JPH11251865A - Surface acoustic wave device - Google Patents

Abstract

(57) [Problem] To provide a surface acoustic wave device capable of improving the filter characteristics in a pass band and reducing the size, height, and weight. SOLUTION: A surface acoustic wave element T provided with an excitation electrode (2) for generating a surface acoustic wave on a piezoelectric substrate 1 is placed on an insulating substrate 4 with the surface on which the excitation electrode (2) is disposed below. Then
Ground electrode 5 connected to excitation electrode (2) in insulating substrate 4
Is provided facing the excitation electrode (2),
When the distance D between the excitation electrode (2) and the ground electrode 5 is D> S /
(1.75 × 10 ⁴ × λ). Here, S is the total area of the electrodes of the surface acoustic wave element, and λ is the wavelength of the surface acoustic wave.

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 suitable as a resonator and a frequency band filter incorporated in a mobile radio device such as an automobile telephone and a portable telephone.

[0002]

2. Description of the Related Art Conventional general surface acoustic waves (Surface Ac
FIG. 10 and FIG. 11 are schematic cross-sectional views of a oustic wave (hereinafter abbreviated as SAW) filter. A SAW filter J1 shown in FIG. 10 includes a SAW element including an excitation electrode 32 for generating a surface acoustic wave on a piezoelectric substrate 31, an input / output electrode pad connected to the excitation electrode 32, and the like. It is mounted on a package described later. That is, the SAW element includes the insulating lower substrate 35 and the frame 3
6, and a package consisting of a lid 37 and the like,
The input / output electrode pads 38 on the SAW element side and the input / output pads 33 formed on the package side are connected by wires 34.

Further, a SAW filter J2 shown in FIG.
Is based on the so-called flip chip method,
The piezoelectric substrate 31 on which the excitation electrode 32 is provided is housed in a package with the surface of the piezoelectric substrate 31 facing down. The excitation electrode 32 of the SAW element is connected to the input / output pad 33 on the package side and a connecting body 39 such as a solder bump. They are electrically connected.

[0004]

At present, in order to reduce the size, height and weight of communication devices in the field of mobile communication, electronic components mounted in the devices are required to have higher performance.
As with equipment, further miniaturization, lower profile, and lighter weight are required.

However, for example, an SA of the type shown in FIG.
In the W filter, the overall size, particularly the package height, is determined in consideration of the loop height of the wire tension and this dimensional intersection, so that a reduction in height cannot be expected at all (for example, 0.3 to 0.6 mm only in the height of the cavity). Degree is required).

For this reason, the type using the flip chip method is expected to be able to achieve a low profile.
As shown in FIG. 12 showing the filter characteristics (| S21 |),
At present, satisfactory characteristics have not been obtained within a high-frequency pass band used in communication equipment. That is, as shown in part E in FIG. 12, the distortion of the characteristics of the pass band is large, and the so-called in-band deviation is too large.
It is considered that this has a large influence on the SAW element due to an external electromagnetic wave generated by the flip chip structure and unnecessary stray capacitance.

Accordingly, an object of the present invention is to provide a surface acoustic wave device capable of improving the filter characteristics in a pass band and reducing the size, height, and weight.

[0008]

In order to solve the above-mentioned problems, a surface acoustic wave device according to the present invention comprises a surface acoustic wave element having an excitation electrode for generating a surface acoustic wave on a piezoelectric substrate. The mounting surface is placed on the insulating substrate below, and a ground electrode connected to the excitation electrode is provided in the insulating substrate so as to face the excitation electrode, and a distance between the excitation electrode and the ground electrode is set. D is D> S / (1.75 × 10 ⁴ × λ)
Was satisfied. Here, S is the total area of the electrodes of the surface acoustic wave element, and λ is the wavelength of the surface acoustic wave.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of a surface acoustic wave device according to the present invention will be described in detail with reference to the drawings.

As shown in FIG. 1, the SAW element T of the present invention
Are excitation electrodes for generating surface acoustic waves on a piezoelectric substrate 1 made of lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO ₃ ), quartz, or lithium tetraborate (Li ₂ B ₄ O ₇ ). (For example, a comb-shaped I
DT electrode), a ladder-like reflector, and the like, and an electrode 2 is formed thereon.
A 2 ° Y cut-X propagation LiTaO ₃ single crystal substrate is used, and the IDT electrodes and reflectors are formed to a thickness of about 4300 ° using photolithography technology which is a fine processing technology. The pitch between electrodes is designed to be 2.13 μm on average.
Further, the design of the filter is based on a so-called ladder type SAW in which the above-described IDT electrode and a SAW resonator having reflectors on both sides in the propagation direction are connected to a ladder type circuit.
It is configured to be a filter, but is not limited to this.

