JP2002353768A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave element with an excellent power withstanding performance by forming an electrode excellent in resistance to stress migration performance onto a piezoelectric substrate. SOLUTION: A background electrode layer 5 whose major component is Ti or Cr is formed as an electrode 3 formed on the piezoelectric substrate 2 and an aluminum electrode layer 4 whose major component is aluminum is formed on the electrode layer 5. An epitaxially grown orientation film is provided to the aluminum electrode layer 4 and the background electrode layer 5, the (111) plane of the crystal of the aluminum electrode layer 4 and the (001) or (100) plane of the crystal of the background electrode layer 5 are oriented in parallel with the (001) plane of the crystal of the piezoelectric substrate 2.

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 such as a surface acoustic wave resonator or a surface acoustic wave filter, and more particularly to an improvement in a crystal structure of an electrode of a surface acoustic wave device. .

[0002]

2. Description of the Related Art As is well known, a surface acoustic wave element is an electronic component using a surface acoustic wave in which mechanical vibration energy propagates only in the vicinity of a solid surface, and is generally a piezoelectric substrate having piezoelectricity. And formed on this piezoelectric substrate,
It comprises electrodes such as interdigital electrodes and / or grating electrodes for applying signals.

[0003] In such a surface acoustic wave device, it is common to use, as an electrode material, a material mainly composed of Al having a low electric resistivity and a small specific gravity, that is, Al alone or an Al-based alloy. .

However, Al has poor resistance to stress migration, and when a large amount of power is applied, hillocks and voids are generated in the electrodes, and eventually the electrodes are short-circuited or disconnected, and the surface acoustic wave element may be destroyed. is there.

[0005] In order to solve the above-mentioned problem, Japanese Patent Laid-Open Publication No. Hei 7-162255 discloses a method for improving power durability by using ion beam sputtering as a method of forming an electrode and improving crystal orientation. (First prior art).

Japanese Patent Laid-Open No. 3-485 discloses a method in which the crystal orientation is oriented in a certain direction by epitaxially growing Al, thereby improving power durability.
No. 11 (second prior art).

On the other hand, it is described in Japanese Patent Application Laid-Open No. 6-6173 that the smaller the crystal grain, the more excellent the power durability of the electrode is (third prior art).

[0008]

However, the first and third prior arts have a problem that the power durability is insufficient when used for high frequency and high power applications.

[0009] The second prior art is applicable only to substantially quartz substrate, therefore, the piezoelectric property is large, widely used in filters and the like, LiNbO ₃
Alternatively, there is a problem that it is difficult to obtain an epitaxial film having good crystallinity on a LiTaO ₃ substrate.

An object of the present invention is to provide a surface acoustic wave device which can solve the above-mentioned problems.

[0011]

According to the present invention, there is provided an elastic surface comprising a piezoelectric substrate and an electrode formed on the piezoelectric substrate, the electrode comprising an Al electrode layer containing Al as a main component. In order to solve the above-mentioned technical problems, the Al electrode layer is composed of an epitaxially grown alignment film, and the relationship between the Al electrode layer and the plane orientation of the epitaxial growth between the piezoelectric substrate and the Al electrode layer. The crystal forming the layer is characterized in that the (111) plane of the crystal forming the piezoelectric substrate and the (001) plane of the crystal forming the piezoelectric substrate are grown in parallel with each other.

In the present invention, (11) of the Al electrode layer
Regarding the epitaxial relationship between the 1) plane and the in-plane orientation of the (001) plane of the piezoelectric substrate, preferably, the Al electrode layer has a <011> direction of the crystal forming the Al electrode layer and a <100> direction of the crystal forming the piezoelectric substrate. > The crystal grows while maintaining a relationship in which the directions coincide with each other.

As described above, the Al electrode layer is preferably made of a polycrystalline thin film having a twin structure as well as being made of an epitaxially grown alignment film.

In the present invention, the electrode preferably further includes a base electrode layer provided between the Al electrode layer and the piezoelectric substrate for improving the crystallinity of the Al electrode layer. This base electrode layer preferably contains at least one of Ti and Cr as a main component.

