JP2000106520A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the surface acoustic wave device where a drift of a resonance frequency is effectively suppressed even after a surface acoustic wave element is mounted in a ceramic package and a metallic cover of the device is sealed. SOLUTION: In the surface acoustic wave device where a surface acoustic wave element 3 is contained in a ceramic package 2 with a cavity 21 formed thereto and an opening of the cavity 21 is sealed air-tightly by a metallic cover 1 by means of seam wielding, and a buffer layer 4 made of a silicone resin whose thickness is 50 μm or over is interposed between the surface acoustic wave element 3 and a bottom face of the cavity 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device, and more particularly to a surface-mountable surface acoustic wave device in which a metal lid is sealed to a ceramic package by seam welding.

[0002]

2. Description of the Related Art As a conventional surface acoustic wave device 60, for example, as shown in FIG. 6, a silicone resin adhesive 65 is instilled on a base substrate 62 made of, for example, metal or ceramic. Place the surface acoustic wave element 63,
The adhesive 65 is heated and cured in a state in which the adhesive 65 is slightly pressed to protrude from the periphery of the surface acoustic wave element 63.

Further, terminal electrodes provided on a base substrate 62 are connected by wire bonding 66. At this time, the silicone resin adhesive 65 is interposed between the base substrate 62 and the surface acoustic wave element 63. When mounting with the normal silicone resin adhesive 65, the thickness of the adhesive is about 10 to 30 μm. there were. In addition, the thickness varies depending on the method of pressing by the assembling machine.

The surface acoustic wave device has been pursued to have a higher frequency and a smaller size. Instead of using a base substrate, a housing-like ceramic package is used. After the surface acoustic wave element is housed in the cavity, the opening is closed by a metal cover. A structure in which the body is seam-welded is employed.

The outer dimensions of the ceramic package are, for example, 2 m.
A small shape of about m × 3 mm or less has been studied, and its rigidity has been significantly reduced.

[0006]

In such a surface acoustic wave device having a sealing structure, a frequency change may occur before and after sealing with a metal lid.

The present inventor investigated the correlation between the thickness of the silicone adhesive layer and the frequency drift in order to investigate the cause.
As a result, the result was as shown in FIG. still,
The measurement is a 443 MHz surface acoustic wave resonator.

From this, it can be seen that there is almost no change when the thickness of the adhesive layer is sufficiently large, whereas the frequency drift is large when the thickness of the adhesive layer is small, and the drift reaches 150 ppm at around 10 μm.

It is presumed from this that frequency drift occurs due to the following reasons. That is, a difference in thermal expansion between the ceramic package and the metal lid occurs at the time of seam welding, and when the ceramic package returns to room temperature, mechanical stress is inherent in the ceramic package and the ceramic package is deformed.

When this mechanical stress or deformation causes a mechanical stress or deformation to the surface acoustic wave element via the thin adhesive layer, and a frequency drift occurs to change the propagation condition of the surface acoustic wave element. Think.

If the thickness of the adhesive layer can be strictly controlled in mounting on a ceramic package, the drift amount can be managed, but it is actually difficult.

For example, a drift of a standard deviation σ = 20 kHz with respect to a resonance frequency occurs in a 443 MHz surface acoustic wave resonance element, and it is necessary to suppress the drift to at least １ ／ or less.

FIG. 5 shows that the drift is small when the thickness of the silicone adhesive is sufficiently increased.

As a method of bonding using an adhesive, a method having low fluidity and high thixotropy is employed. However, it is difficult and time-consuming to supply such an adhesive with a dispenser, and the production efficiency is significantly reduced.

When a surface acoustic wave element is placed on such an adhesive, the adhesive does not spread well, so that a gap is easily generated between the bottom surface of the cavity and the back surface of the surface acoustic wave element. If there is a gap at the position corresponding to the bonding pad of the surface acoustic wave element, sufficient ultrasonic energy cannot be supplied to the bonding pad when bonding the bonding wires by applying ultrasonic waves, and the connection reliability is greatly reduced. Resulting in.

Considering various drift factors in terms of characteristics,
Frequency variation must be minimized. If the frequency variation exceeds the standard, it is discarded as a frequency shift failure. This means a decrease in manufacturing yield. In particular, since the characteristic is defective after sealing, the defect at this stage cannot be reproduced, resulting in a large loss in mass production.

