This study investigates novel concepts of micromechanical bulk acoustic wave sensors for advanced atomic force microscopy (AFM), using micromachined silicon resonators, which are analyzed with regard to their performance as compared to conventional AFM sensors. Conventional AFM systems use a cantilever resonator for sensing the surface forces of the sample.
Since a laser is used to detect the cantilevers movement, a certain minimum area for reflecting the laser beam is required. These restrictions in the cantilever scalability limits the resonant frequency and quality factor of the resonator and thus the overall performance and resolution of a conventional AFM system. To overcome these limitations and to improve atomic force microscopy, new sensor concepts are proposed.
First an analysis of different extensional mode resonator geometries is conducted to determine the dependency of the shape and dimensions on the stiffness, resonant frequency and displacement for the use as AFM sensor.
Based on that a two resonator system is introduced, consisting of a flexural mode resonator acting as sensing unit with design specifications oriented on an AFM cantilever and of an additional bulk mode resonator with high resonant frequency and high quality factor detecting the movement of the first resonator. They are coupled electrostatically, where a DC potential between the two resonators and a variation in gap width, caused by the oscillation of the flexural mode resonator, modulate the pre-stress in the bulk mode resonator resulting in a frequency shift, which is then detected capacitively.
Finite-element simulations are conducted to determine the sensitivity of the system for different resonator geometries, dimensions and DC potentials between the two resonators, as well as the thermal noise and thus the detection limit of the system. As a second design, a variation of the existing sensor is proposed using a mechanical spring system to couple the two resonators.
The benchmark criteria of these novel concepts is that it should be possible to detect a force in the range of the Brownian motion at room temperature. Summarizing the results it can be concluded that a ring shaped geometry is the most suitable for a single bulk acoustic wave resonator sensor for AFM applications, as it achieves highest displacement for a given size, resonant frequency and quality factor.
These findings can also be used to improve the electrostatically coupled resonator system and to reduce the DC potential needed between the two resonators to avoid pull-in and at the same time still achieve good sensor sensitivity.
Author: Wagner, Stefan