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Optical and electrical properties of amorphous carbon films deposited using filtered cathodic vacuum arc with pulse biasing
Thin Solid Films 447 – 448 (2004) 148–152
Optical and electrical properties of amorphous carbon films deposited using filtered cathodic vacuum arc with pulse biasing J.Y. Szea,*, B.K. Taya, D. Sheejaa, S.P. Laua, Y.Q. Fub, Daniel H.C. Chuac, W.I. Milnec a
School of Electrical and Electronic Engineering, NTU, Block S1-B3- 01, Singapore 639798, Singapore b School of Mechanical and Production Engineering, NTU, Singapore c Engineering Department, University of Cambridge, Cambridge CB2 1PZ, UK
Abstract Passive devices using metal containing amorphous (a-C) films have been successfully fabricated. However, difficulties in the etching of these films as well as their inferior inertness compared to pure a-C films led us to study the electrical and optical properties of pure a-C films. The films were deposited using a filtered cathodic vacuum arc system (FCVA) in conjunction with high substrate pulse biasing. It is possible to control the sp2 content and hence the properties, by varying the substrate pulse bias voltage. In this study, the a-C films were prepared by varying the high substrate bias between 3 and 11 kV using a Plasma immersion ion implantation (PI3 ) system. Characterization of these samples gives us an indication about the suitability of the films for integrated passive devices and other applications. Four-point probe measurement has been carried out to study the resistivity of the films deposited on quartz and SiO2 . The resistivity decreases with increasing pulse bias voltage, which is likely attributed to the sp2 fraction in the film as well as the substrates’ resistivity. The sp2 content in the films is estimated using XPS and Raman spectroscopy. Optical properties of the films are characterized using spectroscopic phase-modulated ellipsometry. The band gap decreases from 2.3 to 1.49 with increasing bias voltage. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon; Filtered cathodic vacuum arc; Plasma ion immersion implantation; Electrical and optical properties
1. Introduction In today’s demand for smaller and complex electronic functions, research is constantly being done on thin film integrated passive technologies to exploit their advantages of being highly integrated and reduced power consumption. The types of thin film materials most commonly used in resistors are carbon, tantalum nitride and nichrome. Nichrome has the advantages of being precise and thermally stable. However, it faces problems of susceptibility to chemical and electrolytic corrosion. Carbon type resistors, on the other hand, have a poor temperature coefficient so they are not suited for precision applications requiring little resistance change over temperature. Tantalum is considered as relatively expensive compared to carbon or nichrome. Thus, other types of suitable materials including diamond-like films (taC) have been studied as alternatives in thin film resis*Corresponding author. Tel.: q65-67906127; fax: q65-67933318. E-mail address: [email protected] (J.Y. Sze).
tors. However, the extremely high electrical resistivity, in the region of 109 V-cm w1x, had restricted the application of ta-C. This had led to studying on using metal containing amorphous carbon (a-C:metal) films. Metal containing a-C films, however, have a poorer etching rate compared to pure amorphous carbon (a-C) films. The etching rate, using reactive ion etching (RIE) of a-C:Ti films reached only a maximum of 6 nmymin while the etching rate of a-C film was approximately 30 nmymin w2x. Moreover, the resistivity values obtained tend to be more suitable for resistors of exceptionally high value as the resistivity values are in the range of 96–1000 Vysq for a-C:Ti w3x. In recent years, the use of applied high substrate bias (in the range of negative kV) to deposit a-C films has been shown to result in good tribological properties and high adhesive strength w4x. By changing the pulse width and frequency of the PI3, reduction of compressive stress was achieved in order to grow thick film w5x. These improvements show that the use of PI3 together with FCVA can be investigated further to understand the
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01090-3
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
149
effects of high substrate biasing on the electrical and optical properties of a-C films. If the range of desired resistivity can be achieved, these films will become better alternative thin film resistors. 