Patent Publication Number: US-2011074515-A1

Title: Piezoelectric resonator, oscillator and oscillator package

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application Nos. 2009-228971 filed on Sep. 30, 2009 and 2010-049883 filed on Mar. 5, 2010, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surface mount device (SMD) type piezoelectric resonator, and more particularly to a piezoelectric resonator for a small package having dimensions of 3.2 mm×2.5 mm or smaller. 
     2. Description of the Related Art 
     In recent years, in a mobile phone or a personal digital assistant device, a piezoelectric resonator using quartz or the like is used as a time source, a timing source of a control signal or the like, a reference signal source, or the like. There are known various piezoelectric resonators of this type. As one of the piezoelectric resonators, a surface mount device type piezoelectric resonator is known. As this type of piezoelectric resonator, there is generally known a structure including a base substrate on which internal electrodes are formed, and a piezoelectric resonator chip is fixed onto the base substrate with a conductive adhesive and is sealed with a lid substrate. In particular, a through-silicon via method is developed in recent years, and a mounting form is developed in which passive components are mounted on an LSI to have a stack structure (see Japanese Patent Application Laid-open No. 2004-193909). 
     Here, the piezoelectric resonator having the structure in which the piezoelectric resonator chip is fixed onto the silicon substrate is described briefly. As illustrated in  FIG. 9 , a piezoelectric resonator  200  includes a piezoelectric resonator chip  203 , a silicon substrate  201 , and a lid substrate  202  that has a recess and is bonded to a bonding surface  208  of the base substrate  201 . 
     The piezoelectric resonator chip  203  is an AT-cut resonator chip made of quartz that is a piezoelectric material. There are formed electrodes  205   a  and  205   b  of the resonator chip that sandwich the piezoelectric resonator chip  203  and are patterned so as to vibrate the piezoelectric resonator chip  203 . The piezoelectric resonator chip  203  is bonded to an internal electrode  207   a  formed on the silicon substrate  201  via the electrodes  205   a  and  205   b  of the resonator chip, and a conductive adhesive  204 . 
     A recess is formed in the lid substrate  202 , which seals the silicon substrate  201  so as to form a cavity  209 . The piezoelectric resonator chip  203  is housed in the cavity  209 . 
     On the back surface of the silicon substrate  201 , there are formed external electrodes  206   a  and  206   b.  Of the external electrodes, the external electrode  206   a  is electrically connected to the electrode  205   b  of the resonator chip on one side of the piezoelectric resonator chip  203  via a through-silicon via  210   a , the internal electrode  207   a , and the conductive adhesive  204 . Although not illustrated, the external electrode  206   b  is electrically connected to the electrode  205   a  of the resonator chip on the other side of the piezoelectric resonator chip  203  via a through-silicon via (not shown), an internal electrode (not shown), and a conductive adhesive (not shown). 
     The lid substrate  202  is constituted of a glass substrate or a metal substrate and is bonded to the bonding surface  208  of the silicon substrate  201  by anode bonding or Au—Sn welding, so as to seal the cavity  209 . 
     However, there is a following problem remaining in the conventional piezoelectric resonator  200 . 
     The silicon substrate  201  has a linear expansion coefficient of approximately 2.8 ppm/K. In contrast, a linear expansion coefficient of the AT-cut resonator chip as the piezoelectric resonator chip  203  is 13.7×10 −6  per degrees centigrade in the X axis direction, 9.6×10 −6  per degrees centigrade in the Y axis direction, and 11.6×10 −6  per degrees centigrade in the Z′ axis direction, although the values are different a little depending on documents or the like. Therefore, if the piezoelectric resonator chip  203  is mounted on the silicon substrate  201 , a large stress is applied to the piezoelectric resonator chip  203  because of a large difference of linear expansion coefficient between the silicon substrate  201  and the piezoelectric resonator chip  203 . If a stress is applied to the piezoelectric resonator chip  203 , a temperature characteristic changes largely so that a performance thereof is deteriorated. 
     SUMMARY OF THE INVENTION 
     The present invention is created in view of this circumstance, and it is an object of the present invention to provide a piezoelectric resonator having a structure in which a piezoelectric resonator chip is mounted on a silicon substrate without deteriorating a characteristic. 
     In order to solve the above-mentioned problem, the present invention provides the following means. 
     The present invention provides a piezoelectric resonator include: a silicon substrate; a linear expansion coefficient adjusting film formed on a front surface of the silicon substrate; a pair of mount electrodes formed on the linear expansion coefficient adjusting film; and an AT-cut type piezoelectric resonator chip that is bonded to the pair of mount electrodes to be supported, in which a linear expansion coefficient of the linear expansion coefficient adjusting film is 13×10 −6  per degrees centigrade or larger, and a product of the linear expansion coefficient and a Young&#39;s modulus of the linear expansion coefficient adjusting film is 1.95 MPa per degrees centigrade or larger. Here, the piezoelectric resonator chip is preferably bump-bonded to the pair of mount electrode to be supported. Further, the linear expansion coefficient adjusting film may be a metal film, and the piezoelectric resonator may further include an insulator film formed between the linear expansion coefficient adjusting film and the mount electrode. For example, the linear expansion coefficient adjusting film is made of Ni or Cu. Further, the linear expansion coefficient adjusting film may be a quartz film, and the piezoelectric resonator may further include an insulator film formed between the linear expansion coefficient adjusting film and the mount electrode. 
