Abstract:
Provided are a piezoelectric micro-acoustic transducer and a method of fabricating the same. In the piezoelectric micro-acoustic transducer, a diaphragm is divided into a first region and a second region. The first region may be formed of a material capable of maximizing the exciting force, and the second region may be formed of a material having less initial stress and a lower Young&#39;s modulus than the first region. Also, the second region has a corrugated shape.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-part of prior U.S. application Ser. No. 12/430,652, filed on Apr. 27, 2009, which claims priority from Korean Patent Application No. 10-2008-0094096, filed on Sep. 25, 2008, the disclosures of which are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND 
     1. Field 
     One or more embodiments relate to a microspeaker, and more particularly, to a micro-electro-mechanical systems (MEMS)-based piezoelectric microspeaker and a method of fabricating the same. 
     2. Description of the Related Art 
     The piezoelectric effect is the reversible conversion of mechanical energy into electrical energy using a piezoelectric material. In other words, the piezoelectric effect is a phenomenon in which a potential difference is generated when pressure or vibration is applied to a piezoelectric material, and the piezoelectric material deforms or vibrates when a potential difference is applied. 
     Piezoelectric speakers use the principle of applying a potential difference to a piezoelectric material to deform or vibrate the piezoelectric material and generating sound according to the vibration. 
     With the rapid progress of personal mobile communication, research on a subminiature acoustic transducer has been carried out for several decades. In particular, piezoelectric microspeakers have been researched due to their simple structures and ability to operate at low voltage. 
     In general, a piezoelectric microspeaker includes a piezoelectric plate on both sides of which electrode layers are formed, and a diaphragm which is not piezoelectric. When voltage is applied through the electrode layers, the piezoelectric plate is deformed, which causes the diaphragm to vibrate and generate sound. 
     However, since the piezoelectric microspeaker has a lower sound output level than a voice coil microspeaker, there are few cases of it being put to practical use. Thus, a piezoelectric microspeaker which has a small size and a high sound output level is needed. 
     SUMMARY 
     A piezoelectric micro-acoustic transducer according to an embodiment includes a substrate and a diaphragm. A hole is formed through the substrate in its thickness direction. The diaphragm is formed on the substrate and covers the hole, and includes a first vibration layer formed in a first region corresponding to a center of the hole and a second vibration layer formed at least in a second region corresponding to an edge of the hole, formed of a material having a lower Young&#39;s modulus than that of the first vibration layer, and having a corrugated shape. 
     The piezoelectric micro-acoustic transducer further includes a piezoelectric driver for driving the diaphragm, and a feeder cable unit for supplying a voltage or current to the piezoelectric driver. The feeder cable unit connects the piezoelectric driver to an external power supply through the second region. The feeder cable unit also has a corrugated shape corresponding to the corrugated shape of the second vibration layer. 
     A method of fabricating a piezoelectric micro-acoustic transducer according to another embodiment includes: forming a first vibration layer on an upper side of a substrate; etching a part of the first vibration layer to form a corrugated region including at least one trench; forming a second vibration layer having a Young&#39;s modulus lower than that of the first vibration layer in the at least one trench; and forming a hole in the substrate by etching the a lower side of the substrate until the first vibration layer and the second vibration layer are exposed. 
     The method further includes, before forming the second vibration layer, forming: a first electrode layer on the first vibration layer, forming a first feeder cable unit electrically connected to the first electrode unit, forming a piezoelectric layer on the first electrode layer, forming a second electrode layer on the piezoelectric layer, and forming a second feeder cable unit electrically connected to the second electrode layer, wherein the first feeder cable unit and the second feeder cable unit each have a corrugated shape corresponding to the at least one trench. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the inventive concept. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will become apparent and more readily appreciated from the following description of embodiments taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a plan view of a piezoelectric microspeaker according to an exemplary embodiment. 
         FIG. 2  is a cross-sectional view of the piezoelectric microspeaker of the embodiment of  FIG. 1 . 
         FIGS. 3(A) to 3(G)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker of the embodiment of  FIG. 1 . 
         FIG. 4  is a plan view of a piezoelectric microspeaker according to a another exemplary embodiment. 
         FIG. 5  is a cross-sectional view of the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 4 . 
         FIGS. 6(A) to 6(G)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 4 . 
         FIG. 7  is a plan view of a piezoelectric microspeaker according to another exemplary embodiment. 
