BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a piezoelectric resonator. In particular, the present invention relates to a piezoelectric resonator including an acoustic reflector portion to acoustically separate a vibration portion, which includes a piezoelectric thin film sandwiched between a pair of electrodes, from a substrate and to support the vibration portion. 
         [0003]    2. Description of the Related Art 
         [0004]    Previously, piezoelectric resonators (Bulk Acoustic Wave resonators or BAW resonators) taking advantage of thickness vibration of a piezoelectric thin film and piezoelectric filters formed from the piezoelectric resonators include a type provided with a cavity and a type provided with an acoustic reflector portion to acoustically separate a vibration portion, which includes a piezoelectric thin film sandwiched between a pair of electrodes, from a substrate and to support the vibration portion. 
         [0005]    Regarding the latter type, as in a piezoelectric resonator  100  indicated by a sectional view shown in  FIG. 8 , an acoustic reflector portion  122  is disposed between a vibration portion  120 , in which a piezoelectric thin film  116  is sandwiched between a pair of electrodes  115  and  117 , and a substrate  112 . Regarding the acoustic reflector portion  122 , low acoustic impedance layers  114   a  and  114   b  formed from a material having relatively low acoustic impedance and high acoustic impedance layers  113   a  and  113   b  formed from a material having relatively high acoustic impedance are laminated alternately. The acoustic reflector portion  122  reflects vibration from the vibration portion  120  and acoustically separates the vibration portion  120  from the substrate  112  in such a way that the vibration is not transferred to the substrate  112 . For example, a metal material (W, Mo) is used for a high acoustic impedance layer, and a dielectric material (SiO 2 , Si 3 N 4 ) is used for a low acoustic impedance layer (refer to Japanese Unexamined Patent Application Publication No. 2002-251190, for example). 
         [0006]    Furthermore, as is indicated by a sectional view shown in  FIG. 9 , the document Proceedings of The 8th International Symposium on Sputtering and Plasma Process, “Properties of Sputter Deposited AlN, Mo, W and SiO 2  Thin-Films for Bulk-Acoustic-Wave Applications on 200 mm Si Substrates” discloses that SiO 2  is used for a low acoustic impedance layer, W is used for a high acoustic impedance layer, and AlN is used for a seed layer. 
         [0007]    Regarding the piezoelectric resonator of Japanese Unexamined Patent Application Publication No. 2002-251190, in the case where the metal (for example, W) serving as the high acoustic impedance layer is etched, there is a possibility that the low acoustic impedance layer thereunder is also etched. If a height difference formed around a portion, in which the electrodes are opposed to each other, of the vibration portion increases because of such over etching, a break in the electrode occurs easily. Furthermore, in the case where the film thickness of the high acoustic impedance layer is large, the risk of peeling of the film and cracking of the substrate increases. Consequently, it is difficult to conduct production with stable quality. 
         [0008]    Regarding the publication “Properties of Sputter Deposited AlN, Mo, W and SiO 2  Thin-Films for Bulk-Acoustic-Wave Applications on 200 mm Si Substrates,” the W film is specified to have a tensile stress for the purpose of stress adjustment of the acoustic reflector portion. The W film having the tensile stress exhibits poor crystallinity and tends to become a non-dense film, so that the resonance characteristics are degraded. 
       SUMMARY OF THE INVENTION 
       [0009]    In order to solve the above-described problems, preferred embodiments of the present invention provide a piezoelectric resonator with stable quality and improved resonance characteristics. 
