(12) United States Patent
`Rich et al.
`
`USOO6926670B2
`(10) Patent No.:
`US 6,926,670 B2
`(45) Date of Patent:
`Aug. 9, 2005
`
`(54) WIRELESS MEMS CAPACITIVE SENSOR
`FOR PHYSIOLOGIC PARAMETER
`MEASUREMENT
`
`(75) Inventors: Collin A. Rich, Ypsilanti, MI (US);
`Yafan Zhang, Plymouth, MI (US);
`Nader Najafi, Ann Arbor, MI (US);
`Matthew Z. Straayer, Ann Arbor, MI
`(US); Sonbol Massoud-Ansari, Ann
`Arbor, MI (US)
`(73) Assignee: Integrated Sensing Systems, Inc.,
`Ypsilanti, MI (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 392 days.
`
`(*) Notice:
`
`(21) Appl. No.: 10/054,330
`(22) Filed:
`Jan. 22, 2002
`(65)
`Prior Publication Data
`US 2002/0151816 A1 Oct. 17, 2002
`Related U.S. Application Data
`(60) Provisional application No. 60/263,327, filed on Jan. 22,
`2001, and provisional application No. 60/278,634, filed on
`Mar. 26, 2001.
`(51) Int. Cl. .................................................. A61B 8/14
`(52) U.S. Cl. ....................................................... 600/459
`(58) Field of Search ................................. 600/437-472;
`73/625-633; 367/7, 11, 130, 138; 607/36-38,
`1, 2, 60; 128/916
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,337.747. A
`
`8/1994 Neftel
`
`8/2000 Barber et al. ................. 455/73
`6,101,371. A
`3/2001 Darrow et al.
`6,201,980 B1
`7/2001 Han et al.
`6,268,161 B1
`6,328,699 B1 12/2001 Eigler et al.
`6,567,703 B1 * 5/2003 Thompson et al. - - - - - - - - - - - 607/60
`FOREIGN PATENT DOCUMENTS
`wooi.
`122.
`WOOO/30534
`6/2000
`
`wo
`WO
`
`OTHER PUBLICATIONS
`A Passive Wireless Integrated Humidity Sensor, Timothy
`Harpster et al. 2001, pp. 553–557.
`Electrodeposited Copper Inductors for Intraocular preSSure
`Telemetry; R. Puers et al., 2001 pp. 124-129.
`Hermetically Sealed Inductor-Capacitor (LC) Resonator for
`Remote Pressure Monitoring; Eun-Chul Park et al.; Sep. 8,
`1998; pp. 7124–7128.
`Micromachined Planar Inductors on Silicon Wafers for
`MEMS Applications; Chong H. Ahn et al.
`* cited by examiner
`Primary Examiner Ali Imam
`(74) Attorney, Agent, or Firm-Brinks Hofer Gilson &
`Lione
`ABSTRACT
`(57)
`The present invention relates to an implantable microfabri
`cated Sensor device and System for measuring a physiologic
`parameter of interest within a patient. The implantable
`device is micro electromechanical system (MEMS) device
`and includes a Substrate having an integrated inductor and at
`least one Sensor formed thereon. A plurality of conductive
`paths electrically connect the integrated inductor with the
`Sensor. Cooperatively, the integrated inductor, Sensor and
`conductive paths defining an LC tank resonator.
`
`31 Claims, 9 Drawing Sheets
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`US 6,926,670 B2
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`WIRELESS MEMS CAPACTIVE SENSOR
`FOR PHYSIOLOGIC PARAMETER
`MEASUREMENT
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`This application claims priority to prior U.S. provisional
`application No. 60/263,327 (filed Jan. 22, 2001) and U.S.
`provisional application No. 60/278,634 (filed Mar. 26,
`2001).
`
`BACKGROUND OF THE INVENTION
`
`2
`Bullara (U.S. Pat. No. 4,127,110), and Dunphy (U.S. Pat.
