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Boron carbonitride films deposited by pulsed laser ablation
Applied Surface Science 133 Ž1998. 239–242
Boron carbonitride films deposited by pulsed laser ablation A. Perrone a
a,)
, A.P. Caricato a , A. Luches a , M. Dinescu b, C. Ghica b, V. Sandu b, A. Andrei c
UniÕersity of Lecce and Istituto Nazionale Fisica della Materia, Department of Physics, 73100-Lecce, Italy b Institute of Atomic Physics, PO Box MG 16, RO 76900, Bucharest, Romania c Institute of Nuclear Research, Pitesti, Romania Received 27 May 1997; accepted 19 February 1998
Abstract Boron carbonitride ŽBCN. thin films were deposited on Si Ž100. substrates at room temperature by sequential pulsed laser ablation ŽPLA. of graphite and hexagonal boron nitride Žh-BN. targets in vacuum and in nitrogen atmosphere in the pressure range 1–100 Pa. Different analysis techniques as transmission electron microscopy ŽTEM., X-ray diffraction ŽXRD. and X-ray photoelectron spectroscopy ŽXPS. pointed out the synthesis of h-BCN and c-BCN. The grain size of the crystalline c-BCN phase was estimated to be in the range 30–80 nm. The size of the crystallites in h-BCN phase was 4.6 m m, with a transversal dimension of about 30 nm. Complementary microhardness measurements evidenced the high microhardness Žvalues up to 2.9 GPa. of the deposited films. q 1998 Elsevier Science B.V. All rights reserved. PACS: 8115; 6855; 0780; 3280F Keywords: Laser ablation deposition; Thin films; Hard materials
1. Introduction BCN films have been studied in the last years due to the wide range of prospective applications as light elements based coatings. Mechanical properties comparable to those of diamond-like structures make them useful for wear protection applications w1,2x and the high optical transparency can be used as mask substrates for X-ray lithography w3x. Previously, BCN thin films were obtained by using several deposition methods w4–9x, but most of them were amorphous and hydrogenated. The first c-BCN )
Corresponding author. Tel.: q39-832-320501; fax: q39-832320505; e-mail: [email protected].
films were deposited by Loeffler et al. w10x using r.f. plasma-assisted chemical vapor deposition and by Doll w11x using laser ablation. In this paper we report the successful growth of c-BCN and h-BCN thin films by PLA of a target formed of 2 semidisks: one of h-BN and the other of graphite, with the substrate at room temperature.
2. Experimental apparatus Only a basic outline of our experimental apparatus is given here, since it has already been described elsewhere w12x. The irradiations were performed using a Lambda Physik LPX 315i XeCl excimer laser
0169-4332r98r$ – see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 0 7 - 4
A. Perrone et al.r Applied Surface Science 133 (1998) 239–242
240
Table 1 A summary of the samples used in this experiment Sample
1
2
3
4
PN 2 ŽPa.
10y5
1
5
10
5 100
Ž l s 308 nm, t FW HM s 30 ns.. Series of 10,000 pulses at a repetition rate of 10 Hz were directed to the target. The fluence was set at 5 Jrcm2 . A 30-cm focal length lens was used to focus the laser radiation on the target which continually rotated Ž3 Hz. to reduce the effect of crating on the target surface. The target was prepared as a disk, combining together 2 semi-disk, one of h-BN and one of C Žgraphite.. Si Ž100. etched in dilute HF acid was used as substrate. The target-substrate distance was fixed at 4 cm. The stainless steel deposition chamber was evacuated by a turbomolecular pump down to 10y5 Pa and then filled with N2 at different pressures Ž1 to 100 Pa.. Table 1 presents a summary of the samples used in this investigation, with the indication of the working pressures.
