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Nanoporosity in plasma deposited amorphous carbon films investigated by small-angle X-ray scattering
Diamond and Related Materials 11 (2002) 1946–1951
Nanoporosity in plasma deposited amorphous carbon films investigated by small-angle X-ray scattering L.G. Jacobsohna, G. Capotea, M.E.H. Maia da Costaa, D.F. Franceschinib, F.L. Freire Jr.a,* a
´ ´ ´ ˆ de Sao ˜ Vicente, 225-Gavea, ´ Departamento de Fısica, Pontifıcia Universidade Catolica do Rio de Janeiro, Rua Marques Caixa Postal 38071, 22452-970, Rio de Janeiro, RJ, Brazil b ´ ˆ ´ RJ, Brazil Instituto de Fısica, Universidade Federal Fluminense, Av. Litoranea syn, 24210-340, Niteroi, Received 22 April 2002; received in revised form 23 September 2002; accepted 6 October 2002
Abstract Amorphous carbon films were deposited by plasma enhanced chemical vapor deposition (PECVD) and d.c.-magnetron sputtering and their porosity was investigated by small-angle X-ray scattering (SAXS). In the case of sputtered films, the X-ray scattering intensity increased with the argon pressure used for the film deposition, while the atomic density decreased. The analysis of the SAXS results was performed using the GNOM code assuming a distribution of spherical pores. This analysis suggested that the maximum of these distributions occur for a radius value below 1 nm. The films deposited at 0.17 Pa were essentially pore-free. As the Ar pressure increases, the pore size distribution widens and the volume occupied by the pores increases. A direct relation between the atomic density of carbon films deposited by sputtering and the pore volume fraction was also obtained. The low scattering intensity observed for the films deposited by PECVD showed that they were compact and homogeneous regardless of the self-bias voltage employed in the range between y100 V and y500 V. 䊚 2002 Published by Elsevier Science B.V. Keywords: Amorphous carbon; Amorphous hydrogenated carbon; Microstructure; Deposition
1. Introduction Amorphous carbon films stands out due to its unique set of properties like high hardness, chemical inertness, low friction and high wear resistance w1x. According to Robertson, the amorphous carbon network can be described by small sp2-hybridized carbon clusters bonded to one another by sp3 carbon atoms, with the structural arrangement and the film properties mainly determined by the sp2 ysp3 ratio and the hydrogen content w2x. Amorphous carbon films can be synthesized by many different ways. Among them, we highlight plasma enhanced chemical vapor deposition (PECVD) and sputtering techniques. In common to these techniques is an intense bombardment of the surface of the growing film. In the case of sputtered films, bombardment is due to *Corresponding author. Tel.: q55-21-3114-1272; fax: q55-213114-1275. E-mail address: [email protected] (F.L. Freire Jr.).
both neutrals backscattered on the target and plasma ions when a negative electrical voltage is applied to the substrate holder. In the case of PECVD, the radiofrequency applied to the substrate holder induces a d.c. self-bias voltage that accelerates the plasma ions towards the film. Since the formation mechanism of sp3-hybridized carbon atoms is due to the high local density created by the penetration of a carbon-containing ion into the amorphous network w3x, for both techniques a full spectrum of materials from graphite- and polymerlike to diamond-like can be obtained by changing the deposition conditions. In particular, the energy of the bombarding species appears to be the most important parameter. In fact, while low bombarding energies, below ;100 eV, cannot induce the formation of sp3hybridized carbons w4x, bombarding energies above ;400 eV promote the generation of too many defects in the carbon matrix and lead to a defective graphitelike material. As a consequence, only a relatively narrow window of energies promotes the deposition of diamondlike films w2,4,5x.
0925-9635/02/$ - see front matter 䊚 2002 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 2 1 3 - 3
L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
1947
The occurrence of porosity (voids) in vapor deposited films is a natural consequence of the low mobility of adatoms (surface and bulk mobilities) and of the shadowing mechanism and it can be a drawback to many applications. In the case of protective coatings, porosity may lead to the oxygen diffusion and degradation of the protected layer. Also, the use of amorphous carbon films as an electrode material requires a pore free structure w6x. In order to avoid porosity one has to enhance adatom mobility. This can be achieved by low energy ion bombardment during deposition or by higher substrate temperatures w7,8x. In the case of amorphous carbon materials, high substrate temperatures are not desired since they lead to graphitic materials w9,10x. These graphitic materials, that are similar to those obtained by post-deposition annealing treatment, have weak mechanical properties and cannot be used as protective coatings w11x. The shadowing mechanism arises from geometric constraints imposed by the roughness of the growing film and the line-of-sight of the impinging atoms. It is specially influenced by the direction of the trajectory of the incoming species, the closer it is to the normal of the film surface, the less the porosity is w7,12x. The investigation of porosity in the structural arrangement of amorphous carbon based materials has been carried out mainly by positron annihilation spectroscopy (PAS) w13–16x and gas effusion w17–20x measurements. In the first technique, positronium lifetime can be correlated with the size of subnanometric voids while variable energy positron beams can provide a depth distribution of empty volumes like, point defects, vacancies and small voids. Gas effusion measurements provide information about the structure of voids, if they are isolated or make an interconnected network. It is the aim of this work to present results of small-angle X-ray scattering analysis (SAXS) of sputtered and PECVD deposited amorphous carbon films and, since SAXS results are sensitive to the size and shape distributions of the voids in the material, to discuss the results in the context of previous investigations of the porosity of these materials.
