U.S. patent application number 09/844218 was filed with the patent office on 2002-10-31 for method and system for wafer-level tuning of bulk acoustic wave resonators and filters.
This patent application is currently assigned to Nokia Corporation. Invention is credited to Ella, Juha, Kaitila, Jyrki, Tikka, Pasi.
Application Number | 20020158702 09/844218 |
Document ID | / |
Family ID | 25292149 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020158702 |
Kind Code |
A1 |
Tikka, Pasi ; et
al. |
October 31, 2002 |
METHOD AND SYSTEM FOR WAFER-LEVEL TUNING OF BULK ACOUSTIC WAVE
RESONATORS AND FILTERS
Abstract
A method and system for tuning a bulk acoustic wave device at
wafer level, wherein the thickness of topmost layer of the device
is non-umiform. The thickness non-unifornity causes the resonant
frequency of the device to vary from one location to another
location of the topmost layer. A laser beam, operatively connected
to a beam moving mechanism, is used to locally trim the topmost
layer, one location at a time.
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
|
Assignee: |
Nokia Corporation
|
Family ID: |
25292149 |
Appl. No.: |
09/844218 |
Filed: |
April 27, 2001 |
Current U.S.
Class: |
332/100 ;
310/312 |
Current CPC
Class: |
H03H 3/04 20130101; H03H
3/013 20130101 |
Class at
Publication: |
332/100 ;
310/312 |
International
Class: |
H01L 041/08 |
Claims
What is claimed is:
1. A method of tuning a bulk acoustic wave device comprising a
plurality of acoustic wave generating and controlling layers formed
on a substrate, wherein the bulk acoustic wave device has a surface
layer and a surface layer thickness having a non-uniformity profile
defining a plurality of locations at which the layer surface
requires thickness adjustment, and wherein the bulk acoustic wave
device has an operating frequency which varies partly with the
surface layer thickness, and the operation frequency can be
adjusted by adjusting the surface layer thickness, said method
comprising the steps of: providing an etching medium over the
surface layer at one section thereof for adjusting locally the
thickness of at least one location of the surface layer at a time;
relocating the etching medium, relative to the device, to another
section of the surface layer for adjusting the thickness of at
least one other location of the surface layer; and repeating the
relocating step if at least one remaining location of the surface
layer requires thickness adjustment.
2. The method of claim 1, wherein the etching medium is a laser
beam.
3. The method of claim 2, wherein the surface layer comprises a
plurality of individual bulk acoustic wave components each having a
top surface located at one of said locations, and wherein the laser
beam is used to trim the top surface of the individual bulk
acoustic wave components, one at a time.
4. The method of claim 3, wherein the top surface comprises a
piezoelectric layer.
5. The method of claim 3, wherein the top surface comprises a
passivation layer.
6. The method of claim 3, wherein the top surface comprises an
electrode layer.
7. The method of claim 3, wherein the top surface comprises an
electrode layer and a piezoelectric layer.
8. The method of claim 7, wherein the top surface further comprises
a passivation layer overlying at least a part of the electrode
layer.
9. The method of claim 3, wherein the individual bulk acoustic wave
components comprise one or more resonators.
10. The method of claim 3, wherein the individual bulk acoustic
wave components comprise one or more filters.
11. The method of claim 3, wherein the individual bulk acoustic
wave components comprise one or more stacked crystal filters.
12. The method of claim 1, further comprising the step of mapping
the thickness non-uniformity of the surface layer prior to
adjusting the thickness thereof.
13. A system for tuning a bulk acoustic wave device comprising a
plurality of acoustic wave generating and controlling layers formed
on a substrate, wherein the bulk acoustic wave device has a surface
layer made of a surface material and a surface layer thickness
having a non-uniformity profile defining a plurality of locations
at which the surface layer requires thickness adjustment, and
wherein the bulk acoustic wave device has an operating frequency,
which varies partly with the surface layer thickness, and the
operation frequency can be adjusted by adjusting the surface layer
thickness, said system comprising: means, positioned above a
section of the surface layer, for adjusting locally the thickness
of at least one location of the surface layer at a time; and means
for relocating the thickness adjusting means, relative to the
device, to another section of the surface layer for adjusting the
thickness of at least one other location of the surface layer.
14. The system of claim 13, wherein the thickness adjusting means
comprises a laser for providing a laser beam for removing the
surface material.
14. The system of claim 13 further comprises means for mapping the
thickness non-uniformity profile of the surface layer prior to
adjusting the thickness thereof.