2. Experimental details The carbon plasma was produced from a vacuum arc spot on a 60-mm in diameter cathode target. The cathode target has a 99.99% pure carbon content with the macroparticle and neutrals in the plasma filtered out by the FCVA system. The FCVA system has been described in detail elsewhere w6x. All depositions were carried out at room temperature and the substrate holder was negatively biased using the PI3 system w5x. While the frequency and pulse width was fixed at 600 Hz and 25 ms, respectively, the voltage was varied from y3 to y11 kV for each set of substrates. The samples were deposited for 3 min. Three types of substrates were used. They were highly doped nqq Si N111M, SiO2 and quartz (fused silica). The SiO2 used were polished silicon wafers of 0.62-mm average thickness coated with 500 nm of SiO2. They are studied since SiO2 is a common interlayer dielectric material in the semiconductor industry. A surface profiler was then used to determine the film thickness after deposition. The thickness varied from 41 to 80 nm with noticeably thinner films being deposited on quartz. The sheet resistivity of the films deposited on silicon dioxide and quartz were measured using the Alessi C4s collinear four-point probe (Cascade Microtech Inc.) with 1.3 mm probe spacing. The voltage was measured via the inner probes while a small current (approx. nA) was forced through the pair of outer probes. The sheet resistivity was calculated using rss C*(VyI) where C is the correction factor of 4.4516 w7x. The structure changes in the films were evaluated by Micro Raman Spectroscopy (Raman Imaging Microscope, Renishaw) that uses an Arq ion green laser (514.5 nm). X-ray photoelectron spectroscopy (XPS) was used to estimate the relative concentrations of sp3 and sp2 hybrids in a-C films w8,9x. The C 1s photoemission spectrum was deconvoluted into three components. The components were mixtures of a Gaussian and a Lorentzian distribution. The Gaussian component accounted for the instrumental energy resolution together with the chemical disorder, and the Lorentzian for the lifetime of the photoionization process. The two main components were found at 284.4 and 285.2 eV, and were attributed to sp3 and sp2 hybridized carbon atoms w16x. A third peak was determined at approximately 286.0 eV and was attributed to some C–O contamination formed at the surface of the samples due to exposure to air w9x. To measure the optical properties of the films, spectroscopic phase-modulated ellipsometry was performed
Fig. 1. Sheet resistivity of a-C films on SiO2 and quartz substrate bias.
on a-C films deposited on silicon substrates. The fourlayer model described by Shi et al. w10,11x was used to fit our experimental data and to determine the five parameters of the Forouhi and Bloomer (F.B.) optical dispersion model which had been shown to be appropriate for amorphous diamond-like carbon films w10– 12x. The mechanism for optical absorption is the interband excitations between the bonding and antibonding p bands. The model enabled the determination of the imaginary part of the refractive index (k) over the range of interest and the determination of the real part (n) within an integral constant via the Kramer–Kronig relation w12x. The four-layer model comprised of (SiN111M)y(graded a-C:Si)y(a-C)y(a-Cqvoid). The graded layer consists of a mixture of a-C and silicon simulating carbon ion bombardment into the surface of the silicon substrate due to high ion energy and to account for surface roughness effect, the top layer included a void in the aC films. The five parameters are A, B, C, Eg and n(`) and they follow their physical significance as described in many reviews w10–13x. The spectrum range was measured from 250 to 800 nm with an angle of incidence of 708. 3. Experimental results 3.1. Electrical resistivity vs. varying bias Fig. 1 shows a plot of sheet resistivity rs as a function of substrate bias. It was found that rs decreased from 26.9 MVysq to 7.4 MVysq for quartz with the increase of bias voltage from y3 to y11 kV. However, the sheet resistivity for SiO2 varies greatly from 12 MVysq to 6.8 kVysq in the same voltage range. It is also noted that with the increase in substrate bias, the slopes of the
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
150
Fig. 2. Raman spectra obtained for a-C films on Si at varying applied bias.
two curves decrease at different rates. This leads to the possibility that the layer of SiO2, which is relatively more conducting than quartz, affects the resistivity of the a-C film. The bulk resistivity of the films was determined by rsrs *t, where t is the thickness of the film. The resistivity values of a-C films on SiO2 decrease from 63 to 0.058 V-cm, with increasing in bias voltage. 3.2. Microstructure spectroscopy
of
the
films
using
Raman
From Fig. 2, it can be clearly seen that the peak at 970 cmy1, due to the second phonon scattering of the silicon substrate, is not observable when the substrate bias was more than y5 kV. This indicates a reduction in transparency of the films and an increase in sp2 bonds w5x. There is, however, not a significant change in the peaks between y3 and y5 kV. The spectra were fitted with two Gaussian components—disorder (‘D’) and graphite (‘G’) peaks w5,14,15x. Data from the Raman spectra showed that the G-peak width and D-peak width did not vary significantly. Fig. 3 shows that the calculated ratio (ID yIG) of the films increases with increasing substrate bias. The increase in ID yIG ratio was expected along with a decrease in the sp3 ysp2 ratio in the films as the substrate bias increased as reported in previous experiments w5x. In particular, films deposited on SiO2 had the greatest change in ID yIG ratio. The difference in ID yIG ratio evidently indicates how the microstructure of the films deposited on different substrates would differ from each other.
