Abstract:
A piezoelectric microspeaker fabricated by a method including: forming a lower drive unit by forming a first drive electrode by depositing and etching a first thin conductive layer on a substrate, forming a first piezoelectric plate by depositing and etching a first piezoelectric layer on the first drive electrode, and forming a first common electrode by depositing and etching a second conductive layer on the first piezoelectric plate; after forming the lower drive unit, forming a diaphragm by depositing a non-conductive layer on the first common electrode; and forming an upper drive unit by forming a second common electrode by depositing and etching a third conductive layer on the diaphragm, forming a second piezoelectric plate by depositing and etching a second piezoelectric layer on the second common electrode, and forming a second drive electrode by depositing and etching a fourth conductive layer on the second piezoelectric plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2008-0092844, filed on Sep. 22, 2008, the disclosure of which is incorporated herein in its 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 thereto. 
     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 having an electrode layer formed on each side, and a non-piezoelectric diaphragm. 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 method of fabricating a piezoelectric microspeaker, according to an embodiment, includes forming a lower drive unit by forming a first drive electrode by depositing and etching a thin first conductive layer on a substrate, forming a first piezoelectric plate by depositing and etching a thin first piezoelectric layer on the first drive electrode, and forming a first common electrode by depositing and etching a thin second conductive layer on the first piezoelectric plate; a diaphragm forming step of forming a diaphragm by depositing and etching a thin non-conductive layer on the first common electrode; and an upper drive unit forming step of: forming a second common electrode by depositing and etching a thin third conductive layer on the diaphragm, forming a second piezoelectric plate by depositing and etching a second thin piezoelectric layer on the second common electrode, and forming a second drive electrode by depositing and etching a thin fourth conductive layer on the second piezoelectric plate. 
     A method of fabricating a piezoelectric microspeaker, according to another embodiment, includes: forming a lower drive unit by depositing and etching a thin first conductive layer and a thin first piezoelectric layer on a substrate in sequence; forming a diaphragm by depositing and etching a thin second conductive layer on the lower drive unit; and forming an upper drive unit by depositing and etching a thin second piezoelectric layer and a thin third conductive layer on the diaphragm in sequence. 
     The method may further include: before forming the lower drive unit, etching a part of the substrate to form a cavity in which the lower drive unit is subsequently formed. 
     A piezoelectric microspeaker, according to another embodiment, is fabricated according to one of the above-described methods. 
     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 embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above 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 cross-sectional view of a piezoelectric microspeaker according to an embodiment. 
         FIGS. 2A to 2E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to an embodiment. 
         FIG. 3  is a cross-sectional view of a piezoelectric microspeaker according to another embodiment. 
         FIGS. 4A to 4E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to another embodiment. 
         FIG. 5  is a cross-sectional view of a piezoelectric microspeaker according to yet another embodiment. 
         FIGS. 6A to 6E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another embodiment. 
         FIG. 7  is a cross-sectional view of a piezoelectric microspeaker according to yet another embodiment. 
         FIGS. 8A to 8E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are described more fully hereinafter with reference to the accompanying drawings, in which embodiments are shown. The 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 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 may 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 cross-sectional view of a piezoelectric microspeaker according to an embodiment. 
     Referring to  FIG. 1 , the piezoelectric microspeaker according to this embodiment includes a diaphragm  101 , a lower drive unit  102  and an upper drive unit  103 . The lower drive unit  102  and the upper drive unit  103  may be symmetrically formed with respect to the diaphragm  101 . The lower and upper drive units  102  and  103  may include drive electrodes  201  and  301  connected to a drive power source  110 , piezoelectric plates  202  and  302  which deform according to an applied voltage, and common electrodes  203  and  303  connected to a common power source  120 . 
     The first drive electrode  201  and the second drive electrode  301  may be connected with each other and to the drive power source  110 , and the first common electrode  203  and the second common electrode  303  may be connected with each other and to the common power source. For example, the drive power source may be an alternating current (AC) power source  110 , and the common power source  120  may be ground which provides a reference value of voltage generated from the AC power source  110 . 
