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Enhancement of the mechanical properties of the carbon nitride thin films by doping
Carbon 42 (2004) 1107–1111 www.elsevier.com/locate/carbon
Enhancement of the mechanical properties of the carbon nitride thin films by doping D. Sarangi, R. Sanjines, A. Karimi
*
Institut de Physique de la Matiere Complexe (IPMC-FSB), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Available online 6 February 2004
Abstract Silicon doped carbon nitride (CNx ) thin films were synthesized by hot-filament plasma enhanced chemical vapor deposition (HFPECVD) technique. Slightly doped films showed enhanced mechanical behavior in terms of hardness with reference to undoped films as revealed by nanoindentation measurement. A 26 GPa hardness for the doped CNx was found without the film delamination from the substrate in comparison with 15 GPa for undoped CNx film. Substrate temperature influenced the mechanical properties of the deposited film. With the increase of substrate temperature from 50 to 650 °C, the hardness of the films increased about three times. The observed enhancement of the mechanical property was well supported by the optical measurement. With the increase of Si doping concentrations the film hardness decreases. The role of Si for the film hardening was discussed. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nitride; B. Doping; C. AFM, FTIR
1. Introduction Due to the failure of synthesizing predicted superhard crystalline carbon nitride (CNx ) thin films, lot of efforts have been generated towards amorphous or nanocrystalline CNx or related material. Apart from very good mechanical properties of the CNx films they also offer some major drawbacks like thermal instability, high friction coefficients in humid atmospheres, low amounts of nitrogen (N) incorporation (<40 at.%) etc. Silicon (Si) incorporation into the CNx matrix has been demonstrated to be effective in reducing friction coefficient of these coatings in a humid atmosphere, outstanding oxidation resistance at elevated temperature and many other improved properties suited for applications. Nevertheless, Si incorporation promotes the formation of crystallinity [1]. The present work has been focused on the effect of Si doping on the mechanical properties of the CNx films. First we have made a comparative study of the mechanical properties of Si doped and undoped CNx films. Recording better performance for doped films a * Corresponding author. Tel.: +41-21-693-3395; fax: +41-21-6934470. E-mail addresses: debajyoti.sarangi@epfl.ch (D. Sarangi), ayat. karimi@epfl.ch (A. Karimi).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2003.12.050
detailed investigation of the doped films were carried out involving nanoindentation, Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM). The bonding relationship with mechanical properties has been established and a phase diagram has been proposed based on these observations. 2. Experimental details CNx films doped with silicon were grown on Si (1 0 0) substrates by plasma enhanced hot-filament chemical vapor deposition (PE HF-CVD) technique. The cleaned Si samples were kept on the molybdenum (Mo) plate below the tungsten (W) filament. The distance between the substrates and the filament was adjusted and kept about 4 mm. Acetylene (C2 H2 ), ammonia (NH3 ) and hydrogen (H2 ) were feed into the chamber through the high precision mass flow controller (MFC) keeping the flow rate 10; 20 and 10 sccm, respectively, where as silane (SiH4 ) was used as dopant gas. The flow ratio of [SiH4 ]/ [C2 H2 ] was varied from 0.01 to 1. The substrate temperature was varied from 50 to 650 °C and the filament temperature was kept constant to 1800 °C. A positive dc bias of 50 V and a negative dc bias of 500 V were applied to the filament and to the substrate, respectively. The deposition pressure was kept fixed at 250 Pa.
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D. Sarangi et al. / Carbon 42 (2004) 1107–1111
The hardness and the elastic modulus of the films were measured by nanoindentation using continuous stiffness measurements (CSM) [2] by a nanoindenter XP system (XP-MTS) with a Berkovich style diamond tip. In all CSM depth-sensing tests, a total of nine indents were averaged to determine the mean hardness and elastic modulus values for statistical purposes, and the load (displacement) resolution was 50 mN. The surface morphology was determined by AFM in contact mode (Nanoscope Dimension 3000, Digital Instruments). The bonding characteristics of these films were investigated by FTIR spectroscopy using a Nicolet-Nexus spectrometer.
