Wen P. Lin

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A flip-chip interconnect process at 180^°C using the silver-indium (Ag-In) binary system is reported. An array of 50 × 50 flip-chip joints with 100 μm pitch and 40 μm joint diameter was fabricated. Each joint has the column structure of Ag/(Ag)/Ag₂In/(Ag) that connects the silicon (Si) chip to the copper (Cu) substrate. The joint height is approximately 50 μm. In this structure, Ag₂In is a dominating intermetallic compound in the Ag-In system with melting temperature of 660^°C. (Ag) is a solid solution phase of Ag with In composition up to 20 at.%. It has a solidus temperature range of 695 to 962^°C depending on In composition. In long-term operation, (Ag)/Ag₂In/(Ag) is expected to gradually convert to a single (Ag) phase, which is more reliable. Thus, the flip-chip joints will get better in use. In fabrication, 50 × 50 Ag columns were made on Si wafer coated with chromium (Cr) and gold (Au). The Cu substrate was electroplated with Ag(10 μm)/In(5 μm)/Ag(thin). Si chips with Ag columns were bonded to Cu substrates at 180^°C for 5 min. No flux was used. Cross-sectional scanning electron microscopy images show that all 50 Ag columns in one row are well bonded to the Cu substrate without visible voids or cracks. Energy-dispersive X-ray spectroscopy data indicate that the resulting column structure is Ag/(Ag)/Ag₂In/(Ag). The process temperature of this new interconnect method is 80 ^°C below the typical reflow temperature of tin-based lead-free solders.

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Summary In this study, 6 mm 6 mm Ag foils were bonded directly onalumina substrates which were precoated with TiW and Au with-out any bonding medium such as solder. This is made possible bysolid state bonding theory where Ag atoms and Au atoms arebrought within atomic distance so that they can share electrons.The close proximity of Ag and Au is achieved by deformationwith static pressure. Since Ag and Au are ductile, only 1000 psiand 260 C are required. SEM images show that the Ag foil iswell bonded to the Au layer on alumina. Five bonded sampleswent through shear test. The shear strength measured far exceedsthe strength requirement specified in MIL-STD-883G standard.This bonding technology can serve as an alternative to DBC orDBA technology on applications where Ag is preferred over Cudue to its ductility to manage CTE mismatch and Ag is preferredover Al owing to its higher thermal conductivity. References [1] Yoshino, Y., 1989, “Role of Oxygen in Bonding Copper to Alumina,” J. Am.Ceram. Soc., 72(8), pp. 1322–1327.[2] Dupont, L., Khatir, Z., Lefebvre, S., and Bontemps, S., 2006, “Effects of Metal-lization Thickness of Ceramic Substrates on the Reliability of Power Assem-blies Under High Temperature Cycling,” Microelectron. Reliab., 46(9–11), pp.1766–1771.[3] Yoshino, Y., Ohtsu, H., and Shibata, T., 1992, “Thermally Induced Failure ofCopper-Bonded Alumina Substrates for Electronic Packaging,” J. Am. Ceram.Soc., 75(12), pp. 3353–3357.[4] Schulz-Harder, J., 2001, “HPS DBC Substrates for High ReliableApplications,” Proceedings of IMAPS Nordic, Oslo, Norway.[5] Schulz-Harder, J., 1997, “Reliability of Direct Copper Bonded (DBC) Sub-strates,” Proceedings of ISHM 11th European Microelectronic Conference,Venice, Italy.[6] Schulz-Harder, J., 2003, “Advantages and New Development of Direct BondedCopper Substrates,” Microelectron. Reliab., 43(3), pp. 359–365.[7] Cusano, D. A., Loughran, J. A., and Sun, S. E., 1976, “Direct Bonding of Met-als to Ceramics and Metals,” U.S. Patent No. 3,994,430.[8] Dalgleish, B. J., Trumble, K. P., and Evans, A. G., 1989, “The Strength andFracture of Alumina Bonded With Aluminum Alloys,” Acta Metallic., 37(7),pp. 1923–1931.[9] Ning, X. S., Lin, Y., Xu, W., Peng, R., Zhou, H., and Chen, K., 2003,“Development of a Directly Bonded Aluminum=Alumina Power ElectronicSubstrate,” Mater. Sci. Eng. B, 99(1–3), pp. 479–482.[10] Knoll, H., Weidenauer, W., Ingram, P., Bennemann, S., Brand, S., and Petzold,M., 2010, “Ceramic Substrates With Aluminum Metallization for PowerApplication,” Proceedings of IEEE Electronic System-Integration TechnologyConference, Berlin, Germany, pp. 1–5.[11] Lei, T. G., Calata, J. N., Ngo, K. D. T., and Lu, G. Q., 2009, “Effects of LargeTemperature Cycling Range on Direct Bond Aluminum Substrate,” IEEETrans. Device Mater. Reliab., 9(4), pp. 563–568.[12] Lee, C. C., Wang, D. T., and Choi, W. S., 2006, “Design and Construction of aCompact Vacuum Furnace for Scientific Research,” Rev. Sci. Instrum., 77(12),p. 125104.[13] Wang, P. J., Kim, J. S., and Lee, C. C., 2008, “Direct Laminating SilverFoils on Copper Substrate,” J. Mater. Sci.: Mater. Electron., 19(11), pp.1097–1099.[14] Available online: http://www.q-tech.com/assets/tests/std883_2019.pdf

