Development of novel high frequency Si, Si3N4, and graphene based micro-and nano-electromechanical systems (MEMS and NEMS) requires suitable characterization methods with nanoscale spatial resolution, high frequency (HF) response and high sensitivity. As spatial resolution of existing methods such as Laser Doppler Vibrometry (LDV) is limited by the light wavelength to the micrometre scale , it is tempting to use atomic force microscope (AFM) techniques offering nanoscale resolution. Here we use AFM to analyse the vibrations of nanoscale thin membranes over the frequency range from kHz to several MHz using both linear and nonlinear mechanisms for their excitation and detection.
Our model system is a Si3N4 membrane (200 nm thickness, 500x500 um2, Agar Scientific) on a Si substrate. The AFM (Multimode, Nanoscope 8, Bruker) was modified with a piezoceramic transducer driven by the function generator to excite sample vibrations from kHz to about 10 MHz, with the resulting cantilever deflection detected by a standard lock-in-amplifier. The reference optical vibrometry (OFV-2670 and UHF-120, Polytec) found the membrane fundamental vibrational mode at ~250 KHz suggesting it to be under high tensile stress.
The core idea of our study was to explore the possibility of detection HF membrane vibrations via AFM and effect of the probing tip contact on the resonance frequency. We used three AFM modes: 1) Force Modulation Microscopy (FMM) with tip vibrations detected at the excitation frequency, 2) nonlinear off-resonance regime where HF sample vibration is modulated at low frequency, and cantilever response measured at the modulation frequency (Ultrasonic Force Microscopy, UFM ), 3) UFM resonance regime, where the modulation frequency was around the membrane resonance (M-UFM).
While the edge of a membrane was not detectable via topography, it was clearly visible in all ultrasonic modes. FMM mapping at swept excitation frequency showed that the cantilever-tip loading of the membrane increasingly shifted the resonance frequency down as the tip moved towards the centre, with the maximum response reached at a certain distance from the edge, suggesting an optimum position for the detection of vibrations. In M-UFM mode we found that the membrane resonance was also detectable, even though there was no resonance frequency component in the driving oscillation spectrum. We attributed this to the nonlinear nature of the tip-membrane interaction that produced the localised force at the resonance modulation frequency. This study shows that ultrasonic AFM modes will allow the exploration of the vibration of MEMS/NEMS structures of sub-um dimensions including 2D materials based NEMS.
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