Tuning piezoresistive transduction in nanomechanical resonators by geometrical asymmetries

Abstract

The effect of geometrical asymmetries on the piezoresistive transduction in suspended double clamped beam nanomechanical resonators is investigated. Tapered silicon nano-beams, fabricated using a fast and flexible prototyping method, are employed to determine how the asymmetry affects the transduced piezoresistive signal for different mechanical resonant modes. This effect is attributed to the modulation of the strain in pre-strained double clamped beams, and it is confirmed by means of finite element simulations. V C 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4928709]Resonating nanomechanical structures are attractive building blocks for the realization of high performance sensors and integrated oscillators.1 For example, a double clamped nanometric beam (DCB) (a silicon nanowire or a carbon nanotube) presents very large resonance frequency and extremely low mass, which are both highly convenient to detect minuscule quantities of mass.2,3 Also, DCB 4 or more complex structures 5 can be integrated within CMOS circuits to build high frequency oscillators that eventually may substitute present hybrid and heterogeneous integration approaches for signal processing.The practical application of nanomechanical resonators requires an efficient method for detecting their oscillation, which becomes increasingly difficult as the dimensions of the structures shrink. Electrical read-out of the oscillation is one of the most straightforward ways of obtaining compact systems, enabling the possibility of simultaneous detection of a large number of resonating elements. Electrical transduction can be implemented, for example, by capacitive, 5 piezo-electric, 6 or piezo-resistive 7 read-out. Compared to other methods, piezoresistive read-out presents the advantage of easy implementation, since the beam itself acts as a piezoresistive gauge, simplifying device design and fabrication.While the so called giant piezoresistance effect has enabled piezoresistive read-out in DCB silicon nanowires fabricated by bottom-up methods, 8,9 it has not been observed in top-down fabricated DCBs. Recently, it has been shown that geometrical asymmetries present in DCBs cause an enhancement of the piezoresistive transduction, allowing to obtain large read-out electrical signals from silicon nanowire resonators obtained by top-down and bottom-up fabrication methods.10 Here, we show that piezo-resistive read-out signals in DCB nanomechanical resonators can be tuned by engineering the asymmetry of the device, and in particular, by inducing deliberate asymmetries that enhance the transduction of specific resonant modes.We have focused the present study on DCB nanomechanical resonators fabricated by a fast, accurate, and flexible prototyping method based on the combination of focused ion beam (FIB) exposure and wet silicon etching, 11 which is compatible with CMOS technology. 12 Next, we present the main details of the fabrication process, the electrical characterization setup, and the experimental results that show the influence of the geometri...

Multi-Frequency Resonance Behaviour of a Si Fractal NEMS Resonator

“…The samples have been directly patterned by focused ion beam (FIB) implantation of Ga+ at 30 keV, beam current of 10 pA and dose of 1 × 10 16 per cm 2 in a Cross-Beam system (1560xB from Zeiss, Oberkochen, Germany). The ion beam has been controlled with the nanolithography kit Elphy Quantum (from Raith, Dortmund, Germany) to enable the definition of different geometries [11]. It is remarkable to mention that this resist free lithographic step permits to modify the crystallinity of the irradiated silicon creating an etching mask that can be functional from the electrical point of view with the appropriate treatment [35].…”

“…Nanobeams became functionalised for wide variety of mass detectors of extremely small objects of interest in Physics, Chemistry and Biology [1,[7][8][9][10]. Moreover, there is sufficiently good prediction of nanobeams' resonant frequency and modeshapes by using beam models or finite element method (FEM) softwares such as ANSYS or COMSOL [11,12]. That is why nanobeam resonators are the most developed so far and most of the materials suitable for nanoresonator fabrication had already been tested as a nanobeam resonator [13,14].…”

Evaluating the compressive stress generated during fabrication of Si doubly clamped nanobeams with AFM

“…The method is described in details elsewhere. 11,12 With this technique, it is possible to create suspended cantilevers, doubly clamped beams, 13 nanopillars, 14 and nanomechanical resonators. 13 The starting substrates are silicon on insulator wafers (h110i silicon device layer orientation and thickness 2 6 0.5 lm) while the wet etching employed is a tetramethilammonium hydroxide (TMAH) etching (25% solution at 80 C) that removes crystalline silicon anisotropically.…”

“…11,12 With this technique, it is possible to create suspended cantilevers, doubly clamped beams, 13 nanopillars, 14 and nanomechanical resonators. 13 The starting substrates are silicon on insulator wafers (h110i silicon device layer orientation and thickness 2 6 0.5 lm) while the wet etching employed is a tetramethilammonium hydroxide (TMAH) etching (25% solution at 80 C) that removes crystalline silicon anisotropically. Amorphous implanted areas defined by FIB are etchresistant, and moreover, we can exploit the selective etching a) Electronic mail: matteo.lorenzoni@imb-cnm.csic.es rate of TMAH with respect to the crystalline direction to obtain released or nonreleased structures according to their specific orientation.…”

“…Finally, it is worth to compare values of r 1 to the outcome of simulations performed in Ref. 13 on beams similar to the ones studied in this work. The simulated model has been varied to fit the experimental values of the resonance frequencies.…”