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Formation and characterization of chromium carbide films deposited using a 90° bend magnetic filtered cathodic vacuum arc system
Surface & Coatings Technology 200 (2006) 5052 – 5057 www.elsevier.com/locate/surfcoat
Formation and characterization of chromium carbide films deposited using a 90- bend magnetic filtered cathodic vacuum arc system Chun-Chun Lin, Wei-Jen Hsieh, Jain-Hong Lin, Uei-Shin Chen, Xing-Jian Guo, Han C. Shih * Department of Material Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC Received 1 March 2005; accepted in revised form 18 May 2005 Available online 26 August 2005
Abstract Deposition of chromium carbides films using a 90- bend filtered cathodic vacuum arc (FCVA) system is highly promising for industrial applications, because chromium carbide films uncontaminated by macroparticles exhibit excellent characteristics. Chromium carbide films were synthesized from a Cr target (99.95%) and C2H2/Ar gases, beginning with deposition parameters of the deposition temperature of ambient temperature, 300 and 500 -C and negative bias voltages ranging from 50 to 550 V. The microstructure of crystalline chromium carbide was investigated using glancing incident X-ray diffraction and cross-sectional transmission electron microscopy. The atomic concentrations of C and Cr in chromium carbide thin films deposited at various temperatures were measured by AES and the nature of the chemical bonding of the elements was elucidated by XPS. Our findings revealed that the total C – Cr bond contents depended on the deposition temperature. As the substrate bias voltage was increased from 50 to 550 V, the phase was transformed from amorphous to crystalline Cr3C2; a crystalline Cr23C6 phase also appeared at higher negative substrate bias voltages (< 250 V). The effects of the deposition parameters on the formation of chromium carbides were investigated in this study. D 2005 Elsevier B.V. All rights reserved. Keywords: Cr3C2; Cr23C6; FCVA; AES; XPS; XRD; TEM
1. Introduction Chromium-based coatings are promising for tribological use as an alternative to TiN [1,2]. The deposition of the chromium nitride coatings is well understood [3 –7], and few reports on the deposition of chromium carbide coatings, especially those prepared by filtered cathodic vacuum arc (FCVA) deposition, have been published [8]. FCVA technology is an efficient physical vapor deposition technique for industrial production since it provides a fully ionized and dissociated flow of the target metal vapor with highly concentrated activated reactive species [9]. The extent of metal ionization in this technique is generally ¨ 100% [10]. High-quality films uncontaminated by macro-
* Corresponding author. Tel.: +886 3 5715131 3845; fax: +886 3 5710290. E-mail address: [email protected] (H.C. Shih). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.027
particles have been developed using a magnetic filter to guide the plasma efficiently to the substrate, producing a coating that is free of macroparticles [11 – 13]. This investigation was to compare various conditions for depositing chromium carbide coatings using the FCVA system. Two deposition parameters (deposition temperature and substrate bias voltage) were varied during chromium carbide films. The nanostructure of the chromium carbide film was characterized to determine the optimum values of the deposition parameters for industrial applications.
2. Experimental The base pressure of the chamber was 6.6 10 3 Pa, using a diffusion pump backed with a mechanical pump in the FCVA system. The setup of the FCVA system with dual filtered source is shown in Fig. 1. In this study, one filtered source was used to deposit chromium carbide. A 99.95 wt.%
C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
Vacuum chamber Base pressure of 5 E-5 torr
5053
D.C. cathode arc source
Cathode
Trigger
Thermocouple
Substrate
Hot plate 50°C - 750°C 90° -bend magnetic filter Pulsed-dc bias power supply Fig. 1. Setup of the 90- bend magnetic FCVA system.
