JPH07135443A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To provide a surface acoustic wave device without deterioration in an insertion loss having an electrode wiring with excellent power withstanding performance. CONSTITUTION:A base film 2 of a metal or a compound including boron (B), carbon (C), silicon (Si), germanium (Ge), silicon carbide (SiO), boron nitride (BN) and silicon nitride (Si3N4) or the like whose thickness is 1-30Angstrom and an electrode wiring 3 made of an Al or Al alloy film having a uniaxial orientation structure of (111) whose film thickness is 500-3000Angstrom are formed on the surface of a piezoelectric substrate 1 in the surface acoustic wave device and the wiring 3 is processed interdigitally by the photolithography with precision.

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.

[0002]

2. Description of the Related Art Conventionally, surface acoustic wave devices (hereinafter referred to as SAW filters), which are widely used as filters in mobile communications, optical communications, etc., are generally made of quartz, lithium tantalate, lithium niobate, and tetranitrate. The interdigital transducer and the like are arranged and formed on a single crystal piezoelectric substrate such as lithium borate or on a piezoelectric thin film in which a zinc oxide film is formed on a sapphire substrate. As the electrode material of this interdigital electrode, an aluminum alloy to which aluminum (Al) or copper (Cu) and silicon (Si), titanium (Ti), palladium (Pd) and the like are added in a small amount is used. Further, the electrode metal in the SAW filter is required to have excellent fine workability, have a small specific gravity in order to reduce the electrode load mass effect, and have a low electric resistance in order to reduce the insertion loss. Therefore, metallic materials other than Al or Al alloys are not suitable.

[0003]

The conventional SAW described above is used.
In the filter, when a high-power signal is input to the SAW filter, the Al electrode receives a large stress due to the vibration of the piezoelectric substrate due to the surface acoustic waves, which causes a so-called stress migration in which Al atoms move. It has been known. When this fault occurs, SAW including electrical short circuit, increase of insertion loss, decrease of Q value, etc.
The filter characteristics deteriorate. Moreover, the frequency of occurrence of this stress migration increases as the frequency of the SAW filter increases, which is a major obstacle to increasing the frequency of the SAW filter.

As a countermeasure against the stress migration of the electrodes of this SAW filter and the electromigration of the LSI wiring, in Japanese Patent Application No. 4-75183, titanium, vanadium, iron, cobalt, nickel and copper having a film thickness of 1 to 500 angstroms are disclosed. Also, a wiring made of Al or Al alloy having a (111) uniaxially oriented structure formed on a metal underlayer of yttrium has been proposed. In the SAW filter electrode of this structure, (11
1) Although the reliability of the power resistance is improved as compared with the conventional Al electrode having a polycrystalline structure because of the orientation, the metal of the base film is Al formed on this metal film. There is a drawback in that the electric resistance of the electrode increases due to diffusion into the film and alloying with Al, resulting in an increase in insertion loss of the SAW filter.

The present invention is intended to solve the above-mentioned drawbacks of the conventional example, and its purpose is to:
An object of the present invention is to provide a surface acoustic wave device including an Al electrode having excellent electric power resistance and low electric resistance.

[0006]

A surface acoustic wave device according to the present invention comprises a single crystal piezoelectric substrate or a piezoelectric substrate including a piezoelectric thin film, and electrode wiring made of aluminum or aluminum alloy provided on the surface. In the piezoelectric thin film, high wettability to the single crystal piezoelectric substrate or piezoelectric thin film and aluminum,
And a base film having a film thickness of 1 to 30 angstroms, which is composed of a metal or a compound having a low reactivity with aluminum, and (111) uniaxially oriented structure aluminum or aluminum alloy formed on the base film. And the electrode wiring to be formed.

