Patent Publication Number: US-8526642-B2

Title: Piezoelectric micro speaker including weight attached to vibrating membrane and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2009-0091148, filed on Sep. 25, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments relate to piezoelectric micro speakers, and more particularly, to piezoelectric micro speakers including a weight attached to a vibrating membrane and methods of manufacturing the same. 
     2. Description of the Related Art 
     As terminals for personal voice communication and data communication have developed, amounts of data to be transmitted and received has continuously increased, while the terminals are required to be small and multi-functional. 
     In order to satisfy this requirement, research has been conducted on an acoustic device using micro electro-mechanical system (MEMS) technology. In particular, MEMS and semiconductor technologies make it possible to manufacture a micro speaker with a small size and low cost according to a package process and to easily integrate the micro speaker with a peripheral circuit. 
     Micro speakers using MEMS technology are mainly divided into electrostatic micro speakers, electromagnetic micro speakers, and piezoelectric micro speakers. In particular, a piezoelectric micro speaker may be driven at a lower voltage than in an electrostatic micro speaker, may have a simpler and slimmer structure than an electromagnetic micro speaker. 
     SUMMARY 
     Provided are piezoelectric micro speakers including weight attached to a vibrating membrane and methods of manufacturing the same. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     According to one or more embodiments, a piezoelectric micro speaker includes: a substrate having a cavity therein; a diaphragm that is disposed on the substrate, the diaphragm including a vibrating membrane that overlaps the cavity; a piezoelectric actuator that is disposed on the vibrating membrane; and a weight that is disposed in the cavity and attached to a center portion of the vibrating membrane. 
     The weight may have a substantially columnar shape, and a center of the weight may be disposed on a center line of the cavity. The weight and the substrate may be formed of the same material, and a length of the weight may be equal to or smaller than a thickness of the substrate. The weight may have a substantially cylindrical shape, and a diameter of the weight may be between about 50 μm and about 1000 μm. 
     The piezoelectric actuator may include a first electrode layer disposed on the vibrating membrane, a piezoelectric layer disposed on the first electrode layer, and a second electrode layer disposed on the piezoelectric layer. A first lead line that is connected to the first electrode layer and a second lead line that is connected to the second electrode layer may be formed on the diaphragm, a first electrode pad may be connected to an end of the first lead line and a second electrode pad may be connected to an end of the second lead line. The vibrating membrane of the diaphragm may include a first vibrating membrane formed over a center of the cavity, and a second vibrating membrane formed over an edge of the cavity and formed of a different material from the first vibrating membrane, wherein the piezoelectric actuator is formed on the first vibrating membrane, and the weight is attached to the center of the first vibrating membrane. 
     The second vibrating membrane may be formed of a material having a lower modulus of elasticity than the first vibrating membrane, such as a polymer thin film. The second vibrating membrane may also be disposed on the upper surface of the piezoelectric actuator and on the upper surface of the diaphragm outside the cavity. 
     According to one or more embodiments, a method of manufacturing a piezoelectric micro speaker includes: forming a diaphragm, including a vibrating membrane, on a first side of a substrate; forming a piezoelectric actuator on the vibrating membrane; and forming a cavity passing through the substrate in a thickness direction by etching a surface of a second side of the substrate, opposite the first side, until the vibrating membrane is exposed, and forming a weight disposed in the cavity and attached to a center portion of the vibrating membrane. 
     A center of the weight may be disposed on a center line of the cavity. The weight may be formed of the same material as the substrate, and the length of the weight may be equal to or smaller than the thickness of the substrate. The weight may have a substantially cylindrical shape, and the diameter thereof may be between about 50 μm and about 1000 μm. The piezoelectric actuator may include a first electrode layer, a piezoelectric layer, and a second electrode layer that are sequentially formed on the vibrating membrane. The forming of the piezoelectric actuator may include: forming, on the diaphragm, a first lead line that is connected to the first electrode layer and a second lead line that is connected to the second electrode layer, and forming a first electrode pad at an end of the first lead line and forming a second electrode pad at an end of the second lead line. 
