U.S. patent number RE41,813 [Application Number 12/276,318] was granted by the patent office on 2010-10-12 for piezoelectric thin-film resonator and filter using the same.
This patent grant is currently assigned to Fujitsu Media Devices Limited, Taiyo Yuden Co., Ltd.. Invention is credited to Masafumi Iwaki, Tsutomu Miyashita, Tokihiro Nishihara, Takeshi Sakashita, Tsuyoshi Yokoyama.
United States Patent |
RE41,813 |
Nishihara , et al. |
October 12, 2010 |
**Please see images for:
( Certificate of Correction ) ** |
Piezoelectric thin-film resonator and filter using the same
Abstract
A piezoelectric thin-film resonator includes a substrate, a
lower electrode arranged on the substrate, a piezoelectric film
arranged on the lower electrode, and an upper electrode arranged on
the piezoelectric film. A region in which the upper electrode
overlaps with the lower electrode through the piezoelectric film
has an elliptical shape, and a condition such that 1<a/b<1.9
is satisfied where a is a main axis of the elliptical shape, and b
is a sub axis thereof.
Inventors: |
Nishihara; Tokihiro (Kanagawa,
JP), Yokoyama; Tsuyoshi (Kanagawa, JP),
Sakashita; Takeshi (Kanagawa, JP), Iwaki;
Masafumi (Kanagawa, JP), Miyashita; Tsutomu
(Kanagawa, JP) |
Assignee: |
Taiyo Yuden Co., Ltd. (Tokyo,
JP)
Fujitsu Media Devices Limited (Yokohama, JP)
|
Family
ID: |
34463400 |
Appl.
No.: |
12/276,318 |
Filed: |
November 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
10966035 |
Oct 18, 2004 |
07211931 |
May 1, 2007 |
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Current U.S.
Class: |
310/324;
310/366 |
Current CPC
Class: |
H03H
9/174 (20130101); H03H 9/132 (20130101); H03H
9/02133 (20130101); H03H 9/564 (20130101); H03H
9/568 (20130101) |
Current International
Class: |
H01L
41/08 (20060101) |
Field of
Search: |
;310/320,366,365,324,311,363 ;333/187,189 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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6-295181 |
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Oct 1994 |
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JP |
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8-148968 |
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Jun 1996 |
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JP |
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2003-133892 |
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May 2003 |
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JP |
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2003-204239 |
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Jul 2003 |
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JP |
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2003-289235 |
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Oct 2003 |
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JP |
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2004-200843 |
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Jul 2004 |
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JP |
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2005-33379 |
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Feb 2005 |
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JP |
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2005-109678 |
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Apr 2005 |
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JP |
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2005-236518 |
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Sep 2005 |
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JP |
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2006-157259 |
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Jun 2006 |
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JP |
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WO 03/041273 |
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May 2003 |
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WO |
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Primary Examiner: Benson; Walter
Assistant Examiner: Addison; Karen B
Attorney, Agent or Firm: Arent Fox LLP
Claims
What is claimed is:
1. A piezoelectric thin-film resonator comprising: a substrate; a
lower electrode arranged on the substrate; a piezoelectric film
arranged on the lower electrode; and an upper electrode arranged on
the piezoelectric film, a cavity being provided in the substrate
and being located below the lower electrode in a region in which
the upper electrode overlaps with the lower electrode through the
piezoelectric film, the region and a cross-section of the cavity
having an elliptical shape, 1<a/b<1.9 is satisfied, where a
is a main axis of the elliptical shape, and b is a sub axis
thereof.
2. The piezoelectric thin-film resonator as claimed in claim 1,
wherein the cavity and the lower electrode has a relationship of
size that satisfies 0.9<b'/b<1.5 where b' is a sub axis of
the cavity.
3. The piezoelectric thin-film resonator as claimed in claim 1,
wherein one of the main axis and the sub axis is substantially
parallel to a current direction.
4. The piezoelectric thin-film resonator as claimed in claim 1,
wherein the upper electrode and the lower electrode are
substantially symmetric about an axis of the elliptical shape
perpendicular to a current direction within a range equal to half
the length of the sub axis in the current direction.
5. A piezoelectric thin-film resonator comprising: a substrate; a
lower electrode arranged on the substrate; a piezoelectric film
arranged on the lower electrode; and an upper electrode arranged on
the piezoelectric film, a cavity provided in the substrate and
located below the lower electrode in a region in which the upper
electrode overlaps with the lower electrode through the
piezoelectric film, a membrane that includes the upper electrode
and the lower electrode being formed above the cavity and being
curved outwards, the membrane having a maximum height that is
measured from a surface of the substrate and is at least 1.5 times
the thickness of the membrane.