As described above, when a base electrode layer is provided,
The base electrode layer is composed of an epitaxially grown alignment film, and the relationship between the plane orientation of the epitaxial growth of the Al electrode layer, the base electrode layer, and the piezoelectric substrate depends on the (111) plane of the crystal constituting the Al electrode layer and the base electrode layer. (00
It is preferable that the (1) plane or the (100) plane is parallel to the (001) plane of the crystal constituting the piezoelectric substrate.

In the above case, regarding the epitaxial relationship of the in-plane orientation between the (111) plane of the Al electrode layer and the (001) plane or the (100) plane of the base electrode layer and the (001) plane of the piezoelectric substrate, the Al electrode layer <01 of the crystal constituting
It is more preferable that the crystal is grown while maintaining the same relationship between the <1> direction, the <100> direction of the crystal constituting the base electrode layer, and the <100> direction of the crystal constituting the piezoelectric substrate.

Further, it is more preferable that both the Al electrode layer and the base electrode layer are made of not only an oriented film grown epitaxially but also a polycrystalline thin film having a twin structure.

In a more specific embodiment of the present invention, the piezoelectric substrate is made of a 64 ° YX cut single crystal of LiNbO ₃ , and the crystal constituting the Al electrode layer has a (111) plane. Normal direction and Z of the crystal constituting the piezoelectric substrate
It has a crystal orientation that is oriented in a certain direction so that the axis substantially matches, that is, substantially matches.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been made based on the following findings.

That is, the electrode mainly composed of Al is
It is known that by forming a three-axis oriented film in which three crystal axes 1 are aligned in a certain direction, the stress migration resistance is improved. However, the electromechanical coupling coefficient is large, and LiNbO 2 widely used in filters and the like is used.
Conventionally, on a _three- substrate or LiTaO ₃ substrate, it has not been possible to form a triaxially oriented film having good crystallinity as described above.

Therefore, for example, on a LiNbO ₃ substrate,
By forming a Ti film as a base electrode layer and depositing Al thereon, an Al film epitaxially grown on a LiNbO ₃ substrate could be formed. The crystallinity of this Al film is extremely good, and the (111) axis of Al is 64 ° YX
A special orientation corresponding to the Z-axis direction of the cut LiNbO ₃ substrate was shown.

The epitaxially grown Al
A detailed examination of the crystal structure of the alignment film revealed that the film had a twin structure. The twin structure has both a feature that mechanical strength is high because the direction of the crystal axis is not uniform and a feature that grain boundary diffusion hardly occurs because of high orientation. Therefore, it has been found that the surface acoustic wave device having the electrode formed with such an Al alignment film can significantly improve the power durability.

FIG. 1 is a sectional view showing a part of a surface acoustic wave device 1 according to an embodiment of the present invention, and shows a portion where an electrode 3 is formed on a piezoelectric substrate 2.

The piezoelectric substrate 2 is made of a piezoelectric material such as LiNbO ₃ single crystal. The electrode 3 includes an Al electrode layer 4 containing Al as a main component, and a base electrode layer 5 for improving the crystallinity of Al is provided between the Al electrode layer 4 and the piezoelectric substrate 2. Can be Base electrode layer 5 contains, for example, at least one of Ti and Cr as a main component.

Although not shown, an electrically insulating protective film covering the surface and side surfaces of the electrode 3 may be further formed.

The piezoelectric substrate 2 is preferably 64 °
A substrate made of YX cut LiNbO ₃ single crystal is used. Therefore, the Y-axis direction and the Z-axis direction of the crystal of the piezoelectric substrate 2 are respectively oriented in the directions indicated by arrows in FIG. The X-axis direction is a direction perpendicular to the paper surface.

In forming the electrodes 3 on the piezoelectric substrate 2, a pretreatment is performed by, for example, ion etching. This is for removing a damaged layer having a thickness of several nm generated on the surface of the piezoelectric substrate 2 by polishing or the like.
Thereby, a crystal plane capable of epitaxial growth can be exposed on the surface of the piezoelectric substrate 2.

