U.S. patent application number 09/845096 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 by reducing thickness non-uniformity.
This patent application is currently assigned to Nokia Corporation. Invention is credited to Ella, Juha, Kaitila, Jyrki, Tikka, Pasi.
Application Number | 20020158714 09/845096 |
Document ID | / |
Family ID | 25294383 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020158714 |
Kind Code |
A1 |
Kaitila, Jyrki ; et
al. |
October 31, 2002 |
METHOD AND SYSTEM FOR WAFER-LEVEL TUNING OF BULK ACOUSTIC WAVE
RESONATORS AND FILTERS BY REDUCING THICKNESS NON-UNIFORMITY
Abstract
A method and system for tuning a bulk acoustic wave device at
wafer level by reducing the thickness non-uniformity of the topmost
surface of the device using a chemical vapor deposition process. A
light beam is used to enhance the deposition of material on the
topmost surface at one local location at a time. Alternatively, an
electrode is used to produce plasma for locally enhancing the vapor
deposition process. A moving mechanism is used to move the light
beam or the electrode to different locations for reducing the
thickness non-uniformity until the resonance frequency of the
device falls within specification.
Inventors: |
Kaitila, Jyrki; (Helsinki,
FI) ; Tikka, Pasi; (Helsinki, FI) ; Ella,
Juha; (Halikko, 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: |
25294383 |
Appl. No.: |
09/845096 |
Filed: |
April 27, 2001 |
Current U.S.
Class: |
333/188 ;
333/189; 333/191 |
Current CPC
Class: |
Y10T 29/42 20150115;
H03H 3/013 20130101 |
Class at
Publication: |
333/188 ;
333/189; 333/191 |
International
Class: |
H03H 009/56; H03H
009/58 |
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, 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 in a chemical vapor deposition apparatus, wherein
the chemical vapor deposition apparatus contains a gaseous
precursor for depositing material over the surface layer for
reducing the thickness non-uniformity, said method comprising the
steps of: providing an enhancing medium for locally enhancing the
deposition of material on the surface layer at a location; and
relocating the enhancing medium in a lateral direction relative to
the surface layer to at least one different location for locally
enhancing the deposition of material on the surface layer at said
at least one different location.
2. The method of claim 1, wherein the enhancing medium is a light
beam for use in a photon-assisted chemical vapor deposition
process.
3. The method of claim 1, wherein the enhancing medium is a plasma
for use in a plasma-induced chemical vapor deposition process.
4. The method of claim 1, wherein the enhancing medium is a plasma
for use in a plasma-enhanced chemical vapor deposition process.
5. The method of claim 1, wherein the local enhancing of the
deposition of material on the surface layer at each location is
carried out in a time period, wherein the time period is based
partly on the thickness non-uniformity profile of the surface
layer.
6. The method of claim 1, wherein the surface layer comprises a
piezoelectric layer.
7. The method of claim 1, wherein the surface layer comprises a
passivation layer.
8. The method of claim 1, wherein the surface layer comprises an
electrode layer.
9. The method of claim 1, wherein the surface layer comprises a
plurality of individual bulk acoustic wave components.
10. The method of claim 9, wherein the individual bulk acoustic
wave components comprise one or more resonators.
11. The method of claim 9, wherein the individual bulk acoustic
wave components comprise one or more filters.
12. The method of claim 9, wherein the individual bulk acoustic
wave components comprise one or more stacked crystal filters.
13. The method of claim 1, wherein the surface layer is made of a
layer material, and the gaseous precursor comprises the layer
material.
14. The method of claim 1, wherein the surface layer is made of a
layer material, and the gaseous precursor comprises a material
different from the layer material.
15. The method of claim 1, further comprising the step of mapping
the device surface for determining the non-uniformity profile
across the device surface.
16. The method of claim 15, wherein the mapping step is carried out
by measuring the resonant frequency of the device.
17. The method of claim 15, wherein the mapping step is carried out
by measuring the thickness of the device.
