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TlBr-TlI systems
Thin Solid Films, 232 (1993) 87-93
87
Ageing effects in Me/T1Br-TII systems A. Simanovskis and S. V. Stolyarova Institute of Physics, Latvian Academy of Sciences, Salaspils, LV2169 (Latvia) (Received September 18, 1992; accepted January 19, 1993)
Abstract A wide range of metallic film/thallium bromide-iodide crystal juvenile contacts have been investigated by adhesion and electrical resistance measurements, and Auger depth profiling. It is shown that the behaviour of these systems both in a vacuum and under atmospheric conditions depends on the mutual chemical activity of the film and the substrate components. Interdiffusion and reactions in active systems were observed in a vacuum but no metals exhibited adhesion failure. Chemical reactions between metallic films, atmospheric components and thallium bromide-iodide occurring in some systems after exposure to air caused adhesion failure. The difference between normal electrode potentials of TI and condensed metals appeared to be a good parameter for forecasting the system stability under different conditions.
1. Introduction The possibility of applying a thin film package depends not only on the initial characteristics of the package but also on its behaviour during ageing under different conditions. The stability and integrity of such systems are determined by the processes of diffusion, solid state reactions, recrystallization, etc., which can develop across the interface of a film and a substrate. For the purpose of designing and fabricating thin film devices (acoustic and acousto-optical, for example) with fixed parameters, conditions affecting the abovementioned processes and their effect on the parameters must be known. Moreover,-the investigation of interaction processes in thin film systems also indicates an academic interest in the physics of the contact interaction. It is known that Gibbs' energy may serve as a good criterion of possible interactions on the contact of two solids [1]. In many cases a negative value of Gibbs' energy of a substitutional reaction between a thin metallic film and a substrate means strong adhesion. No additional treatment, e.g., by heating, for such systems is needed to achieve a high adhesion value if the contact between the film and the substrate is free from contaminants (juvenile contact) [2]. If interdiffusion and compound formation is possible, the durability of film/substrate joint is determined by a new system: film/interphase/substrate [3-8]. Recently we have reported on the adhesion of thin metallic films to non-metallic substrates [2]. Considering the data obtained, we came to a conclusion that chemical forces are dominant in metal film adhesion to
0040-6090/93/$6.00
TIBr-TII crystals. If we look a t Gibbs' energy of a substitutional reaction between condensed metals and T1Br-TII, we find that only for T i and Cr from the systems investigated earlier is there some possibility of a substitutional reaction. However, higher adhesion was found for inactive Ag and Au but not for Cr and Ti. It can be assumed that the high adhesion energy was due to the chemisorption by the non-saturated bonds of thallium halide crystal. The lower adhesion of active metals was possibly due to the interphase formation. The durability of the joint in this case was determined by the properties of the metal film/interphase/thallium halide system. The object of the present work was to answer this question and to investigate interaction processes in the Me/TIBr-TII systems during ageing under different conditions. For the metal film/halide systems, thallium bromideiodide was used as it is a good substrate material because of the metals available which exhibit different chemical activities with the possibility of a substitutional reaction with a halide crystal.
2. Experimental details It should be noted that the controllable conditions on the contact between a film and a substrate are of great importance in our investigations. The absence of adsorbates or byphases on the as-prepared film/substrate interface makes it possible to observe contact interaction in its "pure" form. Therefore, the application of juvenile contacts is a good method for studying different
© 1993 - - Elsevier Sequoia. All rights reserved
88
A. Simanovskis, S. V. Stolyarooa / Ageing in Me/TIBr-Tll systems
interaction phenomena in solid state systems, namely in metallic film/halide systems, and it has been used in the present work. Thin polycrystalline metal films deposited onto juvenile (free from contaminants) thallium bromide-iodide crystal surfaces have been investigated. The juvenile contacts were obtained by cutting the crystal along the (100) plane in a flow of metal vapour using techniques described in ref. 6. Microhardness, adhesion and electrical resistivity investigations have shown that the crystal cutting itself does not cause any marked changes in the related parameters, compared with those after the special polishing procedure required for preparing the optical crystals. Moreover, we compare systems which have undergone the same mechanical treatment. Thermal evaporation of metals in a vacuum of 10-4 Pa was used and the deposition rate was 5 n m s -1. The film thickness was 50 nm for Auger depth profiling, 100 nm for electrical resistance measurements and 250 nm for the adhesion tests. The difference in film thicknesses required for different investigations is admissible because a quantitative comparison between the same parameters of adequately prepared systems was made. The samples were aged at room temperature in a vacuum of 10-2 Pa or in air with a relative humidity of (55 + 5)%. The humidity was measured by a BM-2 standard device. The adhesive strength was measured by the direct pull technique using pins of diameter 3 mm which were glued to the film with an epoxy resin. The period between film deposition and pin attachment and that between air admission and pin attachment was taken as the ageing time in a vacuum and in air respectively. Pins were fastened to vacuum-aged films in ambient air, in which case the air exposure did not exceed 30 s. The samples were then held at room temperature for 18-20 h and at 100 °C for 4 h. Hence the adhesion test was performed 24 h after the pins were attached. The relative error of the adhesive strength arithmetic average values was less than 25% with a reliability coefficient of 0.9. The initial adhesion values tro were obtained by attaching the pins to the films that had been relaxed in a vacuum for 1 h in order to decrease the point scattering. We must note that it is difficult to give an exact definition of the initial adhesion values obtained if we consider the procedure of pin attachment and measurement of the adhesion on the one hand, and the difference in the activity of the systems and their sensibility to the ambient influence on the other hand. Henceforth, we have taken these factors into account when comparing the systems and drawing the conclusions. The heating activates interaction processes in the systems under investigation, and as a consequence the measured adhesion value is "shifted" along the time axis. As seen below, the short exposure to air required for pin attachment to vacuum-aged samples appeared to be Critical
for only a few systems. For the rest, the adhesion values obtained are lowered by an amount which can be approximately evaluated from the adhesion dependance on ageing time in air. The Auger depth profiling of the samples was performed by use of a Varian cylindrical mirror analyser. The energy of the primary electrons was 2 keV. Etching was carried out using 1.5keVAr ÷ ions. The total etching depth was determined using a Model 201 profilograph. Identification of the Auger spectra was performed in accordance with the Handbook of Auger Electron Spectroscopy [9]. The relative elemental sensitivity factors and standards both from ref. 9 and those obtained by taking Auger spectra of T1Br, TIC1, TIBrTII were used for a quantitative analysis. The four point probe compensation method and a d.c. resistance bridge MO-62 were used for the film resistance measurements. The maximum error of relative measurements did not exceed 0.1%.
