U.S. patent application number 10/764520 was filed with the patent office on 2004-09-23 for thin-film piezoelectric resonator, band-pass filter and method of making thin-film piezoelectric resonator.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Kimachi, Rei, Miyashita, Tsutomu, Nishihara, Tokihiro, Sakashita, Takeshi, Yokoyama, Tsuyoshi.
Application Number | 20040185594 10/764520 |
Document ID | / |
Family ID | 26624138 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040185594 |
Kind Code |
A1 |
Nishihara, Tokihiro ; et
al. |
September 23, 2004 |
Thin-film piezoelectric resonator, band-pass filter and method of
making thin-film piezoelectric resonator
Abstract
A thin-film piezo-resonator includes a silicon substrate and a
resonator assembly. The substrate is formed with a cavity or
through-hole which is opened in the upper and the lower surfaces of
the substrate. The resonator assembly, disposed at a location
corresponding to the cavity, is composed of a first electrode
contacting the upper surface of the substrate, a piezoelectric
layer formed on the first electrode and a second electrode formed
on the piezoelectric layer. The cavity has a side surface extending
in a substantially perpendicular direction to the first
surface.
Inventors: |
Nishihara, Tokihiro;
(Kawasaki-shi, JP) ; Sakashita, Takeshi;
(Kawasaki-shi, JP) ; Kimachi, Rei; (Kawasaki-shi,
JP) ; Yokoyama, Tsuyoshi; (Kawasaki-shi, JP) ;
Miyashita, Tsutomu; (Kawasaki-shi, JP) |
Correspondence
Address: |
ARMSTRONG, KRATZ, QUINTOS, HANSON & BROOKS, LLP
1725 K STREET, NW
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
Fujitsu Media Devices Limited
Suzaka-Shi
JP
|
Family ID: |
26624138 |
Appl. No.: |
10/764520 |
Filed: |
January 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10764520 |
Jan 27, 2004 |
|
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|
10117219 |
Apr 8, 2002 |
|
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6734763 |
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Current U.S.
Class: |
438/53 |
Current CPC
Class: |
H03H 9/174 20130101;
H03H 9/564 20130101; Y10T 29/42 20150115; H03H 3/02 20130101; H03H
9/568 20130101; H03H 9/173 20130101 |
Class at
Publication: |
438/053 |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 26, 2001 |
JP |
2001-329308 |
Jan 23, 2002 |
JP |
2002-13984 |
Claims
1. A thin-film piezo-resonator comprising: a substrate having a
first surface and a second surface opposite to said first surface,
the substrate being formed with a cavity that has a first opening
in said first surface and a second opening in said second surface;
and a resonator assembly including an exciter composed of a first
electrode contacting said first surface, a piezoelectric layer on
the first electrode and a second electrode on the piezoelectric
layer, the assembly being disposed at a location corresponding to
the cavity; wherein the cavity includes a side surface extending in
a substantially perpendicular direction to said first surface.
2. The resonator according to claim 1, wherein the first electrode
comprises a uniaxially oriented single-layer conductive member or
uniaxially oriented multi-layer conductive member.
3. The resonator according to claim 2, wherein the piezoelectric
layer is uniaxially oriented.
4. The resonator according to claim 1, wherein the substrate is a
(111)-cut silicon substrate, said first surface and said second
surface each being a (111) surface.
5. The resonator according to claim 4, wherein the first electrode
comprises a single conductive layer containing either one of
(111)-uniaxially oriented Al and (111)-uniaxially oriented Cu.
6. The resonator according to claim 4, wherein the first electrode
comprises a stack of uniaxially oriented conductive layers
including a first conductive layer held in contact with said first
surface, the first conductive layer containing either one of
(111)-uniaxially oriented Al and (111)-uniaxially oriented Cu.
7. The resonator according to claim 4, wherein the first electrode
has a two-layer structure comprising a first conductive layer and a
second conductive layer, the first conductive layer held in contact
with said first surface and containing either one of
(111)-uniaxially oriented Al and (111)-uniaxially oriented Cu, the
second conductive layer containing (110)-uniaxially oriented
Mo.
8. The resonator according to claim 4, wherein the piezoelectric
layer is made of either one of (002)-uniaxially oriented AlN and
(002)-uniaxially oriented ZnO.
9. The resonator according to claim 1, further comprising a cover
substrate bonded to said second surface of the substrate so as to
close the cavity.
10. The resonator according to claim 1, wherein each of the first
and the second openings has a circular or oval configuration.
11. The resonator according to claim 1, wherein each of the first
electrode and the piezoelectric layer includes a portion exposed to
the cavity.
12. The resonator according to claim 11, wherein the exposed
portion of the first electrode and the exposed portion of the
piezoelectric layer are made of a material which is not etched by a
fluorine gas.
13. A thin-film piezo-resonator comprising: a (111)-cut silicon
substrate; a first electrode formed on the substrate and containing
either one of Al and Cu; a piezoelectric layer formed on the first
electrode and containing either one of AlN and ZnO; and a second
electrode formed on the piezoelectric layer; wherein the silicon
substrate includes a first surface which is a (111) surface, the
first electrode being held in contact with said first surface.
14. A thin-film piezo-resonator comprising: a substrate having a
first surface and a second surface opposite to said first surface,
the substrate being formed with a cavity that has a first opening
in said first surface; and a resonator assembly including a first
electrode contacting said first surface, a piezoelectric layer on
the first electrode and a second electrode on the piezoelectric
layer, the assembly being disposed at a location corresponding to
the cavity; wherein each of the first electrode and the
piezoelectric layer includes a portion exposed to the cavity.
15. The resonator according to claim 1, wherein the first and the
second electrodes comprise first and second exciter portions,
respectively, that define the exciter, the first opening being
greater in area than the exciter portions by a factor of
1.about.2.25.
16. The resonator according to claim 15, wherein the first exciter
portion and the second exciter portion are substantially identical
in shape.
17. The resonator according to claim 15, wherein each of the first
and the second exciter portions is at least partially circular or
oval.
18. A filter comprising: a substrate having a first surface and a
second surface opposite to said first surface, the substrate being
formed with a plurality of cavities spaced from each other; a first
electrode pattern held in contact with said first surface; a
piezoelectric layer on the first electrode pattern; a second
electrode pattern on the piezoelectric layer; and a plurality of
resonator assemblies provided by a combination of the first
electrode pattern, the piezoelectric layer and the second electrode
pattern, each of the resonator assemblies corresponding in position
to one of the cavities; wherein each of the cavities has a side
surface extending in a substantially perpendicular direction to
said first surface.
19. The filter according to claim 18, wherein each of the cavities
includes a first opening in said first surface and a second opening
in said second surface, a distance between adjoining first openings
being no greater than 420 .mu.m.
20. The filter according to claim 18, wherein said plurality of
resonator assemblies include a first group of resonator assemblies
connected in series and a second group of resonator assemblies
connected in parallel.
21. The filter according to claim 18, wherein the first electrode
pattern and the piezoelectric layer are exposed to one of the
cavities.
