JP3364037B2 - Surface acoustic wave device - Google Patents

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 using a lithium tetraborate single crystal (Li ₂ B ₄ O ₇ ).

[0002]

2. Description of the Related Art A surface acoustic wave device is a circuit element that performs signal processing by converting an electric signal into a surface wave, and is used for a filter, a resonator, a delay line and the like. Usually, a metal electrode called an interdigital transducer (IDT, comb-shaped electrode, interdigital transducer) is provided on an elastic substrate (piezoelectric substrate) having piezoelectricity to convert an electric signal into a surface wave and back conversion. There is. The characteristic of the surface acoustic wave device depends on the propagation characteristic of the surface acoustic wave propagating through the piezoelectric substrate. In particular, in order to cope with the high frequency of the surface acoustic wave device, a piezoelectric substrate having a high surface acoustic wave propagation speed is required.

As a substrate material used in surface acoustic wave devices
For crystal, lithium tantalate (LiTaO₃), Ni
Lithium obate (LiNbO₃), Lithium tetraborate
(Li ₂B_FourO₇) Etc. are known. Also an elastic surface
Rayleigh waves are used as surface acoustic waves used in wave devices.
(Rayleigh Wave) and leaky waves (Le
aky Wave, pseudo surface acoustic wave, leaky surface acoustic wave)
Is mainly known.

The Rayleigh wave is a surface wave propagating on the surface of an elastic body, and propagates without its energy being diffused into the piezoelectric substrate, that is, theoretically without propagation loss. As a substrate material used in a surface acoustic wave device using Rayleigh waves, an ST-cut quartz crystal having a propagation velocity of 3100 m / sec, an X-112 ° Y LiTaO film having a propagation velocity of 3300 m / sec.
₃ , 4000m / sec 128 ° Y-X LiNbO
₃ , 3400 m / sec 45 ° X-Z Li ₂ B ₄ O
There is ₇ .

On the other hand, utilization of a surface acoustic wave (leakage surface acoustic wave), which is called a leaky wave and propagates while radiating energy in the depth direction of the elastic body, has been studied. The leaky wave propagates faster than the Rayleigh wave. Generally, leaky waves have large propagation loss due to radiation and cannot be used in surface acoustic wave devices, but they can be used because the propagation loss is relatively small at a particular cutting angle and propagation direction. For example, LS with a propagation velocity of 3900 m / sec
T-cut crystal, 4200 m / sec 36 ° Y-X L
iTaO ₃ , 4500 m / sec 41 ° Y-X Li
NbO ₃ , 4500 m / sec 64 ° Y-X LiN
bO _{3 and the} like are known.

Further, the longitudinal leaky wave is accompanied by a propagation loss like the ordinary leaky wave. However, the bulk wave propagating in the same direction propagates on the surface of the piezoelectric substrate at a speed between a transverse wave and a longitudinal wave, and therefore has a characteristic that the propagation speed is higher than that of a normal leaky wave. The extraction angle and the propagation direction of lithium tetraborate are Euler angles (0 ° to 45 °).
, 44 ° to 50 °, 80 ° to 90 °) and their equivalent range, it is known that surface acoustic waves of high propagation velocity and low propagation loss exist on the free surface and the metal surface. . This longitudinal wave type leaky wave is 6000-7200m
Since it is a very high speed of / sec, it is expected to realize a high frequency surface acoustic wave device (Japanese Patent Laid-Open No. 06-112763, European Patent Application Publication No. 560634, Soviet Physics Crystallogr).
aphy, vol.37, No.2, pp.220-223, 1992).

[0007]

At least one IDT (interdigital transducer) is required for use as a surface acoustic wave device. As the IDT, a double electrode IDT and a single electrode IDT are generally widely used. In the double electrode IDT, the polarities of two electrode fingers are the same, and the electrode fingers are arranged in the same cycle. Since the multiple reflection that occurs inside the IDT can be suppressed, the frequency response is less likely to be distorted.

On the other hand, the single electrode IDT is arranged in the same cycle so that electrode fingers having different polarities are interleaved with each other, and when processing signals of the same frequency, it is twice as long as the electrode cycle of the double electrode IDT. Can be Therefore, it is easy to form an electrode on the piezoelectric substrate, and the single electrode IDT is used in the surface acoustic wave device for high frequencies.

By the way, when electrodes such as IDTs are formed on a piezoelectric substrate, it is expected that the propagation characteristics of surface acoustic waves will change due to the effect of mass loading of the electrodes. Therefore, there should be an optimum electrode finger film thickness and electrode finger width of the IDT in order to obtain good propagation characteristics of surface acoustic waves. For example,
A lithium tetraborate single crystal is used as the piezoelectric substrate, and the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are 0 ° to 45 °, 44 ° to 50 °, 80 ° in the Euler angle display.
When the surface acoustic wave is a longitudinal wave type leaky wave, the propagation loss is relatively small in the aluminum film propagation path in which the aluminum film is formed on the entire surface when the normalized film thickness is about 3%. Become. However, in the aluminum double electrode IDT, the propagation loss is very large when the normalized film thickness is about 3%, and when the normalized film thickness is about 2%,
Propagation loss is the smallest.

