Abstract
Highly sensitive conversion of motion into readable electrical signals is a crucial and challenging issue for nanomechanical resonators. Efficient transduction is particularly difficult to realize in devices of low dimensionality, such as beam resonators based on carbon nanotubes or silicon nanowires, where mechanical vibrations combine very high frequencies with miniscule amplitudes. Here we describe an enhanced piezoresistive transduction mechanism based on the asymmetry of the beam shape at rest. We show that this mechanism enables highly sensitive linear detection of the vibration of lowresistivity silicon beams without the need of exceptionally large piezoresistive coefficients. The general application of this effect is demonstrated by detecting multipleorder modes of silicon nanowire resonators made by either topdown or bottomup fabrication methods. These results reveal a promising approach for practical applications of the simplest mechanical resonators, facilitating its manufacturability by very largescale integration technologies.
Introduction
Doubly clamped beams based on nanostructures such as carbon nanotubes (CNTs)^{1} and silicon nanowires (SiNWs)^{2,3} are excellent building blocks for the realization of nanomechanical resonators with ultrahighperformance characteristics. Such devices have provided functional features at the verge of fundamental limits in several contexts, ranging from physical sensing^{4,5,6} to signal processing^{7,8}. CNT resonators are at the forefront of recent advances in extreme performance, but their unique features degrade drastically at room temperature^{9}. Moreover, CNT device fabrication technologies are still far from the largescale/highyield production requirements for realworld applications. On the other hand, SiNW resonators have shown lower performance levels but better possibilities for room temperature operation. And a doubly clamped SiNW resonator is the simplest resonant nanostructure that can be monolithically fabricated. This structural simplicity enables their fast, reproducible and lowcost fabrication by very largescale integration (VLSI) methods at the wafer scale^{10}. However, one of the most limiting difficulties for the practical development of batchfabricated SiNW resonators with performance characteristics approaching those of CNTs is to establish highly sensitive transduction schemes that efficiently convert mechanical vibrations into functional electrical signals. The small amplitude of the mechanical vibrations, in combination with their very high frequency and the natural tendency of highaspect ratio beams to operate in a nonlinear regime, represents formidable obstacles for such purpose^{11,12}. The application of the most sensitive transduction schemes reported so far for nanomechanical resonators, such as piezoelectric^{13}, electrothermal^{14} or optomechanical^{15}, results either impossible or impractical for simple beams made of silicon with dimensions deep down in the nanoscale and resonant frequencies in the very highfrequency range.
Electromechanical transduction is the alternative of choice whenever nanomechanical resonators can benefit from integration with onchip electronics that incorporate additional functions. Integration allows multiplexing, signal conditioning and amplification, facilitates portability and enables multisensing platforms^{16}. In general, different mechanisms can be used for electromechanical transduction. For CNT resonators, the most widely used method is modulation of the CNT conductivity induced by capacitive coupling with a gate located at close proximity^{1}, requiring the use of semiconducting CNTs. The measurement of capacitance variation of the oscillating structure with respect to a fixed electrode has also been widely used in siliconbased mesoscopic resonators^{17,18}. However, this approach presents enormous difficulties for very small devices. It requires the realization of very small actuation/readout gaps, and even then, the minuscule change in motional capacitance to be measured is hindered by parasitic capacitances.
Although modulation of the conductivity by capacitive transduction can also be implemented in SiNW resonators^{3,19}, piezoresistive transduction offers a more straightforward solution a priori, as silicon is a piezoresistive material. For example, piezoresistive transduction has been implemented in Ushaped micromechanical resonators with longitudinal resonant modes over 1 GHz (ref. 20). For the case of doubly clamped SiNW resonators, the whole beam body can operate as a piezoresistive gauge for flexural modes, so that transduction is provided by the change of resistance of the beam induced by its periodic elongation as it vibrates (Fig. 1). Nevertheless, the minute amplitude typically produced for the mechanical vibration implies that this elongation is too small to result in signals that can be measured by simple means if the piezoresistive coefficients of bulk Si are considered. However, the piezoresistive coefficients of Si have been found to increase considerably in bottomupsynthesized SiNWs^{21}. In addition, anomalously enhanced piezoresistive behaviour in highly strained, undoped SiNWs has been reported^{22}. The socalled giant piezoresistance effect has indeed been explored for electrical readout of highfrequency SiNW resonators^{2}. Several studies in topdown SiNWs have not been able to find giant values of piezoresistance coefficients^{23,24}.
