- Email: [email protected]
DLC coating of a few microns in thickness on three-dimensional substrates
ARTICLE IN PRESS
Vacuum 73 (2004) 541–547
DLC coating of a few microns in thickness on three-dimensional substrates Yoshihiro Okaa, Yoshimi Nishimuraa,b, Mitsuyasu Yatsuzukaa,* a
Himeji Institute of Technology, Graduate School of Engineering, 2167 Shosha, Himeji, Hyogo 671-2201, Japan b Kurita Seisakusho Co., Ltd., Ujitawara Industrial Estate, Ujitawara-cho, Kyoto 610-0221, Japan
Abstract Uniform coating of the thick diamond-like carbon (DLC) on three-dimensional substrates was studied using a hybrid process of plasma-based ion implantation and deposition (PBIID). In the PBIID system, an RF pulse for plasma generation and a negative high-voltage pulse for ion implantation were supplied to a substrate through a single electrical feedthrough. Then the high-density plasma was produced along the substrate, leading to the formation of uniform coating film and the ion implantation on three-dimensional substrates. Ion implantation by PBII technique served to the reduction in compressive residual stress of DLC film and the formation of mixing layer between the DLC film and the substrate. As a result, the thick and good-adhesive DLC film with more than a few mm in thickness was almost uniformly prepared on the surface of cylindrical pipe, triangle prism, and trench structure. The deposition rate was approximately 1 mm/h using the toluene as a precursor gas. The hardness of the DLC films was found to be about 12 GPa by nano-indentation method. r 2004 Elsevier Ltd. All rights reserved. Keywords: Plasma-based ion implantation (PBII); Plasma CVD; Hybrid process; Diamond-like carbon (DLC); Three-dimensional substrate
1. Introduction Diamond-like carbon (DLC) films are well known for their excellent tribological properties such as a low coefficient of friction, strong wear resistance, and high hardness that are very attractive for industrial applications. However, DLC films have not been used widely for industrial mechanical parts because of their poor uniformity *Corresponding author. Tel.: +81-792-67-4862; fax: +81792-67-8868. E-mail address: [email protected] (M. Yatsuzuka).
in thickness on three-dimensional substrates and their weak adhesion to substrate material. Carbon plasmas generated by vacuum arc, sputtering, and inductive coupling of RF are usually produced remotely from the substrate, then the plasma stream in the one direction occurs from the plasma source to the substrate. In such case, the uniform coating of DLC film might be difficult on the whole circumference of the three-dimensional substrate with complicated shapes. The main cause of poor adhesion of thick DLC film is the presence of strong comprehensive residual stress of more than several GPa in DLC films [1–3]. The key technologies for good uniformity and strong
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.085
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Y. Oka et al. / Vacuum 73 (2004) 541–547
adhesion of thick DLC coating films are the generation of uniform plasma around the substrate and the reduction of residual stress. A hybrid processing system of plasma-based ion implantation [4] and deposition (PBIID) was developed to prepare uniform DLC coating on three-dimensional substrates [5–7]. In this system, the RF pulse for plasma generation was supplied to the substrate together with the negative highvoltage pulse for ion implantation through a single electrical feed-through. As the substrate itself was worked as an RF antenna, the plasma having the largest density around the substrate was produced [5]. In addition, ions were directly accelerated to implant from the plasma to the substrate. Then the hybrid PBIID system supplying RF and highvoltage pulses to the substrate enabled us to realize uniform deposition and ion implantation to threedimensional substrates. It is known that the local heating of substrate by ion beam implantation relaxes residual compressive stress in DLC films [2,3,8]. The authors observed the remarkable reduction of the residual stress of the DLC film using the hybrid PBIID system [7]. Methane (CH4) and acetylene (C2H2) plasmas were used for the formation of a thicker graded interface, which was similar to the DLC coating processing of LANL group [9]. Use of toluene (C6H5CH3) as a precursor gas enabled the high-speed deposition rate as well as the preparation of thick goodadhesion DLC film with the lower residual stress of approximately 0.35 GPa [10]. In this work, we present the preparation of the thick DLC film on the surface of a cylindrical pipe, three triangular prisms with the different wedge angles of 30 , 60 , and 90 , and three type of rectangular trenches with different aspect ratios using the hybrid process of PBIID and the toluene gas plasma. And we report on the different profiles of film thickness at the center and the edge of trench. The adhesion strength and the hardness of DLC film are also presented.
2. Experimental A schematic diagram of the hybrid PBIID system is shown in Fig. 1. In this system, the
Fig. 1. A schematic diagram of hybrid process of plasma-based ion implantation and deposition (PBIID) system.
