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Microstructural determination of fast diffusing species in thin film diffusion couples
Thin Solid Filrns, 88 (1982) 285-290
285
PREPARATION AND CHARACTERIZATION
M I C R O S T R U C T U R A L D E T E R M I N A T I O N O F FAST D I F F U S I N G SPECIES IN T H I N F I L M D I F F U S I O N C O U P L E S S. NAKAHARA AND R. J. McCOY Bell Laboratories, Murray Hill, NJ07974 (U.S.A.) (Received September 14, 1981; accepted October 30, 1981)
A transmission electron microscope was used to study the room temperature interdiffusion of Pd/Sn thin film couples. It was found that the interdiffusion proceeds by the rapid diffusion of palladium into tin, followed by the formation of the intermetallic compound PdSn4. The microstructure of PdSn4 shows that the compound is formed inside the original large tin grains. Furthermore, the solid state structural transformation of tin into PdSn 4 involves only the positioning of palladium atoms in the interstitial sites together with the associated faultings and thus can take place readily without extensive modifications to the original tin structure. In the Pd/Sn diffusion couple, therefore, palladium is the faster diffuser. The detection of such a simple structural transformation can in principle permit the determination of the direction of net atomic flow which in turn provides information on the relative magnitude of the diffusivities of the components in thin film diffusion couples.
I. INTRODUCTION
Kirkendall voids I are generally formed in a diffusion couple of components A and B at the side of the fast diffusing species, say A, during interdiffusion, if the bulk diffusivities DA and DR satisfy the condition D A >> Da. Such void formation has previously been observed in various .bulk and thin film diffusion couples. The relative location of Kirkendall voids generally permits the identification of the fast diffusing species in diffusion couples. Such an analysis, however, can be more complicated in thin films, since the presence of a relatively high density of grain boundaries causes fast grain boundary diffusion to operate in addition to bulk diffusion. Grain boundary diffusion may even cause the material flow direction to be opposite to that predicted by a bulk diffusion mechanism, as observed recently in Au/Sn thin film diffusion couples 2. Furthermore, in thin films the detection of Kirkendall voids at the exact location along the thickness direction is not easily made, because sites for void nucleation may even be at the surfaces rather than inside the film because of the proximity of the film surfaces. As an alternative method for determining a fast diffusing species, we have found that in some thin film couples, which form intermetallic compounds at relatively low 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
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s. N A K A H A R A , R. J. M c C O Y
temperatures, compounds are formed within the microstructure of the slow diffusing species. In other words, intermetallic compounds are formed inside the grains of a slow diffusing species and the morphology of the intermetallic compounds closely replicates the original grain structure of this species. This preferential intermetallic nucleation can be readily recognized by transmission electron microscopy (TEM), if the microstructures of two couples, i.e. their grain sizes, differ considerably. An example of this method will be illustrated from recent interdiffusion studies in Pd/Sn thin film diffusion couples. 2.
EXPERIMENTAL DETAILS
Thin palladium and tin films were deposited onto cleaved KCI{001} and NaCI{001 } substrates held at room temperature. Depositions were made using an electron beam evaporation technique in a vacuum of about 10 - 6 Torr. Either the palladium or the tin film was first deposited onto the substrate and then the second film was deposited in such a way that it was slightly offset 2 with respect to the first film. In this way, it was possible to obtain pure palladium and tin films as well as Pd/Sn sandwiched films for structural comparison. For T E M observations, the films were stripped off the soluble substrates in distilled water and placed on a 3 mm microscope grid. TEM micrographs were taken using a JEM 200 electron microscope operated at 200 kV. In order to determine the crystal structure and nature of the intermetallic compounds resulting from interdiffusion, computer-simulated electron diffraction patterns 3 were used. 3.
