JP3840852B2 - Surface acoustic wave device and 2-port surface acoustic wave resonator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is an improvement of a surface acoustic wave device and a two-port surface acoustic wave resonator, in particular, improves the temperature characteristics of the surface acoustic wave device and operates stably with respect to temperature changes. The present invention relates to a surface wave resonator.
[0002]
[Prior art]
In recent years, surface acoustic wave devices have been used as resonators and bandpass filters in electronic parts and communication parts such as mobile phones and television receivers. Here, FIG. 7 is a plan view showing a conventional surface acoustic wave resonator, and the surface acoustic wave resonator 1 will be described with reference to FIG.
[0003]
A surface acoustic wave resonator 1 shown in FIG. 7 includes a piezoelectric substrate 2, an interdigital transducer (hereinafter referred to as “IDT”) 3, reflectors 4, and 4. The piezoelectric substrate 2 is formed of, for example, a quartz plate. An IDT 3 and reflectors 4 and 4 are formed on the piezoelectric substrate 2 by forming a conductive metal film by thin film forming means such as vapor deposition or sputtering, and then formed by photolithography. ing. The IDT 3 has comb-shaped electrodes 3 a and 3 b in which a plurality of electrode fingers are formed at a predetermined pitch PT, and the IDT 3 is electrically connected to the external terminal 5. Reflectors 4 and 4 are formed by a plurality of electrode fingers at a predetermined pitch PR on both sides of the IDT 3 in the direction of the arrow X, and the reflectors 4 and 4 are formed by providing a predetermined distance L1 and L2 from the IDT 3. Yes. The reflectors 4 and 4 reflect the propagated surface acoustic wave, and the energy of the surface acoustic wave is confined in the reflectors 4 and 4.
[0004]
When an electric signal is supplied to the IDT 3, a surface acoustic wave propagates in the direction of the arrow X due to the piezoelectric effect. The surface acoustic wave is reflected in multiple stages by the reflectors 4 and 4, and a resonance frequency corresponding to the pitch of the electrode fingers of the IDT 3 and the reflectors 4 and 4 can be obtained.
[0005]
By the way, the resonance frequency of the surface acoustic wave resonator 1 shown in FIG. 7 depends on the pitch of the electrode fingers of the IDT 3 and the reflectors 4 and 4, the interval between the IDT 3 and the reflectors 4 and 4, and the like. Therefore, when the predetermined resonance frequency is obtained for the surface acoustic wave resonator 1, it is performed by changing the parameters described above.
[0006]
FIG. 8 is a graph showing the radiation conductance and reflection coefficient in IDT3. In the normalized reflection coefficient | Γ _IDT | of the _{IDT 3} in FIG. 8, the lower limit frequency of the stop band is f _l , the upper limit frequency of the stop band is f _u , and the reflection coefficient | Γ _IDT | is the center of the stop band. Maximum at frequency. On the other hand, the frequency f _Tl to normalized radiation conductance G _a / G _N is the maximum IDT3 Among the stop band is formed near the lower stopband frequency f _l.
[0007]
FIG. 9 is a graph showing a design method for setting the IDT 3 and the reflectors 4 and 4 so that the insertion loss is minimized in the surface acoustic wave resonator 1 of FIG. First, the electrode finger pitch PT in the IDT 3 is substantially equal to the electrode finger pitch PR of the reflectors 4 and 4 (PT = PR), and the distances L1 and L2 between the IDT 3 and the reflectors 4 and 4 are respectively λ / 4 (λ is the wavelength of the surface acoustic wave) is set.
[0008]
First, in FIG. 9 (A), IDT 3 is normalized radiation conductance G _a / G _N is set to be the greatest at f _Tl. On the other hand, the reflection coefficient of the reflector 4, 4 | gamma _Ref | is set such that in FIG. 9 (B) | Γ _Ref | is the f _R frequencies as a maximum. Here, since the pitches, widths, and film thicknesses of the electrode fingers of the reflectors 4 and 4 are set substantially the same as those of the IDT 3, the reflection coefficient | Γ _Ref | of the reflectors 4 and 4 is the reflection coefficient | Γ of the IDT. _It is almost the same as _IDT |. Accordingly, as shown in FIG. 9 (C), f _Tl is the reflection coefficient of the reflector 4, 4 | is set near the lower stop band f _l in | gamma _Ref.
