JP3505449B2 - Substrate for surface acoustic wave device and surface acoustic wave device - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to surface acoustic waves (Surf).
ACE Acoustic Wave (SAW) device.

[0002]

2. Description of the Related Art SAW devices include SAW resonators, transversal SAW filters and resonator SAWs.
There are filters, etc., which are used properly according to the purpose. Hereinafter, for example, a SAW filter will be described as one of the SAW devices.

FIG. 21 is a perspective view schematically showing the structure of a SAW filter. Referring to FIG. 21, the basic structure of the SAW filter 15 is the piezoelectric substrate 12 and the piezoelectric substrate 1
It has a four-terminal structure having a pair of comb-shaped electrodes 13 for exciting and receiving a surface wave formed on the surface of 2. In addition, such an electrode 13 is used as an interdigital electrode (interd
igital electrode), this kind of conversion element is IDT
(Interdigital transducer).

Generally, when an impulse voltage is applied to the comb-shaped electrodes 13 for excitation, distortions of opposite phases are generated between the adjacent electrodes 13 due to the piezoelectric effect, as shown in FIG.
W is excited. The SAW propagates on the surface of the piezoelectric substrate 12, and the surface charge is generated on the piezoelectric substrate 12 due to the distortion caused by the surface wave, and is extracted as an electric signal by the comb-shaped electrode 13 for reception.

Conventionally, a SAW device such as such a SAW filter 15 has a piezoelectric substrate 12 as shown in FIG.
The structure is such that electrodes corresponding to each device are arranged on the surface of the. The characteristics of the SAW device 15 largely depend on the characteristics of the piezoelectric substrate 12, and the piezoelectric substrate 12 is also used properly according to the application. Representative piezoelectric substrate 1 in Table 1
2 shows the material and the characteristics of SAW propagating on the piezoelectric substrate 12.

[0006]

[Table 1]

From Table 1, it can be seen that the quartz substrate has a small temperature characteristic and is good, but has a small electromechanical coupling coefficient (K ² ). In addition, although K ² is large in the LN substrate, the temperature coefficient of delay time (Temperature Coefficient of Delaytim
e: Temperature characteristics such as TCD) are poor. On the other hand, Li ₂ B ₄ O ₇
The substrate has characteristics intermediate between those of the quartz substrate and the LN substrate. As described above, each substrate has its advantages and disadvantages, and is properly used according to the application of the device. In recent years, with the development of video devices such as TVs and communication devices such as mobile phones, SAW devices used therein are also required to have better characteristics than ever before.

The Euler angles in Table 1 will be described below with reference to FIG. Referring to FIG. 24, first, an axis obtained by rotating the X axis by φ in the Y axis direction with the Z axis as the rotation axis is referred to as the first axis. Next, an axis obtained by rotating the Z axis counterclockwise by θ with the first axis as the rotation axis is referred to as the second axis. A substrate is obtained by cutting the second axis as a normal line in a plane orientation including the first axis. In the substrate cut in the above plane orientation, an axis obtained by rotating the first axis counterclockwise by ψ is used as the third axis, and the third axis is used as the SAW propagation direction. The axis orthogonal to the third axis on the surface is the fourth axis. In this way, the Euler angles (φ, θ, ψ) are defined.

[0009]

Generally, K of a piezoelectric substrate is used.
^It is said that the larger the ² is, the easier it is to design a device with a wider bandwidth. On the other hand, the center frequency f of the SAW device
₀ is determined by f ₀ = V / λ (V: surface acoustic wave propagation velocity, λ: IDT electrode pitch). Therefore, in order to manufacture a device having the same center frequency f ₀ , simply using a Li ₂ B ₄ O ₇ substrate having a higher propagation velocity V than a quartz substrate or the like increases the electrode pitch λ of the IDT. And consequently the SAW device itself must be large. That is, it can be said that a substrate having a slow propagation speed V is advantageous for downsizing the SAW device itself.

Therefore, it is an object of the present invention to obtain a SAW device substrate having a K ² intermediate between the quartz substrate and the LN substrate and having a slow propagation speed advantageous for downsizing the SAW device.

[0011]

In order to achieve the above object, in the present invention, a layer of Li ₂ B ₄ O ₇ is formed on a glass surface to form a Li ₂ B ₄ O ₇ / glass structure substrate. The above-mentioned object is achieved by. In the above Li ₂ B ₄ O ₇ / glass structure substrate, more preferably the above Li ₂ B ₄ O
₇ is Euler angle display (0-45 °, 85-95 °, 8
5 to 95 °). More preferably, the KH parameter is set to about 0.5 to obtain a SAW device substrate having a slower propagation velocity.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(Arrangement of Apparatus) FIG. 1 is a schematic sectional view showing the arrangement of a SAW device according to an embodiment of the present invention. With reference to FIG. 1, the SAW device 5 has a glass substrate 1, a piezoelectric substrate 2, and an electrode 3. The piezoelectric substrate 2 is formed on the glass substrate 1 and is made of Li ₂ B ₄
It consists of O ₇ . Further, an electrode 3 having an arrangement according to the device is formed on the piezoelectric substrate 2. It should be noted that the portion composed of the glass substrate 1 and the piezoelectric substrate 2 is SAW
It is referred to as a device substrate 6.

