JPH08237057A - Surface acoustic wave device and its frequency characteristic adjustment method - Google Patents

Abstract

PURPOSE: To set the center frequency of a surface acoustic wave device to a desired frequency. CONSTITUTION: A dielectric film 20 and an electrode use metallic thin film 21 are formed on an interdigital transducer IDT 15 on a piezoelectric substrate 11. The piezoelectric substrate 11 is mounted on a metallic stem and a drive DC voltage is applied between the electrode use metallic thin film 21 and the stem. An electrostatic attractive force is exerted between the electrode use metallic thin film 21 and the stem by the drive DC voltage and the surface pressure of the piezoelectric substrate 11 and the IDT 15 is increased by the electrostatic attractive force. The propagation speed of the surface acoustic wave is changed by the increase in the surface pressure to obtain a desired center frequency.

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 such as a surface acoustic wave filter and a frequency characteristic adjusting method thereof.

[0002]

2. Description of the Related Art Generally, a surface acoustic wave device uses a surface acoustic wave (Surface Acoustic Wave;
It has an interdigital transducer (hereinafter referred to as IDT) for exciting SAW). By processing the IDT,
Various properties and functions are brought to the surface acoustic wave device. Conventionally, the surface acoustic wave device has mainly been a surface acoustic wave filter, but nowadays, surface acoustic wave resonators are also widely used, and the range of the surface acoustic wave device is wide. In addition to this, the surface acoustic wave convolver also has an IDT and is therefore one of the surface acoustic wave devices.

FIG. 2 is a structural view showing a conventional surface acoustic wave device, and a surface acoustic wave resonator is shown as an example.
This surface acoustic wave resonator is made of LiTaO ₃ or LiNbO ₃
Etc. have a piezoelectric substrate 1. An underlay 2 of aluminum or a thin film containing aluminum as a main material is formed on a piezoelectric substrate 1, and an input terminal 3 and an output terminal 4 are formed on the underlay 2. Input terminal 3 and output terminal 4
Is a gold thin film bonding pad. Input terminal 3
The output terminal 4 is connected to the I 2 for SAW excitation through the underlay 2.
Each is connected to DT5. The IDT 5 is composed of an electrode finger 5a connected to the input terminal 3 and an electrode finger 5b connected to the output terminal 4. The IDT 5 was produced at the same time as the underlay 2, and the film thickness and material of the IDT 5 are the same as those of the underlay 2. Reflectors 6 are formed on both sides of the IDT 5. The reflector 6 is also manufactured at the same time as the IDT 5, and the film thickness and material of the reflector 6 are the same as those of the IDT 5. The reflectors 6 are each connected to the bonding pad 8 of the gold thin film for the ground wire via the aluminum pattern 7. Depending on the application, the reflector 6 may be unnecessary. The shape and arrangement of the IDT 5 vary depending on the purpose of use, and there are some variations, but the configuration of the other surface acoustic wave device is basically the same as that of FIG.

[0004]

However, the conventional surface acoustic wave device has the following problems. Generally, various surface acoustic wave devices are designed to function in a specific frequency band. The center frequency F _c of the frequency band, the electrode fingers 5a, 5b of the pitch, i.e. the distance between the electrode fingers, and the thickness thereof come determined by the SAW propagation velocity of the piezoelectric substrate 1. Type of piezoelectric substrate 1 used (eg, LiTaO ₃ , LiNbO ₃ , crystal, etc.) and IDT
When the material of No. 5 is determined, the interval between the electrodes and the film thickness thereof are also naturally determined. In the manufacturing process, the distance between the electrode fingers 5a and 5b of the IDT 5 is determined before the film thickness, so that the film thickness of the IDT 5 finally influences the center frequency F _c . When the film thickness of the IDT 5 becomes thicker than the designed value, the SAW propagation speed becomes slow and the center frequency F _c moves to the low frequency side. On the contrary, when the thickness is smaller than the design value, the SAW propagation speed increases, and the center frequency F _c moves to the high frequency side. Therefore, in order to obtain the desired center frequency F _c , it depends on how accurately the film thickness of the IDT 5 can be formed.