pure Cr target with a diameter of 4 in. was employed. The metal vapor from the target passed through a 90- bend magnetic filter (120 Gauss) in order to filter the macroparticles. The distance between the exit of the filtered source and the substrate holder was 15cm. The p-type (100) silicon substrates with dimensions of 20 mm 20 mm were ultrasonically cleaned in acetone, then in alcohol for 15 min, and finally loaded into the substrate holder. Prior to coating, substrate surfaces were cleaned by bombardment with argon for 5 min at a pressure of 1.3 Pa and a substrate bias of 950 V to remove any native oxides. A pure Cr film (5 nm) was firstly deposited onto the substrate at 150 V for 2 min. Subsequently, the substrate holder was heated by a hot plate with a thermocouple nearby to monitor the substrate temperature. Table 1 summarizes the deposition parameters. The structures of amorphous and crystalline chromium carbide films were investigated using glancing incident Xray diffraction and cross-sectional transmission electron microscopy (TEM). A Mac Science MXP18 diffractometer was used with Cu Ka radiation (k = 0.154 nm) generated at 40 kV and 150 mA at a grazing incidence of 0.5-. A Philips Table 1 Deposition parameters of chromium carbide films using 90- bend magnetic filtered cathodic vacuum arc Working pressure (Pa) 53310
1
Deposition temperature (-C)
Target current (A)
Reactive gas (sccm)
Substrate bias (V)
Ambient temperature 300 500
60
C2H2 /Ar (60:10)
150
50V to 550
Time (min) 30
CM2 TEM at 200 kV was employed to examine the nanostructure. The surface morphology as well as the thickness was measured using FE-SEM (JEOL JSM6500F) at 15 kV. Auger electron spectroscopy (AES) was performed using a Perkin Elmer (670 PHI-Xi) analyzer with a primary electron beam of 3 KeV and a beam current of 25 mA. The spectra were obtained at a resolution of 0.1 eV. The samples were sputter-etched using an ion gun with 3 keVAr+ to obtain the depth profile. These depth profiles reveal a mixture of Cr, C and Si and the concentration of each element of the chromium carbide thin films determined from the AES spectra according to the published relative sensitivity factors [14]. X-ray photoelectron spectrum (XPS) was employed to determine the chemical state of the films and was obtained using an ESCA PHI 1600 spectrometer with Mg Ka radiation (1253.6 eV). The data were taken at a constant analyzer pass energy of 117.4 eV in a vacuum system with a base pressure of 10 8 Pa. All the samples were argon-etched at 3 keV with a current equal to 1 `ıA, at a pressure of 10 6 Pa for 2 min. Discrete scans in the ranges 280– 290 eV for C and 570– 600 eV for Cr were recorded.
3. Results and discussion 3.1. Effect of deposition temperature Elevated temperatures are employed in the thin film deposition processes for three basic reasons: (1) to control the morphology of the growing film; (2) to improve film adhesion, and (3) to activate the reaction between the metal and the reaction gas in reactive deposition. Therefore,
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elevated substrate temperatures are normally viewed as an important parameter in the thin film process [15].
(a) 100
300oC -150V
3.1.1. SEM observation Fig. 2b and c present the cross-sectional morphologies of chromium carbide films deposited at 300 and 500 -C,
(a) Ambient temperature -150V
Atomic Concentration (%)
90 80 70
Cr C
60 50 40 30 20 10
Si
0 0
2
4
6
Chromium carbides
8
10
12
14
16
18
20
Sputter time (min)
(b) 100
500oC -150V
Substrate
100nm
(b) 300ºC -150V
Atomic Concentration (%)
90 80
Cr
70 C
60 50 40 30 20 10 Si
0 0
2
4
6
8
10
12
14
16
18
20
Sputter time (min)
Chromium carbides
Fig. 3. AES depth profile of chromium carbide film deposited at two different deposition temperature: (a) T = 300 -C, (b) T = 500 -C on the Si wafer. The thickness of the film is 500 nm.
Void
Substrate
100nm
(c) 500ºC -150V
Chromium carbides
Substrate
100nm
Fig. 2. Cross-section SEM images of the amorphous chromium carbide film deposited at the substrate bias of 150 V at (a) ambient temperature, (b) 300 -C, and (c) 500 -C.
respectively. The thickness of the chromium carbide film is independent of the deposition temperature; ¨ 500 nm in each case. As the deposition temperature increases from 300 to 500 -C, the morphology of the growing film becomes denser and the microstructure becomes more compact. The void-free and columnar structure is attributed to the activation of the reaction between metal and gas during the deposition process [15]. No macroparticle was observed, due to the 90- bend magnetic filter. However, the film cannot be easily deposited at ambient temperature (without heating) because the coating adhesion is weak and also the compressive stress will increase [15], as shown in Fig. 2a. Chromium carbides have three well known phases, i.e., Cr3C2 (rhombic), Cr7C3 (hexagonal) and Cr23C6 (cubic) which exhibit superior characteristics of good resistance to wear and corrosion, high hardness, a high melting point and oxidation-resistance at high temperature [8,16]. These carbides have great potential for replacing electroplated hard chrome as a premier coating for molding industry [17 – 19]. Glancing incident X-ray diffraction was utilized to identify the crystallographic
C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
(a)
orientation of chromium carbide films deposited at 300 and 500 -C, but no diffraction peaks were discernible and should therefore be consistent with the amorphous phase.
ambient temp. 282
284
286
288
290
Binding energy (eV)
(b) Cr2p
Cr 2p3/2 576.6 eV
500oC
Intensity (arb. unit)
Cr 2p1/2
300oC
ambient temp.
