US20060174712A1 - Hermetic chamber with electrical feedthroughs - Google Patents

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 dates of provisional U.S. Patent Application Ser. No. 60/651,670, filed Feb. 10, 2005, and provisional U.S. Patent Application Ser. No. 60/653,868, filed Feb. 17, 2005.
BACKGROUND OF THE INVENTION
• 1. 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.
• 2. Description of the Prior Art
• 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. 6,855,115, 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, 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 ultraminiature 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'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 inFIGS. 1-5. One method for creating a buried feedthrough is as follows: ametal10 is deposited ontosubstrate12 in a predefined pattern, as shown inFIG. 1. Aninsulating layer14 is deposited on top of the metal layer, as shown inFIG. 2, and thisinsulating layer14 is polished to planarize this surface. InFIG. 3 an etchant has been used to expose the metal layer at input andoutput sites16,18 for the feedthroughs. InFIG. 4, anothersubstrate20 is bonded on top of this structure, forming ahermetic cavity22. A eutectic bonding method is illustrated, which involves the use ofgold deposits24 interposed between theinsulating layer14 and theupper substrate20 to bond the upper substrate to the insulating layer. InFIG. 5 theupper substrate20 is machined to expose theexternal feedthrough18. An electrical component can now be connected to theexternal feedthrough18, whereupon electrical communication is established throughmetal10 to theinternal feedthrough16 and the interior of the hermetically sealedchamber22.
• This prior art buried feedthrough suffers a number of disadvantages. First, there are numerous material interfaces: aninterface30 between thelower substrate12 and themetal10; aninterface32 between themetal12 and the insulatinglayer14, aninterface34 between the insulatinglayer14 and thegold24; and aninterface36 between thegold24 and theupper substrate20, all of which create potential paths for infusion into or effusion out of thehermetic chamber22. 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 tometal10 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.
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 ofFIG. 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 ofFIGS. 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.
DETAILED DESCRIPTION OF THE INVENTION
• Referring now to the drawings, in which like numerals indicate like elements throughout the several views,FIG. 6 illustrates asensor50 that includes apressure cavity body51 defining aninternal pressure chamber52. One of the walls defining thepressure cavity52 comprises adeflectable region54 configured to deflect under a physiologically relevant range of pressure. In a preferred embodiment, a wall of thepressure cavity body51 is thinned relative to other walls of the pressure cavity body to form thedeflectable region54. Thesensor50 can be fabricated using micro-machining techniques and is small, accurate, precise, durable, robust, biocompatible, and insensitive to changes in body chemistry or biology. Additionally, thesensor50 can incorporate radiopaque features to enable fluoroscopic visualization during placement within the body. Thesensor50 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 oflower electrodes56,57 located on afirst wall58 of thechamber52. The twolower electrodes56,57 are electrically isolated from one another. Athird electrode60 is disposed on anopposite wall62 of thepressure cavity52 in parallel, spaced apart relation to thelower electrodes56,57. Theupper electrode60 is mechanically coupled to thedeflectable region54. As ambient pressure increases, thedeflectable region54 moves inward, displacing theupper electrode60 toward thelower electrodes56,57, 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 and57, although other configurations are possible and obvious to one skilled in the art.
• The lower portion of thepressure cavity52 comprisespassages64,65 that traverse the hermeticpressure cavity body51 and are in contact with theelectrodes56,57. As shown inFIG. 6,electrical contact pads66,67 can be formed on the back side of theelectrodes56,57 and extend to the exterior of the housing, thereby providing a region on the exterior of thesensor50 configured with sufficient dimensions so as to allow for a means for connection with external electronics. As an alternative, thepassages64,65 can be filled with an electrically conductive material, withcontact pads66,67 in electrical communication with theelectrodes56,57 by way of theconductive material68. Theelectrode56, thepassage64, and, if present, theelectrical contact pad66 and any electricallyconductive material68 filling thepassage64 comprises a first electrical feedthrough70. Theelectrode57, thepassage65, and, if present, theelectrical contact pad67 and any electricallyconductive material68 filling thepassage65 comprises a second electrical feedthrough71.
• It is a preferred embodiment of this invention that the metal-fused silica interface between thelower electrodes56,57 and the interior surface of thepressure cavity body51 be hermetic. Theelectrical contact pads66,67 can occupy either all or part of thepassages64,65. 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-filledfeedthrough passage64,65, a void inside the feedthrough passages and above theelectrical contact pads66,67 will remain. In order to fill these voids and to enhance the strength of the feedthroughs70,71, any remaining space in thepassages64,65 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, thepads66,67 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. Theelectrical contact pads66,67 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.