As a result of removing the affected layer, as shown in FIG. 2, the surface of the piezoelectric substrate 2 has a very small step-like structure with the Z surface 6 as a terrace. The outermost surface of the Z plane, that is, the (001) plane 6, has an oxygen atom 7 as schematically shown by a white circle in FIG.
It is in a state of being arranged at an interval of 2.972 °.

FIG. 4A shows that the piezoelectric substrate 2 made of LiNbO ₃ has the Z-plane, ie, the (001) plane 6 on the outermost surface.
The state where the oxygen atoms 7 are arranged is shown. The oxygen atoms 7 are arranged at an interval of 2.972 °.

Next, the base electrode layer 5 is formed on the Z surface 6 of the piezoelectric substrate 2 on which the oxygen atoms 7 are arranged as described above. To form the base electrode layer 5, for example, when a film of Ti having a hexagonal close-packed structure with a minimum interatomic distance of 2.920 ° is formed, a hatched circle having a relatively high concentration in FIG. As shown in (1), epitaxial growth is performed in a direction in which the (001) plane of the crystal of Ti atoms 8 is parallel to the Z plane of the piezoelectric substrate 2, that is, the (001) plane 6.

FIG. 4B shows the (0) of the crystal of Ti atom 8.
01) plane is shown. Since the Ti atoms 8 are easily linked to the oxygen atoms 7 shown in FIG. 4A, the distance between the oxygen atoms 7 on the LiNbO ₃ substrate as the piezoelectric substrate 2 shown in FIG. Epitaxial growth is performed at a close interval of 2.951 °. Therefore, Ti
In the base electrode layer 5 composed of the atoms 8, Al
Good crystallinity can be obtained as compared with the Al electrode layer 4 when the electrode layer 4 is formed directly on the piezoelectric substrate 2.

In forming the base electrode layer 5 described above, a method of forming the base electrode layer 5 by a vacuum deposition method at a temperature of 100 ° C. or less is applied. In this vacuum deposition method, 100 ° C.
When a higher temperature is applied, the orientation direction of the Ti atoms 8 changes, and therefore, in forming the Al electrode layer 4 described later, Al
The (111) or (110) plane of the crystal changes so as to grow perpendicular to the piezoelectric substrate 2, and it is difficult to obtain good crystallinity.

Next, the Al electrode layer 4 is formed on the base electrode layer 5. More specifically, the minimum interatomic distance is 2.864
When Al having a face-centered cubic structure is formed on the base electrode layer 5 on which the Ti atoms 8 are arranged by Å, as shown schematically in FIG. The (111) plane of the crystal of Al atom 9 is Ti (00).
1) Epitaxial growth is made parallel to the plane. The Al electrode layer 4 thus obtained exhibits excellent migration resistance.

Further, as shown in FIG. 3B, depending on how the Al atoms 9 enter, the piezoelectric substrate 2 is rotated by 180 ° with respect to the axis extending in the Z-axis direction as the rotation axis.
In some cases, the Al electrode layer 4 having a crystal structure having a seed crystal orientation is formed. Such a crystal structure is generally called twin. The above two types of crystal orientations are respectively
Appearing with a probability of 1/2, the obtained Al electrode layer 4 becomes a polycrystal having a crystal grain boundary, that is, a twin plane at a position indicated by a thick broken line 10.

Although FIG. 3B shows only one atomic layer of Ti atoms 8 for simplicity of illustration, several to several hundred atomic layers are actually formed.

In FIG. 3B, (20)
The directions 0), (020) and (002) are indicated by arrows. Actually, these axes are not on the paper surface of FIG. 3C, but are directed to the near side from the paper surface of about 35 °.

Thus, as shown in FIG.
Piezoelectric substrate 2 composed of a YN-cut LiNbO ₃ substrate
On the Z plane, that is, (11) parallel to the (001) plane 6,
1) An Al electrode layer 4 having a grown surface can be obtained.

