Patent Publication Number: US-2012025336-A1

Title: Converter module and method of manufacturing the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation application of PCT application No. PCT/JP2010/001034 filed on Feb. 18, 2010, designating the United States of America. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a converter module including a converter, such as a sound pressure sensor or a pressure sensor, and to a method of manufacturing the converter module. 
     (2) Description of the Related Art 
     Conventionally, a converter module including a converter, such as a silicon microphone or a pressure sensor, detect pressure fluctuations of, for example, sound by sensing vibration of a diaphragm included in a sound pressure sensor chip or a pressure sensor chip, as disclosed in Japanese Unexamined Patent Application Publication Nos. 2004-537182 and 2007-263677. Hereafter, these references are referred to as Patent Reference 1 and Patent Reference 2, respectively. 
       FIG. 21  is a cross-sectional diagram of a conventional converter module  500  disclosed in Patent Reference 1. As shown, in the conventional converter module  500 , a converter  501  and a semiconductor substrate  503  are implemented on a main surface of a circuit substrate  504 . The converter  501  has a diaphragm  502 , and the semiconductor substrate  503  controls the converter  501 . Moreover, a cavity part  507  is formed in the circuit substrate  504 , immediately below the diaphragm  502 . The converter  501  and the semiconductor substrate  503  are covered with a shielding cap  505 . With this arrangement, the diaphragm  502  vibrates in response to a sound wave transmitted via a sound hole  506  penetrating the shielding cap  505 . Then, the converter module  500  detects the pressure fluctuations of the sound wave from the vibration of the diaphragm  502 . 
     Here, when the cavity part  507  is small in volume, the air resistance of the cavity part  507  is large, which makes it hard for the diaphragm to vibrate. As a result, the amount of displacement of the diaphragm  502  is small, meaning that the pressure fluctuations cannot be detected with accuracy. 
     On this account, in order for the diaphragm  502  to vibrate, it is necessary for the cavity part  507  to have an adequate volume. Moreover, the volume of the cavity part  507  needs to be changed as appropriate according to the characteristics of the converter  501 . 
     In the case of the conventional converter module  500 , the cavity part  507  is formed to be recessed from the main surface of the circuit substrate  504  so that the volume of the cavity part  507  is increased. However, since the converter  501  and the semiconductor substrate  503  are arranged side by side on the circuit substrate  504 , the circuit substrate  504  is large in area size, which leads to a problem that it is difficult to downsize the converter module  500 . 
     To address this problem, Patent Reference 2 discloses a converter module in which a semiconductor substrate, a converter, a shielding cap are laminated on a circuit substrate. In this disclosed example, a cavity part is formed to be recessed from a main surface of the semiconductor substrate so that the volume of the cavity part located immediately below a diaphragm is not decreased. 
     SUMMARY OF THE INVENTION 
     In the conventional converter module having the laminated structure as disclosed in Patent Reference 2, the circuit substrate, the semiconductor substrate, the converter, and the shielding cap are laminated in order from bottom to top. Therefore, the converter module is thick, and a reduction in profile of the converter module is difficult. 
     Examples of the method to reduce the profile of the converter module having the above configuration include (1) thinning the converter and (2) thinning the semiconductor substrate. However, the method (1) causes concern that the strength of the converter is compromised due to the thinned converter. The method (2) also causes concern that the pressure fluctuations cannot be detected with accuracy because it is hard for the diaphragm to vibrate due to the reduced volume of the cavity part. 
     Moreover, the conventional configuration has another problem with the detection sensitivity of the converter module. The detection sensitivity of the converter module can be expressed quantitatively by Equation 1 below, where “Sen” represents the detection sensitivity. 
     
       
         
           
             
               
                 
                   Sen 
                   = 
                   
                     
                       ES 
                       d 
                     
                      
                     
                       C 
                       ges 
                     
                      
                     
                       1 
                       
                         1 
                         + 
                         
                           
                             ω 
                             2 
                           
                            
                           
                             M 
                             0 
                           
                            
                           
                             C 
                             ges 
                           
                         
                         - 
                         
                           jω 
                            
                           
                               
                           
                            
                           
                             r 
                             0 
                           
                            
                           
                             C 
                             ges 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
     Here, “C ges ” is expressed by Equation 2 below. 
     
       
         
           
             
               
                 
                   
                     C 
                     ges 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           1 
                           
                             C 
                             m 
                           
                         
                         + 
                         
                           1 
                           
                             C 
                             v 
                           
                         
                       
                       ) 
                     
                     
                       - 
                       1 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
     In Equation 1 above: “E” represents an electric field; “S” represents an area size of the diaphragm; “d” represents a distance between two pairs of diaphragms; “C ges ” represents combined compliance; “C m ” represents compliance of the diaphragm; “C v ” represents compliance of the cavity part; “M 0 ” represents a mass of the diaphragm; and “r 0 ” represents a radiation impedance. When a resonant frequency of the diaphragm is adequately low, Equation 1 is chanced into Equation 3 as follows. 
     
       
         
           
             
               
                 
                   Sen 
                   = 
                   
                     
                       
                         ES 
                         d 
                       
                        
                       
                         C 
                         ges 
                       
                     
                     = 
                     
                       
                         ES 
                         d 
                       
                        
                       
                         
                           ( 
                           
                             
                               1 
                               
                                 C 
                                 m 
                               
                             
                             + 
                             
                               1 
                               
                                 C 
                                 v 
                               
                             
                           
                           ) 
                         
                         
                           - 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
     The compliance C m  of the diaphragm is expressed by Equation 4 below. 
     
       
         
           
             
               
                 
                   
                     C 
                     m 
                   
                   = 
                   
                     1 
                     k 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
     In Equation 4, “k” represents a spring constant of the diaphragm. 
     The compliance C v  of the cavity part is expressed by Equation 5 below. 
     
       
         
           
             
               
                 
                   
                     C 
                     v 
                   
                   = 
                   
                     V 
                     
                       
                         S 
                         2 
                       
                        
                       γ 
                        
                       
                           
                       
                        
                       
                         P 
                         0 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   5 
                 
               
             
           
         
       
     
