Bonding Technique Using Micro-Scaled Silver-Oxide Particles for In-Situ Formation of Silver Nanoparticles

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1 Materials Transactions, Vol. 49, No. 12 (28) pp to 288 #28 The Japan Institute of Metals Bonding Technique Using Micro-Scaled Silver-Oxide Particles for In-Situ Formation of Silver Nanoparticles Toshiaki Morita 1, Yusuke Yasuda 1, Eiichi Ide 1, Yusuke Akada 2; * and Akio Hirose 2 1 Materials Research Laboratory, Hitachi Ltd., Hitachi , Japan 2 Division of Materials and Manufacturing Science, Osaka University, Suita , Japan We investigated a new bonding technique utilizing micro-scaled silver-oxide (Ag 2 O) particles. The results of our investigations revealed that bonding between electrodes using for semiconductor modules can be accomplished by adding myristyl alcohol to silver-oxide particles, followed by heating the mixture in air at 3 C under a pressure of 2.5 MPa. Since this bonding technique produces silver particles with a size of a few nanometers when the silver oxide is reduced by the presence of the alcohol, low-temperature sintering and bonding can be achieved. [doi:1.232/matertrans.mra28269] (Received August 12, 28; Accepted September 22, 28; Published November 12, 28) Keywords: silver oxide, reduction, nanoparticle, bonding technique, epitaxial growth 1. Introduction The development of hybrid electric vehicles (HEVs) is continuing to intensify with their potential for environmental protection and energy saving, and it is projected that the market for such vehicles will grow to one million units by the year 21. 1) However, it is necessary to develop mounting technologies for the inverter units of the drive systems in HEVs that can handle high-temperature environments such as those around the engine and those resulting from increased current capacity. Although silicon carbide (SiC) is one of the most promising materials for the next generation of devices for handling temperature increases and drive temperatures over 25 C, it is necessary to develop a new packaging technology for these higher temperatures. One serious problem arises with high-temperature packaging technologies, i.e., ensuring the long-term reliability of the metallic bonds between semiconductor-chip-mounted parts and various kinds of wiring connections. In particular, in addition to the fact that semiconductor-chip-mounted components (i.e., die-bonded parts) generate ever more heat and thus need ever better thermal resistance, the discharge characteristics of certain parts must be maintained to enable chips to operate stably. Consequently, bonding technology that can handle these circumstances must be developed. It is forecast that SiC devices which are expected to be used in environments at 25 C and over will be able to solve this problem of reduced reliability due to the remelting and intermetallic-compound growth that plagues bonding with conventional tin-based solders. 2) It is also known that when the size of fine metallic particles reaches several nanometers, their apparent melting point becomes lower than that of their bulk material. 3,4) This is assumed to be caused by a phenomenon whereby, when particle size decreases, the surface energy corresponding to the particle size (i.e., surface tension) becomes enormous and, as a result, particles that contact one another quickly fuse together (Fig. 1). 5) This phenomenon can be used to bond materials composed of particles with temperatures below *Graduate Student, Osaka University Layer of organic material Heating Nanoparticle Film disconnection Fig. 1 Heating Fusion mechanism for nanoparticles. Fusion their own melting point, i.e., the bonds that are formed are irreversible and they will not remelt (before the melting point of the particulate material). For example, if materials with comparatively high melting points, like silver, gold, copper, and nickel, are available (as opposed to low-melting-point (18 3 C) materials like the tin-lead-based (Sn-Pb) solder that has been used as nanoparticles up to now), these materials can be used in bonding technology for hightemperature packaging, since no melting will take place after the bond has been formed. Furthermore, the structure of bonds formed by nanoparticles is metallic; 6 8) thus, in addition to good thermal resistance, they have excellent discharge (thermal diffusion) characteristics. This means that nanoparticle-bonding techniques for die-bonded parts would represent high-valueadded technology that could attain hitherto unattainable heatresistance and radiation characteristics. Such a technique can be expected have applications not only to SiC devices (where high thermal resistance is the target) but also to existing silicon devices (where high levels of thermal radiation are required). The main disadvantage of this technique is that a certain thickness is necessary for the nanoparticle bonding, which necessitates the use of large quantities of expensive nanoparticle material. Development of a similar bonding technique that employs inexpensive materials is thus a worthy challenge. It is known that silver oxide reduces when simply heated in air, that is, without the use of a reducing atmosphere. 9) If

