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Metastable phases formation in Cu–Nb films by ion-beam-assisted deposition
Nuclear Instruments and Methods in Physics Research B 183 (2001) 311±317
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Metastable phases formation in Cu±Nb ®lms by ion-beam-assisted deposition F. Zeng, B. Zhao, F. Pan
*
Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 25 January 2001; received in revised form 11 June 2001
Abstract The Cu±Nb alloy ®lms with positive heat of mixing were prepared by ion-beam-assisted deposition (IBAD) technique. A new Nb-rich fcc phase and an amorphous phase, with composition range around 65±75% Nb fraction, were obtained in Cu±Nb ®lms deposited under Ar ion bombardments with energy below 7 keV. The supersaturated bcc phase was also formed in Cu20 Nb80 ®lms by IBAD. The formation mechanism of metastable phases in Cu±Nb system is discussed in terms of the procedure far from equilibrium and compared with that in Fe±Zr system by IBAD. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion-beam-assisted deposition; Amorphous; Cu; Nb; Metastable phase
1. Introduction In the last two decades, ion-beam mixing (IM) was employed to investigate the microstructure evolution of thin solid ®lms [1,2]. From a physical point of view, IM process is a procedure far from equilibrium. It is applicable to trace the phase evolution experimentally by monitoring energy deposited on the ®lm for IM. As derivation from IM technique, ion-beam-assisted deposition (IBAD) technique was widely used to prepare various ®lms and coatings [3±5]. Processing by IBAD combines the attractive features of ion im*
Corresponding author. Tel.: +86-10-6277-2907; fax: +8610-6277-1160. E-mail address: [email protected] (F. Pan).
plantation and physical vapor deposition. The kinetics of IBAD is similar to that of IM, while the ion energy it used is quite lower and the deposition conditions can be changed conveniently by adjusting bombardment energy, the irradiation dose and deposition velocity. It therefore may be a suitable method for phase evolution investigation. In our recent study, it was found that the metastable alloys including amorphous and crystalline phase could be obtained in Fe±Zr system with a negative heat of mixing by IBAD technique [6]. Known from IM of multilayers, the thermodynamics and kinetics of metastable phases formation for systems with positive heat of mixing are much dierent from those for systems with negative heat of mixing [1]. Formation mechanisms of metastable phases in both systems by IBAD
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F. Zeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 311±317
should be much dierent from each other too. Since no experimental data are available on this aspect, research interest is to investigate the possibility and mechanism of metastable alloys formation in binary systems with positive heat of mixing by IBAD. The mixing heat of the Cu±Nb system selected in this paper is about +4 kJ/mol calculated by the formation heat model proposed by Miedema et al. [7]. No intermediate phase exists according to the phase diagram. When the ambient temperature was below 300 K, the Cu±Nb system, around the composition ranging from 25% to 50% of Cu, could be amorphized by IM upon 500 keV Xe with a ¯ux of 2 1016 ions=cm2 [8]. In this paper, the metastable phases in the Cu±Nb system is obtained by IBAD method and the formation mechanisms of these are discussed based on procedure far from equilibrium in IBAD. Results also are compared with that of the previous Fe±Zr system [6]. 2. Experimental procedure The IBAD apparatus consists of an ion-beam source and an e-beam evaporator with the sketch map presented in [6]. The samples were prepared by alternate electron beam deposition of pure Cu (99.99%) and Nb (99.99%) at the rate of 0.1 nm/s on the NaCl single crystal chips with freshly cleaved surfaces in an IBAD system. The Ar (99.999%) ion-beam bombardment during the vaporization process was provided by the Kaufman ion source with 8-cm diameter beam. The incident ion-beam was normal to the surface of the substrate. The bombardment energies were selected from 0 to 11 keV and the beam current density was about 12 lA/cm2 . The background vacuum was 1 10 4 Pa, and the partial pressure of Ar was about 4 10 3 Pa during IBAD. All depositions were performed at ambient substrate temperature. The thickness of Cu and Nb was realtime monitored by quartz crystal oscillator. The total thickness of the ®lms was about 25 nm, in favor of transmission electron microscopy (TEM) observation. The modulation wavelengths of Cu/ Nb multilayers were about 4 nm, which were
around the project ranges calculated by SRIM [9] under the available energy provided by the system. Three stoichiometries were chosen for this study, i.e. 65%, 75% and 80% of Nb composition. The compositions of the ®lms were controlled by adjusting the relative layer thickness of constituent metals and were later con®rmed by energy-dispersive spectroscopy (EDS) and Rutherford back scattering (RBS) with an experimental error of 5%. After the deposition, the NaCl substrates were dissolved in deionized water and the alloy ®lms were held on the Cu grids. The structural details of the samples were analyzed by selected area electron diraction (SAD) analysis in TEM.
3. Experimental results Fig. 1(a) is a typical SAD pattern of the asdeposited Cu25 Nb75 multilayers, showing the crystalline structure of Cu and Nb. The diraction ring of Cu(1 1 1) is clear and those of Nb are broad indicating Nb as not well crystallized. Samples of the same con®guration were prepared by IBAD with various bombardment energies. The SAD patterns of all ion-bombarded samples revealed that microstructures of the ®lms changed with the ion energies. Fig. 1(b) shows the SAD pattern of
Fig. 1. TEM±SAD patterns of the Cu25 Nb75 ®lms: (a) as-deposited ®lm and ®lm prepared by IBAD with Ar energy of (b) 2 keV, (c) 3 keV, (d) 4 keV, respectively.
F. Zeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 311±317
Cu25 Nb75 alloy ®lm prepared by IBAD with Ar ion energy of 2 keV. Comparing Fig. 1(b) with (a), it was found that, by means of ion bombardment with 2 keV during deposition, the crystalline diffraction lines of the as-deposited ®lm changed completely into a set of new phase with fcc structure, i.e. a new fcc phase was obtained in the Cu25 Nb75 ®lm by IBAD with Ar ion energy of 2 keV. The corresponding crystallographic distances were measured and listed in Table 1. The lattice constant of the new fcc phase is 0:428 0:005 nm. To our knowledge, the Nb-rich new phase with fcc structure in the Cu±Nb system has not been reported previously. In the SAD pattern of the Cu25 Nb75 ®lm with ion energy of 3 keV shown in Fig. 1(c), the fcc diraction lines disappear and change into a set of amorphous halo. It indicated that amorphous phase was obtained in the Cu25 Nb75 ®lm by IBAD with Ar ion energy of 3 keV. By increasing the ion bombardment energy up to 4 keV, the amorphous phase began to decompose into Cu and Nb phase shown in SAD pattern of Fig. 1(d). In this ®gure, the diraction rings of Nb begin to be stronger and that of Cu discernable. This result reveals that both the new fcc and the amorphous phase are unstable and will be transformed lastly into Cu and Nb phase under Ar ion energy higher than 4 keV. To study the composition range of the metastable phases obtained in Cu25 Nb75 ®lms by IBAD, the Nb fraction of the ®lms was adjusted to 65% and 80%, respectively. The Nb-rich fcc phase, with the same microstructure as that shown in Fig. 1(b),
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was also completely formed in Cu35 Nb65 ®lms when the ®lm was prepared under Ar ion energy of 3 keV. The fully amorphous phase was not obtained when the Ar ion energy was increased up to 4 keV. From the SAD pattern of Cu35 Nb65 sample with 4 keV Ar bombardment, the amorphous halo is present, but the line of Cu(1 1 1) is slightly visible beside it. It means that if the composition of Cu and the Ar ion energy were further increased, the amorphous phase would not be formed and the fcc phase might not be obtained. Actually, the ®lms of Cu25 Nb75 and Cu35 Nb65 prepared by IBAD with 7 keV Ar ion energy consisted of Cu and Nb phases, respectively, as results listed in Table 3. In Cu20 Nb80 ®lms prepared by IBAD, a bcc phase was formed when the ®lms were deposited under 4 and 7 keV Ar ion bombardment, respectively. Fig. 2(a) is the diraction pattern of the as-deposited Cu20 Nb80 multilayers and Fig. 2(b) is that of the Cu20 Nb80 ®lm deposited under 4 keV ion bombardment. It can be shown in Fig. 2(b) that the Cu20 Nb80 ®lm deposited under 4 keV ion bombardment is a bcc phase. The indices results of the bcc phase are listed in Table 2. The lattice constant of the bcc phase is 0:340 0:005 nm. This result indicates that the bcc phase of the Cu20 Nb80 ®lm prepared by IBAD with 4 keV ion energy is the supersaturated solid solution of Cu and Nb.
Table 1 Indices results of the diraction lines of Fig. 1(b) Series number
d (obs.) (10 1 nm)
Visual intensitya
Indices
1 2 3 4 5 6 7 8 9
2.475 2.144 1.520 1.298 1.244 1.077 0.988 0.880 0.825
S S S M W W M W W
111 200 220 311 222 400 331, 420 422 333
a
S: strong; M: medium; W: weak.
Fig. 2. TEM±SAD pattern of the Cu20 Nb80 ®lms: (a) as-deposited ®lm and (b) ®lm prepared by IBAD with Ar energy of 4 keV.
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Table 2 Indices results of the diraction lines of Fig. 2(b) Series number
d (obs.) (10 1 nm)
Visual intensitya
Indices
1 2 3 4 5 6
2.401 1.691 1.380 1.195 1.067 0.980
S M S M W W
110 200 211 220 310 222
a
S: strong; M: medium; W: weak.
Table 3 Microstructures of Cu±Nb ®lms by IBAD with various ionbeam energy
As-deposited 2 keV 3 keV 4 keV 7 keV a
Cu20 Nb80
Cu25 Nb75
Cu35 Nb65
Cu + Nb Cu + Nb Cu + Nb bcc bcc
Cu + Nb fcc Aa Cu + Nb Cu + Nb
Cu + Nb Cu + Nb fcc A + Cu Cu + Nb
A: amorphous.