As shown in FIGS. 2 to 5, the surface acoustic wave element T has a piezoelectric substrate 1 having an electrode 2 such as an excitation electrode formed in a package made of an insulating substrate such as alumina. And completed as a SAW device F. The package includes a lower substrate 4 which is an insulating substrate, a lid 3 and the like, and a plurality of electrode patterns 8 (for example, for an input terminal, a first output terminal, a second output terminal, and a grounding device) are formed on the lower substrate 4. 4) are formed. Of these electrode patterns, the ground electrode pattern 8a is embedded in the lower substrate 4 as shown in FIGS. 3A, 4 and 5 so as to face the electrode 2 including the excitation electrode of the SAW element. Connected to the grounded rectangular ground electrode 5. In other words, the propagation surface of the SAW element T is accurately aligned with the electrode surface of the ground electrode 5 so as to cut off external radio waves. For example, two ceramic sheets each having a thickness of about 0.2 mm are laminated and fired, and are manufactured by printing Ag-Pd on a lower one of the two layers for manufacturing a ground electrode and then firing. The ground electrode 5 is formed by opening a via hole (portion 9 in the figure) having a diameter of about 100 μm in the sheet stacked thereon so that the ground electrode 5 can be easily connected to the grounding electrode pattern. I do.

The lower substrate 4 and the electrodes 2 such as excitation electrodes provided on the SAW element T are electrically connected to the respective electrode patterns 8 of the lower substrate 4 by bumps 6 made of, for example, Au. Here, the connection between the electrode 2 and the electrode pattern 8 is desirably performed as follows. That is, when the position between the SAW element and the ground electrode 5 is not accurate (for example, about 300 μm
(Even to some degree), the stray capacitance may increase and the in-band deviation may increase. In order to prevent this as much as possible, as shown in FIG. 6, a concave portion 11 is provided on the bump bonding surface of the lower substrate 4 to realize accurate positioning, thereby reducing the variation of the parasitic capacitance. For example, the diameter of the extraction electrode 12 connected to each electrode pattern 8 of the lower substrate 4 is set to about 150 μm, and the Au electrode is formed so as to form a recess having a thickness of 30 μm.
Is designed at a position where the bump 6 just enters. A conductive paste 10 which is a bonding material is applied in advance to a portion to be bonded, and the bump 6 and the lead electrode 12 are securely bonded. The joining material may be solder, an anisotropic conductive film, or the like.

The lid 3 and the lower substrate 4 are connected with an adhesive 7 such as epoxy resin, glass, solder, or the like. In addition,
Seam welding can also be performed without using the adhesive 7. Further, a glass ceramic having a low dielectric constant may be used as a material of the package. Alternatively, a heat-resistant epoxy resin material obtained by disposing a metal so as to form an electrode and injection molding may be used.

The distance D between the surface of the electrode 2 including the excitation electrode and the ground electrode 5 is, for example, about 0.2 mm. This is because D> S determined in consideration of the influence of the ground electrode 5 and the SAW element.
/(1.75×10 ⁴ × λ). Here, S is the total area of the electrode 2, the area including the excitation electrode of the SAW element and all other electrode patterns, and λ is the wavelength of the surface acoustic wave. For example, λ is 4.26 μm, S
(= L in FIG. 1 (0.8 mm) × W in FIG. 1
If (1.4 mm) is 1.12 mm ² , D is about 0.15
It can be seen that it is preferable to set it larger than mm. When the in-band deviation was measured by changing D, as shown in FIG. 9, it was found that the in-band deviation was smaller than 1 dB at 0.15 mm or more, and the relational expression of D was derived from this result. Things. However, D is 0.15 to achieve the low profile.
A range of about 0.5 mm is appropriate.

FIG. 7 shows the filter characteristics (| S21 |) in the SAW device having the above configuration. It can be seen that the in-band deviation of the filter characteristics is smaller than the filter characteristics diagram (FIG. 12) of the conventional flip chip. Thus, according to the surface acoustic wave device of the present invention, a reduction in size and height can be realized without impairing the filter characteristics.

Hereinafter, the effect of embedding a ground electrode in the insulating substrate and making an appropriate space between the ground electrode and an electrode of a SAW element such as an excitation electrode will be considered.