22. A filter comprising: a substrate having a first surface and a
second surface opposite to said first surface, the substrate being
formed with a plurality of cavities each including a first opening
in said first surface and a second opening in said second surface;
a first electrode pattern held in contact with said first surface;
a piezoelectric layer on the first electrode pattern; a second
electrode pattern on the piezoelectric layer; and a plurality of
exciters provided by a combination of the first electrode pattern,
the piezoelectric layer and the second electrode pattern, each of
the exciters corresponding in position to one of the cavities;
wherein the first electrode pattern and the piezoelectric layer
each include a portion exposed to one of the cavities.
23. The filter according to claim 22, wherein the exposed portions
of the first electrode pattern and the piezoelectric layer are made
of a material which is not etched by a fluorine gas.
24. The filter according to claim 22, wherein each of the exciters
is defined by a first exciter portion and a second exciter portion
contained respectively in the first electrode pattern and the
second electrode pattern, the first exciter portion and the second
exciter portion being substantially identical in shape.
25. The filter according to claim 24, wherein the first opening of
the cavity corresponding to said each exciter is greater in area
than the exciter portions by a factor of 1.about.2.25.
26. The filter according to claim 22, wherein the first and the
second openings of each cavity are circular or oval.
27. A method of making a thin-film piezo-resonator comprising steps
of: preparing a substrate including a first surface and a second
surface opposite to said first surface; forming a resonator
assembly which includes a first electrode held in contact with said
first surface, a piezoelectric layer formed on the first electrode
and a second electrode formed on the piezoelectric layer; and
forming a cavity by dry-etching the substrate, the cavity being
disposed at a location corresponding to the resonator assembly, the
cavity being opened in said first surface and said second surface;
wherein the cavity includes a side surface extending in a
substantially perpendicular direction to said first surface.
28. The method according to claim 27, wherein the dry etching is
Deep-RIE.
29. The method according to claim 27, further comprising the step
of bonding a cover substrate to said second surface so as to close
the cavity.
30. The method according to claim 27, wherein a groove for dividing
the substrate is also formed by etching at the cavity-forming
step.
31. A method of making a thin-film piezo-resonator comprising steps
of: preparing a substrate including a first surface and a second
surface opposite to said first surface; forming a resonator
assembly which includes a first electrode held in contact with said
first surface, a piezoelectric layer formed on the first electrode
and a second electrode formed on the piezoelectric layer; and
forming a cavity by dry-etching the substrate, the cavity being
disposed at a location corresponding to the resonator assembly, the
cavity being opened in said first surface and said second surface;
wherein the first electrode and the piezoelectric layer are
partially exposed to the cavity at the cavity-forming step.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a thin-film piezoelectric
resonator and a method of making the same. The present invention
also relates to a band-pass filter utilizing a thin-film
piezoelectric resonator (called "piezo-resonator" below).
[0003] 2. Description of the Related Art
[0004] With the rapid spread of mobile telecommunications equipment
such as portable telephones, small and light band-pass filters, as
well as resonators needed to make such filters, are in great
demand. As known in the art, thin-film piezo-resonators are
suitable for producing high-power filters.
[0005] A typical thin-film piezo-resonator includes a substrate and
a resonator assembly mounted on the substrate. The resonator
assembly includes a piezoelectric film and a pair of thin
electrodes sandwiching the piezoelectric film from above and below.
The substrate is formed with a cavity below the lower electrode of
the resonator assembly.
[0006] When an AC voltage is applied to the upper and the lower
electrode of the piezo-resonator, the sandwiched piezoelectric film
vibrates in its thickness direction (which is known as the inverse
piezoelectric effect). On the other hand, by the direct
piezoelectric effect, a mechanical vibration or elastic wave is
converted into a corresponding electrical signal. The elastic wave
is a longitudinal wave whose main displacement occurs in the
thickness direction of the piezoelectric film. In such a thin-film
piezo-resonator, the resonator assembly resonates when its
thickness H is equal to n/2 times the wavelength of the elastic
wave (where n is an integer). Supposing that the propagation
velocity of the elastic wave is V (which depends on the material
used), the resonance frequency F is expressed by a formula F=nV/2H.
This means that a piezo-resonator having desired frequency
characteristics can be obtained by adjusting the thickness H of the
resonator assembly. Further, by connecting such resonators in a
ladder configuration, it is possible to produce a band-pass filter
which allows only those electric waves lying within a certain
frequency range to pass.
[0007] In the above-described thin-film piezo-resonator, desired
resonance characteristics are attained by providing a cavity or
hole below the lower electrode. Techniques suitable to making such
a cavity are disclosed in "ZnO/SiO.sub.2-DIAPHRAGM COMPOSITE
RESONATOR ON A SILICON WAFER" (K. NAKAMURA, ELECTRONICS LETTERS 9
Jul. 1981 Vol. 17 No. 14 p507-509), JP-A-60(1985)-189307,
JP-A-2000-69594, U.S. Pat. No. 6,060,818 and U.S. Pat. No.
5,587,620 for example.
[0008] FIG. 20 shows, in section, a thin-film piezo-resonator
disclosed in the above-mentioned "ZnO/SiO.sub.2-DIAPHRAGM COMPOSITE
RESONATOR ON A SILICON WAFER". The thin-film piezo-resonator
(generally indicated by reference numeral 700) includes a (100)-cut
silicon substrate 710 and a resonator assembly 720 supported by the
substrate 710. The resonator assembly 720 is made up of a lower
electrode 721, a piezoelectric film 722, and an upper electrode
723. The silicon substrate 710 has an upper surface upon which a
SiO.sub.2 film 711 is formed by thermal oxidation. The resonator
assembly 720 is placed directly on the SiO.sub.2 film 711. The
silicon substrate 710 is formed with a cavity 710a whose upper
opening is closed by the SiO.sub.2 film 711. The cavity 710a can be
produced by anisotropic etching with respect to the (100) surface
of the silicon substrate. The anisotropic etching is performed from
below the silicon substrate 710 with the use of KOH solution or EDP
solution (ethylenediamine+pyrocatechol+water) for example.
[0009] The above anisotropic etching relies upon the fact that the
etching rate with respect to the (100) surface of the substrate 710
is significantly greater than the etching rate with respect to the
(111) surface. Therefore, the resonator assembly is to be provided
only on the (100) surface of the substrate 710. However, such
positional limitation is disadvantageous since it restricts the
option of the material suitable for making the components of the
resonator assembly 720, while also depriving the resonator assembly
components of freedom of orientation. Another disadvantage is that
the lower opening of the cavity 710a tends to be unduly large due
to the nonupright side wall 710a' of the cavity 710a. In the
illustrated arrangement, the side wall 710a' corresponds to the
(111) surface of the substrate 710 and has an inclination of 54.7
degrees with respect to the (100) surface of the silicon substrate
710. Due to this, the cavity 710a has a large opening in the bottom
surface of the silicon substrate 710. For instance, when the
substrate 710 has a thickness of 300 .mu.m, the lower opening of
the cavity 710a is larger than the upper opening by more than 420
.mu.m. Unfavorably, such a large opening of the cavity 710a reduces
the mechanical strength of the piezo-resonator 700. In addition, it
contributes to an increase in the overall size of the
piezo-resonator 700. With the use of such oversize
piezo-resonators, a compact band-pass filter cannot be obtained.