3. Results and discussion 3.1. Nanoindentation measurement It is difficult to achieve hard CNx films thick enough supported by the substrate due to its very high values of compressive stress. Films are peel off from the substrate once the thickness crosses the limit. In the present investigation we have found 15 GPa hardness for the stable CNx film. Incorporation of little Si into the matrix of CNx has been found to demonstrate enhanced mechanical performance. Nanoindentation has been used to show this behavior. Fig. 1 shows the typical load–displacement curve (for one indent only, others show similar behavior) for the Si doped and undoped CNx film. Both are prepared around 550 °C. The load–displacement curve contains lots of information about the material property, the deformation mechanism that the system is undergoing. Thus it can be regarded as a ‘‘fingerprint’’ for material
property identification. The hardness and the elastic recovery property of Si doped CNx films are better than CNx films as clearly visible in the load–displacement curve (Fig. 1). The characteristic ‘‘pop-ins’’ are visible in all loading curves. Similarly the characteristic ‘‘kickback’’ is more clearly visible in undoped CNx film. ‘‘Pop-ins’’ generally occurs at higher loads and it is due to plastic deformation or crack propagation in the substrate. Similarly ‘‘kick-backs’’ are associated with the formation of radial cracks or phase change in Si substrate. The hardness (H ) and elastic modulus (E) of the doped and undoped CNx films extracted from load– displacement curve are shown in Figs. 2 and 3 as a function of penetration depth. The hardness shows slightly decreasing tendency with the penetration depth. On the other hand, the elastic modulus shows almost constant behavior with the depth. It is clear from the Fig. 2 that there is about 40% enhancement of the hardness for the Si doped CNx films. But the enhancement of elastic modulus as shown in Fig. 3 is less than 10% i.e. a large H =E value. Therefore, incorporation of Si into the CNx matrix enhanced the mechanical performance maintaining elasticity. 3.2. Effect of growth temperature The Si incorporation is found to be beneficial for hardening the CNx matrix. To have better feeling we have grown some films with different elevated temperature (50–650 °C) keeping [SiH4 ]/[C2 H2 ] flow ratio fixed at 0.01. Fig. 4 shows the variation of the hardness and elastic modulus of these films with substrate temperature. Soft polymeric behavior are observed for the films
25 250
20
Doped Si doped
150
100 Undoped
Hardness (H, GPa)
Load On Sample (mN)
200
15
Undoped
10
5
50
0
0 0
200
400
600
800
1000
Displacement Into Surface (nm) Fig. 1. Typical load–displacement curve for the Si doped and undoped CNx film.
0
200
400
600
800
1000
Displacement Into Surface (nm) Fig. 2. Variation of the hardness (H) with the penetration depth for the doped and undoped CNx films.
D. Sarangi et al. / Carbon 42 (2004) 1107–1111
3.3. Effect of [SiH4 ]/[C2 H2 ] flow ratio
250
Doped
Elastic Modulus (E, GPa)
200
Undoped
150
100
50
0
0
200
400
600
800
1000
Displacement Into Surface(nm) Fig. 3. Variation of the elastic modulus (E) with the penetration depth for the doped and undoped CNx films.
