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Study on Vanadium Films Deposited on Concave Object by Conventional Direct Current and High Power Pulsed Magnetron Sputtering
Rare Metal Materials and Engineering Volume 42, Issue 12, December 2013 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2013, 42(12): 2437-2441.
ARTICLE
Study on Vanadium Films Deposited on Concave Object by Conventional Direct Current and High Power Pulsed Magnetron Sputtering Li Chunwei1,3, Gong Chunzhi1, 1
Tian Xiubo1,
Liu Tianwei2,
Qin Jianwei2,
Yang Jingjing1,
Yang Shiqin1 2
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China; State Key Laboratory of 3
Surface Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China; Northeast Forestry University, Harbin 150040, China
Abstract: High power pulsed magnetron sputtering (HPPMS) is a novel tool to fabricate films with high quality. In this paper, vanadium films on concave object have been deposited by HPPMS and conventional direct current magnetron sputtering (DCMS) under the condition of the same average power. The plasma composition, crystalline structure, surface morphology and film thickness have been investigated. The results show that the plasma produced by HPPMS is composed of Ar(1+), V(0) and a certain amount of V(1+). In contrast, the plasma produced by DCMS is composed of Ar(1+), V(0) and a very small amount of V(1+). Both films fabricated by HPPMS and DCMS demonstrate the similar microstructures. The HPPMS vanadium films are dense and flat on the top surface while the surface of DCMS vanadium films presents very sharp peak with larger height. The DCMS vanadium films exhibit a porous columnar grain structure. In contrast, the HPPMS vanadium films have slightly columnar and denser structure. The thickness of the HPPMS vanadium films is less than that of DCMS vanadium films. Compared with the surface on the top, the thickness of the DCMS vanadium films is decreased to about 32% at the side wall and to about 55% at the bottom. However, the HPPMS vanadium films can reach a thickness of about 35% at the side wall and 69% at the bottom relative to that on the top surface. HPPMS shows a better uniformity in the film thickness on concave object. Key words: high power pulsed magnetron sputtering; concave object; vanadium film; uniformity; film thickness
There are many practical components with concave surfaces such as mold, gear and cylinders, etc in industrial applications[1]. Physical vapor deposition (PVD) is usually applied to improve surface properties including wear-resistance[2], corrosion-resistance[3] and so on. Vacuum arc deposition is one of widely utilized PVD technique. However, the macroparticle from the cathode degrade the surface properties of deposited films[4,5]. As an alternative PVD technique, Magnetron sputtering (MS) is also used frequently in industries[6,7]. The adhesion between the film and substrate is usually poor due to the
lower ionization rate of sputtered metal particles[8,9]. High power pulse magnetron sputtering (HPPMS) is an ionized physical vapor deposition technique[10,11] first reported by Kouznetsov et al. HPPMS is gaining increasing interest in many applications due to its higher power density of several kW/cm2 and higher electronic densities of 1018/m3 compared to direct current magnetron sputtering (DCMS)[12,13]. HPPMS may produce the films with high adhesion force to the substrate[14], high film densification[15], better interface engineering through ion irradiation[16] and so on. However, the uni-
Received date: December 20, 2012 Foundation item: National Natural Science Foundation of China (51175118); Project supported by the open foundation of Science and Technology on Surface Physics and Chemistry Laboratory (SPC201104) Corresponding author: Tian Xiubo, Ph. D., Professor, State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, P. R. China, Tel: 0086-451-86418784, E-mail: [email protected] Copyright © 2013, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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formity of metal thin films on concave object by HPPMS technique has seldom been investigated. Since vanadium has many advantages including high temperature resistance, hydrochloric acid and sulfuric resistance, vanadium films have been deposited on concave object to achieve isolation from the surrounding environment[17]. In the present paper, vanadium thin films on concave object have been deposited by HPPMS and DCMS processes, respectively. The plasma composition, crystalline structure, surface morphology and film thickness have been investigated. The advantages of the HPPMS over DCMS have been presented.