2 2876 T. Morita, Y. Yasuda, E. Ide, Y. Akada and A. Hirose Small Cu disk 5 mm diameter 2 mm thick Ag 2 O particles (1µm thick) Pressures Heating in air Shear tool Large Cu disk 1 mm diameter 5 mm thick Fig. 2 the silver produced during reduction is in the form of fine particles, such as those of nanometer size, the abovedescribed bonding could occur. Moreover, since silver oxide is comparatively inexpensive, we considered it could be developed into an inexpensive bonding technique. This research focuses on the reduction of silver oxide. We set our objectives as determining the bonding capability and changes of state of silver-oxide particles during reduction and analyzing the structures of the interface of bonds by using partner electrodes. 2. Experimental Procedure 3 µm Appearance of silver-oxide (Ag 2 O) particles used in testing. 2.1 Particle materials used in bonds To focus on the low-temperature reduction silver oxide and to test bonds made with silver oxide, we prepared silveroxide (Ag 2 O) particles (Wako Pure Chemical Industries, Ltd.) with a particle diameter of 2 to 3 mm, as shown in Fig. 2. We added alcohol (primary alcohol: myristyl alcohol (C 14 H 24 OH) melting point 38 C, made by Wako Pure Chemical Industries, Ltd.) to promote the reduction of the silver oxide, and we tested the resulting mixture. The amount of myristyl alcohol added to the silver-oxide particles was 1 mass%. 2.2 Examination of behavior of silver-oxide particles before and after reduction Thermophysical analysis of silver-oxide particles Since it is thought that the bonding process is closely related to the thermal-decomposition temperature of silveroxide particles, we performed a thermogravimetric/differential thermal analysis (TG-DTA) using a TG/DTA62 (Seiko Instruments) on silver-oxide particles to which myristyl alcohol had been added. All measurements were done in air with a heating rate of 1 C/min Observation of changes of state of silver-oxide particles To investigate the states of silver oxide before and after the peak temperature that was observed by DTA, on either side of the peak temperature (which was observed by DTA), we performed X-ray diffraction (XRD) analysis using a Philips Fig. 3 Specifications of shear-test sample and schematic of shear-test method. PW 34/6 X Pert Pro X-ray diffractometer with a Cu-K tube. We also created specimens that had been cooled rapidly by immersion in ice water after being heated to a predetermined temperature, observed the resulting surfaces with a scanning electron microscope (SEM), and speculated on the reduction of silver oxide and the behavior of the silver particles after the reduction. 2.3 Discussion of bonding structure Bond-strength evaluation We evaluated the bond strength between layers of silveroxide particles and other materials according to their shear strength. Perspective views of a shear test piece are shown in Fig. 3 (JIS Z3198-5). The material bonded to the test piece was anoxic copper (JIS alloy No. C12), nickel plating with a thickness of 2 mm was applied by electrolysis to the copper surface, and then.5-mm-thick silver or gold plating was applied over the nickel plating. The silver-oxide bond material was painted onto each test piece in the regions shown in gray in Fig. 3. The paint thickness was approximately 1 mm. Bonding was then implemented by simultaneous application of heat and pressure in air. We used a SS-1KP bond tester (Seishin Trading, maximum load: 1 kg) for the shear tests. The rate of shear was 3 mm/min, and the maximum load during fracture was measured by applying a shearing force to a small copper disk, as shown in Fig. 3, to fracture the bond. The shear strength was taken as the maximum load divided by the surface area of the bond (equivalent to the surface area of the small copper disk) Observation of bond microstructure First, we observed a cross-section through each bond by SEM. The cross-sectional samples used for the SEM observation were finished by buffing with diamond paste (.25 mm particle diameter) using a special polishing oil. They were then subjected to argon-ion milling (at accelerating voltage of 15 kv) for 18 s. To obtain more detailed images of the condition of the bond between silver oxide and copper, we observed the bond interface with a transmission electron microscope (TEM). The samples for observing the cross-sections of bonds by TEM were milled using focused-ion-beam (FIB) milling and an observation device (Hitachi FB-2A). The milled thinfilm samples were then mounted on a cutout mesh and observed by TEM (Hitachi H9, acceleration voltage:

3 Bonding Using Silver-Oxide Particles for Formation of Silver Nanoparticles 2877 DTA (mv / g) DTA (mv / g) (a) (b) Temperature, T / C TG (mass %) TG (mass %) Fig. 4 TG-DTA examination of change-of-state temperature during heating: (a) silver-oxide particles alone and (b) silver-oxide particles with the addition of 1 mass% myristyl alcohol. DTA (mv/g) Temperature, T / C TG (mass%) Fig. 5 TG-DTA measurements of silver-oxide (Ag 2 O) particles with added myristyl alcohol (15 to 3 C). Intensity (a/b. unit) 18 C Cu Ag Ag 14 C Cu Ag 2 O Ag 2 O 3 kv). When necessary, an ion-milling method was also used to make the observation samples even thinner θ (degree) Results and Discussion 3.1 Decomposition temperature of silver oxide The TG-DTA curves shown in Fig. 4 are for (a) silveroxide particles alone and (b) silver-oxide particles with 1 mass% added myristyl alcohol (called silver-oxide bond material hereafter). For silver-oxide particles alone, we ascertained the presence of an endothermic peak around 4 C in the DTA curve, and we also noted a weight loss of approximately 8 mass% from the TG curve. This weight loss is thought to be because the silver oxide particles were reduced and silver was produced. With the silver-oxide bond material, on the other hand, we observed an exothermic peak in the vicinity of 15 C in the DTA curve and ascertained that there was a weight loss of approximately 2 mass% from the TG curve. The results of TG-DTA measurements on the silver-oxide bond material from 15 to 3 C are shown in Fig. 5. In addition to the exothermal reaction and weight loss around 15 C that were mentioned above, we also found further exothermic peaks around 25 and 28 C in the DTA curve. These exothermic peaks at 15, 25, and 28 C are referred to below as the first, second, and third peaks, respectively. The results of X-ray diffraction on the silver-oxide bond material before and after the first peak are shown in Fig. 6. We ascertained that there are only silver-oxide peaks at 14 C, i.e., before the first peak. However, these silver-oxide peaks disappeared at 18 C, after the first peak, and only a silver peak was detected. These results suggest that the first peak and weight reduction at 15 C are due to the reduction Fig. 6 X-ray diffraction on silver-oxide (Ag 2 O) particles with added myristyl alcohol. reaction of silver oxide. They also suggest that the reduction temperature of silver-oxide particles alone when heated in air, which is approximately 4 C, can be lowered to approximately 15 C by the addition of myristyl alcohol. It is thought that the 1st peak of silver-oxide bond material of Fig. 4(b) is the combustion heat by oxidization of alcohol. Although outside temperature is 15 C, when the combustion reaction by oxidization of alcohol is produced, it is thought that exothermic temperature is high temperature rather than 4 C which is the reduction temperature of the silver-oxide particle alone. Furthermore, it is thought that the reduction reaction of silver oxide has arisen at this time. 3.2 Decomposition of silver-oxide and sintering behavior The surface conditions of the silver-oxide bond material before and after the first peak temperature in Fig. 4 are shown in Figs. 7(a) and 7(b). In (a), 14 C, i.e., before the first peak temperature, the decomposition reaction of the silver oxide did not occur, and there was no change in the surface of the silver-oxide particles. In (b), 18 C, i.e., after the first peak temperature, it was confirmed that a large number of granular bumps in the order of 1 nm formed on the surface of the silver-oxide particles. These bumps are thought to be silver produced by the reduction reaction of the silver oxide. Since we found a large number of nanometer-sized silver particles

