Abstract:
Micro-Electro-Mechanical System (MEMS) Variable Capacitor Apparatus and Related Methods. According to one embodiment, a MEMS variable capacitor is provided. The variable capacitor can include first and second electrodes being spaced apart, and at least one of the electrodes being movable when a voltage is applied across the first and second electrodes. The variable capacitor can also include a first conductive plate attached to and electrically isolated from the first electrode. Furthermore, the variable capacitor can include a second conductive plate attached to the second electrode and spaced from the first conductive plate for movement of at least one of the plates with respect to the other plate upon application of voltage across the first and second electrodes to change the capacitance between the first and second plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This nonprovisional application claims the benefit of U.S. Provisional Application No. 60/388,490, filed Jun. 13, 2002, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present subject matter relates generally to micro-electromechanical systems (MEMS) apparatuses and methods. More particularly, the present subject matter relates to variable capacitor apparatuses and related methods utilizing MEMS technology.  
         BACKGROUND ART  
         [0003]    Micro-electro-mechanical systems (MEMS) apparatuses and methods are presently being developed for a wide variety of applications in view of the size, cost and power consumption advantages provided by these devices. Specifically, a variable capacitor, also known as a varactor, can be fabricated utilizing MEMS technology. Typically, a variable capacitor includes an interelectrode spacing between a pair of electrodes that can be controllably varied in order to selectively vary the capacitance between the electrodes. In this regard, conventional MEMS variable capacitors include a pair of electrodes, one of which is typically disposed upon and fixed to the substrate and the other of which is typically carried on a movable actuator or driver. In accordance with MEMS technology, the movable actuator is typically formed by micromachining the substrate such that very small and very precisely defined actuators can be constructed.  
           [0004]    As appreciated by persons skilled in the art, many types of MEMS variable capacitors and related devices can be fabricated by either bulk or surface micromachining techniques. Bulk micromachining generally involves sculpting one or more sides of a substrate to form desired three dimensional structures and devices in the same substrate material. The substrate is composed of a material that is readily available in bulk form, and thus ordinarily is silicon or glass. Wet and/or dry etching techniques are employed in association with etch masks and etch stops to form the microstructures. Etching is typically performed through the backside of the substrate. The etching technique can generally be either isotropic or anisotropic in nature. Isotropic etching is insensitive to the crystal orientation of the planes of the material being etched (e.g., the etching of silicon by using a nitric acid as the etchant). Anisotropic etchants, such as potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), and ethylenediamine pyrochatechol (EDP), selectively attack different crystallographic orientations at different rates, and thus can be used to define relatively accurate sidewalls in the etch pits being created. Etch masks and etch stops are used to prevent predetermined regions of the substrate from being etched.  
           [0005]    On the other hand, surface micromachining generally involves forming three-dimensional structures by depositing a number of different thin films on the top of a silicon wafer, but without sculpting the wafer itself. The films usually serve as either structural or sacrificial layers. Structural layers are frequently composed of polysilicon, silicon nitride, silicon dioxide, silicon carbide, or aluminum. Sacrificial layers are frequently composed of polysilicon, photoresist material, polimide, metals, or various types of oxides, such as PSG (phosphosilicate glass) and LTO (low-temperautre oxide). Successive deposition, etching, and patterning procedures are carried out to arrive at the desired microstructure. In a typical surface micromachining process, a silicon substrate is coated with an isolation layer, and a sacrificial layer is deposited on the coated substrate. Windows are opened in the sacrificial layer, and a structural layer is then deposited and etched. The sacrificial layer is then selectively etched to form a free-standing, movable microstructure such as a beam or a cantilever out of the structural layer. The microstructure is ordinarily anchored to the silicon substrate, and can be designed to be movable in response to an input from an appropriate actuating mechanism.  
