Piezoelectric vibrator

A piezoelectric oscillator is provided, which has a through electrode providing reliable conduction between a piezoelectric vibrating piece and an external electrode with rare occurrence of a large stress caused by temperature variation in processing or deformation of a mounted base substrate, while the hermeticity of a cavity is maintained. A piezoelectric oscillator having a piezoelectric vibrating piece sealed in a cavity defined between a base substrate and a lid substrate includes a through electrode disposed in a through hole penetrating through the base substrate, and the through electrode has a glass frit filled in the through hole and fired and a core formed of a material containing only iron and nickel and disposed in the through hole together with the glass frit. The values of the thermal expansion coefficients of the base substrate, the glass frit, and the core are set as: base substrate glass≧frit>core.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2009-010184 filed on Jan. 20, 2009, the entire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric vibrator, and more specifically to a surface mount piezoelectric vibrator in which a piezoelectric vibrating piece is sealed in a cavity defined between two bonded substrates.

2. Description of the Related Art

In recent years, mobile telephones and portable information terminal devices employ a piezoelectric vibrator using quartz crystal as a time source, a timing source of control signals, and a reference signal source. As this type of piezoelectric vibrator, various ones are known. As one of them, there is a piezoelectric vibrator of a surface mount device (SMD).

As a typical surface mount piezoelectric vibrator, a three layer structure type is generally known in which a piezoelectric substrate formed with a piezoelectric vibrating piece is sandwiched between a base substrate and a lid substrate from above and below. In this case, the piezoelectric vibrator is accommodated in a cavity (airtight chamber) defined between the base substrate and the lid substrate. In addition, in recent years, two layer structure types are also developed, different from the three layer structure type.

The two layer structure type piezoelectric vibrator is formed to have a two layer structure by directly bonding a base substrate to a lid substrate, in which a piezoelectric vibrating piece is accommodated in a cavity defined between these two substrates. The two layer structure type piezoelectric vibrator is excellent for preferable use in that its thickness can be reduced as compared with three layer structure ones. As one of those two layer structure type piezoelectric vibrators, such a piezoelectric vibrator is known that a conductive member that is formed so as to penetrate through a base substrate is used to bring a piezoelectric vibrating piece and an external electrode formed on a base substrate into conduction (for example, see JP-A-2001-267190 (Patent Document 1) and JP-A-2007-328941 (Patent Document 2)).

In the piezoelectric vibrators described in Patent Document 1 and Patent Document 2, a through hole is formed in an insulating base substrate formed of ceramics or glass. Then, the conductive member is buried in the through hole such that the through hole is blocked. The conductive member is electrically connected to the external electrode formed on the undersurface of the base substrate as well as electrically connected to the piezoelectric vibrating piece accommodated in a cavity.

In the piezoelectric vibrator, the conductive member serves for two main roles: a) blocking the through hole to maintain hermeticity in the cavity, and b) bringing the piezoelectric vibrating piece and the external electrode into conduction. When the through hole is insufficiently contacted with the conductive member, hermeticity in the cavity is sometimes impaired. In addition, when electrical connection of the conductive member to the piezoelectric vibrating piece or to the external electrode is insufficient, this poor electrical connection causes malfunction of the piezoelectric vibrating piece. Therefore, in order to eliminate such failure, it is important to form the conductive member in such a way that the conductive member fully blocks the through hole as firmly contacted with the inner surface of the through hole as well as no dent is made on the surface.

When a conductive paste is used as a conductive member, it is necessary that the conductive paste is buried in the through hole, fired, and cured. However, when the conductive paste is fired, organic substances contained in the conductive paste are evaporated and lost, and thus the volume after fired is generally reduced as compared with the volume before fired (for example, when Ag paste is used as a conductive paste, about 20% of the volume is reduced). On this account, even though a conductive paste is used to form a conductive member, a dent occurs in the surface, or in the worst case, a through hole is formed at the center. Consequently, a problem arises that hermeticity in the cavity is impaired, or continuity between the piezoelectric vibrating piece and the external electrode is impaired.

