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Morphology and composition of ArF excimer laser deposited carbon nitride films as determined by analytical TEM
Applied Surface Science 186 (2002) 502±506
Morphology and composition of ArF excimer laser deposited carbon nitride ®lms as determined by analytical TEM O. Gesztia, G. RadnoÂczia, I. BertoÂtib, T. SzoÈreÂnyic,d,*, F. Antonic, E. Fogarassyc a
Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary Chemical Research Center of the Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary c CNRS-PHASE, BP 20, 67037 Strasbourg Cedex 2, France d Research Group on Laser Physics, Hungarian Academy of Sciences, P.O. Box 406, H-6701 Szeged, Hungary b
Abstract Carbon nitride ®lms deposited onto room temperature silicon substrates by ArF excimer laser ablation of a graphite target in nitrogen atmosphere have been investigated after ¯oating off in transmission electron microscope (TEM). All ®lms fabricated in the 1±100 Pa N2 pressure and 1±10 J cm 2 ¯uence domain are amorphous. It is the nitrogen pressure that governs both the composition and the morphology of the ®lms, the effect of the laser ¯uence being weaker. The morphologies range from dense ®lms grown in 1±5 Pa N2, to structures composed of carbon nitride clusters and voids between them, obtained at and above 50 Pa N2. Ablation with pulses of high ¯uence (10 J cm 2) results in more compact ®lms. The oxygen content determined by energy dispersive X-ray spectrometry well correlates with the compactness of the ®lms, being signi®cantly higher in those fabricated at higher pressures and composed of clusters. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon nitride; PLD; Ablation; Thin ®lms; Microstructure
1. Introduction The interest in describing and understanding the great variety of puzzling properties of the amorphous carbon nitrides, a-CNx is continuously growing. The fabrication of carbon nitrides is a big challenge, indeed, since a high degree in tuning the structural, optical and electronic characteristics is possible, depending on the preparation conditions. Ablation of a graphite target in nitrogen atmosphere is a well-established technique for production of carbon
* Corresponding author. Present address: Research Group on Laser Physics, Hungarian Academy of Sciences, P.O. Box 406, H-6701 Szeged, Hungary. Tel.: 36-62-544274; fax: 36-62-544658. E-mail address: [email protected] (T. SzoÈreÂnyi).
nitride ®lms [1±15], mentioning only very recent papers. The principal process parameters are the pressure of the reactive atmosphere and the laser ¯uence [1±15]. Motivated by the race for the synthesis of the superhard C3N4 phase, in the majority of the papers the analysis is restricted to the effect of process parameters on the chemical composition, most notably the nitrogen content, and the chemical structure of the ®lms. Less attention has been paid to the concomitant changes in the microstructure [4,7]. Systematic mapping of the chemical composition and surface morphology of carbon nitride ®lms deposited in the 1±100 Pa N2 pressure and 1±10 J cm 2 ¯uence window by ex situ XPS and SEM AFM, respectively, revealed a direct correlation between surface structure and oxygen content: extremely smooth, structureless surface and low oxygen content
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 5 8 - 9
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
characterised all ®lms deposited at the low pressure end of the process window, while those fabricated in 50 Pa N2 were porous, consisting of loosely connected, sub-micrometer dimension structures with voids between them and contained more than 10 at.% oxygen [13]. The dependence of the amount and depth distribution of hydrogen in the same ®lms as a function of process parameters pointed also to structural rearrangements [16]. The structural characterisation of these ®lms has recently been completed by a transmission electron microscope (TEM) study, the results of which are summarised here. Since previous studies revealed that when ablating with an ArF excimer laser, the increase in the ¯uence from 1 to 3 J cm 2 had apparently no strong effect on the chemical composition of the carbon nitride ®lms deposited between 5 and 50 Pa N2, while approaching 10 J cm 2 the N/C ratio suddenly started to grow [13], the TEM analysis was restricted to samples deposited at different pressures while ®xing the ¯uence of the excimer laser pulses at 1, 7.5 and 10 J cm 2. 2. Experimental The deposition chamber was pumped by a diffusion pump to a base pressure of 1 2 10 5 Pa, and back®lled with ultra-high purity (99.999%) nitrogen of 1± 100 Pa during ®lm deposition. The beam of the ArF excimer laser (Lambda Physik, l 193 nm, output energy max. 240 mJ, 22 ns pulse duration) was focused at approx 458 onto the surface of a high purity graphite target, rotating with 1 rpm. The ¯uence on the target surface was adjusted between 1 and 10 J/cm2 by changing the output energy of the laser, while keeping the ablated area constant at 1.2 mm2. Due to uncertainties in measuring the pulse energy and spot dimensions, the experimental error in the absolute values of the ¯uence is 20% at best. Films of 50±500 nm thickness were deposited on n-type Si(1 0 0) wafers held at room temperature, at a targetto-substrate distance of 30 mm and 10 Hz pulse repetition rate. Samples for TEM investigation were detached from their substrates in 10±50% HF or HF±HNO3 solution. Structural characterisation was performed by TEM using a Philips CM20 microscope, operated at 200 kV.