with different microstructures and properties, as discussed earlier w21x. Hydrogenated amorphous carbon (a-C:H) films were deposited by plasma decomposition of methane atmospheres on Si (100) and 11-mm-thick Al substrates as a function of the self-bias voltage (VB). For self-bias voltages up to y350 V, the temperature of the substrate holder was kept below 40 8C, while for VBsy500 V it was approximately 90 8C. The desired VB value was adjusted by means of the r.f. power input, which ranged from 4 to 53 W. The deposition pressure was 10 Pa and the influx of gas was 3 sccm. Details of the deposition conditions can be found elsewhere w22x. The films obtained by both techniques, sputtering and PECVD, have a final thickness of 300 nm. The chemical composition was determined by IBA: Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) employing a 4MV Van de Graaff accelerator KN-4000 from High Voltage Engineering Corp. For the RBS measurements, a 2-MeV Heq beam was used with the particle detector positioned at 1658 with respect to the incident beam. ERDA measurements determined the hydrogen content using a 2.2-MeV Heq beam with the detector positioned at 308, while the sample was tilted by 758 with respect to the incident beam. The atomic density was inferred by combining the areal atomic density provided by IBA and the thickness obtained by stylus profilometry. The experimental errors in the density values are approximately 0.15=1023 atomsycm3. Small angle X-ray scattering (SAXS) experiments were performed using the dedicated SAXS beamline at the National Synchrotron Light Laboratory (LNLS Campinas, Brazil) which is equipped with an asymmetrically cut and bent (111) silicon monochromator, yielding a ˚ monochromatic (ls1.608 A) and a horizontally focused beam w23x. The incident beam direction is parallel to the surface normal of the samples. A vertical positive-sensitive X-ray detector positioned at 1 m away from the sample and a multichannel analyzer was used to record the SAXS spectra as a function of the modulus of the scattering vector q, defined as:
2. Experimental procedures
qsŽ4pyl.sinu,
Amorphous carbon (a-C) films were deposited by d.c.-magnetron sputtering on Si (100) and 11-mm-thick aluminum foil substrates. The former substrates were used for ion beam analyses (IBA) and the films deposited onto Al were used for small-angle X-ray scattering experiments. The copper water-cooled substrate holder was electrically grounded and the depositions were carried out with the substrates at room temperature, as measured by a thermocouple. The films were deposited at several Ar pressures from 0.17 to 1.4 Pa in order to change the bombardment regime what leads to films
where u is half of the scattering angle. The maximum ˚ y1. scattering vector value was 0.5 A Each SAXS spectrum corresponds to a data collection time of 300 and 600 s, for sputtered and PECVD deposited samples, respectively. In order to improve the statistics of the SAXS scattering curves, I(q), the average of several spectra was taken. I(q) takes into account the normalization by the incident beam intensity and was corrected by sample absorption and the contribution due to spurious X-ray scattering. Because of the small size of the incident beam cross-section at the detection plane, no mathematical desmearing of the experimental
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L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
1948
Table 2 Deposition rate, r.f. power, chemical composition and atomic density of PECVD deposited a-C:H films
SAXS intensity function was needed. For all measurements, a stack of each film was employed to achieve a final film thickness of ;2 mm. The SAXS results were analyzed by employing the GNOM code w24x.