350
30
300 250
Hardness
20
200 15 150 10
Elastic Modulus 5
100
Elastic Modulus (E, GPa)
Hardness (H, GPa)
25
50
0 0
1109
100
200
300
400
500
600
0 700
As discussed in the previous section, the doped films are harder at 650 °C. How the [SiH4 ]/[C2 H2 ] flow ratio influences the material mechanical properties is the matter of study for this section. Though the hardness has been regarded as a primary material property to define wear resistance, there is strong evidence to suggest that the elastic modulus can also have an important influence on wear behavior. The wear behavior is typically characterized by a long elastic strain to failure, which can be described in terms of the ratio between the hardness (H ) and the elastic modulus (E). The ratio H =E is now treated as one of the parameter in the so called ‘‘plasticity index’’ and is now widely used and quoted as a valuable measure in determining the limit of elastic behavior in a surface contact [4]. Therefore we present the following result with respect to H =E. Fig. 5 shows the variation of the H =E of doped CNx films with varying [SiH4 ]/[C2 H2 ] flow ratio. The values of H =E decreases with the increase of flow ratio and leading to saturation for higher flow ratio. The decrease of the H =E with the increase of flow ratio can be explained by the formation of Si–NH–Si cross links in the deposited materials and the saturation is due to huge influence of SiH4 resulting material matrix without carbon incorporation. This is due to the week bond strength of H2 Si– H (384 kJ mol 1 ), resulting easier dissociation of SiH4 . Therefore low amount of SiH4 flow is the best for enhanced tribological behavior for the films. 3.4. FTIR measurement A good correlation between bonding and mechanical properties has been established through FTIR
o
Substrate Temperature ( C)
0.13
Fig. 4. Variation of the hardness (H ) and the elastic modulus (E) of the Si doped CNx films with the substrate temperature. 0.12
0.11
H/E
grown at lower temperature (<150 °C). With the increase of substrate temperature the films hardness increases and reaches to 26 GPa at 650 °C. The elastic modulus of the corresponding film shows similar behavior and increase linearly from 55 GPa to about 200 GPa. This is the clear indication of the compaction of the films at elevated temperature. Similar hardening behavior with temperature were observed by Jiang et al. [3] for their C–N films deposited by pulsed high temperature plasma. The adatoms and the radicals, responsible for the formation of film, are more mobile at elevated temperature, which in turn make the film more compacts, void free and thus increase the hardness and elastic modulus.
0.1
0.09
0.08 0
0.2
0.4
0.6
0.8
1
1.2
[SiH4]/[C2H2] Flow Ratio Fig. 5. Variation of the H=E of Si doped CNx films with the varying [SiH4 ]/[C2 H2 ] flow ratio.
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3.5. Surface and phase diagram
30
Hardness (H, GPa)
25
20
15
10 2120
2110
2100
2090
2080
2070
2060
2050
-1
Wavenumber (cm ) Fig. 6. Variation of the hardness (H ) with the shift of Si–H peak position due to the change in [SiH4 ]/[C2 H2 ] flow ratio for Si doped CNx films.
measurement. In general the Si–Hn stretching band centered about 2000–2190 cm 1 . We have observed systematic change of the peak position well correlated with material hardness. Fig. 6 shows the variation of the hardness with the shift of Si–H peak position due to the change in [SiH4 ]/[C2 H2 ] flow ratio for Si doped CNx films deposited at 650 °C. The peak position of Si–H stretching band is highly sensitive with the SiH4 partial pressure, which in turn control the mechanical properties of the films. The shift of Si–H stretching band towards higher wave number for less [SiH4 ]/[C2 H2 ] flow ratio is attributed due to the electronegativity difference (Si––1.8, H––2.1, C––2.5 and N––2.5) between next nearest neighbor groups or atoms in the network structure, resulting substitution of Si by C or/and N in the nearest environment of the Si–H bond. In the region where SiH4 flow is very high, the large number of active Si species does not allow any kind of substitution. The Si–H stretching peak position towards higher wavenumber indicates the enhanced mechanical properties of the films.
