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Structure and properties of carbon films prepared by pulsed vacuum arc deposition
292
Surface and Coatings Technology, 47 (1991) 292—298
Structure and properties of carbon films prepared by pulsed vacuum arc deposition E. I. Tochitsky, A. V. Stanishevskii, I. A. Kapustin, V. V. Akulich and 0. V. Selifanov “Plasmoteg” Engineering Centre, B.S.S.R. Academy of Sciences, 22 Logoiski Traki, Minsk-90 (U.S.S.R.)
Abstract Diamond-like carbon films prepared by the pulsed vacuum arc discharge method are characterized by high growth rates and large coating areas. The films obtained demonstrate good adhesion to different substrate materials, continuity for small thicknesses, high microhardness and electric resistivity as well as low impurity content. Analysis of the films’ structure and phase composition has shown that, depending on the deposition conditions, the structure in the short-range order varies considerably and corresponds to that of diamond, carbyne and graphite crystals. It is found that the non-homogeneity of the plasmoid density and composition results in structural non-uniformities in the film microstructure. Electrical, mechanical and optical properties of the films correlate with the short-range structural order. It is shown that the proposed method of film condensation allows effective control of the film properties and structure as well as good reproducibility of the required properties, which is particularly useful in industrial applications.
1.
Introduction
Among numerous methods for the formation of carbon films with amorphous and partially crystalline structures in vacuum [1], the method of plasma condensation from a pulsed arc discharge of the erosion type is relatively novel and unstudied although its main principles have been known for more than two decades. Simple realization of the method, the possibility of depositing coatings on substrates of large size and intricate shape, easy control of the process and its stability in time are all features that make this method highly promising for industrial applications. As regards the physical principles underlying the method, they are very similar to those of a more common technique which employs a stationary arc discharge [2]. However, the former differs by the pulsed nature of plasma creation, a particle flow density two or three orders of magnitude higher, and a wider range of average particle energy variation [3]. In addition, in the pulsed method the plasmoids have a more complex structure, which markedly influences the process of carbon film condensation. Elsevier Sequoia/Printed in The Netherlands
293
mjese lap acceec~a pQwe~Qnd ~,~ces
cAan~sv
peasnia flak’
suñs~.pa~es
vactium pump Fig. 1. Schematic representation of the experimental apparatus for preparing carbon films by pulsed vacuum arc deposition.
2. Sample preparation In this paper we present generalized results of experiments on the condensation of carbon films onto various substrates such as metals, semiconductors, glass and polymers. Figure 1 gives an illustration ofthe experimental set-up. In the course of investigations we varied the pulsed discharge power (5—150 J) and the duration (50—2500 its) and repetition rate (0.1— 50 Hz) of the pulses as well as the position of the substrate inside the chamber with respect to the plasma accelerator. A high purity graphite was used as the starting material. During film deposition the pressure in the chamber was maintained at 1O~—i0~Pa and the substrate temperature was lower than 373 K. Continuous solid layers were obtained with good adhesion to substrate, their thickness ranging from several monolayers to 10 ~m. The layer properties and chemical and phase compositions were studied by optical and electron spectroscopy, electron microscopy and other techniques.
3. Results It is found that the total amount of impurities is less than 1 at.% for all samples. Most of the impurity atoms, mainly oxygen, are contained in the surface layer up to 5 nm thick. Figures 2 and 3 give JR and X-ray photoelectron spectroscopy (XPS) data showing the impurity types and contents in the films. Furthermore, mass spectra of secondary ions point to the traces of potassium, sodium, calcium and nitrogen. Such small amounts of impurities must have significant effect on the film properties as compared with the influence exerted by structural features.
294
~
4
~
4
5
d
7
~
t,,rn’i
Fig. 2. JR spectrum of a diamond-like carbon film 1 ~im thick on a silicon substrate. Arrows indicate absorption of CH bonds. Fig. 3. XPS depth profiles of a diamond-like carbon film (15 nm) on a silicon substrate.