           [0006]    MEMS variable capacitors have been fabricated that include a movable, conductive plate that is suspended above first and second coplanar electrodes. The variable capacitor operates by applying a voltage across the first electrode and the movable plate so that the plate is deflected towards the first electrode by electrostatic attraction. As the movable plate moves, the spacing between the second electrode and the movable plate changes, thus changing the capacitance value between the second electrode and the plate. A signal line is usually connected to the second electrode and the plate to sense the change in capacitance for use in various Radio Frequency functions. One problem with this configuration is that the voltage supply is electrically connected to the signal line through the plate that can result in undesirable noise/interference or degradation of the signal on the signal line. Thus, this configuration may require additional components to combine/separate the signal and actuation voltage, leading to a more complex and costly implementation.  
           [0007]    Other known MEMS variable capacitors provide parallel-plate electrodes that move linearly. The electrodes of these variable capacitors are subject to suddenly “snapping down” towards one another after moving close enough to one another. These types of variable capacitors are also subject to microphonics and stiction problems.  
           [0008]    Some MEMS variable capacitors are based upon electro-thermally actuated parallel-plate design. These types of variable capacitors are subject to reduced power handling capability due to gap reduction and the likelihood for breakdown occurrence. These variable capacitors also consume excessive power, especially if the electro-thermal actuation must be applied continuously to maintain the capacitance value.  
           [0009]    Other MEMS variable capacitors utilize a massively-parallel, interdigited-comb device for actuation. These variable capacitors are so sensitive to parasitic substrate capacitance that they require either a high-resistivity substrate such as glass or the removal of the substrate beneath the MEMS device. Thus, this type of variable capacitor is not readily integrated into a conventional integrated circuit (IC) process. Additionally, the MEMS device is physically large because the capacitance dependence on the overlap of comb fingers requires large aspect ratios. These devices require excessive space and cause a low resonant frequency resulting to shock and vibration problems.  
           [0010]    Therefore, it is desirable to provide MEMS variable capacitor apparatuses and related methods that improve upon aforementioned designs.  
         SUMMARY  
         [0011]    According to one embodiment, a micro-electro-mechanical system (MEMS) variable capacitor and method can be provided including first and second electrodes being spaced apart, and at least one of the electrodes being movable when a voltage is applied across the first and second electrodes. The variable capacitor can also include a first and second conductive plate. The first conductive plate can be attached to and electrically isolated from the first electrode. The second conductive plate can be attached to the second electrode and spaced from the first conductive plate for movement of at least one of the plates with respect to the other plate upon application of voltage across the first and second electrodes to change the capacitance between the first and second plates.  
           [0012]    According to a second embodiment, a micro-electro-mechanical system (MEMS) variable capacitor and method can be provided including first and second electrodes being spaced apart, and at least one of the electrodes being movable when a voltage is applied across the first and second electrodes. The variable capacitor can also include a torsional beam for providing resistance to the rotation of the first and second electrodes with respect to one another. Furthermore, the variable capacitor can include a first and second conductive plate. The first conductive plate can include a non-conductive section for attachment to and electrical isolation from the first electrode. The second conductive plate can be attached to the second electrode and spaced from the first conductive plate for movement of the second conductive plate with respect to the first conductive plate and a change in the capacitance between the first and second conductive plates when voltage is applied across the first and second electrodes.  
           [0013]    According to a third embodiment, a variable capacitor apparatus and method can be provided including a plurality of micro-electro-mechanical system (MEMS) variable capacitors. The variable capacitors can include first and second electrodes being spaced apart, and at least one of the electrodes being movable when a voltage is applied across the first and second electrodes. The variable capacitors can also include a torsional beam for providing resistance to the rotational of the first and second electrodes with respect to one another. Furthermore, the variable capacitors can include a first and second conductive plate. The first conductive plate can be attached to and electrically isolated from the first electrode. The second conductive plate can be attached to the second electrode and spaced from the first conductive plate for movement of the second conductive plate with respect to the first conductive plate and a change in the capacitance between the first and second conductive plates when voltage is applied across the first and second electrodes. The variable capacitor apparatus can also include first and second conductive lines. The first conductive lines can be attached to the first and second electrodes of the variable capacitors for attachment to a voltage supply. The second conductive lines can be attached to the first and second conductive plates of the variable capacitors for attachment to an AC signal.  