         FIG. 8  is a cross-sectional view of the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 7 . 
         FIGS. 9(A) to 9(F)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 7 . 
         FIG. 10  is a plan view of a piezoelectric microspeaker according to another exemplary embodiment. 
         FIG. 11  is a cross-sectional view of the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 10 . 
         FIGS. 12(A) to 12(F)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 10 . 
         FIG. 13  is a plan view of a piezoelectric microspeaker according to another exemplary embodiment. 
         FIG. 14  is a cross-sectional view of the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 13 . 
         FIGS. 15(A) to 15(E)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 13 . 
         FIG. 16  is a plan view of a piezoelectric microspeaker according to another exemplary embodiment. 
         FIG. 17  is a cross-sectional view of the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 16 . 
         FIGS. 18(A) to 18(F)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to the exemplary embodiment of  FIG. 16 . 
         FIGS. 19(A) to 19(C)  show a piezoelectric microspeaker according to another exemplary embodiment. 
         FIGS. 20(A) to 20(E)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to the exemplary embodiment of the  FIGS. 20(A) to 20(E) . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described more fully hereinafter with reference to the accompanying drawings. This general inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the general inventive concept to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. 
       FIG. 1  is a plan view of a piezoelectric microspeaker according to an embodiment, and  FIG. 2  is a cross-sectional view taken along line A-B of  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , the piezoelectric microspeaker according to this embodiment may include a piezoelectric plate  101  which deforms according to a voltage applied thereto, upper and lower electrodes  102  and  103 , and a diaphragm  104  which vibrates due to deformation of the piezoelectric plate  101 . 
     When voltage is applied to the piezoelectric plate  101  through the upper and lower electrodes  102  and  103 , the piezoelectric plate  101  deforms according to the voltage. Deformation of the piezoelectric plate  101  causes the diaphragm  104  to vibrate and generate sound. 
     The diaphragm  104  may include a first region  201  and a second region  202 . For example, the first region  201  may be directly under the piezoelectric plate  101 , and the second region  202  may be the whole or a part of the diaphragm  104  excluding the first region  201 . 
     The first region  201  and the second region  202  may be formed of materials having different Young&#39;s moduli. For example, the first region  201  may be formed of a material having a Young&#39;s modulus similar to that of the piezoelectric plate  101 , and the second region  202  may be formed of a material having a Young&#39;s modulus lower than that of the first region  201 . 
     For example, the piezoelectric plate  101  may be formed of a thin aluminum nitride (AlN) layer or a thin zinc oxide (ZnO) layer having a Young&#39;s modulus of about 50 Gpa to 500 Gpa. The first region  201  of the diaphragm  104  may be formed of silicon nitride (SiN) having a Young&#39;s modulus similar to that of the piezoelectric plate  101 , and the second region  202  of the diaphragm  104  may be formed of a thin polymer layer  105  having a Young&#39;s modulus of about 100 Mpa to 5 Gpa. 
     In the piezoelectric microspeaker according to this embodiment, the center of the diaphragm  104  is formed of a material having a similar Young&#39;s modulus to the piezoelectric plate  101 , and the edge of the diaphragm  104  is formed of a soft material having a lower Young&#39;s modulus than the center. Thus, the piezoelectric microspeaker according to this embodiment may be called a microspeaker having a soft edge. 
     Since the region of the diaphragm  104  directly under the piezoelectric plate  101  is formed of the material having a Young&#39;s modulus similar to that of the piezoelectric plate  101  and the other region of the diaphragm  104  is formed of the material having a Young&#39;s modulus lower than that of the region, deformation efficiency of the diaphragm  104  can be improved, and an output sound pressure level in a low-frequency band can be increased by reducing structural stiffness. 
       FIGS. 3(A) to 3(G)  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to an embodiment. These may be an example of a method of fabricating the piezoelectric microspeaker of  FIG. 2 . 
     The method of fabricating the piezoelectric microspeaker according to this embodiment of the present invention will be described below with reference to  FIGS. 2 and 3A  to  3 G. 
     First, as illustrated in  FIG. 3(A) , the diaphragm  104  is formed on a silicon substrate  106 . For example, the diaphragm  104  may be formed by depositing low-stress silicon nitride to a thickness of about 0.5 μm to 3 μm using a chemical vapor deposition (CVD) process. 