3. Experimental results The crystallinity, composition, morphology and chemical bonding of our films were investigated by XRD, XPS and TEM analyses. The results of all these analysis techniques indicate that the sample deposited in vacuum mainly contains a mixture of h-BCN and c-BCN, the rest of the film appearing to be amorphous, or with some crystalline h-BN. The XPS analyses were carried out with a VG ESCALAB MKII spectrometer using Al K a Ž1486.6 eV. X-ray radiation. The spectra calibration was realized with the Ag M 4 NN Ž1128.7 eV. line. Fig. 1 shows a typical XPS spectrum for B Ž1s. as recorded for the film deposited in vacuum Žsample 1.. The results of the deconvolution procedure show the presence of 3 different possible boron chemical states, centered at 188.0, 189.8 and 192.0 eV. The chemical shift of the pure boron–nitride bond Ž189.8 eV. w13x toward lower energy corresponds to the B bonded in a BCN2 or more probably in a BNC 2 compound. In fact, it is known that the peak B Ž1s. in the ŽBB 3 . structure is shifted by y4 eV with the
Fig. 1. XPS results for B Ž1s. peak of the film deposited in vacuum.
respect to the reference position from 189.8 eV corresponding to the case BN3 , while the peak corresponding to BC 3 is shifted by only y2.5 eV. The BNC 2 structure is closer to the BC 3 structure than to the BN3 structure. This allows us to assign the peak shifted by y1.8 eV to the B in BNC 2 . The component at 192.0 eV can be assigned to B–O bonding w14x. Fig. 2 shows the XPS of the N Ž1s. line shape for the same sample. The deconvolution gives 3 different structures with peaks at 398.0, 399.3 and 401.5 eV. The N Ž1s. peak of B–N bonding appears at 398 eV w15,16x. In the case of C–N bonding, the N Ž1s.
Fig. 2. XPS results for N Ž1s. peak of the film deposited in vacuum.
A. Perrone et al.r Applied Surface Science 133 (1998) 239–242
241
is displaced by q1.3 eV, thus the component centered at 399.3 eV can be assigned to C–N bonding. Similar XPS spectra were obtained for the samples deposited in N2 atmosphere. However, we noticed a decrease of the B concentration and consequently the BCN amount in the films with increasing N2 pressure. The oxygen incorporated in the films was Žin atomic %. of the order of 4–5% and was determined from the oxygen bonds to nitrogen, boron and carbon appearing in the XPS spectra corresponding to N Ž1s., B Ž1s. and C Ž1s. spectral regions. The main part was coming from the B–O bonds. The reactivity of B respect to O is very high and the B–O bonds were observed even if the oxygen partial pressure in the vacuum chamber was very low. The N–O peak can be considered as surface pollution w13x. The oxygen concentration seems to be relatively high
Fig. 4. Ža. Chain of h-BCN having a total length of about 4.6 m m and a transversal dimension of about 30 nm. Žb. Diffraction rings of the h-BCN phase.
Fig. 3. Ža. Conglomerate of cubic BCN with crystallite size between 30 and 80 nm. Žb. Selected area electron diffraction image of a polycrystalline cubic BN0.26 C 0.74 of the sample 1.
because the composition and chemical bonds were studied without sputtering. The crystallinity of the films was investigated by XRD and TEM analyses. The TEM images and diffraction patterns have been obtained by a JEOL 200 CX microscope operating at 200 kV. A conglomerate of cubic BCN with crystallite size between 30 and 80 nm is presented in Fig. 3a and b shows a selected area electron diffraction image of a polycrystalline cubic BN0.26 C 0.74 conglomerate of the sample 1. The stoichiometric coefficients were determined using the observed perfect matching between the experimental electron diffraction pattern and the data in ASTM book ŽAmerican Society for Testing and Materials.. Fig. 4a shows a chain of hexagonal BCN having a total length of about 4.6 m m and a transversal dimension of about 30 nm.
