrimethylsilane
1. Introduction Chemical vapour deposition of polymeric films and coatings via laser-induced gas phase polymerization of organic and organometallic compounds in the gas phase has been recently demonstrated. It has been achieved via (a) polymerization of short-lived species (e.g. silenes w1x, silanone w2x, methanimine w3x pyridinyl w4x radicals) generated in high concentrations by IR or UV laserinduced decomposition of gaseous precursors, or via (b) UV laser production of high concentrations of reactive species by excitation and intermolecular reaction of gaseous methyl acrylate w5x, acrolein w6x, carbon disulphide w7x and mixtures of methyl acrylate and carbon disulfide w8,9x. Recently, we have reported on the efficient lowpressure UV laser-induced gas-phase polymerization of * Corresponding author. Tel.: q420-2-20390308; fax: q420-220920661. E-mail address: [email protected] (J. Pola).
two terminal organosilicon alkynes. The ArF laser irradiation into 3-propynyloxytrimethylysilane w10x and ethynyltrimethylsilane w11x afforded respective formation of thin poly(trimethylsilyloxyhydrocarbon- and poly(trimethylsilylhydrocarbon)-based films. The latter process and the films were described and discussed only for the low-pressure (5 torr) of the parent ETS. This paper deals with UV laser irradiation of ethynyltrimethylsilane (ETS) (100 torr) at atmospheric pressure of helium and reveals this process as a very efficient technique for facile production of polycarbosilane films adhesive to substrates maintained at room temperature. 2. Experimental The UV laser photolysis of ETS (100 torr) in He (total pressure 760 torr) was carried out in a reactor (140 ml in volume) which consisted of two orthogonally positioned Pyrex tubes, one fitted with two quartz plates and the other furnished with two NaCl windows. ETS in He was irradiated at a repetition frequency of 10 Hz
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surface of samples which could form during their transport from the reaction cell to the spectrometer. ETS was commercial sample (Aldrich) distilled prior to use. 3. Results and discussion Gaseous ETS shows absorptivity at 193 nm 4.0=10y3 torry1 cmy1 (Fig. 1) and efficiently absorbs ArF laser radiation to yield gaseous products and an intense white fog observed throughout all the inside of the reactor. Upon the first several pulses into ETS in He, intense currents of the white fog arise in the area of the laser beam behind the reactor entrance window, move within the laser beam area farther to the centre of the reactor and within seconds fill all the reactor volume. The fog descending on the reactor bottom produces transparent films. Fig. 1. UVyVIS absorption spectrum of the deposited film (a) and of ETS (b).
by pulses from a Lambda Physik LPX 210I (ArF) laser operating at an incident energy of 18, 37 and 67 mJy cm2 effective on the area of 2.6 cm2. The laser pulse energy was measured with a pyroelectric joulemeter (Gentec ED-500). The progress of the photolysis was monitored by FTIR spectroscopy (a Shimadzu FTIR 4000 spectrometer) using absorption band of ETS at 660 cmy1 and by gas-chromatography (Gasukuro Kogyo model 370 chromatograph equipped with a packed 2-m long Unipak S SUS column, programmed temperature 30–1508C, FID detector, helium carrier gas). The volatile photolytic products were identified by GCyMS spectrometry (a Shimadzu QP 1000 spectrometer) and their amounts were determined by the gas chromatography. Properties of the deposited films were measured by the FTIR spectroscopy, scanning electron microscopy (a Philips XL30 CP Scanning Electron Microscope instrument) and by X-ray photoelectron spectroscopy (ESCA 310, Gammadata Scienta) and X-ray excited Auger electron spectroscopy (ESCA3 MkII, VG Scientific). The electron spectra measurements were performed in a vacuum of 10y9 mbar, using Al Ka radiation. The spectra of Si 2p, C 1s and O 1s photoelectrons and C KLL Auger electrons were recorded. The Si KLL electrons were excited using bremsstrahlung radiation produced by unmonochromatized X-ray source. The core level binding energies as well as the kinetic energies of Auger electrons were determined with an accuracy of "0.2 eV. The surface composition of the samples was determined before and after mild ion etching (Ar ions, Es5 keV, Is20 A, ts3 min). The aim of the latter procedure was to remove the oxidized layer on the
