Upon the dawn of the new millennium, a new generation of energy revolution is incoming. With the advent and development of wide bandgap (WBG) semiconductor materials, i.e., silicon carbide (SiC) and gallium nitride (GaN), the figure of merits of high-temperature power electronics have been largely improved by breaking the fundamental physical boundaries set by traditional silicon-based semiconductor devices. In order to fulfill the full potential of WBG-based technology, it is indeed essential and necessary to develop compatible electronic packaging materials that can sustain high operating temperature. Silver-indium (Ag-In) solid solution is such one of the promising candidates of metallic material for high-temperature and power electronics.

In the content of this dissertation, the author had systematically investigated multi-physical material properties of the Ag-In and other Ag-based solid solution. First of all, the author had discovered that Ag-In solid solution possessed the superior mechanical properties, i.e., low yield strength, high mechanical strength, and high ductility, which had proved to be an ideal combination for solid-state bonding applications. Secondly, the author had also discovered that Ag-In solid solution exhibited a great anti-tarnishing property, which could potentially solve the problematic tarnishing issues involved in Ag and its alloy applications. Moreover, a quantitative and theoretical model had been established by the author to explain the underlying anti-tarnishing mechanism within the context of HSAB principle, conceptual DFT formalization and Hammer-Nørskov (HN) d-band model, using an original semi-quantum-mechanical approach. Furthermore, the abnormal phenomenon that had been found in Ag-In system, namely, solid solution softening, was further studied and discussed in Ag-Al, Ag-Ga, and Ag-Sn binary systems. In addition, various physical properties of Ag-In alloys, i.e., thermal, electrical and optical properties, were studied to enable to the multi-physical co-designs, leading to Ag-In alloys potential various applications. Based on the findings above, an advanced solid-state bonding technology had been proposed using Ag-In solid solution for high-temperature power electronics applications. Finally, the concluding remarks and future research perspectives were discussed, where a unified multi-physical theorem and material co-design principles for electronic packaging could be developed in the future study.

Despite the superior physical properties of silver (Ag) and its alloys such as high electrical and thermal conductivities and high ductility, their applications in electronics have been quite limited due to corrosion and tarnishing concerns. To expand their applications in electronic packaging, developing new materials and processes and investigating new materials properties are essential. This PhD research aims to design and grow novel Ag alloys and address the intermetallic compound (IMC) issues associated with Ag wire bonds, followed by producing Ag and alloys in various forms for solid state bonding processes valuable for high temperature electronics.

Firstly, the growth of homogeneous Ag solid solution phase with zinc (Zn) is conducted at two different compositions and their mechanical properties are evaluated by tensile test. According to the experimental results, Ag solid solution phase with Zn at either composition show tempered yield strength, high tensile strength and large uniform strain compared to those of Ag. Moreover, the tarnish resistance is improved with the alloying of zinc. Secondly, Ag3Al has been reported to be the weakest part in the Ag-Al wire bonds due to its low toughness and low corrosion resistance. A method to suppress Ag3Al through alloying indium (In) into Ag is devised. The working mechanism is studied through transmission electron microscopy (TEM) and thermodynamic modeling. Reaction kinetics and evolution of defects are further discussed.

A new approach for die attachment through solid-state bonding using fine-grained Ag foils is developed. The bonding is conducted at 300 ℃, assisted by low pressure. The strength of the joints far exceeds the requirement specified in military standards. No degradation can be observed after aging test. This approach avoids the oxidation issue on bonding interfaces in present sintered-silver technique.

Lastly, an Ag-Ag direct bonding process is invented using in-situ decomposition of silver oxide. The bonding is performed at 210 ℃ under 0.1 torr vacuum and assisted by only 1.4 MPa pressure. Encouragingly, the resulting Ag-Ag joints are strong with only sparse voids in nano-scale. Cross-sectional study shows that the bonding is formed by grain growth across the original interface. No residual oxygen is trapped. This new process is a breakthrough in Ag-Ag bonding at low temperature and low pressure that produce pure Ag joints. Its applications are wide open.

