Submicron Patterned Anodic Oxidation of Aluminum Thin Films
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Submicron Patterned Anodic Oxidation of Aluminum Thin Films Qiyu Huang, Whye-Kei Lye, David M. Longo1, Michael L. Reed Department of Electrical Engineering, University of Virginia, Charlottesville, VA 22904, U.S.A. 1 Department of Material Science and Engineering, University of Virginia, Charlottesville, VA 22904, U.S.A. ABSTRACT 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.
INTRODUCTION Porous alumina formed by anodic oxidation exhibits pores with straight sidewalls and high aspect ratios that have controllable areal density (n) and pore diameters (d). These parameters can be expressed as [6]: V n = 1.6 × 1012 ⋅ exp(−(4.76 ⋅ * )) cm-2 (EQ. 1) V V d = 3.64 + 18.9 ⋅ * nm (EQ. 2) V where V * , the critical voltage, determined empirically by [6]: V * = 4.2 + 20.5 ⋅ c( H + ) (EQ. 3) 3 + Here, c(H ) (mol/dm ) is the proton concentration in the bulk of the etch solution. Anodization provides the basis for the inexpensive, high throughput microfabrication of structures with near vertical sidewalls [2]. Previous work has demonstrated high-aspect-ratio microstructures using the property of straight porous aluminum oxide sidewalls [2][3]. Tan et al [2] and A.P. Li et al [5] anodized aluminum prior to patterning the porous aluminum oxide. The exposed part is removed by pore expansion. Traditional lithography was used for patterning, as such the resolution is also limited by factors as in all other traditional lithography methods. D9.49.1
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(d) O2 plasma is used to clear polymer residue left in printed holes (a) Printing to polymer layer. Piezo Actuator
(e) Masked anodic oxidation on aluminum. The patterned polymer layer serves as an anodization barrier.
(b) Transfer of features by embossing.
(f) Removing anodized aluminum oxide by pore expansion. The pattern has been transferred to the al
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