Abstract:
A capacitive transducer includes a substrate having a first surface and a second surface. The first surface of the substrate defines a first plane. The substrate has a cavity with an interior peripheral edge. The cavity extends between the first surface and the second surface. A body is provided that has an exterior peripheral edge. The body is parallel to the first plane and at least partially blocking the cavity. The body is connected to the substrate by resilient hinges such that, upon the application of a force, the body moves perpendicular to the first plane. A first set of comb fingers is mounted to the substrate. The first set of comb fingers is connected to a first electrical connection. A second set of comb fingers is mounted to the body and extends past the exterior peripheral edge of the body. The second set of comb fingers is connected to a second electrical connection that is isolated from the first connection. The first set of comb fingers and the second set of comb finger are interdigitated such that as the body moves, the first set of comb fingers and the second set of comb finger maintain a relative spacing. The first set of comb fingers and the second set of comb fingers define a capacitance. The capacitance is related to the relative position of the first set of comb drive fingers and the second set of comb drive fingers.

Description:
[0001]    This application claims priority from U.S. Provisional Application Ser. No. 60/861,108 filed Nov. 28, 2006. 
     
    
     FIELD 
       [0002]    The present patent document relates to a monolithic capacitive transducer, such as a miniature capacitive transducer. 
       BACKGROUND 
       [0003]    Miniature micromachined microphones have gained great popularity in a variety of applications. Because of its sub-mm size, low cost for mass production, lower power consumption, higher sensitivity and reliability, it is widely recognized as the next generation product to replace the conventional electrets condenser microphone (ECM) in applications such as hearing aids, cell phones, PDAs, laptops, MP3, digital cameras etc. Among all the micromachined microphones, the capacitive condenser type of microphone has many advantages over other technical approaches such as piezoelectric or magnetic type micromachined microphone for its smaller size and higher sensitivity etc. 
         [0004]    The micromachined condenser microphone typically consists of an acoustic pressure sensing element, generally a variable capacitor, and a preamplifier IC circuit. One prior art example of a condenser microphones with a parallel plate capacitor is disclosed in U.S. patent publication no. 2006/0093170 (Zhe et al.) entitled “Backplateless silicon microphone”. The prior art suffers from some or all of shortcomings mentioned below due to the structure and sensing motion of the parallel plate variable capacitor. 
         [0005]    First of all, residual film stress on the diaphragm reduces the sensitivity of the microphone. Since the compliant diaphragm is usually made of a thin film of dielectric and electrically conductive materials, it is very difficult to control or reduce its residual stress because the residual stress is present after the film formation. Stress on the diaphragm has a direct impact on the sensitivity of the microphone. Compressive residual stress results in a defective, buckled diaphragm. Tensile stress severely decreases the sensitivity of the microphone, or totally ruptures the diaphragm at the worst cases. 
         [0006]    Secondly, stiction between a flexible diaphragm and a rigid backplate can result in either a faulty device during microfabrication or malfunction during operation. When the gap between the compliant diaphragm and the backplate is on the order of several microns, the diaphragm will adhere to the fixed backplate with a larger probability because the surface to volume ratio increases and surface forces, which are responsible for stiction, are correspondingly higher. Stiction could prevent the successful releasing of the suspended compliant diaphragm during the wet process of the sacrificial layer etching, leading to permanent adhesion to the fixed backplate. During the operation, if the microphone is exposed to a humid environment, water vapor can condense and form a water film on the diaphragm and backplate surfaces. When the gap between the two surfaces decreases during operation and the water film of one surface touches the counter surface, the two surfaces will stick together. 
         [0007]    Thirdly, “squeeze film” air damping affects the high frequency response, and contributes noise to the microphone output by generating pressure fluctuations in the microphone structure. For the sub-mm-sized capacitive condenser microphone, the air gap must be scaled down to several microns to keep the capacitance value in a range which can drive the input of the buffer amplifier effectively. However, as the air gap is reduced, the “squeeze film” damping effects due to the viscous flow of air trapped between the diaphragm and backplate increases rapidly. “Squeeze film” air damping can also impact the sensitivity of the microphone. 