     Subsequently, as illustrated in  FIG. 3(B) , the lower electrode  103  is formed on the diaphragm  104 . For example, the lower electrode  103  may be formed by depositing a metal, such as Au, Mo, Cu or Al, to a thickness of about 0.1 μm to 3 μm using sputtering or evaporation, and patterning the deposited layer. 
     Subsequently, as illustrated in  FIG. 3(C) , the piezoelectric plate  101  is formed on the lower electrode  103 . For example, the piezoelectric plate  101  may be formed by depositing a piezoelectric material, such as AlN or ZnO, to a thickness of about 0.1 μm to 3 μm using a sputtering process and patterning the deposited layer. 
     Subsequently, as illustrated in  FIG. 3(D) , the upper electrode  102  is formed on the piezoelectric plate  101 . For example, the upper electrode  102  may be formed by depositing a metal, such as Au, Mo, Cu or Al, to a thickness of about 0.1 μm to 3 μm using sputtering or evaporation, and patterning the deposited layer. 
     Subsequently, as illustrated in  FIG. 3(E) , a part of the diaphragm  104  is removed. For example, the piezoelectric plate  101  and the upper and lower electrodes  102  and  103  are covered with an etch mask, a non-covered part of the diaphragm  104  is selectively etched, and thus the part of the diaphragm  104  can be removed. Here, the removed part of the diaphragm  104  may be the whole or a part of the diaphragm  104  excluding a region directly under the piezoelectric plate  101 , and provides a space in which the above-mentioned second region  202  will be formed. 
     Subsequently, as illustrated in  FIG. 3(F) , the thin polymer layer  105  is deposited on the entire substrate  106  including a region from which the part of the diaphragm  104  is removed, and is selectively removed. For example, parylene is deposited to a thickness of about 0.5 μm to 10 μm, and then the deposited parylene can be selectively removed by O 2  plasma etching using photoresist as an etch mask. Here, parylene deposited on the upper electrode  102  is removed to expose the upper electrode  102  to the outside. 
     Finally, as illustrated in  FIG. 3(G) , a part of the substrate  106  is etched from the lower side to release the diaphragm  104 . 
       FIG. 4  is a plan view of a piezoelectric microspeaker according to another embodiment, and  FIG. 5  is a cross-sectional view taken along line A-B of  FIG. 4 . 
     Referring to  FIGS. 4 and 5 , the piezoelectric microspeaker according to this embodiment includes a piezoelectric plate  101 , upper and lower electrodes  102  and  103 , and a diaphragm  104 . The diaphragm  104  includes a first region  201  and a second region  202  having different Young&#39;s moduli. The first region  201  may be formed of a material having a Young&#39;s modulus similar to that of the piezoelectric plate  101 , and the second region  202  may be formed of a material having a Young&#39;s modulus lower than that of the first region  201 . This is the same as described with reference to  FIGS. 1 and 2 . 
     However, while the thin polymer layer  105  deposited on the upper electrode  102  is selectively removed to externally expose the upper electrode  102  to the outside of the structure of  FIG. 2 , the upper electrode  102  is not externally exposed to the outside of the structure of  FIG. 5 . 
       FIGS. 6(A) to 6(G)  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to another embodiment. This may be an example of a method of fabricating the piezoelectric microspeaker of  FIG. 5 . 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 6(A) to 6(G) . 
     First, as illustrated in  FIG. 6(A) , the diaphragm  104  is formed on a silicon substrate  106 . For example, the diaphragm  104  may be formed by depositing low-stress silicon nitride to a thickness of about 0.5 μm to 3 μm using a CVD process. 
     Subsequently, as illustrated in  FIG. 6(B) , the lower electrode  103  is formed on the diaphragm  104 . For example, the lower electrode  103  may be formed by depositing a metal, such as Au, Mo, Cu or Al, to a thickness of about 0.1 μm to 3 μm using sputtering or evaporation, and patterning the deposited layer. 
     Subsequently, as illustrated in  FIG. 6(C) , the piezoelectric plate  101  is formed on the lower electrode  103 . For example, the piezoelectric plate  101  may be formed by depositing a piezoelectric material, such as AlN or ZnO, to a thickness of about 0.1 μm to 3 μm using a sputtering process and patterning the deposited layer. 
     Subsequently, as illustrated in  FIG. 6(D) , the upper electrode  102  is formed on the piezoelectric plate  101 . For example, the upper electrode  102  may be formed by depositing a metal, such as Au, Mo, Cu or Al, to a thickness of about 0.1 μm to 3 μm using sputtering or evaporation, and patterning the deposited layer. 