VB (yV)
Power (W)
Deposition rate (nmymin)
3. Results and discussion 100 200 350 500
3.1. Sample characterization The deposition rate, chemical composition and atomic density of sputtered a-C films are shown in Table 1 as functions of the Ar pressure. Hydrogen was not deliberately incorporated, but it is present in all films and its concentration increases at higher Ar pressures. Previous work showed that hydrogen is mostly bonded to sp3hybridized carbon atoms and that it probably originated in the exposition of the samples to air w21,25x. This is supported by the simultaneous observation of the increase of the oxygen contamination. The density of the a-C films strongly decreases from 1.4=1023 to 1.0=1023 atomsycm3 as the Ar pressure increases. This behavior is associated with a change in the deposition regime from ballistic that occurs at low Ar pressures, to a diffusive one at high Ar pressures w21x. In the ballistic regime, the sputtered carbon atoms reach the substrate along straight paths and with higher energies when compared to the carbon atoms deposited in the diffusive regime since in the latest case the carbon atoms suffer several collisions with Ar atoms before reaching the substrate. Higher porosity is expected in films deposited in the diffusive regime. The films obtained from lowest Ar pressure depositions are hard (film hardness of 15 GPa), stressed (internal stress of 3 GPa) and dense, while those obtained at the highest Ar pressures are soft (hardness of 2 GPa) and relaxed (internal stress of 0.7 GPa) and have an optical gap (1.3 eV) and a refractive index (ns2.03) which suggest a polymer-like character for these films w21x. Table 2 presents the input power, deposition rate, chemical composition and atomic density of PECVD deposited films as functions of the self-bias voltage. The increase of the deposition rate is a consequence of the stronger decomposition of the precursor atmosphere induced by the higher power input necessary to achieve
4 12 26 53
3.5 7.4 13 24
Composition (%) C
H
82.5 82 85 87.5
17.5 18 15 12.5
Atomic density (=1023 atomsycm3) 1.3 1.2 1.3 1.3
higher self-bias voltages. In these films, the hydrogen content slightly decreases for higher VB values, but the atomic density remains constant within the experimental error. In this range of self-bias voltages the films are always hard and stressed. In fact, for these self-bias values, the film hardness is higher than 12 GPa and the internal stress higher than 2 GPa. On the other side, the Raman spectra taken from them are typical of diamondlike films w22x. 3.2. Porosity characterization The investigation of the presence of pores in amorphous carbon films was carried out by means of small angle X-ray scattering (SAXS) measurements. The Xray scattering arises from inhomogeneities in the electron density (refractive index) within the material and the scattering curves I(q) depend on the shape and size of the inhomogeneities. I(q) is the same for pores and particles of the same size and shape, provided the difference in electron density between both phases is the same (Babinet principle). In the case of a polydisperse system, i.e. when there is a distribution of scattering objects with different sizes, the only possible analysis of the scattering data is by modeling the scattering objects and carrying out numerical simulation together with experimental data fitting. In our case, the scattering originates from the difference of the electron densities of the homogeneous carbon matrix and the pores (voids). For the analysis, which was carried out by employing the GNOM code, a distribution of spherical pores was assumed.
Table 1 Deposition rate, chemical composition, atomic density and invariant Q of sputtered a-C films Composition (%)
Ar pressure (Pa)
Deposition rate (nmymin)
C
H
O
0.17 0.36 0.7 1.1 1.4
0.55 0.55 0.63 0.89 0.91
95.5 96 93.6 83.9 82.9
4 4 5.9 11 12
– – 0.4 5 5
See text for details.
Ar
Atomic density (=1023 atomsycm3)
Q (arbitrary units)
0.5 – 0.1 0.1 0.1
1.4 1.4 1.3 1.1 1.0
0.16 3.8 7.2 14 16
L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
1949
is nearly zero for radii greater than ;3 nm, films deposited with the highest Ar pressure (1.4 Pa) present an open structure where pores with radii as high as 8 nm exist. These results are in agreement with previous gas effusion measurements that showed that films deposited at high Ar pressures present a network of interconnected pores that allows the effusion of relatively large molecules at temperatures as low as ;250 8C w20x. The analysis of the SAXS data suggests that the pore volumetric fraction distribution has its maximum below 1 nm. A measure of the total volume fraction occupied by the scattering objects can be obtained by calculating the invariant Q. The invariant Q is defined by w26x: `
|
Qs
IŽq.q2dq
(2)
0
Fig. 1. X-Ray scattering curves as functions of the scattering vector, obtained from sputter-deposited a-C films at several Ar deposition pressures, as indicated. The continuous lines correspond to the results of numerical simulations using the GNOM code and assuming a distribution of spherical pores.
The results of SAXS measurements of the sputtered a-C films are presented in Fig. 1, where the strong increase of the scattering intensity for higher Ar pressures indicates a progressive increase of the presence of pores in the atomic arrangement. In particular, the film deposited at the lowest Ar pressure presents nearly no scattering and is essentially pore-free. In addition to the experimental data, a continuous line is also shown for each spectrum of Fig. 1. These curves correspond to the simulated scattering curves obtained by the GNOM code assuming a distribution of spherical pores. The good agreement between the experimental data and the computer simulations supports the model employed. Fig. 2 presents the pore volumetric fraction distribution as a function of the pore radius as obtained by the analysis of the experimental data by using the GNOM code w24x. As it is well known w26x, it is not possible to obtain a complete volumetric fraction distribution due to the limited range of q values experimentally available. With our experimental set-up, radii smaller than approximately 1 nm cannot be measured. Nevertheless, it was possible to identify clear changes in the pore distribution as the deposition condition, i.e. the Ar pressure, was changed. For the lowest Ar pressure (0.17 Pa), films are nearly pore-free. As the Ar pressure increases, the volumetric fraction of pores increases and the pores size distribution widens. In fact, while films deposited in Ar pressure of 0.36 Pa present a narrower distribution that
where q is the scattering vector defined by Eq. (1) and I(q) is the SAXS intensity. The values of Q quoted in Table 1 show that there is a straight relationship between the atomic density and the pore volume fraction. In the case of PECVD deposited films, the X-ray scattering curves obtained from the a-C:H films deposited at VBsy100 V and from the aluminum foil substrate are shown in Fig. 3 as a function of the scattering vector. All the SAXS spectra taken from different a-C:H films are identical to these two presented in Fig. 3. While the results obtained from the other
Fig. 2. Volumetric fraction distribution of spherical pores as a function of the pore radius for several deposition conditions of sputtered a-C films. These results were obtained by numerical simulation and experimental data fitting by using the GNOM code.
