Abstract:
An electron transfer device is implemented in a structure which is readily capable of achieving charge transfer cycle frequencies in the range of several hundred MHz or more and which can be formed by conventional semiconductor integrated circuit manufacturing processes. The device includes a substrate having a horizontal extent and a pillar on the substrate extending from the substrate vertically with respect to the horizontal extent of the substrate. The pillar is formed to vibrate laterally with respect to the vertical length of the pillar at a resonant frequency which can be several hundred MHz. Drain and source electrodes extend from the substrate vertically with respect to the horizontal extent of the substrate, and have innermost ends on opposite sides of the pillar. The pillar is free to vibrate laterally back and forth between the innermost ends of the drain and source electrodes to transfer charge between the electrodes.

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 10/833,344, now issued as U.S. Pat. No. 6,946,693, filed on Apr. 27, 2004, and issued Sep. 20, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to the field of micro- and nanoelectromechanical systems and to electron transfer devices implemented in such systems. 
     BACKGROUND OF THE INVENTION 
     A classical system in which electrical charge can be transferred or shuttled mechanically uses two large capacitor plates and a metalized ball (or particle) suspended between the plates. Charging one of the plates (and grounding the other) will attract the ball to the charged electrode via the Coulomb force and upon contact with the plate the ball will exchange electrons with the plate. The acquired excess charge in the ball will in turn accelerate the ball in the electric field between the plates to the grounded electrode, where the excess electrons are dumped upon contact. Depending on the system geometry (plate and ball size and distance between the plates), applied voltage, and the manner in which the ball is suspended, the ball will cycle back and forth between the plates at a frequency which may reach a few kHz. The result is an effective transfer of charge (i.e., a current flow) between the plates. 
     The integration of a mechanical degree of freedom in nanostructures utilized in nanoelectromechanical systems (NEMS) opens a potential wide range of applications in, for example, sensors and communication electronics. Mechanical resonators manufactured with nanometer dimensions have been formed to oscillate at radio frequencies up to the range of GHz. See, e.g., R. H. Blick, et al., J. Phys.: Cond. Mat. 14 (2002) R905; X. M. H. Huang, et al., Nature (London) 421 (2003) 496. Significant progress has been made with regard to the control of non-linear dynamics and dissipation of NEMS at radiofrequencies. D. V. Scheible, et al., Appl. Phys. Lett. 81 (2002) 1884; D. V. Scheible, et al., Appl. Phys. Lett. 82 (2003) 3333; F. W. Beil, et al., Phys. Rev. Lett. (2004) (submitted for publication). NEMS also have promise for providing experimental insight into the quantum aspects of mechanical systems. 
     Efforts have been made to implement a mechanical charge shuttle into NEMS structures by combining a nanomechanical resonator with a single-electron transistor (SET). Such nanomechanical charge shuttles were formed by combining cantilevers with SETs fabricated on the tips of the cantilevers to form a so-called electromechanical SET (or EMSET). A. Erbe, et al., Appl. Phys. Lett. 73 (1998) 3751; A. Erbe, et al., Phys. Rev. Lett. 87 (2001) 096106. These devices were achieved using traditional NEMS fabrication utilizing optical and electron beam lithography of layered silicon substrates incorporating a sacrificial layer. Experiments with these devices showed electron tunneling that was mechanically chopped at radio frequencies. It would be desirable to be able to increase the operating frequency preferably to several hundred MHz and into the GHz range, and to be able to fabricate charge transfer devices utilizing semiconductor processing techniques that are more compatible with standard integrated circuit manufacturing. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an electron transfer device is implemented in a structure which is readily capable of achieving charge transfer cycle frequencies in the range of several hundred MHz or more. Furthermore, the devices in accordance with the invention can be formed by conventional semiconductor integrated circuit manufacturing processes without utilizing a sacrificial layer during processing. In addition, the devices of the invention can be formed to have a very small footprint or area on the semiconductor chip, allowing for dense packing of an array of such structures. The devices are well suited to be incorporated as circuit elements, such as rectifiers and transistors, as well as in various gas and radiation sensors. 
