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Study of the sp2-to-sp3 ratio of dual-ion-beam sputtered hydrogenated amorphous carbon films
Surface and Coatings Technology, 47 (1991) 287—291
287
Study of the sp2-to-sp3 ratio of dual-ion-beam sputtered hydrogenated amorphous carbon films S.
Scaglione
Laboratorio Film Sottili, Comitato nazionale per la ricera e per lo suiluppo deli’ Energia Nucleare e delie Energia Alternative, Via Anguillarese 301, 00060 Rome (Italy)
R. Giorgi and J. C. Lascovich Laboratorio Superfici e Metailurgia Fisica, Comitato nazionale per Ia ricera e per lo sviluppo deli’ Energia Nucleare e delle Energia Alternative, Via Anguiliarese 301, 00060 Rome (Italy)
G. Ottaviani Dipartmento di Fisica, Universita’ d Modena, Via Campi 213/A, 41100 Modena (Italy)
Abstract Hydrogenated amorphous carbon (a-C:H) thin films have been deposited with a dual-ion-beam sputtering system. The first argon ion beam impinges on the pure pyrolitic graphite target and the second (composed of an ion mixing of argon and hydrogen) directly on the growing film. The sp2 percentages of the films have been deduced from X-ray induced Auger electron spectra. Increasing the hydrogen ion beam energy causes the sp2 percentage to increase whereas on the contrary the Knoop hardness decreases. Moreover a rise in a-C:H hydrogen content causes an increase in the number of sp3 sites.
1. Introduction Hydrogenated amorphous carbon (a-C:H) is a material with an unusual combination of optical and mechanical properties. In fact, high hardness values and good optical properties (high transparency in the JR region) could be obtained. A good review on optical and mechanical properties and on useful deposition processes of a-C:H thin film can be found in ref. 1. In Robertson’s [2] model the a-C:H can be thought of as composed of a mixture of sp3 (tetrahedral configuration) and sp2 (two-dimensional graphite layered structure) sites. The relative concentrations of sp2 and sp3 are a key question in order to understand the bonding and the structure of the amorphous network. A quantitative analysis can be carried out observing the IR absorption spectra of a-C:H films and measuring the area of the absorption band corresponding to vibrational frequencies active in the IR [3, 4]. The hydrogen content seems to play a crucial role in the optical electrical and mechanical properties [5] even if the role of the hydrogen is not completely clear [6]. Jn this paper, a-C:H thin films produced by the dual-ion-beam-sputtering technique have been studied. An evaluation of the sp2 percentage has been 0257-8972/91/$3.50
(~Elsevier Sequoia/Printed in The Netherlands
288 TABLE 1 Typical deposition parameters required to obtain hydrogenated amorphous carbon films Eb (eV) (mA) (standard cm3 min’/~Hz (standard cm3 min1) ~back (mbar) ‘3work (mbar) T~~b ‘b 4~Ar
300—800 47 (2/4.5) —(3/20) 3 x iO~ 4 x i0~ Room temperature
performed by the X-ray induced Auger electron spectroscopy (XAES) and has been related to the hydrogen content and to the energy with which the hydrogen ions impinge on the growing film. Knoop hardness measurements show that harder samples have a lower concentration of sp2 sites.