18. 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 and a surface layer thickness having a non-uniformity
profile, 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 in a chemical vapor deposition apparatus, wherein
the chemical vapor deposition apparatus contains a gaseous
precursor for depositing material over the surface layer for
reducing the thickness non-uniformity, said system comprising:
means, for providing an enhancing medium for locally enhancing the
deposition of material on the surface layer at a location; and a
moving mechanism, operatively connected to the providing means, for
relocating the enhancing medium in a lateral direction relative to
the surface layer to at least one different location for locally
enhancing the deposition of material on the surface layer at said
at least one different location.
19. The system of claim 18, wherein the enhancing medium comprises
a laser beam for use in a photon-assisted chemical vapor deposition
process and the providing means comprises a laser for providing the
laser beam.
20. The system of claim 18, wherein the enhancing medium comprises
a light beam for use in a photo-assisted chemical vapor deposition
process and the providing means comprises a light source.
21. The system of claim 18, wherein the enhancing medium comprises
a plasma for use in a plasma-assisted chemical vapor deposition
process and the providing means comprises an electrode to produce a
plurality of ions for providing the plasma.
22. The system of claim 18, further comprising a mapping mechanism
for obtaining the non-uniformity profile by mapping the device
surface.
23. The system of claim 22, wherein the mapping mechanism comprises
a frequency measurement device for determining the local resonant
frequency of the device across the device surface.
24. The system of claim 22, wherein the mapping mechanism comprises
a thickness measurement device.
25. The system of claim 18, wherein said relocating means has a
control mechanism for relocating the enhancing medium based on the
non-uniformity profile.
26. The system of claim 18, wherein the device comprises of a
plurality of individual chips each having a chip surface area, and
wherein the enhancing medium defines a spot size larger than the
chip surface area.
27. The system of claim 18, wherein the device comprises a
plurality of individual chips, each having a chip surface area, and
wherein the enhancing medium defines a spot size smaller than the
chip surface area.
28. The system of claim 18, further comprising a software program
for controlling said relocating means according to the thickness
non-uniformity profile for reducing surface non-uniformity.
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/acoustic wave (produced by the RF signal) 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 (an 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), tungsten (W), copper (Cu), Nickel
(Ni), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co),
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 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 in 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 the thickness of each
layer in the device must be controlled in the same 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. For that reason, manufacturers of
FBAR-based devices use wafers of 4-inches or less in diameter for
device fabrication. 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 adding material to 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, stack 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, or an electrode layer, and one or more layers
may be added onto the topmost layer after the thickness of the
topmost layer is adjusted.
[0006] Thus, according to the first aspect of the present
invention, 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, 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 in a chemical vapor
deposition apparatus, wherein the chemical vapor deposition
apparatus contains a gaseous precursor for depositing material on
the surface layer for reducing the thickness non-uniformity. The
method comprises the steps of:
[0007] providing an enhancing medium for locally enhancing the
deposition of material on the surface layer at a location; and
[0008] relocating the enhancing medium in a lateral direction
relative to the surface layer to at least one different location
for locally enhancing the deposition of material on the surface at
said at least one different location.
[0009] Preferably, the enhancing medium is a light beam for use in
a photon-assisted chemical vapor deposition process. It is also
possible that the enhancing medium is a plasma for use in a
plasma-induced or plasma-assisted chemical vapor deposition
process.
[0010] Preferably, the enhancing medium is located at one location
within a time period, and the time period is based on the thickness
non-uniformity of the surface layer.
[0011] The surface layer may comprises a plurality of individual
components and the individual components may be resonators,
filters, stacked crystal filters or a combination thereof
[0012] It is preferable to adjust the thickness non-uniformity of
the piezoelectric layer of the device to tune the bulk acoustic
wave device, but it is also possible to change the thickness of the
electrode or the passivation layer overlapping an active area of
the device..
[0013] 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 be
necessary to adjust the thickness of the top electrode layer using
the same method or other 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.
