WO2012137027A1

WO2012137027A1 - Surface acoustic wave resonator

Info

Publication number: WO2012137027A1
Application number: PCT/IB2011/000814
Authority: WO
Inventors: Viktor Plesski; Valery GRIGOREVSKIY
Original assignee: Gvr Trade Sa
Priority date: 2011-04-07
Filing date: 2011-04-07
Publication date: 2012-10-11

Abstract

Surface acoustic wave resonator and surface acoustic wave oscillator. A surface acoustic wave resonator includes: an IDT (3) which is disposed on a quartz crystal substrate (2) with orientation described by Euler angles of (-1°<φ<1°, 110°<θ<145°, 35°<|ψ|<50°) and which excites a surface acoustic wave in an upper mode of a stop band; and grooves(2) are formed by recessing the quartz crystal substrate; and the IDT fingers (6) are positioned at bottoms of the grooves in such a way that the fingers contact the left and the right side walls of the grooves, and contact the groove bottoms, wherein the following expressions: 0<h_Al/λ<0.05, h_GR>h_AI, where h_GR represents a depth of the grooves (9), h_Al represents a thickness of the IDT fingers (6), are satisfied and when a duty factor of the IDT is η and set to satisfy the following expression 0.5≤η≤0.75.

Description

Surface acoustic wave resonator

Description

Background

1. Technical field to which invention relates

The present invention relates to an electronic device, such as a surface acoustic wave (SAW) resonator and a surface acoustic wave oscillator having the surface acoustic wave resonator, and more particularly, to a surface acoustic wave resonator in which grooves are formed in a substrate surface and fingers of an IDT are positioned at the groove bottoms.

2. Indication of the background art

Frequency stable and compact generators of electrical sinusoidal signals are applied in a large variety of electronic systems. Electronic generators often include quartz resonators to obtain the high short-term frequency stability. Additionally, in the most of applications the high temperature stability is necessary to provide small frequency deviations due to changes of ambient temperature.

In a surface acoustic wave (SAW) device (such as a SAW resonator), a variation in frequency temperature characteristics is greatly affected by a cut angle of a quartz crystal substrate, by a stop band of the SAW, and a configuration of an interdigital transducer (IDT).

2.1 Specific documents reflecting background art

Recent Japanese Patent JP-A-11-214958 discloses a configuration of a SAW resonator, in which and upper mode and a lower mode of a SAW as well as a standing wave distribution in the upper mode and the lower mode of the stop band exist. A SAW device employing an ST-cut of quartz crystal substrate, in which grooves are disposed between electrode fingers of an IDT or between conductor strips of a reflector, which is described in Japanese Patent JP-A-57-5418.

Another Japanese Patent No.3851336 discloses that a configuration for setting a curve representing the frequency temperature characteristic as a third-order polynomial curve is used in the SAW device employing so-called "LST-cut" quartz crystal substrate. It is also disclosed that any substrate with a cut angle having a frequency-temperature characteristic represented by a third-order polynomial curve could not be discovered in a SAW device employing Rayleigh waves.

However, Japanese patent JP-A-2002-100959 discloses that a rotational Y-cut X- propagation quartz crystal substrate is employed and that the frequency-temperature characteristic is more improved than that in the case where the resonance in the lower mode of the stop band is used, by using the resonance in the upper end of the stop band.

Also Japanese patents JP-A-2006-148622, JP-A-2007-208871 , JP-A-2007-267033, and JP-A-2002-100959 disclose that the upper mode of the stop band has a frequency-temperature characteristic more excellent than that in the lower mode of the stop band of the Rayleigh SAW.

Finally, Japanese patents No.2009-045359, No.2009-050112, and No.2009-285224, and the Patent US-2010/0219913 A1 disclose that the upper mode of the stop band of Rayleigh waves has a frequency-temperature characteristic that is described by a third order polynomial curve with maximum relative frequency deviations ±20 ppm in a SAW resonator with grooves disposed between electrode fingers of an IDT or between conductor strips of a reflector.

