JP2004207913A - Crystal vibrator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for enhancing a frequency-temperature characteristic and a CI-temperature characteristic of a high frequency piezoelectric vibrator. <P>SOLUTION: The piezoelectric vibrator is configured by disposing opposed electrodes on both principal sides of a piezoelectric substrate and by employing a plate back amount greater than that expressed in a relation of Δ=(2×F)<SP>1/2</SP>-3, where Δ(%) is a plate back amount by the electrodes and F(MHz) is a resonance frequency of the piezoelectric vibrator. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a crystal resonator, and more particularly to a high-frequency crystal resonator having improved frequency-temperature characteristics and CI-temperature characteristics.
[0002]
[Prior art]
2. Description of the Related Art Piezoelectric vibrators are widely used from communication devices to electronic devices because of their small size, small aging, easy acquisition of high-accuracy and high-stable frequencies, and the like. Among them, an AT-cut crystal resonator whose frequency-temperature characteristic exhibits a cubic curve is combined with a wide-range temperature compensation circuit as a TCXO (Temperature-Compensated Crystal Oscillator) for a mobile phone and housed in a constant temperature bath and an OCXO for a base station. (Highly stable crystal oscillator). The AT-cut quartz plate is also used as a piezoelectric substrate of an MCF (monolithic crystal filter) for IF of a mobile phone.
[0003]
Many researchers and engineers have long been involved in improving the frequency-temperature characteristics and the CI-temperature characteristics (CI is the crystal impedance and the effective resistance Rr of a crystal oscillator) and the Q value of an AT-cut quartz oscillator. On the other hand, in the AT-cut quartz resonator used for the filter, the biggest problem is how to suppress a plurality of unnecessary vibrations, that is, so-called inharmonic modes, generated near the high frequency of the main vibration.
As means for realizing good frequency-temperature characteristics and CI-temperature characteristics, in the low-frequency band crystal resonator of 10 MHz or less, the contour shape of the crystal plate is processed so that the vibration region of the main vibration is concentrated at the center of the substrate. A method has been devised. In the case of a crystal resonator of 10 MHz or more, the dimensions of the electrodes are made smaller in inverse proportion to the frequency, so that a good frequency-temperature characteristic and CI-temperature characteristic, or a crystal vibration in which the inharmonic mode near the resonance of the main vibration is suppressed. A method to realize the child has been experimentally found. However, the experiment had to be repeated every time the frequency changed, which took a lot of time.
[0004]
As described above, the improvement of the frequency-temperature characteristics and the CI-temperature characteristics of the crystal unit and the improvement of the spurious suppression near the resonance frequency have been mainly obtained through repeated experimental methods, but since the publication of the energy confinement theory, The optimal design of the crystal resonator (or crystal resonator) has been found by simple calculations. So that the energy confinement theory of well-known, the main vibration of the wave number k _{0 (also} referred to as a propagation constant) to the imaginary, and the wave number of the higher-order inharmonic mode _{k i (i = 1,2, ··} ) to the real number Thus, the confinement coefficient P (P = μ (a / h) √Δ, μ is the anisotropy constant, a is the size of the electrode, h is the thickness of the piezoelectric substrate, and Δ is the amount of frequency reduction in the excitation electrode portion ( is defined by the plate a back _{amount) Δ = (f s -f 0} ) / f 0. where f _s is the cutoff frequency, f ₀ of the piezoelectric substrate set the cut-off frequency) of the electrode portion, confined This is a method of determining the electrode size a and the plateback amount Δ based on the coefficient P. As described above, when the respective wave numbers k ₀ and k _i are set, the vibration energy of the main vibration is confined in the vicinity of the electrode, and the vibration energy of the higher-order inharmonic mode propagates to the periphery of the quartz plate, and the quartz oscillator It is the theory that only the main vibration can be strongly excited because the vibration energy leaks to the supporting portion and deteriorates.
