Abstract:
An acoustic pressure type sensor fabricated on a supporting substrate is disclosed. The acoustic sensor is fabricated by depositing and etching a number of thin films on the supporting substrate and by machining the supporting substrate. The resulting structure contains a pressure sensitive, electrically conductive diaphragm positioned at a distance from an electrically conductive fixed electrode. In operation, the diaphragm deflects in response to an acoustic pressure and the corresponding change of electrical capacitance between the diaphragm and the fixed electrode is detected using an electrical circuit. Two or more such acoustic sensors are combined on the same supporting substrate with an interaural flexible mechanical connection, to form a directional sensor with a small surface area.

Description:
This application is a divisional of Application Ser. No. 10/302,839, filed Nov. 25, 2002, now U.S. Pat. No. 7,146,016 which claims the benefit of Provisional Application Ser. No. 60/333,125, filed Nov. 27, 2001, the entire contents of which are hereby incorporated by reference in this application. 
   FIELD OF THE INVENTION 
   The present invention relates to the field of acoustic transducers, and more specifically to a microfabricated airborne acoustic microphone using micro electro-mechanical systems (MEMS). 
   BACKGROUND OF THE INVENTION 
   The emergence of micro fabrication technology has led to a number of developments in the field of airborne acoustic sensors (microphones). Traditionally, the most common technology for microphones has been the use of an electret to detect a deflection of a diaphragm caused by a differential acoustic pressure. Electrets are insulators (such as Teflon or Mylar) on which an electrical charge is trapped. An electret is used in a microphone to produce the necessary electrical field in the air gap between the electrically conductive movable diaphragm and fixed electrode to detect the deflection of the diaphragm. Alternatively, a DC potential between the diaphragm and fixed electrode may be applied from an external source to create the electrical field. This latter device is referred to as a condenser microphone. 
   A common problem with electret microphones is leakage of electrical charge from the electret, which directly affects the sensitivity of the microphone. This problem is especially severe at elevated temperature and humidity levels. It is inherently difficult to protect the microphone, since it must be exposed to the environment to detect an acoustic signal. Condenser microphones do not suffer from this problem; however due to the large air gaps in devices made with traditional fabrication methods, the external DC potentials required are in the order of hundreds of Volts, which is difficult to realize in battery powered systems. 
   With the utilization of micro fabrication technology, it is possible to significantly reduce the dimensions, including the air gap, of a microphone. With micro fabrication technology, condenser microphone structures can be fabricated which only require an external DC potential of 5-20 Volts. There are several key motivations for the development of MEMS microphones, the most important of which are: improvement of device ruggedness in system assemblies, miniaturization, improvement of performance and manufacturability of existing devices, and potential monolithic integration with semiconductor electronics. 
   An important limiting parameter for the performance, specifically sensitivity, of micro fabricated microphones is the mechanical sensitivity of the diaphragm in the device. As the device is scaled down, the microphone sensitivity increases linearly as the air gap decreases, but this is counteracted by a decrease which goes to the fourth power of the diaphragm size. The mechanical sensitivity of the diaphragm is determined by the material properties (such as Young&#39;s modulus and Poisson&#39;s ratio), thickness, and any intrinsic stress in the diaphragm. It is therefore very important to maximize the diaphragm sensitivity by making it very thin and with a minimal amount of intrinsic stress. In micro fabrication, it is difficult to control the intrinsic stress levels in materials, hence special attention is required to solve this problem. In the prior art, the stress problem has been addressed by using low-stress materials, such as single crystal silicon, polycrystalline silicon and silicon germanium for the diaphragm. Alternatively, the intrinsic stress can be relieved by creating a compliant suspension between the diaphragm and the supporting substrate, which allows the diaphragm to expand and contract. 