2.2 Technical problem to be solved

The present invention seeks to provide an improved SAW device from the point of view of an excellent frequency-temperature characteristic that is described by a third order-polynomial curve, obtaining a high Q-factor, high power handling capability, low sensitivity to electrostatic discharge (ESD), and the convenience of fabrication to obtain higher resonance frequencies using the modern optical lift-off

photolithography. 3. Summary of the invention

According to the present invention there is provided a SAW resonator fabricated on a quartz crystal substrate , and more particularly, a surface acoustic wave resonator in which grooves are formed in a substrate surface, and fingers of an IDT as well as conducting strips of reflectors are positioned at the groove bottoms in such a way that the fingers mechanically contact the left, the right side walls of the grooves, and contact the groove bottoms, and a surface acoustic wave oscillator exploiting said surface acoustic wave resonator.

3.1 Brief description of drawings

Embodiments of the present invention which now is described, by way of example, with reference to the accompanying drawings in which:

FIGs.lA, 1B, and 1C are diagrams illustrating a configuration of a SAW device according to an embodiment of the invention.

FIG.2 is a graph illustrating the dependence of the first and second temperature coefficients of frequency on the duty factor for a SAW resonator with recessed electrodes.

FIG.3 is a graph illustrating the frequency dependence of the admittance of a SAW resonator at the upper mode and a lower mode of a stop band.

FIG.4 is an example of a frequency-temperature dependence of the resonance frequency of proposed resonator that is described by a third order polynomial curve.

FIG.5 is diagram illustrating the SAW reflection characteristics of the IDT and the reflector.

FIG.6 is diagram illustrating the configuration of a SAW oscillator according to an embodiment of the invention. 3.2 Detailed description of the preferred embodiment

First, a surface acoustic wave (SAW) resonator according to a first embodiment of the invention will be described with reference to FIGs. 1A, 1B, and 1C. FIG.1A is a plan view of the SAW resonator, FIG.1 B is a partially enlarged sectional view, and FIG.1C is an enlarged view illustrating the details of FIG.1 B.

The SAW resonator 1 according to this embodiment basically includes a quartz crystal substrate 2, an IDT 3, and two reflectors 4. The quartz crystal substrate 2 has crystal axes which are expressed by an X axis (electrical axis), a Y axis (mechanical axis), and a Z axis (optical axis).

In this embodiment, an Y-rotated and in-plane rotational cut quartz crystal substrate with Euler angles of (-1°<φ<1°, 110°<θ<145°, 35°<|ψ|<50°) is employed as the quartz crystal substrate 2. The Euler angles, which define an orientation of the crystal substrate, will be described now. A substrate with the Euler angles of (0°, 0°, 0°) is a Z-cut substrate having a main plane perpendicular to the Z-axis. Here, the first angle φ of the Euler angles (φ, θ, ψ), is associated with a first rotation of the Z- cut substrate. The Z-cut substrate is rotated about the Z-axis from the +X-axis direction to the +Y-axis, if the angle φ is positive.

The Euler angle Θ is associated with a second rotation which is carried out after the first rotation of the Z-cut substrate. The second rotation angle Θ assumes a rotation direction about the X -axis after the first rotation from the +Y'-axis after the first rotation to the direction of +Z-axis, if the angle Θ is positive,

The cut plane of a piezoelectric substrate is determined by the first rotation angle φ and the second rotation angle Θ.

The Euler angle ψ is associated with a third rotation which is carried out after the second rotation of the Z-cut substrate. The angle ψ describes a rotating direction about the Z'-axis after the second rotation from the +X'-axis after the second rotation to the +Y'-axis after the second rotation, if the angle ψ is a positive rotating angle. The propagation direction of the SAW is determined by the third rotation angle ψ about the Z'-axis after the second rotation.

The IDT 3 includes a pair of bas bars 5 and electrodes 6, which are connected periodically and alternatively to the bas bars. Here, the electrode fingers 6 are arranged in a direction perpendicular to the X'-axis in which the surface acoustic wave is propagated. The SAW excited in the SAW resonator 1 having the above- mentioned configuration is a Rayleigh type SAW and has a vibration displacement component in both the Z'-axis after the third rotation and the X -axis after the third rotation. In this way, by deviation of the propagation direction of the SAW from the X- axis which is the crystal axis of quartz crystal, one can change the phase of the reflection coefficient of the recessed electrodes and, thus, it is possible to excite the SAW in the upper mode of the stop band.

