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Bonding of Al to Al2O3 via Al–Cu eutectic method
Materials and Design 87 (2015) 619–624
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Bonding of Al to Al2O3 via Al–Cu eutectic method Pengfei Zhang 1, Jun Fang 1, Renli Fu ⁎, Xiguang Gu, Meng Fei College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e
i n f o
Article history: Received 19 May 2015 Received in revised form 12 August 2015 Accepted 14 August 2015 Available online 19 August 2015 Keywords: DAB substrate Al–Cu eutectic KF–AlF3 flux Diffusion Thermal properties
a b s t r a c t The Al oxidation layer in the manufactures of direct aluminum bonded Al2O3 substrates (DAB) has been a longterm trouble for industries. In this work we propose a new method for fabricating the DAB substrates with no requirement of high vacuum or active O2-getters. The new method comprises two stages: (i) Cu-film is bonded onto Al2O3 ceramic surface via DBC method; (ii) Al foil is joined to the DBC substrate by Al–Cu eutectic method at 600 °C in pure N2 atmosphere. KF–AlF3 flux was used to disrupt the Al–oxide layer on the surface of Al foil. The wetting ability was significantly enhanced due to the diffusion of Cu into Al and the dissolving of Al. The final contact angle is achieved of 22.10°. Microstructure and composition of the interface between Al and Al2O3 substrate were analyzed. The XRD, SEM and EDS results show that two new phases Al2Cu and CuAlO2 were formed, leading to a strong bonding along the interface. The thermal cycling reliability and adhesion strength of DAB substrates were also evaluated. The results show that the DAB substrates can satisfy application requirements completely. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Strong and stable joint between Al2O3 and Al remains a difficult technological challenge although they are eagerly required in numerous advanced applications, especially in the field of high power semiconductor devices [1,2]. With the broad use of those high power semiconductor devices [1–4], such as silicon controlled rectifiers (SCR), power transistors, insulated gate bipolar transistors (IGBT), metal oxide semiconductor field effect transistors (MOSFET), power rectifier and power regulators, researchers are highly motivated and stimulated to develop a new method for fabricating high quality Al2O3/Al joints [1,2,5]. In the field of high power electronic circuits and electronic components, direct aluminum bonded (DAB) substrates have been proposed as superior alternatives of the conventional direct bonded copper (DBC) substrates [1,2]. Comparing with DBC substrates [2], DAB substrates have more stably thermal cycling reliability [3], thus leading to longer life-time devices [6,7]. In conjunction with strong adhesion at the ceramic/metal interface, DAB substrates feature excellent thermal fatigue resistance, thermal stability, and low weight [8–10]. In DAB substrates, the strength of the ceramic/metal joint is a crucially important parameter and is mainly determined by the solidceramic/liquid-metal wettability [11–14]. With regard to the affinity at ⁎ Corresponding author. E-mail address: [email protected] (R. Fu). 1 These authors contributed equally to this work and should be considered co-first authors.
http://dx.doi.org/10.1016/j.matdes.2015.08.065 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
the ceramic/metal interface, the Al2O3/Al compositions seem to be an ideal system as Al exists in both metallic and ceramic phases [11]. On the other hand, it has been widely reported that the easy oxidation of pure Al results in an Al-oxide layer, which in turn acts as a barrier and impedes the establishment of an intimate Al2O3/Al interface [9]. Therefore, improving the wettability at the ceramic/metal interface is an urgent task and triggers an increasing research interest in Al2O3/Al system [15,16]. Wettability in Al2O3/Al system can be improved in many ways [9,17, 18]. One is by incorporating O2-getters. The O2-getters are required to have higher chemical affinity with oxygen, e.g. Ti [10,19], thus inhibiting the oxidation of the pure Al and the formation of the Al-oxide layer. In this way, the pure Al metal can directly wet the surface of Al2O3 and forms strong and stable joints along the ceramic/metal interface. Another way out is to apply elevated temperatures and high vacuum or highly inert atmosphere to disrupt the readily formed Al-oxide layer on the surface of pure Al metal [1,8–10,20]. Both the aforementioned methods require highly reactive O2-getters, or high vacuum/highly inert atmosphere [21,22]. They can be achieved by facilitating expensive and complex equipment. Hence, those approaches are attractive only in laboratory and small-scale production. For large industrial scale production, the poor wettability has to be tackled in a more feasible way, both effectively and economically, as long as the fabrication conditions and equipment are in concern. In this paper, we attempt a new method to fabricate DAB substrates. The new method comprises two stages. At the first stage, the Cu-film is bonded onto Al2O3 ceramic surface via DBC method [23]. Afterwards, the Al foil (1 mm) is joined to the DBC substrate by Al–Cu eutectic
