Uniformity of current density distribution at pattern scale during electrochemical micromachining by porous-type anodization

Porous-type anodization of patterned Al-0.5% Cu films and subsequent chemical etching of porous Al2O3 can be considered as a sub-category of electrochemical micromachining. This method results in metallic pillars separated by micro-grooves, whose dimensions are defined by the design of anodization masks and a degree of anisotropy of porous-type anodization. The pillars have a trapezoidal shape with concave slopes due to undercutting, which results from the lateral pore growth under the anodization mask. In addition, the shape of metallic pillars is affected by the distribution of current density at the pattern scale during porous-type anodization. The areas with a low coverage of anodization mask of barrier Al2O3 attract a high current density and develop thick layers of porous Al2O3. Chemical etching of porous Al2O3 reveals deep micro-grooves formed in these areas. These experimental results are explained by the nonuniform distribution of current density at the pattern scale, which results from the nonuniform coverage of patterned features. During porous-type anodization of patterned films the current is limited by the rate of ion migration through the scalloped layer of barrier Al2O3 and follows the pathway of least resistance, as defined by the anodization mask coverage. (c) 2006 The Electrochemical Society.