Sputtering Processes for Bulk Acoustic Wave Filters
Yury Oshmyansky
Agilent
39201 Cherry St.
Newark, CA 94560

John D. Larson
Ph.D.
Agilent Laboratories,
3500 Deer Creek
Palo Alto, CA 94304

Richard Ruby
Ph.D.
Agilent
39201 Cherry St.
Newark, CA
94560

Sergey Mishin
Advanced Modular
Sputtering, Inc.
7920 Winchester
CircleGoleta, CA 93117
smishin@amsincorp.com

INTRODUCTION
Bulk Acoustic Wave (BAW) filters, which are synthesized from Film Bulk Acoustic
Resonators (FBAR), have been called "a technology of the future and likely to always stay thatway", for the last thirty years. Much R&D has been done on FBARs over this period, in both
university and industrial settings [1-6]. Even though several companies were able to produce
reasonable quality BAW filters for military applications, the cost barrier has kept such filters frombecoming a high-volume consumer commodity item. In contrast, Surface Acoustic Wave Filters(SAW) [7] were able to be manufactured in high volumes and hence became the filter technology
of choice for miniaturization. Agilent Technologies has recently shifted this paradigm by shipping
production quantities of cellular phone duplexers during the second half of 2002. Duplexers
contain two FBAR filters that must meet the most stringent specifications of any handset filter
application.
FBAR filter technology is based on thin films of piezoelectrically active materials, such asaluminum nitride (AlN) or zinc oxide (ZnO), and of suitable electrode materials, such asaluminum or molybdenum. For successful, high-volume manufacture of RF and microwave
filters, stringent control of both the material parameters – dielectric, mechanical, & piezoelectric and the thickness of the layers is very important. One of the keys to the success was a custom
sputtering system from Advanced Modular Sputtering Inc. (AMS), Goleta, CA (Figure 1). In
resonator applications, frequency control is very important to yielding good product. The FBAR
resonator frequencies are set by the thickness of the piezoelectric & electrode films, which mustbe accurate to ~ 0.2%, an astonishing level in the thin-film industry! Satisfying this requirement
was a major impetus to develop a custom tool. Capable of depositing metal & piezoelectric filmsof sufficiently well controlled material properties and thickness, the AMS deposition systemenables wafer level yields at the 50% level.
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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

Figure 1 – AMS Custom Cluster Tool for Sputter Deposition.
DEVICE DESCRIPTIONFBAR filters [8] are realized in a ladder topology, as a series of electrically connected, airsuspended membrane-type resonators, made of piezoelectric aluminum nitride sandwiched
between two molybdenum metal electrodes. Figure 2 provides a sketch of the structure and theresulting RF filter characteristics. There are two critical aspects to making FBAR filters: 1)
performance and 2) the cost of the filter.

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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

FBAR Resonator & Filter Definitions
Series
fo

fo

Shunt

fo - ∆ffo

fo - ∆f

fo - ∆f

1000|Zseries||Zshunt|

FBAR Resonator Response|Z| vs Frequency|Z| [Ω]

100

10

1
1750
0

∆f
1800

1850

1900

I.L.
BW

-10

|s21| [dB]

-20

1950

2000

Roll-offfo-30

Filter Response|S21| vs Frequency

-40
-50
-60
1750

1800

1850

1900

Frequency [GHz]

1950

2000

Figure 2 – Illustrating the FBAR resonator ladder filter topology (top panel). The series
and shunt resonators exhibit impedance vs. frequency characteristics (middle panel),
which produce the filter pass-band response for an RF filter (bottom panel).
Filter performance has three main components: 1) in-band insertion loss and bandwidth, 2)
roll-off rate from in-band to out-of-band states, & 3) out of-band rejection (see lower panelof Figure 2). The electro-acoustic coupling coefficient kt2, and the quality factor or “Q” of
AlN, are the key material parameters in determining the filter response. The coupling
constant kt2 determines the insertion loss and bandwidth of the filter, while Q controls theroll-off of the filter (roll-off is defined by the maximum insertion loss in-band and theminimum rejection allowed at the out-of-band corner).
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AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

Figures 3 and 4 illustrate the typical low insertion loss and good out-of-band performancefor FBAR filters made from films deposited with the AMS sputtering system.

Tx Passband

Rx Passband

Attenuation [dB]

0
-10
-20
-30
-40
-50
-60

Return Loss [dB]

0
-4
-8
-12
-16
-20

Figure 3 and 4 - Measured performance of an Agilent HPM7903 FBAR duplexer,
circa August 2000. Both Attenuation (Insertion Loss) and Return Loss |S11| at the
Rx and Tx ports are shown.

