JP3379383B2 - 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 having a reduced size in the field of resonators and resonator filters constructed using surface acoustic waves, transversal filters and the like. Regarding

[0002]

2. Description of the Related Art A resonator and a resonator type filter are known as a first type of a conventional surface acoustic wave device. These are composed of at least one interdigital electrode (hereinafter abbreviated as IDT) and at least one pair of reflectors (Japanese Patent Publication No. 7-73177, Japanese Patent Publication No. 4-52).
005). In addition, JP-A-58-1241
Japanese Patent Laid-Open No. 8 discloses an example of a transverse multimode filter in which the resonator having only the IDT and a resonator thereof are combined. Furthermore, Japanese Patent Application Laid-Open No. 6-85602 discloses an example of an IDT type resonator having a configuration of only an IDT.

As the second type of surface acoustic wave device,
ID composed of double split electrodes (1/8 wavelength electrode)
There is a transversal type filter using T (Japanese Patent Publication No. 2-45368).

[0004]

However, in the case of the prior arts of Japanese Patent Publication No. 7-73177 and Japanese Patent Publication No. 4-52005, which belong to the above-mentioned first type, resonance is caused only by the use of the reflector. The size of the surface acoustic wave propagation direction in the child- and resonator-type filters has become longer, and the elements required for the intermediate frequency filter (IF filter) used in the GSM-type or PHS-type mobile phones, which have made remarkable progress in recent years, There is a problem that the size cannot be reduced, and there is a problem with the conventional Japanese Patent Laid-Open Nos. 58-12418 and 6-856.
In the so-called IDT resonator technology disclosed in Japanese Patent Publication No. 02-202, the Q value of the resonator (numerical value indicating the resonance sharpness) is as small as several thousand, a sufficiently small equivalent series resistance value cannot be obtained, and the insertion loss of the filter is small. Has the drawback that it becomes large.

Further, in the prior art belonging to the above-mentioned second type such as a transversal type filter (Japanese Patent Publication No. 2-45368), since the electrode fingers of 1/8 wavelength are used, the element When the frequency is as high as 400 MHz, the electrode finger width is about 1 μm, and there is a drawback that the manufacturing apparatus such as an expensive exposure apparatus is required for manufacturing, resulting in high cost.

Therefore, the present invention solves such a problem, and a first object thereof is to further reduce the size of a surface acoustic wave device such as a resonator and a resonator type filter without impairing its performance. Is to measure. The second purpose is to use the electrode fingers having the same quarter wavelength width as that of the resonator, thereby eliminating the need for an expensive manufacturing apparatus such as an exposure apparatus and having the same level of performance as the conventional transversal. Type and IDT type filters are provided to the market at low cost.

[0007]

Means for Solving the Problems (1) In a surface acoustic wave device according to the present invention, metal conductors of positive and negative polarities are alternately parallel to each other on a piezoelectric flat plate orthogonal to the phase propagation direction of the surface acoustic wave. in the surface acoustic wave device using an arranged interdigital electrodes are formed resonator, the interdigital transducer into three regions
Divide the conductor width of the electrode fingers into
1/4 of the wavelength λ of the surface acoustic wave used
The electrode fingers of the interdigital transducers in each region are
And the period length PTc of the electrode fingers in the region near the center is set to be different from the period length PTs of the electrode fingers in the regions on both sides by 2% or less. When the coefficient a representing the dispersion relation related to the propagation of the surface acoustic wave by the interdigital transducer is negative, PTc
<Frequency rising type satisfying the condition of PTs, and the coefficient a
Is positive, it is a frequency drop type that satisfies the condition of PTc> PTs, and each of the resonance peak and the anti-resonance peak of the main resonance characteristic of the resonator is monomodal.

[0008]

[0009]

[0010]

[0011]

[0012]

[0013]

( 2 ) In the above (1), the piezoelectric plate is a 128 ° Y-cut X-propagation LiNbO ₃ substrate ,
The ratio H / λ of the thickness H of the electrode finger to the wavelength λ of the surface acoustic wave is set in the range of 4 to 6%, and PTc / PTs = 1.
It is characterized in that it is set to 008 to 1.02.

[0015]

( 3 ) In the above ( 2 ), the number M of pairs of electrode fingers of the interdigital transducer is set to 50 to 250 pairs.

( 4 ) In the above (1), the piezoelectric substrate is made of quartz, and the ratio H / λ of the thickness H of the electrode finger to the wavelength λ of the surface acoustic wave is in the range of 2 to 5%.
It is characterized in that / PTc is changed from 1.007 to 1.02.

