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Ion beam mixing in the AuCu-Al bilayer system
Nuclear Instruments and Methods in Physics Research B19/20 (1987) 623-625 North-Holland, Amsterdam
623
ION BEAM MIXING IN THE AuCu-AI BILAYER SYSTEM Y A N G G e n Qing, S.U. C A M P I S A N O and E. R I M I N I Dipartimento di Fisica dell'UniversitY, Corso Itafia 57, 95129 Catania, Italy
Thin films of CuxAut-x alloys covered with a thin aluminum layer have been bombarded with Xe ÷ ions to produce ion beam mixing. The rates of mixing were measured by 2 MeV He + backscattering as a function of the composition. Higher rates were obtained for Au rich alloys (x < 0.5). Phases have been identified by X-ray diffraction analysis. Mixtures of Au2A1 and AuAI2 have been detected for Au rich alloys while A14Cu9 has been detected for Cu rich alloys. In the middle range of concentration no known phase was observed. The detected phases and the mixing rates are discussed within the framework of thermodynamic calculations of the free energy gain for compound formation.
1. Introduction
2. Experimental
Ion beam mixing is a violent way of producing atom migration over distances of the order of 10 nm. Several systems in either bilayer or multilayers structure have been studied. For the case of bilayers the results may be divided in two groups: the first one includes all the systems which show the formation of the same compounds observed after conventional thermal annealing. In this group we may allocate almost all the metal-silicon and a large number of metal-metal systems [1]. The second group includes all the bilayers in which the compounds observed after ion beam mixing are rectastable phases, i.e. they do not appear after thermal annealing of the corresponding system and they decompose into equilibrium phases after suitable thermal processing. Some of the phases belonging to this second group have been observed after non-equilibrium processing such as plat cooling etc. The A u - G e system [2], e.g. belongs to the second group. It is thus important to understand how the thermodynamic constraints are involved in the phase formation process during ion beam mixing. For pure bilayers it has been shown [3] that the mixing rate is related to the cohesive energy of the corresponding elements. It has been also demonstrated that the thermal reaction rate in the CuxAUl_x-A1 bilayer, x ranging from 0 to 1, depends on thermodynamic constraints and on kinetics [4]. In fact the free energy gain for compound formation depends on the initial alloy composition. The result demonstrated that kinetics dominates over thermodynamics: AuA1 compounds were observed also in a concentration regime where the free energy gain is larger for the formation of CuA1 compounds. On the basis of those results we have investigated atom migration and phase formation in the CuxAUl_x-A1 bilayer for x ranging from 0 to 1 after Xe ion beam bombardment.
Copper gold alloys have been prepared by vacuum codeposition of pure Cu and Au metals onto glass substrates. The A u / C u ratio was changed at each deposition so to cover a wide range of concentrations. The residual pressure was 10 -6 TOll" during deposition. The films were then alloyed at 350 ° C for 2 h and the formation of an homogeneous solid solution was tested by the sharpness of the X-ray diffraction peaks. Before A1 deposition the alloy surface layer was removed by reverse sputtering in a 0.03 Tort purified Ar atmosphere. Pure A1 was then vacuum deposited. The specimens were implanted with Xe ions at 320 keV and for doses up to 2 × 1016 //cm 2. The beam current density was about 1 /~A/cm 2 and no intentional temperature control was used. The samples have been analyzed by 2.0 MeV backscattering and X-ray diffraction techniques.
0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
V. ION BEAM MIXING
3. Results and discussion Irradiation of the pseudo-binary CuxAu~_x-Al bilayer system with Xe ions at energy large enough to reach the interface produce atom migration as evidenced by the backscattering spectra reported in fig. 1. The analyses refer to muo.73Cuo.27-ml and to mu0.37Cu0.63-ml samples respectively. In both cases for doses in the 1016//cm 2 range detectable migration is evidenced by the formation of plateaux in the Cu and Au signals and by tails in the A1 signal. The width of the plateaux in the Cu and Au portions of the spectra are comparable and both elements are revealed at energies larger than the as-deposited samples indicating that both Cu and Au migrate toward the surface by approximately the same distance. This is the first important difference with the thermal annealing case [4]
624
Yang Gen Qing et aL / Mixing in the AuCu-AI bilaver svstem
")
13 =. 20MeV 90
Au°~3Cue~7
ul g Ov
/*5
- -
0 9, lO'~/crn:
.....
