JPH10303681A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase suppression effect on discharge by forming a surface acoustic wave SAW resonator, a wiring pattern, and a conductive film for a lead-out pad electrode on a piezoelectric substrate to larger than specific area, and providing a protection film on the resonator. SOLUTION: The area of inter-digital IDT electrodes 2a and 2b, a reflector 2c, a pad electrode 4, and a conductive film for a wiring pattern 5 on one main surface of a piezoelectric substrate 1 is set to >=225% of the area of the main area of the substrate 1. Consequently, the area of the conductive film which is fast in the speed of acquisition of electric charges suspended in air becomes large and electric charges on the conductive film and substrate 1 are neutralized fast. Further, the protection film is lower in the generation rate of discharge when it is provided only on the SAW resonator 2 than when the protection film is provided over the entire the main surface of the substrate 1 or not provided at all. The protection film 6 is easy to short between electrode fingers when the resistance along the film is <=10<9> Ω and decreases in the electric charge neutralizing operation and a semiconductive resistance of approximately 10<9> to 10<13> Ω is used.

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 built in a mobile communication device such as a portable telephone.

[0002]

2. Description of the Related Art Conventional surface acoustic waves (Surface Acoustic W)
ave, the following abbreviated as SAW) shows the kind of an example of a SAW filter F ₁ of the apparatus in FIG. The figure is a plan view of a basic configuration of the SAW filter F _1. A pair of comb-shaped electrodes (Inter Digitail T) is provided on the main surface of the piezoelectric substrate 1 such as LiTaO _3.
One or a plurality of SAW resonators 2a and 2b and reflectors 2c and 2c are formed, and one or a plurality of SAW resonators 2 are formed, for example, as shown in FIG. Construct a SAW filter. Each SAW resonator 2 is connected by a wiring pattern 5, and a pad electrode (hereinafter, referred to as a pad electrode) 4 for external derivation to be connected to a wire for input / output and a ground is provided at a predetermined portion of the wiring pattern 5. Is provided.

Further, during heat treatment such as packaging and wire bonding, the pyroelectric effect of the piezoelectric substrate 1 causes
Discharge occurs between the electrode fingers of the DT electrodes 2a and 2b and the reflectors 2c and 2c, and the purpose of preventing the electrode fingers from being damaged is to prevent conductive foreign matter from entering between the electrode fingers and shorting between the electrode fingers. For the purpose of prevention, and also for the purpose of preventing the electrode fingers from being corroded by the influence of humidity, an insulating protective film 6 of SiO ₂ or the like is provided on the entire surface other than the pad electrode 4.

[0004]

However, as described above, since the piezoelectric substrate 1 of LiTaO ₃ or the like has pyroelectricity, the electrode fingers are damaged during heat treatment at about 80 ° C.
There is a problem that the SAW resonator does not function.
In order to solve this problem, the present applicant has set the area of the conductive film for the SAW resonator 2, the wiring pattern 5, and the pad electrode 4 to be 25% or more of the SAW filter forming surface of the piezoelectric substrate 1,
It has been proposed that the discharge between the electrode fingers can be reduced (Japanese Patent Application No. Hei 8
-12857). This is because the piezoelectric substrate 1
This is because the speed at which the charges generated on the surface combine with and neutralize the floating charges in the atmosphere is faster in the conductive film made of Al or the like than in the piezoelectric substrate 1. Therefore, the discharge between the electrode fingers can be reduced by increasing the areas of the electrode fingers, the pad electrodes 4 and the wiring patterns 5 as much as possible.

[0005] On the other hand, in the SAW filter manufacturing process, 0. There is a problem that conductive foreign substances of several μm and several μm order intrude between the electrode fingers and short-circuit between the electrode fingers, and furthermore, there is a problem that the electrode fingers having a fine structure are easily corroded when the humidity is high. These problems have been dealt with by providing an insulating protective film such as SiO _{2 on the} surface of the piezoelectric substrate 1.

However, since the rate of neutralization of the charge on the protective film such as SiO ₂ is slower than the rate of neutralization on the electrode finger, covering the electrode finger with the protective film does not reduce the generation of discharge. It was the opposite effect. Therefore, if the area of the electrode finger, the pad electrode 4 and the wiring pattern 5 is increased in order to suppress the discharge, the contradiction that the discharge easily occurs when the protective film for protecting the electrode finger is provided. Had occurred.