6. A filter comprising a plurality of piezoelectric thin-film
resonators, at least one of the thin-film resonators comprising: a
substrate; a lower electrode arranged on the substrate; a
piezoelectric film arranged on the lower electrode; and an upper
electrode arranged on the piezoelectric film, a cavity being
provided in the substrate and being located below the lower
electrode in a region in which the upper electrode overlaps with
the lower electrode through the piezoelectric film, the region and
a cross-section of the cavity having an elliptical shape,
1<a/b<1.9 being satisfied where a is a main axis of the
elliptical shape and b is a sub axis thereof.
7. A filter comprising a plurality of piezoelectric thin-film
resonators, at least one of the thin-film resonators comprising: a
substrate; a lower electrode arranged on the substrate; a
piezoelectric film arranged on the lower electrode; and an upper
electrode arranged on the piezoelectric film, a cavity provided in
the substrate and located below the lower electrode in a region in
which the upper electrode overlaps with the lower electrode through
the piezoelectric film, a membrane that includes the upper
electrode and the lower electrode being formed above the cavity and
is curved outwards, the membrane having a maximum height that is
measured from a surface of the substrate and is at least 1.5 times
the thickness of the membrane.
.Iadd.8. A piezoelectric thin-film resonator comprising: a
substrate; a lower electrode arranged on the substrate; a
piezoelectric film arranged on the lower electrode; and an upper
electrode arranged on the piezoelectric film, a cavity being
located below the lower electrode in a region in which the upper
electrode overlaps with the lower electrode through the
piezoelectric film, the region and a cross-section of the cavity
having an elliptical shape, 1<a/b<1.9 is satisfied, where a
is a main axis of the elliptical shape, and b is a sub axis
thereof..Iaddend.
.Iadd.9. The piezoelectric thin-film resonator as claimed in claim
8, wherein the cavity and the lower electrode has a relationship of
size that satisfies 0.9<b'/b<1.5 where b' is a sub axis of
the cavity..Iaddend.
.Iadd.10. The piezoelectric thin-film resonator as claimed in claim
8, wherein one of the main axis and the sub axis is substantially
parallel to a current direction..Iaddend.
.Iadd.11. The piezoelectric thin-film resonator as claimed in claim
8, wherein the upper electrode and the lower electrode are
substantially symmetric about an axis of the elliptical shape
perpendicular to a current direction within a range equal to half
the length of the sub axis in the current direction..Iaddend.
.Iadd.12. A piezoelectric thin-film resonator comprising: a
substrate; a lower electrode arranged on the substrate; a
piezoelectric film arranged on the lower electrode; and an upper
electrode arranged on the piezoelectric film, a cavity located
below the lower electrode in a region in which the upper electrode
overlaps with the lower electrode through the piezoelectric film, a
membrane that includes the upper electrode and the lower electrode
being formed above the cavity and being curved outwards, the
membrane having a maximum height that is measured from a surface of
the substrate and is at least 1.5 times the thickness of the
membrane..Iaddend.
.Iadd.13. A filter comprising a plurality of piezoelectric
thin-film resonators, at least one of the thin-film resonators
comprising: a substrate; a lower electrode arranged on the
substrate; a piezoelectric film arranged on the lower electrode;
and an upper electrode arranged on the piezoelectric film, a cavity
being located below the lower electrode in a region in which the
upper electrode overlaps with the lower electrode through the
piezoelectric film, the region and a cross-section of the cavity
having an elliptical shape, 1<a/b<1.9 being satisfied where a
is a main axis of the elliptical shape and b is a sub axis
thereof..Iaddend.
.Iadd.14. A filter comprising a plurality of piezoelectric
thin-film resonators, at least one of the thin-film resonators
comprising: a substrate; a lower electrode arranged on the
substrate; a piezoelectric film arranged on the lower electrode;
and an upper electrode arranged on the piezoelectric film, a cavity
located below the lower electrode in a region in which the upper
electrode overlaps with the lower electrode through the
piezoelectric film, a membrane that includes the upper electrode
and the lower electrode being formed above the cavity and is curved
outwards, the membrane having a maximum height that is measured
from a surface of the substrate and is at least 1.5 times the
thickness of the membrane..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to piezoelectric thin-film
resonator and a filter using the same.
2. Description of the Related Art
Wireless devices as represented by mobile telephones have spread
rapidly, and there has been an increasing demand for a downsized
and lightweight resonator and a filter equipped with the same. A
dielectric substance and a surface acoustic wave have been used
extensively so far; however, the piezoelectric thin-film resonator
and the filter equipped with the same have excellent high frequency
characteristics, can be downsized, and can be incorporated into a
monolithic circuit. Therefore, the piezoelectric thin-film
resonator and the filter using the same are attracting
attention.