         [0010]    A piezoelectric resonator according to a preferred embodiment of the present invention includes a substrate, a vibration portion including a piezoelectric thin film sandwiched between a pair of electrodes, and an acoustic reflector portion disposed between the substrate and the vibration portion. The acoustic reflector portion includes a plurality of low acoustic impedance layers made of a material having relatively low acoustic impedance and a plurality of high acoustic impedance layers made of a material having relatively high acoustic impedance, the low and high acoustic impedance layers being disposed alternately between the substrate and the vibration portion, and at least one adjustment layer disposed between one of the high acoustic impedance layer and one of the low acoustic impedance layer and having an acoustic impedance value between the acoustic impedance of the high acoustic impedance layers and the acoustic impedance of the low acoustic impedance layers. The low acoustic impedance layers and the high acoustic impedance layers have compressive stresses, and the at least one adjustment layer has a tensile stress. 
         [0011]    In the above-described configuration, the low acoustic impedance layer and the high acoustic impedance layer have the compressive stresses and, therefore, the films become dense and the crystallinity is good as compared with those in the case of the tensile stress. Consequently, as for an elastic wave, the loss is low, unnecessary scattering of the elastic wave is reduced and minimized, and the resonance characteristics of the piezoelectric resonator are improved. 
         [0012]    If an absolute value of compressive stress is large, film peeling occurs easily. However, a total stress of the acoustic reflector portion can be adjusted by the at least one adjustment layer having the tensile stress. Consequently, an occurrence of film peeling can be prevented. Furthermore, the crystallinity of the high acoustic impedance layer and the amount of over etching of the low acoustic impedance layer can be adjusted by the at least one adjustment layer. 
         [0013]    Accordingly, the quality of the piezoelectric resonator becomes stable and the resonance characteristics of the piezoelectric resonator can be improved. 
         [0014]    Preferably, the acoustic reflector portion between the substrate and the vibration portion includes adhesive layers disposed between the high acoustic impedance layers and the adjustment layers. 
         [0015]    In this case, the adhesion between the adjustment layer and the high acoustic impedance layer is improved and, thereby, film peeling and substrate cracking do not occur easily. Consequently, the quality of the piezoelectric resonator becomes more stable. 
         [0016]    Preferably, the adjustment layer is made of a material that is resistant to etching with a fluorine based gas. 
         [0017]    In this case, in etching of the high acoustic impedance layer, etching is stopped at the adjustment layer, and it is possible that the low acoustic impedance layer thereunder is not etched. Consequently, a dimension of a height difference formed around a portion, in which the electrodes are opposed to each other, of the vibration portion can be minimized and an occurrence of a break in the electrode can be prevented. As a result, the quality of the piezoelectric resonator becomes even more stable. 
         [0018]    Preferably, silicon oxide is used for the low acoustic impedance layer, tungsten is used for the high acoustic impedance layer, and aluminum nitride is used for the adjustment layer, for example. 
         [0019]    In this case, a piezoelectric resonator having stabilized quality and improved resonance characteristics can be produced easily. Aluminum nitride is suitable for the adjustment layer because of a wide range of adjustment of stress while the crystallinity is maintained. 
