Electrodeposition of nickel-copper alloys and nickel-copper-alumina nanocomposites into deep recesses for MEMS

Electrodeposition is an important component in the fabrication of micro electro mechanical systems (MEMS). Nickel is the most commonly used material to produce three dimensional microstructures and few material alternatives have been demonstrated. In this dissertation, electrodeposited Ni-Cu alloys and nanocomposites are investigated as possible replacements for nickel in microsystems. Ni-Cu alloys are attractive for their corrosion resistance, magnetic and thermophysical properties. Alumina nanoparticulates included into metal matrices improve hardness and tribology of deposits. The Ni-Cu alloys and Ni-Cu-g-Al2O3 nanocomposites were electrodeposited from a citrate electrolyte, both at low and high pH. Electrodeposition experiments were performed in recessed microelectrodes 500 mm thick and also on rotating cylinder electrodes. Recessed electrodes were produced by x-ray synchrotron radiation at the Center for Advanced Microstructures and Devices (CAMD). The concentration of copper in the electrolyte was much lower than the nickel concentration to ensure diffusional control. In the microstructure, the copper concentration in the deposit increased along the height, leading to a graded microstructure. This is indicative of a changing boundary layer and a transient process. The addition of alumina nanoparticles in the electrolyte led to an enhancement of copper concentration in the deposit, resulting from an enhancement of its mass transport rate. Two numerical models were developed to describe the steady state and non-steady state deposition processes. The effect of alumina on the metal deposition partial currents and side reactions is simulated by using a surface coverage model. Rotating cylinder experiments and simulation are used to extract kinetic and diffusional parameters of the nickel and copper species. On the recessed electrodes a transient model taking into account the time dependence of concentration is developed. The rise of surface pH, concentration gradients and buffering effects of the complexing agents are explained by the non-steady state model