U.S. patent application number 09/755954 was filed with the patent office on 2002-07-11 for monolithic fbar duplexer and method of making the same.
Invention is credited to Ella, Juha, Kaitila, Jyrki, Tikka, Pasi.
Application Number | 20020089393 09/755954 |
Document ID | / |
Family ID | 25041387 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020089393 |
Kind Code |
A1 |
Tikka, Pasi ; et
al. |
July 11, 2002 |
MONOLITHIC FBAR DUPLEXER AND METHOD OF MAKING THE SAME
Abstract
A monolithic bulk acoustic wave (BAW) duplexer, and a method for
fabricating same, the duplexer having a transmitter section as a
first component filter and a receiver section as a second component
filter, both component filters fabricated on a single substrate and
both including at least one shunt BAW resonator and one series BAW
resonator, each BAW resonator including a resonator section atop an
isolation structure provided so as to separate the resonator
section from the substrate, including: a patterned bottom electrode
material for use as the bottom electrode of each of the resonators
of the duplexer; a patterned piezoelectric material for use as the
piezolayer of each of the resonators of the duplexer; a patterned
top electrode material for use as the top electrode of each of the
resonators of the duplexer; a tuning layer for the shunt resonator
of each of the two component duplexer filters; and a tuning layer
for both the series and shunt resonators of one of the two
component duplexer filters. In some applications, each isolation
structure is an acoustic mirror. Also in some applications, the
duplexer further includes at least one planar spiral inductor
provided in the course of depositing one or another layer of
material in building up the duplexer, the planar spiral inductor
having coils spiraling outward substantially in a plane from an
innermost coil to an outermost coil.
Inventors: |
Tikka, Pasi; (Helsinki,
FI) ; Ella, Juha; (Halikko, FI) ; Kaitila,
Jyrki; (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
|
Family ID: |
25041387 |
Appl. No.: |
09/755954 |
Filed: |
January 5, 2001 |
Current U.S.
Class: |
333/133 ;
29/25.35; 310/324; 333/191 |
Current CPC
Class: |
Y10T 29/42 20150115;
H03H 9/706 20130101; H03H 9/0571 20130101 |
Class at
Publication: |
333/133 ;
310/324; 333/191; 29/25.35 |
International
Class: |
H03H 009/56; H03H
009/205 |
Claims
What is claimed is:
1. A method of fabricating a monolithic bulk acoustic wave (BAW)
duplexer having a transmitter section as a first component filter
and a receiver section as a second component filter, both component
filters fabricated on a single substrate and both including at
least one shunt BAW resonator and one series BAW resonator, each
BAW resonator including a resonator section atop an isolation
structure provided so as to separate the resonator section from the
substrate, the method comprising the steps of: a) depositing and
patterning bottom electrode material for use as the bottom
electrode of each of the resonators of the duplexer; b) depositing
and patterning piezoelectric material for use as the piezolayer of
each of the resonators of the duplexer; c) depositing and
patterning top electrode material for use as the top electrode of
each of the resonators of the duplexer; d) providing a tuning layer
for the shunt resonator of each of the two component duplexer
filters; and e) providing a tuning layer for both the series and
shunt resonators of one of the two component duplexer filters.
2. The method of claim 1, wherein the shunt tuning layer is
deposited in a location in each shunt resonator selected from the
group consisting of: between the mirror and the bottom electrode;
between the bottom electrode and the piezolayer; between the top
electrode and the piezolayer; and on top of the top electrode.
3. The method of claim 1, wherein the tuning layer for both the
series and shunt resonators of one of the two component filters is
deposited in a location, in both the series and shunt resonators of
the component filter, selected from the group consisting of:
between the mirror and the bottom electrode; between the bottom
electrode and the piezolayer; between the top electrode and the
piezolayer; and on top of the top electrode.
4. The method of claim 1, wherein each isolation structure is an
acoustic mirror.