It is known that the propagation characteristics differ depending on the electrode structure formed on the piezoelectric substrate as described above (1994 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, SA-11-).
8). However, the Euler angle display (0 ° to 45
(°, 45 ° to 50 °, 80 ° to 90 °), regarding the structure such as the optimum electrode finger thickness and electrode finger width of the single electrode IDT that gives good propagation characteristics of surface acoustic waves, No knowledge was obtained.

An object of the present invention is to show the extraction angle and the propagation direction of lithium tetraborate in the Euler angle display (0 ° to 45 °).
, 44 ° to 50 °, 80 ° to 90 °) and its equivalent range, and an elastic surface that forms an IDT within a range equivalent to it and is faster than a fast transverse wave of a bulk wave propagating in the same direction and does not exceed a longitudinal wave. An object of the present invention is to provide an optimum electrode structure of a single electrode IDT that exhibits good propagation characteristics for a surface acoustic wave device that utilizes waves.

[0012]

In order to achieve the above object, a surface acoustic wave device according to the present invention comprises a piezoelectric substrate made of lithium tetraborate single crystal and a surface acoustic wave formed on the surface of the piezoelectric substrate. In a surface acoustic wave device having an electrode for exciting, receiving, reflecting, and propagating, the electrode is
The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and the cut-out angle and the elastic surface of the surface of the piezoelectric substrate. The wave propagation direction is formed so as to be in the Euler angle display (0 ° to 45 °, 44 ° to 50 °, 80 ° to 90 °) and in a range equivalent thereto, and the period of the electrode fingers is P, The normalized film thickness h / λ of the electrode obtained by normalizing the film thickness h of the electrode with the wavelength λ of the surface acoustic wave, where M is the width of the electrode finger, is expressed by the following equation: 0.029-0.097 × (M /P)+0.082×(M/P) ² ≦ (h / λ) ≦ 0.033−0.059 × (M / P) + 0.050 × (M / P) ² The velocity of surface acoustic waves is higher than the velocity of fast transverse waves of bulk waves propagating in the same direction, and does not exceed the velocity of longitudinal waves. And wherein the door.

In order to achieve the above object, a surface acoustic wave device according to the present invention is a piezoelectric substrate made of lithium tetraborate single crystal, and is formed on the surface of the piezoelectric substrate to excite, receive, and receive surface acoustic waves. In a surface acoustic wave device having an electrode for reflecting and propagating, the electrode is formed of a metal containing aluminum as a main component, and the electrode is 1
The piezoelectric element has a pair of comb-shaped electrodes arranged such that electrode fingers of each of the electrodes are interleaved with each other, and the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are indicated by an oiler angle (0 ° to 4 °).
5 °, 44 ° to 50 °, 80 ° to 90 °) and a range equivalent thereto, where P is the period of the electrode fingers and M is the width of the electrode fingers. The normalized film thickness h / λ of the electrode in which h is normalized by the wavelength λ of the surface acoustic wave is expressed by the following equation: 0.006−0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030−0.060 × (M / P) + 0.051 × (M / P) ² within the range, and the velocity of the surface acoustic wave is a bulk wave propagating in the same direction. Is higher than the speed of the fast transverse wave and does not exceed the speed of the longitudinal wave.

Further, the surface acoustic wave device according to the present invention comprises a piezoelectric substrate made of lithium tetraborate single crystal, and a surface acoustic wave formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and reflecting surface acoustic waves.
In a surface acoustic wave device having an electrode for propagating, the electrode is formed of a metal having aluminum as a main component, and the electrode is arranged in such a manner that one electrode finger is inserted into the other. Of the piezoelectric substrate, and the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are (0 ° -40 °, 44 ° -50 °, 8 °).
0 ° to 90 °) and an equivalent range thereof, the period of the electrode fingers is P, and the width of the electrode fingers is M.
The normalized film thickness h / λ of the electrode obtained by normalizing the film thickness h of the electrode with the wavelength λ of the surface acoustic wave is expressed by the following equation: 0.029−0.097 × (M / P) +0.082 × (M / P) ² ≦ (h / λ) ≦ 0.033−0.059 × (M / P) + 0.050 × (M / P) ² and the velocity of the surface acoustic wave is It is characterized in that it is higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction and does not exceed the velocity of the longitudinal wave.

In order to achieve the above object, a piezoelectric substrate made of lithium tetraborate single crystal and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves are provided. In the surface acoustic wave device having, the electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes. The cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are Euler angles (0 ° to 40 °, 44 ° to 50 °, 80 ° to
90 °) and its equivalent range, and the thickness h of the electrode is standardized by the wavelength λ of the surface acoustic wave, where P is the period of the electrode finger and M is the width of the electrode finger. The normalized film thickness h / λ of the electrode is 0.006−0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030-0. 0.060 × (M / P) + 0.051 × (M / P) ² and the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction. Characterized by not exceeding speed.

In order to achieve the above object, a piezoelectric substrate made of lithium tetraborate single crystal and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves are provided. In the surface acoustic wave device having, the electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes. The cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are displayed as oiler angles (0 ° to 5 °, 44 ° to 50 °, 80 ° to 9 °).
0 °) and a range equivalent thereto, and the electrode film period h is standardized by the wavelength λ of the surface acoustic wave, where P is the period of the electrode fingers and M is the width of the electrode fingers. The normalized film thickness h / λ of the electrode was 0.029−0.097 × (M / P) + 0.082 × (M / P) ² ≦ (h / λ) ≦ 0.033-0. Within the range of 0.059 × (M / P) + 0.050 × (M / P) ² , the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and the longitudinal wave Characterized by not exceeding speed.

