Flat packaged piezoelectric device using a glass-ceramic composite material comprising forsterite as ceramics
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of application Ser. No. 08/146,575 filed Oct. 29, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a glass-ceramic composite material for use in a flat packaged piezoelectric device. This invention relates also to the flat packaged piezoelectric device. 
     A flat packaged piezoelectric device of the type described is, for example, a crystal oscillator unit or an SAW (surface acoustic wave) resonator unit and is for use in surface mounting. 
     A conventional flat packaged piezoelectric device comprises a base member, a pair of electrode pad parts formed on the base member, and a piezoelectric oscillator piece of, for example, a quartz piece. A pair of electrodes are formed on both surfaces of the piezoelectric oscillator piece and have parts fixed to the electrode pad parts. A cap member is sealed air-tight to the base member to enclose the piezoelectric oscillator piece in cooperation with the base member. 
     As will later be described, the piezoelectric oscillator piece and the base and the cap members have a difference in thermal expansion coefficients. This gives rise to a strain after cooling down from a raised temperature to result in a variation in the resonance frequency of the crystal oscillator unit and to make it impossible to attain a desired frequency characteristic. 
     Moreover, use of a resilient supporter as intermediary for connection of the piezoelectric oscillator piece to the base and the cap members in solving the above-described problem, results in other problems, such as an increased manufacturing cost and a thicker thickness of the flat packaged piezoelectric device as will also later be described. 
     Still further, alumina (Al 2  O 3 ) for use as a principal material of each of the base and the cap members must be sintered at a temperature of 1500° to 1600° C. As a consequence, internal conductors for wiring in the flat packaged piezoelectric device must be made of tungsten (W), molybdenum (Mo), and a like metal having a high melting point if the conductors should concurrently be sintered. Such a metal of the high melting point has a low electric conductivity and is not adapted to soldering. Nickel (Ni) plating and/or gold (Au) plating are therefore indispensable. As a result, great amounts of process and cost are necessary on manufacture. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a glass-ceramic composite material used in a flat packaged piezoelectric device, which material makes it possible to attain a desired frequency characteristic of the flat packaged piezoelectric device. 
     It is another object of this invention to provide a glass-ceramic composite material of the type described, which can be manufactured by a reduced amount of process and at a low cost. 
     It is a further object of this invention to provide a flat packaged piezoelectric device which can attain a desired frequency characteristic. 
     It is a still further object of this invention to provide a flat packaged piezoelectric device of the type described, which has a decreased manufacturing cost. 
     Other objects of this invention will become clear as the description proceeds. 
     According to an aspect of this invention, there is provided a glass-ceramic composite material comprising a glass component and a ceramic component. The glass component has a thermal expansion coefficient between 100×10 -7  /°C. and 150×10 -7  /°C., both inclusive. The ceramic component is 30 to 70% by weight of forsterite (2MgO.SiO 2 ) which is dispersed in the glass component. 
     A flat packaged piezoelectric device to which this invention is applicable comprises a base member, a pair of electrode pad parts formed on the base member, a piezoelectric oscillator piece, a pair of electrodes formed on both surfaces of the piezoelectric oscillator piece and having parts fixed to the electrode pad parts, and a cap member sealed air-tight to the base member to enclose the piezoelectric oscillator piece in cooperation with the base member. According to another aspect of this invention, each of the base and the cap members is made of a glass-ceramic composite material comprising a glass component and a ceramic component. The glass component has a thermal expansion coefficient between 100×10 -7  /°C. and 150×10 -7  /°C., both inclusive. The ceramic component is 30 to 70% by weight of forsterite (2MgO.SiO 2 ) which is dispersed in the glass component. 
     According to the invention each of the base and the cap members is formed of a material having a thermal expansion coefficient similar to that of a piezoelectric oscillator piece, such as a quartz piece, so that the piezoelectric oscillator piece may be fixed directly to the package without intermediary of a supporting member. As the material for the base and the cap members, a composite material of a ceramic component and a glass component is used wherein 30 to 70% by weight of forsterite is dispersed in the glass component. 