     In Equation 5, “V” represents a volume of the cavity part, “y” represents a specific heat resistance of air, and “P 0 ” represents a normal atmospheric pressure. 
     As can be understood from Equation 4, C m  increases as the diaphragm becomes softer. Moreover, as can be understood from Equation 5, C v  increases as the volume of the cavity part is increased. 
     By increasing each value of the parameters in Equation 3, the detection sensitivity Sen can be improved. To be more specific, one of the values of the electric field E, the area size S, the diaphragm compliance C m , and the cavity compliance C v  may be increased. Alternatively, the distance d between the diaphragms may be reduced. 
     However, there is a limit to use the electric field E, and there is also a limit to the distance d between the two pairs of diaphragms in the manufacturing process. Thus, in order to improve the detection sensitivity, it is necessary to (A) increase the area size S of the diaphragm or (B) increase the cavity compliance C v  by increasing the cavity volume V. 
     In the case of (A), however, the converter needs to be increased in size so as to increase the area size S of the diaphragm. When the area size of the diaphragm is increased, this means that the detection sensitivity decreases because the cavity compliance decreases as can be understood from Equation 5. In order to address this, the recessed part of the semiconductor substrate needs to be increased in size so as to increase the area size of the diaphragm, and the volume of the cavity part also needs to be increased. 
     In the case of (B), the recessed part needs to be increased in size by increasing the size of the semiconductor substrate, as is the case with (A). That is to say, as the detection sensitivity of the converter is increased, the size of the semiconductor substrate is accordingly increased. On this account, when a converter module is designed, the size of a semiconductor substrate is determined according to the detection sensitivity of a converter. 
     The present invention is conceived in view of the aforementioned conventional problem, and has an object to provide a converter module and a method of manufacturing the same capable of easily achieving miniaturization and profile reduction without decreasing the pressure detection sensitivity. 
     In order to achieve the above object, the converter module according to an aspect of the present invention is a converter module including: a converter which converts vibration of a diaphragm into an electric signal; and a semiconductor substrate which processes the electric signal obtained as a result of the conversion performed by the converter, wherein the converter includes: a base including a cavity part having an opening in a first main surface of the base; and the diaphragm which is arranged on the first main surface to cover the opening of the cavity part and converts the vibration into the electric signal, and the semiconductor substrate is formed as a part of the base. 
     With this configuration, since the semiconductor substrate is formed as a part of the base in which the cavity part is formed, the volume of the cavity part can be adequately ensured without having to increase the converter module in thickness. Thus, the pressure detection sensitivity does not decrease. As compared with the configuration where a semiconductor substrate and a converter are arranged side by side or are laminated, the converter module can be miniaturized and reduced in profile more easily. 
     Moreover, a part of a side surface of the semiconductor substrate may face the cavity part. 
     With this, the cavity part can be increased in volume and, therefore, the accuracy of the pressure detection can be improved more. 
     Furthermore, a part of a side surface of the base and a part of a side surface of the semiconductor substrate may be in one plane. 
     With this, the converter modules can be easily diced from an array in which a plurality of converters are formed. This allows the converter module to be manufactured at lower cost. 
     Moreover, the base may include a recessed part having an opening in a second main surface of the base, the second main surface being opposite to the first main surface, and the semiconductor substrate may be formed in the recessed part. 
     With this, the diaphragm can be supported precisely, and the strength of the converter can also be maintained. Thus, the accuracy of the pressure detection can be improved. 
     Furthermore, the base may include a through area penetrating from the first main surface to a second main surface opposite to the first main surface, and the semiconductor substrate may be formed in the through area. 
     With this, the cavity part and the through area can be formed at one time. This can simplify the manufacturing process and, thus, the converter module can be manufactured at low cost. Moreover, since the electrode part connecting the diaphragm and the semiconductor substrate can be shortened, parasitic resistance caused by the electrode part can be reduced. 
     Moreover, the second main surface of the base and a main surface of the semiconductor substrate may be in one plane. 
     With this, even when the semiconductor substrate is formed as a part of the base, the thickness of the base does not change and thus the converter module can be reduced in profile. 
     Furthermore, the converter module may further include: a first insulating layer formed between the semiconductor substrate and the base; and a first penetrating electrode penetrating each of the base and the first insulating layer in a thickness direction and electrically connecting the diaphragm and the semiconductor substrate. 
     With this, current is prevented from leaking from the first penetrating electrode. 
     Moreover, the converter module may further include a protection layer including a hole penetrating in a thickness direction, wherein the protection layer is arranged above the diaphragm so that the opening of the cavity part is located immediately below the hole. 
     With this, since the hole is formed immediately above the diaphragm, air flow such as a sound wave can be controlled to head for the diaphragm. As a result, the diaphragm can vibrate with accuracy. 
     Furthermore, the protection layer may include electrical wiring for transmitting, to an external source, the electric signal processed by the semiconductor substrate, and the converter module may further include a second penetrating electrode penetrating the base in a thickness direction and electrically connecting the semiconductor substrate and the electrical wiring. 
     With this, the electric signal processed by the semiconductor substrate can be easily extracted outside. 
     Moreover, the converter module may further include: a second insulating layer formed between the first main surface of the base or the diaphragm and the protection layer; and an external electrode penetrating the second insulating layer and electrically connecting the second penetrating electrode and the electrical wiring. 
     With this, current is prevented from leaking from the second penetrating electrode. 
     Furthermore, the converter module may further include a shielding cap protecting a second main surface of the base and a side surface of the base, the second main surface being opposite to the first main surface. 
     With this, the converter module can be protected from an external shock or from noise caused by an electromagnetic wave or the like. 
     Moreover, the converter module may further include: a circuit substrate which includes electrical wiring for transmitting, to an external source, the electric signal processed by the semiconductor substrate and is formed on a second main surface of the base, the second main surface being opposite to the first main surface; and a third penetrating electrode penetrating the semiconductor substrate in a thickness direction and electrically connecting the semiconductor substrate and the electrical wiring. 
     With this, since the hole is formed immediately above the diaphragm, air flow such as a sound wave can be controlled to head for the diaphragm. As a result, the diaphragm can vibrate with accuracy. Moreover, the electric signal processed by the semiconductor substrate can be easily extracted outside. 
     Furthermore, the protection layer may be a shielding cap further protecting the base by covering a side surface of the base. 
     With this, the converter module can be protected from an external shock or from noise caused by an electromagnetic wave or the like. 
     Moreover, the method of manufacturing the converter module according to another aspect of the present invention is a method of manufacturing a converter module including a converter which converts vibration of a diaphragm into an electric signal and a semiconductor substrate which processes the electric signal obtained as a result of the conversion performed by the converter, the converter including: a base; and the diaphragm which is arranged on a first main surface of the base and converts the vibration into the electric signal, and the method including: etching the base from a second main surface opposite to the first main surface to form a cavity part in an first area immediately below the diaphragm and a recessed part in a second area different from the first area, the cavity part penetrating the base in a thickness direction; and bonding the semiconductor substrate to the recessed part formed in the etching. 
     With this configuration, since the semiconductor substrate is formed as a part of the base, the volume of the cavity part can be adequately ensured without having to increase the converter module in thickness. Thus, the pressure detection sensitivity does not decrease. As compared with the configuration where a semiconductor substrate and a converter are arranged side by side or are laminated, the converter module can be miniaturized and reduced in profile more easily. 
     Furthermore, in the etching, the cavity part and the recessed part may be formed in each of a plurality of converters formed in an array so that the formed recessed parts of two adjacent converters are adjacent to each other, each of the converters including the base and the diaphragm, in the bonding, two semiconductor substrates may be respectively bonded, at one time, to the adjacent recessed parts formed in the etching, and the method may further include dicing the array so that the plurality of converters are individually separated. 
     With this, the converter modules can be easily diced from an array in which a plurality of converters are formed. This allows the converter module to be manufactured at lower cost. 