[0009]
Thereafter, in order to reduce the insertion loss, the setting of the IDT 3 or the reflectors 4 and 4 is changed so that f _Tl and f _R are substantially matched. Specifically, for example, by decreasing the pitch PT of the _{IDT 3} and increasing the frequency f _Tl at which the radiative conductance is maximized, or by increasing the pitch PR of the reflectors 4 and 4 and decreasing the frequency f _R , f _Tl and f _R are made to substantially coincide (f _Tl = f _R ). As a result, the insertion loss of the surface acoustic wave resonator 1 can be minimized.
[0010]
By the way, generally the temperature characteristic of a surface acoustic wave resonator is shown by Formula (1).
[0011]
Δf = a (T−T _o ) + b (T−T _o ) ² (1) where Δf is the amount of change in frequency (ppm) between the temperature T and the apex temperature T _o , and a is the primary temperature coefficient (ppm / ° C.), b is the secondary temperature coefficient (ppm / ° C. ^2), T is the temperature, T _o denotes the temperature (peak temperature) the frequency is maximum.
[0012]
When the piezoelectric substrate 2 is formed of a quartz plate having a so-called ST cut (Euler angles (φ, θ, ψ) = (0 °, 120 ° to 130 °, 0 °)), the primary constant a = 0.0 The secondary constant b = −0.034, which is shown in FIG. In FIG. 10, the temperature characteristic draws an upwardly convex parabola (secondary curve).
[0013]
[Problems to be solved by the invention]
The surface acoustic wave resonator 1 as shown in FIG. 7 is desired to have stable frequency characteristics with respect to changes in temperature, and it is necessary to reduce the amount of frequency change Δf with respect to changes in temperature. . Therefore, the secondary temperature coefficient b shown in FIG. 10 is made closer to 0 so that the frequency change amount Δf with respect to the change in temperature (operating temperature) when the surface acoustic wave resonator 1 is actually used approaches 0. There is a need to improve surface acoustic wave resonators.
[0014]
An object of the present invention is to provide a surface acoustic wave device and a two-port surface acoustic wave resonator that solve the above problems, improve the temperature characteristics of the surface acoustic wave device, and operate stably with respect to temperature changes. It is.
[0015]
[Means for Solving the Problems]
According to the first aspect of the present invention, a plurality of electrode fingers are formed at a predetermined pitch on a piezoelectric substrate, and a first interdigital electrode that converts an electrical signal into a surface acoustic wave;
A plurality of electrode fingers are formed at a predetermined pitch, each having a predetermined interval on both ends of the first interdigital electrode, and a surface acoustic wave propagating from the first interdigital electrode is transmitted as an electric signal. A second interdigital electrode and a third interdigital electrode that convert to
A plurality of electrode fingers are formed at a predetermined pitch, have a predetermined distance from the second interdigital electrode and the third interdigital electrode, and a first reflection for confining the surface acoustic wave energy. And a second reflector,
The distance between the first interdigital electrode and the second interdigital electrode is SP1, the distance between the first interdigital electrode and the third interdigital electrode is SP2, the second interdigital electrode and the first reflection. When the distance of the vessel is L1, the distance between the third interdigital electrode and the second reflector is L2, and the wavelength of the surface acoustic wave is λ,
SP1 = SP2 = nλ (n is a natural number)
Or
SP1 = SP2 = nλ−1 / 2λ (n is a natural number)
And
L1 = L2 = 1 / 4λ
Is formed to satisfy
The first interdigital electrode, the second interdigital electrode, the third interdigital electrode, the first reflector, and the second reflector have a frequency at which a radiation conductance is maximized, and the first reflector, The second reflector is formed such that the frequency at which the reflection coefficient of the second reflector is maximum coincides,
The piezoelectric substrate has Euler angles (φ, θ, ψ) = (0 °, 120 ° to 130 °, 0 °), (0 °, 123 °, 42.95 °) or (0 °, 127. 5 °, 44 °) or (0 °, 96.51 °, 32.43 °), which is achieved by a two-port surface acoustic wave resonator characterized by comprising a quartz plate formed by a cut. The
[0016]
According to the above configuration, the second-order temperature coefficient in the temperature characteristics of the 2-port surface acoustic wave resonator can be brought close to 0, and the amount of change in the frequency characteristics with respect to the temperature change of the 2-port surface acoustic wave resonator can be reduced. it can.