           [0014]    It is therefore an object to provide novel MEMS variable capacitor apparatuses and related methods. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Exemplary embodiments will now be explained with reference to the accompanying drawings, of which:  
         [0016]    [0016]FIG. 1 is a side view of an exemplary MEMS variable capacitor;  
         [0017]    [0017]FIG. 2 is a side view of the variable capacitor of FIG. 1 in a position such that the movable electrode is deflected closer to the stationary electrode;  
         [0018]    [0018]FIG. 3 is a perspective view of the variable capacitor of FIG. 1;  
         [0019]    [0019]FIG. 4 is a top view of the variable capacitor of FIG. 1;  
         [0020]    [0020]FIG. 5 is a side view of another exemplary MEMS variable capacitor;  
         [0021]    [0021]FIG. 6 is a block diagram of an array of identical MEMS variable capacitors;  
         [0022]    [0022]FIG. 7 is a block diagram of another array of different MEMS variable capacitors;  
         [0023]    [0023]FIGS. 8 and 9 are perspective views of a MEMS variable capacitor; and  
         [0024]    [0024]FIG. 10 is a bottom view of the cell layout of a MEMS variable capacitor. 
     
    
     DETAILED DESCRIPTION  
       [0025]    It is understood that when a component such as a layer, substrate, contact, interconnect, electrode, conductive plate, or conductive line is referred to herein as being deposited or formed “on” another component, that component can be directly on the other component or, alternatively, intervening components (for example, one or more buffer or transition layers, interlayers, electrodes or contacts) can also be present. Furthermore, it is understood that the terms “disposed on”, “attached to” and “formed on” are used interchangeably to describe how a given component is positioned or situated in relation to another component. Therefore, it will be understood that the terms “disposed on”, “attached to” and “formed on” do not introduce any limitations relating to particular methods of material transport, deposition, or fabrication.  
         [0026]    Contacts, interconnects, electrodes, conductive plates, conductive lines, and other various conductive elements of various metals can be formed by sputtering, CVD, or evaporation. If gold, nickel or Permalloy™ (Ni x Fe y ) is employed as the metal element, an electroplating process can be carried out to transport the material to a desired surface. The chemical solutions used in the electroplating of various metals are generally known. Some metals, such as gold, might require an appropriate intermediate adhesion layer to prevent peeling. Examples of adhesion material often used include chromium, titanium, or an alloy such as titanium-tungsten (TiW). Some metal combinations can require a diffusion barrier to prevent a chromium adhesion layer from diffusing through gold. Examples of diffusion barriers between gold and chromium would include platinum or nickel.  
         [0027]    Conventional lithographic techniques can be employed in accordance with micromachining of the variable capacitors. Accordingly, basic lithographic process steps such as photoresist application, optical exposure, and the use of developers are not described in detail herein.  
         [0028]    Similarly, generally known-etching processes can be employed to selectively remove material or regions of material. An imaged photoresist layer is ordinarily used as a masking template. A pattern can be etched directly into the bulk of a substrate, or into a thin film or layer that is then used as a mask for subsequent etching steps.  
         [0029]    The type of etching process employed in a particular fabrication step (e.g., wet, dry, isotropic, anisotropic, anisotropic-orientation dependent), the etch rate, and the type of etchant used will depend on the composition of material to be removed, the composition of any masking or etch-stop layer to be used, and the profile of the etched region to be formed. As examples, poly-etch (HF:HNO 3 :CH 3 COOH) can generally be used for isotropic wet etching. Hydroxides of alkali metals (e.g., KOH), simple ammonium hydroxide (NH 4 OH), quaternary (tetramethl) ammonium hydroxide ((CH 3 ) 4  NOH, also known commercially as TMAH), and ethylenediamine mixed with pyrochatechol in water (EDP) can be used for anisotropic wet etching to fabricate V-shaped or tapered grooves, trenches or cavities. Silicon nitride is typically used as the masking material against ethcing by KOH, and thus can be used in conjunction with the selective etching of silicon. Silicon dioxide is slowly etched by KOH, and thus can be used as a masking layer if the etch time is short. While KOH will etch undoped silicon, heavily doped (p++) silicon can be used as an etch-stop against KOH as well as the alkaline etchants and EDP. The preferred metal used to form contacts and interconnects is gold, which is resistant to EDP. The adhesion layer applied in connection with forming a gold component (e.g., chromium) is also resistant to EDP.  
