Abstract:
A micro acoustic transducer and manufacturing method are provided. Firstly, a substrate having one first and second cavities is provided. Then, a backplate with a plurality of acoustic holes is formed on the substrate, and a diaphragm is formed on the backplate. An air gap is formed between the backplate and the diaphragm. The air gap, second cavity, and first cavity are communicated with each other through the acoustic holes. A plurality of rings is formed around the diaphragm. These rings are used to hitch pillars formed on the substrate or fasteners can be formed on the substrate for fastening the diaphragm on fastener holes. Through the arrangement of the rings or fasteners used as the boundary structure of the diaphragm, the mechanical sensitivity of the diaphragm is improved. Moreover, the backplate is supported by a single crystal structure formed by etching the substrate such that the stability is promoted.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims priorities under 35 U.S.C. §119(a) on Patent Application No(s). 095100667 and 095138475 filed in Taiwan, R.O.C. on Jan. 6, 2006 and Oct. 18, 2006, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an acoustic structure, and more particularly, to a micro acoustic transducer and a manufacturing method therefor. 
     2. Related Art 
     Micro acoustic transducers developed are mainly applied in various acoustic receivers and it has become an object of the design thereof to pursue characteristics such as small volume, low power consumption, and high sensitivity. Further, according to the result of theoretical modeling, it can be known that residual stress has a significant impact on the mechanical sensitivity of a diaphragm in an acoustic transducer. Under the influence of the residual stress, the boundary conditions of the diaphragm must be changed or a folding structure must be formed, so as to enhance the mechanical sensitivity of the diaphragm. 
     In U.S. Pat. No. 5,146,435, a basic microphone structure design declared as an acoustic transducer is included. The structure includes a perforated plate and a movable plate, wherein a dielectric fluid is contained there-between. The dielectric fluid is gas medium, air, or liquid, while the perforated plate and the movable plate are supported by a structure that functions as a spring. The shapes of the structures of the spring and the plate can be defined through a development process. The acoustic transducer can be combined with an oscillator circuit, such that the change in capacitance caused by the change in the space between the plates can be used as the base of the measurement of the acoustic transducer. 
     U.S. Pat. No. 5,163,329 discloses a sacrificial layer deposited between the diaphragm and a silicon substrate, such that the sacrificial layer and the substrate are etched by an etchant through etch holes to form a cavity structure. 
     In addition, U.S. Pat. No. 6,535,460 discloses an acoustic transducer, which comprises a substrate, a backplate, and a thin film structure. The backplate comprises a flat surface having a hole with an aspect ratio and a support structure. The support structure of the backplate is a continuous structure or bump. The floating thin film structure is supported by the support structure and fixedly spaced from the backplate. As such, when an acoustic wave reaches, the floating thin film structure moves freely in the direction perpendicular to the plane. 
     However, according to the result of theoretical modeling, residual stress plays a significant impact on the mechanical sensitivity of a diaphragm in an acoustic transducer. Under the influence of the residual stress, the boundary conditions of the diaphragm must be changed or a folding structure must be formed, so as to enhance the mechanical sensitivity of the diaphragm; therefore, how to provide a structure and method for a diaphragm to achieve a better stress-releasing effect and improve the property of a microphone component become an important issue. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a micro acoustic transducer to overcome the technology problems in prior art. A structure of rings is used as a boundary condition to enhance the mechanical sensitivity of a thin film. Besides, a substrate of a single crystal support structure is formed on the backplate structure to support the backplate, thereby enhancing firmness and solving the problems existing in the prior art. 
     An object of the invention is to provide a method of manufacturing micro acoustic transducer to overcome another technology problems in prior art. In order to enhance the stability of the structure of the backplate, the invention provides a method to define acoustic hole regions with sacrificial layers, such that the substrate is etched into a single crystal support structure to support the backplate. 
     An object of the invention is to provide a micro acoustic transducer which utilizes a fastening structure for releasing stress and restricting diaphragm to overcome another technology problems in prior art. 