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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

The first key innovation is the use of molybdenum for the electrodes [9,10]. Molybdenum
is used to minimize the acoustic attenuation and provide good electrical conductivity, both ofwhich improve Q and boost kt2. The second is the ability to sputter AlN films with independent
control of residual stress and kt2, at high deposition rates.
In wireless and microwave applications, different filter characteristics are needed. For
example, in filters intended for transmit (Tx) chain usage, very rapid roll-off from in-band to outof-band regions & low insertion loss in-band are important. Producing FBAR material with higheffective kt2 and Q helps achieve the desired response.
In order to compare FBAR filter performance with respect to competing technologies anddesign methodologies, it is useful to use the kt2 *Q product as a figure of merit. FBARs exhibit
both a series and a parallel resonance (see the middle panel of Figure 2), with a Q factor definedfor each. Typically, the parallel quality factor Qp is lower than the often-quoted series quality
factor, Qs. However, for filter applications, the parallel Qp is much more critical and is the one wequote here. There are many published papers claiming high coupling coefficient and Q for
FBARs [11,12], kt2 *Qp products ~ 5 to 20 are typical numbers, but these are not adequate forgood PCS duplexer performance. For producing commercially competitive parts, the kt2 *Qp
product needs to be above 50, a typical result produced by the AMS sputtering system.
An FBAR resonator may be constructed either as a freestanding membrane technology, or
as a solidly mounted resonator (SMR) [4]. In the SMR, a series of acoustical mismatching layers
are used to prevent the sound wave from penetrating into the substrate material where Q lowering
dissipation would occur. The choice of a "free" standing membrane gives the best Q due to the
air/resonator interface, and lack of additional layers in which the sound wave may be attenuated.
Also a freestanding membrane eliminates the need to deposit multiple layers, the deposition andcontrol of which would create a barrier for high-volume manufacturing throughput.
A freestanding membrane may be fabricated by providing a hole in the silicon with asacrificial support structure on the silicon substrate that is etched out after the processing is done.
This leaves the membrane supported at the edges over a shallow hole (~2 – 4 microns deep) in thesilicon wafer. A potential problem with a freestanding membrane is that it may buckle inwards
toward the bottom of the hole and actually come in contact with the substrate, a process which canalso lower the Q. Avoiding this problem requires that stress in the films be negligible or slightly
compressive, user selectable, in order to force the membrane to stay flat or bow up away from thebottom of the hole. An alternative choice is to etch the silicon wafer away from the back [12].
While this approach eliminates the possibility of the membrane touching the substrate, itmechanically weakens the wafer and hence is considered unsuited for production.
Turning again to the dual questions of quality & competitive pricing, several companies
have recently demonstrated adequate quality Tx and Rx SAW filters in the 900 MHz cellular
phone bands, and Rx filters in the 1900 MHz PCS bands. On the other hand, power-handlingproblems [13] have limited SAW filters in Tx applications at 1900 MHz. Although FBAR filters
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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

have demonstrated much better power-handling characteristics [14], they still have to compete in
the market with SAW filters that are manufactured for 5 – 10 cents each (in quantities of millions)for RF cellular phone applications. Thus, cost of manufacturing FBAR filters is an even more
important consideration than pure performance of the parts.
Fast and reproducible deposition of high-quality AlN films is paramount for low-cost
manufacturing of FBARs. Several techniques [15,16] such as CVD, metallic Al deposition innitrogen, and RF sputtering of Al in nitrogen, have been demonstrated for growing single crystal
or polycrystalline AlN with a well-oriented crystallographic c-axis. This is the main prerequisite
to obtaining acceptable piezoelectric properties. However, for FBAR production, reactive
sputtering at moderate temperatures has proven to be the most practical technique.
ALUMINUM NITRIDE DEPOSITION
In over a decade of developing aluminum nitride deposition techniques for FBARs, wehave evaluated four different "cluster-tool" sputtering systems and one batch system, all fromrenowned thin-film companies. None of these systems have satisfied all of the requirements for
production quantities of FBARs, especially the control of the electro-acoustic coupling constant at
a high and consistent value, and adequate throughput leading to competitive production costs.
Following is a list of the sputtering system requirements for producing micron thickness, highly
piezoelectric aluminum nitride films or high-quality molybdenum films in a production
environment:
•
•
•
•
•
•

System must have very low levels of oxygen and water during deposition,
Aluminum nitride must be sputtered at as high a power as possible to increase adatom
mobility, leading to high film quality,
Substrate must be at as high a temperature as possible to improve adatom mobility, with thechamber walls and shields at low temperature to prevent outgassing of undesirable gases,
Intrinsic electro-acoustic coupling kt2, wafer-to-wafer and across-wafer, must be reproducible
and repeatable throughout the target life.
Uniformity of film thickness across the wafer and repeatability of thickness wafer-to-wafer
must be controllable throughout the target life. It must be better than 0.2% one sigma for thefilter yield to be 50%.
AlN deposition rates of 700 to 1000A/minute are necessary to produce filter products at a low
enough cost to be competitive with other filters on the market.