RESULTS AND DISCUSSION
Figure 1 shows the microstructure of 500/~ of palladium deposited onto 500/~ of tin. Here, dark regions (denoted by S) represent the morphology of the original 500 A tin islands, which are now alloyed by the diffusion of palladium. The lateral diffusion of palladium into these islands forms Kirkendall voids around the islands as indicated by the symbol V. N o regions V are observed in the pure palladium and tin films on either side of the Pd/Sn sandwich. The void regions effectively delineate the remaining pure palladium (P) and prevent further diffusion of the palladium. An electron diffraction pattern from this film has indeed shown that the film consists of pure palladium and the intermetallic compound PdSn,~ only. The single intermetallic compound observed is consistent with the work of Tu 4. Therefore, the dark island regions have transformed completely from tin into PdSn4. Figure 2 is a schematic illustration showing how the Kirkendall voids are formed in Pd/Sn thin film couples. As seen in the figure, tin nucleates as large (0.11 gm) islands (B) and then palladium (A) is deposited onto the tin film. Fine-grained palladium then diffuses into the tin islands via a bulk diffusion mechanism. The lateral palladium diffusion involves the formation of voids (V) particularly in the regions of the tin island perimeter. Note also that the volume of the tin islands will increase slightly to accommodate the addition of palladium. Consequently, the intermetallic compound PdSn 4 (AB) is formed within the tin islands and the boundary becomes a palladium-depleted region, i.e. a void. It is then clear that pure
D E T E R M I N A T I O N O F FAST D I F F U S I O N SPECIES IN T H I N FILM C O U P L E S
287
Fig. 1. Microstructures of a 500 ~ palladium film deposited onto 500/~ tin islands (S), showing void regions (V) around the tin islands. P denotes regions of palladium grains.
INTERDIFFUSION ( D A >> D B)
AB " I N T E R M E T A L L I C
COMPOUND
V : VOID
Fig. 2. A schematic illustration showing how Kirkendall voids (V) can be formed at the perimeters of islands (B) in a bimetallicA/B filmas a result of interdiffusion. See text for details. palladium is left between the islands. It should be pointed out here that, for the formation of such voids to cause the isolation of the islands, the palladium film thickness has to be relatively small. This type of void formation has previously been reported by N a k a h a r a and McCoy 2 in Au/Sn thin film couples. We also found a large number of much smaller voids (about 25/~) lying in the grain boundaries of the pure palladium regions. This indicates that the rapid diffusion of palladium into tin primarily takes place along the palladium grain boundaries, thus leaving microscopic voids at the grain boundaries. In other words, grain boundary self-diffusion in palladium greatly assists the diffusion of palladium atoms into tin grains. In addition to the Kirkendall void formation around the periphery of the tin islands, we found interesting microstructural changes in the intermetallic compound PdSn 4. Figure 3 shows the structure of 500/~ of tin deposited onto 500/~ of palladium. Here, we see large islands of PdSn 4 having black-white lamellar fringes
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s. NAKAHARA, R. J. McCOY
Fig. 3. The formation of PdSn4 intermetallic c o m p o u n d (marked with two black arrows) inside the original tin grains as a result of room temperature interdiffusion in an Sn(500 ,~)/Pd(500/~) bimetallic film. Note that the deposition order is reversed from that of Fig. 1.
