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Investigations on electrical properties of a-C:H thin films deposited in a Microwave Multipolar Plasma reactor excited at Distributed Electron Cyclotron Resonance
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Diamond & Related Materials 17 (2008) 1710 – 1715 www.elsevier.com/locate/diamond
Investigations on electrical properties of a-C:H thin films deposited in a Microwave Multipolar Plasma reactor excited at Distributed Electron Cyclotron Resonance M. Kihel a , R. Clergereaux b,⁎, D. Escaich b , M. Calafat b , P. Raynaud b , S. Sahli a , Y. Segui b a
b
Laboratoire de Microsystèmes et Instrumentation, Université Mentouri de Constantine, Algeria Universités de Toulouse- LAPLACE- CNRS, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse cedex, France Available online 24 January 2008
Abstract Amorphous hydrogenated carbon (a-C:H) thin films have been deposited from pure methane discharges in Microwave Multipolar Plasma excited at Distributed Electron Cyclotron Resonance (MMP-DECR). Investigations on the effect of process parameters on the film physicochemical and electrical properties have been carried out. The plasma discharge power has a significant effect on the film density and on the concentrations of sp3- and sp2-hybridized carbon atoms. These latter (sp3 and sp2 fractions) are also dependant on the deposition time. A low sp2 fraction was obtained in films deposited at high plasma discharge power and/or long deposition time. Moreover, increasing the plasma discharge power leads to less dense films. The film permittivity at 1 kHz is ranging from 3.8 to 2.3 depending on the two process parameters. This evolution of the dielectric constant is correlated to the film density and structure. The current voltage characteristics I (V) of Metal-Insulating-Metal structures using the different films suggest that the carrier transport in a-C:H thin films deposited by MMP-DECR is limited by a space charge conduction mechanism. © 2008 Elsevier B.V. All rights reserved. Keywords: Amorphous hydrogenated carbon; Plasma CVD; Electrical properties characterization; Electrical conductivity
1. Introduction Amorphous hydrogenated carbon layers (a-C:H) are frequently deposited by plasma enhanced chemical vapor deposition processes (PE-CVD). This technology is very promising because of its high control of film quality, its easy integration in current technologies, its low cost, high efficiency and reproducibility. Recently, it has been shown that such layers have various properties [1,2]. As for example, a-C:H thin films deposited in radiofrequency (RF) plasma reach low dielectric constant values [3–5]. Indeed, these films can be deposited with a wide range of structure (from soft-polymer-like carbon and graphitic forms to hard a-C:H and diamond like carbon (DLC) [8,9])
⁎ Corresponding author. Tel.: +33 5 61 55 67 97; fax: +33 5 61 55 64 52. E-mail address: [email protected] (R. Clergereaux). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.01.036
leading to a wide range of dielectric constant values [3– 5,10,11]. Then, as such layers can be easily patterned, they could be used in microelectronic [6,7]. Unfortunately, RF plasma deposited a-C:H films show a low thermal resistance [4]. In contrast to RF plasma processes, Microwave Multipolar Plasma excited at Distributed Electron Cyclotron Resonance (MMP-DECR) are really interesting as in this type of discharge, it is possible to uncouple the effect of the different deposition mechanisms as the monomer dissociation, the chemical reactions of radicals and the effect of ions and then to reach a wider range of physico-chemical structures. Moreover, in this kind of discharge, low dielectric constant materials ( b 3) have been reported with high thermal stability ( N 400 °C) [12]. Some works have been reported on a-C:H thin films deposited in MMP-DECR, where the effects of different plasma parameters such as the gas mixture, the discharge power and the bias
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substrate have been pointed out [13–16] and, in a recent work, we have found that the plasma discharge power and the deposition time have a significant effect on the film structure [14]. In this paper, we have carried out some investigations on the evolution of electrical properties as function of the deposition conditions in order to show some links between the film structure and the electrical properties. 2. Experimental a-C:H thin films were deposited in a MMP-DECR reactor described elsewhere [17]. The films were deposited from pure methane injected at a pressure of about 0.1 Pa. The microwave power was varied from 120 to 800 W and the deposition times were 5, 15 and 30 min. The two types of substrates ((100) intrinsic silicon wafers and aluminum metallized glass) were not cooled down during the deposition process and they were kept at a floating potential during the process. The films deposited on silicon substrates have been used for Infra Red Spectroscopy analysis (carried out on a Bio-Rad FTS60A spectrometer in the absorption mode), for density measurements (using a microbalance) and for thickness and optical indexes measurements (using a spectroscopic ellipsometer Sopra GES-5 in the range of 0.25–2 μm in a micro-spot configuration). Electrical measurements have been carried out on Metal-Insulator-Metal (MIM) structures, formed by evaporating aluminum electrodes on a-C:H films deposited on the metallized glass substrates. The dielectric constant was measured by a LCZ meter (HP4280A) working in the frequency range of 20 Hz–1 MHz and the electrical current measurements have been recorded using an electrometer (Keithley 6512). 