Abstract:
A pressure cavity is durable, stable, and biocompatible and configured in such a way that it constitutes pico to nanoliter-scale volume. The pressure cavity is hermetically sealed from the exterior environment while maintaining the ability to communicate with other devices. Micromachined, hermetically-sealed sensors are configured to receive power and return information through direct electrical contact with external electronics. The pressure cavity and sensor components disposed therein are hermetically sealed from the ambient in order to reduce drift and instability within the sensor. The sensor is designed for harsh and biological environments, e.g. intracorporeal implantation and in vivo use. Additionally, novel manufacturing methods are employed to construct the sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is entitled to the filing date of provisional U.S. Patent Application Ser. No. 60/801,134, filed May 15, 2006 and is a continuation-in-part of U.S. application Ser. No. 11/314,046, filed Dec. 20, 2005, which is entitled to the filing date of 60/651,670, filed Feb. 10, 2005 and 60/653,868, filed Feb. 17, 2005. 

   FIELD OF THE INVENTION 
   The present invention relates to micromachinable, pico- to nanoliter-volume, hermetic packaging that incorporates reliable 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 
   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. 
   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; and U.S. patent application Ser. No. 11/157,375, filed Jun. 21, 2005; U.S. patent application Ser. No. 11/314,696 filed Dec. 20, 2005; and U.S. patent application Ser. No. 11/402,439 filed Apr. 12, 2006 all of which are incorporated herein by reference. 
   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. 
   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. An acceptable level of hermeticity is therefore a rate of fluid ingress or egress that changes the pressure in the internal reference volume (a.k.a. pressure chamber) by an amount preferably less than 10 percent of the external pressure being sensed, more preferably less than 5 percent, and most preferably less than 1 percent over the accumulated time over which the measurements will be taken. In many biological applications, an acceptable pressure change in the pressure chamber is on the order of 1.5 mm Hg/year. 
   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. 
   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 contacts through the cavity walls. As is known in the art, such feedthrough points 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, one of the major challenges to enabling the use of such devices in applications where they are physically connected to other devices has been the creation of reliable hermetic packaging that provides feedthroughs that enable exchange of power and information with external electronics. 
   Design criteria for ultra miniature packaging that overcomes the aforementioned shortcomings are as follows: The packaging must exhibit long term hermeticity (on the order of the life of the sensor, which in some cases can exceed tens of years). Feedthroughs must be provided through the hermetic package that do not introduce new or unnecessary potential modes of failure. The feedthroughs will constitute a necessary material interface, but all other interfaces can and should be eliminated. In other words, the number and area of material interfaces should be minimized to reduce the potential for breach of hermeticity. The materials selected must be compatible with the processes used to fabricate the package as well as sufficiently robust to resist deleterious corrosion and biocompatible to minimize the body&#39;s immune response. Finally, the packaging should be amenable to batch fabrication. 
   In the past, many methods for creating such hermetic packages have been proposed. One approach used in the past to create the pressure cavity is anodic bonding to create a silicon-to-glass seal. A borosilicate glass is required for this method. Another technique utilized in the creation of hermetic packages is eutectic bonding to create a silicon to metal hermetic seal, e.g. Au to Si. Both of these bonding methods used to create the pressure cavity introduce a large area along the perimeter of the material interface of the pressure cavity package which presents opportunity for failure, e.g. through corrosion. These methods for creating the pressure cavity do not minimize the area of the material interface as is desirable. A desirable improvement to the construction of the pressure cavity would minimize the material interface to the hermetic electrical feedthroughs, and, even further, minimize the number and area of material interfaces in those feedthroughs. 
   Previous attempts to create hermetic feedthroughs also fall short of the above-stated requirements. Many prior art hermetic feedthroughs are too large and not amenable to the required miniaturization for pico- to nanoliter volume packaging achievable by MEMS or similar approaches. Furthermore, earlier attempts to create feedthroughs in pico to nanoliter packaging are prone to corrosion because of the materials used in construction or are sufficiently complicated that they introduce more material interfaces than are necessary. A representative feedthrough approach, known as a “buried” feedthrough, is illustrated in  FIGS. 1-5 . One method for creating a buried feedthrough is as follows: a metal  10  is deposited onto substrate  12  in a predefined pattern, as shown in  FIG. 1 . An insulating layer  14  is deposited on top of the metal layer, as shown in  FIG. 2 , and this insulating layer  14  is polished to planarize this surface. In  FIG. 3  an etchant has been used to expose the metal layer at input and output sites  16 ,  18  for the feedthroughs. In  FIG. 4 , another substrate  20  is bonded on top of this structure, forming a hermetic cavity  22 . A eutectic bonding method is illustrated, which involves the use of gold deposits  24  interposed between the insulating layer  14  and the upper substrate  20  to bond the upper substrate to the insulating layer. In  FIG. 5  the upper substrate  20  is machined to expose the external feedthrough  18 . An electrical conductor can now be connected to the external feedthrough  18 , whereupon it is conducted through the metal  10  to the internal feedthrough  16  and thus to a location within the hermetically sealed chamber  22 . 
   This prior art buried feedthrough suffers a number of disadvantages. First, there are numerous material interfaces: an interface  30  between the lower substrate  12  and the metal  10 ; an interface  32  between the metal  12  and the insulating layer  14 , an interface  34  between the insulating layer  14  and the gold  24 ; and an interface  36  between the gold  24  and the upper substrate  20 , all of which create potential paths for infusion into or effusion out of the hermetic chamber  22 . The creation of this buried feedthrough also introduces increased processing steps. Further, the insulating layer material is cited as being prone to corrosion in certain environments, e.g. the human body. Corrosion issues may be further exacerbated by the application of electrical bias to metal  10  which may be required in certain applications. Thus prior art hermetic feedthroughs fall short of meeting the constraints outlined above. 
   Also, many prior art attempts to provide pressure sensors utilize silicon as a substrate material. If the package is implanted in vivo, silicon is not an optimal material choice. Silicon invokes an undesirable immune response over other, more inert materials such as fused silica. If silicon is used, a coating must be applied to ensure biocompatibility. Such a coating increases the package size, thereby decreasing the benefits of miniaturization, and introduces an undesirable additional processing step in the manufacture of the package. 
   Additionally, prior art devices commonly employ the use of borosilicate glass as part of the pressure cavity. The ions in borosilicate glass constitute an impurity in the glass. The barrier to diffusion of water decreases as the purity of glass decreases. This makes use of impure glass undesirable in such applications. 
   Thus a need exists for hermetic pico to nanoliter packaging with electrical feedthroughs for use in biological environments, such packaging being constructed of high-purity materials and having a reduced number and area of material interfaces. 
