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Aluminides and silicides formation by ion beam mixing of multilayers
Nuclear Instruments and Methods in Physics Research B74 (1993) 98-104 North-Holland
NIIM B
Beam Interactions with Materials&Atoms
Aluminides and silicides formation by ion beam mixing of multilayers * I.J.R. Baumvol Institute de F&a, UFRGS, 91501470 Port0 Alegre, RS, Brad
The formation of transition metal-aluminides and silicides by ion beam mixing of thin films multilayered structures is presented here. The present stage of the capability of predicting the phase to be formed when a certain multilayer structure, with a certain imposition, is sub~tted to ion beam mixing under certain bombar~ent ~n~tions is discussed. Examples existing in the literature of transition metal-aluminium and transition metal-silicon multilayers submitted to ion beam mixing are analysed and a more detailed discussion of the multilayered Fe-Al system with Fe and Al thicknesses varied such as to cover most of the concentration intervals is presented.
Transition metal silicides and aluminides are largely involved in the present stage of silicon technology for integrated circuits. In the past several years, the formation of silicides and aluminides by ion beam mixing (IBM) and thermal annealing (TA) was intensively studied [1,2]. The study of the formation of transition metal silicides by IBM demonstrated that for bombardments of the substrate at room temperature most of the phase sequences observed in IBM coincided with the phase sequences observed in silicides formed by thermal annealing [I], independent of the atomic number of the bombarding species. Increasing the total bombardment dose would have the same effect on silicide phase sequences as increasing time or temperature in furnace annealing. IBM of transition metalaluminum thin film structures at room temperature or above also showed many analogies with the formation of equiIib~um phases in bulk or thin film metallur~, the latter being expressed by various different rules for phase formation, like the Walser-BenC rule [3], the extended Ben6 rule [4] and the approach put forward by Tu and GGsele [51. Also early verified was the importance of the mobility of the species ~ntribut~g to grow the phases under study. This means that not only the~odynami~ but also kinetics determine the formation of phases in IBM. Examples of this are the studies of gold and chromium aluminides formation by IBM and TA [6-81 and the formation of nickel and cobalt silicides by IBM and TA [9,X@ Based on these empirical facts Lau and collaborators [ll] proposed that equilibrium phase diagrams are * Work supported in part by CNPq and FAPERGS, Brazil. Old-583X/93/$06.#
an adequate guide to predict the phases formed by IBM. A further step was proposed by Johnson and collaborators [12], that most of the IBM process develops during the thermal spike stage of the collision cascade. At that stage, the formulation of rules to predict phase formation by IBM was precluded by two main obstacles: i) the meaning of eq~librium thermod~~its or equilibrium phase diagrams in the case of a solid phase reaction in thin films was not clearly understood. The rules from refs. [3-5] could not predict many of the observed phases in thin film structures submitted to furnace anneahng; ii) the physical nature of the thermal spike was not known in detai1 as well as the conditions (especially substrate temperature) for subsequent diffusion of defects outside the cascade volume. The concept of thermal spike was clarified and one can certainly refer to the excellent review by Cheng [13] for understanding the thermodynamic aspects of thermal spikes, the conditions for their fo~ation and the critical temperature T, for the onset of temperature-dependent ion mixing (i.e. outdiffusion of defects). Following the understanding of the thermal spikes, the thermodynamics and kinetics mechanisms determining phase transformations induced by ion irradiation could be put on a far more clear basis, and we refer to the also excellent review by Nastasi and Mayer [14] to establish a thermodynamic hierarchy between amorphous, metastable and stable phases formed under ion bombardment, as well as the transformation kinetic paths to reach these phases for each IBM situation (substrate temperature, bombarding species, sample composition, structure of the phase, etc.). According to this hierarchy, amorphous phases can be predicted to form at low temperatures IBM in a wide range of concentrations. In fact, as pointed out by Liu 1151the
0 1993 - Elsevier Science Publishers B.V. All rights reserved
99
I.J.R. Baumvol / Aluminides and silicides formation
only concentration intervals where amorphous phases will not form in low temperature ion beam mixing are the regions where solid solutions with simple structures (bee, fee or hcp) exist. The formation of metastable crystalline phases should occur over intermediate substrate temperatures and stable crystalline phases are expected at higher substrate temperatures. The criteria for low, intermediate and high temperatures in IBM should be correlated to T,, the critical temperature for the onset of diffusion of defects to the surroundings of the cascade volume. Finally the prediction of phase formation sequence and phase stability in binary thin film systems submitted to thermal annealing was elucidated by Pretorius et al. [7], introducing the concept of effective heat of formation. Based on this innovative concept, Pretorius et al. established rules for first phase formation and subsequent phases formed in binary thin film systems. These rules incorporated most of the expertise of the previously existing ones, being also able to predict many of the so far unexplained phases formed in furnace annealing of thin film bilayers. It is noteworthy that the key feature in the concept of effective heat of formation is the effective concentration of atoms at the growing interface. In other words, the thermodynamic driving force for a certain reaction is not just the classical heat of formation of the reaction (AH), but rather an effective heat of formation (AH’) which is
the product of the heat of formation per gram atom (AH), and the total number of atoms used in the reaction. effec. cont. lim. element AH’=
AH
camp. cont. lim. element ’
In the case of phases formed by thermal annealing of thin film structures it is assumed that the largest mobility of the reacting species occurs at the lowest eutectic temperature in the equilibrium phase diagram and the AH’ rules are: i) the first compound phase to form is the phase with the most negative, effective heat of formation at the concentration of the lowest temperature eutectic (liquidus) of the binary system; and ii) after first phase formation, the next phase to form at the interface between the compound phase and the remaining element is the next phase richer in the unreacted element which has the most negative heat of formation. The concept of effective heat of formation privileges the role of the mobile species in phase formation and as such it is very adequate to be used in predicting the formation of phases in IBM of thin film structures. Illustrative examples existing in the literature will be analysed or eventually reanalyzed here. A detailed discussion of the prediction of the phases formed in IBM of Fe/Al multilayers of several different relative thicknesses, bombarded with different ionic species at
800 600
GOLD
CONCENTRATION (a)
(at%)
Cr CONCENTRATION
(at %I
b)
Fig. 1. Phase diagram and effective heat of formation diagram of the Au-Al binary system (from ref. [7]). (b> Phase diagram and effective heat of formation diagram of the AI-Cr binary system (from ref. 181).The dashed vertical lines represent the concentrations at the lowest temperatures eutectic (liquidus). IV. MATERIALS SCIENCE
I.J.R Baumvol / Aluminides and silicides formation
100
several different substrate temperatures from 77 to 473 K will be made. The choice of the Fe-Al system seems very fortunate because the phases formed by IBM of the Fe/Al multilayers include amorphous, icosahedral metastable and crystalline stable phases, constituting an exhaustive test of the present capabilities of predicting phases on IBM of multilayers.
