Micro-structured nanotowers are mapped and modified with an Atomic Force Microscope (AFM). By using increased forces acting between the cantilever-tip and the substrate in suitable experimental procedures, nanotowers can be broken selectively, which makes it possible to write patterns. The fracture forces FN and FL give an indication for the nature of the interface and permit the study to the tribology on the nanometre scale.

Nanotowers can be manufactured with lithography for example by using an electron beam to write structures directly into resist materials. One class of nanotowers which have been prepared consists of amorphous SiO2 on silicon with dimensions of a few nanometres and shall serve as the object of investigation (Fig.1). The interface height can be adjusted by controlling the duration of the etch process and co-determines the initiation of fracture of the towers (Fig.2). If tower dimensions reach below the distance between defects characteristic to the material, the majority of nanotowers will be defect free, thus allowing for the analysis of the defects' influence on the fracture behaviour [1].

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Fig.1: Schematic design of nanotowers with different cross section.	Fig.2:   AFM picture of SiO2 nanotowers on silicon, on which certain towers were removed, in order to write LMN.

The AFM is operated in the Contact mode (CM-AFM) and serves as a measuring instrument (topography, FL) and also as a modification tool. A static and also a dynamic modification mode is used to break towers. With the static modification mode a normal force FN of approximately 25 nN is exerted (Imaging mode: FN = 4 nN), maintaining a constant height. In the dynamic mode the cantilever is excited to oscillate (ν = 1.75 kHz) with an amplitude of 25 nm and a normal force FN of 8 nN [2].
With the procedure described above also erosion and corrosion processes can be examined on the scale of individual towers. Due to the small dimensions such slow procedures can be monitored in reasonable time and under controlled conditions. This is often impossible or difficult with larger real structures and has left to the failure of engineered structures due to slow corrosion processes.
A field with nanotowers can be used as a so-called WORM (Write Once, Read Many). Information can be stored by selective breaking of the towers. The stored information can be readout nearly unlimited times in the imaging mode. The structures can be configured for higher information density or complexity if several interfaces are employed [3].
In the current project, corrosion and different adhesion problems are examined. Technology relevant samples are provided by the companies Geberit and Hightec MC. The main goal is to establish this technology as an alternative to conventional adhesion tests e.g. Scotch-tape test, Peel test, and micro tensile test.

References

[1] B. Baumeister, T. A. Jung, E. Meyer, Applied Physics Letters, 78(17), 2485-2487 (2001)
[2] B. Baumeister, T. A. Jung, E. Meyer, Tribology Letters, 11(2), 107-110 (2001)
[3] US Patent: No. 6,819,588 B2

Contacts
A. Kaufmann: andre.kaufmannpsi.ch
Dr. H. Schift: helmut.schiftpsi.ch
Dr. T.A. Jung: thomas.jungpsi.ch

Academic Partner
Prof. Dr. E. Meyer, University of Basel

Financial Support
This project is co-funded by PSI and the Swiss Nanoscience Institute through the ‘Nano-Argovia’ program.

Note
For the deposition of CaCO3 this project closely interacts with the University of Applied Sciences, Winterthur represented by the Institute of Materials and Process Engineering headed by Prof. Dr. M. Hirayama and with the Commission for Technology and Innovation (CTI) of Switzerland.