Pillar-Cavity Copper-Copper Bonding for Face-to-Face Carbon Nanotube Synthesis

The bonding process is a critical step in the synthesis of face-to-face interlaced carbon nanotubes. Copper-copper bonding has emerged as a promising alternative to traditional silicon dioxide to silicon dioxide bonding due to its advantages, including a reduced bonding area requirement and greater flexibility in chip-to-chip bonding. However, the use of formic acid in copper-copper bonding can adversely affect the catalyst employed in carbon nanotube synthesis. This paper presents a modified technique for achieving high-quality Cu-Cu bonding without the need for formic acid. We introduce the innovative use of pillar-cavity pairs, designed to increase contact pressure at the Cu-Cu interface during the bonding process. Additionally, a copper base is employed to enhance the adhesion strength between the electroplated copper pillars and the substrate. Our experimental results demonstrate that the bonded chips can withstand subsequent processes, including double-side-dicing and high-temperature treatments, highlighting the robustness of this bonding approach. The bonding strength is quantitatively evaluated through shear tests, confirming the reliability and stability of the Cu-Cu bonds formed using this technique. This technology exhibits promising potential for use in the future synthesis of face-to-face interlaced carbon nanotubes.

I. Introduction

The interlacing of carbon nanotubes between parallelly fixed substrates offers a promising approach to enhance thermal conductivity in 3D packaging, particularly between stacked chips. This innovative technology relies on a series of key processes, including catalyst patterning, substrate bonding, and carbon nanotube synthesis. This unique approach enables the face-to-face growth of nanotubes in the space between the bonded substrates [1], resulting in the formation of interlaced connections that significantly reduce thermal resistance between the two substrates. It is important to note that as the interlaced carbon nanotubes grow, they exert pressure that could potentially separate the two substrates. Therefore, establishing a robust and enduring bond between the substrates prior to carbon nanotube synthesis is of paramount importance. This ensures the substrates remain securely bonded throughout the nanotube growth process.