RF pulse for plasma generation was supplied to the substrate together with a negative highvoltage pulse for ion implantation through a single electrical feed-through using a joint matching circuit for both pulses. As the substrate itself was used as an RF antenna, the uniform plasma was generated around the substrate and the largest plasma density appeared near the substrate [5]. The RF power supply had the maximum power of 3 kW, the pulse duration tRF of 5–50 ms, the frequency of 13.56 MHz, and the repetition rate f of 1–4 kHz. A negative highvoltage pulse train with the voltage of 5 to 20 kV, the duration of 1–5 ms, and the same repetition rate as the RF pulse was applied to the substrate at the time of 50 ms later after each RF pulse. The precursor gases used in the present experiment were the hydrocarbon gases, such as CH4, C2H2, and C6H5CH3. The procedure of DLC film preparation was the four steps as follows: The first step was the sputter-cleaning process using the plasma of mixed gases of Ar and CH4 (50:50%) with the negative high-voltage pulse (voltage Vb= 10 kV, duration t=5 ms, repetition rate f=1 kHz). In the second step, the carbon ion implantation with the negative high-voltage pulse (Vb= 20 kV, t=3 ms, f=1 kHz) was performed with the CH4 plasma to make carbon anchors inside the substrate and in the third step the C2H2 plasma was used to produce a
ARTICLE IN PRESS Y. Oka et al. / Vacuum 73 (2004) 541–547
3. Results and discussion 3.1. Residual stress The residual stress of DLC films prepared using C2H2 and C6H5CH3 plasmas was determined from the curvature of its substrate. Fig. 2 shows the residual compressive stress in the DLC film as a function of negative pulse voltage at the fourth step for deposition process, where tRF=50 ms, t=5 ms, T=2 h and the solid points and the circles indicate the residual stress of DLC films prepared using C2H2 and C6H5CH3 plasmas, respectively. As seen in Fig. 2, the compressive residual stress in the DLC film prepared using the C2H2 plasma is about 1.55 GPa with no negative high-voltage pulse. The residual stress considerably decreases with increasing the negative high-voltage during deposition and reduces to about 0.2 GPa at the negative high-voltage of 15 kV. The observed reduction of residual stress might be resulted from the local heating of substrate by deposition of ion beam energy [8]. On the other hand, the residual stress of DLC film prepared with the C6H5CH3 plasma is about 0.35 GPa and little dependent on the negative high-voltage, indicating that ion implantation is almost ineffective for a case of the C6H5CH3 plasma. The observation of plasma
2 Compressive stress (GPa)
graded mixing layer between the coating film and the substrate using the negative high-voltage pulse (Vb= 20 kV, t=3 ms, f=1 kHz). When skipping the second and the third steps, it was hard to obtain good adhesion of DLC film. The last fourth step was the rapid deposition process for production of thick film using the C6H5CH3 plasma and the negative high-voltage pulse of medium voltage (Vb= 10 kV, t=5 ms) and a high repetition rate (f=4 kHz). The gas pressure was p=0.3–0.5 Pa in the second and third steps and p=1.0 Pa during deposition of the fourth step. In the present experimental conditions, at the fourth step, the glow discharge was generated by the negative high-voltage pulse around the substrate. The typical deposition rate was approximately 1 mm/h using the C6H5CH3 plasma for deposition. Three kinds of substrates of aluminum-alloy (A5052) were used for DLC coating; a cylindrical pipe with a diameter of 25 mm and a length of 150 mm, three triangular prisms with each wedge angle (c=30 , 60 , and 90 ) and a length of 90 mm, and three type trenches with the different aspect ratio. The trench A has the aspect ratio of 0.5 and the trench width of 2 cm. The trench B has the aspect ratio of 1 and the trench width of 1 cm. The trench C has the aspect ratio of 2 and the trench width of 1 cm. All trenches have the same top width of 1 cm and the length of 4 cm. The substrate surface was polished to a mirror finish before DLC coating and set at the center of vacuum chamber. The film thickness was measured with the crosssectional SEM observation. The residual stress in each film was determined from the curvature of quartz glass plate (length of 25 mm, width of 5 mm and thickness of 0.5 mm) using Stoney’s equation, where the curvature was measured at both before and after film preparation using a stylus profilometer (Dektak3, ULVAC). The strength of adhesion was evaluated by the REVETEST scratch-testing equipment (CSEM). The damaged region along the scratch track was observed with a CCD microscope (VH-8000, KEYENCE). The hardness was measured with a nano-indentor (ENT1100, ELIONIX).