         [0008]    Fourthly, the “pull-in” effect of the diaphragm reduces the DC bias voltage, which therefore lowers the sensitivity of the microphone. A higher DC bias voltage between diaphragm and backplate yields higher sensitivity. A higher DC bias voltage will create a larger attractive electrostatic force between the diaphragm and backplate. However, in some prior art examples, the gap between the diaphragm and backplate is reduced to several microns, and the mechanical compliance of the diaphragm is kept fairly low in order to have some deflection under certain sound pressure level. Larger attractive electrostatic force can overcome the mechanical restoring force of the diaphragm, and can pull the compliant diaphragm over the small gap to touch the backplate. This phenomenon is called the “pull-in” effect. 
         [0009]    Fifthly, a sub-mm-sized diaphragm that is fully constrained by the surrounding frame reduces the sensitivity of the microphone. The compliance of the diaphragm tends to decease very rapidly with the decreasing size for a given diaphragm material and thickness. The mechanical compliance/stiffness of the diaphragm for the sound pressure scales as the fourth power of the diaphragm size. 
         [0010]    Sixthly, the small air gap and compliant diaphragm of parallel plate type capacitive condenser microphones can&#39;t provide a large dynamic range as higher sound pressure levels could drive the flexible diaphragm to contact the backplate across the small air gap. 
         [0011]    Seventhly, the parasitic capacitance between the flexible diaphragm and rigid fixed backplate degrades the microphone performance. The capacitance between the diaphragm and backplate has two parts. The first part varies with acoustical signal and is desirable for microphone. The second part is a parasitic capacitance which does not vary with acoustical signal. The parasitic capacitance degrades the performance and should be minimized. However, the parasitic capacitance is related to the construction of the parallel plate type of silicon microphone in the prior arts. 
         [0012]    Last but not least, the parallel plate type capacitive condenser microphone is fairly complicated and costly for manufacturing. So far, the prior art has been unable to provide an economic manufacturing method for the mass production of microphones. Some manufacturing methods of sensing elements disclosed in the prior art are not compatible with standard IC CMOS process, resulting in larger hybrid package and higher manufacturing cost. 
       SUMMARY 
       [0013]    A capacitive transducer includes a substrate having a first surface and a second surface. The first surface of the substrate defines a first plane. The substrate has a cavity with an interior peripheral edge. The cavity extends between the first surface and the second surface. A body is provided that has an exterior peripheral edge. The body is parallel to the first plane and at least partially blocking the cavity. The body is connected to the substrate by resilient hinges such that, upon the application of a force, the body moves perpendicular to the first plane. A first set of comb fingers is mounted to the substrate. The first set of comb fingers is connected to a first electrical connection. A second set of comb fingers is mounted to the body and extends past the exterior peripheral edge of the body. The second set of comb fingers is connected to a second electrical connection that is isolated from the first connection. The first set of comb fingers and the second set of comb finger are interdigitated such that as the body moves, the first set of comb fingers and the second set of comb finger maintain a relative spacing. The first set of comb fingers and the second set of comb fingers define a capacitance. The capacitance is related to the relative position of the first set of comb drive fingers and the second set of comb drive fingers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein: 
           [0015]      FIG. 1  is a cross sectional perspective view of the microphone according to a first embodiment; 
           [0016]      FIG. 2  is a cross sectional view showing the structure of the SOI wafer for the transducer according to a first embodiment; 
           [0017]      FIG. 3  is a cross sectional view of the SOI wafer after deposition of layer of oxide on its top and bottom sides according to a first embodiment; 
           [0018]      FIG. 4  is a cross sectional view of the SOI wafer after anisotropic silicon etching of backside cavity and oxide etching from its backside; 
           [0019]      FIG. 5  is a perspective view of the SOI wafer after patterning and etching the oxide on its front side according to a first embodiment; 
           [0020]      FIG. 6   a  is a perspective view of the SOI wafer in  FIG. 5  after patterning of a layer of photoresist; 