   The idea of suspension is attractive, since it will not only allow relief of any intrinsic stress in the diaphragm, but also decouple the diaphragm from any stress induced due to mismatch of thermal expansion between the diaphragm and the substrate, as well as any stress stemming from the mounting of the substrate in a package. There is, however, some undesirable features associated with prior art devices. 
   One prior art microphone device  210  shown in  FIG. 1  contains a diaphragm  211  supported by four or more springs  212 , which are all formed from a silicon substrate  213 . However, to realize springs  212  and diaphragm  211 , a number of slots  214  must be etched in diaphragm  211 , which leads to an acoustical bypass, or leakage, of diaphragm  211 . As a result, the low-frequency roll-off of microphone  210  is directly determined by these slots, the dimensions of which are difficult to control. Furthermore, since the motion of diaphragm  211  is set by the stiffness of suspension springs  212 , it is important to control tightly the physical dimensions of these springs. 
   An alternative microphone device  220  shown in  FIG. 2  is a variation of the design in which the diaphragm  221  is suspended in a single point  222  (see  FIG. 2 ) or along a straight line around which diaphragm  221  can freely expand or contract. Since the air gap in this type of structure not only defines the distance between movable diaphragm  221  and the fixed counter electrode  223 , but also the acoustical leakage resistance in the device it must be tightly controlled. As the diaphragm in this device is essentially a cantilever with one end fixed and the other end free to move, any intrinsic stress gradient in the diaphragm material will cause diaphragm  221  to bend, leading to a change of the air gap in the device, and therefore, the sensitivity and roll-off frequency. This problem is especially important if the diaphragm is composed of more than one material, which may induce a stress gradient by mismatch of thermal expansion in the different materials. Therefore, to realize a suspended diaphragm structure such as that shown in  FIG. 2 , precise control of dimensions and material stress gradients is required. In another prior art design without suspension in which the diaphragm is loosely confined between the substrate and a lateral restraint, there is no suspension force to release the diaphragm from the substrate, and thus, it is important to avoid stiction in the device during the release of the diaphragm. Unfortunately, surface forces and associated stiction is a predominant effect in micro fabrication due to the extremely smooth surfaces in the device. 
   Microphones with directional properties are desirable in many applications to lower background noise levels and, in some systems, to enable determination of sound source location. A fundamental limitation on the directivity of a single pressure type microphone is that the size of the sound detecting diaphragm must be comparable to the wavelength of the sound of interest to achieve significant directivity. For human speech and hearing, which is centered around a wavelength of approximately 156 mm, this requires diaphragms of unrealistic sizes. Alternatively, as shown in  FIG. 3   a , a pressure type microphone  230  can be combined with a pressure gradient microphone in a single structure to achieve a directional response. Such microphones are known as first order gradient microphones. By carefully adjusting the volume of the air cavity  231 , the acoustic resistance through the screen  232 , and the acoustical path length from the front of the diaphragm  233  to the screen  232 , a directivity pattern known as a cardioid pattern can be achieved (see  FIG. 3   b ). The directivity pattern is depicted for a sound source location  235  at the angle θ from a principal direction  234 . Microphone  230  has maximum response on the principal axis  234  of microphone  230  and a null response at ±180° from principal axis  234 , the principal axis being perpendicular to diaphragm  233 . The condition which must be met to achieve the cardioid pattern shown in  FIG. 3   b  is given by:
 
