JP2000232332A - Surface mounted piezoelectric resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate defects such as generation of internal stress due to thermal distortion at a crystal blank plate constituting a crystal vibrating element by excessive heating, the fluctuation of a resonance frequency by the oxidation of an Ag layer constituting a pad electrode and the generation of stress inside of a crystal blank plate by the generation of the camber of a package at the time of thermo-compression bonding and fixing Au bumps formed on Ag pad electrodes provided at a crystal vibrating element to Au internal terminals within a ceramic package. SOLUTION: In this piezoelectric resonator which electrically and mechanically connects a piezoelectric vibrating element 1 to the inside of a surface mounted package 2 in a cantilevered state and connects Ag pad electrodes 16, 17 on one surface of the element 1 and Au metallized internal terminals 24, 25 on a bottom surface in the surface mounted package by the Au bumps 40, connection of the Au motallized internal terminals and the Au bumps is executed by a thermo-compression bonding method by low-temperature heating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface mount type piezoelectric resonator used as a vibrator, a filter, and the like. In particular, the present invention has a problem when a gold bump is used as a means for connecting a piezoelectric vibrating element in a package. The present invention relates to a surface-mount type piezoelectric resonator which has solved various problems.

[0002]

2. Description of the Related Art In recent years, mobile communication devices such as mobile phones have
While miniaturization and weight reduction are progressing, demands for higher functionality are increasing. In order to meet the demand for the increase in the number of components and the miniaturization due to the sophistication, it is important to reduce the area of the printed circuit board that composes the electrical components and to effectively use the board area by increasing the density of mounted components. It has become so. Quartz resonators (vibrators and filters), which are frequency control devices used in mobile communication equipment and transmission communication equipment, are no exception, and surface-mount types are the mainstream for high-density mounting. Demands are growing. FIG. 6 is a perspective view of an AT-cut crystal vibrating element having an ultra-thin portion for the purpose of increasing the frequency. This quartz-crystal vibrating element 1 is a vibrator utilizing a fundamental wave thickness shear vibration wave of an AT-cut crystal element plate. Since the resonance frequency is inversely proportional to the plate thickness, in order to increase the frequency while maintaining the mechanical strength, one main surface of the quartz crystal plate constituting the crystal unit 1 is recessed by etching. The bottom surface of the concave portion 13 is an ultra-thin vibrating portion 13a, and a thick annular surrounding portion 14 that supports the entire outer periphery of the vibrating portion 13a is integrated. Further, a pad electrode 17 is formed by gold mask evaporation or photolithography on one main surface of the quartz crystal plate in addition to the main surface electrode 11 and the lead electrode 15 and the pad electrode 16 extending therefrom. . The pad electrode 17
May be electrically connected to the lead electrode 18 extending from the back electrode 12 similarly formed on the other main surface via the side surface of the quartz crystal plate.

FIG. 7 is a sectional view showing the structure of a surface-mount type crystal unit using the crystal unit 1 described above. After the crystal resonator element 1 shown in FIG. 6 is housed in the ceramic package 2, the upper opening of the ceramic package 2 is hermetically sealed with a metal top cover. In the structure of this type of crystal resonator, it is important to select a method for electrically and mechanically connecting the crystal resonator to the ceramic package in order to stabilize the frequency. Conventionally, the internal terminals 24 and 25 on the inner bottom surface of the ceramic package 2 and the pad electrodes 16 and 17 on the quartz-crystal vibrating element side were conventionally connected using a conductive adhesive. But,
Since the conductive adhesive needs to be cured by heating, distortion occurs due to the shrinkage of the adhesive during the heating and curing, and the stress caused by this distortion is applied to the quartz crystal vibrating element. Stress accumulates in the quartz resonator, causing its frequency to fluctuate and its stabilization to be impaired. Also,
Stress is applied to the crystal vibrating element due to the distortion caused by the difference in the coefficient of thermal expansion between the crystal element plate constituting the crystal vibrating element and the ceramic package.