     Subsequently, as illustrated in  FIG. 6(E) , a part of the diaphragm  104  is removed. For example, the piezoelectric plate  101  and the upper and lower electrodes  102  and  103  are covered with an etch mask, a non-covered part of the diaphragm  104  is selectively etched, and thus the part of the diaphragm  104  can be removed. Here, the removed part of the diaphragm  104  may be the whole or a part of the diaphragm  104  excluding a region directly under the piezoelectric plate  101 , and provides a space in which the above-mentioned second region  202  will be formed. 
     Subsequently, as illustrated in  FIG. 6(F) , a thin polymer layer  105  is deposited on the entire substrate  106  including a region from which the part of the diaphragm  104  is removed, and is selectively removed. For example, parylene is deposited to a thickness of about 0.5 μm to 10 μm, and then the deposited parylene can be selectively removed by O 2  plasma etching using photoresist as an etch mask. Here, parylene deposited on the upper electrode  102  is not etched so as not to expose the upper electrode  102  to the outside. 
     Finally, as illustrated in  FIG. 6(G) , a part of the substrate  106  is etched from the lower side to release the diaphragm  104 . 
       FIG. 7  is a plan view of a piezoelectric microspeaker according to yet another embodiment, and  FIG. 8  is a cross-sectional view taken along line A-B of  FIG. 7 . 
     Referring to  FIGS. 7 and 8 , the piezoelectric microspeaker according to this embodiment includes a piezoelectric plate  101 , upper and lower electrodes  102  and  103 , and a diaphragm  104 . The diaphragm  104  includes a first region  201  and a second region  202  having different Young&#39;s moduli. The first region  201  may be formed of a material having a Young&#39;s modulus similar to that of the piezoelectric plate  101 , and the second region  202  may be formed of a material having a Young&#39;s modulus lower than that of the first region  201 . For example, the second region  202  may be understood as a region from which a part of the diaphragm  104  is removed and filled with a thin polymer layer  105 . 
       FIGS. 9(A) to 9(F)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to yet another embodiment. This may be an example of a method of fabricating the piezoelectric microspeaker of  FIG. 8 . 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 9(A) to 9(F) . 
     First, as illustrated in  FIG. 9(A) , an etch stop layer  107  is formed on a substrate  106 , and the diaphragm  104  is formed on the substrate  106 . Here, the diaphragm  104  may be formed by depositing low-stress silicon nitride. 
     Subsequently, as illustrated in  FIG. 9(B) , the lower electrode  103  is formed by depositing and etching a thin metal layer on the diaphragm  104 , the piezoelectric plate  101  is formed by depositing and etching a thin piezoelectric layer on the lower electrode  103 , and then the upper electrode  102  is formed by again depositing and etching a thin metal layer on the piezoelectric plate  101 . 
     Subsequently, as illustrated in  FIG. 9(C) , the thin polymer layer  105  is deposited on the entire substrate  106  and selectively removed. At this time, the removed part may include a part on the upper electrode  102 . The thin polymer layer  105  may be a thin parylene layer having a lower Young&#39;s modulus than the piezoelectric plate  101 . 
     Subsequently, as illustrated in  FIG. 9(D) , a part of the substrate  106  is etched from the lower side to release the etch stop layer  107  and the diaphragm  104 . 
     Subsequently, as illustrated in  FIG. 9(E) , a part of the diaphragm  104  is removed. For example, the diaphragm  104  excluding a part on which the etch stop layer  107  is formed can be removed by etching the diaphragm  104  from the lower side of the substrate  106 . 
     Finally, as illustrated in  FIG. 9(F) , the etch stop layer  107  is removed. 
       FIG. 10  is a plan view of a piezoelectric microspeaker according to yet another embodiment, and  FIG. 11  is a cross-sectional view taken along line A-B of  FIG. 10 . 
     Referring to  FIGS. 10 and 11 , the piezoelectric microspeaker according to this embodiment includes a piezoelectric plate  101 , upper and lower electrodes  102  and  103 , and a diaphragm  104 . The diaphragm  104  includes a first region  201  and a second region  202  having different Young&#39;s moduli. The first region  201  may be formed of a material having a Young&#39;s modulus similar to that of the piezoelectric plate  101 , and the second region  202  may be formed of a material having a Young&#39;s modulus lower than that of the first region  201 . This is the same as described with reference to  FIGS. 7 and 8 . 
