CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to the following commonly owned co-pending U.S. patent application: 
         [0002]    Provisional Application Ser. No. 61/473,217, “Differential Microphone with Sealed Backside Cavities and Diaphragms Coupled to a Rocking Structure Thereby Providing Resistance to Deflection Under Atmospheric Pressure and Providing a Directional Response to Sound Pressure,” filed Apr. 8, 2011, and claims the benefit of its earlier filing date under 35 U.S.C. §119(e). 
     
    
     GOVERNMENT INTERESTS 
       [0003]    This invention was made with government support under DC009721 awarded by the National Institutes of Health. The U.S. Government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0004]    The present invention relates generally to miniature microphones, and more particularly to a micromachined differential microphone with sealed backside cavities where the diaphragms are coupled to a rocking structure thereby providing resistance to deflection under external atmospheric pressure and providing a directional response to small dynamic sound pressure. 
       BACKGROUND 
       [0005]    Miniature microphones, which may be used in various applications (e.g., cellular phones, laptop computers, portable consumer electronics, hearing aids), typically include a membrane and a rigid back electrode in close proximity to form a capacitor with a gap. Incoming sound induces vibrations in the compliant membrane and these vibrations change the capacitance of the structure which can be sensed with electronics. Typically, the structure of the microphone contains a large backside cavity and a small pressure release hole. The pressure release hole allows the large atmospheric pressure to reach the backside of the membrane. While the membrane compliance is designed to resolve dynamic pressure vibrations with magnitudes of 1 μPa to 1 Pa, atmospheric pressure is approximately 100 kPa (about a factor of 10 5  times larger). Without a pressure release, it is challenging to design compliant membranes that do not collapse under atmospheric pressure. 
         [0006]    Recently, microelectromechanical systems (MEMS) processing has been utilized to fabricate miniature microphones. However, most miniature microphones using MEMS processing use a deep reactive ion etch step through the entire silicon substrate, thereby preventing CMOS compatibility. If, however, miniature microphones could use MEMS processing without the use of the deep reactive ion etch step, then miniature microphones could be manufactured with CMOS compatible processes which have a significant cost advantage over other processes. 
         [0007]    Furthermore, there is a desire to create a vacuum sealed microphone. By removing air from the gap, a microphone with much lower self-noise (which results in higher fidelity) can be fabricated with a potentially better frequency response. However, a very stiff diaphragm would be required to prevent the structure from collapsing under external atmospheric pressure, and such a structure would have poor sensitivity to small sound pressure due to its stiffness. Such structures have been manufactured using MEMS processing to realize ultrasound sensors but not functional microphones. 
       BRIEF SUMMARY 
       [0008]    In one embodiment of the present invention, a microphone comprises a first and a second diaphragm, where the first and second diaphragms form a top layer of a first and a second backside sealed cavity. The microphone further comprises a rocking structure coupled to the first and second diaphragms, where the rocking structure rotates on a pivot and where the rocking structure is placed external to the first and second backside sealed cavities. 
         [0009]    In another embodiment of the present invention, a microphone comprises a diaphragm, where the diaphragm forms a top layer of a backside sealed cavity. The microphone further comprises a rocking structure coupled to the diaphragm, where the rocking structure rotates on a pivot and where the rocking structure is placed internal in the backside sealed cavity. 
         [0010]    In another embodiment of the present invention, a method for fabricating a microphone comprises depositing and patterning a first structural layer to form a first and a second electrode on a substrate. The method further comprises depositing and patterning a first sacrificial layer onto the patterned first structural layer. Additionally, the method comprises performing a dimpled cut in the first sacrificial layer used to create a pivot, where the dimpled cut etches the first sacrificial layer in a manner that leaves a portion of the first sacrificial layer on the substrate. The method further comprises depositing and patterning a second structural layer on the patterned first sacrificial layer to form a first and a second diaphragm, the pivot and a bottom layer of a rocking structure. In addition, the method comprises depositing and patterning additional structural layers to form other layers of the rocking structure. 