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L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
ments of plasma deposited a-C:H w13,14x and a-C w15x films showed the existence of pores within the amorphous structure which, in the former case, were estimated to have radius of approximately 0.5 nm w14x. The discussion above seems to suggest that PAS be a more sensitive technique than SAXS to detect the presence of nanopores in these materials. In fact, it is well known that positron techniques are very sensitive for studying open volume defects like vacancies, voids or nanopores w27x. 4. Conclusions
Fig. 3. X-Ray scattering curve obtained from an a-C:H film deposited at VBsy100 V. The Al substrate scattering intensity is shown for comparison.
films are not shown, the spectrum obtained for the Al substrate is shown for comparison purposes. As can be seen, a much smaller scattering intensity is found when the a-C:H SAXS spectra are compared with the scattering spectra obtained from the sputtered films. In fact, ˚ y1 they can since they are already zero for qf0.02 A only be compared with the scattering curve from the sputtered film deposited in a 0.17-Pa Ar atmosphere. The a-C:H films have negligible X-ray scattering intensity, as confirmed by the scattering intensity of the Al substrate shown in Fig. 3, what provides evidence of a homogeneous amorphous arrangement that is pore-free or that the pore fraction is below the sensitivity of the SAXS measurements. These results can be attributed to the more energetic ion bombardment that occurs during PECVD film deposition in comparison with the carbon sputtering deposition. The higher bombardment enhances more effectively adatom mobility and suppresses pore formation. The constant behavior of the density of aC:H films as a function of the deposition condition, i.e. VB variation, compared with the density variation observed in sputtered films, seems to confirm this interpretation. In addition, previous reported gas effusion measurements of the PECVD deposited a-C:H films showed that they were dense and pore-free w16x. On the other hand, despite the mutual support between the results obtained by three different techniques (SAXS, gas effusion and density measurements), works based on positron annihilation spectroscopy (PAS) measure-
The presence of nanopores in hydrogenated and nonhydrogenated amorphous carbon films was investigated by small-angle X-ray scattering. The films deposited by d.c.-magnetron sputtering in a 0.17-Pa Ar atmosphere were pore-free, while for higher Ar pressures volume fraction occupied by the pores increases and the size distribution of pores widens when the Ar deposition pressure increases. The maximum of this distribution is expected to be for a radius value below 1 nm. A straight relationship between the film atomic density and the pore volume fraction was determined. On the other hand, a-C:H films obtained by PECVD in a range of self-bias voltages from y100 V to y500 V were structurally homogeneous and their SAXS scattering curves reveal that the pore (voids) density was below the detection limit of our experimental set-up. Acknowledgments This work is partially supported by the Brazilian Agencies: CAPES, CNPq and FAPERJ, and by the National Synchrotron Light Laboratory, LNLS, under projects SAS 420y98 and 763y00. The assistance of G. Kellermann during the SAXS measurements is acknowledged. References w1x A. Grill, Diamond Relat. Mater. 8 (1999) 428. w2x J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. w3x Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 (1990) 10468. w4x Y. Lifshitz, Diamond Relat. Mater. 5 (1996) 388. w5x R.G. Lacerda, F.C. Marques, F.L. Freire Jr., Diamond Relat. Mater. 8 (1999) 495. w6x A. Zeng, E. Liu, I.F. Annergren, et al., Diamond Relat. Mater. 11 (2002) 160. w7x A.G. Dirks, H.J. Leamy, Thin Solid Films 47 (1977) 219. w8x K.H. Muller, ¨ J. Appl. Phys. 59 (1986) 2803. w9x E. Mounier, F. Bertin, M. Adamik, Y. Pauleau, P.B. Barna, Diamond Relat. Mater. 5 (1996) 1509. w10x R. Gago, O.S. Garrido, A.C. Font, et al., Thin Solid Films 338 (1999) 88. w11x L.G. Jacobsohn, R. Prioli, F.L. Freire Jr., G. Mariotto, Y.W. Chung, M.M. Lacerda, Diamond Relat. Mater. 9 (2000) 680.
L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951 w12x D. Henderson, M.H. Brodsky, P. Chaudhari, Appl. Phys. Lett. 25 (1974) 641. w13x C.L. Wang, Y. Kobayashi, R. Katoh, R. Suzuki, T. Ohdaira, J. Appl. Phys. 90 (2001) 404. w14x G. Kogel, ¨ ¨ ¨ D. Schodlbauer, W. Triftshauser, J. Winter, Phys. Rev. Lett. 60 (1988) 1550. w15x F.L. Freire Jr., D.F. Franceschini, C.A. Achete, et al., J. Appl. Phys. 81 (1997) 2451. w16x F.L. Freire Jr., C.A. Achete, R.S. Brusa, G. Mariotto, X.T. Teng, A. Zecca, Solid State Commun. 91 (1994) 965. w17x C.h. Wild, P. Koidl, Appl. Phys. Lett. 51 (1987) 1506. w18x X. Jiang, W. Beyer, K. Reichelt, J. Appl. Phys. 68 (1990) 1378. w19x Y. Bounuh, J. Spousta, M. Clin, et al., Diamond Relat. Mater. 5 (1996) 453.