Δl=cC A R A ,
 
where Δl is the acoustical path between diaphragm  233  and screen  232 , c is the speed of sound in air (344 m/s), C A  is the acoustical compliance of the air cavity  231 , and R A  is the acoustical resistance of screen  232 . For very small devices, it is difficult and costly to manufacture screen material with high enough acoustical resistance to meet the condition above. Secondly, since the lower roll-off frequency of the microphone is given approximately by:
 
               f   low     =       1     2   ⁢   π   ⁢           ⁢     R   A     ⁢     C   A         =     c     2   ⁢   πΔ   ⁢           ⁢   l           ,         
the lower roll-off frequency of microphone  230  increases as the exterior dimensions decrease. As a result, most first order gradient directional microphones have a sloped frequency response, since in most cases the frequency of interest for detection is smaller than the roll-off frequency. Such microphones are typically referred to as having a ski-slope response.
 
   A common method employed to improve the frequency response of a directional microphone is to increase the effective acoustical path Δl by design of the microphone package.  FIG. 4  shows a microphone package  240  with two air cavities  241  in which acoustical inlets  242  and  246  for the front and back of the microphone diaphragm  243  are further separated by tubes  247  mounted on the microphone package. The microphone shown in  FIG. 3   a  employs only one damping screen  232 ; however, if a second damping screen  246  is added in front of diaphragm  243 , the frequency response can be leveled when compared to the structure of  FIG. 3   a  with one damping screen. The device of  FIG. 4  has symmetric acoustic paths and resistances on both sides of single diaphragm  243 . 
   Another approach to achieve directivity is to implement a so-called second order gradient (SOG) microphone, in which the difference in arrival time of the incoming acoustic wave is enhanced by electronic or acoustical means. The principal idea of the SOG microphone is illustrated in  FIG. 5  and  FIG. 6 . In  FIG. 5 , the typical electronic implementation of an SOG microphone is shown with an array  250  of four omni directional microphones  251 , and a complex summing network  252 . A specific time delay τ and τ′ is added electronically to the microphone signals and the signals are subtracted in the network  252 . As a result, the output signal  253  of summing network  252  is the sum of the signal from microphone M 1  and the signal from microphone M 4  delayed by τ+τ′ minus the sum of the signal from microphone M 2  delayed by τ′ and the signal from microphone M 3  delayed by τ. If τ and τ′ are chosen such that τ=2τ′, the delays will make the microphone array  250  behave as if the distance between each microphone is increased by c*τ′ where c is the speed of sound in air (344 m/s). In other words, a small array can be made to behave like a larger array, with better directivity, by adding delays to each microphone signal. The disadvantages of adding electronic delays are the number of external components needed to realize the functionality, and the need for finely tuned and matched microphones in the array. 
   It is also possible to achieve the desired delay by acoustical means. Such an implementation is shown in  FIG. 6 , in which a first order gradient microphone  261  is connected to acoustic paths  262 - 265  of different lengths which has openings  266 - 269  with impedance matched acoustic resistances. In operation, the acoustic paths  262 - 265  act as delay lines, and by adjusting the length of the paths, a directional response similar to the electronic system of  FIG. 5  can be realized. A common drawback of all approaches described above is the relative bulkiness of the devices, which does not lend itself well to miniaturization due to fundamental limitations in the underlying physics upon which these devices are based. 
   An alternative detection principle has been found in nature in auditory organs of the  Ormia ochracea  parasitoid fly. This insect uses hearing to locate sounds produced by crickets, and has been shown to possess a remarkable directional hearing ability. An impressive feat considering the distance between the eardrums in the insect is only approximately 2% of the wavelength of the sound of interest (4.8 kHz). It has been shown that complex interaction between the two eardrums through mechanical coupling greatly enhances the directional response of each eardrum. A single diaphragm solution with properties similar to the second order mechanical system of the hearing organs in the fly has been suggested in the prior art. The single diaphragm contains a number of corrugations to create resonance modes similar to the dominant vibration modes in the hearing organs of the fly. Unfortunately, the micro fabrication of a single diaphragm with these properties is difficult and problems with stress and stress gradients in the diaphragm material, leading to intrinsic curling and deflection, complicates the matter in a similar fashion as described earlier. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide an acoustic transducer with a suspended diaphragm which has a good low-frequency response and in which the suspension structure has little or no influence on the sensitivity of the device. 
   It is another object of the present invention to minimize the influence of stiction on the performance of said acoustical transducer. 
   It is a further object of the present invention to provide such an acoustic transducer in which the sensitivity to stress gradients in the diaphragm material is minimal. 
   It is another object of the present invention to provide a batch fabrication process for the acoustic transducer that can be performed using micro machining processes. 
   It is yet a further object of the present invention to provide a fabrication process with a minimal number of layers and masks to reduce the fabrication cost. 
