U.S. patent application number 10/909335 was filed with the patent office on 2005-08-04 for film bulk acoustic wave resonator, film bulk acoustic wave resonator filter and method of manufacturing film bulk acoustic wave resonator.
This patent application is currently assigned to Hitachi Media Electronics Co., Ltd.. Invention is credited to Asai, Kengo, Hikita, Mitsutaka, Isobe, Atsushi, Matsumoto, Hisanori.
Application Number | 20050168102 10/909335 |
Document ID | / |
Family ID | 34805884 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050168102 |
Kind Code |
A1 |
Matsumoto, Hisanori ; et
al. |
August 4, 2005 |
Film bulk acoustic wave resonator, film bulk acoustic wave
resonator filter and method of manufacturing film bulk acoustic
wave resonator
Abstract
The present invention provides a film bulk acoustic wave
resonator (FBAR) filter that can keep the Q factor high. The FBAR
filter comprises a first FBAR with a first resonant frequency and a
second FBAR with a second resonant frequency, formed on a same
substrate. The FBAR filter has such a structure that a first
underlayer is formed between the substrate and a first bottom
electrode layer and a second underlayer is formed between the
substrate and a second bottom electrode layer, the first underlayer
thickness being different from the second underlayer thickness.
Inventors: |
Matsumoto, Hisanori;
(Kokubunji, JP) ; Asai, Kengo; (Hachioji, JP)
; Isobe, Atsushi; (Kodaira, JP) ; Hikita,
Mitsutaka; (Hachioji, JP) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE
SUITE 500
MCLEAN
VA
22102-3833
US
|
Assignee: |
Hitachi Media Electronics Co.,
Ltd.
|
Family ID: |
34805884 |
Appl. No.: |
10/909335 |
Filed: |
August 3, 2004 |
Current U.S.
Class: |
310/312 |
Current CPC
Class: |
H03H 3/04 20130101; H03H
2003/023 20130101; H03H 9/174 20130101; H03H 9/02157 20130101; H03H
2003/0471 20130101; H03H 9/564 20130101; H03H 9/568 20130101; H03H
2003/0442 20130101 |
Class at
Publication: |
310/312 |
International
Class: |
H01L 041/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2004 |
JP |
2004-027590 |
Claims
What is claimed is:
1. A film bulk acoustic wave resonator comprising: a substrate
having a cavity; an underlayer formed on said substrate to cover
said cavity; a bottom electrode layer formed on top of said
underlayer; a piezoelectric layer formed on top of said bottom
electrode layer; and a top electrode layer formed on top of said
piezoelectric layer, wherein said resonator's resonant frequency
can be adjusted, depending on the thickness of said underlayer over
said cavity.
2. The film bulk acoustic wave resonator according to claim 1,
wherein at least either of said bottom electrode layer and said top
electrode layer is made of a material which consists chiefly of at
least one of the following: molybdenum and tungsten.
3. The film bulk acoustic wave resonator according to claim wherein
said underlayer is made of a material which consists chiefly of at
least one of the following: silicon oxide, silicon nitride,
alumina, aluminum nitride, and silicon carbide.
4. A film bulk acoustic wave resonator filter including a plurality
of film bulk acoustic wave resonators comprising: a substrate
having a plurality of cavities; an underlayer formed on said
substrate to cover said cavities; a bottom electrode layer formed
on top of said underlayer; a piezoelectric layer formed on top of
said bottom electrode layer; and a top electrode layer formed on
top of said piezoelectric layer, wherein the underlayer thickness
in a first region where a first resonator out of said plurality of
resonators is formed differs from the underlayer thickness in a
second region where a second resonator out of said plurality of
resonators is formed.
5. The film bulk acoustic wave resonator filter according to claim
4, wherein at least either of said bottom electrode layer and said
top electrode layer is made of a material which consists chiefly of
at least one of the following: molybdenum and tungsten.
6. The film bulk acoustic wave resonator filter according to claim
4, wherein said underlayer is made of a material which consists
chiefly of at least one of the following: silicon oxide, silicon
nitride, alumina, aluminum nitride, and silicon carbide.
7. A transmitting and receiving filter complex including a
plurality of film bulk acoustic wave resonators comprising: a
substrate having a plurality of cavities; an underlayer formed on
said substrate to cover said cavities; a bottom electrode layer
formed on top of said underlayer; a piezoelectric layer formed on
top of said bottom electrode layer; and a top electrode layer
formed on top of said piezoelectric layer, wherein said plurality
of resonators' resonant frequencies can be adjusted, depending on
the thickness of said underlayer over said cavities, wherein a
first resonator out of said plurality of resonators and a second
resonator out of said plurality of resonators constitute a
transmitting filter and a third resonator out of said plurality of
resonators and a fourth resonator out of said plurality of
resonators constitute a receiving filter, and wherein the
underlayer thickness in a first region where said first resonator
is formed, the underlayer thickness in a second region where said
second resonator is formed, the underlayer thickness in a third
region where said third resonator is formed, and the underlayer
thickness in a fourth region where said fourth resonator is formed
differ one another.