( 5 ) In the above ( 4 ), the number M of pairs of electrode fingers of the interdigital transducer is set to 100 to 300 pairs.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to FIG.
It will be explained step by step.

(Embodiment 1) FIG. 1 is an embodiment showing a plan view of an electrode pattern used in the surface acoustic wave device of the present invention. The names of the respective parts in FIG. 1 are as follows: 100 is a piezoelectric flat plate, 101 is a positive power feeding conductor, 102 is a negative power feeding conductor, 103 and 104 are IDT electrode fingers, and the respective feeding conductors 101 and 102 are Connected 105 and 106 are pads for external conduction, 107, 108, 109 and 1
10 and the like are division lines that make up the IDT electrode finger cross width W (X), 1
Reference numeral 11 is a Y axis that is a direction orthogonal to the X axis of 112, and 112 is an X axis that indicates the phase propagation direction of the surface acoustic wave. The piezoelectric plate 100 is composed of a single crystal having crystallinity such as quartz, lithium tantalate, lithium niobate, etc., whose surface is mirror-polished, and a substrate on which a piezoelectric thin film such as ZnO is formed. I formed on the above 100
The conductor pattern such as DT is formed by forming a thin metal film having conductivity such as aluminum and gold by means such as vapor deposition and sputtering, and then forming a pattern by the photolithography technique. The IDT is a positive power feeding conductor 1
01 and the feeding conductor 102 having a negative polarity, and the electrode fingers 103 having a positive polarity and the electrode fingers 104 having a negative polarity, which are connected to these, the electrode finger group uses a surface acoustic wave (Rayleigh wave or leaky wave, etc.) to be used. ) Phase progression direction (longitudinal direction X
A large number are arranged in parallel and periodically at right angles to the (axis). By the dividing lines 107 and 110 of the IDT, the crossing width of the positive and negative electrode fingers is zero. In the IDT dividing lines 108 and 109, W (X), which is the width dimension at which the positive and negative electrode fingers intersect, is COS (KX).
Has a weighting of. Furthermore, the electrode finger cycle length PT (X) forming the IDT is stepwise changed by the X coordinate position according to a specific rule in FIG. 2 described later.

(Embodiment 2) Next, referring to FIG. 2, another embodiment of the present invention will be used.
It shows a specific setting example of (X). The names of the respective parts in the figure are a piezoelectric plate 200, an area 201 surrounded by a dotted line is an IDT SAW resonator, and 202 and 209 are pads for external connection. The area surrounded by the broken lines 203 and 204 is an area where the electrode finger crossing width is zero.
The area indicated by the broken line 14 is an area having a uniform electrode finger crossing width. 205 and 206 and the like are electrode fingers, and 207 and 208 and the like are feeding conductors. 212 step function FP
(X) may be named a frequency potential function that determines the electrode finger length of the IDT, and the curve V (X) of 213 is the vibration displacement function obtained in the embodiment of FIG.

The frequency potential function FP of 212
(X) is determined by the type and material of the piezoelectric plate 200. In the case of the example of FIG. 2, the setting is made in the case of a crystal ST cut-X propagation substrate, and the period length PT of the electrode finger is set.
(X) is set in a step-like manner such that it is shorter toward the center of the X axis. The above PT (X) is set by the following equation (1), where V _S is the velocity of the surface acoustic wave when there is no aluminum film.

[0023]

[Equation 1]

The reference frequency f _{R0 in} equation (1) is
In the IDT-type SAW resonator having the shape shown in FIG. 2 which occurs when PT (X) = PT ₀ (constant), the center of the reflection characteristic created by the electrode guider group generated by short-circuiting between the positive and negative electrode pads 202 and 209. Frequency.

The above FP (X) has a dimension of frequency change rate.

Further, ζi expressed by the frequency change rate in FIG.
(I = 0 to 3) is represented by the formula fi = V _S / (2 · PTi
Using the frequency given by (X)),

[0027]

[Equation 2]

Is given by In particular, ζ ₀ corresponds to the frequency f ₀ when there is no aluminum film, and f ₀ = V _S /
It is given by (2 · PT ₀ ). The vibration displacement V (X) obtained by FP (X) in FIG. 2 has a displacement of a cosine function (COS (KX)) shape that is maximum at the center of the IDT resonator, and is so-called in the X-axis direction. Vibration energy trapping phenomenon has been realized. The reason why this phenomenon occurs will be described with reference to FIG.