   It is still another object of the present invention to provide a fabrication process that will allow the mechanical coupling of two or more such acoustic transducers to realize a directional acoustic transducer. 
   It is still a further object of the present invention to provide a directional acoustic transducer in which the response is designed and optimized for human communication. 
   The present invention results from the realization that, by preventing the suspension structure from controlling the movement of the diaphragm in response to an applied sound pressure, it is possible to eliminate the dependency of the microphone performance on the exact stiffness, and hence dimensional variation, of the suspension structure. Furthermore, if a suspension structure is used which has a restoring force equal to or greater than the surface contact forces in the device, problems associated with stiction can be greatly reduced. A second important realization is that an acoustic seal can be created by utilizing the electrostatic force generated between the diaphragm and fixed counter electrode during operation of the microphone, thereby improving the low-frequency performance of the microphone over other suspended diaphragm structures in the prior art. A third important realization is that by providing for a mechanical flexible connection between two or more such diaphragms, a microphone with directional response may be realized. 
   The present invention comprises a fixed counter electrode formed on a supporting substrate and a diaphragm attached to said supporting substrate by a number of suspension structures. The diaphragm contains an annular indentation along the periphery, which serves to provide a predefined standoff between the diaphragm and the fixed counter electrode when an external DC bias voltage is applied. It also provides an acoustic seal between the front and back of the diaphragm to enable low frequency acoustical response of the diaphragm. The fixed counter electrode contains a number of holes to allow the air in the gap between the fixed electrode and the diaphragm to escape, thereby reducing the acoustical damping in the device. The suspension structure is designed, such that the restoring force overcomes any surface forces from the mechanical contact in the device. In operation, an electrical bias voltage is applied between the fixed counter electrode and the conductive diaphragm, or conductive layer on the diaphragm, to establish an electrical field in the air gap. The electrostatic force associated with the electrical field overcomes the restoring force of the suspension structure, causing the diaphragm to be pulled towards the supporting substrate. The diaphragm makes physical contact with the supporting substrate at the annular indentation, which sets the initial operational air gap in the microphone. An incident sound pressure wave will produce a pressure differential over the diaphragm, causing it to deflect from its initial position. The change in electrical capacitance between the diaphragm and fixed counter electrode is detected with an electronic circuit. The detection circuit may be integrated in the supporting substrate. 
   The present invention also comprises the combination of two or more such microphones on a single supporting substrate, in which the diaphragms are mechanically connected by a centrally supported beam formed in the supporting substrate. In operation, an incoming sound pressure wave will produce a pressure differential on each of the diaphragms, causing them to deflect from their initial state. However, due to the mechanical connection between the diaphragms, the force from the acoustic pressure on each diaphragm is transferred to the other diaphragm(s). If the incoming sound pressure wave is completely in phase on all diaphragms, there will be a condition of force balance in the system, and the compliance of the mechanical beam will determine the deflection of the diaphragms. If the detected incoming sound pressure waves are not in phase, which is the case if the sound source is not located on the principal axis of the microphone, the mechanical interaction between the diaphragms and the mechanical coupling beam will determine the response of each diaphragm. The compliance of the diaphragms and the mechanical coupling beam must be adjusted together with the acoustic damping of the diaphragms to achieve the desired frequency and directional response of the microphone. The acoustic damping of the diaphragms is determined by the height of the operational air gap and the size and density of holes in the fixed counter electrodes. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-sectional view of a prior art microphone with a suspended diaphragm. 
       FIG. 2  is a cross-sectional view of a prior art microphone with a diaphragm suspended in a single central point. 
       FIGS. 3   a  and  3   b  are a conceptual view and a directional response curve of a first order gradient microphone with a single damping screen. 
       FIG. 4  is a conceptual view of a prior art first order gradient microphone with damping screens on both sides of the diaphragm. 
       FIG. 5  is a functional diagram of a prior art second order gradient microphone with electronic time delays. 
       FIG. 6  is a top plan view of a prior art second order gradient microphone with acoustical delay lines. 
       FIG. 7  is a perspective view of a microphone structure according to the present invention partially cut away. 
       FIG. 8  is a cross-sectional view of the microphone structure of  FIG. 7  according to the present invention. 
       FIG. 9  is a top plan view of a directional microphone structure according to the present invention. 
       FIG. 10  is a cross-sectional view of a directional microphone according to the present invention taken along the section line A-A in  FIG. 9 . 
