Convective-diffusive mass transfer inside complex micro-molds during electrodeposition

During fabrication, local variations in convective-diffusive mass transfer and current distribution can give rise to undesirable variations in material composition for electrodeposited microelectromechanical systems (MEMS). Local transport characteristics are normally difficult to analyze in complex microstructures, especially the convective-diffusive conditions present during device fabrication. To tackle this problem, we have developed an experimental method called "Process Archaeology." The general approach involves the electrodeposition of NiFe alloys from an electrolyte formulated to give a local composition that is very sensitive to the local mass transfer rate. Spatially-resolved measurements of the alloy composition and growth rate are accomplished by combining scanning electron microscopy and energy dispersive x-ray spectroscopy. Using a simple inverse problem formulation, these experimentally measured quantities are related to the local convective-diffusive mass transfer coefficient (k) and the local Sherwood number present during fabrication. Here the Sherwood number is defined as Sh=kH/D, where H is the thickness of the lithographic mask and D is the diffusivity of a key transport-controlled species. Electrodeposited microgears are used as a test bed for demonstrating "Process Archaeology." Sherwood number images show the wide ranges of mass transfer rates and the complex mixing features present during microgear fabrication, especially in the region downstream of the microgear's central hub.