   SUMMARY OF THE INVENTION 
   The present invention comprises a micromachinable, hermetic, pico to nanoliter-volume pressure cavity. Such a pressure cavity utilizes high-purity materials and provides reliable electrical feedthroughs. The pressure cavity is constructed of a ceramic material and is optionally fused together so that there is no interface of material where two substrates have been joined to create a cavity. Furthermore, feedthroughs establishing electrical communication within said cavity are formed in at least one of the substrates. The feedthroughs themselves are configured in such a way that the number and area of material interfaces is minimized. Such feedthroughs constitute the only site for material interface in the sensor package, thereby decreasing the number of potential leak sites in and increasing the reliability of the hermetic package. Pressure cavities and sensors of the present invention are 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. 
   The present invention further comprises a sensor 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 sensor is configured to measure one or more physical properties such as pressure or temperature. Communication between the sensor and another device can be established by, e.g., using wires fixed to bonding pads on the exterior of the sensor packaging that are configured so that they are in electrical contact with the hermetic feedthroughs. As another example, the hermetic electrical feedthrough can have a wire extending from the feedthrough, and contact with the pressure cavity can be accomplished via connection with this wire. Devices in electrical communication with sensors according to the present invention may be either implanted or external to the body. Sensors of this invention are sufficiently small to allow for incorporation into medical leads or catheters that are twelve French or smaller, preferably six French or smaller, without causing abrupt changes in geometry of the lead or catheter, and require minimal power to perform their intended function. 
   In one embodiment of the invention, a wired sensor ascending to the present invention comprises a hermetic pressure cavity. The pressure cavity further comprises a capacitor configured so that the characteristic capacitance value of the capacitor varies in response to a physical property, or changes in a physical property, of 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. Additionally, electrical feedthroughs are formed through the substrate defining the pressure cavity and allow for the sensor to receive power and signals, and return information to either implanted or extracorporeal external electronics. 
   In another embodiment of the invention, a wired sensor according to the present invention comprises a hermetic pressure cavity. The pressure cavity further comprises a Wheatstone bridge configured so that the resistance value of said bridge varies in response to a physical property, or changes in a physical property, of a patient. The pressure cavity has at least one deflectable region in mechanical communication with at least one of the resistors comprising the bridge. Additionally, electrical feedthroughs are formed through the substrate and allow for the sensor to receive power and signals, and return information to external electronics. It is a further aspect of this invention that only a portion of the Wheatstone bridge be located within the pressure cavity, the other portion being contained within external electronics. 
   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. The variable capacitive or resistive element and the on-board electronics can be maintained in separate cavities in electrical communication with one another by hermetic feedthroughs formed through a middle substrate. Feedthroughs establishing electrical communication with the sensor exterior may be configured so that moisture does not affect the electronics over the life of the sensor and, optionally, are also hermetic. This configuration offers the advantage that the feedthroughs to the on-board electronics act as a redundant barrier to any potential breach of the hermeticity of the pressure cavity. Alternatively, the capacitor and on-board electronics can be contained within a single hermetic cavity. This configuration offers the advantage of decreased manufacturing steps, thereby lowering the overall cost to produce the sensor. In either case, electrical feedthroughs, which are themselves optionally hermetic, formed through the substrates comprising the external walls allow for the sensor to receive power and return information to external electronics. 
   In yet another embodiment, a device of this invention comprises: a housing having walls defining a chamber, a first one of said walls defining said chamber comprising an exterior wall of said housing; a chip bearing electronics located within said housing and comprising at least one wire bond for enabling electrical communication to said electronics; a passage through said first one of said walls placing the chamber of said housing in communication with the ambient; an electrode hermetically imposed over said passage within said chamber of said housing, whereby said passage is hermetically sealed; and means for establishing electrical connection between said wire bond and said electrode; whereby said chamber is hermetically sealed; and whereby an external electrical device can be placed in electrical communication with said electrode through said passage. 
   In another embodiment, a device further comprises a second electrode deposited on said first one of said walls within said chamber; a chip further comprises a second wire bond for enabling electrical communication to said electronics; and means are provided for establishing electrical connection between said second wire bond and said second electrode. 
   In yet another embodiment, a device further comprises a second one of said walls defining said chamber located opposite said first wall, said second one of said walls comprising an exterior wall of said housing and a deflectable region; a third electrode deposited on said second one of said walls within said chamber in said deflectable region; a fourth electrode deposited on said first one of said walls and a third wire bond operatively associated with said chip, means for establishing electrical connection between said third wire bond and said fourth electrode; a fifth electrode is deposited on said first one of said walls and a fourth wire bond operatively is associated with said chip; and means for establishing electrical connection between said fourth wire bond and said fifth electrode; wherein said third, fourth and fifth electrodes comprise a capacitor. 
   In yet another embodiment, a method of manufacturing a device for securing micro-devices comprises: selecting a first substrate having a height less than the height of the micro-device; cutting holes through the first substrate shaped to conform to the configuration of a micro-device intended to be held there within; placing a second substrate in contact with the first substrate and bonding the first and second substrates; and placing at least one micro-device into a corresponding recess. 
   In another embodiment, a system for securing micro-devices, comprises: a device comprising a plurality of recesses, each recess shaped to house a micro-device; at least one micro-device housed in one of said recesses; and a mask; wherein said device protects said at least one micro-device during processing or shipping. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a first step in manufacturing a PRIOR ART hermetic chamber with electrical feedthroughs. 
       FIG. 2  is a schematic representation of a second step in manufacturing a PRIOR ART hermetic chamber with electrical feedthroughs. 
       FIG. 3  is a schematic representation of a third step in manufacturing a PRIOR ART hermetic chamber with electrical feedthroughs. 
       FIG. 4  is a schematic representation of a fourth step in manufacturing a PRIOR ART hermetic chamber with electrical feedthroughs. 
       FIG. 5  is a schematic representation of a completed PRIOR ART hermetic chamber with electrical feedthroughs. 
       FIG. 6  is a schematic representation of a hermetic chamber with electrical feedthroughs according to a disclosed embodiment of the present invention. 
       FIGS. 7-25  are schematic representation of the steps in manufacturing the hermetic chamber of  FIG. 6 . 
       FIG. 26  is a schematic representation of a hermetic chamber with electrical feedthroughs according to a second disclosed embodiment of the present invention. 
       FIG. 27  is an electrical schematic of a piezoresistive transduction scheme for measuring changes in the position of the deflectable region in the pressure cavity of the hermetic chambers of  FIGS. 6 and 26 . 
       FIG. 28  is a schematic representation of a hermetic chamber with electrical feedthroughs according to a third disclosed embodiment of the present invention. 
       FIG. 29  is a cutaway view of a fourth disclosed embodiment of the present invention as seen along line  29 - 29  of  FIG. 30 . 