The piezoelectric thin-film resonator may be categorized into FBAR
(Film Bulk Acoustic Resonator) type and SMR (Solidly Mounted
Resonator) type. The FBAR type includes main components on a
substrate from the top, namely, an upper electrode, a piezoelectric
film, and a lower electrode. There is a cavity below the lower
electrode that is overlapped with the upper electrode through the
piezoelectric film. The cavity is defined by wet etching a
sacrifice layer on the surface of the silicon substrate, wet or dry
etching from the backside of the silicon substrate, or the like. In
the present description, a membrane is defined as a film-laminated
structure that is located above the cavity and a main component
composed of the lower electrode, piezoelectric film and the upper
electrode. The SMR type employs an acoustic reflector instead of
the cavity, the acoustic reflector being composed of films having
high and low acoustic impedances alternately laminated with a film
thickness of .lamda./4 where .lamda. is a wavelength of an elastic
wave. When a high-frequency electric signal is applied across the
upper electrode and the lower electrode, an elastic wave is excited
inside the piezoelectric film sandwiched between the upper
electrode and the lower electrode, due to the inverse piezoelectric
effect. Meanwhile, a distortion generated by the elastic wave is
converted into an electric signal due to piezoelectric effect. The
elastic wave is totally reflected by the surfaces of the upper and
lower electrodes that respectively interface with air, and it is
thus converted into a thickness-extensional wave having a main
displacement in the thickness direction. In the above-mentioned
structure, a resonance occurs at frequencies at which the total
thickness H of the membrane is equal to integer multiples (n times)
of half the wavelength of the elastic wave. When the propagation
velocity, which depends on materials, is denoted as V, the
resonance frequency F is described as F=nV/2H. The resonator and
the filter having desired frequency characteristics can be produced
by utilizing the resonance and controlling the resonance frequency
with the film thickness.
Materials for the electrodes may, for example, be aluminum (Al),
copper (Cu), molybdic (Mo), tungsten (W), tantalum (Ta), platinum
(Pt), rhodium (Ru), or iridium (Ir). Materials for piezoelectric
films may, for example, be aluminum nitride (AlN), zinc oxide
(ZnO), lead zirconate titanate (PZT), or lead titanate
(PbTiO.sub.3). The substrate may be made of silicon, glass, or the
like.
However, in addition to the thickness-extensional wave, the
above-mentioned piezoelectric thin-film resonator has undesired
waves of the lateral mode that are propagated in parallel with the
electrode surface, and are reflected by the interfaces or an edge
of the cavity. This adversely generates an unnecessary spurious
component in the impedance characteristics of the resonator, or a
ripple in the passband of the filter. This causes a problem in an
application. In order to suppress such adverse affects caused by
the lateral mode wave, U.S. Pat. No. 6,150,703 (hereinafter
referred to as Document 1) and U.S. Pat. No. 6,215,375 (hereinafter
referred to as Document 2) disclose piezoelectric thin-film
resonators having electrodes including non-square and irregular
polygons in which any two sides are not parallel. In the proposed
piezoelectric thin-film resonators, the lateral mode waves
reflected by any points are reflected and travel in different
directions from the previous directions. Thus, the lateral mode
waves do not resonate, so that the above-mentioned problem can be
solved effectively. In addition, in order to solve a similar
problem, Japanese Patent Application Publication No. 2003-133892
(hereinafter referred to as Document 3) discloses a piezoelectric
thin-film resonator having an upper electrode of elliptical shape.
The upper electrode satisfies 1.9<a/b<5.0, where a is the
main axis of the elliptical shape, and b is the sub axis
thereof.
The structures and configurations of Documents 1, 2 and 3 are
certainly effective in solving the above-mentioned problems.
However, the proposed structures and configurations degrade the
strength of the membrane or the productivity of the cavity to the
contrary. This will be described below. The thickness of the
membrane, which depends on the sound speed of the material, is as
very thin as approximately 0.5 to 3 .mu.m in a wireless system
having a frequency range of 900 MHz to 5 GHz. An unexpected
external force easily damages the membrane, and it is thus
important to consider the technique to improve the strength.
One solution is to reduce the damage of the membrane caused by
internal stress by reducing the internal stress of each film at the
time of forming the film. However, the inventors' study shows that
piezoelectricity is improved when compression stress is exerted on
the piezoelectric film, and a resonance characteristic having a
large electromechanical coupling coefficient (K.sup.2) is
obtainable. From this viewpoint, the membrane having compression
stress is very effective if a technique to achieve a desired
strength of the membrane is available. One of the effective methods
is to design the membrane so that stress is evenly applied to the
membrane or the membrane is not damaged easily by the same internal
stress. Unfortunately, any one of Documents 1, 2, and 3 has a
structurally unbalanced symmetry, and the force applied to the
membrane is not equal. Thus, the membrane is easily distorted and
damaged. This results in a serious problem that resonance
characteristics and filter characteristics show large
irregularity.