503
X-ray microanalysis was made by a germanium detector NORAN EDS, attached to the electron microscope. The surface chemical composition of the ®lms was determined by ex situ X-ray photoelectron spectroscopy (XPS). The spectra were recorded by a Kratos XSAM 800 spectrometer in ®xed-retarding-ratio mode FRR 20 by using Mg Ka1,2 (1253.6 eV) excitation. The overall experimental resolution was 0.75 eV. The mass density of the ®lms was calculated from the areal density of the carbon, nitrogen and oxygen atoms, as given by nuclear reaction analysis (NRA), and ®lm thickness, measured by a stylus pro®lometer. 3. Results and discussion Seven ®lms, sampling the whole process window have been analysed. All samples investigated are characterised by an inhomogeneous amorphous structure. The degree of inhomogeneity is controlled by the N2 pressure and laser ¯uence. In line with all but one [4] reports on room temperature PLD ®lms, neither individual (micro)crystallites nor polycrystalline areas have been detected in the amorphous matrix. Selected-area diffraction patterns revealed that particulates sitting on the ®lm surface were typically graphite pieces. The evolution of ®lm microstructure with N2 pressure is shown in Fig. 1 for samples selected from the 1 J cm 2 series. At 1 Pa, morphologically uniform, continuous ®lms are grown (Fig. 1, top left). The amorphous structure is characterised by inhomogeneities at several tens to 100 nm level and a relatively rough surface. An increase in the N2 pressure from 1 to 2 Pa results in an exactly twofold increase in the nitrogen content (Table 1). While the pattern changes and the contrast of the inhomogeneities becomes apparently more pronounced, the feature size remains approximately the same (Fig. 1, top right). Although the apparent ordering suggests crystallinity, the ®lms proved to be totally amorphous by electron diffraction (Fig. 1), and no sign of any crystalline phase could be detected. The apparent regularity of the structures raises the question whether their origin could eventually be the substrate. Although this possibility cannot explicitly be excluded, the fact that all ®lms possessing entirely different structure patterns (Fig. 1) were deposited onto substrates originating from the
504
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
Fig. 1. TEM micrographs taken on ®lms deposited by ablating a graphite target with ArF excimer laser pulses of 1 J cm left), 2 (top right), 5 (bottom left) and 50 Pa (bottom right) nitrogen. Each picture is 1 1 mm2 in size.
