Abstract:
A monolithic silicon microphone including a first backplate, a second backplate and a diaphragm displaced between said first backplate and said second backplate. Said first backplate is supported by a silicon substrate with one or more perforation holes. Said second substrate is attached to a perforated plate which itself is supported on said substrate. Said monolithic silicon microphone has integrated signal conditioning circuit, and is said diaphragm, said first backplate, said second backplate, and said signal conditioning circuit are electrically interconnected. Signals from said diaphragm, said first backplate, and said second backplate are fed into said signal conditioning circuit, and are amplified differentially.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    U.S. Pat. Nos. 5,146,435; 5,452,268; 5,619,476; 5,870,351; 5,894,452; 6,493,288; 6,535,460; 6,847,090; 6,870,937; 7,166,910; 7,202,101; 7,221,767; 2007/0278601. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    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; 5452,268, 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. 
         [0003]    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  3  suspended over a conductive backplate  2  that is perforated with acoustic holes  4 . The membrane  3  is supported by insulative piers  5  to keep a predetermined distance from the backplate  2 . The backplate  2  itself is supported on a silicon substrate  1 . Sound detection is possible when the impinging pressure wave vibrates the membrane  3 , thus changing the capacitance of the transducer. Under normal operation, the change in capacitance of the condenser microphone die  10  is detected by measuring the output current under constant-voltage bias. Acoustic holes  4  are also used to equalize the pressure in the back chamber  6  to the ambient pressure to prevent fluctuations in atmospheric pressure from collapsing the membrane  3  against the backplate  2 . The micromachined microphones are typically attached to a PCB board  8  to seal the back chamber of  6 . 
         [0004]    In actual applications, the microphone die  10  will need to be packaged into an environmentally protective enclosure such that it can be put into the electronic devices such as cell phones. There are many publications dealing with this type of packaging scheme. For example, U.S. Pat. No. 6,781,231 to Minervini, et al. discloses a microelectromechanical system package having a microelectromechanical system microphone, a substrate, and a cover. The substrate has a surface for supporting the microelectromechanical microphone. The cover includes a conductive layer having a center portion bounded by a peripheral edge portion. A housing is formed by connecting the peripheral edge portion of the cover to the substrate. The center portion of the cover is spaced from the surface of the substrate to accommodate the microelectromechanical system microphone. The housing includes an acoustic port for allowing an acoustic signal to reach the microelectromechanical system microphone. 
         [0005]    U.S. Pat. No. 7,166,910 to Minervini et al. discloses a silicon condenser microphone package. The silicon condenser microphone package comprises a transducer unit, a substrate, and a cover. The substrate includes an upper surface having a recess formed therein. The transducer unit is attached to the upper surface of the substrate and overlaps at least a portion of the recess wherein a back volume of the transducer unit is formed between the transducer unit and the substrate. The cover is placed over the transducer unit and includes an aperture. 
         [0006]    The typical layout of this type of packaging is shown in  FIG. 2 . Where the micromachined microphone die  10  is attached to a PCB board  8 . Also attached to the PCB board  8  is an ASIC die  14 . Wire bond  15  is used to establish the electrical connection between ASIC  14  and microphone die  10 . A mechanical cavity  16  is formed with housing wall  11  and cover  12 . There is an acoustic hole  13  on the housing cover  12  to allow the passage of acoustic signal to the microphone die  10 . Conductive pads  17  are attached to the backside of PCB board  8  such that the packaged microphone as shown in  FIG. 2  can be surface mounted to the main board of an electronic device. 
         [0007]    According to the teachings of U.S. Pat. Nos. 6,781,231 and 7,166,910, housing wall  11  and cover  12  are themselves conductive or have conductive layers in between such that an electromagnetic shielding is formed to protect the microphone die from picking up electromagnetic interferences. The housing wall  11  and cover  12  form a complete grounding circuit with ground electrode in PCB  8 . 