[0005]
FIGS. 5A and 5B are diagrams showing a configuration of a conventional AT-cut crystal resonator, in which FIG. 5A is a plan view, and FIG. 5B is a cross-sectional view along QQ. Opposite electrodes 12 and 13 are arranged substantially at the center of the main surface of the AT-cut quartz substrate 11, and lead electrodes 14 and 15 extend from the electrodes 12 and 13 toward the ends of the substrate 11, respectively. A vibration element is formed.
As described above, the plate back amount Δ in the electrode portion of the crystal unit is determined by the cut-off frequency f ₀ of the region where the electrodes 12 and 13 are attached, and the cut-off frequency f _{0 of} the region where the crystal substrate where no electrode is formed is exposed. than the frequency f _s, can be determined by _{Δ = (f s -f 0)} / f 0.
In the case of a crystal resonator having a structure as shown in FIG. It has been generally accepted that the plateback amount Δ is set within a range from several% to about 2% as an upper limit.
[0006]
FIG. 6 is a cross-sectional view showing another configuration example of a conventional AT-cut quartz resonator, in which one of the opposing electrodes is a full-surface electrode 13 '. Full-surface electrode 13 'is the plate back amount delta, without also involved in the equivalent inductance L ₁ or the like, the electrode 12' only that load weight and size of is be related. Therefore, in order to realize a confinement coefficient P equivalent to that of FIG. 5, if the size of the electrode 12 and the electrode 12 ′ is the same, the thickness h ″ of the electrode 12 ′ The thickness may be twice the thickness h '.
[0007]
However, in recent years, with the increase in the frequency of the carrier frequency of the wireless device and the frequency of the carrier wave of the transmission device, a demand for a further increase in the frequency of the crystal oscillator has increased. As is well known, in an AT-cut quartz resonator, the resonance frequency is inversely proportional to the thickness of the piezoelectric substrate. Therefore, in order to obtain a high-frequency quartz resonator having a fundamental resonance frequency exceeding 50 MHz, the quartz plate is thinned to the order of microns. There is a need to. When the quartz plate is thinned down to the micron order, its processing becomes more difficult, and its holding and handling are not easy.
Therefore, as one of means for realizing a thin AT-cut plate, a part of one main surface of the AT-cut quartz plate is depressed by means of etching or the like to form a thin plate-shaped thin portion (vibrating portion) and the thin wall. A high-frequency crystal resonator that realizes a high-frequency fundamental wave resonance frequency of more than 50 MHz while maintaining the mechanical strength of the thin-walled part by integrally forming a thick annular surrounding part that supports the periphery of the part Has been put to practical use.
[0008]
FIG. 7A is a plan view showing the configuration of the above-described high-frequency crystal resonator, FIG. 7B is a rear view, and FIG. 7C is a cross-sectional view taken along QQ. One main surface is kept flat, and the other main surface is provided with an electrode 23 on the flat surface side of the AT-cut quartz plate 21 in which the recessed portion 22 is formed by means of etching or the like. 24 and connected to a pad electrode 25 provided at the end of the quartz plate 21. Further, an electrode 26 is arranged on the recessed surface side 22 so as to face the electrode 23, a lead electrode 27 extends from the electrode 26, and is connected to a pad electrode 28 provided at an end of the quartz plate 21, and Construct a vibrator.
[0009]
A fundamental high-frequency crystal resonator having a structure as shown in FIG. 7 was prototyped by setting the fundamental wave resonance frequency of the crystal resonator to 60 MHz. Five samples were made with the size of the quartz plate 21 being 2.3 mm × 2.3 mm, the size of the recess 22 being 1.0 mm × 1.0 mm, and the diameter thereof being 0.8 mmφ using a circular electrode.
As described above, when the fundamental frequency is increased, the thickness of the quartz substrate is reduced to the order of microns. Then, the electrode film becomes thinner accordingly, but if the electrode film is thinner, the effective resistance is deteriorated due to ohmic crossing of the electrode film. Thus, the plateback amount Δ is set to the upper limit of 2%, and a light metal, here, aluminum is used as the electrode material in order to increase the film thickness.
