Patent Publication Number: US-2018041842-A1

Title: Mems microphone element and manufacturing method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a national stage application, filed under 35 U.S.C. §371, of International Application No. PCT/CN2015/096915, filed on Dec. 10, 2015, which claims priorities to Chinese Application No. 201510288675.5 filed on May 29, 2015, the content of which is hereby incorporated reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to microphones, in particular to a differential-capacitance type micro electro-mechanical systems (MEMS) microphone element and a manufacturing method thereof. 
     BACKGROUND OF THE INVENTION 
     A MEMS microphone is a electroacoustic transducer manufactured by using a micromechanical machining technology, and it has advantages of small size, good frequency response characteristics, low noise, etc. With the trends of developing miniaturized and thinner electronic devices, the MEMS microphone is more and more widely applied to these devices. 
     A current MEMS microphone product contains a MEMS chip substrate on capacitance detection and an ASIC chip, a capacitance of the MEMS chip will generate corresponding changes along with the difference of input sound signals, and then the ASIC chip is used to process and output a changed capacitance signal, such that sound is pickup. The MEMS chip generally includes a base having a back cavity and a parallel plate capacitor disposed on the base and consisting of back pole plates and vibrating diaphragm. The vibrating diaphragm receives external sound signals and vibrates, such that the parallel plate capacitor generates a changed electrical signal, thereby realizing an acoustic-electrical conversion function. 
     Problems of the technical solution above are that single capacitance detection cannot filter external interference signals, a noise level of output signals is affected, and a signal-noise ratio is reduced. 
     If the MEMS microphone is designed into traditional differential-capacitance detection, and a three-layer film structure is adopted, wherein a upper layer and a lower layer serve as back pole plates, a middle layer serves as a vibrating diaphragm, and the vibrating diaphragm forms a capacitor together with the upper and lower layers of back pole plates respectively, and these two capacitors form the differential capacitor. When sound waves act on the vibrating diaphragm in the middle position, the vibrating diaphragm vibrates up and down, resulting in an increase of one capacitance of the differential capacitors while resulting in a decrease of the other, realizing differential detection on the sound waves is realized. But this solution has problems that the process is relatively complex, and intervals between the upper and lower back pole plates and the vibration are difficult to control. As a result, it is very difficult to make static capacitance and sensitivity of the differential capacitor same, resulting in weakening a differential effect and in deviating from the original purpose. 
     Therefore, there is a demand in the art that a new solution for a differential-capacitance type micro electro-mechanical systems (MEMS) microphone element and a manufacturing method thereof shall be proposed to address at least one of the problems in the prior art. 
     SUMMARY OF THE INVENTION 
     One object of this invention is to provide a differential-capacitance type MEMS microphone element with better performance. 
     According to a first aspect of the present invention, there is provided a MEMS microphone element, comprising a base, the base being provided with a first opening and a second opening which run through from top to bottom; and a first capacitor and a second capacitor disposed on the base in parallel, the first capacitor being disposed on the first opening, and the second capacitor being disposed on the second opening, wherein the first capacitor comprises a first back pole plate located below, and a first vibrating diaphragm located above and opposite to the first back pole plate, the second capacitor comprises a second back pole plate located above, and a second vibrating diaphragm located below and opposite to the second back pole plate; and the first capacitor and the second capacitor form differential capacitors together. 
     Alternatively or optionally, the first vibrating diaphragm and the second back pole plate are made of the same material, and the first back pole plate and the second vibrating diaphragm are made of the same material. 
     Alternatively or optionally, the first vibrating diaphragm and the second vibrating diaphragm are electrically connected together to serve as a shared movable pole plate of the differential capacitors. 
     Alternatively or optionally, sensing parts of the first back pole plate and the second back pole plate are respectively provided with a plurality of through holes, and the first vibrating diaphragm and the second vibrating diaphragm are provided with a through hole at their central positions respectively. 
     Alternatively or optionally, the first back pole plate, the first vibrating diaphragm, the second back pole plate and the second vibrating diaphragm are made of any one of the following materials: polycrystalline silicon, silicon nitride attached with a polycrystalline silicon layer, and silicon nitride attached with a metal layer. 
     Alternatively or optionally, the MEMS microphone element is suitable for two kinds of product structures in which a sound signal enters into the MEMS microphone element from above or below. 