3.1. Gaseous products The gaseous products are methane, ethane, ethene, ethyne, propyne, C4-hydrocarbons and trimethylsilane (Fig. 2). Their amounts (in moleymole of depleted ETS = 100)-methane (10–25), ethane (5–7), ethene (;1), ethyne (2–3), C3H4 (8–10) and trimethylsilane (;1) are not too considerably affected by the photolysis progress and do not noticeably differ from those observed earlier in the low pressure photolysis w11x. The energy delivered by the photons at 193 nm corresponds to ca. 620 kJ moley1, which is much w12x in excess of the energy needed for the cleavage of the Si–C (ca. 370 kJ moley1) and C–H (ca. 410 kJ moley1) bonds. The observed gaseous products reveal some extent of cleavage of the Si–C bond in ETS and recombination of and H-abstraction by the formed ethynyl and methyl radicals (Scheme 1). The low yields of the gaseous hydrocarbon products can account for ca. 20% of ETS depletion. The very
Fig. 2. Typical GC trace of volatile products. 1, methane; 2, ethane; 3, ethene; 4, ethyne, 5, propyne; 6, C4 hydrocarbons; 7, trimethylsilane; 8, ETS.
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131
Scheme 1.
low yield of trimethylsilane and the absence of other volatile silicon-containing products indicate very efficient sink reactions for the assumed (CH3)3Si. and.Si(CH3)C^CH radicals leading to the observed polymeric aerosol particles. We note that the relative efficiency for the formation of the gaseous and of the solid products is comparable to that found w11x for the low-pressure photolysis. 3.2. Properties of coatings The coatings properties are not noticeably affected by the laser fluence used. The coatings are transparent and survive the tape test when deposited to surface of metals (Cu, Al), glass and quartz. Their IR absorption spectrum wwavenumberycmy1, (normalized absorptivity)x-698 (0.10), 803 (0.96), 845 (0.90), 1049 (1.0), 1250 (0.48), 1414 (0.10), 1625 (0.14), 1728 (0.22), 2890 (0.12), 2950 (0.22) and 3288 (0.04) can be assigned, in the given order, to n(SiyC), r(CH3Si), n(Si–O), d(CH3Si), n(C_C), n(C_O), n(C^CH), n(Csp3yH) and n(CspyH) vibrations (Fig. 3). This pattern is in line with the absence or a small amount of H atoms bonded to sp2 carbon and with H atoms mostly bonded to sp3 carbon. UVyVIS spectrum of the films (Fig. 1) shows maximum absorption at 200 nm, and is tailing to almost 500 nm, which is in accord with some degree of unsaturation. In Figs. 4 and 5 typical Si 2p and Si KLL spectra are shown for the deposits and for the reference sample of
Fig. 3. FTIR spectrum of the deposited film.
Fig. 4. Si 2p core level spectra of (1) as received laser deposit, (2) deposit after ion sputtering, (3) polycarbosilane and (4) polycarbosilane after ion sputtering.
authentic polycarbosilane (PCS) in which only SiyC, CyC and CyH single bonds are involved, (yMe2SiMe2SiCH2CH2y)n . The measured Si 2p binding energies, Auger parameters (defined as the sum of the binding energy of Si 2p electrons) and kinetic energies of Si KLL Auger electrons) along with the surface composition are displayed in Table 1. Binding energy of O 1s electrons is the same for all samples, 532.4 eV. The presence of relatively high concentration of oxygen in the as received as well as in the sputtered
Fig. 5. Spectra of Si KLL electrons. Definitions as in Fig. 3.