3. Results and discussion 3.1. Physical and chemical analysis Fig. 1a and b show the variation of the deposition rate, the hydrogen concentration and the fraction of sp2-hybridized carbon with the discharge power and the deposition time. The hydrogen concentration has been calculated from IR spectra in the range of 2800–3200 cm− 1 (linked to CHx bonds) [18], whereas the sp2 fraction was calculated from the optical indexes found by ellipsometry [19]. It appears that an increase of the microwave power and the deposition time produces less sp2hybridized material. Otherwise, in contrast with long deposition time, the hydrogen concentration increases with the microwave power. This implies that the film density decreases with the plasma power as it is shown in Fig. 1c. The film density value is ranging from 0.96 to 1.15 g cm− 3. Indeed, the increase of the sp3 CHx species leads to the formation of discontinuous carbon chain causing a porous structure which translates into a decrease of the film density [10]. Thus, microwave power and deposition time have pronounced effects on the film composition. This evolution can be linked to plasma–surface interaction phenomena occurring during the film growth. Increasing the
Fig. 1. Deposition rate and concentration of species in the deposited films as functions of (a) discharge power and (b) deposition time. (c) Represents films density variation as function of the discharge power.
microwave power leads to an increase of the degree of methane dissociation [20] and then to an increase of the hydrogen atom concentration in the discharge. As the substrate is not cooled down, the substrate temperature increases due to ion bombardment during film growth. Then, an erosion mechanism induced by hydrogen atoms can take place [21] as it appears in Fig. 1b in the reduction of the deposition rate for long processes at 800 W (the deposition rate varies from 47 to 116 Å/min). The existence of the erosion mechanism is confirmed by the decrease of the films density with the increase of the discharge power (Fig. 1c): the values of the films density are relative to films deposited during 60 min and, in the case of the deposition process at 800 W, there was no film on the substrate at this deposition time (absence of data for this power). The erosion mechanism leads to chemical reactions on the surface (mainly on sp2 CC bonds)
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to create sp3 CHx bonds in the film [14,20]. Therefore, the hydrogen concentration and the sp3 fraction increase, whereas the concentration of sp2 species decreases during film growth. From the modification of the microwave plasma power and the deposition time, it is then possible to reach a wide range of a-C:H film structure. 3.2. Dielectric properties The dielectric constant variation versus the frequency is plotted in Fig. 2a for five different conditions (120 W, 15 min/ 200 W, 15 min/800 W, 5 min/800 W, 15 min and 800 W, 30 min). For all these different films, dielectric constant values are below 4. For films deposited during 15 min, the dielectric constant decreases when the power increases from 120 to
200 W. As it has been published [22], this reduction is well correlated with the increase of sp3 fraction. However, when the power reached 800 W, the dielectric constant increases. This behavior may be related to the formation of defects and dangling bonds in the film induced by hydrogen erosion and that can lead to the ageing of the films obtained in this condition. Moreover, a minimum value (2.3) has been reached for films deposited at 800 W during 30 min. In order to understand the effect of film structure on its electrical properties, we define two characteristic parameters of the film structure: C sp2/C sp3 and C sp2/CH ratios. Fig. 2b and c show that depending on the microwave power, the dielectric constant follows the evolution of these two parameters. A shift towards the lower value of these ratios is observed when the microwave power increases. This shift can be assigned to the reduction of the film density. Therefore, the decrease of the dielectric constant value may result not only from the decrease of the species concentration having high polarization [10,23], but also from the decrease of the film density. It appears then that the low-k property of a-C:H thin films is highly linked to the deposition process. The increase of the microwave power induces a reduction of the sp2 fraction and an increase of the hydrogen concentration followed by a reduction of the density. Otherwise, as the deposition time increases, the substrate heating activates the erosion mechanism leading to a reduction of the concentration of sp2 carbon and hydrogen. Fig. 3a shows the variation of the dielectric losses versus the frequency of films deposited in different deposition conditions. The microwave discharge power has a small effect on the dielectric loss values. However, the value of the dielectric losses decreases from 1.10− 2 to 5.10− 3 when the deposition time is varied from 5 to 30 min. The dielectric losses evolution versus the frequency – slope of −1 for low frequencies (lower than about 5 · 102 Hz), of 1 for high frequencies (over a few 104 Hz) and a minimum values for medium frequencies – shows that the response of the MIM structure is well simulated by an equivalent electrical circuit composed of a capacitance Cp in parallel with a resistance Rp in serial with a contact resistance r. Thus, one can represent the dielectric losses by the following equation: tan d ¼ Rp Cp x
Fig. 2. (a) Dielectric constant frequencies evolution for a-C:H deposited with different discharge power and deposition times. (b) and (c) represents the dielectric constant variation as a function of Csp2/Csp3ratio and (c) Csp2/CH ratio respectively.