8. The transmitting and receiving filter complex according to claim
7, wherein at least either of said bottom electrode layer and said
top electrode layer is made of a material which consists chiefly of
at least one of the following: molybdenum and tungsten.
9. The transmitting and receiving filter complex according to claim
7, wherein said underlayer is made of a material which consists
chiefly of at least one of the following: silicon oxide, silicon
nitride, alumina, aluminum nitride, and silicon carbide.
10. A method of manufacturing a film bulk acoustic wave resonator
filter comprising a plurality of film bulk acoustic wave resonators
on a single substrate, comprising steps of: forming a trench with a
depth on said single substrate, the depth corresponding to a
difference between a first resonator's resonant frequency and a
second resonator's resonant frequency out of said plurality of
resonators; and depositing an underlayer over said single
substrate.
11. The method of manufacturing the film bulk acoustic wave
resonator filter according to claim 10, further comprising steps
of: smoothing to make said underlayer surface in a first region
where said first resonator is formed substantially flush with said
underlayer surface in a second region where said second resonator
is formed depositing a bottom electrode layer over the surface and
patterning said bottom electrode layer, thereby forming a first
bottom electrode layer on top of the underlayer in said first
region and a second bottom electrode layer on top of the underlayer
in said second region; depositing a piezoelectric layer over the
surface; depositing a top electrode layer over the surface and
patterning said top electrode layer, thereby forming a first top
electrode layer on top of the piezoelectric layer in said first
region and a second top electrode layer on top of the piezoelectric
layer in said second region; and from the back side of said single
substrate, forming a first cavity in said first region and a second
cavity in said second region by trenching, wherein said step of
depositing said underlayer over said single substrate deposits said
underlayer over said single substrate in which said trench has been
formed.
12. The method of manufacturing the film bulk acoustic wave
resonator filter according to claim 10, further comprising steps
of: depositing a bottom electrode layer over the surface and
patterning said bottom electrode layer, thereby forming a first
bottom electrode layer on top of the underlayer in said first
region and a second bottom electrode layer on top of said
underlayer in said second region where said second resonator is
formed; depositing a piezoelectric layer over the surface;
depositing a top electrode layer over the surface and patterning
said top electrode layer, thereby forming said first region's top
electrode layer and said second region's top electrode layer; and
from the back side of said single substrate, forming a first cavity
in said first region and a second cavity in said second region by
trenching, wherein said film bulk acoustic wave resonator filter
comprises the plurality of resonators with different underlayer
thicknesses on said single substrate, and wherein said step of
forming the trench forms the trench in said underlayer in the first
region where said first resonator is formed.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese
application serial no. JP 2004-027590 filed on Feb. 0.4, 2004, the
content of which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to film bulk acoustic wave
resonator filters employing film bulk acoustic wave resonators
(hereinafter referred to as FBARs) and a manufacturing method
thereof.
[0004] 2. Description of the Prior Art
[0005] With the proliferation of mobile communication typified by
mobile telephony, there is an increasing need for radio frequency
filters for several hundred MHz to several GHz. Among many types of
filter technologies, a filter that is constructed from FBARs has
features of excellence in the following respects: (1) higher
frequency operation, (2) smaller construction, (3) temperature
characteristics, and (4) high voltage immunity.
[0006] A basic FBAR structure is as follows: a piezoelectric layer
is sandwiched between a bottom electrode layer and a top electrode
layer and an underlayer supports this sandwich over a cavity (for
example, refer to non-patent document 1). By applying an electric
signal between the two electrodes isolated by the piezoelectric
layer, bulk acoustic waves are excited in the piezoelectric layer.
At this time, the path of the bulk acoustic waves is a diaphragm
structure consisting of the sandwich and the underlayer. Because
the top and bottom of the diaphragm structure are acoustically
isolated by air, the bulk acoustic waves reflect from the
interfaces between the diaphragm structure and the air, which
prevents the acoustic energy from leaking out of the diaphragm
structure.
[0007] To facilitate fabricating a number of FBARs with different
resonant frequencies on a single substrate, the following improved
FBAR structures are known: a structure in which a loading electrode
is added to the bottom surface of the bottom electrode layer (for
example, refer to patent document 1); a structure in which a
loading layer is added to the surface of the piezoelectric layer
(for example, refer to patent document 2); and a structure in which
a loading electrode is added to the surface of the top electrode
layer (for example, refer to patent document 3).