570
575
580
585
590
595
600
Binding energy (eV)
Substrate
Intensity (arb. units)
Cr3C2(011)
Fig. 4. XPS C 1S (a) and Cr 2p (b) spectra of the chromium carbide film deposited at ambient temperature, 300 -C and 500 -C.
Cr3C2(290)
300oC
-C-C285.2 eV
3.1.2. AES and XPS analyses AES was combined with ion sputtering to determine the concentration profiles of amorphous chromium carbide films. Fig. 3a and b show the resulting concentration depth profiles of chromium carbide films deposited at 300 and 500 -C. AES analysis indicates that the film deposited at 300 -C has a higher carbon content, in which the amorphous carbon film generated by C2H2 decomposition and the Cr-embedded amorphous carbon film exhibit very poor adhesion strength [8]. Whereas the deposition temperature increased to 500 -C, the predominant carbon deposition process evolved into a reactive PVD process, in which the Cr plasma reacted with carbon radicals to form the chromium carbide film [8]. The chemical bonding of the elements (C and Cr) of the chromium carbide film was subsequently determined by XPS. Fig. 4a and b display the XPS spectra associated with the C1s and the Cr2p signals at various deposition temperatures. The binding energies of the spectral components of the C1s bond shifted from 285.2 to 284.3 eV as the deposition temperature increases from ambient temperature to 500 -C; the first binding energy was close to 285.0 eV, which is attributed to C – C bonds. The negative shift (0.9 eV) is associated with the carbon – metal interaction, which is indicated by the appearance of a low energy line, as compared with the binding energy of 285.2 eV, in the spectrum [20]. The Cr2p3/2 binding energy is 576.6 eV, characterizing the formation of Cr2O3 [20, 21]. As the temperature increases to 500, the Cr2p signal becomes
Cr3C2(361)
500oC
Cr3C2(360)
Cr
Cr3C2(180)
-C-C-C-
Cr3C2(311)
Intensity (arb. unit)
C1s
280
5055
-550V -450V -350V -150V -50V 30
40
50
60
70
80
2θ Fig. 5. X-ray diffraction patterns of the Cr3C2 deposited at 500 -C with varying substrate bias voltages.
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C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
weak, as indicated in Fig. 3b. Evidently, raising the deposition temperature vaporizes the moisture in the chamber, thereby reducing the extent of formation of oxide layers on the top of the chromium carbide coatings. However, the XRD results did not reveal the Cr2O3 peak; the surface layer of Cr2O3, detected by XPS under such oxidation conditions, is rather thin. The deposition temperature increases to 500 -C to promote the synthesis of the crystallized chromium carbide film. Accordingly, deposition was performed at 500 -C by altering the bias voltage from 50 to 550 V.
(a)
3.2. Effect of substrate bias voltage 3.2.1. XRD results Fig. 5 presents the XRD patterns of the Cr3C2 deposited at 500 -C on the (100) silicon wafer at various substrate
(a) (b)
Chromium carbide film
Cr interlayer
Si substrate
1 nm
(b)
Fig. 7. Cross-section TEM micrograph and SAD pattern from an area that covers the top of the chromium carbide film deposited at (a) 250 V and (b) 550 V.
Cr (110)
Fig. 6. (a) Cross-section TEM micrograph and (b) SAD pattern of the crystallized chromium carbide/Cr deposited on the Si wafer.
biases from 50 to 550 V. The JCPDS database of the Cr3C2 phase (No.35-0804) in the e`/2e` mode was employed to establish the crystallinity of the film. At biases of 50 and 150 V, no diffraction peaks were observed, suggesting that only the amorphous phase was formed. Increasing the negative substrate bias causes not only the peaks of (180) and (361) to appear and dominate, but also minor signals from (311), (361) and (290) to be obtained from the Cr2C3 films prepared at 350 and 450 V.
