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Structure and electrical properties of a-C:H thin films deposited by RF sputtering
Diamond & Related Materials 19 (2010) 695–698
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Structure and electrical properties of a-C:H thin films deposited by RF sputtering M. Yamazato ⁎, I. Mizuma, A. Higa Department of Electrical and Electronics Engineering, University of the Ryukyus, 1, Senbaru, Nishihara, Okinawa 903-0213, Japan
a r t i c l e
i n f o
Available online 1 February 2010 Keywords: Amorphous hydrogenated carbon Sputtering Bonding configurations Optical properties Electrical properties
a b s t r a c t We investigated the film structure and the electrical properties of hydrogenated amorphous carbon (a-C:H) thin films. a-C:H thin films were prepared by RF magnetron sputtering. Two different RF power sources of 13.56 MHz and 60 MHz were used to deposit the a-C:H films. The bonding hydrogen concentration varied from 1.6 × 1022 cm− 3 to 8.6 × 1022 cm− 3. The concentration of incorporated hydrogen atoms varied from 18 to 57 at.%. The optical gap increased from 1.58 eV to 2.56 eV with increasing the hydrogen concentration. The resistivity increased from 1013 Ω cm to 1015 Ω cm with increasing the hydrogen concentration. The permittivity measured at 1 MHz decreased from 5.6 to 2.3 with increasing the hydrogen concentration. These results suggest that the film structure and electrical properties can be controlled by the hydrogen concentration. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated amorphous carbon (a-C:H) films have unique properties such as high hardness, transparency, and chemical inertness [1–3]. These unique properties are suitable for a hard coating film on mechanical tools, a wear-resistant film on hard disks and a transparency coating film on optical components. For the electrical device applications, the control of the electrical properties such as resistivity and permittivity is required. On the other hand, the properties of a-C:H film strongly depend on the film structure. We have already reported that the structure of a-C:H films can be changed by the control of the bonding hydrogen concentration [4–7]. However, the relationship with the structure and the electrical properties of the RF sputtered a-C:H films is not clear. In this work, we investigated the relationship with the film structure and the electrical property.
measurement of capacitance of these cells by LCZ meter at 1 MHz. The film density was estimated by sink–float method. The ESR spectra were measured with an X-band spectrometer. The spin densities in the deposited films were estimated by comparison with DPPH standard sample. 3. Results and discussion Fig. 1 shows the typical IR absorption spectra around 2900 cm− 1 of the deposited a-C:H film. This band is composed of C(sp2)–H(3000 and 3050 cm − 1 ), C(sp 3 )–H(2890 cm − 1 ), C(sp 2 )–H 2 (2850 and 2920 cm− 1) and C(sp3)–H3(2870 and 2960 cm− 1) stretching absorption peaks. The bonding hydrogen concentration (nH) of the deposited a-C:H films corresponds to IR absorption integrated intensity in this region. The integrated intensity of the absorption band (IH), which is calculated by
2. Experimental −1
3200 cm
The a-C:H films were deposited in H2/He plasmas by the RF magnetron sputtering system. Two different RF power sources of 13.56 MHz and 60 MHz were used. The ratio of the H2 flow rate to the sum of the H2 and He flow rates (RH) was varied from 0.1 to 1.0%. The bonding hydrogen concentrations and the chemical states were evaluated using a FT-IR (Jasco FT/IR-300). The bonding structure of carbon atoms was estimated with a micro-Raman spectrometer (Jasco JRS-SYSTEM1000). The optical gap was estimated by the transmittance spectrum, which was measured with UV–visible spectrometer (Jasco V-550). The permittivity of the deposited film was estimated by