     An electron transfer device in accordance with the invention includes a substrate having a horizontal extent and a pillar on the substrate extending from the substrate vertically with respect to the horizontal extent of the substrate. The pillar is formed to vibrate laterally with respect to the vertical length of the pillar and has a fundamental vibration mode with a resonant frequency. The pillar includes an electrical conductor that is electrically insulated from the substrate. Drain and source electrodes extend from the substrate vertically with respect to the horizontal extent of the substrate, with each of the source and drain electrodes having innermost ends which are on opposite sides of the pillar and which are spaced from the pillar. Each of the source and drain electrodes includes an electrical conductor therein that is electrically insulated from the electrical conductor of the pillar and the electrical conductor of the other electrode. The pillar is free to vibrate laterally back and forth between the innermost ends of the drain and source electrodes so as to bring the electrical conductor of the pillar alternately closer to one of the drain and source electrodes and then closer to the other of the drain and source electrodes during each cycle as the pillar vibrates. 
     The substrate is preferably an insulator, and the pillar and drain and source electrodes are preferably formed on the substrate of a layer of semiconductor and a layer of conductive metal on the layer of semiconductor. For example, the devices may be formed on a semiconductor on insulator (SOI) wafer wherein the substrate is the insulating layer (e.g., silicon dioxide), and the semiconductor is crystalline silicon. The conductor may be a layer of metal, such as gold, or a highly doped semiconductor. The resonant frequency of the device may be selected by appropriate choice of dimensions for the device. Typical dimensions for the pillar include a vertical height in the range of 100 nm to 2,000 nm and horizontal dimensions (cross-sectional width or diameter) in the range of 20 nm to 200 nm. However, other structural materials and other dimensions may also be utilized. For example, using appropriate mounting methods, nanotubes may be used as vertical pillars and can have smaller diameters, e.g., in the range of 5 nm. The innermost ends of the drain and source electrodes are typically spaced from the pillar a distance in the range of 5 nm to 50 nm. The pillar may have a region below the top of the pillar which has reduced horizontal dimensions so as to form a waist in the pillar which can be utilized to select the resonant frequency of the pillar. To provide asymmetry, one of the drain and source electrodes is preferably more closely spaced to the pillar than the other of the electrodes. An AC source may be electrically connected to the source and drain electrodes to provide AC voltage thereto to excite vibration of the pillar. A DC source may also be coupled to one of the drain and source electrodes to provide a DC bias voltage thereto. The AC source provides an AC voltage that is preferably at or near the resonant frequency of the pillar. For example, the vertical and horizontal dimensions of the pillar are readily selected such that the pillar has a fundamental lateral vibration mode with a resonant frequency in the range of 10 MHz to 1 GHz or even greater. 
     The device of the invention can also include a gate electrode on the substrate that has an innermost end adjacent to and spaced from the pillar at a position between the innermost ends of the drain and source electrodes. The gate electrode preferably extends from the substrate vertically with respect to the horizontal extent of the substrate, and has an electrical conductor therein that is electrically insulated from the conductor of the pillar and the conductors of the drain and source electrodes. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a simplified plan view of an electromechanical charge transfer device formed in accordance with the invention. 
         FIG. 2  is a cross-sectional view of the device of  FIG. 1  taken generally along the line  2 — 2  of  FIG. 1 . 
         FIG. 3  is a perspective view of the device of the invention with the electrical circuit connections to the device shown schematically. 
         FIG. 4  is a schematic circuit diagram of the device of the invention illustrating the equivalent circuit elements of the device. 
         FIG. 5  is a graph of spectral drain current I D  versus source frequency f for an exemplary device formed in accordance with the invention having a resonant frequency of 369 MHz. 
         FIG. 6  is a graph of spectral drain current I D  versus source frequency f for another exemplary device in accordance with the invention having a resonant frequency of 399 MHz. 
         FIG. 7  is a graph of phase relation and source-drain bias for the device of the invention at an oscillation frequency f which is less than the resonant frequency f h . 
         FIG. 8  is a graph of phase relation and source-drain bias for the device of the invention at an oscillation frequency f equal to the resonant frequency f h . 
