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Analysis of asymmetric resonance response of thermally excited silicon micro-cantilevers for mass-sensitive nanoparticle detection

: Bertke, M.; Hamdana, G.; Wu, W.; Wasisto, H.S.; Uhde, E.; Peiner, E.


Journal of micromechanics and microengineering 27 (2017), No.6, Art. 064001, 10 pp.
ISSN: 0960-1317
ISSN: 1361-6439
Micromechanics and Microsystems Europe Workshop (MME) <27, 2016, Cork>
Journal Article, Conference Paper
Fraunhofer WKI ()
Fano resonance; asymmetric resonance curve; nanoparticle; detector; NEMS / MEMS; thermal actuator; quality factor

In this paper, the asymmetric resonance frequency (f ₀) responses of thermally in-plane excited silicon cantilevers for a pocket-sized, cantilever-based airborne nanoparticle detector (Cantor) are analysed. By measuring the shift of f ₀ caused by the deposition of nanoparticles (NPs), the cantilevers are used as a microbalance. The cantilever sensors are low cost manufactured from silicon by bulk-micromachining techniques and contain an integrated p-type heating actuator and a sensing piezoresistive Wheatstone bridge. f ₀ is tracked by a homemade phase-locked loop (PPL) for real-time measurements. To optimize the sensor performance, a new cantilever geometry was designed, fabricated and characterized by its frequency responses. The most significant characterisation parameters of our application are f ₀ and the quality factor (Q), which have high influences on sensitivity and efficiency of the NP detector. Regarding the asymmetric resonance signal, a novel fitting function based on the Fano resonancereplacing the conventionally used function of the simple harmonic oscillator and a method to calculate Q by its fitting parameters were developed for a quantitative evaluation. To obtain a better understanding of the resonance behaviours, we analysed the origin of the asymmetric line shapes. Therefore, we compared the frequency response of the on-chip thermal excitation with an external excitation using an in-plane piezo actuator. In correspondence to the Fano effect, we could reconstruct the measured resonance curves by coupling two signals with constant amplitude and the expected signal of the cantilever, respectively. Moreover, the phase of the measurement signal can be analysed by this method, which is important to understand the locking process of the PLL circuit. Besides the frequency analysis, experimental results and calibration measurements with different particle types are presented. Using the described analysis method, decentresults to optimize a next generation of Cantor are expected.