⁎ Corresponding author. Tel.: +81 98 895 8679; fax: +81 98 895 8708. E-mail address: [email protected] (M. Yamazato). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.01.035
IH = ∫2700 cm−1
αðvÞ dv; v
nH = As × IH
ð1Þ
where As is a normalization factor proportional to the inverse of the absorption strength. The approximate value of nH can be calculated from IH. Fig. 2 shows the dependence of IH of the deposited film by 13.56 MHz sputtering on the relative H2 gas flow rate RH for each total gas pressure PT. As shown in Fig. 2, the IH of the films deposited at each PT had a peak around the RH = 1% and the IH increased with the PT. The IH of the deposited film by 60 MHz sputtering showed a same tendency. We have already reported that the optical gap has a maximum peak and the spin density has a minimum peak around the RH = 1.0%. Also, if the films showed the same IH, the film properties (optical gap, spin density, hardness, and I(D)/I(G) of Raman spectrum)
696
M. Yamazato et al. / Diamond & Related Materials 19 (2010) 695–698
For investigation of the fraction of CHn bonds, the IR spectra of deposited films were fitted using seven Gaussian curves as shown in Fig. 1. The absorption coefficient of sp3CHn was increased with IH compared with that of sp2CH, so we investigate the sp3CHn in more detail. The fraction of sp3CHn R(sp3) of the deposited film was estimated following the equation: 3
∑ Iðsp3 CHn Þ
3
i=1
Rðsp Þ =
ð3Þ
3
Iðsp CHÞ + ∑ Iðsp3 CHi Þ 2
i=1
and ratios of each sp3CHn to total sp3CHn were estimated following the equation: 3
Rðsp CHn Þ =
Iðsp3 CHn Þ 3
;
ðn = 1; 2; 3Þ:
ð4Þ
∑ Iðsp3 CHi Þ
i=1
Fig. 1. IR absorption spectra around 2900 cm
−1
of the deposited a-C:H film.
have about same values in spite of the films deposited at different RF power frequencies [4–6]. Therefore, we chose the IH as the fundamental index for the film property. The IH of 13.56 MHz sputtered film varied from 16 to 42, while that of 60 MHz sputtered film varied from 49 to 86. When As = 1.0 × 1021 cm− 2 is assumed [8], nH of deposited films varied from 1.6 × 1022 cm− 3 to 8.6 × 1022 cm− 3. If the deposited film was composed of only carbon and hydrogen elements, the approximate value of hydrogen atomic concentration (NH) can be calculated by the following equation: NH =
nH +
nH df −MH × nH MC
Fig. 3a showed the relationship between the fraction of sp3CHn R (sp3) and the IH. The fraction of sp3CHn showed a slight increase with increasing the IH. This result indicates that the fraction of sp3CHn increases with the hydrogen concentration in the deposited film. We have already reported the analysis of the Raman spectra of sputtered a-C:H films. I(D)/I(G) of Raman spectra decreased with increasing IH, so the sp3 fraction of the deposited film increased with IH [4,6,7]. The
ð2Þ
where MH and MC are the mass of the hydrogen atom and the carbon atom. From Eq. (2), NH was varied in wide range from 18 to 57 at.%. However, this estimation is based on the simple model of IR absorption. It is noted that the different C–H stretching modes have different absorption strengths in hydrocarbon molecules [9–11]. For a more accurate estimation, appropriate As is needed to multiply each absorption band in Eq. (1).
Fig. 2. Dependence of IH of the deposited film on the relative H2 gas flow rate RH for each total gas pressure PT.
Fig. 3. (a). Relationship between the fraction of sp3CHn R(sp3) and integrated intensity IH of the deposited film. (b). Integrated intensity IH dependence of the fractions of sp3CH. (c). Integrated intensity IH dependence of the fractions of sp3CH2. (d). Integrated intensity IH dependence of the fractions of sp3CH3.