In the conventional surface acoustic wave device, the IDT5
In most cases, is formed by vapor deposition. It may also be formed by sputtering. IDT5 by vapor deposition
The film quality is higher when the film is formed, and the film quality is higher than when the film is formed by sputtering. Among the vapor deposition machines currently on the market, high performance ones can form an IDT5 film with an accuracy of about 5%. For example, 36 ° YXcu
When the IDT ₅ whose main material is aluminum is formed on the piezoelectric substrate 1 of LiTaO ₃ of t, the desired center frequency F _c is 83 with respect to 835 MHz even if the vapor deposition machine is used.
Only surface acoustic wave devices with a center frequency of 5 ± 2 MHz can be obtained. If this accuracy is acceptable, it is necessary, but if not, the frequency characteristics need to be adjusted. There are basically two methods for adjusting the frequency characteristics. That is,
The frequency characteristic is adjusted depending on whether the method is used when the center frequency F _c is moving to the low range or the method used when the center frequency F _c is moving to the high range.

Since the film thickness of the IDT 5 is thicker than the design value when the center frequency F _c is moving to the low range, the film thickness is gradually reduced by dry etching to adjust the center frequency F _c to a desired value. . When the center frequency F _c is moving to a high range, the film thickness of the IDT 5 is thinner than the designed value.
An insulating film such as SiO ₂ is formed on the DT 5 by sputtering or the like.
ZnO is deposited to be slightly thicker. Then, the insulating film is removed by dry etching or wet etching,
Adjust the center frequency F _c to the desired value. However, in the case of wet etching, since the etching rate is high, ID
Care must be taken that T5 is not etched. In any case, these two methods of adjusting the frequency characteristics take time and effort, and the cost of the surface acoustic wave device is inevitably increased.

[0007]

In order to solve the above-mentioned problems, the first invention is formed on a piezoelectric substrate in a comb shape,
In a surface acoustic wave device including a transducer that excites a surface acoustic wave based on a given high-frequency signal, a dielectric film deposited on the transducer, and an electrode metal thin film formed on the dielectric film. Is provided. According to a second invention, the surface acoustic wave device of the first invention is mounted on a metal plate or a metal thin film, and a variable DC voltage is applied between the metal plate or the metal thin film and the metal thin film for electrodes to generate electrostatic force. Is generated and the surface pressure of the piezoelectric substrate is continuously controlled. According to a third invention, the surface acoustic wave device of the first invention is mounted on a metal plate or a metal thin film, and a plurality of voltage values are selected between the metal plate or the metal thin film and the electrode metal thin film. A voltage source that outputs a voltage value is connected to generate an electrostatic force based on the selected voltage value to control the surface pressure of the piezoelectric substrate stepwise.

[0008]