• Thepressure cavity52 is hermetic for the following reasons. First, thepressure cavity body51 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 thepassages64,65, which themselves are hermetically sealed. One reason for the hermeticity of thepassages64,65 is that theelectrodes56,57 are hermetically imposed onto thewall58 over the feedthroughs. Theelectrodes56,57 (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, thematerial68 filling thepassages64,65 is itself capable of hermetic sealing such that the interface between the material68 and the material defining the feedthrough passages is also hermetic. Thus gas or fluid would have to pass through or around thematerial68 in thepassages64,65 and pass through or around theelectrodes56,57 before it could enter the pressure chamber and compromise its integrity. And finally, thepassages64,65 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 10⁻⁶to 10⁻⁹square meters.
• A disclosed method of fabricating thesensor50 depicted inFIG. 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 inFIGS. 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 CO₂laser 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 feedthroughs70,71 created in thesensor body51 that allow for communication between components inside the hermetically-sealedsensor50 and external electrical components. The feedthroughs70,71 are small, thereby minimizing the area of interface. Such feedthroughs interface with the substrate at areas ranging from 10⁻⁶to 10⁻⁹square meters.
• The manufacturing of thesensor50 depicted inFIG. 6 from the substrate (a.k.a. wafer) level to the final device is described in greater detail below. For clarity, the manufacture of thesensor50 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 inFIGS. 7-20 and described as follows: Referring first toFIG. 7, a thinmetallic film100 is deposited at the surface of a fusedsilica substrate102 using a physical vapor deposition system (e.g., an electron-beam evaporator, filament evaporator, or plasma assisted sputterer). Thisthin film layer100 will form a mask used to create a recessed region in the upper surface of thesubstrate102. The nature and thickness of themetal layer100 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 inFIG. 8, alayer104 of photoresist is formed atop thethin metal film100 andsubstrate102. Then, as shown inFIG. 9, amask106 having a rectangular opening is positioned over thephotoresist layer104, and ultraviolet light, indicated by thearrows107, is directed through themask106 onto the exposed portions of thephotoresist layer104. The exposed photoresist defining the body of the rectangular region is removed via the appropriate etchants, as illustrated inFIG. 10.
• Referring now toFIG. 11, etchants are used to etch away the rectangular portion of the thinmetallic film100 exposed through the patternedphotoresist layer104. When the remaining photoresist material is removed, such as by using an appropriate organic solvent, thesubstrate102 is left with ametallic mask108 defining arectangle110, as illustrated inFIG. 12.
• A glass etchant is now used to etch the portion of the upper surface of thesubstrate102 that is exposed through themask108. To accomplish this, thesubstrate102 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. Thesubstrate102 is removed from the acid when the substrate has been etched to the desired depth, usually on the order of 1-3 micrometers. The resultingetched substrate112 with rectangular recessedregion114 is shown inFIG. 13. Then, as shown inFIG. 14, themask108 is removed from the etchedsubstrate112 using proper selective etchants and solvents.
• The etchedsubstrate112 is now primed for creation of electrodes at the bottom of the recessedregion114. As shown inFIG. 15, a thinfilm metal layer120 is deposited onto the upper surface of the etchedsubstrate112. For the purposes of illustration, this thinfilm metal layer120 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. Thethin film layer120 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, themetal layer120 does not protrude above the level of the original surface of the patterned side of the substrate.
• Referring now toFIG. 16, a layer ofphotoresist122 is deposited over the surface of themetal layer120. Amask124 is positioned over thephotoresist layer122, and ultraviolet light, indicated by thearrows125, is directed onto the exposed portions of the photoresist layer, as shown inFIG. 17. Then, as illustrated inFIG. 18, the exposed photoresist is removed, leaving amask126 of photoresist material formed on the upper surface of themetal layer120.
• Next, the portions of themetal layer120 exposed through themask126 are etched away, as illustrated inFIG. 19. In this instance, the patterns defined by the remainingphotoresist126 represent two side-by-side rectangles whose in-plane, overall foot print is smaller than that of the recessedregion114. 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 therectangular trench114. Subsequently, thephotoresist mask126 is removed with appropriate organic solvents.
• At this point, as depicted inFIG. 20, the etchedlower substrate112 is patterned with arectangular trench114 etched into its upper surface, and the base of the rectangular trench contains side-by-side, spaced apartmetal electrodes56 and57 of known thickness. The difference between the height of the upper surface of either electrode, H₁, and depth D₁of thetrench114 created in thelower substrate102, 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, N₂). One example of an intermetallic compound is the Ti—O—Si system, where titanium is deposited onto a SiO₂substrate. 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 SiO₂in 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 toFIG. 21, anupper substrate150 is micromachined using the same sequence of steps described above to create arectangular trench152 in the fused silica, and theelectrode60 is created using the same photolithographic process as those described for thelower substrate102. 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 thissubstrate150, one continuous rectangle is patterned that maintains a border at least one micrometer thick separating theelectrode60 from the perimeter of therectangular trench152.