M. Yamazato et al. / Diamond & Related Materials 19 (2010) 695–698
Fig. 4. Relationship between spin density and integrated intensity IH.
result of Raman analysis corresponds to the increase of the sp3CHn fraction. Fig. 3b, c and d showed the integrated intensity dependence of the fractions of sp3CH, sp3CH2, and sp3CH3, respectively. In these figures, the fraction of sp3CH was decreased, while that of sp3CH3 increased with increasing the IH. On the other hand, the sp3CH2 fraction was almost kept constant. These results suggest that when the amount of hydrogen atoms in the deposited film increased, most hydrogen atoms would be combined with carbon atoms. Fig. 4 showed the relationship between spin density and IH. The spin density was decreased from 5 × 1020 cm− 3 to 6 × 1018 cm− 3 with increasing the IH. IR and Raman spectra showed that the sp3 carbon fraction increased with IH. The decrease in spin density is due to the termination of dangling bonds with hydrogen atoms. Fig. 5 showed the integrated intensity dependence of the Tauc gap of the deposited film. In this figure, the Tauc gap was increased with IH, and changed from 1.58 to 2.56 eV. Also, the electrical resistivity increased from 1013 Ω cm to 1015 Ω cm with increasing the IH. The optical gap depends on the density of states of electrons (DOS), and the DOS is affected by clusters of sp2 carbon atoms and unpaired electrons [12–16]. The optical band gap becomes small for the a-C:H film which is mainly composed of sp2 carbon atoms, because the energy gap between π and π* bands is lower than that between σ and
Fig. 5. Integrated intensity dependence of the Tauc gap of the deposited film.
697
σ* bands. Therefore, the increase of sp3 carbon fraction caused the increase of optical gap. Also, this IH dependence of the optical gap could be explained by the influence of the unpaired electrons. The unpaired electrons in films originate from defects in π bonds. These defects in π bonds cause defect states around the Fermi level and cause the optical band gap to become smaller [17]. As shown in Fig. 4, spin density decreased with increasing IH. Therefore, the optical gap increased with the increasing IH due to the hydrogen atoms terminated by the unpaired electrons. Robertson reported that the ESR defect density of a-C:H films deposited by PECVD tends to decrease as the band gap increases, and the defect density was varied from about 1017 to 1021 cm− 3 [10,18,19]. From Figs. 4 and 5, it is seen that the spin density of films deposited by sputtering have same tendency, and the values of spin density indicate about same range of films deposited by PECVD. These results suggested that the structure of density of state for sputtered a-C:H films is a similar structure to that of films deposited by PECVD. Fig. 6a showed the relationship between permittivity and film density, and Fig. 6b showed the relationship between permittivity and integrated intensity. From Fig. 6a, the permittivity decreased from 5.6 to 2.3, when the film density was decreased from 1.51 to 1.43 g/cm3. In this result, the permittivity was proportional to the film density. Casiraghi et al. reported the relationship with the square of reflective index and the density of the films by PECVD method [20]. The square of reflective index corresponds to the permittivity. In their report, the permittivity decreased with increasing the film density, like our result. However, the range of permittivity was 3–4 at the film density of 1.43 g/cm3 for PECVD film, while that was 2.3 in our result. This suggests that the RF sputtering is appropriate for the low-k a-C:H deposition. On the other hand, from Fig. 6b, the permittivity decreased with increasing the IH. S.P. Louh et al. reported that the orientational polarization is reduced by the increase of the symmetric sp3CH3 and sp2CH [21]. Therefore, the increase of the sp3CH3 fraction is one reason for the decrease in the permittivity. Also, we have reported that the trans-polyacetylene content increases with the hydrogen content in
Fig. 6. (a). Relationship between permittivity and film density. (b). Relationship between permittivity and integrated intensity.
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M. Yamazato et al. / Diamond & Related Materials 19 (2010) 695–698
the a-C:H films, and the length of trans-PA chain increases with the hydrogen content [6]. Furthermore, the termination structure was increased with hydrogen content. These results indicated that the fraction of well cross-linked structure was decreased with the increase of the hydrogen content. Therefore, the film density and the permittivity decreased in the high hydrogen content region due to the film that has higher volume of free-space. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8]
Hydrogenated amorphous carbon thin films were deposited in H2/He plasmas by the RF magnetron sputtering system. The bonding hydrogen concentration varied from 1.6 × 1022 cm− 3 to 8.6 × 1022 cm− 3, and this range of IH corresponds to incorporated hydrogen atoms which varied from 18 to 57 at.%. The fraction of sp3C in the deposited film increased with hydrogen concentration. The spin density of film was decreased with increasing the IH, and varied from 5 ×1020 cm− 3 to 6× 1018 cm− 3. The number of defect was reduced by the termination of unpaired electrons by hydrogen atoms. The optical gap was increased with hydrogen content, because the sp3C carbon increased and the number of defect was decreased. The permittivity was decreased from 5.6 to 2.3 with hydrogen content due to the increase of sp3CH and the decrease of film density.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Acknowledgement This research was partially supported by Grant-in-aid for Young Scientists (B) (21760051).