same wafer suggests that the structure formation should rather be connected to growth inhomogeneities evolving as a result of incorporation of a relatively small amount of nitrogen into the carbon matrix. Further increase in the nitrogen content (Table 1) leads to a signi®cant change in the microstructure. The micrographs taken on ®lms deposited at 5 and 50 Pa (Fig. 1, bottom left and right, respectively)
2
¯uence in 1 (top
suggest two processes contributing to ®lm formation: one producing a matrix, which is continuous and homogeneous at this magni®cation, by deposition of atomic species, and another resulting in the formation of nodules in this matrix. Nodule formation may be triggered by clusters formed in the ambient and embedded into the matrix after landing on the growing ®lm surface. Interestingly, the dimension of the
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
505
Table 1 Nitrogen pressure: p(N2), laser ¯uence: F, and ®lm composition in terms of atomic concentrations of carbon: cC, nitrogen: cN, and oxygen: cO, as determined by XPS and EDS, respectively p(N2) (Pa)
F (J cm 2)
cC (XPS) (at.%)
cN (XPS) (at.%)
cO (XPS) (at.%)
cC (EDS) (at.%)
cN (EDS) (at.%)
cO (EDS) (at.%)
1 2 5 50 5 100
1 1 1 1 10 7.5
86.9 82.6 72.9 62.0 70.8 61.9
6.1 11.8 18.3 26.1 24.5 24.1
7.0 5.6 8.8 11.9 4.7 14.0
96.1 93.1 83.9 ± 74.8 71.8
3.6 6.5 14.5 ± 24.6 23.1
0.3 0.4 1.6 ± 0.6 5.1
obtained when using 1 J cm 2 pulses. No (structural) features but a few nodules can be seen. When depositing in the 50±100 Pa pressure domain, however, even high ¯uence processing cannot help anymore. The ®lms grown at 100 Pa are composed of carbon nitride cluster agglomerates formed in the ambient and collected on the substrate forming a very porous structure (Fig. 2, right). The dimension of both the clusters in the agglomerates and the voids between them ranges from several tens to more than 100 nm. Again, although the apparent ordering suggests crystallinity, the clusters proved to be totally amorphous by electron diffraction, and no sign of any crystalline phase could be detected.
nodules scatters again between several tens and 100 nm, their actual dimension and their number density being controlled by the pressure. Luches and coworkers [4] reported a new carbon nitride phase observed in ®lms fabricated at room temperature by XeCl excimer laser ablation of a graphite target at 12 J cm 2. Consequently, the samples of the 10 J cm 2 series have been analysed taking special care. The results con®rmed that at not too high N2 pressures, high ¯uence processing is advantageous in terms of both nitrogen content and ®lm morphology. As shown in Fig. 2 left, ablation with 10 J cm 2 pulses at 5 Pa led to a notable improvement in ®lm homogeneity as compared to that
Fig. 2. TEM micrographs taken on ®lms deposited with 10 J cm Each picture is 1 1 mm2 in size.
2
pulses in 5 Pa N2 (left) and with 7.5 J cm
2
pulses in 50 Pa N2 (right).
506
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
The results of parallel X-ray microanalysis con®rmed and complemented the XPS results. As shown in Table 1, the oxygen content measured within the whole ®lm volume by EDS (taking into account the actual thickness of the ®lms, as determined by pro®lometry, while neglecting the changes in the mass density) follows exactly the trend of surface oxygen content determined by ex situ XPS. (EDS data for the ®lm fabricated in 50 Pa N2 with 1 J cm 2 pulses are missing because the sample spoiled under the electron beam.) The EDS ®gures are, however, signi®cantly lower than those determined by XPS, the difference being less in the case of porous ®lms. The difference between the surface and volume oxygen content substantiates that oxygen uptake occurs when the ®lms are exposed to the atmosphere. The af®nity of the ®lms towards ex situ oxygen uptake is determined by their microstructure which is controlled mainly by the N2 pressure: porous ®lms absorb more oxygen and/or water than the more compact layers do. The results of the TEM analysis outlined above explain the dependence of the mass density of the ®lms on N2 pressure. The ®lms grown at and below 5 Pa are relatively compact with r-values above 2 g/ cm3. The sudden decrease in mass density of ®lms deposited above 5 Pa down to r 1:05 g=cm3 is a direct consequence of changes in the microstructure resulting in increased porosity, which supports enhanced ex situ incorporation of contaminations [13,16] into the network. 4. Conclusions When deposition parameters change from low nitrogen pressure (and low ¯uence) through higher nitrogen pressure (or higher ¯uence) to high pressure (and high ¯uence), the morphology of the ®lms changes from a rather inhomogeneous one through a fairly homogeneous one to a structure consisting of cluster agglomerates and voids, accordingly. The oxygen content determined by energy dispersive X-ray spectrometry correlates well with the compactness of the ®lms. It is signi®cantly higher in ®lms, fabricated at higher pressures and composed of clusters and voids, revealing that the oxygen content is a sensitive indicator of the changes in ®lm microstructure, indeed.