       FIG. 30  is a cutaway view as seen along line  30 - 30  of  FIG. 29 . 
       FIG. 31  is a schematic representation of a fixture with recesses according to an embodiment of the present invention. 
       FIG. 32  is a partial side view in cross section of the fixture of  FIG. 31 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, in which like numerals indicate like elements throughout the several views,  FIG. 6  illustrates a sensor  50  that includes a pressure cavity body  51  defining an internal pressure chamber  52 . One of the walls defining the pressure cavity  52  comprises a deflectable region  54  configured to deflect under a physiologically relevant range of pressure. In a preferred embodiment, a wall of the pressure cavity body  51  is thinned relative to other walls of the pressure cavity body to form the deflectable region  54 . The sensor  50  can be fabricated using micromachining techniques and is small, accurate, precise, durable, robust, biocompatible, and insensitive to changes in body chemistry or biology. Additionally, the sensor  50  can incorporate radiopaque features to enable fluoroscopic visualization during placement within the body. The sensor  50  is preferably formed using electrically insulating materials, particularly biocompatible ceramics, as substrate materials. Suitable materials are selected from a group comprising glass, fused silica, sapphire, quartz, or silicon. In one embodiment, fused silica is the substrate material. 
   A capacitor comprises a pair of lower electrodes  56 ,  57  located on a first wall  58  of the chamber  52 . The two lower electrodes  56 ,  57  are electrically isolated from one another. A third electrode  60  is disposed on an opposite wall  62  of the pressure cavity  52  in parallel, spaced apart relation to the lower electrodes  56 ,  57 . The upper electrode  60  is mechanically coupled to the deflectable region  54 . As ambient pressure increases, the deflectable region  54  moves inward, displacing the upper electrode  60  toward the lower electrodes  56 ,  57 , thereby changing the characteristic capacitance value of the capacitor. 
   The capacitor configuration depicted here is one example where the lower capacitor electrode consists of two electrically isolated regions,  56  and  57 , although other configurations are possible and obvious to one skilled in the art. 
   The lower portion of the pressure cavity  52  comprises passages  64 ,  65  that traverse the hermetic pressure cavity body  51  and are in contact with the electrodes  56 ,  57 . As shown in  FIG. 6 , electrical contact pads  66 ,  67  can be formed on the back side of the electrodes  56 ,  57  and extend to the exterior of the housing, thereby providing a region on the exterior of the sensor  50  configured with sufficient dimensions so as to allow for a means for connection with external electronics. As an alternative, the passages  64 ,  65  can be filled with an electrically conductive material, with contact pads  66 ,  67  in electrical communication with the electrodes  56 ,  57  by way of the conductive material  68 . The electrode  56 , the passage  64 , and, if present, the electrical contact pad  66  and any electrically conductive material  68  filling the passage  64  comprises a first electrical feedthrough  70 . The electrode  57 , the passage  65 , and, if present, the electrical contact pad  67  and any electrically conductive material  68  filling the passage  65  comprises a second electrical feedthrough  71 . 
   It is a preferred embodiment of this invention that the metal-fused silica interface between the lower electrodes  56 ,  57  and the interior surface of the pressure cavity body  51  be hermetic. The electrical contact pads  66 ,  67  can occupy either all or part of the passages  64 ,  65 . A variety of metal deposition techniques can be used (e.g., electroplating, use of molten metal, or PVD) depending on the choice of metal and desired material properties. In the case of a partially-filled feedthrough passage  64 ,  65 , a void inside the feedthrough passages and above the electrical contact pads  66 ,  67  will remain. In order to fill these voids and to enhance the strength of the feedthroughs  70 ,  71 , any remaining space in the passages  64 ,  65  can be filled with a ceramic material. Glass frit is one example of a ceramic material that can be used to fill the remaining space and heated sufficiently that the material flows, thereby eliminating any voids in the ceramic material. In the case of metal-filled feedthrough cavities, the pads  66 ,  67  on the exterior of the package are formed by, e.g., fusion bonding, low pressure plasma spray, laser welding, electroplating or PVD, depending on the choice of metal and the desired material properties. The electrical contact pads  66 ,  67  provide a site to connect to external electronics. 
   Suitable non-refractory metals for the electrical feedthroughs include gold, platinum, nickel, and silver and alloys thereof. Suitable refractory metals include niobium, titanium, tungsten, tantalum, molybdenum, chromium, and a platinum/iridium alloy and alloys thereof. If refractory metals are used to construct the feedthroughs, either alternating or direct current may be used to bias the sensors by external electronics. If any other metals are used, the sensors should be biased under AC power to prevent the onset of bias-induced corrosion. 
   The pressure cavity  52  is hermetic for the following reasons. First, the pressure cavity body  51  is formed of a hermetic material and is a unitary structure, meaning there are no seams or bi-material joints that can form a potential path for gas or fluid intrusion into the pressure chamber other than the passages  64 ,  65 , which themselves are hermetically sealed. One reason for the hermeticity of the passages  64 ,  65  is that the electrodes  56 ,  57  are hermetically imposed onto the wall  58  over the feedthroughs. The electrodes  56 ,  57  (along with any other metallic structure fixed to the ceramic substrate) optionally form an intermetallic compound. An intermetallic compound is formed between a metal and a substrate when chemical reactions take place that result in the formation of covalent bonds between two or more elements, with at least one of the elements coming from the substrate and one from the metal. Optionally, the material  68  filling the passages  64 ,  65  is itself capable of hermetic sealing such that the interface between the material  68  and the material defining the feedthrough passages is also hermetic. Thus gas or fluid would have to pass through or around the material  68  in the passages  64 ,  65  and pass through or around the electrodes  56 ,  57  before it could enter the pressure chamber and compromise its integrity. And finally, the passages  64 ,  65  are small, thereby minimizing the area of interface and reducing the probability of flaw creation and propagation. In the disclosed embodiments, the passages have cross-sectional areas ranging from 10 −6  to 10 −9  square meters. 
   A disclosed method of fabricating the sensor  50  depicted in  FIG. 6  is based on the micromachining of two substrates that are subsequently brought into contact and cut into individual sensors. The manufacturing process described herein and illustrated in  FIGS. 7-25  comprises a series of etching, deposition and patterning processes to create depressions and electrodes on the surfaces of the substrates. More specifically, a first substrate is subjected to a series of processes to create local depressions of known depth and to deposit and pattern thin film electrode(s) at the bottom of the depressions. Next, a second substrate is subjected to similar processing as the first substrate to create complementing electrode(s) whose overall footprint and in-plane position correspond to the footprint and in-plane position(s) of the electrode(s) on the first substrate. Creation of depressions in the surface of the second substrate is optional and depends on the desired final configuration of the sensor. The first substrate is then subjected to additional processing on the side of the substrate opposite the previously formed electrode(s) to physically remove material through the entire thickness of the substrate to create the passages that are the first step in creating electrically conductive feedthroughs that allow for electrical communication with the hermetic cavity. The configuration of the electrodes and the passages can be altered to provide for a variety of configurations, such modifications providing manufacturing and/or performance advantages. The two substrates are then brought into intimate contact with the electrodes facing one another. The substrates form a temporary bond due to the presence of Van der Waals forces. The electrodes on opposing 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 two-substrate stack. 
