Mechanically strengthened graphene-Cu composite with reduced thermal expansion towards interconnect applications

High-density integration technologies with copper (Cu) through-silicon via (TSV) have emerged as viable alternatives for achieving the requisite integration densities for the portable electronics and micro-electro-mechanical systems (MEMSs) package. However, significant thermo-mechanical stresses can be introduced in integrated structures during the manufacturing process due to mismatches of thermal expansion and the mechanical properties between Cu and silicon (Si). The high-density integration demands an interconnection material with a strong mechanical strength and small thermal expansion mismatch. In this study, a novel electroplating method is developed for the synthesis of a graphene-copper (G-Cu) composite with electrochemically exfoliated graphenes. The fabrication and evaluation of the G-Cu composite microstructures, including the microcantilevers and micromirrors supported by the composite, are reported. We evaluated not only the micromechanical properties of the G-Cu composite based on in-situ mechanical resonant frequency measurements using a laser Doppler vibrometer but also the coefficients of thermal expansion (CTE) of the composite based on curvature radius measurements at a temperature range of 20–200 °C. The Young’s modulus and shear modulus of the composite are approximately 123 and 51 GPa, which are 1.25 times greater and 1.22 times greater, respectively, than those of pure Cu due to the reinforcement of graphene. The G-Cu composite exhibits a 23% lower CTE than Cu without sacrificing electrical conductivity. These results show that the mechanically strengthened G-Cu composite with reduced thermal expansion is an ideal and reliable interconnection material instead of Cu for complex integration structures.
Circuits: Copper-graphene interconnects A graphene-copper composite exhibits a coefficient of thermal expansion closer to silicon, as compared to pure copper, suggesting its use for interconnects on circuit boards. Copper is widely used for interconnects in silicon-based electronics due to its high electrical conductivity. However, mismatch in thermal expansion coefficient between copper and silicon generates large internal stresses, potentially causing failure in the long-term. Now, a team led by Takahito Ono from Tohoku University reinforce copper with graphene flakes via an electroplating fabrication approach, achieving higher stiffness and a coefficient of thermal expansion 23% lower than pure copper. These results suggest that this material might enable high-density circuits that can resist internal stresses as a result of thermal expansion.