     When the power sources  110  and  120  are connected as described above, electric fields are generated between the first drive electrode  201  and the first common electrode  203  and between the second drive electrode  301  and the second common electrode  303 , respectively. According to the generated electric fields, the piezoelectric plates  202  and  302  may deform, and the deformation may be applied to the diaphragm  101 . For example, when an AC voltage is applied to the piezoelectric microspeaker according to this embodiment, the piezoelectric plates  202  and  302  repeatedly contract and expand according to a change in voltage, which causes the diaphragm  101  to vibrate and generate sound. 
     Here, the electric fields generated between the first drive electrode  201  and the first common electrode  203  and between the second drive electrode  301  and the second common electrode  303  point in opposite directions. For example, a downward electric field may be generated between the first drive electrode  201  and the first common electrode  203 , and an upward electric field may be generated between the second drive electrode  301  and the second common electrode  303 . Thus, the deformation directions of the first piezoelectric plate  202  and the second piezoelectric plate  302  are also opposite to each other. Assuming that the first and second piezoelectric plates  202  and  302  have, for example, a disk shape, the second piezoelectric plate  302  may contract toward its center when the first piezoelectric plate  202  expands from its center to the periphery. Due to the piezoelectric plates  202  and  302  deforming in opposite directions under and on the diaphragm  101 , respectively, a deformation efficiency of the diaphragm  101  can be improved. 
     In the piezoelectric microspeaker of  FIG. 1  according to this embodiment, the diaphragm  101  and the drive units  102  and  103  may be formed by depositing various materials in the form of thin layers on a substrate  104  and etching the deposited layers in specific shapes using a semiconductor fabrication process. 
       FIGS. 2(   a ) to  2 ( e ) are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to an embodiment. This may serve as an example of a method of fabricating the piezoelectric microspeaker of  FIG. 1 . 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 1 and 2A  to  2 E. 
     First, as illustrated in  FIG. 2A , an insulating layer  105  is formed by oxidizing the upper surface of the substrate  104  or depositing and etching a thin insulating layer on the substrate  104 . The insulating layer  105  provides insulation between the lower drive unit  102  to be formed and the substrate  104 . 
     Subsequently, as illustrated in  FIG. 2B , the first drive electrode  201 , the first piezoelectric plate  202  and the first common electrode  203  are formed by depositing and etching a thin conductive layer, a thin piezoelectric layer, and a thin conductive layer in sequence. The first drive electrode  201 , the first piezoelectric plate  202 , and the first common electrode  203  may constitute the lower drive unit  102  formed under the diaphragm  101 . 
     Subsequently, as illustrated in  FIG. 2C , the diaphragm  101  is formed by depositing and etching a thin polymer layer. At this time, the center of the diaphragm  101  can be formed to protrude according to the shape of the lower drive unit  102 . 
     Subsequently, as illustrated in  FIG. 2D , the second common electrode  303 , the second piezoelectric plate  302  and the second drive electrode  301  are formed by depositing and etching a thin conductive layer, a thin piezoelectric layer and a thin conductive layer in sequence. The second common electrode  303 , the second piezoelectric plate  302  and the second drive electrode  301  may constitute the upper drive unit  103  formed on the diaphragm  101 . In addition, in this process, the first drive electrode  201  can be electrically connected with the second drive electrode  301 , and the first common electrode  203  can be electrically connected with the second common electrode  303 . 
     Finally, as illustrated in  FIG. 2E , the diaphragm  101  is released by etching through the lower side of the substrate  104 . 
       FIG. 3  is a cross-sectional view of a piezoelectric microspeaker according to another embodiment. 
     Referring to  FIG. 3 , in the piezoelectric microspeaker according to this embodiment, a lower drive unit  1102  and an upper drive unit  1103  may be symmetrically formed with respect to a diaphragm  1101 . The lower and upper drive units  1102  and  1103  may include drive electrodes  1201  and  1301  connected to a drive power source  110 , piezoelectric plates  1202  and  1302  deforming according to voltage, and common electrodes  1203  and  1303  connected to a common power source  120 . 
     The structure of  FIG. 3  is different from the structure of  FIG. 1  in that the diaphragm  1101  is formed to be flat. More specifically, while the center of the diaphragm  101  protrudes in the structure of  FIG. 1  such that the edge of the diaphragm  101  is disposed at the same level as the lower drive unit  102 , the lower drive unit  1102  is formed at a relatively lower level in the structure of  FIG. 3  such that the edge and center of the diaphragm  1101  are at the same level. 