In order to achieve the above object, a piezoelectric substrate made of lithium tetraborate single crystal and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves are provided. In the surface acoustic wave device having, the electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes. The cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are displayed as oiler angles (0 ° to 5 °, 44 ° to 50 °, 80 ° to 9 °).
0 °) and a range equivalent thereto, and the electrode film period h is standardized by the wavelength λ of the surface acoustic wave, where P is the period of the electrode fingers and M is the width of the electrode fingers. The normalized film thickness h / λ of the electrode is 0.006−0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030-0. 0.060 × (M / P) + 0.051 × (M / P) ² and the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction. Characterized by not exceeding speed.

[0018]

The present inventor has found that the extraction angle and the propagation direction of lithium tetraborate are Euler angles (0 ° to 45 °, 44 °).
Surface acoustic wave which forms a single electrode IDT within a range of 50 to 50 degrees and 80 to 90 degrees and its equivalent and has a propagation velocity faster than a fast transverse wave of a bulk wave propagating in the same direction and does not exceed a longitudinal wave. The optimum electrode finger width and electrode finger film thickness of the single electrode IDT were obtained by simulation for the surface acoustic wave device using the.

A simulation was performed using the model shown in FIG. Electrode fingers are formed at a pitch P on the piezoelectric substrate. The width of the electrode finger is M and the film thickness is h. Electrode fingers having different polarities are alternately arranged one by one. Figure 1
In the propagation characteristics of the surface acoustic wave of the IDT of the model shown in (1), the first-order black reflection occurs due to the periodic perturbation effect by the electrode fingers (strips), and frequency dispersion occurs in the propagation constant κ (wave number). First, the frequency dispersion of this propagation constant κ is simulated. The displacement U _{i of the} surface acoustic wave and the electrostatic displacement φ are represented by the sum of the following spatial harmonics using the Floquet theorem.

[0020]

[Equation 1]

[0021]

[Equation 2] Where the damping constant α^{(m, n)}And the amplitude constant β_i ^{(m, n)}Is a biography
The transport constant κ and the angular frequency ω are set, and for each order m, the
Quasi-electrostatic approximation of the dynamic equation and Maxwell's equation
It can be obtained by solving the load equation. Also space
Harmonic amplitude constant A^{(m, n)}Is the following formula (1) and formula (2)
It is obtained by giving the boundary condition of. Elastic boundaries
As a field condition, the displacement U under the strip₁, U₂, U₃
And x₃Stress in direction T_3jAre continuous and between strips
Is x ₃Stress in direction T_3jIs zero, and the electrical boundary condition is
, The electrostatic potential φ is constant under the strip,
X between groups₃Density D in the direction₃Applies continuous.
In the model shown in Fig. 1, when the electrical terminals are opened
(Open strip) has zero total charge on the strip, short circuit
In case of (short-circuit strip), electrostatic potential φ on the strip
Gives zero. From the above simulation,
The propagation constant κ for the wave number ω can be obtained. Na
Note that the spatial harmonic order m is a sufficiently large finite number.
I am simulating.

Generally, when a periodic perturbation by a strip is applied, the condition of black reflection with a first propagation constant κ (Re (κ) = π / P, where P is the strip period length)
A stop band that is a frequency band that satisfies
Let f _1s (bottom) and f _2s (upper end) be frequencies at both ends of the stop band for the short-circuited strip string, and f _1o (bottom end) and f _2o (upper end) be frequencies at both ends of the stop band for the open strip string. Bandwidth (f _{2s
-F 1s), (f 2o -f} 1o) it is proportional to the amount of reflection of one per strip.

Here, either one of the stop band of the short strip strip and the stop band of the open strip strip always coincides with the Rayleigh wave, but does not necessarily coincide with the leaky wave or the longitudinal wave type leaky wave. Indicate values close to each other. The imaginary part component (attenuation component) of the propagation constant κ at the end of the stop band is 0 in the case of a Rayleigh wave in which no propagation loss is calculated (Im (κ) = 0), but it is a leaky wave or a longitudinal wave type. It does not become 0 in case of leaky wave (Im
(Κ) ≠ 0).

From the speed of the portion without the strip (free portion), the speed of the portion with the strip (metal portion), and the frequency of the stop band end in the short-circuit strip row and the open strip row, as a design method of the surface acoustic wave. It is known that the parameters required for the widely used Smith's cross-field model can be extracted (eg, M. Koshiba and S. Mitobe, "Equivalent.
Networks for SAW Gratings ", IEEE Trans. Ultrason.
Ferro. Freq. Contr., 35, pp.531-535, (1988); Inagawa, Koshiba, "Theoretical Derivation of Equivalent Circuit Constants of Surface Acoustic Wave Interdigital Electrodes", Theory of Communications (C), J73-CI , pp.731-737,
(1990)).