Specifically, when the thickness of the substrate 710 is 300 .mu.m,
the lower opening of the cavity 710a is larger than the upper
opening by more than 420 .mu.m, as noted above. Thus, the distance
between the neighboring upper openings should be more than 420
.mu.m. Further, as the distance between the adjacent upper openings
increases, the length of the wiring pattern for connecting the
adjacent resonator assemblies should also increase. This leads to
an increase in the resistance of the wiring pattern. A greater
resistance of the wiring pattern can be a major factor that
prevents the improvement of the filter characteristics in a
high-frequency band.
[0010] FIG. 21 shows a thin-film piezo-resonator disclosed in
JP-A-60-189307. The piezo-resonator 800 includes a substrate 810,
and a resonator assembly 820 which is made up of a lower electrode
821, a piezoelectric film 822, and an upper electrode 823. A cavity
830 is provided between the substrate 810 and the resonator
assembly 820. According to the Japanese document, the
piezo-resonator 800 is fabricated in the following manner. First, a
sacrifice layer for the cavity 830 is formed in a pattern on the
substrate 810. Next, a SiO.sub.2 film 840 is formed on the
sacrifice layer 840 in a manner such that part of the sacrifice
layer is exposed. Then, the resonator assembly 820 is provided on
the SiO.sub.2 film 840. Finally, the sacrifice layer is removed by
wet etching, so that the cavity 830 appears below the resonator
assembly 820. According to this method, the cavity 830 is kept from
becoming too large with respect to the resonator assembly 820.
[0011] In the thin-film piezo-resonator utilizing a longitudinal
vibration in the thickness direction, a high-orientation
piezoelectric film is required to provide excellent resonance
characteristics. According to the technique disclosed in
JP-A-60-189303, however, it is difficult to give a sufficiently
high orientation to the piezoelectric film 822. The cavity 830
below the resonator assembly 820 has a length L15, which needs to
be at least a few micron meters when a twist and oscillation
displacement of the resonator assembly 820 are taken into
consideration. Unfavorably, the sacrifice layer, formed to have a
thickness corresponding to the length L15, has a greater surface
roughness than that of the silicon substrate 810. This deteriorates
the orientation of the lower electrode 821 and the piezoelectric
film 822 both of which are grown on the sacrifice layer via the
SiO.sub.2 film 840. As a result, it is difficult to obtain good
resonance characteristics with the thin-film piezo-resonator.
[0012] FIG. 22 is a sectional view of a thin-film piezo-resonator
disclosed in JP-A-2000-69594. The thin-film piezo-resonator 900
includes a silicon substrate 910, and a resonator assembly 920 made
up of a lower electrode 921, a piezoelectric film 922 and an upper
electrode 923. A cavity 910a is provided below the resonator
assembly 920. According to this document, to make the thin-film
piezo-resonator 900, the silicon substrate 910 is etched to form a
recess serving as the cavity 910a. Then, a SiO.sub.2 film 930 is
formed by thermal oxidation on a surface of the silicon substrate
910. Next, material is deposited in the cavity 910a to form a
sacrifice layer. After the depositing, the surface of the sacrifice
layer is polished and cleaned. Next, the resonator assembly 920 is
formed on the sacrifice layer in a manner such that the sacrifice
layer is partially exposed. Finally, the sacrifice layer is removed
by wet etching.
[0013] However, the method disclosed in JP-A-2000-69594 includes a
large number of steps such as the step of depositing the sacrifice
layer in the cavity 910a, the step of polishing the sacrifice layer
and so on. Therefore, it is difficult to manufacture the thin-film
piezo-resonator at a low cost and at a high yield.
SUMMARY OF THE INVENTION
[0014] The present invention has been proposed under the
circumstances described above. It is, therefore, an object of the
present invention to solve or reduce the above conventional
problems, and to provide a thin-film piezo-resonator suitable for
miniaturization and having a highly oriented piezoelectric film, to
provide a band-pass filter including this thin-film
piezo-resonator, and to provide a method of making such a thin-film
piezo-resonator.
[0015] According to a first aspect of the present invention, there
is provided a thin-film piezo-resonator comprising: a substrate
having a first surface and a second surface opposite to the first
surface, the substrate being formed with a cavity that has a first
opening in the first surface and a second opening in the second
surface; and a resonator assembly including an exciter composed of
a first electrode contacting the first surface, a piezoelectric
layer on the first electrode and a second electrode on the
piezoelectric layer, the assembly being disposed at a location
corresponding to the cavity. The cavity includes a side surface
extending in a substantially perpendicular direction to the first
surface of the substrate.
[0016] In this specification, the "exciter" refers to the
overlapping region of the first and the second electrodes (or
electrode patterns) and the piezoelectric layer.
[0017] With the above arrangements, it is possible to fabricate a
thin-film piezoelectric resonator that is compact and exhibits
excellent resonance characteristics. The compactness results from
the cavity that penetrates the substrate in a non-flaring manner,
with an uniformly upright side surface. Such a cavity may be
produced by dry etching such as Deep-RIE (Reactive Ion Etching),
regardless of the cut condition of the substrate. With the use of
such compact resonators, a compact filter can be obtained. Further,
since the cut condition of the substrate does not affect the
formation of the cavity, the most suitable cut surface can be
realized in the substrate. The free selectability of the cut
surface facilitates the forming of a highly oriented first
electrode (lower electrode) thereon. This allows a highly oriented
piezoelectric layer to be formed on the first electrode.
Accordingly, it is possible to produce a thin-film piezo-resonator
with excellent resonance characteristics.
[0018] The first electrode and the second electrode may be formed
of e.g. aluminum (Al), copper (Cu), molybdenum (Mo), chromium (Cr),
titanium (Ti) and platinum (Pt). The piezoelectric layer may be
formed of e.g. aluminum nitride (AlN), zinc oxide (ZnO), lead
zirconate titanate (PZT), and lead titanate (PbTi.sub.3). The
substrate may be made of silicon.
[0019] Preferably, the first electrode may include a uniaxially
oriented single-layer conductive member or uniaxially oriented
multi-layer conductive member. In addition, the piezoelectric layer
may also be uniaxially oriented. Preferably, the substrate may be a
(111)-cut silicon substrate, so that its first and second surfaces
are a (111) surface. These arrangements are preferable in providing
a highly oriented piezoelectric layer.
[0020] Preferably, the first electrode may include a single
conductive layer containing either one of (111)-uniaxially oriented
Al and (111)-uniaxially oriented Cu. Or, the first electrode may
include a stack of uniaxially oriented conductive layers including
a first conductive layer held in contact with said first surface,
the first conductive layer containing either one of
(111)-uniaxially oriented Al and (111)-uniaxially oriented Cu. Or,
the first electrode may have a two-layer structure including a
first conductive layer and a second conductive layer, where the
first conductive layer, held in contact with the first surface of
the substrate, contains either one of (111)-uniaxially oriented Al
and (111)-uniaxially oriented Cu, while the second conductive layer
contains (110)-uniaxially oriented Mo.
[0021] With the above arrangement, it is possible to form a highly
oriented first electrode on the (111)-cut silicon substrate.
[0022] Preferably, the piezoelectric layer may be made of either
one of (002)-uniaxially oriented AlN and (002)-uniaxially oriented
ZnO for high orientation.