Preferably, the cavity has the same shape as that of the region in
which the upper electrode overlaps with the lower electrode, and
has a similar size to that of the region. If the size of the cavity
is much bigger than that of the overlapping region, the membrane
will be easily damaged. Thus, it is not recommended. In addition,
the productivity of the cavities disclosed in Documents 1 through 3
is not good. The cavities described in Documents 1 and 2 have
corners. The cavity described in Document 3 has an elliptical shape
with a ratio a/b as large as 1.9<a/b<5.0 where the length of
the main axis is denoted as a and that of the sub axis is denoted
as b. That is, the desired shape of the cavity is not obtainable
because the etching velocity is low at the corners of the cavity.
The lower electrode disclosed in Document 3 has a considerably
large size, as compared to that of the upper electrode. This
results in stray capacitance between the overlapping extensions of
the upper electrode and the lower electrode, and degrades the
electromechanical coupling coefficient (K.sup.2).
SUMMARY OF THE INVENTION
The present invention has been made in view of the above
circumstances and provides a piezoelectric thin-film resonator and
a filter using the same.
More specifically, the present invention provides a piezoelectric
thin-film resonator and a filter equipped with the same that show
little irregularity in characteristics, by employing a structure
that makes it possible to suppress the adverse affects caused by
the lateral mode waves and to achieve a sufficient strength of the
membrane and excellent productivity of the cavity.
Another object of the present invention is to provide a
piezoelectric thin-film resonator and a filter equipped with the
same having a large electromechanical coupling coefficient
(K.sup.2) by the use of a film having a desired compression
stress.
According to an aspect of the present invention, there is provided
a piezoelectric thin-film resonator including a substrate, a lower
electrode arranged on the substrate, a piezoelectric film arranged
on the lower electrode, and an upper electrode arranged on the
piezoelectric film, in which a region in which the upper electrode
overlaps with the lower electrode through the piezoelectric film
has an elliptical shape, and 1<a/b<1.9 is satisfied, where a
is a main axis of the elliptical shape, and b is a sub axis
thereof.
A cavity may be formed in the substrate and located below the
region having the elliptical shape.
According to another aspect of the present invention, there is
provided a piezoelectric thin-film resonator comprising a
substrate, a lower electrode arranged on the substrate, a
piezoelectric film arranged on the lower electrode, and an upper
electrode arranged on the piezoelectric film. A cavity is provided
in the substrate and is located under the lower electrode in a
region in which the upper electrode overlaps with the lower
electrode through the piezoelectric film. A membrane that includes
the upper electrode and the lower electrode is formed above the
cavity and is curved outwards. The membrane has a maximum height
that is measured from a surface of the substrate and is at least
1.5 times the thickness of the membrane.
According to a further aspect of the present invention, there is
provided a filter with any of the above-mentioned piezoelectric
thin-film resonators.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will be described in
detail with reference to the following figures, wherein:
FIG. 1A shows a plane view of a piezoelectric thin-film resonator
in accordance with the present invention;
FIG. 1B shows a cross-section view taken along a line
I.sub.B--I.sub.B shown in FIG. 1A;
FIGS. 2A and 2B show elliptical shapes having a different ratio of
an axis a to an axis b;
FIG. 3A shows a plane view of a filter;
FIG. 3B shows a cross-section view taken along a line
III.sub.B--III.sub.B shown in FIG. 3A;
FIG. 4 shows a circuit diagram of the filter shown in FIGS. 3A and
3B;
FIG. 5 shows a graph illustrating band characteristics of the
filter with four axis ratios of an elliptical shape;
FIG. 6 shows b'/b dependency of a resonant resistance in accordance
with a second embodiment of the present invention;
FIGS. 7A and 7B show plane views of a piezoelectric thin-film
resonator in accordance with a third embodiment of the present
invention;
FIG. 7C shows a comparative example of FIGS. 7A and 7B;
FIG. 8A shows a plane view of a piezoelectric thin-film resonator
in accordance with a fourth embodiment of the present
invention;
FIG. 8B shows a comparative example of FIG. 8A;
FIG. 9 shows a relationship between internal stress and
electromechanical coupling coefficient;
FIG. 10 shows a cross-section view of a membrane of a piezoelectric
thin-film resonator after a cavity is formed; and
FIG. 11 shows a graph illustrating a relationship between
parameters U and M shown in FIG. 10.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A description will now be given, with reference to the accompanying
drawings, of embodiments of the present invention.
First Embodiment
FIG. 1A shows a plain view of a piezoelectric thin-film resonator
in accordance with a first embodiment of the present invention.