     The present invention provides an oscillator including: the piezoelectric resonator according the present invention; and an oscillation circuit formed on a front surface of a silicon substrate. Here, the oscillator may further include a protection film formed between a linear expansion coefficient adjusting film and the silicon substrate. Further, the present invention provides an oscillator including: the piezoelectric resonator according to the present invention; and an oscillation circuit formed on a back surface of a silicon substrate. Here, the oscillator may further include a through-silicon via that penetrates the silicon substrate in a thickness direction so as to connect a mount electrode to the oscillation circuit. 
     The present invention provides an oscillator package including: the oscillator according to the present invention; a package base substrate on which the oscillator is mounted; and a lid substrate that is bonded to a front surface of the package base substrate so that a cavity for housing a piezoelectric resonator chip is formed between the package base substrate and the lid substrate. Further, the present invention provides an oscillator package including: the oscillator according to the present invention; and a lid substrate that is bonded to a front surface of a silicon substrate so that a cavity for housing a piezoelectric resonator chip is formed between the silicon substrate and the lid substrate. 
     According to the present invention, on the front surface of the silicon substrate, there is formed the linear expansion coefficient adjusting film having the linear expansion coefficient of 13×10 −6  per degrees centigrade or larger, and the product of the linear expansion coefficient and the Young&#39;s modulus of 1.95 MPa per degrees centigrade or larger. Therefore, the linear expansion coefficient of the piezoelectric resonator chip may be close to the linear expansion coefficient of the multilayered structural member including the silicon substrate and the linear expansion coefficient adjusting film. Therefore, a stress due to a difference of thermal expansion between the substrate and the piezoelectric resonator chip caused by a change in ambient temperature is reduced. In other words, the deterioration of characteristic due to the temperature characteristic is reduced, so that an effect of reducing a frequency hop or shift from a theoretical curve of a frequency to temperature characteristic is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a top view of a piezoelectric resonator of Embodiment 1 according to the present invention; 
         FIG. 2  is a cross section taken along the line A-A of the piezoelectric resonator of Embodiment 1; 
         FIG. 3  is a top view of an oscillator of Embodiment 2 according to the present invention; 
         FIG. 4  is a cross section taken along the line B-B of the oscillator of Embodiment 2; 
         FIG. 5  is a cross section illustrating a packaging form of the oscillator of Embodiment 2; 
         FIG. 6  is a cross section of an oscillator of Embodiment 3 according to the present invention; 
         FIG. 7  is a cross section illustrating a packaging form of the oscillator of Embodiment 3; 
         FIG. 8  is a cross section illustrating a packaging form of an oscillator of Embodiment 4 according to the present invention; and 
         FIG. 9  is a cross section of a conventional piezoelectric resonator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, Embodiments 1 to 4 of a piezoelectric resonator, an oscillator and an oscillator package according to the present invention are described with reference to  FIGS. 1 to 8 . 
     Embodiment 1  
     A piezoelectric resonator  1  is, as illustrated in  FIGS. 1 and 2 , a piezoelectric resonator including a silicon substrate  2  on which a piezoelectric resonator chip  7  is mounted via a metal film (linear expansion coefficient adjusting film)  3  and an insulator film  4 . Note that the insulator film  4  is omitted in  FIG. 1  for easy recognition. 
       FIGS. 1 and 2  are referred to for the following description. The piezoelectric resonator chip  7  is an AT-cut type resonator chip in a rectangular thin plate shape formed of quartz as a piezoelectric material, which vibrates when a predetermined voltage is applied. Rectangular electrodes  10  and  11  of the resonator chip are formed respectively on a front surface and a back surface of the piezoelectric resonator chip  7  so as to be opposed to each other in the middle portion. The electrode  10  of the resonator chip formed on the front surface of the piezoelectric resonator chip  7  is connected to a lead electrode  8  at one end portion (lower end portion in  FIG. 1 ) of one short side (left short side in  FIG. 1 ). The lead electrode  8  is formed at one end region (lower region in  FIG. 1 ) of one short side region (left short side region in  FIG. 1 ) on the front surface of the piezoelectric resonator chip  7  and is connected to the electrode  10  of the resonator chip. In addition, this lead electrode  8  is electrically connected to a mount electrode  12  of the resonator chip formed on the back surface of the piezoelectric resonator chip  7  via a side electrode formed on a side face of one short side of the piezoelectric resonator chip  7 . The mount electrode  12  of the resonator chip is formed in one end region (lower region in  FIG. 1 ) of one short side region (left short side region in  FIG. 1 ) on the back surface of the piezoelectric resonator chip  7 . The electrode  11  of the resonator chip formed on the back surface of the piezoelectric resonator chip  7  is connected to a lead electrode  9  at the other end portion (upper end portion in  FIG. 1 ) of the one short side (left short side in  FIG. 1 ). The lead electrode  9  is formed in the other end region (upper region in  FIG. 1 ) of the one short side region (left short side region in  FIG. 1 ) on the back surface of the piezoelectric resonator chip  7  and is connected to the electrode  11  of the resonator chip. In addition, this lead electrode  9  is electrically connected to a mount electrode  13  of the resonator chip formed on the back surface of the piezoelectric resonator chip  7 . The mount electrode  13  of the resonator chip is formed in the other end region (upper region in  FIG. 1 ) of the one short side region (left short side region in  FIG. 1 ) on the back surface of the piezoelectric resonator chip  7 . The mount electrodes  12  and  13  of the resonator chip are formed respectively in one end region and the other end region of the one short side region on the back surface of the piezoelectric resonator chip  7 , and are respectively connected to bumps  14  and  15  for supporting the piezoelectric resonator chip  7  at two points in the one short side region. The bumps  14  and  15  are respectively connected to mount electrodes  5  and  6  that are formed on the silicon substrate  2  via a metal film  3  and an insulator film  4  that are described later. 