543
C2H2 C6H5CH3 1
0 0
5 10 15 Negative pulsed voltage (kV)
Fig. 2. The residual compressive stress of DLC film as a function of negative pulse voltage at the fourth step for deposition process, where the solid points and the circles indicate the residual stress of DLC films prepared using C2H2 and C6H5CH3 plasmas, respectively.
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Current (A)
Voltage (kV)
luminescence indicated the generation of a new discharge around the substrate at the same time with the application of negative high-voltage pulse in the fourth step of film preparation procedure [11]. The generation of glow discharge by the negative high-voltage pulse is also confirmed with the waveform of current through the substrate at the time of application of negative high-voltage pulse as shown in Fig. 3, where the pulse voltage was 10 kV, the repetition was 1 kHz and the toluene gas pressure was 1 Pa. The plasma generation by the negative high-voltage pulse leads to the disappearance of large ion-sheath potential around the substrate, so that there is little ion implantation as well as non-reduction of residual stress. In either case, the observed residual stress of the DLC film prepared by the present experimental conditions is almost two orders less than that of other methods [1,2]. Both the mixing layer formation and the reduction of residual stress by carbon-ion implantation could be useful for the sufficient-adhesive
0
-10
V = -10 kV
High-voltage pulse
coating of thick DLC film with more than a few mm in thickness on the aluminum alloy substrate. 3.2. Thickness profile on the trench surface Fig. 4 shows the SEM micrograph of crosssection of film on the center of top surface of trench A, where the deposition time of the fourth step was 4 h. As seen in Fig. 4, the film thickness is approximately 4.2 mm, indicating the deposition speed of about 1 mm/h. From the similar results of thickness measurement, the thickness profiles on the three-dimensional substrates were plotted. Fig. 5 shows the thickness profile of the cylindrical pipe surface at the various positions from the pipe edge, where the deposition time was 3 h, z is the distance from the pipe edge, and y is the deflection angle indicating the position on the circumference. As seen in Fig. 5, the DLC film of about 2 mm in thickness is very uniformly coated on the whole surface of cylindrical pipe. The thickness profile of DLC film on the triangle prism surface of different wedge angle (c=30 , 60 , and 90 ) is shown in Fig. 6, where the deposition time was 4 h, the tip of wedge is the origin of position, and the result at the origin was obtained at the position of 0.1 mm from the wedge tip. Fig. 6 indicates the almost uniform profile in film thickness even in the region around the wedge tip.
0 -5
RF pulse
DLC film
-10 0
50 Time (µs)
100
Fig. 3. The time history of negative high-voltage and current through the substrate at the fourth step for film preparation procedure, where the voltage and repetition of negative highvoltage pulse were 10 kV and 1 kHz, respectively, and the toluene gas pressure was 1 Pa.The waveform of current suggests the generation of glow discharge at the time of application of negative-high-voltage pulse.
Substrate
Fig. 4. Cross-sectional SEM micrograph of DLC coating film on the center of top surface of trench, where the deposition time of the fourth step was 4 h.
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3
545
2
2
1
1
0
3 2 1 0 z = 65 mm 3
Film thickness (µm)
Film thickness (µm)
0 z = 5 mm
ψ = 30°
2 1 0
ψ = 60°
2
2
1
1 0 z = 145 mm 45 90 135 180 225 270 315 Position on the circumference θ (°)
Fig. 5. Film thickness profile in the azimuthal direction on the cylindrical pipe circumference at the various axial positions.
Although not shown here, the film thickness on the trench A was almost uniform on each surface of top, sidewall, and bottom [10]. Fig. 7 shows the thickness profile of film at the center and the 3 mm from the edge for the trench B, where the deposition time was 4 h. As seen in Fig. 7, at the center of trench the film thickness on the top surface is about 1.9 times of that at the sidewall and 1.3 times of bottom. Although the thickness profile at the 3 mm from the edge is similar to the profile at the center, but the difference in thickness at each side becomes small. Fig. 8 shows the thickness profile of DLC film on the surface of trench C. In this case, at the trench center, the film thickness on the top surface is about two times of the sidewall one, while the thickness of bottom is slightly larger than the top one. However, it is found in Fig. 8 that the film thickness on the bottom at the trench edge is lower than that at the trench center. These results suggest that the negative-high-voltage pulse applied to the sub-
ψ = 90° -10 -8 -6 -4 -2 0 2 4 6 8 10 Position (mm)
Fig. 6. Film thickness profile on the triangular prism with different wedge angles.
