MATERIALS FORUM VOLUME 32 - 2008 Edited by J.M. Cairney, S.P. Ringer and R. Wuhrer © Institute of Materials Engineering Australasia Ltd

ALUMINA – COPPER EUTECTIC BOND STRENGTH: CONTRIBUTION OF PREOXIDATION, CUPROUS OXIDES PARTICLES, AND PORES H. Ghasemi 1*, A. H. Kokabi 1, M. A. Faghihi Sani 1, and Z. Riazi 2 1

Department of Materials Science and Engineering, Sharif University of Technology, Iran 2 Bonab Research Center, Bonab *email: [email protected]

ABSTRACT The influences of cupric oxide layer thickness, cuprous oxide particles, and pores on mechanical properties and microstructure of alumina-copper eutectic bond have been investigated. The furnace atmosphere in the first stage was argon gas with 2 × 10-6 atm oxygen partial pressure. In the second stage, the furnace atmosphere was same as the first stage unless that in cooling between 900-1000 °C, the hydrogen gas was purged in furnace atmosphere. Finally, in the last stage a vacuum furnace with 5 × 10-8 atm pressure was chosen for bonding procedure. Peel strength of first stage specimens shows that cupric oxide layer with 320 ± 25 nm thick generates the maximum peel strength (13.1 ± 0.3 kg/cm) in joint interface. In the second stage, by using the hydrogen gas, a joint interface free of any cuprous oxide particle was established. In this case, engrossingly the joint strength has increased to 17.1 ± 0.2 kg/cm. Finally, the bonding process in vacuum furnace indicates that the furnace gas does not have considerable effect on joint interface pores. Furthermore, bonding process in vacuum furnace reduces the peel strength of joint due to formation of more pores. Thorough study of pores formation is presented. KEYWORDS: Alumina-Copper, Bonding, Peel Strength effect on peel strength, Yoshino 2 reported that the voids in the interface can be formed due to release of oxygen gas in the liquid copper; by increasing the thickness of cupric oxide layer, their size will be larger. However, Seager et al. 6 have found that smaller pores (1-3 µm) may be pullouts of the Cu2O particles observed on alumina fracture surface, while the large pores observed by Yoshino are the result of argon entrapment in liquid copper during processing. On the effect of pores on mechanical strength, Reimanis 9 have observed that crackfront perturbation occurs when the crack tip is in contact with a pore; the crack front is drawn into the pore and causes debonding of the regions immediately surrounding the pore. Despite several investigations, the accurate contribution of preoxidation, cuprous oxide particles, and pores on peel strength of this bond is unclear. The goal of current research is to shed more light on the effect of cupric oxide layer thickness, cuprous oxides, and pores on peel strength of alumina-copper eutectic bond. Besides, the sources of pores formation have investigated comprehensively.

1. INTRODUCTION One of the methods for joining copper and alumina is gas-metal eutectic bonding. The bonding of copper to alumina is carried out at 1075 ± 2 °C in a properly controlled oxygen-containing atmosphere 1. The role of oxygen in mentioned method is improving wetting of liquid copper to alumina and hence it is necessary to achieve good spreading of the liquid along the bond interface 2. So far, the mechanical properties of mentioned bond and occurred reactions in bond interface have been studied extensively 2-8. Yoshino 2 have found that the peel strength of alumina-copper bond is primarily affected by dissolved-oxygen concentration. He reported that the cuprous oxides and voids have, also, contributions to true bond strength. Thus, oxygen seems to affect the peel strength of alumina/copper bond principally in two ways: through cohesive bond energy and through formation of cuprous oxide. The measured bond strength is the combination of these effects. He predicted that by removing the oxide particles from the interface, the peel strength of the joint can be raised up to 15 Kg/cm. Moreover, Trumble et al. 7 have reported that the Cu2O particles can be reduced after bonding without any loss in bond strength. Following the other researcher investigations, Ning 8 have expressed that a certain thickness of cupric oxide layer can cause the high bonding strength; this thickness is a function of alumina substrate profile. On source of pores formation and their

2. METHODS AND PROCEDURES Experiments were carried out in three stages. In the first stage, the effect of preoxidation on peel strength of alumina-copper bond has been examined. In the second stage, influence of cuprous oxide particles on peel

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strength of alumina-copper bond has been investigated. Finally, in order to decrease the interface pores, the bonding process was performed in a vacuum furnace. All of the alumina-copper bonds began with 99.99% Cu strips and 97% Al2O3. Copper strips and alumina specimens were 350 and 200 mm in length, 25 mm in width, and 0.8 and 1.5 mm thick, respectively. Copper strips were prepared in three steps. Firstly, strips were degreased by scrubbing the joint surfaces in a solution of liquid detergent, washing with clean hot water and drying thoroughly in a steam of hot air. Secondly, strips were immersed in a 25% nitric acid for 30s. Thirdly, strips were dipped in ethyl alcohol followed by clean cold water and drying by hot air. On the other hand, alumina specimens were polished by diamond paste with average particle size of 0.5µm, cleaned by ultrasonic treatment in ethyl alcohol for 15 min and rinsed in distilled water. At the final step, alumina specimens were annealed in air at 1000°C to eliminate any hydroxyl groups.

