U.S. patent application number 09/381167 was filed with the patent office on 2002-02-07 for thin film pietoelectric element.
Invention is credited to MISU, KOICHIRO, NAGATSUKA, TSUTOMU, WADAKA, SHUSOU.
Application Number | 20020014808 09/381167 |
Document ID | / |
Family ID | 26341045 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014808 |
Kind Code |
A1 |
MISU, KOICHIRO ; et
al. |
February 7, 2002 |
THIN FILM PIETOELECTRIC ELEMENT
Abstract
A film bulk acoustic wave device, comprising: a substrate; a
bottom electrode formed on one surface of the substrate; a
piezoelectric film formed on the bottom electrode; and a first top
electrode formed on the piezoelectric film, further comprises a
second top electrode having a larger mass load than the first top
electrode, and formed on the first top electrode on the
piezoelectric film when viewed from the center of the first top
electrode, wherein the piezoelectric film has a
high-band-cut-off-type dispersion characteristic. The cut-off
frequency of a second top electrode portion piezoelectric film
having a large mass load can be lower than the cut-off frequency of
a first top electrode portion piezoelectric film, to thereby trap
the energy of the acoustic wave in a region of the first top
electrode portion side, so that good performance may be
feasible.
Inventors: |
MISU, KOICHIRO; (TOKYO,
JP) ; NAGATSUKA, TSUTOMU; (TOKYO, JP) ;
WADAKA, SHUSOU; (TOKYO, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
220400747
|
Family ID: |
26341045 |
Appl. No.: |
09/381167 |
Filed: |
September 16, 1999 |
PCT Filed: |
August 4, 1998 |
PCT NO: |
PCT/JP98/03471 |
Current U.S.
Class: |
310/312 ;
310/324; 310/330 |
Current CPC
Class: |
H03H 9/564 20130101;
H03H 9/177 20130101; H03H 9/02133 20130101; H03H 9/562 20130101;
H03H 9/174 20130101 |
Class at
Publication: |
310/312 ;
310/324; 310/330 |
International
Class: |
H01L 041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 16, 1998 |
JP |
10-006844 |
Jan 16, 1998 |
JP |
10-006845 |
Claims
1. A film bulk acoustic wave device comprising: a substrate; a
bottom electrode formed on one surface of said substrate; a
piezoelectric film formed on said bottom electrode; and a first top
electrode formed on said piezoelectric film, characterized in that
said film bulk acoustic wave device further comprises a second top
electrode having a larger mass load than said first top electrode,
and formed on said first top electrode at an outer side of said
piezoelectric film when viewed from the center of said first top
electrode, and said piezoelectric film has a high-band-cut-off-type
dispersion characteristic.
2. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said first and second top electrodes are
integrally formed, and said second top electrode has a larger
thickness than that of said first top electrode.
3. A film bulk acoustic wave device as claimed in claim 1,
characterized in that the product of the electrode thickness and
the density of said second top electrode is larger than the product
of the electrode thickness and the density of said first top
electrode.
4. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said second top electrode is laid on a part
of said first top electrode.
5. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said first top electrode and said second top
electrode are connected with each other.
6. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said second top electrode having a narrower
width than that of said first top electrode is laid on said first
top electrode.
7. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said first and second top electrodes are
divided into two.
8. A film bulk acoustic wave device as claimed in claim 7,
characterized in that a third top electrode is formed on said
piezoelectric film between said two divided first top
electrodes.
9. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said substrate is a semiconductor substrate
or a dielectric substrate.
10. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said piezoelectric film has a Poisson ratio
lower than 0.34.
11. A film bulk acoustic wave device as claimed in claim 1,
characterized in that said piezoelectric film contains lead
titanate (PbTiO.sub.3) as a major component.
12. A film bulk acoustic wave device as claimed in claim 1,
characterized in that a dielectric layer is inserted between said
substrate and said bottom electrode.
13. A film bulk acoustic wave device comprising: a substrate; a
bottom electrode formed on one surface of said substrate; a
piezoelectric film formed on said bottom electrode; and a top
electrode formed on said piezoelectric film, characterized in that
said film bulk acoustic wave device further comprises a dielectric
formed at an outer side of said top electrode on said piezoelectric
film when viewed from the center of said top electrode, and said
piezoelectric film has a high-band-cut-off-type dispersion
characteristic.
14. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said dielectric is formed on a part of said
top electrode.
15. A film bulk acoustic wave device as claimed in claim 13,
characterized in that the product of the film thickness and the
density of said dielectric is larger than the product of the
electrode thickness and the density of said top electrode.
16. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said dielectric is laid on a part of said top
electrode.
17. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said top electrode and said dielectric are
connected with each other.
18. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said dielectric having a narrower width than
that of said top electrode is laid on said top electrode.
19. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said top electrode and said dielectric are
divided into two.
20. A film bulk acoustic wave device as claimed in claim 19,
characterized in that a second top electrode is formed on said
piezoelectric film between said two divided first top
electrodes.
21. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said substrate is a semiconductor substrate
or a dielectric substrate.
22. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said piezoelectric film has a Poisson ratio
lower than 0.34.
23. A film bulk acoustic wave device as claimed in claim 13,
characterized in that said piezoelectric film contains lead
titanate (PbTiO.sub.3) as a major component.
24. A film bulk acoustic wave device as claimed in claim 13,
characterized in that a dielectric layer is inserted between said
substrate and said bottom electrode.
Description
TECHNICAL FIELD
[0001] The present invention relates to a film bulk acoustic wave
device, utilizing an acoustic wave, such as a resonator and a
filter.
BACKGROUND ART
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] A film bulk acoustic wave device has characteristics greatly
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.
[0007] 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 7a, while a region on the piezoelectric
film 3 which is outside the top electrode 4 is designated as a
non-electrode portion piezoelectric film 7b. The wave number of the
acoustic waves in the electrode portion piezoelectric film 7a is
given as "k.sub.m", while the wave number of the acoustic waves in
the non-electrode portion piezoelectric film 7b is given as
"k.sub.f".
[0008] 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.
[0009] The electric field applied between the top electrode 4 and
the bottom electrode 2 causes the electrode portion piezoelectric
film 7a 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 7a and the
non-electrode portion piezoelectric film 7b.
[0010] 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 7a, 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 7b.
[0011] 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 7a be "f.sub.0m" while the cut-off frequency of
the acoustic wave propagating within the non-electrode portion
piezoelectric film 7b be "f.sub.0f". In general, the electrode
portion piezoelectric film 7a has a longer distance for the
thickness resonance than the thickness of the non-electrode portion
piezoelectric film 7b 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 7a causes the cut-off
frequency "f.sub.0m" of the electrode portion piezoelectric film 7a
to be lower than the cut-off frequency "f.sub.0f" of the
non-electrode portion piezoelectric film 7b.
[0012] 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).
[0013] 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.sub.m" of the electrode portion piezoelectric film 7a and the
wave number "k.sub.f" of the non-electrode portion piezoelectric
film 7b both belong to the real number, the acoustic wave excited
by the electrode portion piezoelectric film 7a 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 7b as it is.
[0014] On the other hand, in a frequency f intermediate between the
thickness resonant frequency "f.sub.0m" of the electrode portion
piezoelectric film 7a and the thickness resonant frequency
"f.sub.0f" of the non-electrode portion piezoelectric film 7b, the
propagation band is formed within the electrode portion
piezoelectric film 7a, but the rejection band is formed within the
non-electrode portion piezoelectric film 7b. Therefore, the
acoustic wave propagating in the direction parallel to the surface
is reflected on an interface between the electrode portion
piezoelectric film 7a and the non-electrode portion piezoelectric
film 7b, to thereby trap the energy of the acoustic wave in the
electrode portion piezoelectric film 7a. This phenomenon is called
"energy trapping", which can advantageously realize a resonator
having a high Q value representative of the resonance performance
since the energy of the acoustic wave is not escaped from the
electrode portion. Conventional film bulk acoustic wave device
using zinc oxide (ZnO) can utilize the energy trapping to attain a
resonator having a high Q value.
[0015] However, as described in "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. 125-129, 1991 (hereinafter referred to as Reference 4), zinc
oxide (ZnO) has an electromechanical coupling coefficient "k.sub.t"
of as low as about 0.3. In contrast, a lead-based piezoelectric
material widely used in the field of piezoelectric ceramics, such
as lead zirconate titanate (PZT) or lead titanate (PbTiO.sub.3),
has an electromechanical coupling coefficient "k.sub.t" of 0.4-0.8.
A lower electromechanical coupling coefficient causes a higher
capacitance ratio when the material is used for the resonator and a
narrower variable frequency range when, for example, the resonator
is used to constitute a voltage-variable oscillator. Further, when
it constitutes a filter, such a problem occurs that a lower
electromechanical coupling coefficient causes a narrower bandwidth
feasible with the same insertion loss. That is, there is a problem
that material such as zinc oxide having a low electromechanical
coupling coefficient may be largely restrictive in element
performance to make up a device such as a resonator or a
filter.
[0016] As opposed thereto, for example, lead titanate (PbTiO.sub.3)
representing lead-based piezoelectric ceramics has a large
electromechanical coupling coefficient "k.sub.t", which exhibits a
high-band-cut-off-type dispersion characteristic as shown in FIG.
40. That is, the rejection band is formed within the frequency band
higher than the cut-off frequency, and the propagation band is
formed within the frequency band lower than the cut-off frequency.
When a frequency f is higher than the cut-off frequency "f.sub.0m"
of the electrode portion piezoelectric film 7a and is lower than
the cut-off frequency "f.sub.0f" of the non-electrode portion
piezoelectric film 7b, the rejection band is formed within the
electrode portion piezoelectric film 7a, and the propagation band
is formed within the non-electrode portion piezoelectric film 7b.
Therefore, the energy cannot be trapped in the electrode portion
piezoelectric film 7a, permitting the energy of the acoustic wave
excited to propagate and escape toward the non-electrode portion
piezoelectric film 7b. This causes a great loss.
[0017] As a method of realizing a piezoelectric device using such a
high-band-cut-off-type piezoelectric film, a method shown in FIG.
41 is described in Reference 3. That is, a thickness "h.sub.2" of
the electrode portion piezoelectric film 7a is made thinner than a
thickness "h.sub.1" of the non-electrode portion piezoelectric film
7b, where the top electrode 4 and the bottom electrode 2 are
arranged. It will be noted that the conventional piezoelectric
device shown in FIG. 41 by way of example is not a film bulk
acoustic wave device but a piezoelectric device using a sintered
plate material of piezoelectric ceramics, which is used in a
frequency of several tens MHz band or less.
