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Improvement in thermal stability of B–C–N thin films fabricated by magnetron sputtering using graphite and BN co-target
ARTICLE IN PRESS
Vacuum 80 (2006) 748–751 www.elsevier.com/locate/vacuum
Improvement in thermal stability of B–C–N thin films fabricated by magnetron sputtering using graphite and BN co-target H. Yokomichia,, T. Futakuchib, H. Amekurac, N. Kishimotoc a
Department of Electronics and Informatics, Toyama Prefectural University, Kosugi, Toyama 9390398, Japan Machinery and Electronic Research Institute, Toyama Industrial Technology Center, Takada, Toyama 9308511, Japan c Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 3050003, Japan
b
Abstract Thermal stability of amorphous boron–carbon–nitride (B–C–N) films fabricated by magnetron sputtering using a graphite and BN cotarget was studied by X-ray photoelectron spectroscopy, infrared absorption spectroscopy, Raman spectroscopy and scanning electron microscopy. The boron-rich B–C–N films showed the higher thermal stability. These results suggest that an incorporation of B atoms into the amorphous CN networks improves the thermal stability. This improvement can be explained in terms of the creation of boron–nitrogen bonds and/or boron–carbon bonds. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sputtering; Thermal stability; B–C–N films
1. Introduction Boron–carbon–nitride (B–C–N) thin films are attractive materials, because these films have potential applications for surface coatings and electronic devices. Accordingly, these films were prepared by various methods, e.g., thermal chemical vapor deposition (CVD) [1,2], bias-assisted hotfilament CVD [3] and electron-cyclotron-resonance plasma-assisted physical vapor deposition [4]. We have fabricated the amorphous B–C–N films by magnetron sputtering using a graphite and boron nitride (BN) cotarget in Ar or N2 gas atmosphere [5] and by hot-wire CVD using acetylene (C2H2) and boron trichloride (BCl3) gases [6] in order to use surface coatings. An undesired aging effect, e.g., peeling off the substrate after air exposure, was observed for the amorphous carbon nitride (a-CN) films fabricated by magnetron sputtering [7]. However, the boron incorporation into carbon nitride networks, i.e., the creation of B–C–N networks, improved the aging effect Corresponding author. Tel.: +81 766 56 7500; fax: +81 766 56 8021.
E-mail address: [email protected] (H. Yokomichi). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.040
[5]. Since humidity is the main cause for the aging effect, this result indicates that stability of B–C–N films against humidity is higher than that of CN films. High thermal stability is expected on the analogy of improvement against humidity. Thermal stability at high temperatures as well as the stability against humidity is important for surface coatings. Although the a-CN films have poor thermal stability, higher thermal stability is expected for the B–C–N films as mentioned above. In the present study, we examined thermal stability of the B–C–N films fabricated by magnetron sputtering. 2. Experimental Three types of B–C–N films, SP#16, SP#21 and SP#22, were chosen for the present study. The B–C–N films were fabricated by magnetron sputtering using the graphite and BN co-target in Ar or N2 gas atmosphere. The BN targets with a diameter of 5 mm were prepared by pressing hexagonal boron nitride (h-BN) powders. The BN targets were put on the erosion region of the graphite target with a
ARTICLE IN PRESS H. Yokomichi et al. / Vacuum 80 (2006) 748–751
Table 1 The sample fabrication conditions and chemical components for the three B–C–N films Sample number
SP#16 SP#21 SP#22
Sputtering gas
Number of BN pellets
The color of SP#16 sample after annealing drastically changed from dark red to black. Although surface roughness observed by SEM increased for SP#16 after annealing at 300 1C, those of SP#21 and #22 were unchanged after
100 B C
80
N 60 SP#22 40
20
0
0
100
200 300 400 500 600 Annealing Temperature (°C)
800
SP#21
as-depo
(1, 68, 31) (17, 69, 14) (7, 63, 30)
100
500
1000
1500
2000
2500
3000
3500
4000
4500
-1)
Wavenumber (cm
B C
80 Concentration (%)
700
Fig. 2. The changes of boron, carbon and nitrogen concentrations for the SP#22 sample after thermal annealing.