[0008] An FBAR filter employing FBARs for a 5 GHz, high-speed
wireless LAN has lately been made in pubic (for example, refer to
non-patent document 2). In this publication, only a general
description is provided as: "designed topology consists of four
stages of ladder type filters in which mass loads are applied to
shunt resonators only."
[0009] [Patent document 1]
[0010] JP-A No. 2002-299980
[0011] [Patent document 2]
[0012] JP-A No. 2002-299979
[0013] [Patent document 3]
[0014] JP-A No. 2002-335141
[0015] [Non-patent document 1]
[0016] 1994 IEEE International Frequency Control Symposium, pp.
135-138
[0017] [Non-patent document 2]
[0018] 2002 IEEE Ultrasonics Symposium, pp. 969-972
SUMMARY OF THE INVENTION
[0019] The FBAR is a device that converts electrical signals input
thereto into mechanical vibration and resonates at a frequency that
is in inverse proportion to the thickness of its diaphragm
structure. Taking advantage of this property, the FBAR can be used
as a radio frequency resonator in electronic circuitry.
[0020] To fabricate an FBAR filter, it is necessary to electrically
connect two ore more FBARs with different resonant frequencies.
Although the FBARs with two different resonant frequencies are
sufficient for essential filter use, three or more resonators with
different resonant frequencies may be required in designing the
FBAR filter to accommodate a broader frequency range. To construct
a communicating means with both transmitting and receiving
functions, such as a mobile phone, a transmit/receive switching
device with two filters, namely, a transmitting filter for
transmitting frequency and a receiving filter for receiving
frequency must be fabricated.
[0021] From viewpoints of making device size smaller and cost
reduction, FBAR filters are required to be fabricated on a single
substrate and, ultimately, fabricating the transmitting and
receiving filters on the single substrate is hoped for.
[0022] Because the FBAR resonates at a frequency that is one half
the wavelength of the total thickness of its diaphragm structure,
its resonant frequency is clearly determined from the total
thickness of its diaphragm structure, the acoustic velocities of
the underlayer material, bottom electrode layer material,
piezoelectric layer material, and top electrode layer material. In
other words, by changing the total thickness of the diaphragm
structure, the resonant frequency of the FBAR can be controlled. To
construct the FBAR filter or filters on a single substrate, it is
required to change the total thickness of the diaphragm structure
for each FBAR.
[0023] However, the above-mentioned documents disclose nothing
about problems associated with forming a plurality of FBARs with
different thicknesses on a single substrate, resonance
characteristics, and FBAR structures and fabrication procedures to
realize such FBARs without affecting the quality of their main
sandwiches.
[0024] Because the means provided in the patent documents 1 through
3 are all adding a loading layer to the sandwich that is the main
FBAR portion, an additional process is required in forming the
sandwich and it affects the FBAR performance.
[0025] Specifically, after forming the loading layer, the FBAR
device must be removed from vacuum equipment for patterning the
loading layer in air environment. Consequently, the loading layer
surface is oxidized, which causes deteriorating the resonance
characteristics. Because of the additional process of patterning
the loading layer, the number of times the device is exposed to a
photoresist material, developer, etching liquid or gas, etc.
increases. As a result, factors to degrade the film quality of the
sandwich significantly increase, which leads to deteriorating the
resonance characteristics.
[0026] FIG. 12 shows an FBAR equivalent circuit. Here, R.sub.1 is
loss of the bottom electrode layer, R.sub.2 is loss of the
piezoelectric layer, and R.sub.3 is loss of the top electrode
layer. With degradation of the film qualities of these layers,
R.sub.1, R.sub.2, and R.sub.3 increase. Q factor of the resonator
can be expressed by the following equation:
Q=.omega..sub.rL/(R.sub.1+R.sub.2+R.sub.3) Equation 1
[0027] where, .omega..sub.r is a resonant frequency. From equation
1, it is obvious that an increase in the loss of the layers due to
film quality degradation thereof decreases the Q factor of the
FBAR.
[0028] To construct the FBAR filter, it is required to make a
plurality of FBARs resonate at different resonant frequencies and
to prevent the factors to decrease the quality, especially, the Q
factor of the sandwich that is the main FBAR portion from being
introduced during its fabrication.
[0029] An object of the present invention is to provide an FBAR
filter having a structure that prevents the FBAR performance,
especially, the Q factor, from decreasing, and a method of
manufacturing the FBAR filter.