         [0030]    It will be appreciated that electrochemical etching in hydroxide solution can be performed instead of timed wet etching. For example, if a p-type silicon wafer is used as a substrate, an etch-stop can be created by epitaxially growing an n-type silicon end layer to form a p-n junction diode. A voltage is applied between the n-type layer and an electrode disposed in the solution to reverse-bias the p-n junction. As a result, the bulk p-type silicon is etched through a mask down to the p-n junction, stopping at the n-type layer. Furthermore, photovoltaic and galvanic etch-stop techniques are also suitable.  
         [0031]    Dry etching techniques such as plasma-phase etching and reactive ion etching (RIE) can also be used to remove silicon and its oxides and nitrides, as well as various metals. Deep reactive ion etching (DRIE) can be used to anisotropically etch deep, vertical trenches in bulk layers. Silicon dioxide is typically used as an etch-stop against DRIE, and thus structures containing a buried silicon dioxide layer, such as silicon-on-insulator (SOI) wafers, can be used as starting substrates for the fabrication of microstructures.  
         [0032]    An alternative patterning process to etching is the lift-off process. In this case, the conventional photolithography techniques are used for the negative image of the desired pattern. This process is typically used to pattern metals, which are deposited as a continuous film or films when adhesion layers and diffusion barriers are needed. The metal is deposited on the regions where it is to be patterned and on top of the photoresist mask (negative image). The photoresist and metal on top are removed to leave behind the desired pattern of metal.  
         [0033]    As used herein, the term “device” is interpreted to have a meaning interchangeable with the term “component”.  
         [0034]    As used herein, the term “conductive” is generally taken to encompass both conducting and semi-conducting materials.  
         [0035]    Examples of the methods of the present subject matter will now be described with reference to the accompanying drawings.  
         [0036]    Referring to FIGS.  1 - 4 , different views of an exemplary MEMS variable capacitor, generally designated  100 , are illustrated. Referring specifically to FIG. 1, variable capacitor  100  can include a substrate  102  having a pair of spaced-apart pivot posts  104  formed thereon. Substrate  102  can comprise of one or more layers, composites, or other combinations of silicon, alumina, silica, polymers and other suitable substrate materials known to those of ordinary skill in the art. Pivot posts  104  support a non-conductive, structural component  106  for rotational movement of the opposing ends of component  106  about a pivot point  108 . Pivot posts  104  can also include torsional beams  110  for providing resistance to the rotational movement of component  106 .  
         [0037]    Torsional beam  110  can provide vertical stiffness to limit vertical motion of component  106  with respect to substrate  102 . Further, torsional beam  110  can provide torsional softness to ease rotational motion of component  106 . In this embodiment, torsional beam  110  can have a rectangular cross-section and a beam of sufficient length to provide flexibility. Alternatively, torsional beam  110  can have any suitable cross-section shape, dimension, or length. Additionally, torsional beam  110  can include folded springs. Torsional beam  110  can comprise one or more layers of silica, alumina, un-doped semiconductors, polymers, and other non-conductive materials known to those of ordinary skill in the art. Alternatively, beam  110  can comprise of any suitable type of metal, semimetal, doped semiconductor, or other suitable type of conductive material for electrically connecting electrode  116  and/or plate  118  to voltage source  122  or signal line  120 , respectively.  
