Characterization of Cu and SiCn Surface for Hybrid Bonding
Authors: Tanaka Fabiana Lie , Nakayama Kohei , Hayama Kenta , Kamiya Yutetsu , La Mai Thi Ngoc , Inoue Fumihiro 
Published: December 2024

The miniaturization of integrated circuits (ICs) comes with many challenges caused by R&D costs, physical constraints, and connectivity bottlenecks. Consequently, 3D integration and chiplet have gained a lot of interest as an alternative technique to address such problems. It is expected to offer reduced wire lengths for interconnects, low power consumption, and heterogeneous integration capabilities, among other benefits [1]. 3D integration and chiplets require high-density, and fine-pitch interconnects, which make the conventional solder bumps, commonly used as a bonding technique, face reliability issues. Thus, Cu–Cu direct bonding has emerged as a promising bonding method [1-3]. Compared to conventional solder base interconnects, direct Cu-Cu bonding offers the advantages of significantly lower electrical resistivity, ultra fine-pitch (high density), and lower electromigration. Direct bonding interconnection (DBI) is one of the technologies used for hybrid bonding, which combines a dielectric bond with a Cu-Cu bond (metal bond).

The Cu pads, for hybrid bonding, are created by the electrochemical deposition (ECD) method. After the Cu deposition, reducing the roughness and cleaning the Cu surface prior to the bonding process is necessary to achieve high yield and reliability [4]. Therefore, surface planarization is a key factor for Cu-Cu hybrid bonding, and chemical mechanical polishing (CMP) using a chemical slurry can flatten and decrease the roughness of Cu surfaces [2]. However, CMP slurries contain chemical species, such as oxidizers, chelating agents, corrosion inhibitors, surfactants, and pH adjustors that need to be removed by applying a post-CMP (P-CMP) process [5]. For instance, if BTA (common corrosion inhibitor) remains on the Cu surface after the P-CMP process, it might inhibit Cu interdiffusion during hybrid bonding, which may require a high post-bond annealing temperature [6]. P-CMP process has shown that it can remove the contaminants through the cleaning process, plasma activation, and DIW rinsing before hybrid bonding [6].

This work aimed to evaluate pre and post-CMP surfaces for Cu-SiCN hybrid bonding. Alkaline and Acidic cleaning agents were investigated. Fig 1(a) and (b) show the Open Circuit Potential (OCP) data of the Cu surface when an acidic and an alkaline solution was used as a cleaning agent, respectively. In the alkaline solution, Post-CMP Cu and Oxidized Cu samples showed a decreased response over time until they achieved a steady state, progressing to the Pure Cu values. It suggests that the Pure Cu surface did not show CuO film growth, hence, demonstrating a better cleaning potential than the acidic solution. Cu surface before and after the cleaning process was analyzed by X-ray photoelectron spectroscopy (XPS). The organic residue, BTA, could be removed from the surface after the cleaning process by PVA brush using the alkaline solution, though there were remaining residues after cleaning with the acidic solution. The effect of plasma activation was analyzed in order to reduce the removal of Cu residues, as well. On top of the above mentioned, we will analyze the different Cu types for corrosion inhibition.

Figure 1