           [0021]      FIG. 6   b  is an enlarged perspective view of a portion C of the comb fingers and hinge depicted in  FIG. 6   a;    
           [0022]      FIG. 7   a  is a perspective view of the transducer in  FIG. 6  after Reactive Ion Etching (RIE) of a layer of oxide; 
           [0023]      FIG. 7   b  is an enlarged perspective view of a portion D of the comb fingers and hinge depicted in  FIG. 7   a;    
           [0024]      FIG. 8  is a perspective view of the transducer in  FIG. 7  after the first Deep Reactive Ion Etching (DRIE) of silicon; 
           [0025]      FIG. 9   a  is a perspective view of the transducer in  FIG. 8  after removal of photoresist; 
           [0026]      FIG. 9   b  is an enlarged perspective view of a portion E of the comb finger and hinge depicted in  FIG. 9   a;    
           [0027]      FIG. 10   a  is a perspective view of the transducer in  FIG. 9  after the second Deep Reactive Ion Etching (DRIE) of silicon; 
           [0028]      FIG. 10   b  is an enlarged perspective view of a portion F of the comb finger and hinge depicted in  FIG. 1   a;    
           [0029]      FIG. 11  is a perspective view of the transducer in  FIG. 10  after removal of oxide on the front side and partial etching of buried oxide layer to release the diaphragm and movable fingers; 
           [0030]      FIG. 12  is a cross sectional perspective view of the microphone according to a second embodiment; 
           [0031]      FIG. 13  is the top view of the microphone depicted in  FIG. 1  and  FIG. 12 ; 
           [0032]      FIG. 14  is a cross sectional perspective view of the microspeaker with a larger back cavity and taller comb finger according to a third embodiment; 
           [0033]      FIG. 15  is a cross sectional perspective view of the microspeaker with a larger back cavity and taller comb finger according to a fourth embodiment; 
           [0034]      FIG. 16  is a cross sectional perspective view of the microspeaker with a larger back cavity and taller comb finger according to a fifth embodiment; 
           [0035]      FIG. 17  is a cross sectional perspective view of the N type substrate for the transducer according to a sixth embodiment; 
           [0036]      FIG. 18  is a cross sectional perspective view of the N type substrate after P ++  implantation/diffusion, or epitaxial growth of layer a silicon with P doping according to a sixth embodiment; 
           [0037]      FIG. 19  is a cross sectional perspective view of the substrate depicted in  FIG. 18  after anisotropic silicon etching of backside cavity according to a sixth embodiment; 
           [0038]      FIG. 20  is a cross sectional perspective view of the transducer in  FIG. 19  after process of silicon Deep Reactive Ion Etching (DRIE) with the self-align process disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    The device described below is a miniature sub-mm-sized capacitive condenser with higher sensitivity, larger dynamic measurement range that overcomes the disadvantages of the parallel plate condenser silicon microphones disclosed in the prior art. The sensing element structure of the microphone reduces or eliminates residual stress effects, stiction, “squeeze film” air damping, and “pull-in”. This kind of transducer may be used in microphones and microspeakers used for hearing aids, cell phones, PDAs, laptops, MP3 players, digital cameras and other applications. It may also be used as an accelerometer, pressure sensors, actuator for a pump, optical switches, and optical interferometers. The design and fabrication method described below can also be used for miniature low voltage electrostatic driven microspeakers, accelerometers, etc. In one embodiment, the manufacturing method for the sensing and actuation structure is compatible with standard IC COMS process to form a monolithic integrated miniature silicon capacitive transducer. 
         [0040]    The vertical combdrive structure that allows sensing or actuation eliminates residual thin film stress on the diaphragm, “pull-in” effects and “squeeze film” air damping of the parallel plate type of capacitive sensing and actuation in the prior art examples. The working capacitance of the device is achieved by the interdigital vertical comb fingers. The vertical comb finger structure avoids the need of the backplate which attributes microfabrication challenges and performance sacrifices. The teachings herein provides design and microfabrication method for both sub-mm-sized silicon capacitive microphone with higher sensitivity, larger dynamic range, a miniature electrostatic driven microspeaker with low power consumption and low driving voltage, and also a miniature capacitive accelerometer etc. 
         [0041]    The same structure design principle can be used either in the sensing mode which is applicable for a microphone or an accelerometer etc., or in the actuation mode which is applicable for a microspeaker etc. 
