- Email: [email protected]
aluminum interface created by the surface activated bonding method
Nuclear Instruments
and Methods in Physics Research
B 121 (1997) 519-523
Beam Interactions with Materials 8 Atoms
ELSEVIER
TEM investigation of the stainless steel/aluminum interface created by the surface activated bonding method Liu Yang *,l, Naoe Hosoda, Tadatama Suga RCAST, The University of Tokyo. Komaha 4-6-l. Meguro-ku. 153 Tohyo. Japan
Abstract Stainless steel and aluminum were bonded successfully by means of the Surface Activated Bonding (SAB) method at room temperature. High-resolution electron microscopy and micro-EDS were applied to investigate the morphology of the interface thus created. In the as-bonded joints, an intermediate layer of about 10 nm thick was observed between the stainless steel and aluminum. The intermediate layer was composed of mainly silicon and certain amounts of oxygen and carbon according to the micro-EDS analysis. The intermediate layer was considered to be formed from the silicon-enriched stainless steel surface and residual impurities absorbed on the sputtered surfaces of the two components which were bonded. Keywords: Surface activated
bonding;
Interface;
Electron
microscopy;
1. Introduction Surface activated bonding (SAB) [I] is a novel method for the precise joining of dissimilar materials. It is based on the concept that two atomically clean solid surfaces can develop a strong adhesive force between them when they are brought into contact [2]. We have succeeded in bonding a number of combinations of metals, semiconductors, and ceramics [3-51 with this new method. Recently stainless steel and pure aluminum, which are not easy to bond by the conventional diffusion bonding method, have been successfully bonded by this new method [6]. The joints thus obtained have proved to possess a relatively high joint strength after bonding, and to become very weak at the bonding interface after heating to a certain temperature. This unique feature is of particular significance, and eventually leads to the concept of “reversible interconnection” [6]. It is essential to characterize the interface boundary created by the bonding, since the microstructure of the interface will determine, to a large extent, the performance of the interface, which in turn will affect, or even control to a certain extent, the performance of the joints in application to reversible interconnection. This paper presents the
Stainless
results of recent investigations on the interface microstructure of stainless steel/Al joints created by the surface activated bonding method.
2. Experiments The materials used in experiments were pure aluminum (99.99 wt.%) and JIS-SUS304 stainless steel (IKr-8NiISi-ZMn-balanced Fe, wt.%). Preparation of sampIes for bonding and the bonding conditions for the present investigation were the same as described before [6], except that the bonding load applied was larger than before, in order to get a relatively larger bonded area, which made it easier to prepare the specimens for ‘IEM observation. Thin foils for TEM observations were prepared by the conventional ion-beam thinning method. TEM observations and microEDS (Energy Dispersive X-ray Spectroscopy) analysis were carried out on a JEOL-2010 high resolution electron microscope operated at 200 kV.
3. Experimental 3.1.
* Corresponding author. Fax: + 8 l-3-348 l-4489; email: [email protected]. ’ Permanent address: Department of Materials Engineering, Southwest Jiaotong University, Chengdu 6 1003 I, P.R.China. 0168.583X/97/$17.00 Copyright PII SOl68-583X(96)00446-6
steel; Aluminum
results and discussions
Overall morphology
of the interface
Fig. 1 shows the overall view of the cross section of one joint made by the SAB method. The stainless steel and aluminum have been bonded very well. and no macro-defects were observed either by optical or electron micro-
8 1997 Elsevier Science B.V. All rights reserved
VI. NANOSCALE PROCESS/ETCHING
520
L. Yung et al./Nucl. Instr. and Meth. in Phys. Res. B 12I 119971519-523
Fig. 3. High resolution TEM image of the interface boundary in as-bonded joints indicated in Fig. 2, the thickness of the intermediate layer is about 10 nm.
Fig. I. Overall view of the interface boundary in as-bonded stainless steel/aluminum joints created by the surface activated bonding method. (a) Optical and (b) electron microscopy.