         [0011]    In another embodiment of the present invention, a method for fabricating a microphone comprises depositing and patterning a first structural layer to form a first and a second electrode on a substrate and a bottom layer of post structures. The method further comprises depositing and patterning a first sacrificial layer onto the patterned first structural layer. Additionally, the method comprises performing a dimpled cut in the first sacrificial layer used to create a pivot, where the dimpled cut etches the first sacrificial layer in a manner that leaves a portion of the first sacrificial layer on the substrate. Furthermore, the method comprises depositing and patterning a second structural layer on the patterned first sacrificial layer to form the pivot and a bottom layer of a rocking structure. In addition, the method comprises depositing and patterning additional structural layers to form other layers of the rocking structure. 
         [0012]    The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of the present invention that follows may be better understood. Additional features and advantages of the present invention will be described hereinafter which may form the subject of the claims of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
           [0014]      FIG. 1  illustrates a directional microphone configured in accordance with an embodiment of the present invention; 
           [0015]      FIG. 2  illustrates an embodiment of the present invention of a top view of the rocking structure of the directional microphone of  FIG. 1 ; 
           [0016]      FIG. 3  illustrates an alternative embodiment of the present invention of a directional microphone; 
           [0017]      FIGS. 4A-4B  are a flowchart of a method for fabricating the directional microphone of  FIG. 1  in accordance with an embodiment of the present invention; 
           [0018]      FIGS. 5A-5J  depict cross-sectional views of the directional microphone of  FIG. 1  during the fabrication steps described in  FIGS. 4A-4B  in accordance with an embodiment of the present invention; 
           [0019]      FIGS. 6A-6B  are a flowchart of a method for fabricating the directional microphone of  FIG. 3  in accordance with an embodiment of the present invention; 
           [0020]      FIGS. 7A-7J  depict cross-sectional views of the directional microphone of  FIG. 3  during the fabrication steps described in  FIGS. 6A-6B  in accordance with an embodiment of the present invention; 
           [0021]      FIG. 8  illustrates post arranged in a circular manner to support a diaphragm region of the directional microphone of  FIG. 3  in accordance with an embodiment of the present invention; and 
           [0022]      FIGS. 9A-9B  illustrate the process of vacuum sealing the microphone of  FIG. 3  in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without such specific details. 
         [0024]    As stated in the Background section, recently, microelectromechanical systems (MEMS) processing has been used to fabricate miniature microphones. However, most miniature microphones using MEMS processing use the deep reactive ion through-wafer etch step thereby preventing CMOS compatibility. If, however, miniature microphones could use MEMS processing without the use of the through-wafer deep reactive ion etch step, then miniature microphones could be manufactured with CMOS compatible processes which have a significant cost advantage over other processes. Furthermore, there is a desire to create a vacuum sealed microphone. By removing air from the gap, a microphone with much lower self-noise (which results in higher fidelity) can be fabricated with a potentially better frequency response. However, a very stiff diaphragm would be required to prevent the structure from collapsing under external atmospheric pressure, and such a structure would have poor sensitivity to small sound pressure due to its stiffness. 