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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

In over a decade of developing aluminum nitride deposition techniques for FBARs, we haveevaluated four different "cluster-tool" sputtering systems and one batch system, all from renowned
thin-film companies. None of these systems have satisfied all of the requirements for production
quantities of FBARs, especially the control of the electro-acoustic coupling constant at a high andconsistent value, and adequate throughput leading to competitive production costs.
After several years of trying to modify standard systems, it was decided that a custom-built
system designed specifically for aluminum nitride was necessary. Advanced Modular Sputtering(AMS), Inc., Goleta, CA built a unique, AC-magnetron-based, three-chamber cluster tool toprovide the required film characteristics, as well as high throughput. In addition to the uniquehardware features, a completely new software package had to be developed to allow control of thedeposition process as well as running the three chambers simultaneously without compromisingthroughput. A major result of this work is to realize that systems designed for sputtering
conducting thin films, will not work well in producing dielectric films.
For depositing thin films of conducting materials (typically metallic aluminum, or titanium
nitride), most sputtering systems on the market have an optimized DC magnetron source. In sucha system, small changes in magnetic field do not produce significant changes to the film
properties. The material deposited on the shields around the magnetron is conducting, therefore
does not change the plasma characteristics throughout the life of the target, so the film propertiesand deposition rates are not altered significantly. Likewise, the residual stress does not vary
greatly from run to run.
On the other hand the film properties and deposition rates of dielectric materials, likealuminum nitride, are significantly affected by small changes in the magnetic field. The build-up
of dielectrics on the shields causes changes in the uniformity and deposition rate of the film, run
to run, throughout the life of the target. Most systems use “bipolar” DC magnetrons or a standard
dual magnetron with AC power supplies [17,18], either type of which can cause significant
changes in stress, film uniformity, deposition rate, and film quality throughout the target life. The
typical compensation for film stress and uniformity involves adjustment of gas pressure or
substrate bias. The goal is to provide independent control of all sputtering parameters to achieve
the desired AlN material attributes. Since there aren’t enough independent process parameters
available to control all the AlN thin film parameters to the desired specifications, throughout thetarget life, certain sputtering machine parameters were added to the list of possible control
elements.
In this development, magnetic field was optimized to produce a high deposition rate of thealuminum nitride film, while maintaining the highest coupling coefficient and a slightly
compressive stress. The following three figures show the impact of total magnetic field at thetarget rings in the AMS system, on coupling coefficient, deposition rate and stress.

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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

Figure 5 is a graph showing the dependence of kt2 of aluminum nitride on the strength of themagnetic field. Note that the coupling reaches a broad maximum near 290 Gauss, where it isinsensitive to changes in the field at the targets.
Coupling Coefficient vs. Magnetic Field
Fig. 5
8

Coupling Coefficient

7
6
5
4
3
2
1
0
220

240

280

300

360

600

Magnetic Field, Gauss

Figure 5 - Film coupling coefficient dependence as a function of the magnetic fieldstrength. Note: for optimum coupling constant kt2, magnetic field ~ 290 Gauss isrequired.
Magnetic field also acts to decrease the sputter rate. Figure 6 illustrates that at a field of
290 Gauss, which gives optimum kt2 coupling coefficient, the deposition rate drops nearly 2:1
from that experienced at 200 Gauss. This is a major price to be paid, for relative insensitivity tomagnetic field induced changes in kt2.
Residual stress in the films, which is controlled partly by gas pressure, and partly bymagnetic field, must be controlled to obtain membranes that bow upwards and which do not
become disconnected from their perimeter support or develop cracks in the interior. By design, thestress can be made to be almost zero to slightly compressive, at a magnetic field of ~ 280 Gauss.
Figure 7 illustrates the dependence of residual stress on magnetic field.
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Dep. Rate vs. Magnetic Field
Fig. 61400

Dep. Rate, A/min

1200
1000
800
600
400
200
0
220

240

280

300

360

600

Magnetic Field, Gauss

Figure 6 - Deposition rate as function of the magnetic field at the target ring location