2. Ion beam mixing of Al/transition
and the predictability of these phases using the effective heat of mixing rules (AH’ rules) discussed in the previous section. Only a few illustrative examples were chosen, without any intention of making a complete survey of all the results existing in the literature. 2.1. Au/Al
IBM of multilayered samples with 1 X 1015 Xe+ cm-’ at room temperature produced the Au,Al and AuAl, equilibrium compounds in the concentration range between 15 and 70 at% of Au. For concentrations close to either side of the phase diagram solid solutions were produced.
metal multilayers
In this section we present some cases of IBM of aluminum/transition metal multilayered thin film structures with emphasis on the phases that are formed
77
Fe/Al
(nm)
-
THERMAL
1012 Bza .
10/6 PzzzI . . .
:
8x
6x
[6,7]
TOTAL
- 1OOnm
ANNEALING
lO/lO B :
q lo/28
.
lo/42 n
.
iX 3;
. :
2x
Si02
FETAL
523K * (250°C) ry-FE 548K
573K
FETAL
U-FE
FE*AL
w-F;
FEAL~
FEAL?
FEAL~
FEAL?
S.S.(FCC)
S,S,iFCC)
S.S.(FCC)
S.S.
a-FE
~-FE
a-FE
N-FE
FEAL~
FEAL~
FEAL-,
FEi\L_
S,S.(FCC)
S.S.(FCC)
S.S.(FCC)
S.S.iFCC)
~-FE
a-FE
a-FE
C.-FE
FEAL
FEAL
FEAL~
CElr,L,
a-FE
a-FE
S,S,(FCC)
S.S.(FCC)
a-FE
c(-ryF
FE*AL~
FE??L~
598 K
;E;EAL
FEAL
FEAL
W-FE
w-FE
623
FETAL
iEl?L
FEAL.
(350°C)
~--FE
Cl-FE
C:‘E
60min
FE*AI.,
:211.;
lsochronous
Fig. 2. The phases formed after isochronous annealing (1 h) in ultrahigh vacuum at different temperatures of the different multilayered samples of the present work. The initial thickness of the individual Fe and Al layers are quoted in the top part.
I.J.R Baumvol / Aluminidesand silicidesformation
the prediction of the AH’ rules, and also the AuAl, which would not be predicted as a first phase. However, AuAl, is the next phase of the phase sequence predicted by the AH’ rules. The appearance of this phase may be a consequence of the fact that the mobility of both Au and Al are very high and so the used Xef dose (1 x 1015 cm-‘) may be large enough (with the substrate at room temperature) to allow for second phase formation. The Bene rule states that the first phase formed in metal-metal thin-film reactions is the phase immediately adjacent to the lowest-temperature eutectic in the binary phase diagram [4]. If we look at the Au-Al binary phase diagram of fig. la we see that the predicted first phase according to this rule is Au,Al. As
Room temperature ion mixing of bilayers produced a mixed layer with a composition of Au,Al. A prolonged irradiation resulted in the consumption of all the Au film and the formation of AuAl, at the AuAu,Al interface. Under thermal annealing, Au,Al, and Au,Al are formed at annealing temperatures above 80°C. Subsequent annealing resulted in the formation of AuAl,. According to fig. la, the phases formed by thermal annealing, namely Au& and Au,Al, are the ones predicted by the AH’ rules since these phases have the most negative AH’ at the concentration of the lowest eutectic. The IBM bilayers also give the Au&l, phase consistently with the AH’ rules prediction. The IBM multilayers produce the Au,Al phase consistently with
EmID Fe
Fe/Al
Al
(nm)
-
5x 10 l5
tYzzzzi =: ... .
TOTAL
- 1OOnm
Ar+. cm-2 Xe+.cmm2
15~10’~
8x
101
]
q
lYYYz?l i
6x
5x
:
p”““1 :
35,
;X
SiO2
77K
s.s,
S.S.
(BCC)
(BCC)
[l
pKq S.S.(FCC) .-FE
200K
S.S.
S.S.
(CCC)
(BCC)
j--j
S.S.
(FCC)
.,-FE AWOKP ,
S.S.(FCC)
S.S.(FCC) ~&-FE [i&-l S.S.(FCC) -FE
S.S.
300K
ICOS.
