JP2007228012A - Surface acoustic wave resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new type surface acoustic wave resonator, where fifth-order harmonic operation is attained by utilizing a Rayleigh-type surface acoustic waves of, or the like on a piezoelectric plate, such as in-plane rotation ST cut quartz plate. <P>SOLUTION: The surface acoustic wave resonator comprises one interdigital electrode on the piezoelectric plate, and a pair of reflectors arranged at both the sides in the propagation direction. The width dimension in the X direction of an electrode finger in the interdigital electrode is set to LT, and the line width ratio LT/PT for array period length is to be within the range of 1/2±1/12. The width dimension in the X direction in the reflector is set to LR, and the line width ratio LR/PR for array period length PR is to be within the range of 1/2±1/12. The relation for PT is PR<PT with respect to the dimension PR, and the relation between speed V of the surface acoustic waves and the operating frequency (f) is f=5V/(2PT). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave resonator of a new type in which a fifth harmonic wave operation is performed using a Rayleigh type surface acoustic wave on a piezoelectric plate such as a rotated ST-cut quartz plate rotated in-plane.

  Conventionally, a surface acoustic wave resonator constructed using a quartz ST-cut substrate (an example of a piezoelectric flat plate) having piezoelectricity has a frequency temperature characteristic with a zero temperature coefficient and is highly accurate. This is used as an oscillation element of a crystal oscillator for data transmission because it has an advantage that a signal free from jitter and excellent in phase noise can be easily obtained with high reliability and low cost.

  However, in recent years, the signal transmission speed of the network system has been increased to the GHz band, and a crystal oscillator having a higher frequency and higher accuracy has been demanded. In view of this, an ST-cut quartz plate that has been rotated in-plane to obtain a frequency temperature characteristic of about half (accuracy ± 50 ppm) with respect to the accuracy of ST cut ± 100 ppm (range of 0 to 70 ° C.) has recently been attracting attention. There are surface acoustic wave resonators using The substrate uses a Rayleigh surface acoustic wave (for example, Patent Document 1).

  Also for a surface acoustic wave resonator using an ST-cut quartz plate rotated in-plane, as in the conventional ST-cut, interdigital electrodes (hereinafter, referred to as a plurality of parallel conductor electrode fingers made of metal aluminum, for example) are arranged. Abbreviated as "IDT"), and a pair of reflectors on both sides of the electrode conductors made up of a number of strip shapes arranged in parallel and periodically to form a 1-port surface acoustic wave resonance A child can be formed.

JP-A-57-73513

However, since all of the above-described conventional techniques are surface acoustic wave resonators that operate at a fundamental wave, the relationship between the repetition period length P of the electrode finger of the IDT and the conductor strip of the reflector and the operating frequency is If V is the velocity of the surface acoustic wave and f is the operating frequency, the relationship is f = V / (2P). As a result, the operating frequency f is limited to the speeds V and P. If the speed V of the piezoelectric substrate is 3000 m / s and the resolution of the exposure device for forming the electrode pattern is 0.5 × 10 ⁻⁶ m. , P = 1.0 × 10 ⁻⁶ m, and about 1.5 GHz is the limit of high frequency.

  The present invention solves such a problem, and an object of the present invention is to realize a surface acoustic wave resonator capable of harmonic operation at a frequency five times the fundamental wave operating frequency, and to use it in the 1 to 5 GHz band. It is to provide a high-accuracy and low-phase noise SAW oscillator and a voltage-controlled SAW oscillator to the gigabit high-speed wired communication market.

