U.S. patent application number 09/845089 was filed with the patent office on 2002-10-31 for method of tuning baw resonators.
This patent application is currently assigned to Nokia Mobile Phones Ltd.. Invention is credited to Pensala, Tuomas.
Application Number | 20020158716 09/845089 |
Document ID | / |
Family ID | 25294358 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020158716 |
Kind Code |
A1 |
Pensala, Tuomas |
October 31, 2002 |
METHOD OF TUNING BAW RESONATORS
Abstract
A method of tuning a bulk acoustic wave device having a
piezoelectric layer formed between a top electrode and a bottom
electrode, wherein the top electrode has a frame-like structure at
an edge portion for reducing spurious resonance in the electrical
response of the device. The frame-like structure surrounds a center
zone of the top electrode. In order to down-shift the resonance
frequency of the device, a tuning layer is provided on the top
electrode. In particular, the tuning layer is designed such that it
covers at least the entire center zone in order to reduce the
spurious resonance introduced by the tuning layer itself.
Preferably, the tuning layer covers the center zone as well as the
frame-like structure to further reduce the spurious resonance.
Inventors: |
Pensala, Tuomas; (Helsinki,
FI) |
Correspondence
Address: |
WARE FRESSOLA VAN DER SLUYS &
ADOLPHSON, LLP
BRADFORD GREEN BUILDING 5
755 MAIN STREET, P O BOX 224
MONROE
CT
06468
US
|
Assignee: |
Nokia Mobile Phones Ltd.
|
Family ID: |
25294358 |
Appl. No.: |
09/845089 |
Filed: |
April 27, 2001 |
Current U.S.
Class: |
333/195 ;
310/365 |
Current CPC
Class: |
H03H 3/013 20130101 |
Class at
Publication: |
333/195 ;
310/365 |
International
Class: |
H01L 041/08 |
Claims
What is claimed is:
1. A method of tuning a bulk acoustic wave device comprising a
substrate and a plurality of acoustic wave generating and
controlling layers formed on the substrate, wherein the acoustic
wave generating and controlling layers include a piezoelectric
layer formed between a first electrode and a second electrode for
generating piezoelectrically excited acoustic signals, wherein the
first electrode has a frame-like structure at an edge portion of
the first electrode for reducing spurious resonance components in
the acoustic waves, and the frame-like structure surrounds a center
zone having a surface area, and wherein the device has a resonance
frequency which can be down-shifted by modifying the first
electrode, said method comprises the step of providing a tuning
layer on top of the first electrode for modifying the first
electrode such that the tuning layer covers at least the surface
area of the center zone for reducing spurious resonance resulting
from the tuning layer.
2. The method of claim 1, wherein said tuning layer covers
substantially the surface area of the center zone.
3. The method of claim 1, wherein said tuning layer covers at least
part of the frame-like structure.
4. The method of claim 1, wherein said tuning layer covers
substantially all of the frame-like structure.
5. The method of claim 1, wherein the first electrode is made from
a first material, and said tuning layer is made from a second
material different from the first material.
6. The method of claim 1, wherein the first electrode and said
tuning layer are made from the same material.
7. The method of claim 1, wherein the tuning layer is made from an
electrically conducting material.
8. The method of claim 1, wherein the tuning layer is made from an
electrically non-conducting material.
9. A bulk acoustic wave device comprising a substrate and a
plurality of acoustic wave generating and controlling layers formed
on the substrate, wherein the acoustic wave generating and
controlling layers include a piezoelectric layer formed between a
first electrode and a second electrode for generating
piezoelectrically excited acoustic signals, wherein the first
electrode has a frame-like structure at an edge portion of the
first electrode for reducing spurious resonance components in the
acoustic signals and the frame-like structure surrounds a center
zone having a surface area, and wherein the device has a resonance
frequency which can be down-shifted by modifying the first
electrode, said device comprising a tuning layer provided on top of
the first electrode for modifying the first electrode such that the
tuning layer covers at least the surface area of the center
zone.