[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 and a surface layer thickness having a non-uniformity
profile, 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 in a chemical vapor deposition apparatus, wherein
the chemical vapor deposition apparatus contains a gaseous
precursor for depositing material on the surface layer for reducing
the thickness non-uniformity. The system comprises:
[0015] means, for providing an enhancing medium for locally
enhancing the deposition of material on the surface layer at a
location; and
[0016] a moving mechanism, operatively connected to the providing
means, for relocating the enhancing medium in a lateral direction
relative to the surface layer to at least one different location
for locally enhancing the deposition of material on the surface
layer at said at least one different location.
[0017] Preferably, the enhancing medium comprises a light beam for
use in a photon-assisted chemical vapor deposition process, and the
providing means comprises a light source.
[0018] It is possible that the enhancing medium comprises a plasma
for use in a plasma-assisted chemical vapor deposition process, and
the providing means comprises an electrode, operatively connected
to a radio-frequency (RF) source, to produce ions for providing the
plasma.
[0019] Preferably, the dwell time, within which the enhancing
medium is positioned at a location to enhance the deposition, is
based on the thickness non-uniformity of the surface layer. Thus,
it is preferable to have a software program to control the moving
mechanism to relocate the enhancing medium according to the
thickness non-uniformity profile.
[0020] Preferably, the system also comprises a mechanism for
mapping the thickness non-uniformity profile of a device surface
prior to adjusting the thickness of the surface layer. Preferably,
the mapping mechanism comprises a frequency measurement device for
measuring the frequency at different locations of the device
surface. In that case, the device would already have a patterned
top electrode layer. It is also possible to use a thickness
measurement device to determine the thickness non-uniformity
profile of a surface layer.
[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 cross sectional view of a simplified bulk
acoustic wave device illustrating the thickness non-uniformity of
the topmost layer of the device.
[0024] FIG. 3a is a diagrammatic representation illustrating a
system for adding material on the topmost layer of the bulk
acoustic wave device at wafer level, according to the preferred
embodiment of the present invention.
[0025] FIG. 3b is a diagrammatic representation illustrating
another system for adding material on the topmost layer of the bulk
acoustic wave device at wafer level, according to another
embodiment of the present invention.
[0026] FIG. 4 is cross sectional view illustrating the simplified
bulk acoustic wave device after the thickness of the topmost layer
has been adjusted.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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
[0031] FIG. 2 is a cross section view illustrating a simplified
bulk acoustic wave device 20 having a top surface layer 30 and a
plurality of mirror layers 24 formed on a substrate 22. Some of the
mirror layers are patterned. The substrate 22 can be made of Si,
GaAs, glass or other material. The top layer 30 may comprise a
plurality of resonators (or filters) having top and bottom
electrodes, but the top layer 30 may represent a piezoelectric
layer, a bottom electrode layer or a top electrode layer. When a
bulk acoustic wave generating or controlling layer is formed on a
wafer in a thin-film deposition process, the layer is usually
thicker in the center portion than the edge portion (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 adjust part of the top
surface layer 30 so that the non-uniformity profile of the reduced
surface falls within the tolerance. To illustrate the problem
associated with the thickness non-uniformity profile of the top
surface layer, a plurality of resonators (or filters, stacked
crystal filters or a combination thereof) 31-36 with different
thicknesses is shown in FIG. 2. The thickness non-uniformity
profile of the top surface layer 30, however, can be reduced by
increasing the thickness of some of the resonators so that the
frequency of each resonator falls within the tolerance.
