Improving Manufacturability of AlN Deposition Used in Making Bulk
Acoustic Wave Devices
Sergey Mishin, Brian Sylvia and Daniel R. Marx
Advanced Modular Sputtering, Inc. Goleta, CA 93117 USA
Abstract — Bulk acoustic wave (usually referred to as BAW orFBAR) signal filter devices are currently used in more than 50
million CDMA phones produced in the last three years. While itis no longer a great challenge to make limited numbers of wafers
employing reactively sputtered piezoelectric AlN films suitablefor BAW applications, there are several barriers to making themin a high volume. Control of uniformity, deposition rate, andstress through target lifetimes and from target-to-target
variations in are major barriers to high volume production.
Novel tool innovations have been successfully implemented toaddress these issues. Data from production tests show that in-situ
laser interferometric thickness monitoring significantly reduces
wafer-to-wafer thickness variations due target rate roll-off.
Uniform, consistent sputtering target metallurgy is shown to be acritical factor. Use of a unique local enhancement of themagnetron magnetic field to compensate for non-radial
uniformity profiles eliminates changes induced by variations intarget material as well as system asymmetry. Adjustment ofstress is achieved though process gas flow control.
Index Terms — BAW, FBAR, AlN, sputtering.

thousands of wafers and using many targets from different
target vendors, our customers came to us with somewhat
different set of high volume production related issues:• Wafer-to-wafer repeatability is impacted by the firstwafer warm-up effect and deposition rate changes
throughout the target life.
• Cross-wafer thickness uniformity changes throughout
the target life.
• Stress changes throughout target life.
• Target-to-target variations in uniformity, stress, anddeposition rate.

I. INTRODUCTION

II. WAFER-TO-WAFER THICKNESS CONTROL WITH IN-SITU

Bulk acoustic wave (usually referred to as BAW or FBAR)
filters are currently used in more than 50 million CDMAphones produced in the last three years. It is no longer a greatchallenge to make small volume AlN suitable for BAW
applications. On the other hand there are several barriers tomaking them in a high volume. Target-to-target variations in
uniformity, deposition rate, and stress are a major issue.
Changes in the above parameters throughout the target life are
the second major problem.
For high-volume filters, stringent control of both thematerial parameters — dielectric, mechanical andpiezoelectric — and the thickness of the layers are critical.
Following is a list of the sputtering system requirements for
AlN films that most people have as they start developing
BAW filter technology:• Intrinsic electro-acoustic coupling kt2 must be above
6.4%.
• Uniformity of film thickness across the wafer andrepeatability of thickness wafer-to-wafer must becontrollable <0.5% 1σ.
• Stress must be tightly controlled• Deposition rate must be above 700A/min.
Even though it is not a trivial matter, many companies havedemonstrated the above mentioned parameters on a relatively
small number of wafers. It is only after running tens

Using in-situ thickness monitor dramatically reduces
problems with changing deposition rate. Using secondary
magnetic field to compensate for non-radial uniformity
problems takes care of changes induced by both system
asymmetry as well as variations in target material

LASER INTERFEROMETRY. When wafer volumes were low, operators run a test wafer
and used the results to adjust deposition parameters before
each batch of wafers. This wafer also serves as a warm-upwafer. After system sits idle for more than an hour,deposition rate on the first wafer is about 1% lower than on
the next wafer. Deposition rate on the following wafers
stabilizes quickly and if only a handful of wafers are run, theuser can compensate for this by running a test before eachbatch and selecting appropriate parameters to have repeatabledeposition.
In high volume production twenty-five wafers are run
together in one cassette/batch. Most targets experiencedeposition rate roll-off throughout the target life. This can be
as much as 20% from beginning of the target to the end of thetarget. With thick AlN films, in one cassette the deposition
rate can drop as much as 3% from first to the last wafer. Most
systems address this problem by increasing deposition time or
power by a computer-controlled routine based on thehistorical data collected on the system to compensate fordeposition rate decrease. This technique works well for theprocesses that can tolerate 1% 1-σ thickness control.
However, BAW filter production requires better control.

Wafer-to-wafer repeatability below 0.2% can only be
accomplished by in situ film thickness monitoring using laserinterferometer incorporated into the sputtering chamber
design, Figure 1. .

pattern corresponds to 76.5 nm of AlN. With suitableinterpolation, the measurement is made accurately to less than
±0.2%, for an AlN film thickness in the range of 1 to 3 µm.
This approach is not novel in itself. Maydan et.al (1)describes a similar deposition system concept; however, themain problem with the technique is measuring thickness on
patterned wafers. In their approach, a stepper motor isemployed to drive the laser spot to the scribe line to makerepeatable measurements. This is a cumbersome and not very
reliable method. The development introduced here utilizes asoftware algorithm to separate signals from different surfaces
on a patterned wafer and to automatically determine
appropriate thicknesses whether blanket and patterned films
alike. Figure 3 shows wafer-to-wafer repeatability with the insitu laser interferometer of less than 25A for the twoconditions. In comparison, data from a repeatability study
without the laser shows wafer-to-wafer control to be greaterthan 50A.
Laser Interferometer AlN Thickness Control
Set Point = 10,000A and 11,000A

Figure 1. Schematic of in situ laser interferometer.
12000

The laser is aimed at the wafer through a window located in
the base of the magnetron. The incident light beam is reflectedby the growing AlN surface. Both reflections are routed
through to a beam splitter and into a photo detector. During
the AlN film deposition, the interference between the two
reflected light components causes the output current of thephoto detector (vs. time) to exhibit a periodic pattern of
maxima and minima, Figure 2. These can be related to thefilm thickness, index of refraction, and laser free-space
wavelength by standard interferometry equations. With laser
light at 635 nm and an index of refraction in AlN of 2.075, thechange in AlN thickness to move from a maximum tominimum (or minimum to maximum) in the interference

w/o Laser
Std. Dev = 51 A

Nanospec
Thickness (A)11500

11000

With Laser,
Patterned Mo
Std. Dev = 23 A

10500

10000

With Laser,Blanket Mo
Std. Dev = 23 A

9500

9000

123456789101112131415

Wafer

On Blanket Mo

On Patterned Mo

On Blanket without Laser

Figure 3. Wafer-to-wafer repeatability improvement to better
than 25A with in situ laser interferometer.