     Here, the electrodes  10  and  11  of the resonator chip, the mount electrodes  12  and  13  of the resonator chip, the lead electrodes  8  and  9 , and the side electrode are formed of coating of conductive film made of, for example, chrome (Cr), nickel (Ni), gold (Au), aluminum (Al), titanium (Ti) or the like, or a lamination film as a combination of some of the conductive films. 
     On the silicon substrate  2 , the metal film (linear expansion coefficient adjusting film)  3  is formed for reducing a difference between a linear expansion coefficient of the piezoelectric resonator chip  7  and a linear expansion coefficient of the silicon substrate  2 . The metal film  3  is disposed at a position just under the mount electrodes  5  and  6  to which the bumps  14  and  15  are connected, respectively. The metal film  3  is formed to have a size in a plan view, which includes the two mount electrodes  5  and  6  at the same time. In other words, the two mount electrodes  5  and  6  are formed above the one metal film  3 . Note that it is sufficient if the metal film  3  is formed to have a size that is the same as or larger than the size covering the two mount electrodes  5  and  6  at the same time, and so the metal film  3  may be formed over substantially the entire region of the front surface of the silicon substrate  2 . On the metal film  3 , the insulator film  4  is formed which covers the entire metal film  3  so that the metal film  3  does not become conductive to the mount electrodes  5  and  6 . The insulator film  4  is formed on the entire front surface of the silicon substrate  2  in  FIG. 2 , but it is not necessarily required that the entire front surface of the silicon substrate  2  is covered with the insulator film  4 . As long as the state where the metal film  3  is not conductive to the mount electrodes  5  and  6  is secured, it is possible to form the insulator film  4  only in the periphery of the metal film  3 . However, the number of process steps is smaller in the case where the insulator film  4  is formed on the entire front surface of the silicon substrate  2 . 
     The piezoelectric resonator chip  7  is bump-bonded to front surfaces of the mount electrodes  5  and  6  via the bumps  14  and  15  made of gold or other metal as illustrated in  FIGS. 1 and 2 . More specifically, two mount electrodes  12  and  13  of the resonator chip respectively contact with the two bumps  14  and  15  formed on the mount electrodes  5  and  6 , and, in this state, the piezoelectric resonator chip  7  is bump-bonded. Thus, the piezoelectric resonator chip  7  is supported on the front surface of the mount electrodes  5  and  6  with a space corresponding to the thickness of the bumps  14  and  15 , and the mount electrodes  12  and  13  of the resonator chip are electrically connected to the mount electrodes  5  and  6 , respectively. The structure described above is generally referred to as the piezoelectric resonator  1 . 
     Next, the linear expansion coefficient and Young&#39;s modulus of the metal film  3  are described. In this embodiment, the linear expansion coefficient of the metal film  3  is 13×10 −6  per degrees centigrade or larger, and the product of the linear expansion coefficient and Young&#39;s modulus of the metal film  3  is 1.95 MPa per degrees centigrade or larger. This is a characteristic value that is necessarily for absorbing a difference between a linear expansion coefficient of the AT-cut resonator chip used as the piezoelectric resonator chip  7  of this embodiment and a linear expansion coefficient of the silicon substrate  2  as a base substrate. The linear expansion coefficient of the AT-cut resonator chip is 13.7×10 −6  per degrees centigrade in the X axis direction, 9.6×10 −6  per degrees centigrade in the Y axis direction, and 11.6×10 −6  per degrees centigrade in the Z′ axis direction, although those values are little different depending on documents or the like. On the other hand, the linear expansion coefficient of silicon is approximately 2.8×10 −6  per degrees centigrade. Therefore, if the piezoelectric resonator chip  7  is bump-bonded directly to the silicon substrate  2 , a large stress may be applied to the piezoelectric resonator chip  7  because of a large difference between the linear expansion coefficients. If a stress is applied to the piezoelectric resonator chip  7 , a malfunction that the frequency changes largely may occur. Note that, as a method of avoiding the above-mentioned malfunction, there is considered a method of making the substrate for bump-bonding with the piezoelectric resonator chip  7  of the same material as the piezoelectric resonator chip  7 , or a method of bonding the piezoelectric resonator chip  7  with the substrate with a soft material such as a conductive adhesive. However, in the case of, for example, an oscillator that is described later, it is difficult to adopt the method of making the above-mentioned substrate of the same material as the piezoelectric resonator chip  7 . The substrate on which a circuit or the like is formed is usually made of silicon in most cases, and there are few semiconductor materials having the same linear expansion coefficient as the piezoelectric resonator chip  7 . In addition, if a conductive adhesive is used for bonding the piezoelectric resonator chip  7  with the substrate, it is very difficult to apply the conductive adhesive onto a very small bonding area. In addition, an adhesive generates gas with time, which may deteriorate reliability of the resonator. Therefore, in this embodiment, the piezoelectric resonator chip  7  is bump-bonded onto the silicon substrate  2 , and the metal film  3  is disposed under the mount electrodes  5  and  6  for mounting the piezoelectric resonator chip  7  onto the silicon substrate  2 , so as to adjust a difference of linear expansion coefficient between the silicon substrate  2  and the piezoelectric resonator chip  7 . 
     Here, a linear expansion coefficient of two types of materials that are laminated and bonded is calculated. A relational expression (1) of a force is as follows, when a length of the material changes by ΔL in the case where two materials are laminated and bonded. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     F 
                     