[0032] Referring to FIG. 3a, the thickness adjustment system 40,
according to the present invention, comprises a chemical vapor
deposition chamber 60 having an inlet 64 to provide a gaseous
precursor 62 into the chamber 60 and an outlet 66, which is
operatively connected to a pumping system (not shown) to adjust the
pressure in the chamber 60. As it is known in the art, the gaseous
precursor 62 may contain fragments of a desired solid to be
deposited to the surface 30 of a bulk acoustic wave device 20. The
gaseous precursor 62 is usually brought into the deposition chamber
60 through the inlet 64 by a carrier gas that is not to be
deposited. Chemical vapor deposition is commonly used to grow a
thin-film on a substrate. For example, a gaseous precursor
containing silane (SiH4) and either oxygen (O2) or nitrous oxide
(N2O) is used to grow a thin-film of silicon dioxide (SiO2) on a
substrate. Another gaseous precursor containing silane and either
nitrogen (N2) and/or ammonia (NH4) is used to grow a thin-film of
silicon nitride (SiN2) on a substrate. In the deposition chamber,
energy has to be supplied to the deposition process in order to
activate the reactant species. The choice of gaseous precursor 62
depends on the material of the topmost layer 30. Preferably, a
dielectric material is used for the gaseous precursor 62 for adding
material on a passivation layer. With such a layer, no additional
patterning is needed. It is possible to add material on a
piezoelectric layer or other layer by using a suitable gaseous
precursor containing fragmentation of a suitable solid. For
example, it is also possible to deposit tantalum, tungsten and
other refractory metals onto an electrode layer. Generally, two
methods are used to supply the activation energy: one uses heat and
the other uses a radio-frequency (RF) induced plasma. The latter
method is also referred to as plasma-enhanced chemical vapor
deposition (PECVD). Both methods are widely used in the
semiconductor industry to produce a thin-film on an entire wafer. A
variation of the CVD method is the so-called photon assisted CVD or
photon induced CVD, where the reaction is activated by light,
either by heating the substrate surface or dissociating/exciting
the reactant species in the gas phase. Generally, ultraviolet lamps
or lasers are used as light sources. As shown in FIG. 3a, a light
source 42 is used to provide a light beam 44 through a window 61 of
the deposition chamber 60. The size of the light beam 44, or the
spot size of the assisted deposition, can range from a single
resonator (or filter) to a plurality of resonators (or filters).
The selection of the spot size depends partly on the tolerance of
the resonance frequency and the non-uniformity profile 154 of the
wafer or device 20, and partly on the repeatability of the
deposition process. However, the important aspect of the present
invention is that the chemical deposition process can be enhanced
locally so that the thickness of only one section of the wafer is
adjusted at a time, based on the thickness non-uniformity profile
154. Accordingly, a moving mechanism 150 is used to move the light
source 42 in a lateral direction 48 relative to the device surface
30 for relocating the light beam 44 to a different location of the
surface 30. Preferably, the moving mechanism 150 has a control
program 152 to control the dwell time of the light beam 44 at each
location based on the thickness non-uniformity profile 154. It
should be noted that it is also possible to relocate the device 20
while keeping the light beam 44 stationary.
[0033] Alternatively, a radio-frequency (RF) source 52 is used in
the system 50 to provide power to an electrode 54, coupled to
another electrode 55, to assist the chemical vapor deposition
process locally. As shown in FIG. 3b, the electrode 54, which is
placed above the surface 30, is used to cause plasma glow
discharges 56 to form between the electrode 54 and a section of the
surface 30. The electrons excited to high energy states in the
plasma 56 can dissociate and ionize molecules in the gas phase.
These ions are active. Thus, the plasma supplies a particle flow to
be deposited on a section of the surface 30. The moving mechanism
150 can be used to move the electrode 54 in a lateral direction 58,
relative to the surface 30 to change the deposition locations. It
should be noted that it is possible to move the device 20 relative
to the electrode 54 or the light beam 44 (FIG. 3a) to change the
deposition locations.
[0034] FIG. 4 shows the result after the thickness non-uniformity
of the topmost surface 30' is corrected. It should be noted that,
in most cases, the thickness non-uniformity of the topmost surface
does not need to be completely eliminated. Usually, it is only
required to reduce the thickness non-uniformity so that the spread
in the resonant frequency of the bulk acoustic wave device falls
within a given tolerance. As shown, the thickness of the resonators
31, 32, 35 and 36 has been increased. Although the thickness of the
resonators 31-36 may not be the same, the spread in the resonant
frequency of the device falls within the specification.
[0035] Furthermore, it is also possible to add one or more bulk
acoustic wave generating and controlling layers on top of a
thickness-adjusted layer. If the newly-added layer needs thickness
adjustment, the same method, as described in conjunction with FIGS.
3a and 3b, can be applied until a desired product is achieved.
