Fabrication of porous anodic alumina membranes with ultrathick barrier layer

Porous anodic alumina (PAA) with ultrathick barrier layer has been fabricated by two-step anodization. The formation mechanisms of the pear-shape nanopores and complex PAA films were investigated by a combination study of voltage-time characteristics and field-emission scanning electron microscope measurements. In the initial stage of the first step anodization, pear-shape nanopores are formed due to the flow of Al3+ ions to the top of sidewall. In the second step anodization, the barrier-type film develops simultaneously at the oxide/electrolyte and metal/oxide interfaces by Al3+ egress and O2-/OH- ingress under the electric field. Because of the diffusion limits of the electrolytes in the parallel channels and the distorted electric field distribution at the pores bottom, the formation speed of barrier layer will decrease as the nanochannels prolong. Such a process yields complex films, is a promising application for anodic foils in aluminum electrolyte capacitors, and broadens the scope for the application of PAA in nanoelectronic devices. (c) 2008 Elsevier B.V. All rights reserved.