JP2000040938A - Ultra high frequency piezoelectric device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a ultra high frequency crystal resonator which suppresses spurious vibration in the neighborhood of principal vibration by arranging a stepped part, that becomes a boundary between a vibrating part and an annular part which surrounds its periphery at a specified position. SOLUTION: The part of one principal plane of an AT cut quarts substrate 1 is made recessed by etching and the bottom of a recessed part 2 is etched to prescribed thickness to form a ultra thin vibrating part 3, and also a thick annular surrounding part which supports the circumference of the part 3 is formed integrally. A full electrode 5 is provided on the part 2 side and a partial electrode 6, a lead electrode 7 and a pad electrode 8 are formed on a flat side. The displacement of principal vibration (S0 mode) is small by appropriately setting the dimension W of the part 3 and a stepped part 4 which is a boundary between the part 3, so that an annular part is located at a position where the displacement of the primary symmetrical vibrating mode (S1 mode) is sufficiently larger than the displacement of the S0 mode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high frequency piezoelectric device, and more particularly to an ultra-high frequency piezoelectric device in which spurious components near main vibrations are suppressed.

[0002]

2. Description of the Related Art In recent years, the demand for piezoelectric devices used in these apparatuses has been increasing with the increase in the frequency of communication equipment, the speed of data processing, and the increase in capacity. One such ultrahigh-frequency piezoelectric device is an AT-cut ultrahigh-frequency crystal oscillator, whose frequency is practically used up to several hundred MHz. As is well known, the vibration mode of the AT-cut crystal resonator is a thickness shear vibration, and its frequency is inversely proportional to the thickness of the crystal substrate. Therefore, it is necessary to reduce the thickness of the crystal substrate in order to increase the frequency. . However, when the AT-cut quartz substrate is made of a flat plate, the fundamental wave vibration is about 55 MHz (about 30 μm in thickness) in consideration of mechanical strength such as impact resistance.
Is considered to be the upper limit frequency.

[0003] An AT-cut crystal resonator developed to increase the upper limit of the frequency is an ultra-high-frequency AT-cut crystal resonator (hereinafter, referred to as an ultra-high-frequency resonator) manufactured using photolithography and etching. Yes, FIG. 4
(A) is a sectional view showing the configuration, and (b) is a perspective view. First, the manufacturing process of the quartz substrate will be described. In order to increase the frequency while maintaining the mechanical strength, a thickness of about 80 mm is required.
A part of one main surface of the AT-cut quartz substrate 11 of μm is depressed by using a photolithography technique and etching, and the thickness of the bottom surface of the depressed portion 12 is etched to a predetermined thickness. The vibrating portion 13 is formed, and a thick annular surrounding portion that supports the periphery of the vibrating portion 13 is integrally formed. At this time, since the etched plane of the ultra-thin vibrating portion 13 has a property of being etched in parallel with the other main surface of the quartz substrate 11, the vibrating portion 13 having good flatness and parallelism is formed. You. On the other hand, since the quartz substrate 11 has anisotropy in the crystal axis direction, the steps 14a and 14b around the recess 12 have a property of being etched with a different inclination in the thickness direction depending on the crystal axis direction.

After the thickness of the ultra-thin vibrating part 13 is etched to a predetermined thickness, a gold electrode 15
Is attached to form one electrode, and a partial electrode 16, a lead electrode 17 extending from the electrode, and a bonding portion 18 are formed on the plane side by using a photolithography technique. In this example, the crystal axis is set such that the direction in which the lead electrode 17 of the crystal substrate 11 extends is the X axis, and the direction perpendicular thereto is the Z ′ axis.