[0018] FIG. 42 is a graph showing a dispersion characteristic for
describing an operation of the conventional piezoelectric device
shown in FIG. 41. If the piezoelectric film 3 has a uniform
thickness and the thickness is set "h.sub.1" as a whole, the
cut-off frequency of the non-electrode portion piezoelectric film
7b will be "f.sub.0f". The cut-off frequency will be "f.sub.0m1" of
the one formed with the electrodes on both sides, which is lower
than the cut-off frequency "f.sub.0f" of the non-electrode portion
piezoelectric film 7b due to an influence of the mass loads of the
electrodes and the like. In contrast, if the piezoelectric film 3
is made thinner than the non-electrode portion piezoelectric film
7b, the cut-off frequency "f.sub.0m2" of the electrode portion
piezoelectric film 7a will be higher. This is because the resonance
length for the thickness resonance varies depending upon the
thickness "h.sub.2" of the piezoelectric film. A selection of a
suitable thickness "h.sub.2" may allow the cut-off frequency
"f.sub.0m2" of the electrode portion piezoelectric film 7a to be
higher than the cut-off frequency "f.sub.0f" of the non-electrode
portion piezoelectric film 7b.
[0019] In this case, the dispersion characteristic 8 of the
electrode portion piezoelectric film 7a can present the same wave
number and a higher frequency than the dispersion characteristic 9
of the non-electrode portion piezoelectric film 7b. Therefore, the
frequency which is higher than the cut-off frequency "f.sub.0f" of
the non-electrode portion piezoelectric film 7b and which is lower
than the cut-off frequency "f.sub.0m2" of the electrode portion
piezoelectric film 7a provides actions of the electrode portion
piezoelectric film 7a as the propagation band, and the
non-electrode portion piezoelectric film 7b as the rejection band.
That is, the energy of the acoustic wave can be trapped in the
electrode portion piezoelectric film 7a, achieving a piezoelectric
device having a reduced loss.
[0020] FIGS. 43 and 44 are views showing an example of another
conventional piezoelectric device described in, for example,
"Energy-Trapping for Backward-Wave-Mode Thickness-Vibrations by
Controlling Piezoelectric Reaction", Transactions on The Institute
of Electronics, Information and Communication Engineers, '79/1,
Vol. J62-A, No. 1, pp. 8-15 (hereinafter referred to as Reference
5). FIG. 43 is a top plan view, and FIG. 44 is a sectional view.
The top electrode 4 and the bottom electrode 2 which are both round
are depicted herein.
[0021] In this case, again, it is not a film bulk acoustic wave
device but a piezoelectric device using a sintered plate material
of piezoelectric ceramics, which is used in a frequency having
several tens MHz band or less. The electrode portion piezoelectric
film 7a is polarized, while the non-electrode portion piezoelectric
film 7b is not polarized. A capacitor 11 is inserted in series
between the top electrode 4 and an electric terminal 12a.
[0022] FIG. 45 is a graph showing a dispersion characteristic for
describing an operation of the conventional piezoelectric device
shown in FIGS. 43 and 44. In this figure, reference symbol 13
denotes a dispersion characteristic of a non-electrode portion
piezoelectric film which is polarized, 14, a dispersion
characteristic of a non-electrode portion piezoelectric film which
is not polarized, and 15, a dispersion characteristic anticipated
from the electric terminal.
[0023] Many of lead-based piezoelectric ceramics such as lead
titanate (PbTiO.sub.3) can organize polarization within the
piezoelectric film 3 by applying a series voltage to the
piezoelectric film 3 while heating. Therefore, the region of the
piezoelectric film 3 to be polarized can be limited, and FIGS. 43
and 44 shows an example in which the electrode portion
piezoelectric film 7a is only polarized, but the non-electrode
portion piezoelectric film 7b is not polarized. As described in
Reference 5, the cut-off frequency "f.sub.0f2" of the unpolarized
non-electrode portion piezoelectric film 7b is lower in frequency
than the cut-off frequency "f.sub.0f1" of the polarized one,
presenting the dispersion characteristic 13.
[0024] Since, the capacitor 11 having a suitable capacitance is
inserted between the top electrode 4 and the electric terminal 12a,
the thickness resonant frequency "f.sub.0m2" of the electrode
portion piezoelectric film 7a which is anticipated from the
electric terminal 12a can be effectively made higher than the
cut-off frequency "f.sub.0m1" of the electrode portion
piezoelectric film 7a, presenting the dispersion characteristic 15.
Then, the operation for anticipation from the electric terminal 12a
exhibits the dispersion characteristic 15 for the electrode portion
piezoelectric film 7a and the dispersion characteristic 13 for the
non-electrode portion piezoelectric film 7b. Therefore, if a
frequency f is higher than the cut-off frequency "f .sub.0f2" of
the non-electrode portion piezoelectric film 7b, and is lower than
the thickness resonant frequency "f.sub.0m2" of the electrode
portion piezoelectric film 7a which is anticipated from the
electric terminal 12a, the propagation band is formed within the
electrode portion piezoelectric film 7a, and the rejection band is
formed within the non-electrode portion piezoelectric film 7b. As a
result, the energy trapping will be feasible.
[0025] FIGS. 46 and 47 are views showing an example of another
conventional piezoelectric device described in Reference 5, for
example. FIG. 46 is a top plan view, and FIG. 47 is a sectional
view. The top electrode 4 and the bottom electrode 2 which are both
round is depicted herein.
[0026] In this case, again, it is not a film bulk acoustic wave
device but a piezoelectric device using a sintered plate material
of piezoelectric ceramics, which is used in a frequency having
several tens MHz band or less. In these figures, reference symbol
16 denotes a short-circuit electrode, and 17, a short-circuit
electrode portion piezoelectric film. In the example described in
Reference 5, the short-circuit electrode portion piezoelectric film
17 is polarized. The short-circuit electrode 16 is slightly spaced
and arranged on the outer side of the top electrode 4, and the
short-circuit electrode 16 is electrically short-circuited with the
bottom electrode 2. A capacitor 11 is also inserted in series
between the top electrode 4 and an electric terminal 12a.
[0027] FIG. 48 is a graph showing a dispersion characteristic for
describing an operation of the conventional piezoelectric device
shown in FIGS. 46 and 47. In this figure, reference symbol 18
denotes a dispersion characteristic of the short-circuit electrode
portion piezoelectric film 17.
[0028] As described in Reference 5, the cut-off frequency
"f.sub.0f2" of the short-circuit electrode portion piezoelectric
film 17 the surface of which is covered with the short-circuit
electrode 16 and the bottom electrode 2 is reduced in frequency
than the cut-off frequency "f.sub.0f1" of the non-electrode portion
piezoelectric film 7b the surface of which is a free surface,
presenting the dispersion characteristic 18.
[0029] Further, since, the capacitor 11 having a suitable
capacitance is inserted between the top electrode 4 and the
electric terminal 12a, the thickness resonant frequency "f.sub.0m2"
of the electrode portion piezoelectric film 7a which is anticipated
from the electric terminal 12a can be effectively higher than the
cut-off frequency "f.sub.0m1" of the electrode portion
piezoelectric film 7a, presenting the dispersion characteristic 15.
The operation for anticipation from the electric terminal 12a
exhibits the dispersion characteristic 15 for the electrode portion
piezoelectric film 7a and the dispersion characteristic 18 for the
short-circuit electrode portion piezoelectric film 17. Therefore,
if a frequency f is higher than the cut-off frequency "f.sub.0f2"
of the short-circuit electrode portion piezoelectric film 17, and
is lower than the thickness resonant frequency "f.sub.0m2" of the
electrode portion piezoelectric film 7a anticipated from the
electric terminal 12a, the propagation band is formed within the
electrode portion piezoelectric film 7a, and the rejection band is
formed within the short-circuit electrode portion piezoelectric
film 17. As a result, the energy trapping will be feasible.
[0030] In the above-described conventional film bulk acoustic wave
devices, zinc oxide (ZnO) or aluminum nitride (AlN) have used for
piezoelectric films. These piezoelectric films exhibit a
low-band-cut-off-type dispersion characteristic in which a
propagation band is formed within the frequency band higher than
the cut-off frequency, and a rejection band is formed within the
frequency band lower than the cut-off frequency. The energy
trapping would be feasible in which the energy of the acoustic wave
may be trapped in the electrode portion piezoelectric film 7a. A
resonator having a high Q value, a filter with low loss or the like
would also be feasible. However, there is such a problem that zinc
oxide (ZnO) or aluminum nitride (AlN) having a low
electromechanical coupling coefficient may be largely restrictive
in device performance for realizing a resonator or a filter.
[0031] Further, when a piezoelectric device is attained using not a
film bulk acoustic wave device but a lead-based piezoelectric
ceramics having a high electromechanical coupling coefficient,
depending upon the piezoelectric film used, the piezoelectric
device may exhibit a high-band-cut-off-type dispersion
characteristic in which a rejection band is formed within the
frequency band higher than the cut-off frequency. In this case,
since the energy of the acoustic wave cannot be trapped in the
electrode portion piezoelectric film 7a, there is a drawback in
that a piezoelectric device having a sufficient performance would
not be feasible.
[0032] In order to solve the foregoing disadvantage, there has been
known a conventional method of reducing the thickness of the
electrode portion piezoelectric film 7a so as to be thinner than
the non-electrode portion piezoelectric film 7b at the outer side
thereof. This method requires an etching by a liquid or plasma to
be applied to the piezoelectric film 3. Strictly, However, the
piezoelectric film has a polycrystalline structure formed by
gathering fine grains, each of which is a single crystal, and has a
property in that a grain boundary surface of the polycrystalline
grain is exposed upon the etching. A film bulk acoustic wave device
is designed to operate in a frequency ranging from several hundreds
MHz to several GHz or more, with the wavelength of the acoustic
wave in the piezoelectric film being as extremely small as micron
or submicron unit. The piezoelectric film requires the surface
smoothness less than a fraction of the wavelength, so that the
surface smoothness required for the above-described film bulk
acoustic wave device is extremely low. The dimension of the grain
boundary of the above polycrystalline grain is often some tens
microns to some hundreds microns, which is rough compared to the
required surface smoothness. In other words, for the film bulk
acoustic wave device, it is difficult in manufacturing to partially
reduce the thickness of the piezoelectric film.
[0033] As is apparent from FIGS. 35 and 37, the piezoelectric film
3 in the film bulk acoustic wave device is also a critical
structural material for mechanically supporting the vicinity of the
electrode portions so arranged as to be floated by a via hole 5.
