Abstract:
A semiconductor microphone including a silicon substrate having a perimeter; an N-well diffused into the substrate at the perimeter; a deformable diaphragm disposed over at least a portion of the silicon substrate and in contact with at least a portion of the perimeter; and a signal channel in electrical communication with the diaphragm. The signal channel includes a microphone output channel and a feedback output channel. The diaphragm produces an electric signal on the signal channel in response to deformation of the diaphragm and a portion of the electric signal is transmitted on the feedback output channel to the N-well.

Description:
BACKGROUND 
     The present invention relates to circuits for reduction of parasitic capacitance, in particular for use in a CMOS-MEMS microphone. 
     Parasitic capacitances in any electrical system tend to hurt the performance of the system. In the case of microelectromechanical systems (MEMS) based microphones, parasitic capacitances on the MEMS elements/structures results in loss of signal as sound/air pressure is converted to electrical signals which is referred to as loss of sensitivity. While some solutions to this problem have been proposed, these are unsatisfactory because they involve producing additional layers on the device and/or result in additional power consumption. 
     SUMMARY 
     In one embodiment, the invention provides a semiconductor microphone including a silicon substrate having a perimeter; an N-well diffused into the substrate at the perimeter; a deformable diaphragm disposed over at least a portion of the silicon substrate and in contact with at least a portion of the perimeter; and a signal channel in electrical communication with the diaphragm. The signal channel includes a microphone output channel and a feedback output channel. The diaphragm produces an electric signal on the signal channel in response to deformation of the diaphragm and a portion of the electric signal is transmitted on the feedback output channel to the N-well. 
     In another embodiment the invention provides a method of reducing parasitic capacitance in a semiconductor microphone. The method includes steps of providing a silicon substrate having a perimeter; diffusing an N-well into the substrate at the perimeter; disposing a deformable diaphragm over at least a portion of the silicon substrate and in contact with at least a portion of the perimeter; and providing a signal channel in electrical communication with the diaphragm. The signal channel includes a microphone output channel and a feedback output channel. The method further includes the steps of producing an electric signal on the signal channel in response to deformation of the diaphragm and transmitting a portion of the electric signal on the feedback output channel to the N-well to reduce parasitic capacitance. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a diagram of a CMOS-MEMS condenser microphone. 
         FIG. 2  a top cross-sectional view of a CMOS-MEMS condenser microphone through the line  2 - 2 ′ in  FIG. 1 . 
         FIG. 3A  shows an example of a circuit without a feedback branch that could be used with a MEMS device such as a microphone. 
         FIG. 3B  shows an example of a circuit including a feedback branch that could be used with a MEMS device such as a microphone. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1  shows an embodiment of a micro-electromechanical systems (MEMS) based device such as a microphone  10  which includes a movable or deformable diaphragm  20  attached to a back plate  30 , where the back plate  30  is generally stationary, although in some embodiments the back plate  30  is also movable. The diaphragm  20  has a relatively constant amount of charge on it such that movements of the diaphragm  20  relative to the back plate  30  caused by sound waves change the shape and hence charge density and capacitance of the diaphragm  20 . Changes in capacitance are indicative of sensed sounds. When sounds are impinging on the diaphragm  20  of the microphone  10 , the moving diaphragm  20  creates a variable capacitance (C 0 ). The variable capacitance is converted into an analog voltage signal which may in turn be amplified by an on-chip output amplifier and converted into a digital output signal. 
     Nevertheless, not all portions of the diaphragm  20  are freely moveable. The edge regions  22  of the diaphragm  20  are held stationary relative to the device  10  while the more central portions  24  are flexible and move in response to impinging sound waves. The non-moving edge regions  22  of the diaphragm  20  contribute parasitic capacitance (C P ) that cause signal loss from the microphone  10 . 
     To reduce or eliminate the parasitic capacitance C P  caused by the non-moving portion of the diaphragm  20 , embodiments of the disclosed system and method provide for creating a circular N-well  32  around the edges of the back plate  30 , for example by diffusion during the fabrication process. The N-well  32  may be made shallow or deep, depending in part on the structure of the MEMS device and the manufacturing process flow of the device. While the examples herein depict a circular microphone  10  with a correspondingly circular diaphragm  20  and back plate  30 , other shapes are possible, including oval, hemispherical, square, octagonal, and other curved or polygonal shapes. In any event, the amount of parasitic capacitance C P  that can be driven is controlled by extending or shrinking the N-well region underneath the MEMS. 
     During operation, a voltage is applied to the N-well  32  to balance the charge on the opposite side of the diaphragm  20 . In particular, parasitic capacitance C P  is driven by feeding back a portion of the output signal from the diaphragm using a signal channel  40  ( FIG. 1 ), the signal channel  40  including a microphone output channel  42  and a feedback output channel  44 . Since the N-well  32  is aligned with the stationary edge region  22  of the diaphragm  20  ( FIG. 2 ), driving the N-well reduces or eliminates the effects of the parasitic capacitance C P . Signal channel  40  feeds back a portion of the output signal to the N-well  32 . Signal channel  40  can be constructed using known methods.  FIG. 3A  shows an example of a circuit without a feedback branch that could be used with a MEMS device such as a microphone.  FIG. 3B  shows a similar circuit with the addition of a feedback branch in which the output of the buffer feeds back to the N-well to drive the capacitance in that portion of the device. In this case, if the buffer has a gain=1, then the top and bottom plate of the parasitic capacitance C P  move together, hence the effect of C P  as a load is canceled out. In various constructions, the gain of the buffer is greater than or less than 1 depending on the degree of parasitic capacitance in the system and other factors. The buffer is selected to have more than sufficient power to drive the extra C Nwell  capacitance. When the gain is greater than 1, this may lead to a net loss of signal at the output  42 , which might used for attenuation control. 
     In one particular embodiment, the microphone  10  may be constructed using CMOS-based MEMS technology ( FIG. 1 ). In case of CMOS-MEMS microphone  10 , the moving plate or diaphragm  20  may be a circular metal membrane (made from, e.g., the ‘METAL1’ layer) anchored along the perimeter  34  the base plate  30 , where the base plate  30  may be a silicon substrate. A CMOS layer may be integrated into the base plate  30  or provided above the base plate  30 . The diaphragm  20  may be anchored at one or more discrete locations along the perimeter  34  or it may be anchored continuously around the perimeter  34 . As a result of the anchoring, there is little or no motion of the diaphragm  20  at the edge regions  22  while the more central portions  24  are flexible and are permitted to move. 
     In this particular embodiment, the METAL1-based membrane or diaphragm  20  acts as one plate of a capacitor while the silicon substrate/base plate  30  acts as the other plate. As discussed above, motion of the diaphragm  20  creates a change in capacitance. This charge is then translated to an electrical signal based on the conversion of charge (Q=CV). 
     Given that the membrane or diaphragm  20  acts as a capacitor that has a fixed charge of Q, then if the membrane moves (e.g. due to a change in air pressure as a result of impinging sound waves) the capacitance changes and hence voltage changes. This change in voltage creates the electrical output signal indicative of the applied acoustic pressure. 
     The change in voltage from membrane motion can be expressed by the formula: 
     
