Abstract:
An electret transducer with at least one variable area capacitor and a step to provide a contoured region of a rigid capacitor electrode of said variable area capacitor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part application of divisional application U.S. Ser. No. 09/482,119, filed Jan. 13, 2000, of application U.S. Ser. No. 09/037,733, filed Mar. 10, 1998, now U.S. Pat. No. 6,151,967, issued Nov. 28, 2000. This application references art disclosed in U.S. application Ser. No.: 09/794,198, filed Feb. 27, 2001; Ser. No. 09/816,551, filed Mar. 24, 2001; Ser. No. 09/834,691, filed Apr. 13, 2001; Ser. No. 09/866,351, filed May 25, 2001; and Ser. No. 09/954,670, filed Sept. 18, 2001; all continuation-in-part applications of divisional application U.S. Ser. No. 09/482,119. Each disclosure of the foregoing applications are expressly incorporated herein by reference. All of the applications are assigned to the same assignee as the present application. 
     
    
     GOVERNMENT RIGHTS  
       [0002] This invention was made with Government support under contract N00024-97-C-4157 from the Naval Sea Systems Command. The Government has certain rights to this invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention relates capacitive transducers self-biased by a thin-film electret. More specifically, it relates to wide dynamic range force, pressure, and displacement sensors; electrostatic actuators; and acoustic and ultrasonic transceivers with multiple capacitor elements.  
         BACKGROUND OF THE INVENTION  
         [0004]    Simple electrostatic transducers comprise a variable capacitor with two, substantially parallel-plate electrodes: a flexible electrode responsive to a physical effect and a rigid counter-electrode. A change in pressure or force applied to the flexible electrode causes it to deflect. Displacement of the flexible electrode is electrically sensed by detecting a change in capacitance between cooperating electrodes. When a polarization voltage V 0  is applied across the electrodes, the voltage v c (t) across the capacitor for a small diaphragm displacement b(t) can be approximated by,  
                 v   c          (   t   )       ≈         δ        (   t   )         s   0            V   0       ≈         Δ                   C        (   t   )           C   0            V   0                     (     δ        &lt;&lt;     s   0         )               (   1   )                               
 
           [0005]    where C 0  is the quiescent capacitance and s 0  is the equilibrium spacing between the capacitor plates biased with a static potential V o .  
           [0006]    Conventional capacitive microphones utilize a high polarization voltage V 0  to transduce the amplitude of a time varying acoustic pressure p a (t) to an open circuit voltage v c (t) across the capacitor electrodes. The polarization voltage is applied across the microphone capacitor through a high-value charging resistor R that maintains a substantially constant charge Q o =C 0 V 0  on the variable capacitor. The sensitivity S of a microphone in terms of its open-circuit voltage divided by the pressure amplitude of an incident acoustic wave of angular frequency w can be expressed as  
             S   =           v   c          (   t   )         Δ                   p        (   t   )           =         Δ                   C        (   t   )            V   0         C   0       ·       C   0         C   0     +     C   s         ·       jω                   R        (       C   0     +     C   s       )           1   +     jω                   R        (       C   0     +     C   s       )                         (   2   )                               
 
           [0007]    where C s  is the total parasitic capacitance including the input capacitance of amplification electronics. Eq. 2 can be further refined to include additional frequency dependent factors to account for specific electrode geometry and the effective mass and compliance of the diaphragm and surrounding fluids.  
           [0008]    Electret capacitive transducers operate without an external dc polarization voltage. A thin-film electret is affixed to or formed on one capacitor electrode. An electret has a permanent state of electrical polarization that provides an electric field to self-bias a variable capacitor. This permits small electret microphones to be manufactured in high volume for hearing aids and communications equipment by avoiding the cost and complexity of providing a low-noise source of high voltage.  
           [0009]    Many methods are known to provide electrets and electret capacitance microphones. Four references of note are: 1) G. M. Sessler and J. E. West, “Self-Biased Condenser Microphone with High Capacitance,” Acoust. Soc. of Am. J. 34: 1787-1788, 1962; 2) U.S. Pat. No. 3,740,496 of Carlson et al.; U.S. Pat. No. 5,536,982 of Mino et al.; and 4) U.S. Pat. No. 2001/0033670 A1 of Tai et al.  