The single crystal piezoelectric substrate may be made of any one of quartz, lithium tantalate, lithium niobate and lithium tetraborate, and the piezoelectric substrate is formed on a sapphire substrate. You may comprise by the formed zinc oxide film. Further, the base film is a metal having high wettability with respect to the single crystal piezoelectric substrate or the piezoelectric thin film and aluminum and having low reactivity with aluminum, or boron (B), carbon (C), silicon (S).
i), germanium (Ge), silicon carbide (Si
C), boron nitride (BN) and silicon nitride (Si ₃
It may be composed of a compound containing N ₄ ).

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

1 (a) and 1 (b) are schematic views showing a plane and a cross section of the interdigital electrode in the SAW filter of one embodiment of the present invention. As shown in FIG. 1, in the present embodiment, the piezoelectric substrate 1 has a thickness of 1 to 3 on the surface thereof.
0 angstrom metal or compound containing boron (B), carbon (C), silicon (Si), germanium (Ge), silicon carbide (SiC), boron nitride (BN), silicon nitride (Si ₃ N ₄ ), etc. Base film 2 of
With a film thickness of 500 to 3000 angstroms, (11
1) The electrode wiring 3 made of an Al or Al alloy film having a uniaxially oriented structure is formed, and is finely processed by photolithography into a blind shape as shown in the plan view of FIG.

Generally, an Al or Al-based alloy is directly formed on a piezoelectric thin film in which a zinc oxide film is formed on a single crystal piezoelectric substrate such as quartz, lithium tantalate, lithium niobate and lithium tetraborate, or a sapphire substrate. Thermodynamically looking at the process of forming the
Since the interfacial energy between the substrate and l is larger than the sum of the surface energy of the substrate material and the surface energy of Al, Al grows like islands on these substrates,
The film is converted into a film having a polycrystalline structure. On the other hand, a metal or compound M (M = B, which has high wettability to these substrates and Al and has low reactivity with Al,
When C, Si, Ge, SiC, BN, and Si ₃ N ₄ ) are used as the base film and Al is formed on the base film, the effective interface energy between Al and the substrate is Al directly on the substrate.
Since it has a smaller value than that in the case of forming Al, the Al (111) plane having the smallest surface energy on these underlayer films is a uniaxially oriented film in which the Al (111) plane is grown parallel to the substrate surface. The uniaxially oriented structure of the Al film on the underlayer film is sensitive to the film thickness of the underlayer film, and the film thickness is 1 to 30.
When it is not within the range of angstrom, the uniaxially oriented structure is disturbed. The reason for this is that when the film thickness exceeds an appropriate underlayer film thickness, the underlayer film surface is no longer flat, and Al (11
This is due to the mixture of things with directions other than 1).

In the present invention, as described above, on the surface of the piezoelectric substrate 1, a metal of 1 to 30 angstroms, or boron (B), carbon (C), silicon (Si), germanium (Ge), carbonization is used. A base film 2 of a compound containing silicon (SiC), boron nitride (BN), silicon nitride (Si ₃ N ₄ ) and the like, and Al having a film thickness of 500 to 3000 angstroms and a (111) uniaxial orientation structure.
Alternatively, by forming the electrode wiring 3 of an Al alloy film, the metal of the base film or the compound M has a low reactivity with Al, and therefore, even in the process of manufacturing the SAW filter or in the operating environment, Does not cause a diffusion / alloying reaction between the underlayer film of Al and the Al or Al alloy film formed thereon, and thus the electric resistance of the SAW electrode wiring does not increase, thus increasing the insertion loss of the SAW filter. The deterioration of the SAW filter characteristics including is eliminated.