J. Pola et al. / Surface and Coatings Technology 149 (2002) 129–134
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Table 1 Si 2p core level binding energies, Auger parameters (both in eV) and surface stoichiometry for as received and sputtered deposits and authentic sample of PCS. Sample
Si 2p
A
Stoichiometry
Deposit as received sputtered PCS as received sputtered
101.8 101.8 100.7 100.7
1711.3 1713.4 1712.1 1714.5
Si1.0C3.2O1.3 Si1.0C2.7O1.0 Si1.0C3.0O0.1 Si1.0C2.3O0.1
deposits, the higher binding energy of Si 2p electrons and lower value of Auger parameter point to the presence of SiyO bonds, the population of which in the PCS is negligible (Figs. 4 and 5). According to the literature w13–15x the hybridization of carbon atoms can be estimated from the C KLL spectra, namely from the separation between the two major excursions in the C KLL first-derivative spectra. Similar spectra are obtained for the deposit and the PCS sample (Fig. 6). This result indicates that the deposit contains predominantly sp3 hybridized carbon atoms. In addition to removal of the surface layers the ion sputtering induces remarkable changes in chemistry of the samples studied. These changes do not influence the binding energies of the Si 2p electrons (see Fig. 4 and Table 1) but cause significant shifts of the Auger spectra (Fig. 5) and consequently of the values of the Auger parameters. It can be deduced from comparison of the derivative C KLL spectra of the sputtered sample with the spectra of diamond and graphite (Fig. 6) that the prevailing bonding configuration of the carbon atoms in ion irradiated deposits is sp2. Scanning electron microscopy reveals that the films are not formed from distinct several mm-sized agglomerates, which is widespread for laser chemical vapour deposition of polycarbosilanes from other precursors like silacyclobutanes, organylsilanes and siloxanes (e.g. w1,16–19x). Instead, the films have a continuous structure which must be composed of bodies whose size is many times smaller than 1 mm (Fig. 7). The high content of oxygen in the as received and sputtered films reveals that the films are efficiently penetrated by air and react with oxygen. The coatings are insoluble in common organic solvents (chloroform, benzene, acetone, hexane); this feature shows that the deposited polymer possess a very high molecular weight andyor cross-linked structure.
multiple associations of excited ETS molecules and (2) polymerization induced by addition of R(CH3)2Si. and R (R_CH3, HC2) radicals to ETS molecules and ensued by reactions of the formed radical adducts with ETS. These plausible routes are in Scheme 2 designated as A and B, respectively. The once deposited particles must possess some degree of reactivity in order to react with each other to produce continuous films. The stoichiometry of the films ascertained from the XP spectra (Table 1) cannot be in line with the observed C1yC4 hydrocarbons being the only hydrocarbon products. It is conceivable that the SiyC cleavage reactions are of a larger extent than can be deduced from the observed yield of the C1–C4 hydrocarbons and that the photolysis of ETS also affords hydrocarbons with higher molecular weight, which escaped GC detection and were pumped out of the reactor before transferring the films for XPS analysis. 3.4. Efficiency of the deposition process The deposit gradually grows on lower parts of the inside of the batch reactor. It also covers the entrance reactor window and causes a continuous decay of the laser power within the reactor due to its absorption at 193 nm (Fig. 1). This makes difficult to achieve ETS depletion higher than 30% even when the irradiation is directed to both reactor windows. In spite of that, reasonably high yields of the deposit can be obtained in short irradiation periods; thus, e.g. with fluence 67 mJy cm2 and incident area 2.6 cm2, the deposition of ca. 18 mg of the polymer can be achieved within the 5-min
3.3. Mechanism of films formation The gaseous products and the spectral data on solid films indicate that the major step responsible for the deposition is polymerization on the triple bond. The formation of the observed particles in the gas phase can occur by two paths which are (1) polymerization via
Fig. 6. X-Ray excited derivative spectra of C KLL electrons of (1) as received laser deposit, (2) deposit after ion sputtering (3) polycarbosilane, (4) diamond and (5) graphite.