(CCC)
FEAL~
373 K
FEAL~
-FE
473 K Fig. 3. The phases formed by ion beam mixing with 5 X 101’ err-’ Arc and Xef at different substrate temperatures the different multilayers of the present work. In some cases the Xe+ bombardments led to the formation of an amorphous phase only, or FeAl, only, without any evidence of SS (fee) or alpha-Fe; in these cases the corresponding phase is surrounded by a solid-line box. In one case (Fe/Al lo/42 nm, 373 K) the phase formed by Arf bombardment (FeAl,) was different from the one formed by Xe+ bombardment (ices.); in this case the phase formed by Xe+ bombardment is also surrounded by a solid-line box. IV. MATERIALS SCIENCE
102
I.J.R. Baumvol / Aluminides and silicides formation
discussed above, the effective heat of formation rules correctly predict either Au&, or Au,Al as the first phase and AuAl and AuAl, as the following phases. 2.2. Al/Cr
FeAl (nm)
(IO/21
(IOK) (1OilO)
(10/28)w421
[7,8]
Multilayers of Al-Cr at concentrations of AlCr and AlCr,,5 were bombarded at room temperature with Ar ions, resulting in the formation of the equilibrium compound Al,&Jr,, and, for subsequent irradiations, the equilibrium compounds Al&r, and Al&r. Al is the mobile species in the IBM of Al-Cr multilayers. This fact, together with the AH’ diagram of fig. lb reveals that the observed phases in the IBM of Al-Cr multilayers agree well with the predictions of the AH’ rules, which predict also correctly the phases formed by TA.
Fe
20
40
60
80
Atomic Percent Aluminum
0 Al
“’
Fig. 4. The effective heat of formation diagram of the Fe-Al binary system. The thick, solid line on the right represents the concentration at the lowest temperature liquidus.
2.3. Al/ l’i [16] Multilayers of Al/Ti of several different concentrations were submitted to TA and IBM with Xe+ ions. Room temperature irradiations produced amorphous alloys or supersaturated solid solutions depending on the composition of the initial multilayered structure. Ion beam mixing at elevated temperatures (200-400°C) produced the intermetalic compound Al,Ti independent of the multilayer concentration, like the TA of the same multilayers. Al was observed to be the mobile species. The results of IBM and TA of Al/Ti multilayers are again in strict agreement with the thermodynamic and kinetic hierarchies among amorphous, metastable and stable phases, with the Liu conditions for amorphization and with the AH’ rules for first phase formation. 2.4. Al/Fe IBM of multilayers of the metallic binary system Fe-Al was investigated by several authors, using different ionic species. A comprehensive study of the IBM of Fe/Al multilayers covering the whole concentration range, performed with Arf and Xe+ bombardments at substrate temperatures between 77 and 473 K, was given, including also the study of the phases formed by thermal annealing of the same multilayers 1171. The phases that resulted are summarized in figs. 2 and 3. RBS analyses made on the Al,Fe,,, A16,Fe,, and Al,Fe,, multilayers fully confirmed the higher mobility of Al (both in thermal annealing and IBM) and the consumption of the pure Al layers much before the consumption of the pure Fe layer. In the phases formed by TA we notice that the Al-rich side samples (Al,,Fe,, and Al,Fe,) fully confirm the predictions of the AH’ rules, both in the
first phase and phase sequences, according to fig. 4, as FeAl, formed first, followed by the formation of Fe,Al, by means of the consumption of the pure Fe layer. The intermediate concentration samples Al,Fe,, and the samples Al,,Fe,, also confirm the AH’ rule prediction for the first phase (FeAl,), but in the phase sequence the Fe,Al, and FeAl, phases are skipped and the FeAl phase is formed. In the Fe-rich side (Al,,Fe,,) the predictions of the AH’ rules are not confirmed, neither for the first phase nor for the phase sequence. This last fact can be certainly attributed to the large solid solubility of Al in Fe (around 30%) which allows the formation of a solid solution of Al in Fe from where Fe&l grains can precipitate. The IBM of the multilayers in the Al-rich and intermediate concentration ranges (Al,,Fe,, Al,,Fe,, and Al,,Fe,,) confirm the predictions of ref. [7]: i) If the substrate temperature during IBM is low enough to restrict atomic migration after the duration of the spike (which is the case in Fe/Al multilayers below T, = T, = 250 K) the suppressing of the reordering leads to the prediction of an amorphous or metastable crystalline phase, as it is observed; ii) At intermediate substrate temperatures (for Fe/Al multilayers, around 300 K) metastable phases are predicted to occur, and this is indeed what is observed although in some samples (specially the Al,,Fe& stable crystalline phases ar already formed at 300 K; iii) at higher temperatures (above 300 K) crystalline equilibrium compounds are predicted to be formed, since there is mobility enough of the reacting species (at time intervals longer than the duration of the thermal spike) to allow for the thermodynamic driving force to operate in bringing the system to the equilib-
103
I.J.R. Baumvol / Aluminides and silicides formation
rium at the minimum free energy, overcoming the kinetics barriers. The equilibrium phases that are formed after IBM with Ar+ at substrate temperatures of 373 and 473 K confirm the prediction of the AH’ rules both for first-phase and phase sequence (see fig. 4). In the Fe-rich side (Al,Fe, and Al,,Fe,,) solid solutions of Al in Fe are always formed by IBM with Ar+ and Xe+, even at lower substrate temperatures. There is no connection between the AH’ rules and the formation of phases in the Fe-rich compositions side, and this is again a consequence of the high solubility of Al in Fe which is even enhanced by ion bombardment.