In addition, outgas components released from the adhesive in the hermetically sealed space in the package adhere to the surface of the quartz vibrating element, particularly to the electrodes constituting the vibrating section, and the thickness of the vibrating section becomes thicker. It fluctuates and impairs its stabilization (mass loading effect). Variations in the resonance frequency of the crystal unit due to the above factors are caused by various characteristics, such as deterioration and reliability of the frequency temperature characteristics, such as aging characteristics and reflow characteristics (heating caused by reflow mounting the crystal unit on a printed circuit board. The result is that the residual stress in the quartz vibrating element is released to cause frequency fluctuation, etc.). The frequency of the AT-cut quartz-crystal vibrating element becomes higher as the thickness of the quartz plate becomes thinner. However, the influence of the above-mentioned stress and gas adhesion becomes more serious in a high-frequency quartz vibrating element. Therefore, in the case of the crystal vibrating element having the ultra-thin vibrating portion of the type shown in FIG. 6, the problem caused by using the conductive adhesive is more serious. In order to solve the above problems caused by the use of conductive adhesive, the flip chip bonding method, which has been widely used recently as a surface mounting method for semiconductor chips, is also applied to the surface mounting of piezoelectric vibrators such as quartz vibrators. Is being done. FIG. 7 shows a conventional method of connecting a crystal resonator element to the bottom surface of a ceramic package by bumps by applying a flip-chip method. The ceramic package 2 includes a ceramic substrate 21
An annular frame 22 made of ceramic integrated on the outer periphery of the upper surface of the ceramic substrate 21; and a seam ring 23 fixed annularly on the frame 22 for seam welding the upper lid 3.
And has a concave portion for accommodating the crystal resonator element 1 in the center as a whole, and has a box shape having an annular portion on the outer periphery. Internal terminals 24 and 25 formed by gold metallization are exposed on the upper surface of the ceramic substrate 21, and are respectively connected to external terminals (not shown) provided on the package bottom surface and the like.

[0005] The operation of connecting the crystal vibrating element 1 in the package 2 is performed by connecting the pad electrodes 16 and 17 of the crystal vibrating element 1 to the package.
Internal terminals 24, 2 for input / output of the ceramic package 2
5 are connected by thermocompression bonding using the bumps 4 respectively. By the way, the excitation electrodes 11 and 12, the lead electrodes 15 and 18, and the pad electrodes 16 and 17 on the quartz crystal vibrating element are usually formed using an Au vapor-deposited film, and have good conductivity by utilizing the characteristics of Au. When,
The aging property that it is hard to corrode is secured. Au is formed on the pad electrodes 16 and 17 made of such an Au deposited film.
Various problems occur when the bumps 4 are formed. That is, the pad electrodes 16 and 17 as the Au film must be designed to be thin in accordance with the characteristics such as the resonance frequency and the energy confinement of the crystal vibrating element, and the film thickness thereof depends on the characteristics of the crystal vibrating element. It is regulated to the required thickness,
Even if bumps are formed on such a thin Au film, A
The u film is melted by the load and the heat and cannot support the bump completely, so that the bump easily falls off. In other words, Au wire that has been made spherical by heating and melting the tip of the Au wire with a torch is used.
When the thermocompression bonding is performed by applying ultrasonic waves while being in contact with the pad electrode, the Au film is melted by the load at the time of fixing and the bumps are likely to fall off. In order to solve such a problem, even if an attempt is made to increase the thickness of the Au film, there is a limit in increasing the thickness of the Au film in accordance with the characteristics of the crystal unit. Therefore, as shown in FIG. 8, pad electrodes 16 and 17 having a laminated structure having silver Ag as the uppermost layer have come to be used instead of the Au film. These pad electrodes 16 and 17
Has a structure in which a Ni thin film, an Au thin film, a Ni thin film, and an Ag thin film are sequentially laminated and integrated on a quartz crystal plate. According to the pad electrode having the laminated structure, the thin film is determined in advance according to the characteristics of the crystal resonator element. While maintaining the required thickness of the pad electrode at the required thickness, it is possible to prevent melting due to the load and heat at the time of joining the bumps and to prevent the bumps from falling off due to the melting.

