Characterization of Reversed c-axis AlN Thin Films
John D. Larson III (Fellow)1, Sergey Mishin2, Stefan Bader3
1
Wireless Semiconductor R&D, Avago Technologies, San Jose, CA, 95131-1008 (john.larson@avagotech.com),2
Advanced Modular Systems, Goleta, CA, USA, 3Avago Technologies, Fort Collins, CO, USA
Abstract —Background: It is desired to grow AlN in a reversed c-axis
configuration to fabricate R-FBARS (Reversed c-axis Film Bulk Acoustic
Resonator) with reactively sputtered, thin film Aluminum Nitride (AlN).
Previous methods of growing reversed c-axis AlN, result in films withlow electro-acoustic coupling constant, low Q, or inability to withstandthe Avago Technologies FBAR release process.
Conribution/Methods: An AMS Inc. deposition tool, modified to allow
independent control and unique processes, was used to deposit AlN on Al,
Mo or W bottom electrodes used in this study.
AlN films of thickness 1.2 microns were deposited over patterned bottom
electrodes, with additional processing used to reverse the c-axis of the
AlN. A top electrode was deposited, patterned, and either processing
stopped at the transducer point, or the R-FBAR was released from thesilicon wafer with HF acid.
Results: The Avago Technologies Acoustic Imaging Microscopeinterferometer (AIM) is used in point mode to determine the AlN c-axis
orientation. The transducer or R-FBAR is driven with a 40 kHz sine
wave, and the phase of the top surface motion is observed. Both theorientation and the piezoelectric coefficient were determined.
To evaluate the material constants of the AlN for the R-FBAR structure,
the input RF reflection coefficient vs frequency is measured. From a onedimensional Mason model for the R-FBAR stack, the AlN material
parameters – coupling constant kt2, resonant frequency, velocity, andattenuation, were determined by varying them in the model, to backfit themeasured data.
R-FBAR resonators with reversed c-axis orientation, termed type CN(“Compression Negative”), as well as FBARS with normal c-axis
orientation termed Type CP, (“Compression Positive”), were fabricated. A
strong piezocoupling constant was observed, depending on the deposition
parameters used.
The voltage shift (“Voltco”) coefficient of the resonator resonant
frequencies, fS or fP, was observed, compared to the interoferometricobservations, and found to be a reliable indicator of c-axis polarity. For
the FBARS and R-FBARS reported here, for the Type CP films, a Voltco
of +40 kHz/Volt was observed, and for Type CN films, -30 kHz/Volt wasobserved.
Low frequency RBARS and high frequency RSBARS stacked resonatorstructures were fabricated, and preliminary results are given.
Index Terms — Thin film, AlN, piezoelectricity, reversed c-axis.

I. INTRODUCTION
Aluminum nitride is a piezoelectric material in hexagonal
crystal class 6mm. In the sputtered thin film form, AlN grows
preferentially with its polar axis normal to the metal uponwhich it is grown, unless special process steps are employed.
X-ray diffraction reveals that the films have well oriented
crystallites in the (0001) direction, but the crystallographic a
and b axes are randomly oriented. For single crystals, the caxis polarity relative to any (0001) face can be easily
determined, e.g., by etch pit decoration or interferometrically[1,2]. For thin film AlN, the c-axis polarity, relative

to the metal surface on which it is grown, is determined atfilm deposition time and is difficult to change subsequently[35]. Characterizing reversed c-axis piezoelectric films is the
subject of this paper.
There have been numerous methods to grow reversed c-axis
AlN thin films reported in the literature [1, 3-5]. Thesemethods produce encouraging evidence that the c-axis may be
inverted, but generally do not produce high coupling constantmaterial, or are not compatible with the FBAR technology
developed at Avago Technologies [6-8].
In this paper, we will present results for R-FBAR resonators
made from reversed c-axis AlN films in an FBAR format.
II. REVERSED C-AXIS –A. DEFINITIONS OF C-AXIS POLARITY OF ALNWhen discussing thin films with possible positive or
negative c-axis polarity with respect to the growth electrode asa datum surface, one needs a convention as to the c-axis
orientation of the film, the direction of the applied electricfield, and consistent with the observed motion of the film. In
this work, we will always define the bottom electrode, on
which the AlN is grown, to be both the electrical ground &the datum growth surface, so the positive E-field is applied
conventionally as rising toward the driving electrode on top of
the film, and the positive c-axis points up from the datum
surface. As will be shown later, if a positive voltage is applied
to the top electrode of a film with positive c-axis AlN, thenthe film will contract or compress. We term this AlN as TypeCP, “Compression Positive”. Likewise, if the c-axis isnegative or reversed, pointing down toward the datum surface,
the film will compress when driven by a negative voltage on
the top electrode. This is termed to be Type CN AlN,
“Compression Negative”. In this convention, experiment has
shown the Type CP material has the first nitrogen face on thebottom electrode, while Type CN material has the firstaluminum growth face on the bottom electrode. See the 1949
IRE Standards on Piezoelectric Crystals [9] for a discussion ofpiezoelectric sign assignment conventions.
With the Type CP and CN definitions for AlN films, it iseasy to relate measurements of surface motion to the sign andmagnitude of the applied potential. Fig. 1 illustrates theseconventions. We have used the Avago Technologies AIM[10] to carry out such studies in the point mode of operation.
In high Q FBARS (Type CP AlN), or R-FBARS (Type CN
AlN), one can determine the c-axis type by applying a positive
DC voltage (along with the RF voltage), to the top electrode,Type CP AlN (usual polarity of Avago AlN films)+
V-