~ 8,
~ l l * o
0
60
~o'y~:
AS- Deoo$,led
L
,
g~am~]
20He" MeV ~
Auo~,Cuo6~
-~
*m
i 8 ,o'~/cm:
30;
,2
• • • As-oepos,~eo" ~ - "e,X"% Cu
12
15
~8
Energy(MeV)
Fig. 1. Backscattering spectra showing the ion beam mixing among Au, Cu and AI elements for AuomCuo.27-A1 (upper) and Auo.37Cuo.63-Al (lower) samples implanted with 320 keV Xe ÷ at room temperature. The Au/Cu ratio in the reacted layer is almost equal with the ratio in the as-deposited alloy. where only one element (Cu or Au) reacted with the A1 layer while the other was pushed at larger depths by interface segregation mechanisms. In any case the A u / C u ratio in the reacted layer is almost equal (within 20%) with the A u / C u ratio in the as-deposited samples. The reaction rate can thus be measured by determining the amount of one of the element in the mixed layer. The number of reacted Au atoms is reported in fig. 2 for
different starting compositions and as a function of the square root of the Xe dose. A linear trend is evident with larger rates associated to Au rich alloys. For Cu rich samples the rate is very small and only at the higher doses a detectable amount of reaction is observed. Calculations on the energy profile distribution performed as reported previously [5], show that the energy deposited at the interface is almost the same for pure Cu-A1 and Au-A1 samples. Thus the reaction rate should vary linearly from the rate of pure A1-Cu to that of pure A I - A u assuming that only the energy deposited at the interface is the rate controlling parameter. The measured rates are reported in fig. 3 as a function of the composition and a nonlinear trend is observed in the middle range of composition. This decrease of the reaction rate occurs in the same region where a decrease of the partial molar free energy gain takes place. Within the mixed layer we have the presence of three elements: Cu, Au, AI. The backscattering technique can give us information on the average composition but not on the phases. To detect the presence of intermetallic compounds X-ray diffraction analysis has been used. Fig. 4 reports the diffraction patterns obtained from Auo.87 Cu0.13-Al and from Au0.73Cu0.27-A1 (inset) samples bombarded with 320 keV Xe at 1.8 × 1016/cm 2. In both samples there is a clear presence of Au2A1 and AuAI 2. This is in good agreement with results obtained in Au-A1 bilayers after bombardment. The only difference is that in the present case we do not observe the formation of Au 5A12, probably due to the Cu presence. F o r Cu rich alloys the Xe bombardment produces the formation of A14Cu9, as shown by the diffraction pat-
3.0 r
3°l
1
O
tO
AI (95 nm.),~ux Cu~/glass 320keV Xe~ R.T.
zc
/ /
/
, I ,1
-o z0 /
Z n" O~ t-
O O O Measured Predicted -
0
0 Cu
0 ~ 0
I
20
I
40
-
-
-
I
60
I
80
100 Au
Au concentration(at %) •
05
K --0
1.0
t
1.5
(Dose) I/2 (= 1017CM2)1/2 Fig. 2. Number of reacted Au atoms as a function of the square root of the dose for different AuCu alloy composition.
Fig. 3. Plot of the total mixing rate (Au + Cu) measured by RBS technique as a function of the alloy composition. The mixing rate predicted by the ballistic model is indicated by the dotted line. The reduction of the mixing rate in the middle concentration region is associated with a decrease in the partial molar free energy gain for compound formation.
Yang Gen Qing et aL / Mixing in the AuCu-AI bilaver system 320 keV Xe" R.T 18,d0'6/cm2 J AI/Au°nCu°zCglass
F-1
tu
AI/a, Uor5CueB~/gEass
t
J I/
AuAI
320 keV Xe" R T 16,10~6/cm2
[
D t~ v "O
8
625
I IAu0,5 Cuo85
""3'5 .... 3'o....