1
Experiment
The treated sample was a cylinder (Φ100 mm×50 mm) with a concave hole (Φ40 mm×30 mm), the silicon strips and stainless steel strips acted as the samples, which were placed at the top surface, the side wall and the bottom of the concave object (Fig.1). The experiments were performed in the vacuum chamber with a size of Φ400 mm × 400 mm evacuated by a mechanical pump and a turbo-molecular pump (Fig.2). A metal vanadium cathode (50 mm in diameter, 6 mm thickness, and 99.95% purity) was powered by a hybrid pulsed power supply developed in our laboratory[18]. High purity (99.99%) argon was used as the discharge gas in the working chamber. The concave object was located at a distance of 12 cm from the target and no external heating was applied during the process. A base pressure below 5×10-3 Pa was reached prior to all depositions. At first, the substrates were the sputter etched by argon plasma bombardment at a 10 Pa working pressure using a pulsed middle frequency bias of –1000 V (40 kHz and 50% duty cycle) for 30 min. Then the vanadium films were deposited by DCMS and hybrid HPPMS methods with the same average power of 400 W. The processing parameters are listed in Table 1 and Table 2. The plasma composition was diagnosed by Optical Emission Spectroscope (OES, Avantes, Avaspec 3648). The crystalline structure and the preferred growth orientation were examined by an X-ray diffraction (XRD, Bruker, D8) with CuKa radiation. The nano-scale surface morphology and roughness were characterized by atomic force microscopy (AFM, Bruker, AXS Dimension Icon). Scanning electron microscopy was carried out on the cross section of fractured films using FEI Sirion 200 scanning electron microscope.
2
Results and Discussion
2.1 Plasma characteristics By using an optical emission spectroscopy, it is possible to
investigate the changes in the degree of ionization between the different discharge configurations. Fig.3 shows a comparison between the plasma optical emission spectra obtained from a HPPMS discharge and a DCMS discharge. The HPPMS and DCMS discharges were both excited at an average power of 400 W. Argon lines are dominant in region λ~700 nm. The plasma produced by DCMS is composed of Ar(1+), V(0) and a very small amount of V(1+). In contrast, the plasma produced by HPPMS is composed of Ar(1+), V(0) and a certain amount of V(1+), which exhibit a great difference from that of DCMS. To evaluate qualitatively the enhancement of vanadium metal ionization in the HPPMS plasma, we have compared the ratio of the emission intensities of V(1+) at 311.7 and 320.7 nm. V1+ (HPPMS)/V1+ (DCMS) are 4.2 and 3.9 at 311.7 and 320.7 nm, respectively. This effect is caused by in-
Fig.1 Schematic diagram of the three measurement positions of the concave sample
Fig.2 Schematic diagram of the used coating setup Table 1 Experimental parameters for vanadium films fabricated by DCMS DC Power /A /W 1.05 400
Argon flow rate/mL·min-1
DC bias voltage/V
Pressure/ Pa
Time/ min
10
–80
0.55
75
Table 2 Experimental parameters for vanadium films fabricated by HPPMS HPPMS voltage/V 575
HPPMS frequency/Hz 100
HPPMS pulse width/μs 200
DC/A 0.3
Power/ W 400
Argon flow rate/mL·min-1 10
DC bias voltage/V –80
Pressure/ Pa 0.55
Time/ min 75
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60000
V(0)
600000
V(0) Ar(1+) V(0) Ar(1+) V(0)
20000
V(1+) V(1+)
DCMS
Ar
V(1+)
20000 0 200
400
V(0)
40000 V(0) Ar(1+) V(0)
Intensity/cps
HPPMS
Ar
40000
600 800 Wavelength/nm
1000
1200
Fig.3 Optical emission spectroscopies of vanadium discharge for HPPMS and DCMS
