High-Temperature-Resistant Interconnections Formed by Using Nickel Micro-plating and Ni Nano-particles for Power Devices

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1 Kato et al.: High-Temperature-Resistant Interconnections (1/6) [Technical Paper] High-Temperature-Resistant Interconnections Formed by Using Nickel Micro-plating and Ni Nano-particles for Power Devices Noriyuki Kato, Suguru Hashimoto, Tomonori Iizuka, and Kohei Tatsumi Graduate School of Information, Production and Systems, Waseda University, 2-7 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka , Japan (Received August 6, 2013; accepted November 14, 2013) Abstract The improvement of interconnection technology is becoming a top priority for the operation of SiC devices at high temperatures. We proposed a new interconnection method using nickel electroplating to form bonds between chip electrodes and substrate leads. We also newly proposed low-temperature nickel nanoparticle sintering to form die bonding connections. SiC devices assembled with these new connection methods operated successfully in a high-temperature environment of about 300 C. We confirmed that these methods had adequate potential as an advanced heat resistant package in comparison with conventional interconnections. Keywords: High Temperature Resistant Packaging, Power Device, Micro Electro-plating Bonding, SiC Device, Nano-Nickel 1. Introduction To reduce the size and increase the efficiency of power converters mounted in hybrid vehicles or electric vehicles, research on the development of methods to increase the output power density using technology such as silicon carbide (SiC) devices is in progress. High temperature SiC devices require the materials for packaging capable of working at higher temperature than those for Si devices. While studies have been conducted on heat-resistant packaging materials that can replace Al bonding wire or solder materials having low melting points, various problems in alternative methods have been pointed out.[1, 2] Our approach described in this paper is to form the conductive connection with Ni at relatively lower temperature. [3] For the connection of chip electrodes Ni micro-plating bonding (NMPB)[4] was used and die attach connection was carried out by sintering nickel nanoparticles at a low temperature.[5] An example of the concept for these interconnections is shown in Fig. 1. A -electrode A -electrode A -pla ng Ni-pla ng Cu-wire Ni-pla ng Nano-Ni Cu plate Cu plate Fig. 1 Model of bonded SiC diode specimen. 2. Experimental Procedure 2.1 Bonding by nickel micro-plating We used nickel electroplating to form nickel micro-plating bonds. The plating conditions were as follows: Ni plating bath of Watts solutions, bath temperature of 50 C, electrical current density of 5 A/dm 2, and substrate and lead material of mainly copper. The growth rate of copper plating on a flat plate was ranging from 0.75 μm/min to 0.9 μm/min. To simulate bonding between the chip electrodes and the substrate leads of a power device, we brought a copper wire (diameter: 172 μm) into contact with a copper plate and plated the wire and the plate as shown in Fig. 2 (a). To simulate die bonding, we brought the gold surface of a silicon chip with vapor-deposited gold into contact with a lead frame cut into strips, and plated the chip and the lead frame as shown in Fig. 2 (b) lead frame Cu-wire Cu plate 2.7 Si-chip (a) (b) Au-electrode Fig. 2 (a) Copper wire bonded to plate (b) Silicon chip bonded to lead frame. Copyright The Japan Institute of Electronics Packaging 87

2 Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013 To evaluate the high-heat-resistance reliability of the specimens after micro-plating bonding, we heated them at various temperatures between 100 and 500 C in an argon atmosphere for 60 min and then measured the shear strength of the bonds with a shear tester (Rhesca model PTR-01). The specimens were heated to 500 C for accelerated aging testing to evaluate the interface reliability and the change in resistivity. To observe the cross section of the bonded portions we used a scanning electron microscope (SEM Hitachi model S-3400N.). 2.2 Low-temperature nickel nanoparticle sintering We used a 5 wt.% nickel nanoparticle solution as the bonding material. The nickel nanoparticles had a diameter of 20 nm and were dispersed in a solution of isopropyl alcohol (IPA). Since IPA is highly volatile, it evaporated after application, leaving only the nickel nanoparticles. We used an electrostatic atomizer (made by Apic Yamada)[6] to apply the solution. As shown in Fig. 3(a), this device enabled a selective and uniform application of the solution on the specimen surface by means of electrostatic force. For the evaluation of bonding characteristics with Ni nanoparticles we prepared Si dummy chips with size (A) 2.7 mm 2.7 mm and (B) 8.0 mm 8.0 mm. After applying the solution to chip (A), electrodes were placed on chip (B) with their surfaces facing each other and were heated and pressurized in atmosphere to form a bond as shown in Fig. 3(b). The bonding conditions were as follows: the device stage and the pressurizing head were heated to 300 C, and a constant load of 147 N was applied by the pressurizing head. After heating the bonded specimens in an argon atmosphere for 60 min (between room temperature and 500 C), we measured their shear strength. 2.3 Chips used for bonding strength evaluation The chips used for bonding were silicon dummy chips with aluminum film electrodes, and SiC-SBD(Shottky barrier diode) chips (1,200 V, 15 A, mm, made by SiCED Electronics Development GmbH) having an aluminum electrode film on the top surface and a silver film on the back surface. They had a thickness of 365 μm. To study bonding on these aluminum electrode surfaces, we used chips having an electroless nickel plating film formed on an aluminum electrode surface, and chips having vapordeposited nickel and gold on aluminum electrode films. We evaluated nickel nanoparticle bonding strength using bonding between pairs of silicon dummy chips (A) and (B). We applied the nickel nanoparticle solution to the electrode surface and studied on the bonding characteristics between both electrodes. Fig. 3a Schematic diagram for electrostatic atomization. 3. Results and Discussion 3.1 Nickel micro-plating bonding (NMPB) and evaluation Plating bonding study To simulate bonding between the chip electrodes and the substrate leads of a power device, the copper wire was into contact with a copper plate and plated the wire and the plate, as shown in Fig. 2(a). Figure 4 shows the cross section of a copper wire (diameter: 172 μm) bonded to the 147N Nano Ni Chip(A) Chip(A) Heating Stage Chip (B) Fig. 3b Bonding steps of dummy chips with using Ni nano-particle atomization. 88

