Pin J. Wang

Silver Microstructure Control for Fluxless Bonding Success Using Ag-In System

A fluxless bonding process is successfully developed between silicon (Si) chips and copper (Cu) substrates using the silver-indium (Ag-In) binary system. This is a new design concept that utilizes thick Ag plated over the Cu substrate to deal with the large mismatch in coefficient of thermal expansion between semiconductors, such as Si (3 ppm/°C) and Cu (17 ppm/°C). The Ag layer actually becomes a part of the Ag-Cu substrate. Ag is chosen for the cladding because of its superior physical properties of ductility, high electrical conductivity, and high thermal conductivity. Following the thick Ag layer, 5 μm In and 0.1 μm Ag layers are plated. The thin outer Ag layer inhibits oxidation of inner In. After many bonding experiments, we realize that the success of producing a joint relates to the microstructure of the Ag layer. Ag with small grains results in rapid growth of solid Ag2In intermetallic compounds through grain boundary diffusion. Thus, a joint is not obtained because of lack of molten phase (L). To coarsen Ag grains, an annealing step is added to the Ag-plated Cu substrate. This step makes Ag grains 200 times coarser compared to the as-plated Ag. The coarsened microstructure slows down the Ag2In growth. Consequently, the (L) phase stays at the molten state with sufficient time to react with the Ag layer on the Si chip to produce a joint. Nearly perfect joints are produced on Ag-plated Cu substrates. The resulting joints consist of pure Ag, Ag-rich solid solution, Ag2In, and Ag3In. The melting temperature exceeds 650 °C. Using the present process, high temperature joints of high thermal conductivity are made between Si chips and Cu substrates at low bonding temperature (200°C). We foresee the Ag-In system as an important system to explore for various fluxless bonding applications in electronic packaging. This system provides the possibilities of producing joints of wide composition choices and wide melting temperature range. This paper provides preliminary but useful information on how the microstructure of Ag affects the bonding results.

Silver Joints Between Silicon Chips and Copper Substrates Made by Direct Bonding at Low-Temperature

In this paper, pure silver (Ag) joints between silicon (Si) chips and copper (Cu) substrates are produced successfully at a temperature much lower than its melting point. Silver is ductile and has low-yield strength. It can deform to release the shear stress caused by the large mismatch in coefficient of thermal expansion between Si and Cu. Silver also has the highest electrical conductivity and thermal conductivity among metals. As a bonding medium and interconnect material, it can provide the best electrical and thermal performances. In experiments, Ag in the form of foil is chosen as the bonding medium. Prior to bonding, the Si chips are coated with thin Cr and Au layers. The Si chip, Ag foil, and Cu substrate are bonded together in one step. The bonding process is conducted at 250°C in 50-mtorr vacuum environment. There is no molten phase involved during the bonding process. The resulting joints exhibit nearly perfect quality. No voids are observed at the Si/Ag and Ag/Cu bonding interfaces. The bonding strengths at these two bonding interfaces pass MIL-STD-883G standards. We believe that the bonds at the Au/Ag and Ag/Cu interfaces are formed by short-range interdiffusion. Since the melting point of Ag is 961°C , the Ag joints are expected to sustain high-temperature. The 250°C bonding temperature is the typical reflow temperature of Sn3.5Ag solders used in electronic industries. This novel bonding process can be applied to various electronic devices that require high-thermal performance or high-operation temperature. It is particularly valuable to packaging high-temperature devices.

Very High-Temperature Joints Between Si and Ag–Copper Substrate Made at Low Temperature Using InAg System

We present a fluxless bonding process between silicon and Ag-copper dual-layer substrate using electroplated indium/silver solder. The nucleation mechanism of In plated over Ag layer is first investigated. It is interesting to discover that In atoms react with underlying Ag to form AgIn2 compound layer during electroplating. A novel Ag laminating technique on Cu substrates is developed. The Ag cladding functions as a strain buffer to manage the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/degC) and Cu (17 ppm/degC) substrates. To bond Si chips to the Ag layer on copper substrates, In-based alloy (InAg) is used. A fluxless bonding process is developed between Si/Cr/Au/Ag and Cu/Ag/In/Ag. The process is performed in 50-militorr vacuum to suppress solder oxidation. No flux is used. The resulting joints consist of three distinct layers of Ag, Ag2In and Ag. Microstructure and composition of the joints are examined using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). Bonded samples are further annealed to convert the Ag2In phase into solid solution phase (Ag). The joint has a melting temperature above 850degC. This technique presents our success in overcoming the very large CTE mismatch between silicon and copper. It can be applied to mounting numerous high-power silicon devices to Cu substrates for various industrial applications.