Fig. 3. IDyIG ratio of a-C films on quartz, SiO2 and silicon substrates.
show a shift of peaks towards the left indicating an increase in sp2 carbon bonds. The C 1s spectra were deconvoluted into the three components shown in Fig. 5. The fitting of the C 1s peaks was performed using two main components, being a mixture of a Gaussian and a Lorentzian distribution. The first two components were found at approximately 284.4"0.1 eV (sp2 carbon bonds) and 285.2"0.1 eV (sp3 carbon bonds). The sp3 fraction was calculated as the ratio of the area of the sp3 component over the total area of the two components. A decreasing trend of the sp3 fraction was observed, which is plotted in Fig. 6. A third peak (C– O) of much smaller intensity was also detected at 286.0 eV. It was observed that after etching was done, the C–
3.3. C 1s core level spectra analysis using XPS The C 1s spectra of a-C films deposited on Si substrates, after etching, are plotted in Fig. 4. The spectra
Fig. 4. C 1s XPS spectra of a-C films deposited on Si at different negative substrate bias.
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
Fig. 5. Fitting of C 1s XPS spectra at y5 kV.
151
Fig. 7. F.B. optical band gap plotted against the sp3 fraction for a-C films on Si substrates.
From Fig. 6 shows that the sp3 fraction for films deposited on silicon substrates decreases from 35.89 to 28.16% as the substrate bias varied from y3 to y11 kV. For comparison, an a-C film was deposited on Si at floating bias and investigated by XPS. The estimated sp3 fraction was 75.2%. 3.4. Optical band gap using spectroscopic ellipsometry
Fig. 6. Estimated percentage of sp3 fraction in a-C films deposited on Si substrates.
O area as a percentage of the overall area was reduced and the sp3 fraction content increased. This is in agreement with P. Merel et al. w9x who attributed to some C– O contamination formed at the surface of the samples due to air exposure. The sp3 fraction estimated by XPS correlates well with the ID yIG ratio obtained by Raman spectroscopy.
The optical band gap is the measure of the gap between the extended state in the valence band and the conduction band. The optical band gap in the a-C films deposited on Si was found to vary between 1.49 and 2.3 ("0.05) eV. The largest band gap, determined at a negative bias voltage of y3 kV coincided with a larger amount of sp3 bonds found in the film by XPS. The optical band gap’s correlation with the amount of sp3 fraction in the a-C film is plotted in Fig. 7. The five parameters of the F.B. model were fitted using the fourlayer model and are shown in Table 1. 4. Discussion In measuring the electrical resistivity of the a-C films, an interesting comparison was made between the resistivity values obtained by using of two different substrates. The rate of decrease in resistivity values for the
Table 1 Results for F.B. parameters fitted with the four-layer model Film (kV)
A ("0.005)
B ("0.05)
C ("0.5)
Eg ("0.05) (eV)
´(`) ("0.05)
3 5 8 11
0.777 0.792 0.751 0.709
5.459 5.128 4.794 4.610
8.721 7.545 6.705 6.483
2.3 2.14 1.81 1.49
3.713 3.235 3.230 2.896
152
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
SiO2 substrate was higher than that of quartz (Fig. 1). A possible explanation is that the resistivity of quartz is much higher than that of the SiO2 –Si substrates and the surface of the quartz, during deposition, is likely to have positive charging effects during high voltage bias. More of the higher energy ions may be accelerated towards the SiO2 substrate instead, creating more sp2 sites and hence higher conductivity a-C films. This was also reflected by the differences in the microstructures of the a-C films as determined by Raman spectroscopy. Indeed, it was observed that the differences in the number of incoming ions arriving on the substrates could have even affected the thickness and electrical properties of the films. Amorphous carbon films grown on quartz tend to have lower ID yIG ratio and lower deposition rate while having