1
þK xs1 þ rCp x
ð1Þ
where K and s are constants. The dielectric losses data reported in Fig. 3a can easily be described by Eq. (1) (continuous line). It is then possible to evaluate the electrical parameters r and Rp of the film according to the value of Cp calculated from the dielectric constants found in Fig. 2a. The evolution of contact resistance as a function of the film thickness is presented in Fig. 3b. It appears that the two parameters, microwave power and film thickness have an effect on the contact resistance value. This latter varies from 2 to 15 Ω with the surface roughness: surface roughness decrease with deposition time except for long deposition time processes at high microwave plasma power. The electrical conductivity of the films estimated from the value of Rp is reported in Fig. 3c. The films have an insulating
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character with a conductivity value of about 10− 11 Ω− 1 cm− 1. These values are similar to those published for RF plasma deposited a-C:H films [24]. As it is well known [22,25,26], the conductivity of the a-C:H thin films is related to sp2-hybridized carbon fraction. Therefore, as well as for the dielectric constant, the conductivity of our films is strongly influenced by the microwave power and the deposition time.
Fig. 4. Time dependence of the charging and discharging currents.
3.3. Films response to dc excitation The responses of MIM structures to a dc voltage showed the presence of a significant transient electrical current in all samples. In Fig. 4, temporal variation of the charging and discharging currents for films deposited at 800 W during 15 min is plotted. The charging and discharging electrical currents are very different (the absolute value of discharging current is about one decade lower than the charging one). As the superposition of the charging and the discharging currents is generally characteristic of a dipolar mechanism [27], this latter could not explain the transient current measured in our films. The quasi-steady electrical current measured after applying a step voltage during 5 min, varies in the range 10− 12–10− 7 A when the applied electrical field (E) was varied from about 104 to 2.106 V cm− 1. Fig. 5a shows the evolution of the quasisteady electrical current as function of the square root of the applied electrical field for films elaborated at 800 W during 5 min. The variation of the current is nonlinear, this way excluding of any significant existence of Schottky or Pool– Frenkel charge transport mechanisms in our films [28,29]. However, a presence of a space charge mechanism would explain this behavior of the transient current [30]. Plotted in a bilogarithmic axis, the quasi-steady electrical current variation versus the electrical field increases with a slope close to 1 for low electrical field values and a slope varying from 2 to 4 for higher values (Fig. 5b). The evolution of the quasi-steady electrical current with the applied electrical field, observed for all samples elaborated under different conditions, can be described by the relation characterizing a space charge limited mechanism and given by [28,29]:
Fig. 3. (a) Variation of dielectric losses with frequency. Eq. (1) was used to fit the curves (continuous line on Fig. 3a). (b) Represents the contact resistance as function of the film thickness and (c) the conductivity evolution as a function of the sp2 carbon concentration (square represents the conductivity calculated from dielectric loss measurements and circle the one calculated from I (E) curves).
9 V2 J ¼ er e0 A 3 8 d
ð2Þ
where J is the electrical current density, εr the film permittivity, ε0 the vacuum permittivity, μ the carrier mobility, d the film thickness and V the applied voltage.
