Anodization Patterned on Aluminum Surfaces


Gabriel P. López Chenical and Nuclear Engineering, University of New Mexico

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Novel nanofabrication techniques are being explored as alternatives to conventional lithographic techniques to mass produce structures with feature sizes smaller than 100 nm and to minimize costs. Porous anodic aluminum oxide (AAO) is a self-ordered, hexagonal array of straight cylindrical pores with high densities (108–1011 pores/cm2), tunable diameters (5–250 nm), and depths (a few nanometers to hundreds of micrometers). It can be fabricated through electrochemical anodization of aluminum at moderate and constant temperatures of 0–10°C in aqueous acidic electrolytes. Anodic aluminum oxide is optically transparent in the fingerprint UV/vis and IR regions and thermally stable up to 1000°C. Moreover, it has recently been demonstrated to have unique advantages in several technological applications, including nanotemplating, membrane transport, and photonic crystals. Through-hole AAO membranes have been used as evaporation masks to fabricate nanodot arrays and as etching masks to transfer patterns into several substrates including diamond, silicon, and other semiconductors.

The high fragility of through-hole AAO membranes limits their integration into established microfabrication processes where robust membranes are needed. Although an elastic polypropylene support ring that runs around the circumference of AAO membrane permits one to pick up the membrane with membrane tweezers without shattering the membrane, this approach fails when the membrane undergoes a high-temperature and/or an ultrahigh vacuum. To provide additional supports to fragile AAO, one could pattern aluminum surfaces with an anodization barrier prior to anodization. During anodization of the patterned surfaces, the anodization barrier prevents anodization in the patterned areas while the oxide grows in the unpatterned areas. Recently, Huang et al. have reported the first anodization barrier of polymethylmethacrylate deposited on aluminum films. Because of the penetration of the electrolyte through this polymeric layer, the underlying surface was partially anodized to form AAO up to 10-nm deep. In addition, the procedure required optical lithography, polymer nanoprinting, ion-beam exposure, and milling. Because of its excellent adhesion to aluminum surfaces and electrical insulation as well as its inertness to acidic electrolytes, a SiO2 layer prepatterned on aluminum surfaces could provide a good anodization barrier. The patterning of aluminum surfaces with a SiO2 layer can be achieved through a sol–gel coating or a dielectric evaporation.

This paper describes a patterned anodization of bulk aluminum sheets and evaporated aluminum films prepatterned with an anodization barrier of SiO2 through a sol–gel process or a dielectric evaporation. Using a two-step anodization process, we formed highly ordered, uniform, and straight nanopores of AAO in the unpatterned areas, and did not observe any pores in the patterned areas before and after SiO2 was removed. This approach provides intermittent aluminum supports to fragile AAO membranes, allowing facile incorporation of AAO in a robust form into microdevices for microelectronics, microfluidics, and integrated optics.