[0036] Prior to thickness adjustment of a topmost surface layer 30
in a locally-enhanced chemical vapor deposition chamber 60, as
shown in FIGS. 3a and 3b, it is preferred that the thickness
profile 154 be mapped. It is preferable to use a frequency
measurement apparatus 80 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 152, it is possible to calculate the amount of
material to be added on the upper surface 30. As shown in FIG. 5a,
the profile mapping system 70 comprises a frequency measurement
apparatus 80 and a moving mechanism 88 for moving the frequency
measurement apparatus 80 relative to the device 20 for obtaining
the frequency profile 152, of the surface. The moving direction is
denoted by reference numeral 74. From the frequency profile 152, it
is possible to obtain the thickness non-uniformity profile 154
(FIG. 6).
[0037] FIG. 5b is a diagrammatic representation illustrating a
system 72 for mapping a bulk acoustic wave device 20 by measuring
the physical thickness of the device. Instead of a frequency
measurement apparatus 80, a thickness measurement apparatus 82 is
used to measure the thickness of the device 20 and obtain the
thickness profile 154 directly.
[0038] 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.
[0039] FIG. 7 is a flow chart illustrating the process 100 for
tuning a bulk acoustic wave device, according to the present
invention. As shown at step 102, 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 154 is thus obtained. If the surface layer thickness falls
within the tolerance, as determined at step 103, then new layers
may be added on top of the mapped surface, as determined at step
112 and carried out at step 114. Otherwise, the device 20 is placed
in a locally-enhanced chemical vapor deposition chamber 60 (FIGS.
3a & 3b) for thickness adjustment, at step 104. A CVD
enhancement medium is placed at a desired location over the surface
layer 30 for adding material thereon for a period of time, at step
106. After the thickness of the local area is adjusted by the
locally-enhanced chemical vapor deposition process, it is
determined, at step 108, whether the necessary thickness adjustment
of the entire surface is carried out. If the thickness at more
surface areas needs to be adjusted, the enhancement medium is
relocated to a different location, at step 110. If the necessary
thickness adjustment of the entire surface has been carried out, it
is determined, at step 112, whether more layers need to be
fabricated to complete the device. After one or more new layers are
added, at step 114, on top of the adjusted layer, the surface
profile of the device is again mapped, at step 102, to determine
whether the device is made according to the specification.
[0040] 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. The thickness adjustment
process can be separately and sequentially carried out to adjust at
one or more layers of the FBAR-based device. If a material is added
onto a surface layer to tune the frequency and a gaseous precursor
is used to provide the material to be deposited on the device
surface, it is preferable that the gaseous precursor contains the
material of the surface layer. For example, if the topmost surface
layer to be adjusted is the electrode layer made of tantalum or
tungsten, it is preferred that the gaseous precursor contains
fragmentation of tantalum or tungsten. However, the material for
the gaseous precursor and the material for the surface layer may
not be the same. For example, if the topmost surface layer to be
adjusted is a mirror or passivation layer of SiO2, the gaseous
precursor can contain silane (SiH4) and oxygen (O2).
[0041] 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
after the thickness of the respective electrode layer is adjusted.
However, it is also possible to carry out the patterning steps
prior to the thickness adjustment.
[0042] Furthermore, in the chemical vapor deposition method and
system, as described in conjunction with FIGS. 3a and 3b, only one
light beam 44 or electrode 54 is used to enhance the local
deposition of material on the topmost surface at one location at a
time. It is possible to use two or more light beams or electrodes
to enhance the deposition at two or more locations simultaneously
or sequentially. Moreover, the light beam and the plasma can be
relocated by moving the light source (or a light-path modifying
device, such as a reflector) or the electrode. It is possible to
move the bulk acoustic wave device in a lateral direction, relative
to the light beam or plasma.
[0043] Moreover, the device, as described hereinabove, usually
comprises a plurality of individual chips, each having a chip
surface area. In the process, as described in conjunction with
FIGS. 3a and 3b, the light beam or the plasma for locally enhancing
the chemical vapor deposition at a location defines the spot size
for material deposition. The spot size can be smaller or larger
than the chip surface area.
[0044] 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.
[0045] 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.
[0046] 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.
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