Patent Publication Number: US-2019191245-A1

Title: Apparatus and method to bias mems motors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of U.S. patent application Ser. No. 15/421,278 filed Jan. 31, 2017, which claims priority to U.S. Provisional Application No. 62/289,611 filed Feb. 1, 2016, the entire contents each of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This application relates to micro electro mechanical system (MEMS) devices and, more specifically, to electrically biasing these devices. 
     BACKGROUND 
     Different types of acoustic devices have been used through the years. One type of device is a microphone. In a microelectromechanical system (MEMS) microphone, a MEMS die includes at least one diagram and at least one back plate. The MEMS die is supported by a substrate and enclosed by a housing (e.g., a cup or cover with walls). A port may extend through the substrate (for a bottom port device) or through the top of the housing (for a top port device). In any case, sound energy traverses through the port, moves the diaphragm and creates a changing potential of the back plate, which creates an electrical signal. Microphones are deployed in various types of devices such as personal computers or cellular phones. 
     Microphone performance variation can occur due to wide process ranges or sensitivity to process parameters. Additionally, variations in operating environment can translate into different microphone performance requirements depending upon the amplitude and the frequency of the sound present. In previous approaches, there is little done to shape the response of the microphone and thereby address these situations. 
     The problems of previous approaches have resulted in some user dissatisfaction with these previous approaches. 
     SUMMARY 
     One aspect of the disclosure relates to a microphone comprising a micro electro mechanical system (MEMS) motor. The MEMS motor includes a diaphragm, a first back plate, and a second back plate. The diaphragm is formed with a tension caused by a film stress of the diaphragm. The diaphragm is electrically biased according to a voltage to adjust or compensate for the film stress. 
     Another aspect of the disclosure relates to a method. The method includes providing a microphone comprising a back plate and a diaphragm. The diaphragm is formed with a tension caused by a film stress of the diaphragm. The method further includes applying a bias voltage to the diaphragm to adjust or compensate for the film stress. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the following drawings and the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings. 
         FIG. 1  is a side cut-away view of a microphone according to various embodiments. 
         FIG. 2  is a perspective view of a micro electro mechanical system (MEMS) device according to various embodiments. 
         FIG. 3  is a cross-sectional view of the MEMS device of  FIG. 2  according to various embodiments. 
         FIG. 4A  is a block diagram showing four MEMS motors biased in one arrangement according to various embodiments. 
         FIG. 4B  is a block diagram showing four MEMS motors biased in another arrangement according to various embodiments. 
         FIG. 5  is a graph showing sensitivity versus frequency and some of the advantages according to various embodiments. 
         FIG. 6A  is a diagram showing how to adjust the corner frequency of the sensitivity response according to various embodiments. 
         FIG. 6B  is a diagram showing another example how to adjust the corner frequency of the sensitivity response according to various embodiments. 
         FIG. 7  is a side cut-away view of another example of a MEMS device according to various embodiments. 
     
    
    
