Influence of growth conditions and film thickness on the anodization behavior of sputtered aluminum films and the fabrication of nanorod arrays

This paper reports on how both the growth conditions and thickness of aluminum thin films impact the surface morphology and subsequent anodization behavior when sputtered onto Si and glass substrates up to a 1-mm thickness, respectively. Specifically, the effects of oxygen incorporation during sputtering and variations between continuous vs. stepwise deposition are thoroughly investigated here. Previous studies demonstrated the tremendous impact of oxygen concentration while sputtering, which, in turn, allows minimizing the grain sizes for 200-nm-thick sputtered AlOX-films. The then anodized aluminum oxide (AAO) thin films can then be used, for instance to grow quasi defect-free, large-area gold nanorod arrays for plasmonic applications. The challenge when preparing thicker films as emphasized here, is to avoid gradients of oxygen concentration and grain size for any film thickness. We have developed reproducible protocols how to minimize such gradients: the deposition process is purposely interrupted after every 100/200/300-nm AlOX deposition step for approximately 5 min. to allow thermal relaxation under pure nitrogen atmosphere. The subsequent anodization and pore filling with gold then revealed highly improved properties for AAO-films up to the 1-mm thickness, exhibiting a homogenous nanorod distribution with a very low number of defects.