Patent Publication Number: US-2007113664-A1

Title: Capacitive micromachined acoustic transducer

Description:
CROSS REFERENCE  
      This application claims priority from and is a continuation application of a U.S. non-provisional patent application entitled “Capacitive Micromachined Acoustic Transducer” filed on Apr. 13, 2005, having an application Ser. No. 10/907,706. That application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The batch processing of micromachining has led to the emergence of capacitive micromachined transducers. These transducers offer a larger set of parameters for optimization of performance as well as ease of fabrication and electronic integration. The fabrication and operation of micromachined transducers have been described in many publications and patents. For example, U.S. Pat. Nos. 5,619,476, 5,870,351, 5,894,452 and 6,493,288 describe the fabrication of capacitive-type ultrasonic transducers. U.S. Pat. Nos. 5,146,435; 5,452,268, and 6,870,937 also describe micromachined capacitive transducers that are mainly used in the audio range for sound pickups. In most structures, the movable diaphragm of a micromachined transducer is either supported by a substrate or insulative supports such as silicon nitride, silicon oxide and polyamide. The supports engage the edge of membrane, and a voltage is applied between the substrate and a conductive film on the surface of the membrane causes the membrane to vibrate in response to the passing sound waves. In one particular case as described in the U.S. Pat. No. 6,535,460, the diaphragm is suspended to allow it rest freely on the support rings.  
      Many micromachined condenser microphones use a similar membrane structure to that of large measurement microphones and studio recording microphones. One common structure, shown in  FIG. 1 , consists of a conductive membrane  1  suspended over a conductive back-plate  5  that is perforated with acoustic holes  3 . Sound detection is possible when the impinging pressure wave vibrates the membrane  1 , thus changing the capacitance of the transducer  2 . Under normal operation, the change in capacitance of the condenser microphone  2  is detected by measuring the output current  8  under constant-voltage bias. A pressure equalization vent  4  in the back-chamber  7  prevents fluctuations in atmospheric pressure from collapsing the membrane  1  against the back-plate  5 . A precision condenser microphone for measurement or calibration applications is capable of a uniform frequency response due to its relatively large air gap, on the order of 20 μm, behind the membrane. Silicon micromachined microphones, with membrane dimensions of 1-2 mm, require air gaps  6  on the order of a few microns to maintain adequate sensitivity due to the reduced motion that results from a smaller membrane. However, the reduced dimensions of the air gap magnify the effects of squeeze-film damping, introducing frequency-dependent stiffness and loss. This creates undesirable variations in the mechanical response with acoustic frequency. Furthermore, achieving a large dynamic range and a high sensitivity can be conflicting goals, since large sound pressures may cause the membrane to collapse under its voltage bias. This traditional approach suffers from low sensitivity, especially at low frequencies.  
      In order to achieve wide bandwidth and high sensitivity, the development of high-performance diaphragm is of vital importance in the successful realization of condenser microphones. For most very thin diaphragms, however, large residual stress can lead to undesirable effects such as low and irreproducible performances, if the processes cannot accurately be controlled. One technique for acquiring low-stress diaphragms is to use a sandwich structure, in which layers with compressive and tensile stress are combined. Another technique is to use the support structure such as outlined in the U.S. Pat. No. 6,847,090. U.S. Pat. No. 6,535,460 also describes a structure that the membrane is freely suspended to allow it release the mechanical stress. Unfortunately, in this case, the freely suspended membrane will have unstable sensitivity and unwanted lateral movement, resulting in the signal spew and posing the reliability issues.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a micromachined acoustic transducer with micromachined capacitive elements for sensing acoustic waves.  
      It is a further object of the present invention to provide a micromachined acoustic transducer that comprises a perforated plate supported above a substrate.  
      It is another object of the present invention to provide a micromachined acoustic transducer that has shallowly corrugated membrane that is suspended above a substrate.  
      It is a further object of the present invention to provide a micromachined acoustic transducer whose suspended and shallowly corrugated membrane is anchored on the substrate at one or more locations.  
      It is another object of the present invention to provide a micromachined acoustic transducer that has wide bandwidth and high sensitivity, yet its operation is stable and reliable.  
      It is a further object of the present invention to provide a micromachined acoustic transducer that features the mechanism to suppress the unwanted rolling noise at audio band.  
      It is another object of the present invention to provide a micromachined acoustic transducer that has the shallowly corrugated structures that further provide relatively stable sensitivity.  
      The foregoing and other objects of the invention are achieved by a micromachined acoustic transducer including a perforated plate supported above a substrate, a shallowly corrugated membrane that is suspended above the said substrate and the said suspended shallowly corrugated membrane is anchored on the said substrate at one or more locations. Each membrane supports a conductive electrode for movement therewith, whereby each perforated plate forms a capacitor with the conductive electrode. The capacitance of the said capacitor varies with movement of the membrane responsive to the acoustic wave. Conductive lines interconnect said conductive electrodes to provide output signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other objects of the invention will be more clearly understood from the following description when read in conjunction with the accompanying drawings of which:  