3. Ion beam mixing of Si/transition 3.1. Co/Si
metal multilayers
(101
Multilayered structures of Co/% and Co/CoSi/Si were submmited to TA and IBM at differexit substrate temperatures. The first phase formed by TA was always Co,Si and the second phase was CoSi. Co was observed as the mobile species in TA, and so the first and second phases observed agree perfectly with the AH’ rules according to the AH’ diagrams of fig. 5a. In IBM, both CoSi and Co,Si were induced to form. At high substrate temperatures (above 37O”C), the most mobile species was Co and the Co,Si phase is preferably formed as predicted by the AH’ rules. At intermediate temperatures (between 280 and 37O”C), the fluxes of Si and Co atoms are comparable, which means that the effective concentrations of atoms at the growing interfaces are comparable. In this case,
Atomic
Percent
Silicon
20
40
60
80
the AH’ rules predict that the phase to be formed is the one having the most negative AH’ at the concentration of greater mobility of the reacting species, which in this case must\be a Si to Co ratio around l/l. Looking at fig. 5a we notice that the predicted phase is CoSi, which is indeed the one observed experimentally. 3.2. Ni / Si [9] IBM of Ni/Si multilayers with the substrates at room temperature led to the formation of mainly the Ni$ phase. Ni was observed to be the most mobile species in IBM of Ni/Si at room temperature and the prediction of the AH’ rules according to fig. 5b for the first phase is either Ni,Si, or Ni,Si, but as Ni is the most mobile species, the Ni,si phase is formed first. At higher substrate temperatures during IBM the two species, Ni and Si, are mobile and accordingly the two phases predicted by the AH’ rules, namely Ni,Si and Ni,Si,, were observed. Under TA the Ni/Si system always forms the Ni,Si phase consistently with the AH’ rules since Ni was observed to be the most mobile species.
4. Conclusions The structure (amorphous, crystalline metastable or stable) of the phases formed by IBM of multilayers at each substrate temperature can be predicted by the thermodynamic hierarchy established in ref. [14], the kinetic paths to reach these phases and the solubility constraints for amorphization discussed by Liu [15]. The stable compounds of the equilibrium phase
Atomic
100
Si
Percent
Silicon
Ni
(a)
M
Fig. 5. (a) Phase diagram and effective heat of formation diagrams of the Co-Si binary system; (b) Phase diagram and effective heat of formation
diagrams of the Ni-Si
binary system.
IV. MATERIALS
SCIENCE
104
I.J.R. Baumvol / Aluminides and silicides formation
diagram that are formed by IBM of multilayers at temperatures above T, can be predicted by the AilHi rules, which also succeed very well in predicting the phases formed by TA of the same multilayers. As it was exhaustively shown in the several examples discussed in this paper the mobility of the reacting species plays a central role, since it determines the effective concentrations at the growing interfaces. The mobile species in IBM are not necessarily the
same as in TA. This brings several consequences, like the formation of noncongrnent silicide phases in IBM which would not be formed by TA (e.g. Ni,Si,). Another peculiarity of IBM is that it extends the solubility limits, and as a consequence the IBM of certains m~tilayer concentrations forms solid solutions, whereas the TA forms the phases predicted by the hH’ rules, as it was observed in the Al/Fe muitilayers in the Fe-rich side of the concentration range. Apart from the concentration intervals where solid solutions exist, there is no other relationship between the phases that are formed, either amorphous, crystalline, metastable or stable compounds, and the composition of the multilayers as determined by the relative thicknesses of the individual layers. In the same way, the atomic number of the bombarding ions does not have any special influence in the formation of phases. Therefore, as long as thermal spikes are formed nearby the growing interfaces, the substrate temperature, Liu’s rules for amorphization and the effective heat of formation rules allow the prediction of the phases that are formed by ion beam mixing.
References [l] B.Y. Tsaur, Proc. Symp. on Thin Film Interfaces Interactions, eds. J.E.E. Baglin and J.M. Poate, vol. SO-2 (The Electrochemical Society, Princeton, New Jersey, 1980) p. 205. [2] M.A. N&let and S.S. Lau, in: VLSI Electronics: Microstructure Science, ed. N.G. Einspruch, vol. 6, chap. 6 (Academic Press, New York, 1983). [3] R.M. Walser and R.W. Ben&, Appl. Phys. I..&. 28 (1976) [4] i%. Ben& Appl Phys Lett. 41 (1982) 529. [5] U. Giisele ind K.N. Tn; J. Appl. Phys. 53 (1982) 3552. [6] L.S. Hung and J.W. Mayer, Nucl. Instr. and Meth. B7/8 (1985) 676. [7] R. Pretorius, A.M. Vredenberg, F.W. Sans and R. de Reus, J. Appl. Phys. 70 (1991) 3636. [8] H.K. Kim, S.O. Kim, J.H. Song, J.J. Woo and R.J. Smith, Nucl. Instr. and Meth. B59/60 (1991) 554. [9] CA. Hewett, S.S. Lau, I. Suni and D.B. Poker, Nucl. Instr. and Meth. B7/8 (1985) 597. [lo] W. Xia, C.A. Hewett, M. Fernandes, S.S. Lau and D.B. Poker, J. Appl. Phys. 6.5 (1989) 2300. [ll] S.S. Lau, B.X. Liu and M.A. Nicolet, Nucl. Instr. and Meth. 209/210 (1983) 97. [12] W.L. Johnson, Y.T. Cheng, M. van Rossum and M.A. Nicolet, Nucl. Instr. and Meth. B7/8 (1985) 657. [13] Y.T. Cheng, Mater. Sci. Rep. 5 (1990) 45. [14] M. Nastasi and J.W. Mayer, Mater. Sci. Rep. 6 (1991) 1. 1151B.X. Liu, Phys. Status Solidi A94 (1986) 11. 1161 Q.Z. Hong, D.A. Lilienfeld and J.W. Mayer, J. Appl. Phys. 64 (1988) 4478. [17] J.L. Alexandre, M.A.Z. Vasconcellos, S.R. Teixeira and I.J.R. Baumvol, Appl. Phys. A, to be published.