   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. 
   The resulting hermetic package presents electrical feedthroughs  70 ,  71  created in the sensor body  51  that allow for communication between components inside the hermetically-sealed sensor  50  and external electrical components. The feedthroughs  70 ,  71  are small, thereby minimizing the area of interface. Such feedthroughs interface with the substrate at areas ranging from 10 −6  to 10 −9  square meters. 
   For the purpose of illustration, sensors of the present invention and according to  FIG. 6  have been manufactured that displayed 0.1-10 picofarads capacitance and, more particularly, 1-5 picofarads capacitance. Also, sensitivities of the device easily can be, e.g., 0.1 KHz/mmHg. 
   The manufacturing of the sensor  50  depicted in  FIG. 6  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  50  is described on a single-sensor basis, although it will be understood that multiple sensors can be created simultaneously on the substrate in a batch process to increase manufacturing efficiency. 
   The lower substrate is processed to create a recessed region in its surface and thin film electrodes at the bottom surface of each 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. 
   One method for creating the desired recessed region is depicted in  FIGS. 7-20  and described as follows: Referring first to  FIG. 7 , a thin metallic film  100  is deposited at the surface of a fused silica substrate  102  using a physical vapor deposition system (e.g., an electron-beam evaporator, filament evaporator, or plasma assisted sputterer). This thin film layer  100  will form a mask used to create a recessed region in the upper surface of the substrate  102 . The nature and thickness of the metal layer  100  are chosen so that the mask is not altered or destroyed by a glass etchant. For the purpose of illustration, Cr/Au or Cr/Ni are examples of suitable mask materials. A representative Cr/Au mask is 100-200 Angstroms of chromium and 1000-3000 Angstroms of gold. 
   As can be seen in  FIG. 8 , a layer  104  of photoresist is formed atop the thin metal film  100  and substrate  102 . Then, as shown in  FIG. 9 , a mask  106  having a rectangular opening is positioned over the photoresist layer  104 , and ultraviolet light, indicated by the arrows  107 , is directed through the mask  106  onto the exposed portions of the photoresist layer  104 . The exposed photoresist defining the body of the rectangular region is removed via the appropriate etchants, as illustrated in  FIG. 10 . 
   Referring now to  FIG. 11 , etchants are used to etch away the rectangular portion of the thin metallic film  100  exposed through the patterned photoresist layer  104 . When the remaining photoresist material is removed, such as by using an appropriate organic solvent, the substrate  102  is left with a metallic mask  108  defining a rectangle  110 , as illustrated in  FIG. 12 . 
   A glass etchant is now used to etch the portion of the upper surface of the substrate  102  that is exposed through the mask  108 . To accomplish this, the substrate  102  is placed in a fixture that prevents the etchant from contacting the un-masked back side of the substrate and is then submerged in a solution containing hydro-fluoric acid, resulting in etching of the masked substrate only where the fused silica is exposed. The substrate  102  is removed from the acid when the substrate has been etched to the desired depth, usually on the order of 1-3 micrometers. The resulting etched substrate  112  with rectangular recessed region  114  is shown in  FIG. 13 . Then, as shown in  FIG. 14 , the mask  108  is removed from the etched substrate  112  using proper selective etchants and solvents. 
   The etched substrate  112  is now primed for creation of electrodes at the bottom of the recessed region  114 . As shown in  FIG. 15 , a thin film metal layer  120  is deposited onto the upper surface of the etched substrate  112 . For the purposes of illustration, this thin film metal layer  120  can be composed of elemental chromium and gold. A representative Cr/Au layer is a 100-200 Angstrom seed layer of chromium and 1000-3000 Angstroms of gold. The thin film layer  120  can also utilize a Ti seed layer and either a Ni or Pt secondary layer. The thickness of this layer is carefully controlled so that, in this embodiment, the metal layer  120  does not protrude above the level of the original surface of the patterned side of the substrate. 
   Referring now to  FIG. 16 , a layer of photoresist  122  is deposited over the surface of the metal layer  120 . A mask  124  is positioned over the photoresist layer  122 , and ultraviolet light, indicated by the arrows  125 , is directed onto the exposed portions of the photoresist layer, as shown in  FIG. 17 . Then, as illustrated in  FIG. 18 , the exposed photoresist is removed, leaving a mask  126  of photoresist material formed on the upper surface of the metal layer  120 . 
   Next, the portions of the metal layer  120  exposed through the mask  126  are etched away, as illustrated in  FIG. 19 . In this instance, the patterns defined by the remaining photoresist  126  represent two side-by-side rectangles whose in-plane, overall foot print is smaller than that of the recessed region  114 . The rectangles are a few micrometers to tens of micrometers apart and maintain at least a few micrometers wide border separating the rectangles from the perimeter of the rectangular trench  114 . Subsequently, the photoresist mask  126  is removed with appropriate organic solvents. 
   At this point, as depicted in  FIG. 20 , the etched lower substrate  112  is patterned with a rectangular trench  114  etched into its upper surface, and the base of the rectangular trench contains side-by-side, spaced apart metal electrodes  56  and  57  of known thickness. The difference between the height of the upper surface of either electrode, H 1 , and depth D 1  of the trench  114  created in the lower substrate  102 , 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. 
   An optional step involving creation of an intermetallic compound can be performed, e.g., at this step and serves to increase the hermeticity of the metal-substrate interface. An intermetallic compound is created by annealing a metal deposited onto a ceramic substrate at a temperature sufficient to initiate covalent bonding across the substrates. It may be necessary to protect the surface of the metal from oxidation by providing a protective layer to the exposed metal or by performing the annealing step in an inert environment (e.g., vacuum, N 2 ). One example of an intermetallic compound is the Ti—O—Si system, where titanium is deposited onto a SiO 2  substrate. The exposed Ti surfaces are protected from oxidation by a layer of silicon nitride. The metal and underlying ceramic substrate are heated at a ramp rate of, e.g., 4-10 degrees C./minute to between substantially 700 and substantially 1100 degrees C. in order to drive the fusion reaction. The temperature is gradually increased and decreased in order to obviate any potential problems with CTE mismatch between the metal and the substrate. If necessary, the protective layer is then removed. In this Ti—O—Si system, either the Ti dissolves significant amounts of oxygen prior to oxide formation enabling the oxygen to react with Si diffusing to the interface, or the stable oxide evolves from TiO to SiO 2  in the presence of the Ti-rich phases. Other configurations of metals and substrates can be used to achieve the same effect, e.g., W—Si—O, Mo—Si—O, Ta—Si—O, and Ti—Si—N. To carry out this annealing step, one skilled in the art need only reference the ternary phase diagram to determine sufficient annealing temperatures and to discern the relevant properties of the intermetallic compound. 