         [0008]    U.S. Pat. No. 7,221,767 to Mullenborn, et al. discloses a surface mountable acoustic transducer system, comprising one or more transducers, a processing circuit electrically connected to the one or more transducers, and contact points arranged on an exterior surface part of the transducer system. The contact points are adapted to establish electrical connections between the transducer system and an external substrate, the contact points further being adapted to facilitate mounting of the transducer system on the external substrate by conventional surface mounting techniques. In this particular acoustic transducer system, as shown in  FIG. 3 , a microphone die  10  is adapted to a silicon carrier substrate  20  through solder seal ring  19 . An ASIC die  14  is adapted to the same silicon carrier substrate  20  by solder bump  18 . A lid  12  covers both microphone die  10  and ASIC die  14 . One or multiple acoustic holes  13  is open on the lid  12  to allow the passage of acoustic signal to microphone die  10 . Flip chip bonds  17  are attached at the bottom of carrier silicon substrate  17  such that the packaged acoustic transducer system is surface mountable to the main board of an electronic device. 
         [0009]    The above publications teach what is referred to as a “two-chip” solution to make a completely packaged silicon microphone. As we can see from these publications, this solution requires both a micromachined microphone die and an ASIC die that is used for conditioning the signal from the microphone die. Both microphone die and ASIC die are packaged into a mechanical housing to protect them from environment, and for final operation. 
         [0010]    There also examples of an integrated solution, where the microphone die and ASIC die are combined into one signal micromachined die. U.S. Pat. No. 7,202,101 to Gabriel et al. discloses a structure comprised of alternating layers of metal and sacrificial material built up using standard CMOS processing techniques, a process for building such a structure, a process for fabricating devices from such a structure, and the devices fabricated from such a structure. In one embodiment, a first metal layer is carried by a substrate. A first sacrificial layer is carried by the first metal layer. A second metal layer is carried by the sacrificial layer. The second metal layer has a portion forming a micro-machined metal mesh. When the portion of the first sacrificial layer in the area of the micro-machined metal mesh is removed, the micro-machined metal mesh is released and suspended above the first metal layer a height determined by the thickness of the first sacrificial layer. The structure may be varied by providing a base layer of sacrificial material between the surface of the substrate and the first metal layer. In that manner, a portion of the first metal layer may form a micro-machined mesh which is released when a portion of the base sacrificial layer in the area of the micro-machined mesh is removed. Additionally, a second layer of sacrificial material and a third metal layer may be provided. A micro-machined mesh may be formed in a portion of the third metal layer. The structure may be used to construct variable capacitors, switches and, when certain of the meshes are sealed, microspeakers and microphones. 
         [0011]    Although this teaching successfully combines the microphone die and the ASIC die, a packaging scheme similar to that shown in  FIG. 2  is required to make the final microphone unit that can be surface mounted for the end electronic device. As described in US publication No. 2007/0278601, the MEMS device includes a chip carrier having an acoustic port extending from a first surface to a second surface of the chip carrier, a MEMS die disposed on the chip carrier to cover the acoustic port at the first surface of the chip carrier, and an enclosure bonded to the chip carrier and encapsulating the MEMS die. 
         [0012]    In all above mentioned publications, a complicated die-level packaging scheme is required. This packaging scheme involves the need to create an electrically connected enclosure to serve the purposes of environment protection and the shielding of electromagnetic interferences. This type of packaging scheme is not only time consuming, it also involves expensive equipments for performing post processing of silicon wafers. The need of said electrically connected enclosure also limits the size of microphone, making it difficult to be displaced anywhere in the end device system. 
       SUMMARY OF THE INVENTION 
       [0013]    It is an object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element for sensing acoustic waves. 
         [0014]    It is a further object of the present invention to provide a monolithic silicon microphone with integrated electronics to condition the sensed acoustic waves by said integrated micromachined capacitive sensing element. 
         [0015]    It is another object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element and conditioning electronics that is immune to the environmental factors. 
         [0016]    It is a further object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element and conditioning electronics that is immune to electromagnetic interferences. 