as indicated by two black arrows. Similar fringes were observed in the sample from which the micrograph of Fig. 1 was taken. The size and shape of this intermetallic compound are identical with those of the original tin islands as was also the case in Fig. 1. This phase is essentially formed within the original tin islands, indicating that tin atoms did not move significantly, but palladium primarily diffused into the tin islands. In other words, the observations of structural changes in tin islands permits the detection of an unequal flow of atoms in the Pd/Sn thin film couple. The corresponding electron diffraction pattern is shown in Fig. 4. The lower portion of the pattern represents the computer-simulated diffraction pattern which is based on ref. 5. It is noted that extensive streakings seen in the upper experimental pattern are an indication of the presence of faultings in the intermetallic. The crystallography of the faulted PdSn 4 grain can be understood in terms of the recent analysis of AuSn 4 compound formation in Au/Sn thin film couples made by Buene e t al. 6 In their analysis, AuSn4 (Pabe) is initially formed during the room temperature interdiffusion. According to structural data, 7 PdSn 4 has the same space group (Pabc) as that of AuSn4. It is possible to explain the formation of PdSn4 in the same manner as shown by Buene e t al. 6 for constructing AuSn4 from the tetragonal tin unit cell. This compound formation mechanism can result in the faultings shown in Fig. 3. It is now clear that the Pd/Sn diffusion couples exhibited a considerable microstructural (grain size) difference which permits the fast diffusing species to be determined. Palladium consists of much finer grains than tin does. When these sandwiched films are viewed along the direction of their film normal by TEM, each species can be readily identified by their grain structures. Figure 5 is a schematic illustration showing how microstructural changes evolve during interdiffusion in a thin film couple A/B. Here, the species A and B, having the grain sizes d A and ds,
DETERMINATION OF FAST DIFFUSION SPECIES IN THIN FILM COUPLES
289
Fig. 4. An electron diffraction pattern (upper photograph) from the film shown in Fig. 3. The lower portion represents the computer-simulated diffraction pattern of palladium and PdSn 4.
-'~dA H-INTERDIFFUSION (D A >> 0 B)
~dAs
GRAIN SIZE RELATIONSHIP
dAB~
d B >>
dA
Fig. 5. A schematic illustration showing how the PdSn4 intermetallic compound is formed inside the original large tin grains as a result of fast palladium diffusion into tin. represent palladium and tin respectively, whereas AB denotes the intermetallic c o m p o u n d P d S n 4. Since the diffusivity of palladium is m u c h greater than that of tin, i.e. D A >> DB, palladium a t o m s diffuse into the large-grained tin film and form the intermetallic c o m p o u n d PdSn 4 inside the original tin grains. Similar to the case of AuSn 4 6 the formation of the c o m p o u n d does not involve nucleation and growth processes, but proceeds primarily by faulting processes. Consequently, the comp o u n d is seen to extend t h r o u g h o u t the tin grains and thus the microstructure of P d S n 4 becomes similar to that of the original tin. In the light of this microstructural analysis, an attempt was made to study the same diffusion p h e n o m e n a in the wellk n o w n A1/Au thin film couples. Because of a similarity in their grain structures, however, the present m e t h o d has not been successful.
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S. NAKAHARA, R.J. McCOY
It should also be pointed out that, for each palladium and tin grain, grain growth cannot occur at ambient temperatures. Modification ofmicrostructures as a result of the interdiffusion should be occurring only in the formation and subsequent growth of intermetallics. Thus, the location of intermetallics gives the direction of diffusion which effectively determines the relative magnitudes of their diffusivities. 4. CONCLUSION It has been shown that the identification of sites for intermetallic compound formation can conveniently determine the fast diffusing species in Kirkendall diffusion couples in alloy systems where intermetallics are formed. Studies of the room temperature interdiffusion of Pd/Sn thin film couples have demonstrated that palladium atoms first diffuse into tin and form the PdSn4 intermetallic compound within the original tin grains. From this morphological transformation, it has been shown that palladium is the faster diffusing species in Pd/Sn couples. ACKNOWLEDGMENTS
We would like to thank L. B. Hooker for his careful sample preparation and Drs. G. Y. Chin and S. Mahajan for their critical comments on this manuscript. REFERENCES 1 2 3 4 5 6 7
A.D. Smigetskas and E. O. Kirkendall, Trans. AIME, 171 (1947) 130. S. Nakahara and R. J. McCoy, Appl. Phys. Lett., 37 (1980) 42. S. Nakahara, Ultramicroscopy, 5 (1980) 275. K . N . Tu, IBMRes. Rep. RC4185,1973. JCPDS File, Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1978. L. Buene, H. Falkenberg-Arell, J, GjCnnes and J. Taft¢, Thin Solid Films, 67 (1980) 95. W. B. Pearson, Handbook of Lattice Spacings and Structure of Metals, Vol. 2, Pergamon, New York, 1967.