C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
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Further increasing the bias to 550 V results in the appearance of a weak peak of Cr2C3 (011) plane at 2u = 32.58-. The deposit is predominantly highly crystalline phase of Cr2C3.
successfully prepared using a 90- bend magnetic filtered cathodic vacuum arc (FCVA) system, at a high negative substrate bias voltage ( 250 to 550 V).
3.2.2. TEM observation Fig. 6a shows the strong bond between the substrate and the chromium carbide film through the formation of a Cr interlayer. This bond is attributable to the high-energy bombardment with Cr ions which removes the residual oxides or with foreign contaminants from the substrate surface and the heating of the substrate to promote good coating adhesion [15]. Fig. 6b displays the selected area diffraction (SAD) pattern of the Cr3C2/Cr deposited silicon wafer. The diffraction pattern is weak and has a background of single crystal silicon spots, associated with the thin Cr interlayer. The d-spacing calculated from the diffraction rings agrees with the standard d-value of Cr [22]. The presence of such Cr diffraction rings demonstrate that chromium disilicide (CrSi2) had not been produced. Although CrSi2 can be synthesized by solidstate diffusion [23], no such phase could be detected after the Cr interlayer had been deposited by FCVA. While the Cr interlayer was deposited at the ambient temperature (without heating), no sufficient energy was provided for Cr atoms to react further with the Si substrate, which are in good agreement with the result from the AES concentration profiles (Fig. 3a and b). Fig. 7a and b present cross-sectional TEM micrographs and the corresponding SAD pattern from an area that covers the top of the chromium carbide film deposited at 250 and 550 V where Cr3C2 (241), Cr23C6(22¯0) and Cr23C6(02¯2) are observed. Fig. 6(b) shows Cr3C2 (011) and weak substrate signals. The cross-sectional TEM image presents small but equiaxed crystalline domains in the crystallized phase whose nanostructure are fine and fibrous as shown in Fig. 7a and b.
Acknowledgement
4. Conclusions An amorphous chromium carbide film was formed whose thickness is independent of the deposition temperature. However, the Cr– C bonds appear as the deposition temperature from an ambient temperature to 500 -C. The chromium carbide is transformed from amorphous to crystallized phase, as the negative substrate bias voltage increases from 50 to 550 V at 500 -C. Moreover, the crystallized Cr23C6 phase appeared as the negative substrate bias voltage further increases (< 250 V). A compact and dense Cr3C2 film with the fibrous structure of Cr23C6 was
The authors would like to thank the National Science Council of the Republic of China for the support of this research under Contract No. NSC 922216E007049.
References [1] K. Hohnberg, A. Matthews, Coatings Tribology, Tribology Series, vol. 28, Elsevier, Amsterdam, 1994. [2] H. Holleck, J. Vac. Sci. Technol., A, Vac. Surf. Films 4 (6) (1986) 2661. [3] Giinter Berg, Christoph Friedrich, Erhard Broszeit, Berger Christina, Surf. Coat. Technol. 86 – 87 (1996) 184. [4] H. Jensen, U.M. Jensen, G.N. Pedersen, G. Sorensen, Surf. Coat. Technol. 59 (1993) 135. [5] W. Herr, B. Matthes, E. Broszeit, M. Meyer, R. Suchentrnnk, Surf. Coat. Technol. 60 (1993) 428. [6] W. Herr, B. Matthes, E. Broszeit, M. Meyer, R. Suchentrnnk, Surf. Coat. Technol. 57 (1993) 77. [7] B. NavinSek, P. Panjan, A. Cvelbar, Surf. Coat. Technol. 741 (75) (1995) 155. [8] D.Y. Wang, K.W. Weng, C.L. Chang, W.Y. Ho, Surf. Coat. Technol. 120 – 121 (1999) 622. [9] R.L. Boxman, D.M. Sanders, P.J. Martin, Handbook of Vacuum Arc Science and Technology: Fundamentals and Applications, Noyes, Park Ridge, NJ, 1995. [10] M.M.M. Bilek, W.I. Milne, Thin Solid Films 308 – 309 (1997) 79. [11] I.I. Aksenov, V.A. Belous, V.G. Padalka, V.M. Khoroshikh, Sov. J. Plasma Phys. 4 (1978) 425. [12] P.J. Martin, A. Bendavid, Thin Solid Films 394 (2001) 1. [13] M. Hakovirta, V.M. Tiainen, P. Pekko, Diamond Relat. Mater. 