       FIG. 1  is a cross-sectional view of a typical traditionally micromachined microphone.  
       FIG. 2  shows a cross-sectional view of a micromachined acoustic transducer along the line A-A′ in  FIG. 4  according to the preferred embodiment of the present invention.  
       FIG. 3  shows a cross-sectional view of a micromachined acoustic transducer along the line A-A′ in  FIG. 4  according to another preferred embodiment of the present invention.  
       FIG. 4  shows a top plan view of a micromachined acoustic transducer according to the preferred embodiment of the present invention.  
       FIG. 5  shows a top plan view of a micromachined acoustic transducer according to another preferred embodiment of the present invention.  
       FIG. 6  shows a top plan view of a shallowly corrugated membrane according to another preferred embodiment of the present invention.  
       FIG. 7  shows an angled cross-sectional view of a micromachined acoustic transducer when in operation along the lines A-A′ and B-B′ in  FIG. 4  according to the preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      We approach the problem of making a good and practical micromachined acoustic transducer from a different perspective. Our stress releasing technique is to form corrugations in the membrane. The corrugated membrane is capable of releasing the built-in stress during the processing, thereby increasing the mechanical sensitivity of the membrane and reducing the irreproducibility. Compared with the conventional flat diaphragm, the shallowly corrugated membrane has an increased sensitivity, especially for a high residual stress level.  
      Referring now to  FIG. 2 , this is a cross-sectional view of a micromachined acoustic transducer along the line A-A′ in  FIG. 4  according to the preferred embodiment of the present invention. A shallowly corrugated membrane  11  is anchored at one end on the substrate  12 , and loose at the other end. The built-in stress in the membrane  11  is released through corrugation  20  on the membrane  11 . The built-in stress in membrane  11  is further release through the loose end  22  on membrane  11 . A perforated plate  13  is supported on the substrate  12  through anchor  21 . Perforation holes  14  are regular distributed on the plate  13  to allow the passage of acoustic wave. An air gap  25  is formed between the perforated plate  13  and membrane  11 . On the back of perforated plate  13 , electrode  15  forms a capacitor with the membrane  11 . When acoustic wave passes through the perforation holes  14 , the membrane  11  will vibrate in response to the acoustic wave, thereby generating changing capacitance in the capacitor formed by perforated plate  13  and membrane  11 . The electrode  15  can also be placed on top of the perforated plate  13 , as shown in  FIG. 3 .  
      Holes  17  and  18  are formed by the photolithography process. The size of these holes and their relative positions are chosen such that they will form a low-pass filter that allows the passage of slowly varying ambient pressure change across the stack of membrane  11 , air gap  25  and perforated plate  13 . But it will stop the leakage of acoustic signal at desired frequency. Holes  16  are also formed in the photolithography process to help release the sacrificial material under membrane end  22 . On perforated plate  13 , there are a series of spacers  19 . They protects the membrane  111  from collapsing into the perforated plate  13  while in operation in which the membrane  11  will be pulled towards the perforated plate  13  when applied with bias voltage across them. Spacers  19  are discontinuous, but they can also be made continuous to form a ring type structure.  
      Referring to  FIGS. 4 and 5 , membrane  11  is anchored on to the substrate  12  at one or more positions. If it is anchored at the positions shown in  FIG. 4 , it will essentially form a turning fork structure. In another preferred embodiment according to this invention, the membrane  11  is anchored to the substrate  12  at many locations, as shown in  FIG. 5 . Contact pads  24  are used to wire the external circuit to the said micromachined acoustic transducer.  
       FIG. 6  shows the top view of the membrane  11 . At the center region of the membrane  11 , there may exist some holes  23 . These holes are defined to form acoustic filter to a certain frequency or a range of frequencies. The size of holes  23  may be uniform, non-uniform, or a spread according to the filtering needs. The length of corrugation  20  also varies depends on the desired sensitivity requirement. When a suitable length of corrugation  20  is achieved, the membrane  11  may be resting on the spacers  19  under bias voltage applied across the capacitor formed by membrane  11  and perforated plate  13 . In this case, the bending rigidity of the membrane  11  may be largely reduced because the active region of the membrane  11  will be those bounded by the spacers  19 . This essentially reduces “equivalent thickness” of the membrane  11  due to corrugation  20 .  
      For a condenser microphone, the measured sensitivity can be expressed as:  
       Sensitivity   ∝         a   2       8   ·   σ   ·   h       ·       V   b       d   -     d     V   b                 
 
      Where a is the radius of membrane  11 , σ is the residual stress in membrane  11 , h the thickness of membrane  11 , d the air gap  25  distance, and d Vb  is the change of air gap  25  distance under bias voltage V b . When the bias voltage V b  increases, the sensitivity of the microphone also increases. In most of the applications, this is not the desired results. And therefore, the alternative is to increase the bending stress in membrane  11  when the bias voltage increases. Referring to  FIG. 7 , which is the cross sectional view of line BB′ in  FIG. 5 . When in operation, the membrane  11  will be pulled to bend towards the perforated plate  13 . Increasing the bias voltage will result in the membrane  11  bending further. Since the membrane  11  is anchored, further bending of membrane  11  from its desired operation state will result in the increase of bending stress. This will then compensate the sensitivity increase due to the rise of bias voltage.  
      The foregoing descriptions of specific embodiments of the present invention are presented for the purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.