In order to solve this problem, such a method is proposed in which a metal pin is disposed in a through hole, and a paste material such as a glass frit is filled in a clearance between the through hole and the pin and fired to form a through electrode. The through electrode is formed in this manner, whereby the volume is reduced only in the portion of the paste material. Therefore, the time required for the polishing process after that can be shortened, and the through electrode can be efficiently formed.

SUMMARY OF THE INVENTION

In firing the paste material, a base substrate is heated at high temperature with the pin and the paste material disposed. On this account, the volumes of the base substrate, the pin, and the paste material expand according to their thermal expansion coefficients.

Here, preferably, the thermal expansion coefficient of the paste material is generally set greater than the thermal expansion coefficient of the pin. This is advantageous to maintaining the hermeticity of the through hole because the expansion of the paste material exceeds the expansion of the pin to apply compressive force to the pin in firing, when the thermal expansion coefficients are set in this manner.

However, in the case in which the thermal expansion coefficient of the pin is too small, the range of deforming the paste material after fired becomes too large when cooled after the paste material is fired, or when the base substrate is deformed after mounted. As the result, a large stress is sometimes applied to the paste material and the base substrate after fired. Consequently, a problem arises that the piezoelectric vibrator may be damaged.

The invention has been made in the light of the circumstances, and it is an object of the invention to provide a piezoelectric vibrator having a through electrode capable of reliably bringing a piezoelectric vibrating piece and an external electrode into conduction in which a large stress hardly occurs when a component is cooled after the component is fired or when a base substrate is deformed after the base substrate is mounted, and the hermeticity of a cavity is maintained.

The invention is a piezoelectric vibrator having a piezoelectric vibrating piece sealed in a cavity defined between a base substrate and a lid substrate bonded to each other, the piezoelectric vibrator including: a through electrode disposed in a through hole penetrating through the base substrate in its thickness direction, wherein the through electrode has a glass frit filled in the through hole and fired, and a core formed of a material containing only iron and nickel as metals and disposed in the through hole together with the glass frit for bringing the outside of the cavity and the piezoelectric vibrating piece into conduction, and wherein values of thermal expansion coefficients of the base substrate, the glass frit, and the core are set as: the base substrate≧the glass frit>the core.

In accordance with the piezoelectric vibrator according to the invention, because the difference between the thermal expansion coefficients of the glass frit and the base substrate is small, a large stress hardly occurs in the interface between them. In addition, because the thermal expansion coefficient of the core is smaller than the thermal expansion coefficient of the glass frit, the hermeticity of the cavity can be preferably maintained.

The core may be formed of an alloy containing 58 percent by weight of iron and 42 percent by weight of nickel. In this case, the core can be formed of 42 alloy that is an alloy widely used, and the piezoelectric vibrator according to the invention can be readily fabricated at low costs.

In accordance with the piezoelectric vibrator according to the invention, a piezoelectric vibrator can be provided, which has a through electrode capable of reliably bringing a piezoelectric vibrating piece and an external electrode into conduction in which a large stress hardly occurs when a component is cooled after the component is fired or when a base substrate is deformed after the base substrate is mounted, and the hermeticity of a cavity is maintained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the invention will be described with reference toFIGS. 1 to 13.

As shown inFIGS. 1 to 13, a piezoelectric vibrator1according to the embodiment is a surface mount piezoelectric vibrator formed in a box shape of a two layer stack having a base substrate2and a lid substrate3, in which a piezoelectric vibrating piece4is accommodated in a cavity C defined by the base substrate2and the lid substrate3.

The piezoelectric vibrating piece4is a publicly known tuning fork vibrating piece formed of a piezoelectric material such as quartz crystal, lithium tantalate, or lithium niobate, and vibrates when a predetermined voltage is applied.

As shown inFIG. 2, the piezoelectric vibrating piece4has a pair of oscillating arms10and11disposed in parallel with each other, a base portion12that fixes the base end sides of the oscillating arms10and11in one piece, an excitation electrode15that is formed of a first excitation electrode13and a second excitation electrode14and formed on the outer surfaces of the oscillating arms10and11to cause a pair of the oscillating arms10and11to vibrate, and mount electrodes16and17electrically connected to the first excitation electrode13and the second excitation electrode14, respectively.