     As described above, the first drive electrode  1201  and the second drive electrode  1301  may be connected to the drive power source  110 , and the first common electrode  1203  and the second common electrode  1303  may be connected to the common power source  120 . Thus, the deformation directions of the first piezoelectric plate  202  and the second piezoelectric plate  1302  are opposite to each other. 
       FIGS. 4A to 4E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to another embodiment. This may serve as an example of a method of fabricating the piezoelectric microspeaker of  FIG. 3 . 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 3 and 4A  to  4 E. 
     First, as illustrated in  FIG. 4A , a cavity  401  is formed by etching a part of a substrate  1104  to make a space in which the lower drive unit  1102  will be formed, and an insulating layer  1105  is formed by oxidizing the substrate  1104  or by depositing a thin insulating layer on the substrate  1104 . 
     Subsequently, as illustrated in  FIG. 4B , the first drive electrode  1201 , the first piezoelectric plate  1202  and the first common electrode  1203  are formed by depositing and etching a thin conductive layer, a thin piezoelectric layer and a thin conductive layer in sequence. The first drive electrode  1201 , the first piezoelectric plate  1202  and the first common electrode  1203  may constitute the lower drive unit  1102  formed under the diaphragm  1101 . 
     Subsequently, as illustrated in  FIG. 4C , the diaphragm  1101  is formed by depositing and etching a thin polymer layer. At this time, a part of the lower drive unit  1102  is within the cavity  401  of the substrate  1104 , and thus the diaphragm  1101  can be formed to be substantially flat. 
     Subsequently, as illustrated in  FIG. 4D , the second common electrode  1303 , the second piezoelectric plate  1302  and the second drive electrode  1301  are formed by depositing and etching a thin conductive layer, a thin piezoelectric layer and a thin conductive layer in sequence. The second common electrode  1303 , the second piezoelectric plate  1302  and the second drive electrode  1301  may constitute the upper drive unit  1103  formed on the diaphragm  1101 . In addition, in this process, the first drive electrode  1201  can be electrically connected with the second drive electrode  1301 , and the first common electrode  1203  can be electrically connected with the second common electrode  1303 . 
     Finally, as illustrated in  FIG. 4E , the diaphragm  1101  is released by etching through the lower side of the substrate  104 . 
       FIG. 5  is a cross-sectional view of a piezoelectric microspeaker according to yet another embodiment. 
     Referring to  FIG. 5 , in the piezoelectric microspeaker according to this embodiment, a lower drive unit  2102  and an upper drive unit  2103  may be symmetrically formed with respect to a diaphragm  2101 . The lower and upper drive units  2102  and  2103  may include drive electrodes  2201  and  2301  connected to a drive power source  110 , which may be an AC power source, and piezoelectric plates  202  and  302  deforming according to voltage. 
     The diaphragm  2101  is formed of a thin conductive layer, connected to a common power source, and thus can provide ground for drive voltage. 
     For example, when the first drive electrode  2201  and the second drive electrode  2301  are connected to the AC power source  110  and the diaphragm  2101  of the thin conductive layer is connected to ground  120 , electric fields are generated between the first drive electrode  2201  and the diaphragm  2101  and between the second drive electrode  2301  and the diaphragm  2101 , respectively. 
     Here, the electric fields point opposite directions. Thus, the first piezoelectric plate  2202  and the second piezoelectric plate  2302  deform in opposite directions according to the generated electric fields, and the diaphragm  2101  can vibrate according to the deformation. 
       FIGS. 6A to 6E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another embodiment. This may serve as 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. 5 and 6A  to  6 E. 
     First, as illustrated in  FIG. 6A , an insulating layer  2105  is formed by oxidizing the upper surface of a substrate  104  or by depositing a thin insulating layer on the substrate  104 . 
     Subsequently, as illustrated in  FIG. 6B , the first drive electrode  2201  and the first piezoelectric plate  2202  are formed by depositing and etching a thin conductive layer and a thin piezoelectric layer. The first drive electrode  2201  and the first piezoelectric plate  2202  may constitute the lower drive unit  2102 . 