242
A. Perrone et al.r Applied Surface Science 133 (1998) 239–242
Despite the closed resemblance to graphite in morphology and diffraction pattern, the lack of Ž103. diffraction ring and the relative intensities of the diffraction rings suggest the presence of the h-BCN phase ŽFig. 4b.. The growth direction is along the hexagonal c-axis. The TEM analyses of the samples prepared in N2 atmosphere indicate that they are formed of a mix˚ c0 s ture of polycrystalline h-BN Ž a 0 s 2.504 A, ˚ . having a fiber aspect, large monocrystalline 6.656 A flakes Ž1 m m. of h-BN and some amorphous stuff. The XRD analyses on sample 1 confirm the crystalline structure previously evidenced by TEM. Indeed, the diffraction line corresponding to an inter˚ is present in the spectra. It planar distance of 3.42 A corresponds to the Ž002. interplanar distance of a crystalline phase of h-BCN, the same identified by TEM. Another peak, larger, growing from an amorphous matrix, assigned to Ž111. line of the c-BCN ˚ is structure, with the interplanar distance of 2.70 A, also evidenced. Some other crystalline structures corresponding to different crystalline forms of h-BN are also present. In summary, XRD analyses suggest the presence of crystalline phases as h-BCN, c-BCN and h-BN, together with amorphous phases assigned to compounds as CN. The film microhardness values were measured by a Vickers Leitz Durimet. The indentation were performed under the load of 25, 50 and 100 g. The hardness of the films, always higher than 1 GPa, even though presents a general tendency to decrease with increasing nitrogen pressure, the highest hardness value Ž2.9 GPa. was obtained under a load of 25 g and for the sample deposited at 10 Pa.
4. Conclusions Deposition of BCN thin films by pulsed laser ablation, at room temperature, was successfully car-
ried out in these experiments, using a double BNgraphite target. The existence of bonds between all the B, C, N elements is well evidenced by XPS, and confirmed by TEM and XRD analysis. The results indicate that crystals of c-BCN, h-BCN and h-BN are formed. The films are very hard, showing a high microhardness value. In conclusion, PLA is proved to be a promising technique for one step BCN synthesis and deposition. Further parametric studies are in progress, in order to prepare a pure boron carbonitride compound.
References w1x A.Y. Liu, M.L. Cohen, Science 245 Ž1989. 841. w2x R. Riedel, Adv. Mater. 6 Ž1994. 549. w3x A. Weber, U. Bringmann, R. Nikulski, C.-P. Klages, Diamond Rel. Mater. 2 Ž1993. 201. w4x A.R. Badzian, S. Appenheimer, T. Niemyski, E. Olkusnik, Proc. Third Int. Conf. CVD 747 Ž1994. . w5x K.T. Rie, A. Gebauer, C. Pfohl, J. Phys. IV 5 Ž1995. 637. w6x M. Hubacek, T. Sato, J. Solid State Chem. 114 Ž1995. 258. w7x L. Filipozzi, A. Derre, Carbon 33 Ž1995. 1747. w8x K. Tanaka, S. Suchara, Trans. Inst. Electro. Inf. Commun. Eng. C II J78C Ž1995. 384. w9x T.M. Besmann, J. Am. Ceram. Soc. 73 Ž1990. 2498. w10x J. Loeffler, F. Steinbach, J. Bill, J. Mayer, F. Aldinger, Z. Metallk. 87 Ž1996. 170. w11x G.L. Doll, General Motors, US patent Nr. 5.330.611 Jul. 19 Ž1994.. w12x A. Luches, G. Leggieri, M. Martino, A. Perrone G. Majni, P. Mengucci, I.N. Mihailescu, Appl. Surf. Sci. 79r80 Ž1994. 244. w13x F. Saugnac, F. Teyssandyer, A. Marchand, J. Am. Ceram. Soc. 75 Ž1992. 161. w14x L. Lobstein, E. Millon, J.F. Muller, J. Lambert, M. Alnot, J.J. Erhardt, Proc. SPIE 2207 Ž1994. 705. w15x C.D. Wagner, W.M. Riegs, L.E. Davis, J.F. Moulder. In: G.E. Mulienberg ŽEd.., Handbook of X-ray Photoelectron spectroscopy, Perkin-Elmer, 1979. w16x F. Fujimoto, K. Ogata, Jpn. J. Appl. Phys. 32 Ž1993. L420.
