Process for providing a surface acoustic wave device

Abstract

This invention relates to a surface acoustic wave device and a production process thereof. An electrode is formed by alternately laminating a film of an aluminum alloy containing at least copper added thereto and a copper film on a piezoelectric substrate. While the particle size of the multi-layered electrode materials is kept small, the occurrence of voids in the film is prevented and life time of the surface acoustic wave device is elongated.

Description

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, an electrode structure which can be advantageously utilized in a surface acoustic wave device according to the present invention, and the function and effect of such an electrode, will be explained.

Generally, it is believed that a film obtained by adding a small amount (about 3 to 4 wt%) of a different kind of metal to Al has a structure in which an alloy between Al and the different kind of metal exists at a grain boundary of Al.

FIG. 10 is an explanatory view of an Al-Cu film electrode structure. In this drawing, reference numeral 21 denotes an LiTaO₃ substrate, 22 is an Al-Cu film, 23 is Al crystal grains, 24 is a grain boundary, and 25 is CuAl₂. This drawing illustrates an example where the Al-Cu film 22 is deposited on the LiTaO₃ substrate 21 by sputtering or electron beam deposition and is patterned. Basically, it is a polycrystalline structure of Al, wherein a large number of Al crystal grains 23 exist, and CuAl₂ 25 segregates at the grain boundary 24. It is believed that the reason why the Al-Cu film has higher resistance to migration than the Al film is because Cul225 inhibits fluidization of the Al atoms.

A similar effect can be obtained when Ti, Si, etc., is used as the metal to be added to Al, in place of Cu described above.

Next, we consider the case where a Cu film is formed on the upper surface of the Al film having this structure, with reference to FIG. 11.

FIG. 11 is an explanatory view of an Al-Cu/Cu film electrode structure. In the drawing, reference numeral 21 denotes a LiTaO₃ substrate, 22 is an Al-Cu film, 23 is Al crystal grains, 24 is a grain boundary, 25 is CuAl₂ and 26 is a Cu film. The drawing illustrates an example where the Al-Cu alloy film 22 is formed on the LiTaO₃ substrate 21 by sputtering or electron beam deposition, the Cu film 26 is formed on the former and the Cu film 26 is then patterned. This is basically a polycrystalline structure of Al. A large number of Al crystal grains 23 exist, and CuAl₂ 25 segregates between the grain boundary 24, the Al-Cu film 22 and the Cu film 26.

Even when the temperature is as low as below 200°C C. when forming the Cu film 26 on the Al-Cu film 22, a small amount of CuAl₂ 25 is formed on the interface between the grain boundary, the Al-Cu film 22 and the Cu film 26. The reason for this is believed to be as follows. Cu to be sputtered has large kinetic energy and impinges against Al, and the film is formed while Cu imparts kinetic energy to the Al atoms. Therefore, an effect similar to the effect of local heat-treatment occurs, and CuAl₂ is formed on the interface between the Al-Cu alloy film 22 and the Cu film 26. The thickness of Cu-Al₂ on the interface is some dozens of angstroms (Å).

Now, let's consider the case where the Al-Cu alloy film is further formed on the film having the structure shown in FIG. 11.

FIG. 12 is an explanatory view of the CuAl₂ crystal structure. As shown in the drawing, the CuAl₂ crystal has the structure wherein the Cu layers and the Al layers are alternately laminated. Therefore, matching with the Cu film is extremely excellent, and firm bonding can be expected. Because CuAl₂ 25 existing in the Al grain boundary 24 of the Al-Cu film 22 shown in FIG. 11 and CuAl₂ 25 existing on the interface between the Al-Cu film 22 and the Cu film 26 are the same crystal, mutual bonding strength becomes high.

FIG. 13 is an explanatory view of an Al-Cu/Cu/Al-Cu film electrode structure. In this drawing, reference numeral 21 denotes a LiTaO₃ substrate, 22 is an Al-Cu film, 23 is an Al crystal grain, 24 is a grain boundary, 25 is CuAl₂, 26 is a Cu film, 27 is an Al-Cu film, 28 is an Al crystal grain, 29 is a grain boundary, and 30 is CuAl₂. The drawing illustrates an example where the Al-Cu film 22 is formed on the LiTaO₃ substrate 21 by sputtering or electron beam deposition, the Cu film 26 is formed on the former, and the Al-CU film 27 is further formed on the Cu film 26 and is patterned. CuAl₂ 25 is formed in the grain boundary 24 of the Al crystal grains 23 of the Al-Cu film 22,CuAl₂ 30 is formed in the grain boundary 29 of the Al crystal grains 28 of the Al-Cu film 27, and CuAl₂ is further formed between the Cu film 26 and the upper and lower Al-Cu films 22, 27.

