JP2002164765A - Piezoelectric substrate and saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric substrate for a SAW(surface acoustic wave) filter, which sufficiently corresponds to the high frequency and broadband of a communication system and is easy to form. SOLUTION: In this piezoelectric substrate having a substrate and a piezoelectric thin film formed on the substrate, the substrate is composed of a diamond substrate 1, and the piezoelectric thin film is composed of the thin film of Langasite crystals 2, 3 and 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a piezoelectric substrate and a piezoelectric substrate.
AW filter, in particular, a piezoelectric substrate suitable for use in a SAW filter and an SA formed on the piezoelectric substrate
It relates to a W filter.

[0002]

2. Description of the Related Art Communication devices such as portable telephones and PHSs are rapidly spreading. In the field of communication devices which are being digitized, there is a demand for higher performance of elements such as filters and vibrators. I have. Conventionally, for example, SAW (Surface Acoustic Wav) for mobile phones
e) Quartz crystal units are often used for filters.
However, since quartz has insufficient piezoelectric characteristics, it has a disadvantage of a narrow bandwidth, and cannot respond sufficiently to miniaturization.

On the other hand, a piezoelectric substrate for a SAW filter in which a piezoelectric thin film is laminated on diamond has been proposed in order to cope with a higher frequency and a wider band of a communication system. Diamond is a substance having the highest elastic constant, and it is said that by laminating a piezoelectric thin film thereon, a SAW filter substrate compatible with high frequencies can be realized. As a piezoelectric thin film, see, for example,
Zinc oxide (ZnO) disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 7 (July 2000, Vol. 20, No. 7).
aluminum nitride (AlN) disclosed in p28-p35).

[0004]

By the way, a piezoelectric substrate using ZnO or AlN as a piezoelectric thin film has insufficient electric characteristics such as an electromechanical coupling coefficient and a frequency temperature stability, and has a problem in that the piezoelectric thin film cannot be used. There was a problem that the film forming process was complicated.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a piezoelectric substrate for a SAW filter and a SAW filter which can easily be formed and which can sufficiently cope with a higher frequency and a wider band of a communication system.

[0006]

According to a first aspect of the present invention, there is provided a piezoelectric substrate having a substrate and a piezoelectric thin film formed on the substrate, wherein the substrate is a diamond substrate. A piezoelectric substrate, wherein the piezoelectric thin film is made of a langasite-type crystal thin film.

According to a second aspect, in the first aspect, the langasite crystal is a crystal having a stoichiometric composition of La ₃ Ga ₅ SiO _{14 and} a stoichiometric composition of La ₃ Ta _0.5 Ga _5.5 O _14. Or La ₃ Nb _0.5 Ga _5.5 O
A crystal having any one of ₁₄ stoichiometric compositions.

According to a third aspect of the present invention, there is provided a SAW filter having a substrate, a piezoelectric thin film formed on the substrate, and an electrode having a predetermined shape formed on the piezoelectric thin film. A SAW filter comprising a diamond substrate, wherein the piezoelectric thin film comprises a langasite-type crystal thin film.

[0009]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. According to the present invention, a thin film of a langasite-type crystal, which is a high-performance piezoelectric crystal having a very wide frequency band and a low insertion loss, exhibiting a very small frequency temperature dependency by selecting a cut plane as a piezoelectric crystal, is formed on a diamond substrate. It is formed.

<First Embodiment> FIG. 1 is a structural view showing a first embodiment of a piezoelectric substrate according to the present invention. First, powders of lanthanum oxide (La ₂ O ₃ ), gallium oxide (Ga ₂ O ₃ ), and silicon oxide (SiO ₂ ), each having a purity of 4N,
Stoichiometric composition ratio of Langasite-type crystal La ₃ Ga ₅ Si
A powder raw material is obtained by weighing and mixing so that O ₁₄ (the langasite-type crystal of this composition is also simply referred to as LGS). This powdery raw material is filled into an Ir crucible, heated and dissolved by high frequency heating in an atmosphere in which oxygen is mixed with about 1% of nitrogen, and then cooled to obtain an ingot of a langasite-type crystal. This ingot is pulverized with a crusher to produce a langasite crystal powder.

A target is formed from the powder, and a diamond substrate 1 having a diameter of 3 inches is used as a substrate, and a langasite-type crystal (LGS) thin film 2 is formed on the diamond substrate 1 by a high frequency sputtering method. The sputtering conditions at this time are, for example, RF power: 20
0 W, sputtering atmosphere gas: A containing 30% O ₂
r gas, gas pressure: 2 Pa, substrate heating temperature: 300 ° C., film formation time: 100 minutes. Thereby, about 1 μm
An LGS thin film 2 having a thickness of 2 mm is formed on the diamond substrate 1.