In this communication we describe a physical mechanism that provides highly sensitive and linear piezoresistive transduction for Si beam resonators. This mechanism enables detection of the miniscule vibration amplitude of the resonators at high frequency even in the case that the piezoresistive coefficients are comparable to those of bulk silicon. We show that when the vibration of the beam modulates the longitudinal strain with respect to an asymmetric beam shape at rest, then the piezoresistive effect originates both linear and quadratic components in the transduced signals. The transduction mechanism implies that linear transduction is largely amplified as compared with quadratic transduction depending on the magnitude of the asymmetry. This effect can be used for Si beam resonators obtained by diverse fabrication methods, as it is proved here for both bottomup SiNW grown by the vapour–liquid–solid (VLS) mechanism and topdown SiNW defined by conventional cleanroom techniques.
Results
Modelling piezoresistive transduction
The emergence of amplified linear piezoresistive transduction in doubly clamped beam resonators can be demonstrated by considering the following simple model (Fig. 1a). We refer to electromechanical transduction as the relationship between the measured electrical signal and the mechanical vibration amplitude of the beam. We consider a single vibration mode driven at a frequency ω. Then, the beam profile at any time is assumed to be given by two contributions: first, an asymmetric deflection profile at rest as a consequence of static forces acting at the clamps resulting from the fabrication process; second, a vibration mode shape profile that is excited by the electrostatic driving force. Therefore, the beam profile can be written as:
where x is the longitudinal coordinate along the beam, ω is the driving frequency, φ_{0}(x) is the asymmetric beam profile at rest normalized to the maximum deflection at rest d_{0} and φ_{n}(x) is the nthorder vibration mode profile normalized to the maximum vibration amplitude of the mode a_{n}(ω). We consider here that d_{0} is the component of the beam deflection at rest in the vibrational plane of the corresponding mode.
The piezoresistive effect implies the following relationship between the timevarying resistance change ΔR(t) and the total net strain ε(t) along the nanowire (NW) induced by the beam vibration:
where R_{u} is the undeflected beam resistance and G is the piezoresistive gauge factor.
In order to calculate the strain we consider the deflection given by equation (1). The total net strain caused by the beam deflection is then given by:
where prime denotes derivative with respect to the longitudinal spatial coordinate x, and L is the undeflected beam length. By combining equations (1, 2, 3), we obtain the following expression for the timedependent components of the resistance variation (see Supplementary Discussion):
To quantify the effect of the deflection at rest and vibration profile shapes, we would need to solve the Euler–Bernouilli equation for the beam and obtain the profile shapes for a suitable set of boundary conditions. But more importantly, and regardless of the quantitative effect of the profile shapes, equation (4) shows that the timevarying resistance change has two components (Fig. 1b): the first one at frequency ω is proportional to the vibration amplitude times the deflection at rest (linear transduction); the second one at frequency 2ω is proportional to the square of the vibration amplitude (quadratic transduction), and it does not depend on the deflection at rest.
In addition, equation (4) also implies that the presence of a deflection at rest larger than the oscillation amplitude produces an amplification effect on the piezoresistive transduction as compared with the case of a symmetric beam shape at rest. This amplification can be expected to provide measurable electrical signals even in the case that the gauge factor is not significantly larger than that of bulk silicon. The ratio between the linear and quadratic transduction terms in equation (4) provides a measure of the gain factor of this amplification effect:
We can obtain a simplified expression for this gain factor by considering a beam vibration in the fundamental mode (n=1) and a beam deflection profile shape at rest that can be approximated by that of the fundamental mode, so that φ_{0}(x)≈φ_{1}(x). Then .