Under such a condition,CuAl₂ 25, 30 existing in the grain boundaries 24, 29 in the upper and lower Al-Cu films 22, 27 and CuAl₂ existing on the interface between the Al-Cu films 22, 27 and the Cu film 26 are strongly bonded to one another, and the Cu film at the center of the film as a whole functions as the framework, while CuAl₂ existing in the grain boundaries of the upper and lower Al-Cu film has a small bone network structure. Accordingly, a film having high resistance to stress migration can be realized at a low temperature of below 200°C C. When heat-treatment is applied to the film, CuAl₂ on the interface becomes thick, but because the Al crystal grains grow to a large grain size as described already, the resistance to stress migration drops. Accordingly, heat-treatment at a high temperature above 200°C must not be applied.

As described above, the fundamental principle of the present invention lies in that the Al-Cu film and the Cu film are laminated, and the network structure is formed by CuAl₂ formed in the grain boundary of Al in the Al-Cu film with the Cu film being the center, so as to inhibit stress migration.

As described in the afore-mentioned Yuhara et al. reference, also, the fundamental principle of the present invention is based on the concept that the internal stress of the Al alloy film is largely associated with power characteristics (life) of the surface acoustic wave device, power characteristics are high when the stress of the Al alloy film is zero or rather compressive, and power characteristics drop with higher tensile stress.

FIG. 14 is a graph showing the relation between the internal stress of the alloy film and power characteristics of the surface acoustic wave device. This graph cites the data reported previously by Yuhara et al. The axis of abscissa represents the internal stress of the alloy film, and the ordinate represents the stress of the surface acoustic wave device, that is, the tendency of power characteristics. As can be seen from this graph, power characteristics of the surface acoustic wave device are high when the internal stress of the alloy film is zero or compressive, but are deteriorated when the internal stress is tensile.

Accordingly, power characteristics can be improved by arranging the films so that their internal stresses have opposite signals when the multi-layered alloy film is formed, and moreover, the magnitudes of the internal stresses are mutually in equilibrium, in order to regulate the internal stress of the film as a whose to zero or somewhat compressive.

Next, several embodiments of the present invention will be explained with reference to the drawings. It is to be understood that these embodiments are merely illustrative and in no way limit the present invention.

FIG. 4 is an explanatory structural view of a surface acoustic wave device according to an embodiment of the present invention. In the drawing, reference numeral 1 denotes a LiTaO₃ substrate, 2 is an Al-1%Cu film, 3 is Al crystal grains, 4 is a grain boundary, 5 is CuAl₂, 6 is a Cu film, 7 is an Al-1%Cu film, 8 is Al crystal grains, 9 is a grain boundary, and 10 is CuAl₂. In the surface acoustic wave device of this embodiment, a 1,000Å -thick Al-1%Cu film 2 is formed on the LiTaO₃ substrate 1 having a piezoelectric property while the temperature is kept below 200°C C., a 400Å -thick Cu film 6 is formed on the former, and a 1,000Å -thick Al-1%Cu film 7 is formed on the Cu film 6. In this way, a three-layered film having a total thickness of 2,400Å is formed. This three-layered laminate film is patterned to form an interdigital electrode (hereinafter referred to as the "three layered film electrode A").

In the embodiment shown in the drawing, CuAl₂ 5 is formed in the grain boundary 4 of the Al crystal grains 3 of the Al1%Cu film 2, CUAl₂ 10 is formed in the grain boundary 9 of the Al crystal grains 8 of the Al1%Cu film, and CuAl₂ 5, 10 is also formed between the Cu film 6 and the upper and lower Al1%Cu films 2, 7.

To compare with the three-layered electrode A of this embodiment, an interdigital electrode consisting of a 3,200Å -thick Al1%CU single-layered film (hereinafter referred to as the "single-layered film electrode C") is formed on the LiTaO₃ substrate.