2. Experimental details a-C:H samples have been produced by a dual-ion-beam-sputtering system composed of two ion sources (Kaufman type manufactured by Ion Tech) with a beam size of 2.5 cm just outside the grid aperture. The first argon ion beam impinges on the pure pyrolitic graphite target and the second composed of a mixture of argon and hydrogen ions hits directly on the growing film. The gas flows have been accurately controlled with an MKS mass flow controller. Typical deposition parameters are reported in Table 1. An accurate lay-out of the system and a detailed description of the technique can be found in ref. 7. The XAES spectra have been obtained with a Vacuum Generators spectrometer model ESCALAB MK II. The samples were irradiated with an Al Ket source (hv = 1486.6 eV) in the analysis vacuum chamber at a pressure value of 1O~mbar. In order to obtain a derivative spectra, a 23-point Savitzkey—Golay convolution array was used. A second-order polynomial was employed in this analysis. A complete description of the XAES measurements of a-C:H thin films has been reported in ref. 8. The hydrogen contents of the samples were determined by elastic recoil detection analysis performed with a 2.2 MeV Van de Graaff accelerator at the Laboratori Nazionali di Legnaro. The Knoop hardness values were evaluated by measuring the indentation size obtained with a rhombic-based pyramid diamond. The apparatus used for ultramicrohardness measurement was a Micro Duromat 4000 (manufactured by Reichert). The load cell is mounted on an optical microscope. All the samples were deposited on polished silicon with a typical thickness of 2000 A; as a consequence the hardness cannot be measured directly because the indentation depth is more than the film thickness and the dimension of the impression is influenced either by the hardness of the a-C:H film or by the
289
250
280 270 280 290 Kinetic Energy (eV)
300
Fig. 1. XAES derivative spectrum of an a-C:H film. The parameter D has been used to estimate 2 percentage of the analysed sample. sp
hardness of the substrate. Subsequently, in order to model the behaviour of sample composites by a film and a substrate, a simple geometrical model was used [9].
3. Results and discussion An example of the XAES derivative spectrum of an a-C:H thin film is given in Fig. 1, in which the distance D between the maximum of the positive-going excursion and the minimum of the negative-going excursion is indicated. In such a spectrum the maximum (at 253 eV) remains fixed for all analysed samples whereas the minimum can shift towards a higher energy. This shift could be associated with the presence of two peaks located at about 275 and 280 eV. As such peaks are absent in the XAES derivative spectrum of diamond, they can be assigned to a transition involving pit electrons with energies less than 5 eV typical of sp2 coordination [8, 10]. Table 2 gives the sp2 percentages and D values of a-C:H samples deposited with different values of the hydrogen ion beam energy. The sp2 percentages were estimated from a linear interpolation of D between the diamond (14.2 eV; 100% sp3) and the graphite values (22.5 eV; 100% sp2). Table 2 shows how the hydrogen energy increases as the sp2 percentage TABLE 2 sp2 percentage and D values (see Fig. 1) of hydrogenated amorphous carbon samples deposited at different hydrogen ion beam energies Eb (eV)
D (eV)
sp2 (%)
300 500 800 Gun off
18.0 190 19.8 20.4
48 59 68 75
290
2200
£
N
E E
2100
a
‘~2000
1900 (5
a
c 1800 1700
a 1600
•
•
400
I
I
600
800
Energy (cv)
Fig. 2. Behaviour of the Knoop hardness of a-C:H samples deposited at different hydrogen ion beam energies. 3500 3000
-
~
\
~2500~\
\
~ 2000
-
\~ -
1500
N.
Fiooo
-
5000 I
0
5
10 15 20 25 Indentation Size (micron)
30
Fig. 3. Geometrical model of hardness behaviour of sample composed of an a-C:H film and a substrate: curve Hc, measured composite hardness; curve Hs, substrate hardness; curve Hf, film hardness obtained from the difference between the measured hardness values of the composite sample and the bare substrate.
increases. The hydrogen content is about the same for the samples reported in this table (CH = 50 at.%) unless the sample was deposited with the second gun turned off (CH =4 at.%). The hydrogen ion beam energy is a parameter which has an important influence on the hardness of the a-C:H thin film produced by the dual-ionbeam-sputtering technique. Figure 2 shows the behaviour of Knoop hardness of samples deposited at different hydrogen energies. Typical hardness curves of the specimen composed of an a-C:H film and substrate can be seen in Fig. 3. Curve Hf represents the hardness of the film deduced from the hardness difference between the measured values of the composite sample and the bare substrate.