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w20x F.L. Freire Jr., L.G. Jacobsohn, D.F. Franceschini, S.S. Camargo Jr., J. Vac. Sci. Technol. A 18 (2000) 2344. w21x L.G. Jacobsohn, F.L. Freire Jr., J. Vac. Sci. Technol. A 17 (1999) 2841. w22x L.G. Jacobsohn, D.F. Franceschini, M.E.H. Maia da Costa, F.L. Freire Jr., J. Vac. Sci. Technol. A 18 (2000) 2230. w23x G. Kellermann, F. Vicentin, E. Tamura, et al., J. Appl. Cryst. 30 (1993) 880. w24x D.I. Svergun, J. Appl. Cryst. 25 (1992) 495. w25x F. Alvarez, N. Victoria, P. Hammer, F.L. Freire Jr., M.C. dos Santos, Appl. Phys. Lett. 73 (1998) 1065. w26x O. Glatter, O. Kratky (Eds.), Small Angle X-Ray Scattering, Academic Press, New York, 1982. w27x P.J. Shulz, K.G. Lynn, Rev. Mod. Phys. 60 (1988) 701.
Nanoporosity in plasma deposited amorphous carbon films investigated by small-angle X-ray scattering L.G. Jacobsohna, G. Capotea, M.E.H. Maia da Costaa, D.F. Franceschinib, F.L. Freire Jr.a,* a
´ ´ ´ ˆ de Sao ˜ Vicente, 225-Gavea, ´ Departamento de Fısica, Pontifıcia Universidade Catolica do Rio de Janeiro, Rua Marques Caixa Postal 38071, 22452-970, Rio de Janeiro, RJ, Brazil b ´ ˆ ´ RJ, Brazil Instituto de Fısica, Universidade Federal Fluminense, Av. Litoranea syn, 24210-340, Niteroi, Received 22 April 2002; received in revised form 23 September 2002; accepted 6 October 2002
Abstract Amorphous carbon films were deposited by plasma enhanced chemical vapor deposition (PECVD) and d.c.-magnetron sputtering and their porosity was investigated by small-angle X-ray scattering (SAXS). In the case of sputtered films, the X-ray scattering intensity increased with the argon pressure used for the film deposition, while the atomic density decreased. The analysis of the SAXS results was performed using the GNOM code assuming a distribution of spherical pores. This analysis suggested that the maximum of these distributions occur for a radius value below 1 nm. The films deposited at 0.17 Pa were essentially pore-free. As the Ar pressure increases, the pore size distribution widens and the volume occupied by the pores increases. A direct relation between the atomic density of carbon films deposited by sputtering and the pore volume fraction was also obtained. The low scattering intensity observed for the films deposited by PECVD showed that they were compact and homogeneous regardless of the self-bias voltage employed in the range between y100 V and y500 V. 䊚 2002 Published by Elsevier Science B.V. Keywords: Amorphous carbon; Amorphous hydrogenated carbon; Microstructure; Deposition
1. Introduction Amorphous carbon films stands out due to its unique set of properties like high hardness, chemical inertness, low friction and high wear resistance w1x. According to Robertson, the amorphous carbon network can be described by small sp2-hybridized carbon clusters bonded to one another by sp3 carbon atoms, with the structural arrangement and the film properties mainly determined by the sp2 ysp3 ratio and the hydrogen content w2x. Amorphous carbon films can be synthesized by many different ways. Among them, we highlight plasma enhanced chemical vapor deposition (PECVD) and sputtering techniques. In common to these techniques is an intense bombardment of the surface of the growing film. In the case of sputtered films, bombardment is due to *Corresponding author. Tel.: q55-21-3114-1272; fax: q55-213114-1275. E-mail address: [email protected] (F.L. Freire Jr.).
both neutrals backscattered on the target and plasma ions when a negative electrical voltage is applied to the substrate holder. In the case of PECVD, the radiofrequency applied to the substrate holder induces a d.c. self-bias voltage that accelerates the plasma ions towards the film. Since the formation mechanism of sp3-hybridized carbon atoms is due to the high local density created by the penetration of a carbon-containing ion into the amorphous network w3x, for both techniques a full spectrum of materials from graphite- and polymerlike to diamond-like can be obtained by changing the deposition conditions. In particular, the energy of the bombarding species appears to be the most important parameter. In fact, while low bombarding energies, below ;100 eV, cannot induce the formation of sp3hybridized carbons w4x, bombarding energies above ;400 eV promote the generation of too many defects in the carbon matrix and lead to a defective graphitelike material. As a consequence, only a relatively narrow window of energies promotes the deposition of diamondlike films w2,4,5x.
