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A comparative study of nickel silicides and nickel germanides: Phase formation and kinetics
Microelectronic Engineering 83 (2006) 2101–2106 www.elsevier.com/locate/mee
A comparative study of nickel silicides and nickel germanides: Phase formation and kinetics F. Nemouchi b
a,*
, D. Mangelinck a, J.L. La´ba´r b, M. Putero a, C. Bergman a, P. Gas
a
a L2MP, CNRS-UMR 6137, Univ. Paul Ce´zanne, Case 142, 13397 Marseille Cedex 20, France Research Institute for Technical Physics and Materials Science (MFA), H-1121 Budapest, Konkoly-Thege M. u´t 29-33, Hungary
Available online 18 October 2006
Abstract Nickel silicides and nickel germanides are under interest for their microelectronic applications. They are often declared to have the same behavior that is usually observed in thin film reactions: sequential growth of some phases. This paper deals with the comparison of phase growth and kinetics in both systems. Our observations using several techniques show that nickel silicides have a sequential growth of Ni2Si, NiSi and NiSi2 as usually reported, whereas two nickel germanides, Ni5Ge3 and NiGe, grow simultaneously until the total consumption of Ni film. This switch from a sequential to a simultaneous growth is potentially important for the microelectronic industry. The kinetics parameters for the Ni rich phase growth (Ni2Si and Ni5Ge3) have been derived using a linear-parabolic law which takes into account both interfacial and diffusion contributions. The determination of these parameters helps to understand the contribution of interfacial phenomena on the thin film reaction. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Thin film reactions of metal on semiconductor are under interest since 30 years for their applications in microelectronic devices. Indeed, silicides and more recently germanides are used as ohmic contacts or interconnections in CMOS transistors [1]. The continuous scaling down of devices allows to increase the density of transistors but leads to a sensitive increase of the ohmic contact resistance [2]. This problem can be solved by changing the silicide therefore the metal or by adding germanium [3]. At present time NiSi is a good candidate for the sub-65 nm technology because of its low resistivity and also its thermo-kinetic quality [4]: low Si consumption, low temperature formation and the compatibility with the current metallization process. Germanides and especially the NiGe phase is useful for low power consumption or high speed devices as MOSFET transistors [5] due to the high mobility of both holes and electrons in germanium. The knowledge of the mechanisms which occur during Si or Ge metallization is thus *
Corresponding author. E-mail address: [email protected] (F. Nemouchi).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.09.014
important for the integration. Moreover the tendency to shrink the thickness of silicides (or germanides) requires a better understanding of the first stages of reaction and of the growth kinetics. Both systems, Ni–Si and Ni–Ge, are often declared to have the same behavior [6] and several studies report a sequential growth as it is usually the case in thin film formation. Only some compounds from the phase diagram appear in the sequence: Ni2Si, NiSi and NiSi2 for the silicides and only two for the germanides. Although the available reports agree on the second and final phase, NiGe, they disagree on the first phase. Some authors report orthorhombic Ni2Ge [7,8] while others monoclinic Ni5Ge3 [9] or hexagonal Ni3Ge2 [10]. It is also usually reported that silicides and germanides follow a diffusion controlled growth [11,12] excepted NiSi2 [13] whose growth is limited by the nucleation. In this paper, we have studied both systems (Ni–Si and Ni–Ge) in order to compare their phase formation and growth kinetics. The reaction of nanometric Ni films (10– 50 nm) deposited with amorphous and crystalline semiconductors (Si and Ge) have been analyzed by several techniques for phase characterization, kinetics and electrical measurements.