         [0017]    It is another object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element and conditioning electronics that has a movable diaphragm whereas the diaphragm vibrates in response to the impinging acoustic pressure. 
         [0018]    It is a further object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element and conditioning electronics that has two backplates. 
         [0019]    It is another object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element and conditioning electronics whereas said diaphragm is displaced between said two backplates. Said diaphragm is supported above one of said backplates. 
         [0020]    It is a further object of the present invention to provide a monolithic silicon microphone with integrated micromachined capacitive sensing element and conditioning electronics whereas said conditioning electronics processes differential inputs from said micromachined capacitive sensing element. 
         [0021]    The foregoing and other objects of the invention are achieved by a monolithic silicon microphone including a diaphragm displaced between two opposing backplates. A first backplate is supported by the silicon substrate, and a second backplate is suspended above said diaphragm. The suspension for said second backplate also forms an enclosure for said micromachined silicon sensing elements. Said diaphragm is supported by said first backplate. Said first and second backplates have perforation holes allowing the passage of acoustic pressure wave. Said diaphragm forms a first capacitor with said first backplate, and said diaphragm forms a second capacitor with said second backplate. The capacitances of said first and said second capacitors vary with the movement of said diaphragm responsive to the acoustic wave. Said monolithic silicon microphone has integrated signal conditioning electronics. Whereas the capacitance changes from said first and second capacitors are fed into the differential inputs of said signal conditioning electronics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    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: 
           [0023]      FIG. 1  is a cross-sectional view of a prior art micromachined silicon microphone. 
           [0024]      FIG. 2  shows a cross-sectional view of a prior art packaged micromachined silicon microphone. 
           [0025]      FIG. 3  shows a cross-sectional view of another prior art packaged micromachined silicon microphone. 
           [0026]      FIG. 4  shows a schematic drawing of a silicon microphone. 
           [0027]      FIG. 5  shows a schematic drawing of a dual backplate silicon microphone. 
           [0028]      FIG. 6  shows a cross sectional view of monolithic silicon microphone according to the first preferred embodiment of present invention. 
           [0029]      FIG. 7  shows a cross sectional view of monolithic silicon microphone according to the second preferred embodiment of present invention. 
           [0030]      FIG. 8  shows a cross sectional view of monolithic silicon microphone according to the third preferred embodiment of present invention. 
           [0031]      FIG. 9  shows a cross sectional view of monolithic silicon microphone according to the fourth preferred embodiment of present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Cellular telephones typically have a microphone and associated circuitry to convert sound waves into an electronic signal for transmission to another telephone. The circuitry modulates a high frequency radio-frequency (“RF”) carrier signal (e.g., 1 to 2 GHz) with the microphone signal and transmits this modulated carrier signal via an antenna on the telephone. This modulated RF carrier signal is received by a base station (“a cell”) and forwarded to another telephone. 
         [0033]    A cellular telephone typically comprises many physical components packed into a small physical space. Consequently, electromagnetic energy may escape from some of these components and couple into other cellular telephone components, thereby causing noise interference. (Of particular concern is the energy emitted from the telephone&#39;s antenna.) Pickup of noise signals at audio frequencies is particularly troublesome because these noise signals can interfere with the operation of the loudspeaker or microphone. This audio interference can adversely affect the operation of the cellular telephone. A particular problem is the audio interference signal that may be induced by time division interleaving of transmitter signals with receiver signals in the telephone. Such interleaving can be performed by the receiver de-interleave circuit and in the transmitter interleave circuit. For example, transmitter and receiver RF carrier signal interleaving is performed at a 217 Hz rate in a Time Division Multiple Access (“TDMA”) transmitter/receiver of a Global System for Mobile Communications (“GSM”) mobile telephone. Non-linear circuit elements in a cellular telephone can convert the turn-on and turn-off of the telephone&#39;s RF carrier for transmission at the 217 Hz rate into an audio interference signal at 217 Hz. Audio signal noise at this frequency resembles the sound of a bumblebee and is thus known as “bumblebee noise.” Such bumblebee noise can impact the ability of a cellular telephone to function as a voice communication device. 