8 (1999) 1183. [14] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy, 2nd edR, Physical Electronics Industries, Inc, Eden Prairie, MA, 1976. [15] J.L. Vossen, W. Kern, Handbook of Thin Film Processes, vol. II, Academic Press, 1991. [16] Marvin J. Udy, Chromium: Metallurgy of Chromium and its Alloys, vol. II, New York Reinhold Publishing Co., 1956. [17] H. Kleykamp, J. Alloys Compd. 321 (2001) 138. [18] S. Loubie´re, C. Laurent, J.P. Bonino, A. Rousset, Mater. Res. Bull. 30 (1995) 1535. [19] N. Mare´chal, E. Quesnel, Y. Pauleau, J. Mater. Res. 9 (1994) 1820. [20] A.A. Edigaryan, V.A. Safonov, E.N. Lubnin, L.N. Vykhodtseva, G.E. Chusova, Yu.M. Polukarov, Electrochim. Acta 47 (2002) 2775. [21] L. Cunha, M. Andritschky, L. Rebouta, K. Pischow, Surf. Coat. Technol. 116 – 119 (1999) 1152. [22] Powder Diffraction File, Joint Committee on Powder Diffraction Standard, ASTM, Philadelphia, PA, 1996, (Card 06-0694). [23] S. Han, H.Y. Chen, Z.C. Chang, J.H. Lin, C.J. Yang, F.H. Lu, F.S. Shieu, Han C. Shih, Thin Solid Films 436 (2003) 238.
Formation and characterization of chromium carbide films deposited using a 90- bend magnetic filtered cathodic vacuum arc system Chun-Chun Lin, Wei-Jen Hsieh, Jain-Hong Lin, Uei-Shin Chen, Xing-Jian Guo, Han C. Shih * Department of Material Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC Received 1 March 2005; accepted in revised form 18 May 2005 Available online 26 August 2005
Abstract Deposition of chromium carbides films using a 90- bend filtered cathodic vacuum arc (FCVA) system is highly promising for industrial applications, because chromium carbide films uncontaminated by macroparticles exhibit excellent characteristics. Chromium carbide films were synthesized from a Cr target (99.95%) and C2H2/Ar gases, beginning with deposition parameters of the deposition temperature of ambient temperature, 300 and 500 -C and negative bias voltages ranging from 50 to 550 V. The microstructure of crystalline chromium carbide was investigated using glancing incident X-ray diffraction and cross-sectional transmission electron microscopy. The atomic concentrations of C and Cr in chromium carbide thin films deposited at various temperatures were measured by AES and the nature of the chemical bonding of the elements was elucidated by XPS. Our findings revealed that the total C – Cr bond contents depended on the deposition temperature. As the substrate bias voltage was increased from 50 to 550 V, the phase was transformed from amorphous to crystalline Cr3C2; a crystalline Cr23C6 phase also appeared at higher negative substrate bias voltages (< 250 V). The effects of the deposition parameters on the formation of chromium carbides were investigated in this study. D 2005 Elsevier B.V. All rights reserved. Keywords: Cr3C2; Cr23C6; FCVA; AES; XPS; XRD; TEM
1. Introduction Chromium-based coatings are promising for tribological use as an alternative to TiN [1,2]. The deposition of the chromium nitride coatings is well understood [3 –7], and few reports on the deposition of chromium carbide coatings, especially those prepared by filtered cathodic vacuum arc (FCVA) deposition, have been published [8]. FCVA technology is an efficient physical vapor deposition technique for industrial production since it provides a fully ionized and dissociated flow of the target metal vapor with highly concentrated activated reactive species [9]. The extent of metal ionization in this technique is generally ¨ 100% [10]. High-quality films uncontaminated by macro-
* Corresponding author. Tel.: +886 3 5715131 3845; fax: +886 3 5710290. E-mail address: [email protected] (H.C. Shih). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.027
particles have been developed using a magnetic filter to guide the plasma efficiently to the substrate, producing a coating that is free of macroparticles [11 – 13]. This investigation was to compare various conditions for depositing chromium carbide coatings using the FCVA system. Two deposition parameters (deposition temperature and substrate bias voltage) were varied during chromium carbide films. The nanostructure of the chromium carbide film was characterized to determine the optimum values of the deposition parameters for industrial applications.
