Abstract:
An electromagnetically coupled hermetic chamber includes a body defining a hermetic chamber. A distributed LC circuit is disposed within the hermetic chamber and a second conductive structure is attached to the body outside of the hermetic chamber. The distributed LC circuit is electromagnetically coupled to the second conductive structure without direct electrical paths thereby allowing coupling of the distributed LC circuit to external electronics without the need for electrical feedthroughs or vias that could compromise the integrity of the hermetic chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. Utility application Ser. No. 11/402,439, filed Apr. 12, 2006, which claims the benefit of U.S. Provisional Application No. 60/670,549, filed Apr. 12, 2005, which applications are herein incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to sensors comprising hermetic packaging that eliminates the need for electrical feedthroughs, and sensors configured utilizing the same, all of which are intended to perform reliably in harsh and biological environments. 
       BACKGROUND OF THE INVENTION 
       [0003]    Over the past 20 years, advances in the field of microelectronics have enabled the realization of microelectromechanical systems (MEMS) and corresponding batch fabrication techniques. These developments have allowed the creation of sensors and actuators with micrometer-scale features. With the advent of the above-described capability, heretofore implausible applications for sensors and actuators are now significantly closer to commercial realization. 
         [0004]    In parallel, much work has been done in the development of pressure sensors. Pressure sensors are disclosed, for example, in U.S. Pat. No. 6,111,520, issued Aug. 29, 2000; U.S. Pat. No. 6,278,379, issued Aug. 21, 2001; U.S. Pat. No. 6,855,115, issued Feb. 15, 2005; U.S. patent application Ser. No. 10/054,671, filed Jan. 22, 2002; U.S. patent application Ser. No. 10/215,377, filed Aug. 7, 2002; U.S. patent application Ser. No. 10/215,379, filed Aug. 7, 2002; U.S. patent application Ser. No. 10/943,772, filed Sep. 16, 2004; U.S. patent application Ser. No. 11/157, 375, filed Jun. 21, 2005; and U.S. patent application Ser. No. 11/314,046 filed Dec. 20, 2005, all of which are incorporated herein by reference. 
         [0005]    In particular, absolute pressure sensors, in which the pressure external to the sensor is read with respect to an internal pressure reference, are of interest. The internal pressure reference is a volume within the sensor, sealed, which typically contains a number of moles of gas (the number can also be zero, i.e., the pressure reference can be a vacuum, which can be of interest to reduce temperature sensitivity of the pressure reference as known in the art). The external pressure is then read relative to this constant and known internal pressure reference, resulting in measurement of the external absolute pressure. For stability of the pressure reference and assuming the temperature and volume of the reference are invariant or substantially invariant, it is desirable that the number of moles of fluid inside the reference does not change. One method to approach this condition is for the reference volume to be hermetic. 
         [0006]    The term hermetic is generally defined as meaning “being airtight or impervious to air.” In reality, however, all materials are, to a greater or lesser extent, permeable, and hence specifications must define acceptable levels of hermeticity. Acceptable level of hermeticity is therefore a fluid ingress or egress rate which does not change the pressure in the internal reference volume (a.k.a. pressure chamber) by an amount large compared with the pressure of interest being measured over the accumulated time over which the measurements will be taken. An amount large compared with the pressure of interest should be construed to mean a change in the internal reference volume that is less than 10 percent, preferably less than 5 percent, and most preferably less than 1 percent of the external pressure being sensed. In many biological applications, an acceptable pressure change in the pressure chamber is on the order of 0.5-5 mm Hg/year. 
         [0007]    The pressure reference is typically interfaced with a sensing means that can sense deflections of boundaries of the pressure reference when the pressure external to the reference changes. A typical example would be bounding at least one side of the pressure reference with a deflectable diaphragm or plate and measuring the deflection of the diaphragm or plate by use of, among other techniques, a piezoresistive or a capacitance measurement. If the deflection of the diaphragm or plate is sufficiently small, the volume change of the pressure reference does not substantially offset the pressure in the pressure reference. 
