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.

Pulse-Reverse Electrodeposition and Micromachining of Graphene-Nickel Composite: An Efficient Strategy toward High-Performance Microsystem Application.

Graphene reinforced nickel (Ni) is an intriguing nanocomposite with tremendous potential for microelectromechanical system (MEMS) applications by remedying mechanical drawbacks of the metal matrix for device optimization, though very few related works have been reported. In this paper, we developed a pulse-reverse electrodeposition method for synthesizing graphene-Ni (G-Ni) composite microcomponents with high content and homogeneously dispersed graphene filler. While the Vickers hardness is largely enhanced by 2.7-fold after adding graphene, the Young's modulus of composite under dynamic condition shows ∼1.4-fold increase based on the raised resonant frequency of a composite microcantilever array. For the first time, we also demonstrate the application of G-Ni composite in microsystems by fabricating a Si micromirror with the composite supporting beams as well as investigate the long-term stability of the mirror at resonant vibration. Compared with the pure Ni counterpart, the composite mirror shows an apparently lessened fluctuations of resonant frequency and scanning angle due to a suppressed plastic deformation even under the sustaining periodic loading. This can be ascribed to the reduced grain size of Ni matrix and dislocation hindering in the presence of graphene by taking into account the crystalline refinement strengthen mechanism. The rational discussions also imply that the strong interface and efficient load transfer between graphene layers and metal matrix play an important role for improving stiffness in composite. It is believed that a proper design of graphene-metal composite makes it a promising structural material candidate for advanced micromechanical devices.

Microstructuring of carbon nanotubes-nickel nanocomposite.

In this paper, we develop a novel electroplating method for the synthesis of carbon nanotubes (CNTs)-nickel (Ni) nanocomposite, and present the fabrication of a silicon micromirror with the CNTs-Ni nanocomposite beams to evaluate the mechanical stability of the micromirror in terms of resonant frequency. CNTs are pretreated to have positive charges on their surface and added into a Ni electroplating solution to form a CNTs-Ni nanocomposite electroplating suspension. The weight fraction of the CNTs in the electroplated nanocomposite is 2.4 wt%, and the ultramicroindentation hardness is 18.6 GPa. The mechanical strengthening phenomenon is found in the nanocomposite in comparison with a Ni film. Moreover, the addition of CNTs in the nanocomposite beams effectively increases the shear modulus compared with the pure Ni. The maximum variation of the resonant frequency of the micromirror during a long-term stability test is approximately 0.25%, and its scanning angle is approximately 20°. It shows the potential suitability of the CNTs-Ni nanocomposite with proper design for micromechanical element applications.