JPH11308069A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set frequency temperature characteristic to be optimal within a desired temperature range and to remarkably improve yield of a surface acoustic wave device by properly setting film thickness of a silicone dioxide film. SOLUTION: An IDT electrode 3 is arranged along the propagating direction of the surface acoustic wave to be excited by a 45 deg. X cut lithium tetraborate substrate 1 and reflectors 4a, 4b are arranged on both sides of the electrode 3. The IDT electrode 3 is constituted by a pair of comb-line electrodes with plural electrode fingers to be inserted into each other, one comb-like electrode of the IDT electrode 3 is connected to an input terminal IN and the other comb-line electrode is connected to an output terminal OUT. Then only a desired surface acoustic wave among plural surface acoustic waves excited by the IDT electrode 3 is locked between the reflectors 4a, 4b and is actuated as a SAW resonator. Ratio Hs/λ between the film thickness Hs of the silicon dioxide film 2 and the wavelength λ of the surface acoustic wave to be excited in within the range of 5×10<-4> <=HS/λ<=5×10<-3> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device, and more particularly, to a surface acoustic wave device using a lithium tetraborate single crystal piezoelectric substrate with improved frequency temperature characteristics.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave devices (hereinafter referred to as SAs) have been developed.
W devices) are used in many communication fields because of their characteristics such as high performance, small size, and mass productivity, and play a part in the spread of mobile phones and the like. As is well known, various characteristics such as the frequency temperature characteristic and the coupling coefficient of the SAW device are substantially determined by the piezoelectric substrate, the cutting angle, the electrode film thickness and the like used therein. Piezoelectric materials used for SAW devices include single crystals such as quartz, lithium tantalate, and lithium niobate. For example, when an ST-cut quartz substrate is used for a piezoelectric substrate, a relatively narrow band (fractional bandwidth is about 0.05%) ) Is obtained, and a relatively wide band (fractional bandwidth 2 to 2) is obtained by using 36 ° Y-cut lithium tantalate.
It is well known that a resonator type filter (about 3%) can be formed.

However, in recent years, as an IF filter for a cellular phone, which has been rapidly spreading, a center frequency of 100 to 300 MHz is used.
There is an increasing demand for a middle band filter having a z and a bandwidth of about 0.1 to 0.3%. Therefore, a 45 ° X-cut lithium tetraborate substrate having an electromechanical coupling coefficient substantially intermediate between that of the ST-cut quartz substrate and the 36 ° Y-cut lithium tantalate constitutes a resonator-type filter satisfying the specifications of the mid-band filter. It is attracting attention.

[0004]

SUMMARY OF THE INVENTION
As is well known, the frequency-temperature characteristic of the resonator type filter using the 45 ° X-cut lithium tetraborate substrate is expressed by an upwardly convex quadratic curve Δf / f = −3 × 10 ⁻⁸ (T−Tp) ² . Although it has a zero temperature coefficient at the apex temperature Tp (a temperature exhibiting a zero temperature coefficient in the temperature characteristic of the quadratic curve), the apex temperature Tp is higher than room temperature, for example, when the IDT electrode is made of aluminum having a film thickness of 1% λ. Since the peak temperature Tp becomes about 39 ° C. when formed, the amount of frequency change Δf / f due to temperature on the low temperature side becomes large, and the temperature characteristic of the center frequency of the filter is 100 ppm in the temperature range of −20 ° C. to 70 ° C. There was a problem that the specifications could not be satisfied and the product yield was 100 ppm to 150 ppm, and the product yield was significantly deteriorated. The present invention has been made to solve the above-described problem, and has as its object to provide a SAW filter having a medium frequency band of about 0.1 to 0.3% and excellent frequency temperature characteristics.

[0005]

According to a first aspect of the present invention, there is provided a surface acoustic wave device according to the present invention, wherein a silicon dioxide film is formed on a main surface of a lithium tetraborate piezoelectric substrate. In the surface acoustic wave resonator configured by arranging at least one IDT electrode and reflectors on both sides along the propagation direction of the surface wave to be excited, the thickness Hs of the silicon dioxide film and the Ratio Hs to wavelength λ
A surface acoustic wave device wherein / λ is set to 5 × 10 ⁻⁴ ≦ Hs / λ ≦ 5 × 10 ⁻³ . Claim 2
The described invention is the surface acoustic wave device according to claim 1, wherein a plurality of IDT electrodes are arranged close to each other along the propagation direction of the surface acoustic wave to form a longitudinally coupled multimode surface acoustic wave filter. According to a third aspect of the present invention, a laterally coupled multi-mode surface acoustic wave filter is provided in which an IDT electrode and a surface acoustic wave resonator having reflectors arranged on both sides thereof are arranged close to each other in parallel with the propagation direction of the surface acoustic wave. The surface acoustic wave device according to claim 1, wherein:

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1A is a plan view showing a configuration of a SAW device according to the present invention, and FIG. 1B is a diagram showing a part of a cross section taken along line AA. In the SAW device according to the present invention, a thin film 2 of silicon dioxide (SiO ₂ ) is formed on the main surface of a 45 ° X-cut lithium tetraborate substrate 1 by a method such as evaporation, and an IDT electrode 3 is formed on the film 2. ,
The grating reflectors 4a and 4b (hereinafter, referred to as reflectors) are formed by an ordinary photolithography method using an aluminum alloy.