         [0020]    Preferably, titanium is used for the adhesive layer, for example. 
         [0021]    In this case, a piezoelectric resonator having stabilized quality and improved resonance characteristics can be produced easily. 
         [0022]    Thus, a piezoelectric resonator according to a preferred embodiment of the present invention achieves stable quality and improved resonance characteristics. 
         [0023]    Other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a sectional view of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0025]      FIG. 2  is a magnified sectional view of a key portion of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0026]    FIG.  3 ( 1   a )- 3 ( 6   b ) are sectionals views showing production steps of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0027]    FIG.  4 ( 7   b )- 4 ( 11   b ) are sectional views showing production steps of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0028]      FIG. 5  is a sectional view of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0029]      FIG. 6  is a sectional view of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0030]      FIG. 7  is a sectional view of a piezoelectric resonator according to a preferred embodiment of the present invention. 
           [0031]      FIG. 8  is a sectional view of a conventional piezoelectric resonator. 
           [0032]      FIG. 9  is a sectional view of a conventional piezoelectric resonator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    Examples 1 to 4 according to preferred embodiments according to the present invention will be described below with reference to  FIG. 1  to  FIG. 7 . 
       Example 1 
       [0034]    A piezoelectric resonator  10  of Example 1 will be described with reference to  FIG. 1  to  FIG. 4 . 
         [0035]    As indicated by a sectional view shown in  FIG. 1 , the piezoelectric resonator  10  preferably includes a vibration portion  2 , which includes a piezoelectric thin film  16  sandwiched between an upper electrode  17  and a lower electrode  15 , and the vibration portion  2  is acoustically separated from a substrate  12  with an acoustic reflector portion  4  therebetween. 
         [0036]    In the acoustic reflector portion  4 , three layers of low acoustic impedance layers  30 ,  32 , and  34  and two layers of high acoustic impedance layers  20  and  22  are disposed alternately, for example. The first low acoustic impedance layer  30  is in contact with the substrate  12  and the third low acoustic impedance layer  34  is in contact with the lower electrode  15 . 
         [0037]    Furthermore, an adjustment layer  13   s  is disposed between the first high acoustic impedance layer  20  and the first low acoustic impedance layer  30 , and an adjustment layer  13   t  is disposed between the second high acoustic impedance layer  22  and the second low acoustic impedance layer  32 . 
         [0038]    The low acoustic impedance layers  30 ,  32 , and  34  are preferably formed by using silicon oxide (SiO 2 ) or carbon-containing silicon oxide (SiOC), for example. The high acoustic impedance layers  20  and  22  are preferably formed from a metal, e.g., tungsten (W) and molybdenum (Mo), for example. The adjustment layers  13   s  and  13   t  are preferably formed by using aluminum nitride (AlN), alumina (Al 2 O 3 ), chromium oxide (Cr 2 O 3 ), diamond, or diamond-like carbon, for example. 
         [0039]    For example, the low acoustic impedance layers  30 ,  32 , and  34  are formed from SiO 2  (for example, about 820 μm), the high acoustic impedance layers  20  and  22  are formed from W (for example, about 820 μm), and the adjustment layers  13   s  and  13   t  are formed from AlN (for example, about 30 μm to about 200 μm). Table 1 described below shows the acoustic impedance of the material for each layer. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Acoustic impedance 
               