   Referring now to  FIG. 21 , an upper substrate  150  is micromachined using the same sequence of steps described above to create a rectangular trench  152  in the fused silica, and the electrode  60  is created using the same photolithographic process as those described for the lower substrate  102 . The only change to the preparation of the upper substrate is in the pattern transferred to the second layer of photoresist, i.e. the photoresist layer that serves as a mask for creating the metal electrode. On this substrate  150 , one continuous rectangle is patterned that maintains a border at least one micrometer thick separating the electrode  60  from the perimeter of the rectangular trench  152 . 
   As an optional preparatory step for the upper substrate  150 , a blanket etch can be performed on the back side using hydrofluoric acid or any other suitable etchant to form the recess  54  such that overall thickness of the substrate  150  is reduced to a known thickness that lies in the range of 30-100 micrometers. This step serves to increase sensitivity of the deflectable region of the pressure cavity body  51  ( FIG. 6 ). Alternatively, the upper substrate can have an initial thickness in this range, which obviates the need for the above-described step. 
   The substrates  112 ,  150  are then aligned, subjected to bonding, and reduced to the final overall dimension of the sensor as shown in  FIG. 6  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 corresponding rectangular trenches  114 ,  152  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. 22 . 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 electrodes  56 ,  57  and the electrode  60  where the distance between the electrodes is precisely known. Referring to  FIG. 23 , using a CO 2  laser, indicated by the arrows  160 , the sensor is reduced to its final dimensions. The laser cutting process also seamlessly fuses the upper and lower substrates  112 ,  150 . The result of the above steps is depicted in  FIG. 24 . Thus, the rectangular electrodes created combine to form a complete device that displays the electrical attributes of a parallel plate capacitor. 
   With further reference to  FIG. 24 , 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  51  is completely fused, the interior chamber  52  will be hermetic. 
   At some point, the feedthrough passages  64 ,  65  are created by removing material on the lower surface of the pressure cavity body  51  to expose the back side of the capacitor electrodes  56 ,  57 , establishing electrical communication through this location as pictured in  FIG. 25 . This process step can take place after completion of the electrodes  56 ,  57  on a single substrate  112 , after the two substrates  112 ,  150  have been temporarily bonded, or after the sensors  50  have been individualized, depending on manufacturing considerations. Either laser ablation or chemical etching or a combination of the two is performed to remove the glass substrate and to expose a portion of the back side of each of the electrodes  56 ,  57  located on the lower surface of the pressure cavity  51 . In order to provide for electrical contact pads, any number of techniques can be used to deposit a layer of metal into the passages  64 ,  65 . The metal choice and deposition technique cannot be chosen independently from one another, but these combinations, along with their respective advantages and shortcoming, are well-known in the art. For purposes of illustration, techniques such as low-pressure plasma spray, electroplating, or screen printing can be utilized to this end. Optionally, if compatible with the deposition technique chosen and the strength of the exposed electrodes, the metal deposition is performed under vacuum. If the feedthrough passages  64 ,  65  are only partially filled with the electrical contact pad, a ceramic material (e.g., glass frit) can be used to fill the remainder. This would provide mechanical reinforcement to the feedthrough structure. 
   It is a further aspect of this invention to provide for a hermetic sensor that incorporates a pressure cavity and additional electrical components that incorporate the above described advantages, with additional functionality and advantages being provided. A sensor according to the invention, along with desirable modifications, is depicted in  FIG. 26  and is further described below. 
     FIG. 26  shows a sensor  200  comprising a sensor body  202  of fused silica or other suitable material, as discussed above. The sensor body  202  comprises a lower wall  204 , an upper wall  206 , and an intermediate wall  208 . The intermediate wall  208  divides the hollow interior of the sensor body  202  into a lower hermetic chamber (a.k.a. pressure chamber)  210  and an upper chamber  212 . A first electrode  214  is affixed within the lower hermetic chamber  210  to the lower sensor body wall  204 . A second electrode  216  is affixed within the lower hermetic chamber  210  to the intermediate wall  208 . A third electrode  217  is behind and in-plane with the second electrode  216  and is thus not visible in  FIG. 26 . The first electrode  214  is thus arranged in parallel, spaced-apart relation with respect to the second and third electrodes  216 ,  217  so as to form a gap capacitor. A recess is formed in the lower sensor body wall  204 , or the substrate comprising the lower sensor body wall is configured to be sufficiently thin, to form a region  220  that will deflect in response to pressure changes. Because the first electrode  214  is coupled to the deflectable region  220 , the distance between the first electrode  214  changes with respect to the second and third electrodes  216 ,  217  with variations in external pressure. Thus the characteristic capacitance of a capacitor comprising the first, second, and third electrodes  214 ,  216 ,  217  changes with movement of the deflectable region  220 . 
   Also mounted to the intermediate wall  208  within the lower hermetic chamber  210  is a fourth electrode  224 . A fifth electrode  226  is located on the intermediate wall  208  within the upper chamber  212 , which is, optionally, hermetic. A sixth electrode  225  is behind and in-plane with the fifth electrode  226  and is thus not visible in  FIG. 26 . The fourth electrode  224  is disposed in parallel, spaced apart relation with respect to the fifth and sixth electrodes  225 ,  226 , separated by the thickness of the intermediate wall  208 . Because the distance between the fourth electrode  224  and the fifth and sixth electrodes  225 ,  226  remains constant, a capacitive circuit comprising the fourth, fifth, and sixth electrodes provides a fixed reference. In the capacitor configuration described above, an example where the need for feedthroughs into the lower hermetic chamber  210  is eliminated, a capacitor configuration (i.e., a configuration that is physically two capacitors in parallel) that sacrifices capacitance value for ease of manufacture is utilized. Alternative configurations can be provided for, require either one or two feedthroughs into the lower hermetic chamber and are obvious to one skilled in the art. 