                       cross 
                        
                       
                         - 
                       
                        
                       sectional 
                        
                       
                           
                       
                        
                       area 
                     
                   
                   = 
                   
                     E 
                     · 
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         L 
                       
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, F denotes a force applied to the material, E denotes Young&#39;s modulus of the material, L denotes a length of the single material, ΔL denotes a variation of the length of the material when materials are laminated and bonded. In addition, when a denotes a linear expansion coefficient of the material, a length L t  of the material at a certain temperature is expressed as follows. 
       [Expression 2] 
         L   t   a·Δt·L    (2)
 
     Here, Δt denotes a variation of temperature. Therefore, supposing that a material A and a material B are laminated and bonded, and that a length of the laminated and bonded material becomes L x  when the temperature changes by Δt, then the following expressions hold. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       E 
                       A 
                     
                      
                     
                       
                         
                           
                             
                               a 
                               A 
                             
                             · 
                             Δ 
                           
                            
                           
                               
                           
                            
                           
                             t 
                             · 
                             L 
                           
                         
                         - 
                         
                           L 
                           X 
                         
                       
                       
                         
                           
                             a 
                             A 
                           
                           · 
                           Δ 
                         
                          
                         
                             
                         
                          
                         
                           t 
                           · 
                           L 
                         
                       
                     
                   
                   = 
                   
                     
                       F 
                       A 
                     
                     
                       S 
                       A 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       E 
                       B 
                     
                      
                     
                       
                         
                           
                             
                               a 
                               B 
                             
                             · 
                             Δ 
                           
                            
                           
                               
                           
                            
                           
                             t 
                             · 
                             L 
                           
                         
                         - 
                         
                           L 
                           X 
                         
                       
                       
                         
                           
                             a 
                             B 
                           
                           · 
                           Δ 
                         
                          
                         
                             
                         
                          
                         
                           t 
                           · 
                           L 
                         
                       
                     
                   
                   = 
                   
                     
                       F 
                       B 
                     
                     
                       S 
                       B 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Because the material A and the material B are laminated and bonded while forces are balanced, the following expression holds. 
       [Expression 5] 
         F   A   +F   B =0   (5)
 
     From the expressions (3), (4) and (5), the length Lx of the material A is derived as follows, where SA denotes a cross-sectional area of the material A, and SB denotes a cross-sectional area of the material B. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     L 
                     X 
                   
                   = 
                   
                     
                       
                         
                           a 
                           A 
                         
                         · 
                         
                           
                             a 
                             B 
                           
                            
                           
                             ( 
                             
                               
                                 
                                   E 
                                   A 
                                 
                                 · 
                                 
                                   S 
                                   A 
                                 
                               
                               + 
                               
                                 
                                   E 
                                   B 
                                 
                                 · 
                                 
                                   S 
                                   B 
                                 
                               
                             
                             ) 
                           
                         
                       
                       
                         
                           
                             E 
                             A 
                           
                           · 
                           
                             S 
                             A 
                           
                           · 
                           
                             a 
                             B 
                           
                         
                         + 
                         
                           
                             
                               E 
                               B 
                             
                             · 
                             
                               S 
                               B 
                             
                             · 
                             a 
                           
                            
                           
                               
                           
                            
                           A 
                         
                       
                     
                      
                     Δ 
                      
                     
                         
                     
                      
                     
                       t 
                       · 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     From the expression (6), the linear expansion coefficient when the two materials are laminated and bonded is expressed as follows. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     7 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       a 
                       A 
                     
                     · 
                     
                       
                         a 
                         B 
                       
                        
                       
                         ( 
                         
                           
                             
                               E 
                               A 
                             
                             · 
                             
                               S 
                               A 
                             
                           
                           + 
                           
                             
                               E 
                               B 
                             
                             · 
                             
                               S 
                               B 
                             
                           
                         
                         ) 
                       
                     
                   
                   
                     
                       
                         E 
                         A 
                       
                       · 
                       
                         S 
                         A 
                       
                       · 
                       
                         a 
                         B 
                       
                     
                     + 
                     
                       
                         
                           E 
                           B 
                         
                         · 
                         
                           S 
                           B 
                         
                         · 
                         a 
                       
                        
                       
                           
                       