The center frequency F _c or the like, which is the frequency characteristic of the surface acoustic wave device, moves to the low frequency side when the film thickness of the IDT becomes thicker than the design value, and moves to the high frequency side when it becomes thin. The change in the propagation velocity of the SAW excited by the IDT is the cause of the movement of the center frequency F _{c and the} like. That is, as the film thickness of the IDT becomes thicker, the surface pressure of the piezoelectric substrate increases and the SAW
Of the SAW becomes slower, and the center frequency F _c or the like moves to the lower frequency side in inverse proportion to the propagation speed of the SAW. Similarly, IDT
When the film thickness of S becomes thin, the surface pressure of the piezoelectric substrate decreases and S
The propagation speed of the AW becomes faster, and the center frequency F _c, etc. moves to the high frequency side. In other words, it is no exaggeration to say that the surface pressure of the SAW propagation region of the piezoelectric substrate influences the center frequency F _c of the surface acoustic wave device. According to the first to third inventions, since the metal thin film for electrode is provided on the dielectric film on the IDT, when the surface acoustic wave device is mounted on a metal plate or a metal thin film, the metal plate or The metal thin film and the metal film for electrodes form one capacitor. When a DC voltage is applied between the metal plate or the metal thin film and the electrode metal film, an electrostatic force is generated between them to attract each other. That is, the piezoelectric substrate and the IDT are subjected to surface pressure, and the frequency characteristics change. Therefore, the above problem can be solved.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIGS. 1 (1) to 1 (3) are structural views of a surface acoustic wave resonator showing a first embodiment of the present invention. FIG.
FIG. 2B is an AA sectional view of FIG. 1A, and FIG. 3C is an enlarged sectional view. This surface acoustic wave device is a surface acoustic wave resonator, and has a piezoelectric substrate 11 made of a material such as LiTaO ₃ or LiNbO ₃ . An underlay 12 of aluminum or a thin film containing aluminum as a main material is formed on the piezoelectric substrate 11, and an input terminal 13 and an output terminal 14 are formed on the underlay 12. Input terminal 1
3 and the output terminal 14 are gold thin film bonding pads. The input terminal 13 and the output terminal 14 are the underlay 12
Are connected to the IDTs 15 for comb-shaped SAW excitation via the. The film thickness and material of IDT15 is underlay 1
Same as 2. Reflectors 16 are formed on both sides of the IDT 15. The reflector 16 is also manufactured at the same time as the IDT 15, and the film thickness and material of the reflector 16 are the same as those of the IDT 15. Each reflector 16 is connected to a bonding pad 18 of a gold thin film for a ground wire via an aluminum pattern 17.

A dielectric thin film or a piezoelectric thin film which is the dielectric film 20 is formed between and above the electrode fingers of the IDT 15 as shown in (2) and (3) of FIG. A metal thin film 21 for an electrode is formed on the dielectric film 20.
The IDT 15 and ground and the thin film 21 are completely electrically independent, but a gold thin film bonding pad 19 for connecting to an external circuit (not shown) is provided beside the metal thin film 21 for electrodes. As a selection condition of the material of the thin film 21, it is appropriate that the density is low and unnecessary surface pressure is not applied to the IDT. Aluminum or an alloy mainly composed of aluminum can be considered as a material that meets this selection condition. FIG. 3 is a cross-sectional view showing the manufacturing process of FIG. 1, and the manufacturing process of the surface acoustic wave resonator will be described with reference to this drawing.

In the first step, the underlay 12, the IDT 15, the reflector 16 and the pattern 17 shown in FIG. 1 are formed on the piezoelectric substrate 11 by film formation. The standard film thickness of these films is, for example, 1000Å. In the second step, a resist 22 is applied over the entire surface of the underlay 12, the IDT 15, the reflector 16, and the piezoelectric substrate 11 on which the pattern 17 is formed. In the third step, the mask pattern 23
Through. In the fourth step, a pattern is formed by removing unnecessary resist. Then, the fifth
In the step, for example, SiO ₂ , ZnO or Si ₃ N ₄ of the dielectric film 20 is deposited on the sample obtained in the fourth step. The film thickness of the dielectric film 20 is about 1.5 of that of the IDT 15.
It is about 2.0 times, and the method for forming the dielectric film 20 may be a sputtering method. In the sixth step, the electrode thin metal film 21 is laminated on the dielectric film 20 by vapor deposition or sputtering. In the next 7th step,
Unnecessary resist 22 pattern, unnecessary dielectric film 20,
By removing the unnecessary electrode metal thin film 21 by lift-off, the dielectric film 20 and the electrode metal thin film 21 thereon can be formed only on the upper portion of the IDT 15. After the seventh step, the input terminal 13, the output terminal 14, the ground wire bonding pad 18, etc. are formed.