291 TABLE 3 2 percentage, D values and hydrogen contents of samples deposited with different flow ratios sp of argon and hydrogen gases with which the second source has been fed ql~r(standardcm3 min1)/4H
3 min’) 2(standard cm
2/4.5 2/8.0 3/20
sp2
(%)
D (eV)
83 65 59
21.0 19.5 19.0
CH (at. %)
-
15 26 50
From a comparison of Table 2 and Fig. 2, it seems that the hydrogen ion beam energy influences both the microstructure (as the ratio of sp2 to sp3 sites) and the macroscopic properties (such as the hardness value). Table 3 shows the sp2 percentages, D values and hydrogen contents of samples grown at a fixed argon to hydrogen gas flow ratio at the second ion source. As the hydrogen ion beam energy value was the same for all samples (Eb = 500 eV), the data in Table 2 suggest that the hydrogen content as well as the hydrogen energy are two critical parameters for control of the sp2 to sp3 ratio.
References 1 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Jensen (eds.) Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, p. 110. 2 J. Robertson, Adv. Phys., 35 (1986) 315. 3 B. D. Dishler, A. Bubenzer and P. Koidi, Solid State Commun., 48 (1983) 105. 4 B. Dishler, P. Koidl and P. Oelhafen (eds.), Proc. Symp. on Amorphous Hydrogenated Carbon Films, Vol. XVII, Les Editions de Physique, Strasbourg, 1987, p. 189. 5 H. Tsai and D. B. Bogy, J. Vac. Sci. Technol. A, 5 (1987) 327. 6 J. Robertson and E. P. O’Reilly, in P. Koidl and P. Oelhafen (eds.), Proc. Symp. on Amorphous Hydrogenated Carbon Films, Vol. XVII, Les Editions de Physique, Strasbourg, 1987, p. 251. 7 S. Scaglione and G. Emiliani, J. Vac. Sci. Technol. A, 3 (1985) 2702. 8 J. C. Lascovich, R. Giorgi and S. Scaglione, Appl. Surf. Sci., 47(1991) 17. 9 S. Scaglione and G. Emiliani, J. Vac. Sci. Technol. A, 7 (1989) 2303. 10 Y. Mizokawa, T. Miyasato, S. Nakamura, K. M. Geib and C. W. Wilmsen, Surf. Sci., 182(1987) 431.
287
Study of the sp2-to-sp3 ratio of dual-ion-beam sputtered hydrogenated amorphous carbon films S.
Scaglione
Laboratorio Film Sottili, Comitato nazionale per la ricera e per lo suiluppo deli’ Energia Nucleare e delie Energia Alternative, Via Anguillarese 301, 00060 Rome (Italy)
R. Giorgi and J. C. Lascovich Laboratorio Superfici e Metailurgia Fisica, Comitato nazionale per Ia ricera e per lo sviluppo deli’ Energia Nucleare e delle Energia Alternative, Via Anguiliarese 301, 00060 Rome (Italy)
G. Ottaviani Dipartmento di Fisica, Universita’ d Modena, Via Campi 213/A, 41100 Modena (Italy)
Abstract Hydrogenated amorphous carbon (a-C:H) thin films have been deposited with a dual-ion-beam sputtering system. The first argon ion beam impinges on the pure pyrolitic graphite target and the second (composed of an ion mixing of argon and hydrogen) directly on the growing film. The sp2 percentages of the films have been deduced from X-ray induced Auger electron spectra. Increasing the hydrogen ion beam energy causes the sp2 percentage to increase whereas on the contrary the Knoop hardness decreases. Moreover a rise in a-C:H hydrogen content causes an increase in the number of sp3 sites.