• As an optional preparatory step for theupper substrate150, a blanket etch can be performed on the back side using hydrofluoric acid or any other suitable etchant to form therecess54 such that overall thickness of thesubstrate150 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 body51 (FIG. 6). Alternatively, the upper substrate can have an initial thickness in this range, which obviates the need for the above-described step.
• Thesubstrates112,150 are then aligned, subjected to bonding, and reduced to the final overall dimension of the sensor as shown inFIG. 6 according to the following description: Both the upper andlower substrates112,150 are prepared for assembly, e.g., by cleaning. The patterned surfaces of the substrates are faced and aligned so that the correspondingrectangular trenches114,152 created in each substrate are positioned directly on top of one another. The twosubstrates112,150 are brought together and placed in intimate physical contact, as shown inFIG. 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 theelectrodes56,57 and theelectrode60 where the distance between the electrodes is precisely known. Referring toFIG. 23, using a CO₂laser, indicated by thearrows160, the sensor is reduced to its final dimensions. The laser cutting process also seamlessly fuses the upper andlower substrates112,150. The result of the above steps is depicted inFIG. 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 toFIG. 24, the power of the CO₂laser is controlled such that heat damage to the internal components is avoided. Consequently it is possible that some vestige of aseam162 may remain between the upper andlower substrates112,150. So long as the outer periphery of thepressure cavity body51 is completely fused, theinterior chamber52 will be hermetic.
• At some point, thefeedthrough passages64,65 are created by removing material on the lower surface of thepressure cavity body51 to expose the back side of thecapacitor electrodes56,57, establishing electrical communication through this location as pictured inFIG. 25. This process step can take place after completion of theelectrodes56,57 on asingle substrate112, after the twosubstrates112,150 have been temporarily bonded, or after thesensors50 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 theelectrodes56,57 located on the lower surface of thepressure cavity51. In order to provide for electrical contact pads, any number of techniques can be used to deposit a layer of metal into thepassages64,65. 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 thefeedthrough passages64,65 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 inFIG. 26 and is further described below.
• FIG. 26 shows asensor200 comprising asensor body202 of fused silica or other suitable material, as discussed above. Thesensor body202 comprises alower wall204, anupper wall206, and anintermediate wall208. Theintermediate wall208 divides the hollow interior of thesensor body202 into a lower hermetic chamber (a.k.a. pressure chamber)210 and anupper chamber212. Afirst electrode214 is affixed within the lowerhermetic chamber210 to the lowersensor body wall204. A second electrode216 is affixed within the lowerhermetic chamber210 to theintermediate wall208. A third electrode217 is behind and in-plane with the second electrode216 and is thus not visible inFIG. 26. Thefirst electrode214 is thus arranged in parallel, spaced-apart relation with respect to the second and third electrodes216,217 so as to form a gap capacitor. A recess is formed in the lowersensor body wall204, or the substrate comprising the lower sensor body wall is configured to be sufficiently thin, to form aregion220 that will deflect in response to pressure changes. Because thefirst electrode214 is coupled to thedeflectable region220, the distance between thefirst electrode214 changes with respect to the second and third electrodes216,217 with variations in external pressure. Thus the characteristic capacitance of a capacitor comprising the first, second, andthird electrodes214,216,217 changes with movement of thedeflectable region220.
• Also mounted to theintermediate wall208 within the lowerhermetic chamber210 is afourth electrode224. A fifth electrode226 is located on theintermediate wall208 within theupper chamber212, which is, optionally, hermetic. A sixth electrode225 is behind and in-plane with the fifth electrode226 and is thus not visible inFIG. 26. Thefourth electrode224 is disposed in parallel, spaced apart relation with respect to the fifth and sixth electrodes225,226, separated by the thickness of theintermediate wall208. Because the distance between thefourth electrode224 and the fifth and sixth electrodes225,226 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 lowerhermetic chamber210 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 pads230,231 are formed on theintermediate wall208 within the upper hermetic chamber. A first pad230 is located opposite a portion of the second electrode216. A second pad231 is located opposite a portion of the third electrode217 and is behind and in plane with the first pad230 and thus not visible inFIG. 26. A first feedthrough passage236 places the first pad230 and the second electrode216 in communication through theintermediate wall208. A second feedthrough passage237 places the second pad231 and the third electrode217 in communication through theintermediate wall208. The electrical feedthroughs236,237 are filled with a conductive material, such as metal. The second and third electrodes216,217 are hermetically imposed against the openings of the passages236,237. Optionally, the pads230,231 and the medium filling the passages236,237 are hermetic and are hermetically imposed against the openings of the feedthrough passages236,237. At a minimum, this hermetic imposition of electrodes216 and217 renders the feedthroughs hermetic. Optionally, electrical contact pads230,231 and/or the material filling the feedthrough passages236,237 further renders the feedthroughs hermetic.