  (1) A surface acoustic wave resonator according to the present invention includes at least one interdigital electrode for exciting a surface acoustic wave in a phase propagation direction X on a piezoelectric plate and a pair of reflectors disposed on both sides of the propagation direction. The interdigital electrode is composed of a number of electrode fingers arranged substantially orthogonal to the phase propagation direction X of the surface acoustic wave, and the electrode fingers are arranged in the phase propagation direction X direction. When the width dimension is LT and the dimension in the phase propagation direction X direction of the region where no electrode finger is present is ST, the line width ratio LT / PT with respect to PT = LT + ST which is the arrangement period length of the interdigital electrodes is (1/2 The reflector is composed of a number of electrode fingers arranged substantially orthogonal to the phase propagation direction X of the surface acoustic wave, and the width of the electrode fingers in the phase propagation direction X direction Dimension is LR and there is an electrode finger The line width ratio LR / PR with respect to PR = LR + SR, which is the arrangement period length of the reflectors, is in a range of (1/2 ± 1/12), where SR is the dimension in the phase propagation direction X direction of the non-existing region, The relation between the arrangement period length PT of the interdigital electrodes and the arrangement period length PT of the reflectors is PR <PT, and the ratio of the arrangement period length PR of the reflectors to the arrangement period length PT of the interdigital electrodes is There is a difference of 1 to 10%, and the relationship between the velocity V of the surface acoustic wave and the operating frequency f is f = 5 V / (2PT).

  According to the configuration of the above (1), the electrode fingers of the interdigital electrodes and the reflector even when the same film thickness ratio H / λ is present at the operating frequency five times that of the fundamental wave (λ is the wavelength of the surface acoustic wave) Since the reflection coefficient of the conductor strip of about 60% of the fundamental wave can be secured, the vibration energy of the surface acoustic wave can be confined only by increasing the IDT logarithm of about 40% or the number of reflectors. There is an effect that a good surface acoustic wave resonator that operates in the second harmonic can be realized.

  (2) In the surface acoustic wave resonator of the present invention, the piezoelectric plate is made of a single crystal crystal, and the quartz rotating Y plate is rotated counterclockwise around the electric axis (X axis) from θ = 31 degrees to 42 degrees. The electrode fingers are arranged such that there is a phase propagation direction of the surface acoustic wave in the XP direction that is rotated and further rotated about 40 to 46 degrees from the electric axis around the normal of the main plane, The conductor strip of the reflector is made of a metal pattern in which an aluminum thin film is formed by photolithography, and the ratio of the film thickness H of the aluminum thin film to 2PR indicated by the arrangement period length PR of the reflector (0.02). It is characterized by being in the range of ± 0.01).

With the substrate configuration of (2) above, the film thickness ratio H / λ ₀ = H / (2PR) is 0.02 and the reflection coefficient takes a positive value in the fundamental wave operation state. Also, because the wavelength lambda ₃ of the surface acoustic wave becomes lambda _0/5 at the time of the fifth harmonic operation, a negative value is the reflection coefficient because the film thickness ratio H / lambda ₃ is 5 times 0.10 Take. On the other hand, the conditions of the IDT and the electrode circumference base length of the reflector for obtaining a forced energy confinement state require PR <PT when the reflection coefficient is negative, and when the reflection coefficient is positive. PR> PT is a necessary condition. Therefore, the fundamental wave is suppressed, and only the fifth harmonic operation is possible.

  Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

  First, the piezoelectric flat plate used in the present invention will be described. The piezoelectric plate is an in-plane rotated ST-cut quartz plate, and shows a cutting direction from the quartz crystal in FIG. In the figure, reference numeral 501 denotes an electric axis X, 502 which is a fundamental axis of a crystal crystal, 502 denotes a mechanical axis Y, and 503 denotes an optical axis Z. The in-plane rotation ST-cut quartz plate has a Y plate whose principal surface is a surface formed by two axes of the electric axis X and the optical axis Z, and the counterclockwise direction around the electric axis X is θ degrees (particularly a zero temperature coefficient is obtained). In the substrate 500 rotated by θ = 31 degrees to 42 degrees), an orientation in which the in-plane rotation angle ψ from the X axis is ± (40 to 46) degrees around the normal line 507 of the substrate The phase propagation azimuth X ′ (element x-axis 506) is used.

