Abstract:
The present invention relates to a micro electromechanical (MEM) isolator in which an input signal induces an output signal by means of electrically insulating mechanical motion. The MEM isolator device comprises a dielectric moveable platform suspended above a substrate by flexible beams. A drive and a control capacitor each have one electrode supported by the platform and one electrode supported by the substrate. Coupling between electrical and mechanical energies is achieved by providing an input signal to the drive capacitor to induce platform motion. When the input signal is fed to the drive capacitor, it actuates electrostatic motion of the platform resulting in a change in the value of the control capacitance. The change in the control capacitance is converted via a simple electronics circuit into an output that mirrors the input but is electrically isolated therefrom. The advantages of such a device include simple electrical isolation provided by the dielectric platform, built in signal-debounce inherent to the structure mechanics, and economical integration with silicon integrated circuits.

Description:
FIELD OF THE INVENTION 
     The present invention relates to isolator devices and more specifically to micro electromechanical (MEM) isolator devices providing isolation between at least one input signal and an output signal. 
     BACKGROUND OF THE INVENTION 
     Isolator devices (isolators) are often used to provide electrical or acoustical separation between two locations or environments. Isolators are commonly found in factory and telecommunication applications to prevent interaction between circuits on either side of the isolator. 
     Prior art isolators include optical coupling devices that convert an electrical signal to an optical signal and then convert the optical signal back to an electrical output signal. In optical coupling systems, the electrical signal drives a light emitting diode to generate light pulses that are received at a detector. The detector then converts the light pulses back into an electrical signal. Since the detector output is electrically isolated from the diode, transients are not propagated from the input to the output. Unfortunately, optical isolators are expensive to manufacture and operate. Optical isolators use expensive gallium arsenide (GaAs) substrates and exhibit high operating power and limited lifetime. Further, optical isolators are not well suited to analog applications since it is difficult to transfer signal analog voltage signals due to the difficulty in detecting variations in the light pulse responsive to intermediate voltage levels. 
     Isolation transformers are well known and used in many applications since they have low power consumption and provide magnetic (flux) coupling between one or more pairs of circuits without introducing either significant ohmic (conductive) or electrostatic (capacitive) coupling. The isolation transformer has low power requirements, but transformers are heavy, bulky discrete components that depend on separate wire windings on a steel core. Although the windings are electrically insulated from one another, a transformer may transfer high frequency transients from one environment or circuit to the next so the protected environment may require additional protection circuits to filter out the transients. Further, although transformers are able to convert alternating current (AC) to direct current (DC), transformers are unable to provide isolation for DC to DC applications. 
     Capacitive coupling devices also have low power consumption but similarly have large discrete electrical components that add to system size and weight and are susceptible to electrical failure if stressed under certain conditions. Further, capacitive coupling devices inherently couple harmful transients from the input to the output. 
     Further, with many prior art isolators, an additional external circuit is required to debounce the output signal to prevent introduction of spurious noise. This additional circuit increases power consumption and cost to isolate the desired signal. 
     Notwithstanding the problems with the various types of isolators known in the art, isolators are required in a variety of applications. For example, in tele-communication applications, it is common for signals transmitted through a switching network to have voltage and current levels that are incompatible with telephone or computer devices such as modems and similar equipment. Accordingly, an isolator may be used to isolate the head end of the network from remote terminal equipment while still permitting the signal information content to be transmitted between different electrical environments. Further, since environmental factors can affect the signal in transit, there is a need to remove potentially harmful transient signals (such as lightning induced transients) before the terminal equipment is damaged. Further still, it is also desirable to separate circuit grounds so that improperly grounded equipment will not disrupt the operation of the remainder of the switching network. 
     Similar applications arise in the industrial control environment since automation is becoming an increasingly important part of producing high quality products at competitive prices. As processing equipment becomes more sophisticated and interconnected to other equipment by local area networks or other communication means, control and processing information is generated that must be timely transferred to a server or other processing equipment as well as a noise-free ground reference. In general, isolators are often employed where a signal is generated in a first hostile environment and it is necessary to convey the information content of the signals to a second environment without conveying those signal parameters that are potentially damaging. As noted above, another main function of isolators is to isolate grounds between electrical circuits that are in communications with each other. Having separate matched zero potential conductors on one side compared to the noisy ground from the input other is desirable for noise and safety reasons. 