4 2878 T. Morita, Y. Yasuda, E. Ide, Y. Akada and A. Hirose (a) 14 C (b) 18 C 2 nm 2 nm (c) 22 C (d) 26 C 2 nm 2 nm (e) 29 C (f) 5 C 2 nm 1 µm Fig. 7 Heating-induced shape change of silver-oxide (Ag 2 O) particles with added myristyl alcohol: (a) at 14 C, (b) at 18 C, (c) at 22 C, (d) at 26 C, (e) at 29 C, and (f) at 5 C. after the reduction of the silver oxide, we infer that the silver nanoparticles were produced during reduction of the silver oxide. Regarding the second exothermic peak shown in Fig. 4, the surface conditions of the silver-oxide particles before and after that peak are shown in Figs. 7(c) and 7(d), respectively. In (c), 22 C, i.e., before the second peak, the surface of the silver-oxide was covered with silver nanoparticles size of 1 to 5 nm. In (d), 26 C, i.e., after the peak, sintering of the silver particles progressed up to a size of about 2 nm, and the silver nanoparticles that covered the surface layer disappeared, but the silver nanoparticles with diameter of between 1 and 1 nm have clearly survived at the grain boundaries of the sintered particles. The second peak is thought to be an exothermic peak due to the sintering of the silver nanoparticles covering the surface layer. The appearance of the particle surface at 29 and 5 C, i.e., after the third peak in Fig. 4, is shown in Figs. 7(e) and 7(f), respectively. Figure 7(e) shows that at 29 C, after the third peak, the silver nanoparticles seen at 26 C, in Fig. 7(d), no longer exist, so it is concluded from this result that this exothermal reaction had caused the sintering of the silver to proceed until the particle diameters had grown to approximately 5 nm. At 5 C, the grains had coarsened to between 1 and 2 mm. From the above results, it is clear that silver oxide produces silver nanoparticles when subjected to a heated reduction process with an alcohol-based organic compound and that it is possible to form a bond at the location of the silver nanoparticles. Shear strength (MPa) Ag plating Au plating only Ag-oxide particles Bonding temperature, T / C Fig. 8 Bond strength with respect to bonding temperature Bond-strength evaluation The results of evaluating bond strength (i.e., shear) with respect to bonding temperature are shown in Fig. 8. Each test specimen was the test piece shown in Fig. 3 (either silver or gold plating formed on a copper surface); bonding was done in air at a pressure of 2.5 MPa and a predetermined temperature held for 2.5 minutes. For comparison, we also evaluated silver oxide particles alone, without any alcohol. For silver-oxide particles with no added alcohol (silver plating only), we determined that the bond strength remained unchanged at a low level, even when the bonding temperature was increased, so bonding was not possible. With the silver-

5 Bonding Using Silver-Oxide Particles for Formation of Silver Nanoparticles 2879 (a) (b) Ni Ni Au plating Ag plating 1 µm 1 µm Fig. 9 SEM images of cross-sections of bonds formed with silver or gold plating at bonding temperature of 3 C: (a) silver sintering layer and silver-plating interface and (b) silver sintering layer and gold-plating interface. oxide bond material to which an alcohol-based organic material had been added, on the other hand, bonds with respect to silver-plated surfaces exhibited a bond strength of approximately 18 MPa at 25 C and 2 MPa at 3 Cor more. Gold plating displayed a similar tendency to silver plating, with almost equal shear strength. SEM images of cross-sections through bonds on silver and gold plating at formed at a bonding temperature of 3 C are shown in Fig. 9. In both specimens, a few minute voids are scattered within the silver layer (which has been sintered), but each silver sintered layer is densely sintered and composed of grains with size in the order of a few hundred nanometers. These results images closely match those of Fig. 7(e). Given that fact, we consider that we achieved an effect similar to the nanoparticle bonding previously reported 6 8,1,11) by forming the nanoparticles during the reduction of silver oxide. Figure 1 shows TEM images of the interface between the plating film and the silver sintering layer of each of the specimens observed in Fig. 9. It is clear that both (a) silver plating and (b) gold plating are free from defects and good bonding was achieved. With the silver plating, the boundary surface could not be distinguished, and a structure composed of the same crystal grains was formed. With the gold plating, it is clear that the presence of crystal grains aligned in the same direction on the gold plating film as in the silver sintering layer (see circled part in (b)). Electron-beam (e) (111) Ag (a) (b) Au (111) Ag plating 1 nm Au plating 1 nm (f) 2 nm (c) (d) Ag Au 2 nm Fig. 1 TEM images of cross-sections of bonds with silver or gold plating: (a) low-magnification image (Ag/Ag), (b) low-magnification image (Ag/Au), (c) electron-beam-diffraction diagram of area (1) in Fig. 1(b), (d) electron-beam-diffraction diagram of area (2) in Fig. 1(b), and (e) and (f) high-resolution images (Ag/Au).

