JP3429888B2 - Surface acoustic wave device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material forming an interdigital electrode (hereinafter referred to as IDT) of a surface acoustic wave device.

[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. Aluminum is mainly used as an electrode material forming the IDT because of its low resistance, easy microfabrication, and excellent reliability. Further, the IDT mechanically vibrates in proportion to the electric signal due to the piezoelectricity of the substrate.

It is known that the mechanical vibration causes fatigue deterioration of the electrode fingers forming the IDT (stress migration). 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, or use of an aluminum film oriented in a crystal orientation has been studied so as to enhance resistance to stress migration.

As such a surface acoustic wave device, those described in JP-A-3-048511 and JP-A-6-132777 are known. JP-A-3-048
The invention described in Japanese Patent No. 511 is made in order to provide a surface acoustic wave device having excellent stress migration resistance characteristics. In this patent, the invention is disclosed on a 25- to 39-degree rotating Y-cut quartz substrate (311). A surface acoustic wave device having an aluminum film oriented on its surface is disclosed.

The invention described in Japanese Patent Laid-Open No. 6-132777 provides a surface acoustic wave device in which the stress change inside the metal film for electrodes in the surface acoustic wave device is eliminated and the frequency change during the operation of the device is reduced. In this publication, single crystal aluminum electrodes are formed on an LST-cut quartz substrate obtained by cutting a quartz Z plate in a counterclockwise direction in a range of 7 ° to 15 ° around the X axis. A surface acoustic wave device is disclosed.

[0006]

However, when silicon, copper or the like is added to aluminum, fine processing of the electrode may be difficult in some cases. Further, when an aluminum film oriented in a crystal orientation such as the surface acoustic wave device described in the above publication is used, the resistance to stress migration may not be sufficient depending on the piezoelectric substrate.

An object of the present invention is to provide a surface acoustic wave device having sufficient resistance to stress migration when using a quartz substrate.

[0008]

A surface acoustic wave device according to the present invention includes a quartz substrate and an interdigital electrode on the quartz substrate, and the interdigital electrode comprises a single crystal aluminum film oriented in a low index plane. It is a thing. As the low index plane, a (111) plane, a (100) plane, or the like is used. In particular, the quartz substrate is substantially a 35 ° rotated Y-cut quartz, and the single crystal aluminum film is substantially (1).
It is desirable that the crystal substrate is oriented in the (11) plane, or that the quartz substrate is substantially 28-degree rotated Y-cut quartz and the single crystal aluminum film is oriented substantially in the (100) plane.

Further, it is desirable that the amount of carbon in the single crystal aluminum film is 0.01 atom% or less, and the amount of hydrogen and oxygen is 0.5 atom% or less, respectively.

[0010]

According to the present invention, since the single crystal aluminum film oriented on the low index plane on the quartz substrate is used as the electrode, the electric resistance of the electrode is low, and the crystal structure is stable, so that stress migration does not occur. In contrast, it has excellent resistance. Also, rotate about 35 degrees on the Y-cut quartz substrate (11
1) A single crystal aluminum film oriented in the plane and about 2
The single crystal aluminum film oriented on the (100) plane on the 8-degree rotated Y-cut quartz crystal substrate is particularly stable and has particularly high resistance to stress migration.

Further, if aluminum having a carbon content of 0.01 atom% or less and hydrogen and oxygen content of 0.5 atom% or less is used, a low-resistance single crystal aluminum film having good crystallinity can be formed.

[0012]

FIG. 1 shows a surface acoustic wave device according to an embodiment of the present invention.
Will be explained. The surface acoustic wave filter which is the surface acoustic wave device of the present embodiment is a 35 ° rotated Y-cut quartz substrate 1.
Three input IDTs 21, 22, 23 and four output IDTs 24, 25, 26, 27 are alternately arranged on the surface of 0, and these input IDTs 21, 22, 23 and output IDTs 24, 25, 26 are arranged. , 27 is a pair of reflectors 28,
An electrode structure row 20 sandwiched by 29 is formed.
The three input IDTs 21, 22, 23 are connected in parallel,
The four output IDTs 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 quartz 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 openings 21 to 27 have an opening width of 30λ and are composed of 20.5 pairs of electrode fingers, and the reflectors 28 and 29 are composed of 120 electrodes. IDTs 21 to 27 and reflector 2
Reference numerals 8 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 has the same periodic structure as a whole.

Next, a method of manufacturing the surface acoustic wave device of this embodiment will be described with reference to FIG. First, the surface of the 35 ° rotated Y-cut quartz substrate 10 is ultrasonically cleaned with an organic solvent such as acetone or isopropanol, and ashing is performed in oxygen plasma to remove the residual organic substances on the surface. As a result, the organic substances on the substrate surface are oxidized and sufficiently removed (FIG. 2A).