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2. Experimental procedure Ni thin films (10–50 nm) and amorphous semiconductors layers (a-Si and a-Ge) were deposited by e-beam evaporation (vacuum close to 108 mbar) on undoped (1 0 0)Si wafers. Before Ni deposition, polycrystalline silicon or germanium were deposited by chemical vapor deposition (CVD). X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements have been performed in situ and ex situ using a diffractometer equipped with a temperature ˚ ). The XRD spectra chamber and a Cu tube (k = 1.54 A were continuously recorded both in the Bragg–Brentano and thin film geometry. Isothermal in situ XRD experiments were performed at temperatures between 150 and 300 °C (vacuum in the 105 mbar range). Additionally, samples at selected stages of reaction were taken out and carefully examined by long collection time XRD. Some samples were also analyzed using transmission electron microscopy (TEM) in plane views and cross sections with EELS analysis. Differential scanning calorimetry (DSC) was also used for the kinetics and thermodynamics study. The DSC samples have required a special preparation that consists in thinning of the Si wafer by wet chemical etching before Ni and semiconductor deposition. Finally, the sheet resistance (SR) variation was measured in situ with a four points probe system during heat treatment with a constant ramp in vacuum (105 mbar). 3. Experimental results and discussion 3.1. Ni–Si system In Fig. 1a, a view of isothermal annealing at 250 °C of a 50 nm Ni film deposited on a-Si. The diffraction angle (2H) is plotted versus time and the grey levels represent the peak intensities. We can easily label in Fig. 1a the phases which have grown: Ni2Si, NiSi as it is usually observed on nickel
silicide system. These three phases grow sequentially in a good agreement with the literature [12] (NiSi2 which has been also observed is not shown here). Indeed Ni2Si grows by totally consuming the Ni layer before NiSi begins to appear. Then the NiSi formation occurs with the detriment of Ni2Si. Two ‘‘transient phases’’ that have been recently reported [14,15] were also observed (Fig. 1b). In this work, these phases are characterized by one peak that means that they are strongly textured but this peak position does not allow to distinguish them from: Ni3Si2, Ni31Si12 and NiSi. The first transient phase seems to appear before Ni2Si but it only consumes few nickel then it disappears quickly to the benefit of Ni2Si. The second one grows at the end of Ni2Si formation when the Ni layer becomes extremely thin. An ex situ XRD spectra shows that the second phase should be Ni31Si12. In addition, these XRD measurements can be used to determine the formation kinetics of the silicides. We therefore suppose that the intensity is proportional to the thickness (Eq. (1)). This is supported by the XRR experiments that have shown that the growth of nickel silicides (at least Ni2Si and NiSi) occurs in a layer by layer mode [16]. LðtÞ ¼
IðtÞ Lmax I max
ð1Þ
where L is the thickness, Lmax the total thickness, I the intensity, Imax the maximum peak intensity and t the time. Fig. 2 represents the square of the normalized intensities of Ni, Ni2Si and NiSi during the annealing of 50 nm Ni on a-Si at 230 °C. The thicknesses versus the time follow in first approximation a parabolic law (Eq. (2)) that should mean a diffusion controlled growth for Ni2Si and NiSi. LðtÞ2 ¼ K d t K d ¼ K 0 exp
Ea KB T
ð2Þ ð3Þ
Fig. 1. (a) In situ X-ray diffraction measurements of 50 nm Ni films deposited on amorphous Si annealed at 250 °C using Bragg–Brentano geometry. (b) In situ XRD curves during a temperature ramping between 100 °C (scan 1) and 400 °C (scan 30) with a step of 10 °C.
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Table 1 Activation energies of silicides and germanides as function of the substrate
Fig. 2. Fifty nanometers of Ni on a-Si annealed at 230 °C. The square of normalized intensity of 1 1 1 Ni, 0 2 0 Ni2Si and 2 1 1 NiSi XRD peaks are plotted as function of time.
where t is the time, Kd the rate formation, K0 the pre-exponential factor, Ea the activation energy, KB the Boltzman constant and T the temperature. From several annealing, we can determine the activation energy of Ni2Si and NiSi solving the Arrhenius law (Eq. (3)). The average of activation energies of Ni2Si and NiSi is respectively 1.5 eV and 1.6 eV (Table 1) which is in fair agreement with the reported studies [12]. We can note that the activation energy of NiSi is higher than the Ni2Si one, but the orientation or the substrate nature does not seem to play a strong influence on the silicides kinetics. However, using DSC measurements we find that the Ni2Si formation on a-Si can not be described with by a pure parabolic law (diffusion controlled growth) as it is the case for NiSi. Indeed, DSC isochronal measurements can be fitted with a linear law (L(t) = Ki Æ t) meaning that the growth rate is
Phase
Ea of interface (eV)
Ea of diffusion (eV)
Ni2Si on Si(1 0 0) NiSi on Si(1 0 0) Ni2Si on poly-Si NiSi on poly-Si Ni2Si on a-Si NiSi on a-Si Ni5Ge3 on a-Ge NiGe on a-Ge
– – – – 0.8 – 0.9 –
1.5 ± 0.2 1.6 ± 0.25 1.5 ± 0.5 1.54 ± 0.2 1.5 ± 0.28 1.64 ± 0.23 0.8 0.9
independent of the thickness. In order to fit simultaneously the DSC and the XRD measurements (Fig. 3) we use a linear-parabolic law (Eq. (4)) which takes into account both diffusion and interfacial contributions [17]. This linear-parabolic behaviour was already observed by Deal and Grove in case of SiO2 growth [18]. Using Eq. (4) the activation energy of the Ni2Si for the interfacial contribution [19] can thus be determined (Table 1). This interfacial term can correspond to several phenomena: interfacial reaction, diffusion through an interface, stress relaxation, migration of the interface. . . dLðtÞ Ki Kd Dl ¼ dt K iL þ K d K BT
ð4Þ
where Ki and Kd are the interfacial and the diffusion rate, Dl is the difference of chemical potential across the growing phase. 3.2. Ni–Ge system The reaction of Ni with Ge shows rather uncommon behavior for a thin film formation. Indeed, two germanides grow simultaneously in the presence of Ni. In order to unambiguously identify the phases, and to solve the con-
Fig. 3. XRD and DSC fittings of 50 nm Ni film deposited on a-Si using Eq. (4) and activation energies of Table 1. The XRD sample measurements were made during isothermal annealing at 230 °C.