1. Introduction Hydrogenated amorphous carbon (a-C:H) is a material with an unusual combination of optical and mechanical properties. In fact, high hardness values and good optical properties (high transparency in the JR region) could be obtained. A good review on optical and mechanical properties and on useful deposition processes of a-C:H thin film can be found in ref. 1. In Robertson’s [2] model the a-C:H can be thought of as composed of a mixture of sp3 (tetrahedral configuration) and sp2 (two-dimensional graphite layered structure) sites. The relative concentrations of sp2 and sp3 are a key question in order to understand the bonding and the structure of the amorphous network. A quantitative analysis can be carried out observing the IR absorption spectra of a-C:H films and measuring the area of the absorption band corresponding to vibrational frequencies active in the IR [3, 4]. The hydrogen content seems to play a crucial role in the optical electrical and mechanical properties [5] even if the role of the hydrogen is not completely clear [6]. Jn this paper, a-C:H thin films produced by the dual-ion-beam-sputtering technique have been studied. An evaluation of the sp2 percentage has been 0257-8972/91/$3.50
(~Elsevier Sequoia/Printed in The Netherlands
288 TABLE 1 Typical deposition parameters required to obtain hydrogenated amorphous carbon films Eb (eV) (mA) (standard cm3 min’/~Hz (standard cm3 min1) ~back (mbar) ‘3work (mbar) T~~b ‘b 4~Ar
300—800 47 (2/4.5) —(3/20) 3 x iO~ 4 x i0~ Room temperature
performed by the X-ray induced Auger electron spectroscopy (XAES) and has been related to the hydrogen content and to the energy with which the hydrogen ions impinge on the growing film. Knoop hardness measurements show that harder samples have a lower concentration of sp2 sites.
2. Experimental details a-C:H samples have been produced by a dual-ion-beam-sputtering system composed of two ion sources (Kaufman type manufactured by Ion Tech) with a beam size of 2.5 cm just outside the grid aperture. The first argon ion beam impinges on the pure pyrolitic graphite target and the second composed of a mixture of argon and hydrogen ions hits directly on the growing film. The gas flows have been accurately controlled with an MKS mass flow controller. Typical deposition parameters are reported in Table 1. An accurate lay-out of the system and a detailed description of the technique can be found in ref. 7. The XAES spectra have been obtained with a Vacuum Generators spectrometer model ESCALAB MK II. The samples were irradiated with an Al Ket source (hv = 1486.6 eV) in the analysis vacuum chamber at a pressure value of 1O~mbar. In order to obtain a derivative spectra, a 23-point Savitzkey—Golay convolution array was used. A second-order polynomial was employed in this analysis. A complete description of the XAES measurements of a-C:H thin films has been reported in ref. 8. The hydrogen contents of the samples were determined by elastic recoil detection analysis performed with a 2.2 MeV Van de Graaff accelerator at the Laboratori Nazionali di Legnaro. The Knoop hardness values were evaluated by measuring the indentation size obtained with a rhombic-based pyramid diamond. The apparatus used for ultramicrohardness measurement was a Micro Duromat 4000 (manufactured by Reichert). The load cell is mounted on an optical microscope. All the samples were deposited on polished silicon with a typical thickness of 2000 A; as a consequence the hardness cannot be measured directly because the indentation depth is more than the film thickness and the dimension of the impression is influenced either by the hardness of the a-C:H film or by the
289
250
280 270 280 290 Kinetic Energy (eV)
300
Fig. 1. XAES derivative spectrum of an a-C:H film. The parameter D has been used to estimate 2 percentage of the analysed sample. sp
hardness of the substrate. Subsequently, in order to model the behaviour of sample composites by a film and a substrate, a simple geometrical model was used [9].
3. Results and discussion An example of the XAES derivative spectrum of an a-C:H thin film is given in Fig. 1, in which the distance D between the maximum of the positive-going excursion and the minimum of the negative-going excursion is indicated. In such a spectrum the maximum (at 253 eV) remains fixed for all analysed samples whereas the minimum can shift towards a higher energy. This shift could be associated with the presence of two peaks located at about 275 and 280 eV. As such peaks are absent in the XAES derivative spectrum of diamond, they can be assigned to a transition involving pit electrons with energies less than 5 eV typical of sp2 coordination [8, 10]. Table 2 gives the sp2 percentages and D values of a-C:H samples deposited with different values of the hydrogen ion beam energy. The sp2 percentages were estimated from a linear interpolation of D between the diamond (14.2 eV; 100% sp3) and the graphite values (22.5 eV; 100% sp2). Table 2 shows how the hydrogen energy increases as the sp2 percentage TABLE 2 sp2 percentage and D values (see Fig. 1) of hydrogenated amorphous carbon samples deposited at different hydrogen ion beam energies Eb (eV)
D (eV)
sp2 (%)
300 500 800 Gun off
18.0 190 19.8 20.4
48 59 68 75
290
2200
£
N
E E
2100
a
‘~2000
1900 (5
a
c 1800 1700
a 1600
•
•
400
I
I
600
800
Energy (cv)
Fig. 2. Behaviour of the Knoop hardness of a-C:H samples deposited at different hydrogen ion beam energies. 3500 3000
-
~
\
~2500~\
\
~ 2000
-
\~ -
1500
N.