       Sensing Mode 
       [0042]      FIGS. 1 ,  11  and  12  show a suitable example of a device structure design used for sensing, which is useful, for example, as a microphone or accelerometer. In this embodiment, the device has been formed using a SOI (Silicon On Insulator) substrate. The capacitive transducer is made from bulk conductive silicon mounted on a substrate, which is also referred to below as a carrier wafer  12 . The diaphragm  32  is supported by four hinges  29   a ,  29   b ,  29   c  and  29   d  that are connected to the corners of the rectangular diaphragm  32 . The diaphragm  32 , which is the body that moves when a force is applied, is connected to the fixed anchors  37   a ,  37   b ,  37   c  and  37   d  mounted on the substrate, which is a by serpentine shaped silicon hinges  29   a ,  29   b ,  29   c  and  29   d , respectively. The anchors  37   a ,  37   b ,  37   c  and  37   d  are sitting on a dielectrical material layer  11 , such as an oxide. The sensing element is made up of a vertical combdrive structure that includes a first set of fixed comb fingers  35  and a second set of movable lower comb fingers  36 . All the movable comb fingers  36  are formed on the outside edge of the diaphragm  32 . It will be understood that comb fingers  35  and  36  need not be positioned on all sides of the diaphragm  32  as shown. For example, the fingers may be positioned on two parallel edges of the diaphragm  32 . The fixed comb finger structures  35  are built around the diaphragm  32  and fixed on the dielectrical material  11  by anchors  38   a ,  38   b ,  38   c  and  38   d . The diaphragm  32 , hinges  29   a ,  29   b ,  29   c  and  29   d , anchors  37   a ,  37   b ,  37   c ,  37   d ,  38   a ,  38   b ,  38   c  and  38   d , vertical comb fingers  35  and  36  and electrical interconnection structures  39   a ,  39   b  and  39   c  are made of the same layer of electrical conductive material, such as a conductive single crystal silicon  10  which is on the top of a layer dielectrical material  11  that separates the silicon layer  10  from a base substrate  12  in a SOI structure. The electrical interconnection structures  39   a ,  39   b  and  39   c  electrically connect all four fixed comb finger structures  35  which are around the diaphragm  32 , while movable comb finger structures  36  are electrically connected by diaphragm  32 . As such, any anchor  37   a ,  37   b ,  37   c , and  37   d , can be used as an electrical connection point, and any anchor  38   a ,  38   b ,  38   c  and  38   d  can be used as the other electrical connection point for an integrated on-chip IC circuit, or for the wire bonding pads if a hybrid package is required for the transducer. 
         [0043]    As can be seen in  FIG. 1 , the exterior peripheral edge of the diaphragm  32  overlaps the interior peripheral edge of the cavity  40 . When used as a microphone, this overlap of the diaphragm  32  and the carrier wafer  12  is required to create a long air flow path  33  between the diagram  32  with movable fingers  36  and carrier wafer  12  to reduce leakage around the diaphragm  32 . This establishes a reasonably high resistance, which improves the low frequency response of the transducer. Another method of reducing leakage is to coat the diaphragm  32  on the cavity side  40  with a light weight material, such as a polymer (not shown) to reduce the gap between the diagram  32  with movable fingers  36  and carrier wafer  12 . This may be done, for example, by sputtering or other deposition techniques. During deposition, the material may also be deposited on the sides of the cavity. However, this is not undesirable, as it would also reduce the gap. 
         [0044]    The capacitance increases with the number of comb fingers. For a sub-mm-size diaphragm, it is possible to form a sufficient number of movable comb fingers  36  to achieve working capacitance of Pico Farads, which is equivalent to the capacitance offered by parallel plate structure in the prior arts. When the diaphragm  32  is subjected to pressure waves such as acoustic pressure, or accelerations/decelerations, the diaphragm  32  will move up and down in a piston style movement. The serpentine design of the springs  29   a ,  29   b ,  29   c , and  29   d  helps establish a substantially linear movement throughout. The movement of the diaphragm  32  can be detected by monitoring the capacitance changes between the movable fingers  36  and fixed comb fingers  35 . In addition, the capacitance changes between the movable fingers  36  and the fixed carrier wafer  12  may also be measured, which for example increases the sensitivity of the sensor by taking a differential measurement of the changes in capacitance. In addition, since the vertical comb finger structure and flexible hinges are used, the capacitance change is more sensitive to acoustic pressure  34  or accelerations/decelerations due to the fringe effects of the small comb fingers, which also results in higher sensitivity of the transducer. The flexible hinges helps maintain the piston movement of the diaphragm  32  instead of the parabolic deformation of the diaphragm in the prior art. 