FIG. 1B shows a cross-section view taken along a line
I.sub.B--I.sub.B shown in FIG. 1A. The piezoelectric thin-film
resonator shown in FIGS. 1A and 1B includes a substrate 10, a lower
electrode 11 arranged on the substrate 10, a piezoelectric film 12
arranged on the lower electrode 11, and an upper electrode 13
arranged on the piezoelectric film 12. The electrode 10 is made of,
for example, silicon (Si). The lower electrode 11 is made of a
conductive material having a double layer structure of, for
example, ruthenium (Ru) and chromium (Cr). The layer of ruthenium
is arranged on a main surface of the substrate 10. The
piezoelectric film 12 is made of a piezoelectric material, for
example, aluminum nitride (AlN). The upper electrode 13 is made of
a conductive material having a single layer structure of ruthenium
(Ru), for example. For example, the piezoelectric thin-film
resonator that has a resonance frequency of 5.2 GHz may be
configured so that the lower electrode 11 is a Ru (100 nm)/Cu (50
nm) film, and the piezoelectric film of AlN is 400 nm thick, the
upper electrode 13 of Ru being 100 nm thick. The piezoelectric film
12 has an opening, via which the lower electrode 11 is partially
exposed. The exposed portion of the lower electrode 11 is used as a
pad 16. Referring to FIG. 1B, a cavity 15 is formed in the
substrate 10 below a region (resonator) where the upper electrode
13 and the lower electrode 11 overlap through the piezoelectric
film 12. In accordance with the first embodiment of the present
invention, the cavity 15 has substantially perpendicular side
walls, which may be formed by dry-etching the silicon substrate 10
from the backside thereof with fluorine gas. The cross section of
the cavity 15 has an elliptical shape in a direction parallel to
the main surface of the substrate 10, on which the lower electrode
11 and the like are disposed.
The technical merits of the present invention may be obtained by
materials other than the above-mentioned materials of the substrate
10, the upper and lower electrodes 11 and 13, and the piezoelectric
film 12. For example, the materials disclosed in Documents 1, 2,
and 3 may be used. In addition, the cavity 15 as shown in FIG. 1B
penetrates the substrate 10; however, the cavity 15 may be formed
only on the surface of the substrate 10, and may be formed with the
use of a sacrifice layer. Further, it is to be noted that the
above-mentioned membrane is composed of only main component
elements of the piezoelectric thin-film resonator. In practice, the
membrane may have an additional layer or film. For instance, a
dielectric layer may be added below the lower electrode 11 to
reinforce it. Such a dielectric layer may serve as an etching
stopper. Another dielectric layer may be provided on the surface as
a passivation film. Bumps or a conductive layer for wire bonding
may be provided so as to underlie the pads of the electrodes.
As shown in FIG. 1A, a region 14 where the upper electrode 13
overlaps with the lower electrode 11 has an elliptical shape. The
region 14 forms a membrane 14 (resonator). In accordance with the
first embodiment, the elliptical shape of the membrane 14 satisfies
1<a/b<1.9, where a is the main axis of the elliptical-shaped
membrane 14 and b is the sub axis thereof. The above condition is
based on the following study by the inventors.
The inventors found out a problem caused during the process of
forming the cavity 15 when the length ratio of a/b is large and the
elliptical shape is greatly curved. FIGS. 2A and 2B show exemplary
elliptical shapes of the region 14 where the upper electrode 13
overlaps with the lower electrode 11. FIG. 2A shows an elliptical
shape where a/b=1.2 in which a=75.9 .mu.m, and b=63.3 .mu.m. FIG.
2B shows an elliptical shape where a/b=4.0 in which a=138.7 .mu.m
and b=34.7 .mu.m. For the greatly curved elliptical shape of
a/b=4.0, the etching rate considerably decreases in tapered-off
portions indicated by X at the time of dry-etching silicon. Thus,
etching residue is at the tapered-off portions, so that the desired
shape of the cavity cannot be obtained. Alternatively, if silicon
is over-etched to avoid etching residue, a larger amount of
residual will adhere to an area where etching has already been made
or a sidewall of the cavity. This residue will lead to degradation
and irregularity of characteristics. Such affects will occur to the
vicinity of the apexes of a polygon that does not have a shape of
square as disclosed in Documents 1 and 2. This problem can be
solved by forming the elliptical shape having the ratio a/b as
small as possible. The above-mentioned problems can be
substantially ignored by forming the cavity 15 so that
1<a/b<1.9 is satisfied.
In the above-mentioned range of 1<a/b<1.9, it is essential
that a ripple caused by a lateral mode is suppressed to the level
that does not pose a problem. Then, four different filters equipped
with piezoelectric thin-film resonators are made to evaluate the
ripple in a passband. The four filters have the ratios of 1.0 (a
circle), 1.2, 1.9, and 4.0 in the region in which the upper
electrode 13 overlaps the lower electrode 11 through the
piezoelectric film 12. Table 1 shows sizes of the elliptical shape
in series-arm and parallel-arm resonators.