Next, the LGS thin film 2 is subjected to a heat treatment at 900 ° C. for one hour in an oxygen atmosphere. Thereafter, crystal analysis was performed on the LGS thin film by X-ray diffraction. FIG. 2 is a graph showing an X-ray diffraction pattern of the LGS thin film in the first example. FIG. 2 shows seven peaks, which are (110), (200), (111), and (111) of the langasite-type crystal in order from the lower angle side.
The diffraction peaks of the (021), (120), (002), and (211) planes indicate that a langasite-type crystal thin film (ie, La ₃ Ga ₅ SiO ₁₄ thin film 2) is formed.

As described above, the piezoelectric substrate 1 having the La ₃ Ga ₅ SiO ₁₄ thin film 2 formed on the diamond substrate 1
0A is obtained. The La ₃ Ga ₅ SiO ₁₄ thin film 2 exhibits an extremely small frequency-temperature dependency suitable for a SAW filter, has a wide band and low insertion loss, and can be easily formed by a sputtering method. Also, the piezoelectric substrate 1
By forming an electrode portion having a predetermined shape on the La ₃ Ga ₅ SiO ₁₄ thin film 2 of 0 A, an efficient SAW filter corresponding to a high frequency and having small frequency temperature dependency can be formed.

<Second Embodiment> FIG. 3 is a structural view showing a second embodiment of the piezoelectric substrate of the present invention. First, each powder of lanthanum oxide (La ₂ O ₃ ), tantalum oxide (Ta ₂ O ₅ ), and gallium oxide (Ga ₂ O ₃ ), each having a purity of 4N, was mixed with a stoichiometric composition ratio La ₃ Ta of a langasite-type crystal.
_A powder raw material is obtained by weighing and mixing so as to obtain _0.5 Ga _5.5 O ₁₄ (the langasite-type crystal having this composition is simply referred to as LTG). This powder raw material is filled in an Ir crucible, and is heated by high frequency heating in an atmosphere in which oxygen is mixed with nitrogen at about 1%.
The mixture is heated and melted, and then cooled to obtain an ingot of a langasite type crystal. This ingot is pulverized to produce a langasite crystal powder.

A target is formed from the powder, and a diamond substrate 1 having a diameter of 3 inches is used as a substrate. The LTG thin film 3 is formed on the diamond substrate by high frequency sputtering under the same sputtering conditions as in the first embodiment.
To form Thus, an LT having a thickness of about 1 μm
G thin film 3 is formed on diamond substrate 1.

Next, the LTG thin film 3 was subjected to a heat treatment at 900 ° C. for 1 hour in an oxygen atmosphere. Thereafter, crystal analysis was performed on the LTG thin film by X-ray diffraction. FIG. 4 is a graph showing an X-ray diffraction pattern of the LTG thin film in the second example. FIG. 4 shows seven peaks, which are (110), (200), (111), and (111) of the langasite-type crystal in order from the lower angle side.
(021), (120), (002), and (211) are the diffraction peaks on the respective surfaces, indicating that a langasite-type crystal thin film (ie, La ₃ Ta _0.5 Ga _5.5 O ₁₄ thin film 3) is formed. I understand.

As described above, the piezoelectric substrate 10B having the La ₃ Ta _0.5 Ga _5.5 O ₁₄ thin film 3 formed on the diamond substrate 1 is obtained. The La ₃ Ta _0.5 Ga _5.5 O ₁₄ thin film 3 has a very small frequency temperature dependency, is suitable for SAW filters, has a wide band and low insertion loss,
It can be easily formed by a sputtering method. In addition, by forming an electrode portion having a predetermined shape on the La ₃ Ta _0.5 Ga _5.5 O ₁₄ thin film 3 of the piezoelectric substrate 10B, a high-frequency-adaptive, efficient and small temperature-dependent SAW filter is provided. Can be formed.

<Third Embodiment> FIG. 5 shows a piezoelectric substrate according to the present invention.
It is a block diagram showing a third embodiment of the plate. First, all are pure
Lanthanum oxide (La_TwoO_Three), Niobium oxide
(Nb _TwoO_Five), Gallium oxide (Ga_TwoO_Three) Each powder
Stoichiometric composition ratio La of langasite-type crystal_ThreeNb_0.5G
a_5.5O₁₄(Langasite-type crystals of this composition are simply referred to as L
NG) to obtain powdered raw materials.
You. This powder material is filled in an Ir crucible, and oxygen is added to nitrogen.
Heated by high frequency heating in an atmosphere mixed about 1%
Dissolve and then cool to remove the ingot of the langasite-type crystal.
Get This ingot is crushed and langasite crystals
Make powder.