Film Stress vs. Magnetic Field
Fig.7
4.00E+09

Film Stress, Dyne/sq. sm

2.00E+09
0.00E+00-2.00E+09

220

240

280

300

360

600

-4.00E+09-6.00E+09-8.00E+09-1.00E+10-1.20E+10-1.40E+10
Magnetic Field, Gauss

Figure 7 – Film stress as function of the magnetic field strength. The minimum stressoccurs at 280 Gauss where it changes from tensile to compressive.
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AMS

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Uniformity of the film thickness over the entire wafer has a big impact on FBAR wafer
yield. Uniformity is especially sensitive to target profile changes from run to run. Magnetron
sputtering targets erode non-uniformly due to the magnetic field design. As a result, the contour of
the target ring is altered, and changes in the deposition parameters occur. Those more quickly
eroding target areas are commonly referred to as “race-track”. As the “race-track” gets deeper
during target life, the uniformity and rate of deposition changes. In conventional systems with asingle target ring, gas pressure is used to adjust uniformity. Unfortunately, gas pressure also has ahuge impact on the stress in the aluminum nitride films, so is not useful for correcting the run-torun changes in uniformity.
The AMS system uses two magnetron target rings. It was found that using a standard DC
power supply connected between the two magnetron rings allows excellent uniformity control
throughout the target life. Uniformity is thus achieved without altering gas pressure and without
any other significant impact on film properties. Since the DC supply can be controlled from theprocess recipe, adjustments are easily automated through the system computer. The results ofvarying the DC bias power between the two rings, is summarized in Figure 8.
Film Uniformity vs. DC Power
Fig. 84
3.5

One Sigma, %

3
2.5
2
1.5
1
0.5
0
0

50

90

100

120

200

DC Power, W

Figure 8– Uniformity of the deposited film for different amounts of power from thesecondary DC power supply.
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Advanced Modular Sputtering, Inc.
AMS

93 La Patera Ln.
Goletta, CA 93117
Phone: (805) 692-0912
E-mail: ams@amsincorp.com
www.amsincorp.com

Further refinements resulted from running thousands of wafers through the system. Since
the AMS system was specifically designed for an aluminum nitride process, we eliminated theanode, ground and floating potential between the two magnetrons and inside the inner magnetron.
Eliminating most surfaces on which insulating aluminum nitride is unintentionally depositedimproved plasma stability throughout the target life. The magnetic field was further optimized byadjusting the position of the "race-track" on each target to fine-tune stress and film uniformity.
Throughout the target life as the “race-track” gets deeper and the deposition rate
decreases, the deposition time is increased by a computer-controlled routine based on thehistorical data collected on the system. This technique works well for the processes that cantolerate 1% one-sigma thickness control. However FBAR filters require better control.
Wafer-to-wafer repeatability below 0.2% can only be accomplished by in-situ film
thickness monitoring. After a significant amount of experimentation, we found that an in-situ
laser interferometer meets this need. The laser is aimed at the wafer through a window located in
the middle of the magnetron. The incident light beam is reflected by the growing AlN surface aswell as the molybdenum electrode. Both reflections are routed through to a beamsplitter and into a
photo-detector. During the AlN film deposition, the interference between the two reflected lightcomponents causes the output current of the photo-detector (vs. time) to exhibit a periodic patternof maxima and minima. These can be related to the film thickness, index of refraction, and laser
free-space wavelength by standard interferometry equations. With laser light at 635 nm, and anindex of refraction in AlN of 2.075, the change in AlN thickness to move from a maximum tominimum (or minimum to maximum) in the interference pattern corresponds to 76.5 nm of AlN.
With suitable interpolation, the measurement is expected to be made accurately to ~ +/-0.1%, for
an AlN film thickness in the range of one micron.
To summarize the results on AlN deposition, it was found that a magnetic field of 280 to
290 Gauss was the best compromise in this investigation. AlN films were deposited withdeposition rates close to 900 A/min, better than 0.5% uniformity and better than 0.2% run-to-run
repeatability. The films had run-to-run stable coupling coefficient of about 7%, and slightly
compressive stress.
CONCLUSION
After years of development, AMS has successfully demonstrated a deposition system withthe ability to manufacture FBARs in high volume and at low cost. It is found that the magnetic
field applied to the targets has a profound affect on film uniformity, deposition rate, and electroacoustic coupling coefficient. To meet the stringent requirements for high quality FBAR filters,
the AMS deposition system utilizes a combination of an improved reactive sputtering system with
optimization of the magnetic field applied to the targets. The resulting film stacks exhibit
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uniformity of 0.5% across the wafer, better than 0.2% run-to-run repeatability, and high electroacoustic coupling may be achieved at deposition rates of 700 to 1000 Angstroms per minute.

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