8
Film thickness (µm)
0
0
center 3 mm from edge
6 4 2
Top 0
0
Sidewall 10
Bottom 20
30
Position (mm) Fig. 7. Film thickness profile on the trench B with the aspect ratio of 1, where the solid points and the circles indicate the film thickness at the center and the 3 mm from the edge, respectively.
strate leads to the acceleration of secondary electrons trapped in the trench and the generation of plasma inside the trench with the large aspect
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center 3 mm from edge
6
AE signal (arb. unit)
Film thickness (µm)
8
4 2 Top
0 0
Sidewall
10
20 30 Position (mm)
Lc
Bottom
40
Fig. 8. Film thickness profile on the trench C with the aspect ratio of 2, where the solid points and the circles indicate the film thickness at the center and the 3 mm from the edge, respectively.
0
5
10
15
Load (N) Fig. 9. Acoustic emission signal as a function of load of a diamond stylus of the scratch test, indicating the critical load of about 4 N.
ratio. The plasma luminescence inside the trench was observed as mentioned in Section 3.1 [11]. The observed increase in thickness of the bottom at the trench center should be ascribed to the plasma generation inside the trench by hollow cathode effect. 3.3. Adhesion strength and hardness The adhesive strength of DLC coating film on the top surface of trench was evaluated by the scratch test in which the load of a diamond stylus was increased progressively. Fig. 9 shows the acoustic emission in the scratch test as a function of load L of diamond stylus, where the diamond tip radius r=0.2 mm, the scratching speed dx/ dt=10 mm/min, and the loading rate dL/ dt=100 N/min. As seen in Fig. 9, the critical load Lc at which the acoustic emission signal increased suddenly is about 4 N. The critical load usually characterizes the adhesive strength, but it depends on several parameters affected by the testing conditions and the coating-substrate system. As the aluminum alloy substrate is so soft that the deformation of the substrate easily occurs during scratch test, the measured value of 4 N may be not really the actual adhesion strength of DLC film. The scratch track was observed with the CCD
100 µm Fig. 10. Scratch track observed with CCD micrograph.
micrograph, the result of which is shown in Fig. 10. The damaged region is found to be ‘‘scraping’’ and no ‘‘flaking’’ (adhesive failure) of coating film occurs during and after the scratch test. Fig. 11 shows the hardness and the maximum displacement of DLC film with the thickness in 4.7 mm as a function of maximum load of diamond pyramidal indenter. It is found in Fig. 11 that the hardness is 12.2 GPa at the maximum load of 0.5 g. Since the maximum penetration depth of indenter is 0.12 mm corresponding to about 1/40 of film
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Hardness (GPa)
20
8 Hardness Maximum displacement
6
10
4 2
0
100
101 Load (g)
102
0
Maximum displacement (µm)
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Fig. 11. Hardness and maximum displacement of DLC film prepared using C6H5CH3 plasma.
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was about 1.3 times larger than that of bottom and 1.9 times than the sidewall. For aspect ratio of 2, the film thickness of bottom at the trench center was larger than the top one. The observed increase in thickness of bottom might be resulted from the plasma generation inside the trench by hollow cathode effect. The DLC film hardness measured by the nano-indentor was 12 GPa.
Acknowledgements This work was supported by the Cooperation of Innovative Technology and Advanced Research in Evolutional Area (CITY AREA).
References thickness, the observed hardness shows the real hardness of DLC itself with little effect of the substrate material.
4. Conclusion The hybrid process of plasma-based ion implantation and deposition, in which the substrate itself was used as a RF antenna for plasma production, resulted in the thick and uniform DLC coating of 4–5 mm in thickness on the threedimensional substrates. The use of toluene gas and ion implantation reduced the residual compressive stress of DLC film to 0.2–0.35 GPa. The DLC film thickness profile was almost uniform on the whole surfaces of cylindrical pipe and triangular prism. The DLC film with a few mm in thickness was formed on the whole surfaces of the trench with the aspect ratio of 1–2 and the trench width of 1 cm. For the trench with the aspect ratio of 1, the film thickness of top surface at the trench center
[1] McKenzie DR, Muller D, Pailthorpe BA. Phys Rev Lett 1991;67:773–6. [2] Fallon PJ, Veerasamy VS, Davis CA, Robertson J, Amaratunga GAJ, Milne WI, Koskinen J. Phys Rev B 1993;48:4777–82. [3] Ager III JW, Anders S, Anders A, Brown IG. Appl Phys Lett 1995;66:3444–6. [4] Conrad JR, Radtke JL, Dodd RA, Worzala FJ, Tran NC. J Appl Phys 1987;62:4591–6. [5] Nishimura Y, Chayahara A, Horino Y, Yatsuzuka M. Surf Coat Technol 2002;156:50–3. [6] Nishimura Y, Oka Y, Akamatusu H, Azuma K, Yatsuzuka M. Nucl Instrum Methods B 2003;206:696–9. [7] Oka Y, Tao M, Nishimura Y, Azuma K, Yatsuzuka M. Nucl Instrum Methods B 2003;206:700–3. [8] Adler RJ, Picraux ST. Nucl Instrum Methods B 1985;6:123. [9] Walter KC, Nastasi M, Munson C. Surf Coat Technol 1997;93:287–91. [10] Nishimura Y, Ohkawa R, Oka Y, Azuma K, Fujiwara E, Yatsuzuka M. Trans MRS-J 2003;28:449–52. [11] Onoi M, Oka Y, Fujiwara E, Nishimura Y, Azuma K, Yatsuzuka M. Abstract of Seventh Internatinal Workshop on Plasma Based Ion Implantation, San Antonio, 2003, p. 25.