3. Results and discussion
3. I. Ageing in vacuum
Figure 1 shows the change in the adhesive strength of metal film/thallium bromide-iodide juvenile contacts with ageing time, in vacuum. The most stable are Ag, Cu and Au/TIBr-TII systems, which exhibit only a tendency to adhesion growth. Ni, Co, In and Fe films manifest a slight adhesion increase with ageing time, but Ti, Cd and Cr film adhesion rises considerably with time. Relaxation, recrystallization, interdiffusion and chemical reactions in thin metallic film systems bring about an electrical resistance change in the films [10-15].
CI"
60 5
c~t
"n
0.
20
40
60
t,h
Fig. 1. Adhesive strength of Me/TIBr-TII juvenile contacts vs. ageing
time in a vacuum. Crorepresents the initial adhesion values.
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr- TII systems
T A B L E 1. The calculated Gibbs' energy for substitutional reactions of metals with thallium halides. Data taken from ref. 17
R
R0 ~..0
Ti
C~
¸
1.5
C
89
0
I
1
.
2
5
~
~
~
t,h
Reaction
AG (kJ mol-~)
M g + 2 TIBr M g + 2 TII M n + 2 TIBr Mn + 2 TII Ti + 2 T1Br Ti + 3 TIBr Ti+ 2TlI Cr + 2 TIBr Fe + 2 TIBr Co + 2 TIBr Ni + 2 TIBr Cu + T1Br Ag + TIBr A u + TIBr
- 168 - 109 - 33 0 -46 -25 -12 0 96 124 124 16 16 36
Fig. 2. Electrical resistance of metal films deposited onto TIBr-TII juvenile surfaces v s . ageingtime in a vacuum. Ro is the resistivityof as-prepared films. This resistance was measured along with the changes in the systems under investigation from the first minutes of their ageing. We found a resistance decrease in the Ag, Cu and Au films (Fig. 2) which is typical for relaxation processes [ 15, 16]. Here the tendency to adhesion increase in these systems during ageing in vacuum conditions is evidently a result of a decrease in elastic strain. For the Ni, Co, Fe, Cd, Cr, and Ti/TIBr-TII systems which showed a marked adhesion increase with the ageing time in a vacuum, a corresponding resistance increase was observed. The resistance increase cannot be attributed to film metal oxidization by remanent gases. Thus, for example, both easy oxidizing Cu films on TII-TIBr (see Fig. 2) and Co films on SiO2 (Fig. 3) exhibited a resistance decrease under the same conditions. The resistance increase observed could therefore be due to the interdiffusion of condensed metals with thallium bromide-iodide.
1.00
0.99 "--o--1....__o
0.~
0
i
I
1
Z
~,h
Fig. 3. Electrical resistance o f Co film deposited onto SiO 2 vs. time in a vacuum. Ro is the resistivity o f as-prepared film.
ageing
Let us evaluate the possibility of a chemical interaction of a solid solution o f thallium bromide-iodide with given metals. Because of a lack of data for solid solution, substitutional reactions of thallium bromide and thallium iodide with metals are considered using data from ref. 17. Table 1 shows that the Gibbs' energy is negative only for Mg, Mn, and Ti and for Cr it is equal to zero. This means that there is a very low probability of a chemical reaction for other metals. However, we can arrange themetals in order of their chemical activity, which is as follows: N i - C o - F e - C r Ti. We can ascertain (see Fig. 2) that the increase in electrical resistance correlates with the activity of metals. Auger depth profiling of Ni, Cr, Ti/T1Br-TII systems was carried out to find the reason for these changes. Figure 4 shows that diffusion of metals into the substrate crystal and diffusion of T1 into the film has occurred. The width of the interdiffusion zone grows from Ni to Ti showing the corresponding increase in the resistance and adhesion of the films (see Figs. 1 and 2). The adhesion increase with time for Cr exceeds that for Ti possibly due to a decrease in elastic strain since the latter is higher for chromium films than for titanium films [18]. The direct dependence of the adhesion increase on the activity of the deposited metals is evident. Though the film resistance increases over several days the major changes take place during the first 2 h after the deposition. However, a considerable adhesion increase has been observed for up to three days. This discrepancy can be explained as follows. The film resistance essentially depends on the initial interdiffusion and reaction processes between the film and the substrate, but the adhesion is also sensitive to following transformation processes of the non-conductive interlayer phases.
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr-TII systems
90
,:
.l
•~' 0 -I-
80
m
40
.
I 0
,
I
t !
i
~, 40
.<
L 0
~ , i , 20 60 6O 8 0 Etching Depth, nm
Fig. 4. Auger depth profiles of Me/TIBr-TII juvenile contacts after ageing for 10 days.
Thus it has also been ascertained that the adhesion values obtained for the systems in ref. 2 are determined by complicated film/interphase/T1Br-TlI systems. Adhesion measurements for Mg, Mn and A1/TIBrTII systems could not be carried out because of the fast degradation which occurred when the vacuum was broken. Therefore the electrical resistance measurements for A1 and Mn films on thallium bromide-iodide were made under vacuum conditions (Fig. 5). A typical R/Ro decrease for AI film manifested relaxation processes and showed no evidence of any reaction or interdiffusion. The Mn film exhibited some rise in electrical resistance directly after the deposition, followed by a slight decrease. The Gibbs' potential (see Table 1) AG < 0
shows the possibility of a substitutional reaction between Mn and TIBr. As a result of such a reaction the release of TI could occur. Since every Mn atom can displace two TI atoms and the resistivity of TI is an order of magnitude less than that of Mn, the film resistance is expected to decrease and the resistance curve observed can thus be explained. No signs of degradation were observed in vacuum conditions and it is reasonable to regard these systems as vacuum stable with respect to adhesion failure. Thus the results obtained show that the adhesion failure does not occur in a vacuum. Moreover, the intensity of interaction and mass transfer between metal films and thallium bromide-iodide depends on their mutual chemical activity, characterized by Gibbs' potential. It should be stressed, however, that for the systems under investigation the adhesive strength per se does not show direct conformity with the chemical activity of deposited metals, since the chemical interaction introduces new phases in the contact zone, thus changing the contacting objects. 3.2. Ageing in air As mentioned above, the fast degradation of Mg, Mn, AI/TIBr-TII systems was observed when they were exposed to air. Aluminium films exfoliated via blistering, and the trace of "chemical etching" could be seen on the substrate crystal after detaching the film. The electrical resistance rose steeply during the first 20min after the vacuum was broken (Fig. 6). The resistance increased by up to 130% in 25 min. After that the growth rate essentially diminished and during the next hour the resistance increase was only 25%. These observations give evidence that degradation of A1
.B.