[0030] By way of illustration, a typical means of the present
invention is described below. A film bulk acoustic wave resonator
according to the present invention comprises a substrate having a
cavity, an underlayer formed on the substrate to cover the cavity,
a bottom electrode layer formed on top of the underlayer, a
piezoelectric layer formed on top of the bottom electrode layer,
and a top electrode layer formed on top of the piezoelectric layer,
characterized in that its resonant frequency can be adjusted,
depending on the thickness of the underlayer over the cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a cross sectional diagram of an FBAR device as a
first embodiment of the present invention.
[0032] FIG. 2 is a cross sectional diagram set to explain an
exemplary procedure of fabricating the FBAR device shown in FIG. 1
in process sequence.
[0033] FIG. 3 is a cross sectional diagram of an FBAR device as a
second embodiment of the present invention.
[0034] FIG. 4 is a cross sectional-diagram set to explain an
exemplary procedure of fabricating the FBAR device shown in FIG. 3
in process sequence.
[0035] FIG. 5 is a circuit block diagram of a general mobile
phone's front end portion.
[0036] FIG. 6 is a circuit block diagram of a sub-portion
comprising a couple of transmitting and receiving filters
constructed with an array of FBARs in the front end portion shown
in FIG. 5.
[0037] FIG. 7 is an external perspective view schematic of the
transmitting filter made up of FBARs of the present invention,
fabricated on a single substrate.
[0038] FIG. 8 is a cross sectional view of FBARs as an example of
application of the FBAR structure of Embodiment 1.
[0039] FIG. 9 is a cross sectional view of FBARs as an example of
application of the FBAR structure of Embodiment 2.
[0040] FIG. 10 is a cross sectional view of four FBARs to which the
FBAR structure of Embodiment 1 is applied, the four FBARs
constituting the couple of transmitting and receiving filters on
the single substrate.
[0041] FIG. 11 is a cross sectional view of four FBARs to which the
FBAR structure of Embodiment 2 is applied, the four FBARs
constituting the couple of transmitting and receiving filters on
the single substrate.
[0042] FIG. 12 is an FBAR equivalent circuit diagram.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Preferred embodiments of the present invention will be
described hereinafter with reference to the accompanying drawings
and through concrete embodiment examples.
Embodiment 1
[0044] FIG. 1 is a cross sectional diagram of an FBAR device which
is one example f embodiment of the present invention. In FIG. 1, a
region where a first FBAR 11 is formed will be called a first
region and a region where a second FBAR 12 is formed will be called
a second region. The first and second FBARs have their diaphragm
structures which are respectively formed over cavities 9 and 10
formed on the back side of a single substrate. The diaphragm
structure in the first region comprises an underlayer 2, bottom
electrode layer 3, piezoelectric layer 5, and top electrode layer
7. The diaphragm structure in the second region comprises an
underlayer 131, bottom electrode layer 4, piezoelectric layer 6,
and top electrode layer 8.
[0045] The thickness t2 of the underlayer 131 of the diaphragm
structure in the second region is thicker than the thickness t1 of
the underlayer 2 of the diaphragm structure in the first region.
Therefore, the thickness T2 of the diaphragm structure including
the underlayer 131 in the second region is thicker than the
thickness T1 of the diaphragm structure including the underlayer 2
in the first region. As a result, the resonant frequency of the
second FBAR 12 is lower than the resonant frequency of the first
FBAR 11. In this relation, a preferable material of the
piezoelectric layers 5 and 6 is aluminum nitride (AlN).
[0046] The area of the underlayer 131 over the cavity 10 should be
equal to or larger than a section where the bottom electrode layer
4 and the top electrode layer 8 are formed and a complete match
between the above area and the above section is not required. This
is also true for the first region over the cavity 9.
[0047] Although the FBARs with two different resonant frequencies
formed on the single substrate are shown in FIG. 1, it is also
possible to form FBARs with three or more different resonant
frequencies on the single substrate in the same way. For each FBAR,
its resonant frequency depends on the diaphragm thickness including
the underlayer. In the structure of this embodiment, the underlayer
thickness can be changed without affecting the main sandwich
structures and, thus, the FBARs with different frequencies can
easily be constructed on the same chip.
[0048] FIG. 2 is a cross sectional diagram set to explain a
procedure of fabricating the FBAR structure shown in FIG. 1 in
process sequence. Now, it is assumed that the resonant frequencies
of the first and second FBARs are f1 and f2 (f1>f2); for
instance, f1=1.9 GHz, f2=1.75 GHz. By way of example, a case where
AlN is used as the piezoelectric layer material is discussed
herein; of course, however, this is not restrictive.