A pair of reflectors 4 is disposed on the substrate 2 so as to insert the IDT 3 between the reflectors 4 in the propagation direction of the SAW. In the reflectors both ends of periodically positioned conductor strips 8 disposed parallel to the electrode fingers 6 of the IDT 3 are connected to each other by means of connectors 7.

The electrodes 6 of the IDT 3 or the conductor strips 8 of the reflectors 4 having the above-mentioned configuration can be made of aluminum (Al) or an alloy containing Al as a main component.

In the quartz crystal substrate 2 of the SAW resonator 1 having the above-mentioned basic configuration, grooves 9 are formed (FIG.1 B), and the electrode fingers 6 of the IDT 3 and the conductor strips 8 of the reflectors 6 are positioned into the grooves 9.

As to the grooves 9 formed in the quartz crystal substrate 2, it is preferred that the following expression (1 ):

0.01< h_GR/ <0.1 (1 ) where the wavelength of the SAW in the upper mode of the stop band is λ, and the groove depth is h_GR, is satisfied. By setting the groove depth h_GR to this range, the frequency variation in the operating temperature range (-40° C. to +85° C.) can be suppressed to 25 ppm or less as a target value.

The duty factor η is a value obtained by dividing a line width L of each electrode finger 6 by a pitch X/2=L+S between the electrode fingers 6, as shown in FIG.1C. Therefore, the duty factor η can be expressed by the following expression (2). n=lJ(L+S) (2)

In the SAW resonator 1 according to this embodiment, the duty factor η determined in the range expressed by the following expression (3). 0.50 <η < 0.75 (3)

The thickness of the electrode film material (of the IDT 3 or the reflectors 4) in the SAW resonator 1 according to this embodiment can be preferably in the range expressed by the following expressions (4) and (5).

0<h_A <0.05 (4)

It is a goal to improve the frequency-temperature characteristic until the relative frequency variation AF/F₀ to 25 ppm or less in the operating temperature range from -40°C to +85°C for the SAW resonator 1 according to this embodiment having the above-mentioned configuration.

The 1^st order and the second order temperature coefficients (TF1 and TF2) are coefficients in an approximate polynomial curve representing the frequency- temperature characteristic of the SAW resonator. The small absolute value of the 1^st order and the second order temperature coefficients means a small frequency variation and that the frequency-temperature characteristic is improved. Hereinafter, it is shown by simulation that the SAW device having the above-mentioned configuration is able to accomplish the subject of the invention.

In the SAW resonator whose propagation direction is the direction of the crystal X axis using a quartz crystal substrate called an "ST cut", when the operating temperature range is (-40°C +85°C) , the frequency variation AF/F₀ in the operating temperature range is about 125 ppm and the secondary temperature coefficient (TF2 is about -0.032 ppm/°C². In the SAW resonator which is formed using an in-plane rotation ST-cut quartz crystal substrate in which the cut angle of the quartz crystal substrate and the SAW propagation direction are described by Euler angles (0°, 123°, 45°) and the operating temperature range is the same as above, the frequency variation AF/F₀ is about 63 ppm and the secondary temperature coefficient (TF2 is about -0.016 ppm/°C². The primary temperature coefficient is close to zero.

The variation in frequency-temperature characteristic of the SAW resonator 1 is affected by the duty factor η of the electrode fingers 5 or the electrode thickness h_Ai of the IDT 3 and the groove depth h_GR. The SAW resonator 1 according to this embodiment employs the excitation in the upper mode of the stop band. FIG.2 is a graph illustrating the variation of the primary and the secondary temperature coefficients TF1 and TF2 in the resonance of the upper mode when the duty factor η is varied and the SAW is propagated by the quartz crystal substrate 2. In the simulation shown in FIG.2, the SAW is propagated on the quartz crystal substrate 2 with an electrode film of

The Euler angle (0°, 123°, 40.6°) is used as the cut angle of the quartz crystal.

It can be seen from FIG.2 that the secondary temperature coefficient TF2 greatly varies from the plus side to the minus side in the vicinity of the duty factor η of 0.5 to 0.75 in the upper mode of the stop band. The primary temperature coefficient TF1 in the upper mode of the stop band shows a similar behavior that is shifted from the plus side to a minus side, and at the duty factor η=0.63 the frequency temperature characteristic is significantly improved.