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P. Zhang et al. / Materials and Design 87 (2015) 619–624 Table 1 Composition (wt.%) of Cu2O paste. Materials
wt.%
Cu2O powder Alcohol Castor oil Dibutyl phthalate (DBP) Polyethylene glycol (PEG) Polyvinyl butyral (PVB)
60.6 36.4 0.9 0.6 0.9 0.6
method [24] at 600 °C in pure N2 atmosphere. The thermal shock resistance property of the resultant joints was given in this paper to show out the effectiveness of this method. 2. Materials and experimental procedure Highly pure Al foils (N99%, 1 mm thick, Sinopharm Co. Ltd, China), Cu foils (N99%, b0.1 mm thick, Sinopharm Co. Ltd, China), Al2O3 substrates in prism form (N 96%, porosity b 1%, length 20 mm, width 10 mm and thickness 0.7 mm, Yueke Jinghua Electronic Ceramics Co. Ltd, China), KF–AlF3 flux (Sinopharm Co. Ltd, China) and a Cu2O fine powder (44 μm, Shanghai Sinpeuo Fine Chemical Co. Ltd, China) were used. The Cu2O fine powder was dispersed in anethyl alcohol media mixed with dispersant, plasticizer, and adhesive agents. The ratio between ethyl alcohol and the agents is given in Table 1. To avoid the dirty residues on the surface, the substances were cleaned with pure acetone in ultrasonic bath for 30 min at room temperature. The fabricating procedure is as follows: Firstly, a Cu2O-paste was pasted on the surface of the Al2O3 substrate. The pasted-ceramic substrate was pre-thermally treated in air at 1150 °C for 60 min and slowly cooled down to room temperature. Then the Cu-foil was intimately placed on the pasted-ceramic substrate and the whole system was heat treated in a tube furnace under reduction atmosphere (10% H2/90% N2). The second heat treatment profile was as follows: Temperature increased up to 1000 °C with a heating rate of 10 K/min and maintained for 10 min. Then it increased up to the eutectic ([Cu + Cu2O]) temperature of 1070 °C with 2 K/min and maintained for 90 min. Afterwards the sample was slowly cooled down to room temperature with a rate of 2 K/min. The surfaces of the Cu-foil (already adjoined onto the Al2O3substrate) and the Al-foil were cleaned and degreased with the aid of KF–AlF3 flux after getting rid of the oxide layers. The Al-foil was then tightly assembled on the Cu-surface. The sample was placed into a tube-furnace under pure N2-atmosphere (N99.99%) and finally heattreated at 600 °C for 30 min with a rate of 10 K/min followed by a slow cooling process. The characterization of the produced joints was performed on different apparatus for different purposes. Phase crystallographic analysis was performed by X-ray Diffraction (XRD, Bruker D8 Advance, CuKα,
Fig. 2. X-ray diffractogram of the bonding area.
40 kV). Microstructure observations were carried out on polished cross-sections of ceramic/metal interfaces with the aid of Scanning Electron Microscope (SEM, Quanta 200, FEI), equipped with Energy Dispersive Spectroscopy (EDS) apparatus for element analysis. An optical microscope (9XB-PC) was employed as well. For the thermal shock resistance test, the joints were heated up to 500 °C with a rate of 20 K/s and kept for 10 min. Then, the joints were immediately quenched into room-temperature water. After undergoing extreme temperature shocks, substrates themselves may show delamination of the metal layers from the ceramic. If this phenomenon occurs on the DAB substrates, we regard the substrates as having become failed. The process was repeatedly cycled until the joint failed. For the sake of statistics, 20 samples were prepared and tested in the same way.
3. Results and discussion 3.1. The aluminum bonding process With our new fabricating method, the bonding of Al to the Al2O3substrate is realized mainly with two steps: i) the Al2O3 surface is treated with Cu2O and then bonded with Cu through Cu2O–Cu eutectic reaction, resulting with a thin layer of copper bonded on the substrate; ii) the bonding of Al is achieved with the assistance of the interlayer Cu through Cu–Al eutectic process. The Cu-coated Al2O3 substrate and Al boned Al2O3 substrates are shown in Fig. 1. We can see that the continuous copper layer is formed on the surface of Al2O3 ceramic substrate and the aluminum foil boded with Al2O3 substrate trough Al-Cu eutectic succeeds. Fig. 2 shows the X-ray diffraction pattern taken from the side face of the sample. Beyond the initial phases involved in the joint,
Fig. 1. A SEM image of Cu-coated Al2O3 substrate and a photograph of Al boned Al2O3 substrates.
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pattern in Fig. 2, intermetallic compound consists of Al2Cu in the aluminum is convinced. Additionally, through EDS analysis no evidence of oxygen was found in the bulk metal phase. Therefore, it is suggested that the Al2Cu phase in the bulk aluminum and at the interface is formed by the diffusion of the bonded copper to the aluminum side, since the CuAlO2 phase remains adjacent to the Al2O3 substrate. Thus, it can be concluded that the bonding of aluminum to the substrate is accomplished by the diffusion of Cu to Al, and this leads to the formation of Al2Cu and the eutectic reaction between Al and Cu at the interface.