• To provide electrical access to the interior of the sensor, fifth and sixth feedthrough passages240,241 are provided. The passage240 extends from the exterior of the sensor body to theupper chamber212. The passage241 also extends from the exterior of the sensor body to theupper chamber212 but is behind and in plane with the electrical feedthrough240 and thus not visible inFIG. 26. An electrical contact pad242 is located within theupper chamber212 on theintermediate wall208 and is imposed over the passage240. Likewise, an electrical contact pad243 is located within theupper chamber212 on theintermediate wall208 and is imposed over the passage241. The electrical contact pad243 is behind and in plane with the electrical contact pad242 and is therefore not visible inFIG. 26. Electrical contact pads242,243 can be configured to provide a hermetic interface with theintermediate wall208. In the embodiment ofFIG. 26, the passages240,241 are partially filled with a conductive material such as gold, and electrical connection can be made on the exterior of thesensor body202 as described in previous examples. Any remaining voids in the passages240,241 are filled with a material248 such as glass frit, which lo 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 passages240,241 further renders the feedthroughs hermetic.
• Theupper chamber212 contains one or more electrical components such as asilicon chip250 bearing electronics that can act to buffer, to linearize, or otherwise to manipulate the electronic signal from the transducer. Thesilicon chip250 is placed in electrical communication with the electrodes and with an external source by way of the conductive pads230,231,242, and243. 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 inFIG. 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 electrodes216,217 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. Athin metal electrode214, having a thickness of 500 nm and the same overall, in-plane dimensions as electrodes216 and217, is then formed into this recessed region. Theelectrode214 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 theelectrode214 and the electrodes216 and217. Feedthrough passages236,237 are then created from the top surface of the second substrate down to the upper electrodes216 and217, using laser rastering and HF etching. Also,electrode224 and electrodes225,226 are formed on opposite sides of thewall208.
• Conductive pads230,231,242, and243 on the top surface of the second substrate can be formed during the feedthrough fabrication sequence. Thesilicon chip250 is then connected to the conductive pads230,231,242, and243 that were formed during the feedthrough fabrication sequence. A third substrate that has a recess sufficiently deep to contain thesilicon chip250 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 45 mm long and 0.6 mm tall. Passages240 and241 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 inFIG. 27.Sensor300 andASIC310 together comprise an active Wheatstone bridge, which is known in the art for measuring an unknown resistance.Sensor300 comprises a piezoresistor of resistance value R1. Piezoresistors are well known in the art. The other three legs of the Wheatstone bridge compriseresistors312,314, and316 with values R2, R3, and R4 respectively.Voltage320 of value V0 is supplied by a battery (not shown). The circuit operates on the following principle, which discussion is presented for illustrative purposes only. Whenvoltage320 is applied with value V0, and R1, R2, R3 and R4 are all of known values, then the value VS ofvoltage322 may be determined as is well known in the art from knowledge of V0, R1, R2, R3, and R4. However, if the resistance R1 ofsensor300 changes while values R2, R3, and R4 ofresistors312,314, and316 remain unchanged, then the value VS ofvoltage322 will change. As is well known in the art, measurement of the changed value VS ofvoltage322 may then be used to determine the value of resistance R1 of thesensor300. Becausesensor300 comprises a piezoresistor, the value R1 ofsensor300 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 asensor350 that includes apressure cavity body351 defining aninternal pressure chamber352. One of the walls defining thepressure cavity352 comprises adeflectable region354 configured to deflect under a physiologically relevant range of pressure. In a preferred embodiment, a wall of thepressure cavity351 is thinned relative to other walls of the pressure cavity body to form thedeflectable region354.
• A capacitor comprises a singlelower electrode356 located on afirst wall358 of thechamber352. Asecond electrode360 is disposed on anopposite wall362 of thepressure cavity352 in parallel, spaced apart relation to thelower electrode356. Theupper electrode360 is mechanically coupled to thedeflectable region354.
• The lower portion of thepressure cavity352 contains a pair ofpassages364,365 that traverse the hermeticpressure cavity body351. Thefirst passage364 is in contact with thelower electrode356. Thesecond passage365 is in contact with theupper electrode360 by way of an electrode in the form of an electricallyconductive post357 disposed within thepressure cavity352.Electrical contact pads366,367 are formed within thepassages364,365 on the back side of theelectrodes356,357 and extend to the exterior of thehousing351, thereby providing a region on the exterior of thesensor350 configured with sufficient dimensions so as to allow for a means for connection with external electronics.
• 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.
• 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.