Vacuum 73 (2004) 541–547
DLC coating of a few microns in thickness on three-dimensional substrates Yoshihiro Okaa, Yoshimi Nishimuraa,b, Mitsuyasu Yatsuzukaa,* a
Himeji Institute of Technology, Graduate School of Engineering, 2167 Shosha, Himeji, Hyogo 671-2201, Japan b Kurita Seisakusho Co., Ltd., Ujitawara Industrial Estate, Ujitawara-cho, Kyoto 610-0221, Japan
Abstract Uniform coating of the thick diamond-like carbon (DLC) on three-dimensional substrates was studied using a hybrid process of plasma-based ion implantation and deposition (PBIID). In the PBIID system, an RF pulse for plasma generation and a negative high-voltage pulse for ion implantation were supplied to a substrate through a single electrical feedthrough. Then the high-density plasma was produced along the substrate, leading to the formation of uniform coating film and the ion implantation on three-dimensional substrates. Ion implantation by PBII technique served to the reduction in compressive residual stress of DLC film and the formation of mixing layer between the DLC film and the substrate. As a result, the thick and good-adhesive DLC film with more than a few mm in thickness was almost uniformly prepared on the surface of cylindrical pipe, triangle prism, and trench structure. The deposition rate was approximately 1 mm/h using the toluene as a precursor gas. The hardness of the DLC films was found to be about 12 GPa by nano-indentation method. r 2004 Elsevier Ltd. All rights reserved. Keywords: Plasma-based ion implantation (PBII); Plasma CVD; Hybrid process; Diamond-like carbon (DLC); Three-dimensional substrate
1. Introduction Diamond-like carbon (DLC) films are well known for their excellent tribological properties such as a low coefficient of friction, strong wear resistance, and high hardness that are very attractive for industrial applications. However, DLC films have not been used widely for industrial mechanical parts because of their poor uniformity *Corresponding author. Tel.: +81-792-67-4862; fax: +81792-67-8868. E-mail address: [email protected] (M. Yatsuzuka).
in thickness on three-dimensional substrates and their weak adhesion to substrate material. Carbon plasmas generated by vacuum arc, sputtering, and inductive coupling of RF are usually produced remotely from the substrate, then the plasma stream in the one direction occurs from the plasma source to the substrate. In such case, the uniform coating of DLC film might be difficult on the whole circumference of the three-dimensional substrate with complicated shapes. The main cause of poor adhesion of thick DLC film is the presence of strong comprehensive residual stress of more than several GPa in DLC films [1–3]. The key technologies for good uniformity and strong
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.085
ARTICLE IN PRESS 542
Y. Oka et al. / Vacuum 73 (2004) 541–547
adhesion of thick DLC coating films are the generation of uniform plasma around the substrate and the reduction of residual stress. A hybrid processing system of plasma-based ion implantation [4] and deposition (PBIID) was developed to prepare uniform DLC coating on three-dimensional substrates [5–7]. In this system, the RF pulse for plasma generation was supplied to the substrate together with the negative highvoltage pulse for ion implantation through a single electrical feed-through. As the substrate itself was worked as an RF antenna, the plasma having the largest density around the substrate was produced [5]. In addition, ions were directly accelerated to implant from the plasma to the substrate. Then the hybrid PBIID system supplying RF and highvoltage pulses to the substrate enabled us to realize uniform deposition and ion implantation to threedimensional substrates. It is known that the local heating of substrate by ion beam implantation relaxes residual compressive stress in DLC films [2,3,8]. The authors observed the remarkable reduction of the residual stress of the DLC film using the hybrid PBIID system [7]. Methane (CH4) and acetylene (C2H2) plasmas were used for the formation of a thicker graded interface, which was similar to the DLC coating processing of LANL group [9]. Use of toluene (C6H5CH3) as a precursor gas enabled the high-speed deposition rate as well as the preparation of thick goodadhesion DLC film with the lower residual stress of approximately 0.35 GPa [10]. In this work, we present the preparation of the thick DLC film on the surface of a cylindrical pipe, three triangular prisms with the different wedge angles of 30 , 60 , and 90 , and three type of rectangular trenches with different aspect ratios using the hybrid process of PBIID and the toluene gas plasma. And we report on the different profiles of film thickness at the center and the edge of trench. The adhesion strength and the hardness of DLC film are also presented.