               
                   
                 10 10  [g/s · m 2 ] 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aluminum nitride 
                 3.7 
               
               
                   
                 Tungsten 
                 10.0 
               
               
                   
                 Silicon oxide 
                 1.2 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    In the case where AlN is used for the adjustment layers  13   s  and  13   t,  it is desirable that the C axis crystallinity is poor and the piezoelectric property is not provided or is at a very low level. This is because charges generated by the piezoelectric property of AlN are consumed as an ohmic loss in the high acoustic impedance layers  20  and  22 . 
         [0041]    The high acoustic impedance layers  20  and  22  are patterned in such a way as to be disposed merely just below the vibration portion  2  and in the vicinity thereof in order that capacitive coupling with adjacent resonators is prevented. 
         [0042]    For example, in the steps of laminating individual layers on the substrate  12  sequentially, the W films for forming the high acoustic impedance layers  20  and  22  are etched through reactive ion etching (RIE) by using a fluorine based gas and, thereby, the high acoustic impedance layers  20  and  22  are patterned. In the case where the adjustment layers  13   s  and  13   t  are AlN films not easily etched with the fluorine based gas, etching is stopped at the adjustment layers  13   s  and  13   t,  and the SiO 2  films serving as the low acoustic impedance layers  30  and  32  under the adjustment layers  13   s  and  13   t  are not etched. 
         [0043]    The film configuration of the lower electrode  15  is specified to be Pt/Ti/AlCu/Ti. AlCu refers to an alloy of Al and Cu, and it is preferable that the Cu concentration is about 0.5% to about 10%, for example. As for the piezoelectric thin film  16 , AlN is preferably used. The film configuration of the upper electrode  17  is specified to be AlCu/Ti/Pt/Ti. 
         [0044]    Next, an example of methods for manufacturing the piezoelectric resonator  10  will be described with reference to  FIG. 3  and  FIG. 4 . In  FIG. 3  and  FIGS. 4 , ( 1   a ) to ( 11   a ) on the left side are sectional views showing production steps of a piezoelectric resonator of Reference example, and ( 1   b ) to ( 11   b ) on the right side are sectional views showing production steps of the piezoelectric resonator  10  of Example 1. The adjustment layers  13   s  and  13   t  are formed in the piezoelectric resonator  10  of Example 1, whereas an adjustment layer is not formed in the piezoelectric resonator of Reference example. 
         [0045]    Initially, as shown in  FIG. 3  ( 1   a ) and ( 1   b ), the substrate  12  is prepared. As for the substrate  12 , an inexpensive substrate having excellent workability is used. A Si or glass substrate having a flat surface is better. 
         [0046]    Subsequently, as shown in  FIGS. 3  ( 2   a ) and ( 2   b ), a SiO 2  film serving as the first low acoustic impedance layer  30  in the acoustic reflector portion  4  is formed all over the surface of the substrate  12  by using a technique, e.g., a sputtering method or a thermal oxidation method. 
         [0047]    Thereafter, in merely Example 1, as shown in  FIG. 3  ( 3   b ), an AlN film serving as the adjustment layer  13   s  is further formed on the SiO 2  film  30 . 
         [0048]    Then, as shown in  FIGS. 3  ( 4   a ) and ( 4   b ), a W film  20   k  serving as the first high acoustic impedance layer  20  in the acoustic reflector portion is formed on the SiO 2  film  30  or the AlN film  13   s  by using a technique, e.g., a sputtering method. 
         [0049]    Next, as shown in  FIGS. 3  ( 5   a ) and ( 5   b ), the W film other than resonator portion is removed by using patterning through photolithography and etching (for example, reactive ion etching) in such a way that insulation at least between resonators of a filter can be ensured and, thereby, the first high acoustic impedance layer  20  is formed from the remaining W film. 