3.2. Al wettability on copper-coated Al2O3 substrates
Fig. 3. A cross-section image of the Cu-coated Al2O3 joint.
specifically Al2O3, Al and Cu, two more phases CuAlO2 and Al2Cu were formed during the fabricating process. Initially, the thin layer of copper (less than 100 μm) was bonded to the inchoative surface of Al2O3 ceramic with the aid of the Cu2O paste and afterward thermal treating [23]. At first, a chemical interface was formed through the reaction between the Al2O3 ceramic and the Cu2O at 1150 °C, which resulted in the formation of CuAlO2 (as presented in Fig. 2). In addition, the thickness of CuAlO2 is about 10 μm, as shown in Fig. 3. The reaction can be simplified as: Cu2O + Al2O3 = 2CuAlO2. In order to convincingly identify the composition and morphology of intermediate phases, the Al2O3 substrates were dissolved in HNO3, as shown in Fig. 4. We can see that a continuous laminates is deposit at the interface of Cu-coated Al2O3 substrate. EDS analyses indicate that the composition of these laminates is close to 1:1:2 (Fig. 5). Combining the XRD analyses make it possible to identify that CuAlO2 exists at the interface. The copper bonding was realized by the eutectic reaction between Cu2O and Cu. In the bonding process at 1070 °C, the pure copper only engaged in the eutectic reaction, and it transformed from the eutectic liquid to the eutectic compound when the sample cooled down. The aluminum bonding was carried out on the previously copper bonded substrate and heat treated at 600 °C. Fig. 6 shows a typical SEM image of the resultant aluminum bonded sample. A clear bonding interface was observed between the aluminum substrate and the metal Al, as shown in Fig. 6(a). In Fig. 6(b), big Al2Cu clusters, determined from EDS spot-analysis (as shown in Fig. 7), were observed to disperse in the Al body and along the interface. Combing the XRD
Fig. 4. A SEM image of interface in the Cu-coated Al2O3 substrate.
During the aluminum bonding process, the pre-bonded copper foil plays an important role and improves the wettability of aluminum to the alumina ceramic substrate. In Fig. 6(a), it is clearly visible that a thin film was formed at the ceramic/metal interface. The aluminum liquid, which is formed during the Cu–Al eutectic process, lies on the reaction zone but not directly on the solid substrate of Al2O3. The absence of the Cu layer on the alumina surface suggests that wetting of Al2O3 is achieved through chemical reaction and copper diffusion into Al. In order to study the wetting behavior of Al on Cu pre-bonded Al2O3 substrate, a small piece of bulk Al was introduced on top of the Cu pre-bonded Al2O3 substrate. After heating up to 800 °C for 2 min, the bulk Al melts and spreads on the substrate. Figs. 8 and 9 show the wetting behavior of the surface and cross-section of aluminum on prebonded copper Al2O3 substrate. It turns out that the contact angle of the wetting regime is only about 22.10°, much lower than that reported with the traditional bonding methods giving 130° ~ 140° [24,25]. Therefore we can doubtlessly conclude that with our proposed method, the Al wettability in Al/Al2O3 system can be tremendously improved. The images in Fig. 6 clearly show the liquid Al was formed in-situ on Cu-coated alumina substrate at 600 °C. Therefore, the melting of aluminum on the surface and the diffusion of the pre-bonded Cu into the bulk aluminum, resulting in the chemical formation of Al2Cu, contributed to the wetting of Al on the alumina ceramic. Similar phenomenon has been reported and discussed by León et al. [26]. They showed the contact angles immediately decreased to 12.6° within two minutes and the improved wetting was mainly attributed to the dissolution of the copper when in contact with the liquid aluminum drop. As mentioned above, the diffusion of copper to the aluminum metal and the dissolving of aluminum cooperate in improving the wetting. The dissolved aluminum is consumed during the reaction and diffusion of copper. Comparing with the results by León et al. [26], it is found that the Cu–Al contact angle (~ 22.10°) after holding for 30 min in the present work was higher, implying a de-wetting period of the liquid
Fig. 5. EDS analysis of interface in the Cu-coated Al2O3 substrate.
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P. Zhang et al. / Materials and Design 87 (2015) 619–624
Fig. 6. Microstructure of the Al/Al2O3 interface after heat-treatment at 600 °C. The SEM images were taken from the transverse cross section of the samples. The scale in panel (a) is 500 μm, and it was enlarged to 100 μm for the interface area in panel (b).
The advancing front is locally saturated with copper, retarding the dissolution process. Accordingly, wetting initially occurred on copper, which becomes the drive for the spreading of the dissolved aluminum. Therefore, the wetting front retreats to reach a new but higher equilibrium contact angle. 3.3. The effect of KF–AlF3
Fig. 7. EDS analysis of intermetallic compound in the aluminum layer.
from the ceramic surface. Diffusion should be unsteady, since the metal liquid was continuously dissolved from the aluminum surface until the point that it was completely consumed in the surrounding area on the triple line. Moreover, the significant partial enthalpy of mixing of the Al–Cu liquid system, H = − 28 J/mol, leads to the formation of the low melting eutectic composition (548 °C) in the neighborhood on the interface, which could increase the fluidity of the aluminum. It is assumed that the reaction proceeds at an appreciable rate only on the contact line, where the liquid has direct access to the metallic film.