     Thus, whatever the advantages of the prior art isolator devices, there is a need for an inexpensive, lightweight, low power, isolator device that has isolation characteristics comparable to prior art isolator devices. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an isolator based on moveable micro electromechanical (MEM) structures. The general principle is to have at least one electrical input signal control an output signal by way of mechanical motion of an electrically insulating MEM structure. The MEM isolator device comprises a moveable platform suspended above a substrate and a drive and a control capacitor each having a movable electrode supported by the platform and a stationary electrode supported by the substrate. The moveable and stationary electrodes of each capacitor are separated by an air gap so the capacitance changes as a function of the distance between electrodes of the capacitors. By sensing the change in the output capacitance, it is possible to regenerate the input signal at the output. Electrically insulating coupling between the input and the output signal is achieved by converting electrical energy to mechanical energy. This conversion is achieved by providing an input signal to the drive capacitor to induce electrostatic platform motion. This motion induces a corresponding change in the control capacitance that is detected by a control circuit. The control circuit then produces an output signal that follows the input signal but is electrically isolated therefrom. 
     More specifically, the drive capacitor is coupled to a signal source and when a signal applied across the drive capacitor, an electrostatic force is generated causing the electrode mounted on the moveable platform to move either toward or away from the fixed electrode. This electrode motion converts electrical energy into mechanical energy. The mechanical motion of the platform transfers the mechanical energy to the control capacitor and since the value of the capacitance is proportional to the distance (1/d) between the electrodes of the capacitor, the capacitance value increases when the gap spacing is decreased. Similarly, as the gap spacing is increased, the capacitance value of the capacitor will decrease. Since the value of the capacitance is proportional to the distance (1/d), the value of the capacitor increases when the gap spacing is decreased and as the gap spacing is increased the capacitance value of the capacitor will decrease. The change in the value of the control capacitor is measurable and used to determine the magnitude of the input voltage. Advantageously, the isolator of the present invention is capable of isolating both AC and DC signals. 
     In the preferred embodiment, the moveable electrodes of both the drive and the control capacitors are mounted on a common insulating platform so that there is no electrical connection between the drive and control capacitors. The isolator device of the present invention provides isolation voltage levels on the order of several thousands of volts limited only by the breakdown voltage of the insulating material. The isolator of the present invention protects one side of a circuit from harmful signal components while permitting the transfer of the signal content. Further, separate grounds are readily provided with the present invention since the isolator may couple either digital or analog signals from one environment to another. 
     Depending on design parameters, a data rate of ten thousand bits per second (10,000 bps) is readily achieved. However, if significantly higher data rates are necessary, the signal size will degrade since the resonant frequency of the structure is inversely proportional to square root of the mass. Thus, higher signal rates require smaller capacitors resulting in smaller signal size. Compared to optical isolator devices, active isolators or transformers, power consumption is significantly reduced with the present invention with minimal power consumption on the order of only a few microwatts. The isolator device of the present invention may be integrated together with control circuits as a single component using flip-chip technology. Advantageously, the present invention also provides a signal debounce feature that filters spurious high frequency signals. 
     The MEM structure is fabricated using well-known semiconductor processing techniques to define and release the microstructure from single-crystal silicon material resulting in an isolator device that is small and lightweight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a common electronics application for an isolation device in accordance with the present invention. 
     FIG. 2 is a schematic representation of an isolation device in accordance with the present invention. 
     FIG. 3 shows a top view of the isolation device and specifically the suspended platform that supports movable electrodes of a drive and control capacitor in proximity to the respective stationary electrodes. 
     FIG. 4 illustrates one representative embodiment of an isolator device of the present invention configured as a comparator circuit. 
     FIG. 5 is a schematic representation of the isolator device shown in FIG. 4 configured as a comparator. 
     FIG. 6 illustrates a diagrammatic side view of the isolator of FIG. 3 along section line  6 — 6 . 
    