in Figure 2, the variation in thickness in black region is gradual. After that, however, the thickness of CuO layer grows abruptly. In samples which have oxide layer thickness more than black region, there is some discontinuity and crack on oxide layer surface. Furthermore, the adhesion between the cupric oxide layer and copper strips has decreased notably. These discontinuities on cuprous oxide layer surface are due to thickness of CuO layer. The more oxide layer thickness, the more sensivity to thermal expansion coefficient mismatch. Inasmuch as the thermal expansion coefficient of Cu (20 × 10-6 °C-1) and CuO (4.3 × 10-6 °C-1) have great difference, the stresses formed in cooling is high. Thus, these stresses can lead to crack in oxide layer. Due to formation of the crack in oxide layer surface, the copper oxidation rate has been raised suddenly as illustrated in Figure 2. Hence, the preoxidation conditions which create more than approximately 500- 600 nm oxide layer thickness, makes the strips improper for bonding process.

Copper strips were preoxidized in various temperatures and times to obtain a range of cupric oxide (CuO) thickness. The thickness of cupric oxide layer was calculated from the weight difference before and after preoxidation. In the first stage, the sample were bonded in a tube furnace in argon atmosphere (<2 ppm O2). Gas flow in heating and cooling were 150, 200 L/hr, respectively. The samples were heated at a rate of 10 °C/min to 1000 °C, followed by rate of 2 °C/min to 1075 °C where they were held for 1 hr. During this time, alumina-copper bond was formed. After 1 hr, the temperature was decreased slowly (5 °C/min) to 700 °C and then 10 °C/min to 400 °C. The temperature was held for 1 hr in 400 °C to reduce thermal stresses. Finally, the furnace cooled to room temperature. The furnace temperature curve is illustrated in Figure 1.In the second stage, bonding condition was as same as first stage in expect that in cooling between 900 – 1000 °C, the hydrogen gas was purged to furnace atmosphere. In the last stage, bonding process was carried out in a vacuum furnace. The vacuum tube with a mechanical pump and an oil diffusion pump can make an approximately vacuum atmosphere of 5 × 10-8 atm. The oxygen partial pressure of oxygen was measured by CaO-Stabilized ZrO2 oxygen sensor. The peel strength of specimens was measured by Instron Machine Model 1115. The fracture surfaces were analyzed by optical microscopy and scanning electron microscopy.

b.

The peel strengths of first stage specimens are illustrated in Figure 3. As illustrated in Figure 3(a), distinguishable oxide layer thickness causes the highest peel strength (13.1± 0.3 kg/cm) in 2 × 10-6 atm oxygen partial pressure. This optimum thickness is a result of the interaction between wetting of alumina surface and percentage of oxide particles at the interface. Similarly, Figure 3(b) shows an optimum point. By comparison the optimum layer thickness in Figure3 (a) and 3(b), there is approximately 25 nm difference. This difference can be resulted from errors in experiments. Thus, it can be said that 320 ± 25 nm is the optimum oxide layer thickness for alumina-copper bonding in 2 × 10-6 atm oxygen partial pressure. The peel strength difference in these two states is low; it shows that oxidation in different temperatures to obtain certain oxide layer thickness does not affect the peel strength. Both figures 3(a) and 3(b) reveal that before optimum point, the peel strength of specimens has raised steadily. It can be concluded that improved wetting due to increase in the oxygen content of liquid copper has heighten the peel strength. After optimum point, the effect of Cu2O content is predominant and has decreased the bond strength. Figure 3(c) shows the peel strength of specimens oxidized in 400 °C. In contrast to Figure 3(a) and 3(b), in preoxidation time span of this temperature, the peel strength of specimens has decreased slowly. Obviously, in this time span, the Cu2O content of interface layer is predominant parameter to wetting of alumina by liquid copper. The noteworthy point in Figure 3(a) and 3(b) is the rising and falling rate of peel strength versus the oxide layer thickness. It shows that the effect of wetting on peel strength is more significant than effect of

3. RESULTS AND DISCUSSION a.

Peel Strength Measurments

Cupric Oxide Layer Thickness

The thickness of cupric oxide layer formed in various temperatures and times is depicted in Figure 2. As shown

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Cu2O content of interface. This state is clear in Ning investigations 8.