Ro
I "°
"tO5
1.0
0.g5~
t
0.g 0
i 1
I 2
tjh
Fig. 5. Electrical resistanceof Mn and A1 films deposited onto juvenile T]Br-TII surfaces vs. ageing time in a vacuum. ~ is the resistivity of as-deposited films.
0
10
~.0 50 40
50 t,min
Fig. 6. Electrical resistance of Mn and AI films deposited onto juvenile TIBr-TiI surfaces vs. exposure time to air. Ro is the resistivity of the films before breaking the vacuum.
A. Simanovskis, S. K Stolyarova / Ageing in Me/TIBr-TII systems
91
In 1
0
° •
u
11 ~, [~..
u
i ~a--
0
o --~'o..__
Be
.-o
---'~- ........
~
_
n g --~.r
o
.
.
.
.
.
1"~
o-
100
50
Etching
Au
0.~
,Tt
D e p t h , nm
I
Fig. 7. Depth distribution of elements in M n / T I B r - T I I system after ageing in air for 10 days.
film in air took place only in contact with thallium bromide-iodide crystal and stopped after blistering. Mg and Mn films transformed into liquid phase and finally crystallized under the influence of air. The related electrical resistance measurement showed a total loss of metallic conductivity after 15 min. We can suppose that compounds of the type MnBr2 were formed on the contact between Mn film and thallium halide crystal since such a reaction is preferred (AG < 0). Exposure to air led to water adsorption and its interaction with MnBr2. Auger depth profiling for the system Mn/T1Br-TII has shown (Fig. 7) that for 50 nm thick Mn film no boundary between the film and substrate crystal can be detected up to an etching depth of 120 nm. Nearly constant concentrations of Mn, Br, T1, and O occur in this region with the exception of the very surface layer of I0 nm which confirms the suggestion above. Based on the thermodynamic estimation and the data obtained we can conclude that MnBr2.nH20 was formed. The adhesive strength of Ag, Au, Cu, Ni, Co, In, Cr, and Ti films to TIBr-TII during ageing in air was measured. Figure 8 shows a marked increase in the adhesion of Cr and Ti films, though less than seen during ageing in vacuum. Adhesion of In and Co films increased negligibly, but adhesion failure for Ni, Au, Cu and Ag films was observed. The intensity of degradation increased in the sequence N i - A u - C u - A g . The discrepancy of this order, compared with Gibbs' energy of oxidation does not allow us to connect the adhesion failure observed with the oxygen effect. Earlier, in ref. 19, we reported on the Ambient Induced Diffusion (AID) in metal film/halide systems. We found it to be the reason for adhesion failure of Ag films to TIBr-TII [6]. The possibility of AID in a given system can be evaluated by Gibbs' energy of the reaction between condensed metal and atmospheric chlorine contaminants (AG~) and the reaction between the metal chloride formed and the substrate crystal components
0
100
200
~00
t,h
Fig. 8. Adhesive strength o f M e / T I B r - T I I juvenile contacts vs. ageing time in air. a o represents the initial adhesion values before breaking the vacuum.
T A B L E 2. The calculated Gibbs' energy for the reaction o f metals with chlorine (AG~), for the reaction o f related chloride with TII (AG2) and TIBr (AG3) and the change in adhesive strength o f related metallic films during ageing for 24 h under atmospheric conditions. Data for AG are taken from ref. 17
Metal
AGI
AG2
AG3
Au
54.5 25.2 12.6 - 16.4 -2.5 -4.2
84 50.4 54.5 -42.0 - 12.6 - 12.6
6 3 - 2 - 15 -25 -80
(%)
(kJ/mol -I) Ti Co Ni Au Cu Ag
-470 -273 - 260 - 16.8 - 117 - 109
TIBr and TII (AG2 and AG3). For the given metals AG~ < 0 is always true. Thus the AG2 and AGa are the main criteria of AID. We can ascertain that the adhesion decreases correlates with AG2 and AG3 given in Table 2 and with the intensity of AID [19]. Consequently the adhesion failure observed can be attributed mainly to AID processes.
3.3. The stability criterion of Me/TIBr-TII systems Thus we have found that both in vacuum and air conditions the behaviour of the given systems depends on the mutual activity of the condensed metal and thallium bromide-iodide substrate. This activity can be characterized by Gibbs' energy of substitutional reaction of metal with halide. However it should be stressed that thermodynamic estimation does not give the entire description of contact processes in these systems under different conditions. The film and substrate structure, admixtures both in film and substrate, and the amount and type of defects play a significant role in ageing
92
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr-TII systems
processes affecting diffusion kinetics by enhancing or suppressing different mechanisms. However the results obtained allow us to draw some general conclusions and forecast the behaviour of metal/thallium b r o m i d e - i o d i d e systems. Although we have found Gibbs' energy to be a good characteristic for systems under investigation, some difficulties can arise. Often, before the experiment, we do not exactly know what solid state reactions can take place in the given thin film/substrate system and what compounds can be formed. Even if we knew, the Gibbs' energy data are often not available. Since the thin film/substrate system stability largely depends on the possible direction of film-substrate reaction, parameters denoting this tendency can be used. One of the parameters connected with chemical activity of metals is normal electrode potential e °. In this case it is not necessary to know exactly which compounds are formed in solid state reactions. Figure 9 shows a correlation between the stability of Me/T1Br-TII systems in a vacuum and the difference between normal electrode potentials of T1 and condensed metal Ae ° = e°l - e°e. In the region of Ae ° > 0 reactions and interdiffusion are possible and as a consequence A R / R o > 0. For Ae ° < 0, A R / R o also falls below zero thus giving evidence of relaxation processes and an absence of interdiffusion. With respect to the system stability after film deposition, the right-hand side ( A e ° > 0) represents a marked adhesion increase and degradation of the film conductivity while the left-hand side ( A e ° < 0) metals show stabilization of both conductivity and adhesion. Thus we have shown that in vacuum conditions no metals exhibit adhesion decrease. Considering other characteristics, the systems with A e ° < 0 are more stable. The probability of chemical interaction is higher for the systems with A e ° > 0 which leads to both film adhesion and conductivity increasing with ageing time.