B, C and N concentrations (B, C, N)
16 33 33
N2 Ar N2
3. Results and discussion
Concentration (%)
diameter of 100 mm. The picture of the co-target is shown in Ref. [5]. The co-target was sputtered at an rf power of 100 W under the substrate temperature of 30 1C. A total gas flow rate and a pressure during sputtering were 2 sccm and 0.27 Pa, respectively. The details of the sample fabrication are described in Ref. [5]. The boron, carbon and nitrogen concentrations (B, C, N), of SP#16, SP#21 and SP#22 samples before annealing were (1, 68, 31), (17, 69, 14) and (7, 63, 30), respectively. The typical features of the three B–C–N films are as follows: (i) SP#16 with a low boron concentration and a high nitrogen concentration, (ii) SP#21 with a high boron concentration and a medium nitrogen concentration and (iii) SP#22 with a medium boron concentration and a high nitrogen concentration. The fabrication conditions and chemical components for the three B–C–N films were summarized in Table 1. The B–C–N films kept in a sample case were annealed at 300, 500 and 620 1C in a vacuum of 4.0 10 3 Pa for 1 h. The film properties were characterized by X-ray photoelectron spectroscopy (XPS), infrared (IR) absorption spectroscopy, micro-Raman spectroscopy and scanning electron microscopy (SEM).
749
N
620˚C
SP#21
60 SP#21 40
20
0
500 0
100
200
300
400
500
600
700
800
1000
1500
2000
2500
3000
3500
4000
4500
Wavenumber (cm-1)
Annealing Temperature (°C) Fig. 1. The changes of boron, carbon and nitrogen concentrations for the SP#21 sample after thermal annealing.
Fig. 3. IR spectra of the SP#21 sample before and after thermal annealing at 620 1C. Raman G and D bands are indicated by dashed and solid lines, respectively.
ARTICLE IN PRESS H. Yokomichi et al. / Vacuum 80 (2006) 748–751
750
SP#16
SP#21
as-depo
as-depo
0
500
1000
1500
2000
0
500
Wavenumber (cm-1)
0
500
1000
0
500
SP#21
1500
2000
0
500
1500
2000
1500
2000
500°C
1000 Wavenumber (cm-1)
SP#21
620°C
1000
2000
Wavenumber (cm
Wavenumber (cm-1)
SP#16
1500 -1)
500°C
SP#16
1000
1500
2000
Wavenumber (cm-1)
0
500
620°C
1000 Wavenumber (cm-1)
Fig. 4. Micro-Raman spectra of the SP#16 sample before and after thermal annealing. Raman G and D bands are indicated by dashed and solid lines, respectively.
Fig. 5. Micro-Raman spectra of the SP#21 sample before and after thermal annealing. Raman G and D bands are indicated by dashed and solid lines, respectively.
the same annealing. Consequently, SP#16 sample has poor thermal stability. The XPS measurement for SP#16 after annealing could not be performed, because the film after annealing was peeled off the substrate. The changes of (B, C, N) concentrations for SP#21 and SP#22 samples after annealing are shown in Figs. 1 and 2, respectively. These results are not estimated by in situ XPS measurements. Accordingly, the XPS spectra due to oxygen were observed more or less in all the films. However, the existence of oxygen was excluded from the composition, since the XPS measurement is very sensitive to the oxygen adsorption. The carbon and nitrogen concentrations decreased after annealing in the both samples. However, the boron concentration increased, particularly a remarkable increase
was observed in SP#21. These results indicate that carbon and nitrogen atoms are easily effused from the films as compared with boron. The IR spectra for SP#21 sample before and after annealing are shown in Fig. 3. The peaks centered around 1340 and 1550 cm 1 are identified as due to Raman G and D bands, respectively [8]. It is reported that the signal around 1330 cm 1 is due to the sp2 BN vibration mode [9]. Accordingly, the peak around 1340 cm 1 may include the sp2 BN vibration mode. Another signals around 1050 and 1620 cm 1 were observed for the film after 620 1C annealing. An intensity in the signal around 1050 cm 1 increased after annealing, corresponding to the increase in boron concentration after annealing. Thus, the signal around
ARTICLE IN PRESS H. Yokomichi et al. / Vacuum 80 (2006) 748–751
1050 cm 1 is consistent with the sp3 BN characteristic mode [9]. The signal around 1620 cm 1 may be associated with NH2 stretching mode [9]. However, the possibility of the B-related vibration mode may not be excluded [5,6]. The micro-Raman spectra for SP#16 and SP#21 before and after annealing were shown in Figs. 4 and 5, respectively. The signals around 1380 and 1570 cm 1 are attributed to Raman G and D bands, respectively [8]. The significant decrease in Raman G and D bands for the SP#16 after 500 1C annealing indicates the thermal decomposition of the film. However, the intensity of Raman G and D bands increases for the SP#21 and SP#22 after 500 1C annealing, indicating that thermal stability of the B–C–N films, SP#21 and SP#22, is improved by boron incorporation. Furthermore, the linewidth of Raman G and D bands reduced for the SP#21 after annealing. This result may be associated with occurrence of crystallization. Another signals around 480 and 700 cm 1 were observed for the samples after annealing. The signals around 480 and 700 cm 1 are tentatively identified as due to oxygen-related and/or silicon-related vibration modes (silicon substrate was used), taking account of the increase in oxygen concentration and the decrease in film thickness after annealing. 4. Summary Thermal stability of B–C–N films fabricated by sputtering using the graphite and BN co-target was studied for three types of B–C–N films by XPS, IR, micro-Raman and
751
SEM. Although the intensity of Raman G and D bands in Raman spectra for the B–C–N film with a low boron concentration decreased after annealing, that for boronrich B–C–N films, SP#21 and SP#22 increased after annealing. The IR spectra for SP#21 unchanged after annealing. These results indicate that the B–C–N film with a low boron concentration has poorer thermal stability. However, the thermal stability is improved by incorporation of boron into amorphous CN networks. The carbon and boron concentrations decreased, but the boron concentrations increased after annealing. Therefore, this improvement may be explained in terms of the creation of boron–nitrogen bonds and/or boron–carbon bonds.
