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A characterization method for investigating nanomechanical properties of nanosized structural elements such as cantilevers

There is a highly energized scientific and technological interest in the use of micro- and nanoelectromechanical systems. For instance, micro- and nanomechanical sensors together with very sensitive detection schemes have developed as a very exciting field to detect for example specific molecular interactions, cell adhesion and chemical gases.

In order to be able to develop new products and devices based on nanostructures there is a need to investigate the material and mechanical properties at the nanoscale of such devices we have employed a measurement scheme using an AFM making it possible to unambiguously determine the mechanical properties of the nanocantilevers such as the spring constant, (k), as a function of both their length and the applied force. Hitherto, the interest and use of microcantilever’s mechanical properties have been concerned with small deflections whereas here a detailed investigation is presented of the behaviour of nanocantilevers in the regime of large deflections.

The experimental setup to reveal the ductility and the mechanical properties of the fabricated cantilevers was based on an atomic force microscope (AFM), operating in contact mode. The AFM probe employed had a specified spring constant of 0.032N/m. The force conversion factor was determined by measurements on the bulk part of the Cr structure. By using this factor and adjusting the signal to the photodiode, the set point for the feedback loop in the AFM was chosen to correspond to a fixed force (around 8nN). The deflections of the Cr-cantilevers were then measured while scanning the probe along their length as illustrated in figure 1(b). Additional forces, (F), were then applied by changing the feedback set point, and the corresponding deflections were measured.

This method enables the determination of the bending displacement of the cantilevers at varying distances from the base, as a function of applied force. We detect large deflections revealing a high ductility of the cantilevers. Within measurement accuracy, we could not observe any permanent deformation of the cantilevers after the various AFM measurements.

The AFM probe will be in contact with the Cr-cantilever in each point l. This means that we get data for all lengths when scanning along the full length of 2µm. The method is very simple compared to traditional methods based on force activating needles arranged in a scanning electron microscope system where the static deflection of the cantilever is measured as a function of applied force.

For a rectangular beam with one end fixed, the spring constant, k, can be determined by: K=F/z where F is the applied additional force, and z is the resulting deflection of the cantilever as a function.

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22100 LUND
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