         [0008]    These approaches may require an electrical feedthrough to the hermetic environment (e.g., to contact electrodes inside the hermetic pressure reference) for connection to outside electronics to buffer or transmit the signal. Alternatively, electronics may be incorporated within the reference cavity, requiring power to be conducted into the hermetic environment. To maintain stability of the pressure reference, these seals should also be hermetic, resulting in the necessity to develop a feedthrough technology for electrical contacts through the cavity walls. As is known in the art, such feedthrough locations are typically sites for failure of hermeticity. This problem is further exacerbated when miniaturizing the sensor, since the total volume of material available for hermetic sealing shrinks proportionally and the reliability of the feedthrough is also greatly reduced. In the limit of ultraminiaturized sensors, such as those producible using microelectromechanical systems (MEMS) technology, it would be desirable to eliminate these feedthroughs in their entirety while maintaining the ability to supply power and/or exchange information with the circuitry inside the hermetic pressure reference. 
         [0009]    Thus a need exists for sensors comprising hermetic cavities which maintain electrical communication with the ambient without physically breaching the hermetic cavity. 
       SUMMARY 
       [0010]    The present invention is a hermetic cavity in which information and/or energy can be transmitted through the walls of the cavity without the need to form a physical breach of the wall of the cavity. A particularly useful embodiment of the invention is a sensor that is comprised of a sensor body which defines a hermetic cavity. All sensing elements associated with the sensor are located within the hermetic cavity and therefore are hermetically sealed from the surrounding environment, thereby reducing drift and instability of the sensor. Electrical communication between the sensing elements and electronics external to the hermetic chamber is accomplished by means of electromagnetic coupling between two complementary conductors located on opposite sides of at least one wall defining the hermetic cavity. 
         [0011]    Additional circuitry, e.g., sensing circuitry, can be placed in electrical communication or integral with the conductor inside the hermetic cavity and electrically biased when the conductor inside the hermetic cavity (a.k.a., the internal conductor) is energized. Direct electrical contact, e.g., by means of electrical feedthroughs, can be established between the conductor which is external to the hermetic cavity (a.k.a., the external conductor) and attachment means on the external wall of the sensor in order to connect the device to further electronics. 
         [0012]    Sensors of the present invention are entirely self-packaged and maintain electrical communication with the surrounding environment without the need for electrical feedthroughs breaching the hermetic cavity. Elimination of feedthroughs into the hermetic cavity increases reliability and durability of the sensor by eliminating a feature that is frequently cited as a point of failure of hermeticity in such devices. The sensor can be fabricated using high-purity, hermetic and biocompatible materials, e.g., ceramics, metals and polymers. If ceramics are used to construct the sensor body defining the hermetic cavity, the ceramic substrates can be fused together so that there is no interface of material remaining where the substrates have been joined to create a cavity. This eliminates any material interface in the sensor body that could become the site of a potential leak path into the hermetic cavity and, consequently, increases the reliability and durability of the sensor. Alternatively, anodic or eutectic bonding techniques can be utilized to create the hermetic cavity. Furthermore, sensors of the present invention can be manufactured using microelectromechanical systems (MEMS) fabrication techniques, which allow creation of a device that is small, accurate, precise, durable, robust, biocompatible, and insensitive to changes in body chemistry or biology. 
         [0013]    The present invention further comprises a device that can be incorporated into harsh and biological environments. One example of such an environment is a medical lead or catheter implanted, acutely or chronically, into the human body. The present invention is disclosed in the context of a pressure sensor, but it should be understood that the packaging and communication scheme can be utilized in the creation of any device where hermeticity is desirable. Communication between the sensor and another device can be established by, e.g., electrical feedthroughs terminating in bond pads on the exterior of the sensor body or wires protruding from the sensor body, either of which are configured so that they are in electrical contact with and able to electrically bias the external conductor. 
         [0014]    In one embodiment of the invention, inductive coupling is utilized to establish electrical communication with components residing inside the hermetic cavity. The complementary conductors comprise at least two inductors. In a further embodiment, the inductors comprise planar wire spiral inductors. Although the invention is illustrated by means of inductive coupling through the walls of the hermetic chamber, it should be recognized that alternative means of coupling, including but not limited to capacitive, distributed capacitive/inductive, optical, and combinations thereof, may also be utilized. 