A. Grill, Diamond Relat. Mater. 8 (1999) 428. A. Grill, Diamond Relat. Mater. 10 (2001) 234. P.W. May, S. Hohn, W.N. Wang, N.A. Fox, Appl. Phys. Lett. 72 (1998) 2182. T. Oshiro, M. Yamazato, A. Higa, T. Maehama, M. Toguchi, Jpn. J. Appl. Phys. 43 (2004) 2640. T. Oshiro, F. Begum, M. Yamazato, A. Higa, T. Maehama, M. Toguchi, Thin Solid Films 506/507 (2006) 92. T. Oshiro, M. Yamazato, A. Higa, M. Toguchi, Jpn. J. Appl. Phys. 46 (2007) 756. A. Higa, T. Oshiro, Y. Saida, M. Yamazato, M. Toguchi, New Diam. Front. Carbon Technol. 16 (2006) 245. F. Fujimoto, A. Okuta, K. Komaki, Y. Iwata, I. Yamane, H. Yamashita, Y. Hashimoto, Y. Tawada, K. Nishimura, H. Okamoto, Y. Hamakawa, Jpn. J. Appl. Phys. 23 (1984) 810. J. Ristein, R.T. Stief, L. Ley, W. Beyer, J. Appl. Phys. 84 (1998) 3836. J. Robertson, Mater. Sci. Eng., R 37 (2002) 129. W. Jacob, M. Unger, Appl. Phys. Lett. 68 (1996) 475. B. Popescu, C. Verney, E.A. Davis, V. Paret, A. Brunet-Bruneau, J. Non-Cryst. Solids 266–269 (2000) 778. G. Franchini, A. Tagliaferro, B. Popescu, E.A. Davis, J. Non-Cryst. Solids 299–302 (2002) 846. A.C. Ferrari, J. Robertson, Phys. Rev., B 61 (2000) 14095. J.D. Carey, R.D. Forrest, R.U.A. Khan, S.R.P. Silva, Appl. Phys. Lett. 77 (2000) 2006. R.C. Barklie, Diamond Relat. Mater. 10 (2001) 174. J. Robertson, E.P. O'Reilly, Phys. Rev., B 35 (1987) 2946. R.H. Jarman, G.J. Ray, R.W. Stadley, Appl. Phys. Lett. 49 (1986) 1065. N.M.J. Conway, A.C. Ferrari, A.J. Flewitt, J. Roberson, W.I. Miline, A. Tagliaferro, W. Beyer, Diamond Relat. Mater. 9 (2000) 765. C. Casiraghi, F. Piazza, A.C. Ferrari, D. Grambole, J. Robertson, Diamond Relat. Mater. 14 (2005) 1098. S.P. Louh, I.C. Leu, M.H. Hon, Diamond Relat. Mater. 14 (2005) 1005.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Structure and electrical properties of a-C:H thin films deposited by RF sputtering M. Yamazato ⁎, I. Mizuma, A. Higa Department of Electrical and Electronics Engineering, University of the Ryukyus, 1, Senbaru, Nishihara, Okinawa 903-0213, Japan
a r t i c l e
i n f o
Available online 1 February 2010 Keywords: Amorphous hydrogenated carbon Sputtering Bonding configurations Optical properties Electrical properties
a b s t r a c t We investigated the film structure and the electrical properties of hydrogenated amorphous carbon (a-C:H) thin films. a-C:H thin films were prepared by RF magnetron sputtering. Two different RF power sources of 13.56 MHz and 60 MHz were used to deposit the a-C:H films. The bonding hydrogen concentration varied from 1.6 × 1022 cm− 3 to 8.6 × 1022 cm− 3. The concentration of incorporated hydrogen atoms varied from 18 to 57 at.%. The optical gap increased from 1.58 eV to 2.56 eV with increasing the hydrogen concentration. The resistivity increased from 1013 Ω cm to 1015 Ω cm with increasing the hydrogen concentration. The permittivity measured at 1 MHz decreased from 5.6 to 2.3 with increasing the hydrogen concentration. These results suggest that the film structure and electrical properties can be controlled by the hydrogen concentration. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated amorphous carbon (a-C:H) films have unique properties such as high hardness, transparency, and chemical inertness [1–3]. These unique properties are suitable for a hard coating film on mechanical tools, a wear-resistant film on hard disks and a transparency coating film on optical components. For the electrical device applications, the control of the electrical properties such as resistivity and permittivity is required. On the other hand, the properties of a-C:H film strongly depend on the film structure. We have already reported that the structure of a-C:H films can be changed by the control of the bonding hydrogen concentration [4–7]. However, the relationship with the structure and the electrical properties of the RF sputtered a-C:H films is not clear. In this work, we investigated the relationship with the film structure and the electrical property.