5. The method of claim 4, further comprising the step of providing
the acoustic mirror and also providing at least one planar spiral
inductor having coils spiraling outward substantially in a plane
from an innermost coil to an outermost coil, the step of providing
the acoustic mirror and planar spiral inductor comprising in turn
the steps of: a) depositing on the substrate, over the entire
surface of the substrate, a first metallic material; b) removing
the first metallic material everywhere except where the coils are
to be provided, and where the first layer of the acoustic mirror
for each BAW resonator is to be provided; c) depositing a first
dielectric material over the entire exposed surface; d) providing a
via directed to the beginning of the innermost coil; e) depositing
a second metallic material over the entire exposed surface and so
as to fill the via and come into contact with the first metallic
material; and f) removing the second metallic material everywhere
except where an arm connecting the innermost coil to a terminal is
to be provided, and where a second metallic layer of the acoustic
mirror for each BAW resonator is to be provided.
6. A monolithic bulk acoustic wave (BAW) duplexer having a
transmitter section as a first component filter and a receiver
section as a second component filter, both component filters
fabricated on a single substrate and both including at least one
shunt BAW resonator and one series BAW resonator, each BAW
resonator including a resonator section atop an isolation structure
provided so as to separate the resonator section from the
substrate, comprising: a) a patterned bottom electrode material for
use as the bottom electrode of each of the resonators of the
duplexer; b) a patterned piezoelectric material for use as the
piezolayer of each of the resonators of the duplexer; c) a
patterned top electrode material for use as the top electrode of
each of the resonators of the duplexer; d) a tuning layer for the
shunt resonator of each of the two component duplexer filters; and
e) a tuning layer for both the series and shunt resonators of one
of the two component duplexer filters.
7. The monolithic BAW duplexer of claim 6, wherein the shunt tuning
layer is provided in a location in each shunt resonator selected
from the group consisting of: between the mirror and the bottom
electrode; between the bottom electrode and the piezolayer; between
the top electrode and the piezolayer; and on top of the top
electrode.
8. The monolithic BAW duplexer of claim 6, wherein the tuning layer
for both the series and shunt resonators of one of the two
component filters is provided in a location, in both the series and
shunt resonators of the component filter, selected from the group
consisting of: between the mirror and the bottom electrode; between
the bottom electrode and the piezolayer; between the top electrode
and the piezolayer; and on top of the top electrode.
9. The monolithic BAW duplexer of claim 6, wherein each isolation
structure is an acoustic mirror.
10. The monolithic BAW duplexer of claim 6, further comprising at
least one planar spiral inductor included in one or another layer
of material deposited on the single substrate, the planar spiral
inductor having coils spiraling outward substantially in a plane
from an innermost coil to an outermost coil.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to thin film bulk acoustic
wave resonators. More particularly, the present invention relates
to bulk acoustic wave filters and bulk acoustic wave duplexers
fabricated from thin film bulk acoustic wave resonators.
BACKGROUND OF THE INVENTION
[0002] As shown in FIG. 1, a thin film bulk acoustic wave (BAW)
resonator 10 includes a resonator section 11 based on a layer of
piezoelectric material, such as ZnO or AlN, and some include an
acoustic mirror 12 (where others, not shown, called bridge-type BAW
resonators, include a membrane), all mounted on a substrate 14 made
for example from glass. A BAW resonator converts sound waves to
electric signals, and vice versa, and can be used as a filter in
electronic circuits because of its frequency dependent electrical
impedance.
[0003] Typically, the acoustic mirror of an acoustic-mirror type of
BAW resonator is formed from a combination of layers of materials
of differing acoustic impedance. An acoustic mirror is built up on
a substrate of for example glass by depositing its various layers
of different materials so as to form a stack of layers of different
materials on the substrate. Next, a bottom electrode is deposited
on the acoustic mirror, and the piezoelectric material is then
deposited on the bottom electrode forming a so called piezolayer.
Finally, a top electrode is deposited on the piezolayer. The
combination of top and bottom electrodes and the piezolayer is
called the resonator section of the device. The acoustic mirror
serves to reflect acoustic waves created by the piezolayer in
response to a voltage applied across the electrodes, thereby
isolating the substrate from the piezolayer. FIG. 4 shows the cross
sections of two acoustic-mirror type BAW resonators fabricated to
operate as parts of band pass filters at substantially different
frequencies.
