JPH09172351A - Surface acoustic wave device and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an interdigital electrode with a sufficient immunity against stress migration by adopting a single crystal aluminum film for an electrode formed on the surface of a piezoelectric substrate. SOLUTION: Three input IDTs 21, 22, 23 and four output IDTs 24, 25, 26, 27 are arranged alternately on the surface of a piezoelectric substrate 10 made of a lithium tetraborate single crystal (Li2 B4 O7 ) of a (110)-cut (45 deg. rotation cut) and an electrode structural array 20 is formed, in which the input IDTs 21, 22, 23 and the output IDTs 24, 25, 26, 27 are inserted between a couple of reflectors 28, 29. The three input IDTs 21, 22, 23 are connected in parallel with each other and the four output IDTs 24, 25, 26, 27 are connected in parallel with each other. The electrode structural array 20 including the IDTs as above is formed on a single crystal aluminum film 30 on the surface of the piezoelectric substrate 10 in a way that the direction in parallel with the surface is oriented to be the (111)-plane. The film thickness is nearly 100nm.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a surface acoustic wave device using a piezoelectric substrate made of lithium tetraborate single crystal and a method for manufacturing the same.

[0002]

2. Description of the Related Art A surface acoustic wave device uses an IDT (also called a comb-shaped electrode or a comb-shaped electrode) provided on a piezoelectric substrate to mutually transmit an electric signal and a surface acoustic wave propagating on the surface of the piezoelectric substrate. It is a device that performs conversion and uses the surface acoustic waves to perform functions such as a filter, a resonator, and a delay line. Conventionally, aluminum, which has a low resistance, is easy to perform fine processing, and is excellent in reliability, has been mainly used as an electrode material forming the IDT.

In recent years, the range of use of surface acoustic wave devices is MH.
It spreads from the z-band to the GHz band and is becoming more and more high frequency. As the frequency of signals increases, the need for processing higher power signals also increases. For this reason, the electric current and vibration applied to the IDT electrode are further increased, and it is necessary to use an electrode material having excellent power resistance. It is known that the electrodes constituting the IDT are fatigue-degraded (stress migration) due to mechanical vibrations in the surface acoustic wave device. Due to this stress migration, the surface acoustic wave device may deteriorate during use and the initial characteristics may not be exhibited. Therefore, addition of silicon, copper, or the like to aluminum has been studied so as to enhance resistance to stress migration.

[0004]

However, in the above-mentioned conventional surface acoustic wave device, the stress migration resistance is insufficient. This is because the aluminum formed by the vapor deposition method or the magnetron sputtering method has a polycrystalline structure, and thus deterioration of stress migration resistance due to grain boundary diffusion cannot be avoided.

An object of the present invention is to provide a surface acoustic wave device having sufficient resistance to stress migration and a method for manufacturing the same.

[0006]

A surface acoustic wave device according to the present invention comprises a piezoelectric substrate made of lithium tetraborate single crystal,
It is formed on the surface of the piezoelectric substrate, and has an electrode for exciting, receiving, reflecting and propagating a surface acoustic wave, and the electrode,
It is characterized by being formed of a single crystal aluminum film.

According to the present invention, since the electrode is a single crystal aluminum film and there is no crystal grain boundary, stress migration due to grain boundary diffusion does not easily occur even if the current applied to the electrode and vibration increase. The electric resistance of the electrode is low and the crystal structure is stable, and the resistance to stress migration of the surface acoustic wave device can be dramatically improved.

In the above-mentioned surface acoustic wave device, the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are indicated by an oiler angle (0 ° to 45 °, 30 ° to 90 °, 4).
0 ° to 90 °) and its equivalent range. In this case, the single crystal aluminum film is particularly stable and has a particularly high resistance to stress migration.

In the surface acoustic wave device described above, Si, Cu, C are formed on the single crystal aluminum film forming the electrodes.
It is desirable to add at least one substance selected from the group consisting of o, Mo, Hf and B. In this case, the single crystal aluminum film is particularly stable and has a particularly high resistance to stress migration.

