Patent Publication Number: US-8121317-B2

Title: Piezoelectric microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microphone, and more particularly, to a piezoelectric microphone. 
     2. Description of the Related Art 
     Microphones are used to convert sound waves or ultrasonic waves into electric signals. 
     Examples of the microphones include a carbonic microphone, a piezoelectric microphone, a movable-coil microphone, a vibration-diaphragm microphone, a condenser microphone, and a semiconductor microphone. The carbonic microphone operates using the pressure-dependent electric resistance of carbon particles. The movable-coil microphone operates using a current induced by a vibration of a coil attached to a vibration coil. The vibration-diaphragm microphone operates using an induced current generating when a sound wave vibrates a ribbon-shaped diaphragm disposed in a magnetic field. The condenser microphone operates using the concept of a condenser in which thin-vibration plates (fixed electrodes) face each other at a close distance. The semiconductor microphone operates using a stress semiconductor having an electric resistance varying according to a mechanical force applied to the stress semiconductor. 
       FIGS. 1 and 2  illustrate a conventional piezoelectric microphone. In the conventional piezoelectric microphone, a lower electrode  20 , a piezoelectric layer  30  formed of a piezoelectric material such as aluminum nitride AlN and piezoelectric ceramic, and an upper electrode  40  are sequentially formed on a protection layer  10  deposited on a silicon substrate (S). 
     The conventional piezoelectric microphone can be fabricated through a process illustrated in  FIG. 3 . 
     Referring to  FIG. 3 , in operation S 10 , the protection layer  10  is deposited on the silicon substrate (S). In operation S 12 , the lower electrode  20 , the piezoelectric layer  30 , and the upper electrode  40  are sequentially deposited on the protection layer  10 . In operation S 14 , the backside of the silicon substrate (S) is etched by back-side etching to remove a center portion of the silicon substrate (S) until the bottom surface of the protection layer  10  is exposed. 
     The piezoelectric layer  30  generates a piezoelectric signal in proportion to an applied sound pressure, and the piezoelectric signal is transmitted to an external amplifier through the lower and upper electrodes  20  and  40 . 
     Since the lower electrode  20 , the piezoelectric layer  30 , and the upper electrode  40  are formed into a simple parallel plate structure, the voltage level of the piezoelectric signal generated in response to the sound pressure is limited to below a predetermined value. As a result, it is difficult to increase the sensitivity of the piezoelectric microphone. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a piezoelectric microphone that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a piezoelectric microphone including a plurality of variously arranged cells each having a lower electrode, a piezoelectric layer, and an upper electrode. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a piezoelectric microphone including a plurality of cells arranged on a protection layer in various pattern, wherein each of the cell includes a lower electrode, a piezoelectric layer, and an upper electrode. 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1  is a cross-sectional view illustrating a conventional piezoelectric microphone; 
         FIG. 2  is a plan view of the piezoelectric microphone depicted in  FIG. 1 ; 
         FIG. 3  is a view for explaining a manufacturing process of a conventional piezoelectric microphone; 
         FIG. 4  is a cross-sectional view illustrating a piezoelectric microphone according to a first embodiment of the present invention; 
         FIG. 5  is a plan view illustrating the piezoelectric microphone depicted in  FIG. 4 ; 
         FIG. 6  is a view for explaining a manufacturing process of the piezoelectric microphone depicted in  FIG. 4 ; 
         FIG. 7  is a cross-sectional view illustrating a piezoelectric microphone according to a second embodiment of the present invention; 
         FIG. 8  is a plan view illustrating the piezoelectric microphone depicted in  FIG. 7 ; 
         FIG. 9  is a view for explaining a manufacturing process of the piezoelectric microphone depicted in  FIG. 7 ; and 
         FIG. 10  is a plan view illustrating exemplary cell arrangements of a piezoelectric microphone according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     Referring to  FIGS. 4 to 9 , a piezoelectric microphone of the present invention includes a plurality of cells arranged in various patterns on a protection layer  10   a  or  10   b  deposited on a silicon substrate (S). 
     The cells are connected in series. Each cell includes a lower electrode  20   a  or  20   b , a piezoelectric layer  30   a  or  30   b  formed of a piezoelectric material such as aluminum nitride AlN and piezoelectric ceramic, and an upper electrode  40   a  or  40   b  that are formed on the protection layer  10   a  or  10   b.    