In Table 3, we list all microstructure information of ®lms prepared by IBAD with various Ar ion energies. According to the data listed in Table 3, the composition range of the new fcc phase and the amorphous phase is about 65±75% Nb fraction. These metastable phases are generally unstable and only achieved in ®lms by IBAD with lower Ar ion energy. When the Ar ion energy is higher than 4 keV, neither fcc phase nor pure amorphous phase will be formed. To supersaturated solid solution, it is regarded that the solubility of Cu in Nb in Cu±Nb ®lms is about 20% Cu fraction. The lower energy (lower than 4 keV) of Ar ion bombardment is not sucient to mix Cu and Nb atoms together to form solid solution.
4. Discussion In IBAD technique, the atoms are deposited onto the substrate accompanying with ion-beam bombardment. The atoms arriving on the sub-
strate are mixed together by energetic Ar ionbeam. IM process of IBAD is also far from equilibrium and similar with that of multilayer ®lms so that the results in our experiment can be discussed according to the principles achieved previously in IM of multilayers. 4.1. Basic thermodynamic and kinetic conditions for the formation of metastable phases In ion mixing of multilayers in binary metal system, with either positive or negative energy of mixing, the initial energy state of the multilayers is raised by excess interfacial free energy stored in the multilayer ®lms [1,10]. With deliberate designation of modulation length and number of layers, the initial energy state of multilayers can be higher than that of certain metastable phase. Thus free energy dierence between the initial energy state of the multilayers and that of the metastable phases provides the system internal chemical driving force for the state of multilayers to go to the metastable state. When the thermodynamic conditions for the metastable phases are satis®ed, it still requires an external force provided by energetic ion-beam to drive the initial system to jump to the metastable state. In our IBAD experiments of Cu±Nb and previous Fe±Zr system [6], both the thermodynamics and kinetics conditions should also be satis®ed for the metastable phases formation. Concerning thermodynamics, the Gibbs free energies of metastable phases of Cu±Nb and Fe±Zr systems are calculated according to the method proposed by Miedema and Alonso [7,11] as shown in Fig. 3. The energy state that few (or dilute) atoms of one metal deposit on the surface of another metal is also calculated as adsorption enthalpy proposed by Miedema et al. [7] and the results are listed in Table 4. Comparing data listed in Table 4 with free energy diagrams of Fig. 3, one can see that the adsorption enthalpies of either Cu±Nb or Fe±Zr systems are much higher than the free energies of metastable phases in both systems, respectively. It means that the state of few (or dilute) atoms depositing on the ®lm of both systems is a state of high energy. If there is no Ar ion-beam bombardment during the deposition process, the atoms
F. Zeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 311±317
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process. During the collision cascade and relaxation process induced by impacting Ar ion-beam, the system may attain to one of the state either equilibrium or metastable with lower free energy. The span of the collision cascade and relaxation process is very short, lasting only for 10 9 s so that the metastable phases, only disordered or simple crystalline structure phases, such as bcc, fcc and hcp phases, are expected. It is the reason that metastable fcc and amorphous phases in Fe±Zr and fcc, bcc and amorphous phases in Cu±Nb systems are formed in our experiments. Therefore the Ar ion-beam bombardment during the deposition process plays the role of external kinetic force for the formation of metastable phases.
(a)
(b)
4.2. The formation ranges and stability of the metastable phases
Fig. 3. Calculated Gibbs free energy of (a) Cu±Nb and (b) Fe±Zr system. Table 4 Calculated adsorption enthalpy of A on B, i.e. DHAadsorption on B : (kJ/ mol) Surface (B) Atoms (A) Cu Nb Fe Zr
Cu 274.0
Nb
Fe
Zr
35.8 94.3
141.3
of one metal will nucleate and grow up with the supplement atoms from the source, and the energy of the system will decrease subsequently. When the Ar ion-beam bombardment during the deposition is applied, the atoms from the source will be mixed with the ®lm already on the substrate. Normally, as in IM of multilayers, the impacting Ar ionbeam during deposition process also induces the atomic collision cascade and subsequent relaxation
In Cu±Nb system, the formation range of the amorphous phase is narrow by our IBAD method, which is around 75% of Nb fraction. The formation range of the fcc phase is about 65±75% of Nb fraction and that of the bcc phase is about 80% of Nb fraction. To Fe±Zr system, partial amorphous was obtained around Zr fraction of 50% [6]. Although the experimental result of Fe±Zr system is not complete, it is regarded that the formation range of the amorphous phase is somewhat broader than that of Cu±Nb system. The fcc and the amorphous phase of Cu25 Nb75 and Cu35 Nb65 ®lms began to decompose under ion-beam energy higher than 4 keV, while the bcc phase of Cu20 Nb80 ®lm was still formed under 7 keV. To Fe±Zr system, the metastable phases were formed with ion energy more than 4 keV. There are two reasons to explain the phenomena presented above. Firstly, from the view of thermodynamics, the free energy of the amorphous phase in Cu±Nb system is positive while that of Fe±Zr system is negative. Therefore, the composition range of amorphous phase in Fe±Zr system, i.e. about from 80 to 50 at.% of Zr fraction, is broader than that in Cu±Nb system, i.e. around 75 at.% Nb fraction. Secondly, from the view of kinetics, when the energy deposited on the ®lm is very high, the diffusion of Cu atoms in the collision cascade and
