Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-09-16
2002-05-28
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S365000, C310S311000
Reexamination Certificate
active
06396200
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a film bulk acoustic wave device, utilizing an acoustic wave, such as a resonator and a filter.
BACKGROUND ART
A film bulk acoustic wave device is to act as a resonator or a filter utilizing a piezoelectric material that serves to convert between an electric signal and an acoustic wave.
A description will be made of a conventional film bulk acoustic wave device with reference to the drawings.
FIGS. 34 and 35
are views showing a configuration of a conventional film bulk acoustic wave device as described in, for example, “Fundamental mode VHF/UHF bulk acoustic wave resonators and filters on silicon”, 1980, IEEE, Ultrasonics symposium, pp. 829-833 (hereinafter referred to as Reference 1).
FIG. 34
is a top plan view and
FIG. 35
is a sectional view of that shown in FIG.
34
.
FIGS. 36 and 37
are also views showing a configuration of another conventional film bulk acoustic wave device as described in, for example, U.S. Pat. No. 4320365 (hereinafter referred to as Reference 2).
FIG. 36
is a top plan view and
FIG. 37
is a sectional view of that shown in FIG.
36
.
Throughout
FIGS. 34
to
37
, reference symbol
1
denotes a silicon substrate;
2
, a bottom electrode;
3
, a piezoelectric film;
4
, a top electrode;
5
, a via hole; and
6
, an acoustic resonance portion.
A film bulk acoustic wave device has characteristics depending upon the acoustic resonance portion
6
.
FIG. 38
shows in an enlarged manner the acoustic resonance portion
6
shown in FIG.
35
. For the purpose of simplification, the bottom electrode
2
having an equal size to the top electrode
4
is shown herein; however, the actual film bulk acoustic wave device is shown in
FIG. 35
, in which the bottom electrode
2
is different in size from the top electrode
4
.
In
FIG. 38
, a region of the piezoelectric film
3
which is covered with the top electrode
4
is designated as an electrode portion piezoelectric film
7
a
, while a region on the piezoelectric film
3
which is outside the top electrode
4
is designated as a non-electrode portion piezoelectric film
7
b
. The wave number of the acoustic waves in the electrode portion piezoelectric film
7
a
is given as “k
m
”, while the wave number of the acoustic waves in the non-electrode portion piezoelectric film
7
b
is given as “k
f
”.
When an electric signal is applied between the top electrode
4
and the bottom electrode
2
, an electric field is generated between the top electrode
4
and the bottom electrode
2
. Since the piezoelectric film
3
has an expanding/contracting property when an electric field is applied thereto, an elastic vibration is excited so as to correspond to the applied electric signal. In this regard, it depends upon materials of the piezoelectric film
3
used or crystal orientation thereof what vibration component of the elastic vibration is excited. In conventional film bulk acoustic wave devices, zinc oxide (ZnO) or aluminum nitride (AlN) is used for the piezoelectric film
3
.
The electric field applied between the top electrode
4
and the bottom electrode
2
causes the electrode portion piezoelectric film
7
a
to excite the elastic vibration, to thereby excite the acoustic wave propagating in a direction of the thickness and the acoustic wave propagating in a direction parallel to the surface. In this regard, the top of the top electrode
4
and the bottom of the bottom electrode
2
are exposed to the air, so that the acoustic wave propagating in the thickness direction is substantially fully reflected on these surfaces exposed to the air. On the other hand, the acoustic wave propagating in the direction parallel to the surface exhibits different properties in propagation characteristic depending upon the electrode portion piezoelectric film
7
a
and the non-electrode portion piezoelectric film
7
b.
FIG. 39
is a graph showing propagation characteristics of the acoustic wave as described in, for example, “Acoustic Wave Device Technology Handbook”, edited by the 150th Committee of the Acoustic Wave Device Technology, the Japan Society for the Promotion of Science, pp. 82-87, 1991 (hereinafter referred to as Reference 3). In this graph, the x-axis represents the wave number of the acoustic waves propagating in the direction parallel to the surface of the piezoelectric film
3
, in which the region where the wave number is a real number is shown at the right side of the y-axis, and the region where the wave number is an imaginary number is shown at the left side of the y-axis. The y-axis represents frequencies. Reference symbol
8
indicated by the solid line denotes a dispersion characteristic of the acoustic wave propagating within the electrode portion piezoelectric film
7
a
, and reference symbol
9
indicated by the broken line denotes a dispersion characteristic of the acoustic wave propagating within the non-electrode portion piezoelectric film
7
b.
In
FIG. 39
, the wave number belonging to the real number indicates that the wave is in a propagation band where the wave can propagate in the direction parallel to the surface of the piezoelectric film
3
. The wave number belonging to the imaginary number indicates that the wave is in a rejection band where the wave cannot propagate in the direction parallel to the surface of the piezoelectric film
3
. The frequency that intersects the y-axis means a frequency for a resonance in the direction of the thickness of the piezoelectric film
3
, that is, a thickness resonant frequency. The propagation band and the rejection band are bounded by this thickness resonant frequency and separated from each other, and the thickness resonant frequency is thus called a cut-off frequency. It is assumed herein that the cut-off frequency of the acoustic wave propagating within the electrode portion piezoelectric film
7
a
be “f
0m
” while the cut-off frequency of the acoustic wave propagating within the non-electrode portion piezoelectric film
7
b
be “f
0f
”. In general, the electrode portion piezoelectric film
7
a
has a longer distance for the thickness resonance than the thickness of the non-electrode portion piezoelectric film
7
b
by the thicknesses of the top electrode
4
and the bottom electrode
2
. In addition, a large influence of the mass loads of the top electrode
4
and the bottom electrode
2
on the electrode portion piezoelectric film
7
a
causes the cut-off frequency “f
0m
” of the electrode portion piezoelectric film
7
a
to be lower than the cut-off frequency “f
0f
” of the non-electrode portion piezoelectric film
7
b.
The characteristics shown in
FIG. 39
is such that the propagation band is formed within the frequency band higher than the cut-off frequency, and the rejection band is formed within the frequency band lower than the cut-off frequency. This characteristic indicating the rejection band for the frequency lower than the cut-off frequency is called “low-band-cut-off-type dispersion characteristic”. Such a low-band-cut-off-type dispersion characteristic is possessed by conventionally broadly used zinc oxide (ZnO) or aluminum nitride (AlN).
When the piezoelectric film
3
having such a low-band cut-off-type dispersion characteristic is used, the frequency of the acoustic wave to be excited is higher. If the wave number “k
m
” of the electrode portion piezoelectric film
7
a
and the wave number “k
f
” of the non-electrode portion piezoelectric film
7
b
both belong to the real number, the acoustic wave excited by the electrode portion piezoelectric film
7
a
will propagate in the direction parallel to the surface of the piezoelectric film
3
. Then, the acoustic wave will propagate in the non-electrode portion piezoelectric film
7
b
as it is.
On the other hand, in a frequency f intermediate between the thickness resonant frequency “f
0m
” of the electrode portion piezoelectric film
7
a
and the thickness resonant frequency “f
0f
” of the non-electrode portion piezoelectric film
7
b
, the propagation band is formed within the electrode portion piezoelectric film
7
a
, bu
Misu Koichiro
Nagatsuka Tsutomu
Wadaka Shusou
Dougherty Thomas M.
Mitsubishi Denki & Kabushiki Kaisha
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