measurement of capacitance of these cells by LCZ meter at 1 MHz. The film density was estimated by sink–float method. The ESR spectra were measured with an X-band spectrometer. The spin densities in the deposited films were estimated by comparison with DPPH standard sample. 3. Results and discussion Fig. 1 shows the typical IR absorption spectra around 2900 cm− 1 of the deposited a-C:H film. This band is composed of C(sp2)–H(3000 and 3050 cm − 1 ), C(sp 3 )–H(2890 cm − 1 ), C(sp 2 )–H 2 (2850 and 2920 cm− 1) and C(sp3)–H3(2870 and 2960 cm− 1) stretching absorption peaks. The bonding hydrogen concentration (nH) of the deposited a-C:H films corresponds to IR absorption integrated intensity in this region. The integrated intensity of the absorption band (IH), which is calculated by
2. Experimental −1
3200 cm
The a-C:H films were deposited in H2/He plasmas by the RF magnetron sputtering system. Two different RF power sources of 13.56 MHz and 60 MHz were used. The ratio of the H2 flow rate to the sum of the H2 and He flow rates (RH) was varied from 0.1 to 1.0%. The bonding hydrogen concentrations and the chemical states were evaluated using a FT-IR (Jasco FT/IR-300). The bonding structure of carbon atoms was estimated with a micro-Raman spectrometer (Jasco JRS-SYSTEM1000). The optical gap was estimated by the transmittance spectrum, which was measured with UV–visible spectrometer (Jasco V-550). The permittivity of the deposited film was estimated by
⁎ Corresponding author. Tel.: +81 98 895 8679; fax: +81 98 895 8708. E-mail address: [email protected] (M. Yamazato). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.01.035
IH = ∫2700 cm−1
αðvÞ dv; v
nH = As × IH
ð1Þ
where As is a normalization factor proportional to the inverse of the absorption strength. The approximate value of nH can be calculated from IH. Fig. 2 shows the dependence of IH of the deposited film by 13.56 MHz sputtering on the relative H2 gas flow rate RH for each total gas pressure PT. As shown in Fig. 2, the IH of the films deposited at each PT had a peak around the RH = 1% and the IH increased with the PT. The IH of the deposited film by 60 MHz sputtering showed a same tendency. We have already reported that the optical gap has a maximum peak and the spin density has a minimum peak around the RH = 1.0%. Also, if the films showed the same IH, the film properties (optical gap, spin density, hardness, and I(D)/I(G) of Raman spectrum)
696
M. Yamazato et al. / Diamond & Related Materials 19 (2010) 695–698
For investigation of the fraction of CHn bonds, the IR spectra of deposited films were fitted using seven Gaussian curves as shown in Fig. 1. The absorption coefficient of sp3CHn was increased with IH compared with that of sp2CH, so we investigate the sp3CHn in more detail. The fraction of sp3CHn R(sp3) of the deposited film was estimated following the equation: 3
∑ Iðsp3 CHn Þ
3
i=1
Rðsp Þ =
ð3Þ
3
Iðsp CHÞ + ∑ Iðsp3 CHi Þ 2
i=1
and ratios of each sp3CHn to total sp3CHn were estimated following the equation: 3
Rðsp CHn Þ =
Iðsp3 CHn Þ 3
;
ðn = 1; 2; 3Þ:
ð4Þ
∑ Iðsp3 CHi Þ
i=1
Fig. 1. IR absorption spectra around 2900 cm
−1
of the deposited a-C:H film.