Fig. 2. TEM micrographs of the interface boundary in as-bonded stainless steel/aluminum joints; an intermediate layer between stainless steel and aluminum matrix was revealed by means of the selected area diffraction method. (a) Bright field image, (b) dark field image.
scope. The average tensile strength of the joints was about 20 MPa [6]. Under larger magnification, an intermediate layer between the stainless steel and aluminum could be revealed by the selected area diffraction (SAD) method, as shown in Fig. 2, in which (a) is the bright field image, and (b) the dark field image (recorded by the double exposure method) of the stainless steel and aluminum, respectively. This type of intermediate layer is further revealed by means of high resolution transmission electron microscopy (HRTEM) in the next section. 3.2. HRTEM of the interface HFCEM image of the interface area is shown in Fig. 3, in which the lattice images of both stainless steel and aluminum were taken from (11 l]Y_Fe and (I I 18, diffractions, with the specimen being in almost edge on condition. From Fig. 3, it is clear that the stainless steel does not contact directly to the aluminum. Instead, an intermediate layer exists between them. The picture indicates that the thickness of the intermediate layer is about IO nm. It is noted that, though the boundary between the aluminum and the intermediate layer is relatively smooth, the boundary between the stainless steel and the intermediate layer is rather rough, which might be a reflection of the rough surface of the stainless steel prepared by mechanical grinding. In most cases, it was not possible to get the lattice image of both stainless steel and aluminum simultaneously due to inappropriate diffraction orientation. However. the existence of the intermediate layer was sufficiently reproducible.
Fig. 4. (See next page.) Micro-EDS analysis of the intermediate layer and neighboring (a) Aluminum matrix, (b) the intermediate layer, (c) stainless steel matrix.
matrix, taken from 5 nm nominal
illuminated
area.
L. Yang ef ol./Nucl.
Instr. and Merh. in Phys. Res. B 121 (1997) 519-523
As-bmdad
521
SUS/Al-Al-neighbor-to-inter (a)
Al
6000;
5000. C 0 u 4000. n t s 3000-
2000.
1000. 0 o-y -
J h 1
Cr 2
3
4
5
Fe 6
CU 7
8
9
10
As-bonded SUS/Al intermediate layer W
Energy (kev)
As-bonded SUS/Al-SUS-neighbor-to-inter
Energy (kev)
VI. NANOSCALE
PROCESS /ETCHING
522 3.3. Micro-EDS
L. Yang et aI./Nucl. Instr. and Meth. in Phys. Res. B analysis of the intermediate layer
To understand the reasons for the formation of the intermediate layer, micro-EDS analysis was conducted to measure the composition of the intermediate layer and the neighboring matrix. The representative spectra collected from 5 nm nominal illuminated area are shown in Fig. 4a-c. It is indicated that the intermediate layer is mainly composed of silicon, as well as certain amount of oxygen and carbon. Although the exact amount of oxygen and carbon could not be measured by the EDS analysis, the difference between the intermediate layer and the matrix (both stainless steel and aluminum) are quite clear, which in turn could prove the reliability of the micro-EDS method in the present investigation. 3.4. Discussions 3.4.1. The existence of the intermediate layer in joints created by SAB method The existence of the intermediate layer at the interface has been proved throughout the interface, with the specimen in different diffraction conditions. No direct boundary was found. In previous works on other joint couples such as Al/sapphire and Al/Sri created by the surface activated bonding method [3-51, no intermediate layer was observed. For other joints, either a crystalline intermediate layer (e.g. Cu/Sn> or an amorphous layer (e.g. Al/Si and Al/Si,N,) was identified. In all situations, especially for Al/Al joints, the interface morphology was greatly affected by the bonding environments. It is worthwhile to note that an amorphous layer exists at the interface when silicon is involved in the bonded materials. 3.4.2. Formation of the intermediate layer Since the surface activated bonding procedure is conducted entirely at room temperature, and the temperature raise during ion-beam milling in preparing thin foils for TEM is estimated as lower than 300 K, any chemical reaction taking place at the interface can be neglected. It is then reasonable that the presence of the intermediate layer between matrixes in as-bonded joints is the reflection of the surface condition of matrix materials just before bonding. In other words, the intermediate layer should be composed of materials from the surfaces of the two components to be bonded after sputtering. Accordingly, the key point is to find out the sources of silicon as well as oxygen and carbon on the surface of the two components to be bonded. It is considered that the oxygen and carbon were introduced through absorption on the fresh surface of both stainless steel and aluminum, since residual impurities (the most part being water and the rest H, CO and CO, [5]) necessarily existed in the sputtering atmosphere. Although”‘it might be possible that tliey were introduced during ion-beam milling, clear difference
I21 (1997) 519-523
between the intermediate layer and matrixes in peak strength of both carbon and oxygen suggests that the absorption should be at least partly responsible for the oxygen and carbon inside the intermediate layer. Since the aluminum used in the present investigation is of high purity, only the stainless steel could be the possible source of silicon. It is worthwhile to notice that, the sputtering rate of silicon during fast atom beam bombardment (FAB) is much slower than that of iron, chromium, and nickel [7]. This means that after FAB and before bonding, the surface of stainless steel would be greatly enriched with silicon. With this point in mind, the enriched silicon inside the intermediate layer in the as-bonded state is thus acceptable. Therefore, it is this Si-enriched surface, combined with the absorption of the sputtered fresh surface, that constitutes the intermediate layer after they were bonded together. This argument also agrees with the previous results obtained from different joints by the same method, in which the existence of an intermediate layer was always accompanied by the presence of silicon as one of the component or compound of silicon such as Si3N,, as mentioned in Section 3.4.1.