Also, the bonding between the Ag layer on the pad electrode surface layer and the Au bump is based on solid-phase diffusion between different metals, and Ag has a high metal diffusion rate, so that the bonding force with the Au bump is strong. Yes, load resistance and heat resistance also increase.
When the pad electrodes 16 and 17 are formed of a laminated electrode film having an Ag layer as a surface layer, the excitation electrodes 11 and 12 and the lead electrodes 15 and 18 which are collectively formed are similarly formed in a laminated electrode film. Becomes On the other hand, Au film or A
Thermo-compression bonding is known as a method of mounting the crystal resonator element 1 in which the bumps 4 are formed on the pad electrodes 16 and 17 made of a laminated thin film having a g layer in the ceramic package 2. There is a problem that thermal distortion occurs in the quartz resonator due to heating at the time, and the characteristics are deteriorated.
Therefore, the present applicant has proposed a pulse heat type thermocompression bonding method as shown in FIGS. 8 (a) and 8 (b), and has previously filed a patent application. In this thermocompression bonding method, Au is applied to the pad electrodes 16 and 17.
As shown in FIG. 8A, the quartz vibrating element 1 to which the bumps 4 are fixed is sucked and held by a tool 30 such as a vacuum collet preheated to about 250 to 350.degree.
The tool 30 is lowered so that the Au bumps 4 are seated on the internal terminals (Au plating pads) 24 and 25 on the inner bottom surface of the ceramic package 2 preheated to about 250 to 350 ° C. Press with a load. At this time, pulse heat is supplied from both the tool 30 and the hot plate 31 to the crystal vibrating element 1 and the ceramic package 2 so that the internal terminals 24 and 25 and the Au bump 4
And thermocompression bonding instantly.

According to the thermocompression bonding method of the pulse heating type, the heating time can be greatly shortened as compared with the conventional thermocompression bonding method, and the deterioration of the characteristics can be prevented accordingly. . However, although momentary, the quartz resonator is heated to a high temperature, so that residual stress in the base plate is generated due to thermal strain, which adversely affects the resonance frequency. When an Ag layer is used for the surface layer of the pad electrodes 16 and 17 on the quartz oscillator, the Au bump 4 is formed on the Ag layer having a high diffusion speed.
Bonding can be promoted by thermocompression bonding, and it becomes possible to perform thermocompression bonding with a low load, but the heating deteriorates (oxidizes) the Ag layer and increases the weight of the electrode film. This causes a problem that the resonance frequency fluctuates and the package is warped to generate stress inside the quartz crystal plate.

[0008]

The problem to be solved by the present invention is that an Au bump formed on an Ag pad electrode provided on a crystal vibrating element is replaced with an Au bump in a ceramic package.
When fixing by thermocompression bonding to the internal terminals, internal stress due to thermal strain is generated in the quartz crystal plate constituting the crystal vibrating element due to excessive heating, and the resonance frequency is caused by oxidation of the Ag layer constituting the pad electrode. SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface-mount type piezoelectric resonator that can solve problems such as fluctuations of the package and warpage of the package to generate stress inside the quartz crystal plate.

[0009]

According to a first aspect of the present invention, there is provided a piezoelectric resonator for electrically and mechanically connecting a piezoelectric vibrating element to a surface mount type package by cantilever holding. The connection between the Ag pad electrode on one surface of the piezoelectric vibrating element and the Au metallized internal terminal on the inner bottom surface of the surface mount type package is made by an Au bump, and the connection between the Au metallized internal terminal and the Au bump is made. The connection is performed by a thermocompression bonding method using low-temperature heating in combination with ultrasonic waves. The invention of claim 2 is
As the piezoelectric element constituting the piezoelectric element, a piezoelectric element integrally formed with an ultra-thin vibrating part and a thick annular surrounding part supporting the outer periphery of the vibrating part is used. And The invention according to claim 3 is characterized in that a piezoelectric element that operates at a resonance frequency of 300 MHz or more is used as the piezoelectric element constituting the piezoelectric vibration element.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. FIG. 1 is a perspective view of an AT-cut quartz-crystal vibrating element having an ultra-thin portion for the purpose of increasing the frequency, and this quartz-crystal vibrating element 1 is a vibrator utilizing a fundamental thickness-shear vibration wave of an AT-cut quartz crystal plate. Since the resonance frequency is inversely proportional to the plate thickness, in order to increase the frequency while maintaining the mechanical strength, one main surface of the quartz crystal plate constituting the quartz vibrating element 1 is recessed by etching. The bottom surface of the concave portion 13 is an ultra-thin vibrating portion 13a, and a thick annular surrounding portion 14 that supports the entire outer periphery of the vibrating portion 13a is integrated. Furthermore, a main surface electrode (excitation electrode) is formed on one main surface of the quartz crystal plate by mask deposition of a metal having good conductivity or photolithography.
11 and a lead electrode 15 and a pad electrode (Ag pad electrode) 16 extending therefrom, and a pad electrode (Ag
A pad electrode 17 is formed. The pad electrode 17
Is electrically connected to a lead electrode 18 extending from a back electrode (excitation electrode) 12 similarly formed on the other main surface via a side surface of the quartz crystal plate. Each of the main surface electrode 11, the back surface electrode 12, the lead electrode 15, the pad electrodes (Ag pad electrodes) 16, 17 and the lead electrode 18 has a structure in which dissimilar metals are laminated by vapor deposition. A silver Ag film is located. As a configuration of the electrode film having the laminated structure, for example, the Ni film shown in FIG.
What laminated | stacked Au film-Ni film-Ag film in order can be used.