Boron carbonitride films deposited by pulsed laser ablation A. Perrone a
a,)
, A.P. Caricato a , A. Luches a , M. Dinescu b, C. Ghica b, V. Sandu b, A. Andrei c
UniÕersity of Lecce and Istituto Nazionale Fisica della Materia, Department of Physics, 73100-Lecce, Italy b Institute of Atomic Physics, PO Box MG 16, RO 76900, Bucharest, Romania c Institute of Nuclear Research, Pitesti, Romania Received 27 May 1997; accepted 19 February 1998
Abstract Boron carbonitride ŽBCN. thin films were deposited on Si Ž100. substrates at room temperature by sequential pulsed laser ablation ŽPLA. of graphite and hexagonal boron nitride Žh-BN. targets in vacuum and in nitrogen atmosphere in the pressure range 1–100 Pa. Different analysis techniques as transmission electron microscopy ŽTEM., X-ray diffraction ŽXRD. and X-ray photoelectron spectroscopy ŽXPS. pointed out the synthesis of h-BCN and c-BCN. The grain size of the crystalline c-BCN phase was estimated to be in the range 30–80 nm. The size of the crystallites in h-BCN phase was 4.6 m m, with a transversal dimension of about 30 nm. Complementary microhardness measurements evidenced the high microhardness Žvalues up to 2.9 GPa. of the deposited films. q 1998 Elsevier Science B.V. All rights reserved. PACS: 8115; 6855; 0780; 3280F Keywords: Laser ablation deposition; Thin films; Hard materials
1. Introduction BCN films have been studied in the last years due to the wide range of prospective applications as light elements based coatings. Mechanical properties comparable to those of diamond-like structures make them useful for wear protection applications w1,2x and the high optical transparency can be used as mask substrates for X-ray lithography w3x. Previously, BCN thin films were obtained by using several deposition methods w4–9x, but most of them were amorphous and hydrogenated. The first c-BCN )
Corresponding author. Tel.: q39-832-320501; fax: q39-832320505; e-mail: [email protected].
films were deposited by Loeffler et al. w10x using r.f. plasma-assisted chemical vapor deposition and by Doll w11x using laser ablation. In this paper we report the successful growth of c-BCN and h-BCN thin films by PLA of a target formed of 2 semidisks: one of h-BN and the other of graphite, with the substrate at room temperature.
2. Experimental apparatus Only a basic outline of our experimental apparatus is given here, since it has already been described elsewhere w12x. The irradiations were performed using a Lambda Physik LPX 315i XeCl excimer laser
0169-4332r98r$ – see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 0 7 - 4
A. Perrone et al.r Applied Surface Science 133 (1998) 239–242
240
Table 1 A summary of the samples used in this experiment Sample
1
2
3
4
PN 2 ŽPa.
10y5
1
5
10
5 100
Ž l s 308 nm, t FW HM s 30 ns.. Series of 10,000 pulses at a repetition rate of 10 Hz were directed to the target. The fluence was set at 5 Jrcm2 . A 30-cm focal length lens was used to focus the laser radiation on the target which continually rotated Ž3 Hz. to reduce the effect of crating on the target surface. The target was prepared as a disk, combining together 2 semi-disk, one of h-BN and one of C Žgraphite.. Si Ž100. etched in dilute HF acid was used as substrate. The target-substrate distance was fixed at 4 cm. The stainless steel deposition chamber was evacuated by a turbomolecular pump down to 10y5 Pa and then filled with N2 at different pressures Ž1 to 100 Pa.. Table 1 presents a summary of the samples used in this investigation, with the indication of the working pressures.