     However, while the thin polymer layer  105  deposited on the upper electrode  102  is selectively removed to expose the upper electrode  102  to the outside in the structure of  FIG. 8 , the upper electrode  102  is not exposed to the outside in the structure of  FIG. 11 . 
       FIGS. 12(A) to 12(F)  are cross-sectional views illustrating a method of fabricating the piezoelectric microspeaker according to yet another embodiment. 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 12(A) to 12(F) . 
     First, as illustrated in  FIG. 12(A) , an etch stop layer  107  is formed on a substrate  106 , and a diaphragm  104  is formed on the substrate  106 . Here, the diaphragm  104  may be formed by depositing low-stress silicon nitride. 
     Subsequently, as illustrated in  FIG. 12(B) , the lower electrode  103  is formed by depositing and etching a thin metal layer on the diaphragm  104 , the piezoelectric plate  101  is formed by depositing and etching a thin piezoelectric layer on the lower electrode  103 , and then the upper electrode  102  is formed by again depositing and etching a thin metal layer on the piezoelectric plate  101 . 
     Subsequently, as illustrated in  FIG. 12(C) , a thin polymer layer  105  is deposited on the entire substrate  106  and selectively removed. At this time, the thin polymer layer  105  deposited on the upper electrode may not be removed, and thus it is possible not to expose the upper electrode  102  to the outside. The thin polymer layer  105  may be a thin parylene layer having a lower Young&#39;s modulus than the piezoelectric plate  101 . 
     Subsequently, as illustrated in  FIG. 12(D) , a part of the substrate  106  is etched from the lower side to release the etch stop layer  107  and the diaphragm  104 . 
     Subsequently, as illustrated in  FIG. 12(E) , a part of the diaphragm  104  is removed. For example, the diaphragm  104  excluding a part on which the etch stop layer  107  is formed can be removed by etching the diaphragm  104  from the lower side of the substrate  106 . Here, the removed part of the diaphragm  104  may be a space in which the above-mentioned second region  202  will be formed. 
     Finally, as illustrated in  FIG. 12(F) , the etch stop layer  107  is removed. 
       FIG. 13  is a plan view of a piezoelectric microspeaker according to yet another embodiment, and  FIG. 14  is a cross-sectional view taken along line A-B of  FIG. 13 . 
     Referring to  FIGS. 13 and 14 , the piezoelectric microspeaker according to this embodiment includes a piezoelectric plate  101 , upper and lower electrodes  102  and  103 , and a diaphragm  104 . The diaphragm  104  includes a first region  201  and a second region  202  having different Young&#39;s moduli. The first region  201  may be formed of a material having a Young&#39;s modulus similar to that of the piezoelectric plate  101 , and the second region  202  may be formed of a material having a Young&#39;s modulus lower than that of the first region  201 . For example, the second region  202  may be understood as a region from which a part of the diaphragm  104  is removed and filled with a thin polymer layer  105 . 
       FIGS. 15(A) to 15(E)  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another embodiment. 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 15(A) to 15(F) . 
     First, as illustrated in  FIG. 15(A) , the diaphragm  104  is formed on a substrate  106 . For example, the diaphragm  104  may be formed by depositing low-stress silicon nitride to a thickness of about 0.5 μm to 3 μm using a CVD process. 
     Subsequently, as illustrated in  FIG. 15(B) , the lower electrode  103  is formed by depositing and etching a thin metal layer on the diaphragm  104 , the piezoelectric plate  101  is formed by depositing and etching a thin piezoelectric layer on the lower electrode  103 , and then the upper electrode  102  is formed by again depositing and etching a thin metal layer on the piezoelectric plate  101 . 
     Subsequently, as illustrated in  FIG. 15(C) , a part of the substrate  106  is etched from the lower side to release the diaphragm  104 . 
     Subsequently, as illustrated in  FIG. 15(D) , the thin polymer layer  105  is formed through the etched part of the substrate  106 . For example, the thin polymer layer  105  may be formed by depositing parylene having a Young&#39;s modulus lower than that of the piezoelectric plate  101  on the etched part of the substrate  106  and the released diaphragm  104 . 