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troversies Ni rich phases, a long collection time XRD together with TEM analysis were performed after annealing. In Fig. 4b, several peaks are clearly attributed to Ni and NiGe. The remaining lines are compared to spectra of random powder of Ni3Ge2, Ni2Ge and Ni5Ge3. The major lines are the same for all phases which probably drove to the discrepancy in the literature [6–8]. XRD minor lines can only be attributed to Ni5Ge3. In addition, the TEM examination of Fig. 4a shows a stack of three phases which are identified with EELS as being Ni, NiGe and Ni5Ge3. Labels on the line-profile are consistent with quantitative point-analyses. Both measurements are in good agreement. The simultaneous formation Ni5Ge3 and NiGe which is unexpected if we refer to the conventional thin film reactions [20], can be seen in Fig. 4a. There is a phase competition between both phases in presence of Ni. When Ni is totally consumed, NiGe grows faster to the detriment of Ni5Ge3. The same result was obtained on polycrystalline Ge. Although the NiGe–Ge interface is rough, the interfaces between Ni–Ni5Ge3 and Ni5Ge3–NiGe are relatively flat (Fig. 4a). It means that the germanide growth seems to occur in a layer by layer mode. We thus apply Eq. (2) in order to determine the formation rate. Additionally, we fit the XRD measurements with the phase competition law [21,22] (Eq. (5)) generally applied to the bulk phase formation. dL1 K d1 K d2 ¼ a11 a12 dt L1 þ K d1 =K i1 L2 þ K d2 =K i2 dL2 K d1 K d2 ¼ a21 þ a22 dt L1 þ K d1 =K i1 L2 þ K d2 =K i2
ð5Þ
where aIJ are coefficients proportional to the free energy, Kd1, Kd2, Ki1, and Ki2 the interdiffusion coefficients and the interface coefficients for Ni5Ge3 and NiGe respectively. The coefficients and the equations are not detailed in this publication but it is important to note that the increase of the growth rate of each phase depends on the diffusion and interfacial contributions of both phases. Here, we
assume a linear-parabolic growth for Ni5Ge3 and a parabolic growth for NiGe. Several samples were annealed at different temperatures between 160 and 190 °C. The normalized intensity versus time has been plotted (Fig. 5a) and fitted with Eq. (5), based on the hypothesis of a layer by layer growth. The activation energies deduced from the XRD measurements are summarized in Table 1. DSC and in situ SR measurements during an annealing at 10 K/min of a Ni film deposited on a-Ge are compared in Fig. 5b. For the DSC measurement the Ni and aGe thicknesses were adjusted to give the composition NiGe while Ni was deposited on an excess of Ge for the SR measurements. The SR signal is in good agreement with the DSC one if we slightly shift the temperature (this shift is due to slightly different experimental conditions). The peaks below 300 °C represent the phase formation of both Ni5Ge3 and NiGe. The lowest SR is obtained when NiGe has consumed Ni5Ge3. The increase of the SR above 300 °C is certainly due to the crystallization of Ge and to the agglomeration of NiGe film which occurs at a lower temperature than the one of NiSi [23]. 3.3. Ni–Si and Ni–Ge comparison The reactions of Ni films deposited on Si and on Ge display two different behaviors. First, we observed a phase formation at lower temperature for germanides. Indeed, Ni begins to react at 140 °C on Ge and at 210 °C on Si. It is interesting to note that the ratio between the formation temperature and the melting temperature Tf/Tm is about 0.3 (Si: 1414 °C and Ge: 940 °C) for both systems. Secondly, a sequential formation of three main phases (Ni2Si, NiSi then NiSi2) was observed for the silicides whereas nickel germanides have a simultaneous growth (Ni5Ge3 and NiGe in the presence of Ni) contrary to the usual thin film reactions. Thirdly, the activation energies of silicides are higher than that of germanides (Table 1).