In addition, the piezoelectric vibrating piece4according to the embodiment has a groove18formed on both of the main surfaces of a pair of the oscillating arms10and11along the longitudinal direction of the oscillating arms10and11. The groove18is formed from the base end sides of the oscillating arms10and11to near the center part thereof.

The excitation electrode15formed of the first excitation electrode13and the second excitation electrode14is an electrode that causes a pair of the oscillating arms10and11to vibrate at a predetermined resonance frequency in the direction of bringing a pair of the oscillating arms10and11close to each other or separating them from each other, and the excitation electrode15is patterned and formed on the outer surfaces of the oscillating arms10and11as electrically isolated from each other. More specifically, the first excitation electrode13is mainly formed on the groove18of the oscillating arm10and on two side surfaces of the oscillating arm11, and the second excitation electrode14is mainly formed on two side surfaces of the oscillating arm10and on the groove18of the oscillating arm11.

In addition, the first excitation electrode13and the second excitation electrode14are electrically connected to the mount electrodes16and17through lead electrodes19and20, respectively, on both of the main surfaces of the base portion12. Then, a voltage is applied to the piezoelectric vibrating piece4through the mount electrodes16and17.

In addition, the excitation electrode15, the mount electrodes16and17, and the lead electrodes19and20are formed of coatings of conductive films such as chromium (Cr), nickel (Ni), aluminum (Al), or titanium (Ti), for example.

In addition, at the tip ends of a pair of the oscillating arms10and11, a weight metal film21is coated for control of the vibration state (frequency control) such that a pair of the oscillating arms10and11vibrates within a range of a predetermined frequency. In addition, the weight metal film21is separated into a coarse tuning film21afor use in coarsely tuning frequencies, and a fine tuning film21bfor use in finely tuning frequencies. The coarse tuning film21aand the fine tuning film21bare used to perform frequency control, whereby the frequency of a pair of the oscillating arms10and11is allowed to fall in the range of a nominal frequency of a device.

As shown inFIG. 3, the piezoelectric vibrating piece4thus configured is bonded to the top surface of the base substrate2(the surface on the cavity C side) through a bump with the use of a bump B made of, for example, gold. More specifically, on two bumps B formed on routing electrodes36and37, described later, which are patterned on the top surface of the base substrate2, a pair of the mount electrodes16and17is bonded through the bumps as the mount electrodes16and17are contacted with each other. Thus, the piezoelectric vibrating piece4is supported as separated from and floated above the top surface of the base substrate2, and the mount electrodes16and17are electrically connected to the routing electrodes36and37, respectively.

The lid substrate3is a transparent insulating substrate formed of a glass material such as soda-lime glass, and is formed in a plate shape as shown inFIGS. 1 and 3. Then, on the bonding surface side to which the base substrate2is bonded, a rectangular recessed part3ais formed in which the piezoelectric vibrating piece4is accommodated. The recessed part3ais a cavity recessed part to be the cavity C for accommodating the piezoelectric vibrating piece4in the cavity when the two substrates2and3are laid on each other. Then, the lid substrate3is anodically bonded to the base substrate2as the recessed part3ais faced to the top surface of the base substrate2.

The base substrate2is a transparent insulating substrate formed of a glass material similar to the lid substrate3, and is formed in a plate shape in the size to be laid on the lid substrate3as shown inFIGS. 1 to 3.

In the base substrate2, a pair of through holes30and31is formed to penetrate through the base substrate2in the thickness direction. A pair of the through holes30and31is disposed such that they are opened in the cavity C. More specifically, the through hole30is formed at the corresponding position on the base portion12side of the mounted piezoelectric vibrating piece4, and the through hole31is formed at the corresponding position on the tip end sides of the oscillating arms10and11.