,,R/Ro /6o 1,0 2,5 0,5 2,0~Cr
Ti
J
Co"l~.Fe i
n
t
rtl
n
i
t
1,0
TABLE 3. The adhesion stability of juvenile Me/TIBr-TII contacts and corresponding difference of normal potentials between TI and condensed metal. Data for Ae° are taken from ref. 20 Me
Aa/24 h (%) in a vacuum
in air
Mg AI Mn
-
fast degradation
Ti Cr Fe In Co
28 125 8 6 5
Ni Cu Ag Au
2 1 1 1
6 10
Ae°(V) 2.045 1.325 0.715
3 2
1.415 0.375 0.105 0.005 - 0.065
-2 - 25 -80 - 15
-0.105 - 0.680 -1.136 - 1.755
-
N o w let us take up a question of system stability during ageing in atmospheric conditions. F o r better results the adhesion stability of systems with respect to the difference of normal electrode potentials Ae ° is shown in Table 3. It is evident that the stability increases at low absolute values of Ae °. The most stable are Co, I n / T I B r - T I I systems with [Ae°[ --, 0. The growth of [Ae°[ leads to the loss of stability. The sign of Ae ° plays an essential role in forecasting the possible processes which determine the system behaviour under different ageing conditions. At first for A e ° > 0 a direct chemical interaction of condensed metal with thallium halide is possible. Stability of such systems will depend on the effect of the atmosphere upon the products of the primary reaction and every atmospheric component is to be considered. F o r example, Mn, M g / T l B r - T l I systems lost stability due to water vapour influence. In the case of A e ° < 0 (Ni, Cu, Ag, A u / T I B r - T I I ) degradation of the systems occurred due to ambient induced processes-chemical reactions and interdiffusion between atmospheric components, condensed metal, and substrate components. Chlorine and sulphur, and their compounds in the atmosphere initiated the mentioned processes [19]. It should be stressed that in this region of Ae ° a chemical reaction between the film and active atmospheric component is the first stage of degradation. It is only after this that the substrate joins the interaction process.
|
1,5"A E°
1 Fig. 9. Adhesive strength (O) and electrical resistance ( I ) of metal films vs. Ae° after ageingthe Me/TIBr-TIIsystemsin a vacuum for 24 h.
4. Conclusion A summary of the results obtained in this work and earlier allows us to conclude that thin metal film/thai-
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr-Tll systems
lium bromide-iodide systems exhibit a whole spectrum of possible interactions from mutual inactivity and stability to intensive chemical reactions, interdiffusion and fast degradation. Moreover, ambient conditions play an important role in the system behaviour. Thermodynamic estimations have been found to give useful information about possible processes in the systems under consideration. Normal potentials of substrate cation and condensed metals are convenient parameters in forecasting the behaviour of the systems. References 1 R. A. Musin and G. V. Konyushkov, Joining Metals to Ceramics, Mashinostroenie, Moscow, 1991 (in Russian). 2 S. A. Varchenya, A. Simanovskis and S. V. Stolyarova, Thin Solid Films, 164 (1988) 147. 3 D. Gupta and P. S. Ho, Thin Solid Films, 72(1980) 399. 4 H. K. Pulker, A. J. Perry and R. Berger, Surf. TechnoL, 14(1981) 25. 5 D. M. Mattox, Int. J. Hybrid Microelectronics, 4 (19~1) 164. 6 S. V. Stolyarova, A. Simanovskis, V. N. Kovalev and S. A. Varchenya, Thin Solid Films, 177(1989) 181.
93
7 M. M. Chehimi and J. F. Watts, J. Adhesion Sci. Technol., 6 (1992) 377. 8 W. J. Chen and L. J. Chen, J. Appl. Phys., 70(1991) 2628. 9 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach, R. E. Weber (eds.), Handbook of Auger Electron Spectroscopy, Physical Electronics Industries, Eden Prairie, 1976. 10 J. Haber and I. Okonska-Kozlowska, J. Solid State Chem., 35 (1980) 22. 11 A. G. Dirks and J. J. Van Den Brock, Thin Solid Films, 96 (1982) 257. 12 A. E. Curzon and O. Singh. Thin Solid Films, 76(1981) 185. 13 P. Madakson, J. Appl. Phys., 70 (1991) 1374, 1380. 14 D.-Y. Shih, C.-A. Chang, J. Paraszczak, S. Nunes and J. Cataldo, J. Appl. Phys., 70 (1991) 3052. 15 L. S. Palatnik and V. K. Sorokin, Material Science in Microelectronics, Energiya, Moscow, 1978 (in Russian). 16 L. P. Grebennik, V. A. Demidova and G. N. Podus, in Thin Film Obtaining and Properties, Kiev, 1977, p. 57 (in Russian). 17 M. H. Karapet'yants and M. L. Karapet'yants, Basic Thermodynamic Constants of Inorganic and Organic Substances, Khimiya, Moscow, 1968 (in Russian). 18 K. Kinosita, Thin Solid Films, 12 (1972) 17. 19 A. A. Simanovskis, S. V. Stolyarova and G. P. Upit, Thin Solid Films, 97(1982) 301. 20 G. V. Samsonov (ed.), Physical and Chemical Properties of Elements Handbook, Naukova Dumka, Kiev, 1965 (in Russian).