V (1 mol Cu 2 O ) (Eqn 1) = 1.459 V (2 mol Cu ) Inasmuch as the volume ratio is more than one, in place of Cu2O particles, some pore has remained at interface after Cu2O particles reduction. The copper side fracture surface in three stages is illustrated in Figure 7. In contrast to first stage, the pores sizes are diminished in the second stage. On account of the smaller pores in interface, the tolerances in peel strength has diminished which is clear in Figure 6. By precise focus, it can be found that smaller spherical pores are same in two stages. However, larger pores in first stage which have irregular shape are contracted to smaller irregular pores in the second stage.

To achieve more strength in alumina-copper bond, it is desirable to remove cuprous oxides from the interface 2,7. For this reason, in the second stage, in cooling, between 900-1000 °C the hydrogen gas was purged into the furnace atmosphere to remove Cu2O particles from the interface. Figure 4 shows the cross section of aluminacopper joint in the second stage. It, surprisingly, illustrates no trace of interface layer; the joint is directly established between alumina and copper. No sign of any other phase in expect of copper and alumina is detectable at the fracture surfaces. The peel strength measurements of the joint in this stage are shown in Figure 5.

According to Sun & Driscoll investigations 10, the Cu2O content of interface layer after solidification is more than Cu2O content of eutectic point. (The Cu2O content of eutectic point is 4.3% vol.). Moreover, this fact is clear in pictures which other researchers have taken from the alumina-copper bond 5,6,8. So there are some Cu2O particles in liquid copper in bonding temperature. These Cu2O particles can be reduced at bonding temperature in 2 × 10-6 atm oxygen partial pressure. Reduction of Cu2O particles forms O2 (gas) molecules. These molecules can form spherical pores at interface. But this reduction reaction has not completed during the bonding process. The Cu2O particles in interface after solidification are evidence to this conclusion. Thus, until now, there are two sources for pores formation. First one is the O2 molecules that are formed due to reduction of Cu2O particles in liquid copper. Pullout of Cu2O particles from copper fracture side is the second one. Comparing the Cu2O particles on alumina fracture surface with irregular pores size, it is clear that Cu2O particles are smaller than irregular pores on copper fracture side. So, maybe the irregular pores on fracture surface of copper are formed due to several sources.

In addition, Figure 6 depicts the force versus distance curve of maximum peel strength in first and second stage. As shown in Figure 5, in contrast to first stage specimens, the peel strength of the joint has raised considerably in all cupric oxide layer thicknesses. Moreover, the maximum peel strength of specimens has risen to 17.1 ± 0.2 kg/cm; in this stage the optimum CuO layer thickness has changed. The increase in peel strength of the joint is attributed to removing of Cu2O particles from the interface. After solidification, there are some percent of Cu2O particles in interface. If the environment atmosphere permits the Cu2O particles reduce to Cu. By removing the Cu2O particles the Cu2O/Al2O3 and Cu2O/Cu interfaces has substituted by Cu/Al2O3. According to Chiang et al. and Sun and Driscoll investigations 4, 10, the strength of Cu/Al2O3 is more than Cu2O/Cu; hence, due to improvement of alumina-copper contact surface and decrease in stress concentration owing to absence of Cu2O particles, the peel strength has increased. Three main factors determine the peel strength of the joint; wetting, Cu2O particles, and pores. The pores adverse effect on peel strength is low .Since in the second stage Cu2O particles have been removed, wetting parameter is the main affecting parameter on peel strength. It has been proved that after certain oxygen content of liquid copper the wetting angle do not change considerably. Hence, a few decreases in peel strength have occurred after maximum strength in thick CuO layers due to more pores as depicted in Figure 5. The change of CuO optimum layer thickness in this stage was predicted. Due to reduction of Cu2O particles, the effect of wetting is more significant than increasing the pore percentage and sizes with increase in oxygen content of liquid copper; the optimum point has shifted to thicker CuO layers. By reducing one mole of Cu2O, two mole of Cu will appear. The volume ratio of one mole Cu2O to two mole copper is calculated in equation (1).

In view of the fact that furnace atmosphere gas was predicted as one of sources for pores formation 6, In the hope of removing some pores, the final stage of bonding was carried out at vacuum furnace. The vacuum atmosphere was 5 × 10-8 atm. The maximum peel strength of this stage is illustrated in Figure 8. As depicted in Figure 8, the peel strength has decreased and its tolerances are more in comparison with the second stage. In this stage, same as second stage, the interface is free of Cu2O particles as it was predicted due to suitable environment for Cu2O particles reduction. The outstanding point in the third stage is the formation of larger pores on interface (Figure7). Furthermore, the shape of them has changed. Drop in peel strength is related to formation of larger pores, and these pores are cause of more tolerances in peel strength.