/
2;
....
AkCu9 l
1
>-
;<
]3
l~
22
~;
a;
3'6
3~.
3'2
I i I
I ~ I
, I
320 keV Xe" RT 1 8x1016/crn2 t
zl/Aua37Cuo6,/gtass
2®(degrees)
Fig. 4. X-ray diffraction peaks showing the formation of
Cu
Au2Al after implantation of 320 keV Xe + to a dose of 1.8 x
/
/ Au°j7 Cu°6]
1016 ions/cms 2 onto Au0.sTCuo.13-Al samples at room temperature. The formation of AuAl 2 after the bombardment with 320 keV Xe + to 1.8 × 1016 on Au0.73Cuo.27-Al samples at room temperature is indicated by the pattern in the inset. I6
tern reported in fig. 5a for the case of Au0.15Cu0.85-ml implanted with 320 keV Xe at 1.6 x 1016/cn~. This result is also comparable with the pure Cu/A1 bilayer where the same phase is detected after K.r÷ bombardment. Rather puzzling is the behavior in the middle composition range of the AuCu solid solution. Although the shape of the backscattering spectrum in the mixed layer is similar to the case of extreme concentrations, the X-ray diffraction technique, in the Bragg-Brentano geometry we use, is unable to detect either A u - A l or C u - A l compounds. In some cases at intermediate Xe ion doses an unidentified peak occurring from 42.5 to 43 is detected. This peak shifts at larger angles by increasing the ion dose. At the largest dose we observe instead the presence of a pure Cu peak at 43.3, as reported in fig. 5b. The backscattering spectra show that the three elements are present in the mixed layer while X-ray diffraction shows only the presence of A u - A l compounds in the Au rich region, of C u - A l compounds in the Cu rich region and no known phases in the middle range region. The comparison suggests that in the extreme regions the low concentration elements of the alloy is dispersed in the formed compound (e.g. we do not observe a pure Au peak in the ion mixed Cu rich alloy). In the middle concentration region we can infer a demixing of the AuCu solid solution whilst Au and Cu either as pure elements or as known compounds are not present. The middle concentration region is the one for which the free energy gain for the formation of AuAl or of CuA1 compounds is the smaller and thus metastable phases can take over. Moreover a decrease of the reaction rate is observed with respect to the linear trend which is expected if only ballistic effects are dominants.
I at.
I L.2
I z,0
1 38
1 36
2® (degrees)
Fig. 5. (a) X-ray diffraction peaks showing the formation of A14Cu 9 after implantation of 320 keV Xe + to a dose of 1.6 x 1016 ions/cm z onto Au0.15Cu0.85-Al samples at room temperature and, (b) the presence of pure Cu in the sample of Au0.37Cu0.63-Al implanted with 320 keV Xe + to 1.8 X 1016 ions/cm 2 at room temperature.
In conclusion we have demonstrated that in the pseudo-binary AUxCU3_x-Al system ion beam mixing produces phases which are the same of those observed after ion beam mixing of the corresponding A u - A l or Cu-A1 bilayers. In these extreme regions the gain in free energy for the compound formation is rather large. In the middle concentration range, where the free energy gain for the compound formation is quite small, decrease of the mixing rate, demixing effects and, probably, metastable compounds are observed.
References
[1] J.W. Mayer and S.S. Lau, in: Surface Modification and Alloying, eds., J.M. Poate, G. Foti and D.C. Jacobson (Plenum Press, New York 1983) ch. 9. [2] L. Torrisi, S.U. Campisano, Chu Te Chang, A. Trovato and E. Rimini, Nucl. Instr. and Meth. B7/8 (1985) 552. [3} M. van Rossum, Y-T. Cheng, M.A. Nicolet and W.L. Johnson, Appl. Phys. Lett. 46 (1985) 610. [4] S.U. Campisano, E. Costanzo and R. Cristofolini, J. Appl. Phys. 51 (1980) 3730. [5] Chu Te Chang, S.U. Campisano and E. Rimini, J. Appl. Phys. 55 (1984) 3322.