     The forming of the diaphragm may include: forming a first vibrating membrane and forming a trench surrounding the first vibrating membrane, and, after forming the piezoelectric actuator, forming a second vibrating membrane, that is formed of a different material from the first vibrating membrane, in the trench; and the etching may include etching the second side of the substrate such that a center of the cavity is formed under the first vibrating membrane, and an edge of the cavity is formed under the second vibrating membrane. 
     The second vibrating membrane may be formed of a material having a lower modulus of elasticity than that of a material of the first vibrating membrane, a polymer thin film. 
     The forming of the second vibrating membrane may further include: forming the second vibrating membrane on the upper surface of the piezoelectric actuator inside and on the upper surface of the diaphragm outside the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a plan view of a piezoelectric micro speaker, according to an embodiment; 
         FIGS. 2A and 2B  are cross-sectional views of the piezoelectric micro speaker illustrated in  FIG. 1  taken along lines S 1 -S 2  and S 3 -S 4  of  FIG. 1 , respectively; 
         FIG. 3  is a plan view of a piezoelectric micro speaker from which a second vibrating membrane is removed, according to another embodiment; 
         FIGS. 4A and 4B  are cross-sectional views of the piezoelectric micro speaker illustrated in  FIG. 3  taken along lines S 1 -S 2  and S 3 -S 4  of  FIG. 3 , respectively; 
         FIG. 5A  is a graph illustrating a result of simulating variations of a resonance frequency with respect to an increase in the mass of weight of the piezoelectric micro speaker of  FIG. 3  according to an embodiment; 
         FIG. 5B  is a graph illustrating a result of simulating variations of a sound pressure at a frequency of 1 KHz with respect to a diameter in the weight of the piezoelectric micro speaker of  FIG. 3  according to another embodiment; 
         FIGS. 6A through 6C  are cross-sectional views for describing a method of manufacturing the piezoelectric micro speaker illustrated in  FIG. 1 , according to an embodiment; 
         FIGS. 7A and 7B  are cross-sectional views for describing a method of forming a weight illustrated in  FIG. 6C  having a length smaller than a thickness of a substrate, according to an embodiment; and 
         FIGS. 8A through 8E  are cross-sectional views for describing a method of manufacturing the piezoelectric micro speaker illustrated in  FIG. 3 , according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. 
       FIG. 1  is a plan view of a piezoelectric micro speaker, according to an embodiment.  FIGS. 2A and 2B  are cross-sectional views of the piezoelectric micro speaker illustrated in  FIG. 1  taken along lines S 1 -S 2  and S 3 -S 4  of  FIG. 1 , respectively. 
     Referring to  FIGS. 1 ,  2 A, and  2 B, the piezoelectric micro speaker includes a substrate  110  having a cavity  112 , a diaphragm  120  formed on the substrate  110  to cover the cavity  112 , a piezoelectric actuator  130  formed on the diaphragm  120 , and a weight  140  disposed in the cavity  112 . 
     More specifically, the substrate  110  may be formed of a silicon wafer that is finely micromachined. The cavity  112  may be formed to penetrate a predetermined portion of the substrate  110  in a thickness direction and, for example, may be in a cylindrical shape. 
     The diaphragm  120  may be formed having a predetermined thickness on one side of the substrate  110 , and include a vibrating membrane  121  formed on a region corresponding to the cavity  112 . That is, a part of the diaphragm  120  that covers the cavity  112  functions as the vibrating membrane  121 . The diaphragm  120  may be formed of an insulating material such as a silicon nitride, for example, Si 3 N 4 . Accordingly, the vibrating membrane  121  may be formed of the same material as the insulating material. 
     The piezoelectric actuator  130  vibrates the vibrating membrane  121  and may include a first electrode layer  132 , a piezoelectric layer  134 , and a second electrode layer  136  that are sequentially formed on the vibrating membrane  121 . The first electrode layer  132  and the second electrode layer  136  may be formed of conductive metals. The piezoelectric layer  134  may be formed of a piezoelectric material, for example, aluminum nitride (AlN), zinc oxide (ZnO), or lead zirconate titanate (LZT). 