Acknowledgements This research was supported by NATO's Scienti®c Affairs Division in the framework of the Science for Peace Programme (SfP 971934), the EU (ICAICT-2000-70029), the Hungarian Scienti®c Research Fund (OTKA T030424), the Hungarian Ministry of Education (OMFB EU-98-B4/145 and FKFP 0171/ 2001) and the Hungarian Ministry of Education and Le Ministere des Affaires Etrangeres within the frame of the bilateral intergovernmental S&T agreement between Hungary and France (TeÂT F-26/00). T. SzoÈreÂnyi is grateful to the French Ministry of Education, Research and Technology for his visiting professor fellowship. References [1] F. Kokai, K. Yamamoto, Y. Koga, S. Fujiwara, R.B. Heimann, Appl. Phys. A 66 (1998) 403. [2] M.L. De Giorgi, G. Leggieri, A. Luches, M. Martino, A. Perrone, A. Zocco, G. Barucca, G. Majni, E. GyoÈrgy, I.N. Mihailescu, M. Popescu, Appl. Surf. Sci. 127±129 (1998) 481. [3] Y.F. Lu, Z.M. Ren, W.D. Song, D.S.H. Chan, T.S. Low, K. Gamani, G. Chen, K. Li, J. Appl. Phys. 84 (1998) 2909. [4] G. Barucca, G. Majni, P. Mengucci, G. Leggieri, A. Luches, M. Martino, A. Perrone, J. Appl. Phys. 86 (1999) 2014. [5] E. D'Anna, M.L. De Giorgi, A. Luches, M. Martino, A. Perrone, A. Zocco, Thin Solid Films 347 (1999) 72. [6] C. Popov, M. JelõÂnek, B. Ivanov, R.I. Tomov, W. Kulisch, Diamond Relat. Mater. 8 (1999) 577. [7] A. Zocco, A. Perrone, E. D'Anna, G. Leggieri, A. Luches, A. Klini, I. Zergioti, C. Fotakis, Diamond Relat. Mater. 8 (1999) 582. [8] S. Trusso, C. Vasi, F. Neri, Thin Solid Films 355±356 (1999) 219. [9] T. SzoÈreÂnyi, E. Fogarassy, C. Fuchs, J. Hommet, F. Le Normand, Appl. Phys. A 69 (1999) S941. [10] E. Riedo, F. Comin, J. Chevrier, A.M. Bonnot, J. Appl. Phys. 88 (2000) 4365. [11] P. GonzaÂlez, R. Soto, F. Lusquinos, B. LeoÂn, M. PeÂrez-Amor, J. Vac. Sci. Technol. A 18 (2000) 3004. [12] T. SzoÈreÂnyi, C. Fuchs, E. Fogarassy, J. Hommet, F. Le Normand, Surf. Coat. Technol. 125 (2000) 308. [13] T. SzoÈreÂnyi, F. Antoni, E. Fogarassy, I. BertoÂti, Appl. Surf. Sci. 168 (2000) 248. [14] Z. Geretovszky, Z. KaÂntor, I. BertoÂti, T. SzoÈreÂnyi, Appl. Phys. A 70 (2000) 9. [15] E. Riedo, F. Comin, J. Chevrier, F. Schmithusen, S. Decossas, M. Sancrotti, Surf. Coat. Technol. 125 (2000) 124. [16] T. SzoÈreÂnyi, J.-P. Stoquert, J. Perriere, F. Antoni, E. Fogarassy, Diamond Relat. Mater., in press.