Flip-Chip Process Using Ag–In Transient Liquid Phase Reaction

In this research, a fundamental study is conducted to identify the materials and processes for producing bonding/barrier composites on Bi2Te3 for high temperature thermoelectric applications. Pd, Ni, Ni/Au, Ag, and Ti/Au were deposited and evaluated using scanning electron microscope (SEM) and energy dispersive X-ray (EDX). A thermal evaporated, 100nm Ti/100nm Au composite gave the most promising

Solid State Bonding of Silver Foils to Metalized Alumina Substrates at 260°C

Flip-chip interconnect joints of copper/gold (Cu/Au) with 40-μm diameter and 100-μm pitch were made between silicon (Si) chips and Cu substrates using solid-state bonding at 200°C with a static pressure of 250-400 psi (1.7-2.7 MPa). The array of 50 × 50 Cu/Au columns was first created. In fabrication, photoresist with 50 x 50 cavities of 40-μm diameter and 45-μm depth were produced on Si wafers, which were first coated with 30 nm chromium and 100 nm Au films. Cu of 25-μm thickness was electroplated in the cavities, followed by 10 μm of Au. After stripping the photoresist, the array of 50 x 50 Cu/Au columns was obtained on a chip region of the wafer. The 50 x 50 Cu/Au columns on the chip were bonded to a Cu substrate by solidstate bonding. No molten phase was involved and no flux was used. No underfill was applied. The corresponding load for each column was only 0.22-0.35 g. Cross-section scanning electron microscopy images show that Cu/Au columns were well bonded to the Cu substrate. Despite the large mismatch in the coefficient of thermal expansion between Si and Cu, no joint breakage was observed. The pull test was performed and the fracture modes were evaluated. The fracture force and fracture strength obtained were 11.2-14.2 kg and 35-44 MPa (5000-6400 psi), respectively. The measured fracture force is four times larger than the criterion of the pull-off test in MIL-STD-883E.

Bonding/barrier layers on bismuth telluride (Bi 2 Te 3 ) for high temperature applications

In this paper, we will review the silver-indium (Ag-In) phase diagram and explores its unique features that enable the development of new bonding processes for electronic packaging. Its melting range covers 156°C to 952°C, from In melting point to Ag melting point. It consists of three intermetallic compounds (IMC), AgIn₂, and Ag₂In, and Ag₃In, and a solid solution (Ag). Our experimental results do not show the Ag₃In compound. In a Ag-rich joint design, only Ag₂In and (Ag) show up. By further annealing at 250°C for 350 hours or 400°C for 5 hours, Ag₂In converts to (Ag) by reacting with Ag. With the proper structure to begin with, a joint can be made at 180°C that contains only Ag₂In and (Ag), achieving a melting temperature of 600°C. Annealing to convert Ag₂In to (Ag) will increase the melting to higher than 800°C. Accordingly, various processes can be designed and developed for different packaging applications ranging from automotive electronics, high temperature electronics, and oil exploration.

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In this paper, 10μm Ag flip-chip interconnect joints by solid-state bonding was demonstrated between Si chips and Cu substrates. In experiments, an array of 125×125 Ag columns that had 10μm in diameter, 20μm in pitch and 10μm in height was fabricated in one chip region of Si wafers that were first metalized with Cr/Au. The process was performed using solid-state atomic bonding at 250°C with a static pressure of 800 psi (5.5 MPa) for 10 minutes in 0.1 torr vacuum. The corresponding load for each column was 0.044 gm. Cross section SEM images of one row of joints show that the Ag flip-chip joints were bonded to the Cu substrate without voids or breakage. Despite significant coefficient of thermal expansion (CTE) mismatch between Si and Cu, the Si chips did not break from Cu. There are several advantages compared to the popular Sn based Pb-free flip-chip technology: high electrical and thermal conductivities, no IMCs and related issues, no flux issue, high ductility for managing CTE mismatch between chips and packages, high operation temperature, and the possibility for high aspect ratio interconnect.