J. Pola et al. / Surface and Coatings Technology 149 (2002) 129–134
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Fig. 8. Dependence of ETS depletion on number of pulses with laser fluence 18 (j), 37(x) and 67 (d) mJycm2.
quantum yield with higher fluence is in line with some spallation of the deposit from the entrance reactor window at higher fluence. 4. Conclusion
Fig. 7. SEM image of the deposited film. Magnification 2000 = (a) and 160 = (b).
irradiation. The dependence of ETS depletion on number of pulses with laser fluence 18, 37 and 67 mJycm2 (Fig. 8) shows that the ETS depletion from the gas phase is enhanced with increasing fluence. The quantum yields for ETS depletion with the particular fluences were assessed as 0.17, 0.22 and 0.27, respectively. These values are obstructed by absorption of laser pulses in the films deposited on the entrance window and the true values must be definitely higher. The increase of the
The laser photolysis into gaseous ETS is a useful technique for low-temperature chemical vapour deposition of polycarbosilane coatings which may find applications as insulating films in microelectronics. The fact that the UV radiation-excited polymerization of ETS yields organosilicon material not containing a detrimental admixture of a catalystyphotoinitiator, the species normally used to achieve polymerization of alkynes in the condensed phases (e.g. w20–23x), suggests the use of the material for medical applications and optical data storage and processing. Acknowledgements This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic (grant no. ME192) and by the Science and Technology Agency (Japan). The authors thank Prof. L.E. Gusel’nikov for the gift of PCS sample. References
Scheme 2.
w1x J. Pola, Surf. Coat. Technol. 100–101 (1998) 408 and refs. therein. ˇ w2x J. Pola, M. Urbanova, ´ V. Drınek, ´ J. Subrt, H. Beckers, Appl. Organometal. Chem. J. Mater. Chem. 9 (1999) 2429. w3x J. Pola, A. Lycka, L.E. Gusel’nikov, V.V. Volkova, J. Chem. Soc. Chem. Commun. (1992) 20. w4x M. Urbanova, ´ J. Vıtek, ´ Z. Bastl, K. Ubik, J. Pola, J. Mater. Chem. 5 (1995) 849. w5x H. Morita, T. Sadakiyo, J. Photochem. Photobiol. A: Chem. 87 (1995) 163. w6x H. Morita, M. Shinizu, J. Phys. Chem. 99 (1995) 7621.
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w7x A. Matsuzaki, Y. Hamada, H. Morita, T. Matsuzaki, Chem. Phys. Lett. 190 (1992) 337. w8x H. Morita, H. Haga, J. Photopolym. Sci. Technol. 8 (1995) 475. w9x H. Morita, K. Kanazawa, J. Photochem. Photobiol. A: Chem. 112 (1998) 81. w10x J. Pola, H. Morita, Tetrahedron Lett. 38 (1997) 7809. w11x J. Pola, M. Urbanova, ´ Z. Bastl, H. Morita, Macromol. Rapid Commun. 21 (2000) 178. w12x R. Walsh, in: S. Patai, Z. Rappoport (Eds.), The Chemistry of Organic Silicon Compounds, Wiley, Chichester, 1989, p. 5. w13x A.A. Galuska, H.H. Maden, R.E. Allred, Appl. Surf. Sci. 32 (1988) 253. w14x J.M. Lascovich, R. Giorgi, S. Scaglione, Appl. Surf. Sci. 47 (1991) 17. w15x S.T. Jackson, R.G. Nuzzo, Appl. Surf. Sci. 90 (1995) 195.