   Electrical contact pads  230 ,  231  are formed on the intermediate wall  208  within the upper hermetic chamber. A first pad  230  is located opposite a portion of the second electrode  216 . A second pad  231  is located opposite a portion of the third electrode  217  and is behind and in plane with the first pad  230  and thus not visible in  FIG. 26 . A first feedthrough passage  236  places the first pad  230  and the second electrode  216  in communication through the intermediate wall  208 . A second feedthrough passage  237  (not visible in  FIG. 26 ) places the second pad  231  and the third electrode  217  in communication through the intermediate wall  208 . The electrical feedthroughs  236 ,  237  are filled with a conductive material, such as metal. The second and third electrodes  216 ,  217  are hermetically imposed against the openings of the passages  236 ,  237 . Optionally, the pads  230 ,  231  and the medium filling the passages  236 ,  237  are hermetic and are hermetically imposed against the openings of the feedthrough passages  236 ,  237 . At a minimum, this hermetic imposition of electrodes  216  and  217  renders the feedthroughs hermetic. Optionally, electrical contact pads  230 ,  231  and/or the material filling the feedthrough passages  236 ,  237  further renders the feedthroughs hermetic. 
   To provide electrical access to the interior of the sensor, fifth and sixth feedthrough passages  240 ,  241  are provided. The passage  240  extends from the exterior of the sensor body to the upper chamber  212 . The passage  241  also extends from the exterior of the sensor body to the upper chamber  212  but is behind and in plane with the electrical feedthrough  240  and thus not visible in  FIG. 26 . An electrical contact pad  242  is located within the upper chamber  212  on the intermediate wall  208  and is imposed over the passage  240 . Likewise, an electrical contact pad  243  is located within the upper chamber  212  on the intermediate wall  208  and is imposed over the passage  241 . The electrical contact pad  243  is behind and in plane with the electrical contact pad  242  and is therefore not visible in  FIG. 26 . Electrical contact pads  242 ,  243  can be configured to provide a hermetic interface with the intermediate wall  208 . In the embodiment of  FIG. 26 , the passages  240 ,  241  are partially filled with a conductive material such as gold, and electrical connection can be made on the exterior of the sensor body  202  as described in previous examples. Any remaining voids in the passages  240 ,  241  are filled with a material  248  such as glass frit, which fills the space not occupied by the conductive material and enhances the mechanical stability of the feedthrough structure. Optionally, hermetic imposition of the conductive material into the passages  240 ,  241  further renders the feedthroughs hermetic. 
   The upper chamber  212  contains one or more electrical components such as a silicon chip  250  bearing electronics that can act to buffer, to linearize, or otherwise to manipulate the electronic signal from the transducer. The silicon chip  250  is placed in electrical communication with the electrodes and with an external source by way of the conductive pads  230 ,  231 ,  242 , and  243 . In one embodiment (not shown), the electronics comprises an A/D converter placed in series with an additional silicon chip bearing electronics. In this case, an additional set of electrical contact pads are provided that allow electrical communication between the A/D converter and the additional electronics. 
   The fabrication of the sensor depicted in  FIG. 26  is based on the micromachining of three substrates that are subsequently brought into contact and cut into individual sensors. The fabrication of the individual substrates as well as their final assembly is described as follows: The thin metal electrodes  216 ,  217  having overall, in-plane dimensions of 500 micrometers width, 3-4 mm length and 500 nm thickness, are formed within a recessed region of the same dimensions of the electrode that was previously etched into the surface of a first substrate using photolithography and chemical etching as described for previous examples. The metal electrodes are shorter than the depth of the recessed region by 200 nm. A second substrate has a second recessed region formed therein having a depth of 700 nm and the same cross-sectional dimensions as the recessed region in the upper wafer. A thin metal electrode  214 , having a thickness of 500 nm and the same overall, in-plane dimensions as electrodes  216  and  217 , is then formed into this recessed region. The electrode  214  is thinner than the depth of the recessed region by 200 nm. When the first and second substrates are bonded together with their respective recessed regions facing each other, a gap of 400 nm is thereby formed between the electrode  214  and the electrodes  216  and  217 . Feedthrough passages  236 ,  237  are then created from the top surface of the second substrate down to the upper electrodes  216  and  217 , using laser rastering and HF etching. Also, electrode  224  and electrodes  225 ,  226  are formed on opposite sides of the wall  208 . 
   Conductive pads  230 ,  231 ,  242 , and  243  on the top surface of the second substrate can be formed during the feedthrough fabrication sequence. The silicon chip  250  is then connected to the conductive pads  230 ,  231 ,  242 , and  243  that were formed during the feedthrough fabrication sequence. A third substrate that has a recess sufficiently deep to contain the silicon chip  250  and to make contact to the second substrate is added to the assembly. A laser is then used to remove material around the sensor periphery to reduce the sensor to final dimensions. In the disclosed embodiment, the sensor is 750 micrometers wide by 4-5 mm long and 0.6 mm tall. Passages  240  and  241  are then created to allow for conductive communication with external electronics. 
   In an alternative example, a piezoresistive transduction scheme can be utilized to measure changes in the position of the deflectable region in the pressure cavity. One or more piezoresistive elements translate mechanical strain into changes in electrical resistance. The piezoresistor is made of, e.g., polysilicon and formed on the interior of the pressure cavity in lieu of the electrodes in previous examples. The resistance modulation is, e.g., detected through a fully active Wheatstone bridge, as is known in the art. Optimally, the Wheatstone bridge configuration used is one where only one leg of the bridge is fixed to the deflectable region of the pressure cavity. This design reduces the number of feedthroughs to two. 
   One proposed transduction scheme capable of measuring changes in the position of the deflectable region in the pressure cavity is illustrated in  FIG. 27 . Sensor  300  and ASIC  310  together comprise an active Wheatstone bridge, which is known in the art for measuring an unknown resistance. Sensor  300  comprises a piezoresistor of resistance value R 1 . Piezoresistors are well known in the art. The other three legs of the Wheatstone bridge comprise resistors  312 ,  314 , and  316  with values R 2 , R 3 , and R 4  respectively. Voltage  320  of value V 0  is supplied by a battery (not shown). The circuit operates on the following principle, which discussion is presented for illustrative purposes only. When voltage  320  is applied with value V 0 , and R 1 , R 2 , R 3  and R 4  are all of known values, then the value VS of voltage  322  may be determined as is well known in the art from knowledge of V 0 , R 1 , R 2 , R 3 , and R 4 . However, if the resistance R 1  of sensor  300  changes while values R 2 , R 3 , and R 4  of resistors  312 ,  314 , and  316  remain unchanged, then the value VS of voltage  322  will change. As is well known in the art, measurement of the changed value VS of voltage  322  may then be used to determine the value of resistance R 1  of the sensor  300 . Because sensor  300  comprises a piezoresistor, the value R 1  of sensor  300  changes in response to a change in position of the deflectable region in the pressure cavity, and this circuit therefore gives a measurement of that change in position. 
   As previously indicated, various capacitor configurations are possible.  FIG. 28  illustrates a sensor  350  that includes a pressure cavity body  351  defining an internal pressure chamber  352 . One of the walls defining the pressure cavity  352  comprises a deflectable region  354  configured to deflect under a physiologically relevant range of pressure. In a preferred embodiment, a wall of the pressure cavity  351  is thinned relative to other walls of the pressure cavity body to form the deflectable region  354 . 