References [1] Kouvetakis J, Kaner RB, Sattler ML, Martlett N. J Chem Soc Chem Commun 1986:1758. [2] Kaner RB, Kouvetakis J, Warble CE, Sattler ML, Bartlett N. Mater Res Bull 1987;22:399. [3] Yu JE, Wang EG. Appl Phys Lett 1999;74:2948. [4] Tempez A, Badi N, Bensaoula A, Kulik J. J Vac Sci Technol 1998;A16:2896. [5] Yokomichi H, Funakawa T, Masuda A. Vacuum 2002;66:245. [6] Yokomichi H, Futakuchi T, Yasuoka M, Kishimoto N. J Non-Cryst Solids 2004;338–340:509. [7] Yokomichi H, Sakima H, Masuda A. Jpn J Appl Phys 1998;37:4722. [8] Victoria NM, Hammer P, dos Santos MC, Alvarez F. Phys Rev B 2000;61:1083. [9] Laidani N, Andele M, Canteri R, Elia L, Luches A, Martino M, Micheli V, Speranza G. Appl Surf Sci 2000;157:135.
Vacuum 80 (2006) 748–751 www.elsevier.com/locate/vacuum
Improvement in thermal stability of B–C–N thin films fabricated by magnetron sputtering using graphite and BN co-target H. Yokomichia,, T. Futakuchib, H. Amekurac, N. Kishimotoc a
Department of Electronics and Informatics, Toyama Prefectural University, Kosugi, Toyama 9390398, Japan Machinery and Electronic Research Institute, Toyama Industrial Technology Center, Takada, Toyama 9308511, Japan c Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 3050003, Japan
b
Abstract Thermal stability of amorphous boron–carbon–nitride (B–C–N) films fabricated by magnetron sputtering using a graphite and BN cotarget was studied by X-ray photoelectron spectroscopy, infrared absorption spectroscopy, Raman spectroscopy and scanning electron microscopy. The boron-rich B–C–N films showed the higher thermal stability. These results suggest that an incorporation of B atoms into the amorphous CN networks improves the thermal stability. This improvement can be explained in terms of the creation of boron–nitrogen bonds and/or boron–carbon bonds. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sputtering; Thermal stability; B–C–N films
1. Introduction Boron–carbon–nitride (B–C–N) thin films are attractive materials, because these films have potential applications for surface coatings and electronic devices. Accordingly, these films were prepared by various methods, e.g., thermal chemical vapor deposition (CVD) [1,2], bias-assisted hotfilament CVD [3] and electron-cyclotron-resonance plasma-assisted physical vapor deposition [4]. We have fabricated the amorphous B–C–N films by magnetron sputtering using a graphite and boron nitride (BN) cotarget in Ar or N2 gas atmosphere [5] and by hot-wire CVD using acetylene (C2H2) and boron trichloride (BCl3) gases [6] in order to use surface coatings. An undesired aging effect, e.g., peeling off the substrate after air exposure, was observed for the amorphous carbon nitride (a-CN) films fabricated by magnetron sputtering [7]. However, the boron incorporation into carbon nitride networks, i.e., the creation of B–C–N networks, improved the aging effect Corresponding author. Tel.: +81 766 56 7500; fax: +81 766 56 8021.