IDT formed on silicon dioxide thin film 2
The electrode 3 and the reflectors 4a and 4b are described as follows.
The IDT electrode 3 is arranged along the propagation direction of the surface wave excited by the X-cut lithium tetraborate substrate 1, and reflectors 4a and 4b are arranged on both sides of the electrode 3. The IDT electrode 3 is composed of a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed therebetween. One of the IDT electrodes 3 is connected to the input terminal IN, and the other comb-shaped electrode is connected to the output terminal. Connect to OUT. As is well known, of the plurality of surface waves excited by the IDT electrode 3, only a desired surface wave (usually the lowest-order mode) is confined between the reflectors 4a and 4b to operate as a SAW resonator.

[0008] The inventor of the present application has proposed a method as shown in FIG.
By changing the thickness of the silicon dioxide film 2 by changing the thickness of the silicon dioxide film 2 on how the silicon dioxide film 2 formed on the 45 ° X-cut lithium tetraborate substrate 1 affects the frequency temperature characteristics and the Q value of the SAW device. An experiment was performed. FIG.
FIG. 3 is a diagram showing the relationship between the thickness Hs of the silicon dioxide film 2 and the peak temperature Tp. The thickness Hs of the silicon dioxide film 2 is normalized by the wavelength λ of the excited surface wave, the value Hs / λ is plotted on the horizontal axis, and the peak temperature Tp and the peak temperature when the thickness of the silicon dioxide film 2 are set to zero. Difference from Tp ₀ , that is, peak temperature deviation ΔTp =
Is a diagram of a case where the Tp-tp ₀ as ordinate. From FIG. 2, the relationship between the thickness Hs / λ of the silicon dioxide film 2 and the peak temperature Tp is:
As the film thickness Hs / λ increases, the peak temperature Tp moves to the lower temperature side. The relationship between the peak temperature deviation ΔTp and the film thickness Hs / λ can be approximated by the following equation. ΔTp = −1.0423 × 10 ⁶ × (Hs / λ) ² −1.2574 × 10 ³ × (Hs / λ) (1) From this equation, the thickness Hs / λ of the silicon dioxide film 2 is 0.0005.
It can be seen that if the thickness is made thicker, the peak temperature Tp of the frequency temperature characteristic can be shifted to a low temperature side by 1 ° C. or more.

FIG. 3 shows the thickness Hs of the silicon dioxide film 2.
FIG. 4 is a diagram showing the relationship between / λ and the Q value of a SAW resonator, where the Q value decreases as the film thickness Hs / λ increases. From this figure, the relationship between the thickness Hs / λ of the silicon dioxide film 2 and the Q of the SAW device can be approximated by the following equation. Q = −8.3217 × 10 ⁵ × (Hs / λ) + 5.5054 × 10 ³ (2) Generally, a resonator has a figure of merit M of 10 or more expressed by a value obtained by dividing the resonator Q value by its capacitance ratio γ. Is said to be desirable. When a 45 ° X-cut lithium tetraborate substrate is used for a SAW device, the capacitance ratio γ is approximately γ =
Since the value is 125, the Q value is desirably 1250 or more. When the film thickness Hs / λ of the silicon dioxide film 2 corresponding to this value is obtained from FIG. 3, Hs / λ = 0.005 is obtained.

If the thickness Hs / λ of the silicon dioxide film 2 is increased, it is possible to move the peak temperature Tp to the lower temperature side according to the equation (1), but Q increases as the thickness Hs / λ increases.
The value will be degraded. Therefore, the variable range of the thickness Hs / λ of the silicon dioxide
Is 10 or more, and 5 × 10 ⁻⁴ ≦ Hs / λ ≦ 5 × 10 ⁻³ (3) is obtained as the silicon dioxide film 2 necessary for moving the peak temperature Tp to the low temperature side by 1 ° C. or more. If the thickness Hs / λ of the silicon dioxide film 2 is selected in this range, the peak temperature Tp can be moved to a lower temperature side while securing a high Q value (> 1250). For example, FIG.
In the one-port SAW resonator shown in FIG.
Using a 45 ° X-cut lithium tetraborate substrate, the number of IDT electrodes 3 is 17 and the number of electrode fingers of reflectors 4a and 4b is 400.
In the case where the thickness is H, the thickness H / λ of the silicon dioxide film 2 is
When set to 5, the peak temperature Tp is about 22 ° C., and the peak temperature Tp of the SAW resonator without the silicon dioxide film 2 is set.
_The temperature could be shifted to a lower temperature side by about 17 ° C. as compared to ₀ . The Q value of the resonator at this time was about 2200.