A target is formed from this powder, and a diamond substrate 1 having a diameter of 3 inches is used as a substrate. The LNG thin film 4 is formed on the diamond substrate by high frequency sputtering under the same sputtering conditions as in the first embodiment.
To form Thus, the LN having a thickness of about 1 μm
A G thin film 4 is formed on the diamond substrate 1.

Next, the LNG thin film 4 was subjected to a heat treatment at 900 ° C. for 1 hour in an oxygen atmosphere. Thereafter, crystal analysis of the LNG thin film 4 was performed by X-ray diffraction. FIG. 6 is a graph showing an X-ray diffraction pattern of the LNG thin film in the third example. FIG. 6 shows six peaks, which are (110), (200), (111), and (111) of the langasite-type crystal in order from the lower angle side.
(021), (120), and (211) are the diffraction peaks on each plane, and are the langasite-type crystal thin films (that is, La ₃
It can be seen that an Nb _0.5 Ga _5.5 O ₁₄ thin film 4) was formed.

As described above, a piezoelectric substrate 10C having the La ₃ Nb _0.5 Ga _5.5 O ₁₄ thin film 4 formed on the diamond substrate 1 is obtained. The La ₃ Nb _0.5 Ga _5.5 O ₁₄ thin film 4 exhibits an extremely small frequency temperature dependency, is suitable for SAW filters, has a wide band and low insertion loss,
It can be easily formed by a sputtering method. In addition, by forming an electrode portion having a predetermined shape on the La ₃ Nb _0.5 Ga _5.5 O ₁₄ thin film 4 of the piezoelectric substrate 10C, a SAW filter that can handle high frequencies, is efficient, and has little frequency temperature dependence. Can be formed.

The present invention is not limited to the above-described embodiment, and the conditions for the above-mentioned sputtering and heat treatment may be changed as appropriate. Further, although the langasite-type crystal powder is used as the target, it goes without saying that the sputtering process can be improved by using a target formed by firing the langasite-type crystal powder.

[0023]

As described above, according to the first and second aspects of the piezoelectric substrate of the present invention, the substrate is formed of a diamond substrate, and the piezoelectric thin film is formed of a langasite-type crystal thin film. This makes it easy to form a SAW that is fully compatible with high frequency and broadband communication systems.
There is an effect that a piezoelectric substrate for a filter can be provided.

In the SAW filter according to the present invention,
According to the third aspect of the present invention, the substrate is formed of a diamond substrate, and the piezoelectric thin film is formed of a langasite-type crystal thin film. There is an effect that a filter can be provided.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a piezoelectric substrate of the present invention.

FIG. 2 is a graph showing an X-ray diffraction pattern of the LGS thin film in the first embodiment.

FIG. 3 is a configuration diagram showing a second embodiment of the piezoelectric substrate of the present invention.

FIG. 4 is a graph showing an X-ray diffraction pattern of an LTG thin film in a second example.

FIG. 5 is a configuration diagram showing a third embodiment of the piezoelectric substrate of the present invention.

FIG. 6 is a graph showing an X-ray diffraction pattern of an LNG thin film in a third example.

[Explanation of symbols]

1: Diamond substrate, 2: LGS (La ₃ Ga ₅ SiO)
₁₄₎ a thin _{film, 3 ... LGT (La 3 Ga} 0.5 Ta 5.5 O 14) thin _{film, 4 ... LNT (La 3 Nb} 0.5 Ta 5.5 O 14) thin film.

Claims (3)

[Claims] 1. A piezoelectric substrate having a substrate and a piezoelectric thin film formed on the substrate, wherein the substrate is formed of a diamond substrate, and the piezoelectric thin film is formed of a langasite-type crystal thin film. Characteristic piezoelectric substrate. 2. The method according to claim 1, wherein said langasite-type crystal is La ₃ Ga ₅ Si.
A crystal having a stoichiometric composition of O ₁₄ , La ₃ Ta _0.5 Ga
_5.5 Crystals having a stoichiometric composition of O ₁₄ or La ₃ Nb
Piezoelectric substrate according to claim 1, characterized in that either of the crystals having a stoichiometric composition of _0.5 Ga _{5 .5} O _14. 3. A SAW filter having a substrate, a piezoelectric thin film formed on the substrate, and an electrode having a predetermined shape formed on the piezoelectric thin film, wherein the substrate is formed of a diamond substrate, SAW characterized in that said piezoelectric thin film is composed of a langasite-type crystal thin film.
filter.