E-mail address: [email protected] (H. Yokomichi). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.040
[5]. Since humidity is the main cause for the aging effect, this result indicates that stability of B–C–N films against humidity is higher than that of CN films. High thermal stability is expected on the analogy of improvement against humidity. Thermal stability at high temperatures as well as the stability against humidity is important for surface coatings. Although the a-CN films have poor thermal stability, higher thermal stability is expected for the B–C–N films as mentioned above. In the present study, we examined thermal stability of the B–C–N films fabricated by magnetron sputtering. 2. Experimental Three types of B–C–N films, SP#16, SP#21 and SP#22, were chosen for the present study. The B–C–N films were fabricated by magnetron sputtering using the graphite and BN co-target in Ar or N2 gas atmosphere. The BN targets with a diameter of 5 mm were prepared by pressing hexagonal boron nitride (h-BN) powders. The BN targets were put on the erosion region of the graphite target with a
ARTICLE IN PRESS H. Yokomichi et al. / Vacuum 80 (2006) 748–751
Table 1 The sample fabrication conditions and chemical components for the three B–C–N films Sample number
SP#16 SP#21 SP#22
Sputtering gas
Number of BN pellets
The color of SP#16 sample after annealing drastically changed from dark red to black. Although surface roughness observed by SEM increased for SP#16 after annealing at 300 1C, those of SP#21 and #22 were unchanged after
100 B C
80
N 60 SP#22 40
20
0
0
100
200 300 400 500 600 Annealing Temperature (°C)
800
SP#21
as-depo
(1, 68, 31) (17, 69, 14) (7, 63, 30)
100
500
1000
1500
2000
2500
3000
3500
4000
4500
-1)
Wavenumber (cm
B C
80 Concentration (%)
700
Fig. 2. The changes of boron, carbon and nitrogen concentrations for the SP#22 sample after thermal annealing.
B, C and N concentrations (B, C, N)
16 33 33
N2 Ar N2
3. Results and discussion
Concentration (%)
diameter of 100 mm. The picture of the co-target is shown in Ref. [5]. The co-target was sputtered at an rf power of 100 W under the substrate temperature of 30 1C. A total gas flow rate and a pressure during sputtering were 2 sccm and 0.27 Pa, respectively. The details of the sample fabrication are described in Ref. [5]. The boron, carbon and nitrogen concentrations (B, C, N), of SP#16, SP#21 and SP#22 samples before annealing were (1, 68, 31), (17, 69, 14) and (7, 63, 30), respectively. The typical features of the three B–C–N films are as follows: (i) SP#16 with a low boron concentration and a high nitrogen concentration, (ii) SP#21 with a high boron concentration and a medium nitrogen concentration and (iii) SP#22 with a medium boron concentration and a high nitrogen concentration. The fabrication conditions and chemical components for the three B–C–N films were summarized in Table 1. The B–C–N films kept in a sample case were annealed at 300, 500 and 620 1C in a vacuum of 4.0 10 3 Pa for 1 h. The film properties were characterized by X-ray photoelectron spectroscopy (XPS), infrared (IR) absorption spectroscopy, micro-Raman spectroscopy and scanning electron microscopy (SEM).
749
N
620˚C
SP#21
60 SP#21 40
20
0
500 0
100
200
300
400
500
600
700
800
1000
1500
2000
2500
3000
3500
4000
4500
Wavenumber (cm-1)
Annealing Temperature (°C) Fig. 1. The changes of boron, carbon and nitrogen concentrations for the SP#21 sample after thermal annealing.
Fig. 3. IR spectra of the SP#21 sample before and after thermal annealing at 620 1C. Raman G and D bands are indicated by dashed and solid lines, respectively.
ARTICLE IN PRESS H. Yokomichi et al. / Vacuum 80 (2006) 748–751
750
SP#16
SP#21
as-depo
as-depo
0
500
1000
1500
2000
0
500
Wavenumber (cm-1)
0
500
1000
0
500
SP#21
1500
2000
0
500
1500
2000
1500
2000
500°C
1000 Wavenumber (cm-1)
SP#21
620°C
1000
2000
Wavenumber (cm
Wavenumber (cm-1)
SP#16
1500 -1)
500°C
SP#16
1000
1500
2000
Wavenumber (cm-1)
0
500
620°C
1000 Wavenumber (cm-1)
Fig. 4. Micro-Raman spectra of the SP#16 sample before and after thermal annealing. Raman G and D bands are indicated by dashed and solid lines, respectively.