Through the above steps, the structure of the surface acoustic wave resonator shown in FIG. 1 is completed. Here, when the center frequency F _c, which is the frequency characteristic of the surface acoustic wave resonator, is higher than a desired value, adjustment is performed as follows. When mounting a surface acoustic wave resonator chip in a package, an underlying metal film or metal plate is required in both a single package and a common package. Hereinafter, this metal film or metal plate is referred to as an electrode metal film B. In this embodiment for adjusting the center frequency F _c of the surface acoustic wave resonator, the electrode metal film B is used.
The electrode metal film B and the electrode metal thin film 21 are connected to an external DC circuit so that a DC electric field is applied between the electrode metal film 21 and the electrode metal thin film 21. FIG. 4 is a schematic diagram showing the connection between FIG. 1 and an external DC circuit. When the voltage of the variable voltage DC voltage source 30 is applied to the electrode metal film B and the electrode metal thin film 21 of the surface acoustic wave resonator of FIG. 1, a potential difference is generated between the electrode metal film B and the electrode metal thin film 21. Occurs. As a result, an electrostatic force acts between the electrode metal film B and the electrode metal thin film 21 to attract each other.

FIG. 5 is a diagram showing the surface acoustic wave resonator of FIG. 1 mounted on a stem, and FIGS. 6 (1) and 6 (2) are diagrams showing the AA cross section and the BB cross section of FIG. Is. The chip of the surface acoustic wave resonator is mounted on the stem 40 that serves as the metal film B for electrodes. The stem 40 has an input terminal 41 and an output terminal 42. Each terminal 41, 42 is surrounded by glass 43 and insulated from the stem 40 main body. The input terminal 41 is connected to the input terminal 13 of the surface acoustic wave resonator by a bonding wire 44, and the output terminal 42 is connected to the output terminal 14 of the surface acoustic wave resonator by a bonding wire 45. On the other hand, the ground wire bonding pad 18 of the surface acoustic wave resonator is connected to the main body of the stem 40 by the bonding wire 46, and the ground terminal 47 formed on the main body of the stem 40 is led to the outside. Bonding pad 19 for connecting metal thin film 21 for electrodes to the outside
Is connected to a DC voltage terminal 49 by a bonding wire 48, and the DC voltage terminal 49 is connected to a DC voltage source 30 (not shown).
It is connected to the. Glass 4 around the DC voltage terminal 49
Surrounded by 3, the stem 40 and its DC voltage terminal 49 are insulated. The top of the stem 40 is covered with a metal lid 50, and the surface acoustic wave resonator chip is sealed.

Next, the operation of the surface acoustic wave resonator shown in FIGS. 1, 5 and 6 will be described. When a high frequency signal of several MHz or more is input to the input terminal 13 via the input terminal 41, the electrode fingers 15a electrically connected to the input terminal 13
High frequency voltage is applied to. At this time, a high-frequency voltage is inductively generated in the electrode finger 15b connected to the output terminal 14 adjacent to the electrode finger 15a, but since it is delayed in phase, it is between the input terminal 13 and the output terminal 14. Causes a potential difference. As a result, the surface of the piezoelectric substrate 11 under the electrode fingers is distorted, and SAW having the same frequency as the input signal is excited. SAW
By the excitation, the coupling between the electrode fingers 15a and 15b is further strengthened, and a high frequency signal in a certain band is output from the output terminal 14. It has already been described that the band of the high frequency signal output from the output terminal 14 depends on the propagation speed of the excited SAW. The focus of this embodiment is to electrically control the SAW propagation velocity. S of the piezoelectric substrate 11
When the pressure of the surface on which the AW propagates increases, the propagation velocity of the SAW decreases, and the center frequency F _c of the surface acoustic wave device moves to the low frequency side. In this embodiment, since the electrode metal thin film 21 is connected to the variable voltage DC voltage source 30, the drive DC voltage is applied between the electrode metal thin film 21 and the stem 40 which becomes the electrode metal film B. . Therefore, an electrostatic force acts between the metal thin film for electrode 21 and the stem 40 to attract each other. As a result, the ID of the surface acoustic wave resonator
Since T15 and the piezoelectric substrate 11 receive pressure, the SAW propagation speed becomes slow.