FIG. 2 is a sectional view showing the structure of a surface-mount type crystal unit using the crystal unit 1 described above. A structure is provided in which the quartz resonator element 1 shown in FIG. 1 is housed in a ceramic package 2 and the upper opening of the ceramic package 2 is hermetically sealed with a metal top cover. The ceramic package 2 includes a ceramic substrate 21 and a ceramic substrate 21.
Ceramic annular frame 22 integrated on the outer periphery of the upper surface of
And a seam ring 23 fixed in an annular shape on a frame 22 for seam welding the upper lid 3. As a whole, a concave portion for accommodating the crystal resonator element 1 is provided at the center, and an annular It has a box shape having a portion. Au internal terminals 24 and 25 formed by Au metallization such as plating are exposed on the upper surface of the ceramic substrate 21 and are respectively connected to external terminals (not shown) provided on the package bottom surface or the like. The operation of connecting the crystal vibrating element 1 into the package 2 includes the pad electrodes 16 and 17 of the crystal vibrating element 1 and
Au internal terminal 2 for input / output of ceramic package 2
4 and 25 are bonded by using a required number of Au bumps 40 at a relatively low temperature and a low pressure.

This will be described with reference to the thermocompression bonding method using ultrasonic waves according to the present invention shown in FIGS. 3A and 3B. In this thermocompression bonding method, the pad electrode 1 whose surface layer is made of an Ag layer is used.
As shown in FIG. 3 (a), the quartz vibrating element 1 having three Au bumps 40 fixed to 6 and 17 is attracted and held by a tool 45 such as a vacuum collet which is not preliminarily heated as shown in FIG. The ceramic package 2 is constantly preheated on a hot plate 46 set at a low temperature of about 200 ° C. With the temperature of the ceramic package raised to a required temperature, the tool 45 is moved above the ceramic package 2 and further lowered, so that the internal terminals (Au plating pads) on the inner bottom surface of the package are moved.
The Au bumps 40 are seated on the 24 and 25, and the ultrasonic wave is applied for a required time while the tool 45 presses the quartz resonator element 1 with a required load. Specifically, for example, the quartz resonator element 1 is
While applying a pressure of about 0 g / bump, ultrasonic waves having a frequency of about 60 kHz
Apply for about a second. At this time, the pressing direction of the tool 45 is set in advance so that a load is applied only between the bump 40 and the pad electrodes 24 and 25 while the crystal resonator element 1 is maintained in a horizontal posture. By bonding the crystal vibrating element 1 using the Au bumps 40 on the Au metallized internal terminals 24 and 25 using such a thermocompression bonding method combined with ultrasonic waves, the heating temperature at the time of bonding can be greatly reduced. No residual stress occurs in the base plate. Further, since the quartz vibrating element is not particularly heated, the possibility that residual stress is generated in the quartz crystal plate is further reduced. Further, for the same reason, the problem that the Ag film on the quartz crystal vibrating element is altered (oxidized) and the mass of the electrode film is increased, thereby causing fluctuation in the resonance frequency is eliminated. Also, package 2 is placed at 150 to 200 ° C.
Since heating is performed only at a low temperature, the package does not warp, and no internal stress is generated in the crystal resonator element supported in the package. In addition,
Since the connection between the Au bumps 40 and the Au plating pads 24 and 25 is a connection by the same kind of metal diffusion, the bonding strength is slightly lower than the connection between Au and Ag, but the thickness of the Au plating pads 24 and 25 is , Is sufficiently thicker (1 μm) than the film formed by Au vapor deposition, and can withstand thermocompression bonding, so that a sufficient value can be secured as the bonding strength.