15. The system of claim 14, wherein the mapping means comprises a
frequency measurement apparatus for measuring the frequency of the
device at various location thereof for providing the thickness
non-uniformity profile.
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
acoustically generated 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 resonant 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 resonant
frequency using this fabrication method. Fabricating BAW devices by
depositing thin-film layers on passive substrate materials, one can
extend the resonant 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). The
difference between these two types of devices lies mainly in their
structures. An SCF usually has two or more piezoelectric layers and
three or more electrodes, with some electrodes being grounded.
FBARs are usually used in combination to produce passband or
stopband filters. The combination of one series FBAR and one 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, a 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
FIG. 1. As shown in FIG. 1, the FBAR device 1 comprises a substrate
2, a bottom electrode 4, a piezoelectric layer 6, a top electrode 8
and a passivation layer 10. The FBAR device 1 may additionally
include an acoustic mirror layer 12, which is comprised of a layer
16 of high acoustic impedance sandwiched between two layers 14 and
18 of low acoustic impedance. The mirror usually, but not always,
consists of pairs of high and low impedance layers (even number of
layers). Some mirrors consist of two pairs of such layers arranged
in a sequence like SiO2, W, SiO2, W. Instead of the mirror, an FBAR
device may additionally include one or more membrane layers of SiO2
and a sacrificial layer. The substrate 2 can be made from silicon
(Si), silicon dioxide (SiO2), Galium Arsenide (GaAs), glass, or
ceramic materials. The bottom electrode 4 and top electrode 8 can
be made from gold (Au), molybdenum (Mo), aluminum (Al), titanium
(Ti) or other electrically conductive materials. The piezoelectric
layer 6 can be made from zinc oxide (ZnO), zinc sulfide (ZnS),
aluminum nitride (AlN), lithium tantalate (LiTaO.sub.3) or other
members of the so-called lead lanthanum zirconate titanate family.
The passivation layer can be made from SiO2, Si3N4 or polyimide.
The low acoustic impedance layers 14 and 18 can be made from Si,
SiO2, poly-silicon, Al or a polymer. The high acoustic impedance
layer 16 can be made from Au, Mo or tungsten (W), and in some
cases, dielectric such as AlN to make a number of layer pairs. FBAR
ladder filters are typically designed so that the series resonators
yield a series resonance at a frequency that is approximately equal
to, or near, the desired, or designed, center frequency of the
respective filters. Similarly, the shunt, or parallel, resonators
yield a parallel resonance at a frequency slightly offset from the
series FBAR resonance. The series resonators are usually designed
to have their maximum peak in transmission at the center frequency,
so that signals are transmitted through the series resonators. In
contrast, the shunt resonators are designed to have their minimum
in transmission so that signals are not shorted to ground. FBARs
yield parallel resonance and series resonance at frequencies that
differ by an amount that is a function of a piezoelectric coupling
coefficient of the piezoelectric materials used to fabricate the
devices, in addition to other factors such as the types of layers
and other materials employed within in the device. In particular,
FBAR ladder filters yield passbands having bandwidths that are a
function of, for example, the types of materials used to form the
piezoelectric layers of the resonators and the thickness of various
layers in the device.
[0003] The difference in the thickness of various layers in the
device can be achieved during the fabrication of the device.
Presently, FBARs are fabricated on a glass substrate or a silicon
wafer. The various layers in the FBAR-based device are sequentially
formed by thin-film deposition. In an FBAR-based device, the
resonant frequency of the device usually has to be controlled to
within a 0.2-0.5% tolerance. This means that, if no tuning is used,
the thickness of each layer in the device must be controlled in a
similar way. It is known that, however, the deposition of thin-film
layers is difficult to control to yield a thickness within such
tolerance when the area of substrate or wafer is large. With a
small wafer or substrate, certain thickness non-uniformity can be
accepted without losing many components due to the operation
frequency being out of specification. However, fabricating devices
on small wafers or substrates is less cost-effective than doing the
same on large substrates. In the case of using large substrates,
the problem associated with thickness non-uniformity becomes
acute.
[0004] Thus, it is advantageous and desirable to provide a method
and system to solve the problem associated with thickness
non-uniformity in the fabrication of FBAR-based devices on large
substrates or wafers.
