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ISSN Online: 2377-424X

International Heat Transfer Conference 12
August, 18-23, 2002, Grenoble, France

Forced and Buoyancy-Induced Convection in LIGA Electrodeposition

Get access (open in a dialog) DOI: 10.1615/IHTC12.3430
6 pages

Sinopsis

LIGA (a German acronym for lithography, metalforming, and molding) is a process for microdevice fabrication that includes deposition of metal from an aqueous electrolyte bath. An electric potential applied between the anode (source of plating metal) and the cathode (metallized substrate at the bottom of the feature) results in metal ion transport within the bath and deposition (electrochemical reactions) at the cathodic surface. Depletion of heavy metal ions results in density gradients in the electrolyte. The transport processes thus include diffusion, forced and buoyant convection, and ion migration. In the diffusion limited regime, relatively long deposition times (days) can be expected in large aspect ratio microstructures. Stirring of the bath results in a forced convection flow that affects the flow patterns within the features. Griffiths et al. (1998) showed that enhanced transport by buoyancy-induced convection can account for the increased deposition rate that has been measured in LIGA electrodeposition experiments.
In the present work the interacting mechanisms of the moving boundary, the electric potential and ion transport, and electrochemical boundary conditions, with forced flow and buoyancy-induced convection are investigated for the first time with an application to LIGA electrodeposition. The unsteady conservation equations for momentum, mixture mass, and chemical species, along with a Poisson equation describing the electrolyte potential and equations governing mesh motion, are solved using the finite-element formulation (cf. Chen and Evans, 2001). The motion of the non-planar, deposition surface is determined by the local mass balance of the depositing metal, which depends on the rate of reduction of the metal ions. Buoyancy is shown to have a dramatic effect on the species transport to the deposition surface, resulting in a surface that evolves nonuniformly. The applied voltage is shown to be an important parameter, affecting both the rate and uniformity of deposition. The electrodeposition process depends on the bath temperature primarily through the Butler-Volmer relation for the surface chemical kinetics. This work is part of our ongoing efforts to develop a complete modeling capability for the electrodeposition process in LIGA microstructures.