     Therefore, in order to achieve the aforementioned object, the micro acoustic transducer disclosed in the invention includes a substrate with at least one first cavity and one second cavity above the first cavity, wherein the first cavity and the second cavity are communicated with each other; a backplate formed on the substrate, wherein the backplate has a plurality of acoustic holes; a diaphragm formed above the backplate, wherein a plurality of rings is formed around the diaphragm; and a plurality of pillars formed on the substrate, wherein the position of each pillar corresponds to that of each ring, respectively. An air gap is formed between the diaphragm and the backplate. The air gap, the second cavity, and the first cavity are communicated with each other through the acoustic holes. Each of the rings hitches the corresponding pillar, wherein the diameter of each ring is larger than that of each pillar. 
     Such design provides a support condition similar to a free boundary. The pillars are only used to limit the moving range of the diaphragm on the plane, while the support structure of the free boundary is mainly designed for releasing the residual stress generated in the process of the deposition of the diaphragm. 
     On the other hand, the method of manufacturing the micro acoustic transducer disclosed in the invention includes firstly providing a substrate with at least one first cavity and one second cavity, wherein the first cavity and the second cavity are communicated with each other and the second cavity is located on the first cavity; then, forming a backplate with a plurality of acoustic holes on the substrate; forming a diaphragm on the backplate, wherein a plurality of rings are formed around the diaphragm and an air gap is formed between the diaphragm and the backplate; and forming pillars on the substrate, wherein each of the rings hitches each of the pillars correspondingly and the position of each pillar corresponds to that of each ring. 
     The air gap, the second cavity, and the first cavity are communicated with each other through each of the acoustic holes and each of the rings hitches the corresponding pillar, respectively. The diameter of each ring must be larger than that of the corresponding pillar. As such, under the effect of the acoustic wave, the diaphragm vibrates due to the design of the free boundary. 
     In addition, the invention also provides a micro acoustic transducer which utilizes a fastening structure for releasing stress and limiting diaphragm. The micro acoustic transducer utilizing a fastening structure includes a substrate, a backplate, a diaphragm, a plurality of fasteners, and a plurality of supporting element. The substrate has at least one first cavity and a second cavity formed above the first cavity, and the first cavity and the second cavity is communicated with each other. The backplate is formed on the substrate and has a plurality of acoustic holes. The diaphragm is formed above the backplate and a plurality of fastener holes is surrounded on the diaphragm. Besides, the plurality of fasteners is formed on the substrate, and the position of each fastener is corresponding to that of each fastener hole respectively. In addition, the plurality of supporting elements is formed on the diaphragm so as to support the diaphragm on the surface of the backplate; thereby, an air gap is formed between the diaphragm and the backplate. Through the acoustic holes, the air gap, the second cavity, and the first cavity are communicated with each other. And each fastener is fasten to the corresponding fastener hole respectively, so that a gap exists between each fastener hole and each fastener respectively and the gap is provided for diaphragm&#39;s moving. 
     The micro acoustic transducer provided by the invention is directed to enhancing the sensitivity of the micro acoustic transducer. When the acoustic wave is transmitted, the capacitance value changes due to the structural distortion of diaphragm caused by the change of sound pressure, so as to read the signal of the acoustic wave. As for the design of the structure, a diaphragm structure with high sensitivity and a backplate structure that is kept to be a plane are desired to form a capacitor structure with a thin film structure. 
     As for the design of the diaphragm structure, a sacrificial layer is deposited on the under layer of the diaphragm structure. The pillars and a diaphragm structure surrounded by the ring structures are grown above the sacrificial layer. After the sacrificial layer is etched, the diaphragm is released and the diaphragm structure generates the support boundary condition similar to the free boundary through the design of the rings. The pillars are only used to limit the moving range of the diaphragm on the plane, while the support structure of the free boundary is mainly designed for releasing the residual stress generated in the process of depositing the diaphragm. Furthermore, the pillar may have a stop part to prevent the thin film drifting away during etching process. According to the theoretical modeling, the oscillation sensitivity of a diaphragm under no stress is 100 times more than that under residual stress of 100 MPa. 