         [0025]    The principles of the present invention provide embodiments of a differential microphone (also referred to as a pressure gradient microphone) with sealed backside cavities that can be made with MEMS surface micromachining processes without the use of a through-wafer deep reactive ion etch as discussed further below in connection with  FIGS. 1-3 ,  4 A- 4 B,  5 A- 5 J,  6 A- 6 B,  7 A- 7 J,  8  and  9 A- 9 B.  FIG. 1  illustrates one embodiment of a directional microphone with two sealed backside cavities where the motion of two vacuum sealed diaphragms are coupled to an external freely rotating rocking structure.  FIG. 2  illustrates a top view of the rocking structure of the directional microphone of  FIG. 1 .  FIG. 3  illustrates an alternative embodiment of a directional microphone where the rocking structure is contained within a single large vacuum sealed cavity and coupled to two compliant diaphragms along the top of the cavity.  FIGS. 4A-4B  are a flowchart of a method for fabricating the directional microphone of  FIG. 1 .  FIGS. 5A-5J  depict cross-sectional views of the directional microphone of  FIG. 1  during the fabrication steps described in  FIGS. 4A-4B .  FIGS. 6A-6B  are a flowchart of a method for fabricating the directional microphone of  FIG. 3 .  FIGS. 7A-7J  depict cross-sectional views of the directional microphone of  FIG. 3  during the fabrication steps described in  FIGS. 6A-6B .  FIG. 8  illustrates posts arranged in a circular manner to support a diaphragm region of the directional microphone of  FIG. 3 ; and  FIGS. 9A-9B  illustrate the process of vacuum sealing the microphone of  FIG. 3 . 
         [0026]    Referring now to the Figures in detail,  FIG. 1  illustrates an embodiment of the present invention of a directional microphone  100  with sealed backside cavities  101 A,  101 B, each containing an electrode  102 A,  102 B, respectively. Backside cavities  101 A- 101 B may collectively or individually be referred to as backside cavities  101  or backside cavity  101 , respectively. Electrodes  102 A- 102 B may collectively or individually be referred to as electrodes  102  or electrode  102 , respectively. In one embodiment, a diaphragm  103 A,  103 B forms a portion of the topside of backside cavities  101 A,  101 B, respectively. Diaphragms  103 A,  103 B may collectively or individually be referred to as diaphragms  103  or diaphragm  103 , respectively. 
         [0027]    Microphone  100  may further include a rocking structure or beam  104  coupled to diaphragms  103 . Rocking structure  104  is configured to “rock” or rotate on a pivot  105  as discussed further below. The structure of microphone  100  may reside on a substrate  106 . 
         [0028]    In one embodiment, backside cavities  101  are sealed with any gas, including air, and can be sealed under any pressure. In one embodiment, backside cavities  101  are sealed under vacuum so that no gas occupies the cavity. 
         [0029]    In one embodiment, a plurality of capacitors are formed between diaphragms  103  and electrodes  102 . In one embodiment, a portion of the capacitors are used for sensing and a portion of the capacitors are used for electrostatic actuation. 
         [0030]    In one embodiment, rocking structure  104  provides resistance to deflection under external atmospheric pressure and will provide a directional response to small dynamic sound pressure as discussed below. When sound waves, which are small air pressure oscillations, arrive at microphone  100  in the y direction, as labeled in  FIG. 1 , the pressure oscillations impinge on both the right and left diaphragms  103  at the same time. Force is balanced on both sides of rocking structure  104  and there is no induced rocking motion. However, when sound waves arrive from the x direction, as labeled in  FIG. 1 , a pressure imbalance exists between the left and right diaphragms  103  due to the finite time it takes for sound to travel across microphone  100 . This pressure imbalance applies a net moment to rocking structure  104  which in turn forces rocking structure  104  and diaphragms  103  into motion. As a result, rocking structure  104  has an inherently directional response to sound. Such a feature is useful as it can enable applications where one can point a microphone in a direction of interest to attain maximum sensitivity while simultaneously filtering out ambient sounds coming from the side that would otherwise affect speech intelligibility and signal-to-noise ratio (SNR). 
         [0031]    Furthermore, diaphragms  103  are capable of resisting collapse under atmospheric pressure owing to the stiffness provided by rocking structure  104 . In one embodiment, rocking structure  104  can be made completely insensitive to sound by including perforations into the structure of rocking structure  104  as shown in  FIG. 2 . Said perforations also aid in reducing damping of the structure as described below. 
         [0032]      FIG. 2  illustrates the top view  200  of rocking structure  104  of directional microphone  100  in accordance with an embodiment of the present invention. Referring to  FIG. 2 , in conjunction with  FIG. 1 , since rocking structure  104  of directional microphone  100  is external to backside cavities  101 , a small amount of air damping may occur underneath rocking structure  104 . As a result, rocking structure  104  may include perforations  201  as shown in the top view  200  of rocking structure  104 . As a result, air underneath rocking structure  104  can be displaced through these perforations  201  as rocking structure  104  rotates. 