Meanwhile, as described in "Epitaxial Growth of ZnO Thin Films on
c-plane Sapphire Substrates by ECR-assisted MBE Technique", the
150th Committee of the Acoustic Wave Device Technology, the Japan
Society for the Promotion of Science, the 52nd seminar paper, pp.
1-6, 1997 (hereinafter referred to as Reference 6), or "GHz Band
Film Bulk Acoustic Wave Filters Using Reduced Piezoelectric Area",
the 1997 Engineering Sciences Society Event, The Institute of
Electronics, Information and Communication Engineers, A-11-6, p.
163, 1997 (hereinafter referred to as Reference 7), an internal
stress always exists within the piezoelectric film 3, which is
often a compressive stress, and such a stress as to warp the
piezoelectric film thus exists. Therefore, there occurs a following
problem. That is, if the piezoelectric film 3 is so arranged as to
be thinned in part, such a stress is concentrated on an end surface
that is reduced in thickness, to generate a large deformation on
this end surface. This may significantly deteriorate the
performance of the film bulk acoustic wave device, or may cause a
breakage of the film bulk acoustic wave device.
[0034] Such a method has been heretofore known that the electrode
portion piezoelectric film 7a is only polarized, and the
non-electrode portion piezoelectric film 7b is unpolarized while
the capacitor 11 is inserted in series to the top electrode 4, to
thereby effectively make the thickness resonant frequency
"f.sub.0m2" of the electrode portion piezoelectric film 7a
anticipated from the electric terminal 12a higher than the cut-off
frequency "f.sub.0f2" of the non-electrode portion piezoelectric
film 7b. This causes the cut-off frequency "f.sub.0f1" of the
unpolarized non-electrode portion piezoelectric film 7b to be lower
than the cut-off frequency "f.sub.0f1" of the polarized
non-electrode portion piezoelectric film 7b to the extent
necessary. This also requires the thickness resonant frequency
"f.sub.0m2" of the electrode portion piezoelectric film 7a which is
anticipated from the electric terminal 12a to be higher than the
cut-off frequency "f.sub.0f2" of the unpolarized non-electrode
portion piezoelectric film 7b because of the insertion of the
capacitor 11. However, the condition satisfying this is
restrictive. For example, if gold (Au) or platinum (Pt) having a
large mass load effect is used for the top electrode 4, the cut-off
frequency "f.sub.0m1" of the electrode portion piezoelectric film
7a is so lower than the cut-off frequency "f.sub.0f2" of the
non-electrode portion piezoelectric film 7b. Therefore, even with
the insertion of the capacitor 11, it will be difficult to make the
thickness resonant frequency "f.sub.0m2" of the electrode portion
piezoelectric film 7a which is anticipated from the electric
terminal 12a lower than the cut-off frequency "f.sub.0f2" of the
non-electrode portion piezoelectric film 7b.
[0035] In the conventional piezoelectric ceramics manufactured by
sintering ceramics which is broadly used in a low frequency, the
orientation of the polarization within each crystal grain is
disorganized for the unpolarized one, so that the polarization
within each crystal grain is cancelled in the piezoelectric
ceramics as a whole. Thus, no piezoelectric property will be found.
Meanwhile, the piezoelectric film 3 having often the polarization
in a certain orientation as a whole even with a disorganized
polarization within each crystal grain provides a low piezoelectric
property even for the unpolarized one. Accordingly, the
piezoelectric film has a lower characteristic difference between
the unpolarized and polarized piezoelectric thin films than the
piezoelectric ceramics manufactured by sintering ceramics. A
difference in frequency is low between the cut-off frequency
"f.sub.0f2" of the unpolarized non-electrode portion piezoelectric
film 7b and the cut-off frequency "f.sub.0f1" of the polarized
non-electrode portion piezoelectric film 7b. Therefore, it is
difficult to make the thickness resonant frequency "f.sub.0m2" of
the electrode portion piezoelectric film 7a which is anticipated
from the electric terminal 12a lower than the cut-off frequency
"f.sub.0f2" of the non-electrode portion piezoelectric film 7b.
[0036] The method of forming the short-circuit electrode 16 at an
outer side of the top electrode 4 requires for the short-circuit
electrode 16 and the bottom electrode 2 to be electrically
short-circuited with each other. However, for the device
manufactured mainly through the semiconductor process, like the
film bulk acoustic wave device, the method of connecting the
short-circuit electrode 16 and the bottom electrode 2 through a
through hole opened in the piezoelectric film 3 cannot be used as
practical connection means, because the feasible through hole
diameter is large. Also, the method of connection at the end
surface at an outer side of the piezoelectric film 3 is associated
with the following problem. With a distance from the end surface at
the outer side of the piezoelectric film 3 to the end surface on
the top electrode 4 side, the potential at the end surface of the
short-circuit electrode 16 on the top electrode 4 side may not be
set to the ground potential due to a resistance component or a
reactance component of the short-circuit electrode 16. Therefore,
the dispersion characteristic of the short-circuit electrode
portion piezoelectric film 17 may deviate from the dispersion
characteristic when the short-circuit electrode 16 is set to the
ground potential. In particular, this greatly affects the film bulk
acoustic wave device used in a high frequency, and there is
difficulty in that a designed characteristic will not be
feasible.
[0037] The present invention has been made in order to solve the
foregoing problems, and an object of the present invention is to
obtain a film bulk acoustic wave device capable of attaining the
good performance using a piezoelectric film having a large
electromechanical coupling coefficient.
DISCLOSURE OF THE INVENTION
[0038] To begin with, a film bulk acoustic wave device in
accordance with the present invention is designed to have a second
electrode with an increased thickness placed on the top of a top
electrode or the like, and the structure is as follows.
[0039] A film bulk acoustic wave device in accordance with the
present invention, comprising: a substrate; a bottom electrode
formed on one surface of the substrate; a piezoelectric film formed
on the bottom electrode; and a first top electrode formed on the
piezoelectric film, further comprises a second top electrode having
a larger mass load than the first top electrode, which is formed at
an outer side of the first top electrode on the piezoelectric film
when viewed from the center of the first top electrode, in which
the piezoelectric film has a high-band-cut-off-type dispersion
characteristic.
[0040] Further, a film bulk acoustic wave device is provided in
accordance with the present invention, wherein the first and second
top electrodes are integrally formed, and the second top electrode
has a larger thickness than that of the first top electrode.
[0041] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the product of
the electrode thickness and the density of the second top electrode
is larger than the product of the electrode thickness and the
density of the first top electrode.
[0042] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the second top
electrode is laid on a part of the first top electrode.
[0043] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the first top
electrode and the second top electrode are connected with each
other.
[0044] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the second top
electrode having a narrower width than that of the first top
electrode is laid on the first top electrode.
[0045] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the first and
second top electrodes are divided into two.
[0046] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein a third top
electrode is formed on the piezoelectric film between the two
divided first top electrodes.
[0047] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the substrate is
a semiconductor substrate or a dielectric substrate.
[0048] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the piezoelectric
film has a Poisson ratio lower than 0.34.
[0049] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the piezoelectric
film contains lead titanate (PbTiO.sub.3) as a major component.
[0050] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein a dielectric
layer is inserted between the substrate and the bottom
electrode.
[0051] Next, a film bulk acoustic wave device in accordance with
the present invention is designed to have a dielectric placed on
the top of a top electrode or the like, and the structure is as
follows.
[0052] A film bulk acoustic wave device in accordance with the
present invention, comprising: a substrate; a bottom electrode
formed on one surface of the substrate; a piezoelectric film formed
on the bottom electrode; and a top electrode formed on the
piezoelectric film, further comprises a dielectric formed at an
outer side of the top electrode on the piezoelectric film when
viewed from the center of the top electrode, in which the
piezoelectric film has a high-band-cut-off-type dispersion
characteristic.
[0053] Further, a film bulk acoustic wave device is provided in
accordance with the present invention, wherein the dielectric is
formed on a part of the top electrode.
[0054] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the product of
the film thickness and the density of the dielectric is larger than
the product of the electrode thickness and the density of the top
electrode.
[0055] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the dielectric is
laid on a part of the top electrode.
[0056] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the top electrode
and the dielectric are connected with each other.
[0057] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the dielectric
having a narrower width than that of the top electrode is laid on
the top electrode.
[0058] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the top electrode
and the dielectric are divided into two.
[0059] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein a second top
electrode is formed on the piezoelectric film between the two
divided first top electrodes.
[0060] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the substrate is
a semiconductor substrate or a dielectric substrate.
[0061] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the piezoelectric
film has a Poisson ratio lower than 0.34.
[0062] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein the piezoelectric
film contains lea d titanate (PbTiO.sub.3) as a major
component.