`No. 3,958,558) disclose various devices initially intended
`for hydrocephalus applications (but also amenable to others)
`that use LC resonant circuits. The 276, 110, and 558
`patents, although feasible, do not take advantage of recent
`advances in Silicon (or similar) microfabrication technolo
`gies. Kensey (U.S. Pat. No. 6,015.386) discloses an implant
`able device for measuring blood pressure in a vessel of the
`wrist. This device must be “assembled' around the vessel
`being monitored Such that it fully encompasses the vessel,
`which may not be feasible in many cases. In another
`application, Frenkel (U.S. Pat. No. 5,005,577) describes an
`implantable lens for monitoring intraocular pressure. Such a
`device would be advantageous for monitoring elevated eye
`pressures (as is usually the case for glaucoma patients);
`however, the requirement that the eye's crystalline lens be
`replaced will likely limit the general acceptance of this
`device.
`In addition to the aforementioned applications that Specify
`LC resonant circuits, other applications would also benefit
`greatly from such wireless sensing. Han, et al. (U.S. Pat. No.
`6,268,161) describe a wireless implantable glucose (or other
`chemical) Sensor that employs a pressure Sensor as an
`intermediate transducer (in conjunction with a hydrogel)
`from the chemical into the electrical domain.
`The treatment of cardiovascular diseases Such as Chronic
`Heart Failure (CHF) can be greatly improved through con
`tinuous and/or intermittent monitoring of various pressures
`and/or flows in the heart and associated vasculature. Porat
`(U.S. Pat. No. 6,277,078), Eigler (U.S. Pat. No. 6,328,699),
`and Carney (U.S. Pat. No. 5,368,040) each teach different
`modes of monitoring heart performance using WireleSS
`implantable Sensors. In every case, however, what is
`described is a general Scheme of monitoring the heart. The
`existence of a method to construct a Sensor with Sufficient
`size, long-term fidelity, Stability, telemetry range, and bio
`compatibility is noticeably absent in each case, being instead
`Simply assumed. Eigler, et al., come closest to describing a
`Specific device Structure although they disregard the baseline
`and Sensitivity drift issues that must be addressed in a
`long-term implant. Applications for wireleSS Sensors located
`in a stent (e.g., U.S. Pat. No. 6,053,873 by Govari) have also
`been taught, although little acknowledgement is made of the
`difficulty in fabricating a pressure Sensor with telemetry
`means Sufficiently Small to incorporate into a Stent.
`Closed-loop drug delivery Systems, Such as that of Fein
`gold (U.S. Pat. No. 4,871.351) have likewise been taught. As
`with others, Feingold overlooks the difficulty in fabricating
`Sensors that meet the performance requirements needed for
`long-term implantation.
`In nearly all of the aforementioned cases, the disclosed
`devices require a complex electromechanical assembly with
`many dissimilar materials, which will result in Significant
`temperature- and aging-induced drift over time. Such assem
`blies may also be too large for many desirable applications,
`including intraocular pressure monitoring and/or pediatric
`applications. Finally, complex assembly processes will
`make Such devices prohibitively expensive to manufacture
`for widespread use.
`AS an alternative to conventionally fabricated devices,
`microfabricated Sensors have also been proposed. One Such
`device is taught by Darrow (U.S. Pat. No. 6,201.980). Others
`are reported in the literature (see, e.g. Park, et al., Jpn. J.
`Appl. Phys., 37 (1998), pp. 7124-7128; Puers, et al., J.
`Micromech. Microeng. 10 (2000), pp. 124-129; Harpster et
`al., Proc. 14' IEEE Intl. Conf. Microelectromech. Sys.
`(2001), pp. 553–557).
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`Field of the Invention
`The present invention generally relates to the field of
`MEMS (micro-electromechanical systems) sensors and
`more specifically to a wireless MEMS capacitive sensor for
`implantation into the body of a patient to measure one or
`more physiologic parameters.
`A number of different biologic parameters are Strong
`candidates for continuous monitoring. These parameters
`include, but are not limited to blood pressure, blood flow,
`intracranial preSSure, intraocular preSSure, glucose levels,
`etc. Wired Sensors, if used have certain inherent limitations
`because of the passage of wires (or other communication
`“tethers”) through the cutaneous layer. Some limitations
`include the risks of physical injury and infection to the
`patient. Another risk is damage to the device if the wires (the
`communication link) experience excessive pulling forces
`and Separate from the device itself. WireleSS Sensors are
`therefore highly desirable for biologic applications.