Although the present invention has been described above with reference to a SAW resonator which is a basic configuration of a SAW device, the present invention can be applied to, for example, a first-order to third-order longitudinally-coupled dual-mode SAW filter shown in FIG. That is, a silicon dioxide film 2 is formed on a 45 ° X-cut lithium tetraborate substrate 1 and an IDT is formed thereon.
The electrodes 5, 6, 7 are arranged close to each other along the surface wave to be excited, and reflectors 8a, 8b are provided on both sides of the IDT electrodes 5, 6, 7 respectively.
Are arranged to constitute a first-order / third-order longitudinally-coupled dual-mode SAW filter. The frequency temperature characteristic of the primary-tertiary-order longitudinally-coupled dual-mode SAW filter thus configured is such that its peak temperature Tp can be shifted to a lower temperature side by the thickness Hs / λ of the silicon dioxide film 2. , The peak temperature Tp can be set to a desired temperature. The above describes the present invention as primary-
The case where the present invention is applied to a third-order longitudinally-coupled dual-mode SAW filter has been described. However, it goes without saying that the present invention is not limited to this and can be applied to other longitudinally-coupled multi-mode SAW filters.

On the other hand, when the present invention is applied to the laterally coupled dual mode SAW filter shown in FIG. 5, the frequency temperature characteristics of the filter can be improved. As shown in FIG. 5, a silicon dioxide film 2 is formed on a 45 ° X-cut lithium tetraborate substrate 1, and a resonator including an IDT electrode 9 and reflectors 10a and 10b, an IDT electrode 11 and a reflector 12a are formed thereon. , 12b
Side-coupled dual mode SA
Construct a W filter. The frequency temperature characteristic of the laterally coupled dual mode SAW filter thus configured can be moved to a lower temperature side by the thickness Hs / λ of the silicon dioxide film 2, and the peak temperature Tp can be reduced. It can be set to a desired temperature. In the above, the case where the present invention is applied to a laterally coupled dual mode SAW filter has been described. However, it is needless to say that the present invention can be applied to other laterally coupled multimode SAW filters.

[0013]

According to the present invention, as described above, when a surface acoustic wave device is formed using a lithium tetraborate piezoelectric substrate, the frequency temperature of the silicon dioxide film can be appropriately set by setting the thickness of the silicon dioxide film. The characteristics can be set to be optimum within a desired temperature range, and an excellent effect that the yield of the surface acoustic wave device can be greatly improved is exhibited.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a SAW device according to the present invention, wherein (a) is a plan view and (b) is a cross-sectional view.

FIG. 2 Silicon dioxide film thickness Hs / λ and peak temperature deviation ΔTp
FIG.

FIG. 3 is a diagram showing a relationship between a silicon dioxide film thickness Hs / λ and a Q value of a SAW device.

FIG. 4 is a plan view when the present invention is applied to a longitudinally coupled first-order / third-order dual-mode SAW filter.

FIG. 5 is a plan view when the present invention is applied to a cascade-type laterally coupled dual mode SAW filter.

[Description of Signs] 1. Piezoelectric substrate 2 Silicon dioxide film 3, 5, 6, 7, 9, 11 IDT electrode 4a, 4b, 8a, 8b, 10a, 10b, 12a, 1
2b ・ ・ Grating reflector IN ・ ・ Input OUT ・ ・ Output Hs / λ ・ ・ Silicon dioxide film thickness λ ・ ・ Surface wave wavelength Q ・ ・ Q value ΔTp ・ ・ Vertex temperature deviation

Claims (3)

[Claims] 1. A structure in which a silicon dioxide film is formed on a main surface of a lithium tetraborate piezoelectric substrate, and at least one IDT electrode and reflectors are arranged on both sides along a propagation direction of a surface wave excited on the silicon dioxide film. In the surface acoustic wave resonator, the thickness Hs of the silicon dioxide film and the wavelength λ of the surface acoustic wave to be excited
A surface acoustic wave device, wherein the ratio Hs / λ of the surface acoustic wave is 5 × 10 ⁻⁴ ≦ Hs / λ ≦ 5 × 10 ⁻³ . 2. The surface acoustic wave device according to claim 1, wherein a plurality of IDT electrodes are arranged close to each other along the propagation direction of the surface acoustic wave to form a longitudinally coupled multimode surface acoustic wave filter. 3. A transversely coupled multi-mode surface wave filter in which an IDT electrode and a surface acoustic wave resonator having reflectors arranged on both sides thereof are closely arranged in parallel with the propagation direction of the surface acoustic wave. The surface acoustic wave device according to claim 1.