Gallium-based transient liquid phase bonding for Cu-Cu electronic interconnections

The electronics manufacturing industry has been evolving for many decades, primarily dominated by the need for maturation as well as the demands of functionalities. The exploitation of advanced integrated circuits (ICs) and their devices requires not only reliable integration at the level of components and circuitries but also advanced interconnect materials and packaging technology. However, existing processes and interconnect materials used for integration accompanying with a wide range of technologies developed are posing significant challenges in order to meet ever-stringent requirements for high performance and reliability. Therefore, it has become imperative to research and develop more advanced interconnect materials and viable processes to satisfy the ever-growing demands where the systems will have to under harsh, severe, and extreme service conditions. It is also of greater importance to understand the underpinning fundamentals with respect to the influence of the usage and application and their microstructural characteristics of materials on manufacturing, performance and reliability of the integrated system. This study aims to address these challenges by further exploring the transient liquid phase bonding (TLPB) technology, but with a specific focus on the liquid Gallium (Ga) based alloys as interconnect materials.

First, considering the solid-liquid interactions between Ga-base alloys and copper (Cu) as a base or substrate material, examinations on the formation and growth characteristic of Ga-Cu intermetallic compounds (IMCs) are carried out, with the intention of further developing and optimizing bonding processes. This initial investigation on IMC formations encompasses a detailed analysis of the kinetics of IMC grain growth and the affecting factors, linking to their microstructural and morphological evolution. To enhance the wettability of Ga-based solders, an acid treatment method is applied and proven to be effective in improving and accelerating the TLPB process. Consequently, the Cu/Ga-based/Cu joints are successfully formed through the TLPB process under temperatures of 150~350°C and pressures of 0~1MPa. The resultant joints under varying bonding conditions are subsequently evaluated and characterized in terms of mechanical properties, IMC composition, microstructure, and void formation. The findings show that TLPB is effective for Ga-based interconnections with Cu under the temperature range of 150 to 350°C, and overall bonding strength increases with the bonding temperature. It is also as expected that the strength of the joints formed under a pressure of 0.5-1 MPa is improved compared to the same bonding conditions without applied pressure. The application of pressure effectively reduces porosity, leading to higher shear strength.

To reduce the TLPB processing time associated with the Ga-based alloy with a low melting point, this work has developed a novel manufacturing route by employing a specially designed ultrasonic aid system to expedite the TLPB process, using a synthetic Cu-Ga mixture paste. Such composite Cu-Ga paste is composed of microscale Cu powder blended with liquid Ga. The TLPB process is performed and significantly accelerated with the aid of ultrasonic vibration (USV), which can stimulate the interfacial reaction between liquid Ga and solid Cu to achieve a fluxless interconnect under ambient conditions. The ultrasonic-assisted TLPB bonding and its underlying mechanism associated with the formation and performance of resultant joints are subsequently investigated and discussed.

The research has bridged a significant knowledge gap regarding the Ga-Cu interconnection technology and provides numerous valuable insights into an efficient assembly pathway that exhibits feasibility for high-end electronic integration with tremendous potential for future industrial applications.