         [0038]    Substrate  102  can also include a stationary electrode  112  and a stationary conductive plate  114  formed on a surface thereof. Component  106  can include a movable electrode  116  and a movable conductive plate  118  attached to the underside of component  106  and positioned above stationary electrode  112  and stationary conductive plate  114 , respectively. Movable electrode  116  and movable conductive plate  118  can be attached to component  106  on opposite sides of pivot point  108  for moving in opposite directions with respect to substrate  102  on operative rotation of component  106 . Conductive plates  114  and  118  can be electrically connected to a signal line  120  for supplying a signal, typically AC, to variable capacitor  100  from other electrical circuitry (not shown). Signal line  120  can comprise of a highly-conductive metal such as gold, aluminum, silver, copper, or the like. Signal line  120  can be connected to a high-frequency distribution network with minimum fixed capacitance. Typically, the electrical circuitry connected to signal line  120  is sensitive to capacitance of variable capacitor  100 . Electrodes  112  and  116  and conductive plates  114  and  118  can comprise of any suitable type of metal, semi-metal, or doped semiconductor. The conductivity of the material can be sufficient to avoid substantial lateral voltage drops across electrodes  112  and  116  and/or conductive plates  114  and  118  during operation. Voltage drops are mostly a concern for plates  114  and  118  due to the high frequency signals typically applied to the plates causing high currents to flow. Plates  114  and  118  can comprise a highly conductive metal, such as copper, gold, silver, aluminum or the like.  
         [0039]    Stationary electrode  112  and movable electrode  116  can be connected to a voltage supply  122  via conductive lines  124  and  126 , respectively. Conductive lines  124  and  126  can comprise of any suitable conductive material, such as a metal, semi-metal, and doped semiconductors. As shown, voltage supply  122  can produce a voltage across electrodes  112  and  116 . An equal and opposite electrical charge develops on electrodes  112  and  116  on the application of a voltage. The equal and opposite electrical charge causes an electrostatic force to pull movable electrode  116 , and the portion of component  106  to which electrode  116  is attached, towards stationary electrode  112 . Torsional beams  110  produce a biasing force in an opposite direction from the electrostatic force to oppose movement of movable electrode towards stationary electrode  112 . Movable electrode  116  deflects in a rotational direction about pivot point  108  towards stationary electrode  112  when the voltage applied across electrodes  112  and  116  is great enough to overcome the resistive force of torsional beams  110 . As movable electrode  116  is deflected closer to stationary electrode  112 , the resistive force of torsional beams  110  increases. The voltage applied across electrodes  112  and  116  can be increased to deflect movable electrode  116  closer to stationary electrode  112  than another position. Thus, the gap distance between electrode  112  and  116  can be adjusted by controlling the voltage output by voltage supply  122 . The voltage applied by voltage supply  122  can be varied directly by an operator or other suitable electrical circuitry known to those of skill in the art for controlling the voltage output by a voltage supply.  
         [0040]    [0040]FIG. 2 illustrates variable capacitor  100  in a position such that movable electrode  116 , and the portion of component  106  to which electrode  116  is attached, is deflected closer to stationary electrode  112  than shown in FIG. 1. This deflection can be caused by applying a voltage difference to electrodes  112  and  116  from voltage source  122  via conductive lines  124  and  126 . The voltage difference can cause an electrostatic force which attracts electrodes  112  and  116  towards each other. Movable conductive plate  118 , and the portion of component  106  to which plate  118  is attached, moves with respect to stationary conductive plate  114  when movable electrode  116  moves with respect to stationary electrode  112 . As movable electrode  116  moves closer to stationary electrode  112 , movable conductive plate  118  moves away from stationary conductive plate  114 . Thus, a sufficient voltage applied to electrodes  112  and  116  can cause movable conductive plate  118  to move away from stationary conductive plate  114 . The distance between conductive plates  114  and  118  can be adjusted by control of the voltage across electrodes  112  and  116 . Thus, the capacitance of the capacitor formed by conductive plates  114  and  118  can be adjusted by application of voltage across electrodes  112  and  116 .  