In the surface acoustic wave device described above, carbon is contained in the single crystal aluminum film forming the electrode in an amount of 0.01
It is desirable that the content of oxygen is 0.5 atom% or less and that of oxygen is 0.5 atom% or less. If it deviates from this range, the aluminum film becomes a polycrystalline or amorphous film, which is not preferable. A method of manufacturing a surface acoustic wave device according to the present invention comprises forming a single crystal aluminum film by an ion beam sputtering method on a piezoelectric substrate made of lithium tetraborate single crystal, and patterning the single crystal aluminum film to obtain a surface acoustic wave. Is formed so as to form electrodes for exciting, receiving, reflecting and propagating.

According to the present invention, by using the ion beam sputtering method, a single crystal aluminum film having good crystallinity can be easily formed on a piezoelectric substrate made of lithium tetraborate single crystal. In the method of manufacturing a surface acoustic wave device described above, the cut-out angle of the surface of the piezoelectric substrate is (0 ° to 45 °, 30 ° to 90 °,
40 ° to 90 °) and a range equivalent thereto.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave device according to an embodiment of the present invention will be described with reference to FIG. The surface acoustic wave filter, which is the surface acoustic wave device of the present embodiment, is provided on the surface of the piezoelectric substrate 10 made of (110) cut (45 ° rotation X cut) lithium tetraborate single crystal (Li ₂ B ₄ O ₇ ). ,
Three input IDTs 21, 22, 23 and four output I
DTs 24, 25, 26, 27 are arranged alternately, and these input IDTs 21, 22, 23 and output IDTs 24,
An electrode structure row 20 is formed in which 25, 26 and 27 are sandwiched by a pair of reflectors 28 and 29. 3 inputs I
The DTs 21, 22 and 23 are connected in parallel and have four output I's.
The DTs 24, 25, 26, 27 are also connected in parallel.

The electrode structure array 20 including these IDTs is composed of an oriented single crystal aluminum film 30 whose (111) plane is parallel to the surface of the piezoelectric substrate 10. The film thickness of the electrode is about 100 nm, and the width of the electrode finger and its interval are about 500 nm corresponding to ¼ of the surface acoustic wave wavelength corresponding to the pass center frequency of the filter. Each IDT
The aperture width of each of 21 to 27 is 120λ, and is composed of 20.5 pairs of electrode fingers, and the reflectors 28 and 29 are composed of 120 electrodes. The IDTs 21 to 27 and the reflectors 28 and 29 have the same periodic structure. The IDTs and the IDTs and the reflectors and the IDTs are also formed with the same period, and the electrode structure array 20 as a whole has the same periodic structure.

Next, a method of manufacturing a surface acoustic wave device according to an embodiment of the present invention will be described with reference to FIG. First, (1
10) Cut lithium tetraborate single crystal (Li ₂ B ₄ O
The surface of the piezoelectric substrate 10 made of ₇ ) is cleaned with ultrapure water and ultrasonically with an organic solvent such as acetone or isopropyl alcohol (FIG. 2A).

The lithium tetraborate substrate is easily etched by a neutral solution or an acidic solution, and the morphology of the surface of the substrate may change significantly depending on the cleaning method and the cleaning time, which may interfere with the propagation of surface acoustic waves. There is a nature.
As long as the surface acoustic wave device does not cause any trouble, no matter what cleaning method is used to clean the piezoelectric substrate 10, the growth of the single crystal aluminum film is hardly affected.

Next, the piezoelectric substrate 10 is mounted in the film forming chamber of the ion beam sputtering apparatus, and the film forming chamber is vacuumed. The ultimate vacuum in the deposition chamber before sputtering is about 0.1
It is desirable to set it to mPa or less. If the ultimate vacuum in the film forming chamber before sputtering is low, the aluminum particles are contaminated by the residual gas in the film forming chamber, which hinders the formation of aluminum particles in a vacuum and the movement of the aluminum particles on the surface of the piezoelectric substrate 10. It may not cause crystal growth. Therefore, it is desirable that the ultimate vacuum of the film forming chamber is low, for example, about 0.1 mP or less.

