Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-06-28
2003-07-01
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S365000
Reexamination Certificate
active
06586861
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a film bulk acoustic device, using an acoustic wave, such as resonators and filters.
2. Description of the Prior Art
FIGS. 1 and 2
show this type of the conventional film bulk acoustic wave device as described in, for example, the literature “Bulk-Acoustic-Wave Devices using the Second-Order Thickness-Extensional Mode in Thin ZnO—SiO
2
Composite Films on Si”, Proceedings of The Acoustical Society of Japan, pp. 691-692, September-October 1985 (hereinafter, referred to as reference 1), and the literature “Fundamental Bulk Acoustic Resonators in GHz Range”, Proceedings of The Institutes of Electronics, Information and Communication Engineers, vol.81, No.5, pp.468-472, 1998 (hereinafter, referred to as reference 2).
FIG. 1
is a top view, and
FIG. 2
is a cross sectional view taken along line I—I in FIG.
1
. In the figures, reference numeral
1
denotes a silicon (Si) substrate;
2
denotes a silicon oxide (SiO
2
) formed on the silicon substrate
1
;
3
denotes a bottom electrode formed on the silicon oxide
2
;
4
denotes a piezoelectric film composed of zinc oxide (ZnO) formed on the bottom electrode
3
;
5
denotes a top electrode, divided into input-side electrode
5
a
and output-side electrode
5
b
, formed on the piezoelectric film
4
; and
6
denotes a via hole.
When a voltage is applied to the top electrode
5
and the bottom electrode
3
, an electric field is generated in the piezoelectric film
4
. At this time, an acoustic distortion in the piezoelectric film
4
is caused by the electric field. When the applied voltage is a signal of frequency f, this distortion also vibrates at the same frequency f and excites an acoustic wave. In the case of a structure as shown in
FIGS. 1 and 2
, the excited acoustic wave propagates in the thickness direction, and the acoustic wave propagated in the thickness direction is reflected on the respective air-contact surfaces on the surface of the top electrode
5
and the lower surface of the silicon oxide
2
. Therefor, there occurs an acoustic resonance when a gap between the surface of the top electrode
5
and the lower surface of the silicon oxide film
2
is an integer multiple of the half wave length of the acoustic wave.
On the other hand, there also occurs the propagation of the acoustic wave in the direction parallel to the surface in the interior of the piezoelectric film. The acoustic wave in the piezoelectric film
4
at this moment comes to be a cut-off mode at a frequency lower than a certain frequency f
0
, and comes to be a propagation mode at a frequency higher than the frequency f
0
. The frequency f
0
is a cut-off frequency, and corresponds to a frequency in which the thickness 2 h of the piezoelectric film
4
coincides with a half wavelength of the acoustic wave propagating in the thickness direction in the piezoelectric film
4
when the two surfaces of the piezoelectric film
4
are free surfaces.
In the case of the electrode unit in which the top electrode
5
is present on the surface, the cut-off frequency f
m0
of the electrode unit is lower than the cut-off frequency f
f0
of a non-electrode unit provided with only the bottom electrode
3
, due to the electrode thickness and mass load. Therefor, in the range of the frequencies f
f0
to f
m0
, the acoustic wave is the propagation mode due to a higher frequency side than the cut-off frequency f
m0
in the electrode unit, while it is the cut-off mode due to a lower frequency side than the cut-off frequency f
f0
in the non-electrode unit. Hence, the acoustic wave propagating in parallel to the surface of the film
4
results in a state that an energy is trapped in the top electrode
5
.
When the input-side electrode
5
a
and output-side electrode
5
b
of the top electrode
5
are set appropriately at a symmetric mode frequency providing the common potential, and at an asymmetric mode frequency providing potentials different from each other, an electric signal applied to the input-side electrode
5
a
is traveled to the output-side electrode
5
b
with low loss. Thus, the pass band of a filter is created.
The characteristics of such a filter are determined by the thickness 2 h of the piezoelectric film
4
, the thickness of the top electrode
5
, the thickness g of the silicon oxide
2
, the shape of the top electrode
5
, and a gap between the input-side electrode
5
a
and the output-side electrode
5
b.
The reference 1 describes one example in which a normalized thickness (g/h) of the piezoelectric film
4
is 1.54 and 2.4, which is determined by the thickness 2 h of the piezoelectric film
4
and the thickness g of the silicon oxide
2
. The reference 1 also represents that the design is made by focusing the temperature characteristics and the loss of the acoustic wave. In addition, the reference 1 designates to use an acoustic wave of the second mode. The acoustic wave is a fundamental mode (first mode) when a gap between the surface of the top electrode
5
and the back of the silicon oxide
2
is a half wave length of the acoustic wave, and an N-th mode (N: integer) corresponds to an N multiple of the wave length of the fundamental mode.
FIG. 3
shows this type of the conventional film bulk acoustic device described in, for example, JP-A 6-350154 (hereinafter, referred to as reference 3).
FIG. 3
is a cross sectional view. Though the basic structure is the same as that shown in
FIG. 2
, a piezoelectric film
7
is composed of lead titanate-zirconate (PZT); a bottom electrode
3
is composed of a titanium (Ti) film
8
and a platinum (Pt) film
9
; and a top electrode
5
is composed of a titanium film
10
and a gold (Au) film
11
.
The reference 3 describes one example in which a normalized thickness (d/h) of the bottom electrode
3
is 1.0, which is determined by the thickness 2 h of the piezoelectric film
7
and the thickness d of the bottom electrode
3
. In addition, the reference 3 designates that favorable piezoelectric characteristics may be performed by an appropriate composition ratio of lead titanate (PbTiO
3
) and lead zirconate (PbZrO
3
).
The piezoelectric film
7
composed of lead titanate-zirconate excites a thickness extension vibration as a main vibration. In this case, the acoustic wave propagating in parallel with the surface of the piezoelectric film
7
designates the dispersion characteristics as shown in FIG.
4
. In
FIG. 4
, the horizontal axis corresponds to a normalized thickness of the piezoelectric film
7
which multiplies the wave number k of the acoustic wave propagating in parallel with the surface of the film
7
, and the thickness of the piezoelectric film
7
together, that is, a normalized piezoelectric thickness (2 kh), while the vertical axis corresponds to a frequency.
In the figure, reference numeral
12
designates the characteristics of a first mode (TE
1
) of the thickness extension vibration;
13
designates the characteristics of a second mode (TS
2
) of the thickness shear vibration;
14
designates the characteristics of a third mode (TS
3
) of the thickness shear vibration; and
15
designates the characteristics of a second mode (TE
2
) of the thickness extension vibration. A range that the normalized piezoelectric substance thickness is a real number is a range that the acoustic wave is a propagation mode, while a range that the normalized piezoelectric substance thickness is an imaginary number is a range that the acoustic wave is a cut-off mode. In addition, the frequency at the crossing point with the vertical axis, such that the normalized thickness is zero, is a cut-off frequency f
0
.
As is apparent from
FIG. 4
, the first mode (TE
1
) of the thickness extension vibration shows a characteristic that the frequency is made lower as the normalized piezoelectric substance thickness is larger in the vicinity of the vertical axis. In addition to lead zirconate titanate, this is also applied similarly in a piezoelectric film having the thickness extension vibration as a main vibration, constituting
Ibata Koji
Maeda Chisako
Misu Koichiro
Nagatsuka Tsutomu
Uchikawa Fusaoki
Birch Stewart Kolasch & Birch, LLP.
Dougherty Thomas M.
Mitsubishi Denki & Kabushiki Kaisha
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