         [0034]    The bumblebee noise is transmitted through the electromagnetic coupling to the receiving microphone. In operation, a microphone resembles a variable capacitor with antenna. Refer to  FIG. 4  now. This is a schematic drawing of a simplified silicon capacitive microphone. The microphone has a backplate  32  and a diaphragm  33 . In operation, a bias voltage is applied to the microphone. Assuming the diaphragm  33  is connected to the positive lead of bias, and backplate  32  is connected to the negative lead of bias, as shown in  FIG. 4 . When acoustic pressure wave is impinging on the microphone, the diaphragm  33  will deflect up and down in response to the pressure wave, thus changing the capacitance of the capacitor. At the same time, this microphone structure also acts as an antenna to pick the electromagnetic coupling. The antenna length depends on the physical structure of the microphone, e.g., the physical size of diaphragm  33  and backplate  32 . When the diaphragm  33  deflects up and down, its physical size changes very little. And therefore, the electromagnetic coupling to the microphone is considered as a constant number. 
         [0035]    We now refer to  FIG. 5 . This is a schematic of a microphone with two backplates. The diaphragm  33  is sandwiched between a first backplate  32  and a second backplate  34 . A capacitor C 1  is formed by the diaphragm  33  and the first backplate  32 . Similarly, a second capacitor C 2  is formed by the diaphragm  33  and the second backplate  34 . When the acoustic pressure wave impinges on the diaphragm  33 , it deflects up and down. For the purpose of analysis, we assume the diaphragm deflects down, thus the capacitance C 1  increases by an amount q and the capacitance C 2  decreases by an amount q. The coupled electromagnetic signal, however, remains pretty much the same on both C 1  and C 2 . When C 1  and C 2  signals are fed into the signal conditioning circuit as differential inputs, the electromagnetic portion of the C 1  and C 2  will be canceled out as the common mode, while the capacitance change due to acoustic pressure wave will be doubled. 
         [0036]    We now refer to the first embodiment according to the present invention. As shown in  FIG. 6 , the monolithic silicon microphone  50  has silicon substrate  51 . A first backplate  52  is on and supported by said silicon substrate  51 . A diaphragm  53  is suspended on top of said first backplate  52 , and keeps a predetermined separation from said first backplate  52  by using supports  55 . Diaphragm  53  and first backplate  52  forms a cavity  57 . Both substrate  51  and first backplate  52  have perforation holes  54 . 
         [0037]    The substrate  51  also supports spacers  90 , which themselves support a perforated plate  95 . The perforated plate  95  is itself non-conductive, but it has a second backplate  59  on one of its sides. The spacers  90  keep the perforated plate  95  a predetermined separation from the diaphragm  53  such that the separation of diaphragm  53  from the first backplate  52  is similar to the separation of diaphragm  53  from the second backplate  59 . A second cavity  58  is thus formed between the diaphragm  53  and the second backplate  59 . Perforated plate  95  has perforation holes  56  such that acoustic signals can pass through the perforation holes  56  to impinge onto the diaphragm  53 . 
         [0038]    The signal conditioning electronics  80  is located at the other side of silicon substrate  51 . Through wafer via  70  is used to establish electrical connection between the diaphragm  53 , the first backplate  52 , the second backplate  54  and signal conditioning circuit  80 . Solder bumps  60  are attached to the surface of silicon substrate  51  where the signal conditioning circuit  80  is located. 
         [0039]    In a second preferred embodiment according to the present invention as shown in  FIG. 7 , the monolithic silicon microphone  50  has silicon substrate  51 . A first backplate  52  is on and supported by said silicon substrate  51 . A diaphragm  53  is suspended on top of said first backplate  52 , and keeps a predetermined separation from said first backplate  52  by using supports  55 . Diaphragm  53  and first backplate  52  forms a cavity  57 . Both substrate  51  and first backplate  52  have perforation holes  54 . 