4. Conclusions The interface microstructure of the stainless steel/ aluminum joints made by the surface activated bonding method has been studied. Whereas the bonded interface was perfect, an intermediate layer of about IO nm in thickness was found between stainless steel and aluminum in the as-bonded joints. The intermediate layer is composed of mainly silicon and certain amount of oxygen and carbon identified by micro-EDS. The intermediate layer is considered. to be formed from the silicon enriched surface of the stainless steel due to the different sputtering rate for different elements and residual impurities absorbed on the fresh surfaces in the sputtering atmosphere.
Acknowledgements The authors would like to express their appreciation to Mr. Yoshitaka Kyougoku for his kind help in preparing the stainless steel/aluminum joints by the surface activated bonding method.
References
[II T. Suga and K. Miyazawa, Acta Ser. Metall. (1990)
Proc.
Ser.
4
189.
121 K. Miyoshi and D.H.Buckley, Wear 77 (1982) 23. [3] S. Sacre and T.Suga, Trans. Mater. Res. Sot. Japan 16 B (1994) 1201.
L. Yung et uI./Nucl.
Instr. and Mrth. in Phys. Res. B I21 (1997) S/9-523
[4] T. Suga, T. Fujiwaka and G. Sasaki, Proc. 9th Europ. Hybrid Microelectronics Conf. (1993) 323. [5] T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch and G.Elssner, Acta Metall. Mater. 40 (SuppI). (1992) S133. [6] Y. Kyougoku, L. Yang, N. Hosoda and T. Suga, in: Proc. 2nd
523
Symp. on Microjoining and Assembly Technology in Electronics (Japan Welding Society, 1996) p. 233. [7] A. Kinbara, in: Sputtering Phenomenon (Tokyo University Press, Tokyo, 1985) p. IS.
VI. NANOSCALE
PROCESS/ETCHING
and Methods in Physics Research
B 121 (1997) 519-523
Beam Interactions with Materials 8 Atoms
ELSEVIER
TEM investigation of the stainless steel/aluminum interface created by the surface activated bonding method Liu Yang *,l, Naoe Hosoda, Tadatama Suga RCAST, The University of Tokyo. Komaha 4-6-l. Meguro-ku. 153 Tohyo. Japan
Abstract Stainless steel and aluminum were bonded successfully by means of the Surface Activated Bonding (SAB) method at room temperature. High-resolution electron microscopy and micro-EDS were applied to investigate the morphology of the interface thus created. In the as-bonded joints, an intermediate layer of about 10 nm thick was observed between the stainless steel and aluminum. The intermediate layer was composed of mainly silicon and certain amounts of oxygen and carbon according to the micro-EDS analysis. The intermediate layer was considered to be formed from the silicon-enriched stainless steel surface and residual impurities absorbed on the sputtered surfaces of the two components which were bonded. Keywords: Surface activated
bonding;
Interface;
Electron
microscopy;
1. Introduction Surface activated bonding (SAB) [I] is a novel method for the precise joining of dissimilar materials. It is based on the concept that two atomically clean solid surfaces can develop a strong adhesive force between them when they are brought into contact [2]. We have succeeded in bonding a number of combinations of metals, semiconductors, and ceramics [3-51 with this new method. Recently stainless steel and pure aluminum, which are not easy to bond by the conventional diffusion bonding method, have been successfully bonded by this new method [6]. The joints thus obtained have proved to possess a relatively high joint strength after bonding, and to become very weak at the bonding interface after heating to a certain temperature. This unique feature is of particular significance, and eventually leads to the concept of “reversible interconnection” [6]. It is essential to characterize the interface boundary created by the bonding, since the microstructure of the interface will determine, to a large extent, the performance of the interface, which in turn will affect, or even control to a certain extent, the performance of the joints in application to reversible interconnection. This paper presents the
Stainless
results of recent investigations on the interface microstructure of stainless steel/Al joints created by the surface activated bonding method.
