JPH10190407A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave element provided with a small temperature coefficient, a relatively large electromechanical coupling coefficient and a chemically stable substrate material by forming a metallic electrode film in a propagation direction on the specified cut surface of single crystal Langasite. SOLUTION: When a segmenting angle from the single crystal Langasite (La3 Ga5 SiO14 ) and a surface acoustic wave propagation direction are (180 deg.+α, 40 deg.+β, 20 deg.+γ) by Eulerian angle display, a Langasite substrate 10 is formed by '=-2 deg.-+6 deg., β=-4 deg.-+9 deg. and γ=-1 deg.-+9 deg. or a direction equivalent to it. An electrode 20 for excitation is formed on one end of the substrate and the electrode 30 for reception is formed on the other end by a photolithography method. For the thickness (h) of a film, at the time of defining the wavelength of surface acoustic waves as λ, the range of h/λ=0.005 to 0.2 is suitable. For a delay time temperature coefficient, TCF=3ppm/ deg.C is obtained at 25 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter for frequency selection used in communication equipment and the like, and a surface acoustic wave element used as an element such as a resonator used in a high stability oscillator. .

[0002]

2. Description of the Related Art Conventionally, as substrates for surface acoustic wave devices, generally, lithium niobate, lithium tantalate, lithium tetraborate (for example, see Japanese Patent Application Laid-Open No. Sho 60-4131).
Japanese Unexamined Patent Application Publication No. 5 (1999)), a substrate obtained by cutting and polishing a piezoelectric single crystal such as a quartz crystal at an appropriate cut surface is used.

[0003]

The important characteristics of a piezoelectric substrate used for a surface acoustic wave device include a delay time temperature coefficient (TCD) and an electromechanical coupling coefficient (K ² ).
TCD is closer to zero, and as the K ² is large, preferably as a substrate for a surface acoustic wave device. Conventionally, as a substrate for a surface acoustic wave device, lithium niobate (Li) is generally used.
A substrate obtained by cutting and polishing a piezoelectric single crystal such as NbO ₃ ), lithium tantalate (LiTaO ₃ ), lithium tetraborate (Li ₂ B ₄ O ₇ ), or quartz at an appropriate cut surface has been used. LiNbO ₃ (for example, 128 ° Y cut-
X-propagation), although K ² are as large as 5.5%, TCD is 74
Because of the large ppm / ° C., a frequency drift occurs due to a temperature change, and it cannot be used for a filter that requires narrow band characteristics or an oscillator that requires high stability accuracy. In Li ₂ B ₄ O ₇ (for example, 45 ° X cut-Z propagation), K ² is 1% and TCD is 0 pp.
Although the temperature is m / ° C., the substrate is dissolved in water and has deliquescence, so that the process is difficult or the reliability is poor. Moreover, ST-cut quartz is has a zero TCD, since K ² is smaller and 0.1%, it is impossible to obtain a filter broadband.

[0004] In response to demands for higher performance and higher frequencies for surface acoustic wave devices, substrate materials having a TCD close to zero, a larger K ² , and being chemically stable have been required.
In view of the above circumstances, an object of the present invention is to provide a surface acoustic wave device having a small TCD and a relatively large K ² and having a chemically stable substrate material.

[0005]

The surface acoustic wave device of the present invention which achieves the above object is a langasite (La ₃ Ga ₅ S).
iO ₁₄ ) In a surface acoustic wave device in which a metal film for exciting, receiving, or reflecting surface acoustic waves is formed on a single crystal substrate, the cutout angle and surface acoustic wave propagation direction from the single crystal of the langasite substrate are as follows: In the oiler angle display, (18
0 ° + α, 40 ° + β, 20 ° + γ), α =
-2 ° to + 6 °, β = -4 ° to + 9 °, γ = -1 ° to +
The azimuth is 9 ° or an equivalent direction.

The present inventors have proposed a surface acoustic wave device in which a metal film is formed in a propagation direction on a specific cut plane of langasite (La ₃ Ga ₅ SiO ₁₄ ), which is a single crystal having piezoelectricity, with a small temperature coefficient. Have a relatively large electromechanical coupling coefficient and are chemically stable, leading to the present invention.