Morphology and composition of ArF excimer laser deposited carbon nitride ®lms as determined by analytical TEM O. Gesztia, G. RadnoÂczia, I. BertoÂtib, T. SzoÈreÂnyic,d,*, F. Antonic, E. Fogarassyc a
Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary Chemical Research Center of the Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary c CNRS-PHASE, BP 20, 67037 Strasbourg Cedex 2, France d Research Group on Laser Physics, Hungarian Academy of Sciences, P.O. Box 406, H-6701 Szeged, Hungary b
Abstract Carbon nitride ®lms deposited onto room temperature silicon substrates by ArF excimer laser ablation of a graphite target in nitrogen atmosphere have been investigated after ¯oating off in transmission electron microscope (TEM). All ®lms fabricated in the 1±100 Pa N2 pressure and 1±10 J cm 2 ¯uence domain are amorphous. It is the nitrogen pressure that governs both the composition and the morphology of the ®lms, the effect of the laser ¯uence being weaker. The morphologies range from dense ®lms grown in 1±5 Pa N2, to structures composed of carbon nitride clusters and voids between them, obtained at and above 50 Pa N2. Ablation with pulses of high ¯uence (10 J cm 2) results in more compact ®lms. The oxygen content determined by energy dispersive X-ray spectrometry well correlates with the compactness of the ®lms, being signi®cantly higher in those fabricated at higher pressures and composed of clusters. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon nitride; PLD; Ablation; Thin ®lms; Microstructure
1. Introduction The interest in describing and understanding the great variety of puzzling properties of the amorphous carbon nitrides, a-CNx is continuously growing. The fabrication of carbon nitrides is a big challenge, indeed, since a high degree in tuning the structural, optical and electronic characteristics is possible, depending on the preparation conditions. Ablation of a graphite target in nitrogen atmosphere is a well-established technique for production of carbon
* Corresponding author. Present address: Research Group on Laser Physics, Hungarian Academy of Sciences, P.O. Box 406, H-6701 Szeged, Hungary. Tel.: 36-62-544274; fax: 36-62-544658. E-mail address: [email protected] (T. SzoÈreÂnyi).
nitride ®lms [1±15], mentioning only very recent papers. The principal process parameters are the pressure of the reactive atmosphere and the laser ¯uence [1±15]. Motivated by the race for the synthesis of the superhard C3N4 phase, in the majority of the papers the analysis is restricted to the effect of process parameters on the chemical composition, most notably the nitrogen content, and the chemical structure of the ®lms. Less attention has been paid to the concomitant changes in the microstructure [4,7]. Systematic mapping of the chemical composition and surface morphology of carbon nitride ®lms deposited in the 1±100 Pa N2 pressure and 1±10 J cm 2 ¯uence window by ex situ XPS and SEM AFM, respectively, revealed a direct correlation between surface structure and oxygen content: extremely smooth, structureless surface and low oxygen content
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 5 8 - 9
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
characterised all ®lms deposited at the low pressure end of the process window, while those fabricated in 50 Pa N2 were porous, consisting of loosely connected, sub-micrometer dimension structures with voids between them and contained more than 10 at.% oxygen [13]. The dependence of the amount and depth distribution of hydrogen in the same ®lms as a function of process parameters pointed also to structural rearrangements [16]. The structural characterisation of these ®lms has recently been completed by a transmission electron microscope (TEM) study, the results of which are summarised here. Since previous studies revealed that when ablating with an ArF excimer laser, the increase in the ¯uence from 1 to 3 J cm 2 had apparently no strong effect on the chemical composition of the carbon nitride ®lms deposited between 5 and 50 Pa N2, while approaching 10 J cm 2 the N/C ratio suddenly started to grow [13], the TEM analysis was restricted to samples deposited at different pressures while ®xing the ¯uence of the excimer laser pulses at 1, 7.5 and 10 J cm 2. 2. Experimental The deposition chamber was pumped by a diffusion pump to a base pressure of 1 2 10 5 Pa, and back®lled with ultra-high purity (99.999%) nitrogen of 1± 100 Pa during ®lm deposition. The beam of the ArF excimer laser (Lambda Physik, l 193 nm, output energy max. 240 mJ, 22 ns pulse duration) was focused at approx 458 onto the surface of a high purity graphite target, rotating with 1 rpm. The ¯uence on the target surface was adjusted between 1 and 10 J/cm2 by changing the output energy of the laser, while keeping the ablated area constant at 1.2 mm2. Due to uncertainties in measuring the pulse energy and spot dimensions, the experimental error in the absolute values of the ¯uence is 20% at best. Films of 50±500 nm thickness were deposited on n-type Si(1 0 0) wafers held at room temperature, at a targetto-substrate distance of 30 mm and 10 Hz pulse repetition rate. Samples for TEM investigation were detached from their substrates in 10±50% HF or HF±HNO3 solution. Structural characterisation was performed by TEM using a Philips CM20 microscope, operated at 200 kV.