285
PREPARATION AND CHARACTERIZATION
M I C R O S T R U C T U R A L D E T E R M I N A T I O N O F FAST D I F F U S I N G SPECIES IN T H I N F I L M D I F F U S I O N C O U P L E S S. NAKAHARA AND R. J. McCOY Bell Laboratories, Murray Hill, NJ07974 (U.S.A.) (Received September 14, 1981; accepted October 30, 1981)
A transmission electron microscope was used to study the room temperature interdiffusion of Pd/Sn thin film couples. It was found that the interdiffusion proceeds by the rapid diffusion of palladium into tin, followed by the formation of the intermetallic compound PdSn4. The microstructure of PdSn4 shows that the compound is formed inside the original large tin grains. Furthermore, the solid state structural transformation of tin into PdSn 4 involves only the positioning of palladium atoms in the interstitial sites together with the associated faultings and thus can take place readily without extensive modifications to the original tin structure. In the Pd/Sn diffusion couple, therefore, palladium is the faster diffuser. The detection of such a simple structural transformation can in principle permit the determination of the direction of net atomic flow which in turn provides information on the relative magnitude of the diffusivities of the components in thin film diffusion couples.
I. INTRODUCTION
Kirkendall voids I are generally formed in a diffusion couple of components A and B at the side of the fast diffusing species, say A, during interdiffusion, if the bulk diffusivities DA and DR satisfy the condition D A >> Da. Such void formation has previously been observed in various .bulk and thin film diffusion couples. The relative location of Kirkendall voids generally permits the identification of the fast diffusing species in diffusion couples. Such an analysis, however, can be more complicated in thin films, since the presence of a relatively high density of grain boundaries causes fast grain boundary diffusion to operate in addition to bulk diffusion. Grain boundary diffusion may even cause the material flow direction to be opposite to that predicted by a bulk diffusion mechanism, as observed recently in Au/Sn thin film diffusion couples 2. Furthermore, in thin films the detection of Kirkendall voids at the exact location along the thickness direction is not easily made, because sites for void nucleation may even be at the surfaces rather than inside the film because of the proximity of the film surfaces. As an alternative method for determining a fast diffusing species, we have found that in some thin film couples, which form intermetallic compounds at relatively low 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
286
s. N A K A H A R A , R. J. M c C O Y
temperatures, compounds are formed within the microstructure of the slow diffusing species. In other words, intermetallic compounds are formed inside the grains of a slow diffusing species and the morphology of the intermetallic compounds closely replicates the original grain structure of this species. This preferential intermetallic nucleation can be readily recognized by transmission electron microscopy (TEM), if the microstructures of two couples, i.e. their grain sizes, differ considerably. An example of this method will be illustrated from recent interdiffusion studies in Pd/Sn thin film diffusion couples. 2.
EXPERIMENTAL DETAILS
Thin palladium and tin films were deposited onto cleaved KCI{001} and NaCI{001 } substrates held at room temperature. Depositions were made using an electron beam evaporation technique in a vacuum of about 10 - 6 Torr. Either the palladium or the tin film was first deposited onto the substrate and then the second film was deposited in such a way that it was slightly offset 2 with respect to the first film. In this way, it was possible to obtain pure palladium and tin films as well as Pd/Sn sandwiched films for structural comparison. For T E M observations, the films were stripped off the soluble substrates in distilled water and placed on a 3 mm microscope grid. TEM micrographs were taken using a JEM 200 electron microscope operated at 200 kV. In order to determine the crystal structure and nature of the intermetallic compounds resulting from interdiffusion, computer-simulated electron diffraction patterns 3 were used. 3.