Fig. 5. Micro-Raman spectra of the SP#21 sample before and after thermal annealing. Raman G and D bands are indicated by dashed and solid lines, respectively.
the same annealing. Consequently, SP#16 sample has poor thermal stability. The XPS measurement for SP#16 after annealing could not be performed, because the film after annealing was peeled off the substrate. The changes of (B, C, N) concentrations for SP#21 and SP#22 samples after annealing are shown in Figs. 1 and 2, respectively. These results are not estimated by in situ XPS measurements. Accordingly, the XPS spectra due to oxygen were observed more or less in all the films. However, the existence of oxygen was excluded from the composition, since the XPS measurement is very sensitive to the oxygen adsorption. The carbon and nitrogen concentrations decreased after annealing in the both samples. However, the boron concentration increased, particularly a remarkable increase
was observed in SP#21. These results indicate that carbon and nitrogen atoms are easily effused from the films as compared with boron. The IR spectra for SP#21 sample before and after annealing are shown in Fig. 3. The peaks centered around 1340 and 1550 cm 1 are identified as due to Raman G and D bands, respectively [8]. It is reported that the signal around 1330 cm 1 is due to the sp2 BN vibration mode [9]. Accordingly, the peak around 1340 cm 1 may include the sp2 BN vibration mode. Another signals around 1050 and 1620 cm 1 were observed for the film after 620 1C annealing. An intensity in the signal around 1050 cm 1 increased after annealing, corresponding to the increase in boron concentration after annealing. Thus, the signal around
ARTICLE IN PRESS H. Yokomichi et al. / Vacuum 80 (2006) 748–751
1050 cm 1 is consistent with the sp3 BN characteristic mode [9]. The signal around 1620 cm 1 may be associated with NH2 stretching mode [9]. However, the possibility of the B-related vibration mode may not be excluded [5,6]. The micro-Raman spectra for SP#16 and SP#21 before and after annealing were shown in Figs. 4 and 5, respectively. The signals around 1380 and 1570 cm 1 are attributed to Raman G and D bands, respectively [8]. The significant decrease in Raman G and D bands for the SP#16 after 500 1C annealing indicates the thermal decomposition of the film. However, the intensity of Raman G and D bands increases for the SP#21 and SP#22 after 500 1C annealing, indicating that thermal stability of the B–C–N films, SP#21 and SP#22, is improved by boron incorporation. Furthermore, the linewidth of Raman G and D bands reduced for the SP#21 after annealing. This result may be associated with occurrence of crystallization. Another signals around 480 and 700 cm 1 were observed for the samples after annealing. The signals around 480 and 700 cm 1 are tentatively identified as due to oxygen-related and/or silicon-related vibration modes (silicon substrate was used), taking account of the increase in oxygen concentration and the decrease in film thickness after annealing. 4. Summary Thermal stability of B–C–N films fabricated by sputtering using the graphite and BN co-target was studied for three types of B–C–N films by XPS, IR, micro-Raman and
751
SEM. Although the intensity of Raman G and D bands in Raman spectra for the B–C–N film with a low boron concentration decreased after annealing, that for boronrich B–C–N films, SP#21 and SP#22 increased after annealing. The IR spectra for SP#21 unchanged after annealing. These results indicate that the B–C–N film with a low boron concentration has poorer thermal stability. However, the thermal stability is improved by incorporation of boron into amorphous CN networks. The carbon and boron concentrations decreased, but the boron concentrations increased after annealing. Therefore, this improvement may be explained in terms of the creation of boron–nitrogen bonds and/or boron–carbon bonds.
References [1] Kouvetakis J, Kaner RB, Sattler ML, Martlett N. J Chem Soc Chem Commun 1986:1758. [2] Kaner RB, Kouvetakis J, Warble CE, Sattler ML, Bartlett N. Mater Res Bull 1987;22:399. [3] Yu JE, Wang EG. Appl Phys Lett 1999;74:2948. [4] Tempez A, Badi N, Bensaoula A, Kulik J. J Vac Sci Technol 1998;A16:2896. [5] Yokomichi H, Funakawa T, Masuda A. Vacuum 2002;66:245. [6] Yokomichi H, Futakuchi T, Yasuoka M, Kishimoto N. J Non-Cryst Solids 2004;338–340:509. [7] Yokomichi H, Sakima H, Masuda A. Jpn J Appl Phys 1998;37:4722. [8] Victoria NM, Hammer P, dos Santos MC, Alvarez F. Phys Rev B 2000;61:1083. [9] Laidani N, Andele M, Canteri R, Elia L, Luches A, Martino M, Micheli V, Speranza G. Appl Surf Sci 2000;157:135.