NIIM B
Beam Interactions with Materials&Atoms
Aluminides and silicides formation by ion beam mixing of multilayers * I.J.R. Baumvol Institute de F&a, UFRGS, 91501470 Port0 Alegre, RS, Brad
The formation of transition metal-aluminides and silicides by ion beam mixing of thin films multilayered structures is presented here. The present stage of the capability of predicting the phase to be formed when a certain multilayer structure, with a certain imposition, is sub~tted to ion beam mixing under certain bombar~ent ~n~tions is discussed. Examples existing in the literature of transition metal-aluminium and transition metal-silicon multilayers submitted to ion beam mixing are analysed and a more detailed discussion of the multilayered Fe-Al system with Fe and Al thicknesses varied such as to cover most of the concentration intervals is presented.
Transition metal silicides and aluminides are largely involved in the present stage of silicon technology for integrated circuits. In the past several years, the formation of silicides and aluminides by ion beam mixing (IBM) and thermal annealing (TA) was intensively studied [1,2]. The study of the formation of transition metal silicides by IBM demonstrated that for bombardments of the substrate at room temperature most of the phase sequences observed in IBM coincided with the phase sequences observed in silicides formed by thermal annealing [I], independent of the atomic number of the bombarding species. Increasing the total bombardment dose would have the same effect on silicide phase sequences as increasing time or temperature in furnace annealing. IBM of transition metalaluminum thin film structures at room temperature or above also showed many analogies with the formation of equiIib~um phases in bulk or thin film metallur~, the latter being expressed by various different rules for phase formation, like the Walser-BenC rule [3], the extended Ben6 rule [4] and the approach put forward by Tu and GGsele [51. Also early verified was the importance of the mobility of the species ~ntribut~g to grow the phases under study. This means that not only the~odynami~ but also kinetics determine the formation of phases in IBM. Examples of this are the studies of gold and chromium aluminides formation by IBM and TA [6-81 and the formation of nickel and cobalt silicides by IBM and TA [9,X@ Based on these empirical facts Lau and collaborators [ll] proposed that equilibrium phase diagrams are * Work supported in part by CNPq and FAPERGS, Brazil. Old-583X/93/$06.#
an adequate guide to predict the phases formed by IBM. A further step was proposed by Johnson and collaborators [12], that most of the IBM process develops during the thermal spike stage of the collision cascade. At that stage, the formulation of rules to predict phase formation by IBM was precluded by two main obstacles: i) the meaning of eq~librium thermod~~its or equilibrium phase diagrams in the case of a solid phase reaction in thin films was not clearly understood. The rules from refs. [3-5] could not predict many of the observed phases in thin film structures submitted to furnace anneahng; ii) the physical nature of the thermal spike was not known in detai1 as well as the conditions (especially substrate temperature) for subsequent diffusion of defects outside the cascade volume. The concept of thermal spike was clarified and one can certainly refer to the excellent review by Cheng [13] for understanding the thermodynamic aspects of thermal spikes, the conditions for their fo~ation and the critical temperature T, for the onset of temperature-dependent ion mixing (i.e. outdiffusion of defects). Following the understanding of the thermal spikes, the thermodynamics and kinetics mechanisms determining phase transformations induced by ion irradiation could be put on a far more clear basis, and we refer to the also excellent review by Nastasi and Mayer [14] to establish a thermodynamic hierarchy between amorphous, metastable and stable phases formed under ion bombardment, as well as the transformation kinetic paths to reach these phases for each IBM situation (substrate temperature, bombarding species, sample composition, structure of the phase, etc.). According to this hierarchy, amorphous phases can be predicted to form at low temperatures IBM in a wide range of concentrations. In fact, as pointed out by Liu 1151the
0 1993 - Elsevier Science Publishers B.V. All rights reserved
99
I.J.R. Baumvol / Aluminides and silicides formation
only concentration intervals where amorphous phases will not form in low temperature ion beam mixing are the regions where solid solutions with simple structures (bee, fee or hcp) exist. The formation of metastable crystalline phases should occur over intermediate substrate temperatures and stable crystalline phases are expected at higher substrate temperatures. The criteria for low, intermediate and high temperatures in IBM should be correlated to T,, the critical temperature for the onset of diffusion of defects to the surroundings of the cascade volume. Finally the prediction of phase formation sequence and phase stability in binary thin film systems submitted to thermal annealing was elucidated by Pretorius et al. [7], introducing the concept of effective heat of formation. Based on this innovative concept, Pretorius et al. established rules for first phase formation and subsequent phases formed in binary thin film systems. These rules incorporated most of the expertise of the previously existing ones, being also able to predict many of the so far unexplained phases formed in furnace annealing of thin film bilayers. It is noteworthy that the key feature in the concept of effective heat of formation is the effective concentration of atoms at the growing interface. In other words, the thermodynamic driving force for a certain reaction is not just the classical heat of formation of the reaction (AH), but rather an effective heat of formation (AH’) which is
the product of the heat of formation per gram atom (AH), and the total number of atoms used in the reaction. effec. cont. lim. element AH’=
AH
camp. cont. lim. element ’
In the case of phases formed by thermal annealing of thin film structures it is assumed that the largest mobility of the reacting species occurs at the lowest eutectic temperature in the equilibrium phase diagram and the AH’ rules are: i) the first compound phase to form is the phase with the most negative, effective heat of formation at the concentration of the lowest temperature eutectic (liquidus) of the binary system; and ii) after first phase formation, the next phase to form at the interface between the compound phase and the remaining element is the next phase richer in the unreacted element which has the most negative heat of formation. The concept of effective heat of formation privileges the role of the mobile species in phase formation and as such it is very adequate to be used in predicting the formation of phases in IBM of thin film structures. Illustrative examples existing in the literature will be analysed or eventually reanalyzed here. A detailed discussion of the prediction of the phases formed in IBM of Fe/Al multilayers of several different relative thicknesses, bombarded with different ionic species at
800 600
GOLD
CONCENTRATION (a)
(at%)
Cr CONCENTRATION
(at %I
b)
Fig. 1. Phase diagram and effective heat of formation diagram of the Au-Al binary system (from ref. [7]). (b> Phase diagram and effective heat of formation diagram of the AI-Cr binary system (from ref. 181).The dashed vertical lines represent the concentrations at the lowest temperatures eutectic (liquidus). IV. MATERIALS SCIENCE
I.J.R Baumvol / Aluminides and silicides formation
100
several different substrate temperatures from 77 to 473 K will be made. The choice of the Fe-Al system seems very fortunate because the phases formed by IBM of the Fe/Al multilayers include amorphous, icosahedral metastable and crystalline stable phases, constituting an exhaustive test of the present capabilities of predicting phases on IBM of multilayers.
