## 1 Introduction  
3D packaging is becoming a core advanced packaging technology because of its improved performance, effective power delivery, efficient signal integrity, small form factor, and heterogeneous packaging [1; 2; 3]. However, there are still technological challenges to be resolved such as thermal management, design and methodology, yield, electrical testing and reliability. In 3D packaging, there are three ways to stack devices such as die stacking, wafer stacking and package stacking. Die stacking can offer higher yield, but medium performance improvement, and wafer stacking give the highest performance with the lowest cost possible, but a cost with this methodology depends on a compound yield. Lastly, package stacking provides higher yield, but lower performance improvement and the highest cost [1]. In order to construct a stacked device structure, TSV (through Si via) forming, Si wafer thinning, and wafer or chip bonding processes are needed. In many bonding techniques Cu-to-Cu bonding should be developed for high density and performance. The advantages of Cu as a bonding material are its mainstream CMOS material, low resistivity, high thermal conductivity, good resistance to electromigration, and no brittle intermetallic compound (IMC) formation. However, a good Cu diffusion bonding requires high bonding temperature above 400 \({}^{\circ}\)C, which is not practically suitable for a mass production manufacturing.  
Much research on low temperature Cu-to-Cu bonding has been reported [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The research includes coverage of surface activated bonding [4], passivation using a self-assembled monolayer [5], wet cleaning [6; 7], metal passivation with Pd, Mg, Ag, or Au [8; 9; 10; 11], Cu (111) crystal plane studies [12], Cu/SiO\({}_{2}\) hybrid bonding [3; 13], and Cu/polymer hybrid bonding [14]. So far, the direct bonding interconnect (DBI) technique by Ziptronix [13] has been considered the most adoptable Cu bonding process in mass production. Despite the extensive studies, however, a number of weaknesses such as Cu surface oxidation, Cu planarization by CMP (chemical mechanical polishing), and complete decomposition of passivated material still exist.  
In this research, a method to prevent copper oxidation prior to Cu-to-Cu bonding was studied using two-step plasma treatment. The first plasma step was to remove copper oxide and to clean any contaminants by Ar plasma, and then the second plasma step was to form copper nitride with N\({}_{2}\) plasma. The formed copper nitride on the Cu surface was expected to decompose into metallic Cu and nitrogen. The copper nitride formation was carried out in a conventional direct current sputtering technique because it is a simple and highly cost-efficient method. Although a decomposition of copper nitride depends on various deposition and growth techniques, it typically occurs at \(\sim\)300 \({}^{\circ}\)C [15; 16], which can potentially lower the Cu-to-Cu bonding temperature. Previously the Ar plasma effect in two-step plasma treatment process using sputtered Cu thin film has been studied [17]. However, in this study, the effect of two-step plasma treated process on electroplated Cu compared to sputtered Cu was investigated. Since Cu thin films have different microstructure depending on deposition techniques, it is necessary to study the formation of copper nitride on differently deposited Cu thin film. In addition, electroplated Cu thin films are more commonly used in mass production.