     A first lead line  132   a  that is connected to the first electrode layer  132  of the piezoelectric actuator  130  and a second lead line  136   a  that is connected to the second electrode layer  136  of the piezoelectric actuator  130  may be formed on the diaphragm  120 . The first lead line  132   a  and the second lead line  136   a  may be opposite to each other in view of a center of the piezoelectric actuator  130 . A first electrode pad  132   b  is connected to an end of the first lead line  132   a . A second electrode pad  136   b  is connected to an end of the second lead line  136   a.    
     The weight  140  is disposed in the cavity  112  and is attached to a center portion of the lower surface of the vibrating membrane  121 . The weight  140  may have a variety of shapes, for example, a columnar shape. A center of the weight  140  may be disposed on a center line C of the cavity  112 . For example, the weight  140  may have a cylindrical shape. The weight  140  may be formed of the same material as the substrate  110 , and be longer or shorter than the thickness of the substrate  110 . For example, the thickness of the substrate  110  may be about 500 μm. In this case, the length of the weight  10  may be between about 250 μm and about 500 μm. 
     The weight  140  is attached to the center portion of the vibrating membrane  121  where the vibration displacement is the greatest due to the movement of the piezoelectric actuator  130 , which increases the entire mass of the vibrating membrane  121 . Thus, a resonance frequency of the vibrating membrane  121  is reduced, thereby improving the sound pressure at a low frequency band. If the diameter of the weight  140  is reduced, for example, if the diameter is between about 50 μm and about 1000 μm, a contact area between the weight  140  and the vibrating membrane  121  is reduced. Thus, the weight  140  has relatively little influence on the movement of the piezoelectric actuator  130 , which does not disturb the vibration of the vibrating membrane  121 . This will be described in more detail with reference to  FIGS. 5A and 5B . 
       FIG. 3  is a plan view of a piezoelectric micro speaker, according to another embodiment. (A second vibrating membrane  222  of this embodiment is not illustrated in  FIG. 3 .)  FIGS. 4A and 4B  are cross-sectional views of the piezoelectric micro speaker illustrated in  FIG. 3  taken along lines S 1 -S 2  and S 3 -S 4  of  FIG. 3 , respectively. 
     Referring to  FIGS. 3 ,  4 A, and  4 B, the piezoelectric micro speaker includes a diaphragm  220  formed on a substrate  210  to cover a cavity  212 . The diaphragm  220  includes a first vibrating membrane  221  and the second vibrating membrane  222  that are formed in a region corresponding to the cavity  212 . The first vibrating membrane  221  and the second vibrating membrane  222  are formed of different materials. A piezoelectric actuator  230  is formed on the first vibrating membrane  221 . A weight  240  is attached to the center portion of a lower surface of the first vibrating membrane  221 . 
     More specifically, the diaphragm  220  may be formed having a predetermined thickness on one side of the substrate  210 . The first vibrating membrane  221  is formed in a first region A 1  of the diaphragm  220  that is disposed on the center portion of the cavity  212 . The second vibrating membrane  222  is formed in a second region A 2  of the diaphragm  220  that is disposed on the edge of the cavity  212 . That is, the second vibrating membrane  222  is formed to surround the first vibrating membrane  221  from the outside of the first vibrating membrane  221 . The second vibrating membrane  222  is disposed between the diaphragm  220  that is disposed on the substrate  210  and the first vibrating membrane  221  to connect therebetween, thereby supporting the first vibrating membrane  221  and the piezoelectric actuator  230  formed on the first vibrating membrane  221  with respect to the substrate  210 . The second vibrating membrane  222  may also be formed on the second region A 2 , on the upper surface of the piezoelectric actuator  230  in the first region A 1  (inside the second region A 2 ), and on the upper surface of the diaphragm  220  outside the second region A 2 . In this case, an aperture  228  may be formed in the second vibrating membrane  222  in order to externally expose a first electrode pad  232   b  and a second electrode pad  236   b  that will be described later. 
     The first vibrating membrane  221  and the second vibrating membrane  222  may be formed of different materials. The second vibrating membrane  222  may be formed of a soft material having a low modulus of elasticity so that the second vibrating membrane  222  may be more easily deformable than the first vibrating membrane  221 . The first vibrating membrane  221  may be formed of a material having a modulus of elasticity of between about 50 GPa and about 500 GPa, for example, a silicon nitride. The second vibrating membrane  222  may be formed of a material having a modulus of elasticity of between about 1000 MPa and about 5 GPa, for example, a polymer thin film. 