     Subsequently, as illustrated in  FIG. 6C , an insulating layer  105  for insulation between the diaphragm  2101  and the first drive electrode  2201  is formed, and the diaphragm  2101  is formed by depositing and etching a thin conductive layer. At this time, the center of the diaphragm  2101  can be formed to protrude according to the shape of the lower drive unit  102 . 
     Subsequently, as illustrated in  FIG. 6D , the second piezoelectric plate  2302  is formed by depositing and etching a thin piezoelectric layer on the diaphragm  2101 , the insulating layer  2105  for insulation between the diaphragm  2101  and the second drive electrode  2301  is additionally formed, and then the second drive electrode  2301  is formed by depositing and etching a thin conductive layer. The second drive electrode  2301  and the second piezoelectric plate  2302  may constitute the upper drive unit  2103 . In addition, in this process, the first drive electrode  2201  can be electrically connected with the second drive electrode  2301 . 
     Finally, as illustrated in  FIG. 6E , the diaphragm  2101  is released by etching through the lower side of the substrate  104 . 
       FIG. 7  is a cross-sectional view of a piezoelectric microspeaker according to yet another embodiment. 
     Referring to  FIG. 7 , in the piezoelectric microspeaker according to this embodiment, a lower drive unit  3102  and an upper drive unit  3103  may be symmetrically formed with respect to a diaphragm  3101 . The lower and upper drive units  3102  and  103  may include drive electrodes  3201  and  3301  connected to a drive power source  110 , and piezoelectric plates  3202  and  3302  deforming according to voltage, respectively. 
     The structure of  FIG. 7  is different from the structure of  FIG. 5  in that the diaphragm  3101  is formed to be substantially flat. More specifically, while the center of the diaphragm  2101  protrudes in the structure of  FIG. 5  such that the edge of the diaphragm  2101  is disposed at the same level as the lower drive unit  2102 , the lower drive unit  3102  is formed at a relatively lower level in the structure of  FIG. 7  such that the diaphragm  3101  has a generally flat structure. 
     As described above, the first drive electrode  3201  and the second drive electrode  3301  are connected to the drive power source  110 , and the diaphragm  3101  is formed of a thin conductive layer and connected to a common power source  120 . 
       FIGS. 8A to 8E  are cross-sectional views illustrating a method of fabricating a piezoelectric microspeaker according to yet another embodiment. This may serve as an example of a method of fabricating the piezoelectric microspeaker of  FIG. 7 . 
     The method of fabricating the piezoelectric microspeaker according to this embodiment will be described below with reference to  FIGS. 7 and 8A  to  8 E. 
     First, as illustrated in  FIG. 8A , a cavity  3401  is formed by etching a part of a substrate  3104  to make a space in which the lower drive unit  3102  will be formed, and an insulating layer  3105  is formed by oxidizing the substrate  3104  or by depositing a thin insulating layer on the substrate  3104 . 
     Subsequently, as illustrated in  FIG. 8B , the first drive electrode  3201  and the first piezoelectric plate  3202  are formed by depositing and etching a thin conductive layer and a thin piezoelectric layer. The first drive electrode  3201  and the first piezoelectric plate  3202  may constitute the lower drive unit  3102   
     Subsequently, as illustrated in  FIG. 8C , an insulating layer  3105  for insulation between the diaphragm  3101  and the first drive electrode  3201  is formed, and the diaphragm  3101  is formed by depositing and etching a thin conductive layer. At this time, a part of the lower drive unit  3102  is disposed within the cavity  3401  of the substrate  3104 , and thus the diaphragm  3101  can be formed to be generally flat. 
     Subsequently, as illustrated in  FIG. 8D , the second piezoelectric plate  3302  is formed by depositing and etching a thin piezoelectric layer, the insulating layer  3105  for insulation between the diaphragm  3101  and the second drive electrode  3301  is additionally formed, and then the second drive electrode  3301  is formed by depositing and etching a thin conductive layer. The second piezoelectric plate  3302  and the second drive electrode  3301  may constitute the upper drive unit  3103 . In addition, in this process, the first drive electrode  3201  can be electrically connected with the second drive electrode  3301 . 
     Finally, as illustrated in  FIG. 8E , the diaphragm  3101  is released by etching through the lower side of the substrate  104 . 
     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.