    
     DETAILED DESCRIPTION 
     According to the principles of the present invention, a new, small, low power micro electromechanical isolation device is disclosed having good isolation characteristics. Specifically, the present invention provides a device that receives an electrical signal and induces an output signal via an electrically insulating mechanical coupling means. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that these specific details need not be used to practice the present invention. In other instances, well known structures, circuits and interfaces have not been shown in detail in order not to unnecessarily obscure the present invention. 
     Referring now to FIG. 1, a common electronics application is illustrated where isolation of two signals is necessary. An isolator  10  shown coupling signals generated in a field environment  12  to a protected environment  14  provides signal isolation. Field environment  12  is a potentially harsh and noisy environment such as may be experienced in telecommunications or industrial control applications so any signal generated in or transferred through environment  12  could be dirty and susceptible to noise spikes and other induced transients. By way of example, the field environment may comprise a communications network, such as a dial-up telephone system or other telecommunication network, where a switching network establishes a connection between a head end and terminal equipment such as a computer in the protected environment. In another example of the field environment, sensors on the factory floor generate signals that are transferred to the protected environment  14  for monitoring by a server or other similar equipment. 
     During operation, signals generated in environment  12  are applied to the input of isolator  10 . An optional analog to digital converter  16  conditions any analog signal before it is coupled to isolator  10 . In protected environment  14 , electronic system  18  is coupled to the output of isolator  10 . Electronic system  18  may be a microprocessor or a terminal device such as a modem or a telephone. Isolator  10  functions, by way of example, to protect electronic system  18  from harmful transients imposed on the signals by spurious noise or other environmental factors. Isolator  10  may also couple a ground reference from the field to the protected environment so that potentially harmful ground loops or floating grounds will not be transferred to protected environment  12 . 
     FIG. 2 illustrates, in schematic form, an isolator device  10  in accordance with the present invention. Isolator device  10  comprises a drive capacitor, C d , having a fixed electrode  20  coupled to a substrate  22  and a movable electrode  24  coupled to a moveable mass  26 . Electrodes  20  and  24  are separated by an air dielectric. A second movable electrode  28  is also mounted on mass  26 . Electrode  28  in combination with fixed electrode  30  forms a control capacitor, C c . Electrodes  28  and  30  are also separated by an air gap. 
     A generator circuit  32  in the field environment  12  (represented as being to the left of dotted line  34 ) is electrically coupled to fixed electrode  20  of C d . Generator circuit  32  may, by way of example, be a machine on a factory floor that communicates with a remotely located server (not shown) in protected environment  14  (represented as being to the right of dotted line  34 ). 
