Minimization of undercutting in electrochemical micromachining of patterned aluminum–copper films

Abstract

Al–0.5%Cu films patterned with an anodization mask of barrier Al2O3 are micromachined by porous-type anodization followed by chemical dissolution of porous Al2O3. Electrochemical micromachining results in well-defined metallic pillars separated by micro-grooves. The trapezoidal shape of the pillars is due to the lateral pore propagation under the anodization mask. Regardless of undercutting, porous-type anodization shows a higher degree of anisotropy than mostly isotropic wet chemical etching. The vertical growth of pores is accelerated with respect to the lateral propagation of pores by increasing the voltage of porous-type anodization. Consequently, the lateral pore propagation is minimized and for ∼3 µm thick Al–0.5%Cu films the etch factor increases from 2.6 to 4.3 when the voltage is increased from 20 V to 60 V. Comparing the results obtained with the micromachining of ∼3 µm and ∼10 µm thick films at the same voltage, the etch factor decreases as the depth of micro-grooves increases. The etch factor depends on the distribution of secondary current density at the pattern scale during porous-type anodization, which is defined by the anodization mask coverage. A higher degree of anisotropy of porous-type anodization (in comparison to wet chemical etching) allows for more accurate shape control of three-dimensional metallic microstructures.