As a parameter, the acoustic impedance
Mismatch amount ε (= (Zo / Zm) -1; where Zo is the stroke
Acoustic impedance of the part without lip, Zm is streak
Is the acoustic impedance of the part where
Susceptance component B, which represents the amount accumulated
k²I asked. In addition, the stop band in the dispersion characteristics
Smith's cross with the imaginary part Im (κ) of the edge propagation constant κ
Propagation of the stop band edge obtained from the field model
Constant κ_SmithImaginary part of Im (κ_Smith) Matches
By determining the propagation loss,
Up band edge f _s1, F_s2Propagation loss α_s1, Α
_s2And the stop band end f of the open strip row_o1, F_o2
Propagation loss α_o1, Α_o2And asked. Usually elastic table
The pass band of the surface wave filter is the lower end f of the short-circuit strip row._s1
Occurs in Therefore, the propagation loss α_s1Is a surface acoustic wave fill
It has a great influence on the characteristics of the computer and is particularly important.

In the simulation, the cut-out angle and the propagation direction of the lithium tetraborate single crystal substrate are Euler angles, and (0 ° to 45 °, 44 ° to 50 °, 80 ° to 90 °).
°) and a range equivalent thereto, a single electrode IDT made of aluminum is formed on the substrate, and the velocity of the surface acoustic wave is higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction. In a surface acoustic wave device using a longitudinal leaky wave that does not exceed the velocity, a single electrode I
The purpose of the present invention was to calculate the propagation characteristics (stop band edge frequency, propagation loss, electromechanical coupling coefficient) of the longitudinal wave type leaky when the electrode finger width and electrode finger thickness of DT were changed. The simulation result will be described in detail with reference to FIGS.

2 to 6, the cut-out angle of the lithium tetraborate single crystal substrate and the propagation direction of the surface acoustic wave are (0 °, 47.3 °, 90 °) in Euler angles, that is, (0
11) The propagation direction is perpendicular to the X axis of the plane,
The calculation result of the propagation characteristic of the longitudinal leaky wave in the surface acoustic wave device in which the single electrode IDT containing aluminum as a main component is formed is shown. Here, the frequency is P = 50 cm
It was standardized as (λ = 2P = 1m).

FIG. 2 shows the stop band edge frequency (f _1s ) when the electrode finger width (M / P) standardized by the electrode finger pitch P is 0.3 and the electrode finger film thickness h / λ is changed. , F
_2s , f _1o , f _2o ), the frequency of the free part (f _free ) and the frequency of the metal part (f _metal ) (FIG. 2A),
It is a calculation result of propagation loss (α _1s , α _s2 ) (FIG. 2B) and a calculation result of electromechanical coupling coefficient k ² (FIG. 2C).

The electrode finger film thickness h is set to a value twice the electrode period (λ =
When the normalized film thickness h / λ normalized in 2P) is changed from 0.0% to 3.0%, as shown in FIG. 2A, the stop band width (f _2o −f _1o ),
(F _2s −f _1s ) becomes wider, and as shown in FIG.
The electromechanical coupling coefficient k ² increases from 1.0% to 2.6%, and particularly when the normalized film thickness h / λ is about 0.8% or more, the electromechanical coupling coefficient k ² is 1.6% or more. Become. As shown in FIG. 2B, the propagation loss α _1s increases to 0.05 dB / λ or more when the normalized film thickness h / λ is about 2.5% or more. If it is 2.0% or less, it is as small as 0.02 dB / λ or less.

Therefore, when the electrode finger width M / P is 0.3, the electromechanical coupling coefficient k ² is large and the propagation loss is large when the normalized film thickness h / λ is in the range of 0.8 to 2.0%. Get smaller.
Further, the propagation loss α _1s becomes minimum when the normalized film thickness h / λ is around 1.1%, and the normalized film thickness h / λ is 0.30 to 1.6.
In the range of 4%, it is as small as 0.005 dB / λ or less.

FIG. 3 shows the stop band edge frequency (f _1s ) when the electrode finger width (M / P) standardized by the electrode finger pitch P is 0.4 and the electrode finger film thickness h / λ is changed. , F
_2s , f _1o , f _2o ), the frequency of the free part (f _free ) and the frequency of the metal part (f _metal ) (FIG. 3A),
Propagation loss (α _1s, α _s2) of the calculation result (FIG. 3 (b)), an electromechanical coupling coefficient k ² of the calculation result (Figure 3 (c)).

The electrode finger thickness h is set to a value twice the electrode period (λ =
2P), when the normalized film thickness h / λ is changed from 0.0% to 3.0%, as shown in FIG. 3A, the stop band width (f _2o −f _1o ),
(F _2s −f _1s ) becomes wider, and as shown in FIG.
The electromechanical coupling coefficient k ² increases from 1.2% to 2.8%, and especially when the normalized film thickness h / λ is about 0.3% or more, the electromechanical coupling coefficient k ² is 1.6% or more. Become. As shown in FIG. 3B, the propagation loss α _1s increases to 0.05 dB / λ or more when the normalized film thickness h / λ is about 2.1% or more, but the normalized film thickness h / λ is If it is 1.7% or less, it is as small as 0.02 dB / λ or less.

Therefore, when the electrode finger width M / P is 0.4, the electromechanical coupling coefficient k ² is large and the propagation loss is large when the normalized film thickness h / λ is in the range of 0.3 to 1.7%. Get smaller.
Further, the propagation loss α _1s becomes minimum when the normalized film thickness h / λ is around 0.9%, and the normalized film thickness h / λ is 0.27 to 1.4.
In the range of 0%, it is as small as 0.005 dB / λ or less.