In FIG. 3, 300 and 301 are feeding conductors, 305 and 306 are IDT electrode fingers, and the region surrounded by the line 302 is the period length PT ₁ (= L _n-1 +) of the first electrode finger. S
_n-1 ), similarly 303 is an IDT region having a second electrode finger period length PT ₂ (= L _n + S _n ), 30
Reference numeral 4 is an IDT region having a period length PT ₃ (= L _{n + 1} + S _{n + 1} ) of the third electrode finger. Further, ω ₁ , ω ₂ , and ω ₃ described in the figure are angular frequencies (ω ₀ = 2π) at almost the maximum point of the admittance characteristic of the IDT in the corresponding region.
f _T0 ), or what is called the Bragg frequency. The figure enclosed in a frame describes the wave number dispersion curve of each of the IDTs. This characteristic has a vertical axis of angular frequency ω = 2πf axis and a horizontal axis of vibration amplitude V of a surface acoustic wave.
This is illustrated as the wave number K created by (X). Also,
The left half of FIG. 3 represents the imaginary part of K, and the right half represents the real part of K. The wave number dispersion curve 307 is for the IDT region 302, and 308 is for the IDT region 304.
The dispersion curve can be approximated by the following equation (3).

[0030]

[Equation 3]

Here, ω ₀ is the above Bragg frequency, a
Is a coefficient, and γ is a normalized wave number obtained by normalizing K by the wave number k ₀ (= 2π / λ) given by the wavelength λ of the surface acoustic wave (γ = K / k ₀ ). Curves 307 and 308 are when a is negative. It is added that the coefficient a is determined by the type of substrate, the orientation, and the type of surface acoustic wave. A surface acoustic wave whose wave number dispersion curve has an imaginary value cannot be propagated but is reflected, and a surface acoustic wave having a real wave number can be propagated. In the central IDT region 303, the surface acoustic wave having the angular frequency of ω ₂ is radiated to both sides of the X axis. First, the state of the surface acoustic wave traveling to the IDT 302 side is the P2 point of the dispersion curve 307. Since it has a wave number K2 (real number) of, it can propagate. On the other hand, the surface acoustic wave traveling to the area of the IDT 304 on the right side has a wave number K1 (imaginary number) at point P1 of the dispersion curve 308, and therefore cannot propagate and is reflected. Therefore, what can be said from the above is that the coefficient a is negative and I
When the frequency setting state of DT has a relationship of ω ₁ > ω ₂ > ω ₃ , it means that the surface acoustic wave cannot be propagated in the positive direction of the X axis, is reflected, and its amplitude is attenuated. Further pushed this, omega ₁ if you make a _{<ω 2>} ω ₃ relations, surface acoustic wave, will be confined to the central IDT region having an angular frequency of omega _2, 213 V in FIG. 2 of the aforementioned The vibration displacement of (X) is realized. The above ω ₁ , ω ₂ , and ω ₃ are ω ₁ = 2πf _T1, ω ₂
= 2πf _T2, ω ₃ = 2πf _T3, and can be sequentially associated with ζ _2, ζ _1, ζ _{2 in} the relationship of the equation (2).

Next, with reference to FIG. 4, the method of constructing the surface acoustic wave device of the present invention will be described using the example of the 128 ° Y-XLiNbO ₃ substrate as the specific example of the piezoelectric plate in FIGS. 1 and 2. To do. In the IDT of FIG. 4, the electrode finger width L is a quarter wavelength (λ /
4) length.

The horizontal axis of FIG. 4 represents the ratio H / λ of the surface acoustic wave wavelength λ to the aluminum electrode film thickness H, and the vertical axis represents the frequency change rate Δf / f in ppm. The curve 400 shows the frequency f _T0 of the admittance maximum point of the IDT, and the frequency f ₀ = V when the aluminum film is not present.
_The frequency change rate is displayed with _S / (2 · PT ₀ ) as the reference value 0. A curve 401 shows the dependence of the center frequency f _R0 of the reflection characteristic of the IDT on the electrode finger thickness ratio H / λ 2. In 402, the frequency difference between 402 and 401 is
It is drawn so as to be equal to the frequency difference between 401 and 400, and the region sandwiched between 400 and 402 represents a surface acoustic wave reflection region, that is, a stop band. The curves 400 and 401 intersect at approximately H / λ = 3.2, and in this state, the stop band disappears and the surface acoustic wave propagates without being reflected. Therefore, under the condition of this film thickness ratio, the structure of the filter which is a transversal-type or IDT-type element which is difficult to reflect the surface acoustic wave has an electrode finger width L of about a quarter wavelength (λ / 4). Can be achieved in the length of. Effectively, the reflection coefficient is sufficiently small and effective when H / λ is in the range of about 3 to 3.5%. In the region where H / λ is 3.2 or less, f _T0
In the state of <f _R0, the surface acoustic wave in this region has the characteristics of the dispersion curves 403 and 404 that are shown by being superimposed in the figure,
As in the example of the so-called FIG. 2 described above, a frequency rising confinement state is shown. On the contrary, in the region where H / λ is 3.2 or more, f _T0 > f _R0 , and the surface acoustic waves in this region have the characteristics of the dispersion curves 405 and 406 shown in the figure. 2 shows a so-called frequency drop type confinement state, which is the reverse characteristic of the example of FIG. In summary, the above-described embodiment of FIG. 2 corresponds to the above-mentioned frequency rising confinement, and is a construction method corresponding to the region of H / λ <3.2 or less in FIG. When H / λ> 3.2, ζ ₁ in FIG.
Therefore, if ζ ₃ is set to be negative, a frequency drop type energy confinement state can be constructed. Next, the vibration displacement state of the elastic surface device of the present invention thus obtained will be described with reference to FIG.