SUMMARY OF THE INVENTION
[0005] It is a primary object of the present invention to provide a
method and system for achieving a desired resonant frequency of the
bulk acoustic wave device within a given tolerance. This object can
be achieved by reducing the thickness non-uniformity of the device
on a substrate. The thickness non-uniformity can be reduced by
selectively and locally removing material from the topmost surface
layer of the wafer, or die, before the wafer is cut into a
plurality of device chips. Thus, the wafer has one or more bulk
acoustic wave generating and controlling layers formed thereon. In
that context, the bulk acoustic wave device, as described herein,
refers to the entire wafer or substrate that has one or more layers
fabricated thereon to form one or more individual device chips, or
part of such wafer or substrate. Moreover, the bulk acoustic wave
devices referred to herein include bulk acoustic wave resonators,
bulk acoustic wave filters, stacked crystal filters, any
combination of resonators and filters, and the structural
variations of the resonators and filters. Furthermore, although one
or more layers are already formed on the wafer or substrate when
the thickness non-uniformity of the topmost layer is reduced, the
device may or may not have all the necessary layers or the patterns
of the layers. For example, the topmost layer of the device can be
a piezoelectric layer. In that case, one or more layers are added
on to the adjusted layer to complete the device.
[0006] Thus, according to the first aspect of the present
invention, a method of tuning a bulk acoustic wave device made of a
plurality of acoustic wave generating and controlling layers formed
on a substrate, wherein the bulk acoustic wave device has a surface
layer and a surface layer thickness having a non-uniformity profile
defining a plurality of locations at which the surface layer
requires thickness adjustment, and wherein the bulk acoustic wave
device has an operating frequency which varies partly with the
surface layer thickness, and the operation frequency can be
adjusted by adjusting the surface layer thickness. The method
comprises the steps of:
[0007] providing an etching medium over the surface layer at one
section thereof for locally adjusting the thickness of at least one
location of the surface layer at a time;
[0008] relocating the etching medium to another section of the
surface layer for adjusting the thickness of at least one other
location of the surface layer; and
[0009] repeating the relocating step if at least one remaining
location of the surface layer requires thickness adjustment.
[0010] Preferably, the etching medium is a laser beam.
[0011] When the surface layer comprises a plurality of individual
bulk acoustic wave components each having a top surface located at
one of the locations, it is preferable to use a laser beam to trim
the top surface of the individual bulk acoustic wave components,
one at a time. The top surface can be a piezoelectric layer, a top
electrode layer, a bottom electrode layer, a passivation layer
overlapping the active area of the device, or a combination of
thereof. For example, the top surface may include a piezoelectric
layer and a top electrode layer.
[0012] Each of the individual bulk acoustic wave components may be
a part of resonator, filter, stacked crystal filter or a
combination thereof.
[0013] Preferably, the method further comprises the step of mapping
the thickness non-uniformity of the surface layer prior to
adjusting the thickness thereof.
[0014] According to the second aspect of the present invention, a
system for tuning a bulk acoustic wave device made of a plurality
of acoustic wave generating and controlling layers formed on a
substrate, wherein the bulk acoustic wave device has a surface
layer made of a surface material, and a surface layer thickness
having a non-uniformity profile defining a plurality of locations
at which the surface layer requires thickness adjustment, and
wherein the bulk acoustic wave device has an operating frequency,
which varies partly with the surface layer thickness, and the
operation frequency can be adjusted by adjusting the layer
thickness. The system comprises:
[0015] means, positioned above a section of the surface layer, for
adjusting locally the thickness of at least one location of the
surface layer at a time; and
[0016] means for relocating the thickness adjusting means, relative
to the device, to another section of the surface layer for
adjusting the thickness of at least one other location of the
surface layer.
[0017] Preferably, the thickness adjusting means comprises a laser
for providing a laser beam for removing the surface material.
[0018] Preferably, the system further comprises means for mapping
the thickness non-uniformity profile of the surface layer prior to
adjusting the thickness thereof.
[0019] It is understood that if the thickness of the piezoelectric
layer is adjusted according to the above-described method, then a
top electrode layer is deposited on the piezoelectric layer after
the thickness of the piezoelectric layer is adjusted. It may also
be necessary to adjust the thickness of the top electrode layer
using the same method. Additionally, a patterning step is usually
necessary to produce a desired pattern for the electrode layer. The
patterning step can be carried out before or after the thickness of
the electrode layer is adjusted. The patterning step is not part of
the present invention. Furthermore, if a passivation layer is
deposited on top of the top electrode layer, it may be necessary to
adjust the thickness of the passivation layer. Thus, the thickness
adjustment steps, according to the present invention, may be
carried out one or more times for tuning the entire device, if
necessary.