Next, the quartz substrate 10 is attached to an electron beam vacuum vapor deposition apparatus, and the substrate temperature is raised to 400 ° C. or higher and kept for 1 hour or longer while keeping the degree of vacuum of 0.01 mPa or lower, thereby adsorbing the surface. Remove enough water. Alternatively, the adsorbed moisture on the surface may be sufficiently removed by setting the substrate temperature to about 200 ° C. and holding it for 6 hours. Next, a 100-nm-thick single crystal aluminum film 30 is formed by a vacuum evaporation method with a substrate temperature of 100 ° C. and a growth rate of 0.85 nm / sec (FIG. 2B). The electric resistance of the single-crystal aluminum film 30 is 3.0 μΩ · cm, which is a value sufficiently low as that of bulk aluminum.

When this single crystal aluminum film was analyzed by secondary ion mass spectrometry (SIMS), impurities in the aluminum film 30 had a carbon content of 0.01 atom%.
Hereinafter, the amounts of hydrogen and oxygen were each 0.5 atom% or less. When the amounts of carbon, hydrogen and oxygen in the aluminum film 30 are larger than this, the aluminum film 30 becomes a polycrystalline or amorphous film and does not become a single crystal aluminum film regardless of the film forming conditions such as the growth rate and the substrate temperature.

As conditions for vacuum vapor deposition, if the amount of carbon, hydrogen and oxygen in the aluminum film 30 is the above amount or less, the substrate temperature is 100 ° C. or higher and 300 ° C. or lower, and the growth rate is 1 nm / nm. The crystallinity of the single crystal aluminum film is improved by setting it to 0.1 nm / sec or more for 2 seconds or less.
Substrate temperature is less than 100 ° C, growth rate is 1 nm /
When the speed exceeds the second, single crystal aluminum is formed, but the crystallinity of the single crystal aluminum film deteriorates.

When the growth rate is set to a low speed of less than 0.1 nm / sec, or conversely, a high speed of more than 1 nm / sec is obtained,
Single crystal aluminum is formed, but the controllability of film thickness and productivity are significantly reduced. At the beginning of this vacuum deposition,
In metallic aluminum having a face-centered cubic structure, the (111) plane having the lowest surface energy is likely to appear. When the cut surface of the quartz substrate 10 was variously changed and examined, it was found that the (111) plane stably grows in the range of 33 ° rotated Y cut to 37 ° rotated Y cut. This epitaxial orientation is

[0019]

[Equation 1] Therefore, the misfit rate is considered to be about -0.7%. The first line of the above formula 1 indicates the direction of the single crystal aluminum film 30 and the direction of the crystal substrate 10 which are in agreement,
The second line shows that the seven crystal lattices of the single crystal aluminum film 30 and the two crystal lattices of the quartz substrate 10 are in agreement.

The aluminum single crystal orientation film did not grow when the surface of the substrate was not sufficiently cleaned before vacuum deposition, particularly when ashing was not performed in oxygen plasma. It is considered that this is because the stain remains on the surface of the substrate, and growth in the direction other than the (111) plane is likely to occur from the stain and the orientation is not single crystal. 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. 2).
(C)). The width of the electrode fingers of the IDT and the distance between the electrode fingers are 1 / the wavelength of the surface acoustic wave corresponding to the pass center frequency of the filter.
4 is about 500 nm.

Next, the aluminum film 30 in the portion not covered with the resist layer 40 is treated with a phosphoric acid-based etching solution (volume ratio H ₃ PO ₄ : CH ₃ COOH: HNO ₃ : H ₂ O = 1).
Etching is performed by 6: 2: 2: 2) to produce the electrode structure array 20 including the IDT. Finally, the resist film 40
Are removed, and the processing of the electrode structure array 20 including the IDT is completed (FIG. 2D).

In this way, the surface acoustic wave device according to this embodiment can be formed. As another example or comparative example of the present invention, the single crystal aluminum film 3 oriented to the low index plane can be obtained even when the cut angle of the quartz substrate 10 is specified as another angle. As another example, about 2
It was found that on the 8 ° rotation Y-cut quartz substrate 10, the aluminum film having the (100) plane was grown stably. This epitaxial orientation is

[0023]

[Equation 2] Therefore, the misfit rate is considered to be about 1.9%.
The first line of the above formula 2 indicates the direction of the single crystal aluminum film 30 and the direction of the quartz substrate 10 which coincide with each other, and the second line shows the seven crystal lattices of the single crystal aluminum film 30 and the crystal. It is shown that the four crystal lattices of the substrate 10 match. It should be noted that the crystal substrate 10 deviated by about 2 degrees before and after the Y-cut of about 28 degrees, that is, the crystal substrate of the Y-cut of 26 to 30 degrees is also oriented.

