JPS594309A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To improve the temperature stability, by setting the propagating direction of a surface acoustic wave to the azimuth having a sufficiently small delay time temperature coefficient. CONSTITUTION:A lithium borate single crystal substrate is cut in the azimuth of (90+lambda, 90+mu, 90+theta) in the Euler angle representation by changing (lambda, mu, theta). The values of (lambda, mu, theta) are set in the range of Equations 1 and 2 in the representation of an elliptic function. An oscillator is formed by mirror-polishing the substrate for forming electrodes. The delay time temperature coefficient TCD of the oscillator like this is sufficiently small and the surface acoustic wave device having a stable temperature characteristic is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a surface acoustic wave device using a lithium borate single crystal substrate having a small delay time temperature coefficient of surface acoustic waves.

[Technical background of the invention and its problems]

Surface acoustic wave devices are widely applied to various filters, resonators, etc. because a 4m type electrode is provided on a piezoelectric substrate, and arbitrary filter characteristics can be obtained by changing the shape of the surface acoustic wave device.

The requirements for these applications include a large electromechanical coupling coefficient of 2, which indicates the efficiency of conversion into surface acoustic wave energy, and a delay time temperature coefficient TC, which indicates temperature stability.
It is desired that D be small, particularly within 10 ppm/'C for resonator applications. Here, the delay time refers to the time required for a surface acoustic wave to propagate between the input comb-shaped electrode and the output S-shaped electrode provided on the substrate, and the rate at which this changes depending on temperature is called the delay time temperature coefficient TCD. In general surface acoustic wave materials, TCD is several tens to hundreds of ppm/'
It has a value of C. For example, a Y-cut -2 propagation (Y-Z) LIN with a delay time temperature coefficient of -94 pprrv"'O
When applying the bO, surface acoustic wave substrate to a filter with a center frequency of 100 MHz, the temperature change in the center frequency is 3760 ppm, and the frequency width is 0.376 MHz, assuming the operating temperature range is 0 to 40°C. Therefore, it has the disadvantage that it is difficult to apply to filters and resonators whose frequency change width is narrower than this. When used as a narrower filter,
The ambient temperature is higher than room temperature, for example in the range of 25°C to 60'O,
It is used by controlling the heating to a constant temperature in a surrounding area to reduce temperature changes. For these reasons as well, it is desirable that the change in TCD be small.

By the way, lithium borate single crystal (for example L12B4O,
) is a piezoelectric crystal belonging to the cubic fish group 4m and has a melting point of 917°C. It has been reported that this crystal is produced by a pulling method and used as a surface acoustic wave substrate, and has a zero TCD with an (x-Z) substrate and a large coupling coefficient K. However, the results of experiments actually conducted by the present inventors are Z-X"2B4
In the 07 single crystal substrate, TCD zero did not exist at room temperature.

[Purpose of the invention]

An object of the present invention is to provide a surface acoustic wave device using a lithium borate single crystal substrate in which the propagation direction of surface acoustic waves is set in a direction with a sufficiently small TCD.

[Summary of the invention]

The present invention uses a lithium borate single crystal substrate to express the cutting direction and propagation direction of surface acoustic waves in Euler angle notation.

〔Effect of the invention〕

According to the present invention, a surface acoustic wave device is developed which has an electromechanical coupling coefficient #2 which is sufficiently large compared to quartz crystal, etc., and which has excellent temperature stability with a TCD of within ±10 ppm/°C. Therefore, the present invention is extremely useful for resonator applications and narrowband filter applications.

[Embodiments of the invention]

Before explaining the present invention in detail, a general substrate display method using Euler angle display will be explained using FIG.

The surface wave propagation direction is X1m, and the direction perpendicular to the crystal substrate surface is Xl.
, the direction perpendicular to them is X, and the reference orientation (o, o,
o) Assume that xl =x, x, =y.

Take X3=z. First, the surface wave propagation direction X1 is rotated by λ from the X direction toward the Y direction around the Rotated by μ and finally rotated Xl
The representation of the surface acoustic wave propagation direction including the substrate surface orientation obtained by rotating the propagation direction cormorant.

The present invention will be described in detail below using such Euler angle representation. The Ll, B4O, single crystal produced by the pulling method is expressed in Euler angle (90+λ).

It was cut in the directions of 90+μ, 90+θ) while changing λ, μ, and θ, and mirror-polished. Here, (90, 90, 90) in Euler angle representation means X-cut Z propagation. An AA film was formed on the mirror surface by vacuum evaporation, and regular comb-shaped electrodes having 14 pairs of electrodes were formed by photoetching. The electrode period is 82 μm. Delay time temperature coefficient TCD
The measurement was carried out by configuring an oscillator as shown in FIG. That is, 2 on the surface acoustic wave substrate 2 is input to the high frequency amplifier 10.
One of the regular comb-shaped electrodes 31 of the set is connected, and the other electrode 31 is connected to the output side of the high frequency amplifier l. In this case, from one comb-shaped electrode 35 to the other comb-shaped electrode 3. The surface waves are propagated to the amplifier 1. In the amplifier 1, the surface wave signal is amplified and returned to one of the comb-shaped electrodes 31, resulting in positive feedback, so that it oscillates at a frequency corresponding to the period of the regular comb-shaped electrode. Therefore, a part of this output signal is extracted and read by a frequency counter. From the oscillation frequency fo, the following TCD was determined.