w16x J. Pola, E.A. Volnina, L.E. Gusel’nikov, J. Organometal. Chem. 391 (1990) 275. w17x J. Pola, Res. Chem. Intermed. 25 (1999) 351. ˇ w18x J. Pola, J. Vıtek, ´ ´ J. Subrt, Z. Bastl, M. Urbanova, R. Taylor, J. Mater. Chem. 7 (1997) 1415. ˇ w19x M. Urbanova, ´ V. Drınek, ´ J. Subrt, H. Beckers, Appl. Organometal. Chem. 13 (1999) 655. w20x J.M. Ziegler, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 25 (1984) 223. w21x J. Kunzler, V. Percec, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 29 (1988) 219. w22x M.G. Voronkov, V.B. Pukhnarevich, S.P. Sushchinskaya, V.Z. Annenkova, V.M. Annenkova, N.J. Andreeva, J. Polym. Sci. Polym. Chem. 18 (1980) 53. w23x Y. Okano, T. Masuda, T. Higashimura, J. Polym. Sci. Polym. Chem. 22 (1984) 1603.
Atmospheric pressure chemical vapour deposition of polycarbosilane films via UV laser-induced polymerization of ethynyltrimethylsilane d ˇ J. Polaa,*, Z. Bastlb, A. Ouchic, J. Subrt , H. Moritae a
Academy of Sciences of the Czech Republic, Institute of Chemical Process Fundamentals, Laser Chemistry Group, Rozvojova Str., 16502 Prague, Czech Republic b J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague, Czech Republic c National Institute of Materials and Chemical Research, Tsukuba, Ibaraki, 305–8565, Japan d ˇ ˇ Czech Republic Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Rez, e Graduate School of Science and Technology, Chiba University, Chiba 263, Japan Received 6 November 2000; accepted in revised form 13 August 2001
Abstract ArF laser-induced polymerization of gaseous ethynyltrimethylsilane at atmospheric pressure of He represents a convenient way of efficient chemical vapour deposition of polycarbosilane films. The films are produced at ambient temperature of metals, quartz and glass, and are adhesive to these substrates, which makes this process promising for fabrication of protective coatings on thermally unstable surfaces. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Polycarbosilane films; Laser; Chemical vapour deposition; Ethynyltrimethylsilane
1. Introduction Chemical vapour deposition of polymeric films and coatings via laser-induced gas phase polymerization of organic and organometallic compounds in the gas phase has been recently demonstrated. It has been achieved via (a) polymerization of short-lived species (e.g. silenes w1x, silanone w2x, methanimine w3x pyridinyl w4x radicals) generated in high concentrations by IR or UV laserinduced decomposition of gaseous precursors, or via (b) UV laser production of high concentrations of reactive species by excitation and intermolecular reaction of gaseous methyl acrylate w5x, acrolein w6x, carbon disulphide w7x and mixtures of methyl acrylate and carbon disulfide w8,9x. Recently, we have reported on the efficient lowpressure UV laser-induced gas-phase polymerization of * Corresponding author. Tel.: q420-2-20390308; fax: q420-220920661. E-mail address: [email protected] (J. Pola).
two terminal organosilicon alkynes. The ArF laser irradiation into 3-propynyloxytrimethylysilane w10x and ethynyltrimethylsilane w11x afforded respective formation of thin poly(trimethylsilyloxyhydrocarbon- and poly(trimethylsilylhydrocarbon)-based films. The latter process and the films were described and discussed only for the low-pressure (5 torr) of the parent ETS. This paper deals with UV laser irradiation of ethynyltrimethylsilane (ETS) (100 torr) at atmospheric pressure of helium and reveals this process as a very efficient technique for facile production of polycarbosilane films adhesive to substrates maintained at room temperature. 2. Experimental The UV laser photolysis of ETS (100 torr) in He (total pressure 760 torr) was carried out in a reactor (140 ml in volume) which consisted of two orthogonally positioned Pyrex tubes, one fitted with two quartz plates and the other furnished with two NaCl windows. ETS in He was irradiated at a repetition frequency of 10 Hz
0257-8972/01/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 9 5 - 5
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surface of samples which could form during their transport from the reaction cell to the spectrometer. ETS was commercial sample (Aldrich) distilled prior to use. 3. Results and discussion Gaseous ETS shows absorptivity at 193 nm 4.0=10y3 torry1 cmy1 (Fig. 1) and efficiently absorbs ArF laser radiation to yield gaseous products and an intense white fog observed throughout all the inside of the reactor. Upon the first several pulses into ETS in He, intense currents of the white fog arise in the area of the laser beam behind the reactor entrance window, move within the laser beam area farther to the centre of the reactor and within seconds fill all the reactor volume. The fog descending on the reactor bottom produces transparent films. Fig. 1. UVyVIS absorption spectrum of the deposited film (a) and of ETS (b).