At macroscopic scale, morphology of the Si doped CNx films consists mainly of granular structures as observed using AFM. The surface of the films shows the decrease in dimensions of the granular structure with the growth temperature. Again at particular growth temperature, SiH4 partial flow (Si doping percentage) has control over the structure. An increase in dimension of the granular structure has been observed with the increase of [SiH4 ]/[C2 H2 ] flow ratio as shown in Fig. 7. With the increase of Si flow gas phase reaction changes and the contribution of carbon radicals for the film formation decrease. Nevertheless, enhanced Si may also promote crystallinity. The Si doped CNx films grown in the present investigation are amorphous in nature. The amorphous nature is due to relatively low substrate temperature. The present setup does not permit to increase substrate temperature above 650 °C. As reported by other workers, minimum 800 °C is needed to get the crystalline structure. So we can draw roughly a phase diagram about the film property as shown in Fig. 8. The films are polymeric in nature up to 100 °C substrate temperature, above 100 °C and up to 700 °C films are amorphous in nature (confirm by transmission electron microscopy and X-ray diffraction) and hard. Si doping also enhanced the N incorporation into the film [5]. We may expect crystallinity above 800 °C, which need to be investigated. With higher Si flow SiN structure evolves. Several researchers have also investigated the enhancement of the hardness due to Si doping. Little amount of Si incorporation promotes the formation of sp3 bond leading to hardness enhancement, but increased of Si corporation favors more hydrogen resulting reduction of hardness [6]. In another approach sp2 –sp3 hybrid carbon surrounded by Si(C4 n Nn ) domains and bonded at interfaces with the Si centered tetrahedral plays an important role in the high hardness and stiffness [7]. Enhancement of the hardness can be achieved by arresting the dislocation movement. Superlattice thin films and nanocomposite films show enhanced hardness based on this phenomenon. In case of superlattice films [8], the hardness can be enhanced by
Fig. 7. Surface of the Si doped CNx films by AFM with different [SiH4 ]/[C2 H2 ] flow ratio (a) 0.01 (b) 0.05 and (c) 0.2.
D. Sarangi et al. / Carbon 42 (2004) 1107–1111
tribological applications as derived from the H =E analysis. For most durable coating it is necessary to generate large H =E value i.e. sufficiently high hardness but with low elastic modulus. High substrate temperature where nanocomposite behavior supposed to observe is favorable to achieve this value. We have demonstrated a good correlation between mechanical and optical properties of these films. Shifting of Si–H stretching band towards higher wave number is the clear indication of the enhancement of the material hardness. The phase diagram for the CNx films for different grown conditions has been proposed.
100
60
Polymeric Structure (soft)
20
N = 40 at %
Increased dimension of the granular structure
[SiH4]/‚ C2H2] Flow Ratio (%)
SiN structure: Amorphous to crystalline
80
40
1111
Crystalline Structure (expected)
Amorphous Structure (hard)
0 0
200
400
600
800
1000
Acknowledgements
o
Deposition temperature( C) Fig. 8. Proposed phase diagram of the Si doped CNx films.
making multilayer with two materials with minimum thickness that does not enable dislocation activity. In case of nanocomposite films [9], small crystalline grains are separated by amorphous phase preventing dislocation movement. Thus Si incorporation could lead to small crystalline SiN grains surrounded by a-C layer hence the enhancement of their mechanical property.
4. Conclusions The enhancement of the mechanical properties of the Si doped CNx films are observed in compared to undoped film. Systematic investigations for the mechanical and bonding properties of the doped films are carried out. Substrate temperature and SiH4 partial pressure could control the doped film property in an elegant fashion. Very small amount of SiH4 flow is beneficial for
The Swiss National Science Foundation (SNSF) is acknowledged for the financial support of the project. We are grateful to the Centre Interdepartemental de Microscopie Electronique (CIME-EPFL) for access to SEM and TEM facilities.