     Finally, as illustrated in  FIG. 15(E) , a part of the diaphragm  104  is removed. For example, the piezoelectric plate  101  and the upper and lower electrodes  102  and  103  are covered with an etch mask, a non-covered part of the diaphragm  104  is selectively etched, and thus the part of the diaphragm  104  can be removed. Here, the removed part of the diaphragm  104  may be the whole or a part of the diaphragm  104  excluding a region directly under the piezoelectric plate  101 , and may be the above-mentioned second region  202 . 
       FIG. 16  is a plan view of a piezoelectric microspeaker according to yet another embodiment, and  FIG. 17  is a cross-sectional view taken along line A-B of  FIG. 16 . 
     Referring to  FIGS. 16 and 17 , the piezoelectric microspeaker according to this embodiment has the same structure as described with reference to  FIGS. 13 and 14  except that a thin polymer layer  105  is selectively removed. In other words, in the piezoelectric microspeaker according to this embodiment, the thin polymer layer  105  is selectively etched to expose a part of a diaphragm  104 . 
       FIGS. 18(A) to 18(E)  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another embodiment. 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 18(A)  through (F). 
     First, as illustrated in  FIG. 18(A) , the diaphragm  104  is formed on a substrate  106 . For example, the diaphragm  104  may be formed by depositing low-stress silicon nitride to a thickness of about 0.5 μm to 3 μm using a CVD process. 
     Subsequently, as illustrated in  FIG. 18(B) , a lower electrode  103  is formed by depositing and etching a thin metal layer on the diaphragm  104 , a piezoelectric plate  101  is formed by depositing and etching a thin piezoelectric layer on the lower electrode  103 , and then an upper electrode  102  is formed by again depositing and etching a thin metal layer on the piezoelectric plate  101 . 
     Subsequently, as illustrated in  FIG. 18(C) , a part of the substrate  106  is etched from the lower side to release the diaphragm  104 . 
     Subsequently, as illustrated in  FIG. 18(D) , the thin polymer layer  105  is formed through the etched part of the substrate  106 . For example, the thin polymer layer  105  may be formed by depositing parylene having a Young&#39;s modulus lower than that of the piezoelectric plate  101  on the etched part of the substrate  106  and the released diaphragm  104 . 
     Subsequently, as illustrated in  FIG. 18(E) , a part of the diaphragm  104  is removed. For example, the piezoelectric plate  101  and the upper and lower electrodes  102  and  103  are covered with an etch mask, a non-covered part of the diaphragm  104  is selectively etched, and thus the part of the diaphragm  104  can be removed. Here, the removed part of the diaphragm  104  may be the whole or a part of the diaphragm  104  excluding a region directly under the piezoelectric plate  101 , and may be the above-mentioned second region  202 . 
     Finally, as illustrated in  FIG. 18(F) , the thin polymer layer  105  under the diaphragm  104  is removed to expose the diaphragm  104  to the outside. 
       FIGS. 19(A) to 19(C)  show a piezoelectric microspeaker according to yet another embodiment.  FIG. 19(A)  is a plan view of a piezoelectric microspeaker according to this embodiment,  FIG. 19(B)  is a cross-sectional view taken along line S 1 -S 2  of  FIG. 19(A) , and  FIG. 19(C)  is a cross-sectional view taken along line S 3 -S 4  of  FIG. 19(C) . 
     Referring to  FIGS. 19(A) to 19(C) , the piezoelectric microspeaker may include a substrate  1100 , a diaphragm  1200 , a piezoelectric driver  1300 , a feeder cable unit  1400 , and a pad  1500 . 
     In the substrate  1100 , a hole is formed which penetrates through the substrate  1100  in the thickness direction. 
     The diaphragm  1200  is formed on the substrate  1100  and covers the hole. When an external power supply is connected to the pad  1500 , voltage is applied to the piezoelectric driver  1300  through the feeder cable unit  1400  connected with the pad  1500 . The piezoelectric driver  1300  is deformed by the applied voltage, which causes the diaphragm  1200  to vibrate. When the diaphragm  1200  vibrates, sound can be output through the hole formed in the substrate  1100 . 
     The diaphragm  1200  may include a first vibration layer  1201  and a second vibration layer  1202 . 
     The first vibration layer  1201  is formed in a first region A 1  corresponding to the center of the hole of the substrate  1100 . For example, the first vibration layer  1201  may be formed directly under the piezoelectric driver  1300  and directly on the hole of the substrate  1100 . 
     The second vibration layer  1202  is formed in a second region A 2  corresponding to the edge of the hole of the substrate  1100 . For example, the second vibration layer  1202  may be formed to surround the first vibration layer  1201 . 