623
ION BEAM MIXING IN THE AuCu-AI BILAYER SYSTEM Y A N G G e n Qing, S.U. C A M P I S A N O and E. R I M I N I Dipartimento di Fisica dell'UniversitY, Corso Itafia 57, 95129 Catania, Italy
Thin films of CuxAut-x alloys covered with a thin aluminum layer have been bombarded with Xe ÷ ions to produce ion beam mixing. The rates of mixing were measured by 2 MeV He + backscattering as a function of the composition. Higher rates were obtained for Au rich alloys (x < 0.5). Phases have been identified by X-ray diffraction analysis. Mixtures of Au2A1 and AuAI2 have been detected for Au rich alloys while A14Cu9 has been detected for Cu rich alloys. In the middle range of concentration no known phase was observed. The detected phases and the mixing rates are discussed within the framework of thermodynamic calculations of the free energy gain for compound formation.
1. Introduction
2. Experimental
Ion beam mixing is a violent way of producing atom migration over distances of the order of 10 nm. Several systems in either bilayer or multilayers structure have been studied. For the case of bilayers the results may be divided in two groups: the first one includes all the systems which show the formation of the same compounds observed after conventional thermal annealing. In this group we may allocate almost all the metal-silicon and a large number of metal-metal systems [1]. The second group includes all the bilayers in which the compounds observed after ion beam mixing are rectastable phases, i.e. they do not appear after thermal annealing of the corresponding system and they decompose into equilibrium phases after suitable thermal processing. Some of the phases belonging to this second group have been observed after non-equilibrium processing such as plat cooling etc. The A u - G e system [2], e.g. belongs to the second group. It is thus important to understand how the thermodynamic constraints are involved in the phase formation process during ion beam mixing. For pure bilayers it has been shown [3] that the mixing rate is related to the cohesive energy of the corresponding elements. It has been also demonstrated that the thermal reaction rate in the CuxAUl_x-A1 bilayer, x ranging from 0 to 1, depends on thermodynamic constraints and on kinetics [4]. In fact the free energy gain for compound formation depends on the initial alloy composition. The result demonstrated that kinetics dominates over thermodynamics: AuA1 compounds were observed also in a concentration regime where the free energy gain is larger for the formation of CuA1 compounds. On the basis of those results we have investigated atom migration and phase formation in the CuxAUl_x-A1 bilayer for x ranging from 0 to 1 after Xe ion beam bombardment.
Copper gold alloys have been prepared by vacuum codeposition of pure Cu and Au metals onto glass substrates. The A u / C u ratio was changed at each deposition so to cover a wide range of concentrations. The residual pressure was 10 -6 TOll" during deposition. The films were then alloyed at 350 ° C for 2 h and the formation of an homogeneous solid solution was tested by the sharpness of the X-ray diffraction peaks. Before A1 deposition the alloy surface layer was removed by reverse sputtering in a 0.03 Tort purified Ar atmosphere. Pure A1 was then vacuum deposited. The specimens were implanted with Xe ions at 320 keV and for doses up to 2 × 1016 //cm 2. The beam current density was about 1 /~A/cm 2 and no intentional temperature control was used. The samples have been analyzed by 2.0 MeV backscattering and X-ray diffraction techniques.
0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
V. ION BEAM MIXING
3. Results and discussion Irradiation of the pseudo-binary CuxAu~_x-Al bilayer system with Xe ions at energy large enough to reach the interface produce atom migration as evidenced by the backscattering spectra reported in fig. 1. The analyses refer to muo.73Cuo.27-ml and to mu0.37Cu0.63-ml samples respectively. In both cases for doses in the 1016//cm 2 range detectable migration is evidenced by the formation of plateaux in the Cu and Au signals and by tails in the A1 signal. The width of the plateaux in the Cu and Au portions of the spectra are comparable and both elements are revealed at energies larger than the as-deposited samples indicating that both Cu and Au migrate toward the surface by approximately the same distance. This is the first important difference with the thermal annealing case [4]
624
Yang Gen Qing et aL / Mixing in the AuCu-AI bilaver svstem
")
13 =. 20MeV 90
Au°~3Cue~7
ul g Ov
/*5
- -
0 9, lO'~/crn:
.....