[0023] Preferably, the resonator of the present invention may
further include a cover substrate for protection of e.g. the
resonator assembly. The cover substrate may be bonded to the second
surface of the substrate so as to close the cavity.
[0024] Preferably, the first electrode and the piezoelectric layer
may each include a portion exposed to the cavity. Such exposure is
advantageous to providing the resonator with excellent resonance
characteristics.
[0025] According to a second aspect of the present invention, there
is provided a thin-film piezo-resonator that includes: a (111)-cut
silicon substrate; a first electrode formed on the substrate and
containing either one of Al and Cu; a piezoelectric layer formed on
the first electrode and containing either one of AlN and ZnO; and a
second electrode formed on the piezoelectric layer. The silicon
substrate includes a first surface which is a (111) surface. The
first electrode is held in contact with the first surface of the
substrate.
[0026] According to a third aspect of the present invention, there
is provided a thin-film piezo-resonator that includes: a substrate
having a first surface and a second surface opposite to the first
surface, the substrate being formed with a cavity that has a first
opening in the first surface of the substrate; and a resonator
assembly including a first electrode contacting the first surface
of the substrate, a piezoelectric layer on the first electrode and
a second electrode on the piezoelectric layer. The resonator
assembly is disposed at a location corresponding to the cavity.
Each of the first electrode and the piezoelectric layer includes a
portion exposed to the cavity.
[0027] With the above arrangements, the resonator assembly exhibits
better resonance characteristics than when it is isolated from the
cavity. Further, when the opening of the cavity is wide enough to
allow not only the first electrode but also the piezoelectric layer
to be exposed, resonance characteristics such as the minimum
insertion loss or attenuation pole suppression can be improved.
[0028] According to a fourth aspect of the present invention, there
is provided a filter that includes: a substrate having a first
surface and a second surface opposite to the first surface, where
the substrate is formed with a plurality of cavities spaced from
each other; a first electrode pattern held in contact with the
first surface of the substrate; a piezoelectric layer on the first
electrode pattern; a second electrode pattern on the piezoelectric
layer; and a plurality of resonator assemblies provided by a
combination of the first electrode pattern, the piezoelectric layer
and the second electrode pattern, where each of the resonator
assemblies corresponds in position to one of the cavities. Each of
the cavities has a side surface extending in a substantially
perpendicular direction to the first surface of the substrate.
[0029] Preferably, each of the cavities may include a first opening
in the first surface of the substrate and a second opening in the
second surface of the substrate-, where the distance between
adjoining first openings is no greater than 420 .mu.m.
[0030] With the above arrangement, it is possible to provide a
compact filter. Further, since the connecting distance between any
adjoining resonator assemblies can be short, the resistance of the
wiring pattern is also reducible. Advantageously, a filter with a
less resistive wiring pattern exhibits better performance in a
high-frequency band.
[0031] In a conventional filter which includes a silicon substrate
formed with several cavities (each corresponding in position to one
of the piezoelectric resonators), the upper openings of adjoining
cavities should be spaced from each other by more than 420 .mu.m
(supposing that the thickness of the substrate is 300 .mu.m or
more) due to the downward flare of the cavities (see FIG. 20).
According to the present invention, on the other hand, the distance
between adjoining first or upper openings is 420 .mu.m or smaller
by forming each cavity in a manner such that its side surface
extends perpendicularly to the substrate. As a result, the filter
of the present invention can be smaller than a conventional
filter.
[0032] Preferably, the resonator assemblies used for the filter of
the present invention may include a first group of resonator
assemblies connected in series and a second group of resonator
assemblies connected in parallel. This makes the filter a ladder
type.
[0033] Preferably, the first electrode pattern and the
piezoelectric layer may be exposed to one of the cavities.
[0034] According to a fifth aspect of the present invention, there
is provided a filter that includes: a substrate that has a first
surface and a second surface opposite to the first surface and is
formed with a plurality of cavities each including a first opening
in the first surface of the substrate and a second opening in the
second surface of the substrate; a first electrode pattern held in
contact with the first surface of the substrate; a piezoelectric
layer on the first electrode pattern; a second electrode pattern on
the piezoelectric layer; and a plurality of exciters provided by
the combination of the first electrode pattern, the piezoelectric
layer and the second electrode pattern, where each of the exciters
corresponds in position to one of the cavities. The first electrode
pattern and the piezoelectric layer each include a portion exposed
to one of the cavities.
[0035] In the above-mentioned aspects of the present invention, the
first and the second openings of the cavity may preferably be
circular or oval rather than rectangular. This is because a
rectangular opening is more difficult to make than a smoothly
curved opening by dry-etching, since the etching rate for the
corners of the opening tends to be slower than the etching rate for
the other portions. In particular, when several openings of
different sizes are to be made in a single substrate, the
production efficiency is significantly higher in making arcuate
openings than in making rectangular openings.
[0036] As noted above, the first electrode or the piezoelectric
layer may have a portion exposed to a cavity for better resonance
characteristics. Preferably, these exposed portions may be made of
a material which is not etched by a fluorine gas. Examples of such
material are aluminum and copper. With this arrangement, the first
electrode and the piezoelectric layer will not or hardly be damaged
in performing Deep-RIE.
[0037] In this specification, as defined above, an "exciter" is the
overlapping region of the first and the second electrodes (or
electrode patterns) and the piezoelectric layer. More specifically,
the first electrode includes a "first exciter portion" that
overlaps the second electrode. Likewise, the second electrode
includes a "second exciter portion" that overlaps the first
electrode. In symmetry, the first and the second exciter portions
have the same or substantially same configuration. The exciter is
the assembly of the first and the second exciter portions and a
portion of the piezoelectric layer that is sandwiched between the
first and the second exciter portions. Since the first and the
second exciter portions are (substantially) the same in shape, the
desired capacitance is precisely attained between the first and the
second exciter portions. Preferably, each exciter portion as a
whole or in part may be circular or oval.
[0038] In the first to fifth aspects of the present invention, the
area of the first opening of a cavity may preferably be
1.about.2.25 times larger than the area of the above-defined first
or second exciter portion. With this design, the resonator assembly
can exhibit good resonance characteristics, while being prevented
from suffering deformation or damage.
[0039] According to a sixth aspect of the present invention, there
is provided a method of making a thin-film piezo-resonator. The
method includes the steps of: preparing a substrate including a
first surface and a second surface opposite to the first surface;
forming a resonator assembly which includes a first electrode held
in contact with the first surface of the substrate, a piezoelectric
layer formed on the first electrode and a second electrode formed
on the piezoelectric layer; and forming a cavity by dry-etching the
substrate, where the cavity is disposed at a location corresponding
to the resonator assembly, and opened in the first and the second
surfaces of the substrate. The cavity includes a side surface
extending in a substantially perpendicular direction to the first
surface of the substrate.
[0040] According to a seventh aspect of the present invention,
there is provided a method of making a thin-film piezo-resonator.