To compare the effect of stress regulation of the three-layered film electrode, an interdigital electrode (hereinafter referred to as the "three-layered film electrode B") is formed by first forming a 700Å -thick Al-1%Cu film, a 600Å -thick Cu film and a 700Å -thick Al-1%Cu film on the LiTaO₃ substrate in the total thickness of 2,000Å and patterning this three-layered laminate film.

To examine the heat-treatment effect of the three-layered film electrode A, an interdigital electrode (hereinafter referred to the "three-layered film electrode A") is formed by heat-treating the three-layered film electrode A at 400°C C. after the film formation.

The thickness of these electrode films is determined in the following way.

A. As a reference a 3,200Å -thick Al1%CU single layer film will be considered.

When a surface acoustic wave filter is produced using this Al1%Cu single layer film as the electrode by the later-appearing method, a transmission band-pass filter of an NTT specification having 933 MHz as the center frequency can be realized.

In the surface acoustic wave filter, the center frequency changes in accordance with the mass of the electrode due to the mass load effect. Therefore, in order to correctly compare power characteristics when the electrode is changed, it is necessary to bring the mass of the electrode film into conformity with the mass of the electrode of the surface acoustic wave filter using the Al1%Cu single layer film electrode C so as to prevent frequency fluctuation.

The density of Cu is 8.9, the density of Al is 2.7, and the density of the Cu film is about three times the density of Al. Therefore, the masses of the three-layered film electrodes A, B and AA are substantially the same as the mass of the 3,200Å -thick Al1%Cu single layer film electrode as the reference. Accordingly, the surface acoustic wave filters using the three-layered film electrodes A, B and AA exhibit substantially the same characteristics as the 933 MHz filter.

B. The balance of the internal stresses of the multi-layered film electrode must be secured so as to improve power characteristics as already described.

If the substrate temperature and the film formation rate at the time of growth of the multi-layered film are constant, the internal stress of the multi-layered film depends on the film thickness of each layer.

FIG. 5 is a graph useful for explaining the experimental results of the internal stresses of the Al-1%Cu film and the Cu film. In this graph, the abscissa represents the film thickness of the metal film, and the ordinate represents the stress. In the graph, the experimental results of the film thickness of the Al-1%Cu film and the Cu film, and the internal stress are plotted.

When the balance of the internal stress inside the laminate film is taken into consideration, the stress is -6×10⁸ N/m² (the-sign represents the compressive stress and the + sign represents the tensile stress) in the case of the Cu film at a thickness of 400Å , and +2×10⁸ N/m² in the case of the Al-1%Cu film at a film thickness of 1,000Å in the three-layered film electrode A consisting of the Al-1%Cu film/Cu film/Al-1%Cu film. Therefore, the stress is -2×10⁸ N/m² in the three-layered film electrode as a whole, and a weak compressive stress is applied. According to FIG. 14 previously explained, this internal stress -2×10⁸ N/m² is included in a region in which power characteristics of the multi-layered film electrode are not deteriorated.

In the case of the three-layered film electrode B, the stress value is -1×10⁸ N/m² for the Cu film at a thickness of 600Å, and 2×2.5×10⁸ N/m² for each Al-1%Cu film at a thickness of 700Å. The total stress is 4×10⁸ N/m², and is the tensile stress. According to FIG. 14, this internal stress of 4×10⁸ N/m² is included in the region where power characteristics of the multi-layered film electrode are deteriorated.

FIGS. 6 and 7 are explanatory structural views of the surface acoustic wave filter according to one embodiment of the present invention, wherein FIG. 6 is a perspective view and FIG. 7 is an equivalent circuit diagram. In the drawings, symbol T_in denotes an input terminal, T_out is an output terminal, R_p1 is a first parallel resonator, R_p2 is a second parallel resonator, R_p3 is a third parallel resonator, R_s1 is a first series resonator, R_s2 is a second series resonator, and R_p11, R_p12, R_p21, R_p22, R_p31, R_s32, R_s11, R_s12, R_s21 and R_s22 are reflectors.

The surface acoustic wave filter according to this embodiment is described in detail in Japanese Unexamined Patent Publication (Kokai) No. 5-183380 to which reference is hereby made. The multi-layered film interdigital electrode of this embodiment is formed on a 36°CY-X LiTaO₃ piezoelectric substrate of 1.5×2×0.5 mm, and the first series resonator R_s1 and the second series resonator R_s2 are connected in series from the input terminal T_in towards the output terminal T_out. The first, second and third parallel resonators R_p1, R_p2 and R_p3 are grounded from the junction between the input terminal and the first series resonator R_p1, from the junction between the first and second series resonators R_s1, R_s2 and from the junction between the second series resonator R_s2 and the output terminal.