References [1] Chen LC, Chen CK, Weir SL, Hussar DM, Chen KH, Chen YF, et al. Appl Phys Lett 1998;72(19):2463–5. [2] Oliver WC, Pharr GM. J Mater Res 1992;7(6):1564–83. [3] Jiang YB, Zhang HX, Cheng DJ, Yang SZ. Thin Solid Films 2000;360(1–2):52–5. [4] Leyland A, Matthews A. Wear 2000;246(1–2):1–11. [5] Sarangi D, Sanjines R, Karimi A. Thin Solid Films 2004;447– 448:217–22. [6] Zhao JF, Lemoine P, Liu ZH, Quinn JP, Maguire P, McLaughlin JA. Diam Relat Mater 2001;10(3–7):1070–5. [7] Bendeddouche A, Berjoan R, B^eche E, Hiller R. Surf Coat Technol 1999;111(2–3):184–90. [8] Barnett SA, Madan A, Kim I, Martic K. MRS Bull 2003;28(3):169– 72. [9] Patscheider J. MRS Bull 2003;28(3):180–3.
Enhancement of the mechanical properties of the carbon nitride thin films by doping D. Sarangi, R. Sanjines, A. Karimi
*
Institut de Physique de la Matiere Complexe (IPMC-FSB), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Available online 6 February 2004
Abstract Silicon doped carbon nitride (CNx ) thin films were synthesized by hot-filament plasma enhanced chemical vapor deposition (HFPECVD) technique. Slightly doped films showed enhanced mechanical behavior in terms of hardness with reference to undoped films as revealed by nanoindentation measurement. A 26 GPa hardness for the doped CNx was found without the film delamination from the substrate in comparison with 15 GPa for undoped CNx film. Substrate temperature influenced the mechanical properties of the deposited film. With the increase of substrate temperature from 50 to 650 °C, the hardness of the films increased about three times. The observed enhancement of the mechanical property was well supported by the optical measurement. With the increase of Si doping concentrations the film hardness decreases. The role of Si for the film hardening was discussed. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nitride; B. Doping; C. AFM, FTIR
1. Introduction Due to the failure of synthesizing predicted superhard crystalline carbon nitride (CNx ) thin films, lot of efforts have been generated towards amorphous or nanocrystalline CNx or related material. Apart from very good mechanical properties of the CNx films they also offer some major drawbacks like thermal instability, high friction coefficients in humid atmospheres, low amounts of nitrogen (N) incorporation (<40 at.%) etc. Silicon (Si) incorporation into the CNx matrix has been demonstrated to be effective in reducing friction coefficient of these coatings in a humid atmosphere, outstanding oxidation resistance at elevated temperature and many other improved properties suited for applications. Nevertheless, Si incorporation promotes the formation of crystallinity [1]. The present work has been focused on the effect of Si doping on the mechanical properties of the CNx films. First we have made a comparative study of the mechanical properties of Si doped and undoped CNx films. Recording better performance for doped films a * Corresponding author. Tel.: +41-21-693-3395; fax: +41-21-6934470. E-mail addresses: debajyoti.sarangi@epfl.ch (D. Sarangi), ayat. karimi@epfl.ch (A. Karimi).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2003.12.050
detailed investigation of the doped films were carried out involving nanoindentation, Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM). The bonding relationship with mechanical properties has been established and a phase diagram has been proposed based on these observations. 2. Experimental details CNx films doped with silicon were grown on Si (1 0 0) substrates by plasma enhanced hot-filament chemical vapor deposition (PE HF-CVD) technique. The cleaned Si samples were kept on the molybdenum (Mo) plate below the tungsten (W) filament. The distance between the substrates and the filament was adjusted and kept about 4 mm. Acetylene (C2 H2 ), ammonia (NH3 ) and hydrogen (H2 ) were feed into the chamber through the high precision mass flow controller (MFC) keeping the flow rate 10; 20 and 10 sccm, respectively, where as silane (SiH4 ) was used as dopant gas. The flow ratio of [SiH4 ]/ [C2 H2 ] was varied from 0.01 to 1. The substrate temperature was varied from 50 to 650 °C and the filament temperature was kept constant to 1800 °C. A positive dc bias of 50 V and a negative dc bias of 500 V were applied to the filament and to the substrate, respectively. The deposition pressure was kept fixed at 250 Pa.