The higher the driving DC voltage, the higher the IDT
The pressure applied to 15 increases, and the SAW propagation velocity also decreases in inverse proportion to this. That is, the SAW is continuously driven by the driving DC voltage.
By reducing the propagation velocity of, the center frequency F _c of the surface acoustic wave resonator can be continuously lowered to the low frequency side, and the center frequency F _c can be set to a desired value. As described above, in this embodiment, the dielectric film 2 is formed on the IDT 15.
The metal thin film 21 for electrodes is provided with the surface acoustic wave resonator mounted on the stem 40 corresponding to the metal film B for electrodes in FIG. That is, the surface acoustic wave resonator system is housed in one capacitor having the electrode thin metal film 21 and the stem 40 as both electrodes. Therefore, when a potential difference is generated between the electrode metal thin film 21 and the stem 40, they are attracted by an electrostatic force, and pressure is applied to the IDT 15 of the surface acoustic wave resonator.
The same effect can be obtained by increasing the film thickness of DT15.

The electrostatic force F between the electrode metal film B and the electrode metal thin film 21 can be expressed by the following equation (i). ^{F = -εSV 2/2 / d} 2 ··· (i) where, epsilon; is the dielectric constant between the electrode metal films B and the electrode metal film 21, substantially equal to the dielectric constant of the piezoelectric substrate 11. S: The facing area of the electrode metal film B and the electrode metal thin film 21, which is almost equal to the area of the electrode metal thin film 21. d: The distance between the electrode metal film B and the electrode metal thin film 21, which is approximately equal to the thickness of the piezoelectric substrate 11. V: Driving DC voltage applied between the electrode metal film B and the electrode metal thin film 21 The symbol "-" in the formula (i) indicates that the electrostatic force F is an attractive force. The formula (i) is also shown in the following document. Reference; Shigeo Umoto, "Electromagnetics" (Sho 52), Shokodo, P.
103 The force applied to the surface acoustic wave resonator is proportional to the square of the driving DC voltage V applied to the electrode metal film B and the electrode metal thin film 21. That is, even if the voltage V is not so high, a high surface pressure can be applied to the IDT 15 of the surface acoustic wave resonator. The surface pressure P is given by the following equation (ii). ^{P = F / S = -εV 2} /2 / d 2 The ··· (ii), the propagation velocity of the excitation the SAW is inversely proportional to the surface pressure P, the center frequency F _c is the propagation speed of the surface acoustic wave resonator Proportional to. That is, the center frequency F _c of the surface acoustic wave resonator
Is inversely proportional to the surface pressure P. According to the equation (ii), the surface pressure is proportional to the square of the driving DC voltage V, so that the center frequency F
_c will be inversely proportional to the square of the voltage V.

FIGS. 7 (1) and 7 (2) are diagrams showing the frequency characteristics of the surface acoustic wave resonator shown in FIG. 1. FIG. 7 (1) shows the relationship between the driving DC voltage V and the center frequency, and FIG. ) Indicates the variation of the frequency characteristic due to the driving DC voltage V. When the voltage V between the electrode metal thin film 21 and the electrode metal film B is zero, the center frequency of the surface acoustic wave resonator is F _0, which is the center frequency before the voltage V is applied to the surface acoustic wave resonator. . Figure 7
F _{1 in} (1) is the value of the desired center frequency, and V ₁
Is the value of the driving DC voltage at that time. Since the voltage V is continuously variable, the center frequency _Fc is also continuously variable. FIG. 7B shows the insertion loss of the surface acoustic wave resonator. Frequency characteristic of the surface acoustic wave resonator before the application of the driving DC voltage V is a characteristic curve 60, the voltages V ₁
Is applied, a characteristic curve 61 is obtained.