[0005] The vibration mode of the ultrahigh-frequency vibrator is the same as that of the conventional AT.
The vibration mode is the same as the vibration mode of the cut quartz crystal resonator, and there are a thickness shear mode and a thickness twist mode. The former is a vibration mode that displaces in the X-axis direction and propagates in the X-axis direction, and the latter is a vibration mode that displaces in the X-axis direction and propagates in the Z′-axis direction. In the ultrahigh-frequency vibrator having the structure shown in FIG. 4, the energy confinement in any of the above vibration modes depends on the mass load effect of the partial electrode 16. Further, it is known that the displacement distribution of the vibration mode has a cosine shape on the partial electrode 16 and decays exponentially with the distance from the electrode 16. On the partial electrode 16, in addition to the symmetric lowest-order vibration mode (S ₀ mode) that is the thickness-shear main vibration, a number of symmetric vibration modes S _i (i = 1, 2,...) And anti-symmetric vibration modes Ai (i = 0, 1, 2) are excited, and these anharmonic vibration modes (inharmonic modes) are spurious vibration modes with respect to the main vibration.

The theory for unifying the behavior of the main vibration and spurious vibration mode of the AT-cut vibrator is as follows:
This is a so-called energy confinement theory. As one of the results, the following formula is well known for a thickness shear vibration as a design method of an AT-cut crystal resonator in which spurious is suppressed. L ₁ × √Δ / H = 2.8 (1) Further, for thickness torsional vibration, L ₂ × √Δ / H = 2.4 (2). Here, L ₁ and L ₂ are the dimensions of the partial electrode 16 in the X-axis direction and the Z′-axis direction, and H is the thickness of the vibrating portion 13 and the thickness of the electrode film 15 on the side of the recess converted to the thickness of the quartz substrate. It is sum. Further, Δ is the amount of decrease in frequency, and is generally expressed by the following equation, where fs is the cutoff frequency of the quartz substrate, and fe is the cutoff frequency when electrodes are attached. Δ = (fs−fe) / fs (3) However, in the case of an ultrahigh-frequency oscillator having a structure as shown in FIG.
_{If E} is the film thickness of the partial electrode, it can be expressed as Δ = H _E / (H + H _E ).

In a general AT-cut crystal resonator, various constants are set so that only the main vibration is in the confinement mode by using the equations (1) to (3). If the above equation is applied to a vibrator whose frequency is increased to several hundred MHz, the thickness of the vibrating portion 13 becomes extremely thin, so that the electrode dimensions L ₁ and L ₂ of the partial electrode 16 are made extremely small or thickness H or extremely thin _E of, or need a very small value the value of the both. FIG. 5 shows, as various constants of the conventional high-frequency vibrator, the thickness H of the vibrating portion 13 is 2.237 μm, the frequency reduction Δ is 0.17, and the substrate lengths of the vibrating portion on the X axis and the Z ′ axis are shown. W
1 and W2 are 0.92 mm and 1.16 mm, respectively, and the electrode dimensions L ₁ and L ₂ are both set to 0.2 mm. The resonance spectrum is 619 MH. The horizontal axis represents the frequency (MHz), and the vertical axis represents the reactance (Ω) of the ultrahigh frequency vibrator.

[0008]

However, as apparent from FIG. 5, the conventional ultrahigh-frequency vibrator shows that many spurious vibration modes are in an energy confined state. Thickness H _E partial electrodes 16 of the super high frequency oscillator, restrictions on the deposition technique, in terms of reliability and the like of the ohmic loss or aging, can not be thinned below a certain thickness. Also, as the dimensions L ₁ and L ₂ of the partial electrode 16 are reduced, the equivalent inductance of the vibrator increases, which is not suitable for use as an oscillator element. Due to such restrictions, a large number of anharmonic higher-order vibration modes existing in the X-axis direction and the Z′-axis direction become confinement modes, and a large number of strong spurious vibrations are generated. When a high-frequency vibrator in which such a strong spurious vibration exists in the vicinity of the main vibration is used as a vibrating element of an oscillator, abnormal oscillation (a phenomenon of oscillating in a spurious vibration mode instead of oscillating in a main vibration mode) is caused. There was a problem. Further, when the high-frequency vibrator is used for a voltage-controlled crystal oscillator, there is a problem that the frequency variable range is limited due to spurious vibration. The present invention has been made to solve the above problem, and has as its object to provide an ultrahigh-frequency crystal resonator in which spurious vibrations near the main vibration are suppressed.