Fabrication and characterization of the frequency response
We have applied the model described above to analyse the transduction mechanism in highly doped SiNW resonators fabricated by two different methods: bottomup fabrication by VLS growth (Fig. 2a); and topdown fabrication based on photolithography and standard micromachining processes, including sequential cycles of oxidation/oxide etching to reduce the SiNW diameter (Fig. 2b; see Methods). For both bottomup and topdown devices, a sidegate electrode is placed in close proximity to the NW for electrostatic actuation. The resulting gap distance is around 400 nm for bottomup devices and 900 nm for topdown devices. In both cases, the final resistivity of the SiNWs is very low in the range of 10^{−4} Ω m. Each fabrication method produces particular morphological features that affect the behaviour of the SiNWs as nanomechanical resonators. Bottomup SiNWs grow along the <111> direction and present hexagonal section, epitaxial clamps and smooth surface. Under the present growth conditions, they show a slightly tapered geometry. In the case of topdown SiNWs, their crosssection is not regular either, wider near the clamping ends due to the preference of the oxidation process for certain directions of the crystalline structure. Also, the oxidation/etching steps produce an overhanging membrane at the ends of the anchoring structures.
For both bottomup and topdown devices, an asymmetric beam profile at rest is observed by scanning electron microscopy (SEM) and atomic force microscopy. The magnitude of the maximum deflection d_{0} varies from device to device, and the images provided in Supplementary Figs 1–3 show that it lays in the range of 5–50 nm for both types of devices. However, the maximum deflection d_{0} is typically larger for topdown SiNWs, where it systematically reaches a few tens of nanometres. In this case, it is attributed to tensile stress produced by the underetched parts of the anchoring structures that bend up during the fabrication process (see Supplementary Fig. 2). In the case of bottomup NWs, the deformation has a larger devicetodevice variation, and it is attributed to VLS growth effects such as secondary NW growth at the impinging end clamp^{25}. The bottomup devices used for this study had a maximum deflection d_{0} always below 10 nm, although SiNWs with a much larger deflection were sometimes obtained.
The electromechanical response of the fabricated devices has been characterized by three different frequency downmixing detection methods: frequency modulation (FM) demodulation^{8}, twosource/1ω (ref. 1) and twosource/2ω (ref. 2). Details about the experimental implementation of each detection scheme can be found in Methods. FM demodulation provides the higher signaltonoise ratio and signaltobackground ratio measurements, and thus it is used here as the method of reference for characterizing the frequency response of the devices. Although FM demodulation has been demonstrated to provide a linear transduction component, higherorder terms may also contribute to the measured signal^{8}. However, the twosource/1ω and twosource/2ω schemes selectively target the linear and the quadratic components of the resistance variation, respectively, so that they are used here to probe the relative magnitude of these components.
Regardless of the detection method, the electromechanical characterization of the resonators shows unambiguously the position of the resonance peaks and the presence of split modes. Figure 2 shows representative examples of the frequency response of bottomup and topdown SiNW resonators. Remarkably, the resonance peaks of Fig. 2c,d produce larger (cleaner) signals than those reported previously for SiNW resonators^{2,3,19}. Besides the fundamental modes, we find it possible to detect higherorder modes, even at frequencies higher than 500 MHz, for which the vibration amplitude is expected to be very small. Figure 2c shows the frequency response of a bottomup SiNW with a length of 3 μm and an average diameter of around 90 nm. The first three mechanical modes of resonance are detected. Each mode is split in two orthogonal modes corresponding to two preferential resonant directions, which arise from the irregular geometry of the hexagonal crosssection and other structural defects^{26}. A quality factor of 3,600 is estimated for the first mode of resonance. Figure 2d shows the frequency response of a topdown SiNW resonator. We also observe the first three resonant modes, and each one is split in two orthogonal modes similarly to the bottomup resonators. In this case, the two orthogonal modes are more separated in frequency because the crosssectional area of the SiNW is rectangular and the radial asymmetry is larger than that of the bottomup SiNWs. The quality factor of the first mode is around 4,900 for each of the split modes, remarkably larger than the one obtained for the bottomup SiNWs. This indicates that the effect of the overhanging membrane in the clamping is not detrimental to obtain a good mechanical response. To our knowledge, these are the first electrical measurements obtained for the higher modes of a SiNW mechanical resonator fabricated by topdown methods at this dimensions.
We performed finite element simulations using Ansys^{27} to confirm that the measured resonance peaks actually correspond to the first three mechanical modes (see Supplementary Fig. 4). First, finite element simulations were performed for a bottomup SiNWs mechanical resonator made of a crystalline SiNW (crosssectional flattoflat distance of 90 nm, length of 3 μm) following the <111> direction and with perfect clamping to rigid side walls. Table 1 shows that the values of the experimental resonance frequencies are higher than the values obtained from the simulation when absence of stress is considered. This is an indication of the presence of stress/strain in the bottomup SiNW resonators at rest.
The simulations are replicated in the case of SiNW mechanical resonators fabricated by the topdown method. In this case the geometry is more complex, as the resonators present a widening near the clamping and overhangs that are difficult to characterize. A model based on SEM images is built in which both of these effects are taken into account. The simulations and subsequent fitting of the longitudinal stress are performed similarly to the case of the bottomup SiNWs. We find that the experimental frequencies are higher than the simulated ones in the absence of stress (Table 1). By introducing a stress in the simulations, we find that the experimental measurements are coherent with a longitudinal tensile stress of around 500 MPa.
We have used SEM imaging to estimate the actual vibration amplitude of the resonators while vibrating at resonance (see Supplementary Fig. 5). We find that the vibration amplitude is below the resolution limit of the SEM instrument used, which was estimated to be 10 nm. Also, we have performed a theoretical estimation of the vibration amplitude by modelling the capacitance between the NWs and the actuation electrode. Our estimations provide values of the order of 1 nm, which is consistent with previously reported estimations for similar devices electrostatically driven in similar conditions^{2,3}. Although these qualitative considerations only set an upper limit for the amplitude of around 10 nm and a typical order of magnitude of around 1 nm, they show that the vibration amplitude a_{n} is typically much smaller than the maximum deflection at rest d_{0}.
Electromechanical transduction signals in SiNW resonators
In this section we compare the magnitude of the linear and quadratic components of the transduced signals. First, we show that the twosource/1ω setup is sensitive to signals arising from a linear transduction, whereas the twosource/2ω detects the signals originated from the quadratic contribution. By comparing both responses under equal actuation conditions, the magnitude of the gain factor γ can be evaluated.
The current through the resonator as a function of the change of resistance is:
where V_{NW} is the voltage applied to the SiNW and R_{NW} is the total resistance. This approximation is valid when ΔR/R_{u}≪1, and here we estimate typically ΔR/R_{u}≈10^{−3}. The above expression indicates that mixing occurs in general via multiplication of the terms V_{NW}(t) and ΔR(t). Each of the downmixing schemes uses a different signal for V_{NW}(t) to mix with the different components of ΔR(t) given by equation (4), and then produce lowfrequency components of the motional current that can be detected by a lockin amplifier.
The motional contribution to the current assuming piezoresistive transduction can thus be expressed by combining equations (4) and (6) as:
For the twosource/1ω and twosource/2ω detection schemes, V_{NW}(t) is a harmonic signal at frequencies ω+ω_{L} and 2ω+ω_{L}, respectively:
Therefore, mixing with the resonator vibration produces lowfrequency signals at frequency ω_{L} that for each case can be written as:
The ratio of signal level between the two methods is equal to the enhancement factor defined in equation (4). If we assume a beam vibration in the fundamental mode (n=1) (ref. 11) and a beam deflection profile shape at rest that can be approximated by that of the fundamental mode, then equations (10) and (11) can be rewritten as:
so that the ratio between both signals is given by . The estimations provided above for the deflection at rest and vibration amplitude, with d_{0} in the range of 5–50 nm and a_{1} in the order of 1 nm, result in a ratio above 20.
Figure 3a,b shows the frequency response of a bottomup SiNW with a crosssectional flattoflat distance of 52–60 nm using the twosource/1ω and twosource/2ω detection schemes. We show the spectra for the first (Fig. 3a) and second (Fig. 3b) modes. The actuation conditions are equal for both methods, and therefore it can be assumed that the vibration amplitude is similar regardless of the detection method employed. We observe that the electrical signal amplitude is 3–14 times smaller for the twosource/2ω, in agreement with the theoretical estimation provided above. Measurements of the dependence of the frequency response of the NW as a function of the constant (DC) gate voltage confirm that the electrical transduction generates a signal linearly proportional to the vibration amplitude in the case of the twosource/1ω method and quadratic in the case of the twosource/2ω (see Supplementary Fig. 6).
Figure 3c,d shows the measurement of the first mode of a topdown NW using the twosource/1ω and twosource/2ω detection schemes in the outofplane (Fig. 3c) and inplane (Fig. 3d) directions. The SiNW has crosssectional dimensions of 110 nm (width) × 80 nm (thickness), comparable to those of bottomup NWs. As in the case of Fig. 3a,b, we employ the same actuation conditions for both detection methods, so the mechanical oscillation amplitude of the resonator is expected to be the same. It is noticeable that no resonance is detected using the twosource/2ω method, indicating that for topdown devices the ratio between 1ω and 2ω signals is larger than 20, as estimated above.
Similar results are obtained with smaller topdown NWs with a crosssection of 60 nm (thickness) × 72 nm (width), which are shown in Supplementary Fig. 7. This indicates the absence (or very weak) quadratic transduction in topdown SiNWs under the current fabrication conditions. Measurements of the dependence of the frequency response of the vibration amplitude as a function of the gate voltage show that the electrical transduction generates a signal proportional to the amplitude when using the FM and twosource/1ω methods (see Supplementary Fig. 8).
Discussion
In addition to piezoresistance, another possible alternative mechanism responsible for the linear transduction could be conductance modulation due to capacitive coupling induced by the side gate, similar to CNT and graphene resonators^{1,28}, which has also been reported for topdown SiNWs^{3}. However, we discard this mechanism for the devices under consideration here. Static transconductance measurements were performed in bottomup SiNW nanomechanical resonators for gate voltages from −25 to 25 V (see Supplementary Fig. 9). They show a null or very weak conductivity modulation effect, as it is expected from the high doping level of the SiNWs and the largegap distance between the gate and the SiNW. In consequence, the signal that originated from capacitive coupling would be much lower than the signal experimentally obtained by the FM and twosource/1ω detection methods. The expected signal that should be measured by assuming conductance modulation by capacitive coupling would be of 4 pA, almost two orders of magnitude smaller than the one obtained experimentally (Fig. 3a). In the case of the topdownfabricated devices, it is also found that the static and dynamic transconductance is very low, as no appreciable resistance change was observed during static transconductance measurements when varying the gate voltage from −25 to 25 V, and therefore a transduction mechanism based on capacitive coupling cannot explain the high level of the detected signals. We estimate a value for the transconductance of 20 nS V^{−1} from the purely electrical signal measured at a frequency far away from resonance using the twosource/1ω method, which discards also the conductance modulation due to charge induced by the side gate as transduction mechanism (see Supplementary Fig. 10).
The electrical signals obtained with the devices presented here are thus consistent with a purely piezoresistive transduction mechanism that produces linear and quadratic components in the dependence of the resistance variation with the vibration amplitude. From equations (12) and (13) and the estimations made for the deflection at rest and firstmode vibration amplitude (a_{1}≈1 nm, d_{0}=5–50 nm), we infer gauge factor values in the range from a few units to a few hundreds. This is the range expected from a conventional piezoresistive effect in SiNWs depending on the doping level^{24}. Therefore, highsensitivity piezoresistive transduction, enhanced by an asymmetric beam profile at rest, does not require extraordinarily large piezoresistive coefficients as those reported on highresistivity bottomup NWs, which reach up to 1,000 and above^{21}. Nevertheless, larger gauge factors are still expected for bottomup NWs as compared with their topdown counterparts according to the literature^{23,24}. In fact, a larger gauge factor for bottomup SiNWs explains why the quadratic component of the resistance variation with vibration is only detected for these devices (Fig. 3). According to our model, while the deflection at rest represents a gain factor that allows to obtain a large linear transduction regardless of G, it does not modify the quadratic transduction, which depends only on G. In the experiments, bottomup SiNW resonators provide measurable quadratic transduction signals at least for the first mode (Fig. 3a), whereas no quadratic detection is obtained for topdown SiNWs for any mode or vibration plane. This observation is consistent with the lower gauge factor for topdown SiNWs, which implies that the quadratic signal level is below the minimum detectable level in our setup. However, the larger gauge factor of the bottomup SiNWs does not necessarily imply larger linear signals than those of topdown devices because a larger deflection at rest of the later compensates for their lower piezoresistivity. Finally, we remark that our analysis assumes a linear dependence of the resistance variation with the strain. However, the initial strain caused by the asymmetric beam deflection at rest may lead to the onset of some nonlinearity in this dependence, but the fact that the vibration amplitude is very small makes it reasonable to consider a linear dependence for the strain variations caused by such vibrations. And remarkably, even in the case of a nonlinear variation of resistance with strain, an asymmetric beam profile at rest originates a signal that has a linear dependence with the vibration amplitude (see Supplementary Discussion).
In summary, we have described an enhanced piezoresistive transduction mechanism arising from an asymmetric beam shape at rest in doubly clamped SiNW resonators. Regardless of the detection scheme, this asymmetry introduces a linear component in the resistance variation that is proportional to the product of the vibration amplitude and to the magnitude of deflection at rest, while the quadratic component remains independent of this effect. When the deflection at rest is much larger than the vibration amplitude, this asymmetry produces a largely amplified linear response with respect to the quadratic one. This simultaneous linearization and amplification of piezoresistive transduction in Si beam resonators presents crucial implications for practical development of device applications. For instance, it enables detection of multiple modes at very high resonance frequencies that otherwise would not be easily detectable. Note that the detected highorder modes were electrostatically driven by side gates with a geometry not particularly optimized for the expected mode shapes. Multiplemode detection has recently shown to be crucial for simultaneous measurements of mass and adsorption position of biomolecules for masssensing applications^{4} and to quantum nondemolition measurements of the amplitude, making use of the intermodal coupling^{29,30}. In addition, a linear transduction mechanism allows to perform measurements using standard electrical methods, such as network analysers, as well as its incorporation in closedloop oscillator topologies, both of which would be impaired by quadratic transduction. It should also be noted that an asymmetric beam shape in doubly clamped beams of nanometric dimensions is by no means an uncommon phenomena. In CNT resonators, it has been recently shown that such asymmetries are responsible for some particular features of their electromechanical response^{31}. In the case of Si beams, topdown fabrication methods can be engineered to predetermine the amount of beam deflection at rest to target specific readout signal levels. The transduction mechanism described here is indeed particularly relevant for topdownfabricated Si resonators. The lack of an effective transduction mechanism was a limiting factor for exploiting the VLSI fabrication technologies available for such devices. The fact that the mechanism described here does not require exceptionally large piezoresistive factors or any other special material property different than those of doped bulk Si allows its immediate application for devices fabricated by VLSI techniques.
Methods
Fabrication of bottomup SiNWs
The bottomup fabrication method is based on the VLS growth mechanism catalysed by gold nanoparticles, which is described in more detail in ref. 32 and depicted in Fig. 4a. The process is performed on silicononinsulator (SOI) wafers. First, contacting pads, clamping structures and trenches are defined in the SOI layer using photolithography and dry etching. The wafers are cut into chips, each chip containing 392 structures where the nanomechanical resonator will be formed if the SiNW grows at the proper place. Catalyst particles are randomly deposited on the surface of the chip from a gold colloid solution. Then, the SiNWs are grown in the substrates using the VLS method in an atmospheric chemical vapour deposition. As a final step, the devices are doped by annealing the chips in close proximity to a fragment of a boron nitride wafer, so that the final resistivity of the SiNWs is very low in the range of 10^{−4} Ω m. Two terminal characterization of released devices show a linear I/V curve and a typical resistance of 50 kΩ. Using this process, we have obtained resonators made of SiNWs with crosssectional flattoflat distance in the range 50–100 nm and length of 1–3 μm. Some devices incorporate a silicon electrode close to the SiNW for electrostatic actuation. Typically, the gap distance between the gate and the SiNW is around 400 nm. Owing to the random placement of the catalyst material, the fabrication yield is relatively low: based on postgrowth characterization, we have found it to be of roughly 5%, good enough to get a sufficient number of operative devices per chip.
Fabrication of topdown SiNWs
An overview of the fabrication process is shown in Fig. 4b. An SOI wafer is used as a starting substrate. The patterns are defined using optical photolithography (iline stepper), and then transferred to the silicon device layer using reactive ion etching. At this step, SiNWs with crosssectional areas from 350 to 500 nm are obtained corresponding to the minimum feature size obtainable with the optical photolithography equipment. To reduce their crosssections, an oxidation process is performed at 1,000 °C in an O_{2} environment. Afterwards the oxide is removed by a dip in a solution with hydrofluoric acid followed by critical point drying. This cycle is repeated several times until the desired dimensions are obtained. Finally, the SiNWs are doped by performing an annealing in close proximity to a boron nitride wafer, as in the case of the bottomup. Using this processing sequence, SINWs of dimensions comparable to their bottomup counterparts are fabricated with high yield. By adjusting the oxidation steps, we have succeeded in fabricating SiNWs with a range of crosssectional dimensions from 60 to 175 nm, and length of ~\n3 μm. Some devices incorporate a silicon electrode close to the SiNW for electrostatic actuation. Typically, the gap distance between the gate and the SiNW is around 900 nm.
Electrical detection methods
Electrical detection is performed by downmixing radiofrequency methods, in which the highfrequencytransduced signal arising from the motion of the resonator is transferred to lower frequencies, and then detected using a lockin amplifier. The measurements are performed in a vacuum chamber, at a pressure of 1 × 10^{−6} mbar and at room temperature. Three electrical detection setups based on three different downmixing methods have been employed in this work: the twosource/1ω (ref. 1); the twosource/2ω (ref. 2); and the FM demodulation^{8}. Schemes of the experimental setup for each detection method are shown in Fig. 5, and a full description can be found in ref. 33.
Additional information
How to cite this article: Sansa, M. et al. Highsensitivity linear piezoresistive transduction for nanomechanical beam resonators. Nat. Commun. 5:4313 doi: 10.1038/ncomms5313 (2014).
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Sansa, M., FernándezRegúlez, M., San Paulo, Á. & PérezMurano, F. Electrical transduction in nanomechanical resonators based on doubly clamped bottomup silicon nanowires. Appl. Phys. Lett. 101, 243115 (2012).
Acknowledgements
This work was partially funded by the projects SNM (FP7ICT20118), FORCEforFUTURE (CSD201000024), ANEM (TEC200914517C0201), SGRNANOFABRICACION (2009 SGR 265), SiNSoC (MAT201115159E). M.S. acknowledges the FPU grant (Ref. AP200803849). We thank N. Barniol for her support during the measurements, G. Villanueva for his advice and helpful discussions, and O. Beldarrain, M. Duch and M. Gerbolés for their technical support during the fabrication.
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Affiliations
Instituto de Microelectrónica de Barcelona (IMBCNM, CSIC), Campus UAB, 08193 Bellaterra, Spain
 Marc Sansa
 , Marta FernándezRegúlez
 , Jordi Llobet
 , Álvaro San Paulo
 & Francesc PérezMurano
Instituto de Microelectrónica de Madrid (IMM, CSIC), Isaac Newton 8 (PMT), Tres Cantos, 28768 Madrid, Spain
 Álvaro San Paulo
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Contributions
M.S., A.S.P. and F.P.M. conceived the study, designed the experiments, analysed the data and wrote the manuscript. M.S. fabricated the samples and performed the measurements. M.F.R. collaborated in the SiNW growth and provided conceptual advice. J.L. provided technical support.
Competing interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to Francesc PérezMurano.
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