AFM Nanopatterning of Transition Metal Oxide Thin Films

Transition Metal Oxides (TMOs) exhibit exciting physical properties such as superconductivity, high temperature ferromagnetism, colossal magnetoresistance, ferroelectricity, and semiconducting performances. These properties, like the transition from an insulating to a metallic state, can be tuned by chemical doping and by introducing oxygen vacancies. Nanoscale tailoring of the physical properties of oxides and their nanometric confinement is thus fascinating and the Atomic Force Microscope (AFM) is a flexible tool for this purpose. Local Anodic Oxidation (LAO) applied to many conducting oxides like SrTiO3-x epitaxial thin films does not simply restore the oxygen stoichiometry; instead, a chemical decomposition of the crystalline phase occurs, as evidenced by the formation of mounds at the surface. The overgrown features can be selectively removed by wet etching leaving corresponding grooves, thus allowing the direct fabrication of oxide nanostructures like nanowires and side gate field effect transistors [1]. This approach can also be extended to other conducting TMOs like ferromagnetic manganite thin films [2,3]. We observe that the control of the electrical current through the AFM tip during the LAO process gives a better reproducibility and stability on the lithographic process; 150 nm wide patterns for a total length of the order of millimetres can be easily fabricated in the constant current approach [4]; this feature will be thoroughly discussed in the framework of water meniscus dynamics. For the case of nanofabrication on insulating oxides films or oxides whose AFM patterns cannot be selectively removed by wet etching, fabrication of nanomasks is required. We report the AFM fabrication of Mo/poly(methylmethacrylate) (PMMA) nanomasks through multistep processes combining LAO of Mo and dry etching of PMMA [5]. Epitaxial (Fe,Mn)3O4 nanostructures on the 100 nm scale can be easily fabricated by using nanomasks and subsequent wet chemical etching with H3PO4. This process does not alter the transport and ferromagnetic properties of the films. Detailed discussions and comparisons of these two different AFM oxide nanopatterning techniques will be presented. [1] L. Pellegrino et al. Appl. Phys. Lett. 81, 3849 (2002) [2] I. Pallecchi et al. Appl. Phys. Lett. 94 4435 (2003) [3] Y. Yanagisawa et al. Appl. Phys. Lett. 89, 253121 (2006) [4] L. Pellegrino et al. Appl. Phys. Lett. 87 064102 (2005) [5] L. Pellegrino et al. Adv. Mater., 18, 3099-3104 (2006)