[0063] Still further, a film bulk acoustic wave device is provided
in accordance with the present invention, wherein a dielectric
layer is inserted between the substrate and the bottom
electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] FIG. 1 is a view showing the top of a film bulk acoustic
wave device according to Embodiment 1 of the present invention;
[0065] FIG. 2 is a view showing in section the film bulk acoustic
wave device according to Embodiment 1 of the present invention;
[0066] FIG. 3 is an enlarged view of an acoustic resonance portion
shown in FIG. 2;
[0067] FIG. 4 is a graph showing a dispersion characteristic of the
film bulk acoustic wave device shown in FIG. 3;
[0068] FIG. 5 is a view showing the top of a film bulk acoustic
wave device according to Embodiment 2 of the present invention;
[0069] FIG. 6 is a view showing in section the film bulk acoustic
wave device according to Embodiment 2 of the present invention;
[0070] FIG. 7 is an enlarged view of an acoustic resonance portion
shown in FIG. 6;
[0071] FIG. 8 is a view showing a film bulk acoustic wave device
according to Embodiment 3 of the present invention;
[0072] FIG. 9 is a view showing the top of a film bulk acoustic
wave device according to Embodiment 4 of the present invention;
[0073] FIG. 10 is a view showing in section the film bulk acoustic
wave device according to Embodiment 4 of the present invention;
[0074] FIG. 11 is an enlarged view of an acoustic resonance portion
shown in FIG. 10;
[0075] FIG. 12 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device shown in FIG. 11;
[0076] FIG. 13 is a view showing a film bulk acoustic wave device
according to Embodiment 5 of the present invention;
[0077] FIG. 14 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device illustrated in FIG. 13;
[0078] FIG. 15 is a view showing a film bulk acoustic wave device
according to Embodiment 6 of the present invention;
[0079] FIG. 16 is a view showing a film bulk acoustic wave device
according to Embodiment 7 of the present invention;
[0080] FIG. 17 is a view showing a film bulk acoustic wave device
according to Embodiment 8 of the present invention;
[0081] FIG. 18 is a view showing the top of a film bulk acoustic
wave device according to Embodiment 9 of the present invention;
[0082] FIG. 19 is a view showing in section the film bulk acoustic
wave device according to Embodiment 9 of the present invention;
[0083] FIG. 20 is an enlarged view of an acoustic resonance portion
shown in FIG. 19;
[0084] FIG. 21 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device illustrated in FIG. 20;
[0085] FIG. 22 is a view showing the top of a film bulk acoustic
wave device according to Embodiment 10 of the present
invention;
[0086] FIG. 23 is a view showing in section the film bulk acoustic
wave device according to Embodiment 10 of the present
invention;
[0087] FIG. 24 is an enlarged view of an acoustic resonance portion
shown in FIG. 23;
[0088] FIG. 25 is a view showing a film bulk acoustic wave device
according to Embodiment 11 of the present invention;
[0089] FIG. 26 is a view showing the top of a film bulk acoustic
wave device according to Embodiment 12 of the present
invention;
[0090] FIG. 27 is a view showing in section the film bulk acoustic
wave device according to Embodiment 12 of the present
invention;
[0091] FIG. 28 is an enlarged view of an acoustic resonance portion
illustrated in FIG. 27;
[0092] FIG. 29 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device shown in FIG. 28;
[0093] FIG. 30 is a view showing a film bulk acoustic wave device
according to Embodiment 13 of the present invention;
[0094] FIG. 31 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device illustrated in FIG. 30;
[0095] FIG. 32 is a view showing a film bulk acoustic wave device
according to Embodiment 14 of the present invention;
[0096] FIG. 33 is a view showing a film bulk acoustic wave device
according to Embodiment 15 of the present invention;
[0097] FIG. 34 is a view showing the top of a film bulk acoustic
wave device of the prior art;
[0098] FIG. 35 is a view showing in section the film bulk acoustic
wave device illustrated in FIG. 34;
[0099] FIG. 36 is a view showing the top of another film bulk
acoustic wave device of the prior art;
[0100] FIG. 37 is a view showing in section the film bulk acoustic
wave device illustrated in FIG. 36;
[0101] FIG. 38 is an enlarged view of an acoustic resonance portion
shown in FIG. 34;
[0102] FIG. 39 is a graph showing an example of a propagation
characteristic of an acoustic wave;
[0103] FIG. 40 is a graph showing an example of a
high-band-cut-off-type dispersion characteristic;
[0104] FIG. 41 is a view showing in section a piezoelectric device
of the prior art;
[0105] FIG. 42 is a graph showing a dispersion characteristic of
the piezoelectric device illustrated in FIG. 41;
[0106] FIG. 43 is a view showing the top of another piezoelectric
device of the prior art;
[0107] FIG. 44 is a view showing in section the piezoelectric
device illustrated in FIG. 43;
[0108] FIG. 45 is a graph showing a dispersion characteristic of
the piezoelectric device illustrated in FIGS. 43 and 44;
[0109] FIG. 46 is a view showing the top of still another
piezoelectric device of the prior art;
[0110] FIG. 47 is a view showing in section the piezoelectric
device illustrated in FIG. 46; and
[0111] FIG. 48 is a graph showing a dispersion characteristic of
the piezoelectric device illustrated in FIGS. 46 and 47.
BEST MODE FOR CARRYING OUT THE INVENTION
[0112] Embodiments of the present invention will be described below
with reference to the drawings.
[0113] Described first in Embodiments 1 to 8 below is a film bulk
acoustic wave device in which a second top electrode with increased
thickness is formed on the top of a top electrode or the like.
Embodiment 1
[0114] A description will be made with reference to FIGS. 1 to 4 on
a film bulk acoustic wave device according to Embodiment 1 of the
present invention. FIG. 1 is a view showing the top of the film
bulk acoustic wave device according to Embodiment 1 of the present
invention. FIG. 2 is a view showing in section the film bulk
acoustic wave device according to Embodiment 1 of the present
invention. In the drawings, identical symbols denote identical or
corresponding portions.
[0115] In FIGS. 1 and 2, reference symbol 19 denotes a
semiconductor substrate or a dielectric substrate; 2, a bottom
electrode; 3, a piezoelectric film; 4, a top electrode; 5, a via
hole; 6, an acoustic resonance portion; 20, a top electrode formed
by increasing thickness of the top electrode 4; 21, an air bridge;
and 22, a pad.
[0116] The semiconductor substrate or dielectric substrate 19, as
it does not have direct influence on an acoustic characteristic of
the film bulk acoustic wave device, may be either a conventionally
used semiconductor substrate such as a silicon (Si) semiconductor
substrate and a gallium arsenic (GaAs) semiconductor substrate, or
an insulating dielectric substrate having no semiconductor
characteristic such as a glass, sapphire or magnesium oxide (MgO)
substrate. The silicon (Si) substrate, the gallium arsenic (GaAs)
substrate and the magnesium oxide (MgO) substrate in particular
have good characteristic in high-temperature resistance and are
excellent substrate materials. Hereinafter, the semiconductor
substrate or dielectric substrate 19 is generically referred to as
the semiconductor substrate 19.
[0117] The bottom electrode 2 is formed on this semiconductor
substrate 19. In some cases, a dielectric layer made of silicon
oxide (SiO, SiO.sub.2), silicon nitride (SiN) or the like may be
inserted between the semiconductor substrate 19 and the bottom
electrode 2. This may be applied to all Embodiments shown bellow.
On the bottom electrode 2, the piezoelectric film 3 is formed and
the top electrode 4 is further formed on the piezoelectric film
3.
[0118] Though the air bridge 21 is used to electrically connect the
top electrode 4 and the pad 22, the electrode may sometimes be
connected to the pad 22 using a line adhered to the surface of the
piezoelectric film 3.
[0119] The via hole 5 is formed from the back surface of the
semiconductor substrate 19 to obtain a structure in which the
bottom surface of the bottom electrode 2 is exposed to the air. In
the case that the dielectric layer described above is inserted,
there is the choice between removing also the dielectric layer so
that the bottom surface of the bottom electrode 2 is exposed to the
air and exposing to the air the bottom surface of the dielectric
layer. Those structures are the same as in film bulk acoustic wave
devices of the prior art.
[0120] However, this film bulk acoustic wave device according to
Embodiment 1 uses for the piezoelectric film 3 a material having
the high-band-cut-off-type dispersion characteristic, and is formed
with a region for the top electrode 20 with increased thickness on
the outer side of the top electrode 4.
[0121] FIG. 3 is an enlarged view of the acoustic resonance portion
6 shown in FIG. 2. In FIG. 3, reference symbol 23a denotes a
piezoelectric film of the thin top electrode 4 portion; 23b, a
piezoelectric film of the thick top electrode 20 portion; 23c, a
non-electrode portion piezoelectric film; and 26, a reflection
surface of energy energy trapped resonance.
[0122] FIG. 4 is a graph showing a dispersion characteristic of the
film bulk acoustic wave device illustrated in FIG. 3. In FIG. 4,
reference symbol 24 denotes a dispersion characteristic of the thin
top electrode portion piezoelectric film 23a and 25 denotes a
dispersion characteristic of the thick top electrode portion
piezoelectric film 23b.
[0123] The bottom electrode 2 of the film bulk acoustic wave device
shown in FIG. 3 and the bottom electrode 2 of the film bulk
acoustic wave device shown in FIG. 2 are different in structure
from each other. However, the bottom electrode 2 shown in FIG. 3 is
illustrated in a more simplified way and the actual thin film
piezoelectric has the structure shown in FIG. 2.
[0124] Used for the piezoelectric film 3 is a material exhibiting a
high-band-cut-off-type dispersion characteristic. For example, what
exhibits the high-band-cut-off-type dispersion characteristic
includes fundamental thickness longitudinal vibration of a
piezoelectric film of which Poisson ratio .sigma. is 1/3 or less,
in addition to: fundamental thickness longitudinal vibration of
lead titanate (PbTiO.sub.3); ternary thickness shear vibration of
lead zirconate titanate (PZT); and fundamental thickness
longitudinal vibration of lithium tantalate (LiTaO.sub.3) Z plate.
These piezoelectric bodies exhibit extremely large
electromechanical coupling coefficient, leading to realization of
better device performance as compared with the conventional
piezoelectric device that uses zinc oxide (ZnO) or aluminum nitride
(AlN). Incidentally, Poisson ratio .sigma. is one of the material
constants that indicate a characteristic of a piezoelectric
material, and is explained in detail in "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. 10-21, 1991 (hereinafter, referred to as Reference
8).
[0125] The cut-off frequency "f.sub.0m1" of the thin top electrode
portion piezoelectric film 23a is lower than the cut-off frequency
"f.sub.0f" of the non-electrode portion piezoelectric film 23c
because of the thickness and the mass load effect of the top
electrode 4 and the bottom electrode 2.
[0126] When the electrode thickness of the region on the outer side
of the top electrode 4 is increased to form the thick top electrode
20, owing to the thickness and the mass load effect of the top
electrode 20, the cut-off frequency "f.sub.0m2" of the thick top
electrode portion piezoelectric film 23b is further lower than the
cut-off frequency "f.sub.0m1" of the thin top electrode portion
piezoelectric film 23a.
[0127] Therefore, in the frequency band between the cut-off
frequency "f.sub.0m1" of the thin top electrode portion
piezoelectric film 23a and the cut-off frequency "f.sub.0m2" of the
thick top electrode portion piezoelectric film 23b, a propagation
band is formed within the thin top electrode portion piezoelectric
film 23a and the rejection band is formed within the thick top
electrode portion piezoelectric film 23b. The interface between the
thin top electrode portion piezoelectric film 23a and the thick top
electrode portion piezoelectric film 23b thus serves as the
reflection surface 26 to confine energy of an acoustic wave,
thereby realizing energy trapped resonance.
[0128] Accordingly, adopting such a structure makes it possible to
realize energy trapping with the use of the piezoelectric film such
as lead titanate (PbTiO.sub.3), lead zirconate titanate (PZT) and
lithium tantalate (LiTaO.sub.3) which are large in
electromechanical coupling coefficient. Thus may be realized a film
bulk acoustic wave device of better device performance.
