Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-03-08
2002-07-30
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C310S31300R
Reexamination Certificate
active
06426584
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an elastic wave device, which is used in a circuit of a communication apparatus or an electronic apparatus to propagate an elastic wave.
2. Description of Related Art
FIG. 1
shows an example of characteristics of a conventional elastic wave device in which lithium niobate (LiNbO
3
, called LN) is conventionally used. This example is indicated in Published Unexamined Japanese Patent Application H9-167936 (1997) (first literature). In
FIG. 1
, a Y-axis indicates a propagation loss of a surface acoustic wave (called SAW), and a propagation loss per one wavelength (&lgr;) of the surface acoustic wave denoting a propagated distance is indicated by a unit of decibel (dB). An X-axis indicates a normalized electrode thickness (h/&lgr;) normalized by using the wavelength &lgr;. Here, a symbol h indicates an electrode thickness.
The characteristics shown in
FIG. 1
are obtained by setting a propagation direction of the surface acoustic wave to a direction along a crystal X-axis of the lithium niobate and by setting a substrate surface to a plane perpendicular to a &thgr;-rotated Y-axis which is obtained by rotating a crystal Y-axis of the lithium niobate by an angle &thgr; around the crystal x-axis. In particular, the rotation angle &thgr; of the crystal Y-axis ranges from 62 to 74 degrees.
FIG. 2
is a cross sectional view of the conventional elastic wave device. In
FIG. 2
,
1
indicates a lithium niobate (LN) substrate.
2
indicates an electrode which is made of aluminum (Al) and is arranged on the LN substrate
1
. As shown in
FIG. 2
, a plane perpendicular to the &thgr;-rotated Y-axis is set to a surface of the LN substrate
1
, and the characteristics shown in
FIG. 1
are determined in the case where the whole surface of the LN substrate
1
is covered with an electrode material
2
having a thickness h. The electrode
2
is usually made of aluminum (Al). In cases where the plane perpendicular to the &thgr;-rotated Y-axis is set to a surface of the LN substrate
1
and the crystal X-axis of the LN substrate
1
is set to the propagation direction of the surface acoustic wave, the LN substrate
1
is expressed by &thgr;-rotated Y-cut X-propagation lithium niobate and is abbreviated to &thgr;YX-LN or &thgr;YX- LiNbO3.
As is apparent in the characteristics shown in
FIG. 1
, in cases where a cut angle &thgr; (or a rotation angle &thgr;) is, for example, equal to 62 degrees, the propagation loss is minimized in the neighborhood of the normalized electrode thickness (h/&lgr;) set to 0.03. Also, in cases where a cut angle &thgr; is equal to 74 degrees, the propagation loss is minimized in the neighborhood of the normalized electrode thickness (h/&lgr;) set to 0.1. Therefore, in cases where a surface acoustic wave (SAW) device is manufactured on condition that the normalized electrode thickness (h/&lgr;) is higher than 0.05, it is realized that a cut angle &thgr; is higher than 66 degrees to minimize the propagation loss. As is described above, the propagation loss can be minimized by selecting an appropriate combination of the normalized electrode thickness (h/&lgr;) and the cut angle &thgr;, and an insertion loss of the SAW device can be reduced.
Here, several types waves other-than the surface acoustic wave are also called the elastic waves. In cases where the propagation direction is set to the X-axis of the LN substrate
1
and the cut angle &thgr; is set to a value ranging from 62 to 74 degrees, a surface skimming bulk wave (SSBW) denoting a type of bulk wave and a leaky surface acoustic wave (LSAW) are propagated though the surface of the LN substrate
1
. These SSBW and LSAW are disclosed in a literature: “Paper of Institute of Electronics and Communication Engineers of Japan”, 84/1, Vol.J67-C, No.1, pp.158-165 (second literature). However, in this application, the SAW, SSBW and LSAW are generally called the surface acoustic wave SAW except where it is required to distinguish the SAW, SSBW and LSAW from each other.
FIG. 3
is a diagram showing the configuration of a surface acoustic wave (SAW) filter denoting a type of elastic wave device. In
FIG. 3
,
1
indicates a lithium niobate (LN) substrate functioning as a piezo-electric element.
3
indicates an electrode finger.
4
indicates a bonding pad.
5
indicates an input-side inter-digital transducer (IDT), which is composed of the input-side electrode fingers
3
arranged in a comb-like shape, for performing an energy transformation from electricity to surface acoustic wave.
6
indicates an output-side inter-digital transducer (IDT), which is composed of the output-side electrode fingers
3
arranged in a comb-like shape, for performing an energy transformation from surface acoustic wave to electricity.
7
indicates an input terminal.
8
indicates an output terminal. A length of portions of the electrode fingers
3
crossing each other is called an aperture width W, and a maximum value of the aperture width W is called a maximum aperture width WO.
FIG. 4
is a cross sectional view of the SAW filter shown in FIG.
3
. In
FIG. 4
, a symbol “w” indicates an electrode finger width of each electrode finger
3
. A symbol “p” indicates an arrangement interval of each pair of electrode fingers
3
. A symbol “h” indicates an electrode thickness of each electrode finger
3
.
Next, an operation is described.
When an electric signal is supplied to the input terminal
7
, an electric field is generated in a crossing area of each pair of electrode fingers
3
of the input-side IDT
5
. In this case, because the LN substrate
1
functions as a piezo-electric element, distortion is caused in the LN substrate
1
by the electric field. In cases where the electric signal has a frequency f, the distortion caused in the LN substrate
1
is changed with time to oscillate the LN substrate
1
at the frequency f. Therefore, a surface acoustic wave (SAW) is generated in the LN substrate
1
and is propagated through the LN substrate
1
in a direction perpendicular to a longitudinal direction of the electrode fingers
3
. Thereafter, the propagated surface acoustic wave is transformed into an electric signal in the output-side IDT
6
. Here, a process, in which an electric signal is transformed into a surface acoustic wave, and a process, in which a surface acoustic wave is transformed into an electric signal, have a reversible relationship with each other.
As shown in
FIG. 1
, in cases where the cut angle &thgr; is near to 64 degrees and the propagation direction is set to a direction along the X-axis, as is disclosed in the second literature, a displacement component of the surface acoustic wave is parallel to the electrode fingers
3
, and the surface acoustic wave has a directional component parallel to the surface of the LN substrate
1
. The displacement component of the surface acoustic wave depends on a material of the LN substrate
1
, a cut surface of the LN substrate
1
, a cut angle &thgr; of the cut surface and a propagation direction of the surface acoustic wave. The surface acoustic wave oscillated in the input-side. IDT
5
is propagated toward the output-side IDT
6
. In cases where the LN substrate
1
causes a propagation loss to the surface acoustic wave, an electric power of the surface acoustic wave arriving at the output-side IDT
6
becomes lower than that of the surface acoustic wave obtained just after the oscillation of the LN substrate
1
in the input-side IDT
5
. A degree of the propagation loss caused to the surface acoustic wave is almost equal to a value which is obtained by multiplying a standardized distance by a propagation loss per one wavelength shown in FIG.
1
. The standardized distance is obtained by standardizing a distance between the center of the input-side IDT
5
and the center of the output-side IDT
6
with respect to the wavelength of the surface acoustic wave.
Therefore, in cases where the distance between the input-side IDT
5
and the output-side IDT
6
is fixed to a constant value, as the propag
Hashimoto Ken-ya
Misu Koichiro
Murai Kouji
Nagatsuka Tsutomu
Ohmori Tatsuya
Budd Mark O.
Mitsubishi Denki & Kabushiki Kaisha
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