The obtained film structure was mainly disordered. This made it difficult to perform structural studies; therefore, several methods were employed to establish structural features in the short-range order, and the relative content and distribution of atoms with different types of chemical bonds in the film volume. 3.1. Electron microscopy studies Transmission electron microscopy (TEM) and electron diffractometry data indicate that the films represent a uniform condensate which, depending on the deposition conditions, contains 1% 15% of polycrystalline regions with structures typical of different carbon modifications, the size of separate crystallites being equal to 0.1—2 ~tm. These crystalline inclusions belong mainly to ci- and fl-carbyne and rarely to hexagonal graphite and lonsdaleite. Figure 4 is a microphotographic picture of a typical region of the film containing inclusions. The diffraction studies show that depending on the condensation conditions the structure in the short-range order of the disordered phase strongly changes. The electron diffraction patterns usually exhibit from two to six diffuse rings which may differ by their position, intensity and form. Analysis of the patterns point to an average size of the ordered regions of about 0.5—1.5 nm. When the patterns were taken in the microdiffraction regime, the rings broke down into separate spots. This may indicate that the prepared films are highly fine-grained polycrystals. Figure 5 presents the interference intensity functions obtained from the electron diffraction patterns of films with different structures. The intensity functions for diamond and graphite clusters are given in the figure for comparison. One may easily see a satisfactory coincidence of the basic features of the intensity functions for the films and modelled clusters. This points to the predominance of fragments with certain types of bonds in the films prepared. —
295
Fig. 4. (a) TEM image of crystalline inclusions inside the carbon film with TED patterns of (b) a.carbyne and (c) graphite. (Magnification: (a) 20 000 x).
1~ 10
30
50
70
90
S,rxn~
Fig. 5. Functions of electron scattering intensity for graphite cluster containing 20 atoms (curve a), graphite-like film obtained by pulsed deposition (curve b), lonedaleite cluster containing 32 atoms (curve c) and pulsed deposited diamond-like carbon film (curve d). A weak peak at S = 14 nm~ corresponds to d, 10 of a-carbyne.
3.2. Spectroscopic studies The use of XPS and Auger electron spectroscopy has enabled more extensive and refined information to be obtained about the film structure and chemical bonds. Figures 6 and 7 give the electron energy loss spectra and the density of states in the valence band of films a priori having different short-range order structures. The experimental spectra of diamond, graphite and carbyne taken under the same conditions as those of the films are also given. The position of the main excitation maximum of the ir + 0~plasmon on the loss spectra of the carbon films lies within the range 27—31 eV, which is indicative of a high atomic density of the prepared condensate. it is interesting to note that the films prepared under different conditions exhibit a fairly
296
/.(E)
II
~
S
,~..__
I
~
~
S 2i~j,i’
Si 2~~Si
ib
22
F,eV
I’.
\~_
~~\_
~
~O
Ji?
(‘\
Si
~b
ib Ji?
Fig. 6. Electron-energy loss spectra for carbon films and (a) diamond, (b) graphite and (c) carbyne (— — —). Fig. 7. XPS spectra of density of states in the valence band of carbon films (—---—) and (a) diamond, (b) graphite and (c) carbyne (— — —).
good agreement with the spectra of crystalline modifications of carbon having sp3, sp2 and sp types of bonds. The presence of a weak maximum in the vicinity of 5—10 eV due to excitation of the iv plasmon evidences the presence of a component different from diamond in all films. Comparison of the XPS spectra of the density of states in the valence band of the films and different carbon modifications (Fig. 7) reveals such a specific feature as the presence of maxima in the regions 15.5—17 eV, 18.5— 20 eV and especially 24—27 eV simultaneously. These maxima are caused by the presence of structural fragments with the sp type of bond typical of carbyne [4]. It seems that such fragments are contained in all samples studied. The same result has been obtained by mathematical processing of Auger electron spectra using the procedure proposed in ref. 5 (Fig. 8). This technique has also been used to find the relative content of different types of chemical bonds in the films. The results of the conducted analysis have allowed us to establish some dependences of the structural peculiarities of the films on the deposition conditions. For instance, Fig. 9 shows the average size of the ordered regions (microcrystallites) and the relative contents of sp3, sp2 and sp bonds as a function of the discharge power. Analogous dependences have been found as a function of pulse duration and repetition rate. It is found that as the latter parameters increase the common tendency is towards graphitization of the condensate, which is due to rising substrate temperature. 3.3 The film properties The properties of the films vary in a broad range because of strong differences in the film structure. The films exhibit microhardness up to 150 GPa, electrical resistivity up to 10 G( cm and specific mass up to 3 4 g cm3 The films are fairly transparent in the IR region of the spectrum and highly resistant to the effects of aggressive media. The temperature of the onset of the structural transformation is higher than 873 K. Figure 10 presents some properties of the prepared films as compared with those of other carbon materials. Non-shaded areas in the figure refer to the range of a a aa
297
5
tO 13 20 23
E,sV
ID 20 JO #U 30 50 ~
Fig. 8. Density of states in the valence band of the carbon film calculated from the experimental Auger spectrum. 3, sp2 and sp bonds and the sizes of ordered regions as a function of Fig. 9. The number of sp pulsed discharge power.