In general, it is said that the existence of crystal grain boundaries in the Al electrode layer deteriorates the power durability of the surface acoustic wave device. This is due to stress migration
This is because Al self-diffuses through crystal grain boundaries and defects called hillocks and voids grow. However, in the polycrystalline Al electrode layer 4 obtained according to this embodiment, the crystal grain boundaries are smaller than one atomic interval, and self-diffusion through the crystal grain boundaries does not substantially occur.

On the other hand, the mechanical strength of a metal is higher in a polycrystal than in a single crystal. This is due to the plastic deformation mechanism of the metal. In other words, plastic deformation causes slip deformation of the crystal due to external force (vibration due to the piezoelectric effect in the field of surface acoustic wave devices), but in a single crystal, it is caused only by the slip system activity which is most active. On the other hand, polycrystals require a plurality of slip-type activities (reference: Maruzen, Metal Handbook, Revision 5).
Edition, pages 337-343). For this reason, the difficulty of plastic deformation leads to the difficulty of electrode breakdown due to stress migration, and the electrode structure having a small particle size provides high power durability.

From these facts, by forming the Al electrode layer 4 as an orientation film having a twin structure, the effect of preventing the growth of hillocks and voids due to the self-diffusion of the atoms constituting the electrode through the crystal grain boundaries and the plastic deformation It is possible to achieve extremely excellent power durability, which has both high power durability due to the difficulty.

The Al electrode layer 4, the base electrode layer 5, and the piezoelectric substrate 2
As described above, the relationship of the plane orientation of the epitaxial growth with the (111) plane of the crystal constituting the Al electrode layer 4, the (001) plane of the crystal constituting the base electrode layer 5, and the piezoelectric substrate 2.
Are parallel to each other, but the (111) plane of the Al electrode layer 4 and the (0) plane of the base electrode layer 5 are parallel to each other.
With respect to the epitaxial relationship between the (01) plane and the (001) plane of the piezoelectric substrate 2, the <011> direction of the crystal forming the Al electrode layer 4 and the <100> direction of the crystal forming the base electrode layer 5. It is preferable that the crystal is grown while maintaining the same relationship between the crystal constituting the piezoelectric substrate 2 and the <100> direction.

The <011> direction of the Al electrode layer 4 is defined as a direction connecting the centers of the closest atoms in the same element arrangement in the (111) plane of the Al electrode layer 4, and as an example, FIG. The [0-11] direction shown in c) is mentioned.
The <110> direction is an equivalent direction in consideration of symmetry [110], [1-10], [-11].
0], [101], [-101], [10-1], [0
11], [01-1], [0-11], etc.

The <100> direction of the base electrode layer 5 is defined as a direction connecting the centers of the closest atoms in the same element arrangement in the (001) plane of the base electrode layer 5, and as an example, FIG. [100] shown in b).

The <100> direction of the piezoelectric substrate 2 is as follows.
The direction connecting the centers of the closest atoms in the same element arrangement in the (001) plane of the piezoelectric substrate 2 is defined as an example, and [100] shown in FIG.

The <100> directions of the base electrode layer 5 and the piezoelectric substrate 2 are equivalent directions in consideration of symmetry [100], [-100], [010], [0-].
10], [001], [00-1], etc.

In the embodiment described above, the Al electrode layer 4 is
Although the orientation film has a twin structure, LiNbO ₃ or LNbO ₃
In the case of a surface acoustic wave device including a piezoelectric substrate made of a single crystal of iTaO _3, the crystal of the Al electrode layer does not necessarily have to have a twin structure. That is, the Al electrode layer simply has a crystal orientation oriented in a certain direction such that the normal direction of the (111) plane of the Al crystal substantially coincides with the Z axis of the crystal of the piezoelectric substrate. It may be uniaxial or triaxial.

In the above-described embodiment, the piezoelectric substrate 2
As has used a LiNbO ₃ substrate of 64 ° Y-X cut before removing the damaged layer of the surface by treatment, it is possible to expose the epitaxial growth capable crystal surface, the substrate having a different cut angle Is also effective. The same effect can be obtained also in a LiTaO ₃ substrate having a very similar crystal structure. Further, a piezoelectric substrate other than the LiNbO ₃ substrate or the LiTaO ₃ substrate can be used.