2. Ion beam mixing of Al/transition
and the predictability of these phases using the effective heat of mixing rules (AH’ rules) discussed in the previous section. Only a few illustrative examples were chosen, without any intention of making a complete survey of all the results existing in the literature. 2.1. Au/Al
IBM of multilayered samples with 1 X 1015 Xe+ cm-’ at room temperature produced the Au,Al and AuAl, equilibrium compounds in the concentration range between 15 and 70 at% of Au. For concentrations close to either side of the phase diagram solid solutions were produced.
metal multilayers
In this section we present some cases of IBM of aluminum/transition metal multilayered thin film structures with emphasis on the phases that are formed
77
Fe/Al
(nm)
-
THERMAL
1012 Bza .
10/6 PzzzI . . .
:
8x
6x
[6,7]
TOTAL
- 1OOnm
ANNEALING
lO/lO B :
q lo/28
.
lo/42 n
.
iX 3;
. :
2x
Si02
FETAL
523K * (250°C) ry-FE 548K
573K
FETAL
U-FE
FE*AL
w-F;
FEAL~
FEAL?
FEAL~
FEAL?
S.S.(FCC)
S,S,iFCC)
S.S.(FCC)
S.S.
a-FE
~-FE
a-FE
N-FE
FEAL~
FEAL~
FEAL-,
FEi\L_
S,S.(FCC)
S.S.(FCC)
S.S.(FCC)
S.S.iFCC)
~-FE
a-FE
a-FE
C.-FE
FEAL
FEAL
FEAL~
CElr,L,
a-FE
a-FE
S,S,(FCC)
S.S.(FCC)
a-FE
c(-ryF
FE*AL~
FE??L~
598 K
;E;EAL
FEAL
FEAL
W-FE
w-FE
623
FETAL
iEl?L
FEAL.
(350°C)
~--FE
Cl-FE
C:‘E
60min
FE*AI.,
:211.;
lsochronous
Fig. 2. The phases formed after isochronous annealing (1 h) in ultrahigh vacuum at different temperatures of the different multilayered samples of the present work. The initial thickness of the individual Fe and Al layers are quoted in the top part.
I.J.R Baumvol / Aluminidesand silicidesformation
the prediction of the AH’ rules, and also the AuAl, which would not be predicted as a first phase. However, AuAl, is the next phase of the phase sequence predicted by the AH’ rules. The appearance of this phase may be a consequence of the fact that the mobility of both Au and Al are very high and so the used Xef dose (1 x 1015 cm-‘) may be large enough (with the substrate at room temperature) to allow for second phase formation. The Bene rule states that the first phase formed in metal-metal thin-film reactions is the phase immediately adjacent to the lowest-temperature eutectic in the binary phase diagram [4]. If we look at the Au-Al binary phase diagram of fig. la we see that the predicted first phase according to this rule is Au,Al. As
Room temperature ion mixing of bilayers produced a mixed layer with a composition of Au,Al. A prolonged irradiation resulted in the consumption of all the Au film and the formation of AuAl, at the AuAu,Al interface. Under thermal annealing, Au,Al, and Au,Al are formed at annealing temperatures above 80°C. Subsequent annealing resulted in the formation of AuAl,. According to fig. la, the phases formed by thermal annealing, namely Au& and Au,Al, are the ones predicted by the AH’ rules since these phases have the most negative AH’ at the concentration of the lowest eutectic. The IBM bilayers also give the Au&l, phase consistently with the AH’ rules prediction. The IBM multilayers produce the Au,Al phase consistently with
EmID Fe
Fe/Al
Al
(nm)
-
5x 10 l5
tYzzzzi =: ... .
TOTAL
- 1OOnm
Ar+. cm-2 Xe+.cmm2
15~10’~
8x
101
]
q
lYYYz?l i
6x
5x
:
p”““1 :
35,
;X
SiO2
77K
s.s,
S.S.
(BCC)
(BCC)
[l
pKq S.S.(FCC) .-FE
200K
S.S.
S.S.
(CCC)
(BCC)
j--j
S.S.
(FCC)
.,-FE AWOKP ,
S.S.(FCC)
S.S.(FCC) ~&-FE [i&-l S.S.(FCC) -FE
S.S.
300K
ICOS.