     In protected environment  14 , a detector circuit  36  detects changes induced in the capacitance of C c  and generates a signal responsive to the electrical signal generated by circuit  32 . In operation, the input signal generated by circuit  32  actuates electrostatic platform motion of mass  26 . Displacement of mass  26  causes electrode  28  to move thereby inducing a change in the capacitance of C c . Detector circuit  34  detects the change in capacitance of C c  and an output signal is generated in response to the input signal. Since there is no electrical connection between capacitors C d  and C c , the output signal is electrically isolated from the input signal applied to electrode  20 . As should be apparent to one of ordinary skill in the art, the specific details of generator circuit  32  and detector circuit  36  are dependent on the particular application and as such, specific details as to design or operation are considered to be merely an engineering choice. 
     The operating characteristic of isolator device  10  is determined in large part by the electromechanical properties of mass  26 . These characteristics include the maximum rate for data transfer, built-in signal debounce, isolation quality, power consumption and noise immunity. 
     Referring now to FIG. 3, isolator  10  is illustrated in greater detail with a top view of suspended platform  26  and capacitors C d  and C c  shown. The dimensions of isolator  10  are not drawn to scale since the purpose is to merely demonstrate the relationship between various elements of the isolator in a manner that is readily comprehended. Platform  26  is a suspended microstructure supported by beams  38  above a substrate  40 . Platform  26 , capacitors C d  and C c  and beams  38  are formed from a top or cap substrate  46  and coupled to substrate  40 . 
     Beams  38  act as a set of “springs” that flex in response to application of a voltage applied to the fixed electrode  20  of capacitor C d . The dimensions of beams  38 , specifically, the length and width, are determined in part by the particular application. For example, when the expected voltage levels of input signals applied to plate  20  are relatively low (that is, in a range of about zero to 5.0 volts) beams  38  must be sufficiently flexible such that structural motion is actuated in response an applied signal. In one preferred embodiment where the range of input voltages is from zero to 5.0 volts, beams  38  have a spring constant of about one Newton/meter (1.0 Nt/m). To obtain this spring constant, beams  38  will each have a length of about 500 μm, a width of 2 μm and a height of 10 μm. For higher voltages, beams  38  may be designed to be less flexible by increasing the width or decreasing the length. By depositing a layer of metal (See FIG. 4) on top of beams  38 , electrical connection to the moveable electrodes  20  and  24  may be routed to a pad or other circuits. 
     The operating characteristics of isolator  10  also depend on the dimensions of the capacitors C d  and C c . In general, the capacitance (and accordingly the operating characteristic) is determined by the area of the electrodes, that is the respective length, L, and the height, H (see FIG.  6 ), the distance (d 1  and d 2 ) defining the air gap between the electrodes and the dielectric constant of air, ∈ o . By way of illustration, if a 100 fF capacitor is desired for a particular application, each electrode will be about 1000 μm long and 10 μm high with an air gap between the electrodes of about 2 μm. Based on these dimensions, the capacitors in this embodiment will have a drive coefficient of 0.008 μmV 2  where the drive coefficient is a measure of structural movement in response to an applied voltage, the spring constant and capacitance value. More specifically, the drive coefficient defines displacement as a function of the capacitance value and the spring constant and the square of the applied voltage: 
     