0925-9635/02/$ - see front matter 䊚 2002 Published by Elsevier Science B.V. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 2 1 3 - 3
L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
1947
The occurrence of porosity (voids) in vapor deposited films is a natural consequence of the low mobility of adatoms (surface and bulk mobilities) and of the shadowing mechanism and it can be a drawback to many applications. In the case of protective coatings, porosity may lead to the oxygen diffusion and degradation of the protected layer. Also, the use of amorphous carbon films as an electrode material requires a pore free structure w6x. In order to avoid porosity one has to enhance adatom mobility. This can be achieved by low energy ion bombardment during deposition or by higher substrate temperatures w7,8x. In the case of amorphous carbon materials, high substrate temperatures are not desired since they lead to graphitic materials w9,10x. These graphitic materials, that are similar to those obtained by post-deposition annealing treatment, have weak mechanical properties and cannot be used as protective coatings w11x. The shadowing mechanism arises from geometric constraints imposed by the roughness of the growing film and the line-of-sight of the impinging atoms. It is specially influenced by the direction of the trajectory of the incoming species, the closer it is to the normal of the film surface, the less the porosity is w7,12x. The investigation of porosity in the structural arrangement of amorphous carbon based materials has been carried out mainly by positron annihilation spectroscopy (PAS) w13–16x and gas effusion w17–20x measurements. In the first technique, positronium lifetime can be correlated with the size of subnanometric voids while variable energy positron beams can provide a depth distribution of empty volumes like, point defects, vacancies and small voids. Gas effusion measurements provide information about the structure of voids, if they are isolated or make an interconnected network. It is the aim of this work to present results of small-angle X-ray scattering analysis (SAXS) of sputtered and PECVD deposited amorphous carbon films and, since SAXS results are sensitive to the size and shape distributions of the voids in the material, to discuss the results in the context of previous investigations of the porosity of these materials.
with different microstructures and properties, as discussed earlier w21x. Hydrogenated amorphous carbon (a-C:H) films were deposited by plasma decomposition of methane atmospheres on Si (100) and 11-mm-thick Al substrates as a function of the self-bias voltage (VB). For self-bias voltages up to y350 V, the temperature of the substrate holder was kept below 40 8C, while for VBsy500 V it was approximately 90 8C. The desired VB value was adjusted by means of the r.f. power input, which ranged from 4 to 53 W. The deposition pressure was 10 Pa and the influx of gas was 3 sccm. Details of the deposition conditions can be found elsewhere w22x. The films obtained by both techniques, sputtering and PECVD, have a final thickness of 300 nm. The chemical composition was determined by IBA: Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) employing a 4MV Van de Graaff accelerator KN-4000 from High Voltage Engineering Corp. For the RBS measurements, a 2-MeV Heq beam was used with the particle detector positioned at 1658 with respect to the incident beam. ERDA measurements determined the hydrogen content using a 2.2-MeV Heq beam with the detector positioned at 308, while the sample was tilted by 758 with respect to the incident beam. The atomic density was inferred by combining the areal atomic density provided by IBA and the thickness obtained by stylus profilometry. The experimental errors in the density values are approximately 0.15=1023 atomsycm3. Small angle X-ray scattering (SAXS) experiments were performed using the dedicated SAXS beamline at the National Synchrotron Light Laboratory (LNLS Campinas, Brazil) which is equipped with an asymmetrically cut and bent (111) silicon monochromator, yielding a ˚ monochromatic (ls1.608 A) and a horizontally focused beam w23x. The incident beam direction is parallel to the surface normal of the samples. A vertical positive-sensitive X-ray detector positioned at 1 m away from the sample and a multichannel analyzer was used to record the SAXS spectra as a function of the modulus of the scattering vector q, defined as:
2. Experimental procedures
qsŽ4pyl.sinu,
Amorphous carbon (a-C) films were deposited by d.c.-magnetron sputtering on Si (100) and 11-mm-thick aluminum foil substrates. The former substrates were used for ion beam analyses (IBA) and the films deposited onto Al were used for small-angle X-ray scattering experiments. The copper water-cooled substrate holder was electrically grounded and the depositions were carried out with the substrates at room temperature, as measured by a thermocouple. The films were deposited at several Ar pressures from 0.17 to 1.4 Pa in order to change the bombardment regime what leads to films
where u is half of the scattering angle. The maximum ˚ y1. scattering vector value was 0.5 A Each SAXS spectrum corresponds to a data collection time of 300 and 600 s, for sputtered and PECVD deposited samples, respectively. In order to improve the statistics of the SAXS scattering curves, I(q), the average of several spectra was taken. I(q) takes into account the normalization by the incident beam intensity and was corrected by sample absorption and the contribution due to spurious X-ray scattering. Because of the small size of the incident beam cross-section at the detection plane, no mathematical desmearing of the experimental
(1)
L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
1948
Table 2 Deposition rate, r.f. power, chemical composition and atomic density of PECVD deposited a-C:H films
SAXS intensity function was needed. For all measurements, a stack of each film was employed to achieve a final film thickness of ;2 mm. The SAXS results were analyzed by employing the GNOM code w24x.