`A number of proposed Schemes for wireleSS communica
`tion rely on magnetic coupling between an inductor coil
`asSociated with the implanted device and a separate, external
`35
`“readout' coil. For example, one method of wireless com
`munication (well-known to those knowledgeable in the art)
`is that of the LC (inductor-capacitor) tank resonator. In Such
`a device, a Series-parallel connection of a capacitor and
`inductor has a specific resonant frequency, expressed as 1/
`VLC, which can be detected from the impedance of the
`circuit. If one element of the inductor-capacitor pair varies
`with Some physical parameter (e.g. pressure), while the other
`element remains at a known value, the physical parameter
`may be determined from the resonant frequency. For
`example, if the capacitance corresponds to a capacitive
`preSSure Sensor, the capacitance may be back-calculated
`from the resonant frequency and the Sensed pressure may
`then be deduced from the capacitance by means of a
`calibrated pressure-capacitance transfer function.
`The impedance of an LC tank resonator may be measured
`directly or it may also be determined indirectly from the
`impedance of a separate readout coil that is magnetically
`coupled to the internal coil. The latter case is most useful for
`biologic applications Since the Sensing device may be Sub
`cutaneously implanted, while the readout coil may be
`located external to the patient, but in a location that allows
`magnetic coupling between the implanted Sensing device
`and readout coil. It is possible for the readout coil (or coils)
`to Simultaneously excite the resonator of the implanted
`de Vice and Sense the reflected back impedance.
`Consequently, this architecture has the Substantial advantage
`of requiring no internal power Source, which greatly
`improves its prospects for long-term implantation (e.g.
`decades to a human lifetime).
`Such devices have been proposed in various forms for
`many applications. Chubbuck (U.S. Pat. No. 4,026,276),
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`Past efforts to develop wireleSS Sensors have Separately
`located the Sensor and inductor and have been limited to
`implant-readout Separation distances of 1-2 cm at most,
`rendering them impractical for implantation much deeper
`than immediately below the cutaneous layer. This eliminates
`from consideration wireleSS Sensing applications, Such as
`heart ventricle pressure monitoring or intracranial pressure
`monitoring, that inherently require Separation distances in
`the range of 5-10 cm. In the present State-of-the-art, Several
`factors have contributed to this limitation on the Separation
`distance including 1) signal attenuation due to intervening
`tissue, 2) Suboptimal design for magnetic coupling effi
`ciency; and 3) high internal energy losses in the implanted
`device.
`In view of the above and other limitations on the prior art,
`it is apparent that there exists a need for an improved
`wireless MEMS sensor system capable of overcoming the
`limitations of the prior art and optimized for Signal fidelity,
`transmission distance and manufacturability. It is therefore
`an object of the present invention is to provide a wireleSS
`MEMS sensor system in which the sensing device is adapted
`for implantation within the body of patient.
`A further object of this invention is to provide a wireless
`MEMS sensor system in which the separation distance
`between the Sensing device and the readout device is greater
`than 2 cm, thereby allowing for deeper implantation of the
`Sensing device within the body of a patient.
`Still another object of the present invention is to provide
`a wireless MEMS sensor system in which the sensing device
`utilizes an integrated inductor, an inductor microfabricated
`with the sensor itself.
`It is also an object of this invention to provide a wireleSS
`MEMS sensor system in which the sensing device is bat
`teryleSS.
`A further object of the present invention is to provide a
`wireless MEMS sensor system.
`BRIEF SUMMARY OF THE INVENTION
`In overcoming the limitations of the prior art and achiev
`ing the above objects, the present invention provides for a
`wireless MEMS sensor for implantation into the body of a
`patient and which permits implantation at depths greater
`than 2 cm while still readily allowing for reading of the
`Signals from the implanted portion by an external readout
`device.