(CCC)
FEAL~
373 K
FEAL~
-FE
473 K Fig. 3. The phases formed by ion beam mixing with 5 X 101’ err-’ Arc and Xef at different substrate temperatures the different multilayers of the present work. In some cases the Xe+ bombardments led to the formation of an amorphous phase only, or FeAl, only, without any evidence of SS (fee) or alpha-Fe; in these cases the corresponding phase is surrounded by a solid-line box. In one case (Fe/Al lo/42 nm, 373 K) the phase formed by Arf bombardment (FeAl,) was different from the one formed by Xe+ bombardment (ices.); in this case the phase formed by Xe+ bombardment is also surrounded by a solid-line box. IV. MATERIALS SCIENCE
102
I.J.R. Baumvol / Aluminides and silicides formation
discussed above, the effective heat of formation rules correctly predict either Au&, or Au,Al as the first phase and AuAl and AuAl, as the following phases. 2.2. Al/Cr
FeAl (nm)
(IO/21
(IOK) (1OilO)
(10/28)w421
[7,8]
Multilayers of Al-Cr at concentrations of AlCr and AlCr,,5 were bombarded at room temperature with Ar ions, resulting in the formation of the equilibrium compound Al,&Jr,, and, for subsequent irradiations, the equilibrium compounds Al&r, and Al&r. Al is the mobile species in the IBM of Al-Cr multilayers. This fact, together with the AH’ diagram of fig. lb reveals that the observed phases in the IBM of Al-Cr multilayers agree well with the predictions of the AH’ rules, which predict also correctly the phases formed by TA.
Fe
20
40
60
80
Atomic Percent Aluminum
0 Al
“’
Fig. 4. The effective heat of formation diagram of the Fe-Al binary system. The thick, solid line on the right represents the concentration at the lowest temperature liquidus.
2.3. Al/ l’i [16] Multilayers of Al/Ti of several different concentrations were submitted to TA and IBM with Xe+ ions. Room temperature irradiations produced amorphous alloys or supersaturated solid solutions depending on the composition of the initial multilayered structure. Ion beam mixing at elevated temperatures (200-400°C) produced the intermetalic compound Al,Ti independent of the multilayer concentration, like the TA of the same multilayers. Al was observed to be the mobile species. The results of IBM and TA of Al/Ti multilayers are again in strict agreement with the thermodynamic and kinetic hierarchies among amorphous, metastable and stable phases, with the Liu conditions for amorphization and with the AH’ rules for first phase formation. 2.4. Al/Fe IBM of multilayers of the metallic binary system Fe-Al was investigated by several authors, using different ionic species. A comprehensive study of the IBM of Fe/Al multilayers covering the whole concentration range, performed with Arf and Xe+ bombardments at substrate temperatures between 77 and 473 K, was given, including also the study of the phases formed by thermal annealing of the same multilayers 1171. The phases that resulted are summarized in figs. 2 and 3. RBS analyses made on the Al,Fe,,, A16,Fe,, and Al,Fe,, multilayers fully confirmed the higher mobility of Al (both in thermal annealing and IBM) and the consumption of the pure Al layers much before the consumption of the pure Fe layer. In the phases formed by TA we notice that the Al-rich side samples (Al,,Fe,, and Al,Fe,) fully confirm the predictions of the AH’ rules, both in the
first phase and phase sequences, according to fig. 4, as FeAl, formed first, followed by the formation of Fe,Al, by means of the consumption of the pure Fe layer. The intermediate concentration samples Al,Fe,, and the samples Al,,Fe,, also confirm the AH’ rule prediction for the first phase (FeAl,), but in the phase sequence the Fe,Al, and FeAl, phases are skipped and the FeAl phase is formed. In the Fe-rich side (Al,,Fe,,) the predictions of the AH’ rules are not confirmed, neither for the first phase nor for the phase sequence. This last fact can be certainly attributed to the large solid solubility of Al in Fe (around 30%) which allows the formation of a solid solution of Al in Fe from where Fe&l grains can precipitate. The IBM of the multilayers in the Al-rich and intermediate concentration ranges (Al,,Fe,, Al,,Fe,, and Al,,Fe,,) confirm the predictions of ref. [7]: i) If the substrate temperature during IBM is low enough to restrict atomic migration after the duration of the spike (which is the case in Fe/Al multilayers below T, = T, = 250 K) the suppressing of the reordering leads to the prediction of an amorphous or metastable crystalline phase, as it is observed; ii) At intermediate substrate temperatures (for Fe/Al multilayers, around 300 K) metastable phases are predicted to occur, and this is indeed what is observed although in some samples (specially the Al,,Fe& stable crystalline phases ar already formed at 300 K; iii) at higher temperatures (above 300 K) crystalline equilibrium compounds are predicted to be formed, since there is mobility enough of the reacting species (at time intervals longer than the duration of the thermal spike) to allow for the thermodynamic driving force to operate in bringing the system to the equilib-
103
I.J.R. Baumvol / Aluminides and silicides formation
rium at the minimum free energy, overcoming the kinetics barriers. The equilibrium phases that are formed after IBM with Ar+ at substrate temperatures of 373 and 473 K confirm the prediction of the AH’ rules both for first-phase and phase sequence (see fig. 4). In the Fe-rich side (Al,Fe, and Al,,Fe,,) solid solutions of Al in Fe are always formed by IBM with Ar+ and Xe+, even at lower substrate temperatures. There is no connection between the AH’ rules and the formation of phases in the Fe-rich compositions side, and this is again a consequence of the high solubility of Al in Fe which is even enhanced by ion bombardment.