TABLE-US-00001 TABLE 1 Series-arm Parallel-arm resonator resonator
a/b a b a b 1.0 69.4 69.4 49.1 49.1 1.2 75.9 63.3 53.7 44.8 1.9
95.6 50.3 67.6 35.6 4.0 138.7 34.7 98.1 24.5 Unit: .mu.m
FIGS. 3A, 3B, and 4 show the structure of the above-mentioned four
filters. More particularly, FIG. 3A is a plane view of the filter,
and FIG. 3B is a cross-section view taken along a line
III.sub.B--III.sub.B shown in FIG. 3A. FIG. 4 is a circuit diagram
of the filter shown in FIGS. 3A and 3B. In FIGS. 3A and 3B, the
same components and configurations as those of FIGS. 1A and 1B have
the same reference numerals. There is illustrated a ladder-type
filter that includes series-arm resonators S1, S2, S3, and S4 and
parallel-arm resonators P1, P2, and P3 on the substrate 10. In the
filter, four piezoelectric thin-film resonators S1, S2, S3, and S4
are respectively disposed in series arms, and three piezoelectric
thin-film resonators P1, P2, and P3 are respectively disposed in
parallel arms. The fundamental structure of the filter is the same
as that of the aforementioned piezoelectric thin-film resonator. In
practice, an insulating film such as an SiO.sub.2 film
(approximately 90 nm) may be provided on the upper electrodes of
the parallel resonators P1, P2, and P3 in order to decrease the
resonance frequencies of the parallel-arm resonators and to thus
obtain the desirable bandpass filter characteristics. All the
resonators S1 through S4 and P1 through P3 have the same
configurations as those shown in FIGS. 1A and 1B. The substrate 10
is provided commonly to all the resonators S1 through S4 and P1
through P3. Similarly, the piezoelectric film 12 is also commonly
provided to all the resonators S1 through S4 and P1 through P3.
Some of the lower electrode 11 and the upper electrode 13 are
shared by adjacent resonators. For example, the series-arm
resonators S1 and S2 share the same upper electrode 13. In each of
the resonators S1 through S4 and P1 through P3, the cavity 15 is
provided in the substrate 10 below the region where the upper
electrode 13 overlaps with the lower electrode 11 through the
dielectric film 12. The series-arm resonators in the Table 1 are
the series-arm resonator S1 through S4, and the parallel-arm
resonators are P1 through P3. The lower electrode 11 is partially
exposed through the cavities 15, and the exposed portions serve as
pads 17.
FIG. 5 shows band characteristics of the filters, which are
described with parameter S21. The horizontal axis of the graph
denotes frequency (GHz) and the vertical axis denotes attenuation
(dB). Curves C1, C2, C3 and C4 are the band characteristics
observed when the ratio a/b is equal to 1.0, 1.2, 1.9 and 4.0,
respectively. A general filter specification for wireless devices
requires the ripple in the passband to be suppressed to 0.3 dB or
less. All the filters except a/b=1.0 meet the requirement, and it
can be said that the ripple does not have a high dependency on the
ratio a/b. In the case where a/b=4.0, the cavities are not formed
properly in the parallel-arm resonators, and the loss in the
low-frequency side of the passband is degraded. Therefore, by
employing the elliptical shape that satisfies 1<a/b<1.9, the
ripple caused by the lateral mode can be suppressed to a
practically acceptable level to produce the piezoelectric thin-film
resonator and the filter having excellent productivity of the
cavity.
Second Embodiment
A description will now be given of a second embodiment of the
present invention. The second embodiment has a specific
relationship between the shape of elliptical shape and the size of
the membrane 14 used in the first embodiment. The inventors
evaluated any influence on characteristics, when altering the ratio
of b'/b where b is the length of the sub axis of the elliptical
shape in the membrane 14 where the upper electrode 13 overlaps with
the lower electrode 11, and b' is the length of the sub axis of the
cavity 15, as shown in FIGS. 1A and 1B. In the second embodiment,
the size b is fixed and only the size b' is altered. The region
where the upper electrode 13 overlaps with the lower electrode 11
through the dielectric film 12 has an elliptical shape such that
a=60.2 .mu.m and b=50.2 .mu.m (a/b=1.2). The cap 15 has an
elliptical shape that meets a/b=1.2. FIG. 6 shows a b'/b dependency
of the resonant resistance. When b'/b is too small, the resonance
characteristic is degraded because resonant vibration energy
dissipates into the substrate 10. When b'/b is too large, the lower
electrode 11 or the upper electrode 13 may be cracked, and the
resonance characteristic is degraded. The general specification
requires a tolerable range of the resonant resistance equal to four
.OMEGA. or less. The requirement is met when the ratio b'/b falls
in the range of 0.9<b'/b<1.5.