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We performed 40μm silver (Ag)/gold (Au) composite flip-chip interconnects joints between silicon (Si) chips and copper (Cu) substrate using solid-state bonding process at 200°C. 50×50 Ag/Au columns with 40μm in diameter and 100μm pitch were fabricated on a chip region by photolithographic and electroplating processes. Then, the Ag/Au columns were bonding to Cu substrate with fresh surface using solid-state at 200°C for five minutes with a static pressure of 1.7~2.7 MPa (250~400psi). The corresponding load for each column is 0.22~0.35 gm. The five minutes is constrained by the equipment. In theory, bonding should occur in seconds. Cross section SEM images show that Ag/Au column is well bonded to Cu substrate with no void or breakage within it. Ag/Au composite joints well manage the shear strain induced by coefficient of thermal expansion (CTE) mismatch. There is no molten phase during the bonding process. Neither flux nor underfill was used. Compared to solder flip-chip joints, this new process has the reduction of electrical resistance of the joints of the same size by a factor of 6. Pull test was conducted. The fracture force and fracture strength are 6.5~7.3kg and 2,940~3,310psi (20.2~22.8MPa), respectively. The fracture force is 2.5× of the criterion in MIL-STD-883E. The SEM/EDX analysis of the fracture interface showed that fracture of bonding interface is least likely to incur in pull test.

Cu/Au Flip-Chip Joints Made by 200

Since Ag and In react to form compound at room temperature was investigated, but the chemical reactions during bonding are not well understood. The Ag-In phase diagram shows two dominating intermetallic compounds (IMC), AgIn2 and Ag2In. Therefore, based on the following systematic experiments to study the chemical reactions of AgIn system and compound formation. Copper (Cu) substrates were electroplated with 40 μm thick Ag layer, followed by indium layer of 1, 5, 10, and 15 μm, respectively. Thick Ag layer is chosen to prevent In reaction with underlying Cu substrate. The samples were annealed at 180°C in 100 millitorr for 5 minutes to emulate the bonding conditions. The microstructure and the composition on the surface of the samples were evaluated using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) was also employed to identify the IMC on the surface. The samples were then cut in cross section, polished, and studied also by SEM and EDX. The results show that, for the sample with 1μm thick indium, the In layer converts to Ag2In entirely after annealing. For sample with In layer thickness of 5 μm, AgIn2, Ag2In, and silver solid solution (Ag) all exist after annealing. No indium was identified. For sample having 10, and 15μm thick indium, In was detected on the surface, but was not found on the cross section. The polishing process probably bevels the thin In layer on the surface, making it invisible. We next studied what would happen if the Ag was annealed first before plating In. After plating the Ag layer, samples were annealed at 450°C for 3 hours to grow Ag grains. This was followed by plating 10μm In and annealing at 180°C. The result shows AgIn2, Ag2In as well as In. Significant In remains on the surface. The thicknesses of AgIn2 and Ag2In layers are thinner than the sample whose Ag is not annealed.

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40μm silver-silver indium (Ag-AgIn) flip-chip interconnect process at 180°C by transient liquid phase (TLP) bonding process is reported. Array of 50×50 flip-chip joints with 100μm pitch and 40μm joint diameter was fabricated on silicon (Si) and bonded to copper (Cu) substrate at 180°C for 5 minutes with a static pressure of 0.4-0.7MPa (60-100psi). The corresponding load for each joint is 0.05gm. In the bonding, no flux was used. Cross section SEM images show that Ag columns are well bonded to the Cu substrate without visible voids or cracks. EDX data indicate that the resulting column structure is Ag/(Ag)/Ag2In/(Ag). Each joint has height approximately 50μm. In this structure, Ag2In is a dominating intermetallic compound (IMC) in the Ag-In system with melting temperature of 660°C. (Ag) is a solid solution phase of Ag with In composition up to 20 at. %. It has a solidus temperature range of 695 to 962°C depending on In composition. In long-term operation, (Ag)/Ag2In/(Ag) is expected to gradually convert to a single (Ag) phase which is more reliable. Thus, the flip-chip joints will get better in use. The process temperature of this new interconnect method is 80°C below typical reflow temperature of tin-based lead-free solders. The free shear strain of this 50μm Ag-AgIn flipchip interconnect between Si and Cu is around 0.14. This strain value is relatively small for ductile material such as Ag.

Solid-State Bonding Process

Bismuth telluride (Bi 2 Te 3 ) and its alloys are the most commonly used materials for thermoelectric devices. In this research, Bi 2 Te 3 chips of 9mm×9mm were coated with 100nm titanium (Ti) and 100nm gold (Au) as barrier layer and plated with 10μm Ag layer. Alumina substrates with 40nm TiW and 2.5μm Au were plated with 60μm Ag, followed by 5μm In and thin Ag cap layer for oxidation prevention. The Bi 2 Te 3 chips were bonded to alumina substrates at 180oC in 0.1 torr vacuum with 100 psi static pressure. Despite significant difference in coefficient of thermal expansion (CTE) among materials used, the resulting joint did not break. It consists of five regions: Ag, (Ag), Ag 2 In, (Ag), and Ag, and has a melting temperature higher than 660oC. The bonded sample was annealed at 250oC for 200 hours. The barrier layer and the joint remain of high quality without any breakage. After annealing, the Ag 2 In compound region turns to a Ag-rich alloy layer with melting temperature higher than 690oC. Our bonding results demonstrate the superior characteristics of Ag-In system in high temperature applications. The success of this research opens the door of building thermoelectric modules for power generating or cooling applications, which require long-term operations at high temperature on the hot side.