Low temperature Cu-Cu bonding using copper nanoparticles fabricated by high pressure PVD

Zijian Wu,Jian Cai,Jian Cai,Qian Wang,Junqiang Wang,Junqiang Wang

doi:10.1063/1.4978490

Abstract

Copper nanoparticles (Cu NPs) fabricated by physical vapor deposition (PVD) were introduced in Cu-Cu bonding as surface modification. The bonding structure with Ti adhesive/barrier layer and Cu substrate layer was fabricated on both surfaces first. Loose structure with Cu NPs was then deposited by magnetron sputtering in a high pressure environment. Solid state Cu-Cu bonding process was accomplished at 200°C for 3min under the pressure of 20MPa. Die shear test was carried out and an average bonding strength of 36.75MPa was achieved. The analysis of fracture surface revealed a high-reliability bonding structure. According to cross-sectional observations, a void-free intermediate Cu layer with thickness around 10nm was obtained. These results demonstrated that a reliable low temperature time-saving Cu-Cu bonding was realized by Cu NPs between the bonding pairs. This novel bonding method might be one of the most attractive techniques in the application of ultra-fine pitch 3D integration.

Highlights

Three-dimensional (3D) integration is emerging as an attractive technique to overcome the limitations of two-dimensional structure with lower power consumption, higher density and smaller form factor in microelectronic packaging.[1]
The results from the above studies demonstrated that a reliable low temperature and time saving Cu-Cu bonding with Cu NPs as surface modification was achieved in this study
It can be deduced that sputtered Cu atoms ejected from the target have a much smaller mean free path and tend to be more likely colliding with the gas atoms under the high pressure environment

Summary

INTRODUCTION

Three-dimensional (3D) integration is emerging as an attractive technique to overcome the limitations of two-dimensional structure with lower power consumption, higher density and smaller form factor in microelectronic packaging.[1]. There are many studies investigated to overcome these challenges by introducing nanostructures on Cu surface For nanostructures at both surfaces, low temperature sintering can be potentially promoted due to high surface-to-volume ratio of NPs, which may lead to a significant reduction of the bonding temperature.[19] C. I. Wang proposed a low temperature bonding process using Cu nanorod array fabricated by oblique angle deposition.[20,21] the bonding needed a long duration (1hour) process and a high annealing temperature (>300◦C) to promote the grain growth and eliminate voids at the interface. The results from the above studies demonstrated that a reliable low temperature and time saving Cu-Cu bonding with Cu NPs as surface modification was achieved in this study

EXPERIMENTAL

Cu NPs morphology

Bonding strength and fracture surface

CONCLUSIONS