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structure and density resulting from the difference between the plasma deposition processes used. The only obvious change in I (V) curves is the increase of the electrical current when the microwave power increases or when the deposition time is shorter. Circles in Fig. 3c shows the evolution of the electrical conductivity calculated in the linear part of I (V) characteristics as function of the sp2 fraction. Its value is very low (about a few 10− 11 Ω− 1 cm− 1) and close to those calculated from the dielectric losses variation. Therefore, it appears that the two process parameters (microwave power and deposition time) lead to the deposition of films with different electrical properties. 4. Conclusions The structure of films deposited in microwave plasma from methane strongly depends on the deposition conditions. For high microwave powers, the films show a high fraction of sp3hybridized carbon atoms due to an erosion mechanism activated by the increase of the substrate temperature. The two process parameters, deposition time and microwave power, lead to a large range of materials with different structures, composition and density. It has been shown that the electrical properties of the films (dielectric constant, dielectric losses and conductivity) are strongly dependent on three structural parameters: C sp2/C sp3 and C sp2/CH ratios and film density. The study of the film responses to a dc bias voltage shows that the conduction seems to be limited by a space charge mechanism and the conductivity is strongly influenced by the deposition time and the microwave plasma power. a-C:H films deposited by Multipolar Microwave Plasma excited at Distributed Electron Cyclotron Resonance have potential applications in microelectronics. However, it is necessary to carry out further studies on their mechanical properties and thermal stability. Acknowledgements Fig. 5. Quasi-steady electrical current evolution for films deposited at 800 W/ 5 min as a function of (a) the square root of the electrical field, (b) as a function of the electrical field. (c) Represents the electrical current evolution as a function of the film thickness for an electric field of 2.105 V cm− 1.
This work is supported by CMEP-Tassili (04MDU613 project) and CNRS/DPRS programs. References
It appears then that the conduction mechanism in our films may be due to the formation of a space charge. To confirm this hypothesis, we have plotted in Fig. 5c the variation of the current, recorded on samples with different thicknesses and excited by the same electrical field value (E = 2.105 V cm− 1). The electrical current as a function of the thickness varies with a slope of − 3 according to the relation 2. Then, in contrast with literature, where Pool–Frenkel [31] and Fowler–Nordheim mechanism [32] was found, the conduction in the a-C:H thin films deposited in MMP-DECR is space charge limited. This difference in the assignment of the type of the conduction mechanism could be explained by the differences of the films
[1] A. Grill, Thin Solid Films 355–356 (1999) 189. [2] J.E. Bouree, C. Godet, R. Etemadi, B. Drévillon, Synthetic Metals 76/1–3 (1996) 191. [3] A. Grill, Thin Solid Films 398–399 (2001) 527. [4] A. Grill, Diamond and Related Materials 10 (2001) 234. [5] A. Grill, B.S. Meyeron, V.V. Patel, IBM Journal of Research and Development 34 (1990) 849. [6] A.P. Mousinho, R.D. Mansano, M. Massi, J.M. Jaramillo, Diamond and Related Materials 12/3–7 (2003) 1041. [7] J.C. Orlianges, C. Champeaux, A. Catherinot, A. Pothier, P. Blondy, Thin Solid Films 453–454 (2004) 291. [8] F. Piazza, International Journal of Refractory Metals & Hard Materials 24 (2006) 39. [9] J. Robertson, Materials Science and Engineering: R: Reports 37/4–6 (2002) 129.
M. Kihel et al. / Diamond & Related Materials 17 (2008) 1710–1715 [10] S.P. Louh, I.C. Leu, M.H. Hon, Diamond and Related Materials 14 (2005) 1000. [11] R.D. Mansano, U.M. Massi, L.S. Zambom, P. Verdonck, P.M. Nogueira, H.S. Maciel, C. Otani, Thin Solid Films 373 (2000) 243. [12] Y. Xin, S.H. Xu, Z.Y. Ning, X.H. Lu, M.F. Jiang, S. Huang, W. Du, J. Chen, C. Ye, S.H. Cheng, Chinese Physical Letters 20/3 (2003) 423. [13] B.K. Kim, T.A. Grotjohn, Diamond and Related Materials 9 (2000) 37. [14] R. Clergereaux, D. Escaich, I.S.d. Larclause, B. Bernecker, P. Raynaud, Diamond and Related Materials 15/4–8 (2006) 888. [15] B.H. Lung, M.J. Chiang, M.H. Hon, Thin Solid Films 392 (2001) 16. [16] M. Zarrabian, N. Fourches-Coulon, G. Turban, M. Lancin, C. Marhic, Diamond and Related Materials 6 (1997) 542. [17] J. Pelletier, Y. Arnal, B. Petit, C. Pomot, M. Pichot, Journal of Physics. D, Applied Physics 19/5 (1986) 795. [18] N. Basu, J. Dutta, S. Chaudhuri, A.K. Pal, M. Nakayama, Vacuum 47 (1996) 233. [19] N. Savvides, Journal of Applied Physics 59 (1986) 4133. [20] A. Rhallabi, A. Catherinot, IEEE Transactions on plasma science 19 (1991) 270. [21] A. von Keudell, T. Schwarz-Selinger, W. Jacob, Journal of Applied Physics 89/5 (2001) 2979.