Theoretically, the higher the driving DC voltage V, the lower the center frequency F _c unless an accident such as dielectric breakdown occurs. Here, it is possible to move the center frequency F _c of several megahertz with a driving DC voltage V of several volts practically, and the voltage V is such that dielectric breakdown occurs.
The stem 40 of the metal film B for electrodes and the metal thin film 21 for electrodes.
Need not be applied to. However, as shown in FIGS. 7A and 7B, the surface acoustic wave resonator is set so that the desired center frequency F ₁ is always equal to or lower than the center frequency F ₀ when the DC drive voltage V is zero. Must be manufactured. That is, the center frequency F _c can be made lower than the center frequency F ₀ by the DC drive voltage V, but the reverse is not possible. Therefore,
For example, when the IDT 15 is formed by vapor deposition, the center frequency F ₀ is the desired center frequency F _{0 in} consideration of the film forming accuracy of the vapor deposition machine.
_It is necessary to form the film so that it is higher than the value of ₁ . The closer the center frequency F ₀ is to the center frequency F ₁ , the lower the DC drive voltage V is, and unnecessary strain is not applied to the piezoelectric substrate 11. Further, theoretically, since no current flows between the electrode metal thin film 21 and the electrode metal film B, that is, the stem 40, there is no power consumption. That is, in this embodiment, the variable voltage DC voltage source 30 for applying the driving DC voltage is required, but it is possible to surely adjust the center frequency F _c of the surface acoustic wave resonator to the desired value F _1. This is possible and greatly improves the design flexibility and the manufacturing yield. In particular, it is very effective for a surface acoustic wave resonator that requires a highly accurate center frequency.

Second Embodiment A surface acoustic wave resonator used in this embodiment has the same structure as the surface acoustic wave resonator shown in FIG. 1 and is mounted on a stem 40 similar to that shown in FIGS. 5 and 6. The stem 40 corresponding to the electrode metal thin film 21 and the electrode metal film B formed on the surface acoustic wave resonator is the variable voltage DC voltage source 30 of the first embodiment.
Is connected to a DC voltage source that outputs a staircase voltage different from. FIGS. 8 (1) and 8 (2) are diagrams showing the frequency characteristics of the surface acoustic wave resonator of the second embodiment of the present invention. The surface acoustic wave resonance of the second embodiment will be described with reference to this figure. A method of adjusting the frequency characteristic of the child will be described. The DC voltage source has a function of outputting a voltage selected from a plurality of voltages.
The drive voltages V1 and V2 are output at the cycle T as shown in (1). That is, when the time t is between 0 and T ₁ (0 <t <T ₁ ), the voltage is 0, and when the time t is between T ₁ and T ₂ (T ₁ <t <
The driving voltage V _{1 is} output when T ₂ ), and the driving voltage V ₂ is output when the time t is between T ₂ and T ₃ (T ₂ <t <T ₃ ). By outputting such a driving voltage to the electrode metal thin film 21 of the surface acoustic wave resonator, the frequency characteristic of the insertion loss of the surface acoustic wave resonator becomes as shown in (2) of FIG. The center frequency F _c of the wave resonator changes. When the time t is between 0 and T ₁ (0 <t <T ₁ ), the frequency characteristic becomes the characteristic curve 80, and the center frequency becomes F0. When the time t is between T ₁ and T ₂ (T ₁ <t <T ₂ ), the frequency characteristic is a characteristic curve 81, and the center frequency is F ₁
Becomes When the time t is between T ₂ and T ₃ (T ₂ <t <T ₃ ), the frequency characteristic becomes the characteristic curve 82, and the center frequency is F.
It becomes ₂ .