2. Experimental A schematic diagram of the hybrid PBIID system is shown in Fig. 1. In this system, the
Fig. 1. A schematic diagram of hybrid process of plasma-based ion implantation and deposition (PBIID) system.
RF pulse for plasma generation was supplied to the substrate together with a negative highvoltage pulse for ion implantation through a single electrical feed-through using a joint matching circuit for both pulses. As the substrate itself was used as an RF antenna, the uniform plasma was generated around the substrate and the largest plasma density appeared near the substrate [5]. The RF power supply had the maximum power of 3 kW, the pulse duration tRF of 5–50 ms, the frequency of 13.56 MHz, and the repetition rate f of 1–4 kHz. A negative highvoltage pulse train with the voltage of 5 to 20 kV, the duration of 1–5 ms, and the same repetition rate as the RF pulse was applied to the substrate at the time of 50 ms later after each RF pulse. The precursor gases used in the present experiment were the hydrocarbon gases, such as CH4, C2H2, and C6H5CH3. The procedure of DLC film preparation was the four steps as follows: The first step was the sputter-cleaning process using the plasma of mixed gases of Ar and CH4 (50:50%) with the negative high-voltage pulse (voltage Vb= 10 kV, duration t=5 ms, repetition rate f=1 kHz). In the second step, the carbon ion implantation with the negative high-voltage pulse (Vb= 20 kV, t=3 ms, f=1 kHz) was performed with the CH4 plasma to make carbon anchors inside the substrate and in the third step the C2H2 plasma was used to produce a
ARTICLE IN PRESS Y. Oka et al. / Vacuum 73 (2004) 541–547
3. Results and discussion 3.1. Residual stress The residual stress of DLC films prepared using C2H2 and C6H5CH3 plasmas was determined from the curvature of its substrate. Fig. 2 shows the residual compressive stress in the DLC film as a function of negative pulse voltage at the fourth step for deposition process, where tRF=50 ms, t=5 ms, T=2 h and the solid points and the circles indicate the residual stress of DLC films prepared using C2H2 and C6H5CH3 plasmas, respectively. As seen in Fig. 2, the compressive residual stress in the DLC film prepared using the C2H2 plasma is about 1.55 GPa with no negative high-voltage pulse. The residual stress considerably decreases with increasing the negative high-voltage during deposition and reduces to about 0.2 GPa at the negative high-voltage of 15 kV. The observed reduction of residual stress might be resulted from the local heating of substrate by deposition of ion beam energy [8]. On the other hand, the residual stress of DLC film prepared with the C6H5CH3 plasma is about 0.35 GPa and little dependent on the negative high-voltage, indicating that ion implantation is almost ineffective for a case of the C6H5CH3 plasma. The observation of plasma
2 Compressive stress (GPa)
graded mixing layer between the coating film and the substrate using the negative high-voltage pulse (Vb= 20 kV, t=3 ms, f=1 kHz). When skipping the second and the third steps, it was hard to obtain good adhesion of DLC film. The last fourth step was the rapid deposition process for production of thick film using the C6H5CH3 plasma and the negative high-voltage pulse of medium voltage (Vb= 10 kV, t=5 ms) and a high repetition rate (f=4 kHz). The gas pressure was p=0.3–0.5 Pa in the second and third steps and p=1.0 Pa during deposition of the fourth step. In the present experimental conditions, at the fourth step, the glow discharge was generated by the negative high-voltage pulse around the substrate. The typical deposition rate was approximately 1 mm/h using the C6H5CH3 plasma for deposition. Three kinds of substrates of aluminum-alloy (A5052) were used for DLC coating; a cylindrical pipe with a diameter of 25 mm and a length of 150 mm, three triangular prisms with each wedge angle (c=30 , 60 , and 90 ) and a length of 90 mm, and three type trenches with the different aspect ratio. The trench A has the aspect ratio of 0.5 and the trench width of 2 cm. The trench B has the aspect ratio of 1 and the trench width of 1 cm. The trench C has the aspect ratio of 2 and the trench width of 1 cm. All trenches have the same top width of 1 cm and the length of 4 cm. The substrate surface was polished to a mirror finish before DLC coating and set at the center of vacuum chamber. The film thickness was measured with the crosssectional SEM observation. The residual stress in each film was determined from the curvature of quartz glass plate (length of 25 mm, width of 5 mm and thickness of 0.5 mm) using Stoney’s equation, where the curvature was measured at both before and after film preparation using a stylus profilometer (Dektak3, ULVAC). The strength of adhesion was evaluated by the REVETEST scratch-testing equipment (CSEM). The damaged region along the scratch track was observed with a CCD microscope (VH-8000, KEYENCE). The hardness was measured with a nano-indentor (ENT1100, ELIONIX).