(Manufacturing Method of Device) In such a SAW device substrate 6, a Li ₂ B ₄ O ₇ substrate is bonded as the piezoelectric substrate 2 to the surface of the glass substrate 1 or the piezoelectric substrate 2 is bonded to the surface of the glass substrate 1. after forming the Li ₂ B ₄ O ₇ thin film can be obtained by forming a film and patterned to Li ₂ B ₄ O ₇ electrodes 3 on the substrate (thin film) of the piezoelectric substrate 2.

(Material of each part of the apparatus) As the material of the glass substrate 1, for example, quartz glass, aluminosilicate glass, borosilicate glass, soda lime glass, lead silicate glass, etc. can be used. The material of the piezoelectric substrate 2 may be made of Li ₂ B ₄ O ₇ only, or may partially contain Li ₂ B ₄ O ₇ . The substrate combined with the piezoelectric substrate 2 is not limited to the glass substrate, and is made of a material having a slower sound speed than Li ₂ B ₄ O ₇ and having a Li ₂ B
Any material may be used as long as it has a characteristic having a sign opposite to TCD and TCV (temperature coefficient of sound velocity) of ₄ O ₇ . As a material of the electrode 3, for example, aluminum can be used, but the material is not limited to this.

(Verification of Propagation Velocity) Further, as shown in FIG. 1, the thickness of the piezoelectric substrate 2 is H, the pitch of the electrodes 3 is λ, and K is 2π / λ. Hereinafter, the description will be made while paying attention to the Euler angle of Li ₂ B ₄ O ₇ contained in the piezoelectric substrate 2 and the KH that is the product of K and H.

[0017] Conventional, Li ₂ B ₄ O ₇ which is generally used as a substrate is a _{<110> Li 2 B 4 O} 7 substrate, from Table _{1, <110> Li 2 B} 4 O 7 The value of K ^{2 on} the substrate is 0.80%. In Fig. 2, (0 °, θ,
ψ) Li ₂ B ₄ O ₇ / glass structure substrate, K ² is <1
The result of calculating the value of KH when the ratio is 0.80%, which is the same as in the case of the 10> Li ₂ B ₄ O ₇ substrate, is shown. The method of Campbell and Jones was used for the calculation. Calculated Li
The range of KH of ₂ B ₄ O ₇ is 0 ≦ KH ≦ 3.0. The area inside the broken line is the area where K ² is 0.80%, and the area outside the broken line is within the calculation range of 0 ≦ KH ≦ 3.0, where K ² is 0.80%.
It is a smaller azimuth. That is, 60 ° ≦ θ ≦ 120 °
The region of 60 ° ≦ ψ ≦ 120 ° is the region where K ² is 0.80% or more.

The values of KH shown in FIG. 2, that is, the calculation results of propagation velocity and TCD at KH where K ² is 0.80% are shown in FIGS. 3 and 4, respectively. Further, the glass constant used in the calculation is a glass constant generally used for electronic materials. Table 2 shows the glass constants used in the calculation.

[0019]

[Table 2]

From FIG. 3, it can be seen that there is an Euler angle that gives a slower propagation velocity even though K ² = 0.80% as in the <110> Li ₂ B ₄ O ₇ substrate. In particular,
At (0 °, 85-95 °, 85-95 °), the propagation velocity is about 2850 m / s. <110> L
The propagation velocity in the i ₂ B ₄ O ₇ substrate is 3465 from Table 1.
Since it is m / s, (0 °, 85-95 °, 85-95
°) In the Li ₂ B ₄ O ₇ substrate, <110> Li ₂ B ₄
This means that the propagation velocity is about 18% slower than that of the O ₇ substrate. It can be said that this Euler angle (0 °, 85-95 °, 85-95 °) is an excellent azimuth in that the propagation velocity is slow. The value of KH at this Euler angle is about 0.5 from FIG. Further, the TCD value at this Euler angle is -15.1 from the calculation result (see FIG. 4).

