# Low-Temperature Diffusion of Au and Ag Nanolayers for Cu Bonding
###### Abstract
The mechanism of metal-to-metal wafer bonding consists of diffusion of the metal at the bonding interface [3]. Accordingly, for smooth bonding, it is necessary to keep the metal as clean as possible. However, due to the characteristics of Cu, there is a disadvantage in that oxidation occurs very easily in air. Many studies are being conducted in an effort to prevent this [4; 5; 6; 7; 8]. For example, a two-step plasma treatment using Ar and N\({}_{2}\) was applied to the Cu surface to remove Cu oxide and to activate the Cu surface so as to form a nanometer-level Cu nitride (Cu\({}_{4}\)N) layer to serve as a bonding interface [4]. Other researchers attempted to prevent Cu oxidation and to realize pure Cu-to-Cu bonding by depositing nanometer-level metal thin-film passivation layers consisting of, for instance, titanium (Ti), silver (Ag), and gold (Au), on the Cu surface [5; 6; 7]. Practically, to activate Cu diffusion at the bonding interface and also decompose natural Cu oxide films, Cu bonding lasting approximately one hour at a high temperature of at least 400 \({}^{\circ}\)C is necessary [8]. However, a bonding process at a high temperature can subject semiconductor devices to excessive thermal and mechanical stress; thus, low-temperature Cu bonding studies are needed for three-dimensional stacked structures. Cu bonding using a <111> Cu nanotinned structure was reported at about 200 \({}^{\circ}\)C [9], and Cu bonding using a nanolayer of Au was reported at a temperature of 150 \({}^{\circ}\)C [10]. Also, Cu bonding using a nanolayer of Ag was reported at a temperature of 180 \({}^{\circ}\)C [11]. Both studies confirmed that Cu can be bonded at a temperature of less than 200 \({}^{\circ}\)C, demonstrating that the material deposited with a thin-film passivation layer and Cu were mutually diffused to form a bonded interface. The biggest advantage of the method using the metal passivation nanolayer is that only this method enables Cu bonding at a temperature lower than 200 \({}^{\circ}\)C [12]. However, since the two materials are mutually diffused after passivation metal is deposited on Cu, IMC (intermetallic compound) can be generated, which can cause problems with mechanical and electrical reliability [13]. In this study, Cu-to-Cu bonding was evaluated using Au and Ag nanolayers to prevent Cu oxidation on the Cu surface prior to bonding and to induce solid-state diffusion to form a uniform Cu bonding interface. Au is rarely oxidized, prevents Cu from being oxidized, has excellent electrical conductivity, has high chemical stability, and can be planarized by a chemical mechanical polishing (CMP) process [14]. Ag, like Au, has very good electrical conductivity and a damascene process using CMP is possible [15]. However, Ag can be oxidized and has a higher Young's modulus and coefficient of thermal expansion than Au. The material properties of Au and Ag are listed in Table 1. As mentioned above, the objective of employing metal passivation in Cu bonding is to achieve a robust Cu bond at temperatures below 200 \({}^{\circ}\)C, as Cu is not significantly thermally expanded at this range, ensuring a secure bond. So, in this study, bonding experiments were conducted at 180 \({}^{\circ}\)C, and the characteristics between the Au and Ag passivation layers were compared. In addition, the diffusion properties of each material were investigated at various temperatures. Other studies [3; 10; 11; 12; 16; 17] have already reported low-temperature Cu bonding using Au or Ag, but this study aims to investigate diffusion characteristics of evaporated Au and Ag into Cu and identify mechanisms between them.