liv,
R
un~~~Hll~ ~nHhR 06,,cm
abcae
ab
cde
abcde
Fig. 10. Hardness, electric resistivity and mass density of pulse deposited films (columns e) as compared with those of graphite (columns a), diamond (columns b), carbyne (columns c) and hydrogenated a-C (columns d) films.
variation of the film properties. It should be noted that under certain conditions of condensation layers can be prepared with a high hardness but with an electrical resistivity of 0.1—0.01 Q cm. It is found that the film properties do not change after four years of shelf-life. The film properties correlate satisfactorily with the film structure. The properties are mainly affected by the number of sp2 bonds as well as by the size, form and distribution of atomic groups with this type of bond. Figure 11 gives the dependence of the electrical resistivity, transmission and density of the films on the number of atoms with the sp2 type of bond.
4. Conclusions The results of a comprehensive analysis have shown that pulsed condensation of the arc discharge plasma leads to formation of highly dense and fine-grained polycrystalline layers. One of the specific features of the carbon films obtained is the presence of carbon chains with sp2 bonds between atoms. By changing the deposition conditions, films with a predominance of sp3, sp2 or sp types of bonds can be prepared. However, so far we have not been able to obtain films having one specified type of bond and structure. Nevertheless, the film properties are highly reproducible and stable in time. This allows the proposed method to be recommended for industrial applications where different functional layers of carbon are needed.
298 R,
i; B, A
/1
tO
30
JO
~
2
Fig. 11. Electric resistivity, transmittance and specific mass (density) of carbon films vs. sp bond content.
References
Opt.
1 A. V. Balakov and E. A. Konshina, Mekh. Promsi., 9 (1982) 52. 2 V. E. Strelnitskii, V. G. Padalka and S. I. Vakula, Soy. Phys. Tech. Phys., 23 (1978) 1058 (in Russian). 3 B. A. Osadin, Soy. Phys. Tech. Phys., 35(1965) 1321 (in Russian). 4 V. V. Khvostov, M. B. Guseva, V. G. Babayev and 0. Yu. Rylova, Doki. Akad. Nauk S.S.S.R., 293(1987) 393. 5 V. V. Khvostov, V. G. Babayev and M. B. Guseva, Soy. Solid State Phys., 27 (1985) 887 (in Russian).
Surface and Coatings Technology, 47 (1991) 292—298
Structure and properties of carbon films prepared by pulsed vacuum arc deposition E. I. Tochitsky, A. V. Stanishevskii, I. A. Kapustin, V. V. Akulich and 0. V. Selifanov “Plasmoteg” Engineering Centre, B.S.S.R. Academy of Sciences, 22 Logoiski Traki, Minsk-90 (U.S.S.R.)
Abstract Diamond-like carbon films prepared by the pulsed vacuum arc discharge method are characterized by high growth rates and large coating areas. The films obtained demonstrate good adhesion to different substrate materials, continuity for small thicknesses, high microhardness and electric resistivity as well as low impurity content. Analysis of the films’ structure and phase composition has shown that, depending on the deposition conditions, the structure in the short-range order varies considerably and corresponds to that of diamond, carbyne and graphite crystals. It is found that the non-homogeneity of the plasmoid density and composition results in structural non-uniformities in the film microstructure. Electrical, mechanical and optical properties of the films correlate with the short-range structural order. It is shown that the proposed method of film condensation allows effective control of the film properties and structure as well as good reproducibility of the required properties, which is particularly useful in industrial applications.
1.
Introduction
Among numerous methods for the formation of carbon films with amorphous and partially crystalline structures in vacuum [1], the method of plasma condensation from a pulsed arc discharge of the erosion type is relatively novel and unstudied although its main principles have been known for more than two decades. Simple realization of the method, the possibility of depositing coatings on substrates of large size and intricate shape, easy control of the process and its stability in time are all features that make this method highly promising for industrial applications. As regards the physical principles underlying the method, they are very similar to those of a more common technique which employs a stationary arc discharge [2]. However, the former differs by the pulsed nature of plasma creation, a particle flow density two or three orders of magnitude higher, and a wider range of average particle energy variation [3]. In addition, in the pulsed method the plasmoids have a more complex structure, which markedly influences the process of carbon film condensation. Elsevier Sequoia/Printed in The Netherlands
293
mjese lap acceec~a pQwe~Qnd ~,~ces
cAan~sv
peasnia flak’
suñs~.pa~es
vactium pump Fig. 1. Schematic representation of the experimental apparatus for preparing carbon films by pulsed vacuum arc deposition.
