JPH0748626B2 - Frequency adjustment method for surface acoustic wave device - Google Patents

Description

The present invention relates to a method for adjusting the oscillation frequency of a surface acoustic wave device, and particularly for the purpose of facilitating the frequency adjustment of a high-frequency device and improving the accuracy thereof. 36 around the axis in the Z-axis direction
In a surface acoustic wave device in which a transducer and a silicon dioxide film having a refractive index of 1.46 ± 0.01 are provided on the surface of a substrate cut out from a Y plate rotated by a degree, a tantalum pentoxide film is laminated on the silicon dioxide film. At the very least, the tantalum pentoxide film is laminated in such a thickness that the surface acoustic wave frequency of the substrate becomes a predetermined value.

[Industrial application field]

The present invention relates to a surface acoustic wave device used for signal processing and the like in the VHF and UHF frequency bands, and particularly to a frequency adjusting method thereof.

With the recent increase in the speed of information processing equipment and communication equipment, the frequencies used in these devices are shifting to high frequencies. In addition, it is necessary to generate and filter a reference signal at a high frequency.

The surface acoustic wave element is used for these purposes, and in the above-mentioned equipment that requires high frequency accuracy, high accuracy and stabilization of the element are desired.

The lithium tantalate (LiTaO ₃ ) has a relatively high temperature coefficient despite its large coupling coefficient in terms of insertion loss, specific bandwidth, and frequency variable width as VCO using a surface acoustic wave element. Substrate, especially Y-plate (36) rotated in the Z-axis direction about the X-axis of the lithium tantalate single crystal by 36 degrees
A device using a substrate cut out from (Y-XLiTaO ₃ plate) is extremely useful, and silicon dioxide (SiO ₂ ) is laminated on the 36-Y-XLiTaO ₃ plate on which the transducer is formed to obtain temperature characteristics. Attempts have been made to improve.

In such a surface acoustic wave element, the frequency varies depending on the film thickness and the electrode width of the electrode, the change in the film thickness of SiO ₂ , and the like, so that the frequency adjustment is required.

[Conventional technology]

FIG. 4 is a plan view showing the schematic structure of the surface acoustic wave device (a).
And its cross-sectional view (b) and Fig. 5 (a) and (b) are 36 ° Y-
XLiTaO ₃ is an explanatory view of a plate.

In FIG. 4, a surface acoustic wave device 2 obtained by cutting a substrate 3 from a Y plate (wafer) 1 rotated about the X axis of a lithium tantalate single crystal by 36 degrees in the Z axis direction as shown in FIG. A transducer, that is, a drive electrode 4 and a pair of reflective electrodes 5 sandwiching the drive electrode 4 are formed on the upper surface of the substrate 3.

Conventionally, such a frequency adjusting method for the element 2 has been a method in which the electrodes 4 and 5 are thinned by etching, a part of the electrodes 4 and 5 is trimmed, or between the electrodes of the substrate 3 is etched.

Then, in order to improve the temperature characteristics of the surface acoustic wave element 2,
4 (b) to the surface acoustic wave device formed of the SiO ₂ film 6 as indicated by the dashed line, i.e. SiO ₂ with a refractive index of 1.46 ± 0.01 film 6
In the surface acoustic wave elements formed a is irradiated with ultraviolet rays SiO ₂
A frequency adjustment method for modifying the membrane 6 has been reported.

[Problems to be Solved by the Invention]

As described above, the conventional frequency adjusting method for etching the electrodes or the piezoelectric substrate between the electrodes becomes difficult when the electrode pattern forming the transducer is miniaturized, and is efficient for a high-performance predetermined frequency. Moreover, there is a problem that the adjustment cannot be performed with high accuracy.

Furthermore, in the temperature characteristic improving element with the SiO ₂ film deposited,
The conventional method of irradiating the SiO ₂ film with ultraviolet rays has a problem that the temperature characteristic changes simultaneously with the adjustment of the frequency.

