The Effect of an Ag Nanofilm on Low-Temperature Cu/Ag-Ag/Cu Chip Bonding in Air

Yoonho Kim,Sarah Eunkyung Kim,Seungmin Park

doi:10.3390/app11209444

Yoonho Kim, Sarah Eunkyung Kim + Show 1 more

Open Access

https://doi.org/10.3390/app11209444

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Abstract

Low-temperature Cu-Cu bonding technology plays a key role in high-density and high-performance 3D interconnects. Despite the advantages of good electrical and thermal conductivity and the potential for fine pitch patterns, Cu bonding is vulnerable to oxidation and the high temperature of the bonding process. In this study, chip-level Cu bonding using an Ag nanofilm at 150 °C and 180 °C was studied in air, and the effect of the Ag nanofilm was investigated. A 15-nm Ag nanofilm prevented Cu oxidation prior to the Cu bonding process in air. In the bonding process, Cu diffused rapidly to the bonding interface and pure Cu-Cu bonding occurred. However, some Ag was observed at the bonding interface due to the short bonding time of 30 min in the absence of annealing. The shear strength of the Cu/Ag-Ag/Cu bonding interface was measured to be about 23.27 MPa, with some Ag remaining at the interface. This study demonstrated the good bonding quality of Cu bonding using an Ag nanofilm at 150 °C.

Highlights

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
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
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 ◦C, and of forming a natural oxide film even with low oxygen content [3]

Summary

Introduction

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. 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. 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 ◦C, and of forming a natural oxide film even with low oxygen content [3]. To lower the Cu bonding temperature below 200 ◦C, the use of various metal passivation layers has been reported. Cu bonding research using Ag nanoparticles has been conducted because of the nanoparticles’ high specific surface area, high diffusion rate, and low deformation. 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

Materials and Methods

Findings

IEEE Heterogeneous Integration Roadmap