         FIG. 9  is a graph of phase relation and source-drain bias for the device of the invention at an oscillation frequency f which is greater than the resonant frequency f h . 
         FIG. 10  are graphs illustrating the tuning of the current resonance by a superimposed DC bias V. 
         FIG. 11  are graphs of negative (lower trace) and positive (upper trace) peak currents I D   peak  versus DC bias V, with the traces representing a section of the whole set of current spectra at frequencies f 0   −  about equal to 374 MHz and f 0   +  about equal to 382 MHz. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings, an electromechanical charge transfer device in accordance with the invention is shown generally at  20  in  FIG. 1 . The device  20  is formed on a substrate  21  such as a silicon on insulator (SOI) wafer which has a top surface  22 . The substrate  21  extends in two dimensions of horizontal extent which generally but does not necessarily lie in a flat plane. A source electrode  25 , a drain electrode  26 , and a gate electrode  27  are formed on the substrate  21  and generally extend vertically with respect to the horizontal extent of the substrate  21  and thus vertically away from the surface  22  of the substrate. The source electrode  25  has an innermost end  30  and the drain electrode  26  has an innermost end  31  which face each other and are spaced from each other across a gap. A vibratable pillar  33  is formed on the substrate  21  and extends vertically therefrom in the gap between the source and drain electrodes so that the innermost ends  30  and  31  of the source and drain electrodes, respectively, are on opposite sides of and are spaced from the pillar  33 . The gate electrode  27  also has an innermost end  34  which is spaced from the pillar  33  by a small gap at a position between the innermost ends  30  and  31  of the source and drain electrodes. As illustrated in  FIG. 2 , the substrate  21  may include an insulating layer  40 , and the source electrode  25 , drain electrode  26 , and pillar  33  may be formed of a layer of crystalline silicon  42  with a top layer or coating of conductor  44  on each element formed of a conductive metal such as gold. The gate electrode  27  may also be formed in the same manner with the same structure. The insulating layer  40  may comprise the intermediate insulating SiO 2  layer formed on a handle wafer  46  of crystalline silicon, and with the silicon layer  42  formed from the top crystalline silicon layer of a conventional SOI wafer. However, the invention may be formed of other materials including other semiconductor materials, conductors and insulators. Because the substrate layer  40  is formed of an insulating material (e.g., SiO 2 , etc.), the source electrode  25 , drain electrode  26 , gate electrode  27  and pillar  33  are electrically insulated from each other so that current does not flow through the substrate between these structures. However, electrical insulation of these substrates may be provided by insulation in the structures themselves. The invention may be implemented in other material systems that can be micromachined and by other processes. The entire pillar (or electrodes) can be conducting (e.g., heavily doped semiconductor or a metal) or have a conductor layer formed therein, with electrical insulation formed between the pillar and electrodes. 
     A particular advantage of the present invention is the limited number of processing steps required to form the device, and the compatibility of such processing steps with conventional semiconductor integrated circuit manufacturing. The device of the invention as shown in  FIGS. 1 and 2  can be formed in two steps on a standard SOI substrate having an insulating layer  40  with a thin silicon (or other semiconductor) layer  42  formed thereon. In a first step for processing of the SOI wafer, the pattern of the gold layer  44  is defined using, e.g., nano-scale lithography, such as by utilizing e-beam lithography via a scanning electron microscope (SEM). After the patterned gold layers  44  are formed, the wafer is then moved to a reactive ion etcher (RIE) where dry etching is carried out. The etching process attacks the silicon layer  42  that is not covered by the gold layer  44  and mills down the silicon in a direction perpendicular to the chip surface. The lithographically defined gold layer  44  thus acts both as electrical current leads and as an etch mask for the RIE process. In the etching process, the vertical pillar  33  may be formed to have essentially vertical sidewalls or it may have an area of reduced horizontal cross-section to form a waist  48  as shown in  FIG. 2 . The cross-section of the pillar may have a square or other polygonal shape or may be substantially circular. The dimensions of the pillar  33  are selected to allow the pillar to vibrate mechanically in a lateral mode in which the pillar bends so that the top  49  of the pillar and the conductor  44  move closer to or further away from each of the inner ends  30  and  31  alternately, referred to herein as lateral vibration. The formation of the narrowed waist  48  may be selected to select the resonant frequency of the pillar  33 . 