[0049] Accordingly, the thickness T1 of the diaphragm structure
consisting of the sandwich and underlayer, which resonates at the
higher resonant frequency f1 out of the above set resonant
frequencies, is 1800 nm, and the thickness of T2 of the diaphragm
structure, which resonates at the lower resonant frequency f2, is
1950 nm. To yield a higher resonant frequency, it is advisable to
make the thickness of the underlayers 2, 131 thinner or use a
material having a property of a higher acoustic velocity for the
piezoelectric layers 4. 6.
[0050] Then, the FBAR fabrication procedure will be explained
below, according to the process diagram set of FIG. 2.
[0051] First, form a trench 69 with a depth of .DELTA.t (100 nm) on
the surface of the substrate 1 in the second region (see diagrams A
and B in FIG. 2). The depth .DELTA.t corresponds to a difference
.DELTA.f (.DELTA.f=f1-f2) between the resonant frequency of the
first FBAR 11 and the resonant frequency of the second FBAR 12.
[0052] Next, deposit an underlayer (300 nm thick) over the entire
surface of the substrate and form the underlayer 2 in the first
region and the underlayer 131 in the second region (see diagram C
in FIG. 2).
[0053] Next, perform smoothing, for example, with gas cluster ion
beam (GCIB), to make the underlayer 2 surface in the first region
flush with the underlayer 131 surface in the second region (see
diagram Din FIG. 2). This process' forms the underlayers 2, 131
having a thickness difference .DELTA.t (.DELTA.t=t2-t1) which
corresponds to .DELTA.f.
[0054] Next, after depositing a 200-nm thick bottom electrode
layer, form the bottom electrode layer 3 on top of the underlayer 2
in the first region by patterning and form the bottom electrode
layer 4 on top of the underlayer 131 in the second region by
patterning (see diagram E in FIG. 2).
[0055] Next, deposit a 1500-nm thick piezoelectric layer and form
the piezoelectric layer 5 in the first region and the piezoelectric
layer 6 in the second region (see diagram F in FIG. 2).
[0056] Next, after depositing a 200-nm thick top electrode layer,
form the first top electrode layer 7 on top of the piezoelectric
layer 5 in the first region by patterning and form the second top
electrode layer 8 on top of the piezoelectric layer 6 in the second
region (see diagram G in FIG. 2).
[0057] Then, from the back side of the substrate 1, form the cavity
9 in the first region and the cavity 10 in the second region. In
this way, the two diaphragm structures with different thicknesses
shown in FIG. 1 are constructed.
[0058] By fabricating the FBAR device as above, the underlayers
with different thicknesses are formed without affecting the main
sandwich structures and, consequently, the diaphragm structures
with different thicknesses can be achieved. Thus, the FBARs with
different resonant frequencies can easily be constructed on the
single substrate.
[0059] Depositing the bottom and top electrode layers is normally
performed by sputtering and the piezoelectric layer can be grown by
sputtering or chemical vapor deposition (CVD).
[0060] Patterning the layers can be performed by wet or dry
etching. The trench 69 can be formed by GCIB; otherwise, it can be
formed by using other techniques such as dry etching, wet etching,
and ion milling. Smoothing to make the underlayer 2 surface in the
first region flush with the underlayer 131 surface in the second
region can be performed by chemical mechanical polishing (CMP)
besides GCIB. To form the cavities 9 and 10, dry or wet etching can
be used.
[0061] As the material of the bottom and top electrode layers,
preferably, molybdenum (Mo) or tungsten (W) may be used. Because Mo
and W are materials of a high acoustic velocity, they have superior
acoustic wave transmission properties. Another advantage of Mo and
W is that they are easy to process by wet etch patterning. Other
materials such as aluminum (Al), tantalum (Ta), nickel (Ni),
niobium (Nb), gold (Au), platinum (Pt), cupper (Cu), palladium
(Pd), titanium (Ti), and alloys thereof can be used as the material
of the bottom and top electrode layers.
[0062] As the material of the underlayers, preferably, any material
may be selected from the following: silicon oxide (SiO.sub.2),
silicon nitride (Si.sub.3N.sub.4), alumina (Al.sub.2O.sub.3), AlN,
and silicon carbide (SiC). Because the underlayers must support the
sandwiches with hollows being under them, it is desirable that they
have a high mechanical strength. In this respect, SiO.sub.2,
Si.sub.3N.sub.4, Al.sub.2O.sub.3, AlN, and SiC are preferable.
Other materials such as zinc oxide (ZnO), lead zirconate titanate
(PZT), lead titanate (PbTiO.sub.3), and barium titanate
(BaTiO.sub.3) can be used as the material of the underlayers.
[0063] As the material of the piezoelectric layers, besides AlN,
ZnO, PZT, PbTiO.sub.3, and BaTiO.sub.3 can be used. The
aforementioned thicknesses of the layers are exemplary and,
needless to say, can be designed and changed, as appropriate,
according to employed materials and required resonant frequencies.