       
         Drive coefficient=Force/Spring Constant=(0.5*( C/G )* V   2 )/ K   
       
     
     Where “C” represents the capacitor value and “G” represents the gap spacing. For a nominal position, the value for C is about 100 fF, the value for G is about 0.2 μm and the value for the spring constant, K, is 1 Nt/m. Thus for the illustrated embodiment, deflection of the movable platform will be about 0.2 μm in response to application of a drive voltage of 5.0 volts. The corresponding change in capacitance will be about 10 fF for such a displacement. 
     Depending on the intended application, the operating characteristics are readily modifiable for a variety of applications. For industrial applications where expected signal levels will be between zero and 5.0 volt, a spring stiffness of about 2 Nt/m provides a resonance frequency of about 10 Khz and maximum data rate of about 4 Kbytes/sec. To obtain the spring constant of 2 Nt/m, each beam  38  will have a length of about 400 μm and a width of about 2 μm. The height dimension is determined by the thickness of cap substrate  46 , which in the representative embodiment is about 10 μm. The target capacitance of the isolator is about 100 fF derived from the dimensions of electrodes. In the preferred embodiment, the height of the capacitor is about 10 μm, which is determined by the thickness of cap substrate  46 , the length of the electrode plate is about 1000 μm and the gap distance is about 2 μm. For telecommunication applications, expected voltage signals have a much higher range (between 0 and 48 volts) and a stiffer spring constant is required for a 100 fF capacitor. For example, beams  38  will be about 300 μm long with a height and width of about 10 μm and 2 μm respectively. The spring constant is about 40 Nt/m for beams having these dimensions. This isolator device achieves a 40 Kbytes/second maximum data rate and a resonance frequency of about 100 KHz. By selectively designing beams  38  to provide a selected spring constant, the isolator of the present invention can readily be matched to the anticipated signal environment. 
     FIG. 4 illustrates one representative embodiment of a comparator conversion circuit  41  using the isolator device of the present invention. In this embodiment, a first isolator device  42  has input  48  coupled to generator circuit  32  (see e.g., FIG. 2) for receiving an input signal from the field environment. The input signal is applied to the fixed plate of C d . Moveable electrode  26  connects to output  50 , which may be the ground reference provided from field environment  12 . Output  52  couples fixed electrode  30  of C c  to a detector circuit  44 . By way of example, wire bonding or flip chip technologies may be used to couple output  52  to detector circuit  44  to enable compact packaging. Detector circuit  44  includes an operational amplifier  46  having its inverting input  54  coupled to output  52 . As with the above-described embodiments, the suspended platform couples movable electrode of C d  to the movable electrode of C c . 
     Circuit  41  further includes a reference capacitor C r  connected to output  56 . Output  56  is coupled to the non-inverting input  58  of amplifier  46 . Capacitor values are chosen so the capacitance of C c  is slightly larger than the capacitance of the reference capacitor C r . One manner of adjusting the capacitance value of the capacitors is to adjust the dimensions of respective capacitors C r  and C c . Thus when a low signal level is applied to input  48 , the voltage relations at amplifier  46  will be V 54  &gt;V 58 , causing the amplifier output to be at a low signal level. When a logic high signal is applied at input  48 , the change in the voltage at output  52  is higher than the level applied to reference output  56 . Accordingly, the voltage relation at the inputs to amplifier  46  is V 54  &lt;V 58 , and output  60  of amplifier  46  will be at a high voltage level. A reference bias voltage is applied to input  62  such as Vcc or other selected voltage level that will minimize noise margin problems at the input to amplifier of detector  44 . 
     FIG. 5 shows a schematic representation of the isolator device configured as a comparator. When a signal is applied across terminals  48  and  50 , electrostatic displacement of movable electrode of C d  induces movement of platform  26 . Displacement of platform  26  causes a corresponding change in the capacitance of capacitor C c . Accordingly, with a static voltage V hi  applied at a reference terminal  62 , the voltage at each input of amplifier  46  is a function of the ratio of the respective capacitance values. Specifically, the voltage at inputs V 54  and V 58  is approximately: 
       V   54   ˜C   c   /C   gnd   
     
       
         
           V 
           58 
           ˜C 
           r 
           /C 
           gnd 
         
       
     
     where the reference capacitor is coupled to a stationary platform and, accordingly, the value of C r  is static and not affected by the signal applied at the input of isolator  42 . Typical values of C gnd  are approximately 1.0 pF. This value includes capacitance due to wiring and stray capacitance. The resulting voltage change at the output of the operational amplifier will be about 25 mV if C gnd  is about 1.0 pF. 
     The highest rate of data transfer is determined by the mechanical resonance frequency of the composite structure of mass  26  and beams  38 . The resonance frequency is the determined by the following formula: 
     
       
           f   r =( K/M ) ½   
       
     
     The stiffness or spring constant K is determined by the dimensions of beams  38  and the elastic modulus of the material, primarily the semiconductor material comprising the cap substrate  46 . The height, width and length and the elastic modulus of the beam are combined in accordance with the following formula: 
     