     Moreover, the etching may include: performing a first etching on the first area immediately below the diaphragm by etching from the second main surface of the base; and performing a second etching on the first area etched in the performing a first etching and on the second area at one time by etching from the second main surface of the base, to form the cavity part in the first area and the recessed part in the second area. 
     Furthermore, in the performing a first etching, the first area may be etched so that the base immediately below the diaphragm is at least as thick as the semiconductor substrate, and in the performing a second etching, the first area etched in the performing a first etching and the second area may be etched at least as deep as a thickness of the semiconductor substrate. 
     Moreover, in the etching, the cavity part and the recessed part may be formed at one time so that the recessed part penetrates the base in the thickness direction. 
     With this, the cavity part and the recessed part can be formed at one time. This can simplify the manufacturing process and, thus, the converter module can be manufactured at low cost. 
     The converter module according to the present invention can achieve miniaturization and reduction in profile more easily without decreasing the pressure detection sensitivity, as compared with the conventional converter module. 
     FURTHER INFORMATION ABOUT TECHNICAL BACKGROUND TO THIS APPLICATION 
     The disclosure of Japanese Patent Application No. 2009-098482 filed on Apr. 14, 2009 including specification, drawings and claims is incorporated herein by reference in its entirety. 
     The disclosure of PCT application No. PCT/JP2010/001034 filed on Feb. 18, 2010, including specification, drawings and claims is incorporated herein by reference in its entirety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. In the Drawings: 
         FIG. 1  is a perspective view of a converter module in a first embodiment according to the present invention; 
         FIG. 2  is a plan view of the converter module in the first embodiment; 
         FIG. 3  is a cross-sectional view of the converter module in the first embodiment taken along a line A-A in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a configuration of a converter module in a modification of the first embodiment; 
         FIG. 5  is a cross-sectional view showing a manufacturing process of the converter module in the first embodiment; 
         FIG. 6  is a cross-sectional view showing a process of bonding a semiconductor substrate in the manufacturing process of the converter module in the first embodiment; 
         FIG. 7  is a cross-sectional view showing a dicing process in the manufacturing process of the converter module in the first embodiment; 
         FIG. 8  is a cross-sectional view showing another manufacturing process of the converter module in the first embodiment; 
         FIG. 9  is a perspective view of a converter module in a second embodiment according to the present invention; 
         FIG. 10  is a plan view of the converter module in the second embodiment; 
         FIG. 11  is a cross-sectional view of the converter module in the second embodiment taken along a line B-B in  FIG. 10 ; 
         FIG. 12  is a cross-sectional view of a configuration of a converter module in a modification of the second embodiment; 
         FIG. 13  is a perspective view of a converter module in a third embodiment according to the present invention; 
         FIG. 14  is a plan view of the converter module in the third embodiment; 
         FIG. 15  is a cross-sectional view of the converter module in the third embodiment taken along a line C-C in  FIG. 14 ; 
         FIG. 16  is a cross-sectional view of a configuration of a converter module in a modification of the third embodiment; 
         FIG. 17  is a perspective view of a converter module in a fourth embodiment according to the present invention; 
         FIG. 18  is a plan view of the converter module in the fourth embodiment; 
         FIG. 19  is a cross-sectional view of the converter module in the fourth embodiment taken along a line D-D in  FIG. 18 ; 
         FIG. 20  is a cross-sectional view of a configuration of a converter module in a modification of the fourth embodiment; and 
         FIG. 21  is a cross-sectional view of a configuration of a conventional converter module. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a detailed description of embodiments according to the present invention, with reference to the drawings. 
     First Embodiment 
     A converter module in the first embodiment includes a converter and a semiconductor substrate. The converter includes a diaphragm and a base, and converts vibration of the diaphragm into an electric signal. The semiconductor substrate processes the electric signal obtained as a result of the conversion performed by the converter, and is formed as a part of the base. 
     An example of the converter module in the first embodiment is described.  FIG. 1  is a perspective view of a converter module  100  in the first embodiment.  FIG. 2  is a plan view of the converter module  100  in the first embodiment.  FIG. 3  is a cross-sectional view of the converter module  100  in the first embodiment taken along a line A-A in  FIG. 2 . 
     As shown in  FIG. 1 , the converter module  100  includes a converter  110 , a semiconductor substrate  120 , a circuit substrate  130 , and a shielding cap  140 .  FIG. 2  is a plan view of the converter module  100  shown in  FIG. 1 , as viewed from above. 
     The converter  110  includes a diaphragm  111  and a base  112 , and converts vibration of the diaphragm  111  into an electric signal. 
     The diaphragm  111  is formed on a front surface which is a first main surface of the base  112 , i.e., an upper surface of the base  112  shown in  FIG. 1 , to cover an opening of a cavity part  113 . The diaphragm  111  vibrates in response to a sound wave or the like and converts the vibration into an electric signal. For example, the diaphragm  111  is a filmy diaphragm of capacitor type having two parallel flat-plate electrodes. In this case, a distance between the parallel flat-plate electrodes changes as a result of the vibration and, thus, electrostatic capacitance varies according to the change in distance. Then, the diaphragm  111  outputs the variations in electrostatic capacitance as the electric signal. It should be noted that the diaphragm  111  is made of, for example, one of or both polysilicon (Poly-Si) and silicon nitride (SiN). 
     As shown in  FIG. 3 , the base  112  supports the diaphragm  111  formed on the first main surface of the base  112 , and the cavity part  113  having the opening in the front surface of the base  112  is formed immediately below the diaphragm  111 . Moreover, as shown in  FIG. 1 , the base  112  includes a recessed part  118  having an opening in a back surface which is a second main surface of the base  112 , i.e., a lower surface opposite to the front surface of the base  112 . The semiconductor substrate  120  is formed in this recessed part  118 . On the front surface of the base  112 , an electrode area  114  and an electrode area  117  are formed as well. 
     Moreover, the base  112  includes a penetrating electrode  115  for electrically connecting the electrode area  114  and the semiconductor substrate  120 . The base  112  also includes a penetrating electrode  116  for electrically connecting the electrode area  117  and the semiconductor substrate  120 . The electrode area  117  is used for sending, to an external source, the electric signal received from the semiconductor substrate  120 . Here, as shown in  FIG. 3 , a pair of penetrating electrodes  115  and a pair of penetrating electrodes  116  are formed on the base  112 , so that each of the electric signals respectively from the two parallel flat-plate electrodes of the diaphragm  111  is transmitted to the semiconductor substrate  120 . 
     It should be noted that the base  112  is made of, for example, bulk silicon (bulk Si). The thickness of the base  112  shown in the left side of  FIG. 3  is approximately 100 μm to 200 μm, and the thickness of the base  112  having the recessed part  118  as shown in the right side of  FIG. 3  is approximately 50 μm to 100 μm. 
     In the case where, for example, the converter module does not need to be reduced in profile, the thickness of the base  112  may be 200 μm or more, like 500 μm. When the base  112  is thicker, the volume of the cavity part  113  can be increased and the strength of the converter module  100  can also be increased, which is preferable. Moreover, in such a case, a polishing process to thin the base  112  can be omitted, which allows the converter module  100  to be manufactured at low cost. 
     The cavity part  113  is formed immediately below the diaphragm  111  and has the opening in the front surface of the base  112 . It is preferable for the volume of the cavity part  113  to be large in order for the diaphragm  111  to fully vibrate. Although  FIG. 3  shows that side surfaces of the cavity part  113  are sloped, the cavity part  113  is not limited to the shape shown in  FIG. 3  and can be in any shape. For example, the cavity part  113  may have vertical side surfaces and thus be in the shape of a rectangular parallelepiped. 
     The electrode area  114  is an electrode for extracting the electrical signal from the diaphragm  111 . The electrode area  114  electrically connects one of the parallel flat-plate electrodes of the diaphragm  111  and the semiconductor substrate  120 , via the penetrating electrode  115 . The electrode area  114  is made of a metal, such as Poly-Si or aluminum (Al). 
     The penetrating electrode  115  is an example of a first penetrating electrode which electrically connects the diaphragm  111  and the semiconductor substrate  120  and penetrates the base  112  in the thickness direction. To be more specific, the penetrating electrode  115  is a conductive region for electrically connecting the electrode area  114  and the semiconductor substrate  120 , and fills the inside of a through hole formed in the base  112 . The penetrating electrode  115  is made of a metal, such as Poly-Si, Al, titanium (Ti), or copper (Cu). 
     The penetrating electrode  116  is an example of a second penetrating electrode which electrically connects the semiconductor substrate  120  and electrical wiring formed on the circuit substrate  130  and penetrates the base  112  in the thickness direction. To be more specific, the penetrating electrode  116  is a conductive region for electrically connecting the semiconductor substrate  120  and the electrode area  117 , and fills the inside of a through hole formed in the base  112 . The penetrating electrode  116  is made of a metal, such as Poly-Si, Al, Ti, or Cu. 