[0020] It is understood that, the relocation means can also be used
to relocate the device, relative to the thickness adjusting
means.
[0021] The present invention will become apparent upon reading the
description taken in conjunction with FIGS. 2 to 7.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a cross sectional side view of a typical bulk
acoustic wave device illustrating a plurality of bulk acoustic wave
generating and controlling layers.
[0023] FIG. 2 is a diagrammatic representation illustrating a
simplified bulk acoustic wave device comprising a surface layer
with a plurality of locations at which the surface layer requires
thickness adjustment.
[0024] FIG. 3 is a diagrammatic representation illustrating a
system for removing material from the surface layer of the bulk
acoustic wave device at wafer level, according to the preferred
embodiment of the present invention.
[0025] FIG. 4 is a diagrammatic representation illustrating the
simplified bulk acoustic wave device after the thickness
non-uniformity of the surface layer is reduced.
[0026] FIG. 5a is a diagrammatic representation illustrating a
system for mapping the thickness non-uniformity profile of a bulk
acoustic wave device, according to the present invention.
[0027] FIG. 5b is a diagrammatic representation illustrating
another system for mapping the thickness non-uniformity of a bulk
acoustic wave device, according to the present invention.
[0028] FIG. 6 is a thickness chart illustrating the non-uniformity
profile of a wafer with a plurality of bulk acoustic wave
generating and controlling layers fabricated thereon.
[0029] FIG. 7 is a flow chart illustrating the steps for tuning a
bulk acoustic wave device at wafer level, according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] FIG. 2 shows a bulk acoustic wave device 20 at wafer level,
wherein the device 20 has a plurality of bulk acoustic wave
generating and controlling layers formed on a wafer or substrate
22. The topmost layer is denoted by reference numeral 30 and the
underlying layers are collectively denoted by reference numeral 24.
For example, the underlying layers 24 may comprise a plurality of
mirror layers, some of which are patterned. The top layer 30 may
comprise a plurality of individual bulk acoustic components having
top and bottom electrodes, but the top layer 30 may represent a
piezoelectric layer, a bottom electrode layer or a top electrode
layer. If the wafer is sufficiently large and the layers are
fabricated by vacuum deposition, usually the thickness of the
layers is non-uniform (see FIG. 6). The thickness non-uniformity of
the top surface layer 30 may exceed the tolerance allowed for the
spread in the resonant frequency of the device 20. Thus, it is
desirable to remove part of the top surface layer 30 so that the
non-uniformity of the reduced surface falls within the tolerance.
To illustrate the problem associated with the thickness
non-uniformity of the top surface layer 30, a plurality of
individual bulk acoustic wave components 31-36 with different
thickesses are shown in FIG. 2. The individual bulk acoustic wave
components 31-36 can be resonators (or filters, stacked crystal
filters or a combination thereof). The thickness non-uniformity
profile of the top surface layer 30, however, can be reduced by
reducing the thickness of some of the resonators so that the spread
in the resonant frequency falls within the tolerance.
[0031] A system 100 for reducing the thickness non-uniformity
profile of the top surface layer 30 is shown in FIG. 3. As shown, a
laser 110 is used to provide a laser beam to a beam forming device
112 so that the output beam 115 can be used to locally reduce the
thickness of some of the resonators 31-36, one at a time. A beam
moving mechanism 140 is used to move the laser beam 115 in a
lateral direction 117 to different locations for trimming the
resonators. For example, if it is desirable to trim only the
resonators 32-35, then it is only necessary to relocate the laser
beam 115 by the moving mechanism 140 to the corresponding
locations. It is also possible to use a similar moving mechanism to
relocate the device 20 to different locations for trimming, while
keeping the laser beam 115 stationary.
[0032] After being trimmed by the laser beam 115, the reduced top
surface layer is denoted by reference numerals 30', as shown in
FIG. 4. As shown, although the thickness of the resonators 31-36
may not be the same, the spread in the resonant frequency of the
device 20 falls within the specification.
