Cantilever Based Mass Sensing: Alternative Readout and Operation Schemes

Cantilever based mass sensors utilize that a change in vibrating mass will cause a change in the resonant frequency. This can be used for very accurate sensing of adsorption and desorption processes on the cantilever surface. The change in resonant frequency caused by a single molecule depends on various parameters including the vibrating mass of the cantilever and the frequency at which it vibrates. The minimum amount of molecules detectable is highly dependent on the noise of the system as well as the method of readout. The aim of this Ph.D. thesis has been twofold: To develop a readout method suitable for a portable device and to investigate the possibility of enhancing the functionality and sensitivity of cantilever based mass sensors. A readout method based on the hard contact between the cantilever and a biased electrode placed in close proximity to the cantilever is proposed. The viability of the method is shown theoretically, and the output signal is shown to scale very well with the dimensions of the cantilever, and hence should be applicable to nano-scale cantilevers. The hard contact method is proven to work on cantilevers on the micro- and nanoscale with measured resonant frequencies up to 11MHz. Values of the reciprocal frequency resolution as high as 80000 are obtained together with a signal to noise ratio of 108. The result is an almost digital readout, which in turn simplifies the detection of the resonant frequency considerably. An analytical expression is derived relating the mass and position of a particle attached to a cantilever to the resonant frequency. It is shown theoretical possible to find the mass and position of a particle by measurements of the resonant frequency of several bending modes. In the measurements the sensitivity of the cantilever based mass sensor is improved when operated at higher bending modes. By measuring the resonant frequency of several bending modes both the mass and position of an attached gold bead are determined.