The method includes the steps of: preparing a substrate including a
first surface and a second surface opposite to the first surface;
forming a resonator assembly which includes a first electrode held
in contact with the first surface of the substrate, a piezoelectric
layer formed on the first electrode and a second electrode formed
on the piezoelectric layer; and forming a cavity by dry-etching the
substrate, where the cavity is disposed at a location corresponding
to the resonator assembly, and opened in the first and second
surfaces of the substrate. The first electrode and the
piezoelectric layer are caused to be partially exposed to the
cavity at the cavity-forming step.
[0041] In the sixth and the seventh aspects of the present
invention, the dry etching may be Deep-RIE. The method may further
include the step of bonding a cover substrate to the second surface
of the substrate so as to close the cavity. In the method, a groove
for dividing the substrate may also be formed by etching at the
cavity-forming step.
[0042] Other features and advantages of the present invention will
become apparent from the detailed description given below with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 shows a thin-film piezo-resonator according to a
first embodiment of the present invention;
[0044] FIG. 2 is a sectional view taken in lines II-II in FIG.
1;
[0045] FIG. 3 shows a one-port, thin-film piezo-resonator subjected
to a characteristic evaluation measurement;
[0046] FIG. 4 is a graph showing the dependency of minimum
insertion loss on L2/L1;
[0047] FIG. 5 is a graph showing the dependency of attenuation pole
suppression on L2/L1;
[0048] FIGS. 6A.about.6E show several sequential steps for
manufacturing the thin-film piezo-resonator in FIG. 1;
[0049] FIGS. 7A.about.7D show several steps subsequent to the
previous steps shown in FIGS. 6;
[0050] FIG. 8 is a sectional view of a thin-film piezo-resonator
according to a second embodiment of the present invention;
[0051] FIG. 9 is a sectional view of a thin-film piezo-resonator
according to a third embodiment of the present invention;
[0052] FIG. 10 is a plan view showing a thin-film piezo-resonator
according to a fourth embodiment of the present invention;
[0053] FIG. 11 is a sectional view taken along lines XI-XI in FIG.
10;
[0054] FIG. 12 is a plan view showing a band-pass filter according
to a fifth embodiment of the present invention;
[0055] FIG. 13 is a sectional view taken along lines XIII-XIII in
FIG. 12;
[0056] FIG. 14 is a sectional view taken along lines XIV-XIV in
FIG. 12;
[0057] FIG. 15 is a plan view showing a band-pass filter according
to a sixth embodiment of the present invention;
[0058] FIG. 16 is a sectional view taken along lines XVI-XVI in
FIG. 15;
[0059] FIG. 17 is a sectional view taken along lines XVII-XVII in
FIG. 15;
[0060] FIG. 18 is a sectional view taken along lines XVIII-XVIII in
FIG. 15;
[0061] FIG. 19 is a circuit diagram of a band-pass filter of the
present invention;
[0062] FIG. 20 is a sectional view of a conventional thin-film
piezo-resonator;
[0063] FIG. 21 is a sectional view of another conventional
thin-film piezo-resonator; and
[0064] FIG. 22 is a sectional view of another conventional
thin-film piezo-resonator.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0065] The preferred embodiments of the present invention will be
described below with reference to the accompanying drawings.
[0066] FIG. 1 shows a thin-film piezo-resonator 100 according to a
first embodiment of the present invention.
[0067] FIG. 2 is a sectional view taken along lines II-II in FIG.
1.
[0068] The thin-film piezo-resonator 100 includes a silicon
substrate 110, and a resonator assembly 120 formed thereon. The
silicon substrate 110 is a (111)-cut monocrystal silicon substrate,
and has a first surface 111 and a second surface 112 each provided
as a (111) surface. The resonator assembly 120 includes a first
electrode film 121, a second electrode film 122, and a
piezoelectric film 123 sandwiched by these films. According to the
present embodiment, the first electrode film 121 has a thickness of
100 nm, and is made of (111)-uniaxially oriented Al or Cu. The
second electrode film 122 has a thickness of 100 nm, and is made of
Al or Cu. The piezoelectric film 123 has a thickness of 500 nm, and
is provided by (002)-uniaxially oriented AlN or ZnO.
[0069] The silicon substrate 110 is formed with a cavity 110a below
the resonator assembly 120. The cavity 110a penetrates the silicon
substrate 110 vertically with respect to the first surface 111 and
the second surface 112. The cavity 110a has an opening 112a on the
second surface 112, which is identical in area and shape to an
opening 111a on the first surface 111. In the illustrated example,
the openings 111a and 112a are both square. Since the opening 112a
of the cavity 110a i's not significantly larger than the opening
111a, it is possible to design the silicon substrate 110, and hence
the thin-film piezo-resonator 100, to a desirably small size. For
example, if the thin-film piezo-resonator 100 is to have a
resonance frequency of 5 GHz, the resonator assembly 120 should
have an exciter whose length L1 is equal to about 50 .mu.m. The
cavity 110a has a length L2 equal to about 60 .mu.m. The "exciter"
refers to a region overlapped by the first electrode film 121 and
the second electrode film 122 via the piezoelectric film 123. In
the illustrated example, the exciter has a square configuration
whose side has a length L1. According to the present invention, the
exciter may have a rectangular or other configuration. In
conformity with the exciter, the openings 111a, 112a may be
modified in configuration. In the illustrated example, the first
electrode 121 and the piezoelectric film 123 are partially exposed
to the cavity 110a. This provides the piezo-resonator 100 with
excellent resonance characteristics.
[0070] Description will now given to the influence of the length L1
(of the exciter) and the length L2 (of the cavity 110a or the
opening 111a) on the resonance characteristics. A single-port
resonator provided by the thin-film piezo-resonator 100 was
connected to a network analyzer, as shown in FIG. 3. The passing
characteristics (S21 characteristics) were measured. FIG. 4 and
FIG. 5 show the results of the measurement. The thin-film
piezo-resonators 100 used for the measurement ware made up of a
(111)-cut 300 .mu.m-thick silicon substrate 110, and a resonator
assembly 120 which includes a 100 nm-thick first electrode film 121
made of Al, a 500 nm-thick piezoelectric film 123 made of ZnO, and
a 100 nm-thick second electrode film 122 made of Al. With the
piezo-resonators 100, the ratio of the length L1 to the length L2
is not the same. FIG. 4 is a graph showing the dependency of
minimum insertion loss on L2/L1. FIG. 5 is a graph showing the
dependency of attenuation pole suppression on L2/L1.
[0071] According to the graph in FIG. 4, the minimum insertion loss
increases when L2/L1 is smaller than 1, but remains generally
constant when L2/L1 is no smaller than 1. According to the graph in
FIG. 5, the attenuation pole suppression decreases when L2/L1 is
smaller than 1, but remains generally constant when L2/L1 is no
smaller than 1. As suggested by the minimum insertion loss and the
attenuation pole suppression, the resonance characteristics are
excellent when L2/L1 is no smaller than 1. This is because both the
first electrode 121 and the piezoelectric film 123 are partially
exposed to the cavity 110a when L2/L1 is no smaller than 1 (when
L2/L1 is smaller than 1, only the first electrode 121 is exposed to
the cavity 110a). However, when L2 .mu.l is greater than 1.5, the
exciter can readily warp. Unfavorably, this may increase the
alteration factors of the resonance characteristics and also make
the resonator 100 more vulnerable to damage.
