Templated Self-Assembly of Nanoporous Alumina: A Wafer-Level Methodology for Ordered and Aligned Nanostructures (Nano-Wires, Rods, Dots and Tubes)

Nano-sized materials are core building blocks for advanced functional devices such as interconnects, logics, memories, sensors, and displays. Due to their size-sensitive electrical, optical, magnetic and chemical properties, it is desired to fabricate them with controlled size and distribution on the device-applicable substrates. As a strategy, we are developing templated self-assembly methods that combine top-down (lithography) and bottom-up (self-assembly) approaches for fabricating and assembling metallic nano-wires, rods, and dots, for new applications including nano-contacts for devices and interconnects for mixed-material and multifunctional micro- and nano-systems.

Anodic aluminum oxide (AAO) is a self-ordered nanostructured material that is well-suited as a template for use in magnetic, electronic and opto-electronic devices. Under proper anodization conditions, aluminum oxidizes as a porous structure with aligned pores that have close-packed (hexagonal) order at short range, and with pore sizes that can be varied from 7nm-300nm. Their excellent mechanical and thermal stability makes them suitable both as a physical mask for deposition of nanodot catalysts, as well as a supporting template for catalyzed growth of semiconductor nanowires and carbon nanotubes. While short-range pore ordering can be achieved during anodization, domains (<5um) with different repeat directions occur at longer ranges which limit further implementation of novel devices. We have developed a technique to obtain single-domain porous alumina with sub-30nm pore diameter and high aspect ratio (>50:1) on silicon substrates. This is done by anodizing aluminum films deposited on substrates with lithographically defined periodic topography, leading to templated self-assembly of alumina pores that are perfectly ordered over large areas. Pore spacing and ordering symmetry is controlled by the template, and pore diameters can be independently controlled to sub-lithographic length scales over a range of anodization conditions. Topographic templating of long range order in AAO allows independent control of the pore size, spacing and order symmetry in ranges not achievable without templating. Using the perfectly-ordered AAO templates, we have fabricated ordered metallic nanodots, nanorods, and nanotubes as well as well-aligned multi-walled carbon nanotubes on silicon. We are exploring the use of metal/CNT-filled alumina templates as electrical nano-breadboards using dip-pen nanolithography for applications in molecular electronics. These results demonstrate a wafer-scale approach to the control of the size, pitch, ordering symmetry, and position of nanomaterials in a rigid insulating scaffold.