Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-09-27
2003-09-09
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06617752
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators and surface acoustic wave filters, and manufacturing methods therefor, and more particularly, to a surface acoustic wave device using a Shear Horizontal type (“SH-type”) surface acoustic wave and having a structure for reducing a transversal mode spurious ripple, and a manufacturing method therefor.
2. Description of the Related Art
In surface acoustic wave devices, aluminum or alloys including aluminum as a main component have conventionally been widely used as the electrode material of an interdigital transducer (IDT). At least one IDT is disposed on a piezoelectric substrate and reflectors or reflective end surfaces are disposed at both sides of the area where the IDT is located so as to define a resonator or a longitudinally coupled resonator filter.
In such a surface acoustic wave device, it may be possible that the IDT functions as a waveguide to generate a transversal mode wave, and ripples caused by the transversal mode wave are generated in a pass band. To reduce the ripples caused by the transversal mode wave, various methods have been attempted. Those methods include a method for reducing the intersection width of IDTs and a weighting method.
A surface acoustic wave device has also been proposed in Japanese Unexamined patent Application publication No. Hei-11-298290, in which a quartz substrate is used, an IDT made from a metal or an alloy having tantalum (Ta), which has a larger mass than aluminum (Al), as a main component is disposed on the quartz substrate, and an SH-type surface acoustic wave is used. Since the IDT is made from a metal or an alloy having tantalum, which has a large mass, as a main component, the number of the pairs of the electrode fingers of the IDT is as small as 10 to 20, and thereby the surface acoustic wave device is made compact.
When an electrode material having a large mass-load effect, such as a material having Ta as a main component, is used, the sonic speed obtained at the area where an IDT is located becomes much lower than the sonic speed obtained around the area. Therefore, a waveguide effect is very large at the IDT portion.
Consequently, when a longitudinally coupled resonator filter is produced, ripples caused by a transversal mode wave become complicated and very large, as indicated by arrows X in FIG.
13
.
As described above, as methods for removing ripples caused by a transversal mode wave from the pass band of a filter or from the vicinity of a resonant point of a resonator, a method A in which an intersection width is made small and the frequency distance between a basic-mode wave and a transversal mode wave is made large, and a method B in which the intersection width of an IDT is weighted with a cos
2
function to eliminate the transversal mode wave have been conventionally attempted.
In the method A, it is necessary to set the intersection width to 10&lgr; or less, where &lgr; is the wavelength of a surface acoustic wave. When a quartz substrate and an IDT having 10 to 20 pairs of electrode fingers are used to provide a surface acoustic wave device, the input and output impedance exceeds 2 k&OHgr; and is very high, so that the surface acoustic wave device cannot be used for actual products. Therefore, it is necessary to increase the number of the pairs of electrode fingers to reduce the impedance.
More specifically, whereas the surface acoustic wave device disclosed in the above-described publication uses tantalum, which has a large mass, as a main component to form electrodes and allows the number of pairs in IDTs to be reduced, when the method for reducing the intersection width is used, the number of the pairs of electrode fingers needs to be increased to reduce the input and output impedance. Therefore, the surface acoustic wave device cannot be made compact.
In the method B, weighting itself increases a loss of the surface acoustic wave device. In addition, since weighting reduces the area of an intersection-width portion, the impedance of the surface acoustic wave device becomes very high in the same way as in the method A. Therefore, to reduce the impedance, the intersection width needs to be twice as large as the required length. As a result, the surface acoustic wave device cannot be made compact.
In other words, when either the method A or the method B is used, if ripples caused by a transversal mode wave are to be reduced, the advantage of the surface acoustic wave device in reducing the size of the device as disclosed in the above-described publication is prevented from being achieved.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device which has electrodes made from a material having a larger mass-load effect than aluminum, which is made to be very compact, which has a structure that minimizes and eliminates ripples caused by a transversal mode wave, and which uses an SH-type surface acoustic wave, and a manufacturing method therefor.
According to a preferred embodiment of the present invention, a surface acoustic wave device using an SH-type surface acoustic wave includes a quartz substrate, and at least one interdigital transducer disposed on the quartz substrate and made from electrodes having a larger mass-load effect than that of aluminum, wherein the metallization ratio “d” and the normalized film thickness h/&lgr; of the interdigital transducer are within a range such that a ripple caused by a transversal mode wave is about 0.5 dB or less, where “&lgr;” is the wavelength of the surface acoustic wave and “h” indicates the film thickness of the electrodes of the at least one interdigital transducer.
In the surface acoustic wave device, the metallization ratio “d” and the normalized film thickness h/&lgr; of the interdigital transducer are preferably controlled so as to be within specific ranges such that the ripple caused by the transversal mode wave is about 0.5 dB or less. Therefore, even in a case in which an IDT made of electrodes having a larger mass-load effect than that of aluminum is used, and the number of the pairs of electrode fingers is reduced to make the device very compact, the ripple caused by the transversal mode wave is effectively suppressed and eliminated. Consequently, a compact surface acoustic wave device using an SH-type surface acoustic wave and having excellent frequency characteristics is provided.
In the surface acoustic wave device, the interdigital transducer may include at least one electrode layer made from a metal having a larger mass than that of aluminum.
In the surface acoustic wave device, the interdigital transducer may be made from a single metal having a larger mass than that of aluminum.
The above-described advantages are also achieved in another preferred embodiment of the present invention which provides a surface acoustic wave device using an SH-type surface acoustic wave, including a quartz substrate, and at least one interdigital transducer disposed on the quartz substrate and made from tantalum, wherein the normalized film thickness h/&lgr; of the interdigital transducer is within a range from about 0.6d+1.65 to about 0.6d+1.81, where “d” indicates the metallization ratio of the interdigital transducer, “&lgr;” indicates the wavelength of the surface acoustic wave, and “h” indicates the film thickness of the electrodes of the at least one interdigital transducer.
In the surface acoustic wave device according to this preferred embodiment, since at least one interdigital transducer made from tantalum is disposed on the quartz substrate, and the normalized film thickness h/&lgr; of the interdigital transducer falls in a range from about 0.6d+1.65 to about 0.6d+1.81, where “d” indicates the metallization ratio, ripples caused by a transversal mode wave are effectively suppressed. Therefore, even in a case in which at least one IDT made from tantalum is formed, and the number
Fujimoto Koji
Kadota Michio
Nakao Takeshi
Yoneda Toshimaro
Dougherty Thomas M.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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