# Low-Temperature Diffusion of Au and Ag Nanolayers for Cu Bonding
###### Abstract  
With the recent rapid development of IT technology, the demand for multifunctional semiconductor devices capable of high performance has increased rapidly, and the miniaturization of such devices has also faced limitations. To overcome these limitations, various studies have investigated three-dimensional packaging methods of stacking devices, and among them, hybrid bonding is being actively conducted during the bonding process. studies of hybrid bonding during the bonding process are active. In this study, Cu bonding using a nano passivation layer was carried out for Cu/SiO\({}_{2}\) hybrid bonding applications, with Au and Ag deposited on Cu at the nano level and used as a protective layer to prevent Cu oxidation and to achieve low-temperature Cu bonding. Au was deposited at about 12 nm, and Ag was deposited at about 15 nm, with Cu bonding carried out at 180 \({}^{\circ}\)C for 30 min, after which an annealing process was conducted at 200 \({}^{\circ}\)C for one hour. After bonding, the specimen was disced into a 1 cm \(\times\) 1 cm chip, and the bonding interface was analyzed using SEM and TEM. Additionally, the 1 cm \(\times\) 1 cm chip was diced into 2 mm \(\times\) 2 mm specimens to measure the shear strength of the bonded chip, and the average shear strength of Au and Ag was found to be 5.4 and 6.6 MPa, respectively. The degree of diffusion between Au-Cu and Ag-Cu was then investigated; the diffusion activation energy when Au diffuses to Cu was 6369.52 J/mol, and the diffusion activation energy when Ag diffuses to Cu was 17,933.21 J/mol. Cu bonding; 3D packaging; Au nanolayer; Ag nanolayer; hybrid bonding +
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a micron and Cu oxidation during dicing and chip transfer processes. In the context of device stacking, the arrangement methods can be divided into face-to-face bonding and face-to-back bonding. There are many bonding methods for Cu bonding. The surface-activated bonding (SAB) method allows room temperature bonding by removing copper oxide on the surface using Ar plasma and subsequently activating the surface for bonding in ultra-high vacuum conditions. However, due to the demanding nature of ultra-high vacuum processes and the potential reliability degradation from the high-energy impact of Ar plasma, this method has not been widely adopted for mass production. Wet processes using chemical solutions such as acetic acid, sulfuric acid, and hydrochloric acid are also available as an alternative to plasma for copper oxide removal. However, treating completed components with acidic solutions is considered unsuitable for mass production due to the potential performance degradation. An alternative method for achieving low-temperature bonding while preventing copper surface oxidation involves the use of a self-assembled monolayer (SAM). This method entails coating the copper surface with a polymer-based SAM, adsorbing it to the copper surface, and then detaching it during bonding. Although SAMs can prevent copper oxidation, achieving complete detachment may require temperatures close to 250 \({}^{\circ}\)C. The presence of residual SAM on the copper surface or decomposed SAM remnants in the chamber may pose compatibility issues with semiconductor processes. Among various copper bonding methods, the most suitable for semiconductor CMOS devices' mass production and high-performance computing (HPC) applications in next-generation stacked packaging is metal/dielectric hybrid bonding. Metal/dielectric hybrid bonding is gaining increased attention, driven by the rising number of input/output bumps and a rapid reduction in bump pitch. As a metal, copper (Cu) has been used as a bonding material owing to its excellent electrical properties, thermal conductivity, and low cost [2]. As dielectric materials, SiCN, SiO\({}_{2}\), benzocyclobutene (BCB), and polyimide (PI) are commonly used. Since both Au and Ag can be employed in the CMP process, they can be applied in Cu/dielectric hybrid bonding as a metal passivation layer on the Cu surface.