     As explained above, the piezoelectric microphone of the present invention includes the cells. Each of the cells is formed by sequentially stacking the lower electrode  20   a  or  20   b , the piezoelectric layer  30   a  or  30   b , and the upper electrode  40   a  or  40   b  on the protection layer  10   a  or  10   b  deposited on the silicon substrate (S). The cells can be arranged in various patterns. Thereafter, the backside of the silicon substrate (S) is etched by back-side etching to remove a center portion of the silicon substrate (S) until the bottom surface of the protection layer  10   a  or  10   b  is exposed. Exemplary embodiments of the present invention will now be described. 
       FIGS. 4 and 5  are a cross-sectional view and a plan view illustrating a piezoelectric microphone according to a first embodiment of the present invention. Referring to  FIGS. 4 and 5 , the piezoelectric microphone includes two semicircular cells arranged on a protection layer  10   a.    
     The piezoelectric microphone of the current embodiment can be fabricated as shown in  FIG. 6 . 
     Referring to  FIG. 6 , the piezoelectric microphone of the first embodiment is fabricated as follows. In operation S 20 , the protection layer  10   a  is deposited on a silicon substrate (S). In operation S 22 , two semicircular cells are formed on the protection layer  10   a . Each of the two semicircular cells includes a lower electrode  20   a , a piezoelectric layer  30   a , and an upper electrode  40   a  that are sequentially stacked on the protection layer  10   a . In operation S 24 , the backside of the silicon substrate (S) is etched by back-side etching to remove a center portion of the silicon substrate (S) until the bottom surface of the protection layer  10   a  is exposed. 
       FIGS. 7 and 8  are a cross-sectional view and a plan view illustrating a piezoelectric microphone according to a second embodiment of the present invention. Referring to  FIGS. 7 and 8 , the piezoelectric microphone includes two coaxial cells arranged on a protection layer  10   b.    
     The piezoelectric microphone of the current embodiment can be fabricated as shown in  FIG. 9 . 
     Referring to  FIG. 9 , the piezoelectric microphone of the second embodiment is fabricated as follows. In operation S 30 , the protection layer  10   b  is deposited on a silicon substrate (S). In operation S 32 , two coaxial cells are formed on the protection layer  10   a . Each of the two coaxial cells includes a lower electrode  20   b , a piezoelectric layer  30   b , and an upper electrode  40   b  that are sequentially stacked on the protection layer  10   a . The upper electrodes  40   b  of the two coaxial cells are connected using an air bridge  50 . Then, in operation S 34 , the backside of the silicon substrate (S) is etched by back-side etching to remove a center portion of the silicon substrate (S) until the bottom surface of the protection layer  10   b  is exposed. 
       FIG. 10  is a plan view illustrating exemplary cell arrangements of a piezoelectric microphone according to the present invention. 
     For example, referring to  FIG. 10(   a ), two coaxial cells are arranged on a protection layer like those shown in  FIGS. 7 and 8 . In this case, the inner cell having a circular shape has the same area as the outer cell having a ring shape. 
     The piezoelectric microphone of the present invention operates as follows. 
     In each cell of the piezoelectric microphone, the piezoelectric layer  30   a  or  30   b  generates a piezoelectric signal in proportion to an applied sound pressure, and the piezoelectric signal is transmitted to an external amplifier through the lower electrode  20   a  or  20   b  and the upper electrode  40   a  or  40   b.    
     Since the piezoelectric signals of the respective cells are added, the total piezoelectric signal corresponding to the applied sound pressure can have an increased voltage level. 
     For example, when the piezoelectric microphone includes n cells and a sound pressure is applied to the piezoelectric microphone, the n cells generate n piezoelectric signals in response to the sound pressure. Hence, the total piezoelectric signal of the piezoelectric microphone can have the same voltage level as the sum of voltages levels of the n piezoelectric signals. 
     Therefore, according to the present invention, the voltage level of a total piezoelectric signal can be easily increased to a desired level by adjusting the number of the cells of the piezoelectric microphone. Accordingly, the sensitivity of the piezoelectric microphone can be increased. 
     As described above, the piezoelectric microphone of the present invention includes a plurality of cells each having the lower electrode, the piezoelectric layer, and the upper electrode. The cells can be arranged on the protection layer in various patterns. The piezoelectric microphone of the present invention includes the plurality of cells although a piezoelectric microphone of the related art includes a single parallel plate structure including a lower electrode, a piezoelectric layer, and an upper electrode. Therefore, according to the present invention, the voltage level of a piezoelectric signal of the piezoelectric microphone can be easily increased to a desired level by adjusting the number of the cells. Thus, the sensitivity of the piezoelectric microphone can be increased. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.