   A capacitor comprises a single lower electrode  356  located on a first wall  358  of the chamber  352 . A second electrode  360  is disposed on an opposite wall  362  of the pressure cavity  352  in parallel, spaced apart relation to the lower electrode  356 . The upper electrode  360  is mechanically coupled to the deflectable region  354 . 
   The lower portion of the pressure cavity  352  contains a pair of passages  364 ,  365  that traverse the hermetic pressure cavity body  351 . The first passage  364  is in contact with the lower electrode  356 . The second passage  365  is in contact with the upper electrode  360  by way of an electrode in the form of an electrically conductive post  357  disposed within the pressure cavity  352 . Electrical contact pads  366 ,  367  are formed within the passages  364 ,  365  on the back side of the electrodes  356 ,  357  and extend to the exterior of the housing  351 , thereby providing a region on the exterior of the sensor  350  configured with sufficient dimensions so as to allow for a means for connection with external electronics. 
     FIGS. 29 and 30  illustrate a further embodiment of a sensor  500  comprising a sensor body  505  of fused silica, or other suitable material, as discussed above. The sensor body  505  comprises a lower wall  506  and an upper wall  508 . The lower wall  506  further comprises a first trench  514  and a second trench  516  formed within a portion of the first trench  514 . The sensor body  505  further defines a hermetic chamber  510 . Located within the hermetic chamber  510  on the lower wall  506  in the area comprising the first trench  514  are electrode  520  and electrode  522 . The electrode  522  is behind and in-plane with the first electrode  520  and is thus not visible in  FIG. 29 . Another electrode  525  is located within the hermetic chamber  510  on the upper wall  508  and is positioned such that it is in parallel, spaced-apart relation with respect to electrode  520  and electrode  522 . Electrode  520 , electrode  522  and electrode  525  combine to form a gap capacitor. 
   The second trench  516  in the lower wall  506  contains a silicon chip  550  bearing electronics. Located on the opposite side of the first trench  514  from the electrodes  520 ,  522 , and  525  are electrode  555  and electrode  560 . Electrode  560  is behind and in-plane with the electrode  555  and is thus not visible in  FIG. 29 . A conductor  565  places the electrode  520  in electrical communication with a wire bond on the chip  550 . Similarly, a conductor  570  places the electrode  522  in electrical communication with a second wire bond on the chip  550 . The wire bond  570  is behind and in-plane with the wire bond  565  and is thus not visible in  FIG. 29 . A third conductor  575  places the electrode  555  in electrical communication with a third wire bond on the chip  550 . A fourth conductor  580  places the electrode  560  in electrical communication with a fourth wire bond on the chip  550 . 
   A third trench  585  is provided in the upper wall  508 . The upper wall  508  of the sensor  500  acts as a deflective region  625  and is configured to be sufficiently thin to deflect in response to physiologically relevant pressure changes. As explained in previous examples, the electrode  525  is coupled to this deflective region  625  so that the distance between the electrode  525  and the electrodes  520 ,  522  changes with variations in external pressure, thereby changing the characteristic capacitance of the capacitor. 
   The lower wall  506  of the sensor body  505  comprises passages  590  and  595  that traverse the sensor body  505  and are in contact with the electrodes  555 ,  560 . As shown in  FIGS. 29 and 30 , electrical contact pads  600 ,  605  can be formed on the back side of the electrodes  555 ,  560  and extend to the exterior of the housing, thereby providing a region on the exterior of the sensor  500  configured with sufficient dimensions so as to allow for a means for connection with external electronics. As an alternative, the passages  590 ,  595  can be filled with an electrically conductive material, with contact pads  600 ,  605  in electrical communication with the electrodes  555 ,  560  by way of the conductive material  610 . The electrode  555 , the passage  590 , and, if present, the electrical contact pad  600  and any electrically conductive material  610  filling the passage  590  comprises a first electrical feedthrough. The electrode  560 , the passage  595 , and, if present, the electrical contact pad  605  and any electrically conductive material  610  filling the passage  595  comprises a second electrical feedthrough. 
   The fabrication of the sensor  500  depicted in  FIGS. 29 and 30  is based on the micromachining of two substrates that are subsequently brought into contact and cut into individual sensors. The fabrication of the individual sensors as well as their final assembly is described as follows: A lower substrate is provided. Into this lower substrate, a first trench is etched that is 2 mm by 5 mm and 3 micrometers deep using conventional masking processes and wet etching techniques, as described in earlier examples. A second trench is then etched in a portion of the lower substrate using the same methods as used in creation of the first trench. In the disclosed example, the second trench is 100 micrometers deep, 1.3 mm long, and 0.9 mm wide. Next, the four electrodes deposited on the lower wall are formed by conventional masking techniques and thin film techniques as described in previous examples. The chip is then inserted in the recessed cavity and fixed by, e.g., a press fit, adhesive, or eutectic bonding via additional metallic interfaces such as a soldered preforms. After the chip is placed and fixed to the lower substrate, the four conductors associated with the four electrodes deposited on the lower wall are made via conventional techniques, e.g., with wire that is 25 micrometers in diameter. 
   Next, the upper substrate is prepared. To this end, a third trench is etched into this wafer using the same techniques as used to create the first and second trenches of the lower substrate. In the disclosed embodiment, this trench is 2.5 mm long, 1.7 mm wide, and 0.1 mm deep. The electrode deposited on the upper wall is then created using the same techniques referenced in the creation of the electrodes on the lower substrate. This electrode deposited on the upper wall is 1.4 mm by 2 mm and made of 500 nm layer of Chrome/Gold. 
   Subsequent to individual fabrication of the substrates, the upper wafer is oriented with respect to the lower wafer such that the components are aligned as shown in  FIGS. 29 and 30 . The wafers form a temporary bond and are optionally further subjected to a 200 degrees C. oven for approximately two hours in order to increase the bond strength. 
   Passages  590 ,  595  are created through the exterior surface of the lower substrate using a CO 2  laser as described previously. After laser ablation, the passages are optionally subjected to a wet etch. The passages resulting from this process are approximately 200 micrometers at the exterior surface and about 50 micrometers at the interior surface and the back side of the fourth and fifth electrodes are exposed. Next, a metal layer is deposited through a shadow mask using a DC sputterer. Any suitable metal may be used. In one embodiment, the metal used is titanium and the resulting layer is 10 micrometers thick. This titanium layer is intended to establish electrical communication between the interior of the hermetic chamber and the ambient. Furthermore, the titanium layer increases the integrity of the hermetic cavity and provides further corrosion resistance. 