       FIGS. 11 through 22  are cross-sectional views of a microphone structure according to the present invention at different stages of the fabrication process used to make such microphone. 
       FIGS. 23 through 34  are cross-sectional views of a directional microphone structure according to the present invention at different stages of the fabrication process used to make such microphone. 
       FIG. 35  is a cross-sectional view of an interconnection pad according to the present invention. 
       FIG. 36  is a cross-sectional view of an interconnection pad with a solder ball for flip-chip assembly according to the present invention. 
       FIG. 37   a  is a cross-sectional view of a microphone structure assembled on to a package carrier substrate according to the present invention. 
       FIG. 37   b  is a cross-sectional view of a directional microphone structure assembled on to a package carrier substrate according to the present invention. 
       FIG. 38  is a graphic illustration of the first order relationship between diaphragm thickness and size for specific diaphragm resonance frequencies of 20 kHz and 30 kHz. 
       FIG. 39  is a graphic illustration of the first order relationship between initial air gap and diaphragm size for specific diaphragm resonance frequencies of 20 kHz and 30 kHz and a DC bias voltage of 5 V. 
       FIG. 40  is a graphic illustration of the first order relationship between microphone sensitivity and diaphragm size for certain parasitic capacitances associated with a buffer amplifier. 
       FIG. 41  is a conceptual diagram of a mechanical equivalent model for a directional microphone according to the present invention. 
       FIGS. 42   a  and  42   b  are graphic illustrations of the frequency and directional response curve of a directional microphone according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   One embodiment of the microphone structure  10  according to the present invention is shown in the perspective view of  FIG. 7  and the cross-sectional view of  FIG. 8 . An electrically conductive diaphragm  11  is attached to a supporting substrate  19  by a number of flexible suspension structures  13 . Conductive diaphragm  11  can be a single layer conductive material, or be comprised of several layers of which at least one is conductive. Suspension structures  13  are attached to supporting substrate  19  using a conducting anchor material  14 . Diaphragm  11  contains an annular indentation  12  at the perimeter, which in the initial position forms a narrow air gap  16  with supporting substrate  19 . Supporting substrate  19  is coated with an electrically insulating layer  17 , which isolates the conductive diaphragm  11  and a fixed counter electrode. A fixed counter electrode  21  is made of conductive layer  14 , insulator  17 , and a bulk layer  18 . The purpose of bulk layer  18  is to provide sufficient mechanical rigidity to fixed counter electrode  21 . A number of openings  20  are made in fixed counter electrode  21  to allow the air in the gap  15  to escape when diaphragm  11  deflects in response to an acoustical sound pressure. An electronic detection circuit  25  may be formed on substrate  19  before, during, or after the formation of microphone structure  10 . 
   In operation, an electrical DC bias voltage of 1V to 20V is applied between conductive diaphragm  11  and conductive layer  14  on the fixed counter electrode  21  from an external voltage source. The resulting electrostatic attraction force between diaphragm  11  and fixed counter electrode  21  causes diaphragm  11  to deflect at the suspension points  13  until diaphragm  11  makes mechanical contact with supporting substrate  19 , and the narrow gap  16  is closed. Once gap  16  is closed, suspension structures  13  do not deflect further and any additional load (i.e. sound pressure) on the diaphragm will cause it to deflect within annular boundary  12 . The deflection can be detected as a change of capacitance between diaphragm  11  and fixed counter electrode  21 . 
   An embodiment of a directional microphone  100  according to the present invention is shown in top plan view in  FIG. 9  and cross-sectional view in  FIG. 10 . In this embodiment, two diaphragms  111  are combined on a single supporting substrate  119 . Each diaphragm  111  has features as described above for microphone structure  10 , including mechanical attachment springs  113 , annular indentation  112 , fixed counter electrode  121 , acoustic vent holes  120 , electrically insulating layer  117 , and bulk layer  118 . In addition, an indentation  141 , with the same height as annular indentation  112 , is formed in each diaphragm, and a mechanical coupling beam  140  is formed in the bulk layer  118 . Mechanical coupling beam  140  is attached to substrate  119  at the torsional points  142 , allowing beam  140  to swivel out of the plane of substrate  119 . An electronic detection circuit  125  may be formed on substrate  19  before, during, or after the formation of directional microphone structure  100 . 
   In operation, an electrical DC bias voltage of 1V to 20V is applied between each diaphragm  111  and fixed counter electrode  121 . The resulting electrostatic attraction forces causes each diaphragm to be pulled towards substrate  119  until the annular indentations  112  make mechanical contact with substrate  119 . At the same time, indentations  141  will make mechanical contact at each end of mechanical coupling beam  140 . The diaphragms  111  are in this situation coupled mechanically through beam  140 . When an incident sound pressure wave is applied, the deflection of each diaphragm is governed by the dynamic behavior of the mechanical coupling, leading to a directional response of each diaphragm. 