The diaphragm structures of the present FBARs can be fabricated to
have thickness in the range of 300 nm to 7 .mu.m in the state of
the art of fabrication and this technology is applicable for
resonant frequencies of 0.5 GHz to the order of 10 GHz.
Embodiment 2
[0064] FIG. 3 is a cross sectional diagram of an FBAR device which
is another example of embodiment of the present invention. In FIG.
3, a region where a first FBAR 23 is formed will be called a first
region and a region where a second FBAR 24 is formed will be called
a second region. By way Qf example, the same case as for Embodiment
1 is also discussed herein; that is, the resonant frequencies of
the FBARs are 1.9 GHz and 0.1.75 GHz, respectively, and AlN is used
as the piezoelectric layer material. Therefore, the required
thickness T1 of the diaphragm structure in the first region and the
required thickness T2 of the diaphragm structure in the second
region are the same as for Embodiment 1.
[0065] In FIG. 3, the first FBAR 23 and second FBAR 24 are formed
on the single substrate, on the back side of which cavities 21 and
23 are formed thereunder, respectively. The thickness t1 of the
underlayer 14 of the diaphragm structure in the first region is
thinner than the thickness t2 of the underlayer of the diaphragm
structure in the second region. As a result, the resonant frequency
f1 of the first FBAR 23 is higher than the resonant frequency f2 of
the second FBAR 24.
[0066] The area of the thinner underlayer 14 over the cavity 21 in
the first region should be equal to or larger than a section where
a bottom electrode layer 15 and top electrode layer 19 are formed
in the first region and a complete match between the above area and
the above section is not required. This is also true for the second
region over the cavity 22.
[0067] FIG. 4 is a cross sectional diagram set to explain a
procedure of fabricating the FBAR structure shown in FIG. 3 in
process sequence.
[0068] First, deposit a 200-nm thick underlayer over the surface of
the substrate (see diagrams A and B in FIG. 4).
[0069] Next, form a trench 70 with a depth of .DELTA.t
(.DELTA.ft=t2-t1) in the underlayer 14 in the first region (see
diagram C in FIG. 4). The depth .DELTA.t corresponds to a
difference .DELTA.f (.DELTA.f=f1-f2) between the resonant frequency
f1 of the first FBAR 23 and the resonant frequency f2 of the second
FBAR 24.
[0070] Next, after depositing a bottom electrode layer, form the
bottom electrode layer 15 on top of the underlayer 14 in the first
region by patterning and form the bottom electrode layer 16 on top
of the underlayer 132 in the second region (see diagram D in FIG.
4).
[0071] Next, deposit a piezoelectric layer which is on the order of
950 nm thick (see diagram E in FIG. 4).
[0072] Next, after depositing a top electrode layer, form the top
electrode layer 19 on top of the piezoelectric layer 17 in the
first region by patterning and form the top electrode layer 20 on
top of the piezoelectric layer 18 in the second region by
patterning (see diagram F in FIG. 4) Then, from the back side of
the substrate 13, form the cavity 21 in the first region and the
cavity 22 in the second region. In this way, the FBAR structure
shown in FIG. 3 is constructed.
[0073] The trench 70 can be formed by wet etching, dry etching, ion
milling, and GCIB.
[0074] The fabrication procedure of Embodiment 2 is advantageous in
that it dispenses with smoothing by GCIB and, accordingly, the FBAR
device is constructed with a reduced number of processes as
compared with Embodiment 1.
[0075] The trench 70 in the underlayer can also be formed in the
following method: after completely removing the underlayer in the
region where the trench is to be formed until the substrate 13
surface is exposed, deposit the underlayer material over the
surface again. This method neutralizes the effect of reducing the
number of processes. However, its merit is that the underlayer can
be formed to a thickness that is more accurate than in the case of
forming the trench 70 by trenching.
Embodiment 3
[0076] This section describes constructing a filter with FBARs on a
single substrate as a third embodiment example of the
invention.
[0077] FIG. 5 is a block diagram of a general mobile phone's front
end portion. In FIG. 5, reference numeral 130 denotes a phase
shifter which enables an antenna to be shared by a receiving
portion and a transmitting portion. Radio frequency (RF) received
signals Rx received by the antenna ANT pass through the phase
shifter 130 and through a receiving filter 79 which removes image
frequency signals and allows only signals falling within a
predetermined frequency range of receiving bandwidth and input to a
low noise amplifier 128. The RF received signals Rx amplified in
the low noise amplifier 128 are passed to the mobile phone's
internal circuitry comprising a mixer circuit, an intermediate
frequency filter, etc. which are not shown.