by pulses from a Lambda Physik LPX 210I (ArF) laser operating at an incident energy of 18, 37 and 67 mJy cm2 effective on the area of 2.6 cm2. The laser pulse energy was measured with a pyroelectric joulemeter (Gentec ED-500). The progress of the photolysis was monitored by FTIR spectroscopy (a Shimadzu FTIR 4000 spectrometer) using absorption band of ETS at 660 cmy1 and by gas-chromatography (Gasukuro Kogyo model 370 chromatograph equipped with a packed 2-m long Unipak S SUS column, programmed temperature 30–1508C, FID detector, helium carrier gas). The volatile photolytic products were identified by GCyMS spectrometry (a Shimadzu QP 1000 spectrometer) and their amounts were determined by the gas chromatography. Properties of the deposited films were measured by the FTIR spectroscopy, scanning electron microscopy (a Philips XL30 CP Scanning Electron Microscope instrument) and by X-ray photoelectron spectroscopy (ESCA 310, Gammadata Scienta) and X-ray excited Auger electron spectroscopy (ESCA3 MkII, VG Scientific). The electron spectra measurements were performed in a vacuum of 10y9 mbar, using Al Ka radiation. The spectra of Si 2p, C 1s and O 1s photoelectrons and C KLL Auger electrons were recorded. The Si KLL electrons were excited using bremsstrahlung radiation produced by unmonochromatized X-ray source. The core level binding energies as well as the kinetic energies of Auger electrons were determined with an accuracy of "0.2 eV. The surface composition of the samples was determined before and after mild ion etching (Ar ions, Es5 keV, Is20 A, ts3 min). The aim of the latter procedure was to remove the oxidized layer on the
3.1. Gaseous products The gaseous products are methane, ethane, ethene, ethyne, propyne, C4-hydrocarbons and trimethylsilane (Fig. 2). Their amounts (in moleymole of depleted ETS = 100)-methane (10–25), ethane (5–7), ethene (;1), ethyne (2–3), C3H4 (8–10) and trimethylsilane (;1) are not too considerably affected by the photolysis progress and do not noticeably differ from those observed earlier in the low pressure photolysis w11x. The energy delivered by the photons at 193 nm corresponds to ca. 620 kJ moley1, which is much w12x in excess of the energy needed for the cleavage of the Si–C (ca. 370 kJ moley1) and C–H (ca. 410 kJ moley1) bonds. The observed gaseous products reveal some extent of cleavage of the Si–C bond in ETS and recombination of and H-abstraction by the formed ethynyl and methyl radicals (Scheme 1). The low yields of the gaseous hydrocarbon products can account for ca. 20% of ETS depletion. The very
Fig. 2. Typical GC trace of volatile products. 1, methane; 2, ethane; 3, ethene; 4, ethyne, 5, propyne; 6, C4 hydrocarbons; 7, trimethylsilane; 8, ETS.