Similarly, of the Euler angle parameters (φ, θ, ψ) representing the azimuth, φ is fixed at 10 °, and θ, ψ
FIG. 5 shows the result of calculation for the value of KH at which K ² becomes 0.80% when V is changed. Similarly, the propagation velocity is shown in FIG. 6 and the TCD is shown in FIG. Similarly, φ is 20
FIGS. 8 to 10 respectively show the case of fixing at 0 °. Similarly, the case where φ is fixed at 30 ° is shown in FIG.
~ Shown in FIG. 13, respectively. Similarly, FIGS. 14 to 16 show the cases where φ is fixed at 40 °. Similarly, the case where φ is fixed at 45 ° is shown in FIGS. 17 to 19, respectively. φ = 10 °, 20 °, 30 °, 40 °,
In each of 45 °, (φ, 85-95 °, 85
~ 95 °) and (0,85-95 °, 85-95 °) have similar propagation velocities. Therefore, the Euler angle (0
~ 45 °, 85-95 °, 85-95 °), especially KH =
SA with 0.5 of Li ₂ B ₄ O ₇ formed on the surface of a glass substrate
By using the W device substrate, it becomes possible to fabricate a SAW filter smaller than the SAW device using <110> Li ₂ B ₄ O ₇ which has been conventionally used.

From the symmetry of the Li ₂ B ₄ O ₇ crystal, the calculation was 0 ° ≦ φ ≦ 45 °, 0 ° ≦ θ ≦ 180 °, 0 ° ≦
This was performed only in the range of ψ ≦ 180 °.

(Embodiment) An embodiment in which the SAW device substrate according to the present invention is used for a SAW filter will be described. FIG. 1 shows a sectional view of a SAW filter as an example of a SAW device 5 using a SAW device substrate according to the present invention. The glass substrate 1 having the material constants shown in Table 2 and the piezoelectric substrate 2 having a thickness of 6 μm (0 °, 9
(0 °, 90 °) Li ₂ B ₄ O ₇ substrates are laminated by a direct bonding technique, and the electrode 3 is formed on the surface of the Li ₂ B ₄ O ₇ substrate as the piezoelectric substrate 2. Here, the direct bonding is a technique of directly bonding the substrates without an adhesive layer interposed. Specifically, the polished and washed glass substrate 1
Further, the Li ₂ B ₄ O ₇ substrate as the piezoelectric substrate 2 is subjected to a hydrophilic treatment with an ammonia-based aqueous solution, then the substrates are superposed on each other, and the substrates are bonded by hydrogen bonding. In this embodiment, heat treatment is performed after the bonding in order to increase the bonding strength. Li ₂ B ₄ O ₇ / thus obtained
An aluminum film having a thickness of 1000 Å was formed on the SAW device substrate 6 having a glass structure by sputtering, and the electrodes were patterned by photolithography. Through the above steps, a SAW filter, which is a type of SAW device 5, was obtained.

(Results) <110> Li ₂ B ₄ O ₇ substrate and SA of Li ₂ B ₄ O ₇ / glass structure according to the present invention
The frequency characteristics of a SAW filter in which electrodes designed under the same parameter conditions are formed on the W device substrate 6 are shown in comparison with FIG. In FIG. 20, the frequency characteristic 7 of the SAW filter using the Li ₂ B ₄ O ₇ / glass substrate according to the present invention is shown by a solid line, and the frequency characteristic 8 of the SAW filter using the <110> Li ₂ B ₄ O ₇ substrate is shown. Is indicated by a broken line. Here, the electrode pitch λ is set so that KH = 0.5.
Was 75.2 μm. From FIG. 20, comparing the frequency characteristics shows that the resonance frequencies are significantly different. This is because the propagation speed of the SAW filter according to the present invention is slower than the propagation speed of the conventional SAW filter using the <110> Li ₂ B ₄ O ₇ substrate. In order for the SAW filter according to the present invention to have the same frequency band as the conventional SAW filter, it is necessary to reduce the electrode pitch. The size of the filter can be reduced accordingly. Further, comparing the characteristics in the pass band, it can be seen that the insertion loss and the pass band width are comparable, and the SAW filter according to the present invention has a value of K ² which is comparable to that of the conventional SAW filter.

It should be noted that the above-described embodiment disclosed this time is illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description but by the scope of the claims, and includes meaning equivalent to the scope of the claims and all modifications within the scope.

[0026]

As described above, in the surface acoustic wave device 5 according to the present invention, as the piezoelectric substrate 2, Li ₂ which is Euler angle display (0 to 45 °, 85 to 95 °, 85 to 95 °). Since the piezoelectric substrate 2 containing B ₄ O ₇ was used and the film thickness of the piezoelectric substrate 2 and the pitch of the electrodes 3 were determined so that KH was approximately 0.5, the conventional <110> Li ₂ B ₄ O ₇ substrate was used. The propagation speed can be slower than that of the SAW device substrate. As a result, the pitch of the electrodes 3 can be reduced, which is advantageous for downsizing the SAW device.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a configuration of a SAW device according to an embodiment of the present invention.