FIG. 4 shows the stop band edge frequency (f _1s ) when the electrode finger width (M / P) standardized by the electrode finger pitch P is 0.5 and the electrode finger film thickness h / λ is changed. , F
_2s , f _1o , f _2o ), the frequency of the free part (f _free ) and the frequency of the metal part (f _metal ) (FIG. 4A),
Propagation loss (α _1s, α _s2) of the calculation result (FIG. 4 (b)), an electromechanical coupling coefficient k ² of the calculation results (FIG. 4 (c)).

The electrode finger thickness h is set to a value twice the electrode period (λ =
When the normalized film thickness h / λ standardized in 2P) is changed from 0.0% to 3.0%, as shown in FIG. 4A, the stop band width (f _2o −f _1o ),
(F _2s −f _1s ) becomes wider, and as shown in FIG.
The electromechanical coupling coefficient k ² increases from 1.2% to 3.0%, and particularly when the normalized film thickness h / λ is about 0.16% or more, the electromechanical coupling coefficient k ² is 1.6% or more. Become. Figure 4 (b)
As shown in, the propagation loss α _1s increases to 0.05 dB / λ or more when the normalized film thickness h / λ is about 2.0% or more, but the normalized film thickness h / λ is 1.6% or less. Is as small as 0.02 dB / λ or less.

Therefore, when the electrode finger width M / P is 0.5, the normalized film thickness h / λ is in the range of 0.16 to 1.6%,
The electromechanical coupling coefficient k ² is large and the propagation loss is small. The propagation loss α _1s is 0.8 when the normalized film thickness h / λ is 0.8.
%, The minimum, and the normalized film thickness h / λ is 0.23 to
In the range of 1.26%, it is as small as 0.005 dB / λ or less.

FIG. 5 shows the stop band edge frequency (f _1s ) when the electrode finger width (M / P) standardized by the electrode finger pitch P is 0.6 and the electrode finger film thickness h / λ is changed. , F
_2s , f _1o , f _2o ), the calculation result of the frequency of the free part (f _free ) and the frequency of the metal part (f _metal ) (FIG. 5A),
Propagation loss (α _1s, α _s2) of the calculation result (FIG. 5 (b)), an electromechanical coupling coefficient k ² of the calculation result (FIG. 5 (c)).

The electrode finger thickness h is a value twice the electrode period (λ =
When the normalized film thickness h / λ standardized in 2P) is changed from 0.0% to 3.0%, as shown in FIG. 5A, the stop band width (f _2o −f _1o ),
(F _2s −f _1s ) becomes wider, and as shown in FIG.
The electromechanical coupling coefficient k ² increases from 1.4% to 3.0%, and especially when the normalized film thickness h / λ is about 0.13% or more, the electromechanical coupling coefficient k ² is 1.6% or more. Become. Figure 5 (b)
As shown in, the propagation loss α _1s increases to 0.05 dB / λ or more when the normalized film thickness h / λ is about 2.0% or more, but the normalized film thickness h / λ is 1.6% or less. Is as small as 0.02 dB / λ or less.

Therefore, when the electrode finger width M / P is 0.6, the normalized film thickness h / λ is in the range of 0.13 to 1.6%,
The electromechanical coupling coefficient k ² is large and the propagation loss is small. The propagation loss α _1s is 0.8 when the normalized film thickness h / λ is 0.8.
%, The minimum, and the normalized film thickness h / λ is 0.20 to
In the range of 1.22%, it is as small as 0.005 dB / λ or less.

FIG. 6 shows the stop band edge frequency (f _1s ) when the electrode finger width (M / P) standardized by the electrode finger pitch P is 0.7 and the electrode finger film thickness h / λ is changed. , F
_2s , f _1o , f _2o ), the frequency of the free part (f _free ) and the frequency of the metal part (f _metal ) (FIG. _6A ),
Propagation loss (α _1s, α _s2) of the calculation result (FIG. 6 (b)), an electromechanical coupling coefficient k ² of the calculation results (Figure 6 (c)).

The electrode finger thickness h is set to a value twice the electrode period (λ =
When the normalized film thickness h / λ standardized in 2P) is changed from 0.0% to 3.0%, as shown in FIG. _{6A, the} stop band width (f _2o −f for open and short circuits) is changed. _1o ),
(F _2s −f _1s ) becomes wider, and as shown in FIG.
The electromechanical coupling coefficient k ² increases from 1.4% to 3.0%, and particularly when the normalized film thickness h / λ is about 0.15% or more, the electromechanical coupling coefficient k ² is 1.6% or more. Become. Figure 6 (b)
As shown in, the propagation loss α _1s increases to 0.05 dB / λ or more when the normalized film thickness h / λ is about 2.0% or more, but the normalized film thickness h / λ is 1.6% or less. Is as small as 0.02 dB / λ or less.

Therefore, when the electrode finger width M / P is 0.6, the normalized film thickness h / λ is in the range of 0.15 to 1.6%,
The electromechanical coupling coefficient k ² is large and the propagation loss is small. Further, the propagation loss α _1s is 0.9 when the normalized film thickness h / λ is 0.9.
%, The minimum, and the normalized film thickness h / λ is 0.20 to
In the range of 1.29%, it is as small as 0.005 dB / λ or less.