         [0041]    Movable conductive plate  116  can deflect suddenly and fully against contact stationary conductive plate  112  when torsional beams  110  twist beyond a threshold angle. At this threshold, movable conductive plate  118  suddenly moves a maximum distance from stationary conductive plate  114  and the capacitance of variable capacitor  100  is suddenly reduced. This sudden movement typically occurs when movable electrode  116  has been deflected approximately ⅓ of the distance to stationary electrode  112 , i.e., approximately ⅓ of the distance from a neutral position to a position against stationary electrode  112 . Therefore, the voltage input to capacitance ratio is not a continuous relationship after reaching the threshold.  
         [0042]    Structural component  106  can be formed of a non-conductive, dielectric material for electrically isolating movable electrode  116  and movable conductive plate  118 . Furthermore, structural component  106  can comprise a rigid material for providing fixed attachment of movable electrode  116  to movable conductive plate  118 . Alternatively, structural component  106  can comprise any other suitable non-conductive material known to those of skill in the art. Component  106  can comprise of one or more layers of silica, alumina, un-doped semiconductors, polymers, and other non-conductive materials known to those of ordinary skill in the art.  
         [0043]    Referring to FIG. 3, a perspective view of variable capacitor  100  is illustrated. Substrate  102  is not shown. Torsional beam  110  is shown attached and extending from structural component  106 . Referring now to FIG. 4, a top view of variable capacitor  100  is illustrated. Capacitor  100  can include pivot posts  400  and  402  for attaching torsional beams  110  to substrate  102 . Movable electrode  116  and movable conductive plate  118  can be connected by conductive lines  404  and  406 , respectively, across the underside of torsional beam  110  to pivot posts  400  and  402 , respectively. Additionally, conductive lines  404  and  406  can extend along pivot posts  400  and  402 , respectively, for connection to other voltage source  122  and signal line  120 , respectively.  
         [0044]    Referring to FIG. 5, a side view of a MEMS variable capacitor, generally designated  500 , is illustrated. Variable capacitor  500  can include a substrate  502  having a pair of pivot posts  504  attached on a surface thereof. Pivot posts  504  support a structural component  506  for rotational movement of the ends of component  506  about a pivot  508 . Alternatively, structural component  506  can be suspended over substrate  502  by another known component and method known to those of skill in the art. Pivot posts  504  can also include torsional beams  510 . Substrate  502  can also include a stationary electrode  512  and a stationary conductive plate  514  attached to a surface thereof. Component  506  can include a movable electrode  516  attached to the topside of component  506  and positioned above stationary electrode  512 . Component  506  can also include a movable conductive plate  518  attached to the underside of component  506  and positioned above stationary conductive plate  514 . Conductive plates  514  and  518  can be electrically connected to a signal line  520  for supplying a signal, typically AC, to variable capacitor  500  from other electrical circuitry (not shown). Stationary electrode  512  and movable electrode  516  can be connected to a voltage supply  522  for controlling the capacitance of conductive plates  514  and  518 .  
         [0045]    Movable electrode  516  can be attached to the topside of component  506  to prevent electrode  512  and  516  from contacting. Non-conductive component  506  contacts electrode  512  on full deflection of movable electrode  516 . Thus, contact between electrodes  512  and  516  can be prevented. This functions to reduce stiction and prevent electrical shorting. A larger ratio with more linear control is obtained as structural component  506  tilts further before electrostatic pull-in occurs. If structural component  506  is thick enough, the control can be linear over the full range of motion. To achieve linearity as such, the electrical thickness of structural component  506  can be greater than twice the unactuated gap. Electrical thickness is the actual thickness of component  506  divided by the effective dielectric constant of component  506 .  
         [0046]    An array of variable capacitors having different characteristics can be arranged in an array to provide a device having a desired capacitance response to voltage input. Referring to FIG. 6, a block diagram of an array of identical torsional, MEMS variable capacitors is illustrated. The array can include variable capacitors  600 ,  602 ,  604 , and  606 . In this example, there are four variable capacitors in the array. Alternatively, there can be any number of variable capacitors. Each capacitor can be connected across conductive plates, as described above, to signal line  608 . Furthermore, each capacitor can be connected across electrodes, such as electrodes  112  and  116  shown in FIG. 1, to a voltage supply  610 . The spring stiffness of variable capacitors  600 ,  602 ,  604 , and  606  can each be equal to a constant X. Thus, as the applied voltage from voltage supply  610  reaches a threshold, the collective capacitance of variable capacitors  600 ,  602 ,  604 , and  606  suddenly reduces. This sudden reduction in capacitance is due to the attractive force between the electrodes suddenly overcoming the resistive force of the torsional beams, such as torsional beams  110  shown in FIG. 1, at approximately the same time.  