543
C2H2 C6H5CH3 1
0 0
5 10 15 Negative pulsed voltage (kV)
Fig. 2. The residual compressive stress of DLC film as a function of negative pulse voltage at the fourth step for deposition process, where the solid points and the circles indicate the residual stress of DLC films prepared using C2H2 and C6H5CH3 plasmas, respectively.
ARTICLE IN PRESS Y. Oka et al. / Vacuum 73 (2004) 541–547
544
Current (A)
Voltage (kV)
luminescence indicated the generation of a new discharge around the substrate at the same time with the application of negative high-voltage pulse in the fourth step of film preparation procedure [11]. The generation of glow discharge by the negative high-voltage pulse is also confirmed with the waveform of current through the substrate at the time of application of negative high-voltage pulse as shown in Fig. 3, where the pulse voltage was 10 kV, the repetition was 1 kHz and the toluene gas pressure was 1 Pa. The plasma generation by the negative high-voltage pulse leads to the disappearance of large ion-sheath potential around the substrate, so that there is little ion implantation as well as non-reduction of residual stress. In either case, the observed residual stress of the DLC film prepared by the present experimental conditions is almost two orders less than that of other methods [1,2]. Both the mixing layer formation and the reduction of residual stress by carbon-ion implantation could be useful for the sufficient-adhesive
0
-10
V = -10 kV
High-voltage pulse
coating of thick DLC film with more than a few mm in thickness on the aluminum alloy substrate. 3.2. Thickness profile on the trench surface Fig. 4 shows the SEM micrograph of crosssection of film on the center of top surface of trench A, where the deposition time of the fourth step was 4 h. As seen in Fig. 4, the film thickness is approximately 4.2 mm, indicating the deposition speed of about 1 mm/h. From the similar results of thickness measurement, the thickness profiles on the three-dimensional substrates were plotted. Fig. 5 shows the thickness profile of the cylindrical pipe surface at the various positions from the pipe edge, where the deposition time was 3 h, z is the distance from the pipe edge, and y is the deflection angle indicating the position on the circumference. As seen in Fig. 5, the DLC film of about 2 mm in thickness is very uniformly coated on the whole surface of cylindrical pipe. The thickness profile of DLC film on the triangle prism surface of different wedge angle (c=30 , 60 , and 90 ) is shown in Fig. 6, where the deposition time was 4 h, the tip of wedge is the origin of position, and the result at the origin was obtained at the position of 0.1 mm from the wedge tip. Fig. 6 indicates the almost uniform profile in film thickness even in the region around the wedge tip.
0 -5
RF pulse
DLC film
-10 0
50 Time (µs)
100
Fig. 3. The time history of negative high-voltage and current through the substrate at the fourth step for film preparation procedure, where the voltage and repetition of negative highvoltage pulse were 10 kV and 1 kHz, respectively, and the toluene gas pressure was 1 Pa.The waveform of current suggests the generation of glow discharge at the time of application of negative-high-voltage pulse.
Substrate
Fig. 4. Cross-sectional SEM micrograph of DLC coating film on the center of top surface of trench, where the deposition time of the fourth step was 4 h.
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2
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1
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Film thickness (µm)
Film thickness (µm)
0 z = 5 mm
ψ = 30°
2 1 0
ψ = 60°
2
2
1
1 0 z = 145 mm 45 90 135 180 225 270 315 Position on the circumference θ (°)
Fig. 5. Film thickness profile in the azimuthal direction on the cylindrical pipe circumference at the various axial positions.