Fig. 4. (a) TEM examination and EELS analysis after an annealing at 180°C during 110 min and (b) long time collection XRD after an annealing at 150 °C during 8 h of Ni 50 nm on a-Ge.
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Fig. 5. (a) Normalized intensities vs time during 160 °C annealed and fitted using Eq. (5) and (b) sheet resistance and DSC signals annealed at 10 K/min of Ni 50 nm on a-Ge.
In the phase competition model [21,22] one can define a critical thickness below which one of the phases cannot grows. If one uses this model for the growth of Ni2Si and NiSi on one the hand and of Ni5Ge3 and NiGe on the other hand, this means that the monosilicide (and respectively the monogermanide) starts to grow after a critical thickness has been reached by Ni2Si (and Ni5Ge3, respectively). This critical thickness has been estimated about 200 nm [21] for Ni2Si and only 20 nm for Ni5Ge3 from our kinetic parameters. Starting with 50 nm of nickel on silicon it is possible to form 75 nm of Ni2Si which is not thick enough to have phase competition between Ni2Si and NiSi. On the contrary, this Ni film deposited on Ge allows to reach the kinetic equilibrium (i.e. the simultaneous growth) before the Ni film is completely consumed. Nevertheless, this analysis does not take into account the transient phase. These phases as well as the amorphous phase observed during the first stages of reaction [16,23,24] have probably an influence but their contribution is not well understood yet.
point is really interesting for the microelectronic industry, it is balanced by a lower temperature of agglomeration of NiGe (400 °C) compared to the one of NiSi (600 °C). Finally, the formation kinetics of very thin films, especially during fast isochronal annealing, should be not only controlled by diffusion and the interface phenomena should play an important role in the kinetics as shown by the present results. Acknowledgements The authors thank C. Lavoie for the helpful discussions and valuable comments. Shili Zhang is acknowledged for providing the Si and Ge samples and for helpful discussions. L. Ehouarne is also acknowledged for XRR measurements and analysis. J.L.L. acknowledges the financial support of the Hungarian National Science Fund (OTKA T043437) and the Paul Ce´zanne University, Marseille for 2 months of Invited Professorship. References
4. Conclusion Using several experimental techniques, we showed that Ni–Si and Ni–Ge systems present different behaviors concerning the solid state reactions. Indeed, the Ni–Si system has three major phases (Ni2Si, NiSi and NiSi2) that grow sequentially while the Ni–Ge system shows only two phases (Ni5Ge3 and NiGe) that grow simultaneously. We propose that a different critical thickness need to be reached by Ni2Si (200 nm) and Ni5Ge3 (20 nm) before the simultaneous growth can take place. Two transient phases (Ni3Si2 and Ni31Si12) appear and disappear quickly in the sequence of Ni–Si reaction and should play a role in the kinetic. The germanide reactions occur at lower temperatures than silicides: the temperature of NiSi formation is around 250 °C while the formation of NiGe starts at 150 °C. Although this
[1] F.M. d’Heurle, C.S. Peterson, J.E.E. Baglin, S.J. La Placa, C.Y. Wong, J. Appl. Phys. 55 (1982) 4208. [2] J.P. Gambino, E.G. Colgan, Mater. Chem. Phys. 52 (1998) 99. [3] J. Seger, S.-L. Zhang, D. Mangelinck, H.H. Radamson, Appl. Phys. Lett. 81 (2002) 1978. [4] C. Lavoie, C. Cabral Jr., F.M. d’Heurle, J.L. Jordan-Sweet, J.M.E. Harper, J. Electron. Mater. 31 (6) (2002) 597–609. [5] S.-L. Zhang, Microelectron. Eng. 70 (2003) 174. [6] E.D. Marshall, C.S. Wu, C.S. Pai, D.M. Scott, S.S. Lau, Mater. Res. Soc. Symp. Proc. 47 (1985) 161. [7] Y.F. Hsieh, L.J. Chen, E.D. Marshall, S.S. Lau, Thin Solid Films 162 (1988) 287. [8] J. Li, Q.Z. Hong, J.W. Mayer, J. Appl. Phys. 67 (1989) 2506. [9] J. Ken Patterson, B.J. Park, K. Ritley, H.Z. Xiao, L.H. Allen, A. Rockett, Thin Solid Films 253 (1994) 456. [10] L.J. Jin, K.L. Pey, W.K. Choi, E.A. Fitzgerald, D.A. Antoniadis, A.J. Pitera, M.L. Lee, D.Z. Chi, C.H. Tung, Thin Solid Films 462– 463 (2004) 151.