In the present study, the KF–AlF3 flux was used mainly to disrupt Al– oxide layer on the surface. The good wetting regime (Figs. 8 and 9) suggests the absence of the superficial Al–oxide layer on the surface of liquid Al. Assuming that a good wetting and the chemical reaction interface are the key factors for a good bonding strength and long-term stability of the joint, our novel processing method secures the elimination of the presence of the Al–oxide layer during brazing process at temperatures very close to the Al melting point. At this temperature, wetting angles of the Al2O3/Al system are often reported as high, 130°–140° [24,25], precisely due to the presence of that Al-oxide superficial film. At low partial pressure of oxygen and elevated temperatures, Aloxide layer can be disrupted according to the chemical Eq. (1), whereby the liquid Al actually reduces the oxide film resulting in the gaseous sub-oxide Al2O: 4AlðlÞ þ Al2 O3 ðsÞ→3Al2 OðgÞ:
ð1Þ
Nevertheless, 600 °C is seemingly to be low in order to ensure complete reduction of the oxide film on the aluminum surface. Therefore, the absence of the Al–oxide layer in the current study is attributed to the treatment of the KF–AlF3 flux. Brazing took place and disrupted
Fig. 8. The surface of (a) before and (b) after aluminum wetting on pre-bonded copper Al2O3 substrate.
P. Zhang et al. / Materials and Design 87 (2015) 619–624
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Fig. 9. The cross-section of aluminum wetting on pre-bonded copper Al2O3 substrate. Fig. 11. A image of thermal cycling reliability and adhesion strength for DAB substrates.
the Al–oxide layer as it reacted with the species of F−, AlF3− and AlF4− 6 derived from KF–AlF3 and form AlOF2 − or AlO2 − according to the chemical Eqs. (2)–(4): 2−
Al2 O3 þ AlF 6 3− →3AlOF Al2 O3 þ 2F− þ AlF
4−
2−
2AlOF
2−
→AlO
ð2Þ 2−
→3AlOF 4−
þ AlF
:
ð3Þ ð4Þ
Accordingly, the whole process should take place via the following two stages: (i) dissolution of the superficial Al-oxide film due to the treatment with KF–AlF3 flux; (ii) reaction of Al with Cu and Al2O3, which eventually results in a good wetting regime and a strong interface.
adhesive strength depends on the thickness of CuAlO2 as the intermediate layer. Sufficient adhesive strength can be achieved only above a certain thickness of the intermediate layer. However, the effect of Al2Cu on the strength of DAB substrate is not clear at present. Further experimental study is necessary to elucidate its effect. The thermal shock tests results indicate that excellent bonding was achieved in the DAB substrates produced with our new fabrication method. Furthermore, Comparing with commercial DAB substrates, thermal cycling reliability of our DAB substrates is much more stable. This is important in the light of the experimental conditions. The new fabrication method requires no use of high vacuum conditions but just pure flown N2 and relatively low temperature (600 °C). Moreover, the aspect of the reaction zone suggests a rather non-brittle phase is formed since no coarse crystals or holes are formed at the interface. 4. Conclusions
3.4. Thermal shock resistance Due to the structure of DAB substrates, tensile tests are used to measure the adhesion strength, as shown in Fig. 10. The results of thermal shock resistance tests and adhesion tests are shown in Fig. 11. It can be seen that the average value of adhesion strength for all samples is more than 12 MPa. Moreover, there is no dramatic change of these values. Meanwhile, it is also obviously shown that the DAB substrates successfully passed in average 110 thermal shock cycles. Both of them indicate an excellent bonding was obtained in all DAB samples. The good bonding strength of our DAB samples should be attributed to two aspects: i) the formation of the CuAlO2 chemical interface, ii) the good wetting of Al to the substrate. Some workers have also reported the requirement of CuAlO2 for high adhesive strength between Cu film and Al2O3 substrate. For instance, Kim et al. [27] has reported that
A new method for fabricating DAB substrate is proposed, and the new fabrication method produces DAB substrates with excellent bonding property, which will have huge potentials in power electronic packaging applications. (1) Significant improvement in the wetting of aluminum was achieved by applying a coating of Cu on Al2O3 ceramic. It is mainly due to the Cu diffusion into the aluminum and the dissolving of aluminum. Once the copper is completely removed from the ceramic surface, the final contact angle reaches 22.10°. This value is substantially lower than those for pure aluminum on uncoated alumina ceramics (130° ~ 140°). (2) At relatively low temperature (600 °C), the superficial Al-oxide film is disrupted by the treatment with KF–AlF3 flux, which is different from the traditional conditions, i.e. high vacuum conditions and high temperatures. (3) Thermal shock test results showed that the samples successfully passed 110 thermal shock cycles. The average value of adhesion strength for all samples is more than 12 MPa. The performance of thermal cycling reliability is more stable than commercial DBA substrates. The excellent bonding strength was mainly attributed to the chemical interaction interface layer of CuAlO2 and the improved wetting of aluminum to the alumina substrate.
