JPH05183370A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To improve the DC withstand pressure, RF withstand pressure, and electrostatic breaking stress by providing a resistance film which connects the terminal at the side of the IDT reflector and the terminal at the IDT side of the reflector. CONSTITUTION:An input IDT 20 consisting of comb-line electrodes 22 and 24 is provided on a piezoelectric substrate 10. On both sides of the input IDT 20, an IDT 30 consisting of comb-line electrodes 32 and 34 and an output IDT 40 consisting of comb-line electrodes 42 and 44 are formed. Resistance films 70 and 80 are formed in the area causing the destruction of the electrodes. In short, the resistance film 70 is formed to cover the electrodes of the terminal of a reflector 50, the electrodes of the terminal of the electrodes 34 of the output IDT 30 and the electrodes 32 at the terminal of the electrodes unit. The resistance film 80 is formed to cover the electrodes of the terminal of a reflector 60, the electrodes 44 at the terminal of the electrodes unit of the output IDT 40, and the electrodes 42 at the terminal of the electrodes unit. Thus, the destruction of the electrodes in each area and the remarkable deterioration of the element characteristic of the elastic surface wave device can be prevented.

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 in which an IDT sandwiched by a pair of reflectors is formed on a piezoelectric substrate.

[0002]

2. Description of the Related Art A surface acoustic wave device is one in which an IDT sandwiched by a pair of reflectors is formed on a piezoelectric substrate of lithium tetraborate, for example. The IDT and the reflector are made of a conductive material having a low sheet resistance, for example, aluminum. The width of the electrode finger of the IDT and the width of the electrode of the reflector are often set to 1/4 of the wavelength of the surface acoustic wave propagating on the piezoelectric substrate. In the conventional surface acoustic wave device, the propagation velocity of the surface acoustic wave is in the range of 2500 to 4500 m / s, and the frequency band of the surface acoustic wave is 30 to 900 MHz.
Therefore, the wavelength of the surface acoustic wave is about 150 to 2.7 μm. Therefore, the width of the electrode finger of the IDT and the width of the electrode of the reflector are 1/4 of the wavelength of the surface acoustic wave, that is, 3
The thickness is about 7 to 0.7 μm. Therefore, it is known that the surface acoustic wave device has a weak point that the electrode is easily broken electrically.

As a mode in which the electrodes of the surface acoustic wave device are electrically destroyed, when a large DC voltage is applied between the electrodes, and when electrostatic charges are accumulated on the piezoelectric substrate,
Charges may be accumulated on the piezoelectric substrate due to temperature changes during the manufacturing process due to the pyroelectric effect, and the thinner the width of the electrode, the more easily it is destroyed. In recent years, there has been a demand for a surface acoustic wave device in the quasi-microwave band mainly for mobile communication, and it is considered that the width of such a surface acoustic wave device is further reduced to about 0.3 to 0.5 μm. However, it is considered to be more susceptible to electrical breakdown than ever before, which is a very serious problem.

[0004]

A surface acoustic wave device is known in which a resistor is provided between the IDT electrodes in order to prevent such electrical electrode breakdown. However, if a resistor is connected between the IDT electrodes, there is a problem that the insertion loss as a surface acoustic wave device increases and the element characteristics deteriorate.

An object of the present invention is to provide a surface acoustic wave device capable of effectively preventing electrical electrode breakage without significantly deteriorating the element characteristics of the surface acoustic wave device.

[0006]

The inventor of the present application observed in detail the state in which the electrodes of the surface acoustic wave device were destroyed.
The surface acoustic wave device observed by the present inventor is shown in FIG. Input I composed of comb-shaped electrodes 22 and 24 on the piezoelectric substrate 10.
The DT 20 is formed, and the output IDT 30 including the comb-shaped electrodes 32 and 34 and the output IDT 40 including the comb-shaped electrodes 42 and 44 are formed on both sides of the input DT 20. The input IDT 20 and the output IDTs 30 and 40 are
It is sandwiched by a pair of reflectors 50 and 60.

When the breakdown of the electrodes in such a surface acoustic wave device is observed in detail, as shown in FIG. 1, a region A between the end of the output IDT 30 and the end of the reflector 50 is obtained.
It was also found that it mainly occurred in the region B between the output IDT 40 and the end of the reflector 60. Therefore, it was conceived that by disposing the resistance film only in this region, the destruction of the electrode can be effectively prevented and the influence on the element characteristics can be suppressed to the minimum.

That is, as shown in FIG. 2A, the resistance films 70 and 80 are formed in the regions A and B where the electrodes are destroyed. As shown in FIG. 2B, the resistance film 70 includes electrodes at the end of the reflector 50 and the electrode fingers 3 of the output IDT 30.
It is formed so as to cover the electrode at the end of No. 4 and the electrode at the end of the electrode finger 32. As shown in FIG. 2C, the resistance film 80 includes electrodes at the end of the reflector 60 and the output IDT 40.
It is formed so as to cover the electrode at the end of the electrode finger 44 and the electrode at the end of the electrode finger 42.