As is clear from the above description, the electromechanical coupling coefficient k ² is as large as 1.6% or more, and the propagation loss is 0.
The optimum region of the electrode finger width M / P and the normalized film thickness h / λ, which is as small as 02 dB / λ or less, is shown in FIG. 7 as a hatched region. When this optimum region is expressed by a mathematical expression, 0.029-0.097 × (M / P) + 0.082 × (M / P) ² ≦ (h / λ) ≦ 0.033-0.059 × (M / P) + 0.050 × (M / P) ² .

Further, the optimum region of the electrode finger width M / P and the standardized film thickness h / λ in which the propagation loss is as small as 0.005 dB / λ or less is shown in FIG. 8 as a hatched region. When this optimum region is expressed by a mathematical expression, 0.006−0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030−0.060 × (M / P) + 0.051 × (M / P) ² .

When the normalized film thickness h / λ of the electrode finger that minimizes the propagation loss is expressed by a mathematical expression, h / λ = 0.023−0.055 × (M / P) +0.0
It becomes 50 × (M / P) ² .

The lithium tetraborate single crystal has a point group of 4 m.
Since m has the symmetry and the surface acoustic wave has a predetermined symmetry, the direction of the above-mentioned oiler angle display is (0 ° to
360 °, 44 ° to 50 °, 80 ° to 100 °), (0
° ~ 360 °, 44 ° ~ 50 °, -80 ° ~ -100
°), (0 ° to 360 °, −44 ° to −50 °, 80 °
Up to 100 °), (0 ° to 360 °, −44 ° to −50)
(°, −80 ° to −100 °) and the like.

The lithium tetraborate single crystal is (01
1), (255), (356), (231), (34
5) Multiple surfaces that are easy to cut out (0 ° to 40 °, 4
4 ° to 50 °, 80 ° to 90 °), and particularly, the Laue point has a strong diffraction intensity and the precision of the cut surface can be improved by X-ray diffraction (011) is included (0 ° to 5 °, Four
4 ° to 50 °, 80 ° to 90 °) can be preferably used. In addition, (0 ° -40 °, 44 ° -50 °, 80 °-
90 °), (0 ° -5 °, 44 ° -50 °, 80 ° -9
0 °) also includes a lithium tetraborate single crystal and an equivalent range in accordance with the symmetry of surface acoustic waves.

[0048]

EXAMPLE A surface acoustic wave device according to an example of the present invention will be described with reference to FIGS. A surface acoustic wave device according to this example is shown in FIG. The surface acoustic wave device of the present embodiment is a transversal filter, in which an input IDT 22 and an output IDT 23 having the same structure are formed on the surface of a piezoelectric substrate 21 made of a lithium tetraborate single crystal whose main surface is a (011) plane. In the propagation region between the input IDT 22 and the output IDT 23, a short-circuit strip 24 having the same period and the same opening length as the input IDT 22 and the output IDT 23 is formed.

The input IDT 22 and the output IDT 23 are single electrode IDTs arranged in the same cycle so that electrode fingers having different polarities are interleaved with each other.
The electrode finger period (P) is 4 μm, the opening length is 400 μm, and the surface acoustic wave propagation direction is (0 °, 47.3) in the Euler angle display.
And 90 °). The input IDT 22, the output IDT 23, and the shorting strip 24 are
It is formed of an aluminum film having the same thickness, and is formed so that the propagation direction of the surface acoustic wave is perpendicular to the X-axis direction.

The frequencies at both ends of the stop band are measured from the frequencies at both ends of a large attenuation region based on the reflection of the stop band generated in the main lobe of the pass frequency characteristic, and the propagation loss has a propagation path length of 400 μm, 800 μm, 1200.
The electromechanical coupling coefficient k ² was measured by the change in insertion loss at the stop band edge frequency when the value was changed to μm.
It was measured from the input admittance of 2.23.

FIG. 10 shows the measurement results of the frequency at both ends of the stop band when the electrode finger line width (M) is 0.5 with respect to the electrode finger period (P) and the electrode finger thickness is changed (lower end). ●,
Top ▲) and calculation results (solid line) (Fig. 10 (a)), propagation loss measurement results (bottom ●, top ▲) and calculation results (solid line)
(FIG. 10 (b)), the measurement result (◯) and the calculation result (solid line) of the electromechanical coupling coefficient k ² are shown together (FIG. 10 (c)).

As is apparent from FIG. 10, the experimental results and the calculated results show a relatively good agreement, and the propagation loss is very small near the electrode finger thickness of 1%, and the electromechanical coupling coefficient k ² is It was found to be about 2.1%. The present invention is not limited to the above embodiment, and various modifications can be made. For example, the surface acoustic wave device of the present invention may have a structure different from that of the surface acoustic wave device of the above embodiment. For example,
The present invention can be applied to a resonator type filter having an IDT provided between a pair of grating reflectors and a resonator. The present invention can also be applied to a surface acoustic wave device having a structure in which a large number of IDTs are connected in parallel (IIDT structure).