Claims (40)

1. A device comprising:

a housing having walls defining a chamber, a first one of said walls defining said chamber comprising an exterior wall of said housing;

a passage through said first one of said walls placing the chamber of said housing in communication with the ambient; and

an electrode hermetically imposed over said passage within said chamber of said housing, whereby said passage is hermetically sealed;

whereby said chamber is hermetically sealed; and

whereby an external electrical device can be placed in electrical communication with said electrode through said passage.

2. The device ofclaim 1, wherein said housing further comprises a unitary housing.

3. The device ofclaim 1, wherein said housing is comprised of a ceramic material.

4. The device ofclaim 3, wherein said housing is comprised of glass, fused silica, sapphire, quartz or silicon.

5. The device ofclaim 4, wherein the housing is comprised of fused silica.

6. The device ofclaim 1, wherein said passage has a surface area of from 10⁻⁶to 10⁻¹²meters².

7. The device ofclaim 6, wherein said passage has a surface area of from 10⁻⁶to 10⁻⁹meters².

8. The device ofclaim 1, wherein said housing has a volume of from 10⁻⁸to 10⁻¹⁵meters³.

9. The device ofclaim 1, wherein said passage comprises a first passage, and wherein said device further comprises:

a second passage through said first one of said walls defining said chamber placing said chamber of said housing in communication with the ambient; and

a second electrode hermetically imposed over said second passage within said chamber, whereby said second passage is hermetically sealed;

whereby said chamber is hermetically sealed; and

whereby a second electrical feedthrough can be placed in electrical communication with said second electrode through said second passage.

10. The device ofclaim 9, wherein a second one of said walls defining said chamber and opposite said first wall comprises an exterior wall of said housing;

wherein said second one of said walls defining said chamber comprises a deflectable region;

wherein a third electrode is deposited on said second one of said walls within said chamber in said deflectable region; and

wherein said third electrode is placed in electrical communication with one of said first electrode or said second electrode.

11. The device ofclaim 9, wherein one of said first or second electrodes and said third electrode comprise a capacitor.

12. The device ofclaim 9, wherein a third electrode is deposited on an interior wall of said chamber opposite to said first wall;

wherein said first, second, and third electrodes are electrically isolated from one another; and

wherein said interior wall of said chamber opposite to said first wall further comprises a deflectable region.

13. The device ofclaim 12, wherein said first, second, and third electrodes comprise a capacitor.

14. The device ofclaim 9, wherein a second one of said walls defining said chamber and opposite said first wall comprises an exterior wall of said housing;

wherein said second one of said walls defining said chamber comprises a deflectable region;

wherein at least one piezoresistor is deposited on said second one of said walls defining said chamber in said deflectable region thereof;

wherein said first electrode is in electrical contact with a first side of said piezoresistor; and

wherein said second electrode is in electrical contact with a second side of said piezoresistor.