VB (yV)
Power (W)
Deposition rate (nmymin)
3. Results and discussion 100 200 350 500
3.1. Sample characterization The deposition rate, chemical composition and atomic density of sputtered a-C films are shown in Table 1 as functions of the Ar pressure. Hydrogen was not deliberately incorporated, but it is present in all films and its concentration increases at higher Ar pressures. Previous work showed that hydrogen is mostly bonded to sp3hybridized carbon atoms and that it probably originated in the exposition of the samples to air w21,25x. This is supported by the simultaneous observation of the increase of the oxygen contamination. The density of the a-C films strongly decreases from 1.4=1023 to 1.0=1023 atomsycm3 as the Ar pressure increases. This behavior is associated with a change in the deposition regime from ballistic that occurs at low Ar pressures, to a diffusive one at high Ar pressures w21x. In the ballistic regime, the sputtered carbon atoms reach the substrate along straight paths and with higher energies when compared to the carbon atoms deposited in the diffusive regime since in the latest case the carbon atoms suffer several collisions with Ar atoms before reaching the substrate. Higher porosity is expected in films deposited in the diffusive regime. The films obtained from lowest Ar pressure depositions are hard (film hardness of 15 GPa), stressed (internal stress of 3 GPa) and dense, while those obtained at the highest Ar pressures are soft (hardness of 2 GPa) and relaxed (internal stress of 0.7 GPa) and have an optical gap (1.3 eV) and a refractive index (ns2.03) which suggest a polymer-like character for these films w21x. Table 2 presents the input power, deposition rate, chemical composition and atomic density of PECVD deposited films as functions of the self-bias voltage. The increase of the deposition rate is a consequence of the stronger decomposition of the precursor atmosphere induced by the higher power input necessary to achieve
4 12 26 53
3.5 7.4 13 24
Composition (%) C
H
82.5 82 85 87.5
17.5 18 15 12.5
Atomic density (=1023 atomsycm3) 1.3 1.2 1.3 1.3
higher self-bias voltages. In these films, the hydrogen content slightly decreases for higher VB values, but the atomic density remains constant within the experimental error. In this range of self-bias voltages the films are always hard and stressed. In fact, for these self-bias values, the film hardness is higher than 12 GPa and the internal stress higher than 2 GPa. On the other side, the Raman spectra taken from them are typical of diamondlike films w22x. 3.2. Porosity characterization The investigation of the presence of pores in amorphous carbon films was carried out by means of small angle X-ray scattering (SAXS) measurements. The Xray scattering arises from inhomogeneities in the electron density (refractive index) within the material and the scattering curves I(q) depend on the shape and size of the inhomogeneities. I(q) is the same for pores and particles of the same size and shape, provided the difference in electron density between both phases is the same (Babinet principle). In the case of a polydisperse system, i.e. when there is a distribution of scattering objects with different sizes, the only possible analysis of the scattering data is by modeling the scattering objects and carrying out numerical simulation together with experimental data fitting. In our case, the scattering originates from the difference of the electron densities of the homogeneous carbon matrix and the pores (voids). For the analysis, which was carried out by employing the GNOM code, a distribution of spherical pores was assumed.
Table 1 Deposition rate, chemical composition, atomic density and invariant Q of sputtered a-C films Composition (%)
Ar pressure (Pa)
Deposition rate (nmymin)
C
H
O
0.17 0.36 0.7 1.1 1.4
0.55 0.55 0.63 0.89 0.91
95.5 96 93.6 83.9 82.9
4 4 5.9 11 12
– – 0.4 5 5
See text for details.
Ar
Atomic density (=1023 atomsycm3)
Q (arbitrary units)
0.5 – 0.1 0.1 0.1
1.4 1.4 1.3 1.1 1.0
0.16 3.8 7.2 14 16
L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
1949
is nearly zero for radii greater than ;3 nm, films deposited with the highest Ar pressure (1.4 Pa) present an open structure where pores with radii as high as 8 nm exist. These results are in agreement with previous gas effusion measurements that showed that films deposited at high Ar pressures present a network of interconnected pores that allows the effusion of relatively large molecules at temperatures as low as ;250 8C w20x. The analysis of the SAXS data suggests that the pore volumetric fraction distribution has its maximum below 1 nm. A measure of the total volume fraction occupied by the scattering objects can be obtained by calculating the invariant Q. The invariant Q is defined by w26x: `
|
Qs
IŽq.q2dq
(2)
0
Fig. 1. X-Ray scattering curves as functions of the scattering vector, obtained from sputter-deposited a-C films at several Ar deposition pressures, as indicated. The continuous lines correspond to the results of numerical simulations using the GNOM code and assuming a distribution of spherical pores.