[0078] On the other hand, RF transmitting frequency signals Tx
which have been passed from the mobile phone's internal circuitry
not shown are amplified by a power amplifier 129 and pass through a
transmitting filter 48 which allows only signals falling within a
predetermined frequency range of transmitting bandwidth, and, then,
emitted as radio waves from the antenna.
[0079] The above transmitting filter 78 and the receiving filter 79
for RF signals, employed in the front end portion, can be
constructed by an ensemble of a plurality of FBARs.
[0080] In the following, a FBAR filter consisting of a plurality of
FBARs as the present embodiment, which is constructed as the
transmitting filter 78 and the receiving filter 78, will be
explained. By way of example, it is assumed that the transmitting
frequency Tx band ranges from 1.85 to 1.91 GHz and the receiving
frequency Rx band ranges from 1.93 to 1.99 GHz.
[0081] FIG. 6 is a circuit block diagram of a sub-portion
comprising the phase shifter 130, transmitting filter 78, and
receiving filter 79 in the front end portion shown in FIG. 5. The
transmitting filter 78 is made up of an array of FBARs 71-77 within
an upper block marked off by a dotted line and the receiving filter
79 is made up of an array of FBARs 120-126 within a lower block
marked off by a dotted line. For a block 80 marked off by a chain
line in FIG. 6, it will be described later in an Embodiment 4
section.
[0082] By way of example, an external perspective view schematic of
the transmitting filter 78 fabricated on a single substrate is
shown in FIG. 7. Although quadrangular FBARs 71-77 are shown here,
the shape of the FBARs is not limited to quadrangles. The FBARs
71-73 are series resonators and the FBARs 74-77 are shunt
resonators. In FIG. 7, solid lines between the FBARs denote wiring
leads that connect to the top electrodes of the FBARs and dotted
line denote wiring leads that connect to the bottom electrodes of
the FBARs. A quadrangular region 100 is a piezoelectric layer.
[0083] Reference symbol P1 denotes an input wiring pad to which
transmit signals passed from the internal circuitry not shown are
input. This input wiring pad is connected to an input pad P11 of
the filter, which is connected to an FBAR 71 of the transmitting
filter 78, by a bonding wire BW, and further connected via FBARs
72, 73 connected in series by electrode wiring to an output pad P22
of the filter. The output pad P 22 of the filter and a pad P2
connected to the antenna which is not shown are connected by a
bonding wire BW. Wiring pads connected to the top-electrodes of
FBARs 74, 76 and wiring pads connected to the bottom electrodes of
FBARs 75, 77 are connected to ground pads which are not shown by
bonding wires, respectively. In this way, the transmitting filter
78 shown in the circuit diagram of FIG. 6 is formed on the single
substrate.
[0084] FIG. 8 is a cross sectional view of the FBARs with regard to
the section taken along A-A' line shown in FIG. 7, wherein the FBAR
structure of Embodiment 1 is applied to the FBARs. An FBAR 74 is a
shunt resonator, FBAR 71 is a series resonator, and an FBAR 75 is a
shunt resonator. The underlayers 83 in the regions of the FBARs 74
and 75 are thicker than the underlayer 314 of the FBAR 71. Thus,
the resonant frequency of the FBARs 74 and 75 is lower than the
resonant frequency of the FBAR 71. For example, the thickness T1 of
the diaphragm structure of the FBAR 71 which resonates at 1.9 GHz
is 1800 nm and its underlayer thickness t1 is 50 nm. The thickness
T2 of the diaphragm structures of the FBARs 74 and 75 which
resonate at 1.75 GHz is 1950 nm and their underlayer thickness t2
is 200 nm.
[0085] Because the FBARs can be formed without damaging the
sandwiches, the FBAR filter employing these FBARs can achieve a
high Q factor.
[0086] FIG. 9 is a cross sectional view of the FBARs with regard to
the section taken along A-A' line shown in FIG. 7, wherein the FBAR
structure of Embodiment 2 is applied to the FBARs.
[0087] Components corresponding to those shown in FIG. 8 are
assigned the same reference numerals and their detailed explanation
is not repeated for convenience of explanation. As is the case in
FIG. 8, in FIG. 9 as well, the thickness t2 of the underlayers in
the regions of the FBARs 74 and 75 is thicker than the thickness t1
of the underlayer 131 of the FBAR 71. Thus, the resonant frequency
of the FBARs 74 and 75 is lower than the resonant frequency of the
FBAR 71.