[0009]

According to a first aspect of the present invention, there is provided an ultrahigh frequency piezoelectric device comprising a thin vibrating portion and an annular portion surrounding a peripheral edge of the vibrating portion. integrally formed in the piezoelectric substrate, the ultra-high frequency piezoelectric vibrator in which a pair of electrodes facing each other on the surface of the vibrating portion, the displacement of the main vibration serving S ₀ mode excited in the vibrating section is small, ultra characterized by being configured to position the stepped portion displacement of the first order symmetric mode vibration serving S ₁ mode is a boundary between the annular portion and the vibrating portion in larger position than that of the main vibration It is a high-frequency piezoelectric device. According to a second aspect of the present invention, a thin vibrating portion and a thick annular portion surrounding the periphery of the vibrating portion are integrally formed on a piezoelectric substrate, and a pair of electrodes opposed to each other on a surface of the vibrating portion. , The ratio _Li / H of the dimension L _i (i = 1, 2) of the partial electrode to the thickness H of the vibrating portion in the ultrahigh frequency piezoelectric vibrator provided with
From 60 to 150, and the frequency reduction amount Δ from 0.04 to 0.28
Wherein the ratio W ₁ / H of the dimension W ₁ of the vibrating section 3 to the thickness H of the vibrating section is set to be 195 to 215. According to a third aspect of the present invention, a thin vibrating portion and an annular portion surrounding the periphery of the vibrating portion are integrally formed on a piezoelectric substrate, and an electrode having at least one divided electrode on the surface of the vibrating portion is provided. in opposing microwave piezoelectric multimode filter arranged, wherein a two main vibration excited in the vibrating portion S ₀ mode and a ₀ mode displacement is small, a ₁ mode displacement is a first order mode vibration is mainly An ultrahigh-frequency piezoelectric device, wherein a step portion serving as a boundary between the vibrating portion and the annular portion is arranged at a position where the level is larger than that of the vibration. The invention according to claim 4 is the ultrahigh frequency piezoelectric vibrator according to claims 1 or 2, wherein the piezoelectric substrate is a quartz substrate.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 is a view showing a configuration of an ultrahigh-frequency vibrator according to the present invention. FIG. 1 (a) is a sectional view, and FIG. 1 (b) is a perspective view. AT of about 80μm thickness
A part of one main surface of the cut quartz substrate 1 is recessed by using a photolithography technique and etching.
Is etched to a predetermined thickness to form the ultra-thin vibrating portion 3 and integrally form a thick annular surrounding portion that supports the periphery of the vibrating portion 3. As described above, the shape of the concave portion 2 and the flatness and parallelism of the vibrating portion 3 can be formed with good reproducibility using photolithography and etching.

After finishing the thickness of the vibrating portion 3 of the ultrahigh frequency piezoelectric substrate to a predetermined thickness, the entire surface electrode 5 is applied to the concave portion side,
The partial electrode 6 is formed on the flat side by using a photolithography technique.
And a lead electrode 7 and a pad electrode 8 extending from the electrode.
Is formed and housed in a ceramic package or the like,
An ultra-high frequency vibrator is made by conducting the terminals. The feature of the ultrahigh-frequency vibrator according to the present invention shown in FIG.
The dimension L _i of the partial electrode 6 (i = 1, 2 where the suffix 1 represents the X-axis direction and 2 represents the Z′-axis direction), which is the index of suppression of the spurious vibration mode of the cut crystal resonator, and the dimension of the vibrating section 3 W _i (fix 1 i = 1,2 is the X-axis direction, 2 represents the Z 'axis direction) taking into account and, in point greatly suppressed spurious mode of the main vibration vicinity. The principle is the case where the excitation electrodes 10 provided on the piezoelectric substrate 9 having the same thickness as the vibration portion 3 as shown in FIG. 2, the displacement distribution of the S ₀ mode is the main vibration becomes as shown in a, is spurious vibration mode S ₁ mode can be expressed as b. Small displacement of the S ₀ mode in the present invention, S ₁ displacement is sufficiently large position compared to S ₀ mode, i.e. such that the step portion is located is a boundary between the vibrating section and the annular portion in the position of the dotted line in FIG. Constitute.

