Abstract:
A junctionless Nano-Electro-Mechanical (NEM) resonator, comprising a highly doped conductive channel connecting a drain region and a source region; the conduction channel region is movable and the overall structure is fixed at least at these two ends placed on acting the source and drain regions, respectively; at least one fixed gate electrode arranged to control a depletion charge in the highly doped conductive channel thereby modulating dimensions of a cross-section of the highly doped conductive channel. A dimension of the cross-section in the direction of an electrical field that is oriented from the fixed gate electrode to the highly doped conductive channel, is designed in such a way that it can be reduced under the effect of the depletion charge such that a full depletion in the highly doped conductive channel is achievable with the control of the fixed gate electrode.

Description:
This application is the U.S. national phase of International Application No. PCT/IB2013/053113 filed 19 Apr. 2013 which designated the U.S. and claims priority to EP Patent Application No. 12164756.4 filed 19 Apr. 2012, the entire contents of each of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention concerns the field of integrated transduction of nano-electro-mechanical resonators by means of junctionless field effect transistor(s). 
     BACKGROUND OF THE INVENTION 
     In the past decade, Nano-ElectroMechanical Systems (NEMS) have been gaining increasing attention for their superb ability to detect mass and force on the atomic scale. 1-3  The development of sensors based on large-scale integrated NEMS is likely to open up a broad spectrum of applications in science and technology and revolutionize a range of fields from mass spectrometry 4  to biomedical diagnostics. 5  In the present specification, a novel transduction principle in a silicon nanowire electromechanical resonator is shown, which exploits the depletion charge modulation in the self-aligned, junctionless Field Effect Transistor (FET) body as an intrinsic displacement transducer. A mechanical resonance at the very high frequency of 226 MHz is detected in the drain current of the highly doped silicon wire with a cross section of 28×35 nm 2 . The signal gain and tunability inherent to this device can be harnessed to build nano-oscillators, 6  which can be integrated to high densities in silicon-on-insulator (SOI) complementary metal-oxide semiconductor (CMOS) conventional technology and therefore offer unique opportunities for NEMS-based sensor and signal processing systems hybridized with CMOS circuitry on a single chip. 
     All existing NEMS are based on a mechanical transducer, i.e., an input and output element that converts a form of energy into mechanical motion, and vice versa. Numerous mechanisms have been introduced, including electrostatic, 7  electromagnetic, 8  piezoelectric 9  or optical 10  schemes, among others, which can be combined for mechanical actuation and motion detection. In the last decade, mechanical resonators have undergone a continuous reduction in dimensions, reaching molecular levels in the form of carbon nanotubes or graphene 11-12  One reason for this development is that NEMS, because of their inherent properties as mechanical sensors, tremendously benefit from size reduction. 13  The detection of mass and force in the attogram (10 −18  g)— and attonewton (10 −18 N)— range, respectively, has been repeatedly demonstrated 1, 2, 14  To unfold the full potential of these resonators, fabricating and controlling a very large ensemble of NEMS that comprise tens of thousands of resonators, is necessary. Large area technologies that enable the parallel processing of mass information have a great impact on the development in several fields, 15  such as system biology, where the parallel operation of millions of FET-based sensors recently enabled non-optical genome sequencing on-chip. 16  In terms of NEMS, these requirements severely limit the choice of material and of the type of mechanical transducer. Silicon technology remains therefore a promising avenue to follow for NEMS-based systems targeting a high level of integration and complexity. The piezoresistive effect in silicon has been exploited in nanowire resonators operating at very high operating frequencies. However, transduction schemes employed therein required a detection circuitry involving frequency generation at twice the resonator&#39;s natural frequency, 17  or a complex modal shape design of so-called crossbar cantilevers. 18    
     SUMMARY OF INVENTION 
     The present description addresses the creation of electromechanical resonators in form of highly doped, suspended silicon nanowires that exploit the intrinsic gain in a junctionless FET to transduce mechanical motion up to very high frequencies (up to a few gigahertz), such as for example 224 MHz demonstrated in the experimental on-chip prototype. 
     Accordingly, the present invention concerns a mechanical resonator having at least a highly doped moving part which is fixed by at least two ends, at least one electrode to control the depletion charge in said moving part, whereby the size of the doped part of the moving part in the direction of the electrical field is reduced to such dimension that full depletion is achievable with the action of said electrode. 
     In a first aspect the invention provides a junctionless Nano-Electro-Mechanical (NEM) resonator, comprising a highly doped conductive channel connecting a drain and a source electrode and movably fixed by at least two ends acting as said source and drain electrodes, respectively; at least one fixed gate electrode arranged to control a depletion charge in the highly doped conductive channel thereby modulating dimensions of a cross-section of the highly doped conductive channel. A dimension of the cross-section in the direction of an electrical field that is oriented from the fixed gate electrode to the highly doped conductive channel, is designed in such a way that it can be reduced under the effect of the depletion charge such that a full depletion in the highly doped conductive channel is achievable with the control of the fixed gate electrode. 
     In a first preferred embodiment the highly doped conductive channel is one out of the following list: a nano-beam, a nano-wire, a nano-plate, a nano-disk, a nano-membrane. 
     In a second preferred embodiment the highly doped conductive channel or the at least one gate electrode comprises a thermal silicon oxide layer positioned at an interface between the highly doped conductive channel and the at least one fixed gate electrode. 
     In a third preferred embodiment, a volume between the highly doped conductive channel and the at least one fixed gate electrode is partially or totally filled by the thermal silicon oxide layer. 
     In a fourth preferred embodiment, the highly doped conductive channel and/or the at least one gate comprise a surface that was treated through thermal oxidation for passivation, temperature compensation or functionalization. 
     In a fifth preferred embodiment, the junctionless NEM resonator further comprises at least an additional highly doped conductive channel, wherein the highly doped conductive channel and the least one additional highly doped conductive channel are mechanically coupled to each other through elastic fixations. 
     In a sixth preferred embodiment, the at least one fixed gate electrode gate comprises two fixed gates electrodes arranged to independently couple to the highly doped conductive channel. 
     In a seventh preferred embodiment, the at least one gate electrode is made of one out of the following list: highly doped p+ single crystalline silicon, highly doped n+ single crystalline silicon, other semiconductor material, poly-silicon, metal silicide, a metal. 
     In an eighth preferred embodiment, the highly doped conductive channel has a doping concentration of substantially equal to or greater than 2×10 18  cm −3 . 
     In a ninth preferred embodiment, the dimension of the cross-section in the direction of the electrical field that is oriented from the fixed gate electrode to the highly doped conductive channel, in an absence of a depletion charge is substantially 35 nm. 
     In a tenth preferred embodiment, the highly doped conductive channel is positioned over a bulk silicon substrate. 
     In an eleventh preferred embodiment, the highly doped conductive channel is positioned over an electrically isolated substrate. 
     In a second aspect the invention provides a junctionless Nano-Electro-Mechanical (NEM) mixer-filter comprising at least one junctionless NEM resonator wherein a filter shape is given by a mechanical spectrum of the junctionless NEM resonator and wherein the control of the depletion charge in the highly conductive channel generates a current-voltage nonlinearity upon which a mixing of a first signal and a second signals of different frequency is made possible. The first signal (RF) and the second signal (LO) are applied according to
         (i) the first signal (RF) on the gate, the second signal (LO) on the drain (source)   (ii) the first signal (RF) and the second signal (LO) to the gate,       

     A DC bias at the drain (source) is zero in case of a resistive mixing or non-zero is case of an active mixing. 
     In a third aspect the invention provides a junctionless Nano-Electro-Mechanical (NEM) frequency demodulator comprising at least one junctionless NEM resonator wherein a frequency-modulated carrier signal is demodulated and a based-band is recovered via a combination of the mechanical spectral response of the NEM resonator with the said depletion-charge induced current-voltage nonlinearity in the highly doped conductive channel. 
     In a fourth aspect the invention provides a junctionless Nano-Electro-Mechanical (NEM) resonator phase-locked loop comprising at least one junctionless NEM resonator, wherein the property of the junctionless NEM as mixer-filter or as frequency demodulator is used to generate a negative feedback signal with improved signal-to-noise and to realize a phase-locked loop tracking the NEM mechanical oscillation in real-time. 
     In a fifth aspect the invention provides a junctionless Nano-Electro-Mechanical (NEM) electromechanical oscillator comprising at least one junctionless NEM resonator, wherein a frequency determining element is given by a junctionless NEM resonator mechanical design, and wherein the gate controls the depletion charge and so the current in the highly conductive channel, thereby generating an intrinsic voltage or transconductance amplification which is used to partially sustain the mechanical oscillation in a self-sustained closed loop configuration, or fully sustain the mechanical oscillation in a self-oscillating configuration. 
     In a sixth aspect the invention provides a junctionless Nano-Electro-Mechanical (NEM) resonator sensor comprising at least one junctionless NEM resonator, wherein the physical quantity to be measured is displacement, mass, acceleration, torque, heat, temperature, pressure, or else, wherein the dependency on the frequency, the quality factor or the output signal of the junctionless NEM resonator is known, wherein the said intrinsic signal amplification is exploited to enhance the sensor&#39;s output signal-to-noise ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood through the description of preferred embodiments and in reference to the figures, wherein: 
         FIG. 1  contains a three dimensional view of an example embodiment of a device according to the invention; 
         FIG. 2  contains a top view of the possible architecture of the junctionless NEM resonator; 
         FIG. 3  contains a schematic cross-sectional illustration of a basic operating principle; 
         FIG. 4  contains a further schematic cross-sectional illustration of a basic operating principle; 
         FIG. 5  contains still a further schematic cross-sectional illustration of a basic operating principle; 
         FIG. 6  contains a schematic cross-sectional illustration with more details about possible configurations; 
         FIG. 7  shows two examples of a modal shape design; 
         FIG. 8  shows a possible configuration of a junctionless NEM resonator with a number of highly doped conductive channels ( 4  in this figure) and a number of gate electrodes ( 4  in this figure); 
         FIG. 9  shows an example of a closed loop implementation; and 
         FIG. 10  shows a further example of a closed loop implementation. 
         FIG. 11  contains a SEM view of a fabricated flexible air-gap capacitors are used to couple two independent gate electrodes with a nanowire resonator; 
         FIG. 12  shows a TEM cross-section of a single silicon body according to an example embodiment of the invention; 
         FIGS. 13 a  and 13 b    contains plots of static characteristics of a junctionless nano-electromechanical resonator according to an example embodiment of the invention; 
         FIG. 14 a    contains a plot of resonant characteristics of a junctionless nano-electromechanical resonator with 1.7 μm in length; 
         FIG. 14 b    show plots of transistor transconductance versus detected current; 
         FIG. 14 c    contains a plot of detected current signal versus input power; 
         FIG. 15  contains a plot depicting resonant characteristics of a junctionless nano-electromechanical resonator with 1.0 μm in length, 
         FIG. 16  shows an array of a number of junctionless NEM resonators. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A novel transduction principle in a silicon nanowire electromechanical resonator is obtained by exploiting the depletion charge modulation in a self-aligned, junctionless transistor as an intrinsic displacement transducer. A mechanical resonance at the very high frequency of 226 MHz is detected in the drain current of the highly doped silicon wire with a cross-section of only 28×35 nm 2 . In contrast, the transduction mechanism proposed in the present application implies:
     (i) a fundamental simplification in the detection of the mechanical resonance of truly nano-scale, highly doped silicon resonators. So far, piezoresistance in silicon nanowires has been utilized to transduce rather lightly doped mechanical resonators, which required detection circuitry involving frequency generation at twice the resonator&#39;s natural frequency. The trade-off between efficient piezoresistive effect (low doping concentration) and good conductance (high doping concentration) is avoided. The doping concentration can be chosen arbitrarily high, in contrast to reference §C. Whether the junctionless transduction principle is applicable depends on resonator dimensions only. The junctionless principle is the same as described in references §A, §B, however, with focus on creating a transistor, whereas the focus here lays on creating a transduction principle for a mechanical resonator;   (ii) a fundamental simplification in the fabrication process, as the process is self-aligning and does not involve the formation of a semiconductor junction, when compared to any mechanical resonator involving the formation of a suspended transistor body or of a semiconductor junction reported to date, in contrast to reference §C. The junctionless structure eliminates the effect of junction diffusion, as described in references §A, §B, however, here with focus on a resonator, in contrast to references §A, §B. This greatly improves the thermal budget available during the parallel CMOS process on-chip;   (iii) full transistor functionality implemented in a nanowire mechanical resonator at scales below 50 nm. The resonator maintains high tunability, e.g., with respect to signal gain, motional impedance or level of power consumption. The transduction principle is not limited by further dimensional reduction, following a similar argumentation as in references §A, B, but opposed to reference §C, where the formation of a junction, of whatever type, faces limitation of engineering nature (fabrication) and fundamental nature (doping diffusion, doping fluctuation);   (iv) the signal processing based on the transistor can be utilized to provide a feedback mechanism when embedding silicon nanowire resonators in closed loops. The small-signal transistor gain can be harnessed to compensate for mechanical or other losses and simplify sustaining electronics. Heterodyne and homo-dyne mixing of signals can be used to provide a low noise feedback signal to a reference oscillator and allows implementation of frequency tracking loops;   (v) this type of nano-electromechanical system is integrated in silicon-on-insulator (SOI) complementary metal-oxide semiconductor (CMOS) conventional technology, which offers unique opportunities for hybridization with CMOS circuitry on a single chip. It can be therefore used as fundamental unit to build oscillators with very low power consumption, which can be arranged in dense arrays reaching attogram mass resolution—a range highly attractive for miniaturized environmental gas-sensor and neutral species mass-spectrometry systems.   

     REFERENCES §A, §B AND §C MENTIONED ABOVE 
     
         
         [A] EP2161755 A1, Junctionless Metal-Oxide-Semiconductor Transistor, Inventor: COLIGNE JEAN-PIERRE 
         [B] WO2011101463 A1, A Transistor Device, Inventor: COLINGE JEAN-PIERRE 
         [C] WO2010058351 A1, Active Multi Gate Micro-electro-mechanical device with built-in transistor, Inventor: IONESCU MIHAI ADRIAN [CH]; GROGG DANIEL [CH] 
       
    
     In the present specification, the notion of fixed (as in “a moving part which is fixed by at least two ends”) implies a mechanically elastic fixation, which can be for example: free, guided, pinned, clamped, anchored etc. 
     Further, in the present specification, the control (as in “one electrode to control the depletion charge in said moving part”) refers to the charge control within the silicon volume via the electrostatic field effect. 
     A junctionless field effect transistor has been proposed as a digital switch by Colinge et al. 19  suitable for addressing the scaling challenges of multi-gate (nanowire) transistors that arise in terms of engineering super-abrupt junction profiles for high performance FETs on nanometer-thin films. Such devices are highly doped and the ON-state is characterized by a conduction channel in the entire silicon body; by applying a gate bias, the conduction channel can be depleted, and eventually pinch-off the conduction path (OFF-state). According to Colinge et al. 19  the values of high doping in junctionless transistors range from a few 10 18  cm −3  to a few 101 9  cm −3 . This type of transistor has never been proposed as electro-mechanical transducer. 
     In the present description the term high doping is meant to refer to the range of a few 10 18  cm −3  to a few 10 19  cm −3 . 
       FIG. 1  presents an exemplary and schematic illustration of the device according to the invention and its operating principle. In the static regime, the drain current in a junctionless transistor consisting of a highly n-doped nanowire body with lateral gates is given by the expression: 
                     I   D     =     q   ⁢           ⁢   μ   ⁢           ⁢     N   D     ⁢         t   Si     ⁡     (       W   Si     -     2   ⁢           ⁢       W   dep     ⁡     (     V   G     )           )       L     ⁢     V   D               (     eq   .           ⁢   1     )               
where W Si  is the body (lateral) width, N D  the channel doping concentration, t Si  the channel thickness and L the channel lengths. The depletion width W dep  is controlled by the gate voltage and varies at mechanical resonance, thereby modulating the drain current. This is in total contrast with the previously reported resonant body and resonant gate FET, 20-23  where the carrier density in inversion or accumulation layers was modulated to create a low resistivity path in a high resistivity channel region. The transconductance of the junctionless transistor can be then derived as:
 
     
       
         
           
             
               
                 
                   
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     The electromechanical current modulation due to the field effect is composed of
     (i) the modulation of the depletion charge that results from applying an a.c. voltage and maintaining a constant gap, and   (ii) its modulation due to the time-varying gap under constant gate voltage.   

     The total current modulation in the FET in linear operation can be expressed, without loss of generality, as: 11,21   
                     i   FET     =       ∂     I   D       ≈       g   m     ⁡     (         v   ~     g     +         C   eq   ′       C   eq       ·     z   ~     ·     V   G         )                 (     eq   .           ⁢   3     )               
where C eq  is the equivalent gate capacitance, C′ eq  its derivative with respect to the nanowire position, {tilde over (v)} g  the a.c. voltage and z the (time-varying) motion of the nanowire.
 
     The key to fabricating a junctionless NEM resonator is to form a suspended, crystalline silicon structure that is sufficiently thin to fully deplete the transistor channel via the action one or two gate electrode. Because there exists a maximum depletion width in a MOS system, a limit on the channel profile thus exists, otherwise the transistor cannot be turned off. This condition is bound by a combination of the silicon body width and the doping concentration [Sze, S. M.,  Physics of Semiconductor Devices ; J. Wiley &amp; Sons: Hoboken (New Jersey), 3rd Edition, 2007, pp. 326-327]. The maximum silicon body width W si,max  for a double-gate, junctionless resonant-body FET becomes: 
                     W     si   ,   max       ≤     2   ⁢           ⁢     W     depl   ,   max         ≅     2   ⁢           4   ⁢   ɛ   ⁢           ⁢   kT         q   2     ⁢     N   D         ⁢   ln   ⁢       N   D       n   i                     (     eq   .           ⁢   4     )               
where ε is the permittivity, k the Boltzmann constant, T the temperature, q the electron elementary charge, and n i  the intrinsic impurity concentration. Therefore, the transduction principle we propose here is suited solely for a class of ultra-thin silicon resonators and not limited by further dimensional scaling.
 
     In a preferred embodiment, a 35 nm thin device layer on 8″ inch SOI wafers is used to fabricate NEMS based on a typical SOI-release process. 24  The conventional technology readily allows the integration of large arrays of devices with high densities. After two ion implantations with boron (p+) and phosphorus (n+), which define the gate (&gt;1×10 20  cm −3 ) and the channel doping concentration (˜2×10 18  cm −3 ), respectively, the NEMS active area is patterned using a hybrid DUV/e-beam lithography. A structural resolution and lithographic alignment better than 50 nm is achieved throughout the wafer. After release, the nanowire resonators were terminated with a 12 nm thermal oxide, which ensures a low leakage current and improves electromechanical coupling. It is found that, when it comes to fabricating integrated resonators with lateral air-gaps, the junctionless architecture offers the great advantage of enabling self-aligned processes, given that the gate electrodes are specific to the NEM resonator and simultaneously define the transistor channel. This implies that the junctionless FET can simplify the entire fabrication, compared to any electromechanical resonator, involving the formation of a suspended transistor body or semiconducting junction previously reported. 20-23    
     In order to address a single device on-chip, flexible 60 nm air-gap capacitors were used to couple two independent gate electrodes with the nanowire resonator ( FIG. 2 a   ). The resonators have a typical length between 1 and 2 μm, a total final height of 43 nm and a total final width of 67 nm. The silicon body has a cross-section of 28×35 nm 2  ( FIG. 2 b   ) and is fully depletable by the action of the gate electrodes (the maximum depletion depth is estimated to be W dep,max ˜25 nm for a given doping concentration of ˜2×10 18  cm −3 ). Looking at the results, it can be seen that the current-voltage characteristics reveal a transistor with a well-behaved transition from the OFF- to the ON-state. In  FIG. 3 a   , the drain current is plotted versus gate voltage, which was applied symmetrically (V G1 =V G2 ). The transfer curve shows off-currents corresponding to the noise floor of the measurement system (fA) and on/off current ratios beyond 10 6 . Clear exponential dependence is observed in sub-threshold, with a resulting sub-threshold slope of 580 mV dec −1 .  FIG. 3 b    shows the experimental output characteristics, which indicate the linear operating region and the transition to current saturation at higher drain voltages (V D ≈1 V). 
     In our experiment, the full transistor functionality incorporated into the nanowire resonator is exploited to reveal the mechanical resonance. The resonant properties were measured by means of a frequency modulated (FM) actuation scheme, 25  which lends itself to a straightforward experimental implementation. The FET was biased close to the threshold voltage, but with asymmetric gate bias (V G1 =−13 V; V G2 =+4 V). The drain current path was thereby concentrated on the outer edge (also indicated in  FIG. 1 ), where we can expect the strongest mechanically induced current modulation, and hence mechanical displacement gain.  FIG. 4 a    shows the mechanical amplitude spectrum of a junctionless nanowire resonator with a length of 1.7 μm and fundamental resonance at 96 MHz, plotted for different gate voltages of V G1 . Fitting the resonance based on a model considering the FM actuation scheme yielded a quality factor of Q˜320, or an f-Q product in the order of 7×10 11 , close to the highest values previously reported in silicon nanowire beam resonators of similar dimensions. 17  We also measured the detected drain current as a function of the gate bias and compared it to the device transconductance.  FIG. 4 b    shows the correlation from the ON-state to channel pinch-off, with an exponential dependence in sub-threshold. This is expected from transistor mixer theory. 21  The input and output power are linearly related, as can be seen in  FIG. 4 c   . We note that with a frequency tunability of df 0 /dV G ≈0.98 MHz/V, nanowire resonators of this class are excellent candidates for parametric actuation schemes. 26  Furthermore, in contrast to bottom-up fabrication procedures, this approach offers a high degree of freedom, as the mechanical design of the adjacent electrodes can be freely chosen and other modal shapes, such as free-free beams, can be applied to improve performance. 27    FIG. 5  depicts the fundamental resonance at 224 MHz of a 1 μm long device whose frequency is among the highest resonant frequencies measured with silicon nanowire resonators to date. 8,17,28  The resonator has a very small effective mass of 7×10 −18  kg, a measured Q of ˜80 at 1 atm. The mass resolution can be estimated in the attogram range under ambient conditions, based on the practically achievable performance of such resonators demonstrated in previous works. 5,8,14    
       FIGS. 6 to 8  illustrate the basic operating principle in three steps. 
       FIGS. 9 to 11  illustrate three examples of gate electrode configurations. 
       FIGS. 12 and 13  illustrate two modal shape designs. 
       FIGS. 14 and 15  illustrate two closed loop implementations. 
     In the present application, we have demonstrated the implementation and the unique properties of a self-aligned junctionless silicon nanowire electromechanical FET with two lateral 60 nm air-gap gates. The depletion charge modulation can be harnessed to transduce a mechanical motion at very high frequencies and is suited to a class of very scaled (sub-50 nm) silicon nanowire resonators. In broader terms, our results demonstrate that the concepts and technologies that primarily advance the continued scaling of solid-state FETs can be readily applied to create active, nanomechanical resonators. Interfaced with advanced CMOS on a single silicon chip, these devices can be used in complex collective electromechanical signal processing based on millions of resonant transistors. Such systems with high levels of complexity and low power consumption will enable a wide range of applications, from environmental chemical monitoring 29  to label-free bioanalysis. 30    
     Methods 
     Electrical and Mechanical Measurements. 
     The devices were measured in a vacuum-probe station by Cascade/Süss Microtech with RF GSG-probes (Süss Microtech) under high vacuum conditions (&lt;1×10 −5  mbar), using a phase-sensitive lock-in detector by Stanford Research and a RF signal source HP8648D. The device performance as function of pressure was achieved by flooding the chamber progressively with N 2  and monitoring the pressure precisely with a Pfeiffer PKR 251 and a SMC ZSE30 gauge. For DC measurements, an Agilent parametric analyzer 4556C was used with the same set-up. 
     The examples and embodiments illustrated in the present application are examples that should not be construed in a limiting manner and variations are possible within the spirit and scope of the present invention, for example via equivalent means. 
     Also, the different embodiments described herein may be combined together according to circumstances. 
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     All Incorporated by Reference in their Entirety in the Present Application 
     
         
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