Third Embodiment
A description will now be given of a third embodiment of the
present invention. The third embodiment has a specific relationship
between the direction of the current flowing through a
piezoelectric thin-film resonator and the axis direction of the
elliptical shape in the membrane 14 where the upper electrode 13
overlaps with the lower electrode 11 through the dielectric film
12. The inventors studied the relationship for three piezoelectric
thin-film resonators shown in FIGS. 7A, 7B, and 7C. FIG. 7A shows a
case where the sub axis is parallel to the current direction
(hereinafter referred to type A). FIG. 7B shows a case where the
main axis is parallel to the current direction (hereinafter
referred to type B). FIG. 7C shows a case where the main and sub
axes are slanted by 45 degrees to the current direction
(hereinafter referred to type C). The types A and B have
substantially symmetric shapes in which the upper electrode 13 and
the lower electrode 11 are symmetric about an axis P that is
perpendicular to the current direction, within at least a range
equal to half the sub axis, namely b/2. In contrast, the type C
does not have symmetry within the range. The elliptical shape of
the overlapping region has a size such that a=86.1 .mu.m and b=61.5
.mu.m (a/b=1.4). Table 2 shows the lowest insertion losses in the
above-mentioned three piezoelectric thin-film resonators and
irregularity thereof.
TABLE-US-00002 TABLE 2 Lowest insertion Irregularity Type loss
(3.sigma.) A 0.18 dB 3.2% B 0.22 dB 3.7% C 0.35 dB 7.6%
Type C is larger in both lowest insertion loss and irregularity
than those of the types A and B. In terms of warping of the
membrane, the types A and B are finely curved in the shape of a
dome, while the type C is irregularly distorted like a potato chip.
The types A and B are highly symmetric, and get finely curved when
compression stress is applied in parallel with the current
direction through the membrane. However, the type C is not finely
symmetric with respect to the current direction in which stress is
applied, and is irregularly curved. This results in affects on the
above-mentioned characteristics.
Thus, as shown in FIGS. 7A and 7B, it is preferable that the main
or sub axis of the elliptical shape is substantially parallel to
the current direction. In addition, it is preferable that the upper
electrode 13 and the lower electrode 11 have portions that overlap
each other through the dielectric film 12 to form the elliptically
shaped region and are symmetric about the axis P perpendicular to
the current direction of the elliptical shape within the range
equal to at least half the length of the sub axis b.
Referring back to FIG. 3A, on the ladder-type filter, all the
piezoelectric thin-film resonators S1 through S4 and P1 through P3
are so disposed that the sub axis of the elliptical shape defined
by the overlapping portions of the upper electrode 13 and the lower
electrode 11 is substantially parallel to the current direction. It
is also possible to arrange the piezoelectric thin-film resonators
S1 through S4 and P1 through P3 of the ladder-type filter shown in
FIG. 3A so that the main axis of the elliptical shape in each
resonator is substantially parallel to the current direction, as
shown in FIG. 7B. It is still possible to modify the ladder-type
filter shown in FIG. 3A so that it has both the type A in FIG. 7A
and the type B in FIG. 7B.
Fourth Embodiment
A fourth embodiment of the present invention is focused on the
structure of the extensions of the upper electrode 13 and the lower
electrode 11 in which the extensions extend outwardly from the
elliptical shape in which the upper electrode 13 overlaps with the
lower electrode 11 through the dielectric film 12. FIG. 8A shows a
plane view of a piezoelectric thin-film resonator in accordance
with the fourth embodiment. A reference numeral 21 denotes an edge
of the extension of the upper electrode 13. A reference numeral 22
is an extension of the lower electrode 11. In FIG. 8A, the edges
are drawn in thick solid lines in order to emphasize the edges. The
extension 21 of the upper electrode 13 has a shape that the width
becomes larger outwards from the center of the elliptical shape.
The length of the main axis of the elliptical shape is a;
therefore, the width of the extension 21 is larger than a. The
extension 21 is formed integrally with a pad 23. The lower
electrode 11 is exposed through an opening provided in the
piezoelectric film 12. The exposed portion is used as the pad 24.
The extension 22 of the lower electrode 11, similarly to that of
the upper electrode 13, has a shape that the width becomes larger
outwards from the center of the elliptical shape. Here, the length
of the main axis of the elliptical shape is a; therefore, the width
of the extension 22 is larger than a. The extensions 21 and 22 are
thus tapered and are substantially symmetric. The edges 21 and 22
may have a shape of either line or curve. FIG. 8B is a comparative
example. A piezoelectric thin-film resonator of the comparative
example has extensions with a constant width.
The inventors produced the piezoelectric thin-film resonators as
shown in FIGS. 8A and 8B under the following conditions and
examined the strength of a membrane 14. As shown in FIGS. 8A and
8B, the elliptical shape formed by the overlapping portions of the
upper electrode 13 and the lower electrode 11 had a size such that
a=86.1 .mu.m and b=61.5 .mu.m (a/b=1.4). The lower electrode 11 had
a double layer structure of Ru (100 nm) and Cr (50 nm). The
piezoelectric film 12 was made of AlN (400 nm), and the upper
electrode 13 was made of Ru (100 nm). The piezoelectric thin-film
resonators shown in FIGS. 8A and 8B were produced under the
condition that a laminate composed of the above-mentioned
electrodes and film and followed by patterning has internal stress
equal to -1.56 Ga. The inventors found out a great difference in
membrane damage between the two piezoelectric thin-film resonators
after cavities were formed. More specifically, 27 percent of the
membrane shown in FIG. 8B was damaged, while nothing was
substantially damaged in the membrane shown in FIG. 8A. Thus, even
if the films having the same internal stress are used, the membrane
has different strengths that depend on the shape of the extensions.