When a SAW device is mounted on an actual mobile communication device, the device is used after being soldered to a circuit board of the device. If a current flows and the ground electrode is not provided, the SAW device may receive a signal due to a high-frequency current flowing through the circuit board. This noise actually impairs the filter function and reduces the amount of attenuation. Therefore, it is necessary to insert a ground electrode between the SAW element and the mounting substrate in order to shield the electromagnetic waves from the mounting substrate.

A SAW mounted on a ceramic lower substrate used for a conventional flip chip to constitute a SAW element
When the electrical characteristics of the resonator (frequency dependence of impedance (| Z |)) were analyzed, it was found that the impedance characteristics indicated by the dotted line in FIG. 8 were obtained. This is because the anti-resonance frequency of the resonator characteristic shifts from the solid line (designed) characteristic to the broken line characteristic due to the stray capacitance caused by the electrode of the SAW element and the ground electrode. found.

Therefore, as described above, the ground electrode and the SA
By separating the electrode of the W element from the electrode by an appropriate distance and providing a ground electrode in the insulating substrate, a complicated electric circuit between the SAW element and the package generated due to high frequency is formed, and floating components, which are unnecessary components, are generated. It is considered that the circuit was able to sufficiently reduce the capacitance.

The present invention is not limited to the above-described embodiment. For example, the ground electrode may be comb-shaped as shown in FIG. Is improved over the rectangular shape of FIG. It is considered that the characteristics were improved by adopting a shape for further reducing the stray capacitance between the SAW element and the ground electrode. For example, by setting the pattern shape of the ground electrode to a comb-like electrode (two or more electrode fingers) having a pitch of 1 mm or more, it is possible to effectively block electromagnetic waves and improve characteristics.

[0021]

According to the surface acoustic wave device of the present invention, the ground electrode for preventing the influence of external electromagnetic waves is provided in the insulating substrate in a state facing the excitation electrode. The distance between the antenna and the antenna has been optimized, so that even at high frequencies used in communication equipment, the adverse effects of unnecessary stray capacitance due to the ground electrode etc. provided on the insulating substrate side are minimized, and the in-band deviation is minimized. A surface acoustic wave device having desired characteristics can be provided.

Further, since the ground electrode is provided in the insulating substrate, it is possible to provide an excellent surface acoustic wave device that can easily be reduced in size, height, and weight. Become.

[Brief description of the drawings]

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

FIG. 2 is an exploded perspective view illustrating an embodiment of a surface acoustic wave device according to the present invention.

FIGS. 3A and 3B are plan views illustrating the planar shape of a ground electrode.

FIG. 4 is a schematic sectional view taken along line A-A ′ in FIG. 2;

FIG. 5 is a schematic sectional view taken along line B-B ′ in FIG. 2;

FIG. 6 is a partial cross-sectional view illustrating an example of connecting a surface acoustic wave element according to the present invention to a lower substrate.

FIG. 7 is a diagram illustrating frequency characteristics of the surface acoustic wave device according to the present invention.

FIG. 8 is a diagram illustrating impedance characteristics of a resonator.

FIG. 9 is a graph illustrating a relationship between a distance between an excitation electrode and a ground electrode and an in-band deviation amount.

FIG. 10 is a schematic sectional view illustrating an example of a conventional surface acoustic wave device.

FIG. 11 is a schematic sectional view illustrating an example of a conventional surface acoustic wave device.

FIG. 12 is a diagram illustrating frequency characteristics of a conventional surface acoustic wave device.

[Explanation of symbols]

 1: piezoelectric substrate 2: electrode (including excitation electrode etc.) 3: lid 4: lower substrate (insulating substrate) 5: ground electrode 6: bump 7: adhesive 8: electrode pattern T: surface acoustic wave element F: elastic Surface wave device

Claims (1)

[Claims] 1. A surface acoustic wave device comprising: a surface acoustic wave element having an excitation electrode for generating a surface acoustic wave on a lower surface of a piezoelectric substrate; and a surface acoustic wave device mounted on an insulating substrate. A ground electrode connected to the excitation electrode is provided so as to face the excitation electrode, and a distance D between the excitation electrode and the ground electrode is set so as to satisfy the following expression. apparatus. D> S / (1.75 × 10 ⁴ × λ) (where, S: total electrode area of the surface acoustic wave element, λ: wavelength of the surface acoustic wave)