Electrochemical micromachining of aluminium for microfluidic devices

Through-mask electrochemical micromachining (TMEMM) combines photolithographic masking with anodic metal dissolution and thus enables the mass fabrication of microfluidic devices made from metals. To achieve an even removal rate and surface finish, a uniform current distribution is required, which is influenced by mass-transfer effects, surface-film formation, and the mask-opening geometry. This complex interplay was studied, with the aim of machining microchannel plate-inlays for a plate-type microreactor design, with aluminium acting as the anode and phosphoric acid serving as the electrolyte. This metal/electrolyte system was first characterised in terms of surface-film formation via an extensive electrochemical impedance spectroscopy study. A shape-evolution model based on diffusive mass-transfer was then developed for comparison with experimental shape profiles. Thereafter, micromachining experiments were conducted under potentiostatic control in a custom-built batch cell using rotating discs and stationary plates, while varying the hydrodynamic conditions, and thus the conditions of mass transfer. Analysis of this data confirmed the presence of a compact, barrier-type alumina film along the anode surface, the thickness of which was on the order of a few nanometres only and solely depended on the applied anode potential. Uniform film coverage and thickness ensued, resulting in an even surface finish with sub-micrometre surface roughness. The model agreed well with experimental results in regards to channel depth, width and profile, thereby correctly representing the shape-evolution process in quiescent electrolyte. Deviations occurred at high anode potentials due to concurrent oxygen evolution, as well as in agitated electrolyte due to mass-transfer effects. These effects were shown to manifest themselves in different forms depending on the mask-opening size: in the presence of shearing flow, the shape profiles of individual microchannels became displaced and distorted, whereas larger cavities exhibited wedging, that is a decrease in the removal rate in flow direction. As a result, sufficient control of flow conditions and anode potential was identified as a key factor in enabling the machining of well-defined microfluidic devices into aluminium. Based on this work, it is recommended that TMEMM of microfluidic devices proceeds via through-foil etching of thin metal shims, thus following a stacked-shim assembly for the device design. Also, impinging rather than shearing flow should be used during TMEMM. In this manner, non-uniform channel and cavity dimensions could be minimised. Future work should investigate the commercial viability of TMEMM and focus on the use of difficult-to-etch metals. In addition, preliminary work on the formation of catalytic wall coatings made from Au/Al2O3 via electrochemical means should be pursued further