87
Ageing effects in Me/T1Br-TII systems A. Simanovskis and S. V. Stolyarova Institute of Physics, Latvian Academy of Sciences, Salaspils, LV2169 (Latvia) (Received September 18, 1992; accepted January 19, 1993)
Abstract A wide range of metallic film/thallium bromide-iodide crystal juvenile contacts have been investigated by adhesion and electrical resistance measurements, and Auger depth profiling. It is shown that the behaviour of these systems both in a vacuum and under atmospheric conditions depends on the mutual chemical activity of the film and the substrate components. Interdiffusion and reactions in active systems were observed in a vacuum but no metals exhibited adhesion failure. Chemical reactions between metallic films, atmospheric components and thallium bromide-iodide occurring in some systems after exposure to air caused adhesion failure. The difference between normal electrode potentials of TI and condensed metals appeared to be a good parameter for forecasting the system stability under different conditions.
1. Introduction The possibility of applying a thin film package depends not only on the initial characteristics of the package but also on its behaviour during ageing under different conditions. The stability and integrity of such systems are determined by the processes of diffusion, solid state reactions, recrystallization, etc., which can develop across the interface of a film and a substrate. For the purpose of designing and fabricating thin film devices (acoustic and acousto-optical, for example) with fixed parameters, conditions affecting the abovementioned processes and their effect on the parameters must be known. Moreover,-the investigation of interaction processes in thin film systems also indicates an academic interest in the physics of the contact interaction. It is known that Gibbs' energy may serve as a good criterion of possible interactions on the contact of two solids [1]. In many cases a negative value of Gibbs' energy of a substitutional reaction between a thin metallic film and a substrate means strong adhesion. No additional treatment, e.g., by heating, for such systems is needed to achieve a high adhesion value if the contact between the film and the substrate is free from contaminants (juvenile contact) [2]. If interdiffusion and compound formation is possible, the durability of film/substrate joint is determined by a new system: film/interphase/substrate [3-8]. Recently we have reported on the adhesion of thin metallic films to non-metallic substrates [2]. Considering the data obtained, we came to a conclusion that chemical forces are dominant in metal film adhesion to
0040-6090/93/$6.00
TIBr-TII crystals. If we look a t Gibbs' energy of a substitutional reaction between condensed metals and T1Br-TII, we find that only for T i and Cr from the systems investigated earlier is there some possibility of a substitutional reaction. However, higher adhesion was found for inactive Ag and Au but not for Cr and Ti. It can be assumed that the high adhesion energy was due to the chemisorption by the non-saturated bonds of thallium halide crystal. The lower adhesion of active metals was possibly due to the interphase formation. The durability of the joint in this case was determined by the properties of the metal film/interphase/thallium halide system. The object of the present work was to answer this question and to investigate interaction processes in the Me/TIBr-TII systems during ageing under different conditions. For the metal film/halide systems, thallium bromideiodide was used as it is a good substrate material because of the metals available which exhibit different chemical activities with the possibility of a substitutional reaction with a halide crystal.
2. Experimental details It should be noted that the controllable conditions on the contact between a film and a substrate are of great importance in our investigations. The absence of adsorbates or byphases on the as-prepared film/substrate interface makes it possible to observe contact interaction in its "pure" form. Therefore, the application of juvenile contacts is a good method for studying different
© 1993 - - Elsevier Sequoia. All rights reserved
88
A. Simanovskis, S. V. Stolyarooa / Ageing in Me/TIBr-Tll systems
interaction phenomena in solid state systems, namely in metallic film/halide systems, and it has been used in the present work. Thin polycrystalline metal films deposited onto juvenile (free from contaminants) thallium bromide-iodide crystal surfaces have been investigated. The juvenile contacts were obtained by cutting the crystal along the (100) plane in a flow of metal vapour using techniques described in ref. 6. Microhardness, adhesion and electrical resistivity investigations have shown that the crystal cutting itself does not cause any marked changes in the related parameters, compared with those after the special polishing procedure required for preparing the optical crystals. Moreover, we compare systems which have undergone the same mechanical treatment. Thermal evaporation of metals in a vacuum of 10-4 Pa was used and the deposition rate was 5 n m s -1. The film thickness was 50 nm for Auger depth profiling, 100 nm for electrical resistance measurements and 250 nm for the adhesion tests. The difference in film thicknesses required for different investigations is admissible because a quantitative comparison between the same parameters of adequately prepared systems was made. The samples were aged at room temperature in a vacuum of 10-2 Pa or in air with a relative humidity of (55 + 5)%. The humidity was measured by a BM-2 standard device. The adhesive strength was measured by the direct pull technique using pins of diameter 3 mm which were glued to the film with an epoxy resin. The period between film deposition and pin attachment and that between air admission and pin attachment was taken as the ageing time in a vacuum and in air respectively. Pins were fastened to vacuum-aged films in ambient air, in which case the air exposure did not exceed 30 s. The samples were then held at room temperature for 18-20 h and at 100 °C for 4 h. Hence the adhesion test was performed 24 h after the pins were attached. The relative error of the adhesive strength arithmetic average values was less than 25% with a reliability coefficient of 0.9. The initial adhesion values tro were obtained by attaching the pins to the films that had been relaxed in a vacuum for 1 h in order to decrease the point scattering. We must note that it is difficult to give an exact definition of the initial adhesion values obtained if we consider the procedure of pin attachment and measurement of the adhesion on the one hand, and the difference in the activity of the systems and their sensibility to the ambient influence on the other hand. Henceforth, we have taken these factors into account when comparing the systems and drawing the conclusions. The heating activates interaction processes in the systems under investigation, and as a consequence the measured adhesion value is "shifted" along the time axis. As seen below, the short exposure to air required for pin attachment to vacuum-aged samples appeared to be Critical
for only a few systems. For the rest, the adhesion values obtained are lowered by an amount which can be approximately evaluated from the adhesion dependance on ageing time in air. The Auger depth profiling of the samples was performed by use of a Varian cylindrical mirror analyser. The energy of the primary electrons was 2 keV. Etching was carried out using 1.5keVAr ÷ ions. The total etching depth was determined using a Model 201 profilograph. Identification of the Auger spectra was performed in accordance with the Handbook of Auger Electron Spectroscopy [9]. The relative elemental sensitivity factors and standards both from ref. 9 and those obtained by taking Auger spectra of T1Br, TIC1, TIBrTII were used for a quantitative analysis. The four point probe compensation method and a d.c. resistance bridge MO-62 were used for the film resistance measurements. The maximum error of relative measurements did not exceed 0.1%.
