Mapping vibrational modes of Si3N4 membrane - Ultrasonic Force Microscopies vs Laser Doppler Vibrometry

San Juan Mucientes, Marta and Kolosov, Oleg Victor (2017) Mapping vibrational modes of Si3N4 membrane - Ultrasonic Force Microscopies vs Laser Doppler Vibrometry. In: FTS - Christmas Conference 2017, 2017-12-19, Lancaster University.

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Abstract

Devlopment of advanced micro- and nano-electromechanical systems (MEMS and NEMS) requires characterization of their vibrations methods with sub-micrometre spatial resolution, combined with high frequency (HF) response. Whereas optical methods such as Laser Doppler Vibrometry (LDV) are widely used, their light wavelength – linked resolution is inevitably limited to ~1 um [1]. Here we use atomic force microscopy (AFM), well known for its ability to image features down to atomic scale, to analyse the vibrations of nanoscale thin membranes over the frequency range from kHz to several MHz using both linear and nonlinear mechanisms for the excitation and detection of these vibrations. The main target of our study was to explore the possibility to detect membrane HF vibrations via AFM tip contacting membrane locally and, provided such detection is feasible, to study effect of the tip contact on the membrane resonance frequency. We compared three complementary AFM modes: 1) Force Modulation Microscopy (FMM) detecting tip vibration at the excitation frequency, 2) nonlinear off-resonance regime with HF sample vibration modulated at low frequency, and cantilever response measured at this modulation frequency (Ultrasonic Force Microscopy, UFM [2]), 3) UFM resonance regime, with the modulation frequency tuned to the membrane resonance (M-UFM).The analysed MEMS system was 200 nm thick Si3N4 membrane on a Si substrate (500x500 um2, Agar Scientific). The AFM (Multimode, Nanoscope 8, Bruker) was modified with a piezoceramic transducer driven by a lock-in amplifier (HF2LI, Zurich Instrument) working as generator exciting sample vibrations about 10 MHz as well as detecting the resulting cantilever deflection.First, we studied membrane vibration via optical LDV method (OFV-2670 and UHF-120, Polytec) and located the membrane fundamental vibrational mode at ~250 KHz, with estimates of the frequency as a function of stress suggesting it to be under high tensile stress. In AFM all ultrasonic methods (FMM,< UFM, M-UFM) allowed us to map the membrane shape, that was not detectable in the featureless topography images, with UFM providing best resolution. FMM map acquired at varying frequency, indicated the shifts the resonance frequency down by about 12% as the tip moves towards the centre of the membrane that we attribute to the loading of the membrane. Moreover, we found that the maximum response is reached at a certain distance from the edge, suggesting an optimum tip position for the detection of vibrations. In M-UFM mode excitation that contains no spectral component at the membrane resonance, we nevertheless were able to observe the resonance. We attributed this to the nonlinear nature of the tip-membrane interaction that produced the nm localised force at the resonance modulation frequency, opening a new way for the exploration of the vibration of MEMS/NEMS structures of sub-um dimensions including 2D materials based NEMS. [1] Gates RS, Pratt JR, Nanotechnology, 2012, 23(37). [2] Bosse JL et al., Journal of Applied Physics, 2014, 115(14):144304.