     The glass component preferably consists essentially of 50 to 70% by weight of SiO 2 , 3 to 15% by weight of Al 2  O 3 , 0 to 10% by weight of ZnO, 7 to 25% by weight of RO (where RO represents at least one member of CaO, SrO, and BaO), 0 to 8% by weight of B 2  O 3 , and 13 to 27% by weight of R 2  O (where R 2  O represents at least one member of Na 2  O, K 2  O, and Li 2  O). 
     For improving a bend breakage strength to a desired value, the glass component and the forsterite are pulverized into sufficiently fine powder of an average particle diameter between 1 and 3 microns. For further improvement of the bend breakage strength, an additive of 0 to 5% by weight of at least one member of ZrO 2 , TiO 2 , SnO 2 , P 2  O 5 , and MoO 3  is mixed so that the glass is caused to at least partly crystallize during sintering or firing to have a raised bend breakage strength. 
     As a result of the foregoing, the material of the base and the cap members matches the piezoelectric oscillator piece in their thermal expansion coefficients to reduce a residual strain after cooling down from a raised temperature and to suppress a variation in the resonance frequency. Moreover, a sufficient bend breakage strength is achieved. 
     It is possible to lower the sintering temperature of the package and to use Ag, Ag/Pd, Ag/Au, or a like metal of a low melting point even while the internal conductors are simultaneously sintered. These metal materials have a high electric conductivity. Selection of a component ratio in the internal conductors enables to carry out soldering and to do without plating processes. 
     As a consequence, it is rendered possible to provide a flat packaged piezoelectric device in which a piezoelectric oscillator piece is less subjected to a variation in its resonance frequency and which is thinner in thickness and more highly reliable than ever. In addition, it is possible to reduce the cost of manufacture and to shorten hours of manufacture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an exploded perspective view of a conventional crystal oscillator unit; 
     FIG. 2 is an exploded perspective view of another conventional crystal oscillator unit; 
     FIG. 3 is an exploded perspective view of a crystal oscillator unit according to a first embodiment of this invention; 
     FIG. 4 is a diagram illustrative of a degree of influence which is caused by the temperature of heat treatment on the resonance frequency of the crystal oscillator units illustrated in FIGS. 1 and 3; and 
     FIG. 5 is an exploded perspective view of a crystal oscillator unit according to a second embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a conventional crystal oscillator unit which serves as a flat packaged piezoelectric device, will first be described for a better understanding of this invention. 
     In FIG. 1, the crystal oscillator unit is for use in surface mounting and comprises a quarts piece 2 fixed to a base member 1 made principally of alumina. Subsequently, a cap member 3, made principally of alumina, is used as a cover and with air-tight seal achieved by a sealing part 4 of glass of a low melting point. A sealed package 5 is mounted on a circuit base board (not shown) by reflow soldering or a similar method. On sealing the package and mounting it onto the circuit base board, the package is heated to a raised temperature. It is to be noted in this connection that the package 5 and the quarts piece 2 have a difference in thermal expansion coefficients. This results in a strain in the quartz piece 2 after cooled down from the raised temperature to give rise to a variation in the resonance frequency of the quartz piece 2 and to make it impossible to attain a desired frequency characteristic. As a countermeasure, a periodical published in the Japanese language under the title of &#34;Electronics Update&#34;, 1990.4, teaches on pages 83 to 88, the structure which is illustrated in FIG. 1 and in which the quartz piece 2 is fixed to the base member 2 by intermediary of a resilient supporter 6 for subsequent air-tight seal. 
     As depicted in FIG. 2, another conventional crystal oscillator unit is proposed in Pat. Prepub. (A) No. 105,710 of 1990 as follows. Electrode lead parts 7 for the quartz piece 2 are led from a common end of the quartz piece 2 with the quartz piece 2 fixed, without intermediary of a supporter, directly to electrode pad parts 8 formed on the base member 1. Being fixed to the electrode pad parts 8 only at the common end, the quartz piece 2 has a free end and is not subjected to the strain. 