tensive sputtering of metals during HPPMS and their preferential ionization[19]. It is well known that sputtered particles are highly ionized in HPPMS discharges. 2.2 Phase structure To observe the phase composition the vanadium films were analyzed by means of X-ray diffraction as illustrated in Fig.4. The HPPMS and DCMS vanadium films are only highly textured with a preferential orientation in the (111) direction with the corresponding 2θ of ~41.5°. For vanadium films prepared by above two methods, the variations of diffraction peaks intensity are similar. The diffraction peak of (111) is the strongest and the preferred orientation is the most obvious at the top of concave object. The peaks intensities of (111) decrease at the bottom and the side wall. The peaks intensities of (111) are the weakest at the side wall. It is also noted that the diffraction peaks of the DCMS films at the side wall shift to lower diffraction angles as compared to that of the films at the top and
the bottom. It is probably due to the higher stress[20]. The film stress increases with the film growing thinner[21]. The thickness of DC vanadium films at the side wall is thinner and the film stress is higher. 2.3 AFM surface topography analysis Three-dimensional surface morphologies of vanadium films fabricated by HPPMS and DCMS were measured by AFM as demonstrated in Fig.5a~5f, respectively. The surface of vanadium films fabricated by HPPMS are all dense and flat at different positions of the concave sample. In general, the surface morphologies of vanadium films at the top, the side wall and the bottom are much the same, which exhibit a very good uniformity. In contrast, the surface at different positions of DCMS vanadium films on the concave sample show great difference. The film surface presents very sharp peaks with high height. There are much deeper concave zones between dendritic islands. The distribution density of dendritic islands is very large. In order to further study the uniformity of surface morphologies of vanadium films, the mean surface roughness (Ra) was tested. As shown in Fig.6, Ra of vanadium films at the top, the side wall and the bottom are 4.2, 1.68 and 0.542 nm for HPPMS process. In contrast, Ra of DCMS vanadium films at the top, the side wall and the bottom are 12.3, 5.08 and 1.56 nm, respectively. The lower roughness by HPPMS technique may be attributed to the high mobility of the adatom induced by bombardment with highly energetic ions[22]. 2.4 Cross-sectional structure Fig.7a~7f shows the cross-sectional SEM images of the b
a
2.0 μm 1.5 1.0 0.5
DCMS bottom
Intensity/a.u.
Substrate
0.5
1.5
1.0
2.0 μm
2.0 μm 0.5
0.5
HPPMS bottom
HPPMS side wall
HPPMS top
60
1.5
1.0
d
2.0 μm 1.5
V(111)
45
0.5
DCMS side wall
1.0
30
0.5
c
DCMS top
15
2.0 μm 1.5 1.0 2.0 μm
75
90
0.5
1.0
1.5
2.0 μm
1.0 1.5
0.5
1.0
1.5
2.0 μm
f
e
2.0 μm 1.5
2.0 μm 1.5 1.0 0.5
0.5
1.0
1.5
2.0 μm
1.0 0.5
0.5
1.0
1.5
2.0 μm
105
2θ/(°)
Fig.5 Typical three-dimensional AFM images of the as-deposited vanadium films: (a) HPPMS top, (b) DCMS top, (c) HPPMS
Fig.4 XRD patterns of the vanadium films fabricated by HPPMS and DCMS
side wall, (d) DCMS side wall, (e) HPPMS bottom, and (f) DC bottom
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9
6
6
3
3 0
0 Top
Side wall
Bottom
Position Fig.6 Surface roughness of vanadium films fabricated by HPPMS and DCMS a
b
c
d
e