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[22] A.P. Mousinho, R.D. Mansano, P. Verdonck, Diamond and Related Materials 13 (2004) 311. [23] S.P. Louh, M.H. Hon, Diamond and Related Materials 14 (2005) 1815. [24] L.A. Romanko, A.G. Gontar, A.M. Kutsay, S.I. Khandozko, V.Y. Gorokhov, Diamond and Related Materials 9 (2000) 801. [25] D.R. McKenzie, Reports on Progress in Physics 59 (1996) 1611. [26] Y. Lifshitz, Diamond and Related Materials 12 (2003) 130. [27] M. Lamhamdi, J. Guastavino, L. Boudou, Y. Segui, P. Pons, L. Bouscayrol, R. Plana, Microelectronics Reliability 46/9–11 (2006) 1700. [28] T.C. Chang, S.T. Yan, P.T. Liu, Z.W. Lin, H. Aoki, S.M. Sze, Thin Solid Films 447–448 (2004) 516. [29] J.J. O'Dwyer, Theory of Electrical Conduction and Breakdown in Solid Dielectrics, Clarendon Press, Oxford, England, 1973. [30] D.K. Das-Gupta, K. Doughty, R.S. Brockley, Journal of Physics. D, Applied Physics 13/11 (1980) 2101. [31] K. Yokota, Y. Miyoshi, M. Saoyama, Japanese Journal of Applied Physics 45/2A (2006) 860. [32] N. Tomozeiu, A. Hart, B. Kleinsorge, W.I. Milne, Diamond and Related Materials 8 (1999) 522.
Diamond & Related Materials 17 (2008) 1710 – 1715 www.elsevier.com/locate/diamond
Investigations on electrical properties of a-C:H thin films deposited in a Microwave Multipolar Plasma reactor excited at Distributed Electron Cyclotron Resonance M. Kihel a , R. Clergereaux b,⁎, D. Escaich b , M. Calafat b , P. Raynaud b , S. Sahli a , Y. Segui b a
b
Laboratoire de Microsystèmes et Instrumentation, Université Mentouri de Constantine, Algeria Universités de Toulouse- LAPLACE- CNRS, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse cedex, France Available online 24 January 2008
Abstract Amorphous hydrogenated carbon (a-C:H) thin films have been deposited from pure methane discharges in Microwave Multipolar Plasma excited at Distributed Electron Cyclotron Resonance (MMP-DECR). Investigations on the effect of process parameters on the film physicochemical and electrical properties have been carried out. The plasma discharge power has a significant effect on the film density and on the concentrations of sp3- and sp2-hybridized carbon atoms. These latter (sp3 and sp2 fractions) are also dependant on the deposition time. A low sp2 fraction was obtained in films deposited at high plasma discharge power and/or long deposition time. Moreover, increasing the plasma discharge power leads to less dense films. The film permittivity at 1 kHz is ranging from 3.8 to 2.3 depending on the two process parameters. This evolution of the dielectric constant is correlated to the film density and structure. The current voltage characteristics I (V) of Metal-Insulating-Metal structures using the different films suggest that the carrier transport in a-C:H thin films deposited by MMP-DECR is limited by a space charge conduction mechanism. © 2008 Elsevier B.V. All rights reserved. Keywords: Amorphous hydrogenated carbon; Plasma CVD; Electrical properties characterization; Electrical conductivity
1. Introduction Amorphous hydrogenated carbon layers (a-C:H) are frequently deposited by plasma enhanced chemical vapor deposition processes (PE-CVD). This technology is very promising because of its high control of film quality, its easy integration in current technologies, its low cost, high efficiency and reproducibility. Recently, it has been shown that such layers have various properties [1,2]. As for example, a-C:H thin films deposited in radiofrequency (RF) plasma reach low dielectric constant values [3–5]. Indeed, these films can be deposited with a wide range of structure (from soft-polymer-like carbon and graphitic forms to hard a-C:H and diamond like carbon (DLC) [8,9])
⁎ Corresponding author. Tel.: +33 5 61 55 67 97; fax: +33 5 61 55 64 52. E-mail address: [email protected] (R. Clergereaux). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.01.036