3. Ion beam mixing of Si/transition 3.1. Co/Si
metal multilayers
(101
Multilayered structures of Co/% and Co/CoSi/Si were submmited to TA and IBM at differexit substrate temperatures. The first phase formed by TA was always Co,Si and the second phase was CoSi. Co was observed as the mobile species in TA, and so the first and second phases observed agree perfectly with the AH’ rules according to the AH’ diagrams of fig. 5a. In IBM, both CoSi and Co,Si were induced to form. At high substrate temperatures (above 37O”C), the most mobile species was Co and the Co,Si phase is preferably formed as predicted by the AH’ rules. At intermediate temperatures (between 280 and 37O”C), the fluxes of Si and Co atoms are comparable, which means that the effective concentrations of atoms at the growing interfaces are comparable. In this case,
Atomic
Percent
Silicon
20
40
60
80
the AH’ rules predict that the phase to be formed is the one having the most negative AH’ at the concentration of greater mobility of the reacting species, which in this case must\be a Si to Co ratio around l/l. Looking at fig. 5a we notice that the predicted phase is CoSi, which is indeed the one observed experimentally. 3.2. Ni / Si [9] IBM of Ni/Si multilayers with the substrates at room temperature led to the formation of mainly the Ni$ phase. Ni was observed to be the most mobile species in IBM of Ni/Si at room temperature and the prediction of the AH’ rules according to fig. 5b for the first phase is either Ni,Si, or Ni,Si, but as Ni is the most mobile species, the Ni,si phase is formed first. At higher substrate temperatures during IBM the two species, Ni and Si, are mobile and accordingly the two phases predicted by the AH’ rules, namely Ni,Si and Ni,Si,, were observed. Under TA the Ni/Si system always forms the Ni,Si phase consistently with the AH’ rules since Ni was observed to be the most mobile species.
4. Conclusions The structure (amorphous, crystalline metastable or stable) of the phases formed by IBM of multilayers at each substrate temperature can be predicted by the thermodynamic hierarchy established in ref. [14], the kinetic paths to reach these phases and the solubility constraints for amorphization discussed by Liu [15]. The stable compounds of the equilibrium phase
Atomic
100
Si
Percent
Silicon
Ni
(a)
M
Fig. 5. (a) Phase diagram and effective heat of formation diagrams of the Co-Si binary system; (b) Phase diagram and effective heat of formation
diagrams of the Ni-Si
binary system.
IV. MATERIALS
SCIENCE
104
I.J.R. Baumvol / Aluminides and silicides formation
diagram that are formed by IBM of multilayers at temperatures above T, can be predicted by the AilHi rules, which also succeed very well in predicting the phases formed by TA of the same multilayers. As it was exhaustively shown in the several examples discussed in this paper the mobility of the reacting species plays a central role, since it determines the effective concentrations at the growing interfaces. The mobile species in IBM are not necessarily the
same as in TA. This brings several consequences, like the formation of noncongrnent silicide phases in IBM which would not be formed by TA (e.g. Ni,Si,). Another peculiarity of IBM is that it extends the solubility limits, and as a consequence the IBM of certains m~tilayer concentrations forms solid solutions, whereas the TA forms the phases predicted by the hH’ rules, as it was observed in the Al/Fe muitilayers in the Fe-rich side of the concentration range. Apart from the concentration intervals where solid solutions exist, there is no other relationship between the phases that are formed, either amorphous, crystalline, metastable or stable compounds, and the composition of the multilayers as determined by the relative thicknesses of the individual layers. In the same way, the atomic number of the bombarding ions does not have any special influence in the formation of phases. Therefore, as long as thermal spikes are formed nearby the growing interfaces, the substrate temperature, Liu’s rules for amorphization and the effective heat of formation rules allow the prediction of the phases that are formed by ion beam mixing.
References [l] B.Y. Tsaur, Proc. Symp. on Thin Film Interfaces Interactions, eds. J.E.E. Baglin and J.M. Poate, vol. SO-2 (The Electrochemical Society, Princeton, New Jersey, 1980) p. 205. [2] M.A. N&let and S.S. Lau, in: VLSI Electronics: Microstructure Science, ed. N.G. Einspruch, vol. 6, chap. 6 (Academic Press, New York, 1983). [3] R.M. Walser and R.W. Ben&, Appl. Phys. I..&. 28 (1976) [4] i%. Ben& Appl Phys Lett. 41 (1982) 529. [5] U. Giisele ind K.N. Tn; J. Appl. Phys. 53 (1982) 3552. [6] L.S. Hung and J.W. Mayer, Nucl. Instr. and Meth. B7/8 (1985) 676. [7] R. Pretorius, A.M. Vredenberg, F.W. Sans and R. de Reus, J. Appl. Phys. 70 (1991) 3636. [8] H.K. Kim, S.O. Kim, J.H. Song, J.J. Woo and R.J. Smith, Nucl. Instr. and Meth. B59/60 (1991) 554. [9] CA. Hewett, S.S. Lau, I. Suni and D.B. Poker, Nucl. Instr. and Meth. B7/8 (1985) 597. [lo] W. Xia, C.A. Hewett, M. Fernandes, S.S. Lau and D.B. Poker, J. Appl. Phys. 6.5 (1989) 2300. [ll] S.S. Lau, B.X. Liu and M.A. Nicolet, Nucl. Instr. and Meth. 209/210 (1983) 97. [12] W.L. Johnson, Y.T. Cheng, M. van Rossum and M.A. Nicolet, Nucl. Instr. and Meth. B7/8 (1985) 657. [13] Y.T. Cheng, Mater. Sci. Rep. 5 (1990) 45. [14] M. Nastasi and J.W. Mayer, Mater. Sci. Rep. 6 (1991) 1. 1151B.X. Liu, Phys. Status Solidi A94 (1986) 11. 1161 Q.Z. Hong, D.A. Lilienfeld and J.W. Mayer, J. Appl. Phys. 64 (1988) 4478. [17] J.L. Alexandre, M.A.Z. Vasconcellos, S.R. Teixeira and I.J.R. Baumvol, Appl. Phys. A, to be published.