FIG. 2 is a block diagram showing the configuration of a two-dimensional ion beam sputtering apparatus used for forming an electrode film in the present invention. As shown in FIG. 2, the two-dimensional ion beam sputtering apparatus corresponds to the piezoelectric substrate 1, two ion beam sources 4 and 5, an Al or Al alloy target 6, and a metal used for a base film or The target 7 of the compound M, two shutters 8 and 9 for opening and closing the sputtered particle flux that has jumped out of these two targets 6 and 7, and two films for monitoring and controlling the film thickness. Thickness gauge 10 and 1
1 and an electron gun for reflection high-energy electron diffraction (RHEED) for observing the surface structure of a film deposited on a substrate (hereinafter,
RHEED electron gun) 12 and fluorescent screen 13
And is configured. Argon as the sputtering gas is introduced into the two ion beam sources 4 and 5 by the Alcon gas cylinder 14 through the mass flow controllers 15 and 16.

When forming the electrode film in the present invention,
First, after evacuation to 1 × 10 ⁻⁷ Torr, sputtering is performed at an argon pressure of 2 × 10 ⁻⁴ Torr. In addition, a metal or compound M (M = B, C, S
i, Ge, SiC, BN and Si ₃ N ₄ ) base film,
And, the Al or Al alloy film formed thereon was formed under the condition that the beam voltage of the ion source was 1200 V, the beam current was 50 to 65 mA, and the substrate temperature was 30 ° C. (room temperature). Under the above conditions, the sputter rate for the metal or compound for the base film and Al or Al alloy is 0.1 to 2 angstrom /
It was seconds.

For the piezoelectric substrate, rotate 34 to 45 ° Y
Cut quartz substrate, -75 ° rotated Y-cut quartz substrate, X-cut lithium tantalate (LiTaO ₃ : LT) substrate, 36 ° rotated Y-cut LT substrate, 128 ° rotated Y-cut lithium niobate (LiNbO ₃ : Below, L
N) substrate, 41 ° rotation Y-cut LN substrate, 64 °
The rotating Y-cut LN substrate, the 45 ° rotating X-cut lithium tetraborate (Li ₂ B ₄ O ₇ ) substrate, and the zinc oxide (ZnO) film formed on the sapphire substrate were investigated.
Since no significant difference was found in the results, an example in which a 64 ° rotated Y-cut LN substrate is used among the above-mentioned piezoelectric substrates will be described below.

The power resistance of the manufactured SAW filter of the present invention was evaluated by the system and method described in JP-A-3-48511. That is, as shown in FIG. 3, the output signal of the signal generator 17 is power-amplified by the power amplifier 18 and applied to the SAW filter 20 housed in the constant temperature bath 19, and the SA
The power of the output signal of the W filter 20 is measured by the power meter 21.
Measured by The power measurement data from the power meter 21 is input to the computer 22 and fed back to the signal generator 17 via the computer 22. Due to this feedback action, the frequency of the signal output from the signal generator 17 is controlled, and the frequency of the signal applied to the SAW filter 20 is controlled and set so as to always be the peak frequency of the transmission characteristic. Also, the constant temperature bath 19
The internal temperature was set to 85 ° so that the characteristic deterioration of the SAW filter 20 was accelerated. The life of the SAW filter is determined as follows. That is,
The output of the SAW filter 20 before the test and the output at time t after the start of the test are P ₀ and P, respectively.
It is defined that (t) is the life of the SAW filter at the time point when it is 1.0 dB lower than the initial value, that is, when P (t) becomes the value shown in the following equation.

P (t) ≦ P ₀ −1.0 (dB) (1) In this evaluation test, in addition to the above-mentioned evaluation of power resistance, four terminals were stored under high temperature storage. The change in the specific resistance of the electrode film by the method was also measured.

Next, a first embodiment of the present invention will be described based on the above evaluation test results. As a first embodiment of the present invention, a metal or compound M (M = B, C, S
i, Ge, SiC, BN, and Si ₃ N ₄ ) are used as parameters to change the film thickness, and an Al film having a film thickness of 2000 angstrom is formed on the base film.
The surface structure of the Al film was observed by the RHEED electron gun 12. This result is as shown in Table 1 below, and when the underlayer film thickness of the metal or compound M is in the range of 1 to 30 angstroms, the Al film formed thereon has a thickness of (111). ) Has a uniaxially oriented structure, but when there is no underlayer film or when the underlayer film thickness exceeds 30 angstroms, the Al film has a polycrystalline structure or a structure in which the uniaxially oriented structure is broken (polycrystal and orientation are mixed). Structure). Incidentally, in this embodiment, A
I have described the results when I film was formed.
Al-4.5 wt% Cu film or Al instead of the I film
Even when the Al alloy film of −2.0 wt% Si film is used, the same result as in the case of the Al film is obtained.