In addition, in the embodiment, the through hole in a tapered cross section having the diameter being gradually reduced from the undersurface to the top surface of the base substrate2is taken and described as an example. However, the shape of the through hole is not limited to this shape. For example, an approximately cylindrical through hole may be possible whose diameter is equal in the axial direction. In any cases, the shape of the through hole has no particular restrictions as long as the through hole penetrates through the base substrate2.

Then, in a pair of the through holes30and31, a pair of through electrodes32and33is formed to bury the through holes30and31, respectively. As shown inFIG. 3, the through electrodes32and33are each formed of a conical piece6and a core7fixed in one piece to each of the through holes30and31by firing, and serve to fully block the through holes30and31for maintaining hermeticity in the cavity C as well as to bring the routing electrodes36and37and external electrodes38and39, described later, into conduction.

The conical piece6is formed by firing a paste glass frit. Both ends of the conical piece6are almost flat, and the conical piece6is formed to have almost the same thickness as that of the base substrate2, in which the core7is disposed at the center of the conical piece6to penetrate through the conical piece6. In the embodiment, the outside shape of the conical piece6is formed into an approximately truncated cone (in a tapered cross section) as matched with the shapes of the through holes30and31. Then, as shown inFIG. 3, the conical piece6is fired as buried in the through holes30and31, and is firmly fixed to each of the through holes30and31.

The core7is a cylindrical conductive core material formed of 42 alloy, described later, and the core7is formed to have flat ends as similar to the conical piece6and to have almost the same thickness as the thickness of the base substrate2. In addition, as shown inFIG. 3, when the through electrodes32and33are completed, as discussed above, the core7is formed to have the same thickness as the thickness of the base substrate2(including almost the same thickness). However, in the process of fabrication, the length of the core7is shorter than the initial thickness of the base substrate2in the process of fabrication by a predetermined length, 0.02 mm, for example (this will be described in detail in the description of a fabricating method). Then, the core7is positioned in the center of the hole of the conical piece6, and is firmly fixed to the conical piece6by firing the conical piece6.

In addition, the through electrodes32and33are reliably provided with electrical continuity through the conductive core7.

As shown inFIGS. 1 and 3, on the top surface side of the base substrate2(the surface to which the lid substrate3is bonded), a bonding film35for anodic bonding and a pair of the routing electrodes36and37are patterned with a conductive material (for example, aluminum). Among them, the bonding film35is formed along the outer edge of the top surface of the base substrate2such that the bonding film35surrounds the edge of the recessed part3aformed on the lid substrate3.

In addition, each of the routing electrodes36and37is patterned such that in a pair of the through electrodes32and33, the through electrode32is electrically connected to the mount electrode16of the piezoelectric vibrating piece4, and the through electrode33is electrically connected to the mount electrode17of the piezoelectric vibrating piece4.

More specifically, the routing electrode36is formed right above the through electrode32such that the routing electrode36is positioned just below the base portion12of the piezoelectric vibrating piece4. In addition, the routing electrode37is formed such that the routing electrode37is routed from the position adjacent to the routing electrode36to the tip end sides of the oscillating arms10and11along the oscillating arms10and11and then positioned right above the through electrode33.

Then, the bump B is formed on each of a pair of the routing electrodes36and37, and the bumps B are used to mount the piezoelectric vibrating piece4. Thus, the mount electrode16of the piezoelectric vibrating piece4is conducted to the through electrode32through the routing electrode36, and the mount electrode17is conducted to the through electrode33through the routing electrode37.

As shown inFIGS. 1 and 3, on the undersurface of the base substrate2, the external electrodes38and39are formed, which are electrically connected to a pair of the through electrodes32and33, respectively. In other words, the external electrode38is electrically connected to the first excitation electrode13of the piezoelectric vibrating piece4through the through electrode32and through the routing electrode36. In addition, the external electrode39is electrically connected to the second excitation electrode14of the piezoelectric vibrating piece4through the through electrode33and through the routing electrode37. Thus, the external electrodes38and39are contacted with an external substrate, for example, to operably mount the piezoelectric vibrator on that substrate.