Subsequently, the single crystal aluminum film 30 is formed on the surface of the piezoelectric substrate 10 by the ion beam sputtering method.
Of about 100 nm is formed (FIG. 2B). As the film formation conditions for the ion beam sputtering method, the substrate temperature during film formation is about 150 ° C., and the film growth rate is about 1 nm / sec.
When this single crystal aluminum film 30 was analyzed by secondary ion mass spectrometry (SIMS), the aluminum film 3
The impurities in 0 had a carbon content of 5 × 10 ⁻³ atom%, a hydrogen content of 0.1 atom%, and an oxygen content of 0.1 atom%. The carbon content is 0.01 atom% or less, hydrogen,
The amount of oxygen is preferably 0.5 atom% or less, and when the amount of carbon, hydrogen, and oxygen in the aluminum film 30 is larger than this, the aluminum film 30 is polycrystalline or is formed regardless of film forming conditions such as growth rate and substrate temperature. It becomes an amorphous film and does not become a single crystal aluminum film.

The substrate temperature is preferably in the range of room temperature to 300 ° C. as a film forming condition for the ion beam sputtering method. When the substrate temperature is higher than 300 ° C., the crystallinity of the single crystal aluminum film 30 deteriorates. If the substrate temperature is lower than room temperature, a cooling mechanism must be attached to the ion beam sputtering apparatus, which is not preferable.

The film forming rate is preferably within the range of 0.1 to 50 nm / sec. Growth rate of 50 nm /
If the speed exceeds the second, the crystallinity of the single crystal aluminum film 30 deteriorates. If the film formation rate is less than 0.1 nm / sec, it takes a long time to form an electrode, which is not economically preferable. Next, a resist film 40 is applied on the aluminum film 30 and patterned into the shape of the electrode structure array 20 including the IDT (FIG. 2C). The width of the electrode fingers of the IDT and the distance between the electrode fingers are about 500 nm, which corresponds to 1/4 of the surface acoustic wave wavelength corresponding to the pass center frequency of the filter.

Next, the portion of the single crystal aluminum film 30 which is not covered with the resist layer 40 is removed by etching with an alkaline etching solution whose main component is TMAH (trimethylammonium hydride), and the electrode structure array 20 including the IDT is manufactured. (FIG. 2 (d)). Finally, the resist film 40 is removed, and the processing of the electrode structure array 20 including the IDT is completed (FIG. 2 (e)).

In order to investigate the crystallinity of the single crystal aluminum film thus formed according to this example, a high-speed backscattered electron diffraction image was measured. A spot indicating that the thin film was a single crystal was observed, and it was found that the single crystal aluminum thin film was epitaxially grown on the lithium tetraborate substrate. The orientation pattern of the single crystal aluminum film 30 is (11
1) It turned out to be a surface.

Usually, when the adsorption energy between the thin film and the substrate is small, the contact area between the crystal islands and the substrate is small, and the growth of the thin film is three-dimensional. On the other hand, in order to minimize the surface energy of the entire crystal island, in the case of aluminum having the fcc structure, the (111) plane having the smallest surface energy appears preferentially. When the electric resistance of the single crystal aluminum thin film formed according to this example was measured, the electric resistance of this single crystal aluminum film was 3.
It was 0 μΩ · cm. This resistance value is sufficiently low as that of bulk aluminum, and it can be seen that a single crystal aluminum thin film having good crystallinity is formed.

Next, the power resistance of the surface acoustic wave device manufactured according to this example was evaluated. It was evaluated by applying a predetermined electric power to the surface acoustic wave device and measuring the time (MTF) until the electrical characteristics of the surface acoustic wave device deteriorated. The center frequency of the element was adopted as the electrical characteristic of the surface acoustic wave device, and the time until the amount of fluctuation became ± 1000 ppm or more was measured.

In the surface acoustic wave device according to this embodiment, 40
It took 5000 hours for the electrical characteristics to deteriorate with an applied power of 0 mW. As a comparative example, Al-2.5 wt% C having a polycrystalline structure was formed on a lithium tetraborate single crystal substrate by a DC magnetron sputtering method.
The power resistance was similarly measured for the surface acoustic wave device in which a u alloy thin film was formed to a thickness of 100 nm and patterned to the same electrode structure. As a result, in the surface acoustic wave device according to the comparative example, it took 2000 hours for the electrical characteristics to deteriorate with the same applied power of 400 mW.

Therefore, the surface acoustic wave device according to this example was able to improve the power resistance by 2.5 times as compared with the comparative example. The present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above embodiment, the aluminum film is made of high-purity aluminum, but other metals such as Si, Cu, Co, Mo, Hf, and B may be added to the high-purity aluminum.