The resistance films 70 and 80 may be formed on the electrodes as shown in FIG. 1, but the ends of the reflectors 50 and 60 and the ends of the output IDTs 30 and 40 are electrically connected. It may be formed, for example, on the piezoelectric substrate 10 below the electrodes. In this way, the ends of the reflectors 50 and 60 and the output ID
Resistance films 70 and 8 for electrically connecting the ends of T30 and 40
Since 0 is formed, the destruction of the electrodes in the regions A and B is effectively prevented, and the regions A and B are effectively prevented.
Since the other electrodes of the IDTs 20, 30, and 40 other than the above are sufficiently insulated, the element characteristics of the surface acoustic wave device are not significantly deteriorated.

In FIG. 2, the IDT is formed by the resistance films 70 and 80.
Although the electrodes 30 and 40 and the reflectors 50 and 60 are formed so as to cover only the electrodes at the ends, the resistance films 70 and 80 may be formed large so as to cover the electrodes over several pairs of the ends. Further, the resistance films 70 and 80 may be formed so as to connect the ends of the reflectors 50 and 60 and only one of the comb-shaped electrodes 34 and 44.
Next, the resistance values of the resistance films 70 and 80 were considered.

An electrical equivalent circuit of a surface acoustic wave resonator filter, which is a typical example of a surface acoustic wave device, can be represented as shown in FIG. The area inside the broken line is the surface acoustic wave resonator filter, Zs is the equivalent resistance of the IDT of the surface acoustic wave resonator, Zo is the terminal impedance connected to the IDT, and ρ is the resistance value of the resistance film. Insertion loss I in the equivalent circuit represented in this way
L is expressed by the following equation.

IL = 2 / (2 + Zo / Zs + 2Zo / ρ + Zs / ρ) Here, assuming that the insertion loss is ILo when the resistance film is not provided, the following equation is obtained. ILo = 2 / (2 + Zo / Zs) In order to limit the insertion loss IL when the resistance film is provided to the change of the insertion loss when the resistance film is not provided to 10% or less of ILo, the following equation may be established.

0.9 ≦ ILo / IL 0.9 ≦ (2 + Zo / Zs + 2Zo / ρ + Zs / ρ) / (2 + Zo / Zs) When the above equation is modified, the resistance value ρ of the resistance film is as follows. ρ ≧ 9 (Zs + 2Zo) / (2 + Zs / Zo) Therefore, it is desirable to set the resistance value ρ of the resistance film within the range of the above equation.

FIG. 4 shows changes in the insertion loss IL with respect to the resistance value ρ of the resistance film when the terminating impedance Zo is 50Ω and the equivalent resistance Zs is 25Ω. The resistance value ρ of the resistance film is 50
At 0Ω or less, the insertion loss ILo when the resistance film is not provided
Almost the same insertion loss IL as compared to (~ 1.9 dB)
It can be seen that It was found that when the resistance value ρ of the resistance film is 1 MΩ or more, the change of the insertion loss is small, but the electrode is broken.

In the above description, the surface acoustic wave device in which the three IDTs are sandwiched between the pair of reflectors has been described as an example, but the number of IDTs may be one or more.

[0016]

EXAMPLES Example 1 As the piezoelectric substrate 10, a 45 ° rotated X-cut Z-propagation Li ₂ B ₄ O ₇ substrate was used. On the piezoelectric substrate 10, as shown in FIG. 2, the input IDT 2
0, output IDTs 30 and 40, and reflectors 50 and 60 were formed. These IDTs 20, 30, 40, reflectors 50, 6
No. 0 electrode was formed of an aluminum film having a thickness of about 0.2 μm, and each electrode was formed with a width of about 2 μm and an interval of about 2 μm. The input IDT 20 has a logarithm of 30 pairs, and the output IDT 3 has a logarithm.
The number of pairs of 0 and 40 is 34, and the number of electrodes of the reflectors 50 and 60 is 100. The equivalent resistance Zs of the output IDTs 30 and 40 is 20Ω, and the terminating resistance Zo connected to the output IDTs 30 and 40 is 50Ω.

The resistance film 70 that covers the ends of the output IDT 30 and the reflector 50 and the resistance film 80 that covers the ends of the output IDT 40 and the reflector 60 are made of nickel-chromium alloy having a thickness of about 10 μm. Formed of a film. The resistance value ρ of the resistance films 70 and 80 was set to 1 kΩ. The withstand voltage of the surface acoustic wave device of Example 1 was improved to 10 times or more that of the surface acoustic wave device without the resistance films 70 and 80 (Comparative Example 1).