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relaxation process is very fast and the Cu atoms will agglomerate. The atoms of Cu will nucleate and grow up very fast so that the atoms of Cu and Nb will not be mixed together. Therefore, the metastable phases will decompose. The composition limitation and decomposition of amorphous Cu±Nb alloy ®lm were also con®rmed by Andersen et al. [8] and Pan and Liu [12]. Similar phenomena were also observed in Cu±Ta and Cu±W system with positive heat of mixing [13]. To Fe±Zr system, there is an attractive force between Fe and Zr atoms so that movement of either kind of atoms is limited to each other. Thus the metastable phases in Fe±Zr system are more stable. 4.3. Some considerations for kinetic mechanism of metastable phase formation and phase competing in Cu±Nb system In some Nb-, Ta- and Mo-based systems, around composition AB3 (B stands for Nb, Ta or Mo), the fcc phase can be formed in both negative and positive systems by IM method [1]. In kinetics a two-step transition mechanism of bcc±hcp±fcc was proposed for the fcc transition in IM [14]. The relationship of lattice constant pbetween the fcc phase and the bcc phase is afcc 6=2abcc after the two-step transition at last. In our case, no hcp phase is formed prior to the fcc phase, but the fcc phase is based on the bcc structure of Nb. It is applicable to explain the bcc±fcc transition by martensitic transformation, for the period of phase formation is so short that the atom movement requires minimal distance and energy. Several martensitic transformation models has been proposed to explain the bcc±fcc transition [15,16]. It is hard for us to observe the detail process in transformation in experiment so that we cannot judge the actual style of the martensitic transformation in our formation of fcc phase. Generally the (0 1 1) plan of the bcc structure transforms to the (1 1 1) plan of the fcc structure so that the relationship of lattice constant between the bcc p and fcc phase can also be described by afcc p 6=2a bcc in [16]. According to the relation afcc 6=2abcc , the lattice constant of the fcc phase is estimated by using the lattice constant (0.33 nm) obtained in the bcc phase in ®lm with 4 keV ion bombardment. The
estimated result is 0.404 nm that is compatible with the value (0.428 nm) obtained in our experiment. The relationship between the magnitude of the ion-beam energy and the exact phase formation is much complicated. To Cu±Nb system, the bcc phase was not obtained around the composition of 25±35 Cu fraction, and the fcc and the amorphous phase were not formed around Cu20 Nb80 composition. Around the composition of 25±35 Cu fraction, the free energy curve of the bcc phase intersects with that of the amorphous and the fcc phase shown in Fig. 3(a). It suggests the possibility for the formation of the bcc phase, but we did not obtain bcc phase in this range. We consider the following process to explain phase competing in this region. When the energy of ion-beam bombardment is lower, the Cu±Nb system can ful®ll rapid bcc±fcc transition mentioned above with Cu atoms diusing eectively into the Nb matrix. This procedure may be Cu atoms diusion limited. When the energy of ion-beam is increased, the collision cascade eect makes the Nb atoms more active. The atom size of Nb is much bigger than that of Cu while the diusivity of Nb atoms is lower. Thus the displaced Nb atoms hardly recover to form crystalline structure in the short time of relaxation and the disordered, i.e. amorphous, phase is obtained. This procedure can be regarded as Nb atoms diusion limited. 5. Conclusion A new Nb-rich fcc phase and an amorphous phase were obtained in the ®lms prepared by IBAD with Ar ion energy below 7 keV. The composition range of the fcc and the amorphous phase is around 65±75% of Nb fraction. The Nbrich bcc supersaturated solution was also formed in Cu20 Nb80 ®lms by IBAD with Ar ion energy higher than 4 keV. The IBAD process is far from equilibrium as well as IM is. The formation mechanisms of metastable phases can be discussed according to the principles on both thermodynamics and kinetics in the IM of multilayers. Normally, only simple crystalline structure and disordered phase can be formed in such a process
F. Zeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 311±317
far from equilibrium. The kinetic process of the formation of the fcc phase in Cu25 Nb75 can be interpreted with the model suggested by Liu and Zhang [14]. The phase competing is possible due to the atomic diusive behavior under high energy impacting of ion-beam. Acknowledgements This work was supported in part by National Natural Science Foundation of China (No. 19875027), the Ministry of Science and Technology of China through a Grant No. G20000672071, and by the Administration of Tsinghua University. References [1] B.X. Liu, W.S. Lai, Q. Zhang, Mater. Sci. Eng. R 29 (2000) 1.