HPPMS an additional ionization of the metal ions happens leading to larger ion bombardment. The thickness of the deposited vanadium films was determined from the cross-sectional SEM images. Fig.8 shows the thickness of vanadium films on the concave object. Overall, the thicknesses of HPPMS vanadium films are less than those of DCMS. HPPMS was reported to suppress the deposition rate compared to conventional DCMS processes at the same average power[24]. Christie developed a simple pathways model to explain the experimental results that some of ionized particles could be attracted and went back to the target due to the large target potential (~kV) and the low potential on the samples. Consequently, these metal ions flying to the target are not available for the samples and a lower deposition rate is achieved[25,26]. The thicknesses of DCMS vanadium films decreased to about 32% at the side wall and to about 55% at the bottom compared to that of films at the top surface. In contrast, HPPMS vanadium films could reach a thickness of about 35% at the side wall and 69% at the bottom relative to the top surface. This shows the advantage of HPPMS to coat the object with complex geometries. N. Bagcivan et al.[27] also reported that the uniformity of the thickness on the sample like gear wheels can be increased by using HPPMS. 1800 1600 1400 1200 1000 800 600 400 200 0
HPPMS DCMS
Thickness of Vandium Film/nm
9
DCMS, Ra/nm
12
HPPMS, Ra/nm
12
f
Top
Side wall Position
Bottom
Fig.8 Thickness of vanadium films on the concave object surface Fig.7 Cross-sectional SEM images of the as-deposited vanadium films: (a) HPPMS top, (b) DCMS top, (c) HPPMS side wall, (d) DCMS side wall, (e) HPPMS bottom, and (f) DC bottom
as-deposited vanadium films. The DCMS vanadium films exhibit a porous columnar grain structure, which contains long columnar grains and clear grain boundaries throughout the film thickness. In contrast, the HPPMS vanadium films have a slightly columnar structure as well, which exhibits a denser columnar grain structure. The morphology and the density are functions of the energy induced to the substrate[23]. This energy could be introduced thermally or kinetically. By using higher energetic species the adatom mobility increases and is denser, a fine grained microstructure can be formed. Using
3
Conclusions
1) The plasma produced by HPPMS is composed of Ar(1+), V(0) and a certain amount of V(1+). In contrast, the plasma produced by DCMS is composed of Ar(1+), V(0) and a very small amount of V(1+). 2) The HPPMS and DCMS vanadium films are highly textured with a preferential orientation in the (111) direction. 3) The HPPMS vanadium films were all dense and flat with slightly columnar structure. In contrast, DCMS vanadium films present a rough surface of very sharp peak with long columnar grains and clear grain boundaries throughout the film thickness. 4) HPPMS produces thinner vanadium films than DCMS, the films demonstrate a higher uniformity in thickness.
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References 1 Zhu Zongtao, Tian Xiubo, Wang Zhijian et al. Surface and Coatings Technology[J], 2011, 206: 2021 2 Liu Xing, Ma Guojia, Zhang Lin et al. Rare Metal Materials and Engineering[J], 2012, 41(4): 777 (in Chinese) 3 Wang Yujiang, Wei Shicheng, Liu Yi et al. Rare Metal Materials and Engineering[J], 2012, 41(2): 323 (in Chinese) 4 Fu Zhiqiang, Wang Chengbiao, Li Jinli et al. Rare Metal Materials and Engineering[J], 2010, 39(2): 316 (in Chinese) 5 Wei Yongqiang, Gong Chunzhi, Tian Xiubo et al. Rare Metal Materials and Engineering[J], 2009, 38(4): 788 (in Chinese) 6 Xie Tingting, Mao Shoudong, Yu Chao et al. Vacuum[J], 2012, 86: 1583 7 Ganeva M, Pipa A V, Hippler R. Surface and Coatings Technology[J], 2012, 213: 41 8 Lin Jianliang, Zhang Ningyi, Sproul William D et al. Surface and Coatings Technology[J], 2012, 206: 3283 9 Casper P, Holger H, Christina B et al. Surface and Coatings Technology[J], 2011, 205: S119 10 Kouznetsov V, Macák K, Schneider J M et al. Surface and Coatings Technology[J], 1999, 122: 290 11 Li Chunwei, Tian Xiubo, Liu Tianwei et al. Rare Metal Materials and Engineering[J], 2013, 42(1): 109 (in Chinese) 12 Ehiasarian P, Gonzalvo Y A, Whitmore T D. Plasma Processes Polym[J], 2007, 4: 5309 13 Lin J, Moore J, Sproul W et al. Surface and Coatings Technol-