higher resistivity values. As the electrical and optical properties are related to the sp2 clusters present in the film w17,18x, XPS had been carried out to estimate the sp3 fraction. As the substrate bias was increased to y11 kV, the sp3 fraction decreased to only 28.16%. As compared to having 75.2% sp3-bonded atoms at floating bias, the films had undergone graphitization when high bias was applied. It is likely that the high bias had accelerated the incoming ions with an ion energy more than what was needed (100 eV), thus the ions penetrated into the inner layers. This might change some of the present sp3 to sp2 bonds w5x. The graphitization of the films might have occurred in two manners—an increase of the number of sp2 sites or an increase in sp2 cluster size. If it were because of the increase in cluster size, the optical band gap would be expected to be less than 0.5 eV, (i.e. closer to the band gap of graphite). However, the optical band gap in the films was found to be between 1.49 and 2.3. A likely explanation was that there is a very large fraction of sp2 sites that relates to the drop in resistivity and the low sp3 fraction but the cluster size remained small. This reflects the significant optical band gap w19x. When the optical band gap was related to the sp3 fraction in Fig. 7, it could be predicted that as the sp3 fraction increases, the optical band gap would likely saturate at the optical band gap of ta-C films (approx. 2.7 eV) containing approximately 85% of sp3 carbon w8,19x. 5. Conclusions The PI3 technology with FCVA has been successfully used to deposit a-C films of lower resistivity in a more
useful range between 12 MVysq to 6.8 kVysq when SiO2 was used as a substrate. It has been suggested that the improved conductivity is due to the increase in higher energy ions arriving at the substrates that results in an increased amount of sp2 sites. This paper has further shown that different substrate materials will affect the properties of a-C films deposited by PI3 technique. When compared to ta-C films deposited without the use of PI3, the resistivity values had fallen up to 6 times. Considering that sheet resistance for the integrated thin film resistors is presently offered in the widest range from 1 Vysq to 100 kVysq, there is a possibility of using a-C films deposited using high substrate bias for electrical applications. References w1x X. Shi, H. Fu, J.R. Shi, L.K. Cheah, B.K. Tay, P. Hui, J. Phys.: Condens. Matter. 10 (41) (1998) 9293. w2x L.J. Yu, D. Sheeja, B.K. Tay, D.H.C. Chua, W.I. Milne, J. Miao, Y.Q. Fu, Appl. Surf. Sci. (2002) 107–116. w3x Hybrid Microcircuit Technology Handbook, Noyes Publications, Park Ridge, N.J., 1988. w4x D. Sheeja, B.K. Tay, S.P. Lau, X. Shi, Wear 249 (2001) 433–439. w5x D. Sheeja, B.K. Tay, L.J. Yu, S.P. Lau, J.Y. Sze, C.K. Cheong, Thin Solid Films 420–421 (2002) 62–69. w6x X. Shi, B.K. Tay, H.S. Tan, Li. Zhong, Y.Q. Tu, S.R.P. Silva, W.I. Milne, J. Appl. Phys. 79 (1996) 7234–7239. w7x S.M. Sze, VLSI Technology, AT and T Bell Laboratories, Murray Hill, New Jersey. w8x J. Diaz, G. Paolicelli, S. Ferrer, F. Comin, Phys. Rev. B 54 (1996) 8064. w9x P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Appl. Surf. Sci. 136 (1998) 105–110. w10x X. Shi, L.K. Cheah, B.K. Tay, Thin Solid Films 312 (1998) 160–169. w11x B.K. Tay, X. Shi, L.K Cheah, D.I. Flynn, Thin Solid Films 308–309 (1997) 268–272. w12x A.R. Forouhi, I. Bloomer, Phys. Rev B 34 (1986) 7018. w13x A.R. Forouhi, I. Bloomer, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, New York, 1995, p. 151. w14x J.R. Shi, X. Shi, Z. Sun, E. Liu, B.K. Tay, S.P. Lau, Thin Solid Films 366 (2000) 169–174. w15x M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (6) (1994) 3823–3830. w16x R. Haerle, A. Pasquarello, A. Baldereschi, Comput. Mater. Sci. 22 (2001) 67–72. w17x J. Robertson, E.P. O’Reilly, Phys. Rev. B35 (1987) 2946. w18x W. Jacob, W. Moller, Appl. Phys. Lett. 63 (1993) 1771. w19x J. Robertson, Philos. Mag. B 66 (2) (1992) 199.