     According to a second aspect of the present invention, there is provided with a method for manufacturing a MEMS microphone element, which comprises the following steps: S1, providing a substrate; S2, growing a first isolating layer on the substrate; S3, growing a first pole plate material layer on the first isolating layer; patterning and etching the first pole plate material layer to form a back pole plate of a first capacitor, a movable pole plate of a second capacitor and a first isolating groove for isolating the back pole plate of the first capacitor from the movable pole plate of the second capacitor; S4, depositing a second isolating layer on the first pole plate material layer; forming a connecting window on the second isolating layer above the movable pole plate of the second capacitor, for connecting a movable pole plate of the first capacitor with the movable pole plate of the second capacitor; S5, growing a second pole plate material layer on the second isolating layer; patterning and etching the second pole plate material layer to form the movable pole plate of the first capacitor, a back pole plate of the second capacitor and an isolating groove for isolating the movable pole plate of the first capacitor from the back pole plate of the second capacitor; S6, etching the substrate and the first isolating layer to form a first opening below the first capacitor and to form a second opening below the second capacitor, in which the first opening and the second opening run through from top to bottom; and etching the second isolating layer to form a clearance between the back pole plate and movable pole plate of the first and the second capacitor, respectively. 
     Alternatively or optionally, in the step S3, a plurality of through holes are formed in the back pole plate of the first capacitor and a through hole is formed in the central position of the movable pole plate of the second capacitor; and in the step S5, a plurality of through holes are formed in the back pole plate of the second capacitor and a through hole is formed in the central position of the movable pole plate of the first capacitor. 
     Alternatively or optionally; a thickness of the back pole plate of the first capacitor is larger than that of the movable pole plate, and a thickness of the back pole plate of the second capacitor is larger than that of the movable pole plate. 
     Alternatively or optionally, the MEMS microphone element is suitable for two kinds of product structures in which a sound signal enters into the MEMS microphone element from above or below. 
     According to the differential-capacitance type MEMS microphone of the present invention, a pair of differential capacitors is designed side by side, differential detection is realized by two layers of films, and the present invention has the following beneficial effects, 
     1. The differential-capacitance type MEMS microphone is realized, and is favorable for filtering outside electromagnetic and noise interferences, improving a signal-noise ratio and a reception quality of output signals. 
     2. Since the clearances between the back pole plates and the movable pole plates of the differential capacitors are finished in one step, intervals of the differential capacitors can be made totally consistent, improving a differential effect. 
     3. The process flow of manufacturing is simple in flow and easy to control. The process flow is totally compatible with the current process for a single-capacitance type MEMS microphone, and the process needs no change. 
     The inventors of the present invention have found that there is still no single-chip differential capacitor-type MEMS microphone of a dual-layer film structure in the prior art, therefore, the present invention is a new technical solution. Further features of the present invention and advantages thereof will become apparent from the following detailed description of exemplary embodiments according to the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description thereof, serve to explain the principles of the invention. 
         FIGS. 1-2  are schematic diagrams of an embodiment of a MEMS microphone of the present invention. 
         FIGS. 3-4  are schematic diagrams of a differential detection principle of the MEMS microphone of the present invention. 
         FIGS. 5-14  are schematic diagrams of respective stages of a manufacturing process for the MEMS microphone of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments of the present invention will now be described in detail with reference to the drawings. It should be noted that the relative arrangement of the components and steps, the numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present invention unless it is specifically stated otherwise. 
     The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. 
     Techniques, methods and apparatus as known by one of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate. 
     In all of the examples illustrated and discussed herein, any specific values should be interpreted to be illustrative only and non-limiting. Thus, other examples of the exemplary embodiments could have different values. 
     Notice that similar reference numerals and letters refer to similar items in the following figures, and thus once an item is defined in one figure, it is possible that it need not be further discussed for following figures. 
     Specific to the problems mentioned above, the present patent provides a novel differential-capacitance type MEMS microphone. A pair of differential capacitors is designed side by side and is realized by two layers of films, reducing the difficulty of process. 
       FIGS. 1-2  show a basic structure of the present invention, the structure comprising a base  1 , a first capacitor C 1  and a second capacitor C 2  disposed on the base  1  in parallel, wherein the base  1  comprises a substrate  100  and a first isolating layer  200  located on the substrate  100 . The base  1  is provided with a first opening  101  and a second opening  102  which run through from top to bottom; and the first capacitor C 1  is disposed on the first opening  101 , and the second capacitor C 2  is disposed on the second opening  102 . The first capacitor C 1  comprises a first back pole plate  12  located below, and a first vibrating diaphragm  11  above and opposite to the first back pole plate  12 ; and the second capacitor C 2  comprises a second back pole plate  22  located above, and a second vibrating diaphragm  21  located below and opposite to the second back pole plate  22 . A second isolating layer  400  is respectively disposed between the first back pole plate  12  and the first vibrating diaphragm  11 , and between the second pole plate  22  and the second vibrating diaphragm  21 , to form a clearance  109  respectively between the first back pole plate  12  and the first vibrating diaphragm  11 , and between the second pole plate  22  and the second vibrating diaphragm  21 . 