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure. 
     DETAILED DESCRIPTION 
     The present approaches provide for application of different bias voltages for components (e.g., diaphragms) of micro electro mechanical system (MEMS) motors in microphones. The amount of bias (applied voltage to the diaphragm) controls the amount of acoustic signal that can be received and the amount of deflection of the diaphragms. Advantageously, the peak resonance response in the sensitivity response curve of the microphone is reduced. This lowers the total harmonic distortion (THD) and improves the performance of the microphone. 
     Referring now to  FIG. 1 , one example of a microphone  100  is described. The microphone  100  includes a MEMS device  102 , a base  104  (e.g., a printed circuit board), an integrated circuit  106  (e.g., an application specific integrated circuit (ASIC)), a cover  108 , and a port  110  that extends through the base  104 . Although the port  110  extends through the base in this example (making this a bottom port device), it will be appreciated that the port  110  can extend through the cover (making the device a top port device). 
     The MEMS device  102  includes a diaphragm and a back plate. As sound pressure moves the diaphragm, a varying electrical potential with the back plate creates an electrical signal, which is sent to the integrated circuit  106  via wires  112 . The integrated circuit  106  can perform further processing (e.g., noise removal) on the signal. The processed signal can then be sent from the integrated circuit  106  to the base  104 . Pads (not shown) on the base  104  may be coupled to external electronic devices residing in the device where the microphone  100  is disposed. The microphone  100  may be disposed in a variety of different electronic devices such as cellular phones, lap tops, personal computers, tablets, and personal digital assistants to mention a few examples. Other examples are possible. 
     The MEMS device  102  includes multiple MEMS motors. In one aspect, each MEMS motor includes a diaphragm and a back plate. In one example, two MEMS motors may be present. In another examples, four MEMS motors may be present. Other examples are possible. 
     As described herein, the voltage bias applied to each of the diaphragms of the MEMS motors of the MEMS device  102  is different. Advantageously, the peak resonance response in the sensitivity response curve of the microphone  100  is thereby reduced. This lowers the total harmonic distortion (THD) and improves the performance of the microphone  100 . Voltage may be applied to each of the back plates, but this voltage may be the same for each of the MEMS motors. 
     Referring now to  FIG. 2  and  FIG. 3 , one example of biasing multiple MEMS motors is described. 
     A first MEMS motor  202  includes a first diaphragm  204  and a first back plate  206 . A second MEMS motor  222  includes a second diaphragm  224  and a second back plate  226 . The first diaphragm  204 , first back plate  206 , second diaphragm  224 , and second back plate  226  couple to a MEMS substrate or base  212  that has a back hole  214 . 
     A back plate bias voltage  230  is applied to back plates  206 ,  226  via a conductive pad  232  that couples to a conductive element (e.g., trace or wire)  234 . The back plate bias voltage  230  is the same for each back plate  206  and  226 . In one example, the back plate bias voltage is 0 volts. Other examples are possible. In one aspect, the back plate is connected to 0 VDC potential and is what is sensed, while the diaphragms  204  and  224  would have biases V 1  and V 2  separately. As used herein, a “sensed” electrode refers to an electrode from which the electric signal is received. In other configurations, the diaphragms  204  and  224  are connected to 0 VDC potential and two different biases V 1  and V 2  are applied on the back plates  206  and  226  separately. Both back plate and diaphragm wouldn&#39;t be biased by non-zero voltages at the same time. In some embodiments, the back plates  206  and  226  could be shorted together as shown in  FIG. 2  creating one connection (or input) to an amplifier or could connect directly for instance to either a summing or differential amplifier as separate inputs. 
     In some embodiments, a first diaphragm bias voltage  240  is applied to the first diaphragm  204  via a first diaphragm connector  242  and first diaphragm conductive element (e.g., trace or wire)  244 . A second diaphragm bias voltage  250  is applied to the second diaphragm  224  via a second diaphragm connector  252  and second diaphragm conductive element (e.g., trace or wire)  254 . The first diaphragm bias voltage  240  and the second diaphragm bias voltage  250  are different. For example, the first diaphragm bias voltage  240  may be 10 volts and the second diaphragm bias voltage  250  may be 15 volts. Other examples are possible. It will be appreciated that the examples shown here are single motor configurations, they would also apply to multi-motor and/or stacked configurations. 
     The voltages  230 ,  240 , and  250  that are used for biasing may be fixed or dynamically changed. In some embodiments, only the voltages on the non-sensed electrodes would be changed. For example, the voltages  240  and  250  may be dynamic and be changed. The voltages may be changed to adjust the corner frequency of the operation of the microphone. 
     Referring now to  FIG. 4A  and  FIG. 4B , another example of biasing multiple MEMS motors is described. A first MEMS motor  402  includes a first diaphragm  404  and a first back plate  406 . A second MEMS motor  422  includes a second diaphragm  424  and a second back plate  426 . A third MEMS motor  432  includes a third diaphragm  434  and a third back plate  436 . A fourth MEMS motor  442  includes a fourth diaphragm  444  and a fourth back plate  446 . 
     In the examples of  FIGS. 4A and 4B , the back plates  406 ,  426 ,  436 , and  446  are biased with the same voltage (e.g., 0 volts). This voltage is different from any of the biases applied to any of the diaphragms  404 ,  424 ,  434 , and  444 . 
     In the example of  FIG. 4A , the first diaphragm  404  is based at 1•V, the second diaphragm  424  is biased at ½•V, the third diaphragm  434  biased at 1•V, and the fourth diaphragm  444  biased at ½•V. Thus, motor pairs  402 ,  422  are biased at the same voltage as motor pair  432 ,  442 . 
     It will be appreciated that the bias voltages given in  FIG. 4A  and  FIG. 4B  are examples only and that other examples are possible. 
     In the example of  FIG. 4A , the first diaphragm  404  is based at 1•V, the second diaphragm  424  is biased at ½•V, the third diaphragm  434  biased at ¼•V, and the fourth diaphragm  444  biased at ⅛•V. Thus, motor pairs  402 ,  422 ,  432 , and  442  are all biased at different voltages. 
     In some embodiments, the example of  FIG. 4B  misaligns all of the diaphragm resonances since all of the voltages are different, but it would also be less sensitive. The example of  FIG. 4A  is more sensitive, but some of the resonances would align. 
     Referring now to  FIG. 5 , one example of a graph showing some of the advantages of the present approaches is described. This shows results with a first MEMS motor (that includes a first diaphragm and a first back plate) and a second MEMS motor (that includes a second diaphragm and a second back plate). 
     A first curve  502  shows sensitivity (measured in dB) versus frequency (measured in Hz) when both diaphragms are biased at the same potential. It can be seen that there is a large peak  503 . This large peak  503  is not good or desirable for performance because it can overload the microphone circuit or other electronics downstream. 
     A second curve  504  shows sensitivity (measured in dB) versus frequency (measured in Hz) when the diaphragms are biased at different potentials. In one aspect, the first diaphragm may be biased at 10 volts and the second diaphragm may be biased at 20 volts. The peak is split in two. This is advantageous because the energy of the transducer is not focused in a narrow region, which prevents overload. 
     It can be seen that sensitivity can be controlled in regions  506  and  508  of the sensitivity curve  504 . The exact amount of sensitivity provided may in part depend upon the amount of bias applied to each of the diaphragms and the difference between the biases applied. As can be seen, if region  508  is a region of ultrasonic sensitivity, the sensitivity in that region is reduced by application of the present approaches. 
     It will also be appreciated that the present approaches can be used to vary the corner frequency (fc) of curve  504 . The corner frequency fc is the frequency where a 3 db drop occurs from the constant portion  507  of the curve  504 . The corner frequency fc may be varied during manufacturing to bring it into compliance with a product specification. The corner frequency fc may also be varied in the field after manufacturing when wind noise is an overloading input to prevent clipping and distortion. The corner frequency may be also varied in the field after manufacturing to move it down for customer algorithms that require a constant phase and/or high signal-to-noise ratios at low frequencies. 
     When a vent hole (also known as a pierce hole) is used, the proximity of the hole in the diaphragm to the back plate affects the acoustic resistance of the microphone. Varying the bias affects the diaphragm position and consequently varying the bias varies the corner frequency. 
       FIGS. 6A and 6B  show a MEMS motor  602  with a back plate  604  and a diaphragm  606 . The bias applied to the diaphragm (that has a vent or pierce hole  612 ) is variable and adjustable. The corner frequency (fc) is given by 
     
