Simulation of electrodeposition in ultra-deep microcavities

LIGA (an acronym from German words for lithography, electroplating, and molding) process is a promising new technique for producing non-silicon based MEMS devices, such as high aspect-ratio metal micro-devices. Typically, it is a three-step process consisting of X-ray lithography, resist development, and electroplating. The present work is focused on the electroplating step of the LIGA process. Traditional electroplating practices have not proven very effective in filling the deep cavities characteristic of LIGA molds. Non-uniformities occur during the filling process and are likely related to the transport phenomena inside cavities. Ion transport may either be due to forced convection stirring, or due to buoyancy driven by concentration gradients. For a specific bath composition, there are homogeneous dissociation reactions between species in the bath, which are infinitely fast such that equilibrium is reached; and finite rate surface reactions on the electrode, through which the desired materials are depleted. Fluid flow, mass transport and electrodeposition reactions are coupled together in the process. Ions in solution are depleted at the electrode until the whole cavity is filled up. The interface between the solid and fluid phase moves continuously as the cavity fills. The work presented here is the development a CFD model to simulate and analyze the whole electroplating process. For this purpose, the whole work has been divided into two parts. First we analyze flow and electrodeposition reactions in a deep cavity of fixed depth. Second we develop an appropriate method to study in detail the filling process. The thesis addresses basic problems in the modeling of electroplating process, including: (1) Understanding bottom-driven natural convection in deep cavities over a range of Schmidt (Prandtl) and Rayleigh numbers. (2) Development of a solution algorithm for computing fast equilibrium reactions in the bulk electrolyte coupled with finite rate electrolyte reactions at the electrode surface. (3) Coupling fluid flow and electrodeposition reactions for the prediction of transport effects on electrodeposition in deep fixed domains. (4) Development of a fixed grid algorithm for electrofilling in the presence of fluid flow and electrodeposition reactions.