[0072] Therefore, L2/L1 must be no smaller than 1, but as small as
possible in consideration of the accuracy of the manufacturing
process. Preferably, L2/L1 should be in a range of 1.about.1.5.
[0073] For the measurements, the exciter has a square configuration
whose side is L1 in length, and the opening 111a also has a square
configuration whose side is L2 in length. Since the preferable
value of the L2/L1 is in a range of 1.about.1.5, the areal ratio of
the opening 111a to the exciter should be in a range of
1.about.2.25. This holds for a case where neither the opening 111a
nor the exciter is square.
[0074] Next, description will be given to the influence which the
condition of the silicon substrate surface exerts on the
orientation of the first electrode film and the piezoelectric film
formed on the substrate. For comparison, four substrates A, B, C
and D were prepared. The substrate A was a (111)-cut silicon
substrate. The substrate B was a (100)-cut silicon substrate. The
substrate C was a (111)-cut silicon substrate formed with a
5-micrometer-thick SiO.sub.2 film by sputtering. The substrate D
was a (100)-cut silicon substrate formed with a 5-micrometer-thick
SiO.sub.2 film by sputtering. Then, each substrate was formed with
a 100 nm-thick Al film as the first electrode film by sputtering.
Further, an piezoelectric film of ZnO was formed by sputtering on
the first electrode film to have a thickness of 500 .mu.m. Then,
for each substrate, the orientation of the Al and ZnO in the formed
film was checked. Specifically, the orientation was evaluated based
on the FWHM (Full Width at Half Maximum) value of a rocking curve
obtained through X-ray diffraction. A higher FWHM value means a
higher orientation. In order to provide good piezoelectric
characteristics, the piezoelectric film preferably has an FWHM
value smaller than 6 degrees.
[0075] Table 1 shows FWHM values of the (111)-uniaxially oriented
Al and the (002)-uniaxially oriented ZnO for the Al or ZnO films
formed on the substrates A, B, C and D.
1 TABLE 1 Al ZnO Substrate A 0.71.degree. 0.86.degree. Substrate B
13.2.degree. 7.4.degree. Substrate C Nonmeasurable Nonmeasurable
Substrate D Nonmeasurable Nonmeasurable
[0076] As shown in Table 1, in the substrate A, the Al showed a
good (111) uniaxial orientation, and the ZnO showed a good (002)
uniaxial orientation. The "uniaxial orientation" here means that in
a result of a .theta.-2.theta. measurement in the X-ray
diffraction, diffraction peak intensity from undesired crystal
surfaces is not greater than {fraction (1/100)} of diffraction peak
intensity from the desired crystal surface. In the substrate B,
relatively strong diffraction peaks were observed from other
crystal surfaces such as Al (200) and the ZnO (103), showing that
no sufficient uniaxial orientation was realized in the Al-layer as
the first electrode and in the ZnO-layer as the piezoelectric film.
As shown, the substrate A made by (111) cutting has a smaller FWHM
values for Al(111) and ZnO(002) than the substrate B made by (100)
cutting, indicating that the orientation is high in both the
Al-layer as the first electrode and the ZnO layer as the
piezoelectric film. As for the substrates C and D, the SiO.sub.2
film formed on the substrate has a very rough surface, so that the
orientations in the Al and ZnO films cannot be measured
[0077] For the substrates A and B, additional orientation checking
was performed in the same manner as described above. This time, the
substrates A and B were provided with a 100 nm-thick Cu film as the
first electrode and with a 500 nm-thick ZnO film as the
piezoelectric film. The additional checking revealed that in the
substrate A, Cu showed good (111) uniaxial orientation, whereas ZnO
showed good (002) uniaxial orientation. In particular, the ZnO film
as the piezoelectric film showed a FWHM value of 1.8.degree. in the
substrate A and 9.6.degree. in the substrate B. This result
indicates that ZnO as the piezoelectric film in the substrate A has
a higher orientation than ZnO in the substrate B. Further, another
orientation checking was performed for the substrates A and B in
which a 50 nm-thick Al and an additional 100 nm-thick Mo film were
formed as the first electrode, and a 500 nm-thick AlN film was
formed as the piezoelectric film. The results are that in the
substrate A, Al showed good (111) uniaxial orientation, Mo showed
good (110) uniaxial orientation, and AlN showed good (002) uniaxial
orientation. In the substrate B, a diffraction peak from a
crystal-surface such as Mo(211) was observed in the Mo layer of the
first electrode, which reveals that there was not good uniaxial
orientation. In particular, the AlN film formed as the
piezoelectric film showed a FWHM value of 2.1 for the substrate A,
whereas the value was nonmeasurable for the substrate B.
[0078] The above result show that the substrates A and B (that is,
the silicon substrate formed with no SiO.sub.2 film) are more
suitable for the formation of oriented piezoelectric film than the
substrates C and D which are formed with SiO.sub.2 films. Further,
it is understood that the substrate A, a (111)-cut silicon
substrate, is more suitable for the formation of a highly oriented
piezoelectric film than the substrate B, a (100)-cut silicon
substrate.
[0079] FIGS. 6A.about.6E and 7A.about.7D show essential steps in
the manufacturing processes of the piezo-resonator 100 shown in
FIG. 1. The sectional views shown in these figures are taken along
the lines II-II in FIG. 1.
[0080] Specifically, as shown in FIG. 6A, a first electrode film
121 is formed on a silicon wafer 10 by sputtering. The thickness of
the resultant film 121 is 100 nm. The silicon wafer 10 is a
(111)-cut wafer. A first surface 11 and a second surface 12 of the
wafer 10 are a (111) surface. The first electrode film is made of
Al or Cu. Referring to FIG. 6B, dry etching or wet etching is
performed via a patterned resist layer (not shown) to pattern the
first electrode film 121. For the dry etching, use may be made of a
gas mixture of BCl.sub.3 and Cl.sub.2 for Al, and a gas mixture of
Ar and Cl.sub.2 for Cu. In the wet etching, for Al, the etching
solution may be a water solution containing phosphoric acid, acetic
acid and nitric acid, while for Cu, it may be a water solution of
ceric ammonium nitrite. These can be used for the subsequent
etching processes.
[0081] Next, as shown in FIG. 6C, sputtering is performed to form a
piezoelectric film 123 to a thickness of 500 .mu.m, and a second
electrode film 122 to a thickness of 100 nm. The piezoelectric film
123 is made of AlN or ZnO. The second electrode film 122 is made of
Al or Cu. Next, as shown in FIG. 6D, dry etching or wet etching is
performed via a resist pattern, to pattern the second electrode
film 122. Next, as shown in FIG. 6E, dry etching or wet etching is
performed via a mask, to pattern the piezoelectric film 123. In the
wet etching, heated phosphoric acid can be used as the etching
solution for AlN, and a water solution of acetic acid can be used
for ZnO. The patterning of the piezoelectric film 123 leaves a
resonator assembly 120 on each region to be the resonator
device.
[0082] In the above method, the resonator assembly 120 is produced
through a resist film forming step, light exposure to the resist,
and etching for the electrode film and the piezoelectric film.
According to the present invention, alternatively, the resonator
assembly 120 may be produced through a resist film forming step,
light exposure to the resist, an electrode/piezoelectric film
forming step and a lift-off step.