[0018]

[Table 1]

Next, as a second embodiment of the present invention, a metal or compound M (M = B, C, Si, Ge, SiC,
BN and Si ₃ N 4 ), The surface structure of the Al film formed thereon has a polycrystalline structure (without a base), a (111) uniaxially oriented structure (a base film thickness of 1 to 30 Å), and a base film thickness of 30. A SAW filter is constructed by processing a film having a structure in which the orientation exceeds angstroms (structure in which polycrystal and orientation are mixed) into a interdigital electrode by photolithography to form a SAW filter.
A power resistance test was conducted with the applied power to the W filter set to 1 W. The manufactured SAW filter of this embodiment is
The electrode film thickness including the underlying film thickness was 2100 Å, and the insertion loss measured at the center frequency of 836.5 MHz was 6.1 dB. For comparison, a SAW filter having a titanium underlayer was also manufactured,
The same evaluation test of power durability was also performed. As shown in Table 2 below, the Al film has a uniaxially oriented structure of (111), which is a metal or compound M (M = B, C, Si, G).
The use of a base film of e, SiC, BN and Si ₃ N ₄ ) markedly improves the power resistance in this embodiment, as compared with the Al film having a polycrystalline structure or a structure in which the orientation structure is broken. I understand that. However, in the case of a SAW filter having a titanium underlayer, the Al film is (11
1) Even if it becomes a uniaxially oriented structure, the power withstanding life is still metal or compound M (M = B, C, Si, Ge, SiC, B).
Compared with the (111) uniaxially oriented Al film on the N and Si ₃ N ₄ base film, the value is extremely small.

[0020]

[Table 2]

Next, a third embodiment of the present invention will be described. In the above-mentioned second embodiment, Al is formed on the base film.
Although the result of the power resistance of the SAW filter having the film formed thereon has been described, in the present embodiment, instead of the Al film,
The result of the power withstanding life of the SAW filter using the Al-4.5 wt% Cu alloy film will be described. As shown in Table 3 below, the Al-4.5 wt% Cu alloy film is (1
11) Metal or compound M (M
= B, C, Si, Ge, SiC, BN and Si
It can be seen that the use of the ₃ N ₄ ) base film significantly improves the power resistance as compared with the Al-4.5 wt% Cu alloy film having a polycrystalline structure or a structure in which the orientation structure is broken. A having a (111) uniaxially oriented structure on the same underlying film
Comparing the 1 film and the Al-4.5 wt% Cu alloy film, it is found that the power resistance of the latter is remarkably improved. However, in the case of the SAW filter having the titanium base film, the Al-4.5 wt% Cu alloy film is (11
1) Even if it becomes a uniaxially oriented structure, the power withstanding life is still metal or compound M (M = B, C, Si, Ge, SiC, B).
The value is extremely small as compared with the (111) uniaxially oriented Al-4.5 wt% Cu alloy film on the N and Si ₃ N ₄ ) base film.

[0022]

[Table 3]

As is apparent from the second and third embodiments described above, despite having the same (111) uniaxially oriented structure. 1 to 30 Å thick B, C, Si, Ge, SiC, BN and Si ₃ N
_The Al film or Al alloy film formed on the base film of ₄ is
It is superior in power resistance to an Al film or an Al alloy film formed on a titanium base film. In order to investigate this cause, the specific resistance under high temperature storage was measured.
When C, Si, Ge, SiC, BN and Si ₃ N ₄ base films were used, no significant difference due to the difference in these base films was observed. The test results performed for an Al film having a film thickness of 2000 angstrom formed on the titanium underlayer film having an angstrom will be described.