Accordingly, the present invention has been completed in view of the above circumstances, and an object of the present invention is to prevent or suppress short-circuit between electrode fingers due to a pyroelectric effect of a piezoelectric substrate, thereby suppressing the electrode fingers. An object of the present invention is to make it possible to form a protective film to be protected while suppressing discharge.

[0008]

According to a first aspect of the present invention, there is provided a surface acoustic wave device comprising at least one surface acoustic wave resonator having a pair of comb-like electrodes, a wiring pattern, and a conductive electrode for a pad electrode electrode for external derivation. The film has an area of 25 on one main surface of the piezoelectric substrate.
% Or more, and a protective film is provided on the surface acoustic wave resonator. By not providing a protective film on a wiring pattern having a large charge neutralizing effect or on an external connection pad electrode, discharge is prevented. The suppression effect increases. It is preferable to make the protective film semiconductive because the rate of occurrence of discharge is lower than that of the insulating film.

In the surface acoustic wave device according to a second aspect of the present invention, at least one surface acoustic wave resonator having a pair of comb-like electrodes, a wiring pattern and a conductive film for a pad electrode electrode for external derivation are formed by a piezoelectric device. A semiconductive protective film is formed on at least 25% of the area of one main surface of the substrate, and a semiconductive protective film is provided on the entire main surface of the one main surface of the piezoelectric substrate except for a pad electrode electrode for external lead-out. With the use of the protective film having a property, the rate of occurrence of discharge can be significantly reduced as compared with the case of using an insulating property.

[0010]

1 and 2 show a SAW filter F according to the present invention. FIG. 1 and FIG. 2 are plan views showing two basic embodiments of the SAW filter F. FIG.
1 and 2, reference numeral 1 denotes a piezoelectric substrate such as LiTaO ₃ ,
2 is a SAW resonator comprising a pair of IDT electrodes 2a, 2b and reflectors 2c, 2c provided at both ends of the SAW propagation path, 4 is a pad electrode to be connected to input / output and ground wires, 5 is a wiring The patterns 6, 6a are protective films. 1 and 2, the same parts as those in FIG. 3 are denoted by the same reference numerals.

In the first invention, as shown in FIG.
The area of the IDT electrodes 2a, 2b, the electrode fingers of the reflectors 2c, 2c, the pad electrode 4, and the conductive film for the wiring pattern 5 on one main surface of the piezoelectric substrate 1 is determined by the area of the main surface of the piezoelectric substrate 1. By setting the content to 25% or more, the area of the conductive film having a high speed of capturing the charge floating in the air becomes large, and the charge on the conductive film and the piezoelectric substrate 1 is quickly neutralized. Further, when the protective film 6 is provided only on the SAW resonator 2, the discharge generation rate is lower than when the protective film 6 is provided on the entire main surface of the piezoelectric substrate 1 or not provided at all.

The protective film 6 is made of SiO ₂ , Si
_{O, Cr-SiO 2, Cr} -SiO, those insulating SiN or the like, the resistance of preferably film direction (the direction parallel to the film) 1
Intended 0 ⁹ to 10 ¹³ Omega about semiconductive (Hanshirubeden property),
Si, Ta, Mo, Ge, NiCr, NiCr-Si,
Oxide or nitride of Ta, oxide or nitride of Mo, G
An oxide or nitride of e is preferable. 10 between the electrode fingers is likely to short-circuit is less than ⁹ Omega, in 10 ¹³ Omega than decreases the neutralizing effect of the charge. The semiconductive protective film can reduce the rate of occurrence of discharge to about 30% to about 50% as compared with an insulating film. This is because the semiconductive protective film reduces the difference in charge density between the electrode fingers. The protective film 6 is formed by a known thin film forming method such as a vapor deposition method, a sputtering method, and a CVD method.