The horizontal axis 504 in FIG. 5 represents the X coordinate of the surface acoustic wave device, and the vertical axis 503 represents the vibration energy E (X) of the element (proportional to the square of the vibration displacement amplitude U). Curves 500 to 502 in the figure are functions showing the vibration energy distribution for each condition.
The condition is that the IDT 201 of FIG.
The frequency division amount P (frequency potential) of the divided IDT at the center is divided into units of the above | f _T0 −f _R0 | = η, and P = 0 in the case of 500 and P = η in the case of 501.
The case of 0.8η and 502 is the case of setting P = 1.2η. As can be seen from the figure, when P is not zero with respect to the amount of frequency increase 0, the degree of central concentration of vibration energy increases.

A characteristic diagram (FIG. 6) showing the Q value of the resonator which is the surface acoustic wave device of FIG. 2 shows this phenomenon more easily. As is well known, the Q value of a resonator represents the degree of energy loss of the resonator, and the larger Q is, the better the resonator is with less loss. The horizontal axis of FIG. 6 represents the frequency increase amount P of the central IDT, and the vertical axis represents the resonator Q value Q ₀ when P = ₀ as a reference. It can be seen that when η = 8000 ppm and P = 1.2η, a Q value of about four times is obtained. In the case of the crystal ST cut X propagation azimuth, the specific value of P at which the Q value increasing effect is recognized is the ratio H / λ of the film thickness H of the electrode finger to the wavelength λ of the surface acoustic wave.
In the range of 2 to 5%, 7,000 to 20,000 ppm
(At this time, the ratio of the central electrode finger cycle length PTc to the electrode finger cycle lengths PTs on both sides is PT _S /PTc=1.0.
07-1.02). In the case of a 128 ° Y-X LiNbO ₃ substrate, the ratio H / λ of the film thickness H of the electrode finger to the wavelength λ of the surface acoustic wave is set to a range of 4 to 6%, and 8000
Was 20,000 ppm (at this time, the ratio of the central electrode finger cycle length PTc to the electrode finger cycle lengths PTs on both sides was PTc.
/PTs=1.008 to 1.02). Further, the piezoelectric plate is 128 ° Y-cut X propagation, the ratio H / λ of the thickness H of the electrode finger to the wavelength λ of the surface acoustic wave is in the range of 0.5 to 2%, and PTs / PTc = 1. From 1.0 to 1.02, the Q value can be improved. The logarithm of the positive and negative electrode fingers used for the IDT is 128 ° Y-XLiNbO ₃
In the case of the substrate, the range is 50 to 250 pairs, and in the case of the crystal ST cut X propagation direction, the range is 100 to 300 pairs.

Next, a means for suppressing unnecessary resonance, that is, spurious, generated in the surface acoustic wave device of the present invention will be described. As an example, the 128 ° Y-XLiNbO ₃ substrate will be described as an example. In the above substrate, when the electrode film thickness ratio H / λ is set to 0.05 to form a frequency drop type energy confinement type resonator, spurious due to the longitudinal inharmonic mode is located above the S0 mode which is the main resonance mode. Occur. This is shown in FIG. Spurs are modes 804 and 805 indicated by dotted lines in the figure. 802 is the main resonance mode, and 803 is the anti-resonance mode. 800 is a unimodal resonance peak, 801 is unimodal
Is the anti-resonance peak of.