leading to a wide range of dielectric constant values [3– 5,10,11]. Then, as such layers can be easily patterned, they could be used in microelectronic [6,7]. Unfortunately, RF plasma deposited a-C:H films show a low thermal resistance [4]. In contrast to RF plasma processes, Microwave Multipolar Plasma excited at Distributed Electron Cyclotron Resonance (MMP-DECR) are really interesting as in this type of discharge, it is possible to uncouple the effect of the different deposition mechanisms as the monomer dissociation, the chemical reactions of radicals and the effect of ions and then to reach a wider range of physico-chemical structures. Moreover, in this kind of discharge, low dielectric constant materials ( b 3) have been reported with high thermal stability ( N 400 °C) [12]. Some works have been reported on a-C:H thin films deposited in MMP-DECR, where the effects of different plasma parameters such as the gas mixture, the discharge power and the bias
M. Kihel et al. / Diamond & Related Materials 17 (2008) 1710–1715
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substrate have been pointed out [13–16] and, in a recent work, we have found that the plasma discharge power and the deposition time have a significant effect on the film structure [14]. In this paper, we have carried out some investigations on the evolution of electrical properties as function of the deposition conditions in order to show some links between the film structure and the electrical properties. 2. Experimental a-C:H thin films were deposited in a MMP-DECR reactor described elsewhere [17]. The films were deposited from pure methane injected at a pressure of about 0.1 Pa. The microwave power was varied from 120 to 800 W and the deposition times were 5, 15 and 30 min. The two types of substrates ((100) intrinsic silicon wafers and aluminum metallized glass) were not cooled down during the deposition process and they were kept at a floating potential during the process. The films deposited on silicon substrates have been used for Infra Red Spectroscopy analysis (carried out on a Bio-Rad FTS60A spectrometer in the absorption mode), for density measurements (using a microbalance) and for thickness and optical indexes measurements (using a spectroscopic ellipsometer Sopra GES-5 in the range of 0.25–2 μm in a micro-spot configuration). Electrical measurements have been carried out on Metal-Insulator-Metal (MIM) structures, formed by evaporating aluminum electrodes on a-C:H films deposited on the metallized glass substrates. The dielectric constant was measured by a LCZ meter (HP4280A) working in the frequency range of 20 Hz–1 MHz and the electrical current measurements have been recorded using an electrometer (Keithley 6512). 3. Results and discussion 3.1. Physical and chemical analysis Fig. 1a and b show the variation of the deposition rate, the hydrogen concentration and the fraction of sp2-hybridized carbon with the discharge power and the deposition time. The hydrogen concentration has been calculated from IR spectra in the range of 2800–3200 cm− 1 (linked to CHx bonds) [18], whereas the sp2 fraction was calculated from the optical indexes found by ellipsometry [19]. It appears that an increase of the microwave power and the deposition time produces less sp2hybridized material. Otherwise, in contrast with long deposition time, the hydrogen concentration increases with the microwave power. This implies that the film density decreases with the plasma power as it is shown in Fig. 1c. The film density value is ranging from 0.96 to 1.15 g cm− 3. Indeed, the increase of the sp3 CHx species leads to the formation of discontinuous carbon chain causing a porous structure which translates into a decrease of the film density [10]. Thus, microwave power and deposition time have pronounced effects on the film composition. This evolution can be linked to plasma–surface interaction phenomena occurring during the film growth. Increasing the
Fig. 1. Deposition rate and concentration of species in the deposited films as functions of (a) discharge power and (b) deposition time. (c) Represents films density variation as function of the discharge power.