     The above-mentioned diaphragm after releasing the residual stress may use the fastener structure to limit its moving range on the plane, which means that the diaphragm not only may release the residual stress by the mentioned design but also may use the fastener to fix it. Besides, the vibration may be controlled by the supporting element of the diaphragm. 
     On the other hand, in order to achieve the design of the backplate structure that is kept to be a plane, in many conventional arts, the stiffness of the backplate structure is enhanced through folding of the structure, doped silicon used as an etch stop layer, or a single crystal structure of silicon used as the backplate structure. In the invention, after the substrate is etched to a certain depth by backside etching, the sacrificial layer and the substrate are etched on the front side through etch holes to form a backplate support structure with acoustic holes, because the backplate structure supported by the single crystal structure helps to strengthen the stability of the backplate structure. 
     Further scope of applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more fully understood from the detailed description given herein below for illustration only, and which thus is not limitative of the invention, and wherein: 
         FIG. 1  is a schematic structural view of the micro acoustic transducer of a first embodiment of the invention; 
         FIG. 2  is a schematic sectional view of the diaphragm of the micro acoustic transducer of the first embodiment of the invention; 
         FIG. 3  is a schematic sectional view of the backplate of the micro acoustic transducer of the first embodiment of the invention; 
         FIGS. 4A ,  4 B, and  4 C are flow charts of forming the first cavity and the second cavity of the first embodiment of the invention; 
         FIG. 5A  is a sectional structure view of the pillar and the ring of the first embodiment of the invention; 
         FIG. 5B  is a top view of the pillar and the ring of the first embodiment of the invention; 
         FIG. 6  is a stereogram of the micro acoustic transducer with a circular diaphragm of the invention; 
         FIG. 7  is a top view of the micro acoustic transducer of a second embodiment of the invention; 
         FIG. 8  is a schematic sectional view of  FIG. 7 ; 
         FIG. 9  is a stereogram view of  FIG. 7 ; and 
         FIGS. 10A to 10I  are flow charts of the method of manufacturing the micro acoustic transducer provided by the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to make the objects, structures, features, and functions of the invention more comprehensible, preferred embodiments accompanied with figures are described in detail below. Both the foregoing general description about the invention and the following detailed description about the embodiments are exemplary and are intended to explain the principles of the invention, and provide further explanation of the Claims. 
     Referring to  FIG. 1 , it is a schematic structural view of the micro acoustic transducer of a first embodiment of the invention. The micro acoustic transducer comprises a substrate  60  such as a silicon substrate, a backplate  30  formed on the substrate  60 , a diaphragm  10  formed above the backplate  30 , and a plurality of pillars  70  formed on the substrate  60  and around the diaphragm  10 . The shape of the diaphragm  10  is square, circular, finger-like, or any other shape. A plurality of rings  72  is formed around the diaphragm  10  to hitch the pillars. Each ring  72  hitches one corresponding pillar  70 , but does not completely fix the pillar. The diameter of the hole of each ring is larger that that of each pillar, such that the diaphragm  10  is still a free thin film. The pillars  70  are only used to limit the moving range of the diaphragm  10  on the plane. Further, an air gap  20  is formed between the diaphragm  10  and the backplate  30  with multiple acoustic holes  32 . A first cavity  50  and a second cavity  40  are formed in the substrate  60 , and the first cavity  50 , the second cavity  40 , and the air gap  20  are communicated with each other through the acoustic holes  32 . 