[0010]
[Problems to be solved by the invention]
However, as is apparent from FIG. 8A showing the frequency-temperature characteristics of the prototype sample and FIG. 8B showing the CI-temperature characteristics, the frequency-temperature characteristics deviate significantly from the smooth cubic curve, The CI-temperature characteristic has a problem that when the ambient temperature is changed, the flatness of the effective resistance Rr is significantly impaired. The present invention has been made to solve the above problems, and has as its object to provide a high-frequency vibrator with improved frequency-temperature characteristics and CI-temperature characteristics.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 of the crystal resonator according to the present invention is directed to a crystal resonator configured by arranging opposing electrodes on both main surfaces of an AT-cut crystal substrate. When the fundamental wave resonance frequency is F (MHz) and the amount of plate back at the electrode portion is Δ (%),
Δ> (2 · F) ^1/2 −3
A crystal resonator characterized by satisfying a relationship represented by:
According to a second aspect of the present invention, there is provided a quartz resonator having counter electrodes arranged on both main surfaces of an AT-cut quartz substrate, wherein a fundamental wave resonance frequency of the quartz resonator is F (MHz), and a plate in the electrode portion is provided. When the back amount is Δ (%),
(2 · F) ^1/2 −3 <Δ <(13 · F) ^1/2 −3
A crystal resonator characterized by satisfying a relationship represented by:
The invention according to claim 3 is the crystal resonator according to claim 2, wherein the invention is applied to a crystal resonator having a fundamental frequency of 200 MHz or more.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.
FIG. 1A is a plan view showing the configuration of a high-frequency crystal resonator according to the present invention, FIG. 1B is a rear view, and FIG. 1C is a cross-sectional view taken along QQ. One main surface of the quartz plate 1 is kept flat, and the other main surface is provided with an electrode 3 on the flat surface side of the AT-cut quartz plate 1 in which the recess 2 is formed by means such as etching. A lead electrode 4 extends from 3 and is connected to a pad electrode 5 provided at an end of the quartz plate 1. An electrode 6 is arranged on the concave side 2 so as to face the electrode 3, a lead electrode 7 extends from the electrode 6, and is connected to a pad electrode 8 provided at an end of the quartz plate 1, thereby forming a high-frequency crystal. Construct a vibrator.
[0013]
FIG. 2 shows the electrical characteristics of a 60 MHz fundamental wave crystal resonator as a first embodiment according to the present invention. The actual size of the quartz plate 1 is 2.3 mm x 2.3 mm, the size of the recess 2 (vibrating part) is 1.0 mm x 1.0 mm, the diameter of the circular electrodes 3 and 6 is 0.8 mm, As a result, the frequency-temperature characteristics and the CI-temperature characteristics of ten samples were measured. FIG. 2A shows frequency-temperature characteristics, and FIG. 2B shows CI-temperature characteristics.
As is clear from the figure, the frequency-temperature characteristic of the high-frequency crystal resonator according to the present invention exhibits a smooth cubic curve, and the effective resistance Rr (CI) is almost constant even when the ambient temperature is changed. It can be seen that the value is as good as 13Ω. Based on the conventional theory and results, it can be confirmed that the plate resonator is greatly improved even when compared with FIG. 8 of the crystal resonator in which the plate back amount Δ at the electrode portion is set to 2%.
Initially, the inventor of the present application set the plateback amount Δ to a large value of 2%, which was conventionally set to the upper limit of a general numerical value, in order to avoid deterioration of the effective resistance due to ohmic crossing of the electrode film. The inconvenience of the prior art was considered to be due to the fact that the confinement of energy in the electrode became too strong and the vibration estimated to be an unnecessary mode of higher-order contour system vibration and the main vibration were coupled.
However, in practice, in an AT-cut quartz resonator having a high fundamental wave resonance frequency, the characteristics are improved by increasing the plateback amount Δ to a value (8%) which is much larger than the conventional common sense value. This led to the surprising fact.