           [0010]    Prior-art electret transducers are constructed with substantially parallel-plate electrodes. The sensitivity, linearity, and dynamic range of these capacitive transducers are limited by the geometric constraints of parallel-plate construction. The capacitance-displacement sensitivity of a gap-varying capacitor at mid-range signal frequencies is substantially:  
           Δ C/Δs=−εA/s   2   (3)  
           [0011]    The dependency on s 2  results in a non-linear capacitance sensitivity with plate spacing.  
           [0012]    Other disadvantages of prior-art, variable capacitors result from the minimum spacing that can be reliably maintained between parallel spaced electrodes and associated low values of quiescent capacitance. A transducer with low quiescent capacitance has a high source impedance 1/jωC at acoustic frequencies. This generally requires the capacitor voltage v c (t) to be detected by a JFET amplifier. The noise of a FET and high value bias resistors further limit dynamic range at low frequencies. Another disadvantage of small quiescent capacitance is a loss in sensitivity S due to stray capacitance. The total parasitic capacitance C s  of fringing fields, support structure, electrodes, and inputs of electronic circuitry reduces sensitivity S in Eq. 2 by the factor C o /(C o +C s ).  
           [0013]    The spacing between capacitor electrodes limits the maximum displacement of a movable electrode. This displacement is further restricted by the well-known “pull-in” instability occurring at a critical voltage at which the movable electrode deflects by about ⅓ of the undeflected capacitor gap. Precision capacitance accelerometers use electrostatic force-rebalanced feedback to maintain an inertial mass suspended on a flexible electrode at a substantially fixed position to minimize non-linear capacitance response. However, feedback cannot significantly increase sensitivity or avoid the disadvantages of small quiescent capacitance.  
           [0014]    The disadvantages of capacitance transducers with parallel electrodes (with or without an electret) are avoided by the variable-area capacitor embodiments of U.S. Pat. No. 6,151,967 and U.S. patent application Ser. Nos. 09,834,691 and 09/866,351.  
           [0015]    A variable area capacitor (VAC) is referred to herein as a variable capacitor for which a substantial portion of a change in capacitance with electrode displacement is due to an increase in effective electrode area rather than to a change in electrode spacing. The capacitance of this type of VAC increases as an area of fixed capacitive spacing increases between cooperating electrodes while the approach of a movable electrode with respect to a stationary electrode remains small. This increases the effective area A contributing the majority of the capacitance between the electrodes and accommodates large displacements not limited by the dimensions of a narrow air gap. When a rising voltage is applied to a VAC, an electrostatic force of attraction continuously collapses a flexible electrode across a curved surface of a cooperating rigid electrode.  
           [0016]    The large changes in capacitance of an electret VAC, up to 500% and more, can be linearly transduced by circuit inventions disclosed in U.S. patent application Ser. Nos. 09/794,198 and 09/816,551. An electret VAC with a thin flexible diaphragm can be operated as an electrostatic actuator or as an acoustic transmitter. When a variable voltage is applied across a VAC, acoustic or ultrasonic energy couples to the medium in which it is immersed.  
           [0017]    An electret VAC actuator can couple an electrostatic bias force to a load without the application of an external control voltage. When the electrostatic force balanced feedback invention of U.S. patent application Ser. No. 09/866,351 is used for load position control, the gain of the control loop at and around a static equilibrium position is greater than zero. This allows precision control of mechanical components or optical elements at small displacements.  
         SUMMARY OF THE INVENTION  
         [0018]    The dynamic range of electret capacitive transducers can be extended several orders of magnitude using VAC&#39;s with the general construction of the embodiments of U.S. Pat. No. 6,151,967. Electret transducers of the present invention can be constructed in part by methods disclosed in U.S. patent application Ser. No. 09/834,691. The instant invention can be advantageously applied to VAC transducers constructed to detect physical effects including force, pressure, acceleration, and displacement. VAC transducers can be operated as electrostatic actuators, with and without, force-rebalanced feedback control. All aspects of the present invention are applicable to sensors and actuators with multiple VAC elements.  