503
X-ray microanalysis was made by a germanium detector NORAN EDS, attached to the electron microscope. The surface chemical composition of the ®lms was determined by ex situ X-ray photoelectron spectroscopy (XPS). The spectra were recorded by a Kratos XSAM 800 spectrometer in ®xed-retarding-ratio mode FRR 20 by using Mg Ka1,2 (1253.6 eV) excitation. The overall experimental resolution was 0.75 eV. The mass density of the ®lms was calculated from the areal density of the carbon, nitrogen and oxygen atoms, as given by nuclear reaction analysis (NRA), and ®lm thickness, measured by a stylus pro®lometer. 3. Results and discussion Seven ®lms, sampling the whole process window have been analysed. All samples investigated are characterised by an inhomogeneous amorphous structure. The degree of inhomogeneity is controlled by the N2 pressure and laser ¯uence. In line with all but one [4] reports on room temperature PLD ®lms, neither individual (micro)crystallites nor polycrystalline areas have been detected in the amorphous matrix. Selected-area diffraction patterns revealed that particulates sitting on the ®lm surface were typically graphite pieces. The evolution of ®lm microstructure with N2 pressure is shown in Fig. 1 for samples selected from the 1 J cm 2 series. At 1 Pa, morphologically uniform, continuous ®lms are grown (Fig. 1, top left). The amorphous structure is characterised by inhomogeneities at several tens to 100 nm level and a relatively rough surface. An increase in the N2 pressure from 1 to 2 Pa results in an exactly twofold increase in the nitrogen content (Table 1). While the pattern changes and the contrast of the inhomogeneities becomes apparently more pronounced, the feature size remains approximately the same (Fig. 1, top right). Although the apparent ordering suggests crystallinity, the ®lms proved to be totally amorphous by electron diffraction (Fig. 1), and no sign of any crystalline phase could be detected. The apparent regularity of the structures raises the question whether their origin could eventually be the substrate. Although this possibility cannot explicitly be excluded, the fact that all ®lms possessing entirely different structure patterns (Fig. 1) were deposited onto substrates originating from the
504
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
Fig. 1. TEM micrographs taken on ®lms deposited by ablating a graphite target with ArF excimer laser pulses of 1 J cm left), 2 (top right), 5 (bottom left) and 50 Pa (bottom right) nitrogen. Each picture is 1 1 mm2 in size.