     The piezoelectric actuator  230  may include a first electrode layer  232 , a piezoelectric layer  234 , and a second electrode layer  236  that are sequentially formed on the first vibrating membrane  221 . The first electrode layer  232  and the second electrode layer  236  may be formed of conductive metals. The piezoelectric layer  234  may be formed of a piezoelectric material, for example, AlN, ZnO, or LZT. 
     A first lead line  232   a  that is connected to the first electrode layer  232  of the piezoelectric actuator  230  and a second lead line  236   a  that is connected to the second electrode layer  236  of the piezoelectric actuator  230  may be formed on the diaphragm  220 . The first lead line  232   a  and the second lead line  236   a  may be on opposite sides of a center of the piezoelectric actuator  230 . A first electrode pad  232   b  is connected to an end of the first lead line  232   a . A second electrode pad  236   b  is connected to an end of the second lead line  236   a . A supporter  226  that supports the first lead line  232   a  and the second lead line  236   a  may be formed in the second region A 2 . The supporter  226  may be formed of the same material as the first vibrating membrane  221 , and may be formed to connect the first vibrating membrane  221  and the diaphragm  220  disposed on the substrate  210  across the second region A 2 . As described above, the second vibrating membrane  222  connects the diaphragm  220  disposed on the substrate  210  and the first vibrating membrane  221 , whereas the supporter  226  connects the diaphragm  220  disposed on the substrate  210  and the first vibrating membrane  221  in regions corresponding to the areas where the first lead line  232   a  and the second lead line  236   a  are formed. 
     The weight  240  is disposed in the cavity  212  and is attached to the center portion of the lower surface of the first vibrating membrane  221 . The weight  240  is the same as described with reference to  FIGS. 1 and 2B  and thus the detailed description thereof will not be repeated here. 
     As described above, since the weight  240  is attached to the center portion of the lower surface of the first vibrating membrane  221  in the present embodiment with reference to  FIGS. 3 and 4A  and  4 B, the effect can be obtained as described with reference to  FIGS. 1 and 2A  and  2 B. Also, the second vibrating membrane  222  that is formed of a soft material having a relatively low modulus of elasticity is disposed in the second region A 2  of the diaphragm  220  that is disposed in the edge of the cavity  212 , which reduces a structural rigidity of the diaphragm  220  and increases the deformability thereof, thereby improving the sound output. 
       FIG. 5A  is a graph illustrating a result of simulating variations of a resonance frequency with respect to an increase in the mass of weight of the piezoelectric micro speaker of  FIG. 3  according to an embodiment.  FIG. 5B  is a graph illustrating a result of simulating variations of a sound pressure at a frequency of 1 KHz with respect to a diameter in the weight of the piezoelectric micro speaker of  FIG. 3  according to another embodiment of the present invention. 
     Referring to  FIG. 5A , an increase in the mass of the weight results in a reduction in the resonance frequency. Likewise, the reduction in the resonance frequency results in an increase in the sound pressure at a frequency band lower than the resonance frequency. Referring to  FIG. 5B , when the resonance frequency is higher than 1 KHz, if the diameter of the weight is greater than about 1000 μm, an increase in the diameter of the weight results in the reduction in the sound pressure at the frequency of 1 KHz. However, if the diameter of the weight is smaller than about 1000 μm, the sound pressure is high at the frequency of 1 KHz compared to the case where there is no weight. If the diameter of the weight is very small, for example, if the diameter of the weight is smaller than 50 μm, since the mass of the weight is very small, a reduction in the resonance frequency may be expected. Therefore, the diameter of the weight may be appropriately between about 50 μm and about 1000 μm based on the simulation results shown in  FIGS. 5A and 5B . 
     A method of sequentially manufacturing the piezoelectric micro speaker having the above-described structure will now be described. 
       FIGS. 6A through 6C  are cross-sectional views for describing a method of manufacturing the piezoelectric micro speaker illustrated in  FIG. 1 , according to an embodiment. The cross-sectional views are taken along lines S 3 -S 4  of  FIG. 1 . 
