Tailoring the Surface Morphology and Microstructure of Electrodeposited Copper Foil with Organic Additives

Abstract

Copper foil manufactured by high-speed electrochemical deposition (ED) is an essential material for its extensive applications such as the negative current collector for lithium-ion batteries (LIBs) or building up the designed connection pattern on the printed circuit boards (PCBs). Thus, producing copper foil with optimal quality and machinability depending on the end products is necessary. For example, the thickness of Cu foils for LIBs should be thin (< 10 µm) to reduce the volume and weight of batteries but the mechanical properties should remain strong enough to go through the roll-to-roll manufacturing process without fracture. In addition, the surface roughness on both sides should be identically smooth and glossy to provide the coating carbon materials with acceptable wetability and adhesion. On the other hand, Cu foils for the fifth generation (5G) high-frequency wireless devices should be exceedingly smooth because the skin depth of copper caused by the so-called skin effect is merely 0.266 µm when the transmitting frequency of signals is up to 60 GHz. Thus, a great majority of electric current passing through the conductor will flow extremely near the surface. Under such circumstances, coarse surface roughness has a significant detrimental effect on the signal loss of wireless communication devices, and consequently, the surface morphology and mechanical properties of copper foils should be further improved in order to meet the requirements for the next generation electronic devices. The microstructure of copper fabricated by ED depends highly on plating parameters, including current density, convection rate of the electrolyte, pulsed/direct current deposition, or using appropriate organic additives to ameliorate the nucleation behavior of Cu from cupric ions. Manufacturing of ED copper foils ought to operate with very high current density and high temperature to promote the yield rate for mass production, and the gelatin-based additive has been used for a long time. However, grain structure and surface roughness are coarse and mechanical properties are merely acceptable with around 250 MPa tensile strength and a 4% elongation rate for a normal 18 µm foil. In this work, we demonstrate the copper foil fabricated by high-speed electrodeposition with a current density of 450 mA/cm2 (average growth rate > 150 nm/s) from a 54oC bath, which meets the required yield rate for genuine mass-production, and tailoring the microstructure of copper with selective additives. The surface morphology of the foil facing the electrolyte side is smooth by our designed additives, and surface roughness values, Ra and Rq, are less than 100 nm, which are as smooth as foils manufactured via rolling process. In addition, the structure of copper is further tuned by a specific additive, and the grain size of copper is getting smaller as the concentration of this additive is increased. When the concentration of this additive is up to 800 ppm, the average grain size of copper is shrunk from 3 µm to 0.3 µm, and extremely unique copper texture with nearly single (220) orientation is constructed. Owing to such delicate microstructure, considerable improvement in mechanical properties with more than 40 % increase in tensile strength and 4 times increase in the elongation rate compared to the conventional gelatin-based copper foil is achieved. The surface morphology and structure of copper foil are properly characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM) and tensile testing machine, and the applied additive to tailor the structure of copper will be clearly revealed and discussed. Finally, a novel and commercially realizable scheme to fabricate copper foil with ultra smooth surface morphology and ductile mechanical properties by high-speed ED is proposed.