Submicron Patterned Anodic Oxidation of Aluminum Thin Films

Alumina formed by the electrochemical anodization of bulk aluminum has a regular porous structure [1]. Sub-100 nm pores with aspect ratios as high as 1000:1 can easily be formed [2] without elaborate processing. Anodization of aluminum thus provides the basis for the inexpensive, high throughput microfabrication of structures with near vertical sidewalls [2]. In this work we explore the patterned anodic oxidation of deposited aluminum thin films, facilitating the integration of this technique with established microfabrication tools. An anodization barrier of polymethylmethacrylate (PMMA) is deposited onto 300 nm thick aluminum films. The barrier film is subsequently patterned and the exposed aluminum anodized in a 10% sulfuric acid solution. Barrier patterning techniques utilized in this study include optical exposure, ion-beam milling and nano-imprint lithography. Sharp edge definition on micron scale patterns has been achieved using optical methods. Extension of this technique to smaller dimensions by ion-beam milling and nano-imprint lithography is presented. We further report on the observation of contrast reversal of anodization with very thin PMMA barriers, which provides a novel means of pattern transfer. Potential applications and challenges will be discussed.