Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-07-16
2004-01-06
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C310S334000, C310S335000
Reexamination Certificate
active
06674215
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/JP 00/07239 which has an-International filing date of Oct. 18, 2000, which designated the United States of America and was not published in English.
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an acoustic wave apparatus for propagating acoustic waves, used for the circuit of a communication equipment, an electronic device or the like.
2. Background Art
Heretofore, in such an acoustic wave apparatus in which a piezoelectric substrate containing lithium tantalate (LiTaO
3
, referred to as LT hereinafter) has been used, the cut angle &thgr; of the LT substrate has been set equal to 36°. This setting was a result of the calculation that if an electrode was formed on the surface of such a substrate, and the substrate surface was electrically short-circuited, the amount of propagation loss would be reduced to nearly a value of zero.
However, such calculation was made by assuming the establishment of an ideal state where the electrode had no thickness. Consequently, in the actual acoustic wave apparatus comprising an electrode having thickness, there was a possibility that a condition for reducing the amount of propagation loss to a minimum may be different. In addition, the calculation was made by examining the case where the entire surface of the substrate was covered with the electrode. Consequently, in the acoustic wave apparatus comprising electrode fingers cyclically arrayed as in the case of an SAW filter, there was a possibility that a condition for reducing the amount of propagation loss to a minimum might be different.
Thus, in Japanese Patent Application Laid-Open No. 1997-167936 (referred to as a document 1, hereinafter), a condition for reducing the amount of propagation loss to a minimum is examined by taking into consideration the thickness of a grating electrode formed on the surface of the LT substrate.
FIG. 1
shows the result of calculating the amount of propagation loss in a ladder surface acoustic wave filter of the document 1 shown in FIG.
7
. In the drawing, an ordinate indicates the amount of loss made when a surface acoustic wave (referred to as SAW, hereinafter) is propagated per wavelength (&lgr;), i.e., the amount of loss per wavelength (dB/&lgr;). An abscissa indicates a standardized electrode thickness (h/&lgr;), where the thickness h of the electrode is standardized based on the wavelength &lgr; of SAW.
FIG. 1
shows the case where an LT crystal X-axis direction is set as a SAW propagation direction, a surface perpendicular to a “&thgr;-rotated Y” axis obtained by rotating a crystal Y axis by &thgr; around the crystal X axis, is set as a substrate surface, and a cut angle &thgr; is set in the range of 36° to 46°. The LT substrate having the surface perpendicular to the “&thgr;-rotated Y” axis set as its surface and the crystal X-axis direction set as the SAW propagation direction is represented by &thgr;-rotated Y-cut X-propagation lithium tantalate, abbreviated to &thgr;YX-LT, or &thgr;YX-LiTaO
3
. In many cases, the electrode is made of aluminum (Al) or an alloy mainly containing Al.
As shown in
FIG. 1
, if a standardized electrode thickness (h/&lgr;) is &thgr;, the amount of loss per wavelength (dB/&lgr;) is minimum when a cut angle &thgr; is about 36°. This result coincides with that of the conventional calculation, i.e., if the ideal state of the electrode having no thickness is established, the amount of propagation loss is reduced to nearly a value of zero when a cut angle &thgr; is 36°.
In addition, as shown in
FIG. 1
, if a cut angle &thgr; is 40°, the amount of loss per wavelength (dB/&lgr;) is minimum when a standardized electrode thickness (h/&lgr;) is about 0.05. If a cut angle &thgr; is 42°, the amount of loss per wavelength (dB/&lgr;) is minimum when a standardized electrode thickness (h/&lgr;) is about 0.075. Accordingly, in the SAW device realized by setting the standardized electrode thickness (h/&lgr;) in a range above 0.05, a cut angle &thgr; for reducing the amount of propagation loss to a minimum resides in a range above 40°.
As apparent from the foregoing discussion made with reference to
FIG. 1
, it is possible to reduce the amount of propagation loss to a minimum by selecting the proper combination of a standardized electrode thickness (h/&lgr;) with a cut angle &thgr;. As a result, the insertion loss of the SAW device can be reduced. Therefore, in recent years, the LT substrate having a cut angle &thgr; set equal to 42° has been employed.
There are several kinds of acoustic waves. If a cut angle &thgr; is set in the range of about 36° to 46°, and the direction of propagation is a crystal X axis, for example, a surface skimming bulk wave (SSBW), which is a bulk wave propagated along the surface of an LT substrate described in a document: pp. 158-165, “Journal of Institute of Electronics and Communication Engineers of Japan”, Vo 1. J67-C, No. 1, January 1984 (referred to as a document 2, hereinafter), and a leaky surface acoustic wave (LSAW) are propagated. In the present application, these waves are generically termed as SAW, except when the waves are distinguished from each other.
FIG. 2
is an upper surface view showing the constitution of the SAW filter, which is one type of an acoustic wave apparatus. In the drawing, a reference numeral
1
denotes an LT substrate made of a piezoelectric material;
3
an electrode finger;
4
a bonding pad;
5
an input side interdigital transducer (IDT) for performing electric—surface acoustic wave energy conversion;
6
an output side IDT for performing surface acoustic wave—electric energy conversion;
7
an input terminal; and an
8
an output terminal. W 0 represents a maximum value of the length of a portion intersected by the electrode finger
3
.
FIG. 3
is a sectional view of the SAW filter shown in FIG.
2
. In the drawing, a code w represents a width of the electrode finger
3
; p an arraying cycle of electrode fingers
3
; and h a thickness of the electrode finger
3
.
Next, the operation of the SAW filter will be described.
An electric signal applied to the input terminal
7
forms an electric field at the intersection of each electrode finger
3
of the input side IDT
5
. In this case, as the LT substrate
1
is made of the piezoelectric material, the electric field causes distortion. If the input signal has a frequency f, the strain that has been generated is vibrated at the frequency f, converting the signal into SAW. This SAW is then transmitted in a direction perpendicular to the electrode finger
3
. At the output side IDT
6
, the SAW is converted back into the electric signal. The conversion of the electric signal into the SAW, and the conversion of the SAW into the electric signal are reversible to each other.
If a cut angle &thgr; is about 36°, and the propagation direction of the SAW is in a crystal X-axis direction, as described in the document
2
, the displacement component of the SAW has a direction parallel to the electrode finger
3
, and the surface of the LT substrate
1
. Such a displacement component depends on the cut angle &thgr; of the cut surface of the LT substrate
1
and the propagation direction of the SAW.
The SAW excited by the input side IDT
5
is propagated toward the output side IDT
6
. However, if there is propagation loss in the LT substrate
1
, the power of the SAW having reached the output side IDT
6
is smaller than that of the SAW immediately after its excitation by the input side IDT
5
. The amount of the loss is approximately equal to a value obtained by multiplying a distance between the centers of the input side IDT
5
and the output side IDT
6
standardized based on the wavelength &lgr; of the SAW by an attenuation constant &agr;.
Thus, assuming that the distances of the input side IDT
5
and the output side IDT
6
are equal to each other, as the amount of propagation loss in the LT substrate
1
is increased, the amount of insertion loss for the SAW filter is larger. As described in
Hashimoto Ken-ya
Ibata Koji
Misu Koichiro
Murai Kouji
Nagatsuka Tsutomu
Gonzalez Ramirez Julio
Mitsubishi Denki & Kabushiki Kaisha
Ramirez Nestor
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