RESULTS AND DISCUSSION
Figure 1 shows the microstructure of 500/~ of palladium deposited onto 500/~ of tin. Here, dark regions (denoted by S) represent the morphology of the original 500 A tin islands, which are now alloyed by the diffusion of palladium. The lateral diffusion of palladium into these islands forms Kirkendall voids around the islands as indicated by the symbol V. N o regions V are observed in the pure palladium and tin films on either side of the Pd/Sn sandwich. The void regions effectively delineate the remaining pure palladium (P) and prevent further diffusion of the palladium. An electron diffraction pattern from this film has indeed shown that the film consists of pure palladium and the intermetallic compound PdSn,~ only. The single intermetallic compound observed is consistent with the work of Tu 4. Therefore, the dark island regions have transformed completely from tin into PdSn4. Figure 2 is a schematic illustration showing how the Kirkendall voids are formed in Pd/Sn thin film couples. As seen in the figure, tin nucleates as large (0.11 gm) islands (B) and then palladium (A) is deposited onto the tin film. Fine-grained palladium then diffuses into the tin islands via a bulk diffusion mechanism. The lateral palladium diffusion involves the formation of voids (V) particularly in the regions of the tin island perimeter. Note also that the volume of the tin islands will increase slightly to accommodate the addition of palladium. Consequently, the intermetallic compound PdSn 4 (AB) is formed within the tin islands and the boundary becomes a palladium-depleted region, i.e. a void. It is then clear that pure
D E T E R M I N A T I O N O F FAST D I F F U S I O N SPECIES IN T H I N FILM C O U P L E S
287
Fig. 1. Microstructures of a 500 ~ palladium film deposited onto 500/~ tin islands (S), showing void regions (V) around the tin islands. P denotes regions of palladium grains.
INTERDIFFUSION ( D A >> D B)
AB " I N T E R M E T A L L I C
COMPOUND
V : VOID
Fig. 2. A schematic illustration showing how Kirkendall voids (V) can be formed at the perimeters of islands (B) in a bimetallicA/B filmas a result of interdiffusion. See text for details. palladium is left between the islands. It should be pointed out here that, for the formation of such voids to cause the isolation of the islands, the palladium film thickness has to be relatively small. This type of void formation has previously been reported by N a k a h a r a and McCoy 2 in Au/Sn thin film couples. We also found a large number of much smaller voids (about 25/~) lying in the grain boundaries of the pure palladium regions. This indicates that the rapid diffusion of palladium into tin primarily takes place along the palladium grain boundaries, thus leaving microscopic voids at the grain boundaries. In other words, grain boundary self-diffusion in palladium greatly assists the diffusion of palladium atoms into tin grains. In addition to the Kirkendall void formation around the periphery of the tin islands, we found interesting microstructural changes in the intermetallic compound PdSn 4. Figure 3 shows the structure of 500/~ of tin deposited onto 500/~ of palladium. Here, we see large islands of PdSn 4 having black-white lamellar fringes
288
s. NAKAHARA, R. J. McCOY
Fig. 3. The formation of PdSn4 intermetallic c o m p o u n d (marked with two black arrows) inside the original tin grains as a result of room temperature interdiffusion in an Sn(500 ,~)/Pd(500/~) bimetallic film. Note that the deposition order is reversed from that of Fig. 1.