     In the crystal oscillator units illustrated in FIGS. 1 and 2, the package 5 and the quartz piece 2 have a difference in thermal expansion coefficients. This gives rise to a strain after cooling down from a raised temperature to result in a variation in the resonance frequency of the crystal oscillator unit and to make it impossible to attain a desired frequency characteristic as described above. 
     Moreover, use of the resilient supporter 6 as intermediary for connection of the quartz piece 2 to the package 5 in solving the above-described problem, results in other problems, such as an increased manufacturing cost and a thicker thickness of the package 5 as also described above. 
     Still further, alumina (Al 2  O 3 ) for use as a principal material of the package 5 must be sintered at a temperature of 1500° to 1600° C. As a consequence, internal conductors for wiring in the package 5 must be made of tungsten (W), molybdenum (Mo), and a like metal having a high melting point if the conductors should concurrently be sintered. Such a metal of the high melting point has a low electric conductivity and is not adapted to soldering. Nickel (Ni) plating and/or gold (Au) plating are therefore indispensable. As a result, great amounts of process and cost are necessary on manufacture as also mentioned before. 
     Embodiments and examples of this invention will now be described in detail by referring to Tables 1 and 2 and to FIGS. 3 through 5. 
     
                       TABLE 1                                                     
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Exam-  Glass components (percent by weight)                               
ple    SiO.sub.2                                                          
              Al.sub.2 O.sub.3                                            
                        ZnO  B.sub.2 O.sub.3                              
                                     RO   R.sub.2 O                       
______________________________________                                    
1(A)                                                                      
1      63     3         1    0       11   22                              
1(B)                                                                      
1(C)                                                                      
1(D)                                                                      
2(A)                                                                      
2      70     4         5    0.5     7.5  13                              
2(B)                                                                      
2(C)                                                                      
2(D)                                                                      
3(A)                                                                      
3      55     3         10   8       7    17                              
3(B)                                                                      
3(C)                                                                      
3(D)                                                                      
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                Characteristics                                           
Composite ratio   Thermal expansion                                       
                                Bend                                      
(percent by weight)                                                       
                  coefficient   breakage                                  
Exam-         For-          (×10.sup.-7 /°C.)                
                                        strength                          
ple   Glass   sterite ZrO.sub.2                                           
                            (50 to 500° C.)                        
                                        (kg/cm.sup.2)                     
______________________________________                                    
1(A)  50      50      0     120         1800                              
1     49.5    49.5    1.0   118         3200                              
1(B)  47.5    47.5    5.0   115         2900                              
1(C)  69      30      1.0   129         2500                              
1(D)  29      70      1.0   106         2800                              
2(A)  50      50      0     103         1950                              
2     49.5    49.5    1.0   100         3100                              
2(B)  49.0    49.0    2.0    97         2700                              
2(C)  64.5    34.5    1.0   105         2380                              
2(D)  34.5    64.5    1.0   100         2550                              
3(A)  50      50      0     108         1850                              
3     49.5    49.5    1.0   104         3050                              
3(B)  49.0    49.0    2.0   101         2520                              
3(C)  64.5    34.5    1.0   102         2210                              
3(D)  34.5    64.5    1.0   100         2410                              
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                       TABLE 2                                                     
______________________________________                                    
Exam-  Glass components (percent by weight)                               
ple    SiO.sub.2                                                          
              Al.sub.2 O.sub.3                                            
                        ZnO  B.sub.2 O.sub.3                              
                                     RO   R.sub.2 O                       
______________________________________                                    
4(A)                                                                      
4      50     3         5    0       25   17                              
4(B)                                                                      
4(C)                                                                      
4(D)                                                                      
5(A)                                                                      
5      55     3         5    0       10   27                              
5(B)                                                                      
5(C)                                                                      
5(D)                                                                      
6(A)                                                                      
6      50     15        0    0       12   23                              
6(B)                                                                      
6(C)                                                                      
6(D)                                                                      
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                Characteristics                                           
Composite ratio   Thermal expansion                                       
                                Bend                                      
(percent by weight)                                                       
                  coefficient   breakage                                  
Exam-         For-          (×10.sup.-7 /°C.)                
