The potential of an innovation for establishing a simultaneous mechanical, thermal, and electrical connection between two metallic surfaces without requiring a prior time-consuming and expensive surface nanoscopic planarization and without requiring any intermediate conductive material has been explored. The method takes advantage of the intrinsic nanoscopic surface roughness on the interconnecting surfaces: the two surfaces are locked together for electrical interconnection and bonding with a conventional die bonder, and the connection is stabilized by a dielectric adhesive filled into nanoscale valleys on the interconnecting surfaces. This "nano-locking" (NL) method for chip interconnection and bonding is demonstrated by its application for the attachment of high-power GaN-based semiconductor dies to its device substrate. The bond-line thickness of the present NL method achieved is under 100 nm and several hundred times thinner than those achieved using mainstream bonding methods, resulting in a lower overall device thermal resistance and reduced electrical resistance, and thus an improved overall device performance and reliability. Different bond-line thickness strongly influences the overall contact area between the bonding surfaces, and in turn results in different contact resistance of the packaged devices enabled by the NL method and therefore changes the device performance and reliability. The present work opens a new direction for scalable, reliable, and simple nanoscale off-chip electrical interconnection and bonding for nano- and micro-electrical devices. Besides, the present method applies to the bonding of any surfaces with intrinsic or engineered surface nanoscopic structures as well.

Low temperature heterogeneous integration with diamond is the key technology in pushing upwards the high-power limit of a vertically-external-cavity surface-emitting laser (VECSEL). This work successfully accomplished a functional high-power VECSEL-to-diamond device with a modified Ag-In transient liquid phase (TLP) bonding technology. The post-bonding quality of VECSEL epitaxial membrane was thoroughly examined with scanning electron microscopy (SEM), focus ion beam (FIB) and high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Owing to the low-temperature process, thermal-activated diffusion and thermo-mechanical stress have been suppressed to the minimal level within the epitaxial layers while optimizing the heat-spreading capability of the diamond. Interestingly, with experimental and thermodynamic evidences, a distinct nanostructure from spinodal decomposition has been discovered in the Ag-In bonding layer for the first time, whose structural feature is beneficial to the reliability of a VECSEL-to-diamond device. Conceptually, this work opens a new bonding technology category, i.e., Ag-In spinodal bonding.

In the present study, we have successfully demonstrated the heterogeneous integration of chemical vapor deposition (CVD) diamond and copper (Cu), using a fluxless silver–indium (Ag-In) bonding system. CVD diamond heat spreaders and Cu heat sinks are crucial components for efficient heat dissipation in high-power electronics and photonics. The two components were bonded by a multi-layer Ag-In structure. Microstructure of joints revealed isolated voids, caused by undersupply of molten phase (In) during bonding. Thus, the prior-to-bond Ag annealing process was introduced, in order to resolve the undersupply of the molten phase. Annealing the Ag reduced the grain boundary density and subsequently slowed the diffusion of In into Ag. The prior-to-bond annealing of Ag was conducted in air and in vacuum environment and completely eliminated the isolated voids at the original bonding interface, stemming from the undersupply of molten phase. However, unforeseen adverse effects of the prior-to-bond annealing process were also discovered for the first time, which degraded the shear strength of the Ag-In joints. The underlying degradation mechanisms were thoroughly investigated with scanning electron microscopy, energy-dispersive X-ray spectroscopy, focused ion beam, and X-ray photoelectron spectroscopy. Microstructural studies showed that the formation of metallic oxides and metallic sulfides was responsible for the mechanical degradation of the joint for air- and vacuum-annealed specimens, respectively. The underlying reasons for the formation of such mechanically adverse compounds were thoroughly discussed. With the knowledge of the degradation mechanism, this work will pave the path for the future technical advancements of the Ag-In bonding method, which can be utilized for the integration of CVD diamond into the package, thereby improving the thermal performance and the reliability of the high-power electronics and photonics.

In this study we report on successful bonding of chemical vapor deposition (CVD) grown diamond to Cu using a multi-layer Ag-In structure. To manage the large coefficient of thermal expansion (CTE) mismatch between copper and diamond, Ag-rich Ag-In solution is chosen as the final phase in joint. In our previous investigations, we have shown that Ag-In solid solution exhibit superior mechanical properties, such as low yield strength, high tensile strength, and large elongation. Here, we show that by using a fluxless process at vacuum, mechanically robust joints can be formed at 180 °C between copper and diamond. Numerous samples that were bonded with proposed structure show acceptable shear strength and by performing a post bond annealing at 250 °C for 192 hours, we were able to achieve a joint almost fully composed of Ag solid solution with In, with significantly increased shear strength. The deposited multi-layer structure is examined using scanning electron microscopy (SEM) coupled with focused ion beam (FIB) prior to bonding. Following the bonding, samples are sheared and fracture surfaces are examined using energy dispersive X-ray spectroscopy (EDX). Our studies show that Cr/diamond interface, which is the metallization scheme on diamond is a weak interface in the bond design and as the joint becomes stronger by conversion of Ag-In intermetallic compounds into (Ag), more delamination occurs in the Cr/diamond interface. Additionally, it is reported that annealing the Cr/diamond interface can effectively improve its adhesion.

Bismuth (Bi)-based systems are of great interest as it is considered to potentially replace high lead (Pb)-content solders used in high temperature electronics. In particular, molten Bi strongly reacts with Ni to form higher melting point intermetallic compounds (IMCs) via transient liquid phase (TLP), which can offer a joining method alternative to the traditional solder approach. A fundamental understanding of nucleation and growth of intermetallic phases is crucial to create a reliable joint. Two intermetallic phases form between Bi and Ni (Bi3Ni, BiNi). In this study, growth kinetics for Bi3Ni and BiNi was investigated, both of which show a parabolic growth behavior. Bi3Ni exhibits rapid growth and apparent activation energy of 65.5 kJ/mol at lower temperatures (from 160 to 240 °C) and of 132.9 kJ/mol at higher temperatures (> 240 °C), where the transition is likely due to a viscous-flow nature near melting temperature of Bi. On the other hand, BiNi grows at a later stage with a slower rate with the apparent activation energy of 125.6 kJ/mol (from 260 to 340 °C). In addition, based on the formation sequence and growth direction of these IMCs, interdiffusion coefficients for each of these IMCs were determined. Micro-hardness tests show that Bi3Ni is softer and more brittle than BiNi.

Predominant high melting point solders for high temperature electronics contain lead (Pb), which will soon be banned by environmental regulations as in most of consumer electronics. In an effort to replace the Pb-based solders with a new high-temperature capable material, we developed a transient liquid phase (TLP) bonding of bismuth (Bi) and nickel (Ni). A molten Bi (m.p. of 271°C) strongly reacts with Ni to form a Bi 3 Ni or BiNi intermetallic layer, both of which can withstand over 400°C. To study microstructural developments and their influences on reliability performance, the die attached coupons were assembled using Bi-Ni TLP bonds. It was shown that Bi 3 Ni is the first phase to form, after which the diffusion of Bi controls the growth kinetics of this intermetallic phase with the activation energy of 65.5 kJ/mol for the temperature range from 160 to 240°C. The solid-state transformation of Bi 3 Ni to BiNi follows, which is a slower process and only occurs at a long-term aging (activation energy of 17.8 kJ/mol, for the temperature range from 260 to 300°C). When tested at high temperatures (up to 350°C) or exposed under long-term storage at 200°C over 1000 hours, such bonds showed no degradation in die shear strength. It indicates the potential of the Bi-Ni TLB bonds as a Pb-free alternative to replace high-Pb solders for high temperature electronics that operate at 200°C or higher.