         [0015]    In another embodiment of the invention, the hermetic cavity is further configured to be sensitive to a selected range of pressure. The pressure cavity (i.e., this pressure-sensitive hermetic cavity) further comprises a capacitor configured so that the characteristic capacitance value of the capacitor indicates a physical state, or changes in a physical state, within a patient. The electrodes of the capacitor are substantially planar and are arranged substantially parallel to and spaced apart from one another. The pressure cavity has at least one deflectable region in mechanical communication with at least one of the capacitor electrodes. In this embodiment, the capacitor and inductor are realized through mutually-imposed, planar spiral inductor coils located on opposite sides of the pressure cavity with at least a portion of one of the planar spiral coils fixed to the deflective region. The inductance and capacitance of the circuit are distributed across the area of the mutually-imposed coils. The spiral coils can, optionally, terminate in electrodes. This feature increases the capacitance of the circuit and allows the resonant frequency of the circuit to be tuned by varying the size of the electrodes. Another mutually-imposed, planar spiral inductor coil is located external to the pressure cavity in magnetic proximity to the internal spiral inductor coils. This external inductor coil is isolated from the surrounding environment by coating it with a suitable polymer or encasing it in a ceramic material. In either case, wires or electrical feedthroughs terminating in bond pads are provided so that connection of the sensor to other electronics can be established. 
         [0016]    In another embodiment, the hermetic cavity is further configured to be sensitive to a selected range of pressure. The pressure cavity further comprises a capacitor configured so that the characteristic capacitance value of the capacitor indicates a physical state, or changes in a physical state, within a patient. The electrodes of the capacitor are substantially planar and are arranged substantially parallel to and spaced apart from one another. The pressure cavity has at least one deflectable region in mechanical communication with at least one of the capacitor electrodes. In a further embodiment, the LC circuit is realized through connecting the capacitor to a three-dimensional inductor coil. Another inductor coil is located external to the pressure cavity and in magnetic proximity to the internal coil. This external inductor coil is isolated from the surrounding environment by coating it with a suitable polymer or encasing it in a ceramic material. In either case, wires or electrical feedthroughs terminating in bond pads are provided so that connection of the sensor to remote electronics can be established. 
         [0017]    In yet another embodiment, a wired sensor further comprises on-board (i.e., within the sensor package) electronics, e.g., a silicon chip bearing electronics. This embodiment has the advantage of reduced sensitivity to external electromagnetic effects introducing spurious signals on the leads of the previous embodiments, especially if such leads are long. The variable capacitive element and the on-board electronics can be maintained in separate cavities in electrical communication with one another by opposed inductor coils located on either side of a middle substrate. Feedthroughs establishing electrical communication between the interior of the second chamber and the ambient are provided in this case. Such feedthroughs are configured so that moisture does not affect the electronics over the life of the sensor and, optionally, are hermetic. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross-sectional view of a sensor with an electromagnetically-coupled hermetic cavity according to a disclosed embodiment of the present invention, taken along line  1 - 1  of  FIG. 2 . 
           [0019]      FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 . 
           [0020]      FIGS. 3-18  are schematic representation of the steps in manufacturing the sensor of  FIGS. 1 and 2 . 
           [0021]      FIG. 19  is a cross-sectional view of a second embodiment of a sensor in which the sensor is encapsulated by a coating. 
           [0022]      FIG. 20  is a cross-sectional view of a third embodiment of a sensor in which the sensor is substantially completely encapsulated by a  5  coating. 
           [0023]      FIG. 21  is a cross-sectional view of a fourth embodiment of a sensor with an electromagnetically-coupled hermetic cavity according to the present invention. 
           [0024]      FIG. 22  is a cross-sectional view of a fifth embodiment of a sensor with an electromagnetically-coupled hermetic cavity according to the present invention. 
           [0025]      FIG. 23  is a cross-sectional view of a sixth embodiment of a sensor with an electromagnetically-coupled external cavity comprising additional electronics, taken along line  23 - 23  of  FIG. 24 . 
           [0026]      FIG. 24  is a cross-sectional view taken along line  24 - 24  of  FIG. 23 . 
           [0027]      FIG. 25  is a cross-sectional view taken along line  25 - 25  of  FIG. 23 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Referring now to the drawings, in which like numerals indicate like elements throughout the several views,  FIGS. 1 and 2  illustrate a sensor  2  comprising a sensor body  4 . The sensor body  4  is formed from electrically insulating materials, particularly biocompatible ceramics and polymers. Suitable ceramic materials include glass, fused silica, sapphire, quartz, or silicon. Suitable polymeric materials include polyimide, liquid crystal polymer (LCP), urethane, polyester, Teflon, FEP, PTFE, polyamide and silicone rubber, treated or configured such that the permeation of these materials is reduced to a level such that leakage rates are commensurate with the definition of hermeticity provided herein. Additionally, the sensor  2  can incorporate radiopaque features to enable fluoroscopic visualization during placement within the body. 