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[11] C.-D. Lien, M.-A. Nicolet, S.S. Lau, Phys. Stat. Sol. 81 (1984) 123. [12] P. Gas, F. D’Heurle, Numerical data and functional relationships in Science and Technology volume III 33A, Landolt-Bo¨rnstein, Berlin, 1998. [13] F.M. D’Heurle, J. Mater. Res. 3 (1988) 167. [14] C. Lavoie, F.M. d’Heurle, C. Detavernier, C. Cabral Jr., Micro. Eng. 70 (2003) 144. [15] P. Gergaud, C. Rivero, M. Gaillanou, O. Thomas, B. Froment, H. Jaouen, Mater. Sci. Eng. B 114–115 (2004) 67. [16] L. Ehouarne, M. Putero, D. Mangelinck, F. Nemouchi, T. Bigault, E. Ziggler, R. Coppard, Mic. Eng., accepted.
[17] U.R. Evans, Trans. Electrochem. Soc. 46 (1924) 247. [18] B.E. Deal, A. Grove, J. Appl. Phys. 36 (1965) 3770. [19] F. Nemouchi, D. Mangelinck, C. Bergman, P. Gas, U. Smith, Appl. Phys. Lett. 86 (2005) 041903. [20] F. Nemouchi, D. Mangelinck, C. Bergman, G. Clugnet, P. Gas, J.L. La´ba´r, Appl. Phys. Lett. 89 (2006) 131920. [21] U. Go¨sele, K.N. Tu, J. Appl. Phys. 53 (1982) 3252. [22] .C. Johnson, G. Martin, Solid. Stat. Phen. 3–4 (1988) 247. [23] J.A. Kittel et al., Mater. Res. Soc. Symp. Proc. 765 (2003) 267. [24] L.A. Clevenger, C.V. Thompson, J. Appl. Phys. 67 (3) (1990).
A comparative study of nickel silicides and nickel germanides: Phase formation and kinetics F. Nemouchi b
a,*
, D. Mangelinck a, J.L. La´ba´r b, M. Putero a, C. Bergman a, P. Gas
a
a L2MP, CNRS-UMR 6137, Univ. Paul Ce´zanne, Case 142, 13397 Marseille Cedex 20, France Research Institute for Technical Physics and Materials Science (MFA), H-1121 Budapest, Konkoly-Thege M. u´t 29-33, Hungary
Available online 18 October 2006
Abstract Nickel silicides and nickel germanides are under interest for their microelectronic applications. They are often declared to have the same behavior that is usually observed in thin film reactions: sequential growth of some phases. This paper deals with the comparison of phase growth and kinetics in both systems. Our observations using several techniques show that nickel silicides have a sequential growth of Ni2Si, NiSi and NiSi2 as usually reported, whereas two nickel germanides, Ni5Ge3 and NiGe, grow simultaneously until the total consumption of Ni film. This switch from a sequential to a simultaneous growth is potentially important for the microelectronic industry. The kinetics parameters for the Ni rich phase growth (Ni2Si and Ni5Ge3) have been derived using a linear-parabolic law which takes into account both interfacial and diffusion contributions. The determination of these parameters helps to understand the contribution of interfacial phenomena on the thin film reaction. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Thin film reactions of metal on semiconductor are under interest since 30 years for their applications in microelectronic devices. Indeed, silicides and more recently germanides are used as ohmic contacts or interconnections in CMOS transistors [1]. The continuous scaling down of devices allows to increase the density of transistors but leads to a sensitive increase of the ohmic contact resistance [2]. This problem can be solved by changing the silicide therefore the metal or by adding germanium [3]. At present time NiSi is a good candidate for the sub-65 nm technology because of its low resistivity and also its thermo-kinetic quality [4]: low Si consumption, low temperature formation and the compatibility with the current metallization process. Germanides and especially the NiGe phase is useful for low power consumption or high speed devices as MOSFET transistors [5] due to the high mobility of both holes and electrons in germanium. The knowledge of the mechanisms which occur during Si or Ge metallization is thus *
Corresponding author. E-mail address: [email protected] (F. Nemouchi).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.09.014
important for the integration. Moreover the tendency to shrink the thickness of silicides (or germanides) requires a better understanding of the first stages of reaction and of the growth kinetics. Both systems, Ni–Si and Ni–Ge, are often declared to have the same behavior [6] and several studies report a sequential growth as it is usually the case in thin film formation. Only some compounds from the phase diagram appear in the sequence: Ni2Si, NiSi and NiSi2 for the silicides and only two for the germanides. Although the available reports agree on the second and final phase, NiGe, they disagree on the first phase. Some authors report orthorhombic Ni2Ge [7,8] while others monoclinic Ni5Ge3 [9] or hexagonal Ni3Ge2 [10]. It is also usually reported that silicides and germanides follow a diffusion controlled growth [11,12] excepted NiSi2 [13] whose growth is limited by the nucleation. In this paper, we have studied both systems (Ni–Si and Ni–Ge) in order to compare their phase formation and growth kinetics. The reaction of nanometric Ni films (10– 50 nm) deposited with amorphous and crystalline semiconductors (Si and Ge) have been analyzed by several techniques for phase characterization, kinetics and electrical measurements.