         [0045]    The etching cavities  20   a ,  20   b ,  20   c  and  20   d  on the diaphragm  32  are to reduce the mass the diaphragm  32  for better high frequency response. The transducer requires no backplate, since its diaphragm  32  is suspended on the cavity  40  of the carrier wafer  12 . A barometric relief is not needed for the microphone. 
       Actuation Mode 
       [0046]      FIGS. 14 ,  15  and  16  show a device that is designed to be used in the actuation mode, such as for a microspeaker. Similar reference numbers are used to the embodiment described above. A silicon capacitive transducer (microspeaker) comprises a diaphragm  32  supported by four hinges  29   a ,  29   b ,  29   c  and  29   d . The diaphragm  32  is made of the bulk conductive silicon connected to the fixed anchors by serpentine shaped silicon hinges  29   a ,  29   b ,  29   c  and  29   d . The four hinges are connected to the anchor  37   a ,  37   b ,  37   c  and  37   d  which are sitting on the dielectrical material  11 . The actuation element is the vertical combdrive structure and includes movable comb fingers  36  and fixed comb fingers  35 . The movable comb fingers  36  are formed on the outside edge of the diaphragm  32 . The fixed comb finger structures  35  are built around the diaphragm  32  and fixed on the dielectrical material  11  by anchor  38   a ,  38   b ,  38   c  and  38   d . The diaphragm  32 , hinges  29   a ,  29   b ,  29   c  and  29   d , anchors  37   a ,  37   b ,  37   c ,  37   d ,  38   a ,  38   b ,  38   c  and  38   d , vertical comb fingers  35  and  36  and electrical interconnection structure  39   a ,  39   b  and  39   c  are made of the same layer of electrical conductive silicon  10  which is on the top of dielectrical material  11 . The electrical interconnection structure  39   a ,  39   b  and  39   c  electrically connects all four fixed comb finger structures  35  around the diaphragm  32 . Anchor  37  and  38  can be used as the electrical connection points for the integrated on-chip IC circuit, or for the bonding pads if a hybrid package is required for the transducer. For sub-mm-size or mm-size diaphragm, sufficient number of movable comb fingers  36  can be formed on its edge to achieve working capacitance of Pico Farads which is equivalent to the capacitance offered by parallel plate structure in the prior arts. When the actuation voltage is apply between the anchor  37  and  38 , a high density electrical field will be created between the fixed comb fingers  35  and movable fingers  36 . The resulting electrostatic force will actuate the diaphragm  32  to create acoustic pressure wave. The flexible hinges  29   a ,  29   b ,  29   c  and  29   d  will maintain the piston movement of the diaphragm  32  instead of the parabolic deformation which is common to many prior art devices. The etching cavities  20   a ,  20   b ,  20   c  and  20   d  on the diaphragm  32  are to reduce the mass the diaphragm for better high frequency response. The transducer has no backplate since its diaphragm  32  is suspended on the cavity  40  of the carrier wafer  12 . 
         [0047]    By comparing the actuation embodiment described above with the sensing embodiment described previously, certain differences can be noted. In the actuation embodiment, the external peripheral edge is within the interior peripheral edge of the cavity  12 , such that the diaphragm  32  only partially covers the cavity  12 . In addition, the fixed comb fingers  35  are taller than was the case previously. These differences are intended to improve performance in the actuation mode, as will be described in more detail below. 
       Manufacture 
       [0048]      FIG. 2-FIG .  11  show the major process steps that may be used to manufacture either a sensing or actuation device. 
         [0049]    The general steps to manufacture the capacitive transducer include first, applying a first etching mask on a layer that is mounted on a substrate to define the position of one of a movable set of fingers and a fixed set of fingers. The position of the body and the springs may also be defined by the first mask. Next, a second etching mask is applied to define the movable set of fingers, the fixed set of fingers, a body, and springs, the body being connected to the movable set of fingers and the springs, the movable set of fingers being interdigitated with the fixed set of fingers. The second etching mask is then used to etch the layer and the first etching mask. The second etching mask is removed, and the layer is then etched using the first etching mask, such that one of the movable set of fingers and the fixed set of fingers is shorter than the other of the movable set of fingers and the fixed set of fingers. The body, the springs, and the movable set of fingers are then released using etching, such that, upon applying a force to the body, the body moves parallel to the substrate. The variations in this process to obtain the various embodiments will be apparent from the discussion below. 