         [0033]    Additionally, rocking structure  104  may include a design that is triangular in shape, as shown in top view  200  of rocking structure  104 , where rocking structure  104  is wider along pivot  105  and narrower along its edges in order to minimize the moment of inertia about its rotating axis. 
         [0034]    Returning to  FIG. 1 , in one embodiment, microphone  100  may be designed to provide additional resistance to deflection under external atmospheric pressure by placing an electrostatic charge of one type (e.g., positive charge) on diaphragms  103  and placing an electrostatic charge of the same type on electrodes  102  thereby forming an electrostatic repulsion between diaphragms  103  and electrodes  102 . 
         [0035]    In another embodiment, microphone  100  may be designed to provide additional resistance to deflection under external atmospheric pressure by having diaphragms  103  be made out of a magnetic material (e.g., iron, nickel) which are then magnetized thereby generating a magnetic field. When current is run through diaphragms  103 , the magnetic field exerts an additional upward force on diaphragm  103  to assist in preventing collapse under atmospheric pressure. 
         [0036]    As discussed above, when sound waves arrive from the x direction, as labeled in  FIG. 1 , a pressure imbalance exists between the left and right diaphragms  103 A and  103 B due to the finite time it takes for sound to travel across microphone  100 . This pressure imbalance applies a net moment to rocking structure  104  which in turn forces rocking structure  104  and diaphragms  103  into motion. The motion of rocking structure  104  and/or diaphragms  103  can be sensed using any number of transduction principles common to MEMS and acoustic sensors, such as piezoelectric, optical, piezoresistive and capacitive. For example, diaphragms  103  may be made electrically conductive so that parallel plate capacitors are formed by the diaphragms  103  and electrodes  102 . 
         [0037]    An alternative directional microphone where the rocking structure is sealed along with the electrodes in a backside cavity is discussed below in connection with  FIG. 3 . 
         [0038]      FIG. 3  illustrates an alternative embodiment of the present invention of a directional microphone  300  with a sealed backside cavity  301  containing electrodes  302 A,  302 B and a rocking structure  303  configured to “rock” or rotate on a pivot  304 . Electrodes  302 A- 302 B may collectively or individually be referred to as electrodes  302  or electrode  302 , respectively. Rocking structure  303  is connected to diaphragms  305 A,  305 B as shown in  FIG. 3 . Diaphragms  305 A,  305 B may collectively or individually be referred to as diaphragms  305  or diaphragm  305 , respectively. The structure of microphone  300  may reside on a substrate  306 . 
         [0039]    In one embodiment, backside cavity  301  is sealed with any gas, including air, and can be sealed under any pressure. In one embodiment, backside cavity  301  is sealed under vacuum so that no gas occupies the cavity. 
         [0040]    In one embodiment, a plurality of capacitors are formed between rocking structure  303  and electrodes  302 . In one embodiment, a portion of the capacitors are used for sensing and a portion of the capacitors are used for electrostatic actuation. 
         [0041]    As with the case of directional microphone  100  of  FIG. 1 , rocking structure  303  of directional microphone  300  provides resistance to deflection under external atmospheric pressure and will provide a directional response to small dynamic sound pressure as discussed below. When sound waves arrive at microphone  300  in the y direction, as labeled in  FIG. 3 , the pressure oscillations impinge on both the right and left diaphragms  305  at the same time. Force is balanced on both sides of rocking structure  303  and there is no induced rocking motion. However, when sound waves arrive from the x direction, as labeled in  FIG. 3 , a pressure imbalance exists. This pressure imbalance applied to diaphragms  305  in turn applies a net moment to rocking structure  303  which in turn forces rocking structure  303  and diaphragms  305  into motion. As a result, rocking structure  303  and diaphragms  305  have an inherently directional response to sound. 