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D. Sarangi et al. / Carbon 42 (2004) 1107–1111
The hardness and the elastic modulus of the films were measured by nanoindentation using continuous stiffness measurements (CSM) [2] by a nanoindenter XP system (XP-MTS) with a Berkovich style diamond tip. In all CSM depth-sensing tests, a total of nine indents were averaged to determine the mean hardness and elastic modulus values for statistical purposes, and the load (displacement) resolution was 50 mN. The surface morphology was determined by AFM in contact mode (Nanoscope Dimension 3000, Digital Instruments). The bonding characteristics of these films were investigated by FTIR spectroscopy using a Nicolet-Nexus spectrometer.
3. Results and discussion 3.1. Nanoindentation measurement It is difficult to achieve hard CNx films thick enough supported by the substrate due to its very high values of compressive stress. Films are peel off from the substrate once the thickness crosses the limit. In the present investigation we have found 15 GPa hardness for the stable CNx film. Incorporation of little Si into the matrix of CNx has been found to demonstrate enhanced mechanical performance. Nanoindentation has been used to show this behavior. Fig. 1 shows the typical load–displacement curve (for one indent only, others show similar behavior) for the Si doped and undoped CNx film. Both are prepared around 550 °C. The load–displacement curve contains lots of information about the material property, the deformation mechanism that the system is undergoing. Thus it can be regarded as a ‘‘fingerprint’’ for material
property identification. The hardness and the elastic recovery property of Si doped CNx films are better than CNx films as clearly visible in the load–displacement curve (Fig. 1). The characteristic ‘‘pop-ins’’ are visible in all loading curves. Similarly the characteristic ‘‘kickback’’ is more clearly visible in undoped CNx film. ‘‘Pop-ins’’ generally occurs at higher loads and it is due to plastic deformation or crack propagation in the substrate. Similarly ‘‘kick-backs’’ are associated with the formation of radial cracks or phase change in Si substrate. The hardness (H ) and elastic modulus (E) of the doped and undoped CNx films extracted from load– displacement curve are shown in Figs. 2 and 3 as a function of penetration depth. The hardness shows slightly decreasing tendency with the penetration depth. On the other hand, the elastic modulus shows almost constant behavior with the depth. It is clear from the Fig. 2 that there is about 40% enhancement of the hardness for the Si doped CNx films. But the enhancement of elastic modulus as shown in Fig. 3 is less than 10% i.e. a large H =E value. Therefore, incorporation of Si into the CNx matrix enhanced the mechanical performance maintaining elasticity. 3.2. Effect of growth temperature The Si incorporation is found to be beneficial for hardening the CNx matrix. To have better feeling we have grown some films with different elevated temperature (50–650 °C) keeping [SiH4 ]/[C2 H2 ] flow ratio fixed at 0.01. Fig. 4 shows the variation of the hardness and elastic modulus of these films with substrate temperature. Soft polymeric behavior are observed for the films
25 250
20
Doped Si doped
150
100 Undoped
Hardness (H, GPa)
Load On Sample (mN)
200
15
Undoped
10
5
50
0
0 0
200
400
600
800
1000
Displacement Into Surface (nm) Fig. 1. Typical load–displacement curve for the Si doped and undoped CNx film.
0
200
400
600
800
1000
Displacement Into Surface (nm) Fig. 2. Variation of the hardness (H) with the penetration depth for the doped and undoped CNx films.
D. Sarangi et al. / Carbon 42 (2004) 1107–1111
3.3. Effect of [SiH4 ]/[C2 H2 ] flow ratio
250
Doped
Elastic Modulus (E, GPa)
200
Undoped
150
100
50
0
0
200
400
600
800
1000
Displacement Into Surface(nm) Fig. 3. Variation of the elastic modulus (E) with the penetration depth for the doped and undoped CNx films.