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Formation of larger pores can be attributed to furnace atmosphere. There are two reactions which are responsible for larger pores. The reduction of Cu2O particles in liquid copper is the first one; presence of vacuum condition is the second one. As it was mentioned before, there are some Cu2O particles in liquid copper in bonding temperature. Since the furnace oxygen partial pressure is 10-8 atm, these oxide particles reduce more than two previous stages and liberate more O2 gas. Also, Duo to bonding in vacuum furnace environment, the pressure difference between the furnace environment and released O2 (gas) is high; O2 molecules can extend to larger dimensions. Hence, it can be said that the furnace environment gas does not have significant effect on pores formation. In a conclusion, the pores on alumina-copper eutectic bond can be formed due to two reasons. O2 gas release is the first one. Pullout of Cu2O particles is the second one. However, in some pores both of condition was prepared. In these pores, a portion of pore is formed due to gas release and in perimeter of these pores Cu2O particles are present. These Cu2O particles are responsible for irregular shape of these kinds of pores.

Acknowledgement Financial support for this work was provided by Bonab Research Center. The assistance of the following organizations and people in various aspects of the experimental work is greatly appreciated: Research Vicepresidency of SUT, Mr.T.Tohidi, and Mr. M. Naderi.

References 1. 2. 3. 4.

5.

4. CONCLUSION 1.

2.

3. 4. 5.

6.

Creating more than 500-600 nm cuprous oxide layer thickness by preoxidation makes the strips improper for eutectic bonding due to forming cracks in oxide layer and loss of adhesion. The Optimum cuprous oxide layer thickness to provide highest peel strength in alumina-copper eutectic joint in 2 × 10-6 oxygen partial pressure is 320 ± 25 nm which results to 13.1± 0.3 kg/cm peel strength. By removing all of the Cu2O particles from aluminacopper interface, the peel strength has heightened to 17.1 ± 0.2 kg/cm. Furnace gases do not have a significant effect on pores formation in alumina – copper interface. Performing the alumina-copper eutectic bond in vacuum furnace decreases the peel strength and causes more fluctuations in joint strength due to formation of larger pores. The pores at alumina-copper interface are formed due to two reasons. O2 gas release is the first one. Pullout of Cu2O particles is the second one.

6.

7. 8. 9.

10.

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J. F. Burgess and C. A. Neugebauer, “Direct bonding of metals with a metal-gas eutectic,” U.S.Pat.No.3854892, 1974. Yuichi Yoshino, "Role of Oxygen in Bonding Copper to Alumina,” J. Am. Ceram. Soc., 1989, 72(8), 132227. Y. Yoshino, H.Ohtsu, “Interface structure and bond strength of copper-bonded alumina substrates”, J. Am. Ceram. Soc., 1991, Vol. 74, pp. 2184-2188. W. L. Chiang, V. A. Greenhut, D. J. Shanefield, L. A. Johnson and R. L. Moore, “Gas-metal eutectic bonded Cu to Al2O3 substrate-mechanism and substrate additives effect study,” Ceram. Eng. Sci. Proc., 1993, 14(9-10), 802-12. Sung Tae Kim, Chong Hee Kim, "Interfacial reaction product and its Effect on the strength of copper to alumina eutectic bonding,” J. materials science, 1992, 27, 2061-2066. C. W. Seager, K. Kokini, K. Trumble, M. J. M. Krane ,"The influence of CuAlO2 on the strength of eutectically bonded Cu/Al2O3 interfaces,” Scripta Materialia, 2002, 46, 395–400. K. P. Trumble, "Thermodynamic analysis of Aluminate Formation at Fe/Al2O3 and Cu/Al2O3,” Acta metall. mater. , 1992, Vol.40, S105-S110. H. Ning, J. Ma, F. Huang, Y. Wang, “Preoxidation of the Cu layer in direct bonding technology,” Applied surface science, 2003, 211, 250-258. I. E. Reimanis, K. P. Trumble, K. A. Rogers, B. J. Dalgleish, “Influence of the Cu2O and CuAlO2 interphases on crack propagation at Cu/α-Al2O3 interfaces,” J. Am. Ceram. Soc., 1997, 80(2), 424-32. Y. S. Sun and J. C. Driscoll, “A new hybrid power technique utilizing a direct copper to ceramic bond,” IEEE Trans. Electron Devices, 1976, ED-23 8, 96167.

Figure 1.The furnace curve in bonding process

Figure 2.The variation of oxide layer thickness in different preoxidation conditions

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Figure 3.The peel strength of specimens preoxidized in different conditions

Figure 4.The cross section of alumina-copper joint after removing the Cu2O particles

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Figure 5.The peel strength of the second stage specimens

Figure 6.The maximum peel strength in the first and second stages

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Figure 7.The copper side fracture surfaces of three stages

Figure 8.The comparison between maximum peel strength in three stages

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