3. Experimental results The crystallinity, composition, morphology and chemical bonding of our films were investigated by XRD, XPS and TEM analyses. The results of all these analysis techniques indicate that the sample deposited in vacuum mainly contains a mixture of h-BCN and c-BCN, the rest of the film appearing to be amorphous, or with some crystalline h-BN. The XPS analyses were carried out with a VG ESCALAB MKII spectrometer using Al K a Ž1486.6 eV. X-ray radiation. The spectra calibration was realized with the Ag M 4 NN Ž1128.7 eV. line. Fig. 1 shows a typical XPS spectrum for B Ž1s. as recorded for the film deposited in vacuum Žsample 1.. The results of the deconvolution procedure show the presence of 3 different possible boron chemical states, centered at 188.0, 189.8 and 192.0 eV. The chemical shift of the pure boron–nitride bond Ž189.8 eV. w13x toward lower energy corresponds to the B bonded in a BCN2 or more probably in a BNC 2 compound. In fact, it is known that the peak B Ž1s. in the ŽBB 3 . structure is shifted by y4 eV with the
Fig. 1. XPS results for B Ž1s. peak of the film deposited in vacuum.
respect to the reference position from 189.8 eV corresponding to the case BN3 , while the peak corresponding to BC 3 is shifted by only y2.5 eV. The BNC 2 structure is closer to the BC 3 structure than to the BN3 structure. This allows us to assign the peak shifted by y1.8 eV to the B in BNC 2 . The component at 192.0 eV can be assigned to B–O bonding w14x. Fig. 2 shows the XPS of the N Ž1s. line shape for the same sample. The deconvolution gives 3 different structures with peaks at 398.0, 399.3 and 401.5 eV. The N Ž1s. peak of B–N bonding appears at 398 eV w15,16x. In the case of C–N bonding, the N Ž1s.
Fig. 2. XPS results for N Ž1s. peak of the film deposited in vacuum.
A. Perrone et al.r Applied Surface Science 133 (1998) 239–242
241
is displaced by q1.3 eV, thus the component centered at 399.3 eV can be assigned to C–N bonding. Similar XPS spectra were obtained for the samples deposited in N2 atmosphere. However, we noticed a decrease of the B concentration and consequently the BCN amount in the films with increasing N2 pressure. The oxygen incorporated in the films was Žin atomic %. of the order of 4–5% and was determined from the oxygen bonds to nitrogen, boron and carbon appearing in the XPS spectra corresponding to N Ž1s., B Ž1s. and C Ž1s. spectral regions. The main part was coming from the B–O bonds. The reactivity of B respect to O is very high and the B–O bonds were observed even if the oxygen partial pressure in the vacuum chamber was very low. The N–O peak can be considered as surface pollution w13x. The oxygen concentration seems to be relatively high
Fig. 4. Ža. Chain of h-BCN having a total length of about 4.6 m m and a transversal dimension of about 30 nm. Žb. Diffraction rings of the h-BCN phase.
Fig. 3. Ža. Conglomerate of cubic BCN with crystallite size between 30 and 80 nm. Žb. Selected area electron diffraction image of a polycrystalline cubic BN0.26 C 0.74 of the sample 1.
because the composition and chemical bonds were studied without sputtering. The crystallinity of the films was investigated by XRD and TEM analyses. The TEM images and diffraction patterns have been obtained by a JEOL 200 CX microscope operating at 200 kV. A conglomerate of cubic BCN with crystallite size between 30 and 80 nm is presented in Fig. 3a and b shows a selected area electron diffraction image of a polycrystalline cubic BN0.26 C 0.74 conglomerate of the sample 1. The stoichiometric coefficients were determined using the observed perfect matching between the experimental electron diffraction pattern and the data in ASTM book ŽAmerican Society for Testing and Materials.. Fig. 4a shows a chain of hexagonal BCN having a total length of about 4.6 m m and a transversal dimension of about 30 nm.