FIG. 4 is a plot of the change rate of the specific resistance when stored at 200 ° C. against the storage time. In the Al film formed on the titanium underlayer, the specific resistance is In contrast to the increase with the passage of storage time, in the Al film formed on the silicon underlayer film, such a tendency is not observed, and the second and third
It can be considered that the difference in the power withstanding property in the above example is the difference in the presence or absence of an increase in insertion loss due to the presence or absence of an increase in the specific resistance of the electrode film. B, C, Si, G mentioned above
The use of e, SiC, BN and Si ₃ N ₄ underlayers does not cause an increase in the electrode film specific resistance due to these B, C, Si, Ge, SiC, BN and Si ₃ N ₄ etc. It is considered that the diffusion alloying reaction does not occur due to the low reactivity with Al.

[0025]

As described above, the present invention is applied to a surface acoustic wave device constructed by providing electrode wiring made of aluminum or aluminum alloy on a single crystal piezoelectric substrate or a piezoelectric substrate including a piezoelectric thin film. And is formed on the piezoelectric thin film and is made of a metal or a compound that has high wettability with the single crystal piezoelectric substrate or the piezoelectric thin film and aluminum and has low reactivity with aluminum, and has a film thickness of 1 to 30. By providing an electrode wiring formed of aluminum or an aluminum alloy having a uniaxial orientation structure of (111) formed on the underlying film of Angstrom, the power resistance of the electrode of the surface acoustic wave device is provided. Of the SAW filter, and when applied to a SAW filter, There is an effect that it is possible to eliminate the deterioration in the insertion loss.

[Brief description of drawings]

FIG. 1 is a plan view and a sectional view showing a partial structure of an embodiment of the present invention.

FIG. 2 is a diagram showing an outline of an ion beam sputtering apparatus used for an evaluation test of an example of the present invention.

FIG. 3 is a conceptual diagram showing a power withstand evaluation test system according to an embodiment of the present invention.

FIG. 4 is a diagram showing a change in specific resistance value due to a difference in underlying film thickness.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 Underlayer film 3 Electrode wiring 4, 5 Ion beam source 6, 7 Target 8, 9 Shutter -10, 11 Thickness meter 12 RHEED electron gun 13 Fluorescent screen 14 Argon gas cylinder 15, 16 Mass flow controller 17 Signal generator 18 Power amplifier 19 Constant temperature bath 20 SAW filter 21 Power meter 22 Computer

Claims (4)

[Claims] 1. A surface acoustic wave device comprising electrode wiring made of aluminum or aluminum alloy on a single crystal piezoelectric substrate or a piezoelectric substrate including a piezoelectric thin film, wherein the single crystal is formed on the piezoelectric thin film. A base film having a film thickness of 1 to 30 angstroms, which is formed of a metal or a compound having high wettability to a piezoelectric substrate or a piezoelectric thin film and aluminum and having low reactivity with aluminum, and formed on the base film. 2. A surface acoustic wave device comprising: (111) electrode wiring formed of aluminum or an aluminum alloy having a uniaxially oriented structure. 2. The surface acoustic wave device according to claim 1, wherein the single crystal piezoelectric substrate is made of any one of quartz, lithium tantalate, lithium niobate, and lithium tetraborate. 3. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is formed of a zinc oxide film formed on a sapphire substrate. 4. The base film has high wettability with respect to the single crystal piezoelectric substrate or the piezoelectric thin film and aluminum,
A metal having low reactivity with aluminum, or boron (B), carbon (C), silicon (Si), germanium (Ge), silicon carbide (SiC), boron nitride (B)
2. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is composed of a compound containing N) and silicon nitride (Si ₃ N ₄ ).