2. Experimental The base pressure of the chamber was 6.6 10 3 Pa, using a diffusion pump backed with a mechanical pump in the FCVA system. The setup of the FCVA system with dual filtered source is shown in Fig. 1. In this study, one filtered source was used to deposit chromium carbide. A 99.95 wt.%
C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
Vacuum chamber Base pressure of 5 E-5 torr
5053
D.C. cathode arc source
Cathode
Trigger
Thermocouple
Substrate
Hot plate 50°C - 750°C 90° -bend magnetic filter Pulsed-dc bias power supply Fig. 1. Setup of the 90- bend magnetic FCVA system.
pure Cr target with a diameter of 4 in. was employed. The metal vapor from the target passed through a 90- bend magnetic filter (120 Gauss) in order to filter the macroparticles. The distance between the exit of the filtered source and the substrate holder was 15cm. The p-type (100) silicon substrates with dimensions of 20 mm 20 mm were ultrasonically cleaned in acetone, then in alcohol for 15 min, and finally loaded into the substrate holder. Prior to coating, substrate surfaces were cleaned by bombardment with argon for 5 min at a pressure of 1.3 Pa and a substrate bias of 950 V to remove any native oxides. A pure Cr film (5 nm) was firstly deposited onto the substrate at 150 V for 2 min. Subsequently, the substrate holder was heated by a hot plate with a thermocouple nearby to monitor the substrate temperature. Table 1 summarizes the deposition parameters. The structures of amorphous and crystalline chromium carbide films were investigated using glancing incident Xray diffraction and cross-sectional transmission electron microscopy (TEM). A Mac Science MXP18 diffractometer was used with Cu Ka radiation (k = 0.154 nm) generated at 40 kV and 150 mA at a grazing incidence of 0.5-. A Philips Table 1 Deposition parameters of chromium carbide films using 90- bend magnetic filtered cathodic vacuum arc Working pressure (Pa) 53310
1
Deposition temperature (-C)
Target current (A)
Reactive gas (sccm)
Substrate bias (V)
Ambient temperature 300 500
60
C2H2 /Ar (60:10)
150
50V to 550
Time (min) 30
CM2 TEM at 200 kV was employed to examine the nanostructure. The surface morphology as well as the thickness was measured using FE-SEM (JEOL JSM6500F) at 15 kV. Auger electron spectroscopy (AES) was performed using a Perkin Elmer (670 PHI-Xi) analyzer with a primary electron beam of 3 KeV and a beam current of 25 mA. The spectra were obtained at a resolution of 0.1 eV. The samples were sputter-etched using an ion gun with 3 keVAr+ to obtain the depth profile. These depth profiles reveal a mixture of Cr, C and Si and the concentration of each element of the chromium carbide thin films determined from the AES spectra according to the published relative sensitivity factors [14]. X-ray photoelectron spectrum (XPS) was employed to determine the chemical state of the films and was obtained using an ESCA PHI 1600 spectrometer with Mg Ka radiation (1253.6 eV). The data were taken at a constant analyzer pass energy of 117.4 eV in a vacuum system with a base pressure of 10 8 Pa. All the samples were argon-etched at 3 keV with a current equal to 1 `ıA, at a pressure of 10 6 Pa for 2 min. Discrete scans in the ranges 280– 290 eV for C and 570– 600 eV for Cr were recorded.
3. Results and discussion 3.1. Effect of deposition temperature Elevated temperatures are employed in the thin film deposition processes for three basic reasons: (1) to control the morphology of the growing film; (2) to improve film adhesion, and (3) to activate the reaction between the metal and the reaction gas in reactive deposition. Therefore,
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C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
elevated substrate temperatures are normally viewed as an important parameter in the thin film process [15].