In the above embodiment, the cut surface of the lithium tetraborate single crystal substrate is the (110) plane.
Other cutout surfaces are (011), (345),
You may use (255), (231), (356) etc. That is, the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are displayed in the Euler angle (0 ° to 45 °, 3 °
It is preferable that the electrodes are formed within the range of 0 ° to 90 °, 40 ° to 90 °) and the equivalent range.

Particularly, when the Rayleigh wave is used as the surface acoustic wave, the (110) plane is used as the substrate cut-out surface, and when the Rayleigh wave and the leaky wave which are faster than the Rayleigh wave are used as the surface acoustic wave, they are equivalent to the (011) plane. It is desirable that the electrode is formed in a plane. In addition, the present invention provides an IID that is susceptible to stress migration.
It is effective in a surface acoustic wave device having a T structure,
It is also effective for surface acoustic wave devices of other structures such as other transversal filters and resonator structures.

[0028]

As described above, according to the present invention, since the single crystal aluminum film is used as the electrode on the piezoelectric substrate made of lithium tetraborate single crystal, the electric resistance of the electrode is low and the crystal structure is stable. Therefore, an interdigital electrode having excellent resistance to stress migration can be obtained, and a surface acoustic wave device having excellent power resistance can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a surface acoustic wave device according to an embodiment of the present invention.

FIG. 2 is a process sectional view showing the method of manufacturing the surface acoustic wave device according to the embodiment of the present invention.

[Explanation of symbols]

 10 ... Quartz substrate 20 ... Electrode structure sequence 21, 22, 23 ... Input IDT 24, 25, 26, 27 ... Output IDT 28, 29 ... Reflector 30 ... Single crystal aluminum film 40 ... Resist film

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] April 9, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction contents]

According to the present invention, by using the ion beam sputtering method, a single crystal aluminum film having good crystallinity can be easily formed on a piezoelectric substrate made of lithium tetraborate single crystal. In the method of manufacturing a surface acoustic wave device described above, the cut-out angle of the surface of the piezoelectric substrate is (0 ° to 45 °, 30 ° to 90 °,
40 ° to 90 °) and a range equivalent thereto. In the manufacturing method of the surface acoustic wave device described above,
The single crystal aluminum film forming the electrode is
selected from the group consisting of i, Cu, Co, Mo, Hf and B
It is hoped that at least one substance
Good. In the method of manufacturing a surface acoustic wave device described above,
Carbon is contained in the single crystal aluminum film forming the electrode.
0.01 atom% or less, oxygen is 0.5 atom% or less
It is desirable to be included.

Claims (6)

[Claims] 1. A surface acoustic wave device comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and an electrode formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating a surface acoustic wave. A surface acoustic wave device, wherein the electrodes are made of a single crystal aluminum film. 2. The surface acoustic wave device according to claim 1, wherein the cut-out angle of the surface of the piezoelectric substrate and the propagation direction of the surface acoustic wave are in the Euler angle display (0 ° to 45 °, 30 ° to 90 °).
, 40 ° to 90 °) and a range equivalent thereto. 3. The surface acoustic wave device according to claim 1, wherein the single crystal aluminum film forming the electrode has Si and C
A surface acoustic wave device, to which at least one substance selected from the group consisting of u, Co, Mo, Hf, and B is added. 4. The surface acoustic wave device according to claim 1, wherein the single crystal aluminum film forming the electrode contains 0.01 atom% or less of carbon and 0.5 atom% or less of oxygen. Surface acoustic wave device. 5. A method for manufacturing a surface acoustic wave device, comprising: a piezoelectric substrate made of lithium tetraborate single crystal; and electrodes formed on the surface of the piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves. The surface acoustic wave device according to claim 1, further comprising a step of forming a single crystal aluminum film on the piezoelectric substrate by an ion beam sputtering method, and a step of patterning the single crystal aluminum film to form the electrode. Manufacturing method. 6. The method of manufacturing a surface acoustic wave device according to claim 5, wherein the cutout angle of the surface of the piezoelectric substrate is (0
(° -45 °, 30 ° -90 °, 40 ° -90 °) and the equivalent range thereof.