   A preferred fabrication process for the microphone structure  10  according to the present invention is shown in  FIG. 11 to 22 . Firstly, as shown in  FIG. 11 , a bulk layer  18  is formed on a substrate  19 . A preferred method for forming bulk layer  18  is to perform a diffusion of boron in a silicon substrate, forming a region in the substrate in which the boron concentration is higher than 5*10{circumflex over (0)}19 atoms/cmô3. A second preferred method for forming bulk layer  18  is epitaxial growth of a doped silicon layer on a silicon substrate, in which the boron concentration in the grown layer is higher than 5*10ô19 atomns/cm{circumflex over ( 0 )}3. A third preferred method for forming bulk layer  18  is the use of silicon on insulator (SOI) substrates. 
   Subsequently, as shown in  FIG. 12 , substrate  19  is etched to form a number of cavities  20  in bulk layer  18 . Substrate  19  is then covered with sacrificial material  23  as shown in  FIG. 13 , which covers all surfaces and fills out cavities  20 . Preferred sacrificial materials include phosphosilicate glass (PSG) and silicon germanium alloy. Sacrificial layer  23  is then thinned down on the substrate using a planarization method, as shown in  FIG. 14 . A preferred method of planarization is chemical mechanical polishing (CMP). The planarization is performed until the original surface of bulk layer  18  reappears, leaving cavities  20  filled with sacrificial material  23 . Referring now to  FIG. 15 , an electrically insulating layer is subsequently deposited on both sides of the substrate. The layer  17  on the side of substrate  19  with cavities  20  is patterned in a similar pattern as cavities  20 . The layer  17   b  on the opposite side of substrate  19  is left intact for later use as an etch mask. A preferred material for electrically insulating layers  17  and  17   b  is silicon nitride. In  FIG. 16 , an electrically conductive layer  14  is then deposited and patterned to form the fixed counter electrode  21  and the anchor points  14  for suspended diaphragm  11 . Preferred materials for electrically conductive layer  14  include low resistivity polycrystalline silicon, formed by the addition of a dopant, and silicon germanium alloy. In  FIG. 17 , a second sacrificial layer  24  is deposited and patterned, the thickness of which sets the operational air gap in the microphone structure of  FIGS. 7 and 8 . Preferred materials for second sacrificial layer  24  include phosphosilicate glass (PSG) and silicon germanium alloy. Second sacrificial layer is removed only in the diaphragm anchor areas  14  and the area of the annular indentation  12  of the diaphragm. In  FIG. 18 , a third sacrificial layer  25  is deposited and patterned, the thickness of which sets the initial gap between the supporting substrate  19  and the annular indentation  12 . Preferred materials for the third sacrificial layer include phosphosilicate glass (PSG) and silicon germanium alloy. The third sacrificial layer is removed only in the diaphragm anchor areas  14 . 
   As shown in  FIG. 19 , an electrically conductive layer is deposited and patterned to form the diaphragm  11  with annular indentation  12  and suspension structures  13 . It is desirable to use only one material to form diaphragm  11  to minimize curling or warping of the free structure due to stress gradients caused by mismatch of thermal expansion between layers. It is therefore also important that the single material has little or no intrinsic stress and stress gradient. Preferred materials for diaphragm  11  and suspension structures  13  include low resistivity polycrystalline silicon, formed by the addition of a dopant, and silicon germanium alloy. In  FIG. 20 , layer  17   b  on the opposite side of substrate  19  is then patterned to form an opening  22   b , and substrate  19  is etched through to form the cavity  22 . A preferred method to etch substrate  19  is a chemical solution of water and potassium hydroxide (KOH), which has the advantage of etching substrate  19  but not the preferred bulk layer  18  or sacrificial layers  23 ,  24 , and  25 . Therefore, the etching through substrate  19  has a natural termination and does not have to be closely monitored or controlled. A second preferred method to etch substrate  19  is anisotropic reactive ion etching. Finally, the sacrificial layers are removed by wet chemical etching to realize the complete microphone structure  10 . A preferred wet etchant for phosphosilicate glass (PSG) sacrificial layers is hydrofluoric acid (HF). A preferred wet etchant for silicon germanium alloy is hydrogen peroxide. 
   A preferred fabrication process for the directional microphone structure  100  according to the present invention is shown in  FIGS. 23 to 34 . As shown in  FIG. 23 , a bulk layer  118  is formed on a substrate  119  first. A preferred method for forming bulk layer  118  is to perform a diffusion of boron in a silicon substrate, forming a region in the substrate in which the boron concentration is higher than 5*10ô19 atoms/cmô3. A second preferred method for the formation of the bulk layer is epitaxial growth of a doped silicon layer on a silicon substrate, in which the boron concentration in the grown layer is higher than 5*10ô19 atoms/cmô3. A third preferred method for the formation of the bulk layer is the use of silicon on insulator (SOI) substrates. In  FIG. 24 , substrate  119  is etched to form a number of cavities  120  in bulk layer  118 . Cavities  120  are also etched to form the mechanical coupling beam  140  and torsional attachment points  142 . In  FIG. 25 , substrate  119  is then covered with a sacrificial material  123 , which covers all surfaces and fills out cavities  120 . Preferred sacrificial materials include phosphosilicate glass (PSG) and silicon germanium alloy. In  FIG. 26 , sacrificial layer  123  is then thinned down on substrate  119  using a planarization method. A preferred method of planarization is chemical mechanical polishing (CMP). The planarization