2. Experiments The materials used in experiments were pure aluminum (99.99 wt.%) and JIS-SUS304 stainless steel (IKr-8NiISi-ZMn-balanced Fe, wt.%). Preparation of sampIes for bonding and the bonding conditions for the present investigation were the same as described before [6], except that the bonding load applied was larger than before, in order to get a relatively larger bonded area, which made it easier to prepare the specimens for ‘IEM observation. Thin foils for TEM observations were prepared by the conventional ion-beam thinning method. TEM observations and microEDS (Energy Dispersive X-ray Spectroscopy) analysis were carried out on a JEOL-2010 high resolution electron microscope operated at 200 kV.
3. Experimental 3.1.
* Corresponding author. Fax: + 8 l-3-348 l-4489; email: [email protected]. ’ Permanent address: Department of Materials Engineering, Southwest Jiaotong University, Chengdu 6 1003 I, P.R.China. 0168.583X/97/$17.00 Copyright PII SOl68-583X(96)00446-6
steel; Aluminum
results and discussions
Overall morphology
of the interface
Fig. 1 shows the overall view of the cross section of one joint made by the SAB method. The stainless steel and aluminum have been bonded very well. and no macro-defects were observed either by optical or electron micro-
8 1997 Elsevier Science B.V. All rights reserved
VI. NANOSCALE PROCESS/ETCHING
520
L. Yung et al./Nucl. Instr. and Meth. in Phys. Res. B 12I 119971519-523
Fig. 3. High resolution TEM image of the interface boundary in as-bonded joints indicated in Fig. 2, the thickness of the intermediate layer is about 10 nm.
Fig. I. Overall view of the interface boundary in as-bonded stainless steel/aluminum joints created by the surface activated bonding method. (a) Optical and (b) electron microscopy.
Fig. 2. TEM micrographs of the interface boundary in as-bonded stainless steel/aluminum joints; an intermediate layer between stainless steel and aluminum matrix was revealed by means of the selected area diffraction method. (a) Bright field image, (b) dark field image.
scope. The average tensile strength of the joints was about 20 MPa [6]. Under larger magnification, an intermediate layer between the stainless steel and aluminum could be revealed by the selected area diffraction (SAD) method, as shown in Fig. 2, in which (a) is the bright field image, and (b) the dark field image (recorded by the double exposure method) of the stainless steel and aluminum, respectively. This type of intermediate layer is further revealed by means of high resolution transmission electron microscopy (HRTEM) in the next section. 3.2. HRTEM of the interface HFCEM image of the interface area is shown in Fig. 3, in which the lattice images of both stainless steel and aluminum were taken from (11 l]Y_Fe and (I I 18, diffractions, with the specimen being in almost edge on condition. From Fig. 3, it is clear that the stainless steel does not contact directly to the aluminum. Instead, an intermediate layer exists between them. The picture indicates that the thickness of the intermediate layer is about IO nm. It is noted that, though the boundary between the aluminum and the intermediate layer is relatively smooth, the boundary between the stainless steel and the intermediate layer is rather rough, which might be a reflection of the rough surface of the stainless steel prepared by mechanical grinding. In most cases, it was not possible to get the lattice image of both stainless steel and aluminum simultaneously due to inappropriate diffraction orientation. However. the existence of the intermediate layer was sufficiently reproducible.
Fig. 4. (See next page.) Micro-EDS analysis of the intermediate layer and neighboring (a) Aluminum matrix, (b) the intermediate layer, (c) stainless steel matrix.
matrix, taken from 5 nm nominal
illuminated
area.
L. Yang ef ol./Nucl.
Instr. and Merh. in Phys. Res. B 121 (1997) 519-523
As-bmdad
521
SUS/Al-Al-neighbor-to-inter (a)
Al
6000;
5000. C 0 u 4000. n t s 3000-
2000.