Embodiment 2
[0129] A description will be made with reference to FIGS. 5 to 7 on
a film bulk acoustic wave device according to Embodiment 2 of the
present invention. FIG. 5 is a view showing the top of the film
bulk acoustic wave device according to Embodiment 2 of the present
invention. FIG. 6 is a view showing in section the film bulk
acoustic wave device according to Embodiment 2 of the present
invention. FIG. 7 is a view showing in a magnifying manner an
acoustic resonance portion illustrated in FIG. 6. This film bulk
acoustic wave device according to Embodiment 2 is an example of a
filter of two-electrode construction.
[0130] The dispersion characteristic of the film bulk acoustic wave
device in FIG. 7 are the same as the dispersion characteristic
shown in FIG. 4. The piezoelectric film 3 exhibits the
high-band-cut-off-type dispersion characteristic, and the cut-off
frequency "f.sub.0m2" of the thick top electrode portion
piezoelectric film 23b is lower than the cut-off frequency
"f.sub.0m1" of the thin top electrode portion piezoelectric film
23a.
[0131] Therefore, in the frequency band between the cut-off
frequency "f.sub.0m1" of the thin top electrode portion
piezoelectric film 23a and the cut-off frequency "f.sub.0m2" of the
thick top electrode portion piezoelectric film 23b, a propagation
band is formed within the thin top electrode portion piezoelectric
film 23a and the rejection band is formed within the thick top
electrode portion piezoelectric film 23b. The interface between the
thin top electrode portion piezoelectric film 23a and the
piezoelectric film 23b of the thick top electrode portion thus
serves as the reflection surface 26 to confine energy of an
acoustic wave, thereby realizing energy trapped resonance.
[0132] In energy trapped resonance, resonance is produced in a
direction parallel to the surface of the piezoelectric film 3 to
produce, in the case of the filter with the top electrode 4 being
separated into two, symmetric mode resonance with which two top
electrodes 4 have the same potential and asymmetric mode resonance
with which the electrodes 4 have polarity opposite to each other.
When a resonant frequency for producing the symmetric mode
resonance and a resonant frequency for producing the asymmetric
mode resonance are close to each other, coupling takes place
between two top electrodes 4 to exhibit a filter
characteristic.
[0133] Resonance frequencies for two resonance modes depend on the
kind of the piezoelectric film 3 to be used, the space between the
top electrodes 4, the physical dimensions of the thin top
electrodes 4, the physical dimension of the thick top electrode 20
and a material for the electrodes. The filter operation when the
energy trapping is used is described in detail in Reference 3.
[0134] In the case of the filter, if energy trapped resonance is
not produced, insertion loss could be increased and much spurious
could take place at the outside of the bandwidth. However, the use
of the film bulk acoustic wave device as shown in FIG. 7 may
realize energy trapped resonance to allow to use the piezoelectric
film that is large in electromechanical coupling coefficient, and
may realize as well a film bulk acoustic wave device lesser in loss
and better in characteristic.
Embodiment 3
[0135] A description will be made with reference to FIG. 8 on a
film bulk acoustic wave device according to Embodiment 3 of the
present invention. FIG. 8 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 3 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 3 are the same as those in Embodiment 2.
[0136] Unlike the case of Embodiment 2 shown in FIG. 7, the
electrode thickness of a portion on the outer side of the top
electrode 4 is the same as that of the thin top electrode 4. Energy
trapped resonance is produced on the inner side to the reflection
surface 26, and hence the film bulk acoustic wave device shown in
FIG. 8 performs in principle the same operation as the film bulk
acoustic wave device shown in FIG. 7 regardless of energy trapped
resonance, even with the presence of the thin top electrode 4
outside the reflection surface 26.
[0137] In order to partially increase the electrode thickness of
the top electrode 4 as shown in FIGS. 3, 7 and 8, there are a
method in which a top electrode with the thickness of the thick top
electrode 20 is formed in advance to thereafter form the thin top
electrode 4 by a measure such as etching, and a method in which the
thin top electrode 4 is first formed to then additionally form an
electrode on the top electrode to be the thick top electrode
through the lift-off method or the like. Any of these methods may
be applied to the film bulk acoustic wave device according to
Embodiment 3.
Embodiment 4
[0138] A description will be made with reference to FIGS. 9 to 12
on a film bulk acoustic wave device according to Embodiment 4 of
the present invention. FIG. 9 is a view showing the top of the film
bulk acoustic wave device according to Embodiment 4 of the present
invention. FIG. 10 is a view showing in section the film bulk
acoustic wave device according to Embodiment 4 of the present
invention.
[0139] In FIGS. 9 and 10, reference symbol 4 denotes a first top
electrode and reference symbol 27 denotes a second top
electrode.
[0140] FIG. 11 is a view showing in a magnifying manner an acoustic
resonance portion illustrated in FIG. 10. In FIG. 11, reference
symbol 28a denotes a piezoelectric film of the first top electrode
4 portion; 28b, a piezoelectric film of the second top electrode 27
portion; and 28c, a non-electrode portion piezoelectric film.
[0141] FIG. 12 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device illustrated in FIG. 11. In FIG.
12, reference symbol 29 denotes a dispersion characteristic of the
first top electrode portion piezoelectric film 28a, and reference
symbol 30 denotes a dispersion characteristic of the second top
electrode portion piezoelectric film 28b.
[0142] The electrode thickness and the density of the first top
electrode 4 are respectively given as "d.sub.1" and ".rho..sub.1",
and the electrode thickness and the density of the second top
electrode 27 are respectively given as "d.sub.2" and ".rho..sub.2".
When the product of the electrode thickness "d.sub.2" of the second
top electrode 27 and the density ".rho..sub.2" thereof
(d.sub.2.rho..sub.2) is larger than the product of the electrode
thickness "d.sub.1" of the first top electrode 4 and the density
".rho..sub.1" thereof (d.sub.1.rho..sub.1), the cut-off frequency
"f.sub.0m2" of the second top electrode portion piezoelectric film
28b is lower than the cut-off frequency "f.sub.0m1" of the first
top electrode portion piezoelectric film 28a.
[0143] Therefore, energy trapping may be realized in the frequency
band between the the cut-off frequency "f.sub.0m1" of the first top
electrode portion piezoelectric film 28a and the cut-off frequency
"f.sub.0m2" of the second top electrode portion piezoelectric film
28b to realize a film bulk acoustic wave device of good
performance.
[0144] In the case that the second top electrode 27 is made of the
same material as the first top electrode 4, the electrode thickness
"d.sub.2" of the second top electrode 27 needs to be thicker than
the electrode thickness "d.sub.1" of the first top electrode 4.
[0145] On the other hand, with the use of different materials,
when, for instance, the second top electrode 27 has a density
larger than that of the first top electrode 4, the electrode
thickness "d.sub.2" of the second top electrode 27 may be equal to
or thinner than the electrode thickness "d.sub.1" of the first top
electrode 4.
Embodiment 5
[0146] A description will be made with reference to FIGS. 13 and 14
on a film bulk acoustic wave device according to Embodiment 5 of
the present invention. FIG. 13 is a view showing in a magnifying
manner an acoustic resonance portion of the film bulk acoustic wave
device according to Embodiment 5 of the present invention. FIG. 14
is a graph showing a dispersion characteristic of the film bulk
acoustic wave device illustrated in FIG. 13. Other constructions of
the film bulk acoustic wave device according to Embodiment 5 are
the same as those in Embodiment 4.
[0147] In FIG. 13, reference symbol 31 denotes a piezoelectric film
of a region where the first top electrode 4 and the second top
electrode 27 overlap with each other.
[0148] In FIG. 14, reference symbol 32 denotes a dispersion
characteristic of the piezoelectric film 31 of the region where the
first and second electrodes overlap each other.
[0149] In the film bulk acoustic wave device shown in FIG. 13, the
reflection surface 26 of energy trapped resonance corresponds to
the surface below the end surface of the second top electrode 27
overlapping the first top electrode 4. The cut-off frequency
"f.sub.0m2" of the piezoelectric film 31 of the region where the
first and second electrodes overlap each other is a lower frequency
than the cut-off frequency "f.sub.0m1" of the first top electrode
portion piezoelectric film 28a, owing to both the electrode
thickness "d.sub.1" plus the mass load effect of the first top
electrode 4 and the electrode thickness "d.sub.2" plus the mass
load effect of the second top electrode 27.
[0150] Therefore, energy trapping takes place in the frequency band
between the cut-off frequency "f.sub.0m1" of the first top
electrode portion piezoelectric film 28a and the cut-off frequency
"f.sub.0m2" of the piezoelectric film 31 of the region where the
first and second electrodes overlap each other. That is, filter
operation by energy trapping may be realized.
[0151] In the film bulk acoustic wave device shown in FIG. 13, it
is sufficient that the electrode thickness "d.sub.2" and the
density ".rho..sub.2" of the second top electrode 27 can lower the
cut-off frequency "f.sub.0m2" of the piezoelectric film 31 of the
region where the first and second electrodes overlap each other to
the degree required for filter operation, and the limitation put on
the electrode thickness "d.sub.1" and the density ".rho..sub.1" of
the first top electrode 4 is less strict than in the case shown in
FIG. 11. This is because of the structure in which the second top
electrode 27 overlaps the first top electrode 4.
Embodiment 6
[0152] A description will be made with reference to FIG. 15 on a
film bulk acoustic wave device according to Embodiment 6 of the
present invention. FIG. 15 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 6 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 6 are the same as those in Embodiment 4.
[0153] The film bulk acoustic wave device shown in FIG. 15
corresponds to the first top electrode 4 and the second top
electrode 27 in the film bulk acoustic wave device shown in FIG.
11, which are connected with each other. Accordingly, operation
principle of energy trapping is the same as in the case of the
piezoelectric film shown in FIG. 11.
[0154] The mass load by the second top electrode 27 is thus
required to be larger than the mass load by the first top electrode
4. In other words, the product of the electrode thickness "d.sub.2"
of the second top electrode 27 and the density ".rho..sub.2"
thereof (d.sub.2.rho..sub.2) needs to be larger than the product of
the electrode thickness "d.sub.1" of the first top electrode 4 and
the density ".rho..sub.1" thereof (d.sub.1.rho..sub.1).
[0155] At this time, in the case that the first top electrode 4 and
the second top electrode 27 are made of the same material to have
the same density, the electrode thickness "d.sub.2" of the second
top electrode 27 is necessarily larger than the electrode thickness
"d.sub.1" of the first top electrode 4, obtaining the same
structure as that of the piezoelectric film according to Embodiment
2 shown in FIG. 7.
