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High Frequency Cantilevers as a tool in Nanotechnology

E. Farnault*, M. Hoummady, E. Rochat, and H. Fujita

Institute of Industrial Science, University of Tokyo

This is an abstract for a poster to be presented at the Fifth Foresight Conference on Molecular Nanotechnology.

Scanning force microscopy is now a common tool for imaging, modifying, and manipulating samples with nanometer-scale resolution. Despite dynamic modes, including non-contact and intermittent-contact modes, avoid to damage soft surfaces such as organic materials, near-atomic resolution is not reproducible with any kind of samples. In order to combine local and highly sensitive detection, we are developing high frequency microfabricated mechanical oscillators. Some of their advantages are the following:

1. lower minimum detectable force gradient
2. lower settling time
3. lower thermal motion

Small mass cantilevers hold good enough compromise between low stiffness and high frequency. Separate item fabrication techniques often used expansive microelectronics equipment, such as focused-ion-beam or electron-beam systems. In our previous work, we developed an original method based on local electrochemical etching of a tungsten wire. Fabrication of cylindrical nanomechanical oscillator consisting of a ball supported by a neck has been achieved. According to nanocantilever dimensional parameters (neck radius: 25 nm, neck length: 200 nm, ball diameter: 1 micron), the estimated resonant frequency was roughly 300 MHz, which is high enough to foresee applications in molecular nanotechnology.

Regarding batch-processed microsensors, higher harmonics of conventional single crystal silicon cantilevers have been used to operate at higher frequencies. For instance, a 300 kHz-fundamental frequency cantilever is able to vibrate in air at 14 MHz on its 6th flexural mode. For each eigenmode, cantilever dynamics was analyzed at different tip-sample spacing by sweeping the frequency through the cantilever free resonance. Regarding tip-sample interactions, first results showed force gradient detection sensitivity 7-fold increased using the second harmonic. Moreover, measurements using higher flexural modes induce weaker hydrodynamic damping that occurs during relative motion between cantilever and sample. Thus the method overcomes energy dissipation effect that often prevents measurements at the molecular scale in an aqueous environment. The technique described here takes things step by step in order to perfect non conventional optical detection system and control electronics in the frequency range 1 MHz - 100 MHz.

*Corresponding Address:
Etienne Farnault, Laboratory for Integrated Micro-Mechatronic Systems (LIMMS), Institute of Industrial Science, the University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, JAPAN, ph: +81-3-3402-9568, fax: +81-3-3402-9568, e-mail: etienne@cc.iis.u-tokyo.ac.jp
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