           [0019]    Accordingly, the principle object of the present invention is to provide electret sensors and actuators with the low-noise, high capacitive sensitivity, and wide linear dynamic range characteristic of VAC transducers. This object is realized by electrically polarizing a thin dielectric film placed between a flexible electrode and a curved rigid counter-electrode. Non-limiting examples of VAC transducer embodiments in which an electret can be used to provide a self-biased transducer with a predetermined response characteristic are disclosed in U.S. Pat. No. 6,151,967 and U.S. patent application Ser. Nos. 09/834,691, 09/866,351, and 09/954,670.  
           [0020]    More specifically, it is desirable to obtain large, linear changes of voltage v c (t) in Eq. 1 for values of δ         s o  to provide high values of sensitivity S, as defined by Eq. 2.  
           [0021]    Other objects of the present invention are to:  
           [0022]    1. Provide electret VAC transducers with a flexible electrode comprising a simple, edge-supported cantilever beam;  
           [0023]    2. Provide electret VAC transducers with a flexible electrode comprising a diaphragm;  
           [0024]    3. Provide electret VAC transducers with multiple sensing and actuation elements electrically connected in parallel or alternately connected individually;  
           [0025]    4. Provide electret VAC transducers with a response characteristic governed by a predetermined surface contour of a rigid electrode, such as a contour selected to maximize linear dynamic range;  
           [0026]    5. Provide electret VAC transducers with high values of quiescent capacitance to avoid the noise limitations of small capacitors and detection electronics and a reduction in sensitivity due to parasitic capacitance;  
           [0027]    6. Provide electret VAC transducers that employ electrostatic force-rebalanced feedback to sense and control the displacement of an inertial mass, stylus, or optical element.  
           [0028]    7. Provide electret actuators that provide an electrostatic force to mechanically bias a mechanical, electrical, or optical element around a desired position of static equilibrium.  
           [0029]    In accordance with the present invention, transducers including at least one VAC sensing or actuator element is self-biased by a thin electret film. The electret provides a DC polarization voltage across a flexible electrode and a stationary electrode with a surface contoured region. An electrical signal is generated that is substantially linearly proportional to displacement of the flexible sensing electrode over a wide dynamic range.  
           [0030]    The above and other objects and advantages of the present invention will become apparent from consideration of the following description and drawings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0031]    The embodiments of the invention are described by way of non-limiting drawings. The drawings are schematic in nature for clarity of description and thus features shown are not drawn to relative scale; like reference numbers designate similar parts or elements with similar functions.  
         [0032]    [0032]FIG. 1, is a sectional view of an electret VAC transducer with a flexible electrode comprising a cantilever beam;  
         [0033]    [0033]FIG. 2, is a sectional view of an electret VAC transducer with a flexible electrode comprising a diaphragm;  
         [0034]    [0034]FIG. 3, is a sectional view of a VAC transducer with an electret formed over a rigid electrode;  
         [0035]    [0035]FIG. 4 is a top view of a substrate of an electret VAC transducer with an array of dish-shaped depressions;  
         [0036]    [0036]FIG. 5 is an illustration a portion of a substrate of an electret VAC transducer with multiple ridges and valleys;  
         [0037]    [0037]FIG. 6 is a sectional view of a substrate of a VAC transducer with an array of capacitor elements;  
     
    
     DETAILED DESCRIPTION  
       [0038]    [0038]FIG. 1, is a simplified sectional view of an electret VAC transducer generally shown by reference numeral  10  with a flexible electrode  12  comprising a compliant cantilever beam. A thin film electret  14  is formed over a surfaced contoured region  16  of a rigid electrode  18 . An edge region  20  of flexible electrode  12  is affixed to an edge portion  22  of cooperating rigid electrode  18 . Electret  14  maintains a fixed capacitance spacing d between regions of mutually opposed areas of flexible electrode  12  and cooperating rigid electrode  18 . Regions of fixed capacitance spacing increase as flexible electrode  12  deflects in response to an applied stress.  