as indicated by two black arrows. Similar fringes were observed in the sample from which the micrograph of Fig. 1 was taken. The size and shape of this intermetallic compound are identical with those of the original tin islands as was also the case in Fig. 1. This phase is essentially formed within the original tin islands, indicating that tin atoms did not move significantly, but palladium primarily diffused into the tin islands. In other words, the observations of structural changes in tin islands permits the detection of an unequal flow of atoms in the Pd/Sn thin film couple. The corresponding electron diffraction pattern is shown in Fig. 4. The lower portion of the pattern represents the computer-simulated diffraction pattern which is based on ref. 5. It is noted that extensive streakings seen in the upper experimental pattern are an indication of the presence of faultings in the intermetallic. The crystallography of the faulted PdSn 4 grain can be understood in terms of the recent analysis of AuSn 4 compound formation in Au/Sn thin film couples made by Buene e t al. 6 In their analysis, AuSn4 (Pabe) is initially formed during the room temperature interdiffusion. According to structural data, 7 PdSn 4 has the same space group (Pabc) as that of AuSn4. It is possible to explain the formation of PdSn4 in the same manner as shown by Buene e t al. 6 for constructing AuSn4 from the tetragonal tin unit cell. This compound formation mechanism can result in the faultings shown in Fig. 3. It is now clear that the Pd/Sn diffusion couples exhibited a considerable microstructural (grain size) difference which permits the fast diffusing species to be determined. Palladium consists of much finer grains than tin does. When these sandwiched films are viewed along the direction of their film normal by TEM, each species can be readily identified by their grain structures. Figure 5 is a schematic illustration showing how microstructural changes evolve during interdiffusion in a thin film couple A/B. Here, the species A and B, having the grain sizes d A and ds,
DETERMINATION OF FAST DIFFUSION SPECIES IN THIN FILM COUPLES
289
Fig. 4. An electron diffraction pattern (upper photograph) from the film shown in Fig. 3. The lower portion represents the computer-simulated diffraction pattern of palladium and PdSn 4.
-'~dA H-INTERDIFFUSION (D A >> 0 B)
~dAs
GRAIN SIZE RELATIONSHIP
dAB~
d B >>
dA
Fig. 5. A schematic illustration showing how the PdSn4 intermetallic compound is formed inside the original large tin grains as a result of fast palladium diffusion into tin. represent palladium and tin respectively, whereas AB denotes the intermetallic c o m p o u n d P d S n 4. Since the diffusivity of palladium is m u c h greater than that of tin, i.e. D A >> DB, palladium a t o m s diffuse into the large-grained tin film and form the intermetallic c o m p o u n d PdSn 4 inside the original tin grains. Similar to the case of AuSn 4 6 the formation of the c o m p o u n d does not involve nucleation and growth processes, but proceeds primarily by faulting processes. Consequently, the comp o u n d is seen to extend t h r o u g h o u t the tin grains and thus the microstructure of P d S n 4 becomes similar to that of the original tin. In the light of this microstructural analysis, an attempt was made to study the same diffusion p h e n o m e n a in the wellk n o w n A1/Au thin film couples. Because of a similarity in their grain structures, however, the present m e t h o d has not been successful.
290
S. NAKAHARA, R.J. McCOY
It should also be pointed out that, for each palladium and tin grain, grain growth cannot occur at ambient temperatures. Modification ofmicrostructures as a result of the interdiffusion should be occurring only in the formation and subsequent growth of intermetallics. Thus, the location of intermetallics gives the direction of diffusion which effectively determines the relative magnitudes of their diffusivities. 4. CONCLUSION It has been shown that the identification of sites for intermetallic compound formation can conveniently determine the fast diffusing species in Kirkendall diffusion couples in alloy systems where intermetallics are formed. Studies of the room temperature interdiffusion of Pd/Sn thin film couples have demonstrated that palladium atoms first diffuse into tin and form the PdSn4 intermetallic compound within the original tin grains. From this morphological transformation, it has been shown that palladium is the faster diffusing species in Pd/Sn couples. ACKNOWLEDGMENTS
We would like to thank L. B. Hooker for his careful sample preparation and Drs. G. Y. Chin and S. Mahajan for their critical comments on this manuscript. REFERENCES 1 2 3 4 5 6 7
A.D. Smigetskas and E. O. Kirkendall, Trans. AIME, 171 (1947) 130. S. Nakahara and R. J. McCoy, Appl. Phys. Lett., 37 (1980) 42. S. Nakahara, Ultramicroscopy, 5 (1980) 275. K . N . Tu, IBMRes. Rep. RC4185,1973. JCPDS File, Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1978. L. Buene, H. Falkenberg-Arell, J, GjCnnes and J. Taft¢, Thin Solid Films, 67 (1980) 95. W. B. Pearson, Handbook of Lattice Spacings and Structure of Metals, Vol. 2, Pergamon, New York, 1967.