         [0042]    As with the case with microphone  100 , rocking structure  303  can be designed very stiff to resist deflection under atmospheric pressure acting on each diaphragm  305 . Atmospheric pressure is omnidirectional and therefore the atmospheric pressure is balanced on both diaphragms  305 . 
         [0043]    Furthermore, by placing rocking structure  303  inside a cavity  301 , which may be vacuum sealed, the effects of air damping on the motion of rocking structure  303  are eliminated. 
         [0044]    In addition, in one embodiment, rocking structure  303  may include a design that is triangular in shape that is similar to the shape shown in the top view  200  of rocking structure  104  ( FIG. 2 ) in order to minimize the moment of inertia about its rotating axis. Perforations  201  may also be advantageous to further reduce moment of inertia. 
         [0045]    Returning to  FIG. 3 , the operation of microphone  300  is similar in operation to microphone  100  ( FIG. 1 ). For example, the motion of rocking structure  303  and/or diaphragms  305  can be sensed using any number of transduction principles common to MEMS and acoustic sensors, such as piezoelectric, optical, piezoresistive and capacitive. For example, the motion of rocking structure  303  can change a capacitance which can be sensed with electronics. For instance, rocking structure  303  may be made electrically conductive so that parallel plate capacitors are formed by the rocking structure  303  and electrodes  302 . 
         [0046]    As discussed above, microphones  100  and  300  include two diaphragms  103 ,  305  with sealed backside cavities  101 ,  301 . In each microphone  100 ,  300 , diaphragms  103 ,  305  are coupled to a rocking structure  104 ,  303  which will provide resistance to deflection under external atmospheric pressure and will provide a directional response to small dynamic sound pressure. Additionally, each microphone  100 ,  300  can be manufactured using MEMS surface-micromachining processes without the use of the through-wafer deep reactive ion etch to create a backside cavity. In one embodiment, microphones  100 ,  300  can be fabricated using a standard process with alternating sacrificial oxide and polysilicon layers, such as Sandia&#39;s SUMMiT™ V 5-level surface micromachining processes or MEMSCAP&#39;s poly-MUMPs process, as discussed below in connection with  FIGS. 4A-4B ,  5 A- 5 J,  6   a - 6 B,  7 A- 7 J,  8  and  9 A- 9 B. While the following discusses using such a sequence to fabricate microphones  100 ,  300 , the principles of the present invention are not limited to such processes but can include other processes to fabricate microphones  100 ,  300 . Furthermore, the principles of the present invention are not limited to enacting all the steps of a 5-level surface micromachining process. For example, some of the steps described below may be combined or eliminated, such as by depositing a fewer number of polysilicon and/or sacrificial oxide layers to fabricate the microphones  100 ,  300 . 
         [0047]    Referring to  FIGS. 4A-4B ,  FIGS. 4A-4B  are a flowchart of a method  400  for fabricating directional microphone  100  of  FIG. 1 .  FIGS. 4A-4B  will be discussed in conjunction with  FIGS. 5A-5J , which depict cross-sectional views of microphone  100  during the fabrication steps described in  FIGS. 4A-4B  in accordance with an embodiment of the present invention. 
         [0048]    Referring to  FIG. 4A , in conjunction with FIGS.  1  and  5 A- 5 J, in step  401 , a first layer of polysilicon is deposited on substrate  106 . Substrate  106  may be a blank silicon wafer or may be a silicon wafer with electrically insulating layers across its surface, such as silicon dioxide or silicon nitride. In one embodiment, a thickness of approximately 0.3 μm of polysilicon is deposited in step  401 . In step  402 , the first layer of polysilicon is patterned to form electrodes  102 A,  102 B as illustrated in  FIG. 5A . 
         [0049]    In step  403 , a first sacrificial oxide layer is deposited onto the patterned first layer of polysilicon (structure of  FIG. 5A ). In one embodiment, a thickness of approximately 2 μm of sacrificial oxide is deposited in step  403 . In step  404 , the first sacrificial oxide layer  501  is patterned to define the height of the sealed cavities  101  as illustrated in  FIG. 5B . In one embodiment, the patterning of first sacrificial oxide layer  501  may include making a dimpled cut  511  to create pivot  105  as shown in  FIG. 5B . A “dimpled cut”  511 , as used herein, refers to etching the sacrificial oxide layer  501  so that a portion of sacrificial oxide layer  501  remains above substrate  106 . 
         [0050]    In step  405 , a second layer of polysilicon is deposited onto the structure of  FIG. 5B . In one embodiment, a thickness of approximately 1 μm of polysilicon is deposited in step  405 . In step  406 , the second layer of polysilicon is patterned to form diaphragms  103 , pivot  105  and a portion  502  of the lower section of rocking structure  104  as illustrated in  FIG. 5C . 
         [0051]    In step  407 , a second sacrificial oxide layer is deposited onto the structure of  FIG. 5C . In one embodiment, a thickness of approximately 0.3 μm of sacrificial oxide is deposited in step  407 . In step  408 , the second sacrificial oxide layer  503  is patterned as illustrated in  FIG. 5D . 
         [0052]    In step  409 , a third layer of polysilicon is deposited onto the structure of  FIG. 5D . In one embodiment, a thickness of approximately 1.5 μm of polysilicon is deposited in step  409 . In step  410 , the third layer of polysilicon is patterned to form a portion  504  of the lower section of rocking structure  104  as well as posts  505 A,  505 B on top of diaphragms  103 A,  103 B, respectively, as illustrated in  FIG. 5E . Posts  505 A,  505 B are used to connect the diaphragms  103 A,  103 B to rocking structure  104  as shown further below. 
         [0053]    Referring to  FIG. 4B , in conjunction with FIGS.  1  and  5 A- 5 J, in step  411 , a third sacrificial oxide layer is deposited onto the structure of  FIG. 5E . In one embodiment, a thickness of approximately 2 μm of sacrificial oxide is deposited in step  411 . In step  412 , the third sacrificial oxide layer  506  is patterned as illustrated in  FIG. 5F . 
         [0054]    In step  413 , a fourth layer of polysilicon is deposited onto the structure of  FIG. 5F . In one embodiment, a thickness of approximately 2.25 μm of polysilicon is deposited in step  413 . In step  414 , the fourth layer of polysilicon is patterned to form a portion  507  of the upper section of rocking structure  104  as well as form touchdowns  508 A,  508 B to diaphragms  103 A,  103 B as illustrated in  FIG. 5G . 
         [0055]    In step  415 , a fourth sacrificial oxide layer is deposited onto the structure of  FIG. 5G . In one embodiment, a thickness of approximately 2.0 μm of sacrificial oxide is deposited in step  415 . In step  416 , the fourth sacrificial oxide layer  509  is patterned as illustrated in  FIG. 5H . 
         [0056]    In step  417 , a fifth layer of polysilicon is deposited onto the structure of  FIG. 5H . In one embodiment, a thickness of approximately 2.25 μm of polysilicon is deposited in step  417 . In step  418 , the fourth layer of polysilicon is patterned to increase the thickness  510  of the upper section of rocking structure  104  as illustrated in  FIG. 5I . 
         [0057]    In step  419 , a release etch is performed to remove the sacrificial oxide as illustrated in  FIG. 5J . In step  420 , cavities  101  may be vacuum sealed via deposition of a thin material layer (e.g., a metal) which fills small etch holes in diaphragms  103 . This sealing step will be described in greater detail below. Pivot  105  will touch down when microphone  100  deflects under atmospheric pressure when sealed. 
         [0058]    In some implementations, method  400  may include other and/or additional steps that, for clarity, are not depicted. Further, in some implementations, method  400  may be executed in a different order presented and that the order presented in the discussion of  FIGS. 4A-4B  is illustrative. Additionally, in some implementations, certain steps in method  400  may be executed in a substantially simultaneous manner or may be omitted. 
         [0059]    An embodiment of a method for fabricating directional microphone  300  of  FIG. 3  will now be discussed below in connection with  FIGS. 6A-6B ,  7 A- 7 J,  8  and  9 A- 9 B. 
         [0060]      FIGS. 6A-6B  are a flowchart of a method  600  for fabricating directional microphone  300  of  FIG. 3 .  FIGS. 6A-6B  will be discussed in conjunction with  FIGS. 7A-7J , which depict cross-sectional views of microphone  300  during the fabrication steps described in  FIGS. 6A-6B  in accordance with an embodiment of the present invention. 
         [0061]    Referring to  FIG. 6A , in conjunction with FIGS.  3  and  7 A- 7 J, in step  601 , a first layer of polysilicon is deposited on substrate  306 . In one embodiment, a thickness of approximately 0.3 μm of polysilicon is deposited in step  601 . In step  602 , the first layer of polysilicon is patterned to form electrodes  302 A,  302 B and the bottom layer  701 A,  701 B of the post structures as illustrated in  FIG. 7A . 
         [0062]    In step  603 , a first sacrificial oxide layer is deposited onto the patterned first layer of polysilicon (structure of  FIG. 7A ). In one embodiment, a thickness of approximately 2 μm of sacrificial oxide is deposited in step  603 . In step  604 , the first sacrificial oxide layer  702  is patterned as illustrated in  FIG. 7B . In one embodiment, the patterning of first sacrificial oxide layer  702  may include making a dimpled cut  703  to create pivot  304  as shown in  FIG. 7B . A “dimpled cut”  703 , as used herein, refers to etching the sacrificial oxide layer  702  so that a portion of sacrificial oxide layer  702  remains above substrate  306 . 
         [0063]    In step  605 , a second layer of polysilicon is deposited onto the structure of  FIG. 7B . In one embodiment, a thickness of approximately 1 μm of polysilicon is deposited in step  605 . In step  606 , the second layer of polysilicon is patterned to form pivot  304 , a portion  704  of the lower section of rocking structure  303  as well as to add thickness  705 A,  705 B to the post structures as illustrated in  FIG. 7C . 
         [0064]    In step  607 , a second sacrificial oxide layer is deposited onto the structure of  FIG. 7C . In one embodiment, a thickness of approximately 0.3 μm of sacrificial oxide is deposited in step  607 . In step  608 , the second sacrificial oxide layer  706  is patterned as illustrated in  FIG. 7D . 
         [0065]    In step  609 , a third layer of polysilicon is deposited onto the structure of  FIG. 7D . In one embodiment, a thickness of approximately 1.5 μm of polysilicon is deposited in step  609 . In step  610 , the third layer of polysilicon is patterned to form a portion  707  of the lower section of rocking structure  303  as well as to add thickness  708 A,  708 B to the post structures as illustrated in  FIG. 7E . 
         [0066]    Referring to  FIG. 6B , in conjunction with FIGS.  3  and  7 A- 7 J, in step  611 , a third sacrificial oxide layer is deposited onto the structure of  FIG. 7E . In one embodiment, a thickness of approximately 2 μm of sacrificial oxide is deposited in step  611 . In step  612 , the third sacrificial oxide layer  709  is patterned to form posts  710 A,  710 B on post structures  701 ,  705 ,  708  as illustrated in  FIG. 7F . 
         [0067]    In step  613 , a fourth layer of polysilicon is deposited onto the structure of  FIG. 7F . In one embodiment, a thickness of approximately 2.25 μm of polysilicon is deposited in step  613 . In step  614 , the fourth layer of polysilicon is patterned to form a portion  711  of the upper section of rocking structure  303  as well as to add thickness  712 A,  712 B to the post structures as illustrated in  FIG. 7G . 
         [0068]    In step  615 , a fourth sacrificial oxide layer is deposited onto the structure of  FIG. 7G . In one embodiment, a thickness of approximately 2.0 μm of sacrificial oxide is deposited in step  615 . In step  616 , the fourth sacrificial oxide layer  713  is patterned to form a portion  714  of the upper section of rocking structure  303  as well as form to posts  715 A,  715 B on post structures  701 ,  705 ,  708 ,  710 ,  712  as illustrated in  FIG. 7H . 
         [0069]    In step  617 , a fifth layer of polysilicon is deposited onto the structure of  FIG. 7H . In one embodiment, a thickness of approximately 2.25 μm of polysilicon is deposited in step  617 . In step  618 , the fifth layer of polysilicon  716  is patterned to increase the thickness of the upper section of rocking structure  303  and the post structures as illustrated in  FIG. 7I . 
         [0070]    In step  619 , a release etch is performed to remove the sacrificial oxide as illustrated in  FIG. 7J . In step  620 , cavity  301  is vacuum sealed as discussed further below. 
         [0071]    In some implementations, method  600  may include other and/or additional steps that, for clarity, are not depicted. Further, in some implementations, method  600  may be executed in a different order presented and that the order presented in the discussion of  FIGS. 6A-6B  is illustrative. Additionally, in some implementations, certain steps in method  600  may be executed in a substantially simultaneous manner or may be omitted. 
         [0072]    An additional view of the top surface of microphone  300  is provided below in connection with  FIG. 8 .  FIG. 8  illustrates a portion of the top surface of microphone  300  that includes a circular pattern of posts that extends from the top surface of microphone  300  to the substrate in accordance with an embodiment of the present invention. 
         [0073]    Referring to  FIG. 8 , in conjunction with  FIGS. 3 ,  6 A- 6 B and  7 A- 7 J, a diaphragm region  801  is formed by the free membrane between various posts  802 A-D arranged in a circular or any other manner. Posts  802 A-D may collectively or individually be referred to as posts  802  or post  802 , respectively. While  FIG. 8  illustrates four posts  802  arranged in a circular manner, any number of posts  802  may be arranged in a circular manner. In one embodiment, posts  802  extend from the top surface of microphone  300  to the substrate level  306 . Additionally, a post  803  connects the center of diaphragm  305  to rocking structure  303 . Furthermore, a rigid side-wall  804  may surround microphone  300  as illustrated in  FIG. 8 . 
         [0074]      FIGS. 9A and 9B  illustrate the process of vacuum sealing microphone  300  in accordance with an embodiment of the present invention. Referring to  FIG. 9A , in conjunction with  FIGS. 3 ,  6 A- 6 B and  7 A- 7 J, an etch release hole  901  exists at a portion of the top surface layer  716  of microphone  300 . Etch release hole  901  is used so that the sacrificial oxide can be removed in step  619 . A portion of an underlying polysilicon layer (e.g., polysilicon layer  711 ) is structured as a lip  902  that is used to collect a sealant (e.g., a metal applied during a sputtering or evaporation process step) when it is applied to the top surface layer  716  of microphone  300 , thereby forming a sealing layer  903  as illustrated in  FIG. 9B . This same technique is applicable to microphone  100  ( FIG. 1 ), in which case the etch hole should reside on diaphragm  103  ( FIG. 1 ). 
         [0075]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
A vacuum sealed directional microphone and methods for fabricating said vacuum sealed directional microphone. A vacuum sealed directional microphone includes a rocking structure coupled to two vacuum sealed diaphragms which are responsible for collecting incoming sound and deforming under sound pressure. The rocking structure's resistance to bending aids in reducing the deflection of each diaphragm under large atmospheric pressure. Furthermore, the rocking structure exhibits little resistance about its pivot thereby enabling it to freely rotate in response to small pressure gradients characteristic of sound. The backside cavities of such a device can be fabricated without the use of the deep reactive ion etch step thereby allowing such a microphone to be fabricated with a CMOS compatible process.