New Bonding Technique Using Copper Oxide Materials

Toshiaki Morita and Yusuke Yasuda

Hitachi Research Laboratory, Hitachi Ltd., Hitachi 319-1292, Japan

As a bonding technique between semiconductor elements in a semiconductor device and external substrates, a reduction bonding technique has been developed that uses CuO particles several micrometers large. This technique has the following features. (1) CuO particles can be reduced in about 200°C H2 atmosphere. (2) Pure Cu particles about 50 nm in diameter form at this time. (3) The formed Cu particles form a sintered Cu layer. (4) Bonding can be achieved above 350°C. Bonding requires pressurization. The bonding strength (shear breaking strength) of this sintered Cu layer and Ni electrodes used by many semiconductor devices was evaluated. Consequently, bonding strengths were about 20 MPa at pressurization of 1.2 MPa and a heating temperature of 350°C. Moreover, in the bonded interface of a sintered Cu layer and Ni electrodes, the sintered Cu layer formed the hetero-epitaxial layer between Ni electrodes. For this reason, we conﬁrmed that the bonded interface of a sintered Cu layer and Ni electrodes was in a strong bonding state on the metal object. [doi:10.2320/matertrans.M2014399]

(Received November 13, 2014; Accepted March 24, 2015; Published May 8, 2015) Keywords: lead free, copper oxide, reduction, bonding technique, hetero-epitaxial layer

1. Introduction (5) The silver nano particles formed from the silver oxide particle are sintered immediately. In semiconductor devices used in trains, wind power (6) This method forms the bonding layer of pure silver equipment, automobiles, etc., the current density tends to similarly to the silver nano particle bonding method. increase as equipment is miniturized.1) As this happens, the This technique may be able to reduce material cost below operating temperature for semiconductor devices becomes an that of the silver nano particle bonding method. However, elevated temperature. For this reason, the semiconductor silver tends to produce ion migration, so we were anxious devices are demanded to have long-term high reliability in high about the reliability declining. Furthermore, we were also temperature environments.2) Moreover, in new devices such as anxious about the cost of bond material rising due to a sharp silicone carbide (SiC), bond material needs to be developed rise in the raw material price. that resists high temperature.3) As a bond material for high Additionally, copper is comparatively low cost and has temperature environments of semiconductor devices, the high conductivity and high productivity equivalent to those solder material containing much lead has mainly been used. of silver. Furthermore, it also has excellent ion migration However, from the viewpoint of environmental protection, a resistance, and if copper nano particles are used for bonding, bond material needs to be developed that complies with the many problems will become solvable. On the other hand, the RoHS command that no harmful lead be used. copper nano particle has a problem: it is oxidized easily if left To achieve this, a silver-nano-particle bonding technique at room temperature.10) To solve this problem, there is a has been examined.47) This bonding technique, which does method for coating the copper nano particle surface with not use lead, can carry out bonding at a temperature lower silica11) or a polymer.12) However, to remove these coats, they (about 300°C) than the melting point of silver. This bonding must be heated at 400°C or more. For this reason, this temperature is lower than the bonding temperature of lead method was unsuitable for the bonding process of 350°C or solder material (about 350°C). Furthermore, the resulting less required for semiconductor mounting. On the other hand, bonding layer has high radiation and high heat resistance, a synthetic process of copper nano particles using CTAB which are features of silver. For this reason, this bonding (cetyl trimethyl ammonium bromide) of a low-molecule technique has been considered suitable for a high temperature organic material has been reported.13) However, oxidization environment. Furthermore, a part of the protection coat of copper particles has not been preventable. material used to stabilize a silver nano particle discharges On the basis of this situation, a silver oxide reduction a residue into the bonding layer, without its effect being bonding technique is applied in which reducing atmosphere eliminated during the bonding process. We were anxious is used. Developing a reduction bonding method using about this residue corroding the bonding layer. copper-oxide material may make many problems mentioned To solve these problems, a bonding method using a low- above solvable. cost silver oxide particle has been reported.8,9) This technique has the following features. 2. Reduction Characteristics of Copper Oxide (1) It is a bonding method using the silver oxide particles several micrometers large to which high-boiling-point We conducted thermal analyses (thermogravimetry-differ- alcohol is added. ential thermal analysis (TG-DTA)) on Cu2O and CuO (2) Adding this alcohol accelerates the reduction reaction at particles (all were commercial reagents) and investigated low temperature. the reduction temperature. The weight reduction curve of TG- (3) Silver nano particles form at this time. DTA showed that 10% of the reduction rates was set as the (4) The formed silver nano particles do not have an organic reduction start temperature (T (10%)), and 100% of the substance. reduction rates was set as the reduction end temperature New Bonding Technique Using Copper Oxide Materials 879

105 5 Cu2O ΔT 100 4 95 3 T(10%)

90 2 H / mV/g W / mass% T(100%) 85 1 TG, DTA, DTA, 80 0 75 -1 105 5 ΔT CuO 100 4 2μm 95 T(100%) 3 T(10%) H / mV/g 90 2 W / mass% Fig. 2 SEM image of CuO particles.