When the piezoelectric vibrator1thus configured is operated, a predetermined drive voltage is applied to the external electrodes38and39formed on the base substrate2. Therefore, current can be carried through the excitation electrode15formed of the first excitation electrode13and the second excitation electrode14of the piezoelectric vibrating piece4, and a pair of the oscillating arms10and11can vibrate at a predetermined frequency in the direction of bringing a pair of the oscillating arms10and11close to each other or separating them from each other. Then, with the use of the vibrations of a pair of the oscillating arms10and11, the piezoelectric vibrator1can be used as a time source, a timing source of control signals, or a reference signal source.

Next, the fabrication process steps of the through electrodes32and33will be described.

First, a pair of the through holes30and31is formed on the base substrate2such that the through holes30and31penetrate through the base substrate2in the thickness direction. This step may be performed by sandblasting, for example, from the undersurface side of the base substrate2. When this step is performed in this manner, as shown inFIG. 4, the through hole30and31can be formed in a tapered cross section having the diameter being gradually reduced from the undersurface to the top surface of the base substrate2. The through hole30and31are formed and opened in the recessed part3aformed in the lid substrate3such that the through hole30is positioned at the base portion12side of the piezoelectric vibrating piece4and the through hole31is positioned on the tip end sides of the oscillating arms10and11when the base substrate2is laid on the lid substrate3later.

Subsequently, a pin product9having the portion to be the core7is inserted and disposed in each of the through holes30and31, and a paste glass frit6aformed of a glass material is filled in each of the through holes30and31. For the pin product9used at this time, as shown inFIG. 5, preferably, the pin product9has a base part8in a flat plate shape, and the core7formed on the base part8to have the length shorter than the thickness of the base substrate2by a predetermined value, 0.02 mm, for example, along the direction nearly orthogonal to the flat plate surface of the base part8and having a flat tip end.

Subsequently, as shown inFIG. 6, the core7is inserted until the base part8of the pin product9is contacted with the base substrate2. At this time, it is necessary to dispose the pin product9such that the axial direction of the core7is almost matched with the axial directions of the through holes30and31. However, because the pin product9having the core7formed on the base part8is used, the axial direction of the core7can be almost matched with the axial directions of the through holes30and31by a simple work to only push the pin product9until the base part8is contacted with a base substrate wafer40. Therefore, workability in the setting step can be improved.

In addition to this, the base part8is contacted with the surface of the base substrate2, whereby the paste glass frit6acan be reliably filled in the through holes30and31.

Moreover, because the base part8is formed in a flat plate shape, the base substrate2is stable with no wobbles, even though the base substrate2is placed on the flat surface of a desk, for example, after the pin product9is disposed in the through hole until the glass frit6ais fired, described later. Also from this point, workability can be improved.

In filling the glass frit6ain each of the through holes30and31, a little extra amount of the glass frit6ais coated such that the glass frit6ais reliably filled in each of the through holes30and31. Therefore, the glass frit6ais as well coated on the surface of the base substrate2. An extra amount of the glass frit6ais removed before fired, because the time required for polishing work, described later, is prolonged when the glass frit6ais fired in this state. In this work, as shown inFIG. 7, preferably, a resin squeegee45, for example, is used to contact a tip end45aof the squeegee45with the surface of the base substrate2and moved over the surface, thereby removing the glass frit6a. When this is performed as described above, as shown inFIG. 8, an extra glass frit6acan be reliably removed by a simple work. In the embodiment, because the length of the core7of the pin product9is formed shorter than the thickness of the base substrate2by 0.02 mm, the tip end45aof the squeegee45is not contacted with the tip end of the core7, and the core7is prevented from tilting toward the axis of the through hole when the squeegee45passes over the through holes30and31.

In addition, in the case in which the through hole is formed in the shape in the embodiment, as shown inFIG. 6, preferably, the glass frit6ais easily filled in the through hole when the pin product9is inserted from the top surface side of the base substrate2having a smaller diameter of the through hole.