         [0039]    The permanent electrical polarization of electret  14  provides an electric field that applies an electrostatic stress to electrode  12  shown representatively by concentrated force F E . This force of attraction causes electrode  12  to deflect a distance δ 0  from a position indicated by dashed line  24  to the position shown for static force equilibrium where the elastic reaction forces of the beam equal the electrostatic force F E . At this balanced position, transducer  10  has a high value of quiescent capacitance C 0  compared to the quiescent capacitance of a gap varying capacitor of similar size with a parallel electrode spacing equal to δ o . When additional mechanical or electrostatic force is applied to electrode  12 , it deflects to a new position shown representatively by dashed line  26 . The curvature of contoured region  16  can be selected to govern the rate of change in capacitance of transducer  10  with deflection of electrode  12 .  
         [0040]    An electret transducer with substantially equivalent performance to transducer  10  can be provided by applying electret  14  to flexible electrode  12 . For either embodiment, an affixed means selected from the group consisting of an inertial mass, stylus, hinge, optical element, electrical element, mechanical component, and mechanical coupling can be supported or attached to electrode  12 .  
         [0041]    [0041]FIG. 2 is a simplified sectional view of an electret VAC transducer, generally identified by reference numeral  30 , that can be operated for example as a self-biased pressure sensor or noise-cancelling microphone. Transducer  30  includes a flexible sensing diaphragm  32  comprising a metal film electrode  34  deposited on a thin film electret  36 . A cooperating rigid electrode  38  has a radially symmetric surface contoured region  40  formed over and in a central portion of top surface  42  of rigid electrode  38 . Diaphragm  32  is affixed at edge portion  44  to top surface  42 . The profile of contoured region  40  is selected to provide a desired change in capacitance with deflection of diaphragm  32 . A through hole  46  can be optionally used to connect a central portion  48  of contoured region  40  to a bottom surface  50  of substrate  38 . Cavity  46  provides a passage for pressure equalization or a port for differential pressure sensing.  
         [0042]    Electret  36  maintains a substantially fixed capacitive spacing between regions of mutually opposed areas of the two capacitor electrodes that increase with deflection of diaphragm  32 . The electric field of electret  36  causes diaphragm  32  to deflect from its unloaded position to a position shown by dashed line  52 . When additional mechanical or electrostatic force is applied to diaphragm  32 , it deflects further to a new generatrix. The generatrix shown by dot-dashed line  54  is representative of the displacement of diaphragm  32  to uniform applied pressure.  
         [0043]    [0043]FIG. 3 is a sectional view of an electret VAC transducer, generally identified by reference numeral  60  with a flexible electrode comprising a diaphragm  62 . A thin film electret  64  is applied over a rigid electrode  66 . Electrode  66  is formed by depositing a metal film  66 ′ on a surface contoured region  68  formed over and in a portion of top surface  70  of a substrate  72 . For this embodiment, diaphragm  62  can comprise a metal film, a doped or metallized crystalline film, or a metallized polymer film. If an inertial mass  74  shown by a dashed outline is affixed to diaphragm  62 , transducer  60  has the construction of an electret VAC accelerometer which can be operated as a force-rebalanced accelerometer. A displacement of mass  74  due to a force of acceleration is detected by sensing a capacitance change between diaphragm  62  and rigid electrode  66 . This change in capacitance can be detected to provide a feedback control voltage across the same capacitor electrodes to maintain mass  74  at a substantially fixed position by a feedback method such as the one disclosed in U.S. patent application Ser. No. 09/866,351.  
         [0044]    Flexible electrode diaphragm  62  can be formed by evaporating or vacuum sputter depositing a thin metal film of aluminum, copper, gold, titanium, chrome, or a multiple metal film over a diaphragm of polyester, polycarbonate, polyimide, or a fluoropolymer. The diaphragm thickness can range between one to over  50  micrometers. Metallized polyester (PET) and polycarbonate diaphragms can be thermally bonded to polycarbonate or polyethylene terephthalate glycol (PETG) substrates respectively. It is desirable that diaphragm and substrate materials have closely matched coefficients of thermal expansion. Contoured region  68  can be thermally formed or coined for example by pressing a heated, single-point diamond-machined metal master into surface  70 .  
