Etching electrolytic, selective

As discussed in the previous sections, copper can be either electrolytically plated to an entire surface, and fhen photographically defined using etching, or selectively plated. In both cases, the entire surface of the part is electrically at the same potential and is then connected to a power supply during the plating operation. 

Then the anodic alumina layer formed was removed chemically in the selective etchant composed of phosphoric (6 wt.%) and chromic (1.8 wt.%) acids at 60 C. Hemispheric etching pits - replica of the alumina cell bottoms - remain on the surface of the aluminum foil. The second porous anodization of aluminum was made. At this stage, the pores on the aluminum foil surface arise not in random way but at the sites of primary alumina cell Imprints to repeat the cell size. The pore diameter and spacing are dictated by the parameters of the anodization process, specifically by the electrolyte composition and the anodization voltage. The alumina film thickness is defined by the anodization time and the anodization current density. The second stage provides a continuous development of the alumina film. Total etching process takes 10-20h to get pores of approximately 100 pm lengths. 

If the whole semiconductor/electrolyte interface is illuminated uniformly, both conjugate reactions proceed at the same rate over the same areas on the interface. The stationary potential of an illuminated semiconductor is thus a mixed potential. If the surface of a semiconductor, homogeneous in its composition and properties, is illuminated nonuniformly, in the illuminated and nonillumi-nated areas conditions will not be identical for electrochemical reactions. Here the conjugate reactions appear to be spatially separated, so that we can speak about local anodes and cathodes. This situation is deliberately created, for example, for selective light-sensitive etching of semiconductors (see Section V.2). 

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