In the conventional surface acoustic wave device, the thickness of the silicone adhesive layer between the base substrate and the surface acoustic wave element is sometimes reduced, and as a result, a large variation occurs in the drift amount of the resonance frequency after sealing. Was.

The present invention has been devised in view of the above-described problems, and has as its object to provide a resonance frequency even after mounting a surface acoustic wave element and sealing a metal cover. It is an object of the present invention to provide a surface acoustic wave device capable of effectively suppressing the drift amount of the surface acoustic wave.

[0019]

According to the present invention, a surface acoustic wave element is housed in a ceramic package having a cavity formed therein, and the opening of the cavity is hermetically sealed with a metal lid by seam welding. A surface acoustic wave device, wherein a buffer layer made of a silicone resin having a thickness of 50 μm or more is interposed between the surface acoustic wave element and the bottom surface of the cavity.

Preferably, the buffer layer is formed in an inner area of a margin of 0.05 mm or more around the back surface of the surface acoustic wave element.

[0021]

The surface acoustic wave device of the present invention has a thickness of 50 μm at the mounting interface between the ceramic package and the surface acoustic wave element.
A buffer layer made of a silicone resin of m or more is interposed.

Therefore, deformation and stress reaching the surface acoustic wave element from the ceramic package can be stably absorbed. Therefore, drift of the resonance frequency of the surface acoustic wave element can be effectively suppressed.

The buffer layer is formed on the surface acoustic wave element in advance. For example, the buffer layer can be formed with a more uniform thickness by printing or the like.

As a result, no gaps or bubbles are generated between the surface acoustic wave element and the ceramic package, and stable bonding is achieved in the connection between the electrode pad of the surface acoustic wave element and the electrode of the ceramic package with the bonding wire. It becomes possible.

Further, since the formation of the buffer layer on the surface acoustic wave element can be collectively printed by printing, a surface acoustic wave device having improved production efficiency can be obtained.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave device according to the present invention will be described below with reference to the drawings.

FIG. 1 is an external perspective view of the present invention, FIG. 2 is an end view thereof, and FIGS. 3 (a) and 3 (b) are a plan view and a side view of a surface acoustic wave device.

In the figure, 1 is a metal lid, 2 is a housing-shaped ceramic package, 3 is a surface acoustic wave device, 4 is a buffer layer, and 5 is an adhesive layer.

The housing-shaped ceramic package 2 is made of alumina or the like, and has a cavity 21 with an open upper surface. A step 22 is formed on the inner wall surface of the cavity 21, and an electrode pad 23 is formed on the step 22. Further, a terminal electrode 24 is formed on the outer surface of the ceramic package 2,
It is electrically connected to the internal electrode pad 23.

A metal seal ring 25 for seam welding the metal lid 1 is provided around the upper surface opening of the ceramic package 2.

The surface acoustic wave element 3 has an electrode 32 for generating, exciting, and reflecting a surface acoustic wave on a piezoelectric substrate 31. Further, on the piezoelectric substrate 31, an electrode pad portion 33 for connecting to an external circuit is formed.

The buffer layer 4 is made of a silicone resin or the like, and is formed on the back surface of the piezoelectric substrate 1. The thickness of the buffer layer 4 is 50 μm or more. The buffer layer 4
The piezoelectric substrate 1 is formed so as to leave a margin S around the rear surface.

The connection layer 5 joins the back surface of the surface acoustic wave element 3 on which the above-described buffer layer 4 is adhered, to the bottom surface of the cavity 21 and can be exemplified by a low-viscosity silicone resin. .

The metal cover 1 is made of Kovar, 42 alloy or the like, and has a cavity 2 of the ceramic package 1.
1 is hermetically sealed. Specifically, the metal lid 1 is placed on the seam ring 25 around the opening of the ceramic package 1, and a predetermined current is applied from the outer surface side of the joint between the seam ring 25 and the metal lid 1 to perform welding. .

In the surface acoustic wave device having the above-described structure, a silicone adhesive is applied to the bottom surface of the cavity 21 of the ceramic package 2 to become the adhesive layer 5 with a dispenser.

Next, the surface acoustic wave element 3 having the buffer layer 4 adhered to the back surface on the silicone resin adhesive layer 5
After heating and curing, the bonding pads 6 are connected between the electrode pads 33 of the surface acoustic wave element 3 and the electrode pads 23 of the ceramic package 2.