2. Sample preparation In this paper we present generalized results of experiments on the condensation of carbon films onto various substrates such as metals, semiconductors, glass and polymers. Figure 1 gives an illustration ofthe experimental set-up. In the course of investigations we varied the pulsed discharge power (5—150 J) and the duration (50—2500 its) and repetition rate (0.1— 50 Hz) of the pulses as well as the position of the substrate inside the chamber with respect to the plasma accelerator. A high purity graphite was used as the starting material. During film deposition the pressure in the chamber was maintained at 1O~—i0~Pa and the substrate temperature was lower than 373 K. Continuous solid layers were obtained with good adhesion to substrate, their thickness ranging from several monolayers to 10 ~m. The layer properties and chemical and phase compositions were studied by optical and electron spectroscopy, electron microscopy and other techniques.
3. Results It is found that the total amount of impurities is less than 1 at.% for all samples. Most of the impurity atoms, mainly oxygen, are contained in the surface layer up to 5 nm thick. Figures 2 and 3 give JR and X-ray photoelectron spectroscopy (XPS) data showing the impurity types and contents in the films. Furthermore, mass spectra of secondary ions point to the traces of potassium, sodium, calcium and nitrogen. Such small amounts of impurities must have significant effect on the film properties as compared with the influence exerted by structural features.
294
~
4
~
4
5
d
7
~
t,,rn’i
Fig. 2. JR spectrum of a diamond-like carbon film 1 ~im thick on a silicon substrate. Arrows indicate absorption of CH bonds. Fig. 3. XPS depth profiles of a diamond-like carbon film (15 nm) on a silicon substrate.
The obtained film structure was mainly disordered. This made it difficult to perform structural studies; therefore, several methods were employed to establish structural features in the short-range order, and the relative content and distribution of atoms with different types of chemical bonds in the film volume. 3.1. Electron microscopy studies Transmission electron microscopy (TEM) and electron diffractometry data indicate that the films represent a uniform condensate which, depending on the deposition conditions, contains 1% 15% of polycrystalline regions with structures typical of different carbon modifications, the size of separate crystallites being equal to 0.1—2 ~tm. These crystalline inclusions belong mainly to ci- and fl-carbyne and rarely to hexagonal graphite and lonsdaleite. Figure 4 is a microphotographic picture of a typical region of the film containing inclusions. The diffraction studies show that depending on the condensation conditions the structure in the short-range order of the disordered phase strongly changes. The electron diffraction patterns usually exhibit from two to six diffuse rings which may differ by their position, intensity and form. Analysis of the patterns point to an average size of the ordered regions of about 0.5—1.5 nm. When the patterns were taken in the microdiffraction regime, the rings broke down into separate spots. This may indicate that the prepared films are highly fine-grained polycrystals. Figure 5 presents the interference intensity functions obtained from the electron diffraction patterns of films with different structures. The intensity functions for diamond and graphite clusters are given in the figure for comparison. One may easily see a satisfactory coincidence of the basic features of the intensity functions for the films and modelled clusters. This points to the predominance of fragments with certain types of bonds in the films prepared. —
295
Fig. 4. (a) TEM image of crystalline inclusions inside the carbon film with TED patterns of (b) a.carbyne and (c) graphite. (Magnification: (a) 20 000 x).
1~ 10
30
50
70
90
S,rxn~
Fig. 5. Functions of electron scattering intensity for graphite cluster containing 20 atoms (curve a), graphite-like film obtained by pulsed deposition (curve b), lonedaleite cluster containing 32 atoms (curve c) and pulsed deposited diamond-like carbon film (curve d). A weak peak at S = 14 nm~ corresponds to d, 10 of a-carbyne.
3.2. Spectroscopic studies The use of XPS and Auger electron spectroscopy has enabled more extensive and refined information to be obtained about the film structure and chemical bonds. Figures 6 and 7 give the electron energy loss spectra and the density of states in the valence band of films a priori having different short-range order structures. The experimental spectra of diamond, graphite and carbyne taken under the same conditions as those of the films are also given. The position of the main excitation maximum of the ir + 0~plasmon on the loss spectra of the carbon films lies within the range 27—31 eV, which is indicative of a high atomic density of the prepared condensate. it is interesting to note that the films prepared under different conditions exhibit a fairly
296
/.(E)
II
~
S
,~..__
I
~
~
S 2i~j,i’
Si 2~~Si
ib
22
F,eV
I’.
\~_
~~\_
~
~O
Ji?
(‘\
Si
~b
ib Ji?