Although Al is used as the material of the Al electrode layer 4, an additive effective for improving the power resistance, for example,
An alloy in which Cu, Mg, Ni, Mo or the like is added to Al in a small amount may be used.

Although Ti is used as the material of the base electrode layer 5, other metals that are effective in improving the crystallinity of Al, such as Cr, may be used even if an alloy containing Ti as a main component is used. Alternatively, an alloy containing Cr as a main component may be used.

Although ion etching is used for the pretreatment of the piezoelectric substrate 2, other methods such as chemical mechanical polishing and scrubber cleaning may be used.

[0051]

EXPERIMENTAL EXAMPLE In order to produce a surface acoustic wave filter according to an embodiment of the present invention, first, a 64 ° YX cut LiNb
A pretreatment by ion etching was performed on the O ₃ piezoelectric substrate to remove a damaged layer having a thickness of several nm existing on the substrate surface.

Next, a base electrode layer made of Ti is formed to a thickness of 5 nm at a substrate temperature of 50 ° C. by an electron beam evaporation method, and then an Al electrode layer made of Al is formed to a thickness of 200 nm. It was formed so that it might become. In this manner, the Al electrode layer is formed such that the (111) plane of the crystal is perpendicular to the Z axis of LiNbO _{3 on} the piezoelectric substrate, in other words, the Z plane of LiNbO ₃ , that is, (00)
1) Epitaxial growth was performed so as to be parallel to the plane.

Next, the electrodes composed of the base electrode layer and the Al electrode layer described above were processed into an interdigital shape by using a photolithography technique and a dry etching technique to obtain a surface acoustic wave filter according to the example.

FIG. 5 shows an XRD pole figure of the Al electrode layer provided on the electrode according to the above-described embodiment. 6 in FIG.
Each point indicates detection of a reflection signal from the (002) plane of Al. Since the signal detection point shows six-fold symmetry, the Al crystal is 180
It can be seen that it has a twin structure having two kinds of crystal orientations as if rotated by degrees.

As a comparative example, film formation of Ti and Al was performed at a substrate temperature of 20 without performing ion etching treatment.
When performed at 0 ° C., no epitaxial film was obtained, and a (111) plane of Al became a uniaxially oriented film in which it was grown perpendicular to the substrate. FIG. 6 shows an XRD pole figure of this comparative example.

The surface acoustic wave filter according to the example was compared with the surface acoustic wave filter according to the comparative example, and the time required for failure to occur when a constant power was applied was 1000 times that of the surface acoustic wave filter according to the comparative example. It became longer than above.

[0057]

As described above, according to the present invention, the Al electrode layer provided on the electrode formed on the piezoelectric substrate is made of an epitaxially grown alignment film, and the (111) crystal of the Al electrode layer is formed. Since the surface and the (001) plane of the crystal constituting the piezoelectric substrate are oriented parallel to each other, generation of hillocks and voids due to stress migration of the electrodes is suppressed, and the power durability of the surface acoustic wave element is improved. can do.

When the above-mentioned Al electrode layer is formed of a polycrystalline thin film having a twin structure, the effect of preventing the growth of hillocks and voids due to the self-diffusion of the atoms constituting the electrode through the crystal grain boundaries is exhibited. The effect of increasing the power durability due to the difficulty of plastic deformation is exhibited, and the power durability of the surface acoustic wave element can be further enhanced.

Further, if a base electrode layer mainly containing at least one of Ti and Cr is provided between the Al electrode layer and the piezoelectric substrate, A
1 can be further improved in crystallinity.

In this case, when the underlying electrode layer is made of a polycrystalline thin film having a twin structure, similarly to the Al electrode layer, the same as in the case where the Al electrode layer is made of a polycrystalline thin film having a twin structure, as described above. The effect is exhibited, and the power durability of the surface acoustic wave element can be further improved.