350
30
300 250
Hardness
20
200 15 150 10
Elastic Modulus 5
100
Elastic Modulus (E, GPa)
Hardness (H, GPa)
25
50
0 0
1109
100
200
300
400
500
600
0 700
As discussed in the previous section, the doped films are harder at 650 °C. How the [SiH4 ]/[C2 H2 ] flow ratio influences the material mechanical properties is the matter of study for this section. Though the hardness has been regarded as a primary material property to define wear resistance, there is strong evidence to suggest that the elastic modulus can also have an important influence on wear behavior. The wear behavior is typically characterized by a long elastic strain to failure, which can be described in terms of the ratio between the hardness (H ) and the elastic modulus (E). The ratio H =E is now treated as one of the parameter in the so called ‘‘plasticity index’’ and is now widely used and quoted as a valuable measure in determining the limit of elastic behavior in a surface contact [4]. Therefore we present the following result with respect to H =E. Fig. 5 shows the variation of the H =E of doped CNx films with varying [SiH4 ]/[C2 H2 ] flow ratio. The values of H =E decreases with the increase of flow ratio and leading to saturation for higher flow ratio. The decrease of the H =E with the increase of flow ratio can be explained by the formation of Si–NH–Si cross links in the deposited materials and the saturation is due to huge influence of SiH4 resulting material matrix without carbon incorporation. This is due to the week bond strength of H2 Si– H (384 kJ mol 1 ), resulting easier dissociation of SiH4 . Therefore low amount of SiH4 flow is the best for enhanced tribological behavior for the films. 3.4. FTIR measurement A good correlation between bonding and mechanical properties has been established through FTIR
o
Substrate Temperature ( C)
0.13
Fig. 4. Variation of the hardness (H ) and the elastic modulus (E) of the Si doped CNx films with the substrate temperature. 0.12
0.11
H/E
grown at lower temperature (<150 °C). With the increase of substrate temperature the films hardness increases and reaches to 26 GPa at 650 °C. The elastic modulus of the corresponding film shows similar behavior and increase linearly from 55 GPa to about 200 GPa. This is the clear indication of the compaction of the films at elevated temperature. Similar hardening behavior with temperature were observed by Jiang et al. [3] for their C–N films deposited by pulsed high temperature plasma. The adatoms and the radicals, responsible for the formation of film, are more mobile at elevated temperature, which in turn make the film more compacts, void free and thus increase the hardness and elastic modulus.
0.1
0.09
0.08 0
0.2
0.4
0.6
0.8
1
1.2
[SiH4]/[C2H2] Flow Ratio Fig. 5. Variation of the H=E of Si doped CNx films with the varying [SiH4 ]/[C2 H2 ] flow ratio.
1110
D. Sarangi et al. / Carbon 42 (2004) 1107–1111
3.5. Surface and phase diagram
30
Hardness (H, GPa)
25
20
15
10 2120
2110
2100
2090
2080
2070
2060
2050
-1
Wavenumber (cm ) Fig. 6. Variation of the hardness (H ) with the shift of Si–H peak position due to the change in [SiH4 ]/[C2 H2 ] flow ratio for Si doped CNx films.
measurement. In general the Si–Hn stretching band centered about 2000–2190 cm 1 . We have observed systematic change of the peak position well correlated with material hardness. Fig. 6 shows the variation of the hardness with the shift of Si–H peak position due to the change in [SiH4 ]/[C2 H2 ] flow ratio for Si doped CNx films deposited at 650 °C. The peak position of Si–H stretching band is highly sensitive with the SiH4 partial pressure, which in turn control the mechanical properties of the films. The shift of Si–H stretching band towards higher wave number for less [SiH4 ]/[C2 H2 ] flow ratio is attributed due to the electronegativity difference (Si––1.8, H––2.1, C––2.5 and N––2.5) between next nearest neighbor groups or atoms in the network structure, resulting substitution of Si by C or/and N in the nearest environment of the Si–H bond. In the region where SiH4 flow is very high, the large number of active Si species does not allow any kind of substitution. The Si–H stretching peak position towards higher wavenumber indicates the enhanced mechanical properties of the films.