[0004] As mentioned above, besides BAW resonators including
acoustic mirrors, it is known in the art to provide BAW resonators
constructed on a membrane, with an air gap separating the resonator
section from the substrate.
[0005] Both types of BAW resonators are used as components of
filters making up duplexers in e.g. a mobile phone. As illustrated
in FIG. 2, a mobile phone can include a duplexer 21, which in turn
includes a transmitter (TX) filter 22 and a receiver (RX) filter
23. As shown in FIG. 3, such a filter may be a so-called ladder
filter 31. A ladder filter, in general, includes at least one
so-called L-section, an L-section including a series resonator and
a shunt resonator. The filter 31 shown in FIG. 3 is therefore a
ladder filter consisting of two L-sections 32 33 connected in
series, each L-section 32 33 in turn including two resonators, a
series resonator 32a 33a and a shunt resonator 32b 33b, the
resonators of each individual L-section 32 33 being tuned to
slightly different frequencies by fabricating one or more layers of
the pair so as to have a slightly different thickness
(Alternatively, of course, a TX filter 22 or a RX filter 23 of a
duplexer 21 may consist of only a single combination 32 of a series
resonator 32a and shunt resonator 32b, i.e. a single-stage ladder
filter.)
[0006] A ladder filter, sometimes called an impedance element
filter (IEF), generally consists of one or more so-called
L-sections or L-segments, each L-section in turn including one
series and one parallel resonator, and thus consisting of an even
number of resonators. In some applications, however, a filter
consists of an odd number of resonators. For example, a 21/2-stage
filter could have either two series resonators and three shunt
resonators, or three series resonators and two shunt resonators.
The present invention is not restricted to filters having an event
number of resonators.
[0007] Besides a duplexer 21, a mobile phone can also include other
filters (24 of FIG. 2) close in frequency to the duplexer
frequencies.
[0008] The use of BAW resonators as components of filters in
duplexers (then called FBAR duplexers) as taught by the art
involves using two separate substrates for each filter, with
external passive surface mounted devices (SMDs) as components.
[0009] What is needed is a way of fabricating on a single substrate
the two filters for use in an FBAR duplexer. Ideally, other filters
included as a component of the equipment using the duplexer (such
as a mobile phone) and operating at a frequency near the
frequencies of the duplexer could then also be advantageously
fabricated on the same substrate. Such a fabricating would reduce
the size of the equipment owing to the duplexer and filters
operating near the frequencies of the duplexer, and would also, in
principle, reduce the cost of fabricating the equipment.
SUMMARY OF THE INVENTION
[0010] Accordingly, the present invention provides, a monolithic
bulk acoustic wave (BAW) duplexer, and a method for fabricating
same, the duplexer having a transmitter section as a first
component filter and a receiver section as a second component
filter, both component filters fabricated on a single substrate and
both including at least one shunt BAW resonator and one series BAW
resonator, each BAW resonator including a resonator section atop an
isolation structure provided so as to separate the resonator
section from the substrate, including: a patterned bottom electrode
material for use as the bottom electrode of each of the resonators
of the duplexer; a patterned piezoelectric material for use as the
piezolayer of each of the resonators of the duplexer; a patterned
top electrode material for use as the top electrode of each of the
resonators of the duplexer; a tuning layer for the shunt resonator
of each of the two component duplexer filters; and a tuning layer
for both the series and shunt resonators of one of the two
component duplexer filters.
[0011] In a further aspect of the invention, the shunt tuning layer
is provided in a location in each shunt resonator that is either:
between the mirror and the bottom electrode; between the bottom
electrode and the piezolayer; between the top electrode and the
piezolayer; or on top of the top electrode.
[0012] In another, further aspect of the invention, the tuning
layer for both the series and shunt resonators of one of the two
component filters is provided in a location, in both the series and
shunt resonators of the component filter, that is either: between
the mirror and the bottom electrode; between the bottom electrode
and the piezolayer; between the top electrode and the piezolayer;
or on top of the top electrode.
[0013] In some applications, each isolation structure is an
acoustic mirror.