10. The device of claim 9, wherein said tuning layer covers
substantially the surface area of the center zone.
11. The device of claim 9, wherein said tuning layer covers at
least part of the frame-like structure as well as the surface area
of the center zone.
12. The device of claim 9, wherein said tuning layer covers
substantially all of the frame-like structure as well as the
surface area of the center zone.
13. The device of claim 11, further comprising a passivation layer
covering part of the piezoelectric layer for protecting said
device, wherein at least part of the passivation layer is formed
between the piezolectric layer and the frame-like structure, and
wherein the frame-like structure is formed between said at least
part of the passivation layer and said tuning layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to bulk acoustic
wave resonators and filters and, more particularly, to the tuning
of such resonators and filters.
BACKGROUND OF THE INVENTION
[0002] It is known that a bulk acoustic-wave (BAW) device is, in
general, comprised of a piezoelectric layer sandwiched between two
electronically conductive layers that serve as electrodes. When a
radio frequency (RF) signal is applied across the device, it
produces a mechanical wave in the piezoelectric layer. The
fundamental resonance occurs when the wavelength of the mechanical
wave is about twice the thickness of the piezoelectric layer.
Although the resonant frequency of a BAW device also depends on
other factors, the thickness of the piezoelectric layer is the
predominant factor in determining the resonant frequency. As the
thickness of the piezoelectric layer is reduced, the resonance
frequency is increased. BAW devices have traditionally been
fabricated on sheets of quartz crystals. In general, it is
difficult to achieve a device of high resonance frequency using
this fabrication method. When fabricating BAW devices by depositing
thin-film layers on passive substrate materials, one can extend the
resonance frequency to the 0.5-10 GHz range. These types of BAW
devices are commonly referred to as thin-film bulk acoustic
resonators or FBARs. There are primarily two types of FBARs,
namely, BAW resonators and stacked crystal filters (SCFs). An SCF
usually has two or more piezoelectric layers and three or more
electrodes, with some electrodes being grounded. The difference
between these two types of devices lies mainly in their structure.
FBARs are usually used in combination to produce passband or
stopband filters. The combination of one series FBAR and one
parallel, or shunt, FBAR makes up one section of the so-called
ladder filter. The description of ladder filters can be found, for
example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella,
an FBAR-based device may have one or more protective layers
commonly referred to as the passivation layers. A typical
FBAR-based device is shown in FIGS. 1a and 1b. As shown in FIG. 1,
the FBAR device comprises a substrate 110, a bottom electrode 120,
a piezoelectric layer 130, and a top electrode 140. The FBAR device
may additionally include a membrane layer 112 and a sacrificial
layer 114, among others. The substrate can be made from silicon
(Si), silicon dioxide (SiO2), Gallium Arsenide (GaAs), glass or
ceramic materials. The bottom electrode and top electrode can be
made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu),
nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum
(Ta), cobalt (Co), or aluminum (Al). The piezoelectric layer 130
can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum
nitride (AIN), lithium tantalate (LiTaO.sub.3) or other members of
the so-called lead lanthanum zirconate titanate family. The
passivation layer is typically made from a dielectric material,
such as SiO2, Si3N4, or polyimide, to serve as an electrical
insulator and to protect the piezoelectric layer. It should be
noted that the sacrificial layer 114 in a bridge-type BAW device
is, in general, etched away in the final fabrication stages to
create an air interface beneath the device. In a mirror-type BAW
device, there is an acoustic mirror structure beneath the bottom
electrode 120. The mirror structure consists of several layer pairs
of high and low acoustic impedance materials, usually quarter-wave
thick. The bridge-type and the mirror-type BAW devices are known in
the art.