Optical and electrical properties of amorphous carbon films deposited using filtered cathodic vacuum arc with pulse biasing J.Y. Szea,*, B.K. Taya, D. Sheejaa, S.P. Laua, Y.Q. Fub, Daniel H.C. Chuac, W.I. Milnec a
School of Electrical and Electronic Engineering, NTU, Block S1-B3- 01, Singapore 639798, Singapore b School of Mechanical and Production Engineering, NTU, Singapore c Engineering Department, University of Cambridge, Cambridge CB2 1PZ, UK
Abstract Passive devices using metal containing amorphous (a-C) films have been successfully fabricated. However, difficulties in the etching of these films as well as their inferior inertness compared to pure a-C films led us to study the electrical and optical properties of pure a-C films. The films were deposited using a filtered cathodic vacuum arc system (FCVA) in conjunction with high substrate pulse biasing. It is possible to control the sp2 content and hence the properties, by varying the substrate pulse bias voltage. In this study, the a-C films were prepared by varying the high substrate bias between 3 and 11 kV using a Plasma immersion ion implantation (PI3 ) system. Characterization of these samples gives us an indication about the suitability of the films for integrated passive devices and other applications. Four-point probe measurement has been carried out to study the resistivity of the films deposited on quartz and SiO2 . The resistivity decreases with increasing pulse bias voltage, which is likely attributed to the sp2 fraction in the film as well as the substrates’ resistivity. The sp2 content in the films is estimated using XPS and Raman spectroscopy. Optical properties of the films are characterized using spectroscopic phase-modulated ellipsometry. The band gap decreases from 2.3 to 1.49 with increasing bias voltage. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Amorphous carbon; Filtered cathodic vacuum arc; Plasma ion immersion implantation; Electrical and optical properties
1. Introduction In today’s demand for smaller and complex electronic functions, research is constantly being done on thin film integrated passive technologies to exploit their advantages of being highly integrated and reduced power consumption. The types of thin film materials most commonly used in resistors are carbon, tantalum nitride and nichrome. Nichrome has the advantages of being precise and thermally stable. However, it faces problems of susceptibility to chemical and electrolytic corrosion. Carbon type resistors, on the other hand, have a poor temperature coefficient so they are not suited for precision applications requiring little resistance change over temperature. Tantalum is considered as relatively expensive compared to carbon or nichrome. Thus, other types of suitable materials including diamond-like films (taC) have been studied as alternatives in thin film resis*Corresponding author. Tel.: q65-67906127; fax: q65-67933318. E-mail address: [email protected] (J.Y. Sze).
tors. However, the extremely high electrical resistivity, in the region of 109 V-cm w1x, had restricted the application of ta-C. This had led to studying on using metal containing amorphous carbon (a-C:metal) films. Metal containing a-C films, however, have a poorer etching rate compared to pure amorphous carbon (a-C) films. The etching rate, using reactive ion etching (RIE) of a-C:Ti films reached only a maximum of 6 nmymin while the etching rate of a-C film was approximately 30 nmymin w2x. Moreover, the resistivity values obtained tend to be more suitable for resistors of exceptionally high value as the resistivity values are in the range of 96–1000 Vysq for a-C:Ti w3x. In recent years, the use of applied high substrate bias (in the range of negative kV) to deposit a-C films has been shown to result in good tribological properties and high adhesive strength w4x. By changing the pulse width and frequency of the PI3, reduction of compressive stress was achieved in order to grow thick film w5x. These improvements show that the use of PI3 together with FCVA can be investigated further to understand the
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01090-3
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
149
effects of high substrate biasing on the electrical and optical properties of a-C films. If the range of desired resistivity can be achieved, these films will become better alternative thin film resistors. 