Therefore, in order to suppress this spurious,
A weighting function WF (X) =
Considering COS (KX) (see Fig. 1), set the above K to K
FIG. 7 shows the relationship between the phase variation amount Δφ of the impedance of the resonator due to spurious resonance with respect to ξ, where ξ is referred to as a weighted length ratio when given by = π / (ξM). Since the phase fluctuation amount Δφ represents the intensity of spurious mode generation, the smaller Δφ means that the spurious can be suppressed. It is coordinates in the X (= x / λ) axis direction standardized by the wavelength λ of the X-wave surface acoustic wave. The curve 700 in FIG. 7 is the case where the frequency increase amount P = 0, and at this time, ξ = 0.8 is the minimum value, and the curve 70
In the case of 1, P = 0.8η, and at this time 0.65
Was the minimum value. 702 is a case of P = 1.2 (eta), and the minimum value at this time was 0.5.

Therefore, in the case of the 128 ° Y-XLiNbO ₃ substrate, spurious can be suppressed under the condition of P = 0 to 1.2 with respect to the range of ξ = 0.5 to 0.9. The other substrate orientations showed almost the same tendency.

[0039]

As described above, according to the present invention, a resonator is formed only of IDTs without using a reflector, the IDTs are divided into a plurality of pieces, and the electrode finger cycle length of each divided IDT is set. The frequency potential is set so that the vibration energy of the resonator is concentrated and confined in the center according to the property of the piezoelectric substrate using the frequency rising type or the frequency falling type, and the resonance of the main resonance characteristic of the resonator is set. By making each of the peak and the anti-resonance peak unimodal, it is possible to realize an IDT resonator that is smaller in size and has better performance than ever before, so that further downsizing is required for portable communication devices such as PHS and GSM. It can also contribute to downsizing of VCSO and the like.

[0040]

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of a surface acoustic wave device of the present invention.

FIG. 2 is a diagram showing another embodiment of the present invention.

FIG. 3 is a conceptual diagram explaining the operation principle of the surface acoustic wave device of the present invention.

FIG. 4 is a characteristic diagram of a piezoelectric plate according to an embodiment of the present invention.

FIG. 5 is a characteristic diagram of FIGS. 1 and 2 of the present invention.

FIG. 6 is a characteristic diagram of FIGS. 1 and 2 of the present invention.

FIG. 7 is a characteristic diagram of FIG. 1 of the present invention.

FIG. 8 is another characteristic diagram shown in FIG. 1 of the present invention.

[Explanation of symbols]

100 piezoelectric plate 101 Positive feed conductor 102 Negative feeding conductor 103 Positive electrode finger 104 Negative electrode finger

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Claims (5)

(57) [Claims] 1. An elasticity using a resonator in which a comb-shaped electrode in which metal conductors of positive and negative polarities are alternately arranged in parallel and orthogonal to a phase propagation direction of a surface acoustic wave is formed on a piezoelectric flat plate. In the surface acoustic wave device, the comb-shaped electrode is divided into three regions, and in the three regions, a conductive width of an electrode finger is used.
The wavelength of the surface acoustic wave λ is ¼ and the electrode fingers of the interdigital electrodes in each of the divided areas are
It is formed with a constant cycle length, and the cycle length PTc of the electrode fingers in the area near the center is 2% or more with respect to the cycle length PTs of the electrode fingers in the areas on both sides.
Set differently in the inner, the when coefficient a representative of the dispersion relation relating to the propagation of surface acoustic waves by the piezoelectric substrate and the interdigital electrode is negative, the condition is satisfied frequency elevation of PTc <PTs When the coefficient a is positive, it is a frequency drop type that satisfies the condition of PTc> PTs, and each of the resonance peak and the anti-resonance peak of the main resonance characteristic of the resonator is monomodal. Surface acoustic wave device. 2. The piezoelectric flat plate is a 128 ° Y-cut X-propagation LiNbO ₃ substrate , and the ratio H / λ of the surface acoustic wave wavelength λ to the electrode finger thickness H is in the range of 4 to 6%. 2. The surface acoustic wave device according to claim 1, wherein PTc / PTs = 1.008 to 1.02. 3. The number M of electrode fingers of the interdigital transducer is 5
3. The surface acoustic wave device according to claim 2 , wherein the surface acoustic wave device comprises 0 to 250 pairs. 4. The piezoelectric flat plate is quartz, and the ratio H / λ of the film thickness H of the electrode finger to the wavelength λ of the surface acoustic wave.
In the range of 2 to 5%, PTs / PTc = 1.007
2. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is set to 1.02. 5. The number M of electrode fingers of the interdigital transducer is 1
5. The surface acoustic wave device according to claim 4 , wherein the surface acoustic wave device is composed of 00 to 300 pairs.