Embodiment 7
[0156] A description will be made with reference to FIG. 16 on a
film bulk acoustic wave device according to Embodiment 7 of the
present invention. FIG. 16 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 7 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 7 are the same as those in Embodiment 4.
[0157] This Embodiment 7 employs the structure in which the whole
second top electrode 27 is laid on the top electrode 4. Since the
dispersion characteristic of the portion where the second top
electrode 27 is laid on the first top electrode 4 are the same as
the dispersion characteristic shown in FIG. 14, the operation
principle of energy trapping is almost the same as in the case of
the film bulk acoustic wave device according to Embodiment 5 shown
in FIG. 13.
[0158] Therefore in the film bulk acoustic wave device shown in
FIG. 16, as in the case shown in FIG. 13, it is sufficient that the
electrode thickness "d.sub.2" and the density ".rho..sub.2" of the
second top electrode 27 can lower the cut-off frequency "f.sub.0m2"
of the piezoelectric film 31 of the region where the first and
second electrodes overlap each other to the degree required for
filter operation, and the limitation put on the electrode thickness
"d.sub.1" and the density ".rho..sub.1" of the first top electrode
4 is less strict than in the case of Embodiment 6 shown in FIG. 15.
This is because of the structure in which the second top electrode
27 is laid on the first top electrode 4.
Embodiment 8
[0159] A description will be made with reference to FIG. 17 on a
film bulk acoustic wave device according to Embodiment 8 of the
present invention. FIG. 17 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 8 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 8 are the same as those in Embodiment 7.
[0160] In FIG. 17, reference symbol 33 denotes a third top
electrode and reference symbol 34 denotes a third top electrode
portion piezoelectric film.
[0161] The third top electrode 33 is inserted to improve the degree
of freedom in designing the filter characteristic in the film bulk
acoustic wave device. The dispersion characteristic of this third
top electrode portion piezoelectric film 34 is close to the
dispersion characteristic of the first top electrode portion
piezoelectric film 28a. Namely, the cut-off frequency "f.sub.0m3"
of the third top electrode portion piezoelectric film 34 may be
considered as being close to the cut-off frequency "f.sub.0m1" of
the first top electrode portion e piezoelectric film 28a.
[0162] Accordingly, the dispersion characteristic of the
piezoelectric film shown in FIG. 17 may be deduced with the use of
FIG. 14. That is, the reflection surface 26 of energy trapping
corresponds to the surface below and on the inner side to the end
surface of the second top electrode 27 overlapping the first top
electrode 4.
[0163] The present invention is not limited to lead titanate
(PbTiO.sub.3), lead zirconate titanate (PZT), lithium tantalate
(LiTaO.sub.3) and a piezoelectric film with Poisson ratio .sigma.
of 1/3 or less, but may be applied to all the piezoelectric bodies
that exhibit the high-band-cut-off-type dispersion
characteristic.
[0164] The methods shown in FIGS. 11, 13, 15 and 16 may be applied
to the resonator shown in FIGS. 1 and 2.
[0165] The methods shown in FIGS. 7, 8, 11, 13, 15 and 16 may be
applied, as shown in FIG. 17, to the case in which the film bulk
acoustic wave device consists of more electrodes than in the
two-electrode construction.
[0166] Though the air bridge 21 is used to fetch an electric signal
from the top electrode 4 in FIGS. 1, 2 and 3, the way how to fetch
the electric signal from the top electrode 4 is not necessarily
limited to the air bridge 21, but may be a method of constructing a
line on the surface of the piezoelectric film 3 or other arbitrary
methods.
[0167] The via hole 5 shown is pierced from the back surface of the
semiconductor substrate 19, but is not necessarily limited thereto.
It makes no difference when the via hole is formed through a method
in which an aperture is formed below the acoustic resonance portion
6 through anisotropic etching or the like starting from the front
surface of the semiconductor substrate 19, a method in which a
layer-like aperture is formed by removing a previously prepared
thin film with etching or the like, or a method in which several
layers that are different in acoustical property with one another
are layered so that an acoustic wave propagating along the
thickness direction is reflected on that multi-layer structure.
[0168] Subsequently, a description will be made on a film bulk
acoustic wave device provided with a dielectric on the top of the
top electrode or the like in Embodiments 9 to 15 below.
Embodiment 9
[0169] A description will be made with reference to FIGS. 18 to 21
on a film bulk acoustic wave device according to Embodiment 9 of
the present invention. FIG. 18 is a view showing the top of the
film bulk acoustic wave device according to Embodiment 9 of the
present invention. FIG. 19 is a view showing in section the film
bulk acoustic wave device according to Embodiment 9 of the present
invention. In the drawings, identical symbols denote identical or
corresponding portions.
[0170] In FIGS. 18 and 19, reference symbol 19 denotes a
semiconductor substrate or a dielectric substrate; 2, a bottom
electrode; 3, a piezoelectric film; 4, a top electrode; 5, a via
hole; 6, an acoustic resonance portion; 20A, a dielectric; 21, an
air bridge; and 22, a pad.
[0171] The semiconductor substrate or dielectric substrate 19, as
it does not have direct influence on an acoustical characteristic
of the film bulk acoustic wave device, may be either a
conventionally used semiconductor substrate such as a silicon (Si)
semiconductor substrate and a gallium arsenic (GaAs) semiconductor
substrate, or an insulating dielectric substrate having no
semiconductor characteristic such as a glass, sapphire or magnesium
oxide (MgO) substrate. The silicon (Si) substrate, the gallium
arsenic (GaAs) substrate and the magnesium oxide (MgO) substrate in
particular have good characteristic in high-temperature resistance
and are excellent substrate materials. Hereinafter, the
semiconductor substrate or dielectric substrate 19 is generically
referred to as the semiconductor substrate 19.
[0172] The bottom electrode 2 is formed on this semiconductor
substrate 19. In some cases, a dielectric layer made of silicon
oxide (SiO, SiO.sub.2), silicon nitride (SiN) or the like may be
inserted between the semiconductor substrate 19 and the bottom
electrode 2. This may be applied to all Embodiments shown bellow.
On the bottom electrode 2, the piezoelectric film 3 is formed and
the top electrode 4 is further formed on the piezoelectric film
3.
[0173] Though the air bridge 21 is used to electrically connect the
top electrode 4 and the pad 22, the electrode may sometimes be
connected to the pad 22 using a line adhered to the surface of the
piezoelectric film 3.
[0174] The via hole 5 is formed from the back surface of the
semiconductor substrate 19 to obtain a structure in which the
bottom surface of the bottom electrode 2 is exposed to the air. In
the case that the dielectric layer described above is inserted,
there is the choice between removing also the dielectric layer so
that the bottom surface of the bottom electrode 2 is exposed to the
air and exposing to the air the bottom surface of the dielectric
layer. Those structures are the same as in film bulk acoustic wave
devices of the prior art.
[0175] However, this film bulk acoustic wave device according to
Embodiment 9 uses for the piezoelectric film 3 a material having a
high-band-cut-off-type dispersion characteristic, and is formed
with a region for the dielectric 20A on the outer side of the top
electrode 4.
[0176] FIG. 20 is an enlarged view of the acoustic resonance
portion 6 shown in FIG. 19. In FIG. 20, reference symbol 23a
denotes a piezoelectric film of the top electrode 4 portion; 23b, a
piezoelectric film of the dielectric 20A portion; 23c, a
non-electrode portion piezoelectric film; and 26, a reflection
surface of energy trapped resonance.
[0177] FIG. 21 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device illustrated in FIG. 20. In the
drawing, reference symbol 24 denotes a dispersion characteristic of
the top electrode portion piezoelectric film 23a, and reference
symbol 25 denotes a dispersion characteristic of the dielectric
portion piezoelectric film 23b.
[0178] The bottom electrode 2 of the film bulk acoustic wave device
shown in FIG. 20 and the bottom electrode 2 of the film bulk
acoustic wave device shown in FIG. 19 are different in structure
from each other. However, the bottom electrode 2 shown in FIG. 20
is illustrated in a more simplified way. In the film bulk acoustic
wave device shown in FIG. 19, the air bridge 21 and the top
electrode 4 are electrically connected with each other and the
dielectric 20A is not present between the air bridge 21 and the top
electrode 4. The dielectric 20A, on the other hand, seems to be
present in FIG. 20. The actual thin film piezoelectric has the
structure shown in FIG. 19.
[0179] Used for the piezoelectric film 3 is a material exhibiting
the high-band-cut-off-type dispersion characteristic. For example,
what exhibits the high-band-cut-off-type dispersion characteristic
includes fundamental thickness longitudinal vibration of a
piezoelectric film of which Poisson ratio .sigma. is 1/3 or less,
in addition to: fundamental thickness longitudinal vibration of
lead titanate (PbTiO.sub.3); ternary thickness shear vibration of
lead zirconate titanate (PZT); and fundamental thickness
longitudinal vibration of lithium tantalate (LiTaO.sub.3) Z plate.
These piezoelectric bodies exhibit extremely large
electromechanical coupling coefficient, leading to realization of
better device performance as compared with a conventional
piezoelectric device that uses zinc oxide (ZnO) or aluminum nitride
(AlN). Incidentally, Poisson ratio o is one of the material
constants that indicate a characteristic of a piezoelectric
material, and is explained in detail 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. 10-21, 1991 (Reference 8).
[0180] The cut-off frequency "f.sub.0m" of the top electrode
portion piezoelectric film 23a is lower than the cut-off frequency
"f.sub.0f1" of the non-electrode portion piezoelectric film 23c
because of the thickness and the mass load effect of the top
electrode 4 and the bottom electrode 2.
[0181] When the dielectric 20A is formed in the region above the
top electrode 4, owing to the thickness and the mass load effect of
the top electrode 4 and the dielectric 20A, the cut-off frequency
"f.sub.0f2" of the dielectric portion piezoelectric film 23b is
further lower than the cut-off frequency "f.sub.0m" of the top
electrode portion piezoelectric film 23a.
[0182] Therefore, in the frequency band between the cut-off
frequency "f.sub.0m" of the top electrode portion piezoelectric
film 23a and the cut-off frequency "f.sub.0f2" of the dielectric
portion piezoelectric film 23b, a propagation band is formed within
the top electrode portion piezoelectric film 23a and the rejection
band is formed within the dielectric portion piezoelectric film
23b. The interface between the piezoelectric film 23a and the
piezoelectric film 23b of the dielectric portion thus serves as the
reflection surface 26 to confine energy of an acoustic wave,
thereby realizing energy trapped resonance.