The reflectors R_s11, R_s12 are provided to the first series resonator R_s1, and the reflectors R_s21, Rs_s22 are provided to the second series resonator R_s2. The reflectors R_p11, R_p12 are provided to the first parallel resonator R_p1, and the reflectors R_p21, R_p22 are provided to the second parallel resonator R_p2. Further, the reflectors R_p31, R_p32 are provided to the third parallel resonator R_p3.

The 0.5 mm-thick LiTaO₃ piezoelectric substrate is used in such a manner that its 1.5 mm side as the x-axis direction of the crystal axis exists in the transverse direction of the drawing and its 2 mm side exists in the longitudinal direction of the drawing, or in other words, in the propagating direction of the surface acoustic wave. The pitch λ_p of the electrodes of the first parallel resonator R_p1, is set to 4.39μm, its aperture length is set to 160μm, the aperture length of the first series resonator R_s1 is set to 60μm, and the electrode pitch of the second series resonator R_s2 is set to 4.16μm.

FIG. 8 is a graph showing the transmission characteristics of the surface acoustic wave filter according to one embodiment of the present invention. The abscissa in the graph represents frequency (MHz) and the ordinates represents attenuation (dB). As shown in the graph, the surface acoustic wave filter has the characteristics of a band-pass filter having an about 60 MHz pass band in the proximity of 930 MHz. Attenuation in the pass band is 1.5 dB.

The life test of this surface acoustic wave filter is carried out by selecting a frequency, at which power characteristics are the lowest among the pass band, that is, near 950 MHz in this embodiment, and applying a radio frequency power thereto. At this time, the temperature of the filter chip rises somewhat, but an external temperature is controlled in taking such a temperature rise into consideration in advance, and radio frequency power and its life are controlled while the surface temperature of the filter chip is kept constant.

FIG. 9 is a graph showing the power characteristics of the surface acoustic wave filter according to one embodiment of the present invention. The abscissa in the graph represents input power (W) and the ordinate represents mean time to failure (MTTF). The failure is defined by the degradation of 0.3 dB for 1.5 dB insertion loss in the pass band (see, FIG. 8).

Generally, when the MTTF of the electrode of the surface acoustic wave filter relies on the Arrhenius'equation, that is,

1n(MTTF) =A+B/T-n×1n(Pin),

the natural logarithm of the input power (Pin) and the natural logarithm of mean time to failure (MTTF) are expressed by rightwardly descending straight lines. Here, A, B and n are proportional constants.

Besides the surface acoustic wave filter using the electrode of this embodiment, this FIG. 9 shows also the life time of the surface acoustic wave filters using the four kinds of the electrodes described above, respectively. In this measurement, the filter chip temperature T is set to 393 K (120°C C.).

Curve a in FIG. 9 represents the life time of the surface acoustic wave filter using the Al1%Cu/Cu/l-1%Cu film (three-layered film electrode A) which is not heat-treated and has a compressive stress of -2×10⁸ N/m². Curve b represents the life time of the surface acoustic wave filter using the Al1%Cu/Cu/l-1%Cu film (three-layered film electrode B) which is not heat-treated and has a tensile stress of +4×10⁸ N/m². Curve c represents the life time of the surface acoustic wave filter using the Al1%Cu film (single layer film electrode C) which is not heat-treated, and curve d represents the life time of the surface acoustic wave filter using the Al1%Cu/Cu/Al-1%Cu film (three-layered electrode AA) which is heat-treated at 400°C C. By the way, the substrate temperature when forming each film is 120°C C.

In comparison with the surface acoustic wave filter (see curve c) using the conventional Al1%Cu single layer film (single layer film electrode C), the life time of the surface acoustic wave filter (see curve a) using the l-1%Cu/Cu/Al-1%Cu film (three-layered film electrode A) of this embodiment, which is not heat-treated and has the compressive stress of -2×10⁸ N/m² is 120 times.

The life time of the acoustic wave filter (see curve d) having the three-layered film (three-layered film electrode AA) obtained by heat-treating the Al-1%Cu/Cu/Al-1%Cu film (three-layered electrode A) of this embodiment which is not heat-treated and has a compressive stress of -2×10⁸ N/m², becomes drastically short, and is shorter than the life time of the surface acoustic wave filter (see curve c) using the conventional Al-1%Cu single film layer (single layer film electrode).