         [0047]    Referring to FIG. 7, a block diagram of an array of differing MEMS variable capacitors is illustrated. The array can include variable capacitors  700 ,  702 ,  704 , and  706 . In this example, there are four variable capacitors in the array. Alternatively, there can be any number of variable capacitors. Each capacitor can be connected across conductive plates, as described above, to signal line  708 . Furthermore, each capacitor can be connected across electrodes, as described above, to a voltage supply  710 . The torsional beams of variable capacitors  700 ,  702 ,  704 , and  706  can have different “spring” stiffness. Variable capacitors  700 ,  702 ,  704 , and  706  can have a spring constant W, X, Y, and Z, wherein X is greater than W, Y is greater than X, and Z is greater than Y. Therefore, as the applied voltage from voltage supply  710  reaches a threshold, the collective capacitance of variable capacitors  700 ,  702 ,  704 , and  706  suddenly reduces at different applied voltages. Thus, as the applied voltage increases, capacitance of the array suddenly reduces in a cascading fashion according to when the resistive force of a torsional beam is overcome by the attractive force between the electrodes. Alternatively, other characteristics of the different variable capacitors (e.g., electrode size, conductive plate size, placement of the electrode and conductive plate on the surface or structural layer, and the size of the structural layer) in an array can be altered to change the capacitance versus applied voltage function. Furthermore, several of these characteristics known to those of skill in the art can be modified to optimize the capacitance versus applied voltage function. Variable capacitor  500  (shown in FIG. 5) can be used as one or more of variable capacitors  700 ,  702 ,  704 , and  706  for achieving a capacitance versus voltage function for the array having a smoother and wider range of functional dependencies.  
         [0048]    Referring to FIGS. 8 and 9, perspective views of a MEMS variable capacitor, generally designated  800 , are illustrated. Referring specifically to FIG. 8, a movable conductive plate  802  can be located on the top side of a structural layer  804 . Movable conductive plate  802  can include a conductor  806  that extends along the top of the torsional beam for connection to sensing terminals  808 . Other electrical circuitry can connect to sensing terminals  808  for electrical communication with movable conductive plate  802 .  
         [0049]    Referring to FIG. 9, a perspective view of movable electrode  900  from a side opposite that of FIG. 8 is illustrated. The structural layer is not shown in order to show movable electrode  900  located on the bottom side of the structural layer. Movable electrode  900  includes a conductor  902  that extends along the bottom of the torsional beam for connection to sensing terminals  904 . Other electrical circuitry can connect to sensing terminals  904  for electrical communication with movable electrode  900 . The materials for fabricating variable capacitor  800  can be any of the materials previously described.  
         [0050]    Referring to FIG. 10, a view of the bottom plate layer of a MEMS variable capacitor, generally designated  1000 , is illustrated. Variable capacitor  1000  can include positive and negative conductors  1002  and  1004 , respectively, for connection to voltage driving circuitry for stationary electrode  1006  and movable electrode (not shown). Stationary electrode can be in electrical communication with positive conductor  1002 . Movable electrode (not shown) can be in electrical communication with negative conductor  1004 . Variable capacitor  1000  can also include positive and negative conductors  1008  and  1010 , respectively, for connection to sensing circuitry for stationary conductive plate  1012  and movable conductive plate (not shown). Stationary conductive plate  1012  can be in electrical communication with positive conductor  1008 . Movable conductive plate (not shown) can be in electrical communication with negative conductor  1010 .  
         [0051]    It will be understood that various details of the above subject matter can be changed without departing from the scope of the present subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the subject matter is defined by the claims as set forth hereinafter.