[0003] The desired electrical response in an FBAR-based device is
achieved by a shear or longitudinal acoustic wave propagating in
the vertical thickness through the device. Besides these wave
modes, there exist other modes, including other shear modes,
extensional modes and their higher harmonics. However, with respect
to the operation point, the Lamb wave modes in the nearby
frequencies are the unwanted spurious modes that may deteriorate
the electrical response. In quartz crystals, the strength of these
spurious modes is controlled by adjusting the thickness and the
width of the top electrode. In an FBAR-based device, the dimension
in thickness direction is so small that it renders thickness
adjustment difficult and impractical. A possible solution to
resolving the problems associated with the spurious modes is to
thicken the edge of the top electrode. As disclosed in Kaitila et
al. (WO 01/06647 A1, hereafter referred to as Kaitila), a
frame-like structure 150 is formed on top of the top electrode 140
to thicken the edge thereof. As shown in FIGS. 1a and 1b, the
frame-like structure 150 is a rectangular frame for defining a
first zone and a second zone for acoustic wave excitation. The
first zone is the area under the frame-like structure 150, and the
second zone 148 is the area surrounded by the frame-like structure
150. With such a structure, the cut-off frequency of the
piezoelectrically excited wave modes in the first zone and that in
the second zone is different. When the width of the frame-like
structure and the acoustic properties of the layer structure are
properly arranged, the displacement relating to the strongest of
the piezoelectrically excited resonance modes is substantially
uniform in the second zone. The electrical response of an
FBAR-based device with a thickened edge (solid line) and that
without a thickened edge (dashed line) are presented on a Smith
Chart as shown in FIG. 4.
[0004] As it is known in the art, a Smith Chart is a polar plot of
the complex reflection, which represents the ratio of the complex
amplitudes of the backward and forward waves. The Smith Chart helps
translating the reflection coefficient into impedance, and it maps
part of the impedance plane onto a unit circle. In an FBAR-based
resonator, the piston mode is a vibration mode where the vibration
amplitude is practically uniform over the second zone. If a
resonator exhibits the piston mode, the spurious modes become very
weakly excited and the response of the resonator is optimized with
respect to the spurious resonances. In general, when the Smith
Chart shows a clean circle, the structure of the resonator is close
to a piston mode producing structure. Thus, the Smith Chart is a
good indicator of the quality of the resonator response. In FIG. 4,
the outermost circle that touches the square frame of the plot is
the unit circle in the Smith Chart.
[0005] It should be noted that, as disclosed in Kaitila, the
frame-like structure may be circular, square, polygonal, regular or
irregular. Also, the frame-like structure can have different
configurations, as shown in FIGS. 2 and 3, to achieve the piston
mode. As shown in FIGS. 2 and 3, part of the piezoelectric layer
130 is covered by a passivation layer 160, and part of the
passivation layer is sandwiched between the piezoelectric layer 130
and the frame-like structure 150 extended upward from the edge of
the top electrode 140. In FIGS. 2 and 3, the frame-like structure
150 is basically where the top electrode 140 overlaps with the
passivation layer 130. It should be noted that FIG. 1a is a cross
sectional view of a BAW device, as viewed in the lateral direction,
while FIG. 2 and FIG. 3 are cross sectional views of a BAW device,
as viewed in the horizontal direction.
[0006] In FBAR-based ladder filters, the frequency of the shunt
resonators must be downshifted by adding an extra thin-film of a
suitable material to the film stack of the resonator. The added
thin-film is usually referred to as the tuning layer. The thickness
of the tuning layer is determined by the desired frequency shift
and is generally much smaller than the thickness of other layers on
the device. If the shunt resonator in a ladder filter is designed
to operate optimally, with regard to the suppression of the
spurious mode without the tuning layer, adding the tuning layer may
degrade the performance of the resonator by reintroducing the
spurious resonance frequencies.
[0007] Thus, it is advantageous and desirable to provide a method
of tuning the shunt resonator for frequency down-shifting without
substantially degrading the performance of the shunt resonator.
SUMMARY OF THE INVENTION
[0008] It is a primary object of the present invention to provide a
method for tuning a frequency down-shifted bulk acoustic wave
device for reducing spurious resonance due to the frequency
down-shifting, wherein a tuning layer formed on top of the top
electrode of the bulk acoustic wave device is used to lower the
resonant frequency of the device. This object can be achieved by
configuring the tuning layer.