Acknowledgments
Fig. 10. The sample and tensile test used.
The author gratefully acknowledges a project Funded by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Nanjing, China.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Bonding of Al to Al2O3 via Al–Cu eutectic method Pengfei Zhang 1, Jun Fang 1, Renli Fu ⁎, Xiguang Gu, Meng Fei College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
a r t i c l e
i n f o
Article history: Received 19 May 2015 Received in revised form 12 August 2015 Accepted 14 August 2015 Available online 19 August 2015 Keywords: DAB substrate Al–Cu eutectic KF–AlF3 flux Diffusion Thermal properties
a b s t r a c t The Al oxidation layer in the manufactures of direct aluminum bonded Al2O3 substrates (DAB) has been a longterm trouble for industries. In this work we propose a new method for fabricating the DAB substrates with no requirement of high vacuum or active O2-getters. The new method comprises two stages: (i) Cu-film is bonded onto Al2O3 ceramic surface via DBC method; (ii) Al foil is joined to the DBC substrate by Al–Cu eutectic method at 600 °C in pure N2 atmosphere. KF–AlF3 flux was used to disrupt the Al–oxide layer on the surface of Al foil. The wetting ability was significantly enhanced due to the diffusion of Cu into Al and the dissolving of Al. The final contact angle is achieved of 22.10°. Microstructure and composition of the interface between Al and Al2O3 substrate were analyzed. The XRD, SEM and EDS results show that two new phases Al2Cu and CuAlO2 were formed, leading to a strong bonding along the interface. The thermal cycling reliability and adhesion strength of DAB substrates were also evaluated. The results show that the DAB substrates can satisfy application requirements completely. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Strong and stable joint between Al2O3 and Al remains a difficult technological challenge although they are eagerly required in numerous advanced applications, especially in the field of high power semiconductor devices [1,2]. With the broad use of those high power semiconductor devices [1–4], such as silicon controlled rectifiers (SCR), power transistors, insulated gate bipolar transistors (IGBT), metal oxide semiconductor field effect transistors (MOSFET), power rectifier and power regulators, researchers are highly motivated and stimulated to develop a new method for fabricating high quality Al2O3/Al joints [1,2,5]. In the field of high power electronic circuits and electronic components, direct aluminum bonded (DAB) substrates have been proposed as superior alternatives of the conventional direct bonded copper (DBC) substrates [1,2]. Comparing with DBC substrates [2], DAB substrates have more stably thermal cycling reliability [3], thus leading to longer life-time devices [6,7]. In conjunction with strong adhesion at the ceramic/metal interface, DAB substrates feature excellent thermal fatigue resistance, thermal stability, and low weight [8–10]. In DAB substrates, the strength of the ceramic/metal joint is a crucially important parameter and is mainly determined by the solidceramic/liquid-metal wettability [11–14]. With regard to the affinity at ⁎ Corresponding author. E-mail address: [email protected] (R. Fu). 1 These authors contributed equally to this work and should be considered co-first authors.
http://dx.doi.org/10.1016/j.matdes.2015.08.065 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
the ceramic/metal interface, the Al2O3/Al compositions seem to be an ideal system as Al exists in both metallic and ceramic phases [11]. On the other hand, it has been widely reported that the easy oxidation of pure Al results in an Al-oxide layer, which in turn acts as a barrier and impedes the establishment of an intimate Al2O3/Al interface [9]. Therefore, improving the wettability at the ceramic/metal interface is an urgent task and triggers an increasing research interest in Al2O3/Al system [15,16]. Wettability in Al2O3/Al system can be improved in many ways [9,17, 18]. One is by incorporating O2-getters. The O2-getters are required to have higher chemical affinity with oxygen, e.g. Ti [10,19], thus inhibiting the oxidation of the pure Al and the formation of the Al-oxide layer. In this way, the pure Al metal can directly wet the surface of Al2O3 and forms strong and stable joints along the ceramic/metal interface. Another way out is to apply elevated temperatures and high vacuum or highly inert atmosphere to disrupt the readily formed Al-oxide layer on the surface of pure Al metal [1,8–10,20]. Both the aforementioned methods require highly reactive O2-getters, or high vacuum/highly inert atmosphere [21,22]. They can be achieved by facilitating expensive and complex equipment. Hence, those approaches are attractive only in laboratory and small-scale production. For large industrial scale production, the poor wettability has to be tackled in a more feasible way, both effectively and economically, as long as the fabrication conditions and equipment are in concern. In this paper, we attempt a new method to fabricate DAB substrates. The new method comprises two stages. At the first stage, the Cu-film is bonded onto Al2O3 ceramic surface via DBC method [23]. Afterwards, the Al foil (1 mm) is joined to the DBC substrate by Al–Cu eutectic