With such a configuration, the main vibration (S
While the displacement of ₀₎ is sufficiently small, the displacement of the higher-order spurious vibration mode from the first-order mode (S ₁ mode) is relatively larger at the step portion and the outer, higher-order spurious In the mode, irregular reflection occurs at the steps 4a and 4b, and the energy is greatly lost. That is, the confinement coefficient (L _i) obtained from the confinement theory
Calculating the × √Δ / H), becomes the main vibration not only mode confinement to the high-order spurious vibration mode, these spurious vibration is excited in the stress, the dimensions W _i of the vibrating portion 3 than the conventional small and, since it dissipate irregularly reflected higher order spurious vibration modes of the first order symmetric vibration mode (S ₁ mode), only the primary vibration is excited in the stress. By appropriately setting the dimension W _i of the vibrating part 3 in this way, only the main vibration (S ₀ mode) is set as the confinement mode, and the resonance intensity from the symmetric primary vibration mode (S 1 mode) to a higher vibration mode is set. Is a feature of the present invention.

FIG. 3 shows an example of the ultrahigh-frequency vibrator according to the present invention. The thickness H of the vibrating section is 2.237 μm, the frequency reduction is 0.17, and the dimensions W ₁ and W _{2 of the} vibrating section 3 are various constants. Are 0.4 mm and 0.44 mm, respectively, and the dimensions L ₁ and L ₂ of the partial electrode 6 are both 0.2 mm. The resonance frequency is 618 MHz. Although the electrical constants are equivalent to those shown in FIG.
In the frequency spectrum of FIG. 3, the spurious vibration mode near the main vibration is greatly suppressed as compared with that of the conventional frequency spectrum of FIG. What is particularly noteworthy is that the spurious vibration mode near the anti-resonance point is suppressed, so that the spurious vibration mode does not overlap the anti-resonance frequency. Furthermore,
As a result of conducting experiments with various parameters changed, 600M
In the Hz band, when the ratio L _i / H of the dimension L _i (i = 1, 2) of the partial electrode to the thickness H of the vibrating portion is approximately 100 and the frequency decrease Δ is approximately 0.17, the vibration the ratio W ₁ / H dimension W ₁ and the thickness H of the vibrating portion parts 3 195-21
It has been clarified that the spurious in the vicinity of the main vibration is suppressed when the setting is made to be 5. Also, in the case of other resonance frequencies, L _i / H is 60 to 150, and Δ is 0.04.
When it was set to 結果 0.28, it was confirmed that equivalent results were obtained when W ₁ / H was in the range of 195 to 215. Suppressing the spurious vibration mode close to the main vibration in this way not only eliminates problems such as abnormal oscillation when using the ultrahigh-frequency oscillator as an oscillator, but also controls the ultrahigh-frequency oscillator with voltage control. When used as an oscillator element, the frequency fluctuation width can be greatly improved.

In the above description, an example has been given in which the present invention is applied to a 618 MHz band fundamental wave crystal oscillator. However, the present invention is not limited to this, but is applied to an ultra-high frequency oscillator of any frequency. It is possible. Regarding the shape of the concave portion of the crystal resonator, a concave portion may be formed opposite to both surfaces of the crystal substrate, and the ultra-thin vibrating portion or the partial electrode may be circular. In the above description, the entire surface electrode 5 is provided on the concave portion 2 side of the substrate 1 and the partial electrode 6 is provided on the flat side, but it is needless to say that the reverse or both may be partial electrodes. Furthermore, the lead electrode is X
Although extending in the axial direction, it may be in the Z-axis direction, and it goes without saying that any direction is suitable.

It is needless to say that the present invention can be applied to a dual mode filter using two vibration modes. For double-mode filter, in addition to the symmetrical oscillation mode to the one of the electrodes divided electrodes (S _i-mode), antisymmetric vibrational mode (A _i-mode) is also excited.
Accordingly, the diaphragm size of the vibrating unit 2 of the main vibration S _0, A ₀ the vibration displacement mode is sufficiently small, these higher order spurious vibration _{S 1, S 2, S 3} ·· or A _1, A ₂ , A ₃
The vibration displacement may be relatively large, and the dimensions of the vibration portion may be set such that the vibration displacements (vibration energy) reach the step portions 4a and 4b at the ends of the vibration portion 3 and are irregularly reflected.