     The electrode area  117  is an electrode connected to an external electrode  132  so as to send, to the external source, the electric signal processed by the semiconductor substrate  120 . The electrode area  117  electrically connects the penetrating electrode  116  and the external electrode  132 . The electrode area  117  is made of a metal, such as Poly-Si, Al, Ti, or Cu. 
     In  FIG. 3 , the penetrating electrode  115  fills the inside of the through hole. However, the penetrating electrode  115  may be formed only on the inside wall of the through hole so long as the penetrating electrode  115  electrically connects the electrode area  114  and the semiconductor substrate  120 . The same holds for the penetrating electrode  116 . 
     The semiconductor substrate  120  processes the electric signal obtained as a result of the conversion performed by the converter  110 , and is formed as a part of the base  112 . More specifically, as a part of the base  112 , the semiconductor substrate  120  supports the diaphragm  111  as well as forming the cavity part  113 . 
     For example, the semiconductor substrate  120 , which controls the converter  110 , receives the electric signal obtained from the conversion performed by the diaphragm  111  and includes an amplifier circuit for amplifying the received electric signal. The electric signal amplified by the semiconductor substrate  120  is sent to an external source via the external electrode  132 . Moreover, the semiconductor substrate  120  is formed in the recessed part  118  of the base  112 , as shown in  FIG. 1  and  FIG. 3 . 
     Furthermore, a part of a side surface of the semiconductor substrate  120  faces the cavity part  113 . That is to say, the cavity part  113  is formed by the inner walls of the base  112 , the part of the side surface of the semiconductor substrate  120 , and the shielding cap  140 . In addition, a part of an outer side surface of the base  112  and a part of a side surface of the semiconductor substrate  120  are in one plane. 
     It is preferable that the back surface of the base  112  and a back surface of the semiconductor substrate  120  are in one plane. To be more specific, it is preferable for the thickness of the base  112  shown in the left side of  FIG. 3  to be approximately equal to a sum of the thickness of the base  112  having the recessed part  118  as shown in the right side of  FIG. 3  and the thickness of the semiconductor substrate  120 . 
     An insulating film  121  is formed on a front surface of the semiconductor substrate  120 . With this, current is prevented from leaking from the penetrating electrode  115 . The insulating film  121  is made of oxide silicon (SiO 2 ) or SiN, for example. 
     An insulating paste  122  is formed on the insulating film  121 . The insulating paste  122  is made of, for example, an insulating resin, and bonds the semiconductor substrate  120  and the base  112  (i.e., the converter  110 ). When the converter  110  and the semiconductor substrate  120  are bonded, it is preferable not only to bond a main surface of the semiconductor substrate  120  and an exposed back surface of the base  112  where the recessed part  118  is formed, but also to bond the side surface of the semiconductor substrate  120  and the side surface of the base  112  for a stronger bonding as shown in  FIG. 1  and  FIG. 3 . 
     In this way, the converter module  100  includes a first insulating layer formed between the semiconductor substrate  120  and the base  112 . In  FIG. 1  and  FIG. 3 , the first insulating layer corresponds to the insulating film  121  and the insulating paste  122 . Note that each of the penetrating electrodes  115  and  116  penetrates both the insulating film  121  and the insulating paste  122  in their thickness directions, as shown in  FIG. 3 . 
     The circuit substrate  130  is formed on the converter  110  via an insulating sheet  131 . Here, electrical wiring (not illustrated) for an external electric connection may be included in the circuit substrate  130  or may be formed on a front surface of the circuit substrate  130 . More specifically, this electrical wiring is used for sending, to an external source, the electrical signal processed by the semiconductor substrate  120 . The circuit substrate  130  also functions as a protective layer to protect the upper surface of the converter  110 . 
     The insulating sheet  131  is an example of a second insulating layer formed between the front surface of the base  112  or the diaphragm  111  and the circuit substrate  130 . The insulating sheet  131  is made of, for example, an insulating resin, and bonds the circuit substrate  130  and the converter  110 . 
     The external electrode  132  is formed in the insulating sheet  131 . The external electrode  132  penetrates the insulating sheet  131 , and electrically connects the penetrating electrode  116  and the electrical wiring included in the circuit substrate  130 . The external electrode  132  sends, to the circuit substrate  130 , the electric signal received from the semiconductor substrate  120  via the penetrating electrode  116 . For example, the external electrode  132  is made of a lead-free solder material, which is an alloy of tin (Sn), silver (Ag), and Cu (namely, a Sn—Ag—Cu alloy). 
     Here, a sound hole  133  is formed in the circuit substrate  130  and the insulating sheet  131 . The sound hole  133  penetrates both the circuit substrate  130  and the insulating sheet  131  in their thickness directions. The diaphragm  111  is located immediately below the sound hole  133 . In other words, the circuit substrate  130  is located above the diaphragm  111  so that the opening of the cavity part  113  is located immediately below the sound hole  133 . The diaphragm  111  vibrates in response to a sound wave transmitted via the sound hole  133 . The converter module  100  in the first embodiment can detect pressure fluctuations in the sound wave by sensing the vibration of the diaphragm  111 . 
     The shielding cap  140  is formed under the converter  110  to protect the converter  110  from an external shock or from noise caused by an electromagnetic wave or the like. The shielding cap  140  is boned to the back surface of the base  112  and to the back surface of the semiconductor substrate  120 , via a bonding adhesive  141 . 
     Note that the shielding cap  140  may cover not only a back surface of the converter module  100  but also side surfaces of the converter module  100 .  FIG. 4  is a cross-sectional view of a configuration of a converter module  100   a , as a modification of the converter module  100  in the first embodiment. As shown in  FIG. 4 , a shielding cap  140   a  of the converter module  100   a  covers side surfaces of the converter  110  and the semiconductor substrate  120  as well, via a bonding adhesive  141   a . In this way, the shielding cap  140   a  protects the back surface and side surfaces of the base  112 . 
     With this configuration, the converter module  100   a  can be more protected from an external shock or from noise caused by an electromagnetic wave or the like. Moreover, in the case where the shielding cap  140   a  is made of a metal, heat generated by the converter module  100   a  and the semiconductor substrate  120  can be significantly drawn away. This can reduce warpage and thermal noise which may be caused to the converter module  100   a  and the semiconductor substrate  120  by the generated heat, thereby improving the accuracy in detecting the vibration of the diaphragm  111 . Here, the way of applying the bonding adhesive  141   a  can be selected as appropriate. More specifically, the bonding adhesive  141   a  may be fully applied to a surface where the converter  110  and the shielding cap  140   a  meet or may be applied only to a surface where the circuit substrate  130  and the shielding cap  140   a  meet. 
     As described, since the semiconductor substrate  120  is formed in the recessed part  118  of the base  112 , the circuit substrate  130  can be equal in area size to the converter  110 . Thus, as compared with the conventional configuration where the converter  110  and the semiconductor substrate  120  are arranged side by side on the circuit substrate  130 , the converter module  100  can be miniaturized. 
     In addition, the thickness of the converter  110  (i.e., the base  112 ) shown in the left side of  FIG. 3  can be approximately equal to a sum of the thickness of the converter  110  (i.e., the base  112 ) shown in the right side of  FIG. 3  and the thickness of the semiconductor substrate  120 . Thus, as compared with the conventional configuration where the converter  110  is layered on the semiconductor substrate  120 , the converter module  100  can be reduced in profile. Here, the thickness of the converter module  100  depends on the sum of the thicknesses of the shielding cap  140 , the converter  110 , and the circuit substrate  130 . Therefore, the converter module  100  can be reduced in profile in the present embodiment. 
     In the case of the converter module  100  in the first embodiment, the semiconductor substrate  120  is connected in the recessed part  118  of the base  112 . Thus, the sum of the thicknesses of the semiconductor substrate  120  and the converter  110  on one side can be approximately equal to the thickness of the converter  110  on the other side. On account of this, the thickness of the converter module  100  does not need to be reduced and, therefore, the strength of the converter can be maintained. 
     Moreover, in the case of the converter module  100  in the first embodiment, the volume of the cavity part  113  is determined by the thickness of the base  112  and the size of the area surrounded by the base  112  and the semiconductor substrate  120  connected in the recessed part  118 . This means that thinning the semiconductor substrate  120  does not reduce the volume of the cavity part  113  nor decrease the accuracy in detecting the pressure fluctuations. 
     As described earlier, in order to improve the detection sensitivity, it is necessary to (A) increase the area size of the diaphragm  111  or (B) increase the volume of the cavity part  113 . To achieve (A), the converter  110  needs to be increased in size so as to increase the area size of the diaphragm  111 . In the case of the converter module  100  in the first embodiment, since the semiconductor substrate  120  is connected in the recessed part  118  of the base  112 , the volume of the cavity part  113  can be increased without changing the size of the semiconductor substrate  120 . In fact, when the semiconductor substrate  120  is smaller in size, the volume of the cavity part  113  can be increased more. 