620
P. Zhang et al. / Materials and Design 87 (2015) 619–624 Table 1 Composition (wt.%) of Cu2O paste. Materials
wt.%
Cu2O powder Alcohol Castor oil Dibutyl phthalate (DBP) Polyethylene glycol (PEG) Polyvinyl butyral (PVB)
60.6 36.4 0.9 0.6 0.9 0.6
method [24] at 600 °C in pure N2 atmosphere. The thermal shock resistance property of the resultant joints was given in this paper to show out the effectiveness of this method. 2. Materials and experimental procedure Highly pure Al foils (N99%, 1 mm thick, Sinopharm Co. Ltd, China), Cu foils (N99%, b0.1 mm thick, Sinopharm Co. Ltd, China), Al2O3 substrates in prism form (N 96%, porosity b 1%, length 20 mm, width 10 mm and thickness 0.7 mm, Yueke Jinghua Electronic Ceramics Co. Ltd, China), KF–AlF3 flux (Sinopharm Co. Ltd, China) and a Cu2O fine powder (44 μm, Shanghai Sinpeuo Fine Chemical Co. Ltd, China) were used. The Cu2O fine powder was dispersed in anethyl alcohol media mixed with dispersant, plasticizer, and adhesive agents. The ratio between ethyl alcohol and the agents is given in Table 1. To avoid the dirty residues on the surface, the substances were cleaned with pure acetone in ultrasonic bath for 30 min at room temperature. The fabricating procedure is as follows: Firstly, a Cu2O-paste was pasted on the surface of the Al2O3 substrate. The pasted-ceramic substrate was pre-thermally treated in air at 1150 °C for 60 min and slowly cooled down to room temperature. Then the Cu-foil was intimately placed on the pasted-ceramic substrate and the whole system was heat treated in a tube furnace under reduction atmosphere (10% H2/90% N2). The second heat treatment profile was as follows: Temperature increased up to 1000 °C with a heating rate of 10 K/min and maintained for 10 min. Then it increased up to the eutectic ([Cu + Cu2O]) temperature of 1070 °C with 2 K/min and maintained for 90 min. Afterwards the sample was slowly cooled down to room temperature with a rate of 2 K/min. The surfaces of the Cu-foil (already adjoined onto the Al2O3substrate) and the Al-foil were cleaned and degreased with the aid of KF–AlF3 flux after getting rid of the oxide layers. The Al-foil was then tightly assembled on the Cu-surface. The sample was placed into a tube-furnace under pure N2-atmosphere (N99.99%) and finally heattreated at 600 °C for 30 min with a rate of 10 K/min followed by a slow cooling process. The characterization of the produced joints was performed on different apparatus for different purposes. Phase crystallographic analysis was performed by X-ray Diffraction (XRD, Bruker D8 Advance, CuKα,
Fig. 2. X-ray diffractogram of the bonding area.
40 kV). Microstructure observations were carried out on polished cross-sections of ceramic/metal interfaces with the aid of Scanning Electron Microscope (SEM, Quanta 200, FEI), equipped with Energy Dispersive Spectroscopy (EDS) apparatus for element analysis. An optical microscope (9XB-PC) was employed as well. For the thermal shock resistance test, the joints were heated up to 500 °C with a rate of 20 K/s and kept for 10 min. Then, the joints were immediately quenched into room-temperature water. After undergoing extreme temperature shocks, substrates themselves may show delamination of the metal layers from the ceramic. If this phenomenon occurs on the DAB substrates, we regard the substrates as having become failed. The process was repeatedly cycled until the joint failed. For the sake of statistics, 20 samples were prepared and tested in the same way.
3. Results and discussion 3.1. The aluminum bonding process With our new fabricating method, the bonding of Al to the Al2O3substrate is realized mainly with two steps: i) the Al2O3 surface is treated with Cu2O and then bonded with Cu through Cu2O–Cu eutectic reaction, resulting with a thin layer of copper bonded on the substrate; ii) the bonding of Al is achieved with the assistance of the interlayer Cu through Cu–Al eutectic process. The Cu-coated Al2O3 substrate and Al boned Al2O3 substrates are shown in Fig. 1. We can see that the continuous copper layer is formed on the surface of Al2O3 ceramic substrate and the aluminum foil boded with Al2O3 substrate trough Al-Cu eutectic succeeds. Fig. 2 shows the X-ray diffraction pattern taken from the side face of the sample. Beyond the initial phases involved in the joint,
Fig. 1. A SEM image of Cu-coated Al2O3 substrate and a photograph of Al boned Al2O3 substrates.
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pattern in Fig. 2, intermetallic compound consists of Al2Cu in the aluminum is convinced. Additionally, through EDS analysis no evidence of oxygen was found in the bulk metal phase. Therefore, it is suggested that the Al2Cu phase in the bulk aluminum and at the interface is formed by the diffusion of the bonded copper to the aluminum side, since the CuAlO2 phase remains adjacent to the Al2O3 substrate. Thus, it can be concluded that the bonding of aluminum to the substrate is accomplished by the diffusion of Cu to Al, and this leads to the formation of Al2Cu and the eutectic reaction between Al and Cu at the interface.