Embodiment 4
[0088] Embodiment 4 is an embodiment of the invention in which both
transmitting and receiving filters employing FBARs are fabricated
on a single substrate. The couple of transmitting and receiving
filters included in the front end portion circuitry shown in FIG. 5
are fabricated on the single substrate. Therefore, although the
same circuitry as shown in FIG. 6 applies, the couple 80 of the
transmitting filter 78 made up of the FBARs 71-77 and the receiving
filter 79 made up of the FBARs 120-126 in the block marked off by
the chain line in FIG. 6 is formed on the single substrate 1.
[0089] To fabricate the couple 80 of the transmitting filter 78 and
receiving filter 79 on the single substrate, at least four FBARs
that respectively resonate at different frequencies must be
fabricated on the single substrate. FIG. 10 shows a fabrication
example in which four FBARs with different resonant frequencies are
fabricated on a single substrate in accordance with the FBAR
structure shown in Embodiment 1. Here, however, the electrode
wirings between the FBARs are omitted, and this figure shows a
cross sectional view of the four FBARs having different diaphragm
structure thicknesses T1, T2, T3, and T4 including different
underlayer thicknesses t1, t2, t3, and t4 over the cavities 39, 49,
41, and 42, respectively.
[0090] The receiving filter 79 and transmitting filter 78 are
formed on the single substrate 1. Each of both filters comprises
FBARs of diaphragm structures formed to at least two different
thicknesses; that is, referring to FIG. 6, the receiving filter 79
comprises the FBARs 120-122 as series resonators and the FBARs
123-126 as shunt resonators and the transmitting filter 78
comprises the FBARs 71-73 as series resonators and the FBARs 74-77
as shunt resonators. On the assumption that the transmitting
frequency Tx ranges from 1.85 to 1.91 GHz and the receiving
frequency Rx ranges from 1.93 to 1.99 GHz, for the transmitting
filter 78, the diaphragm structures of the FBARs resonating at 1.9
GHz and 1.75 GHz have different thicknesses T3=1800 nm and T4=1950
with different underlayer thicknesses t3=150 nm and t4=300 nm, as
described in Embodiment 3.
[0091] For the receiving filter, the diaphragm structures of the
FBARs resonating at 2 GHz and 1.85 GHz have different thicknesses
T1=1700 nm and T2=1850 with different underlayer thicknesses t1=50
nm and t2=200 nm.
[0092] Referring to FIG. 10, for example, it is advisable to
construct the receiving filter 79 consisting of an FBAR as a series
resonator 43 with the diaphragm thickness T1 and an FBAR as a shunt
resonator 44 with the diaphragm thickness T2 and the transmitting
filter 78 consisting of an FBAR as a series resonator 45 with the
diaphragm thickness T3 and an FBAR as a shunt resonator 46 with the
diaphragm thickness T4 on the single substrate 1.
[0093] To fabricate such four diaphragm structure variants, for
example, in the fabrication process diagrams of FIG. 2 for
Embodiment 1, after forming the trench 69 in process B, it
advisable to add the following process: cover the entire surface
with photoresist, perform patterning, and form a trench with a
depth corresponding to a difference t4-t3 between the above
thicknesses t4 and t3.
[0094] FIG. 11 shows a fabrication example in which four FBARs with
different resonant frequencies are fabricated on the single
substrate 1 in accordance with the FBAR structure shown in
Embodiment 2. Components corresponding to those shown in FIG. 10
are assigned the same reference numerals and their detailed
explanation is not repeated for convenience of explanation.
Referring to FIG. 11, it will be appreciated that the four FBARs
with different thicknesses can be formed on the single substrate to
constitute the receiving filter 79 and transmitting filter 78 in
the same way as in the case of FIG. 10. To fabricate this
structure, for example, in the fabrication process diagrams of FIG.
4, after depositing the underlayer to the thickness t4 for the FBAR
that must resonate at the lowest frequency in process B, the FBARs
can be formed by adding a process of patterning by photolithography
and dry etching which are repeated to form other underlayers to the
required thicknesses t1, t2, and t3 in process C.
[0095] For fabricating three or more FBARs with different resonant
frequencies on a single substrate, it is needless to say that both
the FBAR structures disclosed in Embodiments 1 and 2 may coexist on
the substrate.
[0096] All FBARs fabricated on the single substrate are made
capable of resonating at different frequencies by forming their
underlayers to different thicknesses and, thus, there is no need to
introduce an additional structure into the main sandwich structures
of the FBARs. Therefore, the sandwich fabrication process is easier
as compared with the aforementioned conventional structure.
[0097] While the present invention has been described in
conjunction with its preferred embodiments, it will be appreciated
that the present invention is not limited to the illustrative
embodiments described hereinbefore and may be embodied in other
modified forms without departing from its spirit or essential
characteristics.
[0098] According to the present invention, sandwich quality
degradation which leads to a decrease in FBAR performance,
especially, Q factor can be avoided.
* * * * *