Multimode Air-Coupled Excitation of Micromechanical Structures

The development of an acoustic measurement system for multimode air-coupled excitation and detection of micrometer-scale cantilever structures, which are, for example, used in micro-electromechanical systems (MEMSs), is detailed and reported for the first time. The source of noncontact vibrational excitation is a pulsed acoustic field generated by an air-coupled transducer. In the experimental system, the transient response of the cantilever beam is obtained at various points along the beam axis to extract its resonance frequencies and corresponding mode shapes. We demonstrate that measurable amplitudes of vibrations can be obtained at various excitation levels in the megahertz range, and higher harmonics of vibration of a microbeam can be excited by the air-coupled mechanism from a distance on the order of 10 mm. In the specific utilizations of the reported system, resonance frequencies and mode shapes can be related to the mechanical properties and geometric attributes (dimensions and defects), as well as the residual stress state in a microstructural element using various established computational and experimental inverse techniques. Another potential application area of the reported system is in the sensors for detecting the bending stiffness of deposited films on cantilever oscillators (in addition to its film mass loading) to increase the detection sensitivity and selectivity in a single sensing element.