[0014] In yet still even another aspect of the invention, the
duplexer also includes at least one planar spiral inductor provided
in the course of depositing one or another layer of material in
building up the duplexer, the planar spiral inductor having coils
spiraling outward substantially in a plane from an innermost coil
to an outermost coil.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above and other objects, features and advantages of the
invention will become apparent from a consideration of the
subsequent detailed description presented in connection with
accompanying drawings, in which:
[0016] FIG. 1 is a schematic of an acoustic-mirror type BAW
resonator, according to the prior art;
[0017] FIG. 2 is a schematic of a duplexer in a mobile phone
according to the prior art, showing a transmitter filter and a
receiver filter, and showing an optional additional filter;
[0018] FIG. 3 is a schematic of a filter such as could be used for
any of the filters of the duplexer of FIG. 2, showing a series
combination of individual L-sections to make a ladder filter, each
individual L-section consisting of a series BAW resonator and a
shunt BAW resonator;
[0019] FIG. 4 shows two acoustic-mirror type BAW resonators,
according to the prior art;
[0020] FIG. 5 shows the principal structure of different BAW
resonators that might be used for the transmitter and receiver
filters of a monolithic duplexer, the resonators differing
essentially only by the addition of one or more different tuning
layers;
[0021] FIG. 6 is a flowchart illustrating the method according to
the invention of fabricating a monolithic FBAR duplexer, i.e. of
fabricating all of the BAW resonators of an FBAR duplexer on a
single substrate;
[0022] FIG. 7 shows the same principal structure of different BAW
resonators of a monolithic duplexer as in FIG. 5, except that the
transmitter tuning layer is deposited between the piezolayer and
the bottom electrode, instead of on top of the top electrode;
[0023] FIG. 8 is a flowchart illustrating how to fabricate a
monolithic FBAR duplexer such as illustrated in FIG. 7;
[0024] FIG. 9 is an illustration of one process for fabricating
several acoustic mirror type BAW resonators (forming a three-stage
L-section filter) on a single substrate, and so illustrating the
techniques used in fabricating a monolithic BAW duplexer according
to the invention, the process shown here being one in which, from
among the layers of the acoustic mirror material, only the metallic
layers are patterned (i.e. removed from some selected areas of the
face of the filter), not the dielectric layers (although other
layers making up the filter are also patterned, including the
bottom and top electrode);
[0025] FIG. 10 is a plan view of a portion of a monolithic FBAR
duplexer according to the invention, in an embodiment including a
planar spiral inductor;
[0026] FIG. 11 is an elevation view corresponding to FIG. 10;
and
[0027] FIG. 12 is a flowchart illustrating one method of
fabrication, according to the invention, of a monolithic FBAR
duplexer including a planar spiral inductor.
BEST MODE FOR CARRYING OUT THE INVENTION
[0028] Referring now to FIG. 5, a duplexer according to the
invention includes a receiver filter 51 and a transmitter filter
52. Both filters include at least two resonators, a series
resonator 51a 52a and a shunt resonator 51b 52b. Each resonator of
each filter includes a bottom electrode, a piezolayer, and a top
electrode. Each resonator can be of either the acoustic-mirror type
or the bridge type of resonator and so include additional structure
(not shown). The shunt resonator of each filter includes a shunt
resonator tuning layer that alters the frequency of the shunt
resonator from that of the series resonator. In addition, both the
series and shunt resonator of the transmitter filter include a
transmitter tuning layer that alters the frequency of the
transmitter filter (i.e. its center frequency) from that of the
receiver filter. All four resonators are deposited on some
additional structure and ultimately on a single substrate of for
example glass. Because a single substrate is used, such a duplexer
is called a monolithic duplexer.
[0029] FIG. 6 is a flowchart of a method according to the invention
for fabricating the monolithic FBAR duplexer of FIG. 5. The method
involves depositing the transmitter tuning layer over the entire
surface of the built up substrate, and then removing the tuning
layer from the receiver filter components. The flow chart of FIG.