(a) 100
300oC -150V
3.1.1. SEM observation Fig. 2b and c present the cross-sectional morphologies of chromium carbide films deposited at 300 and 500 -C,
(a) Ambient temperature -150V
Atomic Concentration (%)
90 80 70
Cr C
60 50 40 30 20 10
Si
0 0
2
4
6
Chromium carbides
8
10
12
14
16
18
20
Sputter time (min)
(b) 100
500oC -150V
Substrate
100nm
(b) 300ºC -150V
Atomic Concentration (%)
90 80
Cr
70 C
60 50 40 30 20 10 Si
0 0
2
4
6
8
10
12
14
16
18
20
Sputter time (min)
Chromium carbides
Fig. 3. AES depth profile of chromium carbide film deposited at two different deposition temperature: (a) T = 300 -C, (b) T = 500 -C on the Si wafer. The thickness of the film is 500 nm.
Void
Substrate
100nm
(c) 500ºC -150V
Chromium carbides
Substrate
100nm
Fig. 2. Cross-section SEM images of the amorphous chromium carbide film deposited at the substrate bias of 150 V at (a) ambient temperature, (b) 300 -C, and (c) 500 -C.
respectively. The thickness of the chromium carbide film is independent of the deposition temperature; ¨ 500 nm in each case. As the deposition temperature increases from 300 to 500 -C, the morphology of the growing film becomes denser and the microstructure becomes more compact. The void-free and columnar structure is attributed to the activation of the reaction between metal and gas during the deposition process [15]. No macroparticle was observed, due to the 90- bend magnetic filter. However, the film cannot be easily deposited at ambient temperature (without heating) because the coating adhesion is weak and also the compressive stress will increase [15], as shown in Fig. 2a. Chromium carbides have three well known phases, i.e., Cr3C2 (rhombic), Cr7C3 (hexagonal) and Cr23C6 (cubic) which exhibit superior characteristics of good resistance to wear and corrosion, high hardness, a high melting point and oxidation-resistance at high temperature [8,16]. These carbides have great potential for replacing electroplated hard chrome as a premier coating for molding industry [17 – 19]. Glancing incident X-ray diffraction was utilized to identify the crystallographic
C.-C. Lin et al. / Surface & Coatings Technology 200 (2006) 5052 – 5057
(a)
orientation of chromium carbide films deposited at 300 and 500 -C, but no diffraction peaks were discernible and should therefore be consistent with the amorphous phase.
ambient temp. 282
284
286
288
290
Binding energy (eV)
(b) Cr2p
Cr 2p3/2 576.6 eV
500oC
Intensity (arb. unit)
Cr 2p1/2
300oC
ambient temp.
570
575
580
585
590
595
600
Binding energy (eV)
Substrate
Intensity (arb. units)
Cr3C2(011)
Fig. 4. XPS C 1S (a) and Cr 2p (b) spectra of the chromium carbide film deposited at ambient temperature, 300 -C and 500 -C.
Cr3C2(290)
300oC
-C-C285.2 eV
3.1.2. AES and XPS analyses AES was combined with ion sputtering to determine the concentration profiles of amorphous chromium carbide films. Fig. 3a and b show the resulting concentration depth profiles of chromium carbide films deposited at 300 and 500 -C. AES analysis indicates that the film deposited at 300 -C has a higher carbon content, in which the amorphous carbon film generated by C2H2 decomposition and the Cr-embedded amorphous carbon film exhibit very poor adhesion strength [8]. Whereas the deposition temperature increased to 500 -C, the predominant carbon deposition process evolved into a reactive PVD process, in which the Cr plasma reacted with carbon radicals to form the chromium carbide film [8]. The chemical bonding of the elements (C and Cr) of the chromium carbide film was subsequently determined by XPS. Fig. 4a and b display the XPS spectra associated with the C1s and the Cr2p signals at various deposition temperatures. The binding energies of the spectral components of the C1s bond shifted from 285.2 to 284.3 eV as the deposition temperature increases from ambient temperature to 500 -C; the first binding energy was close to 285.0 eV, which is attributed to C – C bonds. The negative shift (0.9 eV) is associated with the carbon – metal interaction, which is indicated by the appearance of a low energy line, as compared with the binding energy of 285.2 eV, in the spectrum [20]. The Cr2p3/2 binding energy is 576.6 eV, characterizing the formation of Cr2O3 [20, 21]. As the temperature increases to 500, the Cr2p signal becomes
Cr3C2(361)
500oC
Cr3C2(360)
Cr
Cr3C2(180)
-C-C-C-
Cr3C2(311)
Intensity (arb. unit)
C1s
280
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-550V -450V -350V -150V -50V 30
40
50
60
70
80
2θ Fig. 5. X-ray diffraction patterns of the Cr3C2 deposited at 500 -C with varying substrate bias voltages.