[0183] Accordingly, adopting such a structure makes it possible to
realize energy trapping with the use of the piezoelectric film such
as lead titanate (PbTiO.sub.3), lead zirconate titanate (PZT) and
lithium tantalate (LiTaO.sub.3) which are large in
electromechanical coupling coefficient. Thus may be realized a film
bulk acoustic wave device of better device performance.
Embodiment 10
[0184] A description will be made with reference to FIGS. 22 to 24
on a film bulk acoustic wave device according to Embodiment 10 of
the present invention. FIG. 22 is a view showing the top of the
film bulk acoustic wave device according to Embodiment 10 of the
present invention. FIG. 23 is a view showing in section the film
bulk acoustic wave device according to Embodiment 10 of the present
invention. FIG. 24 is a view showing in a magnifying manner an
acoustic resonance portion illustrated in FIG. 23. This film bulk
acoustic wave device according to Embodiment 10 is an example of a
filter of two-electrode construction.
[0185] The dispersion characteristic of the film bulk acoustic wave
device in FIG. 24 are the same as the dispersion characteristic
shown in FIG. 21. The piezoelectric film 3 exhibits the
high-band-cut-off-type dispersion characteristic, and the cut-off
frequency "f.sub.0f2" of the dielectric portion piezoelectric film
23b is lower than the cut-off frequency "f.sub.0m" of the top
electrode portion piezoelectric film 23a.
[0186] Therefore, in the frequency band between the cut-off
frequency "f.sub.0m" of the top electrode portion piezoelectric
film 23a and the cut-off frequency "f.sub.0f2" of the dielectric
portion piezoelectric film 23b, a propagation band is formed within
the top electrode portion piezoelectric film 23a and the rejection
band is formed within the dielectric portion piezoelectric film
23b. The interface between the top electrode portion piezoelectric
film 23a and the dielectric portion piezoelectric film 23b thus
serves as the reflection surface 26 to confine energy of an
acoustic wave, thereby realizing energy trapped resonance.
[0187] In energy trapped resonance, resonance is produced in a
direction parallel to the surface of the piezoelectric film 3 to
produce, in the case of the filter with the top electrode 4 being
separated into two, symmetric mode resonance with which two top
electrodes 4 have the same potential and asymmetric mode resonance
with which the electrodes 4 have polarity opposite to each other.
When a resonant frequency for producing the symmetric mode
resonance and a resonant frequency for producing the asymmetric
mode resonance are close to each other, coupling takes place
between two top electrodes 4 to exhibit a filter
characteristic.
[0188] Resonance frequencies for two resonance modes depend on the
kind of the piezoelectric film 3 to be used, the space between the
top electrodes 4, the physical dimensions of the top electrodes 4,
the physical dimension of the dielectric 20A and materials for the
electrodes and for the dielectric. The filter operation when the
energy trapping is used is described in detail in Reference 3.
[0189] In the case of the filter, if energy trapped resonance is
not produced, insertion loss could be increased and much spurious
could take place at the outside of the bandwidth. However, the use
of the film bulk acoustic wave device as shown in FIG. 24 may
realize energy trapped resonance to allow to use the piezoelectric
film that is large in electromechanical coupling coefficient, and
may realize as well a film bulk acoustic wave device lesser in loss
and better in characteristic.
Embodiment 11
[0190] A description will be made with reference to FIG. 25 on a
film bulk acoustic wave device according to Embodiment 11 of the
present invention. FIG. 25 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 11 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 11 are the same as those in Embodiment 10.
[0191] Unlike the case of Embodiment 10 shown in FIG. 24, the
dielectric 20A is formed inside the outer end of the top electrode
4. Energy trapped resonance is produced on the inner side to the
reflection surface 26, and hence the film bulk acoustic wave device
shown in FIG. 25 performs in principle almost the same operation as
the film bulk acoustic wave device shown in FIG. 24 regardless of
energy trapped resonance, even with the presence of a region having
no dielectric 20A outside the reflection surface 26.
[0192] In order to form the dielectric 20A on the top electrode 4
as shown in FIGS. 20, 24 and 25, there are a method in which the
dielectric 20A is formed in advance on the entire top surface of
the top electrode 4 to remove thereafter unnecessary dielectric 20A
by a measure such as etching, and a method in which the top
electrode 4 is first formed to then restrict through the lift-off
method, masking or the like a region where the dielectric 20A is
formed. Any of these methods may be applied to the film bulk
acoustic wave device according to Embodiment 11.
[0193] The dielectric to be used may be of a silicon-based compound
such as silicon oxide (SiO, SiO.sub.2) and silicon nitride (SiN), a
titanium-based compound such as titanium dioxide, or other
dielectric. It will do as long as it is a dielectric that is
suitable for combining with materials for the respective parts used
in the film bulk acoustic wave device.
Embodiment 12
[0194] A description will be made with reference to FIGS. 26 to 29
on a film bulk acoustic wave device according to Embodiment 12 of
the present invention. FIG. 26 is a view showing the top of the
film bulk acoustic wave device according to Embodiment 12 of the
present invention. FIG. 27 is a view showing in section the film
bulk acoustic wave device according to Embodiment 12 of the present
invention.
[0195] In FIGS. 26 and 27, reference symbol 4 denotes a top
electrode, and reference symbol 27A denotes a dielectric.
[0196] FIG. 28 is a view showing in a magnifying manner an acoustic
resonance portion illustrated in FIG. 27. In FIG. 28, reference
symbol 28a denotes a piezoelectric film of the top electrode 4
portion; 28b, a piezoelectric film of the dielectric 27A portion;
and 28c, a non-electrode portion piezoelectric film.
[0197] FIG. 29 is a graph showing a dispersion characteristic of
the film bulk acoustic wave device illustrated in FIG. 28. In FIG.
29, reference symbol 29 denotes a dispersion characteristic of the
top electrode portion piezoelectric film 28a, and reference symbol
30 denotes a dispersion characteristic of the dielectric portion
piezoelectric film 28b.
[0198] The electrode thickness and the density of the top electrode
4 are respectively given as "d.sub.1" and ".rho..sub.1", and the
film thickness and the density of the dielectric 27A are
respectively given as "d.sub.2" and ".rho..sub.2". When the product
of the film thickness "d.sub.2" of the dielectric 27A and the
density ".rho..sub.2" thereof (d.sub.2.rho..sub.2) is larger than
the product of the electrode thickness "d.sub.1" of the top
electrode 4 and the density ".rho..sub.1" thereof
(d.sub.1.rho..sub.1), the cut-off frequency "f.sub.0f2" of the
dielectric portion piezoelectric film 28b is lower than the cut-off
frequency "f.sub.0m" of the top electrode portion piezoelectric
film 28a.
[0199] Therefore, energy trapping may be realized in the frequency
band between the cut-off frequency "f.sub.0m" of the top electrode
portion piezoelectric film 28a and the cut-off frequency
"f.sub.0f2" of the dielectric portion piezoelectric film 28b to
realize a film bulk acoustic wave device of good performance.
Embodiment 13
[0200] A description will be made with reference to FIGS. 30 and 31
on a film bulk acoustic wave device according to Embodiment 13 of
the present invention. FIG. 30 is a view showing in a magnifying
manner an acoustic resonance portion of the film bulk acoustic wave
device according to Embodiment 13 of the present invention. FIG. 31
is a graph showing a dispersion characteristic of the film bulk
acoustic wave device illustrated in FIG. 30. Other constructions of
the film bulk acoustic wave device according to Embodiment 13 are
the same as those in Embodiment 12.
[0201] In FIG. 30, reference symbol 31 denotes a piezoelectric film
of a region where the top electrode 4 and the dielectric 27A
overlap each other.
[0202] In FIG. 31, reference symbol 32 denotes a dispersion
characteristic of the piezoelectric film 31 of the region where the
top electrode 4 and the dielectric 27A overlap each other.
[0203] In the film bulk acoustic wave device shown in FIG. 30, the
reflection surface 26 of energy trapped resonance corresponds to
the surface below the end surface of the dielectric 27A overlapping
the top electrode 4. The cut-off frequency "f.sub.0f2" of the
piezoelectric film 31 of the region where the top electrode 4 and
the dielectric 27A overlap each other is a lower frequency than the
cut-off frequency "f.sub.0m" of the top electrode portion
piezoelectric film 28a, owing to both the electrode thickness
"d.sub.1" plus the mass load effect of the top electrode 4 and the
film thickness "d.sub.2" plus the mass load effect of the
dielectric 27A.
[0204] Therefore, energy trapping takes place in the frequency band
between the cut-off frequency "f.sub.0m" of the top electrode
portion piezoelectric film 28a and the cut-off frequency
"f.sub.0f2" of the piezoelectric film 31 of the region where the
top electrode 4 and the dielectric 27A overlap each other. That is,
filter operation by energy trapping may be realized.
[0205] In the film bulk acoustic wave device shown in FIG. 30, it
is sufficient that the film thickness "d.sub.2" and the density
".rho..sub.2" of the dielectric 27A can lower the cut-off frequency
"f.sub.0f2" of the piezoelectric film 31 of the region where the
top electrode 4 and the dielectric 27A overlap each other to the
degree required for filter operation, and the limitation put on the
electrode thickness "d.sub.1" and the density ".rho..sub.1" of the
top electrode 4 is less strict than in the case shown in FIG. 28.
This is because of the structure in which the dielectric 27A
overlaps the top electrode 4.
Embodiment 14
[0206] A description will be made with reference to FIG. 32 on a
film bulk acoustic wave device according to Embodiment 14 of the
present invention. FIG. 32 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 14 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 14 are the same as those in Embodiment 12.
[0207] The film bulk acoustic wave device shown in FIG. 32
corresponds to the top electrode 4 and the dielectric 27A in the
film bulk acoustic wave device shown in FIG. 28, which are
connected with each other. Accordingly, operation principle of
energy trapping is the same as in the case of the piezoelectric
film shown in FIG. 28.
[0208] The mass load by the dielectric 27A is thus required to be
larger than the mass load by the top electrode 4. In other words,
the product of the film thickness "d.sub.2" of the dielectric 27A
and the density ".rho..sub.2" thereof (d.sub.2.rho..sub.2) needs to
be larger than the product of the electrode thickness "d.sub.1" of
the top electrode 4 and the density ".rho..sub.1" thereof
(d.sub.1.rho..sub.1).
