Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-07-17
2003-04-08
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S364000, C310S31300R
Reexamination Certificate
active
06545388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, for example, a band-pass filter or a resonator, and the present invention also relates to a method of producing such a surface acoustic wave device.
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been widely used as band-pass filters and resonators. For application of surface acoustic wave devices as band-pass filters, resonators, or other such devices, the most important requirement is that the devices must have good frequency characteristics.
Generally, appropriate materials (electrode materials) for constructing the interdigital transducers (“IDTs”) and reflectors of the surface acoustic wave devices are different from each other, depending on the surface acoustic wave mode to be used and the particular application for the device. For example, as appropriate materials for the IDTs and reflectors of resonators and band-pass filters using Shear Horizontal or SH waves, metals such as Au, W, Ta, and Pt, having a specific gravity of 15 or higher are used in many cases.
Moreover, in the resonators, the band-pass filters, and such devices, in order to suppress vibration modes that deteriorate the characteristics thereof, the thicknesses of the IDT electrodes and reflectors must be within a predetermined range in some cases. If the film thicknesses of the IDT electrodes and the reflectors have predetermined values so that the vibration modes that deteriorate the characteristics can be initially suppressed, the film thicknesses will significantly depart from the values at which the surface acoustic waves can be most effectively vibrated and reflected. Thus, in some cases, a problem arises in that the necessary good characteristic cannot be obtained.
Moreover, if the film-thicknesses of the IDT electrodes and the reflectors are restricted, it will be more difficult to sufficiently reduce the resistances of the IDT electrodes, and also, problematically, the devices cannot be sufficiently adapted for operation at a high frequency.
Referring to production of multiple filters and resonators using the same wafer, it is desirable that the frequency dispersion be made as small as possible. For this purpose, advantageously, materials having a specific gravity of up to 10, such as Ni, Cu, Al, Mg, and Co, are used. However, when these materials are used, it is difficult to obtain filters and resonators having desired characteristics in practice.
Moreover, when filters and resonators including IDT electrodes and reflectors using metals with a specific gravity of 15 or higher, such as Au, W, Ta, and Pt, are produced, efforts are made to form the IDT electrode and reflector films as uniformly as possible so that the frequency dispersion of multiple surface acoustic wave devices (filters, resonators, and other such devices) produced by using the same wafer can be suppressed. However, there is a limit to the reduction in thickness of the IDT electrodes and the reflectors. Practically, the wafer is divided in individual elements, and thereafter, frequency-adjustment is carried out for each of the elements. The frequency-adjustment is made because a slight difference in film thickness between the electrode films constituting the IDT electrodes and the reflectors causes the acoustic velocity to change significantly, thereby increasing the frequency dispersion of the resonators and filters.
In order to achieve the frequency-adjustment, a method of etching the surface of an IDT electrode by using ion beams, a method of forming an insulator film on a substrate and an IDT electrode, a method of etching a substrate or an IDT electrode by RIE, are generally used. Accordingly, a problem arises in that the manufacturing cost is increased due to reduction of throughput, and moreover, the characteristics deteriorate due to damage to the electrodes and the substrates.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which frequency dispersion, influenced by dispersion in film-thickness, is minimized, while suitable excellent resonance characteristics and filter characteristics are ensured. In addition, preferred embodiments of the present invention provide a method of producing such a surface acoustic wave device.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes an interdigital transducer (IDT) electrode, or an IDT electrode and a reflector, disposed on a piezoelectric substrate, each of the IDT electrode and the reflector having a multi-layer film structure including: (a) at least one high specific gravity metal component containing a layer having a film-thickness of at least about 10 nm and including as a major component a metal having a specific gravity of at least about 15; and (b) at least one low specific gravity metal component containing a layer having a film-thickness of at least about 10 nm and including as a major component a metal having a specific gravity of up to about 10 and a bulk resistivity, at 20° C., of up to about 10×10
−8
&OHgr;·m.
By forming an interdigital transducer (IDT) electrode, or the IDT electrode and a reflector on a piezoelectric substrate, each of the IDT electrode and the reflector having a multi-layer film structure including at least one layer containing, as a major component, a metal with a specific gravity of at least about 15 and a film-thickness of at least about 10 nm as a high specific gravity metal component containing layer, and at least one layer including as a major component a metal with a specific gravity of up to about 10 and a bulk resistivity, at 20° C., of up to about 10×10
−8
&OHgr;·m, and a film-thickness of at least about 10 nm as a low specific gravity metal component containing layer, excellent resonator and filter characteristics can be ensured, and also, frequency dispersion, which may be caused by dispersions in film-thickness, is minimized. Thus, it is unnecessary to perform frequency adjustment, and the manufacturing cost is greatly reduced due to the increased throughput. In addition, damage to the substrate, the IDT electrode, and the reflector is prevented, which further enhances the yield.
In particular, since the high specific gravity metal component containing layer is provided, an SH wave can be easily excited and reflected. Moreover, since the low specific gravity metal component containing layer is provided, frequency dispersion, which may occur when a plurality of filters and resonators are produced using the same wafer, is minimized. Thus, surface acoustic wave devices having excellent resonance and filter characteristics is obtained.
In addition, a vibration mode that exerts an undesirable influence over the characteristics is minimized in the film-thickness range in which high excitation and reflection efficiencies can be ensured.
Preferably, the total number of the at least one high specific gravity metal component containing layer and the at least one low specific gravity metal component containing layer which are included in the IDT electrode or the reflector is preferably in the range of from 2 to 10.
By setting the total number of the at least one high specific gravity metal component containing layer and the at least one low specific gravity metal component containing layer to be in the range of from 2 to 10, the surface acoustic wave device having excellent resonance and filter characteristics can be obtained without having to perform excessively complicated lamination processes. Thus, the present invention is highly effective and advantageous.
Also, preferably, the metal having a specific gravity of at least about 15 that constitutes the high specific gravity metal component containing layer is preferably selected from the group consisting of Au, W, Ta, Pt, and alloys containing at least one of the metals, and the metal having a specific gravity of up to about 10 and a bul
Budd Mark O.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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