ogy[J], 2010, 204: 2230 14 Emerlich J, Mraz S, Snyders R et al. Vacuum[J], 2008, 82: 867 15 Paulitsch J, Mayrhofer P H, Munz W D et al. Thin Solid Films[J], 2008, 517: 1239 16 Ehiasarian A P, Muz W D, Hultman L et al. Surface and Coatings Technology[J], 2003, 163: 267 17 Zinkle S J. Fusion Engineering and Design[J], 2005, 74: 31 18 Gui Gang, Tian Xiubo, Zhu Zongtao et al. Chinese Vacuum[J], 2011, 48: 46 (in Chinese) 19 Vitezslav Stranak, Steffen Drache, Robert Bogdanowicz et al. Surface and Coatings Technology[J], 2012, 206: 2801 20 Lin Jianliang, William D Sproul, John J Moore et al. Surface and Coatings Technology[J], 2011, 206: 1780 21 Machunze R, Ehiasarian A P, Tichelaar F D et al. Thin Solid Films[J], 2009, 518: 1561 22 Ehiasarian A P, Eh Hovsepian P, L Hultman et al. Thin Solid Films[J], 2004, 457: 270 23 Thornton J A. Journal of Vacuum Science & Technology[J], 1986, 4: 3059 24 Broitman E, Czigany Zs, Greczynski G et al. Surface and Coatings Technology[J], 2010, 204: 3349 25 Christie D J. Journal of Vacuum Science & Technology[J], 2005, 23: 330 26 Christie D J, Czech. Journal of Physics[J], 2006, 56: B93 27 Bagcivan N, Bobzin K, Thei S. Contributions to Plasma Physics[J], 2012, 52: 601
2441
ARTICLE
Study on Vanadium Films Deposited on Concave Object by Conventional Direct Current and High Power Pulsed Magnetron Sputtering Li Chunwei1,3, Gong Chunzhi1, 1
Tian Xiubo1,
Liu Tianwei2,
Qin Jianwei2,
Yang Jingjing1,
Yang Shiqin1 2
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China; State Key Laboratory of 3
Surface Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China; Northeast Forestry University, Harbin 150040, China
Abstract: High power pulsed magnetron sputtering (HPPMS) is a novel tool to fabricate films with high quality. In this paper, vanadium films on concave object have been deposited by HPPMS and conventional direct current magnetron sputtering (DCMS) under the condition of the same average power. The plasma composition, crystalline structure, surface morphology and film thickness have been investigated. The results show that the plasma produced by HPPMS is composed of Ar(1+), V(0) and a certain amount of V(1+). In contrast, the plasma produced by DCMS is composed of Ar(1+), V(0) and a very small amount of V(1+). Both films fabricated by HPPMS and DCMS demonstrate the similar microstructures. The HPPMS vanadium films are dense and flat on the top surface while the surface of DCMS vanadium films presents very sharp peak with larger height. The DCMS vanadium films exhibit a porous columnar grain structure. In contrast, the HPPMS vanadium films have slightly columnar and denser structure. The thickness of the HPPMS vanadium films is less than that of DCMS vanadium films. Compared with the surface on the top, the thickness of the DCMS vanadium films is decreased to about 32% at the side wall and to about 55% at the bottom. However, the HPPMS vanadium films can reach a thickness of about 35% at the side wall and 69% at the bottom relative to that on the top surface. HPPMS shows a better uniformity in the film thickness on concave object. Key words: high power pulsed magnetron sputtering; concave object; vanadium film; uniformity; film thickness
There are many practical components with concave surfaces such as mold, gear and cylinders, etc in industrial applications[1]. Physical vapor deposition (PVD) is usually applied to improve surface properties including wear-resistance[2], corrosion-resistance[3] and so on. Vacuum arc deposition is one of widely utilized PVD technique. However, the macroparticle from the cathode degrade the surface properties of deposited films[4,5]. As an alternative PVD technique, Magnetron sputtering (MS) is also used frequently in industries[6,7]. The adhesion between the film and substrate is usually poor due to the