6 288 T. Morita, Y. Yasuda, E. Ide, Y. Akada and A. Hirose diffraction patterns taken from the silver sintering layer (1) and the gold plating (2) in Fig. 1(b) are shown in Figs. 1(c) and 1(d), respectively. It is clear that both patterns are the same, and the gold and silver are in the same orientation. Figures 1(e) and 1(f) are high-resolution TEM images of the interface shown in Fig. 1(b) (although the field of view is different). Figure 1(e) shows that the crystal orientations match, and Fig. 1(f) shows that the crystals that can be seen to be bi-crystals were constructed in the same. Since both silver and gold have a face-centered cubic structure, and the difference between their lattice constants is small (silver: 4.86 Å; gold: 4.79 Å), it is thought that the silver nanoparticles grow epitaxially to match the orientation of the gold-plated surface. We think this technique creates strong bonding between silver and the metal by the combustion heat generated during oxidation of alcohol (which is generated by decomposition of the silver-oxide particles as it is heated). 4. Conclusions We developed a new bonding technique that produces silver nanoparticles in-situ from the reduction of silver-oxide particles with a size of a few micrometers. Our investigations on these silver nanoparticles lead to four main conclusions. (1) Silver-oxide particles with an average particle diameter of 2 to 3 mm are mixed with myristyl alcohol and heated in air, they can be reduced at 15 C to produce silver nanoparticles. (2) The silver nanoparticles produced by the reduction process from silver-oxide particles can be sintered at 25 C. (3) Metal bonding between silver or gold is possible at temperatures of over 25 C and a pressure of 2.5 MPa by producing silver nanoparticles created by heating a mixture of silver-oxide particles and myristyl alcohol. (4) Silver nanoparticles grow at the bonding interface with silver or gold, aligned with their crystal orientations. REFERENCES 1) The Newest Investigation Report of Power Device Module and a Relevant Market, and a Future View 27, (Japan Marketing Survey, Tokyo, 27) pp ) T. Morita, R. Kajiwara, I. Ueno and S. Okabe: Jpn. J. Appl. Phys. 47 (28) ) M. Takagi: J. Phys. Soc. Jpn. 9 (1954) ) J. R. Groza and R. J. Dowding: Nanostruct. Mater. 7 (1996) ) G. L. Allen, R. A. Bayles, W. W. Gile and W. A. Jesser: Thin Solid Films 144 (1986) ) A. Hirose, E. Ide and K. F. Kobayashi: Japan Inst. Electronics Packaging 7 (24) ) E. Ide, S. Angata, A. Hirose and K. F. Kobayashi: Acta Mater. 53 (25) ) E. Ide, A. Hirose and K. F. Kobayashi: Mater. Trans. 47 (26) ) T. Morita, M. Kato, J. Onuki, H. Onose, N. Matsuura and S. Sakurada: Jpn. J. Appl. Phys. 38 (1999) ) T. Morita, E. Ide, Y. Yasuda, A. Hirose and K. Kobayashi: Jpn. J. Appl. Phys. 47 (28) ) Y. Akada, H. Tatsumi, T. Yamaguchi, A. Hirose, T. Morita and E. Ide: Mater. Trans. 49 (28)