[Means for Solving the Problems]

According to FIG. 1 showing an embodiment, the method of the present invention aimed at facilitating the frequency adjustment of the surface wave is shown in FIG. 1 and shows that the lithium tantalate single crystal is rotated around the X axis in the Z axis direction.
On the surface of the substrate 3 cut out from the Y plate rotated once, the transducer (driving electrode 4 and reflecting electrode 5) and the refractive index
In a surface acoustic wave element 11 provided with a silicon dioxide film 6 of 1.46 ± 0.01, a tantalum pentoxide film 12 is laminated on the silicon dioxide film 6 so that the frequency of the surface acoustic wave of the substrate 3 becomes a predetermined value. Is to stack the tantalum pentoxide film 12.

[Action]

The sound velocity propagating through the 36 ° Y-XLiTaO ₃ plate is v ₁ , and the refractive index is 1.46.
When the velocity of sound propagating in a silicon dioxide film of ± 0.01 is v ₂ , and the velocity of sound propagating in a silicon pentoxide film having a specific gravity larger than that of silicon dioxide deposited by the electron beam evaporation method is v ₃ ,
The relationship of v ₁ > v ₂ > v ₃ is established between these sound velocities.

Therefore, when a transducer is formed on the surface of a substrate cut out from a 36 ° Y-XLiTaO ₃ plate and a silicon dioxide film for improving temperature characteristics is formed thereon, an appropriate current is applied to the drive electrode of the transducer to generate The velocity of the surface acoustic wave generated becomes slower than that of the case where the silicon dioxide film is not formed, and the acoustic velocity of the surface acoustic wave of the element in which the silicon pentoxide film is formed on the silicon dioxide film becomes further slower.

Therefore, after improving the temperature characteristics and forming the silicon dioxide film so that the oscillation frequency becomes higher than the desired value,
In the surface acoustic wave device in which a silicon pentoxide film having an appropriate thickness is laminated on a silicon dioxide film, the oscillation frequency can be adjusted to a desired value without damaging the temperature characteristics by selecting the thickness of the silicon pentoxide film.

〔Example〕

The frequency adjusting method of the surface acoustic wave device according to the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of a surface acoustic wave device according to an embodiment of the present invention, FIG. 2 is a diagram showing the relationship between the thickness of the tantalum pentoxide film and the rate of change of oscillation frequency, and FIG. It is a figure which shows the relationship between the thickness of the silicon oxide film laminated | stacked instead of the tantalum pentoxide film of FIG. 1, and an oscillation frequency.

In FIG. 1, the surface acoustic wave element 11 is 36 ° Y-XLiTaO.
_The drive electrode 4 is formed at the center of the upper surface of the substrate 3 cut out from the _three plates, and the reflective electrode 5 is provided at the left and right positions of the drive electrode 4.
Is formed, and a silicon dioxide (SiO ₂ ) film 6 having a refractive index of 1.46 ± 0.01 is laminated thereon by plasma CVD, and then tantalum pentoxide (Ta ₂ O 5) having an appropriate thickness is formed for frequency adjustment.
₅ ) The film 12 is laminated on the SiO ₂ film 6 by the electron beam evaporation method. However, the thickness of the SiO ₂ film 6 deposited for improving the temperature characteristics of the element 11 is 3.75 μm in this embodiment.

In FIG. 2, the vertical axis represents the rate of change in the oscillation frequency of the surface acoustic wave device, and the horizontal axis represents Ta ₂ O laminated on the SiO ₂ film of the surface acoustic wave device.
_The surface acoustic wave device 11 having a thickness of ₅ films (Å) and a Ta ₂ O ₅ film 12 having a thickness of 50 Å and an oscillation frequency f ₀ = 175.75 MHz or 176.04 MHz has a frequency change rate of −0.29. %, While a 100 Å-thick Ta ₂ O ₅ film 12 was applied and the oscillation frequency f ₀ = 17.
The surface acoustic wave device 11 having 5.98 MHz or 175.76 MHz has a frequency change rate of about −0.58%.

The Ta ₂ O ₅ film 12 of SiO ₂ film 6 surface acoustic wave element 11 laminated on the frequency by allowed to laminating example the Ta ₂ O ₅ film 12 0.1
The element 11 adjusted by% has a frequency change rate of about ± 250 ppm when it is cooled or heated to ± 25 ° C., that is, when it is cooled to 0 ° C. or heated to 50 ° C. with reference to the frequency at 25 ° C.