[0053]

As described above, according to the present invention, a piezoelectric substrate made of lithium tetraborate single crystal, and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves. In the surface acoustic wave device having:
The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between them. The propagation direction is the oiler angle display (0 ° ~
45 °, 44 ° to 50 °, 80 ° to 90 °) and its equivalent range, the electrode finger period is P, the electrode finger width is M, and the electrode film thickness h is elastic. The normalized film thickness h / λ of the electrode, which is normalized by the wavelength λ of the surface wave, is expressed by the following equation: 0.029-0.097 × (M / P) + 0.082 × (M / P) ² ≦ (h / λ ) ≦ 0.033−0.059 × (M / P) + 0.050 × (M / P) ² so that the electromechanical coupling coefficient k ²
It is possible to realize a surface acoustic wave device having a large propagation loss and a small propagation loss.

Further, according to the present invention, an elastic surface having a piezoelectric substrate made of lithium tetraborate single crystal and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves. In the wave device, the electrodes are formed of a metal containing aluminum as a main component, and the electrodes are
It has a pair of comb-shaped electrodes arranged such that electrode fingers for each of them are interleaved with each other, and the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are indicated by an oiler angle (0 ° to 45 °
°, 44 ° to 50 °, 80 ° to 90 °) and the equivalent range, and the period of the electrode fingers is P,
The normalized film thickness h / λ of the electrode obtained by normalizing the film thickness h of the electrode with the wavelength λ of the surface acoustic wave, where M is the width of the electrode finger, is expressed by the following equation: 0.006-0.013 × (M / P) Since + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030−0.060 × (M / P) + 0.051 × (M / P) ² is satisfied, It is possible to realize a surface acoustic wave device having a very small propagation loss.

[Brief description of drawings]

FIG. 1 is a diagram showing a model used for a simulation of a surface acoustic wave device according to the present invention.

FIG. 2 is a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on a surface of a lithium tetraborate single crystal substrate, and a cutting angle and a propagation direction of the lithium tetraborate single crystal substrate are represented by Euler angles (0 °, 47.3 °, 90
°), the electrode finger width M / P is 0.3, and the electrode finger film thickness h /
It is a graph which shows the simulation result of a stop band edge frequency (the same figure (a)), a propagation loss (the same figure b)), and an electromechanical coupling coefficient (the same figure (c)) when changing (lambda).

FIG. 3 is a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on a surface of a lithium tetraborate single crystal substrate, and a cutting angle and a propagation direction of the lithium tetraborate single crystal substrate are indicated by an oiler angle (0 °, 47.3 °, 90
°), the electrode finger width M / P is 0.4, and the electrode finger film thickness h /
It is a graph which shows the simulation result of a stop band edge frequency (the same figure (a)), a propagation loss (the same figure b)), and an electromechanical coupling coefficient (the same figure (c)) when changing (lambda).

FIG. 4 is a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on a surface of a lithium tetraborate single crystal substrate, and a cutting angle and a propagation direction of the lithium tetraborate single crystal substrate are indicated by Euler angles (0 °, 47.3 °, 90
°), the electrode finger width M / P is 0.5, and the electrode finger film thickness h /
It is a graph which shows the simulation result of a stop band edge frequency (the same figure (a)), a propagation loss (the same figure b)), and an electromechanical coupling coefficient (the same figure (c)) when changing (lambda).

FIG. 5 is a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on the surface of a lithium tetraborate single crystal substrate, and the cutting angle and the propagation direction of the lithium tetraborate single crystal substrate are indicated by Euler angles (0 °, 47.3 °, 90
°), the electrode finger width M / P is 0.6, and the electrode finger film thickness h /
It is a graph which shows the simulation result of a stop band edge frequency (the same figure (a)), a propagation loss (the same figure b)), and an electromechanical coupling coefficient (the same figure (c)) when changing (lambda).

FIG. 6 is a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on the surface of a lithium tetraborate single crystal substrate, and the cutting angle and the propagation direction of the lithium tetraborate single crystal substrate are represented by Euler angles (0 °, 47.3 °, 90
°), the electrode finger width M / P is 0.7, and the electrode finger film thickness h /
It is a graph which shows the simulation result of a stop band edge frequency (the same figure (a)), a propagation loss (the same figure b)), and an electromechanical coupling coefficient (the same figure (c)) when changing (lambda).

FIG. 7 is a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on the surface of a lithium tetraborate single crystal substrate, in which the electromechanical coupling coefficient is large, the propagation loss is small, and the electrode finger width M / P and It is a graph which shows the optimal area | region of normalized film thickness h / (lambda).

FIG. 8 is a diagram illustrating a surface acoustic wave device in which an IDT containing aluminum as a main component is formed on the surface of a lithium tetraborate single crystal substrate.
It is a graph which shows the optimal area | region of P and normalized film thickness h / (lambda).

FIG. 9 is a diagram showing a surface acoustic wave device according to an embodiment of the present invention.

FIG. 10 shows a surface acoustic wave device according to an embodiment of the present invention, in which the frequency at both ends of the stop band (the same figure (a)) and the propagation loss (the same figure (b)) when the electrode finger thickness is changed. FIG. 5 is a graph showing the measurement result and the simulation result of the electromechanical coupling coefficient (FIG. 7C).