1400

III. UNIFORMITY CORRECTIONS DUE TO SPUTTERING TARGETNON-CONSISTENCY

λ/4n = 6380/(4 x 2.075) = 768.7A
1200

Intensity

1000

The sputtering target material typically used in the reactive
deposition of AlN is high purity (99.9995%) Al. As there is
no solute to promote grain refinement in this material, it isdifficult to obtain targets with consistent fine grain
microstructures. Generally, the inconsistency leads to film
thickness profiles that are not radially concentric. This nonconcentricity varies from target to target. To alleviate thisproblem an adjustable local enhancement of the magnet fieldhas been designed enabling one to increase the field over anadjustable arc (φ) positioned at a user-selected angle (θ),

800600400

21 x λ/4n = 21 x 6380/(4 x 2.075) = 16142A
200

Total Deposition = 16322A
2731

2626

2521

2416

2311

2206

2101

1996

1891

1786

1681

1576

1471

1366

1261

1156

946

1051

841

736

631

526

421

316

1

211

106

0

Half Second Intervals

Figure 2.
thickness.

Interference intensity used to calculate film

2

Figure 4. Figures 5a and 5b illustrate the adjustment effect on
improving radial concentricity.
Once concentric, power adjustment to the dual ringmagnetron enables on to reduce radial non-uniformity.
Protocols are being developed to make these adjustments
automatically by connecting the output from thickness
measuring tool such as a 49-point map from a interferometerto the central controlled of the sputtering tool and allowing thecomputer to determine φ and θ and then adjusting the DC
power for radial uniformity.

Figure 5b
Figure 5a and 5b.
Reduction of uniformity nonconcentricity using the localized magnetic field enhancement.

IV. STRESS ADJUSTMENT
A key to easily and automatically controlling stress is todesign a cathode for an AlN process in which a magnetic fieldstrength is identified that yields a “0” stress level and provides
a reasonably good deposition rate, Figure 6. Once this isachieved, one can adjust the film stress simply with theadjustment of gas flow rate. In an piezoelectric AlN process,a N2 rich gas mixture with Ar is utilized. Small adjustmentsin either gas can drive stress from tensile to compressive as
illustrated in Figure 7.

Figure 4. Schematic of range of adjustment of the localized
magnetic field enhancement

Figure 5a4.0E+09
2.0E+09
0.0E+00-2.0E+09-4.0E+09-6.0E+09-8.0E+09-1.0E+10-1.2E+10-1.4E+10-1.6E+100200018001600140012001000
8006004002000100 200 300 400 500 600 700

Dep Rate (A/min)Stress (dynes/cm2)

Film Stress and Deposition Rate
vs. Magnetic Field

Magnetic Field (Gauss)

Figure 6. Plot of film stress and deposition rate as afunction of the magnetic field strength of the magnetron.

3

come up with automated control of all of these parameters tomaintain stress, uniformity, and film thickness throughout thetarget life is almost impossible. But if wafer thickness istaken out of the equation by using laser interferometer tocontrol thickness, this task becomes doable. Inputting
uniformity data into the tool control software enables thecomputer system to determine the optimum position of themagnet enhancement.

Stress (dynes/cm2)

Adjustement of AlN film stress
8.0E+09
6.0E+09
4.0E+09
2.0E+09
0.0E+00-2.0E+09-4.0E+09-6.0E+09-8.0E+09

VI. CONCLUSION-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Two major improvements have been successfully
implemented into AlN deposition system: laser interferometerfor thickness control and the local enhancement of themagnetic field for uniformity control throughout the targetlife. In-situ thickness control is an indispensable feature of a
production worthy AlN deposition machine. It allows user toautomate other things such as stress and cross-wafer
uniformity control through target life and target-to-target. The
position-able and adjustable magnetic field enhancement is
critical to compensate for asymmetrical uniformity problems
that vary target-to-target and throughout the target life.

Adjustment Of Gas flow Rate about the Nominal
N2Ar

Figure 7. Plot of film stress as a function of the gas flowrate.
Protocols are being developed to make these adjustments
automatically by connecting the output from stress measuring
tool to the central control of the sputtering tool and allowing
calculation of the gas adjustments at the same time it
determines φ, θ and DC power for radial uniformity.

References:
1. Maydan et.al patent 4,618,262 “Laser interferometersystem and method for monitoring and controlling ICprocessing”
2. Mishin, us11/085,362

V. DISCUSSION
Most people find that stress control, uniformity, anddeposition rate change throughout target life. Most methods
to control cross-wafer uniformity and stress also impact
deposition rate. Stress can be controlled by controlling
deposition gas flow rate or substrate bias. Both of these will
have impact on cross-wafer uniformity and deposition rate.
Typical round flat planar magnetrons suffer from the factthat once the erosion profile is developed early in target life,
there is limited ability to vary the film thickness profile. Often
the equipment solution is to vary the target to substratedistance. Using distance between wafer and target to control
uniformity impacts deposition rate and stress. Thus, trying to4