     Referring to  FIG. 2 , it is a schematic sectional view of the diaphragm of the micro acoustic transducer of the first embodiment of the invention. As shown in the figure, a diaphragm electrode layer  13  is further formed on the diaphragm  10  and multiple bumps  14  may be formed on the diaphragm  10 . When a sacrificial layer  11  is processed by wet etching, with the bumps  14 , the diaphragm  10  can be prevented from adhering to the backplate  30 . After the first sacrificial layer  11  in the figure is etched, the air gap  20  is formed as shown in  FIG. 1 . Through each of etch holes  12  in the diaphragm  10 , an etchant is poured in, such that the first sacrificial layer  11  is processed by wet etching. 
     Referring to  FIG. 3 , it is a schematic sectional view of the backplate of the micro acoustic transducer provided of the first embodiment of the invention. It can be seen from the figure that a backplate electrode layer  34  is further formed on the backplate  30  and a plurality of acoustic holes  32  is defined in the backplate  30 , wherein the positions of the acoustic holes  32  correspond to the distribution position of a second sacrificial layer  38 . During manufacturing, an etch mask  36  is formed on the surface of the substrate  60  and the material of the etch mask  36  may be silicon nitride or silicon oxide. The distribution shape and position of the etch mask  36  are defined with a mask. After that, the second sacrificial layer  38  is filled in the parts on the substrate where there is no mask layer  36 . When the first etchant flows in through the etch holes  12 , the first sacrificial layer  11  is first etched. Then, the second etchant continues to flow into each of the acoustic holes  32  to etch the second sacrificial layer  38  and then etch a part of the substrate  60  thereunder, thereby forming the second cavity  40 . 
     Referring to  FIGS. 4A ,  4 B, and  4 C, flow charts of forming the first cavity and the second cavity of the first embodiment of the invention are shown. First, the substrate  60  is etched to a certain depth by backside etching to form a first cavity  50 . After that, the first etchant is poured into each of the etch holes  12 , so as to etch the first sacrificial layer  11  by frontside etching. Then, the second etchant continues to flow down through each of the acoustic holes to etch the second sacrificial layer  38  and a part of the substrate  60  thereunder, thereby forming the second cavity  40 . The first cavity  50  must be communicated with the second cavity  40  and the boundary of the first cavity  50  and the second cavity  40  is defined to be a cavity-connecting hole  52 . 
     As shown in  FIG. 5A , it is a sectional structure view of the pillar and the ring of the first embodiment of the invention. As shown in the figure, a pillar protection layer  74  is coated on the outmost of the pillar  70  and a pillar base  76  is under the pillar  70  to serve as the substrate of the pillar  70 . The diameter of the hole of the ring  72  must be larger than that of the pillar  70 . That is, the ring  72  does not closely fit the pillar  70  and a space must be left between the ring  72  and the pillar  70 , such that the diaphragm  10  vibrates under the effect of the acoustic wave. Referring to  FIG. 5B , it is a top view of the pillar and the ring of the first embodiment of the invention. As shown in  FIG. 5B , the pillar  70  does not completely adhere to the ring  72 .  FIG. 6  is a stereogram of the micro acoustic transducer with a circular diaphragm. 
     As shown in  FIG. 7 , it is a top view of the micro acoustic transducer of a second embodiment of the invention,  FIG. 8  is a schematic sectional view of  FIG. 7 , and  FIG. 9  is a stereogram view of  FIG. 7 . The micro acoustic transducer comprises a substrate  60   a  with at least one first cavity  50   a  and one second cavity  40   a  communicated with the first cavity  50   a , a backplate  30   a  formed on the substrate  60   a  with multiple acoustic holes  32   a , a diaphragm  10   a  formed on the backplate  30   a  with a plurality of fastener holes  80  around the diaphragm  10   a , a plurality of fastener  81  formed on the substrate  60   a  and the position of each fastener  81  is corresponding to that of each fastener hole respectively, and a plurality of supporting element formed on the diaphragm  10   a . The supporting element  82  includes a supporting rod  821  formed on the diaphragm  10   a , a supporting pin  822  is vertically extended from the supporting rod  821 , and a fixed end  823  is horizontally extended from the supporting rod  821 . The supporting element  82  may support the diaphragm  10   a  on the surface of the backplate  30   a  in case. In addition, by the structure, an air gap  20   a  is formed between the diaphragm  10   a  and the backplate  30   a . The air gap  20   a , the second cavity  40   a , and the first cavity  50   a  are communicated with each other through the acoustic holes  32   a . Each of the fasteners  81  is fasten to each corresponding fastener hole  80 , wherein the diameter of each fastener hole  80  is larger than that of each fastener  81 , so that a space is provided between each fastener hole  80  and each fastener  81  respectively for the diaphragm  10   a  moving and also the design provides a movement limit structure for the diaphragm  10   a.    
