JPH07212174A - Boundary acoustic wave device - Google Patents

Abstract

(57) [Summary] [Object] To provide a small-sized boundary acoustic wave device having good temperature characteristics and not requiring packaging by a container accompanied by volume increase. [Structure] In order to strongly excite an elastic boundary wave having good temperature characteristics, a θ-rotation Y-cut lithium tantalate single crystal piezoelectric substrate 10 with 100 <θ <124 or 128 <θ <150 is converted to an elastic boundary wave wavelength conversion. The film thickness h / λ is h / λ>
A silicon oxide film 12 of 1 is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to optical, radio or television I.
The present invention relates to a communication filter such as an F (intermediate frequency) filter that is required to be downsized.

[0002]

2. Description of the Related Art In the field of mobile communication, a boundary acoustic wave device is known as a micro filter. The boundary acoustic wave is an elastic wave that can exist near the boundary between the substrate and the non-metal film by forming a sufficiently thick non-metal film on a piezoelectric single crystal substrate under special conditions. For this reason, the boundary acoustic wave device has an advantage that it does not require packaging by a container that causes an increase in volume.

Conventionally reported boundary acoustic wave (BAW)
Is a silicon oxide film (hereinafter abbreviated as SiO ₂ ) / 126 degree rotation Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate (for example, IEE, Transactions on Sonics and Ultrasonics S25). Volume 6 (1978) No. 384
Pages 389 (IEEE Transactions on sonics and u
ltrasonics, SU-25, No.6, (1978) pp.384-389)).

[0004]

SiO _2/126 degree rotated Y-cut X-propagation lithium tantalate single-crystal piezoelectric substrate [SUMMARY OF THE INVENTION] has poor temperature characteristics. Therefore, it has been impossible to fabricate a boundary acoustic wave device that satisfies the system specifications.

An object of the present invention is to provide a BAW device having good temperature characteristics in order to solve the above-mentioned conventional problems.

[0006]

Means for Solving the Problems The above object is to use a θ-degree rotated Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate having 100 <θ <124 or 128 <θ <150 as a piezoelectric substrate, and a SiO ₂ film. Can be achieved by setting the thickness of h / λ> 1.

Here, the θ ° rotation Y-cut lithium tantalate single crystal piezoelectric substrate means the IR axis from the + Y axis of the IRE standard.
Tantalum so that it is approximately perpendicular (within 1 ° error) to a direction tilted θ ° in the E standard + Z axis direction or a direction tilted θ ° in the IRE standard −Z axis to the IRE standard −Z axis direction. A lithium tantalate single crystal piezoelectric substrate obtained by cutting out a lithium acid crystal. Further, h is the thickness of the SiO ₂ film, λ
Represents the period of the comb electrodes.

[0008]

In order to investigate the action of the above structure, the inventors
A boundary acoustic wave device using a TiO ₂ / θ ° rotation Y-cut lithium tantalate single crystal piezoelectric substrate was prepared and its propagation characteristics were investigated. The electrode structure of the produced element is a two-hole resonator having an electrode period of 4 μm, a logarithm of 10 pairs, and an aperture length of 110λ (λ: electrode period). The SiO ₂ film was formed by the RF sputtering method. As a result, the following was found.

Regardless of θ, when h / λ> 1,
The non-defective rate of the boundary acoustic wave device is rapidly improved. This state is shown in FIG.

[0010] In the structure of SiO _2/126 degree rotated Y-cut X-propagation lithium tantalate single-crystal piezoelectric substrate, SiO
_{2 Even if the} film is thickened, the temperature characteristics do not improve.

Even at 100 <θ <150, BAW exists. In particular, 100 <θ <124 or 128 <θ <
In 150, assuming that the thickness of the SiO ₂ film is h, the temperature characteristic becomes extremely good when h / λ is in the vicinity of 1 to 3. This state is shown in FIG. 18 × λ / h / | θ-1 between θ, h and λ
When the relationship of 26 | = 1 holds, the absolute value | TCD | of the first-order temperature coefficient TCD of the delay time change rate at room temperature shows the minimum value.

The electromechanical coupling coefficient becomes maximum near θ = 138. This state is shown in FIG. 130 <θ <1
It exceeds 45% at 45.

From the above, it was found from an experiment that | TCD | exhibits a minimum value when h / λ is in the vicinity of 1 to 3 when 100 <θ <124 or 128 <θ <150. . Therefore, by using this condition, a boundary acoustic wave device having good temperature characteristics can be created.