Fig. 6. Electron-energy loss spectra for carbon films and (a) diamond, (b) graphite and (c) carbyne (— — —). Fig. 7. XPS spectra of density of states in the valence band of carbon films (—---—) and (a) diamond, (b) graphite and (c) carbyne (— — —).
good agreement with the spectra of crystalline modifications of carbon having sp3, sp2 and sp types of bonds. The presence of a weak maximum in the vicinity of 5—10 eV due to excitation of the iv plasmon evidences the presence of a component different from diamond in all films. Comparison of the XPS spectra of the density of states in the valence band of the films and different carbon modifications (Fig. 7) reveals such a specific feature as the presence of maxima in the regions 15.5—17 eV, 18.5— 20 eV and especially 24—27 eV simultaneously. These maxima are caused by the presence of structural fragments with the sp type of bond typical of carbyne [4]. It seems that such fragments are contained in all samples studied. The same result has been obtained by mathematical processing of Auger electron spectra using the procedure proposed in ref. 5 (Fig. 8). This technique has also been used to find the relative content of different types of chemical bonds in the films. The results of the conducted analysis have allowed us to establish some dependences of the structural peculiarities of the films on the deposition conditions. For instance, Fig. 9 shows the average size of the ordered regions (microcrystallites) and the relative contents of sp3, sp2 and sp bonds as a function of the discharge power. Analogous dependences have been found as a function of pulse duration and repetition rate. It is found that as the latter parameters increase the common tendency is towards graphitization of the condensate, which is due to rising substrate temperature. 3.3 The film properties The properties of the films vary in a broad range because of strong differences in the film structure. The films exhibit microhardness up to 150 GPa, electrical resistivity up to 10 G( cm and specific mass up to 3 4 g cm3 The films are fairly transparent in the IR region of the spectrum and highly resistant to the effects of aggressive media. The temperature of the onset of the structural transformation is higher than 873 K. Figure 10 presents some properties of the prepared films as compared with those of other carbon materials. Non-shaded areas in the figure refer to the range of a a aa
297
5
tO 13 20 23
E,sV
ID 20 JO #U 30 50 ~
Fig. 8. Density of states in the valence band of the carbon film calculated from the experimental Auger spectrum. 3, sp2 and sp bonds and the sizes of ordered regions as a function of Fig. 9. The number of sp pulsed discharge power.
liv,
R
un~~~Hll~ ~nHhR 06,,cm
abcae
ab
cde
abcde
Fig. 10. Hardness, electric resistivity and mass density of pulse deposited films (columns e) as compared with those of graphite (columns a), diamond (columns b), carbyne (columns c) and hydrogenated a-C (columns d) films.
variation of the film properties. It should be noted that under certain conditions of condensation layers can be prepared with a high hardness but with an electrical resistivity of 0.1—0.01 Q cm. It is found that the film properties do not change after four years of shelf-life. The film properties correlate satisfactorily with the film structure. The properties are mainly affected by the number of sp2 bonds as well as by the size, form and distribution of atomic groups with this type of bond. Figure 11 gives the dependence of the electrical resistivity, transmission and density of the films on the number of atoms with the sp2 type of bond.
4. Conclusions The results of a comprehensive analysis have shown that pulsed condensation of the arc discharge plasma leads to formation of highly dense and fine-grained polycrystalline layers. One of the specific features of the carbon films obtained is the presence of carbon chains with sp2 bonds between atoms. By changing the deposition conditions, films with a predominance of sp3, sp2 or sp types of bonds can be prepared. However, so far we have not been able to obtain films having one specified type of bond and structure. Nevertheless, the film properties are highly reproducible and stable in time. This allows the proposed method to be recommended for industrial applications where different functional layers of carbon are needed.
298 R,
i; B, A
/1
tO
30
JO
~
2
Fig. 11. Electric resistivity, transmittance and specific mass (density) of carbon films vs. sp bond content.
References
Opt.
1 A. V. Balakov and E. A. Konshina, Mekh. Promsi., 9 (1982) 52. 2 V. E. Strelnitskii, V. G. Padalka and S. I. Vakula, Soy. Phys. Tech. Phys., 23 (1978) 1058 (in Russian). 3 B. A. Osadin, Soy. Phys. Tech. Phys., 35(1965) 1321 (in Russian). 4 V. V. Khvostov, M. B. Guseva, V. G. Babayev and 0. Yu. Rylova, Doki. Akad. Nauk S.S.S.R., 293(1987) 393. 5 V. V. Khvostov, V. G. Babayev and M. B. Guseva, Soy. Solid State Phys., 27 (1985) 887 (in Russian).