is performed until the original surface of bulk layer  118  reappears, leaving the cavities filled with sacrificial material. As shown in  FIG. 27 , an electrically insulating layer is subsequently deposited on both sides of substrate  119 . The layer  117  on the side of substrate  119  with cavities  120  is patterned in a similar pattern as cavities  120 . The layer  117   b  on the opposite side of substrate  119  is left intact for later use as an etch mask. A preferred material for the electrically insulating layer is silicon nitride. In  FIG. 28 , an electrically conductive layer is then deposited and patterned to form fixed counter electrodes  121  and anchor points  114  for suspended diaphragms  111 . Preferred materials for the electrically conductive layer include low resistivity polycrystalline silicon, formed by the addition of a dopant, and silicon germanium alloy. Then, as shown in  FIG. 29 , a second sacrificial layer  124  is deposited and patterned, the thickness of which sets the operational air gaps in the microphone structure  100 . Preferred materials for the second sacrificial layer include phosphosilicate glass (PSG) and silicon germanium alloy. The second sacrificial layer is removed only in the diaphragm anchor areas  114  and the area of the annular and central indentations  112  and  141  of diaphragms  111 . In  FIG. 30 , a third sacrificial layer  125  is deposited and patterned, the thickness of which sets the initial gap between the supporting substrate  119  and the annular and central indentations  112  and  141 . Preferred materials for the third sacrificial layer include phosphosilicate glass (PSG) and silicon germanium alloy. The third sacrificial layer is removed only in the diaphragm anchor areas  114 . In  FIG. 31 , an electrically conductive layer is deposited and patterned to form the diaphragms  111 , with annular indentations  112  and indentations  141 , and suspension structures  113 . It is desirable to use only one material to form the diaphragm to minimize curling or warping of the free structure due to stress gradients caused by mismatch of thermal expansion between layers. It is therefore also important that the single material has little or no intrinsic stress and stress gradient. Preferred materials for the diaphragm and suspension structures  111  and  113  include low resistivity polycrystalline silicon, formed by the addition of a dopant, and silicon germanium alloy. In  FIG. 32 , the layer  117   b  on the opposite side of substrate  119  is then patterned to form an opening  122   b , and in  FIG. 33 , substrate  119  is etched through to form the cavity  122 . A preferred method to etch substrate  119  is a chemical solution of water and potassium hydroxide (KOH), which has the advantage of etching the substrate but not the preferred bulk layer or sacrificial layers. Therefore, the etching through substrate  119  has a natural termination and does not have to be closely monitored or controlled. A second preferred method to etch substrate  119  is anisotropic reactive ion etching. Finally, in  FIG. 34 , sacrificial layers  123 ,  124  and  125  are removed by wet chemical etching to realize the complete microphone structure  100 . A preferred wet etchant for phosphosilicate glass (PSG) sacrificial layers is hydrofluoric acid (HF). A preferred wet etchant for silicon germanium alloy is hydrogen peroxide. 
   Before the removal of sacrificial layers  23 ,  24  and  25  in microphone structure  10  or sacrificial layers  123 ,  124  and  125  in directional microphone structure  100 , a metal may be deposited and patterned to form electrical connection pads  26 , as shown in  FIG. 35 . A preferred metal for connection pads  26  is gold. In addition, as illustrated in  FIG. 36 , a solder ball  27  may be formed on each metal connection pad  26  to facilitate the use of flip-chip bonding methods for the assembly of the microphone substrate into a package. Preferred methods for the formation of solder balls include ball bonding, ball printing, and ball plating. Flip-chip bonding is especially useful to realize a flat rugged package for the microphone. 
   A particularly useful assembly  200  for mounting microphone structure  10 , and  300  for mounting directional microphone structure  100 , on a package carrier substrate  30  is shown in  FIG. 37   a - b . Electrical interconnection is realized using a conductive layer  31  deposited on carrier substrate  30 . An under filling material, or sealant,  32  provides encapsulation of the electrical interconnection between microphone  10  or  100  and carrier substrate  30 , as well as an acoustic seal between the front and back of the microphone structure. Capillary forces that cause the filling of the gap between microphone  10  or  100  and carrier substrate  30  with sealant  32 , are controlled by providing an opening  33  in carrier substrate  30 , which prevents sealant  32  from reaching the movable parts in microphone structure  10  or  100 . The back volume  35  in the pressure type microphone is formed by attaching a hollow cap  34  to carrier substrate  30 . Cap  34  is attached to carrier substrate  30 , such that a hermetic/acoustic seal is achieved, such that the only acoustic leakage path to the back volume  35  is through a small opening made in the diaphragm within the microphone  10  or  100 . This allows tight control of the lower roll-off frequency of the microphone. 
   A number of simple physical relationships can be used to determine the correct dimensions of the microphone to the first order, the most important of which are diaphragm thickness, diaphragm size, and initial air gap between diaphragm and fixed counter electrode. These dimensions are chosen to satisfy important microphone specifications such as resonance frequency, sensitivity, and DC bias voltage. The first mode resonance frequency for a square diaphragm is given by: 
   