By setting 18 × λ / h / | θ-126 | = 1, a device having better temperature characteristics can be manufactured.

By using 130 <θ <145, the greatest piezoelectric effect can be obtained. Therefore, a boundary acoustic wave device with low loss can be manufactured.

Further, regardless of θ, by using h / λ> 1, the mass productivity of the boundary acoustic wave device can be improved.

Further, since the surface of BAW is protected by the non-metal film, physical and chemical deterioration of the electrode does not occur.
Even with respect to external stress, since it is an indirect effect through the non-metal film, even if it is covered with a malt agent or a silicone adhesive is applied to the surface of the non-metal film, the change in characteristics is extremely small. Therefore, it is not necessary to perform packaging using a container having a large volume, and the device can be downsized.

[0018]

EXAMPLE FIG. 1 shows an example of the present invention in which SiO ₂ /
The θ ° rotation Y-cut X-propagation shows the dependence of BAW propagation velocity on the SiO ₂ film thickness in a lithium tantalate single crystal piezoelectric substrate. The SiO ₂ film thickness h is standardized by the BAW wavelength λ (= electrode period). The SiO ₂ film was formed by the RF sputtering method. The created element has h / λ = 0.5, 1,
1.5, 2.0 and θ = 100, 108, 114, 12
0, 126, 132, 138, 144, 150.

When a product is mass-produced, the variation of the propagation velocity is proportional to the variation of the center frequency, and therefore the variation of the propagation velocity is small with respect to the variation of the SiO ₂ film thickness.
This is a very important issue for improving the yield rate. When BAW is used, the change rate of the propagation velocity becomes smaller as the SiO ₂ film becomes thicker at any θ. According to the present invention, if the variation of the film thickness when the SiO ₂ film is formed is ± 5%, the variation of the center frequency of the product can be reduced to 0.15% or less by setting h / λ> 1.

FIG. 2 shows the temperature characteristics of BAW in a SiO ₂ / θ ° rotation Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate. The vertical axis is | TCD |.

100 <θ <124 and 128 <θ <150
Necessarily has a θ that minimizes | TCD |. SiO ₂
Both .theta. Asymptotically approach 126 degrees as the film becomes thicker. Since 126 degrees is a perfect BAW, SiO ₂
Even if the film becomes thick, | TCD | does not become zero, and approaches a constant value. By interpolating the measured values, | TCD |
As a result of examining the relationship between θ and h / λ that minimize
/ H / | θ-126 | = 1.

It is known that the elastic properties of the SiO ₂ film change depending on the manufacturing method. In particular, in the CVD method, the ratio of Si and O can be changed by changing the mixing ratio of the gas source (commonly referred to as SiO _x film, but here, it is also referred to as SiO ₂ film). It is known that the temperature characteristics change greatly. Therefore, R
In addition to the F sputtering method, the atmospheric pressure CVD method and the plasma CVD method were examined. The piezoelectric substrate used had a θ of 144 degrees and Si
The film thickness h / λ of the O ₂ film is 0.5 to 1.

As a result, when the atmospheric pressure CVD method is used, h
/Λ=0.8 to 0.96, which is a value slightly smaller than that of the RF sputtering method, the | TCD | was minimized. When the plasma CVD method was used, | TCD | became the minimum with h / λ = 0.5 to 0.9, which is considerably smaller than that of the RF sputtering method.

As a result of the above, 0.5 <18 × λ / h / | θ
It can be seen that, in the case of the relationship of −126 | <1, it is possible to minimize | TCD | by any of the above methods.
According to the present invention, by having the relationship of θ, h, and λ, | TCD | is small, that is, a boundary acoustic wave device having excellent temperature characteristics can be manufactured.

FIG. 3 shows the electromechanical coupling coefficient (k ² ) of BAW in a SiO ₂ / θ ° rotation Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate. 2 for 130 <θ <145
Exceeds%. According to the present invention, the greatest piezoelectric effect can be obtained by using 130 <θ <145. Therefore, a boundary acoustic wave device with low loss can be manufactured.

In the device using elastic waves, the θ ° rotation Y cut and the θ ° + 180 ° rotation Y cut are completely equivalent due to the symmetry of the crystal of the piezoelectric substrate. Therefore, the above invention
It is obvious that it can also be applied to a + 180 ° rotated Y-cut lithium tantalate single crystal piezoelectric substrate.

