Nanoporous Aluminum Oxide Templates of Arbitrary Thickness on Silicon Carrier Wafer

Abstract

Porous templates of anodized aluminum oxide (AAO) are of interest in terms of photonics, energy storage, and other various nanoscience applications. There is a great interest to grow AAO templates directly on silicon as a carrier wafer, however most attempts rely on film deposition techniques that are generally suited for depositions on the order of 1 um. Thicker aluminum films are desirable in order to achieve better ordering of the AAO pores through two-step anodization and for greater aspect ratio for applications where surface area is the dominant factor such as solar cell and supercapacitor devices. Thick metal films are commonly applied through electrodeposition, however this technique is not suitable for aluminum owing to its high reduction potential above that of water. Aluminum and silicon are known to alloy up to approximately 1 atomic percent silicon in aluminum. Here, AlSi alloys are used as adhesion layers between aluminum foils and silicon wafers as a solution to the thin film problem. High purity aluminum foils and silicon wafers are pressed together as a diffusion couple. The couple is then annealed under argon atmosphere at temperatures from 550oC to 650oC (from below the AlSi eutectic temperature of 577oC to just below the Al melting temperature of 660oC) for various times. Cross sections of these couples are examined under scanning electron microscopy (SEM) to determine the thickness of the diffusion region and to check for the presence of voids. The chemical composition of the diffusion region is investigated through electron probe micro-analysis. Bonded Al-AlSi-Si composites are anodized in either phosphoric or sulfuric acids using a two-step process, and the final AAO morphology is examined under SEM.