       
         
           
             
               
                 ( 
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     c 
                   
                   
                     
                       C 
                       0 
                     
                     + 
                     
                       C 
                       p 
                     
                   
                 
                 ) 
               
               ⁢ 
               
                 V 
                 bias 
               
             
             = 
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               v 
             
           
         
       
     
     where Δc=change in capacitance due to membrane motion; 
     C 0 =initial capacitance before membrane motion; and 
     C P =parasitic capacitance due to membrane edges and all other connections on that node. 
     As discussed above, although the membrane or diaphragm  20  may be a single, unitary structure it nevertheless includes moving and non-moving portions. The edge regions  22  of the diaphragm  20  are non-moving parts due to anchors that secure the diaphragm  20  to the base plate  30 , or substrate. These non-moving portions act as a load, reducing the net change in voltage. The non-moving parts can be modeled as C P  in the formula above. From the formula, it can be seen that C P  reduces the output signal level or sensitivity. 
     Accordingly, the parasitic capacitance can be reduced or eliminated by driving C P  with a portion of the output signal to compensate for the charge on the relatively stationary edge regions  22  of the diaphragm  20 . In the formula above, it can be seen that if the parasitic capacitance C P  changes with Δc, this effectively cancels out the C P  term from the formula because the parasitic capacitor (i.e. the charge on the edge region  22 ) is neither gaining nor losing charge. This in turn results in a sensitivity boost at the output. One solution to this problem has been to introduce another conductive layer which is above the silicon substrate/base plate and below the METAL1 layer, for example a polysilicon layer, however this results in degradation of the output signal. 
     Accordingly, using an N-well structure created beneath the silicon substrate by a diffusion process during fabrication of the device, parasitic capacitance (including MEMS and circuit parasitic capacitances) can be driven without any signal-to-noise penalty and can boost the signal at the output. 
     Thus, the invention provides, among other things, an apparatus and method for driving parasitic capacitances using diffusion regions under a MEMS structure. Various features and advantages of the invention are set forth in the following claims.