Subsequently, the buried filler is fired at a predetermined temperature. Thus, the through holes30and31, the glass frit6aburied in the through holes30and31, and the pin product9disposed in the glass frit6aare fixed to each other. In firing, because the base substrate2is fired as the base part8is contacted with the pin product9, the core7and the through holes30and31can be fixed in one piece to each other, while the axial direction of the core7remains to be almost matched with the axial directions of the through holes30and31. The glass frit6ais fired and solidified to be the conical piece6.

After fired, as shown inFIG. 9, the base part8of the pin product9is polished and removed. Thus, the base part8that serves to position the conical piece6to the core7is removed, and only the core7is fixed and disposed inside the conical piece6. Then, at the same time, the top surface of the base substrate2is polished into a flat surface until the tip end of the core7is exposed. Consequently, as shown inFIG. 10, a pair of the through electrodes32and33having the conical piece6fixed in one piece to the core7is formed on the base substrate2.

At this time, such a scheme may be possible that the base substrate is formed slightly thicker than the thickness when completed, and after polished, the base substrate is formed to have a desired thickness, and the surface of the base substrate2is nearly flush with the surfaces of the through electrodes32and33. Furthermore, such a scheme may be possible that a plurality of pairs of the through holes30and31is formed on a base substrate wafer, through electrodes are formed according to the process steps described above, and then the base substrate wafer is divided and cut to form multiple base substrates2having the through electrodes at one time.

The descriptions above are the fabricating method of the through electrode in the embodiment. In the method described above, because all of the base substrate2, the glass frit6a, and the pin product9are thermally expanded in firing the filler, it is necessary to take account of these expansions and the occurrence of associated stress.

The base substrate2, the glass frit6a, and the pin product9(particularly the core7) are expanded according to their thermal expansion coefficients when fired. Generally, it is considered to be preferable that the thermal expansion coefficients of the base substrate2and the glass frit6aare substantially equal. This is because the difference between the thermal expansions causes rare occurrence of stress in the interface between the base substrate2and the glass frit6a.

On the other hand, it is considered to be preferable that the thermal expansion coefficient of the pin product9is smaller than the thermal expansion coefficient of the glass frit6a. This is because such a relation of force occurs in which the glass frit6apresses the core7when fired (see arrows shown inFIG. 8), and the deterioration of hermeticity of the cavity C hardly occurs, which is caused by producing a space between the glass frit6aand the core7when cooled.

However, when the difference between the thermal expansion coefficients of the glass frit6aand the core7is too large, a large tensile stress occurs particularly in the portion around the core7in the glass frit6a, and consequently, the bending strength is sometimes deteriorated. Therefore, it is considered to be preferable that the value of the thermal expansion coefficient of the core7is a value smaller than the value of the thermal expansion coefficient of the glass frit for use with a small difference.

Then, in order to investigate the relation between the configurations of the base substrate2, the glass frit6a, and the core7and the stress occurring in the piezoelectric vibrator1, simulations were studied. Two types of simulations were performed: the simulation that considers cooling components after fired, and the simulation that considers the state in which a bending stress is applied to a substrate mounted with a fabricated piezoelectric vibrator. The details will be described below.

(1) Cooling Simulation After Fired

As a temperature setting corresponding to firing and cooling components after fired in forming the through electrodes32and33, such a setting was made in which components were cooled from a temperature of 365° C. to a temperature of 25° C. The thermal expansion coefficient of the base substrate2was 8.33 ppm of a typical soda-lime glass, and the values of the thermal expansion coefficients of the core and the glass frit were variously set to analyze the stress occurring on the base substrate side and on the glass frit side because of the temperature variation.

(2) Bend Simulation for a Substrate

FIG. 11is a schematic diagram depicting a bending test method of the resistance of a substrate of a standard surface mount component (JIS C5206.1.4 (1)). In this method, as shown inFIG. 11, a surface mount component101was mounted at the center of the longitudinal direction of a substrate100having the size in the longitudinal direction being 90 millimeters (mm), and both ends of the substrate100in the longitudinal direction were supported by supports102in a predetermined size so as to position the surface mount component101below. Then, a predetermined indenter was used to apply a pressure to the center of the substrate100in the longitudinal direction from above to bend the substrate100such that the center of the substrate100in the longitudinal direction was moved 3 mm below.