Furthermore, the frequency at which the radiation conductance of the first interdigital electrode, the second interdigital electrode, and the third interdigital electrode is maximized and the reflection coefficient of the first reflector and the second reflector are The first interdigital electrode and the reflector are formed so that the maximum frequency is substantially matched. Thus, the insertion loss can be reduced because the reflector reflects the surface acoustic wave excited from the IDT most strongly at the frequency at which the radiation conductance is maximized.
Furthermore, the second-order temperature coefficient can be made close to 0 by using the quartz plate formed by the specific cut, and the amount of change in the frequency characteristic with respect to the temperature change of the 2-port surface acoustic wave resonator can be reduced. Can do.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.
[0029]
FIG. 1 is a plan view showing a preferred embodiment of a 2-port surface acoustic wave resonator according to the present invention. The 2-port surface acoustic wave resonator 10 will be described in detail with reference to FIG.
[0030]
The surface acoustic wave resonator 10 shown in FIG. 1 includes a piezoelectric substrate 11, a first IDT 12, a second IDT 13, a third IDT 14, a reflector 15, and the like. The piezoelectric substrate 11 is made of, for example, a so-called ST cut quartz plate (X-axis propagation cut of 30 ° to 40 ° rotated Y plate). On the piezoelectric substrate 11, the IDTs 12, 13, 14 and the reflectors 15, 15 are formed by photolithography technique after forming Al (aluminum) by thin film forming means such as vapor deposition or sputtering.
[0031]
The first IDT 12 is composed of comb-shaped electrodes 12a and 12b composed of a plurality of electrode fingers, and the comb-shaped electrodes 12a and 12b are formed at a predetermined pitch PT. The comb electrodes 12a and 12b are electrically connected to the input terminals, respectively, and an electric signal is supplied from the input terminals. When an electric signal is supplied from the input terminal, the first IDT 12 outputs a surface acoustic wave due to the piezoelectric effect in the direction of the arrow X.
[0032]
The second IDT 13 and the third IDT 14 are formed at predetermined intervals SP1 and SP2 on both sides in the propagation direction (arrow X direction) of the surface acoustic wave of the first IDT 12, respectively. The second IDT 13 is composed of comb-shaped electrodes 13a and 13b composed of a plurality of electrode fingers, and the comb-shaped electrodes 13a and 13b are formed at a predetermined pitch PT. Similarly, the second IDT 14 is composed of comb-shaped electrodes 14a and 14b composed of a plurality of electrode fingers, and the comb-shaped electrodes 14a and 14b are formed at a predetermined pitch PT. Accordingly, the first IDT 12 to the third IDT 14 are formed to have substantially the same pitch PT. The comb-type electrode 13a and the comb-type electrode 14a, and the comb-type electrode 13b and the comb-type electrode 14b are electrically connected to each other and are connected to the output terminal. The second IDT 13 and the third IDT 14 convert the surface acoustic wave propagating in the arrow X direction into an electrical signal and output it.
[0033]
Reflectors 15 and 15 are formed so as to sandwich the second IDT 13 and the third IDT 14. The second IDT 13 and the reflector 15 are separated by a distance L1, and the third IDT 14 and the reflector 15 are separated by a distance L2. The reflectors 15 and 15 have a plurality of electrode fingers formed at a predetermined pitch PR. The reflectors 15 and 15 reflect the surface acoustic wave propagating in the direction of the arrow X and confine the energy between the reflectors 15 and 15.
[0034]
Here, the interval SP1 and the interval SP2 are formed so as to satisfy the expressions (2) to (4). Note that λ is the wavelength of the surface acoustic wave, and n is a natural number (a positive integer not including 0).
[0035]
SP1 = SP2 = nλ (2)
Or SP1 = SP2 = nλ−1 / 2λ (3)
Further, the distance L1 and the distance L2 are formed so as to satisfy the following expression.
L1 = L2 = 1 / 4λ (4)
[0036]
FIG. 2 is a graph showing the radiation conductance and reflection coefficient | Γ _IDT | of the surface acoustic wave resonator 10 set in the equations (2) to (4). In FIG. 2A, when the pitch PT of the first IDT 12 and the like and the pitch PR of the reflector 15 are substantially the same (PT = PR), the radiation conductance forms peaks P1 and P2 at frequencies f _Tl and f _Tu , respectively. ing. On the other hand, the reflection coefficient of the 1IDT12 | Γ _IDT | the stop band the maximum frequency among the frequency f _u of the frequency f _l was f0 is formed.
