Solder-less Fine Pitch Copper-to-Copper Bonding

Abstract

The industry has been looking to increase pitch scaling through various methods. One of these methods is an alternative to solid-state solder bonding. However, very few processes can compete with solder bonding’s fast speed, low cost, and flexibility. The approach with the biggest potential for connecting fine pitch interconnects in high-density semiconductor packaging is solder-less copper-to-copper direct bonding. There are many benefits to using copper-to-copper direct bonding, including the ability to achieve ultra-fine-pitch (bump pitch 10 μm), higher reliability (both thermo-mechanically and better electromigration behavior), low resistance, as well as the lack of concern for solder hierarchy for multiple reflow. This copper-to-copper direct bonding technology enables a variety of 2.5D/3D advanced packaging architectures and is also suitable for multi-die stacking. In this study, we report the investigation of a direct copper-to-copper bonding methodology and reliability of the bonded components. Our process differs from common hybrid bonding copper-to-copper techniques, in that it is designed for low volume highly complex products. This technology allows a die-to-die and die-to-wafer process using thermocompression bonding under a deoxidizing vapor flow for direct copper-to-copper bonding. The process does not require high temperature oven annealing and can be done in a normal atmospheric environment. The investigated bonding process is ideal for high power, high thermal, high density 3D stacking and high-speed digital communication applications. We demonstrated the bonding procedure using a test vehicle with 40 μm diameter pillars and 80-μm bump pitch. We investigated the effects of bonding force and evaluated joint reliability. Interconnect/bond strength was determined through destructive shear testing and cross-sectional analysis. A scanning electron microscope was used to inspect and examine the bonded joint quality. Finally, completed assemblies were thermal cycled to evaluate degradation of joint strength under induced stress, and resistance measurements were performed to evaluate the electrical performance of the bonded copper joint.