     The same holds for (B). More specifically, the volume of the cavity part  113  can be increased more when the semiconductor substrate  120  is smaller in size, which means that the detection sensitivity can be improved and that the semiconductor substrate  120  can be reduced in size to the limit. Thus, the number of semiconductor substrates  120  per wafer is increased, which allows the converter module  100  to be manufactured at low cost. 
     In addition, the size of the semiconductor substrate  120  in view of the detection sensitivity of the converter module  100  does not need to be considered, and this allows greater flexibility in designing the converter module  100 . 
     Therefore, as compared with the conventional converter module, the converter module  100  described as an example in the first embodiment can more easily achieve miniaturization and profile reduction without decreasing the pressure detection sensitivity. 
     The following describes a method of manufacturing the converter module  100  in the first embodiment, with reference to the drawings. 
     Firstly, a wafer which is an array having a plurality of converters  110  is prepared. Each of the converters  110  includes the electrode area  114 , the diaphragm  111 , and the base  112 . It should be noted that the converters  110  are formed according to the well-known technique. 
     Next, electrode recessed parts  151  and  152  recessed from the front surface of the base  112  in the thickness direction are formed for the penetrating electrodes  115  and  116 , respectively. For example, a dry etching process or a wet etching process may be performed using a resist, a SiO 2  film, a metal film, or the like as a mask. Each depth of the electrode recessed parts  151  and  152  is approximately 50 μm to 100 μm. The electrode recessed parts  151  and  152  may completely penetrate the base  112 . The penetration manner of the electrode recessed parts  151  and  152  may be determined as appropriate, depending on, for example, diameters of the electrode recessed parts  151  and  152 . By this process, a structure shown in (a) of  FIG. 5  is formed. Note that  FIG. 5  shows only one of the converters  110  formed on the wafer. 
     After this, the cavity part  113  and the recessed part  118  are formed. The recessed part  118  is formed for connecting the semiconductor substrate  120 . The cavity part  113  and the recessed part  118  may be separately formed. However, it is preferable for the cavity part  113  and the recessed part  118  to be formed at one time, in terms of the manufacturing accuracy and the reduced number of processes. Here, suppose that the cavity part  113  and the recessed part  118  are formed at one time. In this case, a material used for a protection film and a deposition method are slightly different between the wet etching process and the dry etching process each of which is performed for etching bulk Si of the base  112 . 
     Thus, the following describes the case where the cavity part  113  and the recessed part  118  are formed at one time by the wet etching process. 
     Firstly, a SiN film, and more specifically, a first protection film  153 , is deposited on the back surface of the base  112  by a chemical vapor deposition (CVD) process or by a reactive sputtering process. Here, a SiO 2  film may be deposited instead of the SiN film. 
     The SiO 2  film can be deposited by thermal oxidation. In this case, however, Poly-Si of the diaphragm  111  is also oxidized and, as a result, the diaphragm  111  partially becomes SiO 2 . Then, there is a possibility that the diaphragm  111  becomes thin and brittle at the completion of the converter module  100 . For this reason, it is preferable to perform the CVD process or the reactive sputtering process. 
     After a photosensitive resist is applied on the SiN film (i.e., the first protection film  153 ) by a spin coating process, a patterning process is performed on the resist to form an opening for each of the cavity part  113  and the recessed part  118 . Following this, using the resist as a mask, a reactive ion etching (RIE) process or the wet etching process is performed to remove the SiN film from the back surface of the base  112  at the area where the cavity part  113  is to be formed. Then, after removing the resist, a structure shown in (b) of  FIG. 5  is formed. 
     Next, using the SiN film (i.e., the first protection film  153 ) as the protection film, the wet etching process is performed to make the depth of the recessed part  118  approximately 50 μm to 100 μm. In this case, it is preferable to use a tetramethylammonium hydroxide (TMAH) aqueous solution as etchant in the wet etching process. The TMAH aqueous solution does not burden the electrode area  114  and allows an anisotropic etching process to be performed on Si along the crystal orientation with high accuracy. Then, after removing the SiN film using a phosphate solution or the like, a structure shown in (c) of  FIG. 5  is formed. 
     Following this, a SiO 2  film or a SiN film, and more specifically, a second protection film  154 , is deposited on the back surface of the base  112  by the CVD process or the reactive sputtering process. As a result, a structure shown in (d) of  FIG. 5  is formed. Here, it is preferable for the second protection film  154  to have a sufficient thickness and to be deposited seamlessly so as not to be torn due to the level differences formed in the process shown in (c) of  FIG. 5 . 
     Then, after a photosensitive resist is applied on the SiO 2  film (i.e., the second protection film  154 ) by the spin coating process, the patterning process is performed on the resist to form an opening for each of the cavity part  113  and the recessed part  118 . Following this, using the resist as a mask, the RIE process or the wet etching process is performed on the SiO 2  film according to the trace patterns, so as to form the opening for each of the cavity part  113  and the recessed part  118 . After removing the resist, a structure shown in (e) of  FIG. 5  is formed. 
     Next, using the SiO 2  film (i.e., the second protection film  154 ) as the protection film, the wet etching process is performed to etch the bulk Si from the back surface of the base  112  until the back surface of the diaphragm  111  is exposed. As a result, the cavity part  113  and the recessed part  118  are formed. This etching process also penetrates the electrode recessed parts  151  and  152  (50 μm to 100 μm in depth) formed in the base  112 . Then, after removing the SiO 2  film using a hydrofluoric acid solution or the like, a structure shown in (f) of  FIG. 5  is formed. 
     In this way, the base  112  is etched from the back surface, so that the cavity part  113  is formed in a first area immediately below the diaphragm  111  and that the recessed part  118  is formed in a second area which is different from the first area. 
     Next, the semiconductor substrate  120  is boned to the recessed part  118  formed in the base  112 . To be more specific, since the etched surface of the recessed part  118  is rough, it is preferable for the semiconductor substrate  120  to be bonded to the recessed part  118  via the insulating paste  122 . 
     Here, the semiconductor substrate  120  is to be fitted in the recessed part  118 . On this account, it is preferable for the semiconductor substrate  120  to be previously ground until the resultant thickness is equal to or smaller than a height measured from the etched surface of the recessed part  118  to the back surface of the base  112 . To be more specific, it is preferable for the thickness of the base  112  shown in the left side of  FIG. 3  to be approximately equal to the sum of the thickness of the base  112  having the recessed part  118  as shown in the right side of  FIG. 3  and the thickness of the semiconductor substrate  120 . 
     In addition, the insulating film  121  having an opening for each of the electrode recessed parts  151  and  152  formed in the base  112  is formed on the front surface of the semiconductor substrate  120 . It should be noted that only one of the insulating film  121  and the insulating paste  122  may be formed. Accordingly, a structure shown in (g) of  FIG. 5  is formed. 
     Next, the penetrating electrodes  115  and  116  are respectively formed in the electrode recessed parts  151  and  152  formed in the base  112 , the insulating paste  122 , and the insulating film  121 . Note that it is preferable for the penetrating electrodes  115  and  116  to be formed at one time for the purpose of reducing the number of manufacturing processes. More specifically, an insulating film, such as a SiO 2  film, is formed on the front surface of the converter  110  and inside the electrode recessed parts  151  and  152  by the CVD process or an insulating-paste printing-filling process. 
     Following this, the insulating film formed on the electrode area  114  and the semiconductor substrate  120  at areas corresponding to the bottoms of the electrode recessed parts  151  and  152  is removed by, again, the dry etching process or the wet etching process. Then, a thin metal film is formed on the entire front surface of the converter  110  by a sputtering process or the like. Here, the thin metal film is mainly made of Ti, titanium tungsten (Ti—W), chromium (Cr), or Cu, for example. 
     Then, after a dry-film pasting process or the application of a photosensitive liquid resist by the spin coating process, the patterning process is performed on the resist for the penetrating electrodes  115  and  116  by exposure and development using a photolithographic technique. It should be noted that the thickness of the resist may be determined according to each thickness of the penetrating electrodes  115  and  116  eventually desired. In general, the thickness is approximately 5 μm to 30 μm. Then, using a metal such as Cu, the penetrating electrodes  115  and  116  are formed by an electrolytic plating process. Here, in order to easily establish an electrical connection between the penetrating electrode  116  and the external electrode  132 , the electrode area  117  is formed by the same process as described. After removing the resist, a structure shown in (h) of  FIG. 5  is formed. 