Metal electrodeC-axis
Metal electrode
Silicon wafer

electrodes is always attractive (compressive), regardless of thesign of the applied voltage. As shown in Fig.2, by applying a
positive (or negative) DC bias voltage to the top electrode, adeflection of the membrane toward the bottom electrodeoccurs.
+0-

Metal electrode
Polyimide Film
Silicon wafer

Compression Positive (CP) –Film Compresses whena positive voltage is applied to the top electrode.
A. c-MUT-No Bias

Type CN ( polarity of reversed c-axis Avago AlN films)
V
+

Metal
counter-electrode(in “swimmingpool”)

Metal electrodeC-axisLaser probe from AIM

Metal electrode
Silicon wafer

Compression Negative (CN) –Film Compresses whena negative voltage is applied to the top electrode.
Fig. 1 - Illustrating the conventions for c-axis orientation,
voltage, and compression of AlN films, for Type CP or CN.
and observing the frequency shift of the resonator. For comparison, the c-axis orientation is also characterized by theinterferometer before release. Type CP AlN material exhibits
a positive (upshift) of frequency, while Type CN AlN exhibitsa negative (downshift) of frequency. By introducing theseconventions about growth surfaces and reference potential, we
have created a framework in which one can and unequivocallydetermine the orientation of an AlN film, and can use low
frequency measurements to non-destructively determine caxis orientation & piezoelectric strength.
It is possible to grow either type of c-axis oriented AlN on awafer. It would be useful to grow both types of c-axis oriented
AlN films on a wafer, either one on top of the other or atlateral sites [11] .
II. REVERSED C-AXIS ALNB. MEASUREMENT METHODS –INTERFEROMETER & C-MUT CALIBRATIONTECHNIQUE
To determine the motion (compression or expansion) on
application of an AC voltage to a piezoelectric film resonator,
one needs a reference device that can be excited at low
frequencies, and which can produce a known motion
direction. A capacitive transducer (c-MUT) [12] provides asolution, because the electrostatic force between the

+
VDC+
VAC
-

Metal
electrode
Polyimide
Film
Metal
counter-electrode(in “swimmingpool”)
B. c-MUT-DC Bias + AC Peak Drive
Bias Voltage Applied. Equivalent toCompression.

* c-MUT-Courtesy of Prof. B.T. Khouri-Yakub, Stanford
University, [11].
Fig. 2- Calibration an AC interferometer with a c-MUT transducer todetermine sign and magnitude of the piezoelectric coefficient d33 .

If an AC voltage, e.g., 40 kHz, is superimposed on this DC
potential, then when the AC is in its positive ½ cycle, the sumof the voltages will be even more positive, and the membrane
will deflect even farther toward the bottom electrode. Thisappears to be compressive stress characteristic of a Type CP
piezoelectric. The compressive motion will be in-phase withthe 40 kHz drive, as seen by the AIM.
If the applied AC voltage is in its negative ½ cycle, then thesum of the DC bias voltage and the negative AC voltage willbe less positive, and the membrane will move away from thebottom electrode. This appears to be an expansion, and the
AIM records this as 180 degrees relative to the AC drive.
This technique is used to establish Type CP state, for the
AIM operated in the point mode. The wafer bearing the cMUT is set up on the AIM, a DC voltage on the order of +50
volts applied to the top electrode, and the phase of an applied
AC driving signal (~1 volt peak) is adