Patent Application: US-201314395626-A

Abstract:
a junctionless nano - electro - mechanical 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:
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 . [ 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 . fig1 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 : 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 : 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 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 : 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 ( fig2 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 ( fig2 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 fig3 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 . fig3 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 fig1 ), where we can expect the strongest mechanically induced current modulation , and hence mechanical displacement gain . fig4 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 . fig4 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 fig4 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 fig5 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 fig6 to 8 illustrate the basic operating principle in three steps . fig9 to 11 illustrate three examples of gate electrode configurations . 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 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 - 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