         [0045]    Electret  64  can be formed by electrically polarizing or implanting a charge in a polymer film by one of several well practiced methods used to manufacture electret microphones. The effective polarization voltage of electrets of conventional microphones ranges between 48 and 200 Volts. A lower polarization voltage, 5 to 20 Volts, can be used to deflect a flexible electrode of transducers of the present invention to a desired operating position while still providing high sensitivity compared to prior-art transducers of equivalent size. This advantage arises because of the high electric field across the small effective capacitor gap of a VAC.  
         [0046]    For microscale transducers, diaphragm  62  can be fabricated from metallized thin films of silicon, polycrystalline silicon, silicon dioxide, silicon nitride, or silicon oxynitride. Surface contoured region  68  can be formed in glass, silicon, or crystalline substrates by methods disclosed in U.S. patent application Ser. Nos. 09/834,691 or 09/954,670.  
         [0047]    [0047]FIG. 4 is a top view a substrate  80  of an electret VAC transducer with a surface contoured region  82  comprising an array of dish-shaped depressions  84  formed over and in a portion of top surface  86 . Substrate  80  can be used to construct a capacitive transducer having multiple VAC elements. For diaphragms of equal stiffness, a transducer with multiple capacitor elements has a dynamic response that extends to higher frequencies than a transducer of comparable size with a single capacitor element. Through holes  88  at the center of each capacitor site provide a path for pressure equalization or to minimize the compliance of the fluid volume between the capacitor electrodes.  
         [0048]    [0048]FIG. 5 is an illustration of a portion of a substrate  90  of an electret VAC transducer with a surface contoured region  92  that includes a series of ridges  94  and valleys  96 . Contoured region  92  is formed over and in a portion of top surface  98  of substrate  90 . Through holes  100  can be optionally formed between valleys  96  and bottom surface  102 . Alternately, or in addition to through holes  100 , one or more channels  104  can be formed to connect valleys  96  to an edgewall  106  to increase the compliance of the air or fluid volume between the capacitor electrodes.  
         [0049]    An advantage of using multiple ridges  94  to support a sensing diaphragm is that for a given pressure, a rectangular diaphragm element of width, a, deflects more than a circular element of diameter, a, or a square element with sides of width, a.  
         [0050]    [0050]FIG. 6 is a sectional view of a substrate  120  of an electret transducer that includes an array of active VAC element sites  122 . A surface contoured region  124  is formed over and in a top surface  126  of substrate  120 . For this sectional view, contoured region  124  can include multiple dish-shaped depressions  84  of the type shown in FIG. 4 or multiple ridges  94  and valleys  96  of the type as shown in FIG. 5. Through holes  128  provide a passage between contoured region  124  and bottom surface  130 . A rigid electrode  132  for each VAC element comprises a metal film  132 ′ deposited over contoured region  124 . Metal film  132 ′ is also deposited on walls  134  of through holes  128  and on bottom surface  130 . Metal film  132 ′ can be selectively etched from surface areas  136  of top surface  126  to reduce inactive electrode area. All VAC element sites  122  are electrically connected in parallel if metal film  132 ′ covers bottom surface  130 . This allows all VAC elements to be sensed and activated as a group. Individual or subgroups of VAC elements can be sensed and actuated individually by selectively etching a portion of metal film  132 ′ from bottom surface areas  138  to provide isolated areas  140  to bond electrical terminals.  
         [0051]    A transducer with individually addressable capacitor elements can be used for applications such as acoustic wavefront analysis and imaging. The electret transducers of the present invention with sealed or partially sealed through holes  128  can operated as acoustic or ultrasonic transceivers by capacitively coupling an AC drive voltage across the capacitor electrodes.  
         [0052]    The size of the through holes and channels of the transducers of the present invention can be sized to control fluid flow into and out of the internal volume of the VAC elements. For transducers with arrays of VAC elements, a plate with a matching array of apertures can be attached to the bottom of a substrate. Alternately, a plate of porous material can be used.  
         [0053]    The specific details of the embodiments described above are not intended to limit the scope of the appended claims and their legal equivalents.