FIG. 4 is a diagram showing an improvement effect when the embodiment of the present invention is used, wherein the resonance frequency is 155 MHz.
Fig. 5 shows the measured characteristics of accelerated aging of a quartz resonator manufactured to secure the fundamental vibration of the above. In the case where the quartz vibrating element is formed by bonding the Au bump fixed to the quartz vibrating element to the Au metallized internal terminal by the thermocompression bonding method using ultrasonic waves, the frequency change is ± 1.
ppm or less, and showed about five times the frequency stability as compared with the case of the conductive adhesive. FIG. 5 is a diagram showing the improvement effect when the embodiment of the present invention is used, and shows the reflow measurement characteristics of a quartz resonator manufactured to secure a fundamental wave vibration having a resonance frequency of 155 MHz. . Note that the measured reflow characteristic indicates a characteristic in which, when this crystal resonator is mounted on a printed circuit board by a reflow method, residual stress in the crystal resonator is released by heat applied at the time of reflow, and the frequency fluctuates. According to the present invention, the frequency change was ± 3 ppm, which was found to have about three times the frequency stability as compared with the case using the conductive adhesive. Note that, in the above embodiment, a quartz vibrating element is shown as the piezoelectric vibrating element used for the piezoelectric resonator. However, this is merely an example, and the present invention is applicable to a piezoelectric resonator using all kinds of piezoelectric vibrating elements. Can be applied. Also, as the piezoelectric vibration element to be used, a configuration was shown in which the outer periphery of the ultra-thin vibrating portion was surrounded and integrated by a thick annular surrounding portion, but this is also an example, and the present invention is applicable to any type of piezoelectric plate. The present invention can be applied to a piezoelectric vibrating element using. In particular, as a piezoelectric element constituting a piezoelectric vibration element, 300M
The present invention is effective when a piezoelectric element that operates at a resonance frequency of not less than Hz is used.

[0014]

As described above, according to the present invention, when an Au bump formed on an Ag pad electrode provided on a crystal resonator element is thermocompression-bonded and fixed to an Au internal terminal in a ceramic package, an excessive amount of metal is used. Heating causes internal stress due to thermal strain on the quartz crystal plate constituting the crystal vibrating element, oxidizes the Ag layer constituting the pad electrode, fluctuates the resonance frequency, and causes the package to warp, causing the crystal element to warp. Problems such as generation of stress inside the plate can be solved at once.

[Brief description of the drawings]

FIG. 1 is a perspective view of an AT-cut quartz-crystal vibrating element as one embodiment of the present invention.

FIG. 2 is a sectional view showing a package structure of a piezoelectric resonator as one embodiment of the present invention.

3 (a) and 3 (b) are views for explaining a thermocompression bonding method using ultrasonic waves according to the present invention.

FIG. 4 is a diagram showing an improvement effect when the embodiment of the present invention is used.

FIG. 5 is a diagram showing an improvement effect when the embodiment of the present invention is used.

FIG. 6 is a perspective view showing an example of a conventional piezoelectric vibration element.

FIG. 7 is a cross-sectional view showing a package structure of a conventional piezoelectric resonator.

8 (a) and (b) are views showing a conventional pulse heating type thermocompression bonding method (unknown).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Quartz crystal vibrating element, 2 ceramic package, 3 top lid, 4 conductive adhesives, 11 main surface electrode, 12 back surface electrode, 13 concave portion, 13a vibrating portion, 14 annular surrounding portion, 15 lead electrode, 16, 17 pad electrode ( Ag
Pad electrode), 18 lead electrode, 21 ceramic substrate, 22 annular frame, 23 seam ring, 24, 25
Au internal terminals, 40 Au bumps, 45 tools.

Claims (3)

[Claims] 1. A piezoelectric resonator for electrically and mechanically connecting a piezoelectric vibrating element to a surface-mounted package in a cantilevered manner, comprising an Ag pad electrode on one side of the piezoelectric vibrating element and the surface-mounted package. The connection between the Au metallized internal terminal on the bottom surface of the package and the Au metallized internal terminal is made by an Au bump.
A surface-mount type piezoelectric resonator which is implemented by a thermocompression bonding method using low-temperature heating in combination with ultrasonic waves. 2. A piezoelectric element plate comprising an ultra-thin vibrating part and a thick annular surrounding part supporting an outer periphery of the vibrating part as a piezoelectric element plate constituting the piezoelectric vibrating element. The surface mount type piezoelectric resonator according to claim 1, wherein the piezoelectric resonator is used. 3. The surface mount type piezoelectric resonator according to claim 1, wherein a piezoelectric element that operates at a resonance frequency of 300 MHz or more is used as the piezoelectric element constituting the piezoelectric vibration element. .