     The shuttling of electrons by the pillar  33  can be represented conceptually as shown in the circuit diagrams of  FIGS. 3 and 4 . Charge transfer between pillar and electrode can occur by electron tunneling, field emission, gas ionization, or direct mechanical contact. As known from work on single-electron tunneling, the barriers of the pillar  33  to drain  20  and to source  23  can be represented by a parallel combination of the tunneling resistance R and the junction capacitance C. However, because the barriers are mechanically flexible tunneling barriers, diagonal lines are shown through the R/C boxes in  FIG. 4 . These two lines are not parallel, relating to the fact that once the source junction is tuned mechanically into the open state, the drain junction will be opaque. 
     The device of the invention is well suited to operation in the radio-frequency range. In  FIG. 3  the fabricated device  20  is shown in perspective with the underlying silicon dioxide insulating base  40  supporting the pillar  33  in the center between the drain electrode  26  and the source electrode  25 . The device is controlled by biasing the source  25  and drain  26  and/or the gate  27 . The bias voltage applied to the gate electrode  27  will affect the resonant frequency of the vibrating pillar  33 , and control of the gate bias voltage can thus be used to control the effective current flow through the device. However, the device of the invention may be utilized without a gate  27  or with multiple gates, if desired. The natural resonant frequency of the pillar  33  depends on the pillar height and lateral (horizontal) dimensions, as well as on the material constants and the metallic top layer mass. For pillar heights (distance from the top  49  of the pillar to the junction of the pillar with the substrate) in the range of 100 nm to 2,000 nm, resonant frequencies of 10 MHz to 1 GHz may be readily obtained. The horizontal dimensions of the pillar are typically in the range of 20 nm to 200 nm, and the innermost ends of the drain and source electrodes are typically spaced from the pillar a distance in the range of 5 nm to 50 nm. Excitation of the resonator is most efficiently provided by an AC source  50  and a DC source  51  biasing through the drain and source electrodes. The resulting drain current flow I D  can be measured, for example, by a current/voltage converter  54 . 
     Of significance for the device  20  when functioning as a switch or signal filter is the fact that its throughput resistance is drastically enhanced since only one contact (drain or source electrode) is open during a half-cycle of operation: once the pillar  33  is biased, a certain number of electrons N will flow onto the pillar (depending on the pillar&#39;s electrostatic potential) and jump off the pillar only when the other contact (drain or source electrode) comes into reach during the second half-cycle of the switch revolution. 
     For excitation of the pillar  33  into lateral vibration, an AC voltage may be applied at the source electrode  25  from the source  51 . Application of an AC signal allows one to excite the pillar  33  resonantly in one of its eigenmodes. Gradually accumulating charge on the pillar  33  results in a resonant Coulomb force (RCF) within the AC source-drain electric field. The basic concept underlying the device is the excitation of mechanical oscillation via the excess charge on the pillar. Hitherto employed excitation mechanisms, such as magnetomotive driving, are not suitable for mass-production devices, due to the high magnetic field which is required (typically B˜1-10 T). In addition, the present device can be operated without cryogenic cooling and achieves accurate and well reproducible operation in the non-cryogenic temperature range of 77–300 K and higher. 