     
       
         
           
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                           v 
                           d 
                           2 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ρ 
                       d 
                     
                   
                 
               
               . 
             
           
         
       
     
   
   where h d  is the thickness, α d  is the side length, E d  is Young&#39;s modulus, ν d  is Poisson&#39;s ratio, and ρ d  is the density of the diaphragm. If, for instance, the microphone is designed for a resonance frequency of 20 kHz or 30 kHz, and polycrystalline silicon is being used as diaphragm material, the relationships shown in  FIG. 38  between diaphragm thickness and size are obtained. The initial air gap required between the diaphragm  11  and the fixed counter electrode  21  is determined from the desired operating DC bias voltage and the diaphragm thickness and size relationship described above. The diaphragm  11  is attracted to the fixed counter electrode  21  due to the electrostatic field in the air gap  15 , and an instability exists where the attraction force overcomes the diaphragm restoring force thereby causing a collapse of the structure  10 . Assuming a piston motion of the diaphragm and a stiff fixed counter electrode, the bias voltage at which the collapse occurs is given by:
 
V collapse =K d α d h α   3/2 .
 
   The constant K d  is determined from the relationship between diaphragm thickness and size. The following values apply for a polycrystalline silicon diaphragm: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Resonance frequency 
               K d   
             
             
                 
                 
             
           
           
             
                 
               20 kHz 
               1.51 * 10 12   
             
             
                 
               30 kHz 
               2.78 * 10 12   
             
             
                 
                 
             
           
        
       
     
   
   An empirical rule in condenser microphone design is to use a DC bias voltage, which is approximately 60% of the collapse voltage. Assuming a DC bias voltage of 5V yields the relationship between diaphragm size and initial air gap shown in  FIG. 39 . If the microphone  10  is operated with a buffer amplifier  40 , as shown in  FIG. 40 , any parasitic capacitance between the diaphragm  11  and fixed counter electrode  21  in the microphone  10 , in the connections to the amplifier  40 , and in the amplifier itself must be considered in the design. In  FIG. 40 , the output sensitivity of a microphone  10 , according to the design rules mentioned above, is shown as function of the diaphragm size and parasitic capacitance C p . As can be seen, an optimum size exists for each value of the parasitic capacitance, which is caused by the counteraction of decreasing mechanical diaphragm sensitivity and increasing microphone source capacitance with increasing diaphragm size. The mechanical sensitivity of the diaphragm will decrease with increasing diaphragm size to maintain the relationship shown in,  FIG. 38 . 
   The dynamic behavior is largely determined by the natural first order resonance frequency of the diaphragm  11 , the acoustic streaming resistance of air in the narrow gap  15  between the diaphragm and fixed counter electrode  21 , and any acoustical leakage across the diaphragm  11 . The streaming resistance, and associated damping, in the air gap  15  may be controlled closely by adding a number of openings  20  in the fixed electrode  21 . The number of openings  20  and their location can be tuned to produce an upper corner frequency of the microphone  10  that coincides with the diaphragm resonance frequency to produce a flat frequency response with maximum bandwidth of the microphone. The lower corner frequency is controlled by adding one or more small openings in the diaphragm  11  or the annular indentation  12  to allow a controlled amount of air to bypass the diaphragm. It is thus possible to tightly control the lower corner frequency in a range from at least 300 Hz to less than 1 Hz. 
   When two or more diaphragms  111  are combined with a mechanical coupling beam  140  to form a directional microphone  100 , the coupled mechanical response at each diaphragm must be determined. For the particular preferred embodiment shown in  FIG. 9-10 , a conceptual mechanical diagram (see  FIG. 41 ) can be used for the analysis, in which the two supported diaphragms  51  and  52  are attached to a centrally supported mechanical coupling beam  53 , and where a sound source  50  is located at an angle θ off the principal axis of the microphone. It has been shown that the complex transfer functions for the deflection at the point of attachment to the mechanical coupling beam of each diaphragm can be approximated by: 
   
     
       
         
           
             
               
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                 ( 
                 
                   
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   in which k d  is the flexural stiffness of each diaphragm, k b  is the flexural stiffness of the mechanical coupling beam, c d  is the mechanical damping of each diaphragm, c b  is the mechanical damping of the coupling beam, m is the mass of each diaphragm, and H f1 (ω) and H f2 (ω) are the sound pressure to force transfer functions for each diaphragm, which for harmonic sound pressures are defined by:
 
 H   f1 (ω)= se   iωτ/2   , H   f2 (ω)= se   −iωτ/2 
 
   where s is the area of each diaphragm, and τ is the time delay for the incident sound pressure between the two diaphragms given by:
 
τ= d  sin (θ)/ c 
 
   where d is the separation between each diaphragm/beam attachment point, θ is the angle of incidence of the sound pressure wave as defined in  FIG. 41 , and c is the speed of sound in air (344 m/s). The equations above can be used with the mechanical stiffness and damping information for the diaphragms  111  and connecting beam  140  to maximize the directivity of the microphone  100  at a specific operating frequency. For maximum efficiency for human communication, it is useful to maximize the directivity of microphone  100  at 2.2 kHz, which is the peak of the frequency response of the human ear (as described by the A-weighting function). The frequency and directional response of diaphragm  51  ( FIG. 41 ) for such an optimized design is shown in  FIGS. 42   a  and  42   b . As can be seen, the directional microphone has a cardioid response with a peak sensitivity at θ=90° and a minimum sensitivity at θ=−90°. The separation between incoming signals from these two positions is as high as 36 dB at 2.2 kHz and above 20 dB in the frequency range between 800 Hz and 6 kHz. 
   Although the present invention has been described in terms of particular embodiments and processes, it is not intended that the invention be limited to those disclosed embodiments and processes. Modifications of the embodiments and processes within the spirit of the invention will be apparent to those skilled in the art. The scope of the invention is defined by the claims that follow.