~ 8,
~ l l * o
0
60
~o'y~:
AS- Deoo$,led
L
,
g~am~]
20He" MeV ~
Auo~,Cuo6~
-~
*m
i 8 ,o'~/cm:
30;
,2
• • • As-oepos,~eo" ~ - "e,X"% Cu
12
15
~8
Energy(MeV)
Fig. 1. Backscattering spectra showing the ion beam mixing among Au, Cu and AI elements for AuomCuo.27-A1 (upper) and Auo.37Cuo.63-Al (lower) samples implanted with 320 keV Xe ÷ at room temperature. The Au/Cu ratio in the reacted layer is almost equal with the ratio in the as-deposited alloy. where only one element (Cu or Au) reacted with the A1 layer while the other was pushed at larger depths by interface segregation mechanisms. In any case the A u / C u ratio in the reacted layer is almost equal (within 20%) with the A u / C u ratio in the as-deposited samples. The reaction rate can thus be measured by determining the amount of one of the element in the mixed layer. The number of reacted Au atoms is reported in fig. 2 for
different starting compositions and as a function of the square root of the Xe dose. A linear trend is evident with larger rates associated to Au rich alloys. For Cu rich samples the rate is very small and only at the higher doses a detectable amount of reaction is observed. Calculations on the energy profile distribution performed as reported previously [5], show that the energy deposited at the interface is almost the same for pure Cu-A1 and Au-A1 samples. Thus the reaction rate should vary linearly from the rate of pure A1-Cu to that of pure A I - A u assuming that only the energy deposited at the interface is the rate controlling parameter. The measured rates are reported in fig. 3 as a function of the composition and a nonlinear trend is observed in the middle range of composition. This decrease of the reaction rate occurs in the same region where a decrease of the partial molar free energy gain takes place. Within the mixed layer we have the presence of three elements: Cu, Au, AI. The backscattering technique can give us information on the average composition but not on the phases. To detect the presence of intermetallic compounds X-ray diffraction analysis has been used. Fig. 4 reports the diffraction patterns obtained from Auo.87 Cu0.13-Al and from Au0.73Cu0.27-A1 (inset) samples bombarded with 320 keV Xe at 1.8 × 1016/cm 2. In both samples there is a clear presence of Au2A1 and AuAI 2. This is in good agreement with results obtained in Au-A1 bilayers after bombardment. The only difference is that in the present case we do not observe the formation of Au 5A12, probably due to the Cu presence. F o r Cu rich alloys the Xe bombardment produces the formation of A14Cu9, as shown by the diffraction pat-
3.0 r
3°l
1
O
tO
AI (95 nm.),~ux Cu~/glass 320keV Xe~ R.T.
zc
/ /
/
, I ,1
-o z0 /
Z n" O~ t-
O O O Measured Predicted -
0
0 Cu
0 ~ 0
I
20
I
40
-
-
-
I
60
I
80
100 Au
Au concentration(at %) •
05
K --0
1.0
t
1.5
(Dose) I/2 (= 1017CM2)1/2 Fig. 2. Number of reacted Au atoms as a function of the square root of the dose for different AuCu alloy composition.
Fig. 3. Plot of the total mixing rate (Au + Cu) measured by RBS technique as a function of the alloy composition. The mixing rate predicted by the ballistic model is indicated by the dotted line. The reduction of the mixing rate in the middle concentration region is associated with a decrease in the partial molar free energy gain for compound formation.
Yang Gen Qing et aL / Mixing in the AuCu-AI bilaver system 320 keV Xe" R.T 18,d0'6/cm2 J AI/Au°nCu°zCglass
F-1
tu
AI/a, Uor5CueB~/gEass
t
J I/
AuAI
320 keV Xe" R T 16,10~6/cm2
[
D t~ v "O
8
625
I IAu0,5 Cuo85
""3'5 .... 3'o....