     During a cycle of deflection of the pillar  33 , the pillar exchanges charge carriers with the biased source electrode  25  and N electrons tunnel off the pillar and M electrons tunnel from the electrode to the pillar. The respective amounts N and M are determined by the instantaneous electrical bias. The detected current I D  is the net number of transferred electrons n (=N−M) times the shuttle frequency f, i.e.:
 
 I   D   =gef ( N−M )= gefn    (1)
 
where e is the electric charge and g accounts for a statistical limitation for the case where the pillar deflection is not sufficient for charge transport in each cycle. For the excitation via the resonant Coulomb force it is necessary that the shuttle pillar sustains a certain excess charge during oscillation. This can be obtained by a mechanical asymmetry (e.g., one of the source and drain electrodes is closer to the pillar than the other electrode) which strongly favors one tunneling barrier at which the shuttle pillar is reset to the same charge state each cycle. Assuming m electrons present on the shuttle for the time period when both tunneling barriers are closed, for stable operation the relation (N−M)=n&lt;m must hold. The RCF function F RCF (t) is then calculated via
 
                       F   RCF     ⁡     (   t   )       =     me   ⁢           ⁢           V   ^     AC     ⁢     sin   ⁡     (     2   ⁢   π   ⁢           ⁢   f   ⁢           ⁢   t     )           d   SD                 (   2   )               
where {circumflex over (V)} AC  is the AC voltage amplitude and d SD  is the source-drain distance. A finite element simulation was used to obtain an estimate of the source to pillar capacitance of about C SI =10 aF (attoFarads). This results in a maximum excess charge m=24 electrons, causing, in turn, a force of 7 pN at an incident AC power of P=+5 dBm. Important for the current frequency behavior is the phase relation of the mechanical motion and the electrical AC bias. In the case of a harmonic oscillator, the driven resonator follows the applied force with a phase lag of
 
                       ϕ   h     ⁡     (   f   )       =     arctan   ⁢       Γ   ⁢           ⁢   f         f   h   2     -     f   2                   (   3   )               
wherein the resonance frequency is f h . For excitation of a mechanical eigenmode h of the nanopillar  33  the phase lag equals φ h =π/2, i.e., maximum deflection occurs when N=M. For an ideal system, we accordingly expect a zero net current I D =0 for f=f h . Above and below the resonance frequency f h , the phase lag approaches −π and zero respectively. Depending on the sign of the excess charge m, the result is either a positive current for f&gt;f h  and a negative current for lower frequencies or vice versa.
 
     Basic measurements of the current frequency response have been carried out for two example devices showing similar behavior at slightly different eigenfrequencies f 0 . This variation is explained by the different heights and waistlines of the pillar  33  in the two devices. A finite element simulation showed similar results as the sample devices in absolute resonance frequencies and variations. The current traces I D  for a set of AC powers P are plotted in  FIGS. 5 and 6  for the two sample devices. Highest current amplitudes correspond to a transported net amount of electrons n&lt;2, following Eq. (1), which satisfies well the condition n&lt;m. For the two sample devices the negative and positive peak currents differ by factors of 2 and 5, respectively. Mechanical asymmetry and inhomogeneity of the effective electric field destroy the phase relation of an ideal harmonic oscillator, and hence cause the asymmetric peaks seen in  FIGS. 5 and 6 . 
     The mechanism of RCF can be verified in the present invention via a superposition of a DC bias voltage V onto the AC voltage V AC . This is accomplished by use of an inserted bias-tee  56  as shown in  FIG. 3 . As long as the DC bias is small compared to the AC voltage amplitude {acute over (V)} AC , the main influence of the DC bias is the phase shift between the deflection point and the effective drain/source bias, as illustrated in  FIGS. 7–9 . As a result, switching is accomplished between forward and reverse current peaks in  FIG. 9 , as well as zero current as in  FIG. 8 .  FIG. 10  shows the spectral current for a set of selected DC bias voltages ranging from −70 mV to +40 mV. It is important to note that the observed behavior not only supports RCF, but also demonstrates the tunability of this mechanical device for rectification of high frequency AC signals. A summary of all traces at the lower and upper peak frequencies f 0   ±  reveals that the peaks are not only detuned, but also can give a reverse current response. As  FIG. 11  shows, this is achieved for a high DC bias exceeding 40 mV. In addition to the detuning, a regular step-like structure is observed in both peaks when the DC bias is ramped. The separation of the steps (˜23 mV) corresponds to the pillar-source capacitance; however, the total island capacitance for room temperature operation should only be e 2 /k B T≈3 aF. Hence, the steps can be attributed to a gradual change of the oscillatory mode rather than Coulomb blockade effects. 
     It is understood that the invention is not confined to the embodiments set forth herein for illustration, but embraces all forms thereof as come within the scope of the following claims.