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Fabrication of Cr cantilevers using electron beam lithography (EBL) and or nanoimprint lithography (NIL)

Ultrathin metallic cantilevers having widths in the nanoscale may be employed as extremely sensitive mass sensors with possible single molecular detection ability. However, very few experiments have been performed with cantilevers at this scale. In order to fully take advantage of the properties when the dimensions are in the nanoscale, careful investigations of such properties have to be undertaken. The aim of this work is to investigate thin chromium cantilevers with sub-90 nanometer thickness, and length up to 3 µm.

The nanocantilevers were made by electron beam lithography (EBL), metal lift off, and subsequent reactive ion etching (RIE). Alternatively, a nanoimprint lithography process may be applied involving printing a pattern into a double resist layer scheme and subsequent metal lift-off.

A continuous determination of the local mechanical properties at all lengths was accomplished by employing an atomic force microscope, (AFM), operated in contact mode and by applying incremental forces along the length of the cantilevers. As the AFM probe was scanned along the length of the cantilever, the total bending displacement, (z), of the cantilevers at varying distances from the base, (L), was monitored. By applying incremental forces, (F), the corresponding change in deflection was 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.

For a rectangular beam with one end fixed, the difference in deflection, z, can according to classical mechanics be determined by: z=F(L)^3/3E

F is the force increment, L is the cantilever length, E is the Young’s modulus of elasticity, and I is the moment of inertia.

The result show that the cantilevers deflect more than anticipated compared to calculated deflections, using eq. 1, and the Young’s modulus for bulk chromium of 248GPa.

Further, the thinner cantilevers appear more soft than the thicker ones, indicating an obvious size effect of Young’s modulus. It is clear that such an effect will have important implications on the performance of nanoelectromechanical devices when they become very thin.

[1] S. G. Nilsson, E. -L. Sarwe, and L. Montelius, Appl. Phys. Lett. 83, 990 (2003).
[2] S. G. Nilsson, X. Borrisé, and L. Montelius, to appear in Appl. Phys. Lett. Oct. 18 (2004).

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