Microgalvanic nickel pulse plating process for the fabrication of thermal microactuators

Nickel is a common material in micro fabrication because of its fatigue resistance and its mechanical properties. It is used for instance for thermal actuators, micro-grippers, or RF-switches. The defined electro deposition of the nickel matrix is crucial for the properties and functionality of e.g., thermal actuators. Micro galvanic processes are the basis of this electro deposition, and require knowledge of the electrochemical fundamentals as well as numerical electrochemical process simulation for adjustment. Especially, realization of high aspect ratios requires the application of sophisticated plating techniques such as pulse reverse deposition. The pulse plating process is adjusted using the results of electrochemical numerical simulation routines, visualizing the (local) potential field and the current field line distribution as a function of the applied electrochemical parameters. Compact, completely void free structures can be obtained by applying the developed pulse plating process to Si wafers that are structured with photo resist. Nickel is chosen for electro deposition due to its chemical stability and its hardness. MEMS structures are designed to convert the thermal expansion of the material into an in-plane deflection. A custom made measurement setup, consisting of a sealable chamber, a Peltier element with a temperature control unit, and an optical microscope is used to measure these deflections at different temperatures. A set of cantilever structures with different lengths is used to evaluate the Young’s modulus and the vertical stress gradient of the plated materials. Additional, finite element simulations are carried out to determine the thermal expansion coefficient of the plated Nickel, by fitting the simulation and the measurement results.