microwave power leads to an increase of the degree of methane dissociation [20] and then to an increase of the hydrogen atom concentration in the discharge. As the substrate is not cooled down, the substrate temperature increases due to ion bombardment during film growth. Then, an erosion mechanism induced by hydrogen atoms can take place [21] as it appears in Fig. 1b in the reduction of the deposition rate for long processes at 800 W (the deposition rate varies from 47 to 116 Å/min). The existence of the erosion mechanism is confirmed by the decrease of the films density with the increase of the discharge power (Fig. 1c): the values of the films density are relative to films deposited during 60 min and, in the case of the deposition process at 800 W, there was no film on the substrate at this deposition time (absence of data for this power). The erosion mechanism leads to chemical reactions on the surface (mainly on sp2 CC bonds)
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to create sp3 CHx bonds in the film [14,20]. Therefore, the hydrogen concentration and the sp3 fraction increase, whereas the concentration of sp2 species decreases during film growth. From the modification of the microwave plasma power and the deposition time, it is then possible to reach a wide range of a-C:H film structure. 3.2. Dielectric properties The dielectric constant variation versus the frequency is plotted in Fig. 2a for five different conditions (120 W, 15 min/ 200 W, 15 min/800 W, 5 min/800 W, 15 min and 800 W, 30 min). For all these different films, dielectric constant values are below 4. For films deposited during 15 min, the dielectric constant decreases when the power increases from 120 to
200 W. As it has been published [22], this reduction is well correlated with the increase of sp3 fraction. However, when the power reached 800 W, the dielectric constant increases. This behavior may be related to the formation of defects and dangling bonds in the film induced by hydrogen erosion and that can lead to the ageing of the films obtained in this condition. Moreover, a minimum value (2.3) has been reached for films deposited at 800 W during 30 min. In order to understand the effect of film structure on its electrical properties, we define two characteristic parameters of the film structure: C sp2/C sp3 and C sp2/CH ratios. Fig. 2b and c show that depending on the microwave power, the dielectric constant follows the evolution of these two parameters. A shift towards the lower value of these ratios is observed when the microwave power increases. This shift can be assigned to the reduction of the film density. Therefore, the decrease of the dielectric constant value may result not only from the decrease of the species concentration having high polarization [10,23], but also from the decrease of the film density. It appears then that the low-k property of a-C:H thin films is highly linked to the deposition process. The increase of the microwave power induces a reduction of the sp2 fraction and an increase of the hydrogen concentration followed by a reduction of the density. Otherwise, as the deposition time increases, the substrate heating activates the erosion mechanism leading to a reduction of the concentration of sp2 carbon and hydrogen. Fig. 3a shows the variation of the dielectric losses versus the frequency of films deposited in different deposition conditions. The microwave discharge power has a small effect on the dielectric loss values. However, the value of the dielectric losses decreases from 1.10− 2 to 5.10− 3 when the deposition time is varied from 5 to 30 min. The dielectric losses evolution versus the frequency – slope of −1 for low frequencies (lower than about 5 · 102 Hz), of 1 for high frequencies (over a few 104 Hz) and a minimum values for medium frequencies – shows that the response of the MIM structure is well simulated by an equivalent electrical circuit composed of a capacitance Cp in parallel with a resistance Rp in serial with a contact resistance r. Thus, one can represent the dielectric losses by the following equation: tan d ¼ Rp Cp x
Fig. 2. (a) Dielectric constant frequencies evolution for a-C:H deposited with different discharge power and deposition times. (b) and (c) represents the dielectric constant variation as a function of Csp2/Csp3ratio and (c) Csp2/CH ratio respectively.
1
þK xs1 þ rCp x
ð1Þ
where K and s are constants. The dielectric losses data reported in Fig. 3a can easily be described by Eq. (1) (continuous line). It is then possible to evaluate the electrical parameters r and Rp of the film according to the value of Cp calculated from the dielectric constants found in Fig. 2a. The evolution of contact resistance as a function of the film thickness is presented in Fig. 3b. It appears that the two parameters, microwave power and film thickness have an effect on the contact resistance value. This latter varies from 2 to 15 Ω with the surface roughness: surface roughness decrease with deposition time except for long deposition time processes at high microwave plasma power. The electrical conductivity of the films estimated from the value of Rp is reported in Fig. 3c. The films have an insulating
M. Kihel et al. / Diamond & Related Materials 17 (2008) 1710–1715
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character with a conductivity value of about 10− 11 Ω− 1 cm− 1. These values are similar to those published for RF plasma deposited a-C:H films [24]. As it is well known [22,25,26], the conductivity of the a-C:H thin films is related to sp2-hybridized carbon fraction. Therefore, as well as for the dielectric constant, the conductivity of our films is strongly influenced by the microwave power and the deposition time.
Fig. 4. Time dependence of the charging and discharging currents.