[0037]
On the other hand, as shown in FIG. 2 (B), the reflection coefficient of the reflector 15, 15 | gamma _Ref | is the 1IDT12 reflection coefficient | becomes substantially identical as the | gamma _Ref. This is because the pitch PT of the first IDT 12 to the third IDT and the pitch PR of the reflectors 15 and 15 are formed to be substantially the same (PT = PR). Therefore, the frequency f _Tl peak P1 in radiation conductance frequency (lower stopband frequency) f _l near the lower end of the stop band of the reflector 15 and 15 is the formation, in the stop band of the reflectors 15 and 15 frequency f _Tu of the upper end of the frequency (upper stopband frequency) f _u peak in the vicinity of the radiation conductance P2 is formed.
[0038]
However, if the radiant conductance peaks P1 and P2 are formed in the vicinity of both ends of the stop band, which is a region where the reflectors 15 and 15 reflect the surface acoustic wave to the maximum, the surface acoustic waves emitted from the first IDT 12 are efficiently used. Will not be reflected and insertion loss will increase. Therefore, for example, the resonance of the peak P1 is suppressed and only the resonance of the peak P2 is strongly excited.
[0039]
That is, as shown in FIG. 2C, the frequency f _Tu at the radiation conductance peak P2 of the first IDT 12, the second IDT 13, and the third IDT 14 is substantially the same as the maximum frequency f _R of the stop band of the reflectors 15 and 15. (F _Tu = f _R ) Specifically, first, the wavelength λ and the frequency f of the surface acoustic wave are generally given by the following equations (5) and (6).
[0040]
λ = 2P (5)
f = V / 2P (6)
Here, P is the pitch of each IDT or reflector, and V is the velocity of the surface acoustic wave. According to the equations (5) and (6), the frequency decreases as the pitches PT and PR of the IDTs 12, 13, and 14 and the reflectors 15 and 15 increase, and the frequency increases as the pitches PT and PR decrease. Accordingly, the IDTs 12, 13, and 14 are formed so that the pitch PT of the first IDT 12 to the third IDT 14 is smaller than the pitch PR of the reflectors 15 and 15 (PT <PR). Then, as shown in FIG. 2 (C), the frequency f _{Tu at} the peak P2 of the radiation conductance becomes substantially the same as the maximum frequency f _R of the stop band of the reflectors 15 and 15, and (f _Tu = f _R ), insertion Loss can be kept small. Note that the pitch PR may be changed to reduce the maximum frequency f _R of the stop band of the reflectors 15 and 15 so that f _Tu and f _R coincide with each other.
[0041]
On the other hand, since the radiation conductance peak P1 is formed in the region outside the stop band, the surface acoustic wave having the frequency f _{Tl at} the peak P1 can be prevented from being reflected. FIG. 3 is a graph showing the insertion loss-frequency characteristics of the 2-port surface acoustic wave resonator 10 of FIG. 1. As shown in FIG. 3, the insertion loss is minimum at the frequency f _Tu and the frequency f _It can be seen that the insertion loss increases at _Tl . As a result, the two-port surface acoustic wave resonator 10 having only the frequency f _{Tu as the} resonance frequency can be manufactured.
[0042]
FIG. 5 is a graph showing temperature characteristics of the 2-port surface acoustic wave resonator 10 shown in FIG. As the piezoelectric substrate 11, a so-called ST cut (Euler angle (φ, θ, ψ) = (0 °, 120 ° to 130 °, 0 °)) quartz plate is used. When the equations (2) to (4) are set, the temperature coefficient can be set to the primary temperature coefficient a = 0 and the secondary temperature coefficient b = −0.029 (ppm / ° C. ² ). Therefore, the surface acoustic wave resonator 10 can perform a stable operation with respect to a temperature change as compared with the conventional surface acoustic wave resonator 1.
[0043]
The two-port surface acoustic wave resonator 10 in FIGS. 1 to 5 uses a so-called ST-cut quartz plate as the piezoelectric substrate 11. However, if another quartz plate is used, the secondary temperature coefficient can be made closer to 0 and more Changes in frequency characteristics due to temperature changes can be prevented. FIG. 6 is a table showing the relationship between the material of the piezoelectric substrate 11 and the secondary temperature coefficient.
[0044]
In FIG. 6, θ = 33 ° ST cut, ψ = 42.75 ° in-plane rotation from the X axis (Euler angles (φ, θ, ψ) = (0 °, 123 °, 42.75 °)). In the case where the quartz plate is the piezoelectric substrate 11, the secondary temperature coefficient b of the two-port surface acoustic wave resonator 10 is b = −0.0105 (ppm / ° C. ² ).