lower ionization rate of sputtered metal particles[8,9]. High power pulse magnetron sputtering (HPPMS) is an ionized physical vapor deposition technique[10,11] first reported by Kouznetsov et al. HPPMS is gaining increasing interest in many applications due to its higher power density of several kW/cm2 and higher electronic densities of 1018/m3 compared to direct current magnetron sputtering (DCMS)[12,13]. HPPMS may produce the films with high adhesion force to the substrate[14], high film densification[15], better interface engineering through ion irradiation[16] and so on. However, the uni-
Received date: December 20, 2012 Foundation item: National Natural Science Foundation of China (51175118); Project supported by the open foundation of Science and Technology on Surface Physics and Chemistry Laboratory (SPC201104) Corresponding author: Tian Xiubo, Ph. D., Professor, State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, P. R. China, Tel: 0086-451-86418784, E-mail: [email protected] Copyright © 2013, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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formity of metal thin films on concave object by HPPMS technique has seldom been investigated. Since vanadium has many advantages including high temperature resistance, hydrochloric acid and sulfuric resistance, vanadium films have been deposited on concave object to achieve isolation from the surrounding environment[17]. In the present paper, vanadium thin films on concave object have been deposited by HPPMS and DCMS processes, respectively. The plasma composition, crystalline structure, surface morphology and film thickness have been investigated. The advantages of the HPPMS over DCMS have been presented.
1
Experiment
The treated sample was a cylinder (Φ100 mm×50 mm) with a concave hole (Φ40 mm×30 mm), the silicon strips and stainless steel strips acted as the samples, which were placed at the top surface, the side wall and the bottom of the concave object (Fig.1). The experiments were performed in the vacuum chamber with a size of Φ400 mm × 400 mm evacuated by a mechanical pump and a turbo-molecular pump (Fig.2). A metal vanadium cathode (50 mm in diameter, 6 mm thickness, and 99.95% purity) was powered by a hybrid pulsed power supply developed in our laboratory[18]. High purity (99.99%) argon was used as the discharge gas in the working chamber. The concave object was located at a distance of 12 cm from the target and no external heating was applied during the process. A base pressure below 5×10-3 Pa was reached prior to all depositions. At first, the substrates were the sputter etched by argon plasma bombardment at a 10 Pa working pressure using a pulsed middle frequency bias of –1000 V (40 kHz and 50% duty cycle) for 30 min. Then the vanadium films were deposited by DCMS and hybrid HPPMS methods with the same average power of 400 W. The processing parameters are listed in Table 1 and Table 2. The plasma composition was diagnosed by Optical Emission Spectroscope (OES, Avantes, Avaspec 3648). The crystalline structure and the preferred growth orientation were examined by an X-ray diffraction (XRD, Bruker, D8) with CuKa radiation. The nano-scale surface morphology and roughness were characterized by atomic force microscopy (AFM, Bruker, AXS Dimension Icon). Scanning electron microscopy was carried out on the cross section of fractured films using FEI Sirion 200 scanning electron microscope.
2
Results and Discussion
2.1 Plasma characteristics By using an optical emission spectroscopy, it is possible to
investigate the changes in the degree of ionization between the different discharge configurations. Fig.3 shows a comparison between the plasma optical emission spectra obtained from a HPPMS discharge and a DCMS discharge. The HPPMS and DCMS discharges were both excited at an average power of 400 W. Argon lines are dominant in region λ~700 nm. The plasma produced by DCMS is composed of Ar(1+), V(0) and a very small amount of V(1+). In contrast, the plasma produced by HPPMS is composed of Ar(1+), V(0) and a certain amount of V(1+), which exhibit a great difference from that of DCMS. To evaluate qualitatively the enhancement of vanadium metal ionization in the HPPMS plasma, we have compared the ratio of the emission intensities of V(1+) at 311.7 and 320.7 nm. V1+ (HPPMS)/V1+ (DCMS) are 4.2 and 3.9 at 311.7 and 320.7 nm, respectively. This effect is caused by in-
Fig.1 Schematic diagram of the three measurement positions of the concave sample
Fig.2 Schematic diagram of the used coating setup Table 1 Experimental parameters for vanadium films fabricated by DCMS DC Power /A /W 1.05 400
Argon flow rate/mL·min-1
DC bias voltage/V
Pressure/ Pa
Time/ min
10
–80
0.55
75
Table 2 Experimental parameters for vanadium films fabricated by HPPMS HPPMS voltage/V 575
HPPMS frequency/Hz 100
HPPMS pulse width/μs 200
DC/A 0.3
Power/ W 400
Argon flow rate/mL·min-1 10
DC bias voltage/V –80
Pressure/ Pa 0.55
Time/ min 75
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60000
V(0)
600000
V(0) Ar(1+) V(0) Ar(1+) V(0)
20000
V(1+) V(1+)
DCMS
Ar
V(1+)
20000 0 200
400
V(0)
40000 V(0) Ar(1+) V(0)
Intensity/cps
HPPMS
Ar
40000
600 800 Wavelength/nm
1000
1200
Fig.3 Optical emission spectroscopies of vanadium discharge for HPPMS and DCMS
tensive sputtering of metals during HPPMS and their preferential ionization[19]. It is well known that sputtered particles are highly ionized in HPPMS discharges. 2.2 Phase structure To observe the phase composition the vanadium films were analyzed by means of X-ray diffraction as illustrated in Fig.4. The HPPMS and DCMS vanadium films are only highly textured with a preferential orientation in the (111) direction with the corresponding 2θ of ~41.5°. For vanadium films prepared by above two methods, the variations of diffraction peaks intensity are similar. The diffraction peak of (111) is the strongest and the preferred orientation is the most obvious at the top of concave object. The peaks intensities of (111) decrease at the bottom and the side wall. The peaks intensities of (111) are the weakest at the side wall. It is also noted that the diffraction peaks of the DCMS films at the side wall shift to lower diffraction angles as compared to that of the films at the top and
the bottom. It is probably due to the higher stress[20]. The film stress increases with the film growing thinner[21]. The thickness of DC vanadium films at the side wall is thinner and the film stress is higher. 2.3 AFM surface topography analysis Three-dimensional surface morphologies of vanadium films fabricated by HPPMS and DCMS were measured by AFM as demonstrated in Fig.5a~5f, respectively. The surface of vanadium films fabricated by HPPMS are all dense and flat at different positions of the concave sample. In general, the surface morphologies of vanadium films at the top, the side wall and the bottom are much the same, which exhibit a very good uniformity. In contrast, the surface at different positions of DCMS vanadium films on the concave sample show great difference. The film surface presents very sharp peaks with high height. There are much deeper concave zones between dendritic islands. The distribution density of dendritic islands is very large. In order to further study the uniformity of surface morphologies of vanadium films, the mean surface roughness (Ra) was tested. As shown in Fig.6, Ra of vanadium films at the top, the side wall and the bottom are 4.2, 1.68 and 0.542 nm for HPPMS process. In contrast, Ra of DCMS vanadium films at the top, the side wall and the bottom are 12.3, 5.08 and 1.56 nm, respectively. The lower roughness by HPPMS technique may be attributed to the high mobility of the adatom induced by bombardment with highly energetic ions[22]. 2.4 Cross-sectional structure Fig.7a~7f shows the cross-sectional SEM images of the b
a
2.0 μm 1.5 1.0 0.5
DCMS bottom
Intensity/a.u.
