# The Effect of an Ag Nanofilm on Low-Temperature Cu/Ag-Ag/Cu Chip Bonding in Air
## 1 Introduction  
Although semiconductor devices have achieved remarkable performance improvement and miniaturization of integrated circuits according to Moore's law, semiconductor devices have reached a physical limit in miniaturization, and device scaling means it becomes very difficult to improve the degree of integration due to the rapidly increasing number of inputs/outputs. To improve the degree of integration of semiconductor devices, 3D packaging technologies such as wafer stacking and die stacking are becoming increasingly important, especially in recent heterogeneous integration, and many studies on 3D packaging have been conducted that cover high performance, miniaturization, reliability, and low manufacturing cost [1; 2]. Si forming and filling, wafer thinning, and the bonding process are required for 3D packaging fabrication, and the most important unit process is currently the Cu bonding process due to the fine pitch structure and high-performance demands. Although Cu has a relatively low cost, fine pitch pattern-ability, and good electrical and thermal conductivities, Cu suffers from needing a high bonding temperature, which is typically higher than 400 \({}^{\circ}\)C, and of forming a natural oxide film even with low oxygen content [3]. If the Cu bonding process takes place at above 400 \({}^{\circ}\)C, the device performance can potentially degrade because the device is made of many metal interconnects. In addition, if Cu oxide is formed on the Cu surface before bonding, Cu bonding is not well formed, and electrical failure occurs in the bonded interconnect.  
Many low-temperature Cu bonding studies have been actively published [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. [PERSON] et al., reported the results of low-temperature Cu bonding at 250 \({}^{\circ}\)C after preventing oxidation of the Cu surface using a self-assembled monolayer surface treatment [4], and more recently, the use of a mixed gas plasma of argon (Ar) and nitrogen (N\({}^{2}\)) [5] prior to Cu bonding has been reported to prevent Cu oxidation. Elsewhere, [PERSON] et al., presentedsequential two-step Ar and N\({}^{2}\) plasma surface treatments to form uniform copper nitride (Cu\({}^{4}\)N) as an antioxidant layer on the Cu surface and allow Cu bonding at 260 \({}^{\circ}\)C and 300 \({}^{\circ}\)C [6,7]. To lower the Cu bonding temperature below 200 \({}^{\circ}\)C, the use of various metal passivation layers has been reported. [PERSON] et al., reported oxidation prevention and low-temperature Cu bonding at 180 \({}^{\circ}\)C using an ultra-thin Ti layer [8], and [PERSON] et al., reported Cu bonding at 160 \({}^{\circ}\)C using an ultra-thin Ti layer [9]. [PERSON] et al., reported low-temperature Cu bonding at 140 \({}^{\circ}\)C using an Au passivation layer [10]. [PERSON] et al., presented Cu chip bonding at 180 \({}^{\circ}\)C using an Ag thin film deposited by sputtering [11]. In addition, Cu bonding research using Ag nanoparticles has been conducted because of the nanoparticles' high specific surface area, high diffusion rate, and low deformation. [PERSON] et al., and [PERSON] et al., reported Cu bonding results without voids at the bonding interface at 180 \({}^{\circ}\)C using Ag nanoparticles deposited by pulsed laser deposition [12,13].  
Cu bonding at 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C and solid-state diffusion using an Ag nanofilm without forming an intermetallic compound were studied herein. Ag nanofilm deposited by an evaporation process produces a porous film and has a high surface area, and evaporation is a more CMOS-compatible process than are other nanoparticle or nanotube processes. The solid-state diffusion of the Ag nanofilm was investigated, and low-temperature Cu bonding using an Ag nanofilm was evaluated for 3D system-in-package applications.