Characterization of the Resonant Properties of Multi-layer Cantilever Sensors

Resonant cantilever-based mass sensors utilize the fact that a vibrating beam experiences a shift in resonance frequency when the beam’s mass is altered, e.g. by adsorption of particles. The mass-sensitivity of a cantilever-sensor is defined by the resolution with which this frequency shift can be measured.

A measure of this frequency-resolution is the quality-factor (or Q-factor ), which depends on the energy loss and damping that the vibrating cantilever experiences. The major part of this PhD project has been to design and construct a vacuum chamber in which the environmental conditions such as temperature, pressure and gas constituents can be controlled with sufficient accuracy to perform characterizations of micro- and nanocantilevers.

The vacuum chamber has been fitted with electrical interconnections for cantilever actuation and signal detection; and with flanges for inlet of vaporized chemicals for adsorption and detection experiments. Special equipment has been designed for piezoelectric actuation of passive cantilevers and for video inspection of the cantilever samples inside the chamber.

A laser-optical detection system has been designed and built in conjunction with the vacuum chamber for characterization of the resonant properties (i.e. resonance frequencies and Q-factors) of micro- and nano-cantilevers as well as alternative resonant devices. Upon completion of the chamber- and detection setup, an experimental investigation of resonant silicon dioxide microcantilevers was performed.

The resonance frequency and Q-factor of the fundamental and higher order flexural modes were characterized at different temperatures and pressures and with different thicknesses of a top-surface gold coating. An analytical solution for the flexural eigenmodes of a multi-layered vibrating cantilever was derived and its validity was verified by comparison to finite–element analysis as well as the experimentally measured resonance frequencies in vacuum.