1000. 0 o-y -
J h 1
Cr 2
3
4
5
Fe 6
CU 7
8
9
10
As-bonded SUS/Al intermediate layer W
Energy (kev)
As-bonded SUS/Al-SUS-neighbor-to-inter
Energy (kev)
VI. NANOSCALE
PROCESS /ETCHING
522 3.3. Micro-EDS
L. Yang et aI./Nucl. Instr. and Meth. in Phys. Res. B analysis of the intermediate layer
To understand the reasons for the formation of the intermediate layer, micro-EDS analysis was conducted to measure the composition of the intermediate layer and the neighboring matrix. The representative spectra collected from 5 nm nominal illuminated area are shown in Fig. 4a-c. It is indicated that the intermediate layer is mainly composed of silicon, as well as certain amount of oxygen and carbon. Although the exact amount of oxygen and carbon could not be measured by the EDS analysis, the difference between the intermediate layer and the matrix (both stainless steel and aluminum) are quite clear, which in turn could prove the reliability of the micro-EDS method in the present investigation. 3.4. Discussions 3.4.1. The existence of the intermediate layer in joints created by SAB method The existence of the intermediate layer at the interface has been proved throughout the interface, with the specimen in different diffraction conditions. No direct boundary was found. In previous works on other joint couples such as Al/sapphire and Al/Sri created by the surface activated bonding method [3-51, no intermediate layer was observed. For other joints, either a crystalline intermediate layer (e.g. Cu/Sn> or an amorphous layer (e.g. Al/Si and Al/Si,N,) was identified. In all situations, especially for Al/Al joints, the interface morphology was greatly affected by the bonding environments. It is worthwhile to note that an amorphous layer exists at the interface when silicon is involved in the bonded materials. 3.4.2. Formation of the intermediate layer Since the surface activated bonding procedure is conducted entirely at room temperature, and the temperature raise during ion-beam milling in preparing thin foils for TEM is estimated as lower than 300 K, any chemical reaction taking place at the interface can be neglected. It is then reasonable that the presence of the intermediate layer between matrixes in as-bonded joints is the reflection of the surface condition of matrix materials just before bonding. In other words, the intermediate layer should be composed of materials from the surfaces of the two components to be bonded after sputtering. Accordingly, the key point is to find out the sources of silicon as well as oxygen and carbon on the surface of the two components to be bonded. It is considered that the oxygen and carbon were introduced through absorption on the fresh surface of both stainless steel and aluminum, since residual impurities (the most part being water and the rest H, CO and CO, [5]) necessarily existed in the sputtering atmosphere. Although”‘it might be possible that tliey were introduced during ion-beam milling, clear difference
I21 (1997) 519-523
between the intermediate layer and matrixes in peak strength of both carbon and oxygen suggests that the absorption should be at least partly responsible for the oxygen and carbon inside the intermediate layer. Since the aluminum used in the present investigation is of high purity, only the stainless steel could be the possible source of silicon. It is worthwhile to notice that, the sputtering rate of silicon during fast atom beam bombardment (FAB) is much slower than that of iron, chromium, and nickel [7]. This means that after FAB and before bonding, the surface of stainless steel would be greatly enriched with silicon. With this point in mind, the enriched silicon inside the intermediate layer in the as-bonded state is thus acceptable. Therefore, it is this Si-enriched surface, combined with the absorption of the sputtered fresh surface, that constitutes the intermediate layer after they were bonded together. This argument also agrees with the previous results obtained from different joints by the same method, in which the existence of an intermediate layer was always accompanied by the presence of silicon as one of the component or compound of silicon such as Si3N,, as mentioned in Section 3.4.1.
4. Conclusions The interface microstructure of the stainless steel/ aluminum joints made by the surface activated bonding method has been studied. Whereas the bonded interface was perfect, an intermediate layer of about IO nm in thickness was found between stainless steel and aluminum in the as-bonded joints. The intermediate layer is composed of mainly silicon and certain amount of oxygen and carbon identified by micro-EDS. The intermediate layer is considered. to be formed from the silicon enriched surface of the stainless steel due to the different sputtering rate for different elements and residual impurities absorbed on the fresh surfaces in the sputtering atmosphere.
Acknowledgements The authors would like to express their appreciation to Mr. Yoshitaka Kyougoku for his kind help in preparing the stainless steel/aluminum joints by the surface activated bonding method.
References
[II T. Suga and K. Miyazawa, Acta Ser. Metall. (1990)
Proc.
Ser.
4
189.
121 K. Miyoshi and D.H.Buckley, Wear 77 (1982) 23. [3] S. Sacre and T.Suga, Trans. Mater. Res. Sot. Japan 16 B (1994) 1201.
L. Yung et uI./Nucl.
Instr. and Mrth. in Phys. Res. B I21 (1997) S/9-523
[4] T. Suga, T. Fujiwaka and G. Sasaki, Proc. 9th Europ. Hybrid Microelectronics Conf. (1993) 323. [5] T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch and G.Elssner, Acta Metall. Mater. 40 (SuppI). (1992) S133. [6] Y. Kyougoku, L. Yang, N. Hosoda and T. Suga, in: Proc. 2nd
523
Symp. on Microjoining and Assembly Technology in Electronics (Japan Welding Society, 1996) p. 233. [7] A. Kinbara, in: Sputtering Phenomenon (Tokyo University Press, Tokyo, 1985) p. IS.
VI. NANOSCALE
PROCESS/ETCHING