have about same values in spite of the films deposited at different RF power frequencies [4–6]. Therefore, we chose the IH as the fundamental index for the film property. The IH of 13.56 MHz sputtered film varied from 16 to 42, while that of 60 MHz sputtered film varied from 49 to 86. When As = 1.0 × 1021 cm− 2 is assumed [8], nH of deposited films varied from 1.6 × 1022 cm− 3 to 8.6 × 1022 cm− 3. If the deposited film was composed of only carbon and hydrogen elements, the approximate value of hydrogen atomic concentration (NH) can be calculated by the following equation: NH =
nH +
nH df −MH × nH MC
Fig. 3a showed the relationship between the fraction of sp3CHn R (sp3) and the IH. The fraction of sp3CHn showed a slight increase with increasing the IH. This result indicates that the fraction of sp3CHn increases with the hydrogen concentration in the deposited film. We have already reported the analysis of the Raman spectra of sputtered a-C:H films. I(D)/I(G) of Raman spectra decreased with increasing IH, so the sp3 fraction of the deposited film increased with IH [4,6,7]. The
ð2Þ
where MH and MC are the mass of the hydrogen atom and the carbon atom. From Eq. (2), NH was varied in wide range from 18 to 57 at.%. However, this estimation is based on the simple model of IR absorption. It is noted that the different C–H stretching modes have different absorption strengths in hydrocarbon molecules [9–11]. For a more accurate estimation, appropriate As is needed to multiply each absorption band in Eq. (1).
Fig. 2. Dependence of IH of the deposited film on the relative H2 gas flow rate RH for each total gas pressure PT.
Fig. 3. (a). Relationship between the fraction of sp3CHn R(sp3) and integrated intensity IH of the deposited film. (b). Integrated intensity IH dependence of the fractions of sp3CH. (c). Integrated intensity IH dependence of the fractions of sp3CH2. (d). Integrated intensity IH dependence of the fractions of sp3CH3.
M. Yamazato et al. / Diamond & Related Materials 19 (2010) 695–698
Fig. 4. Relationship between spin density and integrated intensity IH.
result of Raman analysis corresponds to the increase of the sp3CHn fraction. Fig. 3b, c and d showed the integrated intensity dependence of the fractions of sp3CH, sp3CH2, and sp3CH3, respectively. In these figures, the fraction of sp3CH was decreased, while that of sp3CH3 increased with increasing the IH. On the other hand, the sp3CH2 fraction was almost kept constant. These results suggest that when the amount of hydrogen atoms in the deposited film increased, most hydrogen atoms would be combined with carbon atoms. Fig. 4 showed the relationship between spin density and IH. The spin density was decreased from 5 × 1020 cm− 3 to 6 × 1018 cm− 3 with increasing the IH. IR and Raman spectra showed that the sp3 carbon fraction increased with IH. The decrease in spin density is due to the termination of dangling bonds with hydrogen atoms. Fig. 5 showed the integrated intensity dependence of the Tauc gap of the deposited film. In this figure, the Tauc gap was increased with IH, and changed from 1.58 to 2.56 eV. Also, the electrical resistivity increased from 1013 Ω cm to 1015 Ω cm with increasing the IH. The optical gap depends on the density of states of electrons (DOS), and the DOS is affected by clusters of sp2 carbon atoms and unpaired electrons [12–16]. The optical band gap becomes small for the a-C:H film which is mainly composed of sp2 carbon atoms, because the energy gap between π and π* bands is lower than that between σ and
Fig. 5. Integrated intensity dependence of the Tauc gap of the deposited film.