317
[2] B.X. Liu, W.L. Johnson, M.-A. Nicolet, S.S. Lau, Appl. Phys. Lett. 42 (1983) 45. [3] F.A. Smidt, Int. Mater. Sci. Rev. 35 (1990) 61. [4] N. Sonnenberg, A.S. Longo et al., J. Appl. Phys. 74 (2) (1993) 1027. [5] J.K. Hirvonen, Surf. Coat. Technol. 65 (1994) 84. [6] M. Ding, F. Zeng, F. Pan, Nucl. Instr. and Meth. B 170 (2000) 79. [7] A.R. Miedema, A.K. Niessen et al., Cohesion in Metals: Transition Metal Alloys, North-Holland, Amsterdam, 1989. [8] L.A. Andersen, J. Bùttiger, K. Dyrbye, Nucl. Instr. and Meth. B 51 (1990) 125. [9] J.F. Ziegler et al., SRIM, 2000. [10] Z.J. Zhang, B.X. Liu, Phys. Rev. B 51 (1995) 16475. [11] J.M. Lopez, J.A. Alonso, L.J. Gallego, Phys. Rev. B 36 (1987) 3716. [12] F. Pan, B.X. Liu, Chin. J. Mater. Res. 12 (1998) 311. [13] M. Nastasi, F.W. Saris, L.S. Hung, J.W. Mayer, J. Appl. Phys. 58 (1985) 3052. [14] B.X. Liu, Z.J. Zhang, J. Mater. Res. 9 (1994) 357. [15] G.B. Olsen, M. Cohen, Metall. Trans. 7A (1976) 1905. [16] M. Ahler, Z. Metallk. 65 (1974) 636.
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Metastable phases formation in Cu±Nb ®lms by ion-beam-assisted deposition F. Zeng, B. Zhao, F. Pan
*
Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 25 January 2001; received in revised form 11 June 2001
Abstract The Cu±Nb alloy ®lms with positive heat of mixing were prepared by ion-beam-assisted deposition (IBAD) technique. A new Nb-rich fcc phase and an amorphous phase, with composition range around 65±75% Nb fraction, were obtained in Cu±Nb ®lms deposited under Ar ion bombardments with energy below 7 keV. The supersaturated bcc phase was also formed in Cu20 Nb80 ®lms by IBAD. The formation mechanism of metastable phases in Cu±Nb system is discussed in terms of the procedure far from equilibrium and compared with that in Fe±Zr system by IBAD. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion-beam-assisted deposition; Amorphous; Cu; Nb; Metastable phase
1. Introduction In the last two decades, ion-beam mixing (IM) was employed to investigate the microstructure evolution of thin solid ®lms [1,2]. From a physical point of view, IM process is a procedure far from equilibrium. It is applicable to trace the phase evolution experimentally by monitoring energy deposited on the ®lm for IM. As derivation from IM technique, ion-beam-assisted deposition (IBAD) technique was widely used to prepare various ®lms and coatings [3±5]. Processing by IBAD combines the attractive features of ion im*
Corresponding author. Tel.: +86-10-6277-2907; fax: +8610-6277-1160. E-mail address: [email protected] (F. Pan).
plantation and physical vapor deposition. The kinetics of IBAD is similar to that of IM, while the ion energy it used is quite lower and the deposition conditions can be changed conveniently by adjusting bombardment energy, the irradiation dose and deposition velocity. It therefore may be a suitable method for phase evolution investigation. In our recent study, it was found that the metastable alloys including amorphous and crystalline phase could be obtained in Fe±Zr system with a negative heat of mixing by IBAD technique [6]. Known from IM of multilayers, the thermodynamics and kinetics of metastable phases formation for systems with positive heat of mixing are much dierent from those for systems with negative heat of mixing [1]. Formation mechanisms of metastable phases in both systems by IBAD
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 7 4 7 - 9
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F. Zeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 311±317
should be much dierent from each other too. Since no experimental data are available on this aspect, research interest is to investigate the possibility and mechanism of metastable alloys formation in binary systems with positive heat of mixing by IBAD. The mixing heat of the Cu±Nb system selected in this paper is about +4 kJ/mol calculated by the formation heat model proposed by Miedema et al. [7]. No intermediate phase exists according to the phase diagram. When the ambient temperature was below 300 K, the Cu±Nb system, around the composition ranging from 25% to 50% of Cu, could be amorphized by IM upon 500 keV Xe with a ¯ux of 2 1016 ions=cm2 [8]. In this paper, the metastable phases in the Cu±Nb system is obtained by IBAD method and the formation mechanisms of these are discussed based on procedure far from equilibrium in IBAD. Results also are compared with that of the previous Fe±Zr system [6]. 2. Experimental procedure The IBAD apparatus consists of an ion-beam source and an e-beam evaporator with the sketch map presented in [6]. The samples were prepared by alternate electron beam deposition of pure Cu (99.99%) and Nb (99.99%) at the rate of 0.1 nm/s on the NaCl single crystal chips with freshly cleaved surfaces in an IBAD system. The Ar (99.999%) ion-beam bombardment during the vaporization process was provided by the Kaufman ion source with 8-cm diameter beam. The incident ion-beam was normal to the surface of the substrate. The bombardment energies were selected from 0 to 11 keV and the beam current density was about 12 lA/cm2 . The background vacuum was 1 10 4 Pa, and the partial pressure of Ar was about 4 10 3 Pa during IBAD. All depositions were performed at ambient substrate temperature. The thickness of Cu and Nb was realtime monitored by quartz crystal oscillator. The total thickness of the ®lms was about 25 nm, in favor of transmission electron microscopy (TEM) observation. The modulation wavelengths of Cu/ Nb multilayers were about 4 nm, which were
around the project ranges calculated by SRIM [9] under the available energy provided by the system. Three stoichiometries were chosen for this study, i.e. 65%, 75% and 80% of Nb composition. The compositions of the ®lms were controlled by adjusting the relative layer thickness of constituent metals and were later con®rmed by energy-dispersive spectroscopy (EDS) and Rutherford back scattering (RBS) with an experimental error of 5%. After the deposition, the NaCl substrates were dissolved in deionized water and the alloy ®lms were held on the Cu grids. The structural details of the samples were analyzed by selected area electron diraction (SAD) analysis in TEM.
3. Experimental results Fig. 1(a) is a typical SAD pattern of the asdeposited Cu25 Nb75 multilayers, showing the crystalline structure of Cu and Nb. The diraction ring of Cu(1 1 1) is clear and those of Nb are broad indicating Nb as not well crystallized. Samples of the same con®guration were prepared by IBAD with various bombardment energies. The SAD patterns of all ion-bombarded samples revealed that microstructures of the ®lms changed with the ion energies. Fig. 1(b) shows the SAD pattern of
Fig. 1. TEM±SAD patterns of the Cu25 Nb75 ®lms: (a) as-deposited ®lm and ®lm prepared by IBAD with Ar energy of (b) 2 keV, (c) 3 keV, (d) 4 keV, respectively.
F. Zeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 311±317
Cu25 Nb75 alloy ®lm prepared by IBAD with Ar ion energy of 2 keV. Comparing Fig. 1(b) with (a), it was found that, by means of ion bombardment with 2 keV during deposition, the crystalline diffraction lines of the as-deposited ®lm changed completely into a set of new phase with fcc structure, i.e. a new fcc phase was obtained in the Cu25 Nb75 ®lm by IBAD with Ar ion energy of 2 keV. The corresponding crystallographic distances were measured and listed in Table 1. The lattice constant of the new fcc phase is 0:428 0:005 nm. To our knowledge, the Nb-rich new phase with fcc structure in the Cu±Nb system has not been reported previously. In the SAD pattern of the Cu25 Nb75 ®lm with ion energy of 3 keV shown in Fig. 1(c), the fcc diraction lines disappear and change into a set of amorphous halo. It indicated that amorphous phase was obtained in the Cu25 Nb75 ®lm by IBAD with Ar ion energy of 3 keV. By increasing the ion bombardment energy up to 4 keV, the amorphous phase began to decompose into Cu and Nb phase shown in SAD pattern of Fig. 1(d). In this ®gure, the diraction rings of Nb begin to be stronger and that of Cu discernable. This result reveals that both the new fcc and the amorphous phase are unstable and will be transformed lastly into Cu and Nb phase under Ar ion energy higher than 4 keV. To study the composition range of the metastable phases obtained in Cu25 Nb75 ®lms by IBAD, the Nb fraction of the ®lms was adjusted to 65% and 80%, respectively. The Nb-rich fcc phase, with the same microstructure as that shown in Fig. 1(b),
313
was also completely formed in Cu35 Nb65 ®lms when the ®lm was prepared under Ar ion energy of 3 keV. The fully amorphous phase was not obtained when the Ar ion energy was increased up to 4 keV. From the SAD pattern of Cu35 Nb65 sample with 4 keV Ar bombardment, the amorphous halo is present, but the line of Cu(1 1 1) is slightly visible beside it. It means that if the composition of Cu and the Ar ion energy were further increased, the amorphous phase would not be formed and the fcc phase might not be obtained. Actually, the ®lms of Cu25 Nb75 and Cu35 Nb65 prepared by IBAD with 7 keV Ar ion energy consisted of Cu and Nb phases, respectively, as results listed in Table 3. In Cu20 Nb80 ®lms prepared by IBAD, a bcc phase was formed when the ®lms were deposited under 4 and 7 keV Ar ion bombardment, respectively. Fig. 2(a) is the diraction pattern of the as-deposited Cu20 Nb80 multilayers and Fig. 2(b) is that of the Cu20 Nb80 ®lm deposited under 4 keV ion bombardment. It can be shown in Fig. 2(b) that the Cu20 Nb80 ®lm deposited under 4 keV ion bombardment is a bcc phase. The indices results of the bcc phase are listed in Table 2. The lattice constant of the bcc phase is 0:340 0:005 nm. This result indicates that the bcc phase of the Cu20 Nb80 ®lm prepared by IBAD with 4 keV ion energy is the supersaturated solid solution of Cu and Nb.
Table 1 Indices results of the diraction lines of Fig. 1(b) Series number
d (obs.) (10 1 nm)
Visual intensitya
Indices
1 2 3 4 5 6 7 8 9
2.475 2.144 1.520 1.298 1.244 1.077 0.988 0.880 0.825
S S S M W W M W W
111 200 220 311 222 400 331, 420 422 333
a
S: strong; M: medium; W: weak.
Fig. 2. TEM±SAD pattern of the Cu20 Nb80 ®lms: (a) as-deposited ®lm and (b) ®lm prepared by IBAD with Ar energy of 4 keV.
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Table 2 Indices results of the diraction lines of Fig. 2(b) Series number
d (obs.) (10 1 nm)
Visual intensitya
Indices
1 2 3 4 5 6
2.401 1.691 1.380 1.195 1.067 0.980
S M S M W W
110 200 211 220 310 222
a
S: strong; M: medium; W: weak.
