The performance of semiconductors has primarily been achieved through the miniaturization of devices. As devices are miniaturized to the scale of a few nanometers, economic and physical limits are being reached. As the miniaturization of devices, following Moore's Law, approaches its limits, efforts are ongoing to enhance the performance of devices through 3D packaging technologies. Efforts for miniaturization continue, but with the emerge of advanced packaging technologies, performance enhancement through packaging is gaining even more attention.
As the I/O density increases, packaging methods using solder bumps can pose reliability issues. Additionally, there is another issue of increased resistance due to the formation of Inter-Metallic Compound (IMC), leading to decreased efficiency. Therefore, direct bonding of signal transmission lines using Cu is considered important. Furthermore, hybrid bonding technology that simultaneously bonds Cu and dielectrics is also gaining attention1. While Cu is primarily used for signal transmission lines due to its excellent electrical properties, the direct bonding process of Cu faces many challenges, mainly because Cu readily forms a native oxide layer. Copper bonding with a naturally formed oxide layer requires a temperature of 400 °C2, and such high-temperature processes can lead to a degradation in the performance of devices. Therefore, there has been extensive research aimed at overcoming this challenge. The Surface Activated Bonding (SAB) process, which involves activating the copper surface in Ultrahigh Vacuum, has a significant advantage of being a room temperature process3. However, it faces the challenge of being expensive for mass production due to the bonding process taking place in a high vacuum environment. There has been research introducing the use of wet chemicals to remove oxides, facilitating bonding4. This method also has challenge, including the potential for residual contaminants and the possibility of performance degradation during the treatment process. Another method involves the application of Self-Assembled Monolayer (SAM) to inhibit natural oxidation and facilitate bonding5. This method also has the challenge of potentially leaving residual contaminants and requiring additional heat treatment to remove the monolayer. In addition, various research is being conducted, such as using plasma to inhibit native oxide6,7 and altering material properties by controlling the crystal orientation of copper8. Among these efforts, research utilizing a metal passivation layer on top of a copper layer is actively being conducted9,10,11. The metal passivation layer is a method similar to using SAM, inhibiting native oxide and enabling copper bonding. This method has the advantage of enabling copper bonding at relatively low temperatures and being free from residual contamination. Using the Atomic Layer Deposition (ALD) deposition method, there is considerable interest in research focused on selectively depositing passivation layers on specific copper area12,13. While this process method is advantageous for application in hybrid bonding, it has the challenge of relatively high deposition temperatures and the use of oxygen as a reactant, which raises concerns about oxidation.
Previously, a variety of passivation materials, including Au, Cr/Au, Pd, Ag, Pt, and Ti9,10,11,14,15,16,17, have been utilized. However, these materials have been primarily investigated solely for their role in enhancing bonding quality and electrical properties within homogeneous materials. Therefore, rather than concentrating solely on low-temperature metal passivation bonding to enhance bonding performance, this study aims to elucidate the underlying reasons for variances in diffusion behavior observed depending on the metal passivation material. In other words, the primary focus of this paper is to identify the material characteristics and bonding process that exert the most significant influence on Cu-Cu diffusion bonding. In this study, various metal materials were used to compare the differences in diffusion behavior. Noble metal Pt with good Cu diffusion15, Ti that has been studied a lot with excellent diffusion capability16, Ta that is also used as a diffusion barrier18, and Cr that is also used as a wetting layer because of its smooth roughness14,17 were selected. Pt and Ta materials were chosen as exemplary candidates for highlighting disparities between materials with robust diffusion and those with limited diffusion. It is more advantageous for comparison purposes to include materials exhibiting poor diffusion rather than encompassing solely those demonstrating diffusion. This is because identifying the characteristics of materials with limited diffusion and contrasting them with other materials holds paramount importance. This material selection is considered suitable for unraveling the cause of the diffusion difference by comparing materials with various diffusion properties. When selecting passivation materials in much research, there are many options to consider, including cost, suitability for the desired devices, and diffusion behavior based on film characteristics. By comparing and adding factors influencing diffusion behavior, it is expected that this research will provide important guidelines for selecting passivation materials.