Tech Briefs

Porous alumina systems fabricated via a “boot-strapping” method have applications in electronics and sensing.

This project was dedicated to solving basic scientific issues and developing the scientific basis that underlies the improvement of super-dense memories, towards the terabit-per-square-inch goal and the engineering of chemical and biological sensors. Both applications rely on porous materials. Among them, porous alumina has demonstrated to provide major improvements in these two diverse applications.

The following objectives were set: a) to improve the geometry (regularity, shape, decreased size) of the pores, b) to fabricate and study the properties of nanostructured magnets in a variety of configurations, and c) to apply this technology to chemical and biological sensing.

The size distribution of the porous alumina used for masks was improved significantly using a double anodization process. An unconventional “bootstrapping” method was developed, which uses self-supporting porous membranes. This new method enabled fabrication of very well ordered arrays of small magnetic nanodots on different substrates, which could not be done otherwise.

Although the two applications mentioned earlier are quite different, the basis for both of them is a common technology: self-assembled porous materials — in particular, self-assembled porous alumina on a substrate. Major improvements on the preparation and manipulation of porous alumina were made that have given rise to the understanding of nanostructured magnets, microcapillary condensation, and novel properties of confined organic materials.

Different approaches were investigated for the preparation of porous alumina samples to reach better pore regularity and to broaden the possible shapes that can be obtained. Three schemes were investigated: a) electron beam imprint lithography prior to the anodization to confer order and shape to the pores, b) two-stage anodization evaporation at an angle method to create noncircular nanodots, and c) unconventional bootstrapping to confer order to the pore array without the need of expensive nanoimprint masks. These methods permit the fabrication of complex nanostructures in macroscopic areas that can be used for sensing and confinement.

The original bootstrapping method makes use of iterative, anodization-nanoimprint mask preparation to improve the pore array distribution. High-quality, large, self-supported porous alumina was used as a mask, and by means of reactive ion etching, the ordered pattern was transferred to a supported aluminum thin film. A single, short anodization performed on the aluminum film after this process shows a considerable improvement in the ordering and size distribution of the pores. Unlike the original “bootstrapping” method, this one does not require iterations or the preparation of an imprint mask, which quite often breaks. It also presents a very competitive solution with respect to the electron beam imprint since larger an-odization areas can be improved with much more inexpensive techniques.

The magnetism of one-dimensional Fe chains in metallo phthalocyanines organic thin films was investigated. Making use of the particular stacking of these molecules, one-dimensional magnetic chains of variable lengths were grown and studied. In this fashion, competition between intra-chain and inter-chain exchange interactions was determined. This one-dimensional magnetic material is an excellent model system to test and understand the limitations of storage in reduced dimensionality.

This work was done by Ivan K. Schuller of the University of California, San Diego for the Air Force Office of Scientific Research. AFOSR-0007