[0007]

FIG. 1 is a schematic diagram of a transmission type filter. When a pair of comb-shaped electrodes are formed on the langasite single-crystal substrate 10, and a high-frequency voltage is applied to one of them (the excitation electrode 20), the surface acoustic wave is excited and reaches the reception electrode 30. The frequency characteristic between the input and output has a hand-pass filter characteristic, and the temperature characteristic of the filter is determined by the characteristic of the piezoelectric crystal.

As constants required for calculating the characteristics of surface acoustic waves on a langasite single crystal, 20 ° C.
, The density, the elastic constant, the piezoelectric constant, the dielectric constant and the linear expansion coefficient are shown. Table 2 shows the first and second order of each constant.
The following temperature coefficients are given:

[0009]

[Table 1]

[0010]

[Table 2]

Here, by using the constants shown in Tables 1 and 2, a Newton's equation of motion, a piezoelectric equation, and a Maxwell's equation approximated by quasi-electrostatic on the surface of the piezoelectric substrate are coupled and solved. , TCF (TCF = −TCD) is 5 ppm or less, and the range where the electromechanical coupling coefficient K ² becomes 0.3% or more is expressed as (180 ° + α, 40 ° + β, 20 ° + γ), α = −2 ° to + 6 °, β = −4 ° to + 9 °, γ =
It has been found that an orientation from -1 ° to + 9 ° or an equivalent direction is optimal (see FIGS. 2 to 4).

(TCF = (1 / V) (∂V / ∂T) -γ V: Sound velocity of surface acoustic wave at temperature T = 25 ° C. T: Temperature γ: Cut plane considered, thermal expansion in propagation direction Here, assuming that the wavelength of the surface acoustic wave is λ, the thickness h of the electrode material is preferably in the range of h / λ = 0.005 to 0.2. Aluminum is suitable as an electrode material, and gold or aluminum + titanium or aluminum + copper is also suitable as other materials.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The pattern of the transmission type surface acoustic wave filter shown in the schematic diagram of FIG.
°, 20 °) in a cut plane and a propagation direction by a photolithography process. In this case, the line width and the line interval of the comb-shaped electrodes are each 4 μm, the number of pairs of input / output electrodes is 30, and the opening length of the comb-shaped electrodes is 400 μm. The thickness of the aluminum film was set to 2,400 angstroms (h / λ = 1.5%) by a sputtering method. The device was mounted on a metal package, and the transmitting and receiving comb electrodes were taken out by wire bonding and connected to a network analyzer. Then put this device in a thermostat,
When the change of the center frequency in the temperature range of −20 ° C. to 80 ° C. was measured, the characteristics shown in FIG. 5 were obtained, and it was confirmed that TCF at 25 ° C. = 3 ppm / ° C.

[0014]

As described above, according to the present invention,
A surface acoustic wave device having a small delay time temperature coefficient and a relatively large electromechanical coupling coefficient and having a chemically stable substrate material can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a transmission filter.

FIG. 2 is a graph showing changes in K ² and TCD on a (175 ° to 185 °, 40 °, 20 °) cut surface.

FIG. 3 is a graph showing changes in K ² and TCD on a (180 °, 35 ° to 45 °, 20 °) cut surface.

FIG. 4 is a graph showing changes in K ² and TCD on a (180 °, 40 °, 15 ° to 25 °) cut surface.

FIG. 5 is a graph showing the temperature dependence of the center frequency.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Langasite board 20 Excitation electrode 30 Receiving electrode

Claims (1)

[Claims] 1. Langasite (La ₃ Ga ₅ SiO ₁₄ )
In a surface acoustic wave device in which a metal film for exciting, receiving, or reflecting surface acoustic waves is formed on a single crystal substrate,
The cutout angle and the direction of surface acoustic wave propagation from the single crystal of the langasite substrate are expressed by Euler angles as (180 ° +
α, −40 ° + β, 20 ° + γ), α = −2 °
~ + 6 °, β = -4 ° to + 9 °, γ = -1 ° to + 9 °,
Alternatively, a surface acoustic wave device having an equivalent direction.