Fiooo
-
5000 I
0
5
10 15 20 25 Indentation Size (micron)
30
Fig. 3. Geometrical model of hardness behaviour of sample composed of an a-C:H film and a substrate: curve Hc, measured composite hardness; curve Hs, substrate hardness; curve Hf, film hardness obtained from the difference between the measured hardness values of the composite sample and the bare substrate.
increases. The hydrogen content is about the same for the samples reported in this table (CH = 50 at.%) unless the sample was deposited with the second gun turned off (CH =4 at.%). The hydrogen ion beam energy is a parameter which has an important influence on the hardness of the a-C:H thin film produced by the dual-ionbeam-sputtering technique. Figure 2 shows the behaviour of Knoop hardness of samples deposited at different hydrogen energies. Typical hardness curves of the specimen composed of an a-C:H film and substrate can be seen in Fig. 3. Curve Hf represents the hardness of the film deduced from the hardness difference between the measured values of the composite sample and the bare substrate.
291 TABLE 3 2 percentage, D values and hydrogen contents of samples deposited with different flow ratios sp of argon and hydrogen gases with which the second source has been fed ql~r(standardcm3 min1)/4H
3 min’) 2(standard cm
2/4.5 2/8.0 3/20
sp2
(%)
D (eV)
83 65 59
21.0 19.5 19.0
CH (at. %)
-
15 26 50
From a comparison of Table 2 and Fig. 2, it seems that the hydrogen ion beam energy influences both the microstructure (as the ratio of sp2 to sp3 sites) and the macroscopic properties (such as the hardness value). Table 3 shows the sp2 percentages, D values and hydrogen contents of samples grown at a fixed argon to hydrogen gas flow ratio at the second ion source. As the hydrogen ion beam energy value was the same for all samples (Eb = 500 eV), the data in Table 2 suggest that the hydrogen content as well as the hydrogen energy are two critical parameters for control of the sp2 to sp3 ratio.
References 1 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Jensen (eds.) Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, p. 110. 2 J. Robertson, Adv. Phys., 35 (1986) 315. 3 B. D. Dishler, A. Bubenzer and P. Koidi, Solid State Commun., 48 (1983) 105. 4 B. Dishler, P. Koidl and P. Oelhafen (eds.), Proc. Symp. on Amorphous Hydrogenated Carbon Films, Vol. XVII, Les Editions de Physique, Strasbourg, 1987, p. 189. 5 H. Tsai and D. B. Bogy, J. Vac. Sci. Technol. A, 5 (1987) 327. 6 J. Robertson and E. P. O’Reilly, in P. Koidl and P. Oelhafen (eds.), Proc. Symp. on Amorphous Hydrogenated Carbon Films, Vol. XVII, Les Editions de Physique, Strasbourg, 1987, p. 251. 7 S. Scaglione and G. Emiliani, J. Vac. Sci. Technol. A, 3 (1985) 2702. 8 J. C. Lascovich, R. Giorgi and S. Scaglione, Appl. Surf. Sci., 47(1991) 17. 9 S. Scaglione and G. Emiliani, J. Vac. Sci. Technol. A, 7 (1989) 2303. 10 Y. Mizokawa, T. Miyasato, S. Nakamura, K. M. Geib and C. W. Wilmsen, Surf. Sci., 182(1987) 431.