         [0029]    The sensor body  4  defines a hermetic chamber  6 . One of the walls  7  defining the hermetic chamber  6  comprises a deflectable region  8  configured to deflect under a physiologically relevant range of pressure. In one embodiment, the wall  7  of the pressure-sensitive hermetic chamber  6  is thinned relative to other walls of the sensor body  4  to form the deflectable region  8 . 
         [0030]    Within the hermetic chamber  6  of the sensor  2 , a pair of planar spiral coils  56  and  60  are disposed in parallel, spaced-apart relation. The spiral coils  56  and  60  are not DC coupled, i.e., not connected by a conductive trace. The spiral coils  56  and  60  comprise a distributed LC circuit. In a distributed LC circuit, the inductance and capacitance are distributed across the entire planar spiral circuit. The first planar spiral coil  56  is fixed to the upper wall  7  of the hermetic chamber  6 . The second planar spiral coil  60  is oppositely-disposed to the first planar spiral coil  56  and fixed to a lower wall  16  of the hermetic chamber  6 . 
         [0031]    Located on the opposite side of the lower wall  16  is a third planar spiral  64 . The third planar spiral coil  64  is embedded in a ceramic or polymeric material which comprises the secondary sensor body  21  so as to isolate the coil  64  from the surrounding environment and to stabilize it with respect to the second planar spiral coil  60 . A first metal trace  20  and a second metal trace  22  extend from opposing ends of the third planar spiral coil  64  to the exterior of the secondary sensor body  21 . The metal traces  20 ,  22  can be further connected to a bond pad (not shown) on the exterior of the secondary sensor body  21  that will allow for other electronics (not shown) to be placed in electrical communication with the sensor  2 . Alternatively, the metal traces  20 ,  22  can further comprise wires extending from the sensor package to which other electronics (not shown) can be placed in electrical communication with the sensor  2 . 
         [0032]    When the third coil  64  is energized by an external AC signal generator, the three planar spiral coils  56 ,  60 , and  64 , respectively, act as inductors. The circuit contained within the hermetic chamber  6  exhibits the electrical characteristics associated with a standard inductor-capacitor (LC) circuit. In the embodiment discussed herein, the LC circuit is in part distributed as known in the art. If a current is induced in the LC circuit at a particular frequency known in the art as the resonant frequency of the circuit, the resultant energy will be maximally shared between the inductor and capacitor. The result is an energy oscillation that will vary at a specific frequency. This frequency is termed the “resonant frequency” of the circuit, and it can easily be calculated from the circuit&#39;s inductance and capacitance. Therefore, a change in capacitance or inductance will cause the resonant frequency to shift higher or lower, depending upon the change in the value of variable element(s). Further, the value of the resonant frequency can be inferred from the electrical characteristics of external coil  64 , e.g., by monitoring the impedance of coil  64  as a function of frequency as known in the art. Since mechanical deflection of the deflective region  8  alters the value of the characteristic distributed capacitance and inductance and therefore the resonant frequency, and since the deflection of the deflective region  8  is dependant on the external pressure, detection of this resonant frequency using the coil  64  therefore allows determination of the pressure in which the sensor  2  is embedded without the need for direct electrical connection with the circuitry inside the hermetic chamber  6 . 
         [0033]    A disclosed method for fabricating the sensor  2  depicted in  FIGS. 1 and 2  is based on the micromachining of at least two substrates that are subsequently brought into contact and fused together. The manufacturing process described herein and illustrated in  FIGS. 3-18  comprises a series of etching, patterning, and deposition processes to create depressions and planar spiral coils on the surfaces of the substrates. More specifically, a first substrate is subjected to a series of processes to create a local depression of known depth and to deposit a planar spiral coil at the bottom of the depression. Next, a second substrate is subjected to similar processing as the first substrate to create a complementary planar spiral coil whose overall footprint and in-plane position correspond to the footprint and in-plane position of the planar spiral coil on the first substrate. Creation of a depression in the surface of the second substrate is optional and depends on the desired final configuration of the sensor. Optionally, the first substrate is then subjected to additional processing on the side of the substrate opposite the previously formed planar spiral coil to deposit a third planar spiral coil whose overall footprint and in-plane position correspond to the footprint and in-plane position of the planar spiral coil previously deposited on the first substrate. 