         [0050]    In the etching in Reference example, as shown in  FIG. 3  ( 5   a ), regarding the SiO 2  film  30 , a portion  30   x  around the high acoustic impedance layer  20  is etched and the thickness becomes small easily. On the other hand, in Example 1, as shown in FIG. ( 5   b ), the etching is stopped by the AlN film serving as the adjustment layer  13   s  and, therefore, the SiO 2  film  30  does not become thin. 
         [0051]    Thereafter, film formation and the like of the SiO 2  film, the AlN film, and the W film are repeated likewise. 
         [0052]    That is, as shown in  FIGS. 3  ( 6   a ) and ( 6   b ), a SiO 2  film serving as the second low acoustic impedance layer  32  in the acoustic reflector portion  4  is formed. Subsequently, in merely Example 1, as shown in  FIG. 4  ( 7   b ), an AlN film serving as the adjustment layer  13   t  is formed on the SiO 2  film  32 . 
         [0053]    Then, as shown in  FIGS. 4  ( 8   a ) and ( 8   b ), a W film  22   k  serving as the second high acoustic impedance layer  22  in the acoustic reflector portion is formed on the SiO 2  film  32  or the AlN film  13   t.  Next, as shown in  FIGS. 4  ( 9   a ) and ( 9   b ), the W film  22   k  is etched, so that the second high acoustic impedance layer  22  is patterned. 
         [0054]    In the etching in Reference example, as shown in  FIG. 4  ( 9   a ), regarding the SiO 2  film  32 , a portion  32   x  around the high acoustic impedance layer  22  is etched and the thickness becomes small easily. On the other hand, in Example 1, as shown in FIG. ( 9   b ), the etching is stopped by the adjustment layer  13   t  and, therefore, the SiO 2  film  32  does not become thin. 
         [0055]    Subsequently, as shown in  FIGS. 4  ( 10   a ) and ( 10   b ), a SiO 2  film serving as the third low acoustic impedance layer  34  in the acoustic reflector portion is formed. In this manner, the acoustic reflector portion is completed. 
         [0056]    As indicated by arrows  3   a  and  3   b  shown in  FIGS. 4  ( 10   a ) and ( 10   b ), regarding the dimension of the height difference between the periphery portion, in which the SiO 2  films  30 ,  32 , and  34  are stacked, and the center portion, in which the SiO 2  films  30 ,  32 , and  34  and the W films  20  and  22  are stacked, the dimension in Example 1 is smaller than the dimension in the reference example. Regarding the reference example, since this height difference is large, as shown in FIG. ( 11   a ), in the lower electrode  15  formed on the acoustic reflector portion, a break occurs easily at the portion where the height difference occurs. On the other hand, regarding Example 1, since the height of the height difference is minimized, in the lower electrode  15 , a break does not occur easily at the portion where the height difference occurs. 
         [0057]      FIG. 2  is a magnified sectional view of a key portion of the acoustic reflector portion  4  in the piezoelectric resonator  10  of Example 1. Formation is conducted in such a way that the film stresses of the low acoustic impedance layers  30 ,  32 , and  34 , which are the SiO 2  films, and the high acoustic impedance layers  20  and  22 , which are the W films, become compressive stresses, as indicated by arrows  80  and  82  in  FIG. 2 , and the film stresses of the adjustment layers  13   s  and  13   t  become tensile stresses, as indicated by arrows  90 . 
         [0058]    A method for forming the AlN film is described in publicly known documents, for example, K. Umeda et al., Vacuum 80 (2006) p. 658-661. In this document, AlN is formed by an RF magnetron sputtering method. In that case, the stress of the AlN film is adjusted from compressive to tensile by the RF power, the gas pressure, and the substrate bias and, in addition, it is known that the crystallinity of AlN at that time is almost constant. 
         [0059]    In order to improve the crystallinity of the W film, it is necessary to improve the crystallinity of the adjustment layer serving as a substrate therefor. Consequently, the AlN film is suitable for the adjustment layer because of a wide range of adjustment of stress while the crystallinity is maintained. 
         [0060]    Regarding the W film having a film thickness of about 300 nm, for example, experimentally obtained relationships between the stress and the crystallinity and between the stress and the surface roughness are shown in Table 2 described below. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 X-ray rocking curve full 
                 Surface 
               