same wafer suggests that the structure formation should rather be connected to growth inhomogeneities evolving as a result of incorporation of a relatively small amount of nitrogen into the carbon matrix. Further increase in the nitrogen content (Table 1) leads to a signi®cant change in the microstructure. The micrographs taken on ®lms deposited at 5 and 50 Pa (Fig. 1, bottom left and right, respectively)
2
¯uence in 1 (top
suggest two processes contributing to ®lm formation: one producing a matrix, which is continuous and homogeneous at this magni®cation, by deposition of atomic species, and another resulting in the formation of nodules in this matrix. Nodule formation may be triggered by clusters formed in the ambient and embedded into the matrix after landing on the growing ®lm surface. Interestingly, the dimension of the
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
505
Table 1 Nitrogen pressure: p(N2), laser ¯uence: F, and ®lm composition in terms of atomic concentrations of carbon: cC, nitrogen: cN, and oxygen: cO, as determined by XPS and EDS, respectively p(N2) (Pa)
F (J cm 2)
cC (XPS) (at.%)
cN (XPS) (at.%)
cO (XPS) (at.%)
cC (EDS) (at.%)
cN (EDS) (at.%)
cO (EDS) (at.%)
1 2 5 50 5 100
1 1 1 1 10 7.5
86.9 82.6 72.9 62.0 70.8 61.9
6.1 11.8 18.3 26.1 24.5 24.1
7.0 5.6 8.8 11.9 4.7 14.0
96.1 93.1 83.9 ± 74.8 71.8
3.6 6.5 14.5 ± 24.6 23.1
0.3 0.4 1.6 ± 0.6 5.1
obtained when using 1 J cm 2 pulses. No (structural) features but a few nodules can be seen. When depositing in the 50±100 Pa pressure domain, however, even high ¯uence processing cannot help anymore. The ®lms grown at 100 Pa are composed of carbon nitride cluster agglomerates formed in the ambient and collected on the substrate forming a very porous structure (Fig. 2, right). The dimension of both the clusters in the agglomerates and the voids between them ranges from several tens to more than 100 nm. Again, although the apparent ordering suggests crystallinity, the clusters proved to be totally amorphous by electron diffraction, and no sign of any crystalline phase could be detected.
nodules scatters again between several tens and 100 nm, their actual dimension and their number density being controlled by the pressure. Luches and coworkers [4] reported a new carbon nitride phase observed in ®lms fabricated at room temperature by XeCl excimer laser ablation of a graphite target at 12 J cm 2. Consequently, the samples of the 10 J cm 2 series have been analysed taking special care. The results con®rmed that at not too high N2 pressures, high ¯uence processing is advantageous in terms of both nitrogen content and ®lm morphology. As shown in Fig. 2 left, ablation with 10 J cm 2 pulses at 5 Pa led to a notable improvement in ®lm homogeneity as compared to that
Fig. 2. TEM micrographs taken on ®lms deposited with 10 J cm Each picture is 1 1 mm2 in size.
2
pulses in 5 Pa N2 (left) and with 7.5 J cm
2
pulses in 50 Pa N2 (right).
506
O. Geszti et al. / Applied Surface Science 186 (2002) 502±506
The results of parallel X-ray microanalysis con®rmed and complemented the XPS results. As shown in Table 1, the oxygen content measured within the whole ®lm volume by EDS (taking into account the actual thickness of the ®lms, as determined by pro®lometry, while neglecting the changes in the mass density) follows exactly the trend of surface oxygen content determined by ex situ XPS. (EDS data for the ®lm fabricated in 50 Pa N2 with 1 J cm 2 pulses are missing because the sample spoiled under the electron beam.) The EDS ®gures are, however, signi®cantly lower than those determined by XPS, the difference being less in the case of porous ®lms. The difference between the surface and volume oxygen content substantiates that oxygen uptake occurs when the ®lms are exposed to the atmosphere. The af®nity of the ®lms towards ex situ oxygen uptake is determined by their microstructure which is controlled mainly by the N2 pressure: porous ®lms absorb more oxygen and/or water than the more compact layers do. The results of the TEM analysis outlined above explain the dependence of the mass density of the ®lms on N2 pressure. The ®lms grown at and below 5 Pa are relatively compact with r-values above 2 g/ cm3. The sudden decrease in mass density of ®lms deposited above 5 Pa down to r 1:05 g=cm3 is a direct consequence of changes in the microstructure resulting in increased porosity, which supports enhanced ex situ incorporation of contaminations [13,16] into the network. 4. Conclusions When deposition parameters change from low nitrogen pressure (and low ¯uence) through higher nitrogen pressure (or higher ¯uence) to high pressure (and high ¯uence), the morphology of the ®lms changes from a rather inhomogeneous one through a fairly homogeneous one to a structure consisting of cluster agglomerates and voids, accordingly. The oxygen content determined by energy dispersive X-ray spectrometry correlates well with the compactness of the ®lms. It is signi®cantly higher in ®lms, fabricated at higher pressures and composed of clusters and voids, revealing that the oxygen content is a sensitive indicator of the changes in ®lm microstructure, indeed.