         [0034]    Alternatively, a third substrate is subjected to an identical series of preparatory steps as the first two substrates and presents a planar spiral coil that does not protrude past the uppermost surface of the substrate (i.e., it is recessed) and whose overall footprint and in plane position corresponds to the footprint and in-plane position of the planar spiral coil on the first substrate when the substrates are brought into contact. The configuration of the planar spiral coils can be altered to provide for a variety of geometries as manufacturing and/or performance advantages may dictate. The first two substrates are brought into face-to-face contact, forming the chamber, and, optionally, a third substrate is then brought into intimate contact with the back side of first substrate. Alternatively, if the third set of planar coils are plated on the back side of the first substrate, electrical contact with the coils are provided for and the coils are isolated from the surrounding environment by a polymer or ceramic material. The components are aligned as shown in  FIG. 1 . The substrates form a temporary bond because of the presence of Van der Waals forces. The planar spiral coils contained within a chamber formed upon imposition of the first and second substrates are separated by a gap of known value, i.e., the difference between the sum of the depths of the recessed region and the sum of the thicknesses of the electrodes. A laser is then used to excise the sensor into its final overall dimensions from the at least two-substrate stack. 
         [0035]    The laser cutting operation fuses the substrates, hermetically sealing the sensor and trapping air or any other desirable gas in the hermetic cavity of the sensor, or creating a vacuum within the hermetic cavity of the sensor. In one example, a CO 2  laser operating at a peak wavelength of ten microns is used to hermetically seal and to reduce the sensor to its final size. The laser energy is confined to a precise heat effect zone where the substrates are fused, eliminating any material interface between the original substrates. 
         [0036]    The resulting sensor presents three mutually-arranged planar spiral coils that, through inductive coupling, are capable of sensing ambient pressure. More particularly, a change in environmental pressure causes the deflective region to be displaced, and the capacitor electrode fixed to that region moves with respect to the coil formed on the first substrate. Thus, the electrical characteristics (e.g., energy loss, phase change) of the LC circuit located within the hermetic chamber are altered. When an external AC signal source is placed in connection with the third coil, electromagnetic coupling supplies energy to the first two coils, which are located in magnetic proximity to the third coil. The resulting electromagnetically-coupled circuit, i.e., the three mutually-imposed planar spiral coils, will change predictably in response to external changes in pressure. 
         [0037]    The manufacturing of the sensor  2  depicted in  FIGS. 1 and 2  from the substrate (a.k.a. wafer) level to the final device is described in greater detail below. For clarity, the manufacture of the sensor  2  is described on a single-sensor basis, although it will be understood that multiple sensors are preferably created simultaneously on the substrate in a batch process to increase manufacturing efficiency. 
         [0038]    The lower substrate is processed to create a recessed region in its surface and planar spiral coils at the bottom surface of the recessed region. Creation of a recessed region with known geometry comprises the steps of (i) depositing and patterning a mask at the surface of the wafer, (ii) etching the wafer material through openings in the mask, and (iii) removal of the mask. 
         [0039]    One method for creating the desired recessed region is depicted in  FIGS. 3-10  and described as follows. Referring first to  FIG. 3 , a seed layer  100  is deposited on the surface of a fused silica substrate  102  and comprises a 1000-2000 Angstrom layer of Cr/Cu. As can be seen in  FIG. 4 , a layer  104  of photoresist is formed atop the seed layer  100 . Then, as shown in  FIG. 5 , photolithographic techniques are used to reduce the photoresist layer to provide an island  106  of photoresist that defines the perimeter of a desired recessed region. This photoresist is approximately 5-10 micrometers thick, depending on the desired height of the metal to be plated in the subsequent step. 
         [0040]    Next, as shown in  FIG. 6 , a metal (e.g., Ni) is plated to a selected height, in the range of 5-10 micrometers in the present example, to form a second mask  108 . Then, as shown in  FIG. 7 , the photoresist and underlying seed layer are removed via selective etchants and solvents to expose the underlying substrate  102  in a central region  109 , thereby forming a masked substrate  110 . Next, the masked substrate  110  is subjected to further selective etchants such as hydrofluoric acid solutions, to remove the fused silica in the exposed central region  109  to a desired depth, e.g., 70 micrometers, as shown in  FIG. 8 . Then, the second mask  108  and remaining seed layer  100  are removed with selective etchants and solvents to form the etched substrate  112  with recessed region  109 , as shown in  FIG. 9 . 