[0083] Next, as shown in FIG. 7A, a resist 30 is formed on the
first surface 11 of the silicon wafer 10. The resist 30 protects
the resonator assembly 120 in the subsequent steps.
[0084] Next, as shown in FIG. 7B, a resist pattern 20 is formed on
the silicon wafer 10. Specifically, a photo-resist film is formed
on the second surface 12 of the silicon wafer 10. The photo-resist
film is then exposed and developed to leave the resist pattern
20.
[0085] Next, as shown in FIG. 7C, the silicon wafer 10 is subjected
to Deep-RIE, which is a dry etching, via the resist pattern 20.
This step leaves a cavity 110a in each of the resonator device. In
the Deep-RIE, etching and sidewall protection are performed
alternately. For instance, etching with SF.sub.6 gas is performed
for about 10 seconds, to be followed by sidewall protection with
C.sub.4F.sub.8 gas which is performed for about 10 seconds. The
bias applied to the wafer is about 20 W. This forms the cavity 110a
that is generally vertical to the first surface 11 and second
surface 12 of the silicon wafer 10. In the dry etching with the use
of fluoric gas, advantageously, Al, Cu, AlN and ZnO are not etched
away. Thus, the cavity 110a can be produced without damaging the
first electrode 121 and the piezoelectric film 122. In the dry
etching, Al, Cu, AlN and ZnO serve as a stopper for the etching
process.
[0086] Additionally, according to the present invention, in the
step of forming the cavity 110a, split grooves for separating the
wafer into device elements may be formed. This eliminates a dicing
step otherwise necessary for dividing the wafer, thereby
facilitating the manufacturing of the thin-film piezo-resonator
100.
[0087] Referring to FIG. 7D, the resist pattern 20 and the resist
30 are removed. Then, through the dicing step, a plurality of
thin-film piezo-resonators are obtained.
[0088] FIG. 8 is a sectional view of a thin-film piezo-resonator
according to a second embodiment of the present invention. The
piezo-resonator 200 includes a first electrode 221 and a second
electrode 222. As seen from the figure, the first electrode 221 has
a two-fold structure which includes a first conductive layer 221a
and a second conductive layer 221b. The first conductive layer 221a
has a thickness of 50 nm, and is made of (111)-uniaxially oriented
Al or Cu. The second conductive layer 221b has a thickness of 100
nm, and is made of (110)-uniaxially oriented Mo. The second
electrode 222 has a thickness of 100 nm, and is made of Mo.
[0089] To make the piezo-resonator 200, the step shown in FIG. 6A
is replaced by a step in which the first and the second conductive
layers 221a, 221b are formed in a stacking manner. Then, in the
next step (corresponding to the step shown in FIG. 6B), the first
and the second conductive layers 221a, 221b are subjected to
patterning together. The wet etching for this patterning and the
wet etching for patterning the second electrode 222 may be
performed with etchant such as a water solution containing
phosphoric acid, acetic acid and nitric acid. Except for the
above-mentioned difference, the piezo-resonator 200 of the second
embodiment is similar in arrangement to the piezo-resonator 100 of
the first embodiment, and therefore can be fabricated in a similar
manner.
[0090] In the piezo-resonator 200 again, it is possible to form the
highly oriented piezoelectric film 123 of e.g. AlN and ZnO on the
(111)-cut silicon substrate 110.
[0091] According to the present invention, the first electrode may
be composed of a single conductive layer or multiple conductive
layers each of which is uniaxially oriented. In the latter case,
the lowest conductive layer (the one held in direct contact with
the silicon substrate 110) is preferably provided by
(111)-uniaxially oriented Al or Cu.
[0092] FIG. 9 shows, in section, a thin-film piezo-resonator
according to a third embodiment of the present invention. The
illustrated piezo-resonator 300 includes a piezo-resonator 100 as
shown in FIG. 2 and a 200 .mu.m-thick cover substrate 50 attached
to the resonator 100 so as to close the cavity 110a. More
specifically, the cover substrate 50 has an upper surface formed
with a Au--Sn film 51 as a bonding layer. The Au--Sn film 51 has a
thickness of 5 .mu.m and is produced by sputtering for example. The
cover substrate 50 is attached to the second surface 112 of the
silicon substrate 110 via the Au--Sn film 51. The silicon substrate
110 and the cover substrate 50 will be secured to each other after
they are heated up and kept at about 310 C.degree. for 30
minutes.
[0093] The cover substrate structure described above protects the
resonator assembly 120 from being damaged through the resonator
fabrication process. For instance, when the second surface 112 is
to be bonded to a motherboard with conductive paste, the cover
substrate 50 prevents the paste from penetrating into the cavity
110a. Also, the cover substrate 50 keeps the first electrode 121 or
piezoelectric film 123 from being damaged when the resonator 300 is
sucked up by a suction collet for performing flip chip bonding.
[0094] According to the present invention, the bonding material may
not necessarily be Au--Sn film. A different kind of metal material
or resin material (such as epoxy) may be used. Further, without
relying on a bonding paste, direct bonding or anodic bonding may be
employed for fixing the cover substrate 50 to the silicon substrate
110.
[0095] FIGS. 10 and 11 show a thin-film piezoelectric resonator 400
according to a fourth embodiment of the present invention. FIG. 10
is a plan view, while FIG. 11 is a sectional view taken along lines
XI-XI.
[0096] The resonator 400 includes a silicon substrate 410 and a
resonator assembly 420 formed on the substrate. The substrate 410
is a (111)-cut monocrystal silicon substrate and includes a first
surface 411 and a second surface 412 both of which are a (111)
surface. The resonator assembly 420 is composed of a first
electrode 421, a second electrode 422 and a piezoelectric film 423
disposed between these electrodes. The first and the second
electrodes 421, 422 include circular electrode portions 421a and
422a, respectively. As seen from FIG. 11, the circular electrode
portions 421a, 422a are of the same size and aligned vertically,
with the piezoelectric film 423 intervening therebetween. The
combination of the electrode portions 421a and 422a serves as the
exciter of the resonator 400. In the illustrated example, the
diameter L1' of the exciter is about 50 .mu.m. The first electrode
421, the second electrode 422 and the piezoelectric film 423 of the
fourth embodiment are identical in thickness and material to the
counterparts of the resonator 100 according to the first
embodiment.
[0097] The silicon substrate 410 is formed with a cavity 410a
located immediately below the exciter of the resonator 420. The
cavity 410a extends through the substrate 410 perpendicularly to
the first and the second surfaces 411, 412. The cavity 410a has a
first circular opening 411a opened in the first surface 411 and a
second circular opening 412a opened in the second surface 412. In
the illustrated example, the constant diameter L2' of the cavity
410a is about 60 .mu.m. With this arrangement, the resonator 400
can be made compact since the second opening 412a is not larger
than the first opening 411a (cf. the conventional resonator 700 of
FIG. 20). As shown in FIG. 11, the circular electrode portion 421a
and the piezoelectric film 423 are exposed to the cavity 410a,
which is advantageous to provide excellent resonance
characteristics. The resonator 400 may be fabricated in the same
manner as the resonator 100.