FIG.3 is a graph illustrating the resonant behavior in the lower mode and in the upper mode of the stop band in the present embodiment. It can be seen from FIG.3 that the left resonance corresponding to the lower mode is much less pronounced than the right resonance that corresponds to the upper mode.

FIG.4 is a graph in which the simulated frequency-temperature characteristic of the present embodiment is plotted. It can be seen from FIG.4 that the relative frequency variation AF/F₀ in the operating temperature range is equal to or less than about 25ppm. The point of inflection is situated at about the room temperature.

FIG.5 illustrates a possible way of embodiment of a SAW resonator in which only the upper mode is used. On a periodically non-uniform crystal surface a SAW is reflected in a frequency band with a lower end frequency f1 and an upper end frequency f2. The frequency band from f1 to f2 is usually called as a stop band. In this embodiment in order to efficiently trap the energy of the surface acoustic wave excited predominantly in the upper mode of the stop band, the upper end frequency f2iDT of the stop band of the IDT 3 can be set between the lower end frequency f1_REF of the stop band of the reflector 4 and the upper end frequency f2_REF of the stop band of the reflector 4, as shown in FIG. 5. That is, the frequencies can be set to satisfy the following expression (6).

In the case described by expression (6) the reflectors 4 provide high reflectivity, and the energy of the SAW is efficiently trapped in the region between the reflectors 4. In order to fulfill the expression (6), it is necessary to shift the stop band of the reflector 4 to a higher frequency as compared to the stop band of the IDT 3. Specifically, to implement this shift the arrangement period of the conductor strips 8 of the reflector 4 can be smaller than the arrangement period of the electrode fingers 6 of the IDT 3. As another method, the thickness or the duty factor of the conductor strips 8 of the reflector 4 can be set to be different from the thickness or the duty factor of the electrodes 6 of the IDT 3. Additionally, the depth of grooves 9 of the IDT 3 can be set different from the depth of grooves 9 of the reflectors 4. Two or more of the methods can be combined.

A SAW oscillator according to an embodiment of the invention is shown in FIG.6.

It should be clear to the person skilled in the art that other embodiments of this invention are possible, such as using reflectors with the same electrode and groove dimensions as in the IDT, or using long IDT with no reflectors at all. The device can be used not only for stabilization of frequency of oscillator, but as 2-port resonator or a narrow band filter, or as a sensor of physical values, etc. All such embodiments are included in the patent.

Claims

Claims What is claimed is:

1. A surface acoustic wave resonator comprising:

an IDT which is disposed on a quartz crystal substrate with an Euler angle of (- Γ<φ<Γ, 110°<θ<145°, 35°<|ψ|<50°) and which excites a surface acoustic wave in an upper mode of a stop band; and grooves are formed by recessing the quartz crystal substrate; and the said IDT fingers are positioned at bottoms of the grooves in such a way that the fingers mechanically attached to the left, the right side walls of the grooves, and to the the groove bottoms, wherein the following expressions:

0<h_A <0.05

where II_GR represents a depth of the grooves, IIA_I represents a thickness of the IDT fingers, are satisfied and when a duty factor of the IDT is η and set to satisfy the following expression

0.5<η<0.75

2. The surface acoustic wave resonator according to claim 1 , wherein the following expression:

where f2|_DT represents a frequency of the upper mode of the stop band in the IDT, f1 REF represents a frequency of the lower mode of the stop band in reflectors disposed to interpose the IDT there between in a propagation direction of the surface acoustic wave, and f2_REF represents a frequency of the upper mode of the stop band in the reflectors, is satisfied.

3. The surface acoustic wave resonator according to claim 1 , wherein conductor strips of the reflectors are disposed inside grooves, and wherein the depth of the groove is smaller than the depth of the grooves in the IDT providing the high power handling capability of the device and reduced sensitivity to ESD.

4. The surface acoustic wave resonator according to claim 1 , wherein the duty factor of IDT fingers is smaller than the duty factor of conductor strips in reflector.

5. A surface acoustic wave oscillator comprising the surface acoustic wave resonator according to claim 1 and an IC driving the IDT.