3. Results and discussion
3. I. Ageing in vacuum
Figure 1 shows the change in the adhesive strength of metal film/thallium bromide-iodide juvenile contacts with ageing time, in vacuum. The most stable are Ag, Cu and Au/TIBr-TII systems, which exhibit only a tendency to adhesion growth. Ni, Co, In and Fe films manifest a slight adhesion increase with ageing time, but Ti, Cd and Cr film adhesion rises considerably with time. Relaxation, recrystallization, interdiffusion and chemical reactions in thin metallic film systems bring about an electrical resistance change in the films [10-15].
CI"
60 5
c~t
"n
0.
20
40
60
t,h
Fig. 1. Adhesive strength of Me/TIBr-TII juvenile contacts vs. ageing
time in a vacuum. Crorepresents the initial adhesion values.
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr- TII systems
T A B L E 1. The calculated Gibbs' energy for substitutional reactions of metals with thallium halides. Data taken from ref. 17
R
R0 ~..0
Ti
C~
¸
1.5
C
89
0
I
1
.
2
5
~
~
~
t,h
Reaction
AG (kJ mol-~)
M g + 2 TIBr M g + 2 TII M n + 2 TIBr Mn + 2 TII Ti + 2 T1Br Ti + 3 TIBr Ti+ 2TlI Cr + 2 TIBr Fe + 2 TIBr Co + 2 TIBr Ni + 2 TIBr Cu + T1Br Ag + TIBr A u + TIBr
- 168 - 109 - 33 0 -46 -25 -12 0 96 124 124 16 16 36
Fig. 2. Electrical resistance of metal films deposited onto TIBr-TII juvenile surfaces v s . ageingtime in a vacuum. Ro is the resistivityof as-prepared films. This resistance was measured along with the changes in the systems under investigation from the first minutes of their ageing. We found a resistance decrease in the Ag, Cu and Au films (Fig. 2) which is typical for relaxation processes [ 15, 16]. Here the tendency to adhesion increase in these systems during ageing in vacuum conditions is evidently a result of a decrease in elastic strain. For the Ni, Co, Fe, Cd, Cr, and Ti/TIBr-TII systems which showed a marked adhesion increase with the ageing time in a vacuum, a corresponding resistance increase was observed. The resistance increase cannot be attributed to film metal oxidization by remanent gases. Thus, for example, both easy oxidizing Cu films on TII-TIBr (see Fig. 2) and Co films on SiO2 (Fig. 3) exhibited a resistance decrease under the same conditions. The resistance increase observed could therefore be due to the interdiffusion of condensed metals with thallium bromide-iodide.
1.00
0.99 "--o--1....__o
0.~
0
i
I
1
Z
~,h
Fig. 3. Electrical resistance o f Co film deposited onto SiO 2 vs. time in a vacuum. Ro is the resistivity o f as-prepared film.
ageing
Let us evaluate the possibility of a chemical interaction of a solid solution o f thallium bromide-iodide with given metals. Because of a lack of data for solid solution, substitutional reactions of thallium bromide and thallium iodide with metals are considered using data from ref. 17. Table 1 shows that the Gibbs' energy is negative only for Mg, Mn, and Ti and for Cr it is equal to zero. This means that there is a very low probability of a chemical reaction for other metals. However, we can arrange themetals in order of their chemical activity, which is as follows: N i - C o - F e - C r Ti. We can ascertain (see Fig. 2) that the increase in electrical resistance correlates with the activity of metals. Auger depth profiling of Ni, Cr, Ti/T1Br-TII systems was carried out to find the reason for these changes. Figure 4 shows that diffusion of metals into the substrate crystal and diffusion of T1 into the film has occurred. The width of the interdiffusion zone grows from Ni to Ti showing the corresponding increase in the resistance and adhesion of the films (see Figs. 1 and 2). The adhesion increase with time for Cr exceeds that for Ti possibly due to a decrease in elastic strain since the latter is higher for chromium films than for titanium films [18]. The direct dependence of the adhesion increase on the activity of the deposited metals is evident. Though the film resistance increases over several days the major changes take place during the first 2 h after the deposition. However, a considerable adhesion increase has been observed for up to three days. This discrepancy can be explained as follows. The film resistance essentially depends on the initial interdiffusion and reaction processes between the film and the substrate, but the adhesion is also sensitive to following transformation processes of the non-conductive interlayer phases.
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr-TII systems
90
,:
.l
•~' 0 -I-
80
m
40
.
I 0
,
I
t !
i
~, 40
.<
L 0
~ , i , 20 60 6O 8 0 Etching Depth, nm
Fig. 4. Auger depth profiles of Me/TIBr-TII juvenile contacts after ageing for 10 days.
Thus it has also been ascertained that the adhesion values obtained for the systems in ref. 2 are determined by complicated film/interphase/T1Br-TlI systems. Adhesion measurements for Mg, Mn and A1/TIBrTII systems could not be carried out because of the fast degradation which occurred when the vacuum was broken. Therefore the electrical resistance measurements for A1 and Mn films on thallium bromide-iodide were made under vacuum conditions (Fig. 5). A typical R/Ro decrease for AI film manifested relaxation processes and showed no evidence of any reaction or interdiffusion. The Mn film exhibited some rise in electrical resistance directly after the deposition, followed by a slight decrease. The Gibbs' potential (see Table 1) AG < 0
shows the possibility of a substitutional reaction between Mn and TIBr. As a result of such a reaction the release of TI could occur. Since every Mn atom can displace two TI atoms and the resistivity of TI is an order of magnitude less than that of Mn, the film resistance is expected to decrease and the resistance curve observed can thus be explained. No signs of degradation were observed in vacuum conditions and it is reasonable to regard these systems as vacuum stable with respect to adhesion failure. Thus the results obtained show that the adhesion failure does not occur in a vacuum. Moreover, the intensity of interaction and mass transfer between metal films and thallium bromide-iodide depends on their mutual chemical activity, characterized by Gibbs' potential. It should be stressed, however, that for the systems under investigation the adhesive strength per se does not show direct conformity with the chemical activity of deposited metals, since the chemical interaction introduces new phases in the contact zone, thus changing the contacting objects. 3.2. Ageing in air As mentioned above, the fast degradation of Mg, Mn, AI/TIBr-TII systems was observed when they were exposed to air. Aluminium films exfoliated via blistering, and the trace of "chemical etching" could be seen on the substrate crystal after detaching the film. The electrical resistance rose steeply during the first 20min after the vacuum was broken (Fig. 6). The resistance increased by up to 130% in 25 min. After that the growth rate essentially diminished and during the next hour the resistance increase was only 25%. These observations give evidence that degradation of A1
.B.