6, as well as the flowcharts of FIGS. 8 and 12, do not expressly
include many of the details of the fabrication process, such as
cleaning of the wafer (substrate, from which individual chips are
sawed), or depositing possible adhesion layers. The flowcharts of
FIG. 6, 8, and 12 should be understood to be at a general level.
Patterning of each layer after it is deposited and before the next
is deposited, i.e. a layer-by-layer patterning, is preferred, but
patterning can also be done all at once, after all of the layers
are deposited. Also any combination is in principle possible; for
example in some cases two consecutively deposited layers may be
patterned in one step after the deposition.
[0030] Referring now to FIG. 7, another duplexer according to the
invention again includes a receiver filter 71 and a transmitter
filter 72, and both filters again include two resonators, a series
resonator 71a 72a and a shunt resonator 71b 72b, as in the duplexer
illustrated in FIG. 5. However, the duplexer here is made with the
transmitter tuning layer deposited between the piezolayer and the
bottom electrode, instead of on top of the top electrode. FIG. 8 is
a flowchart of a process according to the invention for fabricating
such a monolithic FBAR duplexer. A tuning layer, for either a shunt
resonator or for (both resonators of) the transmitter, can be
either a metal or a dielectric layer. A metal tuning layer must be
patterned (removed from between resonators); otherwise, such a
tuning layer would short out all the resonators.
[0031] In case of a metal tuning layer for tuning either a shunt
resonator or the transmitter, the tuning layer can be in any of
four locations: between the mirror and the bottom electrode;
between the bottom electrode and the piezolayer; between the top
electrode and the piezolayer; or on top of the top electrode. A
metal tuning layer has the added benefit of slightly reducing the
resistive losses of the electrodes.
[0032] On the other hand, if a dielectric material is used for the
tuning layer, then the tuning layer must be under the bottom
electrode or on top of the top electrode; otherwise, such a tuning
layer would reduce the electric field inside the piezolayer, which
would result in a decrease in the coupling coefficient. If such a
tuning layer were used to tune down the transmitter filter, then it
could be left unpatterned in the area of this filter. If, however,
it is placed on top of the top electrode, then it should be
patterned. In case of a monolithic duplexer, it is of course also
possible to combine two such tuning methods, so that for example a
dielectric layer is used to tune the transmitter filter, and a
metal layer is used to tune the shunt resonator of each filter.
[0033] Materials used for tuning layers (either for transmitter
tuning layers or for shunt resonator tuning layers) are either
metals (e.g. aluminum, copper) or dielectrics (e.g. silicon
dioxide, silicon nitride). The removal of material from selected
regions of the wafer is done by methods known in the art. Such
selective material removal, which is usually performed in any
integrated circuit (IC) process, is called patterning.
[0034] Besides the embodiments illustrated in FIGS. 6 and 8, the
invention also comprehends using the same layers in both the
transmitter and receiver filters and adding a tuning layer to the
filter that operates at the lower frequency (usually the
transmitter), as opposed to growing a thick tuning layer over the
entire substrate (as built up by having deposited one or more
layers of the filters), a tuning layer suitable for the lower
frequency part, and then selectively etching away a predetermined
amount of the tuning layer from the higher frequency part.
[0035] Referring now to FIG. 9, an exemplary process is shown for
fabricating a filter, a filter which could be either the TX or RX
filter of a duplexer. What is shown is the fabrication of three
L-sections (connected in series) of a filter on a single substrate,
as opposed to both a TX and a RX filter on the same substrate, but
the techniques of fabrication for the three L-section filter are
the same as for a monolithic BAW duplexer. The fabrication is
similar to the fabrication of normal active IC's. The layers of the
structures being made are deposited on top of each other, and most
of the layers are also patterned using conventional lithography and
etching. (Patterning here means the selective removal of material
from certain regions on the wafer; it does not mean the use of
localized ion beam.) In the method shown in FIG. 9, successive
layers making up the acoustic mirror are deposited in turn, and in
so doing, after depositing each metallic layer and before
depositing the next dielectric layer, the metallic layer material
is selectively removed from between each region where a resonator
section will be built up. The deposition of the metals is usually
done by sputtering; the dielectric layers can be grown for example
by Chemical Vapor Deposition (CVD)or Plasma Enhanced CVD. As
mentioned above, the selective removal (patterning) of the metallic
layers can be done by either wet or dry etching. With either
etching method, it is possible to obtain sloped edges of the
metallic layers, which is helpful in avoiding step coverage
problems. For the dielectric (low acoustic impedance) layers, the
preferred materials are SiO2 or Si3N4, and the preferred materials
for the metallic (high impedance) layers are W or Mo, although
other materials can also be used. When the piezolayer is deposited,
it extends across the entire face of the substrate (on top of the
layers already deposited). The piezolayer can either be left
unpatterned (not selectively removed), for example when fabricating
a two-stage balanced filter (except for providing for vias, i.e.