   The capacitor contained within the individualized sensor made with the above geometry is approximately 5 picofarads. It is obvious to one skilled in the art in light of the present disclosure to modify the spacing between the electrodes as well as the area of the interposed electrodes to increase or decrease the capacitance value. 
   The sensor is then individualized from the two wafer stack using the same method as disclosed in previous embodiments. 
   Regarding the manufacture of the sensor, the overall size of the resultant device can be reduced through use of an anisotropic etching method (e.g., ICP glass etching, ultrasonic glass etching) instead of isotropic wet etching. If the chip utilized in the example above is 800 by 1300 micrometers, the second trench in the lower wafer can very well be merely 810 by 1310 micrometers. Also, the capacitor area can be reduced to 700 microns by 800 microns by reducing the gap between the electrodes. Furthermore, the thickness of the fused silica package can also be reduced to about 100 micrometers by reducing the thickness of the wall surrounding the hermetic cavity. Thus, it follows that the sensor of the present embodiment can be reduced to final overall dimensions of 1 mm by 2.3 mm by 0.6 mm versus the disclosed example that results in a device that is 2 mm by 5 mm by 0.6 mm, as shown in  FIG. 30 . In addition, if no chip is included in the sensor package the sensor (such as that disclosed in  FIG. 1 ) can achieve even smaller geometries. Also, as obvious to one skilled in the art, the aspect ratio (length to width) can be altered and achieve similar results. 
   In an alternative example, a piezoresistive transduction scheme can be utilized to measure changes in the position of the deflectable region in the pressure cavity. One or more piezoresistive elements translate mechanical strain into changes in electrical resistance. The piezoresistor is made of, e.g., polysilicon and formed on the interior of the pressure cavity in lieu of the electrodes in previous examples. The resistance modulation is, e.g., detected through a fully active Wheatstone bridge, as is known in the art. Optimally, the Wheatstone bridge configuration used is one where only one leg of the bridge is fixed to the deflectable region of the pressure cavity. This design reduces the number of feedthroughs to two. 
   While the invention as been illustrated in the context of a biological device, it will be appreciated that the hermetic chamber herein described can be adapted to non-biological applications, for example, industrial applications in which a harsh environment is encountered. 
   Referring now to  FIGS. 31 and 32 , in another aspect of this invention, a fixture is provided to house previously individualized sensors in alignment for subsequent processing or shipping. The fixture  700  includes a plurality of recesses  702 , each adapted to house a sensor, or other suitable micro-device. The fixture can be used, e.g., to position multiple individualized sensors with respect to features on a mask, allowing for contact between the mask and the sensors, and as a shipping container. This fixture is useful for micro-devices which are small and not easily handled or manipulated, such as those described previously. Also, this method will effectively prevent contamination of device surfaces during handling and processing. Furthermore, this fixture enables a batch technique to define features on the surface of individual devices. This feature is particularly useful when deposition of metal on the exterior of such a device would compromise subsequent processes (e.g., coating, bonding) or would prohibit the creation of an operable device due to thermal or chemical considerations. E.g., if fused silica is used to create a sensor according to this invention and the feedthroughs comprise Ti, then the fused silica cannot be cleaned with hydrofluoric acid (HF) as would normally be done. The HF will also etch away the Ti. With this process, the sensors may be individualized, cleaned and placed in this fixture, where the deposition of thick Ti pads would occur after cleaning. The fixture would preserve the cleanliness of the other surfaces so as to allow for coating or subsequent processing while one avoids exposing the Ti pads on the exterior of the fixture to a deleterious substance during manufacture of the device. 
   The accuracy of the features placed on the microfabricated device (e.g., sensor), the deposition method used to define additional features on the device surface, the tolerance of the manufacturing technique used to create the recesses in the fixture and the tolerance of the device itself are all factors in determining the tolerances of the final fixture. Thus, a manufacturing method should be chosen to make the fixture that is capable of defining features that correspond with allowable tolerances of the final device. The fixture can be made out of silicon (sub-micron tolerances), fused silica (10 micron tolerances with laser) or machined metal (1 mil tolerances) or any combination thereof. Also, if a first substrate with holes and a second substrate are fixed together to create the fixture, then the alignment and bonding method used to create the fixture also influences the tolerances of the accuracy of the final features placed on the device. 
   A pick-and-place operation can then be utilized to insert the devices into individual recesses. Then, the substrate can be “capped” with a final substrate to provide a shipping container. 
   Alternatively, or before capping to create a shipping container, a shadow mask is provided in order to facilitate the addition of metal features to the surface of the device. Here, the fixture containing the devices is placed on a first platform. Then, the shadow mask is connected to a base wafer and the mask is aligned. Contact is made between the shadow mask and the fixture by alignment equipment which is well-known in the art. After contact is made, the shadow mask and fixture are secured by some form of mechanical fixation to prevent any relative movement. The machine used to apply the metal to the device is common to microelectronics and utilizes physical vapor deposition (PVD) techniques. These machines are designed to hold wafers from at least 2 in. to about 12 in. in diameter, thus the substrates of the present invention can be wafers of these dimensions. 
   SiO2 FIXTURE EXAMPLE 
   To create a fixture from fused silica, two substrates are utilized and the dimensions of the device to be placed in the fixture are known. A first substrate thickness is selected such that it is less than the height of the device to be placed in the fixture. Then, holes are cut all the way through this first substrate using, e.g., a CO2 laser operating at a peak wavelength of 10 microns. Then a second fused silica substrate is provided and placed in contact with the first substrate. 
   The bonding and subsequent uppermost surface of the fixture should be uniform enough so that the devices are maintained at a certain height above the height of the substrate and no part of the substrate is as tall as or taller than the device. For fused silica, direct bonding (use Van der Waals forces); adhesives; highly localized heat (e.g., a CO2 laser), or a mechanical clamp (in conjunction with the mask) could be used alone or in combination to bond or otherwise fix the two substrates together. 
   Si FIXTURE EXAMPLE 
   For a silicon fixture only one substrate is necessary. The recesses are created by known techniques using inductively-coupled plasma. This method provides a highly reliable and accurate method to create the fixture. Again, the recess dimensions dictate the initial substrate thickness and the dimensions of the recesses created therein. 
   Alternatively, a Si fixture could be constructed with the two wafer process described above the in the SiO2 fixture and bonded with conventional Si bonding methods. All manufacturing techniques used in the construction of this fixture are known to those skilled in the art. 
   METAL MACHINING EXAMPLE 
   Metal machining techniques can also be utilized to create the fixture of the present invention. Such techniques are well-known in the art. 
   Finally, if two substrates are used to construct the fixture combinations such as silicon-fused silica and fused silica-metal can be utilized. Methods to bond these dissimilar substrates are well known in the art. 
   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.