3 Kato et al.: High-Temperature-Resistant Interconnections (3/6) Cu-wire Ni-pla ng Cu plate 5μm Fig. 4 Plating cross section of copper wire bonded to copper plate. Fig. 6 Heating temperature-shear strength characteristic for plated bonding specimens of copper wire and copper plate made using different plating times. Si-chip 50μm Ni-pla ng lead frame Fig. 5 Plating cross section of silicon chip with vapor-deposited gold bonded to lead frame. copper plate by Ni micro-plating. To simulate die bonding, we plated the chip and the lead frame as shown in Fig. 2 (b). Figure 5 shows the observed cross section. Both cross sections indicate no problematic voids or other defects generated by the nickel plating Shear testing To evaluate whether specimens with plated bonds had high heat resistance, we measured their shear strength after heating. We measured specimens in which a copper wire (with a length of 1 mm and diameter of 172 μm) was brought into contact with a copper plate and plated for 15 min or 30 min. The specimens were heated in an argon atmosphere and then shear-tested. We carried out shear testing nine times at each heating temperature and found the average shear strength of the specimens. As shown in Fig. 6, specimens with plating times of both 15 and 30 min exhibited high shear strength after being heated to 500 C. The shear strength of both specimen types increased with an increase in the heating temperature. This finding may have been the result of increasing copper-nickel alloying caused by diffusion, resulting in greater adhesion and strength at the interface. From the results described in 3.1.4, it appeared that no second phase was formed at the Fig. 7 Heating temperature-resistance and heating-temperature-resistivity characteristics for specimens of nickel-plated wire on copper. interface between copper and nickel layers, indicating that no deterioration caused by the formation of brittle phases or intermetallics with voiding occurred in the high-temperature environment Copper-nickel alloy resistance measurement Since the electrical resistance of a copper-nickel alloy layer formed in a high-temperature environment may increase, we measured the change in resistance caused by alloying. We used the four-terminal method to measure the resistance of copper wire specimens (having a length of 30 mm and diameter of 172 μm) plated for 30 min and then heated in an argon atmosphere. After plating, the wire diameter increased to 200 μm, and the distance between the terminals used for the wire resistance measurement was 12.5 mm. Figure 7 shows the results. Diffusion and alloying proceeded with an increase in the temperature, causing an increase in resistance.[7] The increase in resistance even for a heating temperature of 500 C (for 1 h) was a value of around 10%. The diffusion condition of 500 C for 1 h is equivalent to that of 350 C for 4,580 h, assuming that the activation energy for Ni diffusion in Cu is 225 kj/mol.[8] This indicates that problematic defects are not considered to be generated in the interconnection during practical use up to 350 C. 89

4 Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013 (a) Heating condition: no heating (as bonded). (b) Heating condition: heated for 60 min at 300 C. (c) Heating condition: heated for 60 min at 500 C. Fig. 8 SEM images and concentration distribution graphs of cross section of nickel-plated copper plate specimens Analysis of copper-nickel diffusion state To verify the state of copper-nickel diffusion in specimens, after heating, we used a SEM to observe the cross sections of a copper plate covered by Ni plating. We measured concentration profiles of each element in the vicinity of interfaces using an energy-dispersive X-ray spectroscopy (EDX) line analysis. Figure 8 shows the results. The positions of EDX line analysis are indicated within each SEM image and the concentration profiles are displayed in the graphs. A comparison of the graphs of unheated specimens and specimens heated at 300 C in Fig. 8 (a) and (b) reveals almost no change in the concentration distribution. On the other hand the concentration distribution of the specimen heated at 500 C in Fig. 8 (c) shows the formation of an alloy layer of approximately 5 μm. There were no second phase observed, as predicted from a phase diagram of Cu-Ni system. Further, although Kirkendall voids have formed in the copper plate, the results described in Sections and indicate that their effect is not serious. 3.2 Nickel nanoparticle bonding and evaluation As described in Section 3.1.1, plating-based die bonding can be carried out by using a strip or lattice substrate structure, but it is difficult to use for bonding flat plate surfaces together. It has been shown that flat plates can be bonded using low-temperature nickel nanoparticle bonding[5, 9]; hence, we studied the application of this method to die bonding. We brought the electrode surfaces of silicon dummy chips that are (A) mm and (B) mm in size into facing position, bonded them using nickel nanoparticles, and then shear-tested the specimens after heating. We performed shear testing five times at each temperature and plotted the average shear strengths. Figure 9 shows that a slight drop in the shear strength was 90