Fluxless Hermetic Lid Sealing Using Electroplated Sn-Rich Solder

A new fluxless hermetic sealing technique using electroplated Sn-rich soft solder is reported. Specific glass (SCG72) is chosen as lid material to seal alumina packages. It has nominal coefficient of thermal expansion of 7 ppm/deg C, close to that of alumina. Alternatively, sapphire can also be used as the lid material. A thick Sn layer is plated over the Cr/Au patterned glass wafer, followed immediately by thin Au layer. This outer Au layer prevents the inner Sn from oxidation. In bonding, the glass lid is placed over the package rim which has a metallization structure of W/Ni/Au. The fluxless sealing process is performed in vacuum (50 millitorrs) to suppress tin oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15 200. Fluxless bonding is valuable in many hermetic sealing applications such as microelectromechanical systems (MEMS), sensors, photonic devices, and imaging devices. Nearly void-free sealed joints are achieved with Sn-rich composition. Helium leakage tests are performed to evaluate hermetic quality. Scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) is used to evaluate the composition and microstructure of the sealing joints. The results show how the Sn-rich solder reacts with the W/Ni/Au on the package. This new sealing process can be applied to nearly all devices and packages that require hermetic sealing.

Fluxless Bonding of Silicon to Ag–Copper Using In–Ag With Two UBM Designs

A fluxless process of bonding silicon to Ag-cladded copper using electroplated In-Ag multilayer structure is developed. The Ag cladding on the copper substrate is a stress buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/degC) and Cu (17 ppm/degC). To manufacture Ag on copper substrate, two techniques are developed. The first is an electroplating process to fabricate a thick Ag layer. The second technique is a novel laminating process that bonds Ag foil directly on Cu substrate. On Si chips, two underbump metallurgy (UBM) structures are designed, Si/Cr/Au and Si/Cr/Ni/Au. To produce a solder layer, Si chips are electroplated with In followed by thin Ag. The thin Ag cap layer prevents oxidation of the inner In region. To achieve a fluxless feature, the bonding process is performed in a vacuum environment (50 mtorrs) to suppress indium oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15 200. Using Cr/Au UBM structure, the silicon chip was detached from Cu substrate. The broken interface lies between Si/Cr and Ag2In IMC on Cu substrate. Using a new UBM design of Si/Cr/Ni/Au, high-quality joints are produced that comprise of three distinct layers of In7Ni3, Ag2In , and Ag. Microstructure and composition of the joints are studied using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX).

Intermetallic reaction between electroplated indium and silver layers

Reaction of indium (In) and silver (Ag) during the electroplating process of indium over thick silver layer was investigated. It is found that the plated In atoms reacts with Ag to form AgIn2 intermetallic compound at room temperature. Indium is commonly used in electronic industries to bond delicate devices due to its unique ductility and low melting temperature. In this study, copper (Cu) substrates were electroplated with 60µm thick Ag layer, followed by electroplating In layer with thickness of 5µm and 10µm, respectively, at room temperature. To investigate chemical reaction between In and Ag, the microstructure and composition on the surface and cross section of samples were observed using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). X-ray diffraction method (XRD) was also employed for phase identification. All of results clearly indicate that indium atoms react with underlying Ag to form AgIn2 during the plating process. After the sample was stored at room temperature in air for one day, AgIn2 grew to 5µm in thickness. With longer storage time, AgIn2 continued to grow until all indium atoms were consumed. The indium layer, thus, disappeared and could barely be detected by XRD.