     Finally, referring to  FIGS. 10A to 10I , flow charts of the method of manufacturing the micro acoustic transducer provided by the invention are shown. First, a substrate  710  is provided, wherein the substrate  710  is a silicon substrate. Etching masks  712  are coated on the upper surface and the lower surface of the substrate  710 . A part of the etching mask  712  is etched firstly through the definition of the mask, so as to define the positions where the acoustic holes and the first cavity are to be formed. 
     Then, the frontside etching sacrificial layer  714  is filled in the part of the etching mask  712  which has been etched, and the pillar bases  720  are formed on both ends of the substrate  710 . The position of the frontside etching sacrificial layer  714  corresponds to the positions of the acoustic holes. After that, the backplate  716  is formed thereon and defines a plurality of acoustic holes  718 . Furthermore, a backplate electrode layer  722  is further formed on the backplate  716 . 
     Subsequently, an air gap sacrificial layer  726  is coated on the back electrode layer  722  and the pillars  724  of the same material are formed on the pillar bases  720 . Later, the air gap sacrificial layer  726  is etched to form an air gap. 
     Next, a diaphragm  732  is formed on the air gap sacrificial layer  726  and a pillar protection layer  730  of the same material is formed on the surface of the pillars. The structure of the rings  728  is formed around the pillars  724 . After that, a diaphragm electrode layer  734  is further formed on the diaphragm  732 . 
     Finally, the first cavity  738  is formed in the substrate  710  by backside etching. Then, the first etchant is poured into the etch holes to etch the air gap sacrificial layer  726  by frontside etching, so as to form the air gap  742 . The first etchant flows down to etch each of the acoustic holes  718 . Then the second etchant flows into each of the acoustic holes to etch the frontside etching sacrificial layer  714  and a part of the substrate  710  under the frontside etching sacrificial layer  714 , thereby forming the second cavity  740 , wherein the air gap  742 , the first cavity  738 , and the second cavity  740  are communicated with each other. 
     In the above-mentioned method, when a same material is utilized to form the pillar  724  in the pillar bases  720 , the above-mentioned fastener  81  may be formed to replace the pillar  724 . Besides, during the steps of the air gap sacrificial layer  726  coated on the back electrode layer  722  and the structure of the rings  728  being formed around the pillars  724 , the above-mentioned fastener holes  80  may be used to replace the rings. Also, because the fastener  81  is fasten to the fastener hole  80  which has a diameter larger than that of the fastener  81 , the fastener  81  may be used for limiting the range of movement of the diaphragm  10   a.    
     In the invention, the rings hitch the pillars to form the support structures or the fasteners fastens to the fastener holes, thus achieving a diaphragm of releasing residual stress, and improving the performance of the micro acoustic transducer. On the other hand, the backplate structure supported by a single crystal is manufactured by backside silicon substrate etching and frontside sacrificial layer etching. The whole support structure is similar to an interlaced net rack support, thereby enhancing the firmness of the backplate structure. After the silicon substrate is etched to a certain depth by backside etching, the sacrificial layer and the silicon substrate are etched on the front side through the etch holes, so as to form the backplate support structure with the acoustic holes, which can be applied in the acoustic transducer. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.