Patent Publication Number: US-7916879-B2

Title: Electrostatic acoustic transducer based on rolling contact micro actuator

Description:
This application claims priority of U.S. provisional patent application No. 60/751,002 filed Dec. 16, 2005 hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention has applications to the field of acoustic components and transducers, and specifically to the field of acoustic sound generating structures based on micro fabrication. 
     BACKGROUND OF THE INVENTION 
     The realization of sound generating structures based on micro fabrication, or micro electro mechanical systems (MEMS), technology is particularly desirable as the utilization of the high-volume batch fabrication technology may reduce the device size, and improve the device quality, yield, and performance-to-cost ratio of such devices. The fundamental problem with sound generation, in contrast to sound detection, is that the device must provide a certain air volume displacement to generate a certain sound pressure. If the area of the sound generating structure (i.e. diaphragm) is reduced, to reduce the overall device size, the result is that the structure must have a larger displacement to generate the same sound pressure. A consequence of this is that the force necessary to drive the diaphragm increases. This is not easily combined with the reduction of the actuator size, since smaller actuators in general provide less actuation force. This scaling issue has proven prohibitive for micro scale implementations of established electromagnetic actuation principles, which are common in larger conventional acoustic transducers, since the actuation force needed is beyond the reasonable capability of electromagnets with excessive power consumption as a result. 
     There are transduction principles that can generate the necessary forces on the micro scale. The problem is that the force must be generated over a relatively large physical travel of the actuator. This generally disqualifies all piezoelectric actuators, since such devices can generate large strains and forces, but with very limited travel. A more promising actuator technology is based on electrostatic attraction forces that are caused by opposing electrical charges built up on conductive surfaces. Since the electrostatic force is inversely proportional to the square of the distance between the conductors, potentially very large forces can be generated if the conductors are in close proximity. In particular, if an actuator is used in which the conductors come into physical contact, only being separated by a solid insulator, the electrostatic force can be increased substantially if the solid insulator has a high relative permittivity and is very thin. An electrostatic transducer based on an electrostatic actuator principle has been disclosed in U.S. Pat. No. 6,552,469 and is shown in cross-section in  FIG. 1 . This prior art structure involves a micro fabricated cantilever actuator, which is attached to an external membrane with a support brace. The fabrication of such a support brace and membrane would be cumbersome in high-volume manufacturing, and it would be desirable to integrate all structural components to realize a smaller structure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to realize an acoustic transducer structure with an integrated electrostatic actuator. 
     It is a further object of this invention to realize such an electrostatic actuator with as few structural materials as possible to minimize the cost of fabrication. 
     It is a further object of this invention to realize such an electrostatic actuator that can operate at bias voltages below 10V for easy integration in low voltage portable systems. 
     It is a further object of this invention to realize all necessary components of said acoustic transducer structure in a monolithic structure. 
     It is yet a further object of this invention to realize such an acoustic transducer structure in which the electrostatic actuator is fabricated as an integral part of, and is permanently attached to, the diaphragm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a prior art electrostatic acoustic transducer. 
         FIG. 2  is a cross-sectional view of an electrostatic acoustic transducer according to the present invention. 
         FIG. 3  is a three dimensional cut-away view of an electrostatic transducer according to the present invention. 
         FIG. 4  is a cross-sectional view of an electrostatic acoustic transducer according to the present invention in which an initial electrical potential is applied between the counter electrode and the cantilevers causing the tip of the cantilevers to deflect towards the counter electrode. 
         FIG. 5  is a cross-sectional view of an electrostatic acoustic transducer according to the present invention in which an electric potential is applied between the counter electrode and the cantilevers causing the cantilevers to collapse onto the counter electrode, and the diaphragm to deflect towards the counter electrode. 
         FIG. 6  is a graph depicting the relationship between diaphragm center deflection, defined in  FIG. 5 , and applied electric potential for an example electrostatic acoustic transducer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention results from the realization that an electrostatic actuator can be integrated with a sound generating diaphragm in single a micro fabrication process by forming the necessary movable cantilever, or cantilevers, directly on the diaphragm. 
     A preferred embodiment of an acoustic transducer  100  according to the present invention is shown in cross-section in  FIG. 2  and in three dimensional cut-away in  FIG. 3 . In this embodiment, one, or more, cantilevers  102  are formed on the sound generating diaphragm  101 , on base substrate  103 . Preferred materials for sound generating diaphragm  101  include silicon, polycrystalline silicon, silicon dioxide, silicon nitride, and polymer. A preferred material for base substrate  103  is silicon. Cantilevers  102  are electrically conductive, and may be constructed from a single electrically conductive material, or multiple layers of dielectric and electrically conductive materials, such that the surface of the cantilevers facing cap substrate  105  is electrically conductive. The cantilevers are attached in the center of the diaphragm, the diaphragm being attached along, or at, the perimeter to the base substrate. A small initial air gap  104  is formed by micro fabrication between the cantilevers and diaphragm by a sacrificial layer method. A second cap substrate  105 , in which a cavity  106  has been formed, is attached to the base substrate. Preferred methods for the formation of cavity  106  in cap substrate  105  include etching of cap substrate  105  and flow forming by compression stamping of cap substrate  105 . The preferred embodiment comprises an insulating cap substrate on which an electrically conductive counter electrode is formed. In a second preferred embodiment the cap substrate is electrically conducting or semi-conducting and therefore directly forms a counter electrode to cantilevers  102 . Preferred conductive or semi-conductive materials for cap substrate  105  are silicon, nickel, aluminum, stainless steel, or titanium. The cap substrate is coated with electrical insulator  107 , which prevents electrical short circuit during operation of the device. In a second preferred embodiment, electrical insulator  107  is formed on cantilevers  102 . Preferred materials for electrical insulator  107  are silicon dioxide, silicon nitride, or a polymer. A number of openings  108  are formed in cap substrate  105  to allow air to flow to and from the cavity  106 . Preferred methods for the formation of openings  108  in cap substrate  105  include etching or stamp cutting of cap substrate  105 . Preferred methods for the attachment of cap substrate  105  to base substrate  103  include anodic bonding, adhesive bonding, direct bonding, thermo-compression bonding, eutectic bonding, thermo-sonic bonding, microwave bonding, or solder bonding. 
     In  FIG. 4 , the initial operation of the acoustic transducer  100  is shown. An initial electrical potential is applied between the cantilevers  102  and the cap substrate  105 . The resulting electrostatic attraction force causes the cantilevers to deflect towards the cap substrate. If the applied electrical potential is large enough, the cantilevers will deflect so far that the tips of the cantilevers will make initial contact with the insulator layer  107  on the cap substrate. Since the electrostatic force is inversely proportional to the conductor separation and proportional to the dielectric constant of the material between the conductors, the cantilevers will quickly collapse on to the cap substrate, as shown in  FIG. 5 , until a balance is reached between the electrostatic attraction forces and the mechanical restoring forces of the cantilevers and the diaphragm. The nature of the force balance can be analyzed by considering the relaxation of the total stored energy in the acoustic transducer from the diaphragm and cantilever restoring forces, and the electrostatic attraction force. The principle of energy relaxation dictates that the equilibrium of a system is a state in which the stored energy is minimized. The energy consideration of the acoustic transducer according to the present invention yields the following relationship: 
     