holes in a dielectric layer created by patterning the layer so that
when a metal is then deposited on the dielectric layer it will fill
the hole, thus providing a connection through the dielectric
layer), or the piezolayer can be removed everywhere except on top
of each mirror stack.
[0036] Still referring to FIG. 9, as can be seen in the pictures
above (step 2, 4 and 6), the metal layers of the mirrors are
patterned to form separate stacks underneath each individual
resonator. If this were not done then there would be a large
capacitance from the bottom electrode of each resonator to the
metal mirror layers, which would then provide for a parasitic,
capacitive coupling from one resonator to another.
[0037] Still referring to FIG. 9, the tuning of the shunt
resonators (step 12) and the transmit part overall tuning material
can be either a metal or a dielectric layer. If a metal is used,
the tuning layer must be patterned, otherwise it would short out
all the resonators. A metal tuning layer (for either shunt or
transmitter tuning) is typically provided in one or another of four
locations: 1) in an acoustic-mirror type of BAW resonator, between
the mirror and the bottom electrode; 2) between the bottom
electrode and the piezolayer; 3) between the top electrode and the
piezolayer; and 4) on top of the top electrode. A metal tuning
layer is advantageous in that it reduces slightly the resistive
losses of the electrodes.
[0038] If a dielectric material is used for the tuning layer, then
the tuning layer must be located under the bottom electrode or on
top of top electrode, otherwise it would reduce the electric field
inside the piezolayer, thus also decreasing the coupling
coefficient. If such a layer is used to tune down the whole
transmitter filter, then it could be left unpatterned in the area
of this filter. However, if such a tuning layer is provided on top
of the top electrode, then it will have to be patterned to the
extent that vias are provided to electrically connect the
underlying electrodes to the rest of the duplexer circuit. (If a
dielectric tuning layer on top of the top electrode is left fully
unpatterned, then all the metal layers will be covered by it
everywhere. Since some holes through such a tuning layer are needed
to get the electric signal from the outside world to the electrode
metals, at least vias must be patterned at the locations of the
signal pads for wire bonds or flip chip bumps.) For a monolithic
duplexer, it is of course also possible to combine the two tuning
method, by e.g. using a dielectric layer as the transmitter overall
tuning (tuning both the shunt and series resonators of the
transmitter filter) and using a metal layer as the shunt tuning
(tuning the shunt BAW resonator of the transmitter filter and the
shunt BAW resonator of the receiver filter).
[0039] The response of a duplexer can sometimes be improved by an
additional inductance in series with one or more of the shunt
resonators, shifting down the series resonance of these resonators.
(The parallel resonance is not affected.) The prior art teaches the
use of such additional inductance, but not in monolithic form.
Therefore, according further to the invention, besides providing
for both the receiver filter and the transmitter filter of a
duplexer on the same substrate, in some applications one or more
coils (planar spiral inductors) serving as components of the
duplexer are integrated onto the same substrate. A ladder filter
response typically includes two attenuation maxima, i.e. two
so-called notches, one on each side of the passband for the filter.