     Referring to  FIG. 6A , the substrate  110  is prepared. The substrate  110  may be formed of a silicon wafer that is able to be finely micromachined. The diaphragm  120  is formed on a surface of the substrate  110  having a predetermined thickness. More specifically, the diaphragm  120  may be formed by depositing an insulating material such as silicon nitride SixNy, for example, Si 3 N 4  on the surface of the first substrate  110  to a thickness between about 0.5 μm and about 3 μm by using a chemical vapor deposition (CVD) process. A part of the diaphragm  120 , which covers the cavity  112  that is to be formed during an operation described with reference to  FIG. 6C , functions as the vibrating membrane  121 . 
     Referring to  FIG. 6B , the piezoelectric actuator  130  is formed on the vibrating membrane  121  of the diaphragm  120 . The piezoelectric actuator  130  may be formed by sequentially stacking the first electrode layer  132 , the piezoelectric layer  134 , and the second electrode layer  136  on a surface of the vibrating membrane  121 . More specifically, the first electrode layer  132  may be formed by depositing a conductive metallic material such as Cr, Au, Mo, Cu, Al, Ti, or Pt, etc. on the vibrating membrane  121  to a thickness between about 0.1 μm and about 3 μm via evaporation or sputtering, and then, patterning the conductive metallic material to have a predetermined shape. In this regard, the first electrode layer  132  may be a single layer metal film or a multi-layer metal film. Simultaneously with the forming of the first electrode layer  132 , the first lead line  132   a  connected to the first electrode layer  132  and the first electrode pad  132   b  connected to an end of the first lead line  132   a  may be formed on the diaphragm  120 . The piezoelectric layer  134 , which is formed of a piezoelectric material, for example, AlN, ZnO, PZT, PbTi03 or PLT may be formed on the first electrode layer  132  to a thickness between about 0.1 μm and about 3 μm via sputtering or spinning. The piezoelectric layer  134  may be thicker than the first electrode layer  132  to cover the first electrode layer  132  in order to insulate the first electrode layer  132  and the second electrode layer  136  that will be described later. The second electrode layer  136  may be formed on the piezoelectric layer  134  in the same manner as in the method of forming the first electrode layer  132 . In this regard, simultaneously with the forming of the second electrode layer  136 , the second lead line  136   a  connected to the second electrode layer  136  and the second electrode pad  136   b  connected to an end of the second lead line  136   a  may be formed on the diaphragm  120 . The second lead line  136   a  may be disposed to be opposite to the first lead line  132   a  in view of the center of the piezoelectric actuator  130 . 
     Referring to  FIG. 6C , the cavity  112  is formed to pass through the substrate  110  in a thickness direction by etching a surface of another side of the substrate  110  until the vibrating membrane  121  is exposed. In this regard, an etching mask is used so that a portion corresponding to the center of the cavity  112  is etched. In this way, the weight  140  that is in a columnar shape and is attached to the center portion of a lower surface of the vibrating membrane  121  remains in the cavity  112 . The weight  140  may be formed of the same material as the substrate  110 , and have the same thickness and length, for example, about 500 μm, as the substrate  110 . The weight  140  may have a cylindrical shape and the center thereof may be disposed on the center line C of the cavity  112 . 
     The weight  140  may be formed to have a length smaller than the thickness of the substrate  110 .  FIGS. 7A and 7B  are cross-sectional views for describing a method of forming the weight  140  illustrated in  FIG. 6C  having a length smaller than the thickness of the substrate  110 , according to another embodiment. 
     Referring to  FIG. 7A , a first etching mask M 1  is formed on the lower surface of the substrate  110  except a portion of the substrate  110  in which the cavity  112  is to be formed, and the cavity  112  is formed having a predetermined depth by etching the substrate  110 . 
     Thereafter, a second etching mask M 2  is formed on the lower surface of the cavity  112  in which the weight  140  is to be formed, and the substrate  110  is again etched until the vibrating membrane  121  is exposed. In this way, the weight  140  having a length smaller than the thickness of the substrate  110 , for example, a length of about 250 μm, may be formed in the cavity  112 . 
       FIGS. 8A through 8E  are cross-sectional views for describing a method of manufacturing the piezoelectric micro speaker illustrated in  FIG. 3 , according to another embodiment. The cross-sectional views are taken along lines S 1 -S 4  of  FIG. 3 . 