Table 3 Microstructures of Cu±Nb ®lms by IBAD with various ionbeam energy
As-deposited 2 keV 3 keV 4 keV 7 keV a
Cu20 Nb80
Cu25 Nb75
Cu35 Nb65
Cu + Nb Cu + Nb Cu + Nb bcc bcc
Cu + Nb fcc Aa Cu + Nb Cu + Nb
Cu + Nb Cu + Nb fcc A + Cu Cu + Nb
A: amorphous.
In Table 3, we list all microstructure information of ®lms prepared by IBAD with various Ar ion energies. According to the data listed in Table 3, the composition range of the new fcc phase and the amorphous phase is about 65±75% Nb fraction. These metastable phases are generally unstable and only achieved in ®lms by IBAD with lower Ar ion energy. When the Ar ion energy is higher than 4 keV, neither fcc phase nor pure amorphous phase will be formed. To supersaturated solid solution, it is regarded that the solubility of Cu in Nb in Cu±Nb ®lms is about 20% Cu fraction. The lower energy (lower than 4 keV) of Ar ion bombardment is not sucient to mix Cu and Nb atoms together to form solid solution.
4. Discussion In IBAD technique, the atoms are deposited onto the substrate accompanying with ion-beam bombardment. The atoms arriving on the sub-
strate are mixed together by energetic Ar ionbeam. IM process of IBAD is also far from equilibrium and similar with that of multilayer ®lms so that the results in our experiment can be discussed according to the principles achieved previously in IM of multilayers. 4.1. Basic thermodynamic and kinetic conditions for the formation of metastable phases In ion mixing of multilayers in binary metal system, with either positive or negative energy of mixing, the initial energy state of the multilayers is raised by excess interfacial free energy stored in the multilayer ®lms [1,10]. With deliberate designation of modulation length and number of layers, the initial energy state of multilayers can be higher than that of certain metastable phase. Thus free energy dierence between the initial energy state of the multilayers and that of the metastable phases provides the system internal chemical driving force for the state of multilayers to go to the metastable state. When the thermodynamic conditions for the metastable phases are satis®ed, it still requires an external force provided by energetic ion-beam to drive the initial system to jump to the metastable state. In our IBAD experiments of Cu±Nb and previous Fe±Zr system [6], both the thermodynamics and kinetics conditions should also be satis®ed for the metastable phases formation. Concerning thermodynamics, the Gibbs free energies of metastable phases of Cu±Nb and Fe±Zr systems are calculated according to the method proposed by Miedema and Alonso [7,11] as shown in Fig. 3. The energy state that few (or dilute) atoms of one metal deposit on the surface of another metal is also calculated as adsorption enthalpy proposed by Miedema et al. [7] and the results are listed in Table 4. Comparing data listed in Table 4 with free energy diagrams of Fig. 3, one can see that the adsorption enthalpies of either Cu±Nb or Fe±Zr systems are much higher than the free energies of metastable phases in both systems, respectively. It means that the state of few (or dilute) atoms depositing on the ®lm of both systems is a state of high energy. If there is no Ar ion-beam bombardment during the deposition process, the atoms
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process. During the collision cascade and relaxation process induced by impacting Ar ion-beam, the system may attain to one of the state either equilibrium or metastable with lower free energy. The span of the collision cascade and relaxation process is very short, lasting only for 10 9 s so that the metastable phases, only disordered or simple crystalline structure phases, such as bcc, fcc and hcp phases, are expected. It is the reason that metastable fcc and amorphous phases in Fe±Zr and fcc, bcc and amorphous phases in Cu±Nb systems are formed in our experiments. Therefore the Ar ion-beam bombardment during the deposition process plays the role of external kinetic force for the formation of metastable phases.
