Anodization of Al Alloyed with in, Sn and Ag
Authors: Ramsey Keaton , Sides William , Huang Qiang 
Published: February 2020

Since the discovery of self-ordering during the anodization of aluminum[1], it has attracted great deal of interest for its potential application in device fabrication. Tremendous progress has been achieved in understanding the process, controlling the pore size and improving the long-range ordering[2-8]. As the formation of the pore is a synergistic play between the electrochemical oxidation of aluminum and voltage assisted dissolution of the oxide, the self ordering further involves the stress resulted from the volume expansion during oxidation[3,4]. Recently, a study using a thin layer laminated in Al revealed a viscous fluid behavior of the metal during the large stress created by the volume expansion.[8] Different type of chemistries, voltage bias and predesigned voltage profiles have been studied in conjunction with temperature to realize a large variety of special shape profiles in the pores[9-12]. Recently, studies on industrial grade Aluminum with alloying elements has been studied for anodization and slower anodization rate and poorer pore circularity as well as poorer pore ordering were observed[13].
This paper will present some preliminary work of investigating the anodization of a few different Aluminum alloys. The alloying element was evaporated as a thin layer sandwiched between two aluminum layers followed by high temperature annealing. Anodization of the as-deposit laminated layers as well as the annealed layers were carried out. For example, a 100-nm layer of In, Sn and Ag were sandwiched between two 1-um thick Al layers and the cross sectional scanning electron microscopy (SEM) micrographs of the three films after 450 C annealing are shown in Figure 1. Due to the fast inter-diffusion of In with Al, Kirkendall voids were observed in the Al/In/Al system. These voids were even observed before annealing. Due to the high concentration of the alloying elements and their low solubility in Al, precipitation of the alloying elements was observed. However, while the precipitation occurs primarily at the interfaces of the film with ambient or with the substrate for Sn, precipitation in the Al was observed for Ag. Figure 1 also shows the SEM images of the films after anodization. Due to the precipitates of the alloying metal, the direction of the pore growth as well as the morphology of the pore were changed. Figure 2 shows a comparison of pure Al and a sandwich with only 10 nm Ag in between. While the precipitation was not observed and the pore growth direction was not significantly changed at this much lower Ag concentration a change of the roughness of the pores were observed.

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Figure 1