Direct Silver to Copper Bonding Process

A novel process of bonding silver (Ag) foils to copper (Cu) substrates has been developed. This direct bonding method does not use any intermediate layer in between. An important application of this process is electronic packaging where semiconductor device chips are bonded to Cu substrates or Cu electrodes fabricated on substrates. Cu is chosen as the major material for substrates and electrodes due to its high electrical and thermal conductivities, high strength, adequate rigidity, easiness in forging and machining, and low cost. On the other hand, Cu has a large mismatch in the coefficient of thermal expansion with most semiconductors, particularly with silicon. This makes it very difficult to bond large device chips to Cu substrates with a metallic joint. We thus design the Ag-cladded Cu structure to overcome this difficulty. Ag is quite soft and ductile. It can function as a strain buffer between the semiconductor chip and the Cu substrate. Ag also has superior physical properties. It has the highest electrical and thermal conductivities among all metals. In the beginning, we used an electroplating process to produce Ag-cladded Cu substrates. However, it is time consuming and costly to electroplate thick Ag layers. To obtain thick Ag layers (more than 200μm), this new laminating process is developed. The Ag foil is laminated to the Cu substrate directly with a static load of 1000psi at 250°C in a 50mtorr vacuum to suppress oxidation. No bonding medium is used. Scanning electron microscopy images on cross sections of bonded samples exhibit a perfect Ag–Cu bond.

A new bonding technology dealing with large CTE mismatch between large Si chips and Cu substrates

Fluxless bonding between large silicon (Si) chips and copper (Cu) substrates using electroplated indium (In) and silver (Ag) as solders has been successfully developed. The nucleation mechanism in In-Ag system is first studied. It is interesting to discover that In reacts with underlying Ag and forms Agln2 as soon as it is electroplated at room temperature. To reduce stress caused by coefficient of thermal expansion (CTE) mismatch between Si and Cu, a 280 mum thick Ag foil, working as a stress buffer, is directly bonded on Cu substrate at low process temperature of 250degC. It is a typical reflow temperature of lead-free (Pb-free) solders. There are three different bonding structure designs to perform fluxless bonding between Si chips and Ag-cladded Cu substrates in this project. High quality joints are achieved by conducting bonding between Si/Cr/Au/Ag and Cu/Ag/In/Ag in 50 millitorr vacuum. The initial joint is very strong without any voids. It consists of three distinct layers of Ag, Ag2In, and Ag. Further annealing step is employed on the bonded sample to convert Ag2In intermetallic compound (IMC) into (Ag) solid solution phase. The resulting joint comprises (Ag) and pure Ag layers and is expected to sustain high operating temperature up to 850degC. The joints do not contain any IMC layers. Thus, all reliability issues associated with IMCs and IMC growth do not exist anymore. This novel bonding process can be applied to a variety of electronic devices that require high thermal performance or high operating temperature.

Fluxless bonding of Si chips to Ag-copper using electroplated indium and silver structures

In this study, we present fluxless bonding process between silicon and Ag-copper dual-layer substrate using electroplated In/Ag solder. We first study nucleation mechanism of electroplated In over Ag substrate. It is interesting to find that electroplated In reacts with underlying Ag to from AgIn2 IMC layer during electroplating. A novel laminating technique is developed on Cu substrate as cladding layer. The Ag cladding on the copper substrate is a buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3ppm/°C) and Cu (17ppm/°C). To bond Si chips to the Ag layer on copper substrates, In-based alloy (InAg) is used. A fluxless bonding process is designed and developed between Si/Cr/Au/Ag and Cu/Ag/In/Ag. The bonding process is performed in 50-militorr vacuum atmosphere without any flux. The resulting joints consist of three distinct layers, i.e., Ag, Ag2In intermetallic compound and Ag layer. Microstructure and composition of the joints are studied using Scanning Electron Microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). Annealing on the bonding samples is under way to change the three layer joint into two layers of (Ag) and Ag, which would have high re-melting temperature above 700°C. This technique presents our success in overcoming the very large mismatch in thermal expansion between silicon and copper. It can be applied to mounting numerous high power silicon devices to Cu substrate for various industry applications.

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

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