               
                   
                 Film stress 
                 width half maximum 
                 roughness 
               
               
                   
                 Stress (MPa) 
                 FWHM (deg) 
                 Ra (nm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 W film (1) 
                 −2064 
                 2.9 
                 0.3 
               
               
                 W film (2) 
                 −310 
                 10.4 
                 5.0 
               
               
                   
               
             
          
         
       
     
         [0061]    From Table 2, the W film ( 1 ) having a large compressive stress is preferable for the high acoustic impedance layer of the acoustic reflector. 
         [0062]    As an absolute value of compressive stress of the W film becomes large, the film becomes dense, the film surface becomes flat, and the crystallinity becomes good. This is because in the case where the crystallinity is good as described above, as for an elastic wave, the loss is low, unnecessary scattering of the elastic wave is at a low level, and the resonance characteristics of the piezoelectric resonator are improved. 
         [0063]    If the absolute value of compressive stress is large, film peeling occurs easily. However, a total stress of the acoustic reflector portion can be adjusted by the adjustment layer having the tensile stress. Consequently, an occurrence of film peeling can be prevented. 
         [0064]    Therefore, even when a high quality W film having a compressive stress is used for the high acoustic impedance layer of the acoustic reflector portion, the stress of the whole acoustic reflector portion can be relaxed by the AlN adjustment layer having a tensile stress. Furthermore, the crystallinity of the high acoustic impedance layer and the amount of over etching of the low acoustic impedance layer can be adjusted by the adjustment layer. 
         [0065]    Accordingly, the piezoelectric resonator  10  having stabilized quality and improved resonance characteristics can be produced. 
       Example 2 
       [0066]    A piezoelectric resonator  10   a  of Example 2 will be described with reference to  FIG. 5 . 
         [0067]    As indicated by a sectional view shown in  FIG. 5 , the piezoelectric resonator  10   a  of Example 2 preferably has substantially the same configuration as that of the piezoelectric resonator  10  of Example 1 shown in  FIG. 1 , and the same effects as those in Example 1 are obtained. In the following description, the points different from Example 1 will be explained mainly, and the same constituent portions as those in Example 1 are represented by the same reference numerals. 
         [0068]    The piezoelectric resonator  10   a  of Example 2 is different from Example 1 in configuration of an acoustic reflector portion  4   a.  Specifically, in the acoustic reflector portion  4   a  of Example 2, adhesive layers  40  and  42  are disposed between the adjustment layers  13   s  and  13   t  and high acoustic impedance layers  20   a  and  22   a.  The adhesive layers  40  and  42  are patterned through etching or the like at the same time with the high acoustic impedance layers  20   a  and  22   a  thereon. 
         [0069]    For example, in the case where the high acoustic impedance layers  20   a  and  22   a  are W, the low acoustic impedance layers  30   a,    32   a,  and  33   a  are SiO 2 , and the adjustment layers  13   s  and  13   t  are AlN, it is desirable that the adhesive layers  40  and  42  are Ti. 
         [0070]    The adhesion between the high acoustic impedance layers  20   a  and  22   a  and the adjustment layers  13   s  and  13   t  is improved by the adhesive layers  40  and  42 , film peeling and substrate cracking do not occur easily, and the quality of the piezoelectric resonator  10   a  becomes more stable. 
       Example 3 
       [0071]    A piezoelectric resonator  10   b  of Example 3 will be described with reference to  FIG. 6 . 
         [0072]    As indicated by a sectional view shown in  FIG. 6 , the piezoelectric resonator  10   b  of Example 3 preferably has substantially the same configuration as that of the piezoelectric resonator  10  of Example 1 shown in  FIG. 1 , and the same effects as those in Example 1 are obtained. 
         [0073]    As shown in  FIG. 6 , the piezoelectric resonator  10   b  of Example 3 is different from Example 1, and the width of the high acoustic impedance layer  20   b  on the substrate  12  side is larger than the width of the high acoustic impedance layer  22   b  on the vibration portion  2   b  side. Such high acoustic impedance layers  20   b  and  22   b  having different dimensions can be formed by using photomasks having different dimensions for individual high acoustic impedance layers  20   b  and  22   b.    
         [0074]    Consequently, the number of height differences formed on the uppermost surface  4   s  of the acoustic reflector portion  4   b  extending around the vibration portion  2   b  (that is, a surface, which is in contact with the lower electrode  15   b  or the piezoelectric thin film  16   b,  of the low acoustic impedance layer  34   b  farthest from the substrate  12 ) increases from one to two. As a result, the height per height difference becomes small, and occurrences of a break and thickness reduction of the lower electrode  15   b  become more difficult as compared with that in the case where the number of height difference is one. Therefore, the quality of the piezoelectric resonator  10   b  becomes more stable. 
       Example 4 
       [0075]    A piezoelectric resonator  10   c  of Example 4 will be described with reference to  FIG. 7 . 
         [0076]    As indicated by a sectional view shown in  FIG. 7 , the piezoelectric resonator  10   c  of Example 4 has substantially the same configuration as that of the piezoelectric resonator  10   b  of Example 3 shown in  FIG. 6 , and the same effects as those in Example 3 are obtained. 
         [0077]    The point different from Example 3 is that etching end surfaces  20   k  and  22   k  of the high acoustic impedance layers  20   c  and  22   c  are worked into the shape of a curved surface. 
         [0078]    For example, before etching of the high acoustic impedance layers  20   c  and  22   c  is conducted, a mask pattern of photoresist is formed in the following procedure.
   (1) Application of photoresist   (2) Baking (heating)   (3) Exposure   (4) Development   (5) Secondary baking (heating)   
 