Although the resonance spectra of the main vibration and the inharmonic mode are not shown, the largest effective resistance value in the inharmonic mode was three times or more that of the main vibration. Therefore, it has been confirmed that even when the crystal resonator according to the present invention is used for an oscillation circuit, there is no risk of occurrence of a so-called frequency jump phenomenon that oscillates in an inharmonic mode.
[0014]
FIG. 3 shows the electrical characteristics of a 310 MHz fundamental wave crystal resonator as a second embodiment. Assuming that the size of the quartz plate 1 is 1.0 mm × 1.5 mm, the size of the concave portion 2 (vibrating portion) is 0.5 mm × 0.5 mm, the diameter of the electrodes 3 and 6 is 0.2 mmφ, and the plateback amount Δ in the electrode portion is 23%. Samples were prototyped, and their frequency-temperature characteristics and CI-temperature characteristics were measured. FIG. 3A shows frequency-temperature characteristics, and FIG. 3B shows CI-temperature characteristics. As is clear from this figure, the frequency-temperature characteristic exhibits a smooth cubic curve, and the effective resistance Rr (CI) shows a substantially constant value of 26Ω even when the temperature is changed. Various environmental tests were performed using this high-frequency crystal oscillator as a voltage controlled crystal oscillator (VCXO), but no frequency jump phenomenon occurred.
[0015]
Although two examples have been described above, the feature of the present invention lies in that the plateback amount Δ in the electrode portion is greatly increased without being bound by the conventional energy confinement theory or the conventional design results.
Since it is possible to set a large plateback amount Δ, a metal having a large mass can be used as an electrode material, and an electrode can be formed using gold instead of light aluminum. As is well known, gold is a metal that is extremely chemically stable and is the most suitable material as an electrode material, and the temperature of heat treatment and the like during the crystal oscillator manufacturing process can be set to a high temperature. There is also obtained a secondary effect that frequency aging and the like are improved.
[0016]
Furthermore, various experiments were performed on a high-frequency fundamental wave crystal resonator. As a result, no higher-order contour vibration was coupled to the main vibration, and the frequency-temperature characteristics exhibited a smooth cubic curve. It has been found that the characteristics are also substantially flat straight lines. The resonance level of the inharmonic mode was also suppressed to be smaller than that of the main vibration, and it was found that it could be used sufficiently for an oscillator. For example, a prototype of a 600 MHz band fundamental wave crystal resonator was mounted on a VCXO substrate, and frequency-temperature characteristics and a frequency jump phenomenon due to a temperature change were measured. Good results were obtained. The amount of plate back in the electrode portion of the crystal unit in the 600 MHz band exceeded 30%. From these experiments, the relationship between the resonance frequency F (MHz) of the crystal unit and the plateback amount Δ (%) can be expressed by an approximate expression as follows.
Δ ₁ = (2 · F) ^1/2 −3 (1)
[0017]
On the other hand, when the electrode film is thick, distortion of the quartz plate due to electrode film distortion, vibration loss of the electrode film, and the like occur, so that the electrode film cannot be made too thick.
Based on this fact, the inventor of the present application has not confirmed by experiment yet, but the relationship between the fundamental resonance frequency F (MHz) of the crystal resonator and the thickness of the electrode film, that is, the plate back amount Δ (%) is Infer that it should be smaller than the value represented by the following equation.
Δ ₂ = (13 · F) ^1/2 −3 (2)
Figure 4 is a diagram illustrating a relationship between the resonant frequency F of the crystal oscillator (MHz) and the plate back amount delta (%), the lower limit of the delta ₁ represented by Formula 1 plate back amount delta, in Formula 2 It represents delta ₂ indicates the upper limit of the plate back amount delta, if a crystal unit is configured using a delta-F in this range, frequency - both temperature characteristics and CI- temperature characteristics good oscillator obtained.
In the figures, α and β are plots of experimental values of the first and second embodiments.