         [0041]    Now, thick planar spiral coils are created in the bottom of the recessed region  109  of the etched substrate  112 . To this end, as shown in  FIG. 10 , a second seed layer  116  and second phototoresist layer  118  are is deposited on the etched substrate  112 . The photoresist layer  118  is 25-35 micrometers in height, depending on the desired height of the planar coil created in the next step. Then, photolithographic techniques are used to etch away material to create recesses  120  in the photoresist layer, thereby forming a mold defining the desired planar coil pattern, as shown in  FIG. 11 . A thick metal planar spiral  60  is then formed by electroplating Cu to a height of 25-35 micrometers on the exposed seed layer  116 , as shown in  FIG. 12 . The photoresist  118  and seed layer  116  underlying the photoresist are then removed via selective etchants and solvents. 
         [0042]    At this point, as depicted in  FIG. 13 , the etched lower substrate  112  is patterned with a recessed region  114  etched into its upper surface, and the base of the recessed region  114  contains a planar spiral coil  60  of known thickness. The difference between the height of the upper surface of the planar spiral, H 1 , and depth D 1  of the recessed region  114  in lower substrate  112 , is substantially constant (excepting for inherent variations in the substrate and patterned metal), and these dimensions are known with great precision, i.e. fractions of micrometers. 
         [0043]    Referring now to  FIG. 14 , an upper substrate  150  is provided and a second planar spiral  56  is created using the same photolithographic and metal deposition processes as those described for the lower substrate  112 . 
         [0044]    The substrates  112 ,  150  are then aligned, subjected to bonding, and reduced to the final overall dimension of the sensor as shown in  FIG. 1  according to the following description: Both the upper and lower substrates  112 ,  150  are prepared for assembly, e.g., by cleaning. The patterned surfaces of the substrates are faced and aligned so that the planar spiral coils  56 ,  60  created in each substrate are positioned directly on top of one another. The two substrates  112 ,  150  are brought together and placed in intimate physical contact, as shown in  FIG. 15 . A temporary bond is formed because of Van der Waals forces existing between the two substrates. As previously described, a gap is maintained between the first planar spiral coil  56  and the second planar spiral coil  60  where the distance between the planar spiral coils is precisely known. Referring to  FIG. 16 , using a CO 2  laser, indicated by the arrows  160 , the sensor is reduced to its final dimensions. The laser cutting process also fuses the upper and lower substrates  112 ,  150  substantially seamlessly. The pressure cavity body  152  resulting from the above steps is depicted in  FIG. 17 . 
         [0045]    With further reference to  FIG. 17 , the power of the CO 2  laser is controlled such that heat damage to the internal components is avoided. Consequently it is possible that some vestige of a seam  162  may remain between the upper and lower substrates  112 ,  150 . So long as the outer periphery of the pressure cavity body  152  is completely fused, the interior chamber  6  will be hermetic. 
         [0046]    At some point during the process outlined above, the third planar spiral coil is provided. This feature can be achieved two ways: (i) by providing a third substrate with a third planar spiral coil recessed within a depression and adding this substrate to the two wafer stack before they are fused or (ii) by depositing a third planar spiral coil on the bottom of the first substrate  112  and covering the coil with a polymer or ceramic material to isolate it from the surrounding environment (if necessary). 
         [0047]    If the first method is utilized, a third substrate  160  is subjected to identical processing steps as that used in the creation of etched substrate  112 . The resultant third substrate  160  presents a planar spiral coil  64  that does not protrude above the top of a recessed region  164 . This arrangement is shown n  FIG. 18 . This substrate  160  is then aligned and temporarily bonded to the lower surface of substrate  112 . Electrical contact with the third planar spiral coil  64  is made by laser rastering and/or chemical etching through the back side of the third substrate  160  to expose the back side of the coil. Then, electrically conductive material is deposited in the passage, thereby forming electrical feedthroughs  166 . The manufacture of these feedthrough structures is detailed in co-pending U.S. patent application Ser. No. 11/314,046 filed Dec. 20, 2005, which application has previously been incorporated by reference. This step can take place at the substrate level or after the third substrate  160  is bonded to the first two substrates. Also, two or more feedthroughs can be provided as dictated by any additional electronics. This method for providing the third or external coil  164  is particularly desirable when the final device is intended to be implanted chronically in a patient, for it presents inductor coils which will not change position with respect to one another over very long periods of time, thereby minimizing a potential cause of drift within the sensor. 