                        
                       A 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     A linear expansion ratio and Young&#39;s modulus of the silicon substrate  2  is substituted into this expression (7), and further it is supposed that the film thickness of the metal film  3  is 5 μm that is generally used in plating or the like, the thickness of the silicon substrate  2  is 50 μm, and the target linear expansion coefficient is 3.8×10 −6  per degrees centigrade that is one third of the linear expansion coefficient of the quartz resonator chip in the Z′ axis direction. Then, the linear expansion coefficient and the Young&#39;s modulus of the metal film  3  are determined. The linear expansion coefficient of the metal film  3  is 13×10 −6  per degrees centigrade or larger. In addition, when the product of the linear expansion coefficient and the Young&#39;s modulus of the metal film  3  is 1.95 MPa per degrees centigrade or larger, the linear expansion coefficient of 3.8×10 −6  per degrees centigrade is satisfied. Here, the linear expansion coefficient of the silicon substrate  2  is 2.8×10 −6  per degrees centigrade, and the Young&#39;s modulus of the same is 130 GPa. Ni and Cu satisfy the above-mentioned value as a general metal. 
     As described above, it is understood that the metal film  3  formed on the silicon substrate  2  is a material having a characteristic value satisfying the condition that the linear expansion coefficient is 13×10 −6  per degrees centigrade or larger and that the product of the linear expansion coefficient and the Young&#39;s modulus is 1.95 MPa per degrees centigrade or larger. In addition, satisfying metals under this condition are Ni and Cu. In this embodiment, a thickness of the metal film  3  is set to 5 μm, and a thickness of the silicon substrate  2  is set to 50 μm. However, if the thickness of the metal film  3  is set to 5 μm or larger, or if the thickness of the silicon substrate  2  is set to 50 μm or smaller, a limit range of the linear expansion coefficient or the Young&#39;s modulus of the metal film  3  is expanded. However, considering an operable condition, it would be reasonable that the linear expansion coefficient of the metal film  3  should be 13×10 −6  per degrees centigrade or larger and that the product of the linear expansion coefficient and the Young&#39;s modulus of the metal film  3  is 1.95 MPa per degrees centigrade or larger. Note that it is preferable that the linear expansion coefficient of the metal film  3  is 30×10 −6  per degrees centigrade or smaller and that the product of the linear expansion coefficient and the Young&#39;s modulus of the metal film  3  is 2.5 MPa per degrees centigrade or smaller. 
     Because the metal film  3  as the above-mentioned linear expansion coefficient adjusting film is formed, the linear expansion coefficient of the piezoelectric resonator chip  7  may be close to the linear expansion coefficient of the multilayered structural member including the silicon substrate  2 , the metal film  3  and the insulator film  4 . Therefore, the stress due to a difference of thermal expansion between the silicon substrate  2  and the piezoelectric resonator chip  7  caused by a change in ambient temperature may be reduced. In other words, characteristic deterioration due to a temperature characteristic is decreased, so that an effect of reducing a frequency hop or shift from a theoretical curve of a frequency to temperature characteristic may be obtained. 
     In addition, because the bump bonding of the piezoelectric resonator chip  7  is adopted in this embodiment, the conventional problem accompanying downsizing may be solved. More detail description is given as follows. Accompanying downsizing of the piezoelectric resonator, the mount electrodes  12  and  13  of the resonator chip  7  become small, and it is necessary to decrease also the bonding region of the conductive adhesive that is usually used. However, the conductive adhesive has fluidity and is hardly retained within a constant region and may be spread to the entire region. Therefore, it is difficult to decrease the bonding region (i.e., the size of the mount electrodes  12  and  13  of the resonator chip). On the other hand, it is also considerable to increase the size of the mount electrodes  12  and  13  of the resonator chip for securing the bonding region. In this case, however, the electrodes  10  and  11  of the resonator chip may become small so that a region of a vibrating part of the piezoelectric resonator chip  7  may become small, resulting in deterioration of the characteristic. In contrast, this embodiment adopts the bump bonding so that the area of the bonding region may be very small. Therefore, the vibration region of the piezoelectric resonator chip  7  may be large, so it is possible to improve impedance characteristic and to facilitate designing the piezoelectric resonator chip  7 . The bump bonding is ultrasonic bonding of metal and has an effect that the piezoelectric resonator chip  7  and the silicon substrate  2  may be bonded to each other securely to have very high resistance against an impact despite of very small area of the bonding region. Because of such strong bonding, there may be a large stress applied to the piezoelectric resonator chip  7  due to a difference between the linear expansion coefficient of the silicon substrate  21  and the linear expansion coefficient of the piezoelectric resonator chip  7 . However, this problem may be solved by providing the above-mentioned metal film  3 . 