     In the case where the electrode recessed parts  151  and  152  are not filled with the penetrating electrodes  115  and  116 , respectively, a filling layer (not illustrated) may be formed in the electrode recessed parts  151  and  152 . As a filling material, a resin or a metal may be used. 
     For example, when a metal is used for filling, metal plating may be performed by the electrolytic plating process or a metal paste may be mainly used by the printing-filling process or a dipping process. 
     In the case of the electrolytic plating process, it is desirable for the filling to be performed at the same time as when the penetrating electrodes  115  and  116  are formed. In this case, the electrode recessed parts  151  and  152  are completely filled with the filling layers. Suppose here that the filling layers and the penetrating electrodes  115  and  116  are formed separately, for example. In such a case, after the penetrating electrodes  115  and  116  are formed, a mask having an opening for each of the electrode recessed parts  151  and  152  is formed and the filling layer is formed in each of the electrode recessed parts  151  and  152  by the electrolytic plating process. 
     When a resin material is used for filling, a liquid light-curing or thermo-curing resin may be applied by the spin coating process or a resin paste may be applied by the printing-filling process or the dipping process. 
     Next, the sound hole  133  is formed in the circuit substrate  130 . The sound hole  133  penetrates the circuit substrate  130  in the thickness direction, and is approximately equal in volume to the cavity part  113 . It should be noted that the electrical wiring is formed in the circuit substrate  130  at an area where the sound hole  133  is not formed. 
     Moreover, the external electrode  132  is formed on the electrode area  117  (or, the penetrating electrode  116 ) by a solder ball placing process using flux, a solder paste printing process, or an electrolytic plating process. After this, via the insulating sheet  131  having an opening corresponding to the sound hole  133 , the converter  110  and the semiconductor substrate  120  are temporarily fixed on the circuit substrate  130 . 
     Following this, the insulating sheet  131  and the external electrode  132  are heated under pressure. As a result, the converter  110  and the semiconductor substrate  120  sink toward the circuit substrate  130 , and then the circuit substrate  130  and the external electrode  132  can be electrically connected. 
     After this, the shielding cap  140  is bonded to the converter  110  and the semiconductor substrate  120  via the bonding adhesive  141 . Here, the shielding cap  140  is bonded so as to cover each back surface of the converter  110  and the semiconductor substrate  120  to protect the converter module  100  from an external shock or from noise caused by an electromagnetic wave or the like. Without using the bonding adhesive  141 , the shielding cap  140  may be boned to each back surface of the converter  110  and the semiconductor substrate  120  by an ultrasonic thermocompression bonding process. After the processes described thus far, the structures as shown in  FIG. 1  and  FIG. 3  are formed. 
     The semiconductor substrate  120  may be formed in the recessed part  118  one at a time. However, for the purpose of reducing the number of manufacturing processes, it is preferable that a plurality of semiconductor substrates  120  be respectively formed in a plurality of recessed parts  118  of the converters  110  at one time, as shown in  FIG. 6 . 
     To be more specific, the cavity part  113  and the recessed part  118  are formed in each of the converters  110  formed in the array so that the formed recessed parts  118  of two adjacent converters  110  are adjacent to each other. In the first embodiment, the wafer which is an array having the plurality of converters  110  is used to manufacture a plurality of converter modules  100  at one time. Here, the converters  110  are arranged on the wafer so that areas for the recessed parts  118  of the two adjacent converters  110  are adjacent to each other. With this, two semiconductor substrates  120  are bonded to two converters  110 , respectively, at one time as shown in  FIG. 6 . 
     In  FIG. 5 , the inner walls of the base  112  after the etching process (i.e., the side surfaces of the cavity part  113  and the recessed part  118 ) are illustrated as vertical planes for the sake of simplicity. However, when the wet etching process is performed as described above, the inner walls of the base  112  are sloped as shown in  FIG. 3 . 
     Finally, the wafer is divided into the plurality of converter modules  100  using a cutting member  160 , such as a dicing saw or a laser dicer, as shown in  FIG. 7 . 
     Accordingly, the converter module  100  in the first embodiment can be manufactured as shown in  FIG. 1  to  FIG. 3 . 
     As described above, the cavity part  113  and the recessed part  118  are formed by the two etching processes. More specifically, a first etching process is performed on the first area by etching from the back surface of the base  112  to form the cavity part  113  immediately below the diaphragm  111 , and then a second etching process is performed on the first area for the cavity part  113  and the second area for the recessed part  118  at one time by etching from the back surface of the base  112 . Here, by the first etching process, the back surface of the base  112  is etched so that the base  112  in the first area where the cavity part  113  is to be formed is at least as thick as the semiconductor substrate  120 . Moreover, by the second etching process, the back surface of the base  112  is etched at least as deep as the thickness of the semiconductor substrate  120 . In this way, the cavity part  113  and the recessed part  118  are formed. 
     It should be noted that, after the converters  110  and the semiconductor substrates  120  are diced, each of the converters  110  and semiconductor substrates  120  may be picked up to be bonded to the circuit substrate  130 . 
     Moreover, although not illustrated here, each of the external electrode  132  and the insulating sheet  131  may be an anisotropic conductive film. For example, an anisotropic conductive film having an opening for the sound hole  133  is bonded on the circuit substrate  130 , and the converter  110  and the semiconductor substrate  120  are temporarily fixed on this anisotropic conductive film. In this case, the anisotropic conductive film previously has a conductive trace pattern at a place where the penetrating electrode  116  and the electrode area  117  are to be bonded. After this, complete bonding is performed by application of pressure and heat. 
     Furthermore, the insulating paste  122  may be bonded to the base  112  and the semiconductor substrate  120  after the trace patterns for the electrode recessed parts  151  and  152  formed in the base  112  are opened. In addition, it is preferable for the insulating paste  122  not only to bond the main surface of the semiconductor substrate  120  and the back surface of the base  112  where the recessed part  118  is formed, but also to bond the side surface of the semiconductor substrate  120  and the side surface of the base  112  as shown in  FIG. 3 . 
     Moreover, the semiconductor substrate  120  may be bonded after the recessed part  118  is formed, and the cavity part  113  may be formed after the penetrating electrodes  115  and  116  are formed. With this, since the surface of the diaphragm  111  is fully supported by the base  112  before the cavity part  113  is formed, process loads on the diaphragm  111  can be reduced. The process loads include stress of when the semiconductor substrate  120  is bonded, plating stress of when the penetrating electrodes  115  and  116  are formed, and wafer handling. 
     Furthermore, the cavity part  113  and the recessed part  118  may be formed by the dry etching process. The following describes the case where the cavity part  113  and the recessed part  118  are formed by the dry etching process. 
     Firstly, as in the case shown in (a) of  FIG. 5 , the electrode recessed parts  151  and  152  are formed in the converter  110  including the electrode area  114 , the diaphragm  111 , and the base  112 . As a result, a structure shown in (a) of  FIG. 8  is formed. 
     Next, a first protection film  171  which a resist, a SiN film, a SiO 2  film, or a thin metal film is deposited on the back surface of the base  112 . After this, the patterning process is performed on the first protection film  171  for openings of the cavity part  113  and the recessed part  118 . The openings may be formed by either the wet etching process or the dry etching process (RIE process). As a result, a structure shown in (b) of  FIG. 8  is formed. 
     After this, a second protection film  172  is deposited. The second protection film is a resist, a SiN film, a SiO 2  film, or a thin metal film having etching resistance different from the etching resistance of the first protection film  171  deposited in the previous process. As a result, a structure shown in (c) of  FIG. 8  is formed. Then, as in the case above, the wet etching process is performed to form the opening of the cavity part  113  according to the trace pattern. Thus, a structure shown in (d) of  FIG. 8  is formed. 
     Next, the dry etching process (RIE process) is performed on the back surface of the base  112  to make the depth of the recessed part  118  approximately 50 μm to 100 μm. For example, the base  112  is etched by the dry etching process using fluorinated gas. Following this, the second protection film  172  formed on the back surface of the base  112  is removed. As a result, a structure shown in (e) of  FIG. 8  is formed. 
     Finally, the dry etching process is performed to etch the back surface of the base  112  until the back surface of the diaphragm  111  is exposed. As a result, the cavity part  113  and the recessed part  118  are formed. This etching process also penetrates the electrode recessed parts  151  and  152  (50 μm to 100 μm in depth) formed in the base  112 . Then, after removing the first protection film  171 , a structure shown in (f) of  FIG. 8  is formed. 
     The subsequent processes are the same as those performed in the case of the wet etching process, as shown in (g) and (h) of  FIG. 8 . 
     As described thus far, the cavity part  113  and the recessed part  118  can be formed by the dry etching process. It should be noted that since each of the first protection film  171  and the second protection film  172  can be deposited approximately evenly in the example shown in  FIG. 8 , the base  112  can be protected more solidly. 