3.3. Films response to dc excitation The responses of MIM structures to a dc voltage showed the presence of a significant transient electrical current in all samples. In Fig. 4, temporal variation of the charging and discharging currents for films deposited at 800 W during 15 min is plotted. The charging and discharging electrical currents are very different (the absolute value of discharging current is about one decade lower than the charging one). As the superposition of the charging and the discharging currents is generally characteristic of a dipolar mechanism [27], this latter could not explain the transient current measured in our films. The quasi-steady electrical current measured after applying a step voltage during 5 min, varies in the range 10− 12–10− 7 A when the applied electrical field (E) was varied from about 104 to 2.106 V cm− 1. Fig. 5a shows the evolution of the quasisteady electrical current as function of the square root of the applied electrical field for films elaborated at 800 W during 5 min. The variation of the current is nonlinear, this way excluding of any significant existence of Schottky or Pool– Frenkel charge transport mechanisms in our films [28,29]. However, a presence of a space charge mechanism would explain this behavior of the transient current [30]. Plotted in a bilogarithmic axis, the quasi-steady electrical current variation versus the electrical field increases with a slope close to 1 for low electrical field values and a slope varying from 2 to 4 for higher values (Fig. 5b). The evolution of the quasi-steady electrical current with the applied electrical field, observed for all samples elaborated under different conditions, can be described by the relation characterizing a space charge limited mechanism and given by [28,29]:
Fig. 3. (a) Variation of dielectric losses with frequency. Eq. (1) was used to fit the curves (continuous line on Fig. 3a). (b) Represents the contact resistance as function of the film thickness and (c) the conductivity evolution as a function of the sp2 carbon concentration (square represents the conductivity calculated from dielectric loss measurements and circle the one calculated from I (E) curves).
9 V2 J ¼ er e0 A 3 8 d
ð2Þ
where J is the electrical current density, εr the film permittivity, ε0 the vacuum permittivity, μ the carrier mobility, d the film thickness and V the applied voltage.
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structure and density resulting from the difference between the plasma deposition processes used. The only obvious change in I (V) curves is the increase of the electrical current when the microwave power increases or when the deposition time is shorter. Circles in Fig. 3c shows the evolution of the electrical conductivity calculated in the linear part of I (V) characteristics as function of the sp2 fraction. Its value is very low (about a few 10− 11 Ω− 1 cm− 1) and close to those calculated from the dielectric losses variation. Therefore, it appears that the two process parameters (microwave power and deposition time) lead to the deposition of films with different electrical properties. 4. Conclusions The structure of films deposited in microwave plasma from methane strongly depends on the deposition conditions. For high microwave powers, the films show a high fraction of sp3hybridized carbon atoms due to an erosion mechanism activated by the increase of the substrate temperature. The two process parameters, deposition time and microwave power, lead to a large range of materials with different structures, composition and density. It has been shown that the electrical properties of the films (dielectric constant, dielectric losses and conductivity) are strongly dependent on three structural parameters: C sp2/C sp3 and C sp2/CH ratios and film density. The study of the film responses to a dc bias voltage shows that the conduction seems to be limited by a space charge mechanism and the conductivity is strongly influenced by the deposition time and the microwave plasma power. a-C:H films deposited by Multipolar Microwave Plasma excited at Distributed Electron Cyclotron Resonance have potential applications in microelectronics. However, it is necessary to carry out further studies on their mechanical properties and thermal stability. Acknowledgements Fig. 5. Quasi-steady electrical current evolution for films deposited at 800 W/ 5 min as a function of (a) the square root of the electrical field, (b) as a function of the electrical field. (c) Represents the electrical current evolution as a function of the film thickness for an electric field of 2.105 V cm− 1.
This work is supported by CMEP-Tassili (04MDU613 project) and CNRS/DPRS programs. References
It appears then that the conduction mechanism in our films may be due to the formation of a space charge. To confirm this hypothesis, we have plotted in Fig. 5c the variation of the current, recorded on samples with different thicknesses and excited by the same electrical field value (E = 2.105 V cm− 1). The electrical current as a function of the thickness varies with a slope of − 3 according to the relation 2. Then, in contrast with literature, where Pool–Frenkel [31] and Fowler–Nordheim mechanism [32] was found, the conduction in the a-C:H thin films deposited in MMP-DECR is space charge limited. This difference in the assignment of the type of the conduction mechanism could be explained by the differences of the films
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