     In addition, because it needs a long time to cure the conductive adhesive, in the assembly process, it is necessary to retain the piezoelectric resonator chip  7 , or it is necessary to bond the piezoelectric resonator chip  7  in an inclined posture in advance so that the piezoelectric resonator chip  7  becomes parallel to the silicon substrate  2  by the weight of itself when the conductive adhesive is cured. However, such time-consuming bonding method is not necessary if the bump bonding is adopted as this embodiment. In addition, because the piezoelectric resonator chip  7  is supported by the bump bonding with a space from the silicon substrate  2 , a minimum vibration gap that is necessary for vibration may be naturally secured. Therefore, the thickness of the silicon substrate  2  may be as thin as possible. In this point, Embodiment 1 may reduce the thickness of the piezoelectric resonator  1 . Further, because the bump ball does not generate outgas in a long term use, there is obtained an effect that the stable characteristic of the piezoelectric resonator  1  may be maintained in a long term as the effect. Further, because the bump ball does not change with time depending on thermal environment, the stable characteristic of the piezoelectric resonator  1  may be maintained in a long term. 
     Embodiment 2  
     A structure and effects of Embodiment 2 are substantially the same as those described above in Embodiment 1, but Embodiment relates to an oscillator, and an oscillation circuit is formed on silicon substrate  2  as a large difference. Therefore, description of the same structure as Embodiment 1 is omitted, and the difference is described. 
     An oscillator  20  utilizing the piezoelectric resonator  1  of Embodiment 1 described above is illustrated in  FIGS. 3 and 4 . On the front surface of the silicon substrate  21 , there are formed an oscillation circuit  21 A, connection terminals  25  and  26  of the piezoelectric resonator, and electrode terminals  27 ,  28 ,  29  and  30  of the oscillation circuit. In addition, on the front surface of the silicon substrate  21 , there are formed a protection film  22  constituted of an oxide film or a nitride film with openings corresponding to portions of the connection terminals  25  and  26  of the piezoelectric resonator and the electrode terminals  27 ,  28 ,  29  and  30  of the oscillation circuit. The metal film  3  is formed on the protection film  22 , and the insulator film  4  is formed to cover the metal film  3 . The metal film  3  is disposed at a position just below the mount electrodes  5  and  6 , and is formed to have a size that is the same as or larger than the size covering the two mount electrodes  5  and  6  at the same time. The insulator film  4  covers the entire metal film  3  so that the metal film  3  is not conductive to the mount electrodes  5  and  6 . In  FIGS. 3 and 4 , the insulator film  4  is formed only in the periphery of the metal film  3 , but this structure should not be interpreted as a limitation. As long as the state where the metal film  3  and the mount electrodes  5  and  6  are not conductive to each other may be maintained, the insulator film  4  may be formed on the entire substrate except for openings at portions of the connection terminals  25  and  26  of the piezoelectric resonator and the electrode terminals  27 ,  28 ,  29  and  30  of the oscillation circuit. Although depending on a process method, the number of the process steps is fewer if the insulator film  4  is formed on the entire surface of the silicon substrate  21 . 
     When the mount electrodes  5  and  6  are formed on the insulator film  4  and above the metal film  3 , connection electrodes  23  and  24  for connecting the connection terminals  25  and  26  of the piezoelectric resonator on the silicon substrate  21  with the mount electrodes  5  and  6  are formed simultaneously. The piezoelectric resonator chip  7  is bump-bonded on the mount electrodes  5  and  6  by using bumps  14  and  15  made of gold or other metal as illustrated in  FIGS. 3 and 4 . Thus, the piezoelectric resonator chip  7  is supported with a space from the front surface of the mount electrodes  5  and  6  corresponding to the thickness of the bumps  14  and  15 . In addition, the mount electrodes  12  and  13  of the resonator chip, the mount electrodes  5  and  6 , the connection electrodes  23  and  24 , and the connection terminals  25  and  26  of the piezoelectric resonator are electrically connected to each other, respectively. When a voltage is applied to the oscillation circuit electrode terminal, the oscillation circuit  21 A supplies excitation current to the piezoelectric resonator chip  7  via the connection terminal  25  of the piezoelectric resonator, and receives a resonance signal from the connection terminal  26  of the piezoelectric resonator so as to sustain the oscillation. Then, it may be used as a timing source of a control signal or a reference signal source or the like, utilizing the oscillation. 
     In addition,  FIG. 5  illustrates a state of housing the oscillator  20  of Embodiment 2 in a package. The package base substrate  31  is made of ceramic or the like and is formed in a recess shape. The oscillator  20  described above in Embodiment 2 is mounted on the bottom surface of the recess of the package base substrate  31 . Bonding pads  33  and  34  are formed on the front surface of the package base substrate  31  in the region except for the recess, and package external electrodes  37  and  38  are formed on the back surface of the package base substrate  31 . In addition, on the package base substrate  31 , there is also formed an internal electrode pattern for connecting the bonding pads  33  and  34  to the package external electrodes  37  and  38 , respectively. After that, the bonding pads  33  and  34  formed on the package base substrate  31  are bonded to the electrode terminals  27 ,  28 ,  29  and  30  of the oscillation circuit on the silicon substrate  21  with wires  35  and  36  (electrode terminals  28  and  30  of the oscillation circuit are not illustrated in  FIG. 5 ). Thus, a voltage is applied to the oscillation circuit  21 A, and excitation current is supplied to the piezoelectric resonator chip  7 , so that the piezoelectric resonator chip  7  oscillates. The excitation current flowing back from the piezoelectric resonator chip  7  is rectified by the oscillation circuit  21 A, and a signal is output to the outside of the package via the electrode terminals  27  and  29  of the oscillation circuit, the wires  35  and  36 , the bonding pads  33  and  34 , and the package external electrodes  37  and  38 . 