Ro
I "°
"tO5
1.0
0.g5~
t
0.g 0
i 1
I 2
tjh
Fig. 5. Electrical resistanceof Mn and A1 films deposited onto juvenile T]Br-TII surfaces vs. ageing time in a vacuum. ~ is the resistivity of as-deposited films.
0
10
~.0 50 40
50 t,min
Fig. 6. Electrical resistance of Mn and AI films deposited onto juvenile TIBr-TiI surfaces vs. exposure time to air. Ro is the resistivity of the films before breaking the vacuum.
A. Simanovskis, S. K Stolyarova / Ageing in Me/TIBr-TII systems
91
In 1
0
° •
u
11 ~, [~..
u
i ~a--
0
o --~'o..__
Be
.-o
---'~- ........
~
_
n g --~.r
o
.
.
.
.
.
1"~
o-
100
50
Etching
Au
0.~
,Tt
D e p t h , nm
I
Fig. 7. Depth distribution of elements in M n / T I B r - T I I system after ageing in air for 10 days.
film in air took place only in contact with thallium bromide-iodide crystal and stopped after blistering. Mg and Mn films transformed into liquid phase and finally crystallized under the influence of air. The related electrical resistance measurement showed a total loss of metallic conductivity after 15 min. We can suppose that compounds of the type MnBr2 were formed on the contact between Mn film and thallium halide crystal since such a reaction is preferred (AG < 0). Exposure to air led to water adsorption and its interaction with MnBr2. Auger depth profiling for the system Mn/T1Br-TII has shown (Fig. 7) that for 50 nm thick Mn film no boundary between the film and substrate crystal can be detected up to an etching depth of 120 nm. Nearly constant concentrations of Mn, Br, T1, and O occur in this region with the exception of the very surface layer of I0 nm which confirms the suggestion above. Based on the thermodynamic estimation and the data obtained we can conclude that MnBr2.nH20 was formed. The adhesive strength of Ag, Au, Cu, Ni, Co, In, Cr, and Ti films to TIBr-TII during ageing in air was measured. Figure 8 shows a marked increase in the adhesion of Cr and Ti films, though less than seen during ageing in vacuum. Adhesion of In and Co films increased negligibly, but adhesion failure for Ni, Au, Cu and Ag films was observed. The intensity of degradation increased in the sequence N i - A u - C u - A g . The discrepancy of this order, compared with Gibbs' energy of oxidation does not allow us to connect the adhesion failure observed with the oxygen effect. Earlier, in ref. 19, we reported on the Ambient Induced Diffusion (AID) in metal film/halide systems. We found it to be the reason for adhesion failure of Ag films to TIBr-TII [6]. The possibility of AID in a given system can be evaluated by Gibbs' energy of the reaction between condensed metal and atmospheric chlorine contaminants (AG~) and the reaction between the metal chloride formed and the substrate crystal components
0
100
200
~00
t,h
Fig. 8. Adhesive strength o f M e / T I B r - T I I juvenile contacts vs. ageing time in air. a o represents the initial adhesion values before breaking the vacuum.
T A B L E 2. The calculated Gibbs' energy for the reaction o f metals with chlorine (AG~), for the reaction o f related chloride with TII (AG2) and TIBr (AG3) and the change in adhesive strength o f related metallic films during ageing for 24 h under atmospheric conditions. Data for AG are taken from ref. 17
Metal
AGI
AG2
AG3
Au
54.5 25.2 12.6 - 16.4 -2.5 -4.2
84 50.4 54.5 -42.0 - 12.6 - 12.6
6 3 - 2 - 15 -25 -80
(%)
(kJ/mol -I) Ti Co Ni Au Cu Ag
-470 -273 - 260 - 16.8 - 117 - 109
TIBr and TII (AG2 and AG3). For the given metals AG~ < 0 is always true. Thus the AG2 and AGa are the main criteria of AID. We can ascertain that the adhesion decreases correlates with AG2 and AG3 given in Table 2 and with the intensity of AID [19]. Consequently the adhesion failure observed can be attributed mainly to AID processes.
3.3. The stability criterion of Me/TIBr-TII systems Thus we have found that both in vacuum and air conditions the behaviour of the given systems depends on the mutual activity of the condensed metal and thallium bromide-iodide substrate. This activity can be characterized by Gibbs' energy of substitutional reaction of metal with halide. However it should be stressed that thermodynamic estimation does not give the entire description of contact processes in these systems under different conditions. The film and substrate structure, admixtures both in film and substrate, and the amount and type of defects play a significant role in ageing
92
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr-TII systems
processes affecting diffusion kinetics by enhancing or suppressing different mechanisms. However the results obtained allow us to draw some general conclusions and forecast the behaviour of metal/thallium b r o m i d e - i o d i d e systems. Although we have found Gibbs' energy to be a good characteristic for systems under investigation, some difficulties can arise. Often, before the experiment, we do not exactly know what solid state reactions can take place in the given thin film/substrate system and what compounds can be formed. Even if we knew, the Gibbs' energy data are often not available. Since the thin film/substrate system stability largely depends on the possible direction of film-substrate reaction, parameters denoting this tendency can be used. One of the parameters connected with chemical activity of metals is normal electrode potential e °. In this case it is not necessary to know exactly which compounds are formed in solid state reactions. Figure 9 shows a correlation between the stability of Me/T1Br-TII systems in a vacuum and the difference between normal electrode potentials of T1 and condensed metal Ae ° = e°l - e°e. In the region of Ae ° > 0 reactions and interdiffusion are possible and as a consequence A R / R o > 0. For Ae ° < 0, A R / R o also falls below zero thus giving evidence of relaxation processes and an absence of interdiffusion. With respect to the system stability after film deposition, the right-hand side ( A e ° > 0) represents a marked adhesion increase and degradation of the film conductivity while the left-hand side ( A e ° < 0) metals show stabilization of both conductivity and adhesion. Thus we have shown that in vacuum conditions no metals exhibit adhesion decrease. Considering other characteristics, the systems with A e ° < 0 are more stable. The probability of chemical interaction is higher for the systems with A e ° > 0 which leads to both film adhesion and conductivity increasing with ageing time.