[0018]
As the frequency of the fundamental wave crystal resonator is increased, difficulties arise in the etching of the quartz plate. In particular, when the thickness of the concave portion (vibrating portion) is about 7 μm, the parallelism, flatness, and the like between the flat surface that is not etched and the etched surface on the concave side affect the electrical characteristics of the vibrator. When the electrode film is thin, the influence of the substrate tends to appear significantly. Therefore, it is considered that making the plateback amount according to the present invention much larger than the normal design is an effective means for realizing good frequency-temperature characteristics and CI-temperature characteristics. For this reason, the present invention is considered to be a particularly effective means for producing a high-frequency fundamental-wave crystal resonator of 200 MHz or more (corresponding to a thickness of the substrate of about 7 μm) or more.
Also, as is well known, the frequency-temperature characteristic exhibits a curve deviated from an ideal cubic curve as if the cutting angle of the quartz plate was rotated due to the mass effect of the electrode film. In other words, in order to obtain a desired frequency-temperature characteristic exhibiting a desired cubic curve, the cutting angle of the crystal substrate is corrected in advance according to the plateback amount Δ in the same manner as in the conventional case.
[0019]
【The invention's effect】
Since the present invention is configured as described above, the invention described in claim 1 shows an excellent effect of giving a lower limit of the plate back amount to the frequency F when obtaining a good fundamental wave crystal resonator. The invention described in claim 2 shows an excellent effect of giving the upper limit of the plateback amount to the frequency F. The invention described in claim 3 shows an excellent effect of giving an important design guideline in manufacturing a crystal oscillator of 200 MHz or more which is particularly important in practical use.
[Brief description of the drawings]
1A and 1B are diagrams showing a configuration of a high-frequency crystal resonator according to the present invention, wherein FIG. 1A is a plan view, FIG. 1B is a rear view, and FIG.
FIGS. 2A and 2B are diagrams showing electric characteristics of a 60 MHz fundamental wave crystal resonator. FIG. 2A shows frequency-temperature characteristics, and FIG. 2B shows CI-temperature characteristics.
3A and 3B are diagrams showing electric characteristics of a 310 MHz fundamental wave crystal resonator, wherein FIG. 3A shows frequency-temperature characteristics and FIG. 3B shows CI-temperature characteristics.
[4] In relation diagram between the frequency F and the plate back amount delta of the present invention, a plate back amount within the desired range between the curve of the upper delta indicated by lower curve and delta ₂ indicated by delta _1.
5A and 5B are diagrams showing a configuration of a conventional AT-cut crystal resonator, wherein FIG. 5A is a plan view and FIG. 5B is a cross-sectional view.
FIG. 6 is a cross-sectional view showing a configuration of a conventional high-frequency AT-cut quartz resonator.
7A and 7B are diagrams showing a configuration of a conventional fundamental wave crystal resonator, wherein FIG. 7A is a plan view, FIG. 7B is a rear view, and FIG. 7C is a cross-sectional view.
FIGS. 8A and 8B are diagrams showing electric characteristics of a conventional 60 MHz fundamental wave crystal resonator, wherein FIG. 8A shows frequency-temperature characteristics and FIG. 8B shows CI-temperature characteristics.
[Explanation of symbols]
1 ··· Piezoelectric substrate 2 ··· Recess 3, 6 ··· Electrode 4, 7 ··· Lead electrode 5, 8.

Claims (3)

In a quartz oscillator configured by arranging opposed electrodes on both main surfaces of an AT-cut quartz substrate, a fundamental wave resonance frequency of the quartz oscillator is F (MHz), and a plateback amount in the electrode portion is Δ (%). When
Δ> (2 · F) ^1/2 −3
A crystal resonator characterized by satisfying the relationship represented by: In a quartz oscillator configured by arranging opposed electrodes on both main surfaces of an AT-cut quartz substrate, a fundamental wave resonance frequency of the quartz oscillator is F (MHz), and a plateback amount in the electrode portion is Δ (%). When
(2 · F) ^1/2 −3 <Δ <(13 · F) ^1/2 −3
A crystal resonator characterized by satisfying the relationship represented by: 3. The crystal unit according to claim 2, wherein the crystal unit has a fundamental frequency of 200 MHz or more.

Images