     A lid substrate  32  having a recess is bonded to the front surface of the package base substrate  31  on which the oscillator  20  is mounted, in a nitride atmosphere or in a vacuum atmosphere, so that the inside of a cavity  39  of the package is sealed in an airtight manner. 
     Thus, in this embodiment, the silicon substrate  21  on which the oscillation circuit  21 A is formed and the piezoelectric resonator chip  7  are integrated to one unit. Therefore, it is not necessary to provide the mounting space of the silicon substrate  21  on which the oscillation circuit  21 A is formed and the mounting space of the piezoelectric resonator chip  7  individually unlike the conventional oscillator package. Thus, thickness of the oscillator package may be reduced, and cost reduction may also be realized. Further, the piezoelectric resonator chip  7  and the silicon substrate  21  are very close to each other. Therefore, for example, in the case of a temperature compensation type oscillation circuit, the temperature sensor may measure the temperature of the piezoelectric resonator chip  7  more precisely, so that accurate temperature compensation may be performed. 
     Embodiment 3  
     A structure and effects of Embodiment 3 are substantially the same as those described above in Embodiment 1 and Embodiment 2, but Embodiment 3 has a large difference in that the oscillation circuit is formed on the back surface of the silicon substrate, and the piezoelectric resonator chip  7  is mounted on the front surface of the silicon substrate. Therefore, description of the same structure as Embodiment 1 and Embodiment 2 is omitted, and the difference is described.  FIG. 6  is a cross sectional view and illustrates only a part of the connection terminals of the piezoelectric resonator or the electrode terminals of the oscillation circuit, but the actual structure thereof is the same as that of Embodiment 2. 
     In an oscillator  40  illustrated in  FIG. 6 , an oscillation circuit  41 A, a terminal  43  connected with a piezoelectric resonator, and electrode terminals  44  and  45  of the oscillation circuit are formed on the back surface of a silicon substrate  41 . On the back surface of the silicon substrate  41 , there is formed a protection film  42  constituted of an oxide film or a nitride film with openings corresponding to portions of the electrode terminals  44  and  45  of the oscillation circuit. A through-silicon via  47  is formed in the silicon substrate  41  at the portion where the terminal  43  connected with a piezoelectric resonator is formed, so that the terminal  43  connected with a piezoelectric resonator and the through-silicon via  47  are electrically connected to each other. In addition, a connection electrode  46  formed on the front surface of the silicon substrate  41  and the through-silicon via  47  are electrically connected to each other. The metal film  3  is formed on the front surface of the silicon substrate  41 . The metal film  3  is formed at the position just below the mount electrode  5  and has a size that is the same as or larger than the size covering the two mount electrodes at the same time. The insulator film  4  covers the entire metal film  3  so that the metal film  3  and the mount electrode  5  are not conductive to each other. In  FIG. 6 , the entire front surface of the silicon substrate  41  is covered with the insulator film  4 , but it is not necessarily required that the entire surface of the substrate is covered with the insulator film  4 . As long as the state where the metal film  3  and the two mount electrodes  5  are not conductive to each other may be maintained, the insulator film  4  may cover only in the periphery of the metal film  3 . However, the number of process steps is smaller in the case where the insulator film  4  is formed on the entire front surface of the silicon substrate  41 . 
     When the mount electrode  5  is formed on the insulator film  4  and above the metal film  3 , connection electrodes  46  for connecting the through-silicon via  47  of the silicon substrate  41  with the mount electrode  5  is formed simultaneously. The piezoelectric resonator chip  7  is bump-bonded on the front surface of the mount electrode  5  by using bump  14  made of gold or other metal as illustrated in  FIG. 6 . Thus, the piezoelectric resonator chip  7  is supported with a space from the front surface of the mount electrode  5  corresponding to the thickness of the bump  14 . In addition, the mount electrode  12  of the resonator chip, the mount electrode  5 , the connection electrodes  46 , the through-silicon via  47 , and the terminals  43  connected with a piezoelectric resonator are electrically connected to each other, respectively. The structure described above is generally referred to as the oscillator  40 . When a voltage is applied to the electrode terminals  44  and  45  of the oscillation circuit, the oscillation circuit  41 A supplies excitation current to the piezoelectric resonator chip  7  via the terminal  43  connected with a piezoelectric resonator, and receives a resonance signal from the piezoelectric resonator chip  7  so as to sustain the oscillation. Then, it may be used as a timing source of a control signal or a reference signal source or the like, utilizing the oscillation. 
     In addition,  FIG. 7  illustrates a state of housing the oscillator  40  of Embodiment 3 in a package. The package base substrate  51  is made of ceramic or the like and is formed in a plate shape. Bonding pads  52  and  53  for an oscillation circuit are formed on the front surface of the package base substrate  51 , and package external electrodes  54  and  55  are formed on the back surface of the package base substrate  51 . In addition, on the package base substrate  51 , there is also formed an internal electrode pattern for connecting the bonding pads  52  and  53  to the package external electrodes  54  and  55 , respectively. Bumps  56  and  57  are provided to the electrode terminals  44  and  45  of the oscillation circuit on the back surface of the silicon substrate  41 , and the oscillator  40  is flip-chip mounted to the bonding pads  52  and  53  on the package base substrate  51 . Thus, a voltage is applied to the oscillation circuit  41 A, and excitation current is supplied to the piezoelectric resonator chip  7 , so that the piezoelectric resonator chip  7  oscillates. The excitation current flowing back from the piezoelectric resonator chip  7  is rectified by the oscillation circuit  41 A, and a signal is output to the outside of the package via the electrode terminals  45  of the oscillation circuit, the bump  57 , the bonding pad  53 , and the package external electrode  55 . 