85 1 DTA, TG, 80 0 Small Cu disk (Ni plating) 75 -1 5 mm diameter Pressures (1.2MPa) Shear tool 2 mm thick 0 100 200 300 400 500 600 700 Heating Temperature, T / °C in H2 Bond material Fig. 1 TG-DTA results of each oxide particle. (100μm thick)

Large Cu disk (Ni plating) Table 1 Each value of T (10%), T (100%), and "T for each oxide particle. 10 mm diameter 5 mm thick Oxide T (10%) (°C) T (100%) (°C) ¦T (°C) Fig. 3 Speciﬁcations for shear-test sample and schematic of shear-test Cu2O 422 572 150 method. CuO 265 345 80

the ﬁgure. The paint itself was approximately 100 µm thick. Bonding was accomplished by simultaneously applying heat (T (100%)). The results are shown in Fig. 1 and listed in and pressure (1.2 MPa) in a H2 atmosphere, and the bonding Table 1. The analyses were performed in a mixed gas of 3% time was 5 min. We used an SS-100KP bond tester (Seishin hydrogen and 97% nitrogen. Different "T for T (100%) and Shoji, maximum load: 100 kg) for the shear tests. The rate T (10%) are also listed in Table 1. The T (10%) and T of shear was 30 mm/min, and the maximum load during (100%) for the Cu2O were large. On the other hand, "T was fracturing was measured by applying a shearing force to larger for Cu2O than CuO. Consequently, we conﬁrmed that the small Cu disk shown in Fig. 3 to fracture the bond. The 14) CuO was easier to reduce than Cu2O. Using these results, shear strength was taken as this maximum load divided by we were able to compound a copper oxide material with CuO the bond surface area (equivalent to the surface area of the junctions. disk).

3. Experimental Procedure 3.3 Observation of bonding interface A diamond paste (particle diameter: 0.25 µm) was used to 3.1 Composition of CuO particle for bonding finish buffing the cross-sectional samples used for the SEM CuO particles for bonding were synthesized on the basis of observation that were first finished by buffing using polishing a metal salt-base reaction.15) An aqueous solution of NaOH oil and then subjected to Ar-ion milling (at an accelerating (1 M) (19 mL) was added to 981 mL of 0.0102 M Cu(NO3)2 voltage of 15 kV) for 180 s. We observed the interface using aqueous solution under vigorous stirring at 20°C, which TEM to obtain more detailed images of the bond between the resulted in a Cu2+ concentration of 0.01 M in the final Cu and Ni. The samples used for observing the cross-sections solution. The reaction time was 24 h, and a SEM image of of the bonds were milled using a focused ion beam (FIB; these CuO particles is shown in Fig. 2. Hitachi FB-2000A). The milled thin film samples were mounted on a cutout mesh and observed using TEM (Hitachi 3.2 Bonding strength H9000; acceleration voltage: 300 kV). Ion milling was also We investigated the bonding capability of synthetic CuO used when necessary to make the observation samples even particles (bond material) with Ni. The bonding capability was thinner. evaluated as the bond strength under shear. Perspective views of the shearing test piece are shown in Fig. 3 (JIS Z3198-5). 4. Results and Discussion The test piece was a Cu disk that was about 2 µm thick and had electrolysis Ni plating on the surface. The bond material 4.1 Results of bonding strength evaluation was painted onto each test piece within the black regions in Figure 4 shows the relationship between the bonding 880 T. Morita and Y. Yasuda

35 (a) (b) 30 25 High sintering density layer F / MPa Sintered Cu layer 20

15 Ni 10 Ni Cu 30μm 1μm

Shear strength, Shear strength, 5 (c) (d) Cu Cu 0 Interface 200 250 300 350 400 Bonding temperature, T / °C Ni μ Fig. 4 Relationship between bonding temperature and bonding strength. 2 m Fig. 6 Analysis results of bonding cross section to Ni electrode R 400°C ((a) cross-sectional SEM image of bonding area, (b) enlargement of ﬂ 250°C 300°C 350°C 400°C interface, (c) sintered Cu and Ni interface (re ection electron image), and (d) EBSD crystal orientation map).

Cu (a) Vacancy

Interface Large disk Small disk Ni 50nm : Pressurizing area with small disk (b) Fig. 5 State of shearing side of test piece after shearing examination.