As a result of the above consideration, it can be said that the
membrane of FIG. 8A has a less damaged structure than that of FIG.
8B.
The sub axis of the elliptical shape is substantially parallel to
the current direction shown in FIG. 8A. However, even if the main
axis of the elliptical shape is substantially parallel to the
current direction, the same function and effect as mentioned above
are obtainable by arranging the extensions of the upper and lower
electrodes so as to have an increasing width outwards from the
center of the elliptical shape. Preferably, the extension 21 of the
upper electrode 13 and the extension 22 of the lower electrode 11
are formed as shown in FIG. 8A. However, only one of the extensions
21 and 22 may be arranged so that the width becomes larger outwards
from the center of the elliptical shape.
Fifth Embodiment
A fifth embodiment of the present invention has a structure defined
by taking internal stress and resonance characteristic of a film
laminate into consideration. The film laminate is composed of the
lower electrode 11, the piezoelectric film 12, and the upper
electrode 13. The inventors conducted an experiment directed to
investigating the affect of the internal stress on the resonance
characteristic. The piezoelectric thin-film resonator used in the
experiment had the following laminate structure. The lower
electrode 13 had a double layer structure of Ru (100 nm)/Cr (50
nm). The piezoelectric film 12 was AlN and 400 nm thick. The upper
electrode 13 was made of Ru and 100 nm thick. The elliptical shape
of the region defined by overlapping the upper electrode 13
overlaps with the lower electrode 11 had a size such that a=60.2
.mu.m and b=50.2 .mu.m (a/b=1.2). The cavity 15 was a size such
that a=66.2 .mu.m and b=55.2 .mu.m (a/b=1.2). FIG. 9 shows a
relationship between internal stress (GPa) and electromechanical
coupling coefficient (K.sup.2). It is noted that the internal
stress denotes the stress of the above-mentioned film laminate
before patterning. It can be seen from FIG. 9 that the larger the
compression stress, the greater the electromechanical coupling
coefficient (K.sup.2). In the above-mentioned experiment, two films
having two kinds of tensile stress of 0.52 GPa and 0.87 Gpa were
produced. However, both the membranes were broken after the
cavities 15 were defined. A general specification requires an
electromechanical coupling coefficient (K.sup.2) equal to six
percent or higher. Therefore, the film of -0.68 GPa or less in
compression stress is required to satisfy the specification.
In the case where the film with stress is employed, the membrane is
warped after the cavity is provided. Especially, in the case where
the film with compression stress is employed, the membrane 14 is
curved outwards in the opposite side to the cavity 15 after the
cavity 15 is provided, as shown in FIG. 10. FIG. 11 shows a
relationship between U/M and the internal stress, where M is the
thickness of the membrane 14 (650 nm in the fifth embodiment), and
U is the maximum height measured from the surface of the substrate
11 above the membrane 14. It can be seen from FIG. 11 that the
larger the compression stress, the greater the curve of the
membrane 14. According to the above-mentioned result, it is
possible to produce a desired piezoelectric thin-film resonator
with an electromechanical coupling coefficient (K.sup.2) of six
percent or more when U/M is equal to 1.5 or higher.
Sixth Embodiment
A sixth embodiment of the present invention is a piezoelectric
thin-film resonator and a filter device that employs an acoustic
reflector substituted for the cavity 15 located below the membrane
14. The acoustic reflector is composed of high and low acoustic
impedance films that are alternately laminated by the thickness of
.lamda./4, where .lamda. is a wavelength of an elastic wave.
In accordance with the present invention, even with the structure
intended to suppress adverse affects caused by the lateral mode
waves, the piezoelectric thin-film resonator is configured so as to
obtain a sufficient strength and excellent productivity of the
cavity. Thus, the piezoelectric thin-film resonator with less
irregularity in characteristics and the filter thereof are
obtainable. In addition, by utilizing the film having the desired
compression stress, the piezoelectric thin-film resonator with a
large electromechanical coupling coefficient (K.sup.2) and the
filter thereof are obtainable.
The present invention is not limited to the above-mentioned first
embodiment, and other embodiments and modifications may be made
without departing from the scope of the present invention.
The present invention is based on Japanese Patent Application No.
2003-360054 filed on Oct. 20, 2003, the entire disclosure of which
is hereby incorporated by reference.
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