     The second vibration layer  1202  is formed of a material having a lower elastic modulus than the first vibration layer  1201 . For example, the first vibration layer  1201  may be formed of silicon nitride having an elastic modulus, for example a Young&#39;s modulus, of about 50 Gpa to 500 Gpa, and the second vibration layer  1201  may be formed of a thin polymer layer having a Young&#39;s modulus of about 100 Mpa to 5 Gpa. 
     Also, the second vibration layer  1202  may be formed into a corrugated or ripple shape. 
     The piezoelectric driver  1300  may include a first electrode layer  1301 , a piezoelectric layer  1302 , and a second electrode layer  1303  sequentially stacked on the first vibration layer  1201 . The piezoelectric layer  1302  may be formed of a thin AlN layer or a thin ZnO layer having a shape varying according to applied voltage. The first electrode layer  1301  is a lower electrode formed under the piezoelectric layer  1302 , and the second electrode layer  1302  is an upper electrode formed on the piezoelectric layer  1302 . 
     The feeder cable unit  1400  supplies the first electrode layer  1301  and the second electrode layer  1303  with voltage. For example, the feeder cable unit  1400  may include a first feeder cable  1401  connecting the pad  1500  connected with the external power supply and the first electrode layer  1301 , and a second feeder cable  1402  connecting the pad  1500  connected with the external power supply and the second electrode layer  1303 . 
     The respective feeder cables  1401  and  1402  may have a shape similar to that of the second vibration layer  1202 . For example, each of the first feeder cable  1401  and the second feeder cable  1402  may be formed into the corrugated or ripple shape, like the second vibration layer  1202 . 
     When power is applied to the piezoelectric driver  1300 , the first vibration layer  1201  formed directly under the piezoelectric driver  1300  vibrates, and a second vibration layer  1202  around the first vibration layer  1201  supports the first vibration layer  1201 . At this time, since the second vibration layer  1202  has a Young&#39;s modulus lower than that of the first vibration portion  1201 , a mechanical resonant frequency can be lowered, and a sound pressure level of a low-frequency component can be thereby increased. Also, since the second vibration layer  1202  and the feeder cable unit  1400  are formed into a corrugated or ripple shape, it is possible to further lower the resonant frequency and improve durability. 
       FIGS. 20(A) to 20(E)  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another exemplary embodiment of the present invention. These may be an example of a method of fabricating the piezoelectric microspeaker illustrated in  FIGS. 19(A) to 19(C) . 
     Referring to  FIG. 20(A) , the first vibration layer  1201  is formed on one side of the substrate  1100 . For example, the first vibration layer  1201  can be formed by depositing low-stress silicon nitride to a thickness of about 0.5 μm to 3 μm using a CVD process. 
     Referring to  FIG. 20(B) , a corrugated or ripple trench  1600  is formed by etching a part of the first vibration layer  1201 . For example, a part of the first vibration layer  1201  corresponding to the center of the substrate  1100  is covered with a predetermined mask, and the other part of the first vibration layer  1201  is selectively etched to form the trench  1600 . 
     Referring to  FIG. 20(C) , the first electrode layer  1301 , the piezoelectric layer  1302 , and the second electrode layer  1303  are sequentially stacked on the first vibration layer  1201  to form the piezoelectric driver  1300 . At this time, the first feeder cable  1401  having a corrugated or ripple shape corresponding to the trench  1600  and connecting an external power supply and the first electrode layer  1301  and the second feeder cable  1402  having a corrugated or ripple shape corresponding to the trench  1600  and connecting the external power supply and the second electrode layer  1303  may be formed together. 
     Referring to  FIG. 20(D) , the second vibration layer  1202  is formed on the first vibration layer  1201  on which the piezoelectric driver  1300  and the feeder cable unit  1400  are formed. For example, the second vibration layer  1202  can be formed according to a trench shape formed on the first vibration layer  1201  using a thin polymer layer having a Young&#39;s modulus lower than that of the first vibration layer  1201 . 
     Referring to  FIG. 20(E) , a hole is formed in the substrate  1100  by etching the other side of the substrate  1100  until the first vibration layer  1201 , the second vibration layer  1202 , and the feeder cable unit  1400  under the second vibration layer  1202  are exposed. For example, the lower side of the substrate  1100  may be etched to expose the diaphragm  1200  including the first vibration layer  1201  and the second vibration layer  1202 . 
     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.