                                        strength                          
ple   Glass   sterite ZrO.sub.2                                           
                            (50 to 500° C.)                        
                                        (kg/cm.sup.2)                     
______________________________________                                    
4(A)  50      50      0     118         1800                              
4     49.5    49.5    1.0   116         3150                              
4(B)  49.0    49.0    2.0   115         3000                              
4(C)  64.5    34.5    1.0   126         2550                              
4(D)  34.5    64.5    1.0   105         2730                              
5(A)  50      50      0     123         1600                              
5     49.5    49.5    1.0   121         2800                              
5(B)  49.0    49.0    2.0   120         2600                              
5(C)  64.5    34.5    1.0   135         2300                              
5(D)  34.5    64.5    1.0   112         2450                              
6(A)  50      50      0     120         1650                              
6     49.5    49.5    1.0   119         3050                              
6(B)  49.0    49.0    2.0   117         2950                              
6(C)  64.5    34.5    1.0   128         2420                              
6(D)  34.5    64.5    1.0   108         2750                              
______________________________________                                    
 
    
     In the glass compositions of Tables 1 and 2, RO represents at least one oxide selected from CaO, SrO, and BaO. R 2  O represents at least one oxide selected from Na 2  O, K 2  O, and Li 2  O. 
     Glass, ceramics (forsterite), and an additive (zirconia) of Table 1 were first ground and mixed in a wet ball mill, dried to have an average particle diameter between 1 and 3 microns, machine-pestled, powder-mold pressed, and sintered or fired at a temperature between 800° C. and 1000° C. With a rectangular parallelopiped thereafter cut out, the band breakage strength and the thermal expansion coefficient were measured. The results are given in Table 1 and prove that a large thermal expansion coefficient of about 100 to 140×10 -7  /°C. was achieved by a mixture of fine powder of glass and forsterite according to this invention and that addition of zirconia raises the bend breakage strength to more than 2200 kg/cm 2  and, depending on the composition, even more than 3000 kg/cm 2 . It has been confirmed by results of differential thermal analysis and X-ray diffraction that a material sintered with addition of zirconia was partially crystallized. It appears that zirconia serves as an agent for providing seeds of crystallization. 
     In order next to study characteristics of packages for use as crystal oscillator units, the crystal oscillator units were manufactured to have the structure depicted in FIG. 3 as a first embodiment of this invention by using material powder given in Example 1 or Example 1(A) of Table 1 and by manufacturing processes specified in the following. 
     (a) A slurry was prepared by mixing the above-mentioned material, a binder of, for example, polyvinyl butyral, and a solvent of, for example, 2-methoxyethanol. A first green sheet of a thickness between 100 and 300 microns was manufactured by the doctor blade method. 
     (b) Through-holes (not shown) were formed through the first green sheet of (a). Silver (Ag) paste was screen printed to fill the through-holes and to provide internal conductors (not shown). 
     (c) Ag/Pd paste was screen printed on a second green sheet to provide electrode lead-out parts (not shown). 
     (d) Sets of rectangular holes were punched through the second green sheet, each set as a cavity for the quartz piece 2. 
     (e) In the manner which will presently be described, the first and the second green sheets and a dummy green sheet were stacked and hot-pressed at 100° C. and under a pressure between 100 and 200 kg/cm 2 . 
     (f) The above-mentioned laminate was debindered and then sintered at a temperature between 800° C. and 1000° C. 
     (g) The sintered laminate was cut into pieces, each for use as a base member 1 of a package 50 shown in FIG. 3. 
     (h) Mixed powder, used for the base member 1, was shaped by powder pressing and sintered at 800° C. to 1000° C. to provide a cap member 3 of the package 50. 
     (i) A sealing part 4 was preliminarily formed on the base member 1 by using known glass of a low melting point. Into the structure depicted in FIG. 3, electrode pad parts 80 formed on the base member 1 and electrode lead parts 7 of the quartz piece 2 were fixed to the base member 1 by a known electrically conductive adhesive. Subsequently, the cap member 3 was sealed by using the sealing part 4 to complete a flat packaged crystal oscillator unit. 