FIG. 2 is a graph showing the value of KH at which K ^{2 of} (0 °, θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate is 0.80%.

FIG. 3 (0 °, θ, ψ) at KH shown in FIG.
Is a graph showing the propagation velocity of the Li ₂ B ₄ O ₇ / glass structure substrate (m / s).

FIG. 4 is (0 °, θ, ψ) at KH shown in FIG.
Li ₂ B ₄ O ₇ / TCD of glass structure substrate (ppm / ° C)
It is a graph which shows.

FIG. 5 is a graph showing the value of KH at which K ^{2 of} (10 °, θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate is 0.80%.

6 is a graph of KH shown in FIG. 5 (10 °, θ,
ψ) Li ₂ B ₄ O ₇ / glass structure substrate propagation velocity (m /
It is a graph which shows s).

FIG. 7 shows (10 °, θ, at KH shown in FIG.
ψ) Li ₂ B ₄ O ₇ / TCD of glass structure substrate (ppm /
(° C) is a graph showing.

FIG. 8 is a graph showing the value of KH at which K ^{2 of} (20 °, θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate is 0.80%.

9 is a graph of KH (20 °, θ,
ψ) Li ₂ B ₄ O ₇ / glass structure substrate propagation velocity (m /
It is a graph which shows s).

FIG. 10 shows (20 °, θ, at KH shown in FIG.
ψ) Li ₂ B ₄ O ₇ / TCD of glass structure substrate (ppm /
(° C) is a graph showing.

FIG. 11 is a graph showing the value of KH at which K ^{2 of} (30 °, θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate is 0.80%.

FIG. 12 is a graph of KH (30 °,
θ, ψ) Li ₂ B ₄ O ₇ / Propagation velocity of glass structure substrate (m
/ S) is a graph showing.

13 is a graph of KH shown in FIG. 11 (30 °,
θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate TCD (pp
(m / ° C).

FIG. 14 is a graph showing the value of KH at which K ^{2 of} (40 °, θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate is 0.80%.

FIG. 15 is a graph of KH (40 °,
θ, ψ) Li ₂ B ₄ O ₇ / Propagation velocity of glass structure substrate (m
/ S) is a graph showing.

16 is a graph of KH shown in FIG. 14 (40 °,
θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate TCD (pp
(m / ° C).

FIG. 17 is a graph showing KH values at which K ^{2 of} (45 °, θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate is 0.80%.

18 is a graph of KH shown in FIG. 17 (45 °,
θ, ψ) Li ₂ B ₄ O ₇ / Propagation velocity of glass structure substrate (m
/ S) is a graph showing.

FIG. 19 is a graph of KH (45 °,
θ, ψ) Li ₂ B ₄ O ₇ / glass structure substrate TCD (pp
(m / ° C).

FIG. 20: <110> Li ₂ B ₄ O ₇ substrate and Li ₂ B ₄ O ₇
6 is a graph showing frequency characteristics of a SAW filter using a glass structure substrate.

FIG. 21 is a perspective view showing the configuration of a general SAW filter.

22 is a diagram for explaining the operation of the SAW filter shown in FIG. 21. FIG.

FIG. 23 is a sectional view schematically showing a configuration of a SAW device based on a conventional technique.

FIG. 24 is a diagram for explaining an Euler angle.

[Explanation of symbols]

1 glass substrate, 2 piezoelectric substrate (Li₂B_FourO₇), 3,
13 electrodes, 5,15 SAW device, 6 SAW device
Substrate for chair, 7 Li based on the present invention₂B_FourO ₇/ Glass
Frequency characteristics of SAW filter using substrate, 8 <11
0> Li₂B_FourO₇Frequency of SAW filter using substrate
Characteristics, 12 Piezoelectric substrate.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-11-55070 (JP, A) JP-A-6-326553 (JP, A) JP-A-6-144996 (JP, A) Takehiko UNO, Fabrication of Li2B4O7 Piezoelectric Thin Films, Japanese Journal of Applied Physics, 1988, Vol. 27 Supplement 27-1, p. 120-122, Proceedings of 8th Symposium on Ultrasonic Electrics, Tokyo 1987 (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/25 H01L 41/09 H01L 41/09 / 18 H03H 9/145

Claims (2)

(57) [Claims] And 1. A glass substrate, a piezoelectric substrate including a Li ₂ B ₄ O ₇ formed on the glass substrate, the Li ₂
B ₄ O ₇ is Euler angle display (0 to 45 °, 85 to 95
°, surface acoustic wave device wafer you being a 85 to 95 °). 2. When the electrodes are formed on the surface acoustic wave device substrate, the pitch of the electrodes is λ, the thickness of the piezoelectric substrate is H, and K is 2π / λ, K and H The product of and is approximately 0.5.
The surface acoustic wave device according to.