Although not shown here, the film thickness on the trench A was almost uniform on each surface of top, sidewall, and bottom [10]. Fig. 7 shows the thickness profile of film at the center and the 3 mm from the edge for the trench B, where the deposition time was 4 h. As seen in Fig. 7, at the center of trench the film thickness on the top surface is about 1.9 times of that at the sidewall and 1.3 times of bottom. Although the thickness profile at the 3 mm from the edge is similar to the profile at the center, but the difference in thickness at each side becomes small. Fig. 8 shows the thickness profile of DLC film on the surface of trench C. In this case, at the trench center, the film thickness on the top surface is about two times of the sidewall one, while the thickness of bottom is slightly larger than the top one. However, it is found in Fig. 8 that the film thickness on the bottom at the trench edge is lower than that at the trench center. These results suggest that the negative-high-voltage pulse applied to the sub-
ψ = 90° -10 -8 -6 -4 -2 0 2 4 6 8 10 Position (mm)
Fig. 6. Film thickness profile on the triangular prism with different wedge angles.
8
Film thickness (µm)
0
0
center 3 mm from edge
6 4 2
Top 0
0
Sidewall 10
Bottom 20
30
Position (mm) Fig. 7. Film thickness profile on the trench B with the aspect ratio of 1, where the solid points and the circles indicate the film thickness at the center and the 3 mm from the edge, respectively.
strate leads to the acceleration of secondary electrons trapped in the trench and the generation of plasma inside the trench with the large aspect
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center 3 mm from edge
6
AE signal (arb. unit)
Film thickness (µm)
8
4 2 Top
0 0
Sidewall
10
20 30 Position (mm)
Lc
Bottom
40
Fig. 8. Film thickness profile on the trench C with the aspect ratio of 2, where the solid points and the circles indicate the film thickness at the center and the 3 mm from the edge, respectively.
0
5
10
15
Load (N) Fig. 9. Acoustic emission signal as a function of load of a diamond stylus of the scratch test, indicating the critical load of about 4 N.
ratio. The plasma luminescence inside the trench was observed as mentioned in Section 3.1 [11]. The observed increase in thickness of the bottom at the trench center should be ascribed to the plasma generation inside the trench by hollow cathode effect. 3.3. Adhesion strength and hardness The adhesive strength of DLC coating film on the top surface of trench was evaluated by the scratch test in which the load of a diamond stylus was increased progressively. Fig. 9 shows the acoustic emission in the scratch test as a function of load L of diamond stylus, where the diamond tip radius r=0.2 mm, the scratching speed dx/ dt=10 mm/min, and the loading rate dL/ dt=100 N/min. As seen in Fig. 9, the critical load Lc at which the acoustic emission signal increased suddenly is about 4 N. The critical load usually characterizes the adhesive strength, but it depends on several parameters affected by the testing conditions and the coating-substrate system. As the aluminum alloy substrate is so soft that the deformation of the substrate easily occurs during scratch test, the measured value of 4 N may be not really the actual adhesion strength of DLC film. The scratch track was observed with the CCD
100 µm Fig. 10. Scratch track observed with CCD micrograph.
micrograph, the result of which is shown in Fig. 10. The damaged region is found to be ‘‘scraping’’ and no ‘‘flaking’’ (adhesive failure) of coating film occurs during and after the scratch test. Fig. 11 shows the hardness and the maximum displacement of DLC film with the thickness in 4.7 mm as a function of maximum load of diamond pyramidal indenter. It is found in Fig. 11 that the hardness is 12.2 GPa at the maximum load of 0.5 g. Since the maximum penetration depth of indenter is 0.12 mm corresponding to about 1/40 of film
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Fig. 11. Hardness and maximum displacement of DLC film prepared using C6H5CH3 plasma.
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was about 1.3 times larger than that of bottom and 1.9 times than the sidewall. For aspect ratio of 2, the film thickness of bottom at the trench center was larger than the top one. The observed increase in thickness of bottom might be resulted from the plasma generation inside the trench by hollow cathode effect. The DLC film hardness measured by the nano-indentor was 12 GPa.
Acknowledgements This work was supported by the Cooperation of Innovative Technology and Advanced Research in Evolutional Area (CITY AREA).
References thickness, the observed hardness shows the real hardness of DLC itself with little effect of the substrate material.
4. Conclusion The hybrid process of plasma-based ion implantation and deposition, in which the substrate itself was used as a RF antenna for plasma production, resulted in the thick and uniform DLC coating of 4–5 mm in thickness on the threedimensional substrates. The use of toluene gas and ion implantation reduced the residual compressive stress of DLC film to 0.2–0.35 GPa. The DLC film thickness profile was almost uniform on the whole surfaces of cylindrical pipe and triangular prism. The DLC film with a few mm in thickness was formed on the whole surfaces of the trench with the aspect ratio of 1–2 and the trench width of 1 cm. For the trench with the aspect ratio of 1, the film thickness of top surface at the trench center
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