         [0048]    Referring now to  FIG. 19 , if the second method is utilized, the first substrate  112  is subjected to additional photolithography and electrodeposition steps to create the third planar spiral coil  178  directly on the lower surface of the pressure cavity body  152 . Then, at least the lower surface of the device, and optionally the entire sensor, is encapsulated in a polymeric material  180 . Silicone rubber is a preferred material for this purpose. In this case, electrical contact of the third planar spiral coil  178  with external inductor leads  182  can be established before the polymer is applied via, e.g., conventional wire bonding techniques. Alternatively, the terminal ends of the coil can remain exposed or later be exposed, as shown in  FIG. 20 , to accomplish electrical contact subsequent to the application of the polymer coating. 
         [0049]      FIG. 21  illustrates a sensor  80  that is a variation of the sensor  2  depicted in  FIGS. 1 and 2 . In  FIG. 21  the sensor  80  comprises a first planar wire spiral coil  24  and a second planar wire spiral coil  26 . The first spiral coil  24  terminates in a first electrode  28 , and the second spiral coil  26  terminates in a second electrode  30 . Here, electrodes,  28  and  30 , are formed integral with the interior terminal ends of the planar spiral coils  24  and  26 , respectively. The mutually-imposed electrodes  28 ,  30  form a “lumped” capacitor. The addition of this lumped capacitor serves to increase the capacitance and to shift the resonant frequency in a controllable manner. In a variation of this configuration, the planar spiral coils may be fixed so that they do not change position in response to changes in environmental pressure, thereby limiting the deflective region to at least one of the capacitor plates comprising the lumped capacitor. In this configuration, the inductance of the circuit is fixed, and the capacitance is variable. 
         [0050]    Yet another sensor  200  according to the present invention is shown in  FIG. 22 . The sensor comprises an inductor  202  and a capacitor  204  housed within a hermetic chamber  206 . In the sensor  200 , the inductor  202  and  20  capacitor  204  are arranged as a “lumped” LC circuit. In a lumped LC circuit, the inductor and the capacitor comprise two discrete elements. Thus, the inductance value of the circuit is substantially wholly attributable to a discrete inductor component while the capacitance value is substantially wholly attributable to a discrete capacitor component. It is a further aspect of this example that the inductor  202  comprises a three dimensional, helical coil structure, as opposed to the planar coil structure of the prior examples. Further, the capacitor comprises three capacitor plates,  208 ,  210 , and  212 . Either the inductor  202  or the capacitor  204  can be configured to vary in response to external pressure, although it is preferred that the capacitor be configured to do so. To this end, at least one capacitor plate  208  is fixed to a deflective region  214  on an upper substrate  216 . The manufacture of the hermetic chamber  206  is detailed in co-pending U.S. patent application Ser. No. 11/157,375 filed Jun. 21, 2005, previously incorporated herein by reference. To modify the wireless sensor disclosed in the &#39;375 application to the present invention, an external inductor coil  220  is provided. 
         [0051]    In one aspect of this embodiment, a third substrate  222  is provided that comprises a trench  224  containing the external inductor coil  220 . In this case, the coil leads  228 ,  230  are connected to electrical feedthroughs  232 ,  234  traversing the lower wall  238  of the third substrate  222 . Bond pads can be provided for on the exterior surface of the device as a means to connect the sensor to other electronics. The manufacture of these feedthrough structures  232 ,  234  are detailed in co-pending U.S. patent application Ser. No. 11/314,046 filed Dec. 20, 2005, previously incorporated herein by reference. 
         [0052]    Alternatively, the external inductor coil  220  can be attached directly to the lower surface of the sensor and encased in a polymeric material such as polyimide or silicone. In this case, the leads of the coil  220  can be utilized to attach the device to further electronics. Alternatively, separate wires can be connected to the coil  220 , either before or after the coil has been encased in the polymeric material. 