  Further, after the surface of the piezoelectric flat plate 500 is mirror-polished, the electrode fingers of a number of parallel conductors made of, for example, metal aluminum are periodically formed perpendicular to the phase propagation direction X ′ of the Rayleigh type surface acoustic wave. The surface acoustic wave resonator is configured by forming at least one interdigital electrode and forming a pair of reflectors on both sides thereof.

  In order to facilitate understanding of the embodiments of the surface acoustic wave resonator according to the present invention, examples of specific electrode patterns will be described with reference to FIG. 1, and then FIG. 2, FIG. 3, FIG. The operating principle will be explained in detail using.

  FIG. 1 shows an electrode pattern formed on a piezoelectric plate for an embodiment of a surface acoustic wave resonator according to the first aspect of the present invention.

  In FIG. 1, the names of the respective parts are as follows: 100 is a piezoelectric plate made of quartz, 101 and 103 are reflectors, 102 is an interdigital electrode, 104 and 105 are conductor strips constituting the reflector, 106 is a feeding conductor (bus bar) ) 108 is a positive electrode finger connected to 108, and 107 is a negative electrode finger connected to a power supply conductor (busbar) 109. Further, reference numeral 110 on the piezoelectric plate is an X ′ axis that corresponds to the phase propagation direction of the surface acoustic wave to be used (corresponding to 506 in FIG. 5).

  More specifically, in the reflectors 101 and 103 in FIG. 1, the symbol LR is the width (line) dimension of the conductor strip 104, and SR is the width (space) dimension not covered by the conductor. PR is the period length of the conductor strip composed of the sum of LR and SR. Next, in the IDT 102, the symbol LT is the width (line) dimension of the electrode fingers 106 and 107 and the like, and ST is the width (space) dimension not covered by the electrode conductor. PT is the period length of the electrode finger consisting of the sum of LT and ST.

  More specifically, LT / PT defined as the line width ratio in the IDT of 102 is in the range of (1/2 ± 1/12), and in the reflector, the line width ratio LR / PR is ( 1/2 ± 1/12), and the structure has a wide space area. The dimensions PR and PT have a difference of about 1 to 10% which is effective for most substrates because PR <PT. The electrode fingers and the conductor strips of the reflector are made of a metal pattern in which an aluminum thin film is formed by photolithography, and the ratio of the aluminum thin film H to the PR is in the range of (0.02 ± 0.01). .

  In this element having the above configuration, the frequency of the signal applied to the IDT 102 is approximately five times f3 = 5V / (2PT), where f0 = V / (2PT) is the fundamental operating frequency. It is characterized by being operated with

  Next, the operation characteristics of the element of the present invention will be described with reference to FIGS.

  FIG. 2 shows a reflection characteristic 200 of an IDT and a reflector having a periodic structure with 200 electrode fingers or conductor strips as shown in FIG. 1 according to the present invention. FIG. 2 shows the reflection characteristics in the fifth harmonic operating state at the frequency f5. The horizontal axis in FIG. 2 represents the line width ratio L / P of the electrode finger of the IDT and the conductor strip of the reflector. The line width ratio L / P is substantially the same as LT / PT for IDT, and substantially corresponds to LR / PR for a reflector. The vertical axis represents the reflection coefficient of the surface acoustic wave possessed by the IDT and the entire reflector, and indicates Γ, which is the sum of the reflection coefficients r of the individual electrode fingers and conductor strips. As shown in FIG. 2, the reflection coefficient Γ has a maximum value of 0.65 when L / P is about 0.1, 0.3, 0.5, 0.7, and 0.9. Therefore, if L / P is set to a value in the range of approximately (1/2 ± 1/12), the maximum value of the reflection coefficient can be obtained, and the minimum surface acoustic wave resonator can be obtained. On the other hand, FIG. 3 is a characteristic diagram showing a reflection characteristic 300 similar to FIG. 2 except that the operating frequency is the fundamental wave f0, and the maximum value of the reflection coefficient Γ is about 0.997. Show. In this case, the reflection coefficient Γ is maximum when L / P is 0.5.