As described above, in this embodiment, it is possible to give one surface acoustic wave resonator various frequency characteristics by controlling the driving voltage. The present invention is not limited to the above embodiment, and various modifications and applications are possible. Examples of the modifications and applications are as follows. (I) Although the example of the surface acoustic wave resonator is shown in the first and second embodiments, the present invention can adjust the frequency characteristics of all surface acoustic wave devices having an IDT. . Therefore, particularly when a band-pass filter is configured using surface acoustic wave resonators, it becomes easy to make the resonance frequencies of the surface acoustic wave resonators and the antiresonance frequency of the surface acoustic wave resonators have good accuracy. Further, it is possible to widen or narrow the bandwidth of the bandpass filter by controlling the resonance frequency and the antiresonance frequency. (II) In the surface acoustic wave duplexer, a surface acoustic wave filter for transmission and a surface acoustic wave filter for reception which are adjacent to each other in the frequency band are used. The center frequencies F _c of the surface acoustic wave filters are separated from each other by about 30 MHz. Here, if the method of adjusting the center frequency as described in the second embodiment is used, a surface acoustic wave duplexer that does not require a demultiplexing circuit can be obtained by using only the transmitting surface acoustic wave filter and the receiving surface acoustic wave filter. Can be configured. That is, when the surface acoustic wave filter for transmission operates normally, the center frequency F _c1 of the surface acoustic wave filter for reception is moved to a direction not adversely affecting the surface acoustic wave filter for transmission, and conversely, for reception. When the surface acoustic wave filter operates normally, the center frequency _Fc2 of the transmitting surface acoustic wave filter is moved. By doing so, a demultiplexing circuit becomes unnecessary, and a surface acoustic wave duplexer having a lower loss than in the case of using a demultiplexing circuit can be configured. (III) In the first and second embodiments, the portion corresponding to the electrode metal film B is set in the stem 40, but the piezoelectric substrate 1
It is also possible to form a metal film for electrodes on the lower side of 1 and use it as the metal film B for electrodes. In this case, the degree of freedom in selecting the stem material is increased.

[0021]

As described in detail above, the first to third aspects
According to the invention, since the dielectric film and the metal film for electrodes are provided on the IDT, the metal film for electrodes and the metal film or metal plate on which the surface acoustic wave device is mounted are one capacitor. Become. By applying a voltage between the metal film for electrodes and the metal film or the metal plate on which the surface acoustic wave device is mounted, an electrostatic attractive force is generated between them and pressure is applied to the surfaces of the piezoelectric substrate and the IDT, The frequency characteristic of the surface acoustic wave device changes. Therefore, as in the second aspect, by using the variable DC voltage, the center frequency of the surface acoustic wave device can be set to a desired value. Further, as in the third invention, by changing the voltage stepwise, different frequency characteristics can be given to one surface acoustic wave device.

[Brief description of drawings]

FIG. 1 is a structural diagram of a surface acoustic wave resonator showing a first embodiment of the present invention.

FIG. 2 is a structural diagram showing a conventional surface acoustic wave resonator.

FIG. 3 is a cross-sectional view showing the manufacturing process of FIG.

FIG. 4 is a diagram showing a connection between FIG. 1 and an external DC circuit.

5 is a diagram showing the surface acoustic wave resonator of FIG. 1 mounted on a stem.

FIG. 6 is a cross-sectional view of FIG.

FIG. 7 is a diagram showing frequency characteristics of the surface acoustic wave resonator shown in FIG.

FIG. 8 is a diagram showing frequency characteristics of a surface acoustic wave resonator according to a second embodiment of the present invention.

[Explanation of symbols]

 11 Piezoelectric Substrate 12 Underlay 13 Input Terminal 14 Output Terminal 15 IDT 20 Dielectric 21 Electrode Metal Film 30 Variable Voltage DC Voltage Source 40 Stem B Electrode Metal Film

Claims (3)

[Claims] 1. A surface acoustic wave device comprising a transducer, which is formed in a comb shape on a piezoelectric substrate and excites a surface acoustic wave based on a given high frequency signal, comprising: a dielectric film deposited on the transducer; A surface acoustic wave device comprising: a metal thin film for electrodes formed on the dielectric film. 2. The surface acoustic wave device according to claim 1 is mounted on a metal plate or a metal thin film, and a variable DC voltage is applied between the metal plate or the metal thin film and the metal thin film for electrodes to generate electrostatic force. A method for adjusting frequency characteristics of a surface acoustic wave device, which is characterized in that the surface pressure of the piezoelectric substrate is continuously controlled. 3. The surface acoustic wave device according to claim 1 mounted on a metal plate or a metal thin film, and a voltage selected from a plurality of voltage values between the metal plate or the metal thin film and the metal thin film for electrodes. A frequency characteristic adjusting method for a surface acoustic wave device, comprising connecting a voltage source for outputting a value and generating an electrostatic force based on the selected voltage value to control the surface pressure of the piezoelectric substrate stepwise. .