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2. Experimental procedure Ni thin films (10–50 nm) and amorphous semiconductors layers (a-Si and a-Ge) were deposited by e-beam evaporation (vacuum close to 108 mbar) on undoped (1 0 0)Si wafers. Before Ni deposition, polycrystalline silicon or germanium were deposited by chemical vapor deposition (CVD). X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements have been performed in situ and ex situ using a diffractometer equipped with a temperature ˚ ). The XRD spectra chamber and a Cu tube (k = 1.54 A were continuously recorded both in the Bragg–Brentano and thin film geometry. Isothermal in situ XRD experiments were performed at temperatures between 150 and 300 °C (vacuum in the 105 mbar range). Additionally, samples at selected stages of reaction were taken out and carefully examined by long collection time XRD. Some samples were also analyzed using transmission electron microscopy (TEM) in plane views and cross sections with EELS analysis. Differential scanning calorimetry (DSC) was also used for the kinetics and thermodynamics study. The DSC samples have required a special preparation that consists in thinning of the Si wafer by wet chemical etching before Ni and semiconductor deposition. Finally, the sheet resistance (SR) variation was measured in situ with a four points probe system during heat treatment with a constant ramp in vacuum (105 mbar). 3. Experimental results and discussion 3.1. Ni–Si system In Fig. 1a, a view of isothermal annealing at 250 °C of a 50 nm Ni film deposited on a-Si. The diffraction angle (2H) is plotted versus time and the grey levels represent the peak intensities. We can easily label in Fig. 1a the phases which have grown: Ni2Si, NiSi as it is usually observed on nickel
silicide system. These three phases grow sequentially in a good agreement with the literature [12] (NiSi2 which has been also observed is not shown here). Indeed Ni2Si grows by totally consuming the Ni layer before NiSi begins to appear. Then the NiSi formation occurs with the detriment of Ni2Si. Two ‘‘transient phases’’ that have been recently reported [14,15] were also observed (Fig. 1b). In this work, these phases are characterized by one peak that means that they are strongly textured but this peak position does not allow to distinguish them from: Ni3Si2, Ni31Si12 and NiSi. The first transient phase seems to appear before Ni2Si but it only consumes few nickel then it disappears quickly to the benefit of Ni2Si. The second one grows at the end of Ni2Si formation when the Ni layer becomes extremely thin. An ex situ XRD spectra shows that the second phase should be Ni31Si12. In addition, these XRD measurements can be used to determine the formation kinetics of the silicides. We therefore suppose that the intensity is proportional to the thickness (Eq. (1)). This is supported by the XRR experiments that have shown that the growth of nickel silicides (at least Ni2Si and NiSi) occurs in a layer by layer mode [16]. LðtÞ ¼
IðtÞ Lmax I max
ð1Þ
where L is the thickness, Lmax the total thickness, I the intensity, Imax the maximum peak intensity and t the time. Fig. 2 represents the square of the normalized intensities of Ni, Ni2Si and NiSi during the annealing of 50 nm Ni on a-Si at 230 °C. The thicknesses versus the time follow in first approximation a parabolic law (Eq. (2)) that should mean a diffusion controlled growth for Ni2Si and NiSi. LðtÞ2 ¼ K d t K d ¼ K 0 exp
Ea KB T
ð2Þ ð3Þ
Fig. 1. (a) In situ X-ray diffraction measurements of 50 nm Ni films deposited on amorphous Si annealed at 250 °C using Bragg–Brentano geometry. (b) In situ XRD curves during a temperature ramping between 100 °C (scan 1) and 400 °C (scan 30) with a step of 10 °C.