5 Kato et al.: High-Temperature-Resistant Interconnections (5/6) Fig. 9 Heating temperature-shear strength characteristic of specimens bonded using nickel nanoparticles. SiC diode V D I D 2 Ω Variable DC power source 0 15V Fig. 10 Circuit used for measuring diode V-I characteristics. I D [ma] V D [mv] Fig. 11 V-I characteristics of bonded SiC diodes at different heating temperatures. found when the specimens were heated at 500 C, but since the value still remained high (over 10 N). 3.3 High-temperature circuit operation testing and evaluation using SiC diode chips To evaluate whether the bonding technologies that we have discussed are practical for use in power device connections, we carried out high-temperature circuit operation testing on SiC-SBD chips (1,200 V, 15 A, made by SiCED). As shown in Fig. 10, the interconnections were created for the specimen by using nickel micro-plating to bond connections between chip electrodes and leads of a silver-plated copper substrate joined and bonded by a copper wire (using a plating time of 30 min). Nickel nano-particles were used for die bonding, with a surface-processed aluminum electrode bonded to a silver-plated copper substrate. After bonding the specimens, we performed the electrical operation tests while heating each specimen on a hot plate at a different constant temperature (ranging between 20 and 325 C). We then measured the diode voltage (V D ) and current (I D ), varying the power supply voltage (between 0 and 15 V) using the circuit illustrated in Fig. 10. Figure 11 shows the results for different temperatures. In a high-temperature environment of 325 C, we obtained the diode V-I characteristics and did not observe the change in the resistivity caused by the bond deterioration during measurements. With using Ni interconnections newly introduced in this paper the high temperature operation of the SiC diode was first confirmed. 4. Conclusions 1. Our findings demonstrated that nickel micro-plating bonding(nmpb) could be used for bonding chips to substrate electrodes and for die bonding to chip substrates in power devices. We demonstrated that low-temperature bonding ensured bond reliability in high-temperature environments of over 300 C. We observed no bond deterioration during accelerated diffusion tests of bonds formed by coppernickel plating. 2. We used nickel micro-plating bonding and low-temperature nickel nanoparticle sintering to form bonds between SiC power device chips and substrate electrodes and to form die bonding connections, respectively. We tested device operation in a high-temperature environment of about 300 C. 3. Our findings demonstrated that micro-plating-based chip bonding technology could be considered to have adequate potential as a practical, simple mounting technology that is highly heat-resistant and ensures high reliability and low cost. References [1] H. A. Mantooth, M. M. Mojarradi, and R. W. Johnson, Emerging Capabilities in Electronics Technologies for ExtremeEnvironments. Part I - High Temperature Electronics, IEEE Power Electronics Society Newsletter, issue 1, [2] L. Coppola, D. Huff, F. Wang, R. Burgos, and D. Boroyevich, Survey on High Temperature Packaging Materials for SiC-Based Power Electronics, Proc. PESC, Orlando, FL, pp , [3] S. Terashima, Y. Yamamoto, T. Uno, and K. Tatsumi, Significant reduction of wire sweep using Ni plating to realize ultra fine pitch wire bonding, Proceedings of the 52nd Electronic Components and Technology 91

6 Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013 Conference, 52, pp , [4] K. Tatsumi and T. Ando, Plating micro bonding used for Tape Carrier Package, Proc. NIST/IEEE VLSI PACKAGING WORKSHOP, YORKTOWN HEIGHTS, N.Y. 1993, [5] K. Tatsumi et al., Electronic component bonding material, composition for bonding, bonding method, and electronic component, PCT/JP2012/ [6] (accessed Aug 2013). [7] C. Y. Ho, M. W. Ackerman, K. Y. Wu, T. N. Havill, R. H. Bogaard, R. A. Matula, S. G. Oh, and H. M. James, Electrical resistivity of ten selected binary alloy systems, J. Phys. Chem. Ref. Data, Vol. 12, No. 2, pp , [8] Metal data book, edited by Japan Institute of Metals, Maruzen, [9] S. Hashimoto, T. C. Lun, K. Tatsumi, A. Nogami, and Y. Sawa, Study on bonding by using Ni nano particles for high temperature packaging, Proceedings of autumn meeting of the Japan Institute of Metals and Materials 2013, p Noriyuki Kato Suguru Hashimoto Tomonori Iizuka Kohei Tatsumi 92