/
2;
....
AkCu9 l
1
>-
;<
]3
l~
22
~;
a;
3'6
3~.
3'2
I i I
I ~ I
, I
320 keV Xe" RT 1 8x1016/crn2 t
zl/Aua37Cuo6,/gtass
2®(degrees)
Fig. 4. X-ray diffraction peaks showing the formation of
Cu
Au2Al after implantation of 320 keV Xe + to a dose of 1.8 x
/
/ Au°j7 Cu°6]
1016 ions/cms 2 onto Au0.sTCuo.13-Al samples at room temperature. The formation of AuAl 2 after the bombardment with 320 keV Xe + to 1.8 × 1016 on Au0.73Cuo.27-Al samples at room temperature is indicated by the pattern in the inset. I6
tern reported in fig. 5a for the case of Au0.15Cu0.85-ml implanted with 320 keV Xe at 1.6 x 1016/cn~. This result is also comparable with the pure Cu/A1 bilayer where the same phase is detected after K.r÷ bombardment. Rather puzzling is the behavior in the middle composition range of the AuCu solid solution. Although the shape of the backscattering spectrum in the mixed layer is similar to the case of extreme concentrations, the X-ray diffraction technique, in the Bragg-Brentano geometry we use, is unable to detect either A u - A l or C u - A l compounds. In some cases at intermediate Xe ion doses an unidentified peak occurring from 42.5 to 43 is detected. This peak shifts at larger angles by increasing the ion dose. At the largest dose we observe instead the presence of a pure Cu peak at 43.3, as reported in fig. 5b. The backscattering spectra show that the three elements are present in the mixed layer while X-ray diffraction shows only the presence of A u - A l compounds in the Au rich region, of C u - A l compounds in the Cu rich region and no known phases in the middle range region. The comparison suggests that in the extreme regions the low concentration elements of the alloy is dispersed in the formed compound (e.g. we do not observe a pure Au peak in the ion mixed Cu rich alloy). In the middle concentration region we can infer a demixing of the AuCu solid solution whilst Au and Cu either as pure elements or as known compounds are not present. The middle concentration region is the one for which the free energy gain for the formation of AuAl or of CuA1 compounds is the smaller and thus metastable phases can take over. Moreover a decrease of the reaction rate is observed with respect to the linear trend which is expected if only ballistic effects are dominants.
I at.
I L.2
I z,0
1 38
1 36
2® (degrees)
Fig. 5. (a) X-ray diffraction peaks showing the formation of A14Cu 9 after implantation of 320 keV Xe + to a dose of 1.6 x 1016 ions/cm z onto Au0.15Cu0.85-Al samples at room temperature and, (b) the presence of pure Cu in the sample of Au0.37Cu0.63-Al implanted with 320 keV Xe + to 1.8 X 1016 ions/cm 2 at room temperature.
In conclusion we have demonstrated that in the pseudo-binary AUxCU3_x-Al system ion beam mixing produces phases which are the same of those observed after ion beam mixing of the corresponding A u - A l or Cu-A1 bilayers. In these extreme regions the gain in free energy for the compound formation is rather large. In the middle concentration range, where the free energy gain for the compound formation is quite small, decrease of the mixing rate, demixing effects and, probably, metastable compounds are observed.
References
[1] J.W. Mayer and S.S. Lau, in: Surface Modification and Alloying, eds., J.M. Poate, G. Foti and D.C. Jacobson (Plenum Press, New York 1983) ch. 9. [2] L. Torrisi, S.U. Campisano, Chu Te Chang, A. Trovato and E. Rimini, Nucl. Instr. and Meth. B7/8 (1985) 552. [3} M. van Rossum, Y-T. Cheng, M.A. Nicolet and W.L. Johnson, Appl. Phys. Lett. 46 (1985) 610. [4] S.U. Campisano, E. Costanzo and R. Cristofolini, J. Appl. Phys. 51 (1980) 3730. [5] Chu Te Chang, S.U. Campisano and E. Rimini, J. Appl. Phys. 55 (1984) 3322.