J. Pola et al. / Surface and Coatings Technology 149 (2002) 129–134
131
Scheme 1.
low yield of trimethylsilane and the absence of other volatile silicon-containing products indicate very efficient sink reactions for the assumed (CH3)3Si. and.Si(CH3)C^CH radicals leading to the observed polymeric aerosol particles. We note that the relative efficiency for the formation of the gaseous and of the solid products is comparable to that found w11x for the low-pressure photolysis. 3.2. Properties of coatings The coatings properties are not noticeably affected by the laser fluence used. The coatings are transparent and survive the tape test when deposited to surface of metals (Cu, Al), glass and quartz. Their IR absorption spectrum wwavenumberycmy1, (normalized absorptivity)x-698 (0.10), 803 (0.96), 845 (0.90), 1049 (1.0), 1250 (0.48), 1414 (0.10), 1625 (0.14), 1728 (0.22), 2890 (0.12), 2950 (0.22) and 3288 (0.04) can be assigned, in the given order, to n(SiyC), r(CH3Si), n(Si–O), d(CH3Si), n(C_C), n(C_O), n(C^CH), n(Csp3yH) and n(CspyH) vibrations (Fig. 3). This pattern is in line with the absence or a small amount of H atoms bonded to sp2 carbon and with H atoms mostly bonded to sp3 carbon. UVyVIS spectrum of the films (Fig. 1) shows maximum absorption at 200 nm, and is tailing to almost 500 nm, which is in accord with some degree of unsaturation. In Figs. 4 and 5 typical Si 2p and Si KLL spectra are shown for the deposits and for the reference sample of
Fig. 3. FTIR spectrum of the deposited film.
Fig. 4. Si 2p core level spectra of (1) as received laser deposit, (2) deposit after ion sputtering, (3) polycarbosilane and (4) polycarbosilane after ion sputtering.
authentic polycarbosilane (PCS) in which only SiyC, CyC and CyH single bonds are involved, (yMe2SiMe2SiCH2CH2y)n . The measured Si 2p binding energies, Auger parameters (defined as the sum of the binding energy of Si 2p electrons) and kinetic energies of Si KLL Auger electrons) along with the surface composition are displayed in Table 1. Binding energy of O 1s electrons is the same for all samples, 532.4 eV. The presence of relatively high concentration of oxygen in the as received as well as in the sputtered
Fig. 5. Spectra of Si KLL electrons. Definitions as in Fig. 3.
J. Pola et al. / Surface and Coatings Technology 149 (2002) 129–134
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Table 1 Si 2p core level binding energies, Auger parameters (both in eV) and surface stoichiometry for as received and sputtered deposits and authentic sample of PCS. Sample
Si 2p
A
Stoichiometry
Deposit as received sputtered PCS as received sputtered
101.8 101.8 100.7 100.7
1711.3 1713.4 1712.1 1714.5
Si1.0C3.2O1.3 Si1.0C2.7O1.0 Si1.0C3.0O0.1 Si1.0C2.3O0.1
deposits, the higher binding energy of Si 2p electrons and lower value of Auger parameter point to the presence of SiyO bonds, the population of which in the PCS is negligible (Figs. 4 and 5). According to the literature w13–15x the hybridization of carbon atoms can be estimated from the C KLL spectra, namely from the separation between the two major excursions in the C KLL first-derivative spectra. Similar spectra are obtained for the deposit and the PCS sample (Fig. 6). This result indicates that the deposit contains predominantly sp3 hybridized carbon atoms. In addition to removal of the surface layers the ion sputtering induces remarkable changes in chemistry of the samples studied. These changes do not influence the binding energies of the Si 2p electrons (see Fig. 4 and Table 1) but cause significant shifts of the Auger spectra (Fig. 5) and consequently of the values of the Auger parameters. It can be deduced from comparison of the derivative C KLL spectra of the sputtered sample with the spectra of diamond and graphite (Fig. 6) that the prevailing bonding configuration of the carbon atoms in ion irradiated deposits is sp2. Scanning electron microscopy reveals that the films are not formed from distinct several mm-sized agglomerates, which is widespread for laser chemical vapour deposition of polycarbosilanes from other precursors like silacyclobutanes, organylsilanes and siloxanes (e.g. w1,16–19x). Instead, the films have a continuous structure which must be composed of bodies whose size is many times smaller than 1 mm (Fig. 7). The high content of oxygen in the as received and sputtered films reveals that the films are efficiently penetrated by air and react with oxygen. The coatings are insoluble in common organic solvents (chloroform, benzene, acetone, hexane); this feature shows that the deposited polymer possess a very high molecular weight andyor cross-linked structure.