At macroscopic scale, morphology of the Si doped CNx films consists mainly of granular structures as observed using AFM. The surface of the films shows the decrease in dimensions of the granular structure with the growth temperature. Again at particular growth temperature, SiH4 partial flow (Si doping percentage) has control over the structure. An increase in dimension of the granular structure has been observed with the increase of [SiH4 ]/[C2 H2 ] flow ratio as shown in Fig. 7. With the increase of Si flow gas phase reaction changes and the contribution of carbon radicals for the film formation decrease. Nevertheless, enhanced Si may also promote crystallinity. The Si doped CNx films grown in the present investigation are amorphous in nature. The amorphous nature is due to relatively low substrate temperature. The present setup does not permit to increase substrate temperature above 650 °C. As reported by other workers, minimum 800 °C is needed to get the crystalline structure. So we can draw roughly a phase diagram about the film property as shown in Fig. 8. The films are polymeric in nature up to 100 °C substrate temperature, above 100 °C and up to 700 °C films are amorphous in nature (confirm by transmission electron microscopy and X-ray diffraction) and hard. Si doping also enhanced the N incorporation into the film [5]. We may expect crystallinity above 800 °C, which need to be investigated. With higher Si flow SiN structure evolves. Several researchers have also investigated the enhancement of the hardness due to Si doping. Little amount of Si incorporation promotes the formation of sp3 bond leading to hardness enhancement, but increased of Si corporation favors more hydrogen resulting reduction of hardness [6]. In another approach sp2 –sp3 hybrid carbon surrounded by Si(C4 n Nn ) domains and bonded at interfaces with the Si centered tetrahedral plays an important role in the high hardness and stiffness [7]. Enhancement of the hardness can be achieved by arresting the dislocation movement. Superlattice thin films and nanocomposite films show enhanced hardness based on this phenomenon. In case of superlattice films [8], the hardness can be enhanced by
Fig. 7. Surface of the Si doped CNx films by AFM with different [SiH4 ]/[C2 H2 ] flow ratio (a) 0.01 (b) 0.05 and (c) 0.2.
D. Sarangi et al. / Carbon 42 (2004) 1107–1111
tribological applications as derived from the H =E analysis. For most durable coating it is necessary to generate large H =E value i.e. sufficiently high hardness but with low elastic modulus. High substrate temperature where nanocomposite behavior supposed to observe is favorable to achieve this value. We have demonstrated a good correlation between mechanical and optical properties of these films. Shifting of Si–H stretching band towards higher wave number is the clear indication of the enhancement of the material hardness. The phase diagram for the CNx films for different grown conditions has been proposed.
100
60
Polymeric Structure (soft)
20
N = 40 at %
Increased dimension of the granular structure
[SiH4]/‚ C2H2] Flow Ratio (%)
SiN structure: Amorphous to crystalline
80
40
1111
Crystalline Structure (expected)
Amorphous Structure (hard)
0 0
200
400
600
800
1000
Acknowledgements
o
Deposition temperature( C) Fig. 8. Proposed phase diagram of the Si doped CNx films.
making multilayer with two materials with minimum thickness that does not enable dislocation activity. In case of nanocomposite films [9], small crystalline grains are separated by amorphous phase preventing dislocation movement. Thus Si incorporation could lead to small crystalline SiN grains surrounded by a-C layer hence the enhancement of their mechanical property.
4. Conclusions The enhancement of the mechanical properties of the Si doped CNx films are observed in compared to undoped film. Systematic investigations for the mechanical and bonding properties of the doped films are carried out. Substrate temperature and SiH4 partial pressure could control the doped film property in an elegant fashion. Very small amount of SiH4 flow is beneficial for
The Swiss National Science Foundation (SNSF) is acknowledged for the financial support of the project. We are grateful to the Centre Interdepartemental de Microscopie Electronique (CIME-EPFL) for access to SEM and TEM facilities.
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