       
           K=E ×( W   3   ×H )/ L   3   
       
     
     With respect to the mass of the structure, the mass of the beams must be included in the mass calculations for a uniform structure. However, as will be readily apparent, as the platform becomes large when compared to the mass of the beams, the beam effect becomes less significant. It is also noted that metal is the most significant contributor to the mass of the overall structure so electrodes  24  and  28  will contribute a significant portion of the total mass. In the embodiment shown in FIG. 2, the structure has an estimated mass of about 1.0×10 −6  grams (1.0 μg) and a spring constant of 1 Nt/meter. The resulting resonance frequency is about 5 kHz and the isolator device will be operable for input signal frequencies up to about the resonance frequency. For an input signal comprising digital data consisting of 8 bits words, the signal frequency would limit sample rates to about 600 samples per second. If the data consists of 16 bits, the maximum sample rate would be 300 samples per second. 
     One very advantageous feature of the resonance-limited response is that it provides built-in signal debounce since the amplitude of microstructure motion decreases with drive frequency at a rate of 10 dB per octave above the resonant frequency. Thus for the above described structure, with a resonance frequency of 5 kHz, structure motion in response to 40 Khz signals will be 80 dB smaller than for the 1 kHz signals. This damping of high frequency motion (and hence absence of high frequency “bounce” signals) is achieved without external circuitry. 
     The isolation between input and output signals is high with the isolator of the present invention. It should be apparent that the maximum potential difference is set by the breakdown fields of the dielectric between the respective conductors on platform  26 . For typical semiconductor processing parameters, the breakdown value is well in excess of 1.0 Megavolt (MV). With signal line spacing of about 100 μm, this breakdown level permits a maximum input voltage exceeding 10 kV without undesirable coupling to the amplifier  44 . Moreover, since signal isolation is affected by capacitive coupling (typically under 0.001 fF) between adjacent signal lines on platform  26 , the line-to-line impedance at 1 kHz will actually be much greater than a giga-ohm. Moreover, the maximum potential difference may be further improved by increasing line-to-line separation. 
     Power consumption of device  10  is very low. Power use of the mechanical element stems from actuation of the platform motions, achieved by charging up small value capacitors in the range of 100 fF. Typical charging currents are on the order of a microampere (1 μA) and drive voltages are approximately about 50V so power consumption is about 50 μW. The power consumption of the control circuit is also minimal since only one operational amplifier is needed at the output of device  10 . 
     One device characteristic unique to this MEM device of the present invention is potential susceptibility to mechanical noise. This susceptibility stems from the microstructure geometry where the suspended large-mass structure acts as an accelerometer. Thus, acceleration induced mechanical motion may cause a change in the control capacitor that is unrelated to the input signal. Fortunately, this susceptibility can be greatly minimized by using a reference capacitor having geometry nearly identical to that of the control capacitor. Acceleration-related displacements of the reference capacitor will match those of the control capacitor so that the motion induced voltage change at the amplifier inputs results in no net voltage difference. 
     Where the application requires a data rate of approximately 16 Khz (that is, 16 bits of data sampled at a sample rate of 1 kHz), device  10  must have its resonant frequencies over 50 kHz. However, there exits a fundamental tradeoff between maximum response speed and maximum signal level. Since signal level is proportional to capacitor size, the major source of mass that affects the resonant frequency is the size of the drive capacitor. Lumped-parameter expressions for platform frequency, f, and displacement, x, as a function of capacitor dimensions is: 
     
       
           f ˜1/( M ) 0.5 ˜1/( A+ρ*H*L*W ) 0.5   
       
     
     
       
         
           x˜C˜H*L*W 
         
       
     
     Where A is the structure mass from components other than the drive capacitor (that is the platform, beams and control capacitor), ρ represents material density of the drive capacitor and the height, H, length, L, and width, W, of the electrode mounted on the moveable platform. Decreasing the dimensions of the drive capacitor increases the structure&#39;s resonant frequency but diminishes the maximum signal level. Thus, there is a tradeoff between maximum data rate and maximum signal level. 
     Isolator device  10  must be capable of operating within the range of standard acceptable levels characteristic of the logic family. In one preferred embodiment, the logic family is CMOS logic integrated on a common substrate with the isolator device. One potential problem is the range of the signal inputs the device must convert correctly. In particular, the device must recognize voltage inputs from −0.4 V to 1.5 V as a low logic signal and voltage inputs from 4.6 V to 5V as a high logic level. The electrostatic actuation relation suggests that this input voltage range should not be problematic for the proposed device since the actuation force, and hence platform deflection, is proportional to the square of the input voltage. Thus, the smallest value for the ratio of the high displacement to the low displacement will be: 
     
