Multi-Frequency Resonance Behaviour of a Si FractalNEMS Resonator

Novel Si-based nanosize mechanical resonator has been top-down fabricated. The shapeof the resonating body has been numerically derived and consists of seven star-polygons that forma fractal structure. The actual resonator is defined by focused ion-beam implantation on a SOIwafer where its 18 vertices are clamped to nanopillars. The structure is suspended over a 10 mtrench and has width of 12 m. Its thickness of 0.040 m is defined by the fabrication process andprescribes Young's modulus of 76 GPa which is significantly lower than the value of the bulk material.The resonator is excited by the bottom Si-layer and the interferometric characterisation confirmsbroadband frequency response with quality factors of over 800 for several peaks between 2 MHzand 16 MHz. COMSOL FEM software has been used to vary material properties and residual stressin order to fit the eigenfrequencies of the model with the resonance peaks detected experimentally.Further use of the model shows how the symmetry of the device affects the frequency spectrum.Also, by using the FEM model, the possibility for an electrical read out of the device was tested. Theexperimental measurements and simulations proved that the device can resonate at many differentexcitation frequencies allowing multiple operational bands. The size, and the power needed foractuation are comparable with the ones of single beam resonator while the fractal structure allowsmuch larger area for functionalisation.