`In achieving the above, the present invention provides a
`MEMS sensor system having an implantable unit and a
`non-implantable unit. The implantable unit is microfabri
`cated utilizing common microfabricating techniques to pro
`vide a monolithic device, a device where all components are
`located on the same chip. The implanted device includes a
`Substrate on which is formed a capacitive Sensor. The fixed
`electrode of the capacitive Sensor may formed on the Sub
`strate itself, while the moveable electrode of the capacitive
`Sensor is formed as part of a highly doped silicon layer on
`top of the Substrate. Being highly doped, the Silicon layer
`itself operates as the conductive path for the moveable
`electrode. A separate conductive path is provided on the
`Substrate for the fixed electrode.
`In addition to the capacitive Sensor, the implanted Sensing
`device includes an integrally formed inductor. The integral
`inductor includes a magnetic core having at least one plate
`and a coil defining a plurality of turns about the core. One
`end of the coil is coupled to the conductive lead connected
`with the fixed electrode while the other end of the coil is
`electrically coupled to the highly doped Silicon layer,
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`thereby utilizing the Silicon layer as the conductive path to
`the moveable electrode.
`In order to optimize the operation of the inductor and to
`permit greater implantation depths, a novel construction is
`additionally provided for the magnetic core. In general, the
`optimized magnetic core utilizes a pair of plates formed on
`opposing Sides of the Substrate and interconnected by a post
`extending through the Substrate. The windings of the coil, in
`this instance, are provided about the post.
`The external readout device of the present System also
`includes a coil and various Suitable associated components,
`as well known in the field, to enable a determination of the
`preSSure or other physiologic parameter being Sensed by the
`implanted Sensing device. The external readout device may
`Similarly be utilized to power the implanted Sensing device
`and as Such the implanted Sensing device is wireleSS.
`Integrally formed on the implanted device and microfab
`ricated therewith, may be additionally be active circuitry for
`use in conjunction with capacitive Sensor. Locating this
`circuitry as near as possible to the capacitive Sensor mini
`mizes noise and other factors which could lead to a degra
`dation in the received signal and the Sensed measured
`physiologic parameter. AS Such, the active circuitry may be
`integrally microfabricated in the highly doped Silicon layer
`mentioned above.
`Further object and advantages of the present invention
`will become apparent to those skilled in the art from a
`review of the drawings in connection with the following
`description and dependent claims.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic illustration of a wireless MEMS
`Sensor System according the principles of the present inven
`tion;
`FIG. 2 is a graphical illustration of impedance magnitude
`and phase angle near resonance, as Sensed through a readout
`coil;
`FIG. 3 is a croSS-Sectional representation of a Sensing
`device embodying the principles of the present invention.
`FIGS. 4A and 4B are schematic illustrations of the mag
`netic field distribution with FIG. 4A illustrating the magnetic
`field distribution of prior art devices and with FIG. 4B
`illustrating the magnetic field distribution for a Sensing
`device having a magnetic core embodying the principles of
`the present invention;
`FIG. 5 is an enlarged cross-sectional view of the dia
`phragm portion of FIG.3 operating in what is herein referred
`to as a “proximity” mode,
`FIG. 6 is a cross-sectional view similar to that seen in
`FIG. 5 illustrating, however, the diaphragm operating in
`what is herein referred to as a "touch' mode;
`FIG. 7 is a capacitance verSuS pressure curve in the
`proximity and touch modes of operation;
`FIG. 8 is a top plane view of a second embodiment of the
`main electrode in the capacitive Sensor portion of the
`implanted Sensing device according to the principles of the
`present invention;
`FIG. 9 is a diagrammatic illustration of one scheme for
`providing electrically isolated paths for the connections and
`electrodes of the capacitive Sensor portion;
`FIG. 10 is a diagrammatic illustration of another scheme
`for electrically isolating the conductive paths for the con
`nections and contacts of the capacitive Sensor portion;
`FIG. 11 is a cross-sectional view, generally similar to that
`Seen in FIG. 3, further incorporating active circuitry into the
`Sensing device,
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`FIG. 12 is a block diagram illustrating one possible circuit
`implementation of the active circuitry when incorporated
`into the sensing device of the present wireless MEMS
`Sensing System;
`FIG. 13 illustrates one method of mounting, within the