3.2. Al wettability on copper-coated Al2O3 substrates
Fig. 3. A cross-section image of the Cu-coated Al2O3 joint.
specifically Al2O3, Al and Cu, two more phases CuAlO2 and Al2Cu were formed during the fabricating process. Initially, the thin layer of copper (less than 100 μm) was bonded to the inchoative surface of Al2O3 ceramic with the aid of the Cu2O paste and afterward thermal treating [23]. At first, a chemical interface was formed through the reaction between the Al2O3 ceramic and the Cu2O at 1150 °C, which resulted in the formation of CuAlO2 (as presented in Fig. 2). In addition, the thickness of CuAlO2 is about 10 μm, as shown in Fig. 3. The reaction can be simplified as: Cu2O + Al2O3 = 2CuAlO2. In order to convincingly identify the composition and morphology of intermediate phases, the Al2O3 substrates were dissolved in HNO3, as shown in Fig. 4. We can see that a continuous laminates is deposit at the interface of Cu-coated Al2O3 substrate. EDS analyses indicate that the composition of these laminates is close to 1:1:2 (Fig. 5). Combining the XRD analyses make it possible to identify that CuAlO2 exists at the interface. The copper bonding was realized by the eutectic reaction between Cu2O and Cu. In the bonding process at 1070 °C, the pure copper only engaged in the eutectic reaction, and it transformed from the eutectic liquid to the eutectic compound when the sample cooled down. The aluminum bonding was carried out on the previously copper bonded substrate and heat treated at 600 °C. Fig. 6 shows a typical SEM image of the resultant aluminum bonded sample. A clear bonding interface was observed between the aluminum substrate and the metal Al, as shown in Fig. 6(a). In Fig. 6(b), big Al2Cu clusters, determined from EDS spot-analysis (as shown in Fig. 7), were observed to disperse in the Al body and along the interface. Combing the XRD
Fig. 4. A SEM image of interface in the Cu-coated Al2O3 substrate.
During the aluminum bonding process, the pre-bonded copper foil plays an important role and improves the wettability of aluminum to the alumina ceramic substrate. In Fig. 6(a), it is clearly visible that a thin film was formed at the ceramic/metal interface. The aluminum liquid, which is formed during the Cu–Al eutectic process, lies on the reaction zone but not directly on the solid substrate of Al2O3. The absence of the Cu layer on the alumina surface suggests that wetting of Al2O3 is achieved through chemical reaction and copper diffusion into Al. In order to study the wetting behavior of Al on Cu pre-bonded Al2O3 substrate, a small piece of bulk Al was introduced on top of the Cu pre-bonded Al2O3 substrate. After heating up to 800 °C for 2 min, the bulk Al melts and spreads on the substrate. Figs. 8 and 9 show the wetting behavior of the surface and cross-section of aluminum on prebonded copper Al2O3 substrate. It turns out that the contact angle of the wetting regime is only about 22.10°, much lower than that reported with the traditional bonding methods giving 130° ~ 140° [24,25]. Therefore we can doubtlessly conclude that with our proposed method, the Al wettability in Al/Al2O3 system can be tremendously improved. The images in Fig. 6 clearly show the liquid Al was formed in-situ on Cu-coated alumina substrate at 600 °C. Therefore, the melting of aluminum on the surface and the diffusion of the pre-bonded Cu into the bulk aluminum, resulting in the chemical formation of Al2Cu, contributed to the wetting of Al on the alumina ceramic. Similar phenomenon has been reported and discussed by León et al. [26]. They showed the contact angles immediately decreased to 12.6° within two minutes and the improved wetting was mainly attributed to the dissolution of the copper when in contact with the liquid aluminum drop. As mentioned above, the diffusion of copper to the aluminum metal and the dissolving of aluminum cooperate in improving the wetting. The dissolved aluminum is consumed during the reaction and diffusion of copper. Comparing with the results by León et al. [26], it is found that the Cu–Al contact angle (~ 22.10°) after holding for 30 min in the present work was higher, implying a de-wetting period of the liquid
Fig. 5. EDS analysis of interface in the Cu-coated Al2O3 substrate.
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Fig. 6. Microstructure of the Al/Al2O3 interface after heat-treatment at 600 °C. The SEM images were taken from the transverse cross section of the samples. The scale in panel (a) is 500 μm, and it was enlarged to 100 μm for the interface area in panel (b).
The advancing front is locally saturated with copper, retarding the dissolution process. Accordingly, wetting initially occurred on copper, which becomes the drive for the spreading of the dissolved aluminum. Therefore, the wetting front retreats to reach a new but higher equilibrium contact angle. 3.3. The effect of KF–AlF3
Fig. 7. EDS analysis of intermetallic compound in the aluminum layer.
from the ceramic surface. Diffusion should be unsteady, since the metal liquid was continuously dissolved from the aluminum surface until the point that it was completely consumed in the surrounding area on the triple line. Moreover, the significant partial enthalpy of mixing of the Al–Cu liquid system, H = − 28 J/mol, leads to the formation of the low melting eutectic composition (548 °C) in the neighborhood on the interface, which could increase the fluidity of the aluminum. It is assumed that the reaction proceeds at an appreciable rate only on the contact line, where the liquid has direct access to the metallic film.
