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VHF NEMS-CMOS piezoresistive resonators for advanced sensing applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 435501 (http://iopscience.iop.org/0957-4484/25/43/435501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 435501 (8pp)
doi:10.1088/0957-4484/25/43/435501
VHF NEMS-CMOS piezoresistive resonators for advanced sensing applications Julien Arcamone, Cécilia Dupré, Grégory Arndt, Eric Colinet, Sébastien Hentz, Eric Ollier and Laurent Duraffourg Univ. Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, Minatec Campus, 17 rue des Martyrs, F-38054 Grenoble, France E-mail: [email protected] Received 13 May 2014, revised 26 August 2014 Accepted for publication 27 August 2014 Published 7 October 2014 Abstract
This work reports on top-down nanoelectromechanical resonators, which are among the smallest resonators listed in the literature. To overcome the fact that their electromechanical transduction is intrinsically very challenging due to their very high frequency (100 MHz) and ultimate size (each resonator is a 1.2 μm long, 100 nm wide, 20 nm thick silicon beam with 100 nm long and 30 nm wide piezoresistive lateral nanowire gauges), they have been monolithically integrated with an advanced fully depleted SOI CMOS technology. By advantageously combining the unique benefits of nanomechanics and nanoelectronics, this hybrid NEMS-CMOS device paves the way for novel breakthrough applications, such as NEMS-based mass spectrometry or hybrid NEMS/CMOS logic, which cannot be fully implemented without this association. S Online supplementary data available from stacks.iop.org/NANO/25/435501/mmedia Keywords: nanowires, NEMS, CMOS, VHF, top-down technology, fully-depleted SOI
[8], silicon or metallic piezoresistive detection [9–11], transistor-based [12–14], detection, or on magnetomotive detection [15]. In fact, piezoresistive (PZR) detection combines several unique advantages and perspectives of improvement; a number of works, like [10] and [11], demonstrate how piezoresistive nanogauges benefit from favorable scaling laws. Since the cross-sectional area on which the force applied is smaller, the induced stress is magnified (thereby, very thin, sub-100 nm thick NEMS can be operated), and the resulting relative resistance variation is magnified as well. Scaling laws are such that PZR NEMS generate high electrical signals without necessarily relying on a giant piezoresistive effect [16–19]. Another interesting feature is the intrinsic compatibility of CMOS in terms of materials and processes, unlike piezoelectric devices, for example. PZR NEMS can be fabricated with top-down VLSI approaches that enable the implementation of large arrays and provide process repeatability, as required by industries. Whatever the transduction scheme, scaling down the mechanical structure from MEMS to NEMS induces some electrical detection issues when using a direct measurement (without any heterodyning strategy), such as (i) high
Nanoelectromechanical systems (NEMS) constitute a very promising research field both in terms of scientific interest— for instance, for the observation of quantum mechanics phenomena—and from the practical applications point of view. NEMS may enable novel breakthrough applications, mainly in the field of chemical analysis [1, 2], life science [3–5] and computing [6, 7]. In those first two fields (gas sensing and mass spectrometry, respectively), they appear as excellent candidates since they exhibit unprecedented mass resolution, thanks to their inherent low mass, high resonance frequency and good frequency stability. In both fields, recent works [2, 4], have not only confirmed their intrinsic exceptional sensing potential but have also illustrated their increasing maturity as they have been successfully integrated into complex systems. Nevertheless, these new applications usually involve NEMS arrays with a rather large number of electrical contacts per device, making their individual addressing complex. In this context, much work has been devoted to the determination of the best electromechanical transduction scheme at these scales; most of the realizations reported in the literature, which are generally in the 10–100 MHz frequency range, rely on capacitive detection 0957-4484/14/435501+08$33.00
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In this context, this work reports on a number of novelties relative to the literature. It is the first demonstration of single-crystal silicon, piezoresistive NEMS resonators monolithically integrated at the front-end with a CMOS readout circuitry (see figure 1). The NEMS resonator itself has a very high resonance frequency (100 MHz) and is presumably the smallest top-down resonator ever reported (20 nm thick with a 1.2 μm long and 100 nm wide beam, and two lateral 100 nm long and 30 nm wide nanowire gauges). Another remarkable feature is the fact that the CMOS integration has enabled the direct (homodyne) measurement of the 100 MHz frequency response, whereas most realizations rely on more complex heterodyne schemes [23–25], that use bulky off-chip electronics. This work strongly illustrates how NEMS and CMOS are converging in terms of dimensions and processes and how their association can be synergetic to pave the way for major applicative breakthroughs such as mass spectrometry for biomedical analysis and for NEMS-based computing. By combining advanced FDSOI CMOS technology (which is now competing [26, 27], with bulk FinFET for ultimate sub-28 nm CMOS nodes) with very high-frequency NEMS resonators, these NEMS-CMOS devices have risen to the cutting edge of nanoelectronics and nanomechanics. In conclusion, the purpose of this work is to provide a highresolution sensing system for bio- and chemical applications. This demonstrator relies on a pre-CMOS integration approach in which NEMS resonators are fabricated before the CMOS process and released after completing it. The implemented CMOS technology is an advanced FDSOI process [28], developed at CEA-LETI. It is also much easier to get suspended, mobile NEMS structures using an SOI CMOS process instead of a bulk CMOS one. The technological convergence between advanced NEMS, such as the crossbeams reported in this paper, and advanced CMOS technologies in terms of the device’s size and processing tools is noteworthy. In this work, NEMS and the transistors’ channels are both fabricated on the same single-crystal Si layer (SOI top layer), though not simultaneously, by applying the same hybrid DUV/e-beam lithography technique and the same plasma dry etching process. NEMS and their associated amplifying circuitry already have equivalent sizes, with a joint area of about 10 × 20 μm2. In the near future, one can anticipate the demonstration of ultra-dense NEMS-CMOS arrays or ‘imagers’ with 5 × 5 μm2 pixels. The technological process flow starts by doping the 40 nm thick top SOI layer by implantation (1019 at cm−3 of boron, leading to a sheet resistivity of 9 mΩ cm; the value is measured by the van der Pauw method). This step is only performed on NEMS-dedicated areas; the CMOS areas are protected by resist. After patterning the NEMS, they are released by removing the SiO2 sacrificial layer with a stictionfree, vapor-HF (hydrofluoric acid) isotropic etching. Then, the free-standing NEMS are partially oxidized at 1100 °C (figure 2(a)) and encapsulated with both a PolySi sacrificial layer and a high-temperature oxide (HTO) (figure 2(b)). After locally opening this protection layer, the CMOS transistors are fabricated at the front-end level. The back-end levels on both the CMOS transistors and NEMS are performed at the
Figure 1. Tilted SEM images of the NEMS-CMOS device with an
increasing zoom-in: (a) global overview, with the FDSOI CMOS readout circuit on the left and the NEMS area on the right; (b) crosssectional zoom-in (AA’ view) of the NEMS resonator and its electrodes, above which the dielectric passivation layers were locally etched; (c) zoom-in of the NEMS resonator and its nanogauges, inset: TEM cross-section of a nanogauge and its related dimensions.
connection losses due to pad and cable capacitances (their high-resonance frequency, which generally ranges from 1 to 100 MHz, are far higher than the cutoff frequency of the output parasitic low-pass filter) and (ii) an increase of feedthrough parasitic coupling between the resonator input and output. Both issues result in a degradation of the signal-tobackground ratio and possibly of the signal-to-noise ratio. Cointegrating NEMS resonators with a dedicated adjacent CMOS readout and conditioning electronics is a smart and very efficient way to overcome these difficulties since NEMSCMOS devices potentially benefit from a better signal-tonoise ratio (SNR), immunity to external parasitics, higher density and compactness and possibly a lower cost. The integration of NEMS resonators with a standard CMOS technology has already been demonstrated, but in all past examples, either the NEMS resonators were made from other materials as opposed to single-crystal Si (such as poly-Si [20] or back-end metal layers [21]), or the device size was too large to provide a high frequency and sufficiently high mass sensitivities [22]. In summary, ultra-small NEMS devices with a co-integrated readout under the form of compact pixels were yet to be developed. 2
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(1 zg = 10−21 g). The beam has two lateral piezoresistive nanogauges (100 nm long, 30 nm wide; see inset of figure 1(c)) located close to the beam clamping point, which provide an integrated electromechanical transduction of the beam’s mechanical motion (see the left part of figure 3(c)). In this configuration, since both gauges are nominally identical, they exhibit resistance variations ΔR of the same magnitude but of the opposite sign while the beam is under lateral displacement. By considering the clamping point as the NEMS output node VOUT_NEMS, the detection scheme therefore constitutes a half Wheatstone bridge and VOUT_NEMS is given by (see supporting information): VOUT _ NEMS =
( VB2 − VB1)
(
2 + jCL ω 2R ba + R ga + R
)
⎛ ΔR ⎞ 1 ⎟ ×⎜ ⎝ R ⎠ 1 + R ga / R
(1)
whereby R is the gauges’ resistance, assuming identical gauge resistances (RJ = R1 = R2); Rba and Rga correspond, respectively, to the resistance of the silicon beam anchor and to the resistance of the silicon gauges anchor (see Supplementary Information). VB2 and VB1 are the DC bias voltages of the two gauges (see figure 3). ω is the pulsation of the mechanical motion, and CL the parasitic capacitance of the electrical interconnection between NEMS and CMOS. Since the beam and its electrodes are symmetrical with respect to the cantilever main axis, the feedthrough signals between the electrodes and the VOUT_NEMS node are potentially cancelled out on that node by differentially actuating the resonator. In practice, VACT1 and VACT2 have the same DC component and same AC magnitude, but their AC signals are in counterphase. The relative resistance variation at resonance is given by [11]:
Figure 2. Cross-sectional schematic views of the main technological process steps realized on thin SOI wafers (20 nm thick top Si layer); (a) after NEMS patterning, release and thermal oxidation; (b) after [Polysilicon + HTO] encapsulation and before the beginning of the CMOS process; (c) final view of the device after completion of the CMOS process, dry etching of the passivation dielectrics over the NEMS resonators and dry etching of the encapsulating polysilicon to release the mobile structures. It is noteworthy that the top Si layer of such SOI wafers serves as both the NEMS structural layer in dedicated NEMS areas and as transistor channels in CMOS circuits areas; (d) corresponding SEM image of the NEMS area and its related electrical contacts, which interconnect the NEMS to its CMOS readout circuit.
ΔR G αQFACT = π 44 σ J = R E AJ
same time. In this first demonstrator, the minimum feature size for the channel has been set to 300 nm to secure the circuit’s performance. After completion of the CMOS process, intermetallic oxides are etched above the NEMS down to the PolySi encapsulation layer, which is subsequently chemically removed with a CF4-based gas (figures 2(c) and (d)). This process is highly selective since the surrounding SiO2 is etched at a speed 70 times slower than PolySi. By using this approach, intermetallic passivation layers are not degraded during the release and do not require a protection layer, as usually required in N/MEMS-CMOS monolithic integration. More details on the process flow can be found in [29]. The hybrid NEMS-CMOS device (figures 1 and 3) reported in this paper first consists of a 100 MHz nanomechanical resonator. This so-called ‘crossbeam’ [11] (figure 1(c)) is an electrostatically actuated, p-type silicon cantilever beam (1.2 μm long, 100 nm wide, 20 nm thick) operated on its first in-plane flexural mode. With such dimensions, crossbeams resonate around 100 MHz and feature an outstanding mass sensitivity S of ∼0.1 zgHz−1
(2)
whereby AJ is the nanogauges’ cross-sectional area (30 × 20 nm2), Q is the resonator quality factor, α is a unitless factor that accounts for the mechanical lever effect between the equivalent point of the application of FACT and the nanogauges’ position along the beam, and FACT is the applied electrostatic force on the resonator. This force is proportional to the DC and AC components of VACT, VACT_DC and VACT_AC, respectively. The p-type silicon substrate is oriented to get the highest piezoresistive coefficient, i.e. G (for example, [30]). The resonator is designed so that all of the parameters of equation (2) are optimized to maximize ΔR/R: (i) the optimum substrate orientation and type (p) for piezoresistive detection, (ii) the optimum location of the gauges along the beam to increase the lever effect while ensuring a proper orthogonal transmission of the stress and (iii) very aggressive dimensions. The actuation gap is 60 nm, enabling large electrostatic forces with relatively low actuation voltages; since the gauge’s cross-section is small, the induced stress is highly concentrated and magnified. The gauge factor G of such silicon nanowires was measured 3
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Figure 3. (a) SEM picture of the crossbeam NEMS device and its lead silicon resistors; (b) image of the resonance mode of interest (first inplane flexural one) obtained by COMSOL simulation; (c) equivalent electrical schematic of the monolithically integrated NEMS-CMOS device (black squares are the contact pads). The NEMS resonator is polarized by VACT1 and VACT2 and by VB1 and VB2, respectively, the actuation voltages (AC + DC), and the DC bias voltages of the two nanogauges. Nominally, both gauges are identical, i.e. R1 = R2 and ΔR1 = ΔR2. The 10-transistor CMOS readout circuit contains an amplifying stage (M0–M1) DC biased by VAMP and a buffer (M4–M9) DC biased by VDB1, VDB2 and VDB3. The buffer input is adequately DC biased, thanks to a dedicated stage (CDC, M2–M3). The supply voltages (VSS/VDD) are 0 and 1.2 V, respectively. The circuit is designed so that it can load 100 pF at 100 MHz at its output (VOUT). The key parameter is the extremely low connection capacitance CL (∼20 fF) between the NEMS resonator output and the circuit input. As a consequence, the NEMS homodyne response is not attenuated by the low-pass RC filter at the NEMS output; that usually dramatically degrades the response in classical configurations (stand-alone NEMS chip with off-chip readout electronics).
elsewhere [31] at 89. In this case, the lever arm coefficient α is 1.8. For more information on the crossbeam’s design and features, please refer to the Supporting Information. The output node VOUT_NEMS of the nanomechanical resonator is interconnected to an adjacent CMOS circuit; the circuit’s equivalent electrical schematic is depicted in figure 3. Beyond its capability of signal amplification, the main function of this circuit is to adequately interface the NEMS with external electronics by providing efficient impedance matching. The monolithic integration inherently induces a high impedance at the NEMS output as the interconnection capacitance CL is drastically decreased (down to a few tens of fF, which is two orders of magnitude less than in stand-alone devices) such that R + Rga + 2Rba CL ω ≪ 2 (see equation (1)). Consequently, VOUT_NEMS is no longer attenuated by the parasitic RCL low-pass filter at the NEMS output, hence:
(
VOUT _ NEMS =
transistors, is a high input impedance, low-gain (∼1) voltage amplifier. The second one, which comprises M2, M3 and CDC, a decoupling capacitor, constitutes the DC biasing stage of the third block (M4–M9), which is a three-stage buffer to adapt the impedance between the amplifier output and the large capacitances at the buffer output (10 to 100 pF) that arises from bonding pads, wires and connection cables. The typical frequency responses of the buffer and the ensemble buffer + amplifier have been shown in [32]. A very relevant advantage provided by this overall impedance matching of the NEMS with the outside world is that the direct measurement of the resonator is enabled; the electrical signal has the same frequency all along the chain, which simplifies the architecture of the readout circuit. In comparison, the quasiexclusivity of the realizations reported in the literature on the operation of high-frequency NEMS resonators (>20 MHz) is based on implementing heterodyne (mostly downmixing, [11, 23, 25]) detection schemes that require much more complex set-ups and the use of specific functions, such as frequency mixing, that are not straightforward to miniaturize and implement in analog CMOS circuitry. Among the very few other examples of homodyne (direct) detection of VHF resonators, the transduction scheme presented in [19] and related works—thermal actuation and piezoresistive detection
)
VB2 − VB1 ⎛ ΔR ⎞ 1 ⎜ ⎟ . ⎝ ⎠ 2 R 1 + R ga / R
(3)
The CMOS readout circuit is designed to load 100 pF at 100 MHz and contains ten transistors distributed in three blocks (see the transistors’ dimensions in the supporting information). The first one, which comprises the M0 and M1 4
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Figure 4. Homodyne open-loop electrical responses (VOUT node, see figure 3) of a 100 MHz CMOS-integrated crossbeam NEMS resonator (20 nm thick, 1.2 μm long and 100 nm wide beam; the nanogauges’ length and width: 100 and 30 nm). These results were obtained in vacuum for an outstandingly low AC actuation voltage (VACT_AC) magnitude of 2.25 mV rms. (a) Variations of the magnitude response (linear scale) with VACT_DC; (b) Same data as in (a) but with the phase response; (c) Same data as in (a), but the magnitude response is represented in terms of s21 (in dB, log scale) [s21 = 20 log(VOUT/VACT_AC)]; In (a), (b) and (c), the gauges’ DC bias is VB1 − VB2 = 4 V.
—seems a relevant alternative. However [19], involves much bigger devices (three and four decades bigger in area and volume, respectively, than this work); therefore, both works cannot be directly compared. Another feature of the NEMSCMOS device is the very small size of the NEMS + amplifier cell (about 10 × 20 μm2). Although the biasing stage and the buffer represent an area of about 30 × 90 μm2, the amplifier is very compact (10 × 10 μm2), and there is room for further size reductions. This paves the way for ultra-dense arrays of individually addressed NEMS, which will be required in emerging applications such as NEMS-based mass spectrometry. In such cases, each NEMS would be associated to its own amplifier with a buffer that is common to all NEMS. The electrical response of such 100 MHz NEMS-CMOS devices is measured using a direct (homodyne) detection scheme. The resonator is differentially actuated with VACT1 and VACT2, which have equal DC components (see figure 3). Their AC components are in counter-phase, and their magnitudes are finely tuned (one with respect to the other) in order to cancel out the feedthrough signals on the VOUT_NEMS node. Without this operation, we experimentally observe a remnant background signal on VOUT that may completely hide the resonance signal. In most of the tested devices, a 1 dB difference is required to obtain the best balance. If the exact origin of this mismatch is difficult to retrieve, slight
asymmetries in-between the on-chip routing of VACT1 and VACT2 or in-between their respective bonding wires with respect to the cantilever main axis are probably enough to generate this effect at a high frequency (100 MHz). VB1 and VB2 are adequately tuned so that the gate of M1 is DC biased at VOUT_NEMS_DC = 0.7 V, as required for optimum operation of the circuit (M1 was operated in a saturation regime). The effective DC actuation voltage VACT_DC_EFF applied on the resonator is then (VACT_DC − 0.7) V. The CMOS circuit is supplied with 1.2 V and is DC polarized with VAMP = 0.27 V, VDB1 = 0.6 V, VDB2 = 0.5 V and VDB3 = 0.75 V. The frequency response (VOUT) of a CMOS-integrated crossbeam measured in vacuum (10−4 mBar) is depicted in figure 4. After biasing the gauges with VB1 = 2.7 V and VB2 = −1.3 V (i.e. VB1 − VB2 = 4 V), the measured response is plotted in terms of magnitude and phases in figures 4(a) and (b), respectively, for three values of DC actuation voltage VACT_DC (3, 4 and 5 V); the well-known spring-softening effect [33] consequently induces the three resonance frequency peaks. The quality factor Q in a vacuum is estimated at ∼2300 in all of the cases (Q ∼ 400 in air). These peaks feature an excellent signal-tobackground ratio (SBR) and signal-to-noise ratio (SNR), although they are obtained using a very low AC actuation voltage (VACT_AC of 2.25 mV rms), and the gauges’ DC bias and DC actuation voltages are smaller than 5 V. The 5
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frequency response in a linear scale is presented in figure 4(a). For VACT_DC = 3 V, the output voltage VOUT_NEMS at resonance is 38 μV, with a background close to 3 μV; for VACT_DC = 4 V, VOUT_NEMS = 50 μV; and for VACT_DC = 5 V, VOUT_NEMS = 60 μV with a background of ∼4 μV. The experimental magnitudes are close to the expected values estimated by equation (3): 42 μV, 60 μV and 78 μV, which correspond to the cantilever displacements of 0.3 nm, 0.45 nm and 0.6 nm for 3, 4 and 5 V, respectively (see supporting information for more details). Figure 4(c) is an s21 (transmission gain) plot of the same data; the on-resonance gain is as high as −32 db. This result confirms the high electromechanical transduction efficiency of electrostatic actuation based on ultra-thin gaps coupled to monolithic piezoresistive/ CMOS detection. In this first demonstration, the circuit gain is close to one, but a slightly more complex circuit topology would have undoubtedly provided an s21 gain equal or larger than 0 dB, thus enabling the implementation of fully integrated VHF NEMS-CMOS oscillators. The differential actuation with independently tunable AC amplitudes allows balancing of the feedthrough signals and the ability to reach SBR larger than 25 dB at VACT_DC = 5 V. Considering the ultimate dimensions of the resonator, which is presumably the smallest one ever reported using top-down technologies, and its very high operating frequency that usually implies small motional signals and high parasitic coupling, these electrical results are outstanding. In fine, one of the main motivations guiding the development of such monolithic NEMS-CMOS devices is to make resonance frequency-sensitive sensors, with their frequency stability δf/f0 as a key feature. A usual and convenient method to estimate this frequency stability is to measure the Allan deviation [35] σA of the output signal versus integration time τ. For narrow integration bandwidths (1/τ) compared to the resonance frequency f0, and when white noise processes dominate, σA follows the τ−1/2 power law, given by: σ A (τ ) =
1 1/ τ . 2Q SNR
Figure 5. Frequency stability of the NEMS-CMOS output response
(VOUT) measured in terms of the Allan deviation versus integration time τ. The τ−1/2 slope between τ = 1 ms and 500 ms indicates that the dominant source of noise over this range is a white noise process (see supporting information).
limit of detection (LOD) at τ = 500 ms is then around 2 zg, i.e. ∼1.3 kDa (this number is the computation of the theoretical mass sensitivity and the experimentally measured noise data; it still needs to be confirmed in mass detection experiments). This represents a mass sensor range (between the LOD and 1% of the NEMS mass, which is the upper limit for a linear response, i.e. ∼0.1 fg) of more than four orders of magnitude with major applications in biology. This would indeed allow some bio-samples, such as human proteins or protein-based bacteria, to be analyzed cell by cell. In practice, NEMS-based gas or mass sensors require tracking the resonance frequency in real-time in order to capture events of short durations (possibly down to a few tens of ms). This specification is only achieved by operating NEMS in a closed-loop. Therefore, as a first step toward implementing such NEMS-CMOS devices as sensors, figure 6 details how they have been embedded in a homemade, digital phase-locked loop (PLL), which potentially responds faster than one ms. This off-chip, software-based PLL sequentially contains a phase comparator, a corrector using the ‘H-infinity’ approach [36], a voltage-controlled oscillator (VCO) and a phase shifter to differentially actuate the resonator. Relying on the spring-softening effect, the DC actuation voltage VACT_DC is varied on purpose in figure 6(b) (between 4.4 and 4.6 V with a steady value of 4.5 V) to exemplify the real-time monitoring of resonance frequency variations. In conclusion, this work demonstrates unique monolithic NEMS-CMOS devices with an outstanding electrical response despite the ultra-challenging context (very highfrequency and ultimate NEMS dimensions). The reported fabrication process allows combining NEMS resonators with an advanced FDSOI CMOS process, which is now the main alternative to FinFET CMOS technology—the mainstream approach to extend Moore’s law beyond the 28 nm node. This monolithic device constitutes the single demonstration in the literature, both of the direct (homodyne) measurement of
(4)
The Allan deviation was measured on our samples in an open-loop configuration by recording the phase noise with a lock-in amplifier. It is depicted in figure 5 with integration times τ in a range of practical use for most sensing applications (between 1 ms and 1 s). In agreement with equation (4), σA follows the τ−1/2 power law up to approximately 500 ms, indicating that white noise processes dominate down to σA = 2 × 10−7. Above 500 ms, the Allan deviation inflects due to the prevalence of colored noises (in 1/fα). For τ = 500 ms, the thermomechanical noise is estimated at 117 nV (√Hz−1), which corresponds to 1.33 × 10−13 m (√Hz−1) (or a force noise of 1.9 × 10−16 N (√Hz−1). The Johnson noise and the readout electronics’ noise (referred to as its input) are evaluated at 46 nV (√Hz−1) and 10 nV (√Hz−1), respectively. Therefore, the thermomechanical noise is the dominant process in our case (see supporting information for more information). Since the NEMS device of this work has a mass of 6 fg (i.e. a theoretical mass sensitivity of 0.12 zg Hz−1), its 6
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also acknowledge Antoine Niel and Jérémie Ruellan for their punctual support on the experimental characterization.
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Figure 6. Demonstration of a closed-loop operation of this 100 MHz NEMS-CMOS device; the CMOS-integrated resonator is embedded in a PLL so that it continuously oscillates at its resonance frequency (f0). This PLL would constitute the main building block of a NEMSbased, resonance-frequency sensitive sensor that would monitor realtime variations of f0. (a) The PLL chain is located off-chip and controlled via a LabVIEW interface. (b) PLL-based real-time monitoring of f0 over a 150 s timeframe (PLL sampling time of 100 ms and a corrector bandwidth of 1 s), with VACT_AC = the magnitude of 2.85 mV rms and (VB1 − VB2) = 4 V. The resonance frequency is sequentially modified by varying VACT_DC.
VHF (100 MHz) NEMS and of CMOS-integrated NEMS, made of single-crystal Si and with PZR detection. This integration scheme results in a frequency stability (measured in terms of Allan deviation) of 2 × 10−7, which is equivalent to a theoretical limit of mass detection of 1.3 kDa. In practice, this means that NEMS constitute promising analyzers to implement a new generation of mass spectrometers for bio-sample analysis.
Acknowledgments We gratefully acknowledge the financial support from the European Commission through the FP7 NEMSIC project. We 7
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[19]
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of top-down suspended Si nanowires Nanotechnology 22 395701 Hall H J, Rahafrooz A, Brown J J, Bright V M and Pourkamali S 2013 I-shaped thermally actuated VHF resonators with submicron components Sensors Actuators A 195 160–6 Arcamone J, Van Den Boogaart M A F, Serra-Graells F, Fraxedas J, Brugger J and Perez-Murano F 2008 Full-wafer fabrication by nanostencil lithography of micro/ nanomechanical mass sensors monolithically integrated with CMOS Nanotechnology 19 305302 Verd J, Uranga A, Teva J, López J L, Torres F, Esteve J, Abadal G, Perez-Murano F and Barniol N 2006 Integrated CMOS—MEMS with on-chip readout electronics for highfrequency applications IEEE Electron Device Lett. 27 495–7 Arcamone J et al 2012 VLSI platform for the monolithic integration of single-crystal Si NEMS capacitive resonators with low-cost CMOS IEEE Int. Electron Devices Meeting pp 359–62 Bargatin I, Myers E B, Arlett J, Gudlewski B and Roukes M L 2005 Sensitive detection of nanomechanical motion using piezoresistive signal downmixing Appl. Phys. Lett. 86 133109 Gouttenoire V, Barois T, Perisanu S, Leclercq J-L, Purcell S T, Vincent P and Ayari A 2010 Digital and FM demodulation of a doubly clamped single-walled carbon-nanotube oscillator: towards a nanotube cell phone Small 6 1060–5 Sazonova V, Yaish Y, Ustünel H, Roundy D, Arias T A and McEuen P L 2004 A tunable carbon nanotube electromechanical oscillator Nature 431 284–7 Strojwas A J 2013 Is the bulk vs. SOI battle over? Int. Symp. on VLSI Technology, Systems, and Applications pp 1–2 Kuhn K J 2011 Moore’s crystal ball: device physics and technology past the 15 nm generation Microelectron. Eng. 88 1044–9
[28] Faynot O et al 2010 Planar FDSOI technology for sub 22 nm nodes Int. Symp. on VLSI Technology Systems and Applications pp 26–7 [29] Ollier E et al 2012 Ultra-scaled high-frequency single-crystal Si NEMS resonators and their front-end co-integration with CMOS for high sensitivity applications IEEE 25th Int. Conf. on Micro Electro Mechanical Systems pp 1368–71 [30] Kanda Y 1982 A graphical representation of the piezoresistance coefficients in silicon IEEE Trans. Electron Devices 29 64–70 [31] Ouerghi I, Philippe J, Marcoux C, Scheiblin P, Duraffourg L and Ernst T 2014 Gauge factor extraction method for nanowires with a crossbeam-type nano electromechanical system submitted to IEEE J. Microelectromech. Syst. submitted [32] Arndt G, Dupré C, Arcamone J, Cibrario G, Rozeau O, Duraffourg L, Ollier E and Colinet E 2012 Towards ultradense arrays of VHF NEMS with FDSOI-CMOS active pixels for sensing applications Proc IEEE Int. Solid-State Circuits Conf. (Digest of Technical Papers) pp 320–2 [33] Bannon F D, Member S, Clark J R and Nguyen C T 2000 High- Q HF microelectromechanical filters IEEE J. SolidState Circuits 35 512–26 [34] Verd J, Uranga A, Abadal G, Teva J L, Torres F, Lopez J L, Pérez-Murano F, Esteve J and Barniol N 2008 Monolithic CMOS MEMS oscillator circuit for sensing in the attogram range IEEE Electron Device Lett. 29 146–8 [35] Rubiola E 2005 On the measurement of frequency and of its sample variance with high-resolution counters Rev. Sci. Instrum. 76 054703 [36] Kharrat C, Colinet E and Voda A 2008 H∞ loop shaping control for PLL-based mechanical resonance tracking in NEMS resonant mass sensors IEEE Sensors Conf. pp 1135–8
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VHF NEMS-CMOS piezoresistive resonators for advanced sensing applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 435501 (http://iopscience.iop.org/0957-4484/25/43/435501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 435501 (8pp)
doi:10.1088/0957-4484/25/43/435501
VHF NEMS-CMOS piezoresistive resonators for advanced sensing applications Julien Arcamone, Cécilia Dupré, Grégory Arndt, Eric Colinet, Sébastien Hentz, Eric Ollier and Laurent Duraffourg Univ. Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, Minatec Campus, 17 rue des Martyrs, F-38054 Grenoble, France E-mail: [email protected] Received 13 May 2014, revised 26 August 2014 Accepted for publication 27 August 2014 Published 7 October 2014 Abstract
This work reports on top-down nanoelectromechanical resonators, which are among the smallest resonators listed in the literature. To overcome the fact that their electromechanical transduction is intrinsically very challenging due to their very high frequency (100 MHz) and ultimate size (each resonator is a 1.2 μm long, 100 nm wide, 20 nm thick silicon beam with 100 nm long and 30 nm wide piezoresistive lateral nanowire gauges), they have been monolithically integrated with an advanced fully depleted SOI CMOS technology. By advantageously combining the unique benefits of nanomechanics and nanoelectronics, this hybrid NEMS-CMOS device paves the way for novel breakthrough applications, such as NEMS-based mass spectrometry or hybrid NEMS/CMOS logic, which cannot be fully implemented without this association. S Online supplementary data available from stacks.iop.org/NANO/25/435501/mmedia Keywords: nanowires, NEMS, CMOS, VHF, top-down technology, fully-depleted SOI
[8], silicon or metallic piezoresistive detection [9–11], transistor-based [12–14], detection, or on magnetomotive detection [15]. In fact, piezoresistive (PZR) detection combines several unique advantages and perspectives of improvement; a number of works, like [10] and [11], demonstrate how piezoresistive nanogauges benefit from favorable scaling laws. Since the cross-sectional area on which the force applied is smaller, the induced stress is magnified (thereby, very thin, sub-100 nm thick NEMS can be operated), and the resulting relative resistance variation is magnified as well. Scaling laws are such that PZR NEMS generate high electrical signals without necessarily relying on a giant piezoresistive effect [16–19]. Another interesting feature is the intrinsic compatibility of CMOS in terms of materials and processes, unlike piezoelectric devices, for example. PZR NEMS can be fabricated with top-down VLSI approaches that enable the implementation of large arrays and provide process repeatability, as required by industries. Whatever the transduction scheme, scaling down the mechanical structure from MEMS to NEMS induces some electrical detection issues when using a direct measurement (without any heterodyning strategy), such as (i) high
Nanoelectromechanical systems (NEMS) constitute a very promising research field both in terms of scientific interest— for instance, for the observation of quantum mechanics phenomena—and from the practical applications point of view. NEMS may enable novel breakthrough applications, mainly in the field of chemical analysis [1, 2], life science [3–5] and computing [6, 7]. In those first two fields (gas sensing and mass spectrometry, respectively), they appear as excellent candidates since they exhibit unprecedented mass resolution, thanks to their inherent low mass, high resonance frequency and good frequency stability. In both fields, recent works [2, 4], have not only confirmed their intrinsic exceptional sensing potential but have also illustrated their increasing maturity as they have been successfully integrated into complex systems. Nevertheless, these new applications usually involve NEMS arrays with a rather large number of electrical contacts per device, making their individual addressing complex. In this context, much work has been devoted to the determination of the best electromechanical transduction scheme at these scales; most of the realizations reported in the literature, which are generally in the 10–100 MHz frequency range, rely on capacitive detection 0957-4484/14/435501+08$33.00
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In this context, this work reports on a number of novelties relative to the literature. It is the first demonstration of single-crystal silicon, piezoresistive NEMS resonators monolithically integrated at the front-end with a CMOS readout circuitry (see figure 1). The NEMS resonator itself has a very high resonance frequency (100 MHz) and is presumably the smallest top-down resonator ever reported (20 nm thick with a 1.2 μm long and 100 nm wide beam, and two lateral 100 nm long and 30 nm wide nanowire gauges). Another remarkable feature is the fact that the CMOS integration has enabled the direct (homodyne) measurement of the 100 MHz frequency response, whereas most realizations rely on more complex heterodyne schemes [23–25], that use bulky off-chip electronics. This work strongly illustrates how NEMS and CMOS are converging in terms of dimensions and processes and how their association can be synergetic to pave the way for major applicative breakthroughs such as mass spectrometry for biomedical analysis and for NEMS-based computing. By combining advanced FDSOI CMOS technology (which is now competing [26, 27], with bulk FinFET for ultimate sub-28 nm CMOS nodes) with very high-frequency NEMS resonators, these NEMS-CMOS devices have risen to the cutting edge of nanoelectronics and nanomechanics. In conclusion, the purpose of this work is to provide a highresolution sensing system for bio- and chemical applications. This demonstrator relies on a pre-CMOS integration approach in which NEMS resonators are fabricated before the CMOS process and released after completing it. The implemented CMOS technology is an advanced FDSOI process [28], developed at CEA-LETI. It is also much easier to get suspended, mobile NEMS structures using an SOI CMOS process instead of a bulk CMOS one. The technological convergence between advanced NEMS, such as the crossbeams reported in this paper, and advanced CMOS technologies in terms of the device’s size and processing tools is noteworthy. In this work, NEMS and the transistors’ channels are both fabricated on the same single-crystal Si layer (SOI top layer), though not simultaneously, by applying the same hybrid DUV/e-beam lithography technique and the same plasma dry etching process. NEMS and their associated amplifying circuitry already have equivalent sizes, with a joint area of about 10 × 20 μm2. In the near future, one can anticipate the demonstration of ultra-dense NEMS-CMOS arrays or ‘imagers’ with 5 × 5 μm2 pixels. The technological process flow starts by doping the 40 nm thick top SOI layer by implantation (1019 at cm−3 of boron, leading to a sheet resistivity of 9 mΩ cm; the value is measured by the van der Pauw method). This step is only performed on NEMS-dedicated areas; the CMOS areas are protected by resist. After patterning the NEMS, they are released by removing the SiO2 sacrificial layer with a stictionfree, vapor-HF (hydrofluoric acid) isotropic etching. Then, the free-standing NEMS are partially oxidized at 1100 °C (figure 2(a)) and encapsulated with both a PolySi sacrificial layer and a high-temperature oxide (HTO) (figure 2(b)). After locally opening this protection layer, the CMOS transistors are fabricated at the front-end level. The back-end levels on both the CMOS transistors and NEMS are performed at the
Figure 1. Tilted SEM images of the NEMS-CMOS device with an
increasing zoom-in: (a) global overview, with the FDSOI CMOS readout circuit on the left and the NEMS area on the right; (b) crosssectional zoom-in (AA’ view) of the NEMS resonator and its electrodes, above which the dielectric passivation layers were locally etched; (c) zoom-in of the NEMS resonator and its nanogauges, inset: TEM cross-section of a nanogauge and its related dimensions.
connection losses due to pad and cable capacitances (their high-resonance frequency, which generally ranges from 1 to 100 MHz, are far higher than the cutoff frequency of the output parasitic low-pass filter) and (ii) an increase of feedthrough parasitic coupling between the resonator input and output. Both issues result in a degradation of the signal-tobackground ratio and possibly of the signal-to-noise ratio. Cointegrating NEMS resonators with a dedicated adjacent CMOS readout and conditioning electronics is a smart and very efficient way to overcome these difficulties since NEMSCMOS devices potentially benefit from a better signal-tonoise ratio (SNR), immunity to external parasitics, higher density and compactness and possibly a lower cost. The integration of NEMS resonators with a standard CMOS technology has already been demonstrated, but in all past examples, either the NEMS resonators were made from other materials as opposed to single-crystal Si (such as poly-Si [20] or back-end metal layers [21]), or the device size was too large to provide a high frequency and sufficiently high mass sensitivities [22]. In summary, ultra-small NEMS devices with a co-integrated readout under the form of compact pixels were yet to be developed. 2
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(1 zg = 10−21 g). The beam has two lateral piezoresistive nanogauges (100 nm long, 30 nm wide; see inset of figure 1(c)) located close to the beam clamping point, which provide an integrated electromechanical transduction of the beam’s mechanical motion (see the left part of figure 3(c)). In this configuration, since both gauges are nominally identical, they exhibit resistance variations ΔR of the same magnitude but of the opposite sign while the beam is under lateral displacement. By considering the clamping point as the NEMS output node VOUT_NEMS, the detection scheme therefore constitutes a half Wheatstone bridge and VOUT_NEMS is given by (see supporting information): VOUT _ NEMS =
( VB2 − VB1)
(
2 + jCL ω 2R ba + R ga + R
)
⎛ ΔR ⎞ 1 ⎟ ×⎜ ⎝ R ⎠ 1 + R ga / R
(1)
whereby R is the gauges’ resistance, assuming identical gauge resistances (RJ = R1 = R2); Rba and Rga correspond, respectively, to the resistance of the silicon beam anchor and to the resistance of the silicon gauges anchor (see Supplementary Information). VB2 and VB1 are the DC bias voltages of the two gauges (see figure 3). ω is the pulsation of the mechanical motion, and CL the parasitic capacitance of the electrical interconnection between NEMS and CMOS. Since the beam and its electrodes are symmetrical with respect to the cantilever main axis, the feedthrough signals between the electrodes and the VOUT_NEMS node are potentially cancelled out on that node by differentially actuating the resonator. In practice, VACT1 and VACT2 have the same DC component and same AC magnitude, but their AC signals are in counterphase. The relative resistance variation at resonance is given by [11]:
Figure 2. Cross-sectional schematic views of the main technological process steps realized on thin SOI wafers (20 nm thick top Si layer); (a) after NEMS patterning, release and thermal oxidation; (b) after [Polysilicon + HTO] encapsulation and before the beginning of the CMOS process; (c) final view of the device after completion of the CMOS process, dry etching of the passivation dielectrics over the NEMS resonators and dry etching of the encapsulating polysilicon to release the mobile structures. It is noteworthy that the top Si layer of such SOI wafers serves as both the NEMS structural layer in dedicated NEMS areas and as transistor channels in CMOS circuits areas; (d) corresponding SEM image of the NEMS area and its related electrical contacts, which interconnect the NEMS to its CMOS readout circuit.
ΔR G αQFACT = π 44 σ J = R E AJ
same time. In this first demonstrator, the minimum feature size for the channel has been set to 300 nm to secure the circuit’s performance. After completion of the CMOS process, intermetallic oxides are etched above the NEMS down to the PolySi encapsulation layer, which is subsequently chemically removed with a CF4-based gas (figures 2(c) and (d)). This process is highly selective since the surrounding SiO2 is etched at a speed 70 times slower than PolySi. By using this approach, intermetallic passivation layers are not degraded during the release and do not require a protection layer, as usually required in N/MEMS-CMOS monolithic integration. More details on the process flow can be found in [29]. The hybrid NEMS-CMOS device (figures 1 and 3) reported in this paper first consists of a 100 MHz nanomechanical resonator. This so-called ‘crossbeam’ [11] (figure 1(c)) is an electrostatically actuated, p-type silicon cantilever beam (1.2 μm long, 100 nm wide, 20 nm thick) operated on its first in-plane flexural mode. With such dimensions, crossbeams resonate around 100 MHz and feature an outstanding mass sensitivity S of ∼0.1 zgHz−1
(2)
whereby AJ is the nanogauges’ cross-sectional area (30 × 20 nm2), Q is the resonator quality factor, α is a unitless factor that accounts for the mechanical lever effect between the equivalent point of the application of FACT and the nanogauges’ position along the beam, and FACT is the applied electrostatic force on the resonator. This force is proportional to the DC and AC components of VACT, VACT_DC and VACT_AC, respectively. The p-type silicon substrate is oriented to get the highest piezoresistive coefficient, i.e. G (for example, [30]). The resonator is designed so that all of the parameters of equation (2) are optimized to maximize ΔR/R: (i) the optimum substrate orientation and type (p) for piezoresistive detection, (ii) the optimum location of the gauges along the beam to increase the lever effect while ensuring a proper orthogonal transmission of the stress and (iii) very aggressive dimensions. The actuation gap is 60 nm, enabling large electrostatic forces with relatively low actuation voltages; since the gauge’s cross-section is small, the induced stress is highly concentrated and magnified. The gauge factor G of such silicon nanowires was measured 3
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Figure 3. (a) SEM picture of the crossbeam NEMS device and its lead silicon resistors; (b) image of the resonance mode of interest (first inplane flexural one) obtained by COMSOL simulation; (c) equivalent electrical schematic of the monolithically integrated NEMS-CMOS device (black squares are the contact pads). The NEMS resonator is polarized by VACT1 and VACT2 and by VB1 and VB2, respectively, the actuation voltages (AC + DC), and the DC bias voltages of the two nanogauges. Nominally, both gauges are identical, i.e. R1 = R2 and ΔR1 = ΔR2. The 10-transistor CMOS readout circuit contains an amplifying stage (M0–M1) DC biased by VAMP and a buffer (M4–M9) DC biased by VDB1, VDB2 and VDB3. The buffer input is adequately DC biased, thanks to a dedicated stage (CDC, M2–M3). The supply voltages (VSS/VDD) are 0 and 1.2 V, respectively. The circuit is designed so that it can load 100 pF at 100 MHz at its output (VOUT). The key parameter is the extremely low connection capacitance CL (∼20 fF) between the NEMS resonator output and the circuit input. As a consequence, the NEMS homodyne response is not attenuated by the low-pass RC filter at the NEMS output; that usually dramatically degrades the response in classical configurations (stand-alone NEMS chip with off-chip readout electronics).
elsewhere [31] at 89. In this case, the lever arm coefficient α is 1.8. For more information on the crossbeam’s design and features, please refer to the Supporting Information. The output node VOUT_NEMS of the nanomechanical resonator is interconnected to an adjacent CMOS circuit; the circuit’s equivalent electrical schematic is depicted in figure 3. Beyond its capability of signal amplification, the main function of this circuit is to adequately interface the NEMS with external electronics by providing efficient impedance matching. The monolithic integration inherently induces a high impedance at the NEMS output as the interconnection capacitance CL is drastically decreased (down to a few tens of fF, which is two orders of magnitude less than in stand-alone devices) such that R + Rga + 2Rba CL ω ≪ 2 (see equation (1)). Consequently, VOUT_NEMS is no longer attenuated by the parasitic RCL low-pass filter at the NEMS output, hence:
(
VOUT _ NEMS =
transistors, is a high input impedance, low-gain (∼1) voltage amplifier. The second one, which comprises M2, M3 and CDC, a decoupling capacitor, constitutes the DC biasing stage of the third block (M4–M9), which is a three-stage buffer to adapt the impedance between the amplifier output and the large capacitances at the buffer output (10 to 100 pF) that arises from bonding pads, wires and connection cables. The typical frequency responses of the buffer and the ensemble buffer + amplifier have been shown in [32]. A very relevant advantage provided by this overall impedance matching of the NEMS with the outside world is that the direct measurement of the resonator is enabled; the electrical signal has the same frequency all along the chain, which simplifies the architecture of the readout circuit. In comparison, the quasiexclusivity of the realizations reported in the literature on the operation of high-frequency NEMS resonators (>20 MHz) is based on implementing heterodyne (mostly downmixing, [11, 23, 25]) detection schemes that require much more complex set-ups and the use of specific functions, such as frequency mixing, that are not straightforward to miniaturize and implement in analog CMOS circuitry. Among the very few other examples of homodyne (direct) detection of VHF resonators, the transduction scheme presented in [19] and related works—thermal actuation and piezoresistive detection
)
VB2 − VB1 ⎛ ΔR ⎞ 1 ⎜ ⎟ . ⎝ ⎠ 2 R 1 + R ga / R
(3)
The CMOS readout circuit is designed to load 100 pF at 100 MHz and contains ten transistors distributed in three blocks (see the transistors’ dimensions in the supporting information). The first one, which comprises the M0 and M1 4
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Figure 4. Homodyne open-loop electrical responses (VOUT node, see figure 3) of a 100 MHz CMOS-integrated crossbeam NEMS resonator (20 nm thick, 1.2 μm long and 100 nm wide beam; the nanogauges’ length and width: 100 and 30 nm). These results were obtained in vacuum for an outstandingly low AC actuation voltage (VACT_AC) magnitude of 2.25 mV rms. (a) Variations of the magnitude response (linear scale) with VACT_DC; (b) Same data as in (a) but with the phase response; (c) Same data as in (a), but the magnitude response is represented in terms of s21 (in dB, log scale) [s21 = 20 log(VOUT/VACT_AC)]; In (a), (b) and (c), the gauges’ DC bias is VB1 − VB2 = 4 V.
—seems a relevant alternative. However [19], involves much bigger devices (three and four decades bigger in area and volume, respectively, than this work); therefore, both works cannot be directly compared. Another feature of the NEMSCMOS device is the very small size of the NEMS + amplifier cell (about 10 × 20 μm2). Although the biasing stage and the buffer represent an area of about 30 × 90 μm2, the amplifier is very compact (10 × 10 μm2), and there is room for further size reductions. This paves the way for ultra-dense arrays of individually addressed NEMS, which will be required in emerging applications such as NEMS-based mass spectrometry. In such cases, each NEMS would be associated to its own amplifier with a buffer that is common to all NEMS. The electrical response of such 100 MHz NEMS-CMOS devices is measured using a direct (homodyne) detection scheme. The resonator is differentially actuated with VACT1 and VACT2, which have equal DC components (see figure 3). Their AC components are in counter-phase, and their magnitudes are finely tuned (one with respect to the other) in order to cancel out the feedthrough signals on the VOUT_NEMS node. Without this operation, we experimentally observe a remnant background signal on VOUT that may completely hide the resonance signal. In most of the tested devices, a 1 dB difference is required to obtain the best balance. If the exact origin of this mismatch is difficult to retrieve, slight
asymmetries in-between the on-chip routing of VACT1 and VACT2 or in-between their respective bonding wires with respect to the cantilever main axis are probably enough to generate this effect at a high frequency (100 MHz). VB1 and VB2 are adequately tuned so that the gate of M1 is DC biased at VOUT_NEMS_DC = 0.7 V, as required for optimum operation of the circuit (M1 was operated in a saturation regime). The effective DC actuation voltage VACT_DC_EFF applied on the resonator is then (VACT_DC − 0.7) V. The CMOS circuit is supplied with 1.2 V and is DC polarized with VAMP = 0.27 V, VDB1 = 0.6 V, VDB2 = 0.5 V and VDB3 = 0.75 V. The frequency response (VOUT) of a CMOS-integrated crossbeam measured in vacuum (10−4 mBar) is depicted in figure 4. After biasing the gauges with VB1 = 2.7 V and VB2 = −1.3 V (i.e. VB1 − VB2 = 4 V), the measured response is plotted in terms of magnitude and phases in figures 4(a) and (b), respectively, for three values of DC actuation voltage VACT_DC (3, 4 and 5 V); the well-known spring-softening effect [33] consequently induces the three resonance frequency peaks. The quality factor Q in a vacuum is estimated at ∼2300 in all of the cases (Q ∼ 400 in air). These peaks feature an excellent signal-tobackground ratio (SBR) and signal-to-noise ratio (SNR), although they are obtained using a very low AC actuation voltage (VACT_AC of 2.25 mV rms), and the gauges’ DC bias and DC actuation voltages are smaller than 5 V. The 5
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frequency response in a linear scale is presented in figure 4(a). For VACT_DC = 3 V, the output voltage VOUT_NEMS at resonance is 38 μV, with a background close to 3 μV; for VACT_DC = 4 V, VOUT_NEMS = 50 μV; and for VACT_DC = 5 V, VOUT_NEMS = 60 μV with a background of ∼4 μV. The experimental magnitudes are close to the expected values estimated by equation (3): 42 μV, 60 μV and 78 μV, which correspond to the cantilever displacements of 0.3 nm, 0.45 nm and 0.6 nm for 3, 4 and 5 V, respectively (see supporting information for more details). Figure 4(c) is an s21 (transmission gain) plot of the same data; the on-resonance gain is as high as −32 db. This result confirms the high electromechanical transduction efficiency of electrostatic actuation based on ultra-thin gaps coupled to monolithic piezoresistive/ CMOS detection. In this first demonstration, the circuit gain is close to one, but a slightly more complex circuit topology would have undoubtedly provided an s21 gain equal or larger than 0 dB, thus enabling the implementation of fully integrated VHF NEMS-CMOS oscillators. The differential actuation with independently tunable AC amplitudes allows balancing of the feedthrough signals and the ability to reach SBR larger than 25 dB at VACT_DC = 5 V. Considering the ultimate dimensions of the resonator, which is presumably the smallest one ever reported using top-down technologies, and its very high operating frequency that usually implies small motional signals and high parasitic coupling, these electrical results are outstanding. In fine, one of the main motivations guiding the development of such monolithic NEMS-CMOS devices is to make resonance frequency-sensitive sensors, with their frequency stability δf/f0 as a key feature. A usual and convenient method to estimate this frequency stability is to measure the Allan deviation [35] σA of the output signal versus integration time τ. For narrow integration bandwidths (1/τ) compared to the resonance frequency f0, and when white noise processes dominate, σA follows the τ−1/2 power law, given by: σ A (τ ) =
1 1/ τ . 2Q SNR
Figure 5. Frequency stability of the NEMS-CMOS output response
(VOUT) measured in terms of the Allan deviation versus integration time τ. The τ−1/2 slope between τ = 1 ms and 500 ms indicates that the dominant source of noise over this range is a white noise process (see supporting information).
limit of detection (LOD) at τ = 500 ms is then around 2 zg, i.e. ∼1.3 kDa (this number is the computation of the theoretical mass sensitivity and the experimentally measured noise data; it still needs to be confirmed in mass detection experiments). This represents a mass sensor range (between the LOD and 1% of the NEMS mass, which is the upper limit for a linear response, i.e. ∼0.1 fg) of more than four orders of magnitude with major applications in biology. This would indeed allow some bio-samples, such as human proteins or protein-based bacteria, to be analyzed cell by cell. In practice, NEMS-based gas or mass sensors require tracking the resonance frequency in real-time in order to capture events of short durations (possibly down to a few tens of ms). This specification is only achieved by operating NEMS in a closed-loop. Therefore, as a first step toward implementing such NEMS-CMOS devices as sensors, figure 6 details how they have been embedded in a homemade, digital phase-locked loop (PLL), which potentially responds faster than one ms. This off-chip, software-based PLL sequentially contains a phase comparator, a corrector using the ‘H-infinity’ approach [36], a voltage-controlled oscillator (VCO) and a phase shifter to differentially actuate the resonator. Relying on the spring-softening effect, the DC actuation voltage VACT_DC is varied on purpose in figure 6(b) (between 4.4 and 4.6 V with a steady value of 4.5 V) to exemplify the real-time monitoring of resonance frequency variations. In conclusion, this work demonstrates unique monolithic NEMS-CMOS devices with an outstanding electrical response despite the ultra-challenging context (very highfrequency and ultimate NEMS dimensions). The reported fabrication process allows combining NEMS resonators with an advanced FDSOI CMOS process, which is now the main alternative to FinFET CMOS technology—the mainstream approach to extend Moore’s law beyond the 28 nm node. This monolithic device constitutes the single demonstration in the literature, both of the direct (homodyne) measurement of
(4)
The Allan deviation was measured on our samples in an open-loop configuration by recording the phase noise with a lock-in amplifier. It is depicted in figure 5 with integration times τ in a range of practical use for most sensing applications (between 1 ms and 1 s). In agreement with equation (4), σA follows the τ−1/2 power law up to approximately 500 ms, indicating that white noise processes dominate down to σA = 2 × 10−7. Above 500 ms, the Allan deviation inflects due to the prevalence of colored noises (in 1/fα). For τ = 500 ms, the thermomechanical noise is estimated at 117 nV (√Hz−1), which corresponds to 1.33 × 10−13 m (√Hz−1) (or a force noise of 1.9 × 10−16 N (√Hz−1). The Johnson noise and the readout electronics’ noise (referred to as its input) are evaluated at 46 nV (√Hz−1) and 10 nV (√Hz−1), respectively. Therefore, the thermomechanical noise is the dominant process in our case (see supporting information for more information). Since the NEMS device of this work has a mass of 6 fg (i.e. a theoretical mass sensitivity of 0.12 zg Hz−1), its 6
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also acknowledge Antoine Niel and Jérémie Ruellan for their punctual support on the experimental characterization.
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Figure 6. Demonstration of a closed-loop operation of this 100 MHz NEMS-CMOS device; the CMOS-integrated resonator is embedded in a PLL so that it continuously oscillates at its resonance frequency (f0). This PLL would constitute the main building block of a NEMSbased, resonance-frequency sensitive sensor that would monitor realtime variations of f0. (a) The PLL chain is located off-chip and controlled via a LabVIEW interface. (b) PLL-based real-time monitoring of f0 over a 150 s timeframe (PLL sampling time of 100 ms and a corrector bandwidth of 1 s), with VACT_AC = the magnitude of 2.85 mV rms and (VB1 − VB2) = 4 V. The resonance frequency is sequentially modified by varying VACT_DC.
VHF (100 MHz) NEMS and of CMOS-integrated NEMS, made of single-crystal Si and with PZR detection. This integration scheme results in a frequency stability (measured in terms of Allan deviation) of 2 × 10−7, which is equivalent to a theoretical limit of mass detection of 1.3 kDa. In practice, this means that NEMS constitute promising analyzers to implement a new generation of mass spectrometers for bio-sample analysis.
Acknowledgments We gratefully acknowledge the financial support from the European Commission through the FP7 NEMSIC project. We 7
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