2. Experimental details The carbon plasma was produced from a vacuum arc spot on a 60-mm in diameter cathode target. The cathode target has a 99.99% pure carbon content with the macroparticle and neutrals in the plasma filtered out by the FCVA system. The FCVA system has been described in detail elsewhere w6x. All depositions were carried out at room temperature and the substrate holder was negatively biased using the PI3 system w5x. While the frequency and pulse width was fixed at 600 Hz and 25 ms, respectively, the voltage was varied from y3 to y11 kV for each set of substrates. The samples were deposited for 3 min. Three types of substrates were used. They were highly doped nqq Si N111M, SiO2 and quartz (fused silica). The SiO2 used were polished silicon wafers of 0.62-mm average thickness coated with 500 nm of SiO2. They are studied since SiO2 is a common interlayer dielectric material in the semiconductor industry. A surface profiler was then used to determine the film thickness after deposition. The thickness varied from 41 to 80 nm with noticeably thinner films being deposited on quartz. The sheet resistivity of the films deposited on silicon dioxide and quartz were measured using the Alessi C4s collinear four-point probe (Cascade Microtech Inc.) with 1.3 mm probe spacing. The voltage was measured via the inner probes while a small current (approx. nA) was forced through the pair of outer probes. The sheet resistivity was calculated using rss C*(VyI) where C is the correction factor of 4.4516 w7x. The structure changes in the films were evaluated by Micro Raman Spectroscopy (Raman Imaging Microscope, Renishaw) that uses an Arq ion green laser (514.5 nm). X-ray photoelectron spectroscopy (XPS) was used to estimate the relative concentrations of sp3 and sp2 hybrids in a-C films w8,9x. The C 1s photoemission spectrum was deconvoluted into three components. The components were mixtures of a Gaussian and a Lorentzian distribution. The Gaussian component accounted for the instrumental energy resolution together with the chemical disorder, and the Lorentzian for the lifetime of the photoionization process. The two main components were found at 284.4 and 285.2 eV, and were attributed to sp3 and sp2 hybridized carbon atoms w16x. A third peak was determined at approximately 286.0 eV and was attributed to some C–O contamination formed at the surface of the samples due to exposure to air w9x. To measure the optical properties of the films, spectroscopic phase-modulated ellipsometry was performed
Fig. 1. Sheet resistivity of a-C films on SiO2 and quartz substrate bias.
on a-C films deposited on silicon substrates. The fourlayer model described by Shi et al. w10,11x was used to fit our experimental data and to determine the five parameters of the Forouhi and Bloomer (F.B.) optical dispersion model which had been shown to be appropriate for amorphous diamond-like carbon films w10– 12x. The mechanism for optical absorption is the interband excitations between the bonding and antibonding p bands. The model enabled the determination of the imaginary part of the refractive index (k) over the range of interest and the determination of the real part (n) within an integral constant via the Kramer–Kronig relation w12x. The four-layer model comprised of (SiN111M)y(graded a-C:Si)y(a-C)y(a-Cqvoid). The graded layer consists of a mixture of a-C and silicon simulating carbon ion bombardment into the surface of the silicon substrate due to high ion energy and to account for surface roughness effect, the top layer included a void in the aC films. The five parameters are A, B, C, Eg and n(`) and they follow their physical significance as described in many reviews w10–13x. The spectrum range was measured from 250 to 800 nm with an angle of incidence of 708. 3. Experimental results 3.1. Electrical resistivity vs. varying bias Fig. 1 shows a plot of sheet resistivity rs as a function of substrate bias. It was found that rs decreased from 26.9 MVysq to 7.4 MVysq for quartz with the increase of bias voltage from y3 to y11 kV. However, the sheet resistivity for SiO2 varies greatly from 12 MVysq to 6.8 kVysq in the same voltage range. It is also noted that with the increase in substrate bias, the slopes of the
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
150
Fig. 2. Raman spectra obtained for a-C films on Si at varying applied bias.
two curves decrease at different rates. This leads to the possibility that the layer of SiO2, which is relatively more conducting than quartz, affects the resistivity of the a-C film. The bulk resistivity of the films was determined by rsrs *t, where t is the thickness of the film. The resistivity values of a-C films on SiO2 decrease from 63 to 0.058 V-cm, with increasing in bias voltage. 3.2. Microstructure spectroscopy
of
the
films
using
Raman
From Fig. 2, it can be clearly seen that the peak at 970 cmy1, due to the second phonon scattering of the silicon substrate, is not observable when the substrate bias was more than y5 kV. This indicates a reduction in transparency of the films and an increase in sp2 bonds w5x. There is, however, not a significant change in the peaks between y3 and y5 kV. The spectra were fitted with two Gaussian components—disorder (‘D’) and graphite (‘G’) peaks w5,14,15x. Data from the Raman spectra showed that the G-peak width and D-peak width did not vary significantly. Fig. 3 shows that the calculated ratio (ID yIG) of the films increases with increasing substrate bias. The increase in ID yIG ratio was expected along with a decrease in the sp3 ysp2 ratio in the films as the substrate bias increased as reported in previous experiments w5x. In particular, films deposited on SiO2 had the greatest change in ID yIG ratio. The difference in ID yIG ratio evidently indicates how the microstructure of the films deposited on different substrates would differ from each other.
