Ammonia Plasma Surface Treatment for Enhanced Cu–Cu Bonding Reliability for Advanced Packaging Interconnection

By Ho Jeong Jeon and Sang Jeen Hong, Myongji University, Republic of Korea

With the emergence of 3D stacked semiconductor products, such as high-bandwidth memory, bonding-interface reliability cannot be overemphasized. The condition of the surface interface before bonding is important and can substantially affect product reliability. Plasma technology can be used to control the state of a bonding interface, but various factors of interest, such as surface roughness, chemical bonding state, and surface cleanliness, may depend on the type of gaseous plasma. These factors may increase voids at the interface, which can jeopardize the product reliability. In this study, NH₃ plasma surface treatment is investigated and compared with the conventionally preferred surface treatment under Ar plasma. Under the latter method, specific anomalies occurred and led to void formation at the interface during bonding. By contrast, NH₃ plasma treatment maintained higher uniformity, higher overall surface conditions, and a smooth reduction process. Furthermore, the formation of a nitride passivation layer effectively inhibited the oxidation of the metal surface, and the flat surface resulted in the decrease in voids compared with the Ar plasma treatment after the copper–copper bonding. From the experimental analysis, we achieved a 12% reduction in resistance in the samples treated with NH₃ plasma treatment due to the suppression of surface oxidation. However, it is unfortunate that the shear strength in the experimental samples treated with NH₃ plasma treatment needs to be further improved.

1. Introduction

The needs for excellent device density, high signal communication bandwidth, superior performance, and low manufacturing costs persist with the expansion of the role of 3D packaging in the field of semiconductors [1,2]. An example of early 3D packaging technology is stacked chip-scale packaging with copper wires, but an increase in the number of copper wires in a package leads to a power consumption problem, signal loss, and the increased package footprint [3]. Through-silicon vias (TSVs) have been devised to alleviate concerns of multiple wire bonding, but the high cost of TSV fabrication on wafers has hindered the emergence of the application in commercial products [4]. Likewise, direct Cu–Cu bonding with TSV interconnection is a promising interconnection method in 3D packaging [5]. Cu–Cu bonding involves connecting two copper bumps or pads back-to-back in a TSV to interconnect another semiconductor chip, and it can also be used for hybrid bonding along with SiO2–SiO2 bonding [6]. It plays a crucial role in advanced packaging, a stacked high-bandwidth memory (a type of dynamic random-access memory), and backside power delivery networks [7,8].

Cu–Cu bonding technology is currently being investigated in various research directions. Cu–Cu bonding is normally conducted at temperatures exceeding 400 ◦C. Its thermal impact on nearby components can be reduced by decreasing the bonding temperature. This can be achieved via surface-activated bonding using plasma surface treatment [9]. Additionally, various issues related to the bonding interface are being studied, including oxidation and diffusion problems [10,11]. Papers on oxidation problems focus on inhibiting oxidation itself and removing oxide layers through plasma and wet chemical surface treatments. As for diffusion problems, researchers are investigating the application of coatings to the copper surface to promote diffusion during bonding [12–16]. Plasma technology should be used effectively to address these challenges. Ar plasma–based surface treatment has been adopted in many studies; this process is driven by Ar ion sputtering, which physically removes the bond between copper and oxygen. Although this process performs well in surface activation and oxide film removal, it requires appropriate control because it may increase the surface roughness [17].

For successful Cu–Cu bonding, surface conditions should be controlled strictly. The key factors affecting surface conditions include surface roughness, surface chemical state, and surface cleanliness [18]. The surface roughness should be minimized, and any surface oxide layer must be removed. The presence of particles on the surface can disrupt proper bonding and reduce bonding reliability [19]. Plasma processes enable the control of these parameters. Surface roughness reduction decreases the probability of void formation during bonding and increases the contact area between different Cu surfaces, thereby improving the shear strength and electrical properties of devices [20]. The effective control of oxide layers can enhance electrical characteristics and copper atom diffusion. The roughness of a copper surface is improved by adding H2 to Ar plasma during plasma surface treatment [17]. Furthermore, the use of N2 plasma for copper surface treatment effectively suppresses oxidation by forming a passivation layer on the surface [21]. In this study, the copper surface treatment was performed using NH3 plasma to obtain the passivation layer expected from N2 plasma and the improved surface roughness expected from H2 plasma. In the following section, we explain how the samples were prepared, bonded, and tested, including the plasma surface treatment procedure. In Section 3, we present our experimental results and discuss our observations regarding the bonding strength. Finally, the conclusion is presented in Section 4.

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