Cu strength and the bonding temperature for a synthetic CuO particle. The 250°C bonding temperature was not suitable for the bonding. However, the bonding strength rose as the Interface bonding temperature rose to 300°C or more. The bonding strength was about 20 MPa at the bonding temperatures of 350°C and 400°C. We thus determined that the joint was excellent. Ni Figure 5 shows the state on the shearing side of the test piece after the shearing examination. We confirmed that the bond material was unreduced because of the black region at 4nm 250°C. We also determined that the unreduced material was Fig. 7 TEM observation of bonding area using Ni ((a) low magnification not bondable based on the graph shown in Fig. 4. At 300°C image and (b) high resolution image). or more, the bond material was changed into sintered copper. We confirmed that the bond material continued to reduce. Moreover, all the sintered copper remained on the shearing breaking side at 300°C or more. As for breaking while Cu sintering layer of the interface had grown in this shearing, breaking predominantly happened during sintering crystal direction to form a multi-crystal Ni electrode. These of copper. results prove that there is a good metal bond and that the sintering Cu and Ni electrode interface is in a strong 4.2 Observation of detailed bonding structure bonding state. Figure 6 shows SEM images of the cross-sections of the A TEM image of the interface of the sintering Cu and Ni is Cu sintered layer and Ni electrode (Ni plating). The bonding shown in Fig. 7. The low-magnification TEM image (a) temperature was 400°C. The copper sintered layer shows a shows that the crystal growth of the sintering Cu is in the uniform porosity state in the SEM image (a), and image (b) same direction as the crystal direction of the ground Ni shows the Cu sintered layer at the interface with the Ni electrode. Furthermore, the high-resolution image (b) con- electrode in more detail. We understood from this image that firms that the atomic arrangement of Cu and Ni is mostly in a high sintering density layer forms at the interface. We the same direction and that their diffraction patterns were believe that this layer had been strongly bonded to the Ni mostly in agreement. Thus, Cu and Ni can be seen to have an electrode. Subsequently, a crystal direction analysis was epitaxial direction relationship. Therefore, Cu and Ni turned conducted on this portion using electron backscatter out to have a good metal bonding state. In addition, we diffraction (EBSD). Figure 6(d) shows the results of the confirmed that the synthetic CuO material also joins well analysis that support image (c). Image (d) shows that the with the Cu electrode. New Bonding Technique Using Copper Oxide Materials 881

(b) (a)Ni (c) (d) Ni Interface

Cu Cu

1μm

Fig. 8 Analysis results of bonding cross section of Ni electrode ((a) cross-sectional TEM image of bonding area, (b) EBSD crystal orientation map, (c) Cu mapping image, and (d) Ni mapping image).

(a) 150 °C (b) 175 °C

CuO particles Cu particles

Sintered Cu Sintered Cu

500 nm

Fig. 9 State of CuO particles after heating in H2 ((a) 150°C, (b) 175°C, (c) 250°C, and (d) 300°C).

4.3 State of diffusion of bonded interface ensured by the diffusion, a good metal-bonding is assumed to We were able to clarify the state of the crystal grain of the be realizable. sintered Cu and Ni interelectrode. However, the diffusion was not clarified. We thus tried EBSD from the same viewpoint as 4.4 Reduction of CuO particles and sinter-bonding TEM and the elemental mapping by using TEM-EDX. mechanism Figure 8(b) shows the EBSD crystal orientation map from the We investigated the sintering behavior in the reduction same view as in (a), which is a cross-section TEM image. In process of CuO particles. Figure 9 shows the results from addition, the mapping data of (c) Cu and (d) Ni by using a SEM observation of CuO particles heat-treated in H2 at TEM-EDX is shown. As mentioned above, the crystal grew (a) 150°C, (b) 175°C, (c) 250°C, and (d) 300°C. SEM-EDX in the same direction for the Cu and Ni in the bonded was used to confirm the compositions of the particles after interface. each heat treatment. On the other hand, neither Cu nor Ni is diffused mutually At 150°C, the CuO particles formed a rectangular when (a), (c), and (d) are compared for the Cu-Ni crystal configuration. After heat-treating at 175°C, copper particles grain in the same direction. We determined that there was a about 50 nm in diameter formed on the CuO particles. We clear distribution interface of Cu and Ni in the same crystal believe that these copper particles carried out the reduction grain direction. The Cu was distributed in the grain boundary formation from the CuO particles. At 250°C, Cu was formed, of the Ni electrode (Ni plating), and thus, Ni plating film is a and after 300°C heating, the entire structure went through Cu stable thin film. We believe that Cu composed of a sintering substance stratification. Copper is believed to be the sintering layer was easier to diffuse than Ni composed of a plating layer formed of the Cu particles formed when the CuO is film. Moreover, since the bonding structure integrity is reduced. 882 T. Morita and Y. Yasuda