The insertion loss of the surface acoustic wave device of Example 1 is
As shown in FIG. 5, even when compared to the surface acoustic wave device (Comparative Example 1) in which the resistance films 70 and 80 were not provided, there was almost no increase, and no significant deterioration in element characteristics was observed. In the case of the surface acoustic wave device (Comparative Example 2) in which the resistance value ρ of the resistive films 70 and 80 was 220Ω, the breakdown voltage was improved 10 times or more as compared with Comparative Example 1, but as shown in FIG. Is increasing and the device characteristics are deteriorated.

The resistance value ρ of the resistance films 70 and 80 is 10
In the surface acoustic wave device having a resistance of 0 kΩ or more (Comparative Example 3), the element characteristics were hardly deteriorated, but the withstand voltage was not improved. Example 2 As the piezoelectric substrate 10, a 36 ° rotated Y-cut X-propagation LiTaO ₃ substrate was used. On this piezoelectric substrate 10, as shown in FIG. 2, an input IDT 20 and an output IDT
T30, 40 and reflectors 50, 60 were formed. These I
The electrodes of the DTs 20, 30, 40 and the reflectors 50, 60 were formed of an aluminum film having a thickness of about 0.3 μm, and each electrode was formed with a width of about 2 μm and an interval of about 2 μm. I for input
The DT 20 has 24 pairs, the output IDTs 30 and 40 have 20 pairs, and the reflectors 50 and 60 have 120 electrodes. The equivalent resistance Zs of the output IDTs 30 and 40 is 30Ω.
The terminating resistance Zo connected to the output IDTs 30 and 40 is 50Ω.

The resistance film 70 that covers the ends of the output IDT 30 and the reflector 50 and the resistance film 80 that covers the ends of the output IDT 40 and the reflector 60 are nickel-chromium alloy with a thickness of about 10 μm. Formed of a film. The resistance value ρ of the resistance films 70 and 80 was set to 2 kΩ. The withstand voltage of the surface acoustic wave device of Example 2 was improved to 10 times or more that of the surface acoustic wave device without the resistance films 70 and 80 (Comparative Example 4).

The insertion loss of the surface acoustic wave device of Example 2 is
As shown in FIG. 6, even when compared with the surface acoustic wave device (Comparative Example 4) in which the resistance films 70 and 80 were not provided, there was almost no increase, and no significant deterioration in element characteristics was observed. In the case of the surface acoustic wave device (Comparative Example 5) in which the resistance value ρ of the resistive films 70 and 80 was 100Ω, the breakdown voltage was improved 10 times or more as compared with Comparative Example 1, but as shown in FIG. Is increasing and the device characteristics are deteriorated.

The resistance value ρ of the resistance films 70 and 80 is 10
In the surface acoustic wave device having a resistance of 0 kΩ or more (Comparative Example 3), the element characteristics were hardly deteriorated, but the withstand voltage was not improved.

[0023]

As described above, according to the present invention, since the resistive film connecting the end portion of the IDT on the reflector side and the end portion of the reflector on the IDT side is provided, the element characteristics are greatly deteriorated. It is possible to effectively prevent the electrical destruction of the electrode. Therefore, it is possible to overcome the essential weaknesses of the surface acoustic wave device and improve the DC breakdown voltage, the RF breakdown voltage, and the electrostatic breakdown resistance.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional surface acoustic wave device.

FIG. 2 is a diagram showing a surface acoustic wave device according to the present invention.

FIG. 3 is an equivalent circuit diagram of a surface acoustic wave device according to the present invention.

FIG. 4 is a graph showing a change in insertion loss with respect to a resistance value of a resistance film of a surface acoustic wave device according to the present invention.

5 is a graph showing frequency response characteristics of Example 1, Comparative Example 1, and Comparative Example 2. FIG.

6 is a graph showing frequency response characteristics of Example 2, Comparative Example 4, and Comparative Example 5. FIG.

[Explanation of symbols]

 10 ... Piezoelectric substrate 20 ... Input IDTs 22, 24 ... Comb-shaped electrodes 30, 40 ... Output IDTs 32, 34, 42, 44 ... Comb-shaped electrodes 50, 60 ... Reflectors 70, 80 ... Resistive films

Claims (2)

[Claims] 1. A piezoelectric substrate, a pair of reflectors formed on the piezoelectric substrate, at least one IDT formed on the piezoelectric substrate and sandwiched between the pair of reflectors, and the IDT of the IDT. A surface acoustic wave device comprising: a reflector side end portion; and a resistance film that connects the IDT side end portion of the reflector. 2. The surface acoustic wave device according to claim 1, wherein the resistance value ρ of the resistance film is ρ ≧ 9 (Zs + 2Zo) / (2 + Zs / Zo), where Zs is an equivalent resistance of the IDT and Zo is the IDT. Surface acoustic wave device, characterized in that the terminating impedance connected to