[0098] FIGS. 12.about.14 show a band-pass filter 500 according to a
fifth embodiment of the present invention. FIG. 12 is a plan view,
while FIG. 13 is a sectional view taken along lines XIII-XIII in
FIG. 12, and FIG. 14 is a sectional view taken along lines XIV-XIV
in FIG. 12.
[0099] The band-pass filter 500 includes a silicon substrate 110
upon which are provided a first electrode pattern 121, a second
electrode pattern 122 and a piezoelectric film 123 disposed between
the first and the second electrode patterns. These three elements
are so arranged as to form four thin-film piezoelectric resonators
100A (connected in series) and four thin-film piezoelectric
resonators 100B (connected in parallel). Each of the resonators
100A and 100B corresponds to the resonator 100 of the first
embodiment. For connection to an external device, circuit, etc.,
the first electrode pattern 121 includes a pair of exposed terminal
portions 60A and another pair of exposed terminal portions 60B. In
order to make the resonance frequency of the serial resonators 100A
significantly different from that of the parallel resonators 100B,
the parallel resonators 100B may be covered with a 50 nm-thick
aluminum layer formed on the second electrode pattern 122. The
band-pass filter 500 is a ladder type filter in which eight
thin-film piezoelectric resonators 100A, 100B are integrally
provided on a single silicon substrate 110. FIG. 19 is a circuit
diagram of the band-pass filter 500.
[0100] As shown in FIG. 12, each resonator 101A has a square
exciter whose side length L3 is 75 .mu.m. The corresponding cavity
110a, as shown in FIG. 13, has a square opening whose side length
L5 is 80 .mu.m. The clearance L6 between the cavities 110a of the
adjacent resonators 100A is 20 .mu.m. In the resonator 100B, the
side length L4 of the square exciter is 50 .mu.m. The corresponding
cavity 110a, as shown in FIG. 14, has a square opening whose side
length L7 is 55 .mu.m. The clearance L8 between the cavities 110a
of the adjacent resonators 100B is 45 .mu.m. In the band-pass
filter 500, each cavity 110a corresponds in position to the exciter
of a resonator 100A or 100B, wile also extending perpendicularly to
the first surface 111 of the substrate 110. Accordingly, the
respective resonator assemblies 120 (and hence the resonators 100A,
100B) can be disposed closer to each other than is conventionally
possible, which is advantageous to making the band-pass filter
compact. Further, the close arrangement of the resonator assemblies
120 advantageously reduces the resistance of the wiring pattern
between them.
[0101] As shown in FIG. 13, the first electrode pattern 121 and the
piezoelectric film 123 for each resonator 100A are partially
exposed to the corresponding cavity 111a. This holds for each of
the parallel resonators 100B. Such exposure ensures excellent
resonance characteristics for the respective resonators 100A, 100B.
Accordingly, the band-pass filter 500 also has excellent resonance
characteristics. The band-pass filter 500 may be fabricated in
substantially the same manner as the resonator 100 of the first
embodiment. More precisely, the fabrication method of the resonator
100 may be modified in a manner obvious to the person skilled in
the art so that the predetermined number of resonators 100 will be
integrally formed on the same substrate.
[0102] In accordance with the present invention, the first
electrode pattern may be a laminate of a 50 nm-thick aluminum layer
and a 100 nm-thick molybdenum layer, while the second electrode
pattern may be a single, 100 nm-thick molybdenum layer, as in the
resonator 200 of the second embodiment (FIG. 8). Further, as in the
resonator 300 of the third embodiment (FIG. 9), a cover substrate
may be attached to the substrate 110 from below.
[0103] FIGS. 15-18 show a band-pass filter 600 according to a sixth
embodiment of the present invention. FIG. 15 is a plan view, while
FIGS. 16, 17 and 18 are a sectional view taken along lines XVI-XVI,
XVII-XVII and XVIII-XVIII in FIG. 15, respectively.
[0104] The band-pass filter 600 includes a silicon substrate 410
upon which are provided a first electrode pattern 421, a second
electrode pattern 422 and a piezoelectric film 423 disposed between
these electrode patterns. As in the fifth embodiment described
above, the electrode patterns 421, 422 and the piezoelectric film
423 are arranged to produce four serially-connected resonators 400A
and four parallel resonators 400B. Each of the resonators 400A and
400B corresponds to the resonator 400 of the fourth embodiment
(FIGS. 10 and 11). For connection to an external device, a circuit,
etc., the first electrode pattern 421 includes a pair of terminal
portions 70A and another pair of terminal portions 70B. As in the
previous embodiment, preferably the parallel resonators 400B are
covered with a 50 nm-thick aluminum layer (formed on the second
electrode pattern 422) for significantly differentiating the
resonance frequency of the serial resonators 400A from that of the
parallel resonators 400B. The band-pass filter 600 is a ladder type
filter in which eight thin-film piezoelectric resonators 400A, 400B
are integrally provided on a single silicon substrate 410. The
band-pass filter 600 is also represented by the circuit diagram
shown in FIG. 19.
[0105] The diameter L9 (FIG. 15) of the circular exciter of the
resonator 400A is 85 .mu.m, while the diameter L11 (FIG. 16) of the
opening of the corresponding cavity 410a is 90 .mu.m. The distance
L12 (FIG. 16) between the cavities 410a of the adjacent resonators
400A is 20 .mu.m. The length L10 (FIG. 15) of the exciter of the
resonator 400B is 55 .mu.m. As shown in FIGS. 17 and 18, the
diameter L13 of the opening of the corresponding cavity 410a is 60
.mu.m. The distance L14 (FIG. 17) between the cavities 410a of the
adjacent resonators 400B is 50 .mu.m. As in the previous
embodiment, the band-pass filter 600 can be made compact by
disposing the resonator assemblies 420 close to each other.
[0106] As shown in FIGS. 16 and 17, the first electrode pattern 421
and the piezoelectric film 423 for each resonator 400A or 400B are
partially exposed to the corresponding cavity 410a. Such exposure
ensures excellent resonance characteristics for the respective
resonators 400A, 400B. Accordingly, the band-pass filter 600 also
has excellent resonance characteristics. The band-pass filter 600
may be fabricated in substantially the same manner as the resonator
100 of the first embodiment. More precisely, the fabrication method
of the resonator 100 may be modified in a manner obvious to the
person skilled in the art so that the predetermined number of
resonators 400 will be integrally formed on the same substrate.
[0107] In accordance with the present invention, the first
electrode pattern may be a laminate of a 50 nm-thick aluminum layer
and a 100 nm-thick molybdenum layer, while the second electrode
pattern may be a single, 100 nm-thick molybdenum layer, as in the
resonator 200 of the second embodiment (FIG. 8). Further, as in the
resonator 300 of the third embodiment (FIG. 9), a cover substrate
may be attached to the substrate 410 from below. The exciter may be
elliptical or oval rather than circular.
[0108] The fifth and sixth embodiments relate to band-pass filters,
as described above. It should be noted that the thin-film
piezoelectric resonators of the present invention can be used for
providing a high-pass filter or low-pass filter.
[0109] The present invention being thus described, it is obvious
that the same may be varied in many ways. Such variations are not
to be regarded as a departure from the spirit and scope of the
present invention, and all such modifications as would be obvious
to those skilled in the art are intended to be included within the
scope of the following claims.
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