  Although FIG. 2 is obtained at the operating frequency f5, the maximum value of 0.65 is only about a 35% decrease compared to the operating characteristic diagram 3 at the fundamental wave f0, and a surface acoustic wave resonator is obtained. This will not interfere with the configuration. Incidentally, even if the number of electrode fingers or conductor strips is greater than 200, at least the L / P value indicating the maximum value of the reflection characteristic does not change.

  Next, FIG. 4 shows the characteristics of the piezoelectric plate used in the structure of the surface acoustic wave resonator of the present invention. In FIG. 4, the horizontal axis represents the line width ratio L / P, and the vertical axis represents the reflection coefficient r per electrode strip or conductor strip of the reflector in unit%. A white circle point 400 (r = 3%) in the figure is a point indicating the reflection coefficient r when the line width ratio L / P is 0.5 and the film thickness ratio H / λ is 0.02. Reference numeral 401 denotes a point indicating a reflection coefficient r (= 10%) when the line width ratio L / P is 0.5 and the film thickness ratio H / λ is 0.09. A curve 402 is a boundary line where the reflection coefficient r is almost zero, and shows a boundary line between a region where r in the upper right half region 403 takes a negative value and a region where r in the lower left half surface 404 takes a positive value. In the surface acoustic wave resonator according to the present invention, the series equivalent resistance of the resonator in the fundamental wave operating state, that is, the so-called CI value can be increased and suppressed using the characteristics shown in FIG.

  The mechanism is as follows. First, in the fundamental wave operation f0, the film pressure ratio H / λ of the surface acoustic wave resonator of the present invention is set to +0.02 and the line width ratio L / P is set to 0.5 (corresponding to the white circle point 400 in FIG. 4). . At this time, the wavelength λ of the surface acoustic wave is 2PT, which is twice that of PT. Next, when operating at a frequency f5 that is five times f0, the wavelength λ of the surface acoustic wave becomes 1/5 of 2PT, so the film pressure ratio H / λ is 5 / (2PT). Doubled to 0.10 (corresponding to the black dot 401 in FIG. 4). As a result, the reflection coefficient becomes a negative value. From here on, the mechanism is as follows as shown in the schematic diagram of FIG.

  In FIG. 6, the horizontal axis represents the operating frequency f, and the vertical axis represents the relative value of the reflection coefficient and the radiation amplitude of the surface acoustic wave in the IDT or reflector. A solid curve 600 is a reflection characteristic Γ possessed by the IDT, and 601, 602, and 603 indicated by broken lines are radiation characteristics of the surface acoustic wave possessed by the same IDT. A curve 601 disposed below the flat portion of the reflection characteristic 600 is in the vicinity of f0 at the time of fundamental wave operation where the reflection coefficient r has a positive value, and a curve 603 disposed above one flat portion has a negative reflection coefficient r. It is a radiation characteristic of the surface acoustic wave that the same IDT has in the fifth harmonic operation f5 state taking a value.

  In general, regarding the frequency arrangement of the reflection characteristics of the reflectors constituting the surface acoustic wave resonator and the radiation characteristics of the IDT, the reflector assumes the reflection characteristic curve 600 of FIG. It is customary to set so that In other words, the IDT radiation characteristic 602 is arranged almost at the center of the reflection characteristic curve 600 so that the most forced reflection phenomenon is realized and a resonator having a high Q value is obtained. The period length PT of the electrode finger is set. In order to set in this way, in the fundamental wave operating state, the dimension of the electrode period length PT of the IDT is reduced and shifted from the curve 601 to the state 602. Therefore, a relationship of PT <PR is necessary. On the other hand, in the fifth harmonic operation state, the dimension of the electrode period length PT of the IDT is increased and shifted from the curve 603 to the state 602. Therefore, a relationship of PT> PR is necessary.