     Here, it is preferable for the first protection film  171  and the second protection film  172  to be made of different materials. Suppose that the first protection film  171  is a SiN film and that the second protection film  172  is a SiO 2  film. In this case, etchants used in the wet etching process are different between the SiN film and the SiO 2  film. For example, a phosphate solution is used in the wet etching process performed on the SiN film whereas a hydrofluoric acid solution is used in the wet etching process performed on the SiO 2  film. This means that when the SiO 2  film is etched, the SiN film having deposited earlier is hardly etched. On this account, it is preferable to perform the wet etching process for etching the protection films. 
     A method of depositing the protection films is not limited to the method described above. For example, as shown in  FIG. 8 , the two protection films are firstly deposited and, after this, the base  112  may be etched by the wet etching process. 
     Second Embodiment 
     In a converter module in the second embodiment, a through area penetrating from a front surface of a base to a back surface of the base is formed, and a semiconductor substrate is formed in this through area. 
     The following is a description of the converter module in the second embodiment.  FIG. 9  is a perspective view of a converter module  200  in the second embodiment.  FIG. 10  is a plan view of the converter module  200  in the second embodiment.  FIG. 11  is a cross-sectional view of the converter module  200  in the second embodiment taken along a line B-B in  FIG. 10 . 
     As shown in  FIG. 9  to  FIG. 11 , a base  212  included in the converter module  200  is different in shape from the base  112  included in the converter module  100  shown in  FIG. 1  to  FIG. 3  in the first embodiment. In the present embodiment, components identical to those in the first embodiment are assigned the same numerals used in the first embodiment and, therefore, the detailed explanations of these components are omitted and different components are mainly explained. 
     As shown in  FIG. 11 , the base  212  includes a through area  218  (corresponding to the recessed part  118  in the first embodiment) which penetrates from a front surface of the base  212  to a back surface of the base  212 , in the converter module  200  of the second embodiment. Here, the front surface and the back surface of the base  212  may be referred to as a first main surface and a second main surface, respectively. In this through area  218 , a semiconductor substrate  220  is formed. The back surface of the base  212  and a back surface of the semiconductor substrate  220  are in one plane. This means that the thickness of the base  212  is approximately equal to a sum of the thicknesses of the semiconductor substrate  220 , the insulating film  121 , and the insulating paste  122 . 
     Since the base  212  is not formed on the semiconductor substrate  220 , each of the penetrating electrodes  115  and  116  penetrates only a diaphragm  211 , the insulating film  121 , and the insulating paste  122 . In this way, the penetrating electrodes  115  and  116  can be shortened. Therefore, the converter module  200  can reduce parasitic resistance caused by the electrode part more than the converter module  100  in the first embodiment, in addition to the advantageous effect described in the first embodiment. 
     Note that the shielding cap  140  may cover not only a back surface of the converter module  200  but also side surfaces of the converter module  200 .  FIG. 12  is a cross-sectional view of a configuration of a converter module  200   a , as a modification of the converter module  200  in the second embodiment. As shown in  FIG. 12 , a shielding cap  240  of the converter module  200   a  covers side surfaces of a converter  210  and the semiconductor substrate  220  as well, via a bonding adhesive  241 . 
     With this configuration, the converter module  200   a  can be more protected from an external shock or from noise caused by an electromagnetic wave or the like. Here, the way of applying the bonding adhesive  241  can be selected as appropriate. More specifically, the bonding adhesive  241  may be fully applied to a surface where the converter  210  and the shielding cap  240  meet or may be applied only to a surface where the circuit substrate  130  and the shielding cap  240  meet. 
     Next, points of difference between a method of manufacturing the converter module  200  in the second embodiment and the method of manufacturing the converter module  100  in the first embodiment are explained. 
     As mentioned, the base  212  included in the converter module  200  is different in shape from the base  112  included in the converter module  100  in the first embodiment. To be more specific, the area for forming the semiconductor substrate  220  penetrates the base  212  in the converter module  200 . That is, the through area  218  can be formed by the same process performed for forming the cavity part  113 . 
     Thus, as shown in (b) of  FIG. 8 , a protection film which a resist, a SiN film, a SiO 2  film, or a thin metal film is deposited on the back surface of the base  212 . After this, the patterning process is performed on the protection film for openings of the cavity part  113  and the through area  218  (corresponding to the recessed part  118  in the first embodiment). Then, the wet etching process or the dry etching process, for example, is performed to penetrate the base  212  from the back surface to the front surface. 
     With this, the two-step process including lithography and etching to form the cavity part  113  and the through area  218  is not necessary. This can reduce the number of manufacturing processes and, thus, the converter module  200  can be manufactured at low cost. 
     Third Embodiment 
     In a converter module in the third embodiment, a shielding cap having a sound hole is arranged on a front surface of a base or above a diaphragm, and a circuit substrate for transmitting an electric signal to an external source is arranged on a back surface of the base. 
     The following is a description of the converter module in the third embodiment.  FIG. 13  is a perspective view of a converter module  300  in the third embodiment.  FIG. 14  is a plan view of the converter module  300  in the third embodiment.  FIG. 15  is a cross-sectional view of the converter module  300  in the third embodiment taken along a line C-C in  FIG. 14 . 
     As shown in  FIG. 13  to  FIG. 15 , the converter module  300  is different from the converter module  100  in the first embodiment in that a shielding cap  340  is arranged on the front surface of the base  112  and a circuit substrate  330  is arranged on the back surface of the base  112 . In the present embodiment, components identical to those in the first embodiment are assigned the same numerals used in the first embodiment and, therefore, the detailed explanations of these components are omitted and different components are mainly explained. 
     As shown in  FIG. 13  to  FIG. 15 , the shielding cap  340  is formed on the front surface of the converter  110  and above the diaphragm  111 . To be more specific, the shielding cap  340  is bonded to the front surface of the base  112 , the diaphragm  111 , and the electrode area  114  via a bonding adhesive  341 . 
     The shielding cap  340  is an example of a protection film having a sound hole  342  penetrating the protection film in the thickness direction. Here, the shielding cap  340  is formed above the diaphragm  111  so that the opening of the cavity part  113  is located immediately below the sound hole  342 . 
     The circuit substrate  330  is arranged on the back surface of the base  112 . More specifically, the circuit substrate  330  is bonded to the back surface of the base  112  and to a back surface of a semiconductor substrate  320  via an insulating sheet  331 . Moreover, in the circuit substrate  330 , electrical wiring is formed to, for example, extract the electric signal processed by the semiconductor substrate  320 . 
     Moreover, a penetrating electrode  323  is formed inside a through hole penetrating the semiconductor substrate  320  in the thickness direction. The penetrating electrode  323  is electrically connected to the penetrating electrode  115  and extends to a part of the back surface of the semiconductor substrate  320 . The penetrating electrode  323  is an example of a third penetrating electrode which penetrates the semiconductor substrate  320  in the thickness direction and which electrically connects the semiconductor substrate  320  and the electrical wiring formed in the circuit substrate  330 . To be more specific, the penetrating electrode  323  is electrically connected to the electrical wiring formed in the circuit substrate  330  in order to transmit, to an external source, the electric signal processed by the semiconductor substrate  320 . The penetrating electrode  323  is made of a metal, such as Ti or Cu. 
     An external electrode  332  is formed in the insulating sheet  331  to electrically connect the penetrating electrode  323  and the circuit substrate  330 . Here, the external electrode  332  is made of, for example, a lead-free solder material which is a Sn—Ag—Cu alloy. 
     Since the cavity part  113  is formed in an area surrounded by the circuit substrate  330 , the base  112 , and the semiconductor substrate  320  connected in the recessed part  118 , the converter module  300  in the third embodiment can achieve the advantageous effect described in the first embodiment. 
     Note that the shielding cap  340  may cover not only a front surface of the converter module  300  but also side surfaces of the converter module  300 .  FIG. 16  is a cross-sectional view of a configuration of a converter module  300   a , as a modification of the converter module  300  in the third embodiment. As shown in  FIG. 16 , a shielding cap  340   a  of the converter module  300   a  covers side surfaces of the converter  110  and the semiconductor substrate  320  as well, via a bonding adhesive  341   a . In this way, the shielding cap  340   a , which is an example of a protection layer, protects the back surface and side surfaces of the base  112 . 
     With this configuration, the converter module  300   a  can be more protected from an external shock or from noise caused by an electromagnetic wave or the like. Here, the way of applying the bonding adhesive  341   a  can be selected as appropriate. More specifically, the bonding adhesive  341   a  may be fully applied to a surface where the converter  110  and the shielding cap  340   a  meet or may be applied only to a surface where the circuit substrate  330  and the shielding cap  340   a  meet. 