         [0053]    As an important note, the electrical resistance of the circuit is a function of the frequency of the final device. Thin film techniques and electroplating are two effective methods for creating the necessary metal traces comprising the circuitry. The metal deposition technique chosen should be capable of forming “thick” metal traces. The tendency for high-frequency currents to flow on the surface of a conductor is known as the “skin effect.” Skin depth is the distance from the surface of the conductor at which the current flows and increases with decreasing frequency. The term “thick” as used herein should be construed to mean thick with regards to the skin depth of the metal at a frequency of interest. The relationship between skin depth and frequency is: 
         [0000]    
       
         
           
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                   * 
                   σ 
                 
               
             
           
         
       
     
         [0000]    where d is the skin depth, f is the frequency, μ is the magnetic permeability, and σ is the electrical conductivity. Thus, the lower end of the desired range of frequency will determine the minimum skin depth of the wire used in construction of the circuit. If a wire thickness that is insufficient to accommodate the skin depth corresponding to the desired minimum frequency response is used, the resistance of the circuit will necessarily increase. Conversely, use of wire that is too thick needlessly increases the size of the inductor. Thus, the manufacturing technique chosen to form the metal traces comprising the circuit is selected with the above considerations in mind. (One exception is the case of the three dimensional inductor coils which are optionally constructed of pre-fabricated wire that is selected to have the appropriate thickness according to the above considerations). 
         [0054]    In any of the above embodiments, the external inductor coil can be attached to further electronics. For the purpose of illustration, a sensor  250  similar in principle to the sensor  2  of  FIGS. 1 and 2  is shown in  FIGS. 23-25  with additional electronics incorporated into the sensor package. The sensor  250  comprises a sensor body  252  defining a hermetic chamber  254  and an external chamber  256 . The upper wall  260  of the sensor body  252  comprises a deflectable region  262 . An upper coil  264  is mounted within the hermetic chamber  254  to the lower side of the deflectable region. A lower coil  266  is mounted to the lower wall  268  of the hermetic chamber. An external coil  270  is  15  mounted within the external chamber  256  within magnetic proximity to the lower coil  266 . 
         [0055]    Additional electronics are mounted within the external chamber  256  and comprise one or more silicon chips  280  bearing electronics. The additional electronics should be located sufficiently far away from the electromagnetic field to avoid any deleterious, parasitic interaction but should be close enough to preserve the small size of the device. It is possible for the additional electronics to be positioned vertically (not shown) or horizontally (as shown) relative to the inductor element. The leads  282 ,  284  of the inductor coil are placed in electrical communication with the silicon chip  280  via, e.g., conventional wire bonding techniques or by deposition of metal traces configured to provide conductive paths between the components. The silicon chip  280  is fixed in the cavity  256  either mechanically (e.g., press fit), with an adhesive (e.g., epoxy or polyimide), via use of flip chip interconnects, or via eutectic bonding using additional metallic interfaces (i.e., using soldered performs), or other methods known in the art. The silicon chip  280  can be placed in electrical communication with external electronics by way of feedthroughs  286 . 
         [0056]    One example of useful circuitry on the silicon chip  280  would be a high gain amplifier that causes oscillations at the natural resonant frequency of the arrangement of conductors  264 ,  266 . In this approach, the output of the chip  280  consists of a frequency that depends on the external pressure. The geometry of the external chamber  256  needed to contain the electronics  280  and inductor coil  266  can be modified in various ways which are obvious to one skilled in the art. 
         [0057]    It is also possible to contemplate positioning the silicon chip  280  within the hermetic cavity  254  instead of, or in addition to, the external cavity  256 . This may have advantages when a very high degree of compactness is required, and/or if the chip is required to operate in a hermetic environment with no direct feedthroughs. In this embodiment, electrical energy for chip operation can be supplied in a similar fashion to the energizing of conductors  268  and  264 , with the addition of appropriate rectification and filtering circuitry on or adjacent to the chip  280  depending on the mode of energizing utilized. 
         [0058]    Note that the sensors depicted above are all designed for maximum reliability because there are no conductor bonds or joints or feedthroughs breaching the hermetic cavity, and there are no wires connecting the two sides of the LC circuit contained within the pressure cavity. It is possible to provide a wire connecting the two sides of the LC circuit inside the pressure cavity to provide for DC coupling, but it is not necessary, and the absence of such a feature eliminates a potential site for failure of the device. 
         [0059]    Specific embodiments have been described herein, by way of example and for clarity of understanding, and variations and modifications to the present invention may be possible given the disclosure above. Hence the scope of the present invention is limited solely by the appended claims.