As mentioned above, the CuO particles turned out to form 5. Conclusion into approx. 50-nm-diameter Cu particles at 175°C during the reduction process, sintered under heat-treatment at 300°C, (1) The CuO in a copper oxide is easier to reduce than the and became bulk Cu. The Cu particles sintered at the low Cu2O. temperature of 300°C or less are nanometer sized because (2) The reduced CuO and sintered Cu layer can be bonded they are produced using this reduction process. For the to Ni plating ﬁlm. This Cu layer has a hetero-epitaxial bonding method using CuO particles, we found out that orientation relationship with the Ni plating ﬁlm, which metal-bonding can ensure that Ni is used as mentioned above. is believed to cause high bonding strength with the Ni. Moreover, from the EBSD results shown in Figs. 6 and 8, the hetero-epitaxial bonding structure acquired when using REFERENCES Ni is believed to be a characteristic metal bonding structure acquired during the low temperature sintering of metal 1) J. Gobl and J. Faltenbacher: Proc. CIPS 2010, March, 1618, (2010) particles. Nuremberg/Germany. The interdiffusion of Cu and Ni has been widely 2) F. Lang, H. Nakagawa, M. Aoyagi, H. Ohashi and H. Yamaguchi: J. Mater. Sci.: Mater. Electron. 21 (2010) 917925. researched. However, those investigations were done in a 3) F. Lang, H. Yamaguchi, H. Ohashi and H. Sato: J. Electron. Mater. 40 16) high temperature region exceeding 700°C. Moreover, the (2011) 15631571. research on diffusion bonding of Cu and Ni clearly showed 4) E. Ide, S. Angata, A. Hirose and K. F. Kobayashi: Acta Mater. 53 that a good bonding temperature was 700°C or more.17) (2005) 23852393. On the other hand, den Broeder and Nakahara heat-treated 5) E. Ide, A. Hirose and K. F. Kobayashi: Mater. Trans. 47 (2006) 211 217. the Polycrystal diffusion couple of pure Cu and pure Ni and 6) T. Morita, E. Ide, Y. Yasuda, A. Hirose and K. Kobayashi: Jpn. J. Appl. investigated the grain boundary diffusion region experimen- Phys. 47 (2008) 66156622. tally.18) Consequently, in the diffusion couple heat-treated at 7) Y. Yasuda, E. Ide and T. Morita: Jpn. J. Appl. Phys. 48 (2009) 125004. 600°C for 360 minutes, they clearly showed that diffusion of 8) T. Morita, Y. Yasuda, E. Ide, Y. Akada and A. Hirose: Mater. Trans. 49 Cu to the inside of Ni phase is about 1 micrometer. (2008) 2875 2880. 9) Y. Yasuda, E. Ide and T. Morita: Mater. Trans. 54 (2013) 10631065. The above is the result of basing research on the bulk 10) J. Yang, T. Okamoto, R. Ichino, T. Bessho, S. Satake and M. Okido: material of Cu and Ni. A high temperature is needed to bond Chem. Lett. 35 (2006) 648649. Ni to Cu. However, although Cu is the nanometer-sized 11) Y. Kobayashi and T. Sakuraba: Colloids Surf. A Physiochem. Eng. particulate like this research result, in low temperature Asp. 317 (2008) 756759. (400°C) and short time (5 min) conditions, Cu turned out 12) Y. Kobayashi, S. Ishida, K. Ihara, Y. Yasuda, T. Morita and S. Yamada: Colloid Polymer Sci. 287 (2009) 877880. to diffuse about 1 micrometer in the grain boundary of Ni 13) S. H. Wu and D. H. Chen: J. Colloid Interface Sci. 273 (2004) 165169. (Fig. 8). 14) Y. Yasuda, E. Ide and T. Morita: The Open Surface Sci. J. 3 (2011) Because Cu particles are nanometer sized, the surface of 123130. particles is greatly revitalized.6) We think these particles exist 15) Y. Kobayashi, T. Maeda, K. Watanabe, K. Ihara, Y. Yasuda and T. in a condition similar to the liquid phase. When Cu particles Morita: J. Nanopart. Res. 13 (2011) 5365 5372. 16) K. Monma, H. Suto and H. Oikawa: J. Japan Inst. Metals 28 (1964) in such a state come in contact with the bulk Ni, we think Cu 192196. particles diffuse to the grain boundary of the Ni membrane 17) I. Kawakatsu and S. Kitayama: J. Japan Inst. Metals 40 (1976) 96103. like the molten metal. 18) F. J. A. den Broeder and S. Nakahara: Scr. Metal. 17 (1983) 399404.