     The first back pole plate  12  and the second back pole plate  22  are fixed pole plates, and the first vibrating diaphragm  11  and the second vibrating diaphragm  21  are movable pole plates. 
     The first capacitor C 1  and the second capacitor C 2  form a pair of differential capacitors, the first vibrating diaphragm  11  and the second vibrating diaphragm  21  are electrically connected together to serve as a shared movable pole plate of the differential capacitors, the first back pole plate  12 . and the second vibrating diaphragm  21  are isolated by an insulating layer  106 , and the first vibrating diaphragm  11  and the second back pole plate  22  are isolated by an isolating groove  108 . 
     Sensing parts of the first back pole plate  12  and the second back pole plate  22  are respectively provided with a plurality of through holes  104 , the first vibrating diaphragm  11  and the second vibrating diaphragm  21  are provided with a through hole  103  at their central positions respectively, and the through holes  103  and  104  play roles of sound conduction and sound pressure balancing. 
     The first vibrating diaphragm  11  and the second back pole plate  22  are made of the same material, and the first back pole plate  12  and the second vibrating diaphragm  21  are made of the same material. A thickness of the back pole plate  12  of the first capacitor C 1  can be equal to or larger than that of the movable pole plate  11 , and a thickness of the back pole plate  22  of the second capacitor C 2 . can be equal to or larger than that of the movable pole plate  21 . 
     The first back pole plate  12 , the first vibrating diaphragm  11 , the second back pole plate  22  and the second vibrating diaphragm  21  are made of any one of the following materials: polycrystalline silicon, silicon nitride attached with a polycrystalline silicon layer and silicon nitride attached with a metal layer. A material of the first isolating layer  200  for example is silicon oxide. The second isolating layer  400  for example can be an oxide layer, and the insulating layer  106  can be a part of the second isolating layer  400 . 
     From  FIG. 1 , it can be seen that the MEMS microphone element is suitable for a TOP product structure in which a sound signal enters into the MEMS microphone element from above, and is also suitable for a BOTTOM product structure in which a sound signal enters into from below. 
     From  FIGS. 1-2 , it can be seen that according to the present invention, two MEMS structures are disposed in one single chip, In the first capacitor C 1 , the vibrating diaphragm  11  is above and the back pole plate  12  is below; and in the second capacitor C 2 , the vibrating diaphragm  21  is below and the back pole plate  22  is above. The back pole plate  12  of the first capacitor C 1  and the vibrating diaphragm  21  of the second capacitor C 2  are manufactured simultaneously, and are made of the same material, for example, the polycrystalline silicon, or material of silicon nitride attached with a metal layer; and the vibrating diaphragm  11  of the first capacitor C 1  and the back pole plate  22  of the second capacitor  22  are manufactured simultaneously, and are made of the same material, for example, the polycrystalline silicon, The back pole plate  12  of the first capacitor C 1  and the vibrating diaphragm  21  of the second capacitor C 2  are made of a first pole plate material layer  300 , and the vibrating diaphragm  11  of the first capacitor Cl and the back pole plate  22  of the second capacitor C 2  are made of a second pole plate material layer  500 . 
     According to the special process design in the present invention, the vibrating diaphragm of the first capacitor C 1  and the vibrating diaphragm of the second capacitor C 2  are electrically connected together as a shared movable pole plate of the differential capacitors. When sound waves act on, the capacitance of the first capacitor C 1  is increased, then the capacitance of the second capacitor C 2  is reduced, or the capacitance of the first capacitor C 1  is reduced, then the capacitance of the second capacitor C 2  is increased, Specifically speaking, when there are no sound wave actions, C 1 =C 2 =C 0 . When sound waves enter into the microphone from a sound hole, the followings will happen. 
     If a sound pressure has a downward action, as shown in  FIG. 3 , the first vibrating diaphragm  11  moves downwards, resulting in a decrease of an interval between the first vibrating diaphragm  11  and the first back pole plate, and in a increase of the first capacitance C 1 ; and the second vibrating diaphragm  21  also moves downwards, resulting in a increase of an interval between the second vibrating diaphragm  21  and the second back pole plate  22 , and in a decrease of the second capacitance C 2 . Therefore, the first capacitance C 1  is larger than C 0 , and C 0  is larger than the second capacitance C 2 , that is, C 1 &gt;C 0 &gt;C 2 . 
     If the sound pressure has an upward action, as shown in  FIG. 4 , the first vibrating diaphragm  11  moves upwards, resulting in a increase of an interval between the first vibrating diaphragm  11  and the first back pole plate  12 , and in a decrease of the first capacitance C 1 ; and the second vibrating diaphragm  21  also moves upwards, resulting in a decrease of an interval between the second vibrating diaphragm  21  and the second back pole plate  22 , and in a increase of the second capacitance C 2 . Therefore, the first capacitance C 1  is smaller than C 0 , and C 0  is smaller than the second capacitance C 2  that is, C 1 &lt;C 0 &gt;C 2 . 