Fig. 3. IDyIG ratio of a-C films on quartz, SiO2 and silicon substrates.
show a shift of peaks towards the left indicating an increase in sp2 carbon bonds. The C 1s spectra were deconvoluted into the three components shown in Fig. 5. The fitting of the C 1s peaks was performed using two main components, being a mixture of a Gaussian and a Lorentzian distribution. The first two components were found at approximately 284.4"0.1 eV (sp2 carbon bonds) and 285.2"0.1 eV (sp3 carbon bonds). The sp3 fraction was calculated as the ratio of the area of the sp3 component over the total area of the two components. A decreasing trend of the sp3 fraction was observed, which is plotted in Fig. 6. A third peak (C– O) of much smaller intensity was also detected at 286.0 eV. It was observed that after etching was done, the C–
3.3. C 1s core level spectra analysis using XPS The C 1s spectra of a-C films deposited on Si substrates, after etching, are plotted in Fig. 4. The spectra
Fig. 4. C 1s XPS spectra of a-C films deposited on Si at different negative substrate bias.
J.Y. Sze et al. / Thin Solid Films 447 – 448 (2004) 148–152
Fig. 5. Fitting of C 1s XPS spectra at y5 kV.
151
Fig. 7. F.B. optical band gap plotted against the sp3 fraction for a-C films on Si substrates.
From Fig. 6 shows that the sp3 fraction for films deposited on silicon substrates decreases from 35.89 to 28.16% as the substrate bias varied from y3 to y11 kV. For comparison, an a-C film was deposited on Si at floating bias and investigated by XPS. The estimated sp3 fraction was 75.2%. 3.4. Optical band gap using spectroscopic ellipsometry
Fig. 6. Estimated percentage of sp3 fraction in a-C films deposited on Si substrates.
O area as a percentage of the overall area was reduced and the sp3 fraction content increased. This is in agreement with P. Merel et al. w9x who attributed to some C– O contamination formed at the surface of the samples due to air exposure. The sp3 fraction estimated by XPS correlates well with the ID yIG ratio obtained by Raman spectroscopy.
The optical band gap is the measure of the gap between the extended state in the valence band and the conduction band. The optical band gap in the a-C films deposited on Si was found to vary between 1.49 and 2.3 ("0.05) eV. The largest band gap, determined at a negative bias voltage of y3 kV coincided with a larger amount of sp3 bonds found in the film by XPS. The optical band gap’s correlation with the amount of sp3 fraction in the a-C film is plotted in Fig. 7. The five parameters of the F.B. model were fitted using the fourlayer model and are shown in Table 1. 4. Discussion In measuring the electrical resistivity of the a-C films, an interesting comparison was made between the resistivity values obtained by using of two different substrates. The rate of decrease in resistivity values for the
Table 1 Results for F.B. parameters fitted with the four-layer model Film (kV)
A ("0.005)
B ("0.05)
C ("0.5)
Eg ("0.05) (eV)
´(`) ("0.05)
3 5 8 11
0.777 0.792 0.751 0.709
5.459 5.128 4.794 4.610
8.721 7.545 6.705 6.483
2.3 2.14 1.81 1.49
3.713 3.235 3.230 2.896
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SiO2 substrate was higher than that of quartz (Fig. 1). A possible explanation is that the resistivity of quartz is much higher than that of the SiO2 –Si substrates and the surface of the quartz, during deposition, is likely to have positive charging effects during high voltage bias. More of the higher energy ions may be accelerated towards the SiO2 substrate instead, creating more sp2 sites and hence higher conductivity a-C films. This was also reflected by the differences in the microstructures of the a-C films as determined by Raman spectroscopy. Indeed, it was observed that the differences in the number of incoming ions arriving on the substrates could have even affected the thickness and electrical properties of the films. Amorphous carbon films grown on quartz tend to have lower ID yIG ratio and lower deposition rate while having