F. Nemouchi et al. / Microelectronic Engineering 83 (2006) 2101–2106
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Table 1 Activation energies of silicides and germanides as function of the substrate
Fig. 2. Fifty nanometers of Ni on a-Si annealed at 230 °C. The square of normalized intensity of 1 1 1 Ni, 0 2 0 Ni2Si and 2 1 1 NiSi XRD peaks are plotted as function of time.
where t is the time, Kd the rate formation, K0 the pre-exponential factor, Ea the activation energy, KB the Boltzman constant and T the temperature. From several annealing, we can determine the activation energy of Ni2Si and NiSi solving the Arrhenius law (Eq. (3)). The average of activation energies of Ni2Si and NiSi is respectively 1.5 eV and 1.6 eV (Table 1) which is in fair agreement with the reported studies [12]. We can note that the activation energy of NiSi is higher than the Ni2Si one, but the orientation or the substrate nature does not seem to play a strong influence on the silicides kinetics. However, using DSC measurements we find that the Ni2Si formation on a-Si can not be described with by a pure parabolic law (diffusion controlled growth) as it is the case for NiSi. Indeed, DSC isochronal measurements can be fitted with a linear law (L(t) = Ki Æ t) meaning that the growth rate is
Phase
Ea of interface (eV)
Ea of diffusion (eV)
Ni2Si on Si(1 0 0) NiSi on Si(1 0 0) Ni2Si on poly-Si NiSi on poly-Si Ni2Si on a-Si NiSi on a-Si Ni5Ge3 on a-Ge NiGe on a-Ge
– – – – 0.8 – 0.9 –
1.5 ± 0.2 1.6 ± 0.25 1.5 ± 0.5 1.54 ± 0.2 1.5 ± 0.28 1.64 ± 0.23 0.8 0.9
independent of the thickness. In order to fit simultaneously the DSC and the XRD measurements (Fig. 3) we use a linear-parabolic law (Eq. (4)) which takes into account both diffusion and interfacial contributions [17]. This linear-parabolic behaviour was already observed by Deal and Grove in case of SiO2 growth [18]. Using Eq. (4) the activation energy of the Ni2Si for the interfacial contribution [19] can thus be determined (Table 1). This interfacial term can correspond to several phenomena: interfacial reaction, diffusion through an interface, stress relaxation, migration of the interface. . . dLðtÞ Ki Kd Dl ¼ dt K iL þ K d K BT
ð4Þ
where Ki and Kd are the interfacial and the diffusion rate, Dl is the difference of chemical potential across the growing phase. 3.2. Ni–Ge system The reaction of Ni with Ge shows rather uncommon behavior for a thin film formation. Indeed, two germanides grow simultaneously in the presence of Ni. In order to unambiguously identify the phases, and to solve the con-
Fig. 3. XRD and DSC fittings of 50 nm Ni film deposited on a-Si using Eq. (4) and activation energies of Table 1. The XRD sample measurements were made during isothermal annealing at 230 °C.
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troversies Ni rich phases, a long collection time XRD together with TEM analysis were performed after annealing. In Fig. 4b, several peaks are clearly attributed to Ni and NiGe. The remaining lines are compared to spectra of random powder of Ni3Ge2, Ni2Ge and Ni5Ge3. The major lines are the same for all phases which probably drove to the discrepancy in the literature [6–8]. XRD minor lines can only be attributed to Ni5Ge3. In addition, the TEM examination of Fig. 4a shows a stack of three phases which are identified with EELS as being Ni, NiGe and Ni5Ge3. Labels on the line-profile are consistent with quantitative point-analyses. Both measurements are in good agreement. The simultaneous formation Ni5Ge3 and NiGe which is unexpected if we refer to the conventional thin film reactions [20], can be seen in Fig. 4a. There is a phase competition between both phases in presence of Ni. When Ni is totally consumed, NiGe grows faster to the detriment of Ni5Ge3. The same result was obtained on polycrystalline Ge. Although the NiGe–Ge interface is rough, the interfaces between Ni–Ni5Ge3 and Ni5Ge3–NiGe are relatively flat (Fig. 4a). It means that the germanide growth seems to occur in a layer by layer mode. We thus apply Eq. (2) in order to determine the formation rate. Additionally, we fit the XRD measurements with the phase competition law [21,22] (Eq. (5)) generally applied to the bulk phase formation. dL1 K d1 K d2 ¼ a11 a12 dt L1 þ K d1 =K i1 L2 þ K d2 =K i2 dL2 K d1 K d2 ¼ a21 þ a22 dt L1 þ K d1 =K i1 L2 þ K d2 =K i2
ð5Þ
where aIJ are coefficients proportional to the free energy, Kd1, Kd2, Ki1, and Ki2 the interdiffusion coefficients and the interface coefficients for Ni5Ge3 and NiGe respectively. The coefficients and the equations are not detailed in this publication but it is important to note that the increase of the growth rate of each phase depends on the diffusion and interfacial contributions of both phases. Here, we