Further, the life time of the Al1%Cu/Cu/Al-1%Cu film (three-layered film electrode B) which is not heat-treated and has a tensile stress of +4×10⁸ N/m² (see curve b) is improved in comparison with the life time of the surface acoustic wave filter (see curve c) using the conventional Al1%Cu single layer film (single layer film electrode C), but is incomparatively shorter than the life time of the surface acoustic wave filter of this embodiment having the internal stress thereof regulated (see curve a).

The surface acoustic wave filter of this embodiment can provide 200,000 hours as the useful life at the time of input of 1W. Accordingly, the filter can be said to have sufficient power characteristics as an antenna duplexer.

Though a general piezoelectric crystal substrate can be used as the piezoelectric substrate, the piezoelectric materials illustrated in this embodiment, such as LiTaO₃ (36°C Y cut-X propagation), LiNbO₃ (64°C Y cut-X propagation), etc., are effective in order to improve the characteristics of the filter, and the like.

As described above, the present invention employs the multi-layered structure of the Al-Cu film/Cu film/Al-Cu film as the electrode material. Therefore, even in the case of surface acoustic wave devices which cannot be heat-treated at a high temperature due to stress migration, the present invention can drastically improve their power characteristics, and greatly contributes to the improvement in performance of the surface acoustic wave devices such as the surface acoustic wave filters.

Claims

1. A process for producing a surface acoustic wave device having a piezoelectric substrate and an electrode disposed on said substrate, comprising the steps of:

alternately laminating an aluminum copper alloy film and a copper film on said piezoelectric substrate at a temperature not higher than 200°C C. to thereby form a laminate structure having at least three layers, with two aluminum-copper alloy films sandwiching one copper film the aluminum-copper alloy films being polycrystalline films having aluminum crystal grains and cual₂ segregated at a boundary of the aluminum crystal grains;

patterning the resultant laminate structure to form an electrode; and

carrying out subsequent processings while maintaining the temperature of not higher than 200°C C.

2. A process for producing a surface acoustic wave device according to claim 1, wherein the laminate structure consists of two aluminum-copper alloy films sandwiching one copper film.

3. A process for producing a surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is made of a piezoelectric material selected from the group consisting of LiTaO₃ and LiNbO₃.

4. A process for producing a surface acoustic wave device according to claim 1, wherein:

the aluminum-copper alloy films have one of a tensile internal stress and a compressive internal stress,

the copper film has the other of a tensile internal stress and a compressive internal stress, such that the internal stresses of the aluminum copper alloy films and the cooper film have mutually opposite directions, and

the sum of the internal stresses is either zero or compressive.

5. A process for producing a surface acoustic wave device according to claim 1, wherein the aluminum-copper alloy films are polycrystalline films having aluminum crystal grains and cual₂ segregated at a grain boundary thereof.

6. A process for producing a surface acoustic wave device according to claim 1, wherein the aluminum-copper alloy films are formed by sputtering or electron beam deposition.

7. A process for producing a surface acoustic wave device having a piezoelectric substrate and an electrode disposed on said substrate, comprising the steps of:

alternately laminating an aluminum-copper alloy film and a copper film on said piezoelectric substrate at a temperature not higher than 200°C C. to thereby form a laminate structure having at least three layers, with two aluminum-copper alloy films sandwiching one copper film, the laminate having cual₂ formed at the interfaces between said aluminum-copper alloy films and said copper film;

patterning the resultant laminate structure to form an electrode; and

carrying out subsequent processings while maintaining the temperature of not higher than 200°C C.

8. A process for producing a surface acoustic wave device according to claim 7, wherein the piezoelectric substrate is made of a piezoelectric material selected from the group consisting of LiTaO₃ and LiNbO₃.

9. A process for producing a surface acoustic wave device according to claim 7, wherein

the aluminum-copper alloy films have one of a tensile internal stress and a compressive internal stress,

the copper film has the other of a tensile internal stress and a compressive internal stress, such that the internal stresses of the aluminum-copper alloy films and the copper film have mutually opposite directions, and

the sum of the internal stresses is either zero or compressive.

10. A process for producing a surface acoustic wave device according to claim 7, wherein the aluminum-copper alloy films are formed by sputtering or electron beam deposition.