Thereafter, the electrical characteristics are evaluated.
An insulating thin film such as iO ₂ is formed by sputtering, or the piezoelectric substrate is etched by plasma etching or the like, so that a desired frequency characteristic is obtained. Thereafter, the metal lid 1 is welded to the ceramic package 2 by seam welding to complete the surface acoustic wave device shown in FIG.

Here, the surface acoustic wave element 3 is a piezoelectric substrate 3 made of a single crystal material such as LT or quartz or a piezoelectric ceramic.
As shown in FIG. 3 (a), an electrode 32 made of aluminum or the like is formed on the surface of a piezoelectric substrate 31 by means of a cross-finger thin film technique for surface acoustic wave excitation.

As shown in FIG. 3B, a buffer layer 4 is formed on the back surface of the piezoelectric substrate 31. Buffer layer 4
Is a resin having elastic properties such as silicone resin (rubber hardness: 20 to 70 degrees). For example, in the case of a resin having a high hardness after curing such as an epoxy resin or a phenol resin, even if the thickness of the resin layer is changed, the influence of mechanical stress and thermal stress at the time of seam welding of the metal lid 1 is obtained. Can not be absorbed.

On the other hand, the thickness of the buffer layer 4 is 50 μm
As described above, the mechanical stress and the thermal stress during the seam welding of the metal lid 1 can be sufficiently absorbed by the buffer layer 4.

For this purpose, a silicone resin is printed on the back surface of the piezoelectric substrate 31 to a thickness of, for example, about 100 μm, and is cured by heating at 150 ° C. for one hour.

Such a piezoelectric substrate 31 is formed using a large-sized piezoelectric substrate 30 from which a plurality of element regions can be extracted.
Specifically, the buffer layer 4 is formed by printing on the back surface area of each piezoelectric substrate 31. Since the buffer layer 4 is formed by printing as described above, the thickness of the buffer layer 4 can be made uniform, and the surface thereof can be easily flattened. In addition, plastic particles, ceramic powder, and glass particles may be mixed as a filler in order to enhance the thixotropy of the silicone adhesive during printing.

Thus, even if the surface acoustic wave element 3 is adhered to the ceramic package 2 via the adhesive layer 5 by flattening the coating surface of the buffer layer 4, the surface acoustic wave element 3
No air gap or air gap is generated at the interface between the surface acoustic wave device and the ceramic package 2, and the wire bonding to the electrode pad portion 33 on the surface acoustic wave element 3 can be performed stably.

According to the characteristic diagram of FIG. 5, if the buffer layer 4 made of silicone resin is 50 μm or more, the surface acoustic wave element 3 is accommodated in the ceramic package 2 and the metal cover 1 is seam-welded. However, frequency fluctuations due to damping can be effectively suppressed. In this example,
At the time of printing, the thickness is set to 100 μm so as not to fall below 50 μm due to variations in the printing process.

The buffer layer 4 is formed around the back surface of the piezoelectric substrate 31 with a margin S of 0.05 mm or more. Here, it is necessary to control the margin portion 34 of 0.05 mm or more so as not to cover at least the formation region of the electrode pad 33. That is, the width of the margin S is 0.05 m.
m or more and a lower limit determined by the electrode design of the surface acoustic wave element 3 (the formation area of the electrode pad 33).

The width of the margin S is 0.05 mm or more.
The surface acoustic wave element 3 is formed in each region of the large-sized piezoelectric substrate 30, and finally dicing cut is performed. At this time, it is for avoiding a dicing cut portion. Thus, by forming the cut lines 7 and 7 in FIG. 4 so as to be removed, the buffer layer 4 made of a silicone resin is not cut when cutting using a dicing saw. For this reason, dicing cut can be performed at high speed, and the cutting blade is not damaged.

In the concrete bonding between the ceramic package 2 and the surface acoustic wave element 3, a low-viscosity silicone resin is applied as the adhesive layer 5, and the surface acoustic wave element 3 is placed on the applied surface and pressed.

As a result, the silicone resin serving as the adhesive layer 5 is applied to the piezoelectric substrate 31 and the ceramic package so as to surround the interface between the buffer layer 4 and the bottom surface of the cavity 21 of the ceramic package 2 and the periphery of the buffer layer 4. It is formed in a meniscus state between the second cavity portion 21 and the bottom surface. In other words, at least the silicone resin layer of the buffer layer 4 is formed between the surface acoustic wave element 3 and the ceramic package 2, and the buffer layer 4 has a sufficient thickness (5
0 μm or more), the stress such as deformation when the ceramic package 2 is seam-welded with the metal lid 1 causes the buffer layer 4
And the resonance frequency does not change.