In the second invention, as shown in FIG.
Electrode fingers of IDT electrodes 2a, 2b and reflectors 2c, 2c;
The area of the conductive film for the pad electrode 4 and the wiring pattern 5 is
The semiconductive protective film 6a is formed to be 25% or more of the area of one main surface of the piezoelectric substrate 1 and to cover the entire main surface of the piezoelectric substrate 1 excluding the pad electrodes 4, that is, almost the entire main surface of the piezoelectric substrate 1. Is provided. As described above, when the protective film 6a is provided on the entire main surface of the piezoelectric substrate 1 except for the pad electrode 4, the protective film 6a is made of a semiconductive material. In this case, the protective film 6a is semiconductive (semiconductive) as in FIG.
i, Ta, Mo, Ge, NiCr, NiCr-Si, T
oxide or nitride of a, oxide or nitride of Mo, Ge
Of oxide or nitride of the above, and is formed by the above-mentioned thin film forming method.

The thickness of these protective films is preferably in the range of 25 to 750 ° in the case of insulating properties such as SiO _2.
The propagation characteristics of the AW tend to deteriorate. In the case of semiconductivity of Si or the like, the angle is preferably 50 to 750 °, and if it is less than 50 °, the effect as a protective film is hardly provided, and if it exceeds 750 °, the SAW propagation characteristics are liable to deteriorate.

In the present invention, the IDT of the SAW resonator 2
The electrodes 2a and 2b are made of Al or Al alloy (Al-Cu
, Al-Ti system, etc.), and Al is particularly preferred because of its high excitation efficiency and low material cost. The IDT electrodes 2a and 2b are formed by a thin film forming method such as an evaporation method, a sputtering method, or a CVD method.

The logarithm of the IDT electrodes 2a and 2b is 5
About 0 to 200, the width of the electrode finger is about 0.1 to 10.0 μm, the interval between the electrode fingers is about 0.1 to 10.0 μm, the opening width (cross width) of the electrode finger is about 10 to 100 μm, and the IDT electrode It is preferable that the thickness of each of 2a and 2b be about 0.2 to 0.5 μm in order to obtain desired characteristics as a resonator or a filter. If a piezoelectric material such as zinc oxide or aluminum oxide is formed between the electrode fingers of the IDT electrodes 2a and 2b, the resonance efficiency of the SAW is preferably improved.

As the piezoelectric substrate 1 for a SAW filter,
36 ° Y cut-X propagating LiTaO ₃ crystal, 64 ° Y
Cut-X propagating LiNbO ₃ crystal, 45 ° X-cut
A Z-propagating LiB ₄ O ₇ crystal or the like is preferable because it has a large electromechanical coupling coefficient and a small group delay time temperature coefficient. The thickness of the piezoelectric substrate 1 is preferably about 0.3 to 0.5 mm.
If it is less than 3 mm, the piezoelectric substrate becomes brittle, and if it exceeds 0.5 mm, the material cost increases.

Thus, the present invention has the effect of preventing and suppressing short-circuiting between electrode fingers due to the pyroelectric effect of the piezoelectric substrate, and at the same time, forming a protective film for protecting the electrode fingers in a state where discharge is suppressed. Have.

It should be noted that the present invention is not limited to the above embodiment, and various changes may be made without departing from the scope of the present invention.

[0020]

Embodiments of the present invention will be described below. First, the configuration of the SAW filter of the present invention was changed as described below, and the discharge occurrence rate was evaluated.

[1] The area of the conductive film for the IDT electrode, the wiring pattern, and the pad electrode is 38% and 20% of the area of one main surface of the piezoelectric substrate.

[2] Two types of protective films, Si and SiO ₂ , were used.

[3] There are three types of protection film formation portions: one that covers the entire main surface of the piezoelectric substrate other than the pad electrodes, one that includes only the SAW resonator and the reflector, and one that does not.

These various SAW filters are heated at 150 ° C.
Table 1 shows the results of examining the rate of occurrence of discharge by leaving the apparatus on a hot plate for 30 minutes.

[0025]

[Table 1]

In the case where a conventional insulating protective film is provided on the entire surface other than the pad electrode, the discharge occurrence rate is 100%.
In the case of semiconductive Si, the discharge generation rate was significantly reduced.
Further, when the protective film was provided only on the SAW resonator and the reflector, the discharge occurrence rate could be further reduced. This is presumably because the charge neutralizing action of the wiring pattern and the pad electrode greatly contributes to the reduction of the occurrence of discharge.