Acknowledgements This research was supported by NATO's Scienti®c Affairs Division in the framework of the Science for Peace Programme (SfP 971934), the EU (ICAICT-2000-70029), the Hungarian Scienti®c Research Fund (OTKA T030424), the Hungarian Ministry of Education (OMFB EU-98-B4/145 and FKFP 0171/ 2001) and the Hungarian Ministry of Education and Le Ministere des Affaires Etrangeres within the frame of the bilateral intergovernmental S&T agreement between Hungary and France (TeÂT F-26/00). T. SzoÈreÂnyi is grateful to the French Ministry of Education, Research and Technology for his visiting professor fellowship. References [1] F. Kokai, K. Yamamoto, Y. Koga, S. Fujiwara, R.B. Heimann, Appl. Phys. A 66 (1998) 403. [2] M.L. De Giorgi, G. Leggieri, A. Luches, M. Martino, A. Perrone, A. Zocco, G. Barucca, G. Majni, E. GyoÈrgy, I.N. Mihailescu, M. Popescu, Appl. Surf. Sci. 127±129 (1998) 481. [3] Y.F. Lu, Z.M. Ren, W.D. Song, D.S.H. Chan, T.S. Low, K. Gamani, G. Chen, K. Li, J. Appl. Phys. 84 (1998) 2909. [4] G. Barucca, G. Majni, P. Mengucci, G. Leggieri, A. Luches, M. Martino, A. Perrone, J. Appl. Phys. 86 (1999) 2014. [5] E. D'Anna, M.L. De Giorgi, A. Luches, M. Martino, A. Perrone, A. Zocco, Thin Solid Films 347 (1999) 72. [6] C. Popov, M. JelõÂnek, B. Ivanov, R.I. Tomov, W. Kulisch, Diamond Relat. Mater. 8 (1999) 577. [7] A. Zocco, A. Perrone, E. D'Anna, G. Leggieri, A. Luches, A. Klini, I. Zergioti, C. Fotakis, Diamond Relat. Mater. 8 (1999) 582. [8] S. Trusso, C. Vasi, F. Neri, Thin Solid Films 355±356 (1999) 219. [9] T. SzoÈreÂnyi, E. Fogarassy, C. Fuchs, J. Hommet, F. Le Normand, Appl. Phys. A 69 (1999) S941. [10] E. Riedo, F. Comin, J. Chevrier, A.M. Bonnot, J. Appl. Phys. 88 (2000) 4365. [11] P. GonzaÂlez, R. Soto, F. Lusquinos, B. LeoÂn, M. PeÂrez-Amor, J. Vac. Sci. Technol. A 18 (2000) 3004. [12] T. SzoÈreÂnyi, C. Fuchs, E. Fogarassy, J. Hommet, F. Le Normand, Surf. Coat. Technol. 125 (2000) 308. [13] T. SzoÈreÂnyi, F. Antoni, E. Fogarassy, I. BertoÂti, Appl. Surf. Sci. 168 (2000) 248. [14] Z. Geretovszky, Z. KaÂntor, I. BertoÂti, T. SzoÈreÂnyi, Appl. Phys. A 70 (2000) 9. [15] E. Riedo, F. Comin, J. Chevrier, F. Schmithusen, S. Decossas, M. Sancrotti, Surf. Coat. Technol. 125 (2000) 124. [16] T. SzoÈreÂnyi, J.-P. Stoquert, J. Perriere, F. Antoni, E. Fogarassy, Diamond Relat. Mater., in press.