15. The device ofclaim 1, wherein said passage is partially filled with an electrically conductive material such that said electrically conductive material is in electrical contact with said electrode.

16. The device ofclaim 15, wherein the remainder of said passage is filled with a different material.

17. The device ofclaim 16, wherein said different material comprises a ceramic.

18. The device ofclaim 15, wherein said electrically conductive material is hermetic and is hermetically disposed in said passage.

19. The device ofclaim 15, wherein said electrically conductive material comprises a non-refractory metal.

20. The device ofclaim 19, wherein said non-refractory metal is selected from the group consisting of gold, platinum, nickel, and silver and alloys thereof.

21. The device ofclaim 15, wherein said electrically conductive material comprises a refractory metal.

22. The device ofclaim 21, wherein said refractory metal is selected from the group consisting of niobium, titanium, tungsten, tantalum, molybdenum, chromium, and platinum/iridium alloy and alloys thereof.

23. The device ofclaim 1, wherein said passage is completely filled with an electrically conductive material, whereby an external electrical device can be placed in electrical communication with said electrode by electrically connecting said electrical device to said electrically conductive material.

24. The device ofclaim 23, wherein said electrically conductive material is hermetic and is hermetically disposed in said passage.

25. The device ofclaim 23, wherein said electrically conductive material comprises a non-refractory metal.

26. The device ofclaim 25, wherein said non-refractory metal is selected from the group consisting of gold, platinum, nickel, and silver and alloys thereof.

27. The device ofclaim 23, wherein said electrically conductive material comprises a refractory metal.

28. The device ofclaim 27, wherein said refractory metal is selected from the group consisting of niobium, titanium, tungsten, tantalum, molybdenum, chromium, and platinum/iridium alloy and alloys thereof.

29. A device comprising:

a unitary housing having walls defining first and second internal chambers, said first and second chambers having a common wall;

a passage through said common wall; and

an electrode within said first chamber hermetically imposed over said passage, whereby said passage is hermetically sealed;

whereby said first chamber is hermetically sealed from said second chamber; and

whereby said second chamber can be placed in electrical communication with said electrode through said passage.

30. The device ofclaim 29, wherein said passage comprises a first passage, wherein said electrode comprises a first electrode, and wherein said device further comprises:

a second passage through said common wall; and

a second electrode within said first chamber hermetically imposed over said second passage.

31. The device ofclaim 30, wherein said second chamber is at least partially defined by an exterior wall of said unitary housing, and wherein said device further comprises:

a third passage through said exterior wall; and

a third electrode within said second chamber imposed across said third passage;

whereby an external electrical device can be placed in electrical communication with said third electrode through said third passage.

32. The device ofclaim 31, further comprising:

a fourth passage through said exterior wall;

a fourth electrode within said second chamber imposed across said fourth passage,

whereby an external electrical device can be placed in electrical communication with said fourth electrode through said fourth passage.

33. The device ofclaim 32, wherein said third and fourth electrodes comprise hermetic material which is hermetically imposed over said third and fourth passages.

34. The device ofclaim 31, further comprising an electrical circuit disposed within said second chamber and in electrical communication with said first and second electrodes, said electrical circuit being operable to manipulate an electrical signal input through said first and second electrodes.

35. The device ofclaim 34, wherein said electrical circuit being operable to manipulate an electrical signal input through said first and second electrodes comprises an integrated circuit.

36. The device ofclaim 35, wherein said integrated circuit is operable to condition an electrical signal passing between said first and second electrodes.

37. The device ofclaim 35, wherein said integrated circuit is operable to condition an electrical signal passing between said first and second electrodes and external electronics.

38. A device comprising:

a substrate having first and second sides;

a passage through said substrate from said first side to said second side; and

an electrode imposed against said first side of said substrate over said passage;

whereby an electrical device on the second side of said substrate can be placed in electrical communication with said electrode on said first side of said substrate through said passage.

39. The device ofclaim 38, wherein said electrode is hermetically imposed against said first side of said substrate.

40. The device ofclaim 38, further comprising a conductive material located within said passage and in electrical contact with said electrode, whereby an electrical device on the second side of said substrate can be placed in electrical communication with said electrode on said first side of said substrate through said conductive material.

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