assume a linear-parabolic growth for Ni5Ge3 and a parabolic growth for NiGe. Several samples were annealed at different temperatures between 160 and 190 °C. The normalized intensity versus time has been plotted (Fig. 5a) and fitted with Eq. (5), based on the hypothesis of a layer by layer growth. The activation energies deduced from the XRD measurements are summarized in Table 1. DSC and in situ SR measurements during an annealing at 10 K/min of a Ni film deposited on a-Ge are compared in Fig. 5b. For the DSC measurement the Ni and aGe thicknesses were adjusted to give the composition NiGe while Ni was deposited on an excess of Ge for the SR measurements. The SR signal is in good agreement with the DSC one if we slightly shift the temperature (this shift is due to slightly different experimental conditions). The peaks below 300 °C represent the phase formation of both Ni5Ge3 and NiGe. The lowest SR is obtained when NiGe has consumed Ni5Ge3. The increase of the SR above 300 °C is certainly due to the crystallization of Ge and to the agglomeration of NiGe film which occurs at a lower temperature than the one of NiSi [23]. 3.3. Ni–Si and Ni–Ge comparison The reactions of Ni films deposited on Si and on Ge display two different behaviors. First, we observed a phase formation at lower temperature for germanides. Indeed, Ni begins to react at 140 °C on Ge and at 210 °C on Si. It is interesting to note that the ratio between the formation temperature and the melting temperature Tf/Tm is about 0.3 (Si: 1414 °C and Ge: 940 °C) for both systems. Secondly, a sequential formation of three main phases (Ni2Si, NiSi then NiSi2) was observed for the silicides whereas nickel germanides have a simultaneous growth (Ni5Ge3 and NiGe in the presence of Ni) contrary to the usual thin film reactions. Thirdly, the activation energies of silicides are higher than that of germanides (Table 1).
Fig. 4. (a) TEM examination and EELS analysis after an annealing at 180°C during 110 min and (b) long time collection XRD after an annealing at 150 °C during 8 h of Ni 50 nm on a-Ge.
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Fig. 5. (a) Normalized intensities vs time during 160 °C annealed and fitted using Eq. (5) and (b) sheet resistance and DSC signals annealed at 10 K/min of Ni 50 nm on a-Ge.
In the phase competition model [21,22] one can define a critical thickness below which one of the phases cannot grows. If one uses this model for the growth of Ni2Si and NiSi on one the hand and of Ni5Ge3 and NiGe on the other hand, this means that the monosilicide (and respectively the monogermanide) starts to grow after a critical thickness has been reached by Ni2Si (and Ni5Ge3, respectively). This critical thickness has been estimated about 200 nm [21] for Ni2Si and only 20 nm for Ni5Ge3 from our kinetic parameters. Starting with 50 nm of nickel on silicon it is possible to form 75 nm of Ni2Si which is not thick enough to have phase competition between Ni2Si and NiSi. On the contrary, this Ni film deposited on Ge allows to reach the kinetic equilibrium (i.e. the simultaneous growth) before the Ni film is completely consumed. Nevertheless, this analysis does not take into account the transient phase. These phases as well as the amorphous phase observed during the first stages of reaction [16,23,24] have probably an influence but their contribution is not well understood yet.
point is really interesting for the microelectronic industry, it is balanced by a lower temperature of agglomeration of NiGe (400 °C) compared to the one of NiSi (600 °C). Finally, the formation kinetics of very thin films, especially during fast isochronal annealing, should be not only controlled by diffusion and the interface phenomena should play an important role in the kinetics as shown by the present results. Acknowledgements The authors thank C. Lavoie for the helpful discussions and valuable comments. Shili Zhang is acknowledged for providing the Si and Ge samples and for helpful discussions. L. Ehouarne is also acknowledged for XRR measurements and analysis. J.L.L. acknowledges the financial support of the Hungarian National Science Fund (OTKA T043437) and the Paul Ce´zanne University, Marseille for 2 months of Invited Professorship. References
4. Conclusion Using several experimental techniques, we showed that Ni–Si and Ni–Ge systems present different behaviors concerning the solid state reactions. Indeed, the Ni–Si system has three major phases (Ni2Si, NiSi and NiSi2) that grow sequentially while the Ni–Ge system shows only two phases (Ni5Ge3 and NiGe) that grow simultaneously. We propose that a different critical thickness need to be reached by Ni2Si (200 nm) and Ni5Ge3 (20 nm) before the simultaneous growth can take place. Two transient phases (Ni3Si2 and Ni31Si12) appear and disappear quickly in the sequence of Ni–Si reaction and should play a role in the kinetic. The germanide reactions occur at lower temperatures than silicides: the temperature of NiSi formation is around 250 °C while the formation of NiGe starts at 150 °C. Although this
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