Here, the oscillation frequency (Hz) Δf when fea is 25°C:
Temperature change in oscillation frequency ΔT: Temperature change in the surface acoustic wave substrate (°C) Figure 3 (,)'', (b) shows the state in which the orientation of the surface acoustic wave substrate is (90+λ, 90,90) in Euler angle representation. The temperature coefficient of delay time TCD of the surface wave at that time is shown. This means that when λ = 0, it is X-2 propagation, and increasing λ is 2
The propagation is constant and the cut plane rotates from the X axis to the Y axis. As you can see, in the X-Z propagation, T
CD is 11 ppm/'0. As λ increases, TCD increases, and TCD becomes zero when λ is 20°. To achieve within ±10 ppm/°C, the range of λ is from 2° to 386°.

FIGS. 4(-) and 4(b) show the state of (90°90+μ, 90) in Euler angle representation and the temperature coefficient of delay time TCD of the surface wave at that time. This is 647 due to X-Z propagation when μ=O, and increasing μ means defeating the cut plane and the propagation direction. When μ increases, TCD becomes -11ppm/'O
It increases from then on and becomes zero at 15°. ±10 ppm/'o
The value of μ must be within the range from 20' to a O) Figures 5 (&) and (b) are shown in Euler angles (9
0°90.9010) and the surface wave delay time temperature coefficient TCD at that time. This means that when θ=0,
Increasing θ in the force and toz propagation means that the cut plane is the X plane, and the propagation direction of the surface wave is rotated from zIIllI in the X plane. When θ is increased, TCD increases from -1 lppm/°O and becomes zero at 6°.The values of θ that fall within ±10 ppm/'C are from 1° to 11'.

In the above explanation, two of λ, μ, and θ are set to 0, and one of the remaining values is changed, but the isolateral surface becomes an ellipse with respect to λ and μ as shown in FIG. Therefore λ, μ
It can be seen that the range of and θ is preferably between the ellipse amount. This is shown in FIG. Furthermore, although the explanation has been given above assuming that all angles are in the positive direction, the same effect can be obtained when the angles are all in the negative direction.

In addition, in actual manufacturing of the board, it is preferable to pull the crystal along the X axis and cut it out in a direction perpendicular to the pulling direction for ease of operation.From this point of view, Then, in the above-mentioned ellipse representation, it is preferable to select a range on one of the axes λ, μ, and θ.

[Brief explanation of the drawing]

Figure 1 is a diagram to explain the general Euler angle display.
Figure 2 shows the configuration of an oscillator using a surface acoustic wave device. Figure 3 (9) and (b) show the (90+λ.90.90) force, the orientation of the L12B4O7 single crystal substrate, and the delay time temperature. Figures 4 (^) and (b) showing the characteristics are (90, 90 μ, 90) cut L12B4O. Figures 5(a) and 5(b) show the orientation and delay time temperature characteristics of a single crystal substrate. Ll 2B40. A diagram showing the orientation and delay time temperature characteristics of a single crystal substrate, FIG. 6 is a diagram showing a curve of equal delay time temperature coefficients with respect to λ and μ, and FIG. 7 is a diagram showing curves of equal delay time temperature coefficients with respect to λ and μ. FIG. 3 is a diagram showing a curved surface of an equal delay time temperature coefficient with respect to θ. 1... High frequency amplifier, 2... Surface acoustic wave substrate, 31
r32...Regular comb-shaped electrode. Applicant's agent Patent attorney Takehiko Suzue (a) Figure 5 θ (α white diagram)

Claims (4)

[Claims] (1) A surface acoustic wave device using a lithium-lamb borate single crystal substrate, characterized in that the propagation direction of the surface acoustic wave is expressed in Euler angles (90+λ, 90+μ, 90+θ) and is set as a box. Device. (2) μ=θ basin O, λ=2 to 38 or λ=
The surface acoustic wave device according to claim 1, wherein the propagation direction is set in the range of -2 to -38. (3) As μ=λ=0, θ=1 to 11 or θ=
The surface acoustic wave device according to claim 1, wherein the propagation direction is set in the range of -1 to -11. (4) θ; Assuming λ=0, μ=3 to 20 or μ−
The surface acoustic wave device according to claim 1, wherein the propagation direction is set in the range of -3 to -20.