       
         ( x   hi /x lo )min=( V   himin   /V   lomax ) 2 =(4.5/1.5) 2 =9 
       
     
     In general, if the displacement ratio approaches 10 or greater, the displacement is sufficiently robust to generate detectable change for the selected logic family input level signals. It will be apparent to one skilled in the art that the input to the isolator device need only be sufficient to induce motion of the platform and is not intended to be limited to applications of 5.0 volt logic levels. Clearly, the input signal may be an acoustical signal or other signal or condition capable of inducing motion of the platform. 
     One risk that must be addressed in the design of the isolator device of the present invention is achieving control of the mechanical and electrical parameters. Mechanically, the microstructures must have dimensions so that the stationary structure has a control capacitance slightly higher than the reference capacitor, and the displaced structure has a control capacitance smaller than reference capacitor. This means that the capacitor-plate air gaps must be controllable, as well as the structure spring constants. These dimensions are determined by lithography and etching processes, both of which are well-characterized processes. 
     Referring now to FIG. 6, a cut-away side view of the isolator  10  of FIG. 3 is illustrated. Specifically, a substrate  40  supports the entire device. Substrate  40  may be a common silicon device used for processing semiconductor devices. A top substrate  46 , having a dielectric layer of oxide  64  on one side, is disposed on top of substrate  40 . A layer of epoxy  66  couples substrate  40  to top substrate  46 . During processing, substrate  46  is patterned and etched to define the capacitors C d  and C c  and to remove the substrate material from platform  26  as well as between the electrodes of the capacitors. It should be apparent that the height, H, of capacitors C d  and C c  is determined by the thickness of substrate  46 . A selective etch then removes the dielectric layer  64  to define beams  38  and platform  26 . During the etch process the epoxy layer is also removed to free platform  26  and to permit movement. It should be readily apparent that the epoxy layer and the dielectric layer need not be etched in a single etch process step but may in fact comprise two or more mask and etch steps if dictated by a particular design consideration. A layer of metal is then deposited and patterned to form the electrode plates as well as the conductors  68 - 74  that couple the electrodes to the appropriate inputs or outputs (see, e.g., FIG.  4 ). 
     The substrates and technology for fabricating the proposed device is primarily dictated by the need to realize millimeter-sized structures. Such large structures are preferably fabricated from single-crystal substrate material since structures from polycrystalline materials tend to exhibit stress-related non-planar dislocations over temperature or operating extremes. 
     There are two preferred fabrication technologies for fabricating the isolator devices of the present invention. One fabrication technology employs a silicon-on-insulator (SOI) substrate. Such substrates are commercially available from a variety of sources with a 20 μm thick epitaxial silicon layer already applied. Structures defined by the epitaxial silicon are released by a selective wet-chemical etch of the insulator substrate. 
     Another fabrication technology involves bonding two wafers using electronic-grade epoxy adhesive. One wafer is thinned to structure thickness by lapping and polishing while the other wafer serves as a substrate for the structure. Free-standing microstructures are formed by patterning and etching the structure wafer and undercutting these structures by etching the underlying epoxy in an isotropic O2 plasma to release the structure. This fabrication technology is more fully described in U.S. Patent application Ser. No. 09/075,008 filed May 8, 1998 now U.S. Pat. No. 6,159,385, which issued Dec. 12, 2000, entitled Process for Manufacture of Micro Electromechanical Devices Having High Electrical Isolation and originally assigned to Rockwell Science Center LLC, the assignee of the present invention, the disclosure of which is incorporated herein by reference. 
     Although the invention has been described herein with reference to a specific embodiment, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the present invention as defined in the following claims.