[0033] Prior to thickness adjustment of the topmost surface layer
30 of the device 20, as described in conjunction with FIGS. 3 and
4, it is preferred that the thickness profile of the device 20 be
mapped. It is preferable to use a frequency measurement apparatus
172 to perform localized measurement of the resonant frequency of
the device 20. It may be necessary to measure the resonant
frequency of the individual resonators and/or filters of the device
20. It should be noted that, in order to measure the resonant
frequency of those components, it is necessary to form and pattern
the top electrode layer on the wafer. Based on the frequency
profile 190, it is possible to calculate the amount of material to
be removed from the upper surface layer 30. As shown in FIG. 5a,
the profile mapping system 170 comprises a frequency measurement
apparatus 172, and a moving mechanism 180 for moving the frequency
measurement apparatus 172 relative to the device 20 for obtaining
the frequency profile 190 of the surface. The movement direction of
the frequency measurement apparatus 172 is represented by arrow
168. From the frequency profile 190 it is possible to obtain the
thickness non-uniformity profile 192 (FIG. 6).
[0034] FIG. 5b is a diagrammatic representation illustrating a
system 171 for mapping a bulk acoustic wave device 20 by measuring
the physical thickness of the device. Instead of a frequency
measurement apparatus 172, a thickness measurement apparatus 174 is
used to measure the thickness of the device 20 and obtain the
thickness non-uniformity profile 192 directly.
[0035] FIG. 6 is a thickness chart illustrating the non-uniformity
thickness profile of a wafer with a plurality of acoustic wave
generating and controlling layers fabricated thereon. In
particular, FIG. 6 shows the non-uniformity profile of a
piezoelectric (ZnO) layer expressed in terms of nanometers. If the
average thickness is used as a reference, then the thickness
variation across the layer is about .+-.23%. With such a large
variation in thickness, the frequency variation across the wafer is
usually not acceptable. Thus, the device must be tuned by adjusting
the thickness of the device.
[0036] FIG. 7 is a flow chart illustrating the process 200 for
tuning a bulk acoustic wave device, according to the present
invention. As shown at step 202, a frequency measurement apparatus
(FIG. 5a) or a thickness measurement apparatus (FIG. 5b) is used to
map the surface of the device 20. A thickness non-uniformity
profile 192 is thus obtained. If the variations in surface
thickness fall within a given tolerance, as determined at step 204,
then it is not necessary to adjust the thickness of the mapped
surface. However, new layers may be added on top of the mapped
surface, depending on the determining step 212. Otherwise, the
topmost layer 30 of the device 20 is trimmed locally by the laser
beam 115 (FIG. 3), at step 206. At step 208, the laser beam is
relocated relative to the device 20 to locally trim the topmost
layer 30 at another location thereof. At step 210, it is determined
whether the local trimming is completed. At step 212, it is
determined whether more layers need to be fabricated to complete
the device. After one or more new layers are added, at step 214, on
top of the adjusted layer, the surface profile of the device is
again mapped, at step 202, to determine whether the device is made
according to the specification.
[0037] In summary, the present invention discloses a method and
system for tuning the bulk acoustic wave device at a wafer, or die,
level. The method and system, as disclosed, are particularly useful
when the surface area of the wafer is sufficiently large such that
the deposition of thin-film cannot achieve acceptable thickness
uniformity. Tuning the frequency across the wafer by adjusting the
thickness at localized areas of the wafer surface can increase the
yield of the FBAR manufactory process.
[0038] It is known in the art that the fabrication of the top and
bottom electrode layers, in general, involves one or more
additional steps to make a pattern out of each of the electrode
layers. It is preferred that the patterning steps are carried out
before the thickness of the respective electrode layer is adjusted.
However, it is also possible to carry out the patterning steps
after the thickness adjustment.
[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] Furthermore, the thickness non-uniformity, as described
hereinabove, is related to the frequency non-uniformity of the BAW
devices on the wafer. The purpose of trimming the surface layer is
to reduce the frequency non-uniformity of the devices. Thus,
trimming the surface layer does not necessarily make the topmost
layer perfectly even. In other words, even if the topmost layer has
a very uniform thickness, it might be necessary to trim it to
correct for the non-uniformity of one or more of the underlying
layers. For example, if the topmost layer is a top electrode layer
overlying a piezoelectric layer which is not uniform, the purpose
of trimming the top electrode layer is for reducing the frequency
non-uniformity of the devices due to the thickness non-uniformity
of the piezoelectric layer--even if the top electrode layer itself
is uniform. The object of the present invention is to achieve the
desired uniformity of the final frequency of the devices.
Therefore, the surface layer can be a single layer, such as the
top, bottom or piezoelectric layer, but the surface layer can also
be a combination of layers, such as the combination of the top
electrode layer and the piezoelectric layer.
[0041] 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.
* * * * *