FIG. 4, FIG. 5 and FIG. 6 show another embodiment of the present invention. FIG. 4 shows a single aperture resonator, FIG. 6 shows a double aperture resonator, and FIG. 5 is a sectional view. A SiO ₂ film 12 is formed on a lithium tantalate single crystal piezoelectric substrate 10. Between the lithium tantalate single crystal piezoelectric substrate 10 and the SiO ₂ film 12, there is a comb-shaped electrode 11 having an electrode period λ that excites BAW having a wavelength λ.

According to the present invention, since the SiO ₂ film 12 acts as a protective film for the comb-shaped electrode 11, it is not necessary to package a can package or the like in a container having a large volume.

Further, since the propagation direction of BAW can be set to the X-axis direction of the IRE standard, PFA (Power Flow Angle: angle formed by phase velocity and group velocity) can be made zero. This has an advantage that the design can be facilitated because the electrode can be designed without considering the PFA.

[0030]

According to the present invention, if a SiO ₂ film is formed on a θ ° rotated Y-cut lithium tantalate single crystal piezoelectric substrate and the thicknesses of θ and the SiO ₂ film are set to the above-mentioned conditions, temperature characteristics can be improved. It is possible to fabricate a boundary acoustic wave device which is excellent in productivity, small in insertion loss, and excellent in mass productivity.

Further, since the SiO ₂ film acts as a protective film for the comb-shaped electrodes, it is not necessary to package the container with a volume increase, so that a micro-sized boundary acoustic wave device can be realized. Therefore, by applying the boundary acoustic wave device of the present invention to a filter or the like in various communication fields, it becomes possible to realize a compact module and high performance.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the SiO ₂ film thickness dependence of the propagation velocity of a boundary acoustic wave in a SiO ₂ / θ ° rotation Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate.

FIG. 2 is a temperature characteristic diagram of boundary acoustic waves in a SiO ₂ / θ ° rotation Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate.

FIG. 3 is a characteristic diagram showing an electromechanical coupling coefficient of a boundary acoustic wave in a SiO ₂ / θ ° rotation Y-cut X-propagation lithium tantalate single crystal piezoelectric substrate.

FIG. 4 Si single crystal piezoelectric substrate on lithium tantalate
FIG. 3 is a plan view of a one-hole resonator in which an O ₂ film is formed and a comb-shaped electrode is provided on the interface.

FIG. 5: Si on a lithium tantalate single crystal piezoelectric substrate
O ₂ film is formed, cross-sectional view of one opening resonator having a comb electrode on the interface.

FIG. 6 shows a Si single crystal on a lithium tantalate single crystal piezoelectric substrate.
FIG. 3 is a plan view of a two-hole resonator in which an O ₂ film is formed and a comb-shaped electrode is provided on the interface.

[Explanation of symbols]

1 ... Propagation velocity of boundary acoustic wave with θ = 144, 2 ... θ = 13
Propagation velocity of 0 boundary acoustic wave, 3 ... Propagation velocity of boundary acoustic wave of θ = 120, 4 ... Propagation velocity of boundary acoustic wave of θ = 110, 5 Propagation of bulk wave of SiO ₂ obtained from simulation Velocity, 6 ... h / λ = 1 | TCD of boundary acoustic wave
|, 7 ... | TCD |, 8 of the boundary acoustic wave with h / λ = 1.5
... | TCD | of boundary acoustic wave of h / λ = 1, 9 ... h / λ =
2.5 boundary acoustic wave | TCD |, 10 ... Lithium tantalate single crystal piezoelectric substrate, 11 ... Comb type electrode, 12 ... SiO
₂ membranes.

Claims (3)

[Claims] 1. A rotation Y-cut lithium tantalate single crystal piezoelectric substrate having an angle θ, a comb-shaped electrode having an electrode period λ having at least a pair of electrode fingers on a main surface of the piezoelectric substrate, the piezoelectric substrate and the comb. In a boundary acoustic wave device in which a silicon oxide film having a film thickness h is formed on a mold electrode, the cut angle θ of the piezoelectric substrate is 100 <θ <124 or 128 <θ <.
The boundary acoustic wave device is characterized in that it is set to 150 and h / λ> 1. 2. The method according to claim 1, wherein between the film thickness h of the silicon oxide film and the cut angle θ of the piezoelectric substrate, 0.5 <1.
A boundary acoustic wave device having a relationship of 8 × λ / h / | θ-126 | <1. 3. The boundary acoustic wave device according to claim 1, wherein 130 <θ <145.