Further, <01> of the crystal constituting the Al electrode layer
If the <1> direction matches the <100> direction of the crystal constituting the piezoelectric substrate, and if a base electrode layer is provided, the <100> direction of the crystal forming the base electrode layer is further provided.
When the direction matches each of the above-described directions, the stress migration resistance of the electrode is further enhanced, and the power resistance of the surface acoustic wave element can be more reliably improved.

[Brief description of the drawings]

FIG. 1 is a surface acoustic wave device 1 according to an embodiment of the present invention.
It is sectional drawing which shows a part of.

FIG. 2 is a cross-sectional view schematically showing the surface of the piezoelectric substrate 2 shown in FIG. 1, showing a Z surface 6 exposed after removing a work-affected layer on the surface.

FIG. 3 is a plan view of a Z-plane 6 shown in FIG. 2 (a).
Schematically illustrates the oxygen atoms 7 arranged thereon,
(B) schematically shows a Ti atom 8 further arranged thereon and an Al atom 9 arranged further thereon.

FIG. 4 (a) shows the Z plane of LiNbO ₃ , that is, (00)
1) shows oxygen atoms 7 arranged on the surface, and (b)
i shows the (001) plane, and (c) shows the (111) plane of Al.
Plane.

FIG. 5 shows the X of the Al electrode layer according to a specific embodiment of the present invention.
It is an RD pole figure.

FIG. 6 is an XRD pole figure of an Al electrode layer according to a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Surface acoustic wave element 2 Piezoelectric substrate 3 Electrode 4 Al electrode layer 5 Base electrode layer 6 Z-plane, ie, (001) plane of piezoelectric substrate 8 Ti atom 9 Al atom

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazuhiro Inoue 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto F-term in Murata Manufacturing Co., Ltd. (reference) 5J097 AA25 AA31 FF03 GG04 HA02

Claims (9)

[Claims] 1. A surface acoustic wave device comprising: a piezoelectric substrate; and an electrode formed on the piezoelectric substrate, wherein the electrode includes an Al electrode layer containing Al as a main component, and the Al electrode layer Is a surface acoustic wave device comprising an oriented film grown epitaxially, wherein a (111) plane of a crystal constituting the Al electrode layer and a (001) plane of a crystal constituting the piezoelectric substrate are parallel to each other. 2. The method according to claim 1, wherein the crystal forming the Al electrode layer has a <01
The surface acoustic wave device according to claim 1, wherein the <1> direction and the <100> direction of a crystal constituting the piezoelectric substrate are coincident with each other. 3. The surface acoustic wave device according to claim 1, wherein the Al electrode layer is formed of a polycrystalline thin film having a twin structure. 4. The electrode according to claim 1, wherein the electrode further comprises a base electrode layer provided between the Al electrode layer and the piezoelectric substrate for improving crystallinity of the Al electrode layer. A surface acoustic wave device according to any of the above. 5. The surface acoustic wave device according to claim 4, wherein the base electrode layer contains at least one of Ti and Cr as a main component. 6. The base electrode layer is composed of an epitaxially grown alignment film, and has a (111) plane of a crystal forming the Al electrode layer and a (00) plane of a crystal forming the base electrode layer.
The surface acoustic wave device according to claim 4 or 5, wherein the (1) plane or the (100) plane is parallel to the (001) plane of the crystal constituting the piezoelectric substrate. 7. The method according to claim 7, wherein the crystal constituting the Al electrode layer has a <01
1> Direction and <100> of the crystal constituting the base electrode layer
7. The surface acoustic wave device according to claim 6, wherein a direction and a <100> direction of a crystal constituting the piezoelectric substrate coincide with each other. 8. The surface acoustic wave device according to claim 6, wherein said Al electrode layer and said base electrode layer are both formed of a polycrystalline thin film having a twin structure. 9. The piezoelectric substrate is made of a single crystal of LiNbO ₃ cut at 64 ° YX, and the crystal constituting the Al electrode layer is composed of the normal direction of the (111) plane and the piezoelectric substrate. The surface acoustic wave device according to any one of claims 1 to 8, wherein the surface acoustic wave device has a crystal orientation oriented in a certain direction so that the Z axis of the crystal to be formed substantially matches.