       
         
           
             
               
                 
                   V 
                   = 
                   
                     
                       
                         
                           k 
                           
                             2 
                             / 
                             3 
                           
                         
                         ⁢ 
                         
                           
                             3 
                             ⁢ 
                             
                               h 
                               i 
                             
                           
                         
                       
                       
                         
                           N 
                           
                             2 
                             / 
                             3 
                           
                         
                         ⁢ 
                         
                           w 
                           c 
                           
                             2 
                             / 
                             3 
                           
                         
                         ⁢ 
                         
                           E 
                           
                             1 
                             / 
                             6 
                           
                         
                         ⁢ 
                         
                           
                             
                               ɛ 
                               r 
                             
                             ⁢ 
                             
                               ɛ 
                               0 
                             
                             ⁢ 
                             
                               h 
                               c 
                             
                           
                         
                       
                     
                     ⁢ 
                     
                       
                         
                           w 
                           d 
                           
                             2 
                             / 
                             3 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               δ 
                               0 
                             
                             - 
                             
                               w 
                               d 
                             
                           
                           ) 
                         
                       
                       
                         1 
                         / 
                         3 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In which, V is the applied electrical potential, k is the stiffness of diaphragm  101  when loaded by a force in the center, h i  is the thickness of insulator layer  107 , N is the number of cantilevers  102 , w c  is the width of cantilevers  102 , E is the combined Young&#39;s modulus of the cantilever materials, h c  is the thickness of cantilevers  102 , ε r  is the relative permittivity of insulator layer  107 , ε 0  is the permittivity of vacuum, w d  is the center deflection of diaphragm  101  per  FIG. 5 , and δ 0  is the depth of cavity  106  per  FIG. 5 . With this equation, it is possible to establish the diaphragm deflection versus applied electrical potential of the acoustic transducer. To illustrate the function of the acoustic transducer, an example device was analyzed with the following parameters: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 k 
                 26.8 N/m 
               
               
                   
                 E c   
                 160 GPa 
               
               
                   
                 N 
                 8 
               
               
                   
                 h c   
                 2 μm 
               
               
                   
                 w c   
                 150 μm 
               
               
                   
                 ε r   
                 8 
               
               
                   
                 l 
                 2 mm 
               
               
                   
                 δ 0   
                 40 μm 
               
               
                   
                   
               
            
           
         
       
     
     These are dimensions and characteristics that are readily implemented using micro fabrication technology. The diaphragm deflection w d  can be calculated from (1) and is shown as function of the applied electrical potential in  FIG. 6 . The diaphragm stiffness factor k selected in this example is consistent with a 1 μm thick silicon nitride diaphragm and a diameter of 6 mm. 
     If an electrical operating potential of 8 V is selected, according to  FIG. 6  the diaphragm will have a static deflection of ˜12.4 μm. If the electrical potential is now varied, the diaphragm deflection will track the curve shown in  FIG. 6 . In order to generate for instance 108 dB SPL sound pressure in a 2 cc closed volume, the average deflection of the example diaphragm must be 3.44 μm. The volumetric deflection factor for the example diaphragm is 0.286. From this it can concluded the center deflection w d  of the diaphragm must be: 
     
       
         
           
             
               
                 
                   
                     w 
                     d 
                   
                   = 
                   
                     
                       
                         3.44 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         μm 
                       
                       0.286 
                     
                     = 
                     
                       12.0 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       μm 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     From  FIG. 6 , it is evident that such a displacement can be generated with ˜2.4 V positive amplitude, or ˜7 V negative amplitude, from the electrical operating potential of 8 V. 
     While a specific embodiment has been illustrated and described, many variations and modifications in structure and materials may be apparent to those skilled in the art. Such variations shall also be claimed assuming they fall within the scope of the present invention.