The results of SAXS measurements of the sputtered a-C films are presented in Fig. 1, where the strong increase of the scattering intensity for higher Ar pressures indicates a progressive increase of the presence of pores in the atomic arrangement. In particular, the film deposited at the lowest Ar pressure presents nearly no scattering and is essentially pore-free. In addition to the experimental data, a continuous line is also shown for each spectrum of Fig. 1. These curves correspond to the simulated scattering curves obtained by the GNOM code assuming a distribution of spherical pores. The good agreement between the experimental data and the computer simulations supports the model employed. Fig. 2 presents the pore volumetric fraction distribution as a function of the pore radius as obtained by the analysis of the experimental data by using the GNOM code w24x. As it is well known w26x, it is not possible to obtain a complete volumetric fraction distribution due to the limited range of q values experimentally available. With our experimental set-up, radii smaller than approximately 1 nm cannot be measured. Nevertheless, it was possible to identify clear changes in the pore distribution as the deposition condition, i.e. the Ar pressure, was changed. For the lowest Ar pressure (0.17 Pa), films are nearly pore-free. As the Ar pressure increases, the volumetric fraction of pores increases and the pores size distribution widens. In fact, while films deposited in Ar pressure of 0.36 Pa present a narrower distribution that
where q is the scattering vector defined by Eq. (1) and I(q) is the SAXS intensity. The values of Q quoted in Table 1 show that there is a straight relationship between the atomic density and the pore volume fraction. In the case of PECVD deposited films, the X-ray scattering curves obtained from the a-C:H films deposited at VBsy100 V and from the aluminum foil substrate are shown in Fig. 3 as a function of the scattering vector. All the SAXS spectra taken from different a-C:H films are identical to these two presented in Fig. 3. While the results obtained from the other
Fig. 2. Volumetric fraction distribution of spherical pores as a function of the pore radius for several deposition conditions of sputtered a-C films. These results were obtained by numerical simulation and experimental data fitting by using the GNOM code.
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L.G. Jacobsohn et al. / Diamond and Related Materials 11 (2002) 1946–1951
ments of plasma deposited a-C:H w13,14x and a-C w15x films showed the existence of pores within the amorphous structure which, in the former case, were estimated to have radius of approximately 0.5 nm w14x. The discussion above seems to suggest that PAS be a more sensitive technique than SAXS to detect the presence of nanopores in these materials. In fact, it is well known that positron techniques are very sensitive for studying open volume defects like vacancies, voids or nanopores w27x. 4. Conclusions
Fig. 3. X-Ray scattering curve obtained from an a-C:H film deposited at VBsy100 V. The Al substrate scattering intensity is shown for comparison.
films are not shown, the spectrum obtained for the Al substrate is shown for comparison purposes. As can be seen, a much smaller scattering intensity is found when the a-C:H SAXS spectra are compared with the scattering spectra obtained from the sputtered films. In fact, ˚ y1 they can since they are already zero for qf0.02 A only be compared with the scattering curve from the sputtered film deposited in a 0.17-Pa Ar atmosphere. The a-C:H films have negligible X-ray scattering intensity, as confirmed by the scattering intensity of the Al substrate shown in Fig. 3, what provides evidence of a homogeneous amorphous arrangement that is pore-free or that the pore fraction is below the sensitivity of the SAXS measurements. These results can be attributed to the more energetic ion bombardment that occurs during PECVD film deposition in comparison with the carbon sputtering deposition. The higher bombardment enhances more effectively adatom mobility and suppresses pore formation. The constant behavior of the density of aC:H films as a function of the deposition condition, i.e. VB variation, compared with the density variation observed in sputtered films, seems to confirm this interpretation. In addition, previous reported gas effusion measurements of the PECVD deposited a-C:H films showed that they were dense and pore-free w16x. On the other hand, despite the mutual support between the results obtained by three different techniques (SAXS, gas effusion and density measurements), works based on positron annihilation spectroscopy (PAS) measure-
The presence of nanopores in hydrogenated and nonhydrogenated amorphous carbon films was investigated by small-angle X-ray scattering. The films deposited by d.c.-magnetron sputtering in a 0.17-Pa Ar atmosphere were pore-free, while for higher Ar pressures volume fraction occupied by the pores increases and the size distribution of pores widens when the Ar deposition pressure increases. The maximum of this distribution is expected to be for a radius value below 1 nm. A straight relationship between the film atomic density and the pore volume fraction was determined. On the other hand, a-C:H films obtained by PECVD in a range of self-bias voltages from y100 V to y500 V were structurally homogeneous and their SAXS scattering curves reveal that the pore (voids) density was below the detection limit of our experimental set-up. Acknowledgments This work is partially supported by the Brazilian Agencies: CAPES, CNPq and FAPERJ, and by the National Synchrotron Light Laboratory, LNLS, under projects SAS 420y98 and 763y00. The assistance of G. Kellermann during the SAXS measurements is acknowledged. References w1x A. Grill, Diamond Relat. Mater. 8 (1999) 428. w2x J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. w3x Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 (1990) 10468. w4x Y. Lifshitz, Diamond Relat. Mater. 5 (1996) 388. w5x R.G. Lacerda, F.C. Marques, F.L. Freire Jr., Diamond Relat. Mater. 8 (1999) 495. w6x A. Zeng, E. Liu, I.F. Annergren, et al., Diamond Relat. Mater. 11 (2002) 160. w7x A.G. Dirks, H.J. Leamy, Thin Solid Films 47 (1977) 219. w8x K.H. Muller, ¨ J. Appl. Phys. 59 (1986) 2803. w9x E. Mounier, F. Bertin, M. Adamik, Y. Pauleau, P.B. Barna, Diamond Relat. Mater. 5 (1996) 1509. w10x R. Gago, O.S. Garrido, A.C. Font, et al., Thin Solid Films 338 (1999) 88. w11x L.G. Jacobsohn, R. Prioli, F.L. Freire Jr., G. Mariotto, Y.W. Chung, M.M. Lacerda, Diamond Relat. Mater. 9 (2000) 680.
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