Hereinafter, specific examples will be described.

Example 1 A SAW filter F shown in FIG. 1 was manufactured as follows.

(1) 36 ° Y cut as piezoelectric substrate 1
Using a X-propagation LiTaO ₃ crystal, a resist film is applied to one main surface of the piezoelectric substrate 1, and the SAW resonator 2, the pad electrode 4
A negative pattern of the wiring pattern 5 was formed on the resist film by a photolithography method.

(2) On the resist film, a film thickness of 400 n
An about m conductive film (Al) was formed by a vapor deposition method.

(3) Unnecessary portions of the conductive film are lifted off, and the SAW resonator 2, the pad electrode 4, and the wiring pattern 5 are lifted off.
Is 38% of the area of one main surface of the piezoelectric substrate 1, and the IDT
Each of the electrodes 2a and 2b and the reflector 2c has an electrode finger width and electrode finger interval of about 1 μm, and is a 2.5-stage π-type ladder-type SAW filter for the 800 to 900 MHz band.

(4) Further, a resist film was applied on one main surface of the piezoelectric substrate 1, and a negative pattern of the protective film 6 covering only the SAW resonator 2 was formed on the resist film by photolithography.

(5) On this resist film, a film thickness of 250 °
About a Si film was formed by a vapor deposition method, and unnecessary portions of the Si film were removed by lift-off, thereby producing a SAW filter of the present invention.

Then, the SAW filter was left on a hot plate at 150 ° C. for 30 minutes, and the discharge generation rate was measured to be about 7%.

Example 2 A SAW filter was manufactured in the same manner as in Example 1 except that the protective film 6 was made of SiO ₂ . When the discharge occurrence rate of the SAW filter was examined in the same manner as in Example 1, it was about 14%.

Example 3 A SAW filter was manufactured in the same manner as in Example 1, except that a protective film 6a made of Si was provided on the entire main surface of the piezoelectric substrate 1 except for the pad electrode 4 as shown in FIG. did. When the discharge occurrence rate of the SAW filter was examined in the same manner as in Example 1, it was about 35%.

[0037]

According to the present invention, the area of the conductive film is set to 25% or more of the area of one main surface of the piezoelectric substrate 1, and the protective film is provided on the SAW resonator. By setting the semi-conductive protective film on one main surface of the piezoelectric substrate 1 excluding the pad electrode to be 25% or more of the area of one main surface of the piezoelectric substrate 1, the heat treatment such as packaging and wire bonding can be performed. A discharge is generated between the IDT electrode and the electrode finger of the reflector due to the pyroelectric effect of the piezoelectric substrate, and damage to the electrode finger is suppressed or prevented. Further, when the protective film is made of a semiconductive material, there is also an effect that the protective film can be formed in a state where discharge between the electrode fingers is suppressed.

Further, the protective film can prevent conductive foreign matter from entering between the IDT electrodes and short-circuit between the electrode fingers, and prevent corrosion of the electrode fingers due to moisture. As a result, the production yield can be improved. Reliability also increases.

[Brief description of the drawings]

FIG. 1 is a plan view of a basic configuration of a SAW filter F of the present invention.

FIG. 2 is a plan view showing a basic configuration of a SAW filter F according to another embodiment of the present invention.

3 is a plan view of a basic configuration of a conventional SAW filter F _1.

[Explanation of symbols]

 1: piezoelectric substrate 2: SAW resonator 2a: IDT electrode 2b: IDT electrode 2c: reflector 4: pad electrode 5: wiring pattern 6: protective film 6a: protective film

Claims (2)

[Claims] At least one surface acoustic wave resonator having a pair of comb-like electrodes, a conductive pattern for a wiring pattern and a pad electrode for external lead-out are formed on an area of one principal surface of a piezoelectric substrate by 25%.
%, And a protective film is provided on the surface acoustic wave resonator. 2. A piezoelectric substrate comprising: at least one surface acoustic wave resonator having a pair of comb-shaped electrodes;
%, And a semiconductive protective film is provided on an entire surface of one main surface of the piezoelectric substrate except for a pad electrode for leading out to the outside.