The lower notch can be enhanced (widened) by the use of an inductor
to coincide in frequency with the TX band, thus providing more
attenuation from TX to RX. The integration of such an inductor (in
the form of a planar spiral inductor) onto the same substrate as
the other duplexer components (and in particular the BAW
resonators) reduces the overall size of the duplexer. Inductors
made in a thin-film process, i.e. planar spiral inductors, usually
exhibit quite low quality values (Q-values), because of large
resistive losses and parasitic capacitance or other parasitic
elements associated with materials between the inductors and the
substrate. In case of fabricating a BAW duplexer, some of the
metals used in the fabrication can be fairly thick and can be used
in fabricating the notch-enhancing planar spiral inductors so that
the (planar) coils are thicker than in prior art thin-film
fabrication processes. A thicker coil has lower resistance to
current, so that resistive losses are reduced by fabricating such
coils as part of the fabrication of a monolithic duplexer according
to the invention. In addition, BAW devices usually benefit from
having a high resistivity substrate (i.e. the substrate is usually
an electrical insulator), and the resonator sections as well as any
coil deposited on an acoustic mirror-type of structure are fairly
well electrically isolated from the substrate by the mirror
dielectrics, thus reducing the strength of any parasitic elements
down to the substrate and so improving the Q-value of a monolithic
notch-enhancing coil.
[0040] Referring now to FIGS. 10 and 11, a planar spiral inductor
100 of the type provided according to the invention is shown on the
same substrate as a duplexer. FIG. 10 shows a planar coil 100 and
two metallic layers of an acoustic mirror used as an isolation
structure, but for clarity, does not show dielectric layers
deposited between the metallic layers. FIG. 11 shows that the
metallic layers are separated by dielectric material, deposited as
described below in two layers. The planar spiral inductor is shown
as having two terminals 101 102. One of the terminals is connected
to the innermost coil by an arm 103 extending across the outer
coils. The arm 103 must be insulated from the outer coils so that
it does not short out the outer coils where it crosses them. The
arm is therefore provided after laying down a layer of dielectric
material 104 on top of the coils, with a via 110 to allow
connecting the arm to the innermost coil.
[0041] FIG. 12 illustrates the fabrication, according to the
preferred embodiment of the invention, of a monolithic FBAR
duplexer including at least one planar spiral inductor on the same
substrate as the BAW resonators of the duplexer. In the embodiment
of FIG. 12, after depositing the first metallic layer of an
acoustic mirror over the entire surface of the substrate, the
metallic layer is patterned to provide the first mirror layer and
the first layer of the coil, the patterning including: applying
photoresist; exposing and developing it; a subsequent etch step;
and finally removing the photoresist remaining after the etch step.
Next, the dielectric material for use as the first dielectric layer
of the acoustic mirror is deposited over the entire exposed surface
(which in some places is the surface of the substrate, and in
others is a metallic material that is either for use as a metallic
layer of an acoustic mirror, or for use as the planar spiral
inductor coils). This dielectric need not be selectively removed,
and in the preferred embodiment, is not removed anywhere, except
that to provide a connection from a terminal to the innermost coil
(i.e., in FIG. 10, from terminal 102, via arm 103, to the innermost
coil), a via is provided in the dielectric layer. Then a second
metallic material is deposited everywhere, and extends through the
via onto the first metallic material at the beginning of the
innermost coil. The second metallic material should be a type of
metal that will not corrode because of coming into contact with the
metal of the first metallic layer, i.e. ideally, it is the same or
a similar kind of metal as the second metallic material. Using a
patterning process similar to that used for the first metallic
layer, the second metallic material is removed everywhere except
where the arm connecting the innermost coil to a terminal is to be
provided, and where the second metallic layer of the acoustic
mirror for each BAW resonator is to be provided. Fabrication of the
duplexer BAW resonators then continues, layer by layer, patterning
each layer of metallic material and the piezoelectric material, but
leaving as deposited, each layer of dielectric material.
[0042] The planar spiral inductor can also be fabricated from
either the top or bottom electrode metal, or even from some
additional metal not included in the basic process (specifically
such as from gold, which might be deposited on the substrate after
the resonator fabrication).
[0043] It is to be understood that the above-described arrangements
are only illustrative of the application of the principles of the
present invention. Numerous modifications and alternative
arrangements may be devised by those skilled in the art without
departing from the spirit and scope of the present invention, and
the appended claims are intended to cover such modifications and
arrangements.
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