As a comparative example, it was found that the Y-cut quartz substrate 10 rotated by about 32 degrees stably grows the aluminum film having the (210) plane. This epitaxial orientation is

[0025]

[Equation 3] Therefore, it is considered that the misfit rate is about 3.3%. The first line of the above formula 3 indicates the direction of the single crystal aluminum film 30 and the direction of the quartz substrate 10 which are in agreement, and the second line thereof is the three crystal lattices of the single crystal aluminum film 30 and the crystal. It shows that the seven crystal lattices of the substrate 10 match. It should be noted that the quartz substrate 10 whose angle is shifted before and after the Y-cut about 32 degrees, that is, 31 degrees to 3 degrees.
The orientation is the same for a Y-cut crystal substrate rotated by 3 degrees.

Further, as another comparative example, a rotation of about 38 degrees Y
It was found that on the cut quartz substrate 10, the aluminum film having the (311) plane appeared was stably grown. This epitaxial orientation is

[0027]

[Equation 4] Therefore, the misfit rate is considered to be about 0.6%. The first line of the equation 4 indicates the direction of the single crystal aluminum film 30 and the direction of the quartz substrate 10 which are in agreement with each other.
The second line shows that the seven crystal lattices of the single crystal aluminum film 30 and the two crystal lattices of the quartz substrate 10 are in agreement. It should be noted that the crystal substrate 10 whose angle is shifted before and after the Y-cut of about 38 degrees, that is, the crystal substrate of the Y-cut of 37 to 40 degrees is also oriented.

As in the above-mentioned Examples and Comparative Examples, (111) plane, (100) plane, (210) plane, (3)
11) The electric power resistance of the surface acoustic wave filter using the IDT with the aluminum film oriented on the (11) plane was evaluated. It was evaluated by applying a power of about 400 mW to the surface acoustic wave device and measuring the time until the electrical characteristics deteriorate (MTF). The MTF is 2500 hours when the aluminum film oriented in the (111) plane of the example is used and 2000 hours when the aluminum film oriented in the (100) plane of the example is used. However, when the aluminum film oriented to the (210) plane or the (311) plane, which is a comparative example, was used, it was as short as 1800 hours.

As yet another comparative example, a thickness of 100
Similarly, the power resistance of the surface acoustic wave device using a polycrystalline pure aluminum film having a non-nm orientation was measured. In the case of this comparative example, the time until the electrical characteristics deteriorate with the same applied power of about 400 mW is 15
It was 00 hours. As described above, according to the present example, the power resistance is significantly improved as compared with the comparative example, and it is understood that the stress migration resistance is improved.

The present invention is not limited to the above embodiment, but various modifications are possible. For example, in the above embodiment, the aluminum film is made of high-purity aluminum, but Cu, Si, Co, etc., which do not affect the orientation, may be added. Further, the present invention is effective in a surface acoustic wave device having an IIDT structure that is easily affected by stress migration, but is also effective in a surface acoustic wave device having another structure such as another transversal filter or a resonator structure. Is.

The surface acoustic wave filter having the IIDT structure has a plurality of input IDTs and a plurality of output IDTs alternately arranged, and has a plurality of input IDTs and a plurality of output IDTs.
Are surface acoustic wave filters each having a structure connected in parallel.

[0032]

As described above, since the single crystal aluminum film oriented on the low index plane on the quartz substrate is used as the electrode, the electric resistance of the electrode is low, the crystal structure is stable, and it is resistant to stress migration. An interdigital electrode having excellent resistance can be obtained, and a surface acoustic wave device having excellent power resistance can be realized.

[Brief description of 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 a method of manufacturing a surface acoustic wave device according to an embodiment of the present invention.

[Explanation of symbols]

10 ... Crystal substrate 20 ... Electrode structure row 21, 22, 23 ... Input IDT 24, 25, 26, 27 ... Output IDT 28, 29 ... Reflector 30 ... Aluminum film 40 ... Resist film

Continuation of front page (56) Reference JP-A-6-132777 (JP, A) JP-A-5-226337 (JP, A) JP-A-5-183373 (JP, A) JP-A-5-190548 (JP , A) JP-A-3-48511 (JP, A) Eiji Ieki, Atsushi Sakurai, SAW resonator using epitaxial A1 electrode, IEICE Transactions A, February 25, 1993, VOL. J76-A, NO. 2, p. 145-152 (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/145 H03H 9/25

Claims (2)

(57) [Claims] 1. A single crystal aluminum film which includes a substantially 28 ° rotated Y-cut quartz substrate and an interdigital electrode on the quartz substrate, the interdigital electrode being substantially oriented in a (100) plane. A surface acoustic wave device characterized in that 2. The single crystal aluminum film has a carbon content of 0.01 atom% or less and a hydrogen content and an oxygen content of 0.
The surface acoustic wave device according to claim 1 , wherein the surface acoustic wave device is 5 atom% or less.