`body of a patient, a Sensing device embodying the principles
`of the presents invention;
`FIG. 14 illustrates a second embodiment by which a
`Sensing device embodying the principles of the present
`invention may be secured to tissues within the body of a
`patient
`FIGS. 15 and 16 are diagrammatic illustrations of differ
`ent embodiments for locating a Sensing device according to
`the principles of the present invention, within a vessel in the
`body of a patient;
`FIG. 17 illustrates a Sensing device, according to the
`principles of the present invention, encapsulated in a mate
`rial yielding a pellet-like profile for implantation into the
`tissues in the body of a patient;
`FIG. 18 illustrates a Sensing device according to the
`principles of the present invention being located within the
`electrode tip of an implantable Stimulation lead, Such as that
`used for cardiac pacing,
`FIG. 19 illustrates a plurality of sensing devices according
`to the present invention located within a catheter and utilized
`to calculate various physiologic parameters within a vessel
`within the body of a patient;
`FIG. 20 is a schematic illustration of multiple sensors
`being used to measure performance of a component in the
`body or a device mounted within the body of a patient;
`FIG. 21 illustrates a Sensing device according to the
`principles of the present invention being utilized to measure
`preSSure externally through a vessel wall;
`FIG. 22 illustrates a portion of a further embodiment of
`the present invention in which the pressure Sensing features
`of the Sensing device have been augmented over or replaced
`with a structure allowing a parameter other than preSSure to
`be sensed;
`FIG. 23 is schematic perspective view, with portions
`enlarged, illustrating an alternative embodiment for Sensing
`according to the principles of the present invention; and
`FIG. 24 is an embodiment generally similar to that seen
`in FIG. 23 for Sensing according to the principles of the
`present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`In order to provide for battery-less, wireleSS physiologic
`parameter Sensing over Significant distances greater than 2
`cm (e.g. 10 cm), the present invention provides a wireless
`MEMS Sensing System, generally designated at 10 and Seen
`schematically in FIG. 1. The system 10 includes a micro
`fabricated implantable Sensing device 12, optimized for
`coupling with an external readout device 14. The Sensing
`device 12 is provided with an integrated inductor 16 that is
`conductive to the integration of transducers and/or other
`components necessary to construct the WireleSS Sensing
`System 10. AS an example, the preferred embodiment inte
`grates a capacitive pressure Sensor 18 into a common
`substrate 20 with the integrated inductor 16. A second
`inductor 24, in the readout device 14, couples magnetically
`26 with the integrated inductor 16 of the sensing device 12.
`The readout device 14 is constructed according to tech
`niques well known in the industry and in the Sensing field in
`general. AS Such, the readout device 14 is not illustrated or
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`described in great detail. It is noted, however, that the
`readout device 14 may be included, in addition to its
`inductor 24, Signal conditioning, control and analysis cir
`cuitry and Software, display and other hardware and may be
`a Stand alone unit or may be connected to a personal
`computer (PC) or other computer controlled device.
`The magnetic coupling 26 seen in FIG. 1 allows the
`impedance of the LC tank circuit 22 to be sensed by the
`readout device 14. The typical impedance magnitude 28 and
`phase angle 30 near resonance 32, as Sensed through the
`readout coil 14, is seen in FIG. 2. Real-time measurement
`and analysis of this impedance and changes therein allows
`the Sensed pressure to be determined as previously men
`tioned.
`Referring now to FIG. 3, a cross section of a preferred
`embodiment of the Sensing device 12 is illustrated therein.
`The sensing device 12 includes a main substrate 34
`(preferably 7740 Pyrex glass) formed and located within
`recessed regions of the Substrate 34 are those Structures
`forming the integrated inductor 16. The integrated inductor
`16 is seen to include a magnetic core 33 defined by a top
`plate 36, a bottom plate 38 and a post 40 connecting the top
`plate 36 to the bottom plate 38 and being continuous through
`the substrate 34. The plates 36 and 38 and the post 40 are
`preferably constructed of the same material, a ferromagnetic
`material and are monolithic. The integrated conductor 16
`additionally includes a coil 42, preferably composed of
`copper or other high-conductivity material, Successive turns
`of which surround the post 40 of the magnetic core 33.
`In FIG. 3, the coil 42 is seen as being recessed into the top
`plate 36. The coil 42 may additionally be planar or layered
`and preferably wraps as tightly as possible about the post 40.