  What can be said from the above is that if PT> PR is set in order to set the best resonance condition in the fifth harmonic operation state, the IDT radiation characteristic is below the flat part of the reflection characteristic of the reflector in the fundamental wave operation state. In other words, sufficient reflection is not realized and a good resonance phenomenon cannot be obtained. Incidentally, it is known that the third harmonic can be excited by the IDT, and it is added that the third harmonic resonance does not actually occur. Accordingly, the resonance sharpness Q in the fundamental wave f0 is greatly reduced, and the equivalent series resistance CI value is increased several times as much as the CI value of the fifth harmonic operation. As a result, the fundamental wave, the second, third, and fourth harmonics are suppressed, and when the surface acoustic wave resonator of the present invention is used to construct a crystal SAW oscillator, the CI value of the fifth harmonic is small. Only the oscillation in which the fifth harmonic frequency f5 is selected is realized.

The top view which shows the electrode pattern which one Example of the surface acoustic wave resonator of this invention has. The characteristic view which shows the reflective characteristic in the 5th harmonic operation state of the surface acoustic wave resonator of this invention. The characteristic view which shows the reflective characteristic in the fundamental wave operation state of the surface acoustic wave resonator of this invention. The characteristic view which the piezoelectric material flat plate which is a component of the surface acoustic wave resonator of the present invention has. FIG. 3 is a cut orientation view of a piezoelectric plate that is a component of the surface acoustic wave resonator according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The general view explaining the principle of operation of the surface acoustic wave resonator of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... Piezoelectric body plate, 101, 103 ... Reflector, 102 ... Interdigital electrode (IDT).

Claims (2)

A surface acoustic wave resonator comprising at least one interdigital electrode for exciting a surface acoustic wave in a phase propagation direction X on a piezoelectric plate and a pair of reflectors disposed on both sides of the propagation direction,
The interdigital electrode is composed of a number of electrode fingers arranged substantially orthogonal to the phase propagation direction X of the surface acoustic wave, and the width dimension of the electrode finger in the phase propagation direction X direction is LT, and the presence of the electrode finger The line width ratio LT / PT with respect to PT = LT + ST, which is the arrangement period length of the interdigital electrode, is in the range of (1/2 ± 1/12), where ST is the dimension in the phase propagation direction X direction of the non-conducting region.
The reflector includes a large number of electrode fingers arranged substantially orthogonal to the phase propagation direction X of the surface acoustic wave, the width dimension of the electrode finger in the phase propagation direction X direction is LR, and the presence of the electrode finger The line width ratio LR / PR with respect to PR = LR + SR, which is the arrangement period length of the reflectors, is in a range of (1/2 ± 1/12), where SR is the dimension in the phase propagation direction X direction of the region that is not The relation between the arrangement period length PT of the interdigital electrodes and the arrangement period length PT of the reflectors is PR <PT, and the ratio of the arrangement period length PR of the reflectors to the arrangement period length PT of the interdigital electrodes is A surface acoustic wave resonator having a difference of 1 to 10%, and the relationship between the surface acoustic wave velocity V and the operating frequency f is f = 5 V / (2PT). The piezoelectric flat plate is made of a single crystal, and the quartz rotating Y plate is rotated counterclockwise around θ = 31 ° to 42 ° around the electric axis (X axis), and further, electric around the normal of the main plane. The electrode fingers are arranged so that the phase propagation direction of the surface acoustic wave exists in the XP direction rotated from 40 degrees to 46 degrees from the axis,
The conductor strips of the electrode fingers and the reflector are made of a metal pattern in which an aluminum thin film is formed by photolithography, and the ratio of the aluminum thin film H to 2PR indicated by the arrangement period length PR of the reflector is (0 The surface acoustic wave resonator according to claim 1, wherein the surface acoustic wave resonator is in a range of 0.02 ± 0.01).

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