697
σ* bands. Therefore, the increase of sp3 carbon fraction caused the increase of optical gap. Also, this IH dependence of the optical gap could be explained by the influence of the unpaired electrons. The unpaired electrons in films originate from defects in π bonds. These defects in π bonds cause defect states around the Fermi level and cause the optical band gap to become smaller [17]. As shown in Fig. 4, spin density decreased with increasing IH. Therefore, the optical gap increased with the increasing IH due to the hydrogen atoms terminated by the unpaired electrons. Robertson reported that the ESR defect density of a-C:H films deposited by PECVD tends to decrease as the band gap increases, and the defect density was varied from about 1017 to 1021 cm− 3 [10,18,19]. From Figs. 4 and 5, it is seen that the spin density of films deposited by sputtering have same tendency, and the values of spin density indicate about same range of films deposited by PECVD. These results suggested that the structure of density of state for sputtered a-C:H films is a similar structure to that of films deposited by PECVD. Fig. 6a showed the relationship between permittivity and film density, and Fig. 6b showed the relationship between permittivity and integrated intensity. From Fig. 6a, the permittivity decreased from 5.6 to 2.3, when the film density was decreased from 1.51 to 1.43 g/cm3. In this result, the permittivity was proportional to the film density. Casiraghi et al. reported the relationship with the square of reflective index and the density of the films by PECVD method [20]. The square of reflective index corresponds to the permittivity. In their report, the permittivity decreased with increasing the film density, like our result. However, the range of permittivity was 3–4 at the film density of 1.43 g/cm3 for PECVD film, while that was 2.3 in our result. This suggests that the RF sputtering is appropriate for the low-k a-C:H deposition. On the other hand, from Fig. 6b, the permittivity decreased with increasing the IH. S.P. Louh et al. reported that the orientational polarization is reduced by the increase of the symmetric sp3CH3 and sp2CH [21]. Therefore, the increase of the sp3CH3 fraction is one reason for the decrease in the permittivity. Also, we have reported that the trans-polyacetylene content increases with the hydrogen content in
Fig. 6. (a). Relationship between permittivity and film density. (b). Relationship between permittivity and integrated intensity.
698
M. Yamazato et al. / Diamond & Related Materials 19 (2010) 695–698
the a-C:H films, and the length of trans-PA chain increases with the hydrogen content [6]. Furthermore, the termination structure was increased with hydrogen content. These results indicated that the fraction of well cross-linked structure was decreased with the increase of the hydrogen content. Therefore, the film density and the permittivity decreased in the high hydrogen content region due to the film that has higher volume of free-space. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8]
Hydrogenated amorphous carbon thin films were deposited in H2/He plasmas by the RF magnetron sputtering system. The bonding hydrogen concentration varied from 1.6 × 1022 cm− 3 to 8.6 × 1022 cm− 3, and this range of IH corresponds to incorporated hydrogen atoms which varied from 18 to 57 at.%. The fraction of sp3C in the deposited film increased with hydrogen concentration. The spin density of film was decreased with increasing the IH, and varied from 5 ×1020 cm− 3 to 6× 1018 cm− 3. The number of defect was reduced by the termination of unpaired electrons by hydrogen atoms. The optical gap was increased with hydrogen content, because the sp3C carbon increased and the number of defect was decreased. The permittivity was decreased from 5.6 to 2.3 with hydrogen content due to the increase of sp3CH and the decrease of film density.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Acknowledgement This research was partially supported by Grant-in-aid for Young Scientists (B) (21760051).
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