When the ceramic package 2 is mounted on a printed wiring board or the like, the surface acoustic wave element 3 is not affected even when subjected to stress due to thermal expansion / contraction or mechanical attachment. That is, a stable product that is not affected by the mounting of the printed circuit board can be obtained.

When the buffer layer 4 made of a silicone resin and the adhesive layer 5 made of a silicone resin as shown in FIG. 1 have a total thickness of 50 μm or more, the ceramic package 2 is formed by two silicone resin layers. Thus, the stress affecting the surface acoustic wave element 3 can be absorbed.

Here, the thickness of the buffer layer 4 should be strictly changed according to the rigidity of the ceramic package 2, the size of the surface acoustic wave element 3, or seam welding conditions. However, the optimum thickness of the buffer layer 4 must be selected according to the actual shape of the surface acoustic wave element or the ceramic package. However, when the thickness of the buffer layer 4 is approximately 50 μm
By setting m or more, it is possible to effectively prevent frequency fluctuations due to stress generated in the ceramic package 2 and internal stress.

In the above-described embodiment, the surface acoustic wave element 3 having the buffer layer 4 formed on the back surface is elastically attached to the bottom surface of the cavity 21 of the ceramic package 2 with a low-viscosity silicone resin adhesive. Although the surface acoustic wave element 3 is bonded, since the buffer layer 4 is formed on the back surface, it is not necessary to use a silicone resin as a material for the adhesive.
An instant adhesive that adheres to the buffer layer 4 may be used.

The surface acoustic wave element 3 is housed in a housing-shaped ceramic package 1. The surface acoustic wave element 3 having a buffer layer 4 formed on the back surface is mounted on a flat ceramic package (base plate). It may be adhered and covered with a housing-shaped lid.

Further, the above-mentioned buffer layer 4 is used as the surface acoustic wave element 3.
Although the silicone resin is formed by printing on the back surface, a silicone resin sheet having a thickness of 50 μm or more may be used for the buffer layer 4, for example. At this time, a silicone resin sheet may be attached to the ceramic package 2 side, and the surface acoustic wave element 3 may be attached and mounted on the silicone resin sheet via a silicone resin adhesive. May be attached in the state of a large piezoelectric substrate.

[0055]

According to the present invention, a buffer layer of 50 μm or more is formed at the bonding interface between the surface acoustic wave element and the ceramic package. For this reason, the mechanical stress of the ceramic package generated when the metal lid is sealed in the ceramic package or mounted on the printed wiring board is absorbed by the buffer layer made of the silicone resin, and the vibration characteristics of the surface acoustic wave element change. And a highly reliable product can be obtained.

Further, since the buffer layer can be formed on the surface acoustic wave element in advance, the thickness of the buffer layer can be made uniform and flat, thereby enabling more stable wire bonding. Further, since the buffer layer is not formed around the piezoelectric substrate, formation of the surface acoustic wave element, particularly, cutting, can be performed very easily.

[Brief description of the drawings]

FIG. 1 is an external perspective view of a surface acoustic wave device according to the present invention.

FIG. 2 is a sectional view of a surface acoustic wave device according to the present invention.

3A and 3B are diagrams illustrating a surface acoustic wave device according to an embodiment of the present invention, wherein FIG. 3A is a plan view and FIG. 3B is a side view.

FIG. 4 is a rear perspective view of a large-sized piezoelectric substrate in a manufacturing process of the present invention.

FIG. 5 is a characteristic diagram showing a relationship between a buffer layer (thickness of silicone resin) and a change in frequency characteristics.

FIG. 6 is a sectional view of a conventional surface acoustic wave device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Metal lid 2 ... Ceramic package 3 ... Surface acoustic wave element 4 ... Buffer layer 5 ... Adhesive layer 6 ... Bonding wire 7 ... Cut line

Claims (2)

[Claims] 1. A surface acoustic wave device in which a surface acoustic wave element is accommodated in a ceramic package having a cavity formed therein, and an opening of the cavity is hermetically sealed with a metal lid by seam welding. Between the surface acoustic wave element and the bottom surface of the cavity portion,
A surface acoustic wave device characterized by being attached with a buffer layer made of silicone resin having a thickness of 50 μm or more interposed therebetween. 2. The surface acoustic wave device according to claim 1, wherein the buffer layer is formed in an inner region of a margin of 0.05 mm or more around the back surface of the surface acoustic wave element.