higher resistivity values. As the electrical and optical properties are related to the sp2 clusters present in the film w17,18x, XPS had been carried out to estimate the sp3 fraction. As the substrate bias was increased to y11 kV, the sp3 fraction decreased to only 28.16%. As compared to having 75.2% sp3-bonded atoms at floating bias, the films had undergone graphitization when high bias was applied. It is likely that the high bias had accelerated the incoming ions with an ion energy more than what was needed (100 eV), thus the ions penetrated into the inner layers. This might change some of the present sp3 to sp2 bonds w5x. The graphitization of the films might have occurred in two manners—an increase of the number of sp2 sites or an increase in sp2 cluster size. If it were because of the increase in cluster size, the optical band gap would be expected to be less than 0.5 eV, (i.e. closer to the band gap of graphite). However, the optical band gap in the films was found to be between 1.49 and 2.3. A likely explanation was that there is a very large fraction of sp2 sites that relates to the drop in resistivity and the low sp3 fraction but the cluster size remained small. This reflects the significant optical band gap w19x. When the optical band gap was related to the sp3 fraction in Fig. 7, it could be predicted that as the sp3 fraction increases, the optical band gap would likely saturate at the optical band gap of ta-C films (approx. 2.7 eV) containing approximately 85% of sp3 carbon w8,19x. 5. Conclusions The PI3 technology with FCVA has been successfully used to deposit a-C films of lower resistivity in a more
useful range between 12 MVysq to 6.8 kVysq when SiO2 was used as a substrate. It has been suggested that the improved conductivity is due to the increase in higher energy ions arriving at the substrates that results in an increased amount of sp2 sites. This paper has further shown that different substrate materials will affect the properties of a-C films deposited by PI3 technique. When compared to ta-C films deposited without the use of PI3, the resistivity values had fallen up to 6 times. Considering that sheet resistance for the integrated thin film resistors is presently offered in the widest range from 1 Vysq to 100 kVysq, there is a possibility of using a-C films deposited using high substrate bias for electrical applications. References w1x X. Shi, H. Fu, J.R. Shi, L.K. Cheah, B.K. Tay, P. Hui, J. Phys.: Condens. Matter. 10 (41) (1998) 9293. w2x L.J. Yu, D. Sheeja, B.K. Tay, D.H.C. Chua, W.I. Milne, J. Miao, Y.Q. Fu, Appl. Surf. Sci. (2002) 107–116. w3x Hybrid Microcircuit Technology Handbook, Noyes Publications, Park Ridge, N.J., 1988. w4x D. Sheeja, B.K. Tay, S.P. Lau, X. Shi, Wear 249 (2001) 433–439. w5x D. Sheeja, B.K. Tay, L.J. Yu, S.P. Lau, J.Y. Sze, C.K. Cheong, Thin Solid Films 420–421 (2002) 62–69. w6x X. Shi, B.K. Tay, H.S. Tan, Li. Zhong, Y.Q. Tu, S.R.P. Silva, W.I. Milne, J. Appl. Phys. 79 (1996) 7234–7239. w7x S.M. Sze, VLSI Technology, AT and T Bell Laboratories, Murray Hill, New Jersey. w8x J. Diaz, G. Paolicelli, S. Ferrer, F. Comin, Phys. Rev. B 54 (1996) 8064. w9x P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Appl. Surf. Sci. 136 (1998) 105–110. w10x X. Shi, L.K. Cheah, B.K. Tay, Thin Solid Films 312 (1998) 160–169. w11x B.K. Tay, X. Shi, L.K Cheah, D.I. Flynn, Thin Solid Films 308–309 (1997) 268–272. w12x A.R. Forouhi, I. Bloomer, Phys. Rev B 34 (1986) 7018. w13x A.R. Forouhi, I. Bloomer, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, New York, 1995, p. 151. w14x J.R. Shi, X. Shi, Z. Sun, E. Liu, B.K. Tay, S.P. Lau, Thin Solid Films 366 (2000) 169–174. w15x M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (6) (1994) 3823–3830. w16x R. Haerle, A. Pasquarello, A. Baldereschi, Comput. Mater. Sci. 22 (2001) 67–72. w17x J. Robertson, E.P. O’Reilly, Phys. Rev. B35 (1987) 2946. w18x W. Jacob, W. Moller, Appl. Phys. Lett. 63 (1993) 1771. w19x J. Robertson, Philos. Mag. B 66 (2) (1992) 199.