`If the material of the coil 42 has a high electrical resistance
`relative to the material of the core 33, (as in a copper coil and
`NiZn ferrite core system) the core 33, and specifically the
`top plate 36 may be directly deposited on top of the coil 42
`without need for a intermediate insulating layer. If the
`electrical resistance of the coil material relative to the coil
`material is not high, an intermediate insulating layer must be
`included between the Successive turns of the coil 42 and the
`core 33.
`Top and bottom cap layerS 44 and 46 respectively, are
`provided over upper and lower faces 48 and 50 of the
`substrate 20 and over the top and bottom plates 36 and 38 of
`the magnetic core 33. To accommodate any portions of the
`magnetic core 33 that extend Significantly above or below
`the upper and lower faces 48 and 50 of the substrate 20, the
`cap layers 44 and 46 may be provided with recesses 52 and
`54, respectively. Preferably, the cap layers 44 and 46 are of
`monocrystalline Silicon. Other preferred materials include
`polycrystalline Silicon, epitaxially deposited Silicon,
`ceramics, glass, plastics, or other materials that can be
`bonded to lower Substrate and/or are Suitable for fabrication
`of the Sensor diaphragm. In lieu of a monolithic cap layer,
`Several Sub-pieces may be fabricated at Separate process
`Steps, together forming a complete cap layer after processing
`is finished.
`The coupling effectiveness of the integrated inductor 16 is
`a function of the magnetic flux enclosed by the windings of
`the coil 42, therefore the coupling is greatest if the Structure
`of the integrated inductor 16 maximizes the flux encom
`passed by all of the winding loops. FIG. 4A shows sche
`matically the magnetic field distribution 56 in a known
`inductor Structure having a single core layer 58 and associ
`ated windings 60. Schematically shown in FIG. 4b is the
`magnetic field distribution 62 for an inductor structure 16
`
`Abbott
`Exhibit 1001
`Page 013
`
`

`

`7
`having upper and lower plates 36' and 38', connected by a
`post 40' about which windings of a coil 42' are located, as
`generally Seen in the present invention. The design of the
`present invention optimizes the inductor geometry for maxi
`mum field coupling. Placing the plates 36 and 38 on opposite
`sides of the substrate 20, as in FIG. 3, increases the plate
`to-plate spacing. The increased plate spacing creates a
`localized path of least resistance for the free-space magnetic
`field of an external readout coil, causing the magnetic field
`to preferentially pass through the post 40 of the integrated
`inductor's magnetic core 33. This increases device effec
`tiveness Since the coupling efficiency between the Sensor
`and a readout unit increases with the total magnetic flux
`encompassed by the windings of the inductor. A greater
`coupling efficiency increases the maximum Separation dis
`tance between the Sensor and a readout unit.
`The materials used to form the integrated inductor 16
`should be chosen and/or processed to maximize the above
`mentioned effect and to minimize drift in the inductance
`value across time, temperature, package StreSS, and other
`potentially uncontrolled parameters. A high-permeability
`material Such as NiZn ferrite is used to maximize this effect
`on the magnetic field and to minimize drift. Other preferred
`materials include nickel, ferrite, permalloy, or Similar ferrite
`composites.
`To the right of the integrated inductor 16 seen in FIG. 3
`is the capacitive preSSure Sensor 18. The capacitive pressure
`Sensor 18 may be constructed in many forms commonly
`know to those familiar with the art. In the illustrated
`embodiment, the upper cap layer 44 is formed to define a
`diaphragm 64. The diaphragm 64 constitutes and may also
`be referred to as the moveable electrode of the pressure
`sensor 18. The fixed electrode 66 of the pressure sensor 18
`is defined by a conductive layer formed on the upper face 48
`of the Substrate 20, in a position immediately below the
`moveable electrode or diaphragm 64. If desired, a conduc
`tive layer may additionally be located on the underside of the
`moveable electrode 64. To prevent shorting between the
`upper electrode 64 (as defined by either the diaphragm itself
`or the diaphragm and the conductive layer 68) and the lower
`electrode 66, one or both of the electrodes 64 and 66 may be
`provided with a thin dielectric layer (preferably less than
`1000 A) deposited thereon.