[Explanation of symbols]

21 ... Piezoelectric substrate 22 ... Input IDT 23 ... Output IDT 24 ... Shorting strip 25 ... Electrode finger 26 ... Insulating layer 27 ... Insulating layer

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-7-86868 (JP, A) Takahiro Sato, Hidenori Abe, Characteristics of longitudinal wave type Leaky SAW propagating periodic metal grating on Li2B4O7, electron Proceedings of the 1995 IEICE General Conference, March 10, 1995, Fundamentals / Boundaries, p. 373 (A-373) Ryusuke Sato, Hidenori Abe, Propagation Characteristics of Longitudinal Leaky Waves in AI / Li2B4O7 Structure, The Institute of Electronics, Information and Communication Engineers Autumn Meeting 1994-Society Advance Meeting-Lecture Collection, September 1994 5 Day, p. 309 (SA-11-8) (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/145 H03H 9/25

Claims (6)

(57) [Claims] 1. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating a surface acoustic wave. The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and a cutting angle of a surface of the piezoelectric substrate. And the propagation direction of the surface acoustic wave is an oiler angle display (0 ° to 45 °, 44 ° to 50 °, 80 ° to 90 °).
°) and its equivalent range, and the period h of the electrode fingers is P and the width of the electrode fingers is M, and the film thickness h of the electrode is standardized by the wavelength λ of the surface acoustic wave. The normalized film thickness h / λ of the electrode is 0.029−0.097 × (M / P) + 0.082 × (M / P) ² ≦ (h / λ) ≦ 0.033-0. Within the range of 059 × (M / P) + 0.050 × (M / P) ² , the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and the velocity of the longitudinal wave. Surface acoustic wave device characterized by not exceeding 2. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating a surface acoustic wave. The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and a cutting angle of a surface of the piezoelectric substrate. And the propagation direction of the surface acoustic wave is an oiler angle display (0 ° to 45 °, 44 ° to 50 °, 80 ° to 90 °).
°) and its equivalent range, and the period h of the electrode fingers is P and the width of the electrode fingers is M, and the film thickness h of the electrode is standardized by the wavelength λ of the surface acoustic wave. The normalized film thickness h / λ of the electrode is 0.006−0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030-0. It is in the range of 060 × (M / P) + 0.051 × (M / P) ² , and the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and the velocity of the longitudinal wave. Surface acoustic wave device characterized by not exceeding 3. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and electrodes formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves. The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and a cutting angle of a surface of the piezoelectric substrate. And the propagation direction of the surface acoustic wave is the oiler angle display (0 ° to 40 °, 44 ° to 50 °, 80 ° to 90 °).
°) and its equivalent range, and the period h of the electrode fingers is P and the width of the electrode fingers is M, and the film thickness h of the electrode is standardized by the wavelength λ of the surface acoustic wave. The normalized film thickness h / λ of the electrode is 0.029−0.097 × (M / P) + 0.082 × (M / P) ² ≦ (h / λ) ≦ 0.033-0. Within the range of 059 × (M / P) + 0.050 × (M / P) ² , the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and the velocity of the longitudinal wave. Surface acoustic wave device characterized by not exceeding 4. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and electrodes formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves. The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and a cutting angle of a surface of the piezoelectric substrate. And the propagation direction of the surface acoustic wave is the oiler angle display (0 ° to 40 °, 44 ° to 50 °, 80 ° to 90 °).
°) and its equivalent range, and the period h of the electrode fingers is P and the width of the electrode fingers is M, and the film thickness h of the electrode is standardized by the wavelength λ of the surface acoustic wave. The normalized film thickness h / λ of the electrode is 0.006−0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030-0. It is in the range of 060 × (M / P) + 0.051 × (M / P) ² , and the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and the velocity of the longitudinal wave. Surface acoustic wave device characterized by not exceeding 5. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating a surface acoustic wave. The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and a cutting angle of a surface of the piezoelectric substrate. And the propagation direction of the surface acoustic wave is the oiler angle display (0 ° to 5 °, 44 ° to 50 °, 80 ° to 90 °)
And the electrode film formed so as to be in a range equivalent thereto, wherein the film thickness h of the electrode is normalized by the wavelength λ of the surface acoustic wave, where P is the period of the electrode finger and M is the width of the electrode finger. The normalized film thickness h / λ of is 0.029−0.097 × (M / P) + 0.082 × (M / P) ² ≦ (h / λ) ≦ 0.033−0.059 × Within the range of (M / P) + 0.050 × (M / P) ² , the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and exceeds the velocity of the longitudinal wave. A surface acoustic wave device characterized by the absence thereof. 6. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and electrodes formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves. The electrode is formed of a metal containing aluminum as a main component, and the electrode has a pair of comb-shaped electrodes arranged such that one electrode finger is interposed between the electrodes, and a cutting angle of a surface of the piezoelectric substrate. And the propagation direction of the surface acoustic wave is the oiler angle display (0 ° to 5 °, 44 ° to 50 °, 80 ° to 90 °)
And the electrode film formed so as to be in a range equivalent thereto, wherein the film thickness h of the electrode is normalized by the wavelength λ of the surface acoustic wave, where P is the period of the electrode finger and M is the width of the electrode finger. The normalized film thickness h / λ of is 0.006-0.013 × (M / P) + 0.011 × (M / P) ² ≦ (h / λ) ≦ 0.030-0.060 × Within the range of (M / P) + 0.051 × (M / P) ² , the velocity of the surface acoustic wave is equal to or higher than the velocity of the fast transverse wave of the bulk wave propagating in the same direction, and exceeds the velocity of the longitudinal wave. A surface acoustic wave device characterized by the absence thereof.