The Ag solid solution phases investigated here show great potential not only in wire bonding and other mechanical component in electronic devices but also photonic applications. The novel bonding processes achieved in this study should be highly valuable for applications in advanced electronic and photonic packages where performance and reliability are highly preferred.

As the most commonly used electrode, leadframe, and package material in electronic packaging, copper (Cu) has severe oxidation problem, preventing wetting during soldering processes and prohibiting successful wire bonding. Thus leads and bond pads of leadframes and packages are often coated with a passivation layers such as palladium, silver, or tin, which suppresses the growth of copper oxides. In this research, I investigated the use of silver-indium ((Ag)In) solid solution as a new alternative coating material for passivation. First, a reaction study of bulk (Ag)-9.5In solid solution disks with Sn was conducted to evaluate possible intermetallic compounds in the Ag-In-Sn system. Next, (Ag)-9.5In solid solution layer was coated on copper substrates by E-beam evaporation. The (Ag)9.5In-passivated Cu substrates and bare Cu substrates were aged at 150°C for up to 1000 hours in air. No oxide was detectable on (Ag)9.5In-passsivated Cu substrates while thick Cu2O3 was grown on bare Cu. This result illustrates that (Ag)9.5In solid solution is an excellent passivation coating material. Then, (Ag)9.5In-passivated Cu substrates were electroplated with tin. As a reference, bare Cu substrates were also plated with tin. These two different types of samples were reflowed and aged for different amount of time. No flux was used. The reaction results were examined by scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) and compared. The experimental results show that tin wetted and reacted well with (Ag)9.5In-passivated Cu substrates. The growth of intermetallic compound (Cu6Sn5, Cu3Sn) on (Ag)9.5In-passivated Cu substrates is also suppressed compared to the growth on bare Cu substrates. The design of (Ag)In solid solution passivation is an invention and its superior performance is a discovery. It provides an economical, environmentally friendly, and valuable passivation alternative on copper electrodes, leadframes, and packages.

In electronic packaging, silver tarnishing is a critical issue since the corrosive gases could penetrate into the gas-permeable epoxy molding compounds and tarnish the silver-based bonding components [1]. The generated silver sulfide after silver tarnishing reduces the electric conductivity and mechanically reduces the bonding reliability [2]. Hence, to improve the reliability of Ag-based bonding systems in the packaging industry, it is critical to develop an effective method of enhancing the Ag anti-tarnishing property. In Ag-based system, Ag-In alloys were proposed as the promising interconnection material for electronics packaging due to its great bonding ability and superior intrinsic mechanical properties [3]. In this paper, remarkable improvement in the anti-tarnishing property of Ag-In solid solution alloys has been revealed. A brief summary of preparation of the homogeneous Ag-In solid solution phase is illustrated. The crystallographic structure and chemical composition of the Ag-xIn alloys are identified by using X-ray diffraction (XRD) and scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX). In the following sections, quantitative results of the anti-tarnishing property of Ag-In solid solution alloys are exhibited and discussed with pure silver samples in comparison. According to experimental results, the reaction rates between silver and sulfur gases are greatly suppressed in Ag-In alloys. As a result, Ag-In solid solution alloys exhibit excellent anti-tarnishing property, offering an alternative Ag-based bonding material in the future applications in electronic packaging.

Rapid evolution of electronics industry has made the adoption of high performance chips more urgent. In fact, those chips cannot be used unless high temperature die attach materials and methods are available. The conventional die attach methods typically require a processing temperature 20-30 °C above the melting temperature of bonding material, which melts and reacts with to-be–bonded parts to form the bonding joint. For high temperature applications (> 400 °C), the processing temperature will sharply increase along with the increased melting temperature to reach the molten phase. However, such high processing temperature will damage nearly all electronic components. To respond to such challenge, the metal bonding is considered to be a possible solution.