[0009] Thus, according to the first aspect of the present
invention, a method of tuning a bulk acoustic wave device
comprising a substrate and a plurality of acoustic wave generating
and controlling layers formed on the substrate, wherein the
acoustic wave generating and controlling layers include a
piezoelectric layer formed between a first electrode and a second
electrode for generating piezoelectrically excited acoustic
signals, wherein the first electrode has a frame-like structure at
an edge portion of the first electrode for reducing spurious
resonance components in the acoustic waves, and the frame-like
structure surrounds a center zone having a surface area, and
wherein the device has a resonance frequency which can be
down-shifted by modifying the first electrode. The method comprises
the step of providing a tuning layer on top of the first electrode
for modifying the first electrode such that the tuning layer covers
at least the surface area of the center zone for reducing spurious
resonance resulting from the tuning layer.
[0010] Alternatively, the tuning layer also covers at least part of
the frame-like structure.
[0011] Preferably, the tuning layer substantially covers the entire
frame-like structure.
[0012] According to the second aspect of the prevent invention, a
bulk acoustic wave device comprising a substrate and a plurality of
acoustic wave generating and controlling layers formed on the
substrate, wherein the acoustic wave generating and controlling
layers include a piezoelectric layer formed between a first
electrode and a second electrode for generating piezoelectrically
excited acoustic signals, wherein the first electrode has a
frame-like structure at an edge portion of the first electrode for
reducing spurious resonance components in the acoustic signals and
the frame-like structure surrounds a center zone having a surface
area, and wherein the device has a resonance frequency which can be
down-shifted by modifying the first electrode. The device comprises
a tuning layer provided on top of the first electrode for modifying
the first electrode such that the tuning layer covers at least the
surface area of the center zone, and alternatively, at least part
of the frame-like structure as well as the surface area of the
center zone. Preferably, the tuning layer substantially covers the
entire frame-like structure as well as the surface area of the
center zone.
[0013] The present invention will become apparent upon reading the
description taken in conjunction with FIGS. 5 to 9.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1a is a cross sectional view illustrating a bulk
acoustic wave device having a top electrode with a thickened edge
or frame-like structure provided thereon for reducing spurious
resonance.
[0015] FIG. 1b is a top view of the bulk acoustic wave device as
shown in FIG. 1a.
[0016] FIG. 2 is a cross sectional view illustrating another bulk
acoustic wave device having a frame-like structure on top of the
top electrode.
[0017] FIG. 3 is a cross sectional view illustrating yet another
bulk acoustic wave device having a frame-like structure on top of
the top electrode.
[0018] FIG. 4 is a Smith Chart showing the effect of the frame-like
structure on the electrical response of the bulk acoustic wave
device.
[0019] FIG. 5 is a cross sectional view showing a tuning layer used
for frequency downshifting a bulk acoustic wave device, wherein the
surface area of the tuning layer is smaller than the surface area
of the center zone.
[0020] FIG. 6a is a cross sectional view showing a bulk acoustic
wave device having a tuning layer, according to the present
invention.
[0021] FIG. 6b is a cross sectional view showing a bulk acoustic
wave device having a tuning layer, according to another embodiment
of the present invention.
[0022] FIG. 6c is a cross sectional view showing a bulk acoustic
wave device having a tuning layer, according to the preferred
embodiment of the present invention.
[0023] FIG. 7a is a Smith Chart showing the electrical response of
a bulk acoustic wave device including spurious resonance components
due to the tuning layer, wherein the gap between the tuning layer
and the frame-like structure is about 2.4% of the entire width of
the resonator.
[0024] FIG. 7b is a Smith Chart showing the reduction of spurious
resonance by extending the tuning layer toward the frame-like
structure, such that the gap between the tuning layer and the
frame-like structure is about 1.7% of the entire width of the
resonator.