In FIG. 3, the vertical axis is the oscillation frequency (MHz) of the surface acoustic wave device, and the horizontal axis is the thickness (Å) of the silicon oxide film (SiO _x ) laminated on the SiO ₂ film of the surface acoustic wave device. 3.75μ
SiO ₂ film 6 is formed oscillation frequency surface acoustic wave device 178.0MHz of m, the oscillation frequency when forming the SiO _x film having a thickness of 2000Å on the SiO ₂ film 6 is approximately 1.36MHz (0.76%) only Will start to decline.

Therefore, by forming a SiO _x film, the oscillation frequency is increased to 17
It can be seen that a SiO _x film having a thickness of 1500 Å can be laminated to adjust the oscillation frequency of the surface acoustic wave device which was 8.0 MHz to 177 MHz, that is, to reduce the oscillation frequency by 0.56%.

Therefore, the temperature characteristics of the surface acoustic wave element A in which the Ta ₂ O ₅ film 12 is laminated and the frequency is adjusted are compared with the temperature characteristics of the surface acoustic wave element B in which the SiO ₂ film is laminated instead of the Ta ₂ O ₅ film 12. The measurement was performed.

The frequency characteristics of the elements A and B were 25% when the frequency adjustment amount was 0.1% by stacking the Ta ₂ O ₅ film 12 or the SiO ₂ film that was replaced with the Ta ₂ O ₅ film 12.
℃ is the standard temperature of the measurement, -25 ℃ to the standard temperature or 0
The frequency change rate at + 25 ° C. or + 25 ° C., that is, 50 ° C. was measured.

According to the measurement results, the frequency change rates of the elements A and B at 50 ° C. are both 30 ppm, and the element A at 0 ° C.
The frequency change rate of element B is about 50 ppm, whereas the frequency change rate of element B at 0 ° C. is about 120 ppm, demonstrating that element A is superior to element B.

When the amount of change in temperature is increased in the frequency characteristics of the elements A and B, the difference between the elements A and B hardly changes with respect to the + side change, but the difference between the elements A and B with respect to the − side change. The superiority and inferiority due to will expand.

〔The invention's effect〕

As described above, according to the method of the present invention, Ta ₂ O ₅ can be used without touching the transducer (electrode) and the substrate.
When the film is laminated to have an appropriate thickness, for example, the Ta ₂ O ₅ film is laminated so that the oscillation frequency becomes a predetermined value while measuring the oscillation frequency, or when the SiO ₂ film for improving the temperature characteristic is formed. From the measured oscillation frequency to the specified thickness set
By depositing the Ta ₂ O ₅ film, the frequency of the surface wave can be adjusted, and in particular, the productivity of the high frequency element can be improved and an element having excellent frequency characteristics can be provided.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a surface acoustic wave device according to an embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the thickness of the tantalum pentoxide film in FIG. 1 and the rate of change of oscillation frequency. The figure which shows the relationship between the thickness and the oscillation frequency of the silicon oxide film laminated instead of the tantalum pentoxide film of the figure. The figure 4 shows the outline of the surface acoustic wave element. The figure 5 shows the 36 ° Y-XLiTaO ₃ plate. It is an explanatory diagram. In the figure, 1 is a 36 ° Y-XLiTaO ₃ plate 3, 3 is an element substrate, 4,5 is an electrode 6, silicon dioxide film 11 is a surface acoustic wave element 12 is a tantalum pentoxide film.

Continuation of the front page (56) References JP-A-58-33310 (JP, A) JP-A-61-77407 (JP, A) JP-B-55-5924 (JP, B2)

Claims (1)

[Claims] 1. A transducer (4 and 5) having a refractive index of 1.46 ± on a surface of a substrate (3) cut out from a Y plate rotated around the X axis of a lithium tantalate single crystal by 36 degrees in the Z axis direction. In a surface acoustic wave element provided with a silicon dioxide film (6) of 0.01, a tantalum pentoxide film (1) is formed on the silicon dioxide film (6).
2) is laminated, and the tantalum pentoxide film (12) is laminated to a thickness such that the frequency of the surface acoustic wave of the substrate (3) becomes a predetermined value, and a method for adjusting the frequency of a surface acoustic wave element. .