     A lid substrate  58  having a recess is bonded to the front surface of the package base substrate  51  on which the oscillator  40  is mounted, in a nitride atmosphere or in a vacuum atmosphere, so that the inside of a cavity  59  of the package is sealed in an airtight manner. Thus, in this embodiment, the silicon substrate  41  on which the oscillation circuit  41 A is formed and the piezoelectric resonator chip  7  are integrated to one unit. Therefore, it is not necessary to provide the mounting space of the silicon substrate  21  on which the oscillation circuit is formed and the mounting space of the piezoelectric resonator chip individually unlike the conventional oscillator package. Thus, thickness of the oscillator package may be reduced, and cost reduction may also be realized. In addition, a space for wire bonding is not required, and a smaller package may be created. Further, the piezoelectric resonator chip  7  and the silicon substrate  41  are very close to each other. Therefore, for example, in the case of a temperature compensation type oscillation circuit, the temperature sensor may measure the temperature of the piezoelectric resonator chip  7  more precisely, so that accurate temperature compensation may be performed. 
     Embodiment 4  
     As to Embodiment 4 too, overlapping description is omitted, and only a difference is described. 
     Embodiment 4 is a form in which the oscillator  40  of Embodiment 3 is housed in a package as illustrated in  FIG. 8 . A lid substrate  61  formed in a shape having a recess is bonded to the front surface of the silicon substrate  41  of the oscillator  40  so as to enclose the piezoelectric resonator chip  7 . The lid substrate  61  is bonded to the front surface of the silicon substrate  41  in a nitride atmosphere or in a vacuum atmosphere, so that the inside of a cavity  62  of the package is sealed in an airtight manner. The lid substrate  61  is made of metal, semiconductor, or glass. In the case of metal, Au—Sn bonding or the like is used for bonding to the silicon substrate  41 . In the case of semiconductor or glass, anode bonding may be used besides the Au—Sn bonding. 
     Here, an effect of the anode bonding is described. If the anode bonding is used for bonding the silicon substrate  41  to the lid substrate  61 , the silicon substrate  41  on which a plurality of oscillation circuits are arranged like an array and a plurality of piezoelectric resonator chips  7  may be bonded and sealed in an airtight manner at the same time. Very high pressure is usually applied to the silicon substrate  41  in the bonding process. If a plurality of units are bonded simultaneously as the above-mentioned anode bonding, the pressure applied to each of the silicon substrates  41  and the lid substrates  61  in the bonding process is reduced, so that breakage or chipping does not occur even if the bonding surface is small. In particular, according to the experiment result up to now, it is confirmed that if the width of the bonding surface  63  between the silicon substrate  41  and the lid substrate  61  is set to 50 μm or larger, the bonding may be performed while maintaining airtightness of the inside of the cavity  62 . In addition, according to the experiment, even if the width of the bonding surface  63  of the anode bonding is set to 300 μm or larger, no change was observed in characteristics of airtightness and durability. Therefore, it may be said that a desirable range of the width of the bonding surface  63  between the silicon substrate  41  and the lid substrate  61  in this embodiment is 50 to 300 μm. 
     In addition, if the width of the bonding surface  63  is decreased, a size of the cavity  62  may be large so that a size of the piezoelectric resonator chip  7  may be increased. If the size of the piezoelectric resonator chip  7  is increased, crystal impedance that is performance of the piezoelectric resonator chip  7  is decreased so that the performance is improved. Further, in the case of the anode bonding, there is little gas that is generated in the bonding process. Therefore, the airtightness inside the cavity  62  may be maintained, and a characteristic change of the piezoelectric resonator chip  7  with time is decreased. 
     In the manufacturing method of the piezoelectric resonator and the oscillator according to this embodiment, a plurality of piezoelectric resonators or oscillators may be manufactured simultaneously, so cost reduction may be realized. 
     Note that the technical scope of the present invention is not limited to Embodiment 1 to Embodiment 4 described above, which may be modified variously without deviating from the spirit of the present invention. For instance, the case of using the metal film  3  as the linear expansion coefficient adjusting film is described in Embodiments 1 to 4 described above, but this structure should not be interpreted as a limitation. For instance, instead of the metal film  3  in Embodiments 1 to 4, as the linear expansion coefficient adjusting film, it is possible to use a quartz plate made of the same material as the piezoelectric resonator chip  7  made of quartz. In this case, the quartz plate as a linear expansion coefficient adjusting film has the same linear expansion coefficient as the piezoelectric resonator chip  7  made of quartz, as a matter of course. Therefore, the linear expansion coefficient of the piezoelectric resonator chip  7  may be close to the linear expansion coefficient of the multilayered structural member including the silicon substrate  2 , the linear expansion coefficient adjusting film made of the quartz plate, and the insulator film  4 , so that the same effect as Embodiments 1 to 4 described above may be obtained.