[0025] FIG. 7c is a Smith Chart showing the reduction of spurious
resonance by further extending the tuning layer toward the
frame-like structure, such that the gap between the tuning layer
and the frame-like structure is about 0.7% of the entire width of
the resonator.
[0026] FIG. 7d is a Smith Chart showing the reduction of spurious
resonance by further extending the tuning layer to cover the entire
center area.
[0027] FIG. 7e is a Smith Chart showing the reduction of spurious
resonance by extending the tuning layer to cover part of the
frame-like structure.
[0028] FIG. 7f is a Smith Chart showing the reduction of spurious
resonance by extending the tuning layer to cover the entire
frame-like structure.
[0029] FIG. 8a is a cross sectional view showing a variation of the
preferred embodiment of the present invention.
[0030] FIG. 8b is a cross sectional view showing yet another
variation of the preferred embodiment of the present invention.
[0031] FIG. 9 is a Smith Chart showing the electrical response of a
frame-tuned resonator where the frame-like structure in shunt
resonators is designed such that a tuning layer is considered as
part of the resonators, and the electrical response of an optimally
tuned resonator.
DETAILED DESCRIPTION
[0032] FIG. 5 shows a bulk acoustic wave device 10 formed on a
substrate 20 and having a piezoelectric layer 24 sandwiched between
a bottom electrode 22 and a top electrode 26 for generating
piezoelectrically excited acoustic signals. As shown in FIG. 5, the
top electrode 26 has a frame-like structure 30 surrounding a center
zone 28 for reducing spurious components in the acoustic signals.
The surface area of the center zone 28, as surrounded by the
frame-like structure 30, is denoted by SC. The surface area of the
frame-like structure is denoted by SF. A tuning layer 32, having a
thickness TT and a surface area ST, is provided on top of the top
electrode 26 for down-shifting the frequency of the device 10. The
thickness TT is related to the amount of frequency down-shifting.
It has been found that, when the surface area ST of the tuning
layer 32 is smaller than the surface area SC, leaving a gap G, the
electrical response of the device 10 shows a substantial amount of
spurious resonance, as shown in FIG. 7a. Thus, the addition of the
tuning layer 32 on top of the top electrode 26 introduces spurious
resonance components to the piezoelectrically excited acoustic
signals. FIG. 7a shows the spurious resonance when the gap G
between the tuning layer 32 and the frame-like structure 30 is
about 2.4% of the width of the device.
[0033] It has also been found that, when the gap G between the
surface area of the center zone 28 and the surface area ST of the
tuning layer 32 is reduced, the spurious resonance is also reduced.
If the gap G between the tuning layer 32 and the frame-like
structure 30 is reduced to about 1.7% of the width of the device,
the amount of spurious resonance is somewhat reduced, as shown in
FIG. 7b. If the gap G is further reduced to 0.7%, the electrical
response is further improved, as shown in FIG. 7c. When the tuning
layer 32 totally covers the entire surface area of the center zone
28, as shown in FIG. 6a, the resulting Smith Chart is as shown in
FIG. 7d.
[0034] Furthermore, it has been found that if the tuning layer 34
is extended further to cover the surface area SF of the frame-like
structure 30, as shown in FIG. 6b, the spurious resonance in the
electrical response can be further reduced, as shown in FIG. 7e. In
FIG. 6b, the bulk acoustic wave device 12' has a tuning layer 34
which has an extended section 34' to overlap partially with the
frame-like structure 30. The overlapped width of the extended
section 34' is denoted by letter L. When the overlapped width L is
increased to cover the entire surface area of the frame-like
structure 30, as shown in FIG. 6c, the spurious resonance becomes
insignificant. The device 12", as shown in FIG. 6c, represents one
form of the preferred embodiment of the present invention. Other
forms of the preferred embodiment of the present invention are
shown in FIGS. 8a and 8b.