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weak, as indicated in Fig. 3b. Evidently, raising the deposition temperature vaporizes the moisture in the chamber, thereby reducing the extent of formation of oxide layers on the top of the chromium carbide coatings. However, the XRD results did not reveal the Cr2O3 peak; the surface layer of Cr2O3, detected by XPS under such oxidation conditions, is rather thin. The deposition temperature increases to 500 -C to promote the synthesis of the crystallized chromium carbide film. Accordingly, deposition was performed at 500 -C by altering the bias voltage from 50 to 550 V.
(a)
3.2. Effect of substrate bias voltage 3.2.1. XRD results Fig. 5 presents the XRD patterns of the Cr3C2 deposited at 500 -C on the (100) silicon wafer at various substrate
(a) (b)
Chromium carbide film
Cr interlayer
Si substrate
1 nm
(b)
Fig. 7. Cross-section TEM micrograph and SAD pattern from an area that covers the top of the chromium carbide film deposited at (a) 250 V and (b) 550 V.
Cr (110)
Fig. 6. (a) Cross-section TEM micrograph and (b) SAD pattern of the crystallized chromium carbide/Cr deposited on the Si wafer.
biases from 50 to 550 V. The JCPDS database of the Cr3C2 phase (No.35-0804) in the e`/2e` mode was employed to establish the crystallinity of the film. At biases of 50 and 150 V, no diffraction peaks were observed, suggesting that only the amorphous phase was formed. Increasing the negative substrate bias causes not only the peaks of (180) and (361) to appear and dominate, but also minor signals from (311), (361) and (290) to be obtained from the Cr2C3 films prepared at 350 and 450 V.
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Further increasing the bias to 550 V results in the appearance of a weak peak of Cr2C3 (011) plane at 2u = 32.58-. The deposit is predominantly highly crystalline phase of Cr2C3.
successfully prepared using a 90- bend magnetic filtered cathodic vacuum arc (FCVA) system, at a high negative substrate bias voltage ( 250 to 550 V).
3.2.2. TEM observation Fig. 6a shows the strong bond between the substrate and the chromium carbide film through the formation of a Cr interlayer. This bond is attributable to the high-energy bombardment with Cr ions which removes the residual oxides or with foreign contaminants from the substrate surface and the heating of the substrate to promote good coating adhesion [15]. Fig. 6b displays the selected area diffraction (SAD) pattern of the Cr3C2/Cr deposited silicon wafer. The diffraction pattern is weak and has a background of single crystal silicon spots, associated with the thin Cr interlayer. The d-spacing calculated from the diffraction rings agrees with the standard d-value of Cr [22]. The presence of such Cr diffraction rings demonstrate that chromium disilicide (CrSi2) had not been produced. Although CrSi2 can be synthesized by solidstate diffusion [23], no such phase could be detected after the Cr interlayer had been deposited by FCVA. While the Cr interlayer was deposited at the ambient temperature (without heating), no sufficient energy was provided for Cr atoms to react further with the Si substrate, which are in good agreement with the result from the AES concentration profiles (Fig. 3a and b). Fig. 7a and b present cross-sectional TEM micrographs and the corresponding SAD pattern from an area that covers the top of the chromium carbide film deposited at 250 and 550 V where Cr3C2 (241), Cr23C6(22¯0) and Cr23C6(02¯2) are observed. Fig. 6(b) shows Cr3C2 (011) and weak substrate signals. The cross-sectional TEM image presents small but equiaxed crystalline domains in the crystallized phase whose nanostructure are fine and fibrous as shown in Fig. 7a and b.
Acknowledgement
4. Conclusions An amorphous chromium carbide film was formed whose thickness is independent of the deposition temperature. However, the Cr– C bonds appear as the deposition temperature from an ambient temperature to 500 -C. The chromium carbide is transformed from amorphous to crystallized phase, as the negative substrate bias voltage increases from 50 to 550 V at 500 -C. Moreover, the crystallized Cr23C6 phase appeared as the negative substrate bias voltage further increases (< 250 V). A compact and dense Cr3C2 film with the fibrous structure of Cr23C6 was
The authors would like to thank the National Science Council of the Republic of China for the support of this research under Contract No. NSC 922216E007049.
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