FIG. 12is a diagram depicting a model that was used in this simulation as a half size model for the test method described above. In this model, a first end part103A of a substrate103in the size in the longitudinal direction being 45 mm was supported by a support102, and the piezoelectric vibrator1as a surface mount component was mounted on the undersurface of a second end part103B. Then, a predetermined indenter was used to apply a pressure from the top surface of the second end part103B, and the substrate103was bent until the second end part103B was moved 3 mm below. At this point in time, stresses occurring in the base substrate2, the glass frit6a, and the lid substrate3were analyzed.

In all of the simulations, for the core7, three types of cores7were used in total: one formed of an alloy (Kovar) of iron, nickel, and cobalt, and two types of ones formed of alloys containing only iron and nickel as metals (42 alloy: 58 percent by weight (wt %) of iron and 42 wt % of nickel; and 50 alloy: 50 wt % of iron and 50 wt % of nickel). The thermal expansion coefficients were 4.81 ppm for Kovar, 6.7 ppm for 42 alloy, and 9.7 ppm for 50 alloy. Because the thermal expansion coefficient of the glass frit6acan be set finely by changing the compositions, the thermal expansion coefficient was set to various values for analysis. The results of these two types of simulations are shown inFIG. 13.

As shown inFIG. 13, under the conditions of using the Kovar core, in all of the cooling simulations after fired and the bending simulations for a substrate, a large stress of 100 megapascals (MPa) or greater occurred in at least one of the base substrate and the glass frit. It was estimated that this was caused by an overlarge difference between the thermal expansion coefficient of Kovar formed of iron, nickel, and cobalt and the thermal expansion coefficient of frit glass.

In contrast to this, under the conditions of using the 42 alloy core and the 50 alloy core containing only iron and cobalt as metals, although the stress occurring was substantially 100 MPa or below, such a tendency was observed that the thermal expansion coefficient of the glass frit was larger than that of the base substrate and the stress occurring became higher as the difference between these coefficients grew. Under the conditions in which the thermal expansion coefficient of the glass frit was the greatest value, 9.7, a stress of 100 MPa or greater occurred on the glass frit in the bending simulation for a substrate. Therefore, it was considered to be preferable that the thermal expansion coefficient of the glass frit was set equal to the thermal expansion coefficient of the base substrate or below.

In addition, apart from the simulations, when the 42 alloy is compared with the 50 alloy, the thermal expansion coefficient of the 42 alloy can be easily set lower than that of the glass frit, and it is considered to be preferable in viewpoint of maintaining the hermeticity of the cavity C.

Therefore, when this as well as the simulation results were considered, it was thought to be the most preferable that the thermal expansion coefficient of the glass frit6awas set equal to the thermal expansion coefficient of the base substrate2or below and an alloy only containing iron and cobalt as metals, 42 alloy, for example, was used to form the pin product9for making the thermal expansion coefficient of the core7smaller than that of the glass frit6a. In this case, the order of the thermal expansion coefficients is as below:
base substrate 2≧glass frit 6a>core 7.

In consideration of the results, in the piezoelectric vibrator1according to the embodiment, the 42 alloy core is used for the core7, and the order of the thermal expansion coefficients is set as below:
base substrate 2≧glass frit 6a>core 7.

According to the piezoelectric vibrator1thus configured, the hermeticity of the cavity C can be preferably maintained, and damages caused by the occurrence of a large stress when a component is surface mounted can be preferably prevented.

As discussed above, the embodiment of the invention has been described. The technical scope of the invention is not limited to the embodiment above, which can be variously modified within the scope of the teachings of the invention.

For example, in the embodiment, although an example is described that the pin product9including the core7is formed of 42 alloy, other alloys containing only iron and cobalt as metals may be used to form the core7, as long as the relation, the base substrate2≧the glass frit6a>the core7, is held in the thermal expansion coefficients. However, because 42 alloy is an alloy that is widely used and available at low costs, the piezoelectric vibrator according to the invention can be readily fabricated at low costs when 42 alloy is used to form the core7.