(a)
(b)
4.2. The formation ranges and stability of the metastable phases
Fig. 3. Calculated Gibbs free energy of (a) Cu±Nb and (b) Fe±Zr system. Table 4 Calculated adsorption enthalpy of A on B, i.e. DHAadsorption on B : (kJ/ mol) Surface (B) Atoms (A) Cu Nb Fe Zr
Cu 274.0
Nb
Fe
Zr
35.8 94.3
141.3
of one metal will nucleate and grow up with the supplement atoms from the source, and the energy of the system will decrease subsequently. When the Ar ion-beam bombardment during the deposition is applied, the atoms from the source will be mixed with the ®lm already on the substrate. Normally, as in IM of multilayers, the impacting Ar ionbeam during deposition process also induces the atomic collision cascade and subsequent relaxation
In Cu±Nb system, the formation range of the amorphous phase is narrow by our IBAD method, which is around 75% of Nb fraction. The formation range of the fcc phase is about 65±75% of Nb fraction and that of the bcc phase is about 80% of Nb fraction. To Fe±Zr system, partial amorphous was obtained around Zr fraction of 50% [6]. Although the experimental result of Fe±Zr system is not complete, it is regarded that the formation range of the amorphous phase is somewhat broader than that of Cu±Nb system. The fcc and the amorphous phase of Cu25 Nb75 and Cu35 Nb65 ®lms began to decompose under ion-beam energy higher than 4 keV, while the bcc phase of Cu20 Nb80 ®lm was still formed under 7 keV. To Fe±Zr system, the metastable phases were formed with ion energy more than 4 keV. There are two reasons to explain the phenomena presented above. Firstly, from the view of thermodynamics, the free energy of the amorphous phase in Cu±Nb system is positive while that of Fe±Zr system is negative. Therefore, the composition range of amorphous phase in Fe±Zr system, i.e. about from 80 to 50 at.% of Zr fraction, is broader than that in Cu±Nb system, i.e. around 75 at.% Nb fraction. Secondly, from the view of kinetics, when the energy deposited on the ®lm is very high, the diffusion of Cu atoms in the collision cascade and
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relaxation process is very fast and the Cu atoms will agglomerate. The atoms of Cu will nucleate and grow up very fast so that the atoms of Cu and Nb will not be mixed together. Therefore, the metastable phases will decompose. The composition limitation and decomposition of amorphous Cu±Nb alloy ®lm were also con®rmed by Andersen et al. [8] and Pan and Liu [12]. Similar phenomena were also observed in Cu±Ta and Cu±W system with positive heat of mixing [13]. To Fe±Zr system, there is an attractive force between Fe and Zr atoms so that movement of either kind of atoms is limited to each other. Thus the metastable phases in Fe±Zr system are more stable. 4.3. Some considerations for kinetic mechanism of metastable phase formation and phase competing in Cu±Nb system In some Nb-, Ta- and Mo-based systems, around composition AB3 (B stands for Nb, Ta or Mo), the fcc phase can be formed in both negative and positive systems by IM method [1]. In kinetics a two-step transition mechanism of bcc±hcp±fcc was proposed for the fcc transition in IM [14]. The relationship of lattice constant pbetween the fcc phase and the bcc phase is afcc 6=2abcc after the two-step transition at last. In our case, no hcp phase is formed prior to the fcc phase, but the fcc phase is based on the bcc structure of Nb. It is applicable to explain the bcc±fcc transition by martensitic transformation, for the period of phase formation is so short that the atom movement requires minimal distance and energy. Several martensitic transformation models has been proposed to explain the bcc±fcc transition [15,16]. It is hard for us to observe the detail process in transformation in experiment so that we cannot judge the actual style of the martensitic transformation in our formation of fcc phase. Generally the (0 1 1) plan of the bcc structure transforms to the (1 1 1) plan of the fcc structure so that the relationship of lattice constant between the bcc p and fcc phase can also be described by afcc p 6=2a bcc in [16]. According to the relation afcc 6=2abcc , the lattice constant of the fcc phase is estimated by using the lattice constant (0.33 nm) obtained in the bcc phase in ®lm with 4 keV ion bombardment. The
estimated result is 0.404 nm that is compatible with the value (0.428 nm) obtained in our experiment. The relationship between the magnitude of the ion-beam energy and the exact phase formation is much complicated. To Cu±Nb system, the bcc phase was not obtained around the composition of 25±35 Cu fraction, and the fcc and the amorphous phase were not formed around Cu20 Nb80 composition. Around the composition of 25±35 Cu fraction, the free energy curve of the bcc phase intersects with that of the amorphous and the fcc phase shown in Fig. 3(a). It suggests the possibility for the formation of the bcc phase, but we did not obtain bcc phase in this range. We consider the following process to explain phase competing in this region. When the energy of ion-beam bombardment is lower, the Cu±Nb system can ful®ll rapid bcc±fcc transition mentioned above with Cu atoms diusing eectively into the Nb matrix. This procedure may be Cu atoms diusion limited. When the energy of ion-beam is increased, the collision cascade eect makes the Nb atoms more active. The atom size of Nb is much bigger than that of Cu while the diusivity of Nb atoms is lower. Thus the displaced Nb atoms hardly recover to form crystalline structure in the short time of relaxation and the disordered, i.e. amorphous, phase is obtained. This procedure can be regarded as Nb atoms diusion limited. 5. Conclusion A new Nb-rich fcc phase and an amorphous phase were obtained in the ®lms prepared by IBAD with Ar ion energy below 7 keV. The composition range of the fcc and the amorphous phase is around 65±75% of Nb fraction. The Nbrich bcc supersaturated solution was also formed in Cu20 Nb80 ®lms by IBAD with Ar ion energy higher than 4 keV. The IBAD process is far from equilibrium as well as IM is. The formation mechanisms of metastable phases can be discussed according to the principles on both thermodynamics and kinetics in the IM of multilayers. Normally, only simple crystalline structure and disordered phase can be formed in such a process
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far from equilibrium. The kinetic process of the formation of the fcc phase in Cu25 Nb75 can be interpreted with the model suggested by Liu and Zhang [14]. The phase competing is possible due to the atomic diusive behavior under high energy impacting of ion-beam. Acknowledgements This work was supported in part by National Natural Science Foundation of China (No. 19875027), the Ministry of Science and Technology of China through a Grant No. G20000672071, and by the Administration of Tsinghua University. References [1] B.X. Liu, W.S. Lai, Q. Zhang, Mater. Sci. Eng. R 29 (2000) 1.
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