         [0084]    A normal taper is formed on the end surfaces of the photoresist by conducting the secondary baking. The etching end surfaces  20   k  and  22   k  of the high acoustic impedance layers  20   c  and  22   c  can be worked into the shape of a curved surface by conducting etching through the use of the resulting photoresist. 
         [0085]    Regarding the acoustic reflector portion  4   c,  the corner  4   k  of the height difference formed around the vibration portion  2  is worked into the shape of a curved surface. Consequently, a sharp change in angle is eliminated and occurrences of a break and thickness reduction of the lower electrode  15   b  formed thereon become more difficult. 
         [0086]    Therefore, the quality of the piezoelectric resonator  10   c  becomes more stable. 
         [0087]    As described above, the acoustic reflector portion preferably includes the high acoustic impedance layer and the low acoustic impedance layer having the compressive stresses and the adjustment layer having the tensile stress. As a result, the total stress of the acoustic reflector portion can be relaxed and minimized, so that an occurrence of interlayer film peeling can be prevented. 
         [0088]    Since the total stress of the acoustic reflector portion can be relaxed and minimized, warping of a wafer can be reduced. Consequently, in the step of forming a resonator portion, for example, in an exposure apparatus used in a photolithography step, a wafer can be fixed with a vacuum chuck, so that patterning.position accuracy is improved. 
         [0089]    Furthermore, since the total stress of the acoustic reflector portion can be relaxed and minimized, substrate cracking can be prevented and the yield of product can be improved. 
         [0090]    In the case where tungsten (W) or molybdenum (Mo) is preferably used for the high acoustic impedance layer, silicon oxide (SiO 2 ) or carbon-containing silicon oxide (SiOC) is preferably used for the low acoustic impedance layer, for example, and Ti is preferably used for the adhesive layer, every layer can be etched easily with a fluorine based gas, e.g., CF 4 . Consequently, for example, in the case where a laminated structure of W/SiO 2  or a laminated structure of W/Ti/SiO 2  is used for the high acoustic impedance layers and the low acoustic impedance layers disposed alternately in the acoustic reflector portion, when the W layer or the W/Ti layer is etched, the SiO 2  layer serving as a substrate is etched through over etching. In such a case, proceeding of etching into the substrate can be prevented by using a material (for example, AlN) that is not etched easily with the fluorine based gas, as the adjustment layer, between the W film or the W/Ti film and the SiO 2  layer. 
         [0091]    Since over etching can be prevented by using a material (for example, AlN) not etched easily with the fluorine based gas, as the adjustment layer, the height difference of the acoustic reflector portion does not increase and, thereby, an occurrence of a break of the lower electrode can be prevented. 
         [0092]    In the case where the crystallinity of the W film is improved, the compressive stress is enhanced. If the compressive stress is large, film peeling occurs easily. However, in preferred embodiments of the present invention, the total stress can be adjusted by the adjustment layer, so that film peeling does not occur and the W film exhibiting a small elastic wave loss can be used. Consequently, the resonance characteristics are improved. 
         [0093]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
A piezoelectric resonator that achieves stable quality and improved resonance characteristics includes an acoustic reflector portion disposed between a substrate and a vibration portion, which includes a piezoelectric thin film sandwiched between a pair of electrodes, and a plurality of low acoustic impedance layers made of a material having relatively low acoustic impedance and a plurality of high acoustic impedance layers formed made of a material having relatively high acoustic impedance, the acoustic impedance layers being disposed alternately, and adjustment layers, which are disposed between the high acoustic impedance layers and the low acoustic impedance layers on the substrate sides of the high acoustic impedance layers and which have an acoustic impedance value intermediate between that of the high acoustic impedance layers and that of the low acoustic impedance layers. The low acoustic impedance layers and the high acoustic impedance layers have compressive stresses and the adjustment layers have a tensile stress.