     In order to compare with a prior-art manufacture, the crystal oscillator unit of this invention was tested as regards how the resonance frequency was dependent on the temperature of thermal treatment. Forty crystal oscillator units were manufactured. First, their resonance frequency (f0) was measured by a spectrum analyzer. Next, ten-unit sets were subjected to heat treatment at 100° C., 200° C., 300° C., and 400° C. for thirty minutes and then cooled down to room temperature. Their resonance frequencies (f(T)) were measured. Equation 1 was used to calculate Δf/f0. 
     
         Δf/f0=(f(T)-f0)/f0, 
    
     namely, (the resonance frequency after the heat treatment minus the resonance frequency before the heat treatment)/(the resonance frequency before the heat treatment). 
     Results of measurement are depicted in FIG. 4. Results are also depicted in FIG. 4 as regards crystal oscillator units sealed in packages made principally of alumina according to prior art. This shows that the heat treatment resulted in a variation in the resonance frequency, which variation was about a quarter of that caused to prior-type manufactures. 
     Next, like packages 50 for crystal oscillator units were manufactured by using material powder listed in Table 1 and Table 2 as Example 2 or 2(A), Example 3 or 3(A), Example 4 or 4(A), Example 5 or 5(A), and Example 6 or 6(A) and in accordance with the processes specified in conjunction with Example 1 or 1(A). With a quartz piece 2 put in place, each crystal oscillator unit was completed. Evaluation was carried out on the dependency of the resonance frequency on the temperature of heat treatment of such crystal oscillator units in comparison with prior-art manufactures. The results were substantially similar to those made of composite materials of Examples 1 and 1(A) depicted in FIG. 4. 
     Using material powder given in Example 1 or 1(A) of Table 1, packages 10 were manufactured for use as crystal oscillator units of a structure depicted in FIG. 5 as a second embodiment of this invention in accordance with the above-described processes. With an IC 9 for an oscillator circuit and the quartz piece 2 placed in each package 10, such crystal oscillator units were tested in comparison with prior-art manufactures as regards the dependency of its resonance frequency on the temperature of thermal treatment. Results were substantially similar to those described in connection with Examples 1 and (1A) depicted in FIG. 4. 
     Similar results were obtained when fine powder of TiO 2 , SnO 2 , P 2  O 5 , and MoO 3  was singly or jointly used in addition to fine powder of zirconia (ZrO 2 ) used in the above-described examples in the manner listed in Tables 1 and 2 as Examples 1(B) through 1(D), 2(B) through 2(D), 3(B) through 3(D), 4(B) through 4(D), 5(B) through 5(D), and 6(B) through 6(D). Preferably, the fine powder is given an average particle diameter between 0.1 and 1 micron. Results of evaluation are given in Tables 1 and 2. 
     Inasmuch as the thermal expansion coefficient of a piezoelectric oscillator piece, such as the quartz piece, matches the thermal expansion coefficient of the package, it is possible to make the crystal oscillator unit or the like have a frequency characteristic hardly susceptible to thermal treatment during and after seal of the piezoelectric oscillator piece in the package and to give a high reliability to the crystal oscillator units and other piezoelectric devices for surface mounting. It is possible to reduce the cost as well. 
     This invention is widely applicable to a package for a SAW resonator unit and a like piezoelectric device, besides the crystal oscillator unit, for surface mounting. 
     Moreover, it is no more necessary to use the resilient support member for absorbing the strain resulting from the thermal treatment. This makes it possible to give a thin thickness to the package. 
     Furthermore, it is possible to lower the temperature of sintering the package. This reduces the cost of manufacture. In addition, this renders it unnecessary to plate the electrode lead-out parts and thereby to shorten the hours of manufacture. 
     The package of this invention has a high bend breakage strength. Use of fine powder of the ceramics and the glass raises the bend breakage strength. Addition of the additive or additives further raises the bend breakage strength.