242
A. Perrone et al.r Applied Surface Science 133 (1998) 239–242
Despite the closed resemblance to graphite in morphology and diffraction pattern, the lack of Ž103. diffraction ring and the relative intensities of the diffraction rings suggest the presence of the h-BCN phase ŽFig. 4b.. The growth direction is along the hexagonal c-axis. The TEM analyses of the samples prepared in N2 atmosphere indicate that they are formed of a mix˚ c0 s ture of polycrystalline h-BN Ž a 0 s 2.504 A, ˚ . having a fiber aspect, large monocrystalline 6.656 A flakes Ž1 m m. of h-BN and some amorphous stuff. The XRD analyses on sample 1 confirm the crystalline structure previously evidenced by TEM. Indeed, the diffraction line corresponding to an inter˚ is present in the spectra. It planar distance of 3.42 A corresponds to the Ž002. interplanar distance of a crystalline phase of h-BCN, the same identified by TEM. Another peak, larger, growing from an amorphous matrix, assigned to Ž111. line of the c-BCN ˚ is structure, with the interplanar distance of 2.70 A, also evidenced. Some other crystalline structures corresponding to different crystalline forms of h-BN are also present. In summary, XRD analyses suggest the presence of crystalline phases as h-BCN, c-BCN and h-BN, together with amorphous phases assigned to compounds as CN. The film microhardness values were measured by a Vickers Leitz Durimet. The indentation were performed under the load of 25, 50 and 100 g. The hardness of the films, always higher than 1 GPa, even though presents a general tendency to decrease with increasing nitrogen pressure, the highest hardness value Ž2.9 GPa. was obtained under a load of 25 g and for the sample deposited at 10 Pa.
4. Conclusions Deposition of BCN thin films by pulsed laser ablation, at room temperature, was successfully car-
ried out in these experiments, using a double BNgraphite target. The existence of bonds between all the B, C, N elements is well evidenced by XPS, and confirmed by TEM and XRD analysis. The results indicate that crystals of c-BCN, h-BCN and h-BN are formed. The films are very hard, showing a high microhardness value. In conclusion, PLA is proved to be a promising technique for one step BCN synthesis and deposition. Further parametric studies are in progress, in order to prepare a pure boron carbonitride compound.
References w1x A.Y. Liu, M.L. Cohen, Science 245 Ž1989. 841. w2x R. Riedel, Adv. Mater. 6 Ž1994. 549. w3x A. Weber, U. Bringmann, R. Nikulski, C.-P. Klages, Diamond Rel. Mater. 2 Ž1993. 201. w4x A.R. Badzian, S. Appenheimer, T. Niemyski, E. Olkusnik, Proc. Third Int. Conf. CVD 747 Ž1994. . w5x K.T. Rie, A. Gebauer, C. Pfohl, J. Phys. IV 5 Ž1995. 637. w6x M. Hubacek, T. Sato, J. Solid State Chem. 114 Ž1995. 258. w7x L. Filipozzi, A. Derre, Carbon 33 Ž1995. 1747. w8x K. Tanaka, S. Suchara, Trans. Inst. Electro. Inf. Commun. Eng. C II J78C Ž1995. 384. w9x T.M. Besmann, J. Am. Ceram. Soc. 73 Ž1990. 2498. w10x J. Loeffler, F. Steinbach, J. Bill, J. Mayer, F. Aldinger, Z. Metallk. 87 Ž1996. 170. w11x G.L. Doll, General Motors, US patent Nr. 5.330.611 Jul. 19 Ž1994.. w12x A. Luches, G. Leggieri, M. Martino, A. Perrone G. Majni, P. Mengucci, I.N. Mihailescu, Appl. Surf. Sci. 79r80 Ž1994. 244. w13x F. Saugnac, F. Teyssandyer, A. Marchand, J. Am. Ceram. Soc. 75 Ž1992. 161. w14x L. Lobstein, E. Millon, J.F. Muller, J. Lambert, M. Alnot, J.J. Erhardt, Proc. SPIE 2207 Ž1994. 705. w15x C.D. Wagner, W.M. Riegs, L.E. Davis, J.F. Moulder. In: G.E. Mulienberg ŽEd.., Handbook of X-ray Photoelectron spectroscopy, Perkin-Elmer, 1979. w16x F. Fujimoto, K. Ogata, Jpn. J. Appl. Phys. 32 Ž1993. L420.