       
         
           
             
               
                 f 
                 c 
               
               = 
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   
                     R 
                     pierce 
                   
                    
                   
                     C 
                     BV 
                   
                 
               
             
             , 
           
         
       
     
     where R pierce  is the acoustic resistance of the vent or pierce hole and CBV is the acoustic compliance of the back volume. 
     Referring now to  FIG. 6A , a smaller bias (Vbias(1)) (e.g., Vbias(1)=5 volts) makes the diaphragm  606  deflect less and increases the corner frequency cf(1) because a low resistance air path  622  is provided (the diaphragm and back plate are relatively far apart). 
     Referring now to  FIG. 6B , a larger bias (Vbias(2) with Vbias(2)&gt;Vbias(1), e.g., Vbias(2)=20 volts) makes the diaphragm  606  deflect more and decreases the corner frequency cf(2) because a high resistance air path  624  is provided (the diaphragm and back plate are relatively close together). Cf(2) is less than cf(1). 
     Referring now to  FIG. 7 , another example of a MEMS device  700  is described. The MEMS device  700  includes a first back plate  702 , a second back plate  704 , and a diaphragm  706  disposed between the first back plate  702  and the second back plate  704 . A first Vbias  708  is applied between the first back plate  702  and the diaphragm  706 , and a second Vbias  710  is applied between the second back plate  704  and the diaphragm  706 . In one example, the first Vbias  708  and the second Vbias  710  are the same. The diaphragm  706  in one example is a membrane or film that is formed with a film stress. 
     Film stress induces tension on the diaphragm  706 . Increased tension due to the increased film stress results in less deflection of the diaphragm (Δd) for the same sound pressure (ΔP). During manufacturing, the stress can vary substantially. To combat changes in tension due to film stress, the bias can be dynamically changed during or after manufacturing to adjust the sensitivity: 
     Sensitivity is proportional to 
     
       
         
           
             
               
                 
                   
                     V 
                     bias 
                   
                   · 
                   Δ 
                 
                  
                 
                     
                 
                  
                 d 
               
               
                 Δ 
                  
                 
                     
                 
                  
                 
                   P 
                   · 
                   d 
                 
               
             
             , 
           
         
       
     
     where V bias  is the voltage applied to the diaphragm, Δd is the deflection of the diaphragm, ΔP is the change in sound pressure and d is the nominal gap. 
     To take one example, if a change in pressure (ΔP) causes a change in deflection (Δd), then V bias  can be adjusted up or down to maintain the same sensitivity or to maintain a target sensitivity. As mentioned, this adjustment may occur on the fly during or after manufacturing of the microphone. Similar approaches may also be taken to compensate for film stress in microphones with a single back plate and diaphragm. 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). 
     It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). 
     The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.