         [0040]    The substrate  51  also supports spacers  90 , which themselves support a perforated plate  95 . The perforated plate  95  is itself non-conductive, but it has a second backplate  59  on one of its sides. The spacers  90  keep the perforated plate  95  a predetermined separation from the diaphragm  53  such that the separation of diaphragm  53  from the first backplate  52  is similar to the separation of diaphragm  53  from the second backplate  59 . A second cavity  58  is thus formed between the diaphragm  53  and the second backplate  59 . Perforated plate  95  has perforation holes  56  such that acoustic signals can pass through the perforation holes  56  to impinge onto the diaphragm  53 . 
         [0041]    The signal conditioning electronics  80  is located at the other side of silicon substrate  51 . Through wafer via  70  is used to establish electrical connection between the diaphragm  53 , the first backplate  52 , the second backplate  54  and signal conditioning circuit  80 . Solder bumps  60  are attached to the perforated plate  95  for mounting the monolithic silicon microphone  50  according to the second preferred embodiment of the present invention. 
         [0042]    In the third preferred embodiment according to the present invention, as shown in  FIG. 8 , the monolithic silicon microphone  50  has silicon substrate  51 . A first backplate  52  is on and supported by said silicon substrate  51 . A diaphragm  53  is suspended on top of said first backplate  52 , and keeps a predetermined separation from said first backplate  52  by using supports  55 . Diaphragm  53  and first backplate  52  forms a cavity  57 . Both substrate  51  and first backplate  52  have perforation holes  54 . 
         [0043]    The substrate  51  also supports spacers  90 , which themselves support a perforated plate  95 . The perforated plate  95  is itself non-conductive, but it has a second backplate  59  on one of its sides. The spacers  90  keep the perforated plate  95  a predetermined separation from the diaphragm  53  such that the separation of diaphragm  53  from the first backplate  52  is similar to the separation of diaphragm  53  from the second backplate  59 . A second cavity  58  is thus formed between the diaphragm  53  and the second backplate  59 . Perforated plate  95  has perforation holes  56  such that acoustic signals can pass through the perforation holes  56  to impinge onto the diaphragm  53 . 
         [0044]    The signal conditioning electronics  80  is located at the same side of silicon substrate  51  where the silicon sensing elements are. Solder bumps  60  are attached to the other surface of silicon substrate  51  for the mounting of monolithic silicon microphone  50  according to the third preferred embodiment of the present invention. Through wafer via  70  is used to establish electrical connection between the solder bumps  60  and signal conditioning circuit  80 . 
         [0045]    In the fourth preferred embodiment according to the present invention, as shown in  FIG. 9 , the monolithic silicon microphone  50  has silicon substrate  51 . A first backplate  52  is on and supported by said silicon substrate  51 . A diaphragm  53  is suspended on top of said first backplate  52 , and keeps a predetermined separation from said first backplate  52  by using supports  55 . Diaphragm  53  and first backplate  52  forms a cavity  57 . Both substrate  51  and first backplate  52  have perforation holes  54 . 
         [0046]    The substrate  51  also supports spacers  90 , which themselves support a perforated plate  95 . The perforated plate  95  is itself non-conductive, but it has a second backplate  59  on one of its sides. The spacers  90  keep the perforated plate  95  a predetermined separation from the diaphragm  53  such that the separation of diaphragm  53  from the first backplate  52  is similar to the separation of diaphragm  53  from the second backplate  59 . A second cavity  58  is thus formed between the diaphragm  53  and the second backplate  59 . Perforated plate  95  has perforation holes  56  such that acoustic signals can pass through the perforation holes  56  to impinge onto the diaphragm  53 . 
         [0047]    The signal conditioning electronics  80  is located at the same side of silicon substrate  51  where the silicon sensing elements are. Solder bumps  60  are attached to the perforated plate  95  for mounting the monolithic silicon microphone  50  according to the fourth preferred embodiment of the present invention. 
         [0048]    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.