The authors demonstrate a method for the fabrication of in situ ultrathin porous anodic aluminum oxide layers ͑aspect ratioϽ 2:1͒ on Si, which can be directly used as templates for nanodot preparation and for pattern transfer. The regular shape of the aluminum oxide pores is maintained even when the thickness of the aluminum oxide template is reduced to 50 nm. By using these in situ ultrathin templates as lift-off masks, the authors successfully prepared a Ba x Sr 1−x TiO 3 nanodot array on Si surface. Furthermore, these nanotemplates are employed as lithographic masks to transfer the nanopattern into the silicon substrate. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2767768͔Recently, porous anodic aluminum oxide ͑AAO͒ templates have received quite some attention for application in nanowire and nanotube synthesis, due to the ideal pore diameters and high aspect ratio of their channels. 1-3 Generally, AAO templates exhibit columnar pore structure, vertical to the substrate and parallel to each other with pore diameters from several tens to several hundreds nanometers and with an aspect ratio between 10 and 1000 or more. 4-9 Thus, by filling the pores of the AAO templates, arrays of well aligned nanowires and nanotubes with uniform diameter and length can be obtained by electroplating or other growth methods.However, the use of AAO templates for nanodot deposition and nanopattern transfer masks, an interesting approach for the fabrication of nanostructures without the need of costly nanolithography, is often impaired by this high aspect ratio. In fact, for high aspect ratio templates, the filling of the pores during deposition is poorer and dry etching for pattern transfer is more difficult. Furthermore, the channel sidewalls tend to be less smooth, which negatively affect the abovementioned nanostructuring processes. 8 Therefore, it is very important to reduce the aspect ratio of the templates when we employ them for nanostructuring Si substrates. A few methods have been developed to prepare ordered ultrathin AAO mask. 7,8 In these methods, the bulk Al is chosen as starting material, and then after anodization AAO templates are released from the Al substrate and mounted on the surface of Si or SiO 2 substrates to act as deposition masks. However, AAO is like a brittle ceramic film. Great attention needs to be paid in the manipulation. Moreover, these methods are not very compatible with the conventional integrated circuit ͑IC͒ processes, which restrict their application in Si based structures. Thus it would be a great benefit to realize in situ ultrathin AAO mask layers directly on a Si substrate.In this letter, we report on a simple method to fabricate in situ ultrathin AAO templates ͑aspect ratioϽ 2:1͒ on Si substrate. As schematically shown in Figs. 1͑a͒ and 1͑b͒, after the ordered thicker AAO template on Si substrate is obtained by anodization, an additional step is introduced which makes use of the capillary effect ͑antiwetting͒ to only etch the surface of AAO template and not the sidewalls of a͒ E...