Although the above description has been made by taking a quartz substrate as an example of the piezoelectric substrate, the present invention is not limited to quartz, and other piezoelectric materials, langasite, lithium tantalate, niobate tantalate, and tetraborate can be used. It goes without saying that the present invention can be applied to piezoelectric materials such as lithium.

[0017]

As described above, the present invention is constructed as described above. Therefore, in an ultra-high frequency device, the thickness of the diaphragm of the ultra-thin vibrating portion is restricted by the film formation, the reliability of the electrode film, or For ease of use, etc., we found a means to effectively suppress spurious modes even if the electrode film thickness or electrode size is increased beyond the energy confinement coefficient.
An ultra-high frequency piezoelectric device having no spurious components near the main vibration can be realized. If this device is used in a communication device or the like, the performance can be greatly improved.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams showing a configuration of an ultrahigh-frequency crystal resonator according to the present invention, wherein FIG. 1A is a cross-sectional view and FIG. 1B is a perspective view.

FIG. 2 is a cross-sectional view illustrating a displacement portion distribution in a main vibration mode a and a symmetric primary vibration mode b.

FIG. 3 is a diagram showing a frequency spectrum of the ultrahigh frequency crystal resonator according to the present invention.

FIG. 4 is a diagram showing a configuration of a conventional ultra-high frequency crystal resonator.
(A) is a sectional view, and (b) is a perspective view.

FIG. 5 is a diagram showing a frequency spectrum of a conventional ultrahigh-frequency crystal resonator.

[Explanation of symbols]

1. AT cut crystal substrate 2. Depressed part 3. Vibration part 4. Step part 5. Depressed part side electrode (backside electrode) 6. Partial electrode 7. Lead electrode 8. Pad electrode L ₁ , L ₂ ... X-axis and Z′-direction dimensions of the partial electrode W ₁ , W ₂ ... X-axis and Z′-direction dimensions of the diaphragm of the vibrating part HE _E. X, Z '· · · Crystal axis of AT-cut quartz substrate

Claims (4)

[Claims] 1. A thin vibrating portion and a thick annular portion surrounding a peripheral edge of the vibrating portion are integrally formed on a piezoelectric substrate, and a pair of electrodes facing each other are provided on a surface of the vibrating portion. in very high frequency piezoelectric transducer, the vibration in the main are excited in the vibration portion vibrating serving S ₀ mode of the displacement is small, displacement of the first order symmetric mode vibration serving S ₁ mode is greater than that of the main vibration position An ultrahigh frequency piezoelectric device, wherein a step portion serving as a boundary between a portion and the annular portion is arranged. 2. Surrounding a thin vibrating portion and a peripheral edge of the vibrating portion.
And a thick annular portion formed integrally with the piezoelectric substrate,
Ultra high height with a pair of electrodes facing each other on the surface of the vibrating part
In the frequency piezoelectric vibrator, the dimension L of the partial electrode_i(I = 1, 2) and the thickness H of the vibrating part
The ratio L_i/ H is 60 to 150 and the frequency reduction amount Δ is 0.0
4 to 0.28, the dimension W of the vibrating portion 3 ₁And before
Ratio W to thickness H of recording part₁/ H is 195 to 215
Ultra-high frequency piezoelectric device characterized by setting
Su. 3. A thin vibrating portion and a thick annular portion surrounding the periphery of the vibrating portion are integrally formed on a piezoelectric substrate, and an electrode having at least one of the divided electrodes on a surface of the vibrating portion. in opposing microwave piezoelectric multimode filter arranged, wherein a two main vibration excited in the vibrating portion S ₀ mode and a ₀ mode displacement is small, a ₁ mode displacement is a first order mode vibration is mainly An ultrahigh-frequency piezoelectric device, wherein a step portion serving as a boundary between the vibrating portion and the annular portion is arranged at a position where the level is larger than that of the vibration. 4. The ultrahigh frequency piezoelectric device according to claim 1, wherein said piezoelectric substrate is a quartz substrate.