Metal working – Piezoelectric device making
Reissue Patent
2000-01-14
2003-02-25
Hall, Carl E. (Department: 3729)
Metal working
Piezoelectric device making
Reissue Patent
active
RE038002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface acoustic wave device and a production process thereof.
Generally, a surface acoustic wave (SAW) device comprises a piezoelectric substrate and a comb-shaped interdigital electrode disposed on the substrate, for converting a voltage to a surface acoustic wave or vice versa. The function of the surface acoustic wave device is to convert a radio frequency voltage to a surface acoustic wave having a wavelength of about 10
−5
times by using a comb-shaped interdigital electrode, which causes this wave to propagate on the surface of the piezoelectric substrate and converts again the wave to the voltage by the comb-shaped interdigital electrode.
Frequency selectivity can be provided in accordance with the shape of the interdigital electrode during the two conversion operations between the surface acoustic wave and the voltage, and a filter or a resonator can be constituted by utilizing this characteristic property. Because the propagation speed can be retarded to about 10
−5
times that of an electromagnetic wave, the surface acoustic wave device can be used as a delay device.
The application of the surface acoustic wave device to small, economical filters, resonators, delay lines, etc., has already been done by utilizing the functions described above. In other words, the surface acoustic wave device has been applied to IF filters of television sets, resonators of VTR (vide tape recorder) oscillators, VCOs of cordless telephones, and recently, the application has been expanded to RF filters and IF filters of automobile telephones, mobile telephones, and so forth.
To further expand the utilization in this field, it is important to improve a pass band and power characteristics of the surface acoustic wave device. Particularly in the case of the automobile telephones and the mobile telephones, transmission power is relatively great, that is, 0.6 to 3W, and a large RF power is applied to a filter of a front-end portion inside the apparatus, particularly, to an antenna duplexer.
The maximum input power of the surface acoustic wave filter has been about 0.2W up to the present, and the filter lacks sufficient power characteristics. For this reason, a dielectric filter having high power resistance has been used for the antenna duplexer. However, because the dielectric filter is large in scale, it causes a problem when the size of the apparatus is reduced as a whole.
Accordingly, if the power characteristics of the surface acoustic wave device can be improved and the antenna duplexer can be realized by utilizing the surface acoustic wave device, the mobile telephones can be made even smaller, and the effect of utilization in industry becomes greater.
2. Description of the Related Art
The interdigital electrode is used in the surface acoustic wave device as described above, and aluminum (Al) or an aluminum alloy containing a small amount of a different kind of metal (not always a solid solution body in many cases) is generally used because the mass is small and its electrical resistance value is low.
Several proposals have been made for the structure of the antenna duplexer using the surface acoustic wave device. Typical examples are described in Japanese Unexamined Patent Publication (Kokai) Nos. 5-167388 and 5-167389. In order to simplify the filter structure in the duplexer and to secure desired characteristics, Japanese Unexamined Patent Publication (Kokai) No. 5-167388 proposes to constitute a duplexer by using a plurality of band-pass filters each formed by using the surface acoustic wave device. Japanese Unexamined Patent Publication (Kokai) No. 5-167389 proposes to integrate a plurality of surface acoustic wave band-pass filter chips having mutually different center frequency bands and having signal input/output terminals and ground terminals, by storing them in one package so as to minimize the duplexer while keeping excellent isolation.
However, the conventional antenna duplexer does not have characteristics such that the filer can sufficiently withstand the increase of RF power. To evaluate the power resistance or characteristics, the life time at the maximum input power at which the apparatus can be used is generally used as a guideline. The conventional antenna duplexer has a life time of only about 1,600 hours at the 1W input at an environmental temperature of 85° C. (chip temperature of 120° C.) in an accelerated deterioration test stipulated for the mobile telephones of the NTT specification in Japan, for example. These values are not considered sufficient for the life of mobile telephones, and the values of at least twice are believed necessary.
The main factor that determines the useful life of the surface acoustic wave device is power characteristics of electrode fingers of the filter (interdigital electrode fingers IDT), and an aluminum system alloy film containing a trace amount of copper and formed by sputtering, which is well known as being resistant to migration in the field of semiconductor devices, has been used. However, this alloy is not yet sufficient as the electrode material of the surface acoustic wave device used as the antenna duplexer to which a high power load is applied.
Besides the patent references described above, the following reports have been made regarding the methods of improving electric power of the electrode of the surface acoustic wave device.
1. Change of addition metal in aluminum (Al) system alloy:
The use of an aluminum-titanium alloy (AlTi), etc., for example, is described in detail in “Examination of Al System Thin Film Material for SAW Power-Resistant Electrode and Production Method Thereof” (by Yuhara et al.), No. 17th EM Symposium Presume, pp. 7-12. According to this report, the useful life of the surface acoustic wave device can be improved by about 10 times the life of an aluminum-copper (Al-Cu) alloy film by changing the electrode material to an aluminum-titanium (Al-Ti) alloy.
2. Use of aluminum (Al) epitaxial single crystal film:
This method is based on the fact that grain boundary diffusion in stress migration of aluminum (Al) can be restricted by converting the structure to a single crystal, and is reported in papers of the Electronic Data Communication Society, A, Vol. J76-A, No. 2, pp. 145-152 (1993) (by Ieki et al.). According to this report, life time can be improved to 2,000 times that of an aluminum-copper alloy (Al-Cu) film by vacuum evaporation.
In comparison with films formed by sputtering, the useful life of an aluminum-copper alloy (Al-Cu) film formed by vacuum evaporation is much shorter from the beginning (refer to Yuhara et al., and other references), and the improvement in life time is believed to be substantially 20 to 200 times. At present, however, it has been confirmed only that this method can cause epitaxial growth only when the substrate material as the base is quartz, and cannot realize the film when LiTaO
3
or LiNbO
3
, which have been widely used as a substrate material for filters for mobile communication, is employed.
As described above, stress migration in the surface acoustic wave device is analogous to electromigration and stress migration in wiring technology of semiconductor devices, and migration-resistant technology in the semiconductor devices will be useful for the migration-resistant technology in the surface acoustic wave devices. Among them, the following technology has drawn increasing attention.
Namely, it is the method which forms in a laminar form a film of an intermetallic compound of aluminum (Al) and a transition metal between the aluminum (Al) films so as to block electromigration of the aluminum (Al) atoms by the intermetallic compound. This method is reported in U.S. Pat. No. 4,017,890 (J. K. Howard, IBM, Apr. 1977) and in connection with this patent, a report is made by J. K. Howard, J. F. White and P. S. Ho in “J. Appl. Phys., Vol. 49, p. 4083 (1978).
According to these reports, life time becomes maximal when chromium (Cr) is used as the transition metal, and is about 10 times
Ikata Osamu
Matsuda Takashi
Nishihara Tokihiro
Satoh Yoshio
Takamatsu Mitsuo
Fujitsu Limited
Hall Carl E.
Staas & Halsey , LLP
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