,,R/Ro /6o 1,0 2,5 0,5 2,0~Cr
Ti
J
Co"l~.Fe i
n
t
rtl
n
i
t
1,0
TABLE 3. The adhesion stability of juvenile Me/TIBr-TII contacts and corresponding difference of normal potentials between TI and condensed metal. Data for Ae° are taken from ref. 20 Me
Aa/24 h (%) in a vacuum
in air
Mg AI Mn
-
fast degradation
Ti Cr Fe In Co
28 125 8 6 5
Ni Cu Ag Au
2 1 1 1
6 10
Ae°(V) 2.045 1.325 0.715
3 2
1.415 0.375 0.105 0.005 - 0.065
-2 - 25 -80 - 15
-0.105 - 0.680 -1.136 - 1.755
-
N o w let us take up a question of system stability during ageing in atmospheric conditions. F o r better results the adhesion stability of systems with respect to the difference of normal electrode potentials Ae ° is shown in Table 3. It is evident that the stability increases at low absolute values of Ae °. The most stable are Co, I n / T I B r - T I I systems with [Ae°[ --, 0. The growth of [Ae°[ leads to the loss of stability. The sign of Ae ° plays an essential role in forecasting the possible processes which determine the system behaviour under different ageing conditions. At first for A e ° > 0 a direct chemical interaction of condensed metal with thallium halide is possible. Stability of such systems will depend on the effect of the atmosphere upon the products of the primary reaction and every atmospheric component is to be considered. F o r example, Mn, M g / T l B r - T l I systems lost stability due to water vapour influence. In the case of A e ° < 0 (Ni, Cu, Ag, A u / T I B r - T I I ) degradation of the systems occurred due to ambient induced processes-chemical reactions and interdiffusion between atmospheric components, condensed metal, and substrate components. Chlorine and sulphur, and their compounds in the atmosphere initiated the mentioned processes [19]. It should be stressed that in this region of Ae ° a chemical reaction between the film and active atmospheric component is the first stage of degradation. It is only after this that the substrate joins the interaction process.
|
1,5"A E°
1 Fig. 9. Adhesive strength (O) and electrical resistance ( I ) of metal films vs. Ae° after ageingthe Me/TIBr-TIIsystemsin a vacuum for 24 h.
4. Conclusion A summary of the results obtained in this work and earlier allows us to conclude that thin metal film/thai-
A. Simanovskis, S. V. Stolyarova / Ageing in Me/TIBr-Tll systems
lium bromide-iodide systems exhibit a whole spectrum of possible interactions from mutual inactivity and stability to intensive chemical reactions, interdiffusion and fast degradation. Moreover, ambient conditions play an important role in the system behaviour. Thermodynamic estimations have been found to give useful information about possible processes in the systems under consideration. Normal potentials of substrate cation and condensed metals are convenient parameters in forecasting the behaviour of the systems. References 1 R. A. Musin and G. V. Konyushkov, Joining Metals to Ceramics, Mashinostroenie, Moscow, 1991 (in Russian). 2 S. A. Varchenya, A. Simanovskis and S. V. Stolyarova, Thin Solid Films, 164 (1988) 147. 3 D. Gupta and P. S. Ho, Thin Solid Films, 72(1980) 399. 4 H. K. Pulker, A. J. Perry and R. Berger, Surf. TechnoL, 14(1981) 25. 5 D. M. Mattox, Int. J. Hybrid Microelectronics, 4 (19~1) 164. 6 S. V. Stolyarova, A. Simanovskis, V. N. Kovalev and S. A. Varchenya, Thin Solid Films, 177(1989) 181.
93
7 M. M. Chehimi and J. F. Watts, J. Adhesion Sci. Technol., 6 (1992) 377. 8 W. J. Chen and L. J. Chen, J. Appl. Phys., 70(1991) 2628. 9 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach, R. E. Weber (eds.), Handbook of Auger Electron Spectroscopy, Physical Electronics Industries, Eden Prairie, 1976. 10 J. Haber and I. Okonska-Kozlowska, J. Solid State Chem., 35 (1980) 22. 11 A. G. Dirks and J. J. Van Den Brock, Thin Solid Films, 96 (1982) 257. 12 A. E. Curzon and O. Singh. Thin Solid Films, 76(1981) 185. 13 P. Madakson, J. Appl. Phys., 70 (1991) 1374, 1380. 14 D.-Y. Shih, C.-A. Chang, J. Paraszczak, S. Nunes and J. Cataldo, J. Appl. Phys., 70 (1991) 3052. 15 L. S. Palatnik and V. K. Sorokin, Material Science in Microelectronics, Energiya, Moscow, 1978 (in Russian). 16 L. P. Grebennik, V. A. Demidova and G. N. Podus, in Thin Film Obtaining and Properties, Kiev, 1977, p. 57 (in Russian). 17 M. H. Karapet'yants and M. L. Karapet'yants, Basic Thermodynamic Constants of Inorganic and Organic Substances, Khimiya, Moscow, 1968 (in Russian). 18 K. Kinosita, Thin Solid Films, 12 (1972) 17. 19 A. A. Simanovskis, S. V. Stolyarova and G. P. Upit, Thin Solid Films, 97(1982) 301. 20 G. V. Samsonov (ed.), Physical and Chemical Properties of Elements Handbook, Naukova Dumka, Kiev, 1965 (in Russian).