     Next, points of difference between a method of manufacturing the converter module  300  in the third embodiment and the method of manufacturing the converter module  100  in the first embodiment are explained. 
     As mentioned above, the converter module  300  is different from the converter module  100  in the first embodiment in that the shielding cap  340  is arranged on the front surface of the base  112  and the circuit substrate  330  is arranged on the back surface of the base  112 . Moreover, since the circuit substrate  330  is located at a different position from the position of the circuit substrate  130  in the first embodiment, the penetrating electrode  323 , instead of the penetrating electrode  116 , is formed in the semiconductor substrate  320 . On account of this, a process of forming the penetrating electrode and a process of bonding the shielding cap and the circuit substrate are different between the manufacturing methods in the first and third embodiments. The detailed explanation is given as follows. 
     In the process of forming the electrode recessed parts  151  and  152  as shown in (a) of  FIG. 8  in the first embodiment, only the electrode recessed part  151  for forming the penetrating electrode  115  is formed in the base  112  in the present embodiment. Then, after the semiconductor substrate  320  is bonded to the base  112 , the through hole penetrating the semiconductor substrate  320  in the thickness direction is formed and the penetrating electrode  323  is formed in the formed through hole. 
     More specifically, an electrode area  324  is firstly formed on the back surface of the semiconductor substrate  320 . Next, the patterning process is performed on a protection film which a resist, a SiN film, a SiO 2  film, or a thin metal film formed on the back surface of the semiconductor substrate  320 , so as to penetrate immediately below the electrode area  324 . After this, the dry etching process or the wet etching process, for example, is performed. As a result, the through hole is formed in the semiconductor substrate  320 . Following this, the penetrating electrode  323  is formed by the same process performed for forming the penetrating electrode  115 . 
     Next, the sound hole  342  is formed in the shielding cap  340 , and the shielding cap  340  is bonded to the base  112  and the diaphragm  111  via the bonding adhesive  341 . Moreover, the circuit substrate  330  is bonded to the back surfaces of the base  112  and the semiconductor substrate  320  via the insulating sheet  331 . Here, the external electrode  332  is formed in the insulating sheet  331  as in the first embodiment. Although it is desirable for the insulating sheet  331  to have an opening to increase the volume of the cavity part  113 , the opening of the insulating sheet  331  does not necessarily correspond to the trace pattern of the cavity part  113 . 
     The circuit substrate  330  of the converter module  300  in the third embodiment includes no hole, and this allows greater flexibility in designing the converter module  300 . To be more specific, in the first and second embodiments, the electrical wiring formed in the circuit substrate  130  needs to be formed so as not to be cut due to the sound hole  133 . However, this concern does not need to be considered in the case of the converter module  300  in the third embodiment. 
     Fourth Embodiment 
     In a converter module in the fourth embodiment, a through area penetrating from a front surface of a base to a back surface of the base is formed, and a semiconductor substrate is formed in this through area. As in the case of the third embodiment, a shielding cap having a sound hole is arranged on the front surface of the base or above a diaphragm, and a circuit substrate for transmitting an electric signal to an external source is provided on the back surface of the base. 
     The following is a description of the converter module in the fourth embodiment.  FIG. 17  is a perspective view of a converter module  400  in the fourth embodiment.  FIG. 18  is a plan view of the converter module  400  in the fourth embodiment.  FIG. 19  is a cross-sectional view of the converter module  400  in the fourth embodiment taken along a line D-D in  FIG. 18 . 
     As shown in  FIG. 17  to  FIG. 19 , a base  212  included in the converter module  400  is different in shape from the base  112  included in the converter module  300  shown in  FIG. 13  to  FIG. 15  in the third embodiment. In the present embodiment, components identical to those in the third embodiment are assigned the same numerals used in the third embodiment and, therefore, the detailed explanations of these components are omitted and different components are mainly explained. 
     As shown in  FIG. 19 , the base  212  includes a through area  218  (corresponding to the recessed part  118  in the third embodiment) which penetrates from a front surface of the base  212  to a back surface of the base  212 , in the converter module  400  of the fourth embodiment. Here, the front surface and the back surface of the base  212  may be referred to as a first main surface and a second main surface, respectively. In this through area  218 , a semiconductor substrate  420  is formed. The back surface of the base  212  and a back surface of the semiconductor substrate  420  are in one plane. This means that the thickness of the base  212  is approximately equal to a sum of the thicknesses of the semiconductor substrate  420 , the insulating film  121 , and the insulating paste  122 . 
     Since the base  212  is not formed on the semiconductor substrate  420 , the penetrating electrode  115  penetrates only a diaphragm  211 , the insulating film  121 , and the insulating paste  122 . In this way, the penetrating electrode  115  can be shortened. Therefore, the converter module  400  can reduce parasitic resistance caused by the penetrating electrode  115  more than the converter module  300  in the third embodiment, in addition to the advantageous effect described in the third embodiment. 
     Note that the shielding cap  340  may cover not only a back surface of the converter module  400  but also side surfaces of the converter module  400 .  FIG. 20  is a cross-sectional view of a configuration of a converter module  400   a , as a modification of the converter module  400  in the fourth embodiment. As shown in  FIG. 20 , a shielding cap  340   a  of the converter module  400   a  covers side surfaces of a converter  210  and the semiconductor substrate  420  as well, via a bonding adhesive  341   a.    
     With this configuration, the converter module  400   a  can be more protected from an external shock or from noise caused by an electromagnetic wave or the like. Here, the way of applying the bonding adhesive  341   a  can be selected as appropriate. More specifically, the bonding adhesive  341   a  may be fully applied to a surface where the converter  210  and the shielding cap  340   a  meet or may be applied only to a surface where the circuit substrate  330  and the shielding cap  340   a  meet. 
     Next, points of difference between a method of manufacturing the converter module  400  in the fourth embodiment and the method of manufacturing the converter module  300  in the third embodiment are explained. 
     As mentioned, the base  212  included in the converter module  400  is different in shape from the base  112  included in the converter module  300  in the third embodiment. To be more specific, the area for forming the semiconductor substrate  420  penetrates the base  212  in the converter module  400 . That is, the through area  218  can be formed by the same process performed for forming the cavity part  113 . 
     Thus, as shown in (b) of  FIG. 8 , a protection film which a resist, a SiN film, a SiO 2  film, or a thin metal film is deposited on the back surface of the base  212 . After this, the patterning process is performed on the protection film for openings of the cavity part  113  and the through area  218  (corresponding to the recessed part  118  in the first or third embodiment). Then, the wet etching process or the dry etching process, for example, is performed to penetrate the base  212  from the back surface to the front surface. 
     With this, the two-step process including lithography and etching to form the cavity part  113  and the through area  218  is not necessary. This can reduce the number of manufacturing processes and, thus, the converter module  400  can be manufactured at low cost. 
     Although the converter module and the method of manufacturing the same according to the present invention have been fully described by way of embodiments, the present invention is not limited to these embodiments. It is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. 
     For example, since the semiconductor substrate in each of the above embodiments is formed in the recessed part or the through area formed by etching from the back surface of the base, the back surface of the semiconductor substrate and the back surface of the base are in one plane. However, the semiconductor substrate may be arranged in any place as long as the semiconductor substrate is formed as a part of the base. The semiconductor substrate may be exposed on the surface of the base or may be formed inside the base, for instance. In such a case, however, it is preferable that the arrangement of the semiconductor substrate does not result in that the converter is thicker than the base. 
     Moreover, in each of the above embodiments, the cavity part and the recessed part are formed by the etching process. However, a converter previously having the cavity part may be used and only the recessed part may be formed. 
     The converter module according to the present invention can be used as a sound pressure sensor, a pressure sensor, or a flow sensor. When the converter module is used as a flow sensor, a path for gas is formed above the circuit substrate having the sound hole or above the shielding cap, so that the gas is guided into the sound hole. Then, the diaphragm vibrates in response to the guided gas, and the converter module according to the present invention can be used as the flow sensor detecting this vibration. 
     INDUSTRIAL APPLICABILITY 
     The converter module according to the present invention is capable of easily achieving miniaturization and profile reduction without decreasing the pressure detection sensitivity, and is suitable especially for various kinds of sensors, such as a sound pressure sensor, a pressure sensor, and a flow sensor.