multiple associations of excited ETS molecules and (2) polymerization induced by addition of R(CH3)2Si. and R (R_CH3, HC2) radicals to ETS molecules and ensued by reactions of the formed radical adducts with ETS. These plausible routes are in Scheme 2 designated as A and B, respectively. The once deposited particles must possess some degree of reactivity in order to react with each other to produce continuous films. The stoichiometry of the films ascertained from the XP spectra (Table 1) cannot be in line with the observed C1yC4 hydrocarbons being the only hydrocarbon products. It is conceivable that the SiyC cleavage reactions are of a larger extent than can be deduced from the observed yield of the C1–C4 hydrocarbons and that the photolysis of ETS also affords hydrocarbons with higher molecular weight, which escaped GC detection and were pumped out of the reactor before transferring the films for XPS analysis. 3.4. Efficiency of the deposition process The deposit gradually grows on lower parts of the inside of the batch reactor. It also covers the entrance reactor window and causes a continuous decay of the laser power within the reactor due to its absorption at 193 nm (Fig. 1). This makes difficult to achieve ETS depletion higher than 30% even when the irradiation is directed to both reactor windows. In spite of that, reasonably high yields of the deposit can be obtained in short irradiation periods; thus, e.g. with fluence 67 mJy cm2 and incident area 2.6 cm2, the deposition of ca. 18 mg of the polymer can be achieved within the 5-min
3.3. Mechanism of films formation The gaseous products and the spectral data on solid films indicate that the major step responsible for the deposition is polymerization on the triple bond. The formation of the observed particles in the gas phase can occur by two paths which are (1) polymerization via
Fig. 6. X-Ray excited derivative spectra of C KLL electrons of (1) as received laser deposit, (2) deposit after ion sputtering (3) polycarbosilane, (4) diamond and (5) graphite.
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Fig. 8. Dependence of ETS depletion on number of pulses with laser fluence 18 (j), 37(x) and 67 (d) mJycm2.
quantum yield with higher fluence is in line with some spallation of the deposit from the entrance reactor window at higher fluence. 4. Conclusion
Fig. 7. SEM image of the deposited film. Magnification 2000 = (a) and 160 = (b).
irradiation. The dependence of ETS depletion on number of pulses with laser fluence 18, 37 and 67 mJycm2 (Fig. 8) shows that the ETS depletion from the gas phase is enhanced with increasing fluence. The quantum yields for ETS depletion with the particular fluences were assessed as 0.17, 0.22 and 0.27, respectively. These values are obstructed by absorption of laser pulses in the films deposited on the entrance window and the true values must be definitely higher. The increase of the
The laser photolysis into gaseous ETS is a useful technique for low-temperature chemical vapour deposition of polycarbosilane coatings which may find applications as insulating films in microelectronics. The fact that the UV radiation-excited polymerization of ETS yields organosilicon material not containing a detrimental admixture of a catalystyphotoinitiator, the species normally used to achieve polymerization of alkynes in the condensed phases (e.g. w20–23x), suggests the use of the material for medical applications and optical data storage and processing. Acknowledgements This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic (grant no. ME192) and by the Science and Technology Agency (Japan). The authors thank Prof. L.E. Gusel’nikov for the gift of PCS sample. References
Scheme 2.
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