     Referring to  FIG. 8A , a silicon wafer that is able to be finely micromachined is prepared as the substrate  210 . The diaphragm  220  is formed on a surface of the substrate  110  having a predetermined thickness. A method of forming the diaphragm  220  is the same as the method of forming the diaphragm  120  described with reference to  FIG. 6A . 
     Referring to  FIG. 8B , a trench  224  is formed in the second region A 2  disposed in the edge of the cavity  212  that will be formed during an operation described with reference to  FIG. 8E  by etching the diaphragm  220 . Then, the first vibrating membrane  221  that is surrounded by the trench  224  is defined in the first region A 1  disposed in the center of the cavity  212 . In this regard, the trench  224  is not formed in a portion of the second region A 2  in which the first lead line  232   a  and the second lead line  236   a  are to be formed during an operation described with reference to  FIG. 8C , whereas the supporter  226  that supports the first lead line  232   a  and the second lead line  236   a  may remain therein. 
     Referring to  FIG. 8C , the piezoelectric actuator  230  is formed on the first vibrating membrane  221 . The piezoelectric actuator  230  may be formed by sequentially stacking the first electrode layer  232 , the piezoelectric layer  234 , and the second electrode layer  236  on the first vibrating membrane  221 . 
     A method of forming the piezoelectric actuator  230  is the same as the method of forming the piezoelectric actuator  130  described with reference to  FIG. 6B  and thus the detailed description thereof will not be repeated here. 
     Simultaneously with the forming of the first electrode layer  232 , the first lead line  232   a  connected to the first electrode layer  232  and the first electrode pad  232   b  connected to an end of the first lead line  232   a  may be formed on the diaphragm  220 . Simultaneously with the forming of the second electrode layer  236 , the second lead line  236   a  connected to the second electrode layer  236  and the second electrode pad  236   b  connected to an end of the second lead line  236   a  may be formed on the diaphragm  220 . The first lead line  232   a  and the second lead line  236   a  may be formed on the surface of the supporter  226  as described above. 
     Referring to  FIG. 8D , after the piezoelectric actuator  230  is formed, the second vibrating membrane  222  that is formed of a different material from the first vibrating membrane  221  is formed in the trench  224 . The second vibrating membrane  222  may be formed of a soft material having a low modulus of elasticity in order to more easily deform the second vibrating membrane  222  than the first vibrating membrane  221 . More specifically, the first vibrating membrane  221  may be formed of a silicon nitride as described above, and the second vibrating membrane  222  may be formed of a polymer thin film that is deposited to a thickness between about 0.5 μm and about 10 μm, for example. 
     The second vibrating membrane  222  may be formed in the second region A 2 , on the upper surface of the piezoelectric actuator  230  in the first region A 1  (inside the second region A 2 ), and on the upper surface of the diaphragm  220  outside the second region A 2 . In this case, the aperture  228  may be formed in the second vibrating membrane  222  in order to externally expose the first electrode pad  232   b  and the second electrode pad  236   b.    
     Referring to  FIG. 8E , the cavity  212  is formed to pass through the substrate  110  in a thickness direction by etching a surface of another side of the substrate  110  until the first vibrating membrane  221  and the second vibrating membrane  222  are exposed. In this regard, an etching mask may be used so that a portion corresponding to the center of the cavity  212  is not etched. In this way, the weight  140  that is in a columnar shape and is attached to the center portion of a lower surface of the first vibrating membrane  221  remains in the cavity  212 . 
     The weight  240  is the same as the weight  140  described with reference to  FIG. 6C  and thus the detailed description thereof will not be repeated here. The weight  240  may have a length smaller than a thickness of the substrate  210  as described with reference to  FIGS. 7A and 7B . 
     Thus, the piezoelectric micro speaker having a structure in which the first vibrating membrane  221  is disposed in the first region A 1  in the center of the cavity  212 , the second vibrating membrane  222  formed of a soft material is disposed in the second region A 2  in the edge of the cavity  212 , and the weight  240  is attached to the center portion of the lower surface of the first vibrating membrane  221  is completely manufactured. 
     It should be understood that the embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.