[0045]
Also, θ = 37.5 ° rotation Y-cut, ψ = 44 ° in-plane rotation from the X axis (Euler angles (φ, θ, ψ) = (0 °, 127.5 °, 44 °)) When the crystal plate is the piezoelectric substrate 11, the secondary temperature coefficient b is b = −0.0074 (ppm / ° C. ² ).
[0046]
Further, θ = 6.51 ° rotation Y cut (K cut), ψ = 32.43 ° in-plane rotation from the X axis (Euler angles (φ, θ, ψ) = (0 °, 96.51, 32.43) When the piezoelectric plate 11 is used as the quartz plate 11 formed by the above (°)), the secondary temperature coefficient b of the 2-port surface acoustic wave resonator 10 is b = −0.016 (ppm / ° C. ² ). Therefore, by forming the piezoelectric substrate 11 from the above-described material, a surface acoustic wave resonator that is more stable against temperature changes can be supplied.
[Brief description of the drawings]
FIG. 1 is a plan view showing a preferred embodiment of a 2-port surface acoustic wave resonator according to the present invention.
FIG. 2 is a graph showing the radiation conductance and reflection coefficient with respect to the frequency of the 2-port surface acoustic wave resonator of the present invention.
FIG. 3 is a graph showing the radiation conductance and reflection coefficient with respect to the frequency of the 2-port surface acoustic wave resonator of the present invention.
FIG. 4 is a graph showing frequency-insertion loss characteristics in the 2-port surface acoustic wave resonator of the present invention.
FIG. 5 is a graph showing frequency-temperature characteristics of the 2-port surface acoustic wave resonator according to the present invention.
FIG. 6 is a table showing different types of piezoelectric substrates in the two-port surface acoustic wave resonator of the present invention.
FIG. 7 is a plan view showing an example of a conventional surface acoustic wave resonator.
8 is a graph showing the radiation conductance and reflection coefficient with respect to the frequency of the surface acoustic wave resonator shown in FIG.
9 is a graph showing how the resonance frequency of the surface acoustic wave resonator shown in FIG. 7 is optimized.
FIG. 10 is a graph showing temperature characteristics of a conventional surface acoustic wave resonator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... SAW resonator 11 ... Piezoelectric substrate 12 ... 1st IDT (1st interdigital electrode)
13 ... 2nd IDT (2nd interdigital electrode)
14 ... 3rd IDT (3rd interdigital electrode)
15 ... reflector SP1 ... first IDT and second IDT interval SP2 ... first IDT and third IDT interval L1 ... second IDT and reflector distance L2 ... third IDT and reflector distance

Claims (1)

A plurality of electrode fingers formed on the piezoelectric substrate at a predetermined pitch, and a first interdigital electrode for converting an electric signal into a surface acoustic wave;
A plurality of electrode fingers are formed at a predetermined pitch, each having a predetermined interval on both ends of the first interdigital electrode, and a surface acoustic wave propagating from the first interdigital electrode is transmitted as an electric signal. A second interdigital electrode and a third interdigital electrode that convert to
A plurality of electrode fingers are formed at a predetermined pitch, have a predetermined distance from the second interdigital electrode and the third interdigital electrode, and a first reflection for confining the surface acoustic wave energy. And a second reflector,
The distance between the first interdigital electrode and the second interdigital electrode is SP1, the distance between the first interdigital electrode and the third interdigital electrode is SP2, the second interdigital electrode and the first reflection. When the distance of the vessel is L1, the distance between the third interdigital electrode and the second reflector is L2, and the wavelength of the surface acoustic wave is λ,
SP1 = SP2 = nλ (n is a natural number)
Or
SP1 = SP2 = nλ−1 / 2λ (n is a natural number)
And
L1 = L2 = 1 / 4λ
Is formed to satisfy
The first interdigital electrode, the second interdigital electrode, the third interdigital electrode, the first reflector, and the second reflector have a frequency at which a radiation conductance is maximized, and the first reflector, The second reflector is formed such that the frequency at which the reflection coefficient of the second reflector is maximum coincides,
The piezoelectric substrate has Euler angles (φ, θ, ψ) = (0 °, 120 ° to 130 °, 0 °), (0 °, 123 °, 42.95 °) or (0 °, 127. A two-port surface acoustic wave resonator comprising a quartz plate formed by a cut of 5 °, 44 °) or (0 °, 96.51 °, 32.43 °).

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