`To improve performance of the capacitive pressure Sensor
`18, as seen in FIG. 8, one or more secondary electrodes
`designated at 70 may be located about the fixed electrode 66
`near the projected edge of the diaphragm 64 where pressure
`induced deflection of the diaphragm 64 is minimal. The
`Secondary electrodes 70 experience all of the capacitance
`effecting phenomena Seen by the main electrode 66, with the
`exception of any pressure-induced phenomena. The Second
`ary electrodes 70, as Such, operate as reference electrodes
`and by Subtracting the Secondary electrodes capacitive
`measurement from the capacitive measurement of the main
`electrode 66, most or all non-pressure-induced capacitance
`changes (signal drift) may be filtered out. Examples as
`Sources of signal drift, that may be filtered out by this
`method, include thermally induced physical changes and
`parasitics resulting from an environment with changing
`dielectric constant, Such as insertion of the Sensor into tissue.
`In a preferred embodiment, the Secondary (or reference)
`electrodes 70 would require an additional coil, similar to
`construction of the previously mentioned coil 42 to form a
`Separate LC tank circuit. It is noted, that both coils may,
`however, share the same core post 40.
`Under normal operation, pressure applied to the exterior
`or top Surface of the capacitive pressure Sensor 18 causes the
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`US 6,926,670 B2
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`8
`diaphragm 64 (or at least the center portions thereof) to
`deflect downward toward the fixed electrode 66. Because of
`the change in distance between the fixed electrode 66 and the
`moveable electrode 64, a corresponding change will occur in
`the capacitance between the two electrodes. The applied
`preSSure is therefore translated into a capacitance. With this
`in mind, it is seen that the capacitance pressure Sensor 18
`may be operated in either of two modes.
`A first mode, hereinafter referred to as the “proximity”
`mode, is generally Seen in FIG. 5. In this mode of operation,
`the Starting gap between the fixed electrode 66 and the
`moveable electrode 64, as well as the material and physical
`parameters for the diaphragm 64 itself, are chosen Such that
`the fixed electrode 66 and the moveable electrode 64 will be
`Spaced apart from one another over the entire operating
`preSSure range of the Sensor 18. For the Standard equation of
`parallel plate capacitance, C=6A/d, the plate Separation d
`will vary with the applied pressure, while the plate area A
`and the permittivity 6 remain constant.
`In the touch mode of operation, generally Seen in FIG. 6,
`the geometry (e.g., initial gap spacing between the fixed
`electrode 66 and the moveable electrode 64) as well as the
`material and physical parameters of the diaphragm 64 itself,
`are chosen Such that the fixed electrode 66 and the moveable
`electrode 64 will progressively touch each other over the
`operating pressure range of the Sensor 18. Accordingly, the
`area 72 of the fixed electrode 66 and the moveable electrode
`64 in contact with each other will vary with the applied
`preSSure. In the touch mode of operation, the dominant
`capacitance is the capacitance of the regions of the fixed
`electrode 66 and the moveable electrode 64 in contact with
`one another (if the dielectric coating 74 is thin compared to
`the total gap thickness, thereby yielding a relatively Small
`effective plate separation distance d). In the capacitance
`equation mentioned above, plate Separation d and permit
`tivity 6 will remain constant (at approximately that of the
`dielectric thickness) while the plate contact area A varies
`with the applied pressure.
`In the graph of FIG. 7, capacitance-pressure relationship
`in the proximity and touch modes, respectively designated at
`76 and 78, are seen. From a practical standpoint, the
`operational mode may be chosen based upon Sensitivity,
`linearity, and dynamic range requirements. The touch mode
`typically yields higher Sensitivity with a more linear output,
`but involves mechanical contact between Surfaces and there
`fore requires a careful choice of the materials to avoid wear
`induced changes in performance of the pressure Sensor 18.
`To permit the innermost turn of the coil 42 to be electri
`cally connected to the moveable electrode 66, a post 80
`(formed integral with the substrate 20) extends upward
`through the top plate 36 and a conductive trace 82 runs up
`the side of the post 80. The trace 82 b