Embodiment 15
[0209] A description will be made with reference to FIG. 33 on a
film bulk acoustic wave device according to Embodiment 15 of the
present invention. FIG. 33 is a view showing in a magnifying manner
an acoustic resonance portion of the film bulk acoustic wave device
according to Embodiment 15 of the present invention. Other
constructions of the film bulk acoustic wave device according to
Embodiment 15 are the same as those in Embodiment 12.
[0210] In FIG. 33, reference symbol 33 denotes a second top
electrode, and reference symbol 34 denotes a second top electrode
portion piezoelectric film.
[0211] The second top electrode 33 is inserted to improve the
degree of freedom in designing the filter characteristic in the
film bulk acoustic wave device. The dispersion characteristic of
this second top electrode portion piezoelectric film 34 is close to
the dispersion characteristic of the first top electrode portion
piezoelectric film 28a. Namely, the cut-off frequency "f.sub.0f3"
of the second top electrode portion piezoelectric film 34 may be
considered as being close to the cut-off frequency "f.sub.0m" of
the first top electrode portion piezoelectric film 28a.
[0212] Accordingly, the dispersion characteristic of the
piezoelectric film shown in FIG. 33 may be deduced with the use of
FIG. 31. That is, the reflection surface 26 of energy trapping
corresponds to the surface below and on the inner side to the end
surface of the dielectric 27A overlapping the first top electrode
4.
[0213] The present invention is not limited to a piezoelectric film
such as lead titanate (PbTiO.sub.3), lead zirconate titanate (PZT),
lithium tantalate (LiTaO.sub.3) and a piezoelectric film with
Poisson ratio .sigma. of 1/3 or less, but may be applied to all the
piezoelectric bodies that exhibit the high-band-cut-off-type
dispersion characteristic.
[0214] The methods shown in FIGS. 28, 30 and 32 may be applied to
the resonator shown in FIGS. 18 and 19.
[0215] The methods shown in FIGS. 24, 25, 28, 30 and 32 may be
applied, as shown in FIG. 33, to the case in which the film bulk
acoustic wave device consists of more electrodes than in the
two-electrode construction.
[0216] Though the air bridge 21 is used to fetch an electric signal
from the top electrode 4 in FIGS. 18, 19 and 20, the way how to
fetch the electric signal from the top electrode 4 is not
necessarily limited to the air bridge 21 but may be a method of
constructing a line on the surface of the piezoelectric film 3 or
other arbitrary methods.
[0217] The via hole 5 shown is pierced from the back surface of the
semiconductor substrate 19, but is not necessarily limited thereto.
It makes no difference when the via hole is formed through a method
in which an aperture is formed below the acoustic resonance portion
6 through anisotropic etching or the like starting from the front
surface of the semiconductor substrate 19, a method in which a
layer-like aperture is formed by removing a previously prepared
thin film with etching or the like, or a method in which several
layers that are different in acoustical property with one another
are layered so that an acoustic wave propagating along the
thickness direction is reflected on that multi-layer structure.
Industrial Applicability
[0218] To begin with, a film bulk acoustic wave device in
accordance with the present invention is designed to have a second
electrode with an increased thickness placed on the top of a top
electrode or the like, and the structure is as follows.
[0219] As has been described, A film bulk acoustic wave device in
accordance with the present invention, comprising: a substrate; a
bottom electrode formed on one surface of the substrate; a
piezoelectric film formed on the bottom electrode; and a first top
electrode formed on the piezoelectric film, further comprises a
second top electrode having a larger mass load than the first top
electrode, which is formed at an outer side of the first top
electrode on the piezoelectric film when viewed from the center of
the first top electrode, in which the piezoelectric film has a
high-band-cut-off-type dispersion characteristic. The cut-off
frequency of a second top electrode portion piezoelectric film
having a large mass load can be lower than the cut-off frequency of
a first top electrode portion piezoelectric film, to thereby trap
the energy of the acoustic wave in a region of the first top
electrode portion side. This serves such an effect that good
performance may be feasible.
[0220] As has been described, a film bulk acoustic wave device is
further provided in accordance with the present invention, wherein
the first and second top electrodes are integrally formed, and the
second top electrode has a larger thickness than that of the first
top electrode. The cut-off frequency of a second top electrode
portion piezoelectric film having a larger thickness can be lower
than the cut-off frequency of a first top electrode portion
piezoelectric film, to thereby trap the energy of the acoustic wave
in a region of the first top electrode portion side. This serves
such an effect that good performance may be feasible.
[0221] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the product of the electrode thickness and the density of
the second top electrode is larger than the product of the
electrode thickness and the density of the first top electrode. The
cut-off frequency of a second top electrode portion piezoelectric
film with larger product of the electrode thickness and the density
can be lower than the cut-off frequency of a first top electrode
portion piezoelectric film, to thereby trap the energy of the
acoustic wave in a region of the first top electrode portion side.
This serves such an effect that good performance may be
feasible.
[0222] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the second top electrode is laid on a part of the first top
electrode. The cut-off frequency of the piezoelectric film of the
electrode where the second top electrode and the first top
electrode overlap each other can be lower than the cut-off
frequency of a first top electrode portion piezoelectric film, to
thereby trap the energy of the acoustic wave in a region of the
first top electrode portion side. This serves such an effect that
good performance may be feasible.
[0223] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the first top electrode and the second top electrode are
connected with each other. The cut-off frequency of a second top
electrode portion piezoelectric film having a large mass load can
be lower than the cut-off frequency of a first top electrode
portion piezoelectric film, to thereby trap the energy of the
acoustic wave in a region of the first top electrode portion side.
This serves such an effect that good performance may be
feasible.
[0224] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the second top electrode having a narrower width than that
of the first top electrode is laid on the first top electrode. The
cut-off frequency of the piezoelectric film of the electrode where
the second top electrode and the first top electrode overlap each
other can be lower than the cut-off frequency of a first top
electrode portion piezoelectric film, to thereby trap the energy of
the acoustic wave in a region of the first top electrode portion
side. This serves such an effect that good performance may be
feasible.
[0225] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the first and second top electrodes are divided into two.
This serves such an effect that performance of good filter
characteristics may be feasible.
[0226] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein a third top electrode is formed on the piezoelectric film
between the two divided first top electrodes. This serves such
effects that performance of good filter characteristics may be
feasible and that the degree of freedom in designing the filter
characteristics may be improved.
[0227] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the substrate is a semiconductor substrate or a dielectric
substrate. This serves such an effect that good performance may be
feasible.
[0228] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the piezoelectric film has a Poisson ratio lower than 0.34.
This serves such effects that a piezoelectric film having a large
electromechanical coupling coefficient can be used and that good
performance may be feasible.
[0229] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the piezoelectric film contains lead titanate (PbTiO.sub.3)
as a major component. This serves such effects that a piezoelectric
film having a large electromechanical coupling coefficient can be
used and that good performance may be feasible.
[0230] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein a dielectric layer is inserted between the substrate and
the bottom electrode. This serves such an effect that good
performance may be feasible.
[0231] Next, a film bulk acoustic wave device in accordance with
the present invention is designed to have a dielectric placed on
the top of a top electrode or the like, and the structure is as
follows.
[0232] A film bulk acoustic wave device in accordance with the
present invention, comprising: a substrate; a bottom electrode
formed on one surface of the substrate; a piezoelectric film formed
on the bottom electrode; and a top electrode formed on the
piezoelectric film, further comprises a dielectric formed at an
outer side of the top electrode on the piezoelectric film when
viewed from the center of the top electrode, in which the
piezoelectric film has a high-band-cut-off-type dispersion
characteristic. The cut-off frequency of a dielectric portion
piezoelectric film can be lower than the cut-off frequency of a top
electrode portion piezoelectric film, to thereby trap the energy of
the acoustic wave in a region of the top electrode portion side.
This serves such an effect that good performance may be
feasible.
[0233] As has been described, a film bulk acoustic wave device is
further provided in accordance with the present invention, wherein
the dielectric is formed on a part of the top electrode, to thereby
trap the energy of the acoustic wave in a region of the top
electrode portion side. This serves such an effect that good
performance may be feasible.
[0234] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the product of the film thickness and the density of the
dielectric is larger than the product of the electrode thickness
and the density of the top electrode. The cut-off frequency of the
dielectric with larger product of the electrode thickness and the
density can be lower than the cut-off frequency of a top electrode
portion piezoelectric film, to thereby trap the energy of the
acoustic wave in a region of the top electrode portion side. This
serves such an effect that good performance may be feasible.
[0235] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the dielectric is laid on a part of the top electrode. The
cut-off frequency of the piezoelectric film of the electrode where
the dielectric and the top electrode overlap each other can be
lower than the cut-off frequency of a top electrode portion
piezoelectric film, to thereby trap the energy of the acoustic wave
in a region of the top electrode portion side. This serves such an
effect that good performance may be feasible.
[0236] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the top electrode and the dielectric are connected with
each other. The cut-off frequency of a dielectric portion
piezoelectric film can be lower than the cut-off frequency of a top
electrode portion piezoelectric film, to thereby trap the energy of
the acoustic wave in a region of the top electrode portion side.
This serves such an effect that good performance may be
feasible.
[0237] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the dielectric having a narrower width than that of the top
electrode is laid on the top electrode. The cut-off frequency of
the piezoelectric film of the electrode where the dielectric and
the top electrode overlap each other can be lower than the cut-off
frequency of a top electrode portion piezoelectric film, to thereby
trap the energy of the acoustic wave in a region of the top
electrode portion side. This serves such an effect that good
performance may be feasible.
[0238] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the top electrode and the dielectric are divided into two.
This serves such an effect that performance of good filter
characteristics may be feasible.
[0239] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein a second top electrode is formed on the piezoelectric film
between the two divided first top electrodes. This serves such
effects that performance of good filter characteristics may be
feasible and that the degree of freedom in designing the filter
characteristics may be improved.
[0240] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the substrate is a semiconductor substrate or a dielectric
substrate. This serves such an effect that good performance may be
feasible.
[0241] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the piezoelectric film has a Poisson ratio lower than 0.34.
This serves such effects that a piezoelectric film having a large
electromechanical coupling coefficient can be used and that good
performance may be feasible.
[0242] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein the piezoelectric film contains lead titanate (PbTiO.sub.3)
as a major component. This serves such effects that a piezoelectric
film having a large electromechanical coupling coefficient can be
used and that good performance may be feasible.
[0243] As has been described, a film bulk acoustic wave device is
still further provided in accordance with the present invention,
wherein a dielectric layer is inserted between the substrate and
the bottom electrode. This serves such an effect that good
performance may be feasible.
* * * * *