In the present study, the KF–AlF3 flux was used mainly to disrupt Al– oxide layer on the surface. The good wetting regime (Figs. 8 and 9) suggests the absence of the superficial Al–oxide layer on the surface of liquid Al. Assuming that a good wetting and the chemical reaction interface are the key factors for a good bonding strength and long-term stability of the joint, our novel processing method secures the elimination of the presence of the Al–oxide layer during brazing process at temperatures very close to the Al melting point. At this temperature, wetting angles of the Al2O3/Al system are often reported as high, 130°–140° [24,25], precisely due to the presence of that Al-oxide superficial film. At low partial pressure of oxygen and elevated temperatures, Aloxide layer can be disrupted according to the chemical Eq. (1), whereby the liquid Al actually reduces the oxide film resulting in the gaseous sub-oxide Al2O: 4AlðlÞ þ Al2 O3 ðsÞ→3Al2 OðgÞ:
ð1Þ
Nevertheless, 600 °C is seemingly to be low in order to ensure complete reduction of the oxide film on the aluminum surface. Therefore, the absence of the Al–oxide layer in the current study is attributed to the treatment of the KF–AlF3 flux. Brazing took place and disrupted
Fig. 8. The surface of (a) before and (b) after aluminum wetting on pre-bonded copper Al2O3 substrate.
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Fig. 9. The cross-section of aluminum wetting on pre-bonded copper Al2O3 substrate. Fig. 11. A image of thermal cycling reliability and adhesion strength for DAB substrates.
the Al–oxide layer as it reacted with the species of F−, AlF3− and AlF4− 6 derived from KF–AlF3 and form AlOF2 − or AlO2 − according to the chemical Eqs. (2)–(4): 2−
Al2 O3 þ AlF 6 3− →3AlOF Al2 O3 þ 2F− þ AlF
4−
2−
2AlOF
2−
→AlO
ð2Þ 2−
→3AlOF 4−
þ AlF
:
ð3Þ ð4Þ
Accordingly, the whole process should take place via the following two stages: (i) dissolution of the superficial Al-oxide film due to the treatment with KF–AlF3 flux; (ii) reaction of Al with Cu and Al2O3, which eventually results in a good wetting regime and a strong interface.
adhesive strength depends on the thickness of CuAlO2 as the intermediate layer. Sufficient adhesive strength can be achieved only above a certain thickness of the intermediate layer. However, the effect of Al2Cu on the strength of DAB substrate is not clear at present. Further experimental study is necessary to elucidate its effect. The thermal shock tests results indicate that excellent bonding was achieved in the DAB substrates produced with our new fabrication method. Furthermore, Comparing with commercial DAB substrates, thermal cycling reliability of our DAB substrates is much more stable. This is important in the light of the experimental conditions. The new fabrication method requires no use of high vacuum conditions but just pure flown N2 and relatively low temperature (600 °C). Moreover, the aspect of the reaction zone suggests a rather non-brittle phase is formed since no coarse crystals or holes are formed at the interface. 4. Conclusions
3.4. Thermal shock resistance Due to the structure of DAB substrates, tensile tests are used to measure the adhesion strength, as shown in Fig. 10. The results of thermal shock resistance tests and adhesion tests are shown in Fig. 11. It can be seen that the average value of adhesion strength for all samples is more than 12 MPa. Moreover, there is no dramatic change of these values. Meanwhile, it is also obviously shown that the DAB substrates successfully passed in average 110 thermal shock cycles. Both of them indicate an excellent bonding was obtained in all DAB samples. The good bonding strength of our DAB samples should be attributed to two aspects: i) the formation of the CuAlO2 chemical interface, ii) the good wetting of Al to the substrate. Some workers have also reported the requirement of CuAlO2 for high adhesive strength between Cu film and Al2O3 substrate. For instance, Kim et al. [27] has reported that
A new method for fabricating DAB substrate is proposed, and the new fabrication method produces DAB substrates with excellent bonding property, which will have huge potentials in power electronic packaging applications. (1) Significant improvement in the wetting of aluminum was achieved by applying a coating of Cu on Al2O3 ceramic. It is mainly due to the Cu diffusion into the aluminum and the dissolving of aluminum. Once the copper is completely removed from the ceramic surface, the final contact angle reaches 22.10°. This value is substantially lower than those for pure aluminum on uncoated alumina ceramics (130° ~ 140°). (2) At relatively low temperature (600 °C), the superficial Al-oxide film is disrupted by the treatment with KF–AlF3 flux, which is different from the traditional conditions, i.e. high vacuum conditions and high temperatures. (3) Thermal shock test results showed that the samples successfully passed 110 thermal shock cycles. The average value of adhesion strength for all samples is more than 12 MPa. The performance of thermal cycling reliability is more stable than commercial DBA substrates. The excellent bonding strength was mainly attributed to the chemical interaction interface layer of CuAlO2 and the improved wetting of aluminum to the alumina substrate.
Acknowledgments
Fig. 10. The sample and tensile test used.
The author gratefully acknowledges a project Funded by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Nanjing, China.
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