Substrate
0.5
1.5
1.0
2.0 μm
2.0 μm 0.5
0.5
HPPMS bottom
HPPMS side wall
HPPMS top
60
1.5
1.0
d
2.0 μm 1.5
V(111)
45
0.5
DCMS side wall
1.0
30
0.5
c
DCMS top
15
2.0 μm 1.5 1.0 2.0 μm
75
90
0.5
1.0
1.5
2.0 μm
1.0 1.5
0.5
1.0
1.5
2.0 μm
f
e
2.0 μm 1.5
2.0 μm 1.5 1.0 0.5
0.5
1.0
1.5
2.0 μm
1.0 0.5
0.5
1.0
1.5
2.0 μm
105
2θ/(°)
Fig.5 Typical three-dimensional AFM images of the as-deposited vanadium films: (a) HPPMS top, (b) DCMS top, (c) HPPMS
Fig.4 XRD patterns of the vanadium films fabricated by HPPMS and DCMS
side wall, (d) DCMS side wall, (e) HPPMS bottom, and (f) DC bottom
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9
6
6
3
3 0
0 Top
Side wall
Bottom
Position Fig.6 Surface roughness of vanadium films fabricated by HPPMS and DCMS a
b
c
d
e
HPPMS an additional ionization of the metal ions happens leading to larger ion bombardment. The thickness of the deposited vanadium films was determined from the cross-sectional SEM images. Fig.8 shows the thickness of vanadium films on the concave object. Overall, the thicknesses of HPPMS vanadium films are less than those of DCMS. HPPMS was reported to suppress the deposition rate compared to conventional DCMS processes at the same average power[24]. Christie developed a simple pathways model to explain the experimental results that some of ionized particles could be attracted and went back to the target due to the large target potential (~kV) and the low potential on the samples. Consequently, these metal ions flying to the target are not available for the samples and a lower deposition rate is achieved[25,26]. The thicknesses of DCMS vanadium films decreased to about 32% at the side wall and to about 55% at the bottom compared to that of films at the top surface. In contrast, HPPMS vanadium films could reach a thickness of about 35% at the side wall and 69% at the bottom relative to the top surface. This shows the advantage of HPPMS to coat the object with complex geometries. N. Bagcivan et al.[27] also reported that the uniformity of the thickness on the sample like gear wheels can be increased by using HPPMS. 1800 1600 1400 1200 1000 800 600 400 200 0
HPPMS DCMS
Thickness of Vandium Film/nm
9
DCMS, Ra/nm
12
HPPMS, Ra/nm
12
f
Top
Side wall Position
Bottom
Fig.8 Thickness of vanadium films on the concave object surface Fig.7 Cross-sectional SEM images of the as-deposited vanadium films: (a) HPPMS top, (b) DCMS top, (c) HPPMS side wall, (d) DCMS side wall, (e) HPPMS bottom, and (f) DC bottom
as-deposited vanadium films. The DCMS vanadium films exhibit a porous columnar grain structure, which contains long columnar grains and clear grain boundaries throughout the film thickness. In contrast, the HPPMS vanadium films have a slightly columnar structure as well, which exhibits a denser columnar grain structure. The morphology and the density are functions of the energy induced to the substrate[23]. This energy could be introduced thermally or kinetically. By using higher energetic species the adatom mobility increases and is denser, a fine grained microstructure can be formed. Using
3
Conclusions
1) The plasma produced by HPPMS is composed of Ar(1+), V(0) and a certain amount of V(1+). In contrast, the plasma produced by DCMS is composed of Ar(1+), V(0) and a very small amount of V(1+). 2) The HPPMS and DCMS vanadium films are highly textured with a preferential orientation in the (111) direction. 3) The HPPMS vanadium films were all dense and flat with slightly columnar structure. In contrast, DCMS vanadium films present a rough surface of very sharp peak with long columnar grains and clear grain boundaries throughout the film thickness. 4) HPPMS produces thinner vanadium films than DCMS, the films demonstrate a higher uniformity in thickness.
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