[0035] In the bulk acoustic wave device 14, as shown in FIG. 8a,
the transition from the center zone 28 to the frame-like structure
30 of the top electrode 26 is less abrupt than that in the device
12", as shown in FIG. 6c. Accordingly, the transition from the
tuning layer 34 to the extended section 34' is less abrupt than its
counterpart on the device 12", as shown in FIG. 6c. In FIG. 8b, the
frame-like structure 30 is a thickened edge of the top electrode
26. Accordingly, the extended section 34' is formed on top of the
thickened edge 30.
[0036] The method of reducing spurious resonance in the electrical
response due to frequency down-shifting, according to the present
invention, has been described in conjunction with a number of
simple bulk acoustic wave devices as illustrated in FIGS. 6a, 6b,
6c, 8a and 8b. In those devices, there are only three acoustic-wave
generating and controlling layers, namely, the top electrode, the
piezoelectric layer and the bottom electrode. The same method can
also be used on more complex bulk acoustic wave devices having
additional acoustic-wave generating and controlling layers. Also,
the tuning layer as described in conjunction with FIGS. 6a-6c, 8a
and 8b is concerned with the top electrode of a BAW device. It is
possible, however, that the tuning layer is positioned below the
piezoelectric layer or below the bottom electrode. In either case,
the tuning layer should laterally extend over all of the center
area and most of the frame-like structure area. Furthermore, if the
tuning layer is positioned between the piezoelectric layer and the
bottom electrode, it is preferred to use an electrically conductive
tuning layer so that the tuning layer forms part of the bottom
electrode.
[0037] It should be noted that the tuning layer can be made from
the same material as the top electrode, but it can also be made
from a different material. For example, if the top electrode is
made of aluminum, then the tuning layer can be made of aluminum,
molybdenum, tungsten, copper, gold or other electrically conductive
material. Furthermore, the tuning layer can be made of a
non-electrically conductive material such as SiO2, Si3N4, and the
like. The critical points in selecting the tuning material are the
etch selectivity against other layers for the patterning process
and the ability to accurately deposit the correct thickness of the
tuning layer. In general, it is preferable to use a lighter
material for the tuning layer because a thicker layer would be
needed for the same frequency shift. Precise deposition of a
thinner layer is generally more difficult than that of a thicker
layer.
[0038] Furthermore, when the tuning layer 34' and the top electrode
30 are made of the same material, as shown in FIGS. 6c and 8a,
together they appear to be the same as the top electrode 140, as
shown in FIGS. 2 and 3. Thus, it is possible to design a top
electrode that includes a tuning layer as part of the BAW device in
the first place. In a device that consists of series and shunt
resonators, the shunt resonators always require some kind of
tuning. Thus, the frame-like area of the shunt resonators could be
different from that of the series resonators. Accordingly, the
frame-like area of the tuned resonators could also be different
from that of the series resonators. It should be noted that the
dimensions of the frame-like structure on the top electrode,
according to Kaitila, are calculated based on a set of boundary
conditions. If the frame-like structure in shunt resonators is
designed according to Kaitila and considering the tuning layer as a
part of the resonator in the first place, the dimension of the
frame-like structure in these frame-tuned resonators would be
slightly different from the frame-like structure that reproduces
the piston mode. The electrical response of a frametuned resonator
(solid line) is shown in FIG. 9. The non-circularity is about
0.11%. In order to optimize the performance of the shunt resonator,
the width of the frame-like structure can be modified to reproduce
the piston mode (dashed line), as shown in FIG. 9.
[0039] It should be noted that the bulk acoustic wave devices,
according to the present invention, include individual resonators,
stacked crystal filters, ladder filters and the combinations
thereof However, there are other filter types in addition to the
ladder structure that can be constructed from FBARs. All of them
include some resonators, which have to be tuned, but they cannot be
called parallel or shunt resonators in all cases. The balanced
filter is an example of such a filter type.
[0040] Thus, although the invention has been described with respect
to a preferred embodiment thereof, it will be understood by those
skilled in the art that the foregoing and various other changes,
omissions and deviations in the form and detail thereof may be made
without departing from the spirit and scope of this invention.
* * * * *