     According to differential design of the present invention, it is favorable for filtering outside electromagnetic and noise interferences, improving a signal-noise ratio and a reception quality of output signals. 
     A manufacturing method for a MEMS microphone of the present invention is introduced with reference to  FIGS. 5-14 . 
     1) Referring to  FIG. 5 , a substrate  100  is provided. 
     2) Referring to  FIG. 6 , a first isolating layer  200  is grown on the substrate  100 , and the first isolating layer  200  for example selects silicon oxide. 
     3) Referring to  FIG. 7 , a first pole plate material layer  300  is grown on the first isolating layer  200 , and the first pole plate material layer  300  for example selects polycrystalline silicon. 
     4) Referring to  FIG. 8 , the first pole plate material layer  300  is patterned and etched to form a back pole plate  12  of a first capacitor C 1 , a movable pole plate  21  of a second capacitor C 2 , and a first isolating groove  105  for isolating the back pole plate  12  of the first capacitor C 1  from the movable pole plate  21  of the second capacitor C 2 , From  FIG. 8 , it can be seen that a plurality of through holes  104  are formed in the back pole plate  12  of the first capacitor C 1 , and a through hole  103  is formed in the central position of the movable pole plate  21  of the second. capacitor C 2 . 
     5) Referring to  FIG. 9 , a second isolating layer  400  is deposited on the first pole plate material layer  300 , and the second isolating layer  400  for example selects oxide. 
     6) Referring to  FIG. 10 , a connecting window  107  is formed in the second isolating layer  400  above the movable pole plate  21  of the second capacitor C 2 , for connecting a movable pole plate  11  of the first capacitor C 1  with a movable pole plate  21  of the second capacitor C 2 . 
     7) Referring to  FIG. 11 , a second pole plate material layer  500  is directly grown on the second isolating layer  400 , and the second pole plate material layer  500  for example selects the polycrystalline silicon. 
     8) Referring to  FIG. 12 , the second pole plate material layer  500  is patterned and etched to form a movable pole plate  11  of the first capacitor C 1 , a back pole plate  22  of the second capacitor C 2 , and an isolating groove  108  for isolating the movable pole plate  11  of the first capacitor C 1  from the back pole plate  22  of the second capacitor C 2 . From  FIG. 12 , it can be seen that at the connecting window  107 , the movable pole plate  11  of the first capacitor C 1 , that is, the first vibrating diaphragm  11 , and the movable pole plate  21  of the second capacitor, that is, the second vibrating diaphragm  21  can be connected together. 
     9) Referring to  FIG. 13 , the substrate  100  is etched from the lower part by a deep reactive ion etching (DRIP process to form back cavities of a first MEMS structure and a second MEMS structure. 
     10) The structure is released by a two-step release process, thereby finishing the machining of the whole device. Referring to  FIG. 14 , the first isolating layer  200  and the second isolating layer  400  of the first MEMS structure as well as the first isolating layer  200  of the second MEMS structure are etched from the bottom. After this step, a first opening  101  which run through from top to bottom, is formed below the first capacitor C 1 , a second opening  102  which run through from top to bottom, is formed below the second capacitor C 2 , and a clearance  109  is formed between the back pole plate  12  and vibrating diaphragm  11  of the first capacitor C 1 . Then the second isolating layer  400  of the second MEMS structure is etched from the top to form another clearance  109  between the back pole plate  22  and vibrating diaphragm  21  of the second capacitor C 2 . 
     It needs to be noted that the process flow is merely an exemplary flow, and if required, thicknesses of the back pole plates of the first capacitor C 1  and the second capacitor C 2  can be made larger than those of the vibrating diaphragms of the first capacitor C 1  and the second capacitor C 2   
     According to the differential-capacitance type MEMS microphone of the present invention, a pair of differential capacitors is designed side by side, differential detection is realized by two layers of films, and the present invention has the following beneficial effects. 
     1. The differential-capacitance type MEMS microphone is realized, and is favorable for filtering outside electromagnetic and noise interferences, improving a signal-noise ratio and a reception quality of output signals. 
     2. Since the clearances between the back pole plates and the movable pole plates of the differential capacitors are finished in one step, intervals of the differential capacitors can be made totally consistent, improving differential effect. 
     3. The process flow of manufacturing is simple and easy to control. The process flow is totally compatible with the current process for a single-capacitance type MEMS microphone, and the process needs no change. 
     Although some specific embodiments of the present invention have been demonstrated in detail with examples, it should be understood by a person skilled in the art that the above examples are only intended to be illustrative but not to limit the scope of the present invention.