In this dissertation, the solid-state bonding technique is employed for various bonding designs by electroplating Ag-based and In-based systems as bonding media. No molten phase or flux is involved. For most cases, the bonding conditions are conducted at 300 °C with 6.89 MPa (1,000 psi) pressure for a dwell time of 3 min in vacuum. The bonding time, 3 min, is constrained by the furnace. It is worth mentioning that this pressure is less than 10% of what is used in industrial thermo-compression processes.

To begin with, silver (Ag) was chosen as a bonding medium because of its exceptional properties and reasonable price. There are two designs for the Ag layer. For the first design, the 50 μm Ag layer plated on the copper (Cu) substrate is initially annealed at 400 °C for 5 h to increase Ag grain sizes, thereby making it easier to deform during bonding. For the second design, the 10 μm Ag layer is plated on the substrate and followed another 5 μm Ag with cavities. The fundamental concept is to release the thermal induced stress by creating cavities in the Ag layer to allow easier plastic deformation for the bonding medium. The resulting Ag layers are then bonded to the Cu chips. For both designs, all samples are bonded well and pass MIL-STD-883J method 2019.9. The Ag layer is also applied to the Cu wire bonding. To overcome Cu oxidation issue, the bonding surface on the 1 mm Cu wire is plated a 50 μm Ag layer. An annealing step is followed to facilitate the Ag layer easier to deform and conform to the Cu or Si bonding surfaces. In-plane pull test and vertical pull test are performed to measure the breaking force of the wire bonds. For wire-bonds made on the Cu substrate, the breaking forces on in-plane pull test are greater than 20 kg. The breaking forces on vertical pull test are approximately one-half of in-plane pull test results. For wire-bonds made on Si chip, breaking forces are approximately 80% of those made on Cu substrate.

Next, the novel method is to bond the Si chips to Cu substrates directly. This structure design can provide low-resistance paths for electricity and heat. There is no specific joint used in between. The basic concept is to provide room for Cu and Si to deform without restriction by producing trenches inside the Cu substrate. When the bond between two surfaces is formed, the trenches inside the Cu substrate could release the part of thermal stress from the coefficient of thermal expansion CTE mismatch. Therefore, the bond could deal with the large CTE mismatch between Si (3 ppm/°C) and Cu (17 ppm/°C). The cross-sectional images show that the Si chips are well bonded to the Cu substrates without visible voids and cracks. After the bonding process, the simple shear tests are conducted to evaluate the bond strength, while the Si chips are all broken first. It demonstrates that the bond is stronger than the Si chip itself.

Finally, the Ag-rich Ag-In solid solution layers have successfully developed on Si and silicon carbide (SiC) chips. Ag has definite advantages among metals, but it still has its own weaknesses, which can actually or potentially lead to failure. That is Ag can be tarnished not only when exposed to certain corrosive gases such as hydrogen sulfide or sulfur gas but also under normal atmospheric conditions for a long period of time. Ag-In solid solution is further studied while Ag-In binary system has been demonstrated with an exceptional anti-tarnish property. To fabricate the single phase Ag-In solid solution, the Ag/In/Ag multilayers are electroplated on the chips. Importantly, the bottom Ag layer is initially annealed at 350 °C to increase its grain sizes and to reduce grain boundaries, inasmuch as the reaction rate between Ag and In is subject to the microstructure of Ag layer. The In/Ag layers are followed to be plated onto Si chip. To decompose AgIn2 and Ag2In, the two-step annealing process is then performed in a vacuum environment at 180 °C and 350 °C, respectively. After the bonding process, the compositions of resulting joints are measured by SEM/EDX, which is a homogeneous Ag-rich Ag-In solid solution.

In this study, five different bonding designs are reported. All bonds made between the chips and the substrates are fabricated at relatively low processing temperatures. The bonding structures are designed to be used for high operating temperatures. The results are encouraging. These novel methods may bring a chance for those who need die-attach materials for high performance electronic devices under severe environmental conditions in industries.