11. A process for producing a surface acoustic wave device according to claim 7, wherein

the aluminum-copper alloy films are polycrystalline having aluminum crystal grains and cual₂ segregated at a boundary of the aluminum crystal grains, and

the cual₂ segregated at the boundary of the aluminum crystal grains is mutually bonded with the cual₂ formed at the interfaces between the aluminum-copper alloy films and the copper film.

12. A process for producing a surface acoustic wave device having a piezoelectric substrate and an electrode disposed on said substrate, comprising the steps of:

alternately laminating an aluminum-copper alloy film and a copper film on said piezoelectric substrate at a temperature sufficient to produce cual₂, to thereby form a laminate structure having at least three layers, with two aluminum-copper alloy films sandwiching one copper film, the laminate having cual₂ formed at the interfaces between said aluminum-copper alloy films and said copper film;

patterning the resultant laminate structure to form an electrode; and

carrying out subsequent processing while maintaining the temperature at a temperature not higher than 200°C C.

13. A process for producing a surface acoustic wave device according to claim 12, wherein the temperature sufficient to produce cual₂ is not higher than 200°C C.

14. A process for producing a surface acoustic wave device according to claim 12, wherein the piezoelectric substrate is made of a piezoelectric material selected from the group consisting of LiTaO₃ and LiNbO₃.

15. A process for producing a surface acoustic wave device according to claim 12, wherein

the aluminum-copper alloy films have one of a tensile internal stress and a compressive internal stress,

the copper film has the other of a tensile internal stress and a compressive internal stress, such that the internal stresses of the aluminum-copper alloy films and the copper film have mutually opposite directions, and

the sum of the internal stresses is either zero or compressive.

16. A process for producing a surface acoustic wave device according to claim 12, wherein the aluminum-copper alloy films are formed by sputtering or electron beam deposition.

17. A process for producing a surface acoustic wave device according to claim 12, wherein

the aluminum-copper alloy films are polycrystalline having aluminum crystal grains and cual₂ segregated at a boundary of the aluminum crystal grains, and

the cual₂ segregated at the boundary of the aluminum crystal grains is mutually bonded with the cual₂ formed at the interfaces between the aluminum-copper alloy films and the copper film.

18. A process for producing a surface acoustic wave device having a piezoelectric substrate and an electrode disposed on said substrate, comprising the steps of:

alternately laminating an aluminum-copper alloy film and a copper film on said piezoelectric substrate to thereby form a laminate structure having at least three layers, with two aluminum-copper alloy films sandwiching one copper film;

producing a cual₂ layer from copper contained in said copper film at a temperature sufficient to produce cual₂;

patterning the resultant laminate structure to form an electrode; and

carrying out subsequent processing while maintaining the temperature at a temperature not higher than 200°C C.

19. A process for producing a surface acoustic wave device according to claim 18, wherein the temperature sufficient to produce cual₂ is not higher than 200°C C.

20. A process for producing a surface acoustic wave device according to claim 18, wherein the piezoelectric substrate is made of a piezoelectric material selected from the group consisting of LiTaO₃ and LiNbO₃.

21. A process for producing a surface acoustic wave device according to claim 18, wherein

the aluminum-copper alloy films have one of a tensile internal stress and a compressive internal stress,

the copper film has the other of a tensile internal stress and a compressive internal stress, such that the internal stresses of the aluminum-copper alloy films and the copper film have mutually opposite directions, and

the sum of the internal stresses is either zero or compressive.

22. A process for producing a surface acoustic wave device according to claim 18, wherein the aluminum-copper alloy films are formed by sputtering or electron beam deposition.

23. A process for producing a surface acoustic wave device according to claim 18, wherein

the aluminum-copper alloy films are polycrystalline having aluminum crystal grains and AuCl₂ segregated at a boundary of aluminum crystal grains, and

the cual₂ segregated at the boundary of the aluminum crystal grains is mutually bonded with the cual₂ layer.

24. A process for producing a surface acoustic wave device having a piezoelectric substrate and an electrode disposed on said substrate, comprising the steps of:

producing a laminate structure at a temperature sufficient to produce cual₂, the laminate structure having at least three layers, with two aluminum-copper alloy films sandwiching one cual₂ layer;

patterning the resultant laminate structure to form an electrode; and

carrying out subsequent processing while maintaining the temperature at a temperature not higher than 200

°C C.