         [0050]      FIG. 2  shows the wafer for the transducer. The process for making such wafer is not described here. The layer  10  is a layer of conductive material such as preferred single crystalline bulk silicon or low stress polysilicon. The layer  11  is a layer of dielectrical materials such as an oxide or nitride. Carrier wafer  12  material can be regular silicon or glass. The substrate can also be purchased from any SOI (Silicon On Insulator) vendors. Although a variety of materials can be used, SOI type wafer is used for the purpose of explaining the process of a first embodiment. 
         [0051]      FIG. 3  shows the SOI wafer after growing a layer of oxide  13  and  16  on its top and bottom sides. The thermal oxidation process can be used for oxide growth.  FIG. 4  shows the substrate after being subjected to a silicon anisotropic etch in KOH (Potassium hydroxide) or TMAH (Tetramethylammonium hydroxide) and, subsequent oxide etching in a buffered HF (Hydrofluoric acid) solution with the top side of the SOI wafer protected. The cavity  14  is formed on the oxide layer  11  and a cavity  40  on the carrier silicon wafer  12 . The cavity  40  can be also etched using any other anisotropic etching methods such as Silicon Deep Reactive Ion Etching (DRIE). 
         [0052]    The critical processing for making the vertical comb driver structure is to guarantee the perfect alignment of movable fingers  36  with the fixed fingers  35 . If they are misaligned, the air gaps between one moveable finger with adjacent two fixed fingers or vice visa will not be equal, which will result in the sideway movement of the movable finger  36  due to unequal electrostatic force on its right and left side. This unexpected sideway movement will cause the malfunction of the comb drive structure. 
         [0053]    The manufacturing steps applies the self alignment process to microfabricate the vertical comb drive structure.  FIG. 5  shows the patterning of the oxide layer on the top side of a SOI wafer. The oxide pattering is done using regular lithography and oxide etching process such as RIE (Reactive ion Etch). The oxide  22  on the movable fingers is patterned. Areas  17   a ,  17   b ,  17   c ,  17   d ,  18   a ,  18   b ,  18   c , and  18   d  are pattered for the anchors  37  and  38 . Areas  21   a ,  21   b  and  21   c  are patterned for the electrical interconnection structure  39 . Areas  201   a ,  201   b ,  201   c  and  201   d  are oxide free for formation silicon cavities  20  on the diaphragm  32 . 
         [0054]      FIG. 6   a  shows the SOI wafer depicted in  FIG. 5  after photoresist patterning. This lithography step defines the shape of movable fingers and hinges, and redefines the oxide pattern depicted in  FIG. 5 . In order to accommodate the possible larger alignment tolerance during the process, the geometry sizes of oxide patterning in  FIG. 5  are larger than the desired device feature sizes. An enlarged perspective view of a portion C of the comb fingers  25  and  27  and hinge  26  depicted in  FIG. 6   a  is shown in  FIG. 6   b . The final shapes of the movable comb fingers  36 , fixed comb fingers  35 , diaphragm  32  and hinges  29   a ,  29   b ,  29   c , and  29   d  are defied precisely by photoresist  25 ,  27 ,  23  and  26  respectively. The extra oxide  24  of oxide layer  13  will be cleared out by a subsequent oxide RIE process.  FIG. 7  shows the SOI wafer in  FIG. 6  after oxide RIE etch process. An enlarged perspective view of a portion D of the comb fingers  25  and  27  and hinge  26  depicted in  FIG. 7   a  is shown in  FIG. 7   b.    
         [0055]    The patterned photoresist layer is used as etching mask material for the first silicon DRIE etching. The oxide layer  11  is used as the etching stop layer for the first silicon DRIE.  FIG. 8  shows the substrate after the first silicon DRIE. 
         [0056]    The photoresist is removed after the first silicon DRIE etching.  FIG. 9   b  shows an enlarged perspective view of a portion E of the comb fingers  28  and  30  and hinge  29   a  depicted in  FIG. 9   a . The hinge  29   a  and fixed comb finger  28  have no oxide on their tops while the movable finger  30  and diaphragm  32  have oxide on their tops for the subsequent second silicon DRIE etching. The second silicon DRIE etching forms the lower fixed comb fingers, flexible hinges  29   a ,  29   b ,  29   c  and  29   d  and cavities  20 .  FIG. 10   b  shows an enlarged perspective view of a portion F of the comb fingers  28  and  30  and a hinge  29   a  depicted in  FIG. 10   a.    
         [0057]    After etched in buffered HF, the diaphragm  32 , hinges  29   a ,  29   b ,  29   c  and  29   d , and movable comb fingers  36  are released form the oxide layer  11 . The finished transducer is shown in  FIG. 11  and  FIG. 1 . The microfabrication processes disclosed here doesn&#39;t include the processes to integrate with standard IC COMS process. However, it is very easy to achieve such integration for the people skill in the art.  FIG. 12  shows a cross sectional perspective view of the microphone according to a second embodiment, where the comb finger configuration is different. In  FIG. 12 , the movable fingers  35  and fixed fingers  36  are offset, whereas in  FIGS. 1 and 11 , the movable fingers  35  are higher than the fixed fingers  36 . Either set of fingers may be taller or positioned above the other. While the offset fingers are more difficult to manufacture, they have a larger effective range motion, or in the actuation mode, require less power. 
         [0058]    Referring to  FIG. 13 , an air gap  41  between comb fingers  35  and  36  and an air gap  42  between hinges  29   a ,  29   b ,  29   c , and  29   d  and comb fingers  36  of around 2 im provides sufficient resistance for a low frequency response of the transducer. If 2 im is achievable for air gaps  41  and  42  with the current microfabrication technology, then the long air flow path  33  shown in between diaphragm  32  and the carrier wafer  12  becomes unnecessary. 
         [0059]    If the device is intended to be used as a microspeaker, a larger translation of the diaphragm  32  during actuation is preferable to create a higher sound wave pressure level from the miniature silicon microspeaker. So a thicker silicon layer  10  as shown in  FIG. 14  should be used to make the larger height difference between the fixed fingers  35  and movable fingers  36 . In doing so, a larger electrostatic force and a correspondingly larger actuated translation movement between the fixed and movable fingers  35  and  36  can be expected. A larger silicon cavity  40  should also be formed in the carrier wafer  12  so that the diaphragm  32  can obtain a larger up and down translational movement without any mechanical obstruction. The miniature microspeaker embodiment is depicted in the  FIG. 14 . 
         [0060]    One of the advantages of silicon microspeaker is that it offers less power consumption due to electrostatic actuation. In addition, for the same hinge design, the driving voltage for the silicon microspeaker can be further lowered by reducing the overlapping region between fixed and movable comb fingers, such that they are offset. The reason for this is that the electrical field in the overlapping region between fixed and movable comb fingers  35  and  36  will prevent the constructive movement of the diaphragm  32 . One way to reduce the overlapping region between fixed and movable comb fingers  35  and  36  is to etch away the lower part of the fixed comb fingers  35  during the fabrication of the engineered SOI wafer. For example, the device layer may be pre etched before it is bonded with a carrier silicon wafer.  FIGS. 15 and 16  depict embodiments where the lower portion of either fixed comb finger  35  or movable finger  36  is etched away. 
         [0061]    Another alternative embodiment is shown in  FIG. 20 , and formed of an N type substrate with P type structures. A regular N type silicon wafer  18  in  FIG. 17  is the starting material for the transducer. A layer of P ++  silicon  49  is formed on top of N type silicon  48  by either epitaxial growth or doping/diffusion or implantation/diffusion as shown in  FIG. 18 . P ++  silicon  49  is used for building the transducer. Referring to  FIG. 191  the P ++  silicon  49  is used as silicon etch stop layer to form diaphragm  50  using silicon anisotropic etching in either KOH or TMAH, which will etch the N type substrate  18 , but not the P++ silicon  49 . 
         [0062]    The embodiment shown in  FIG. 20  is formed using the self alignment process method disclosed above with reference to  FIG. 3-FIG .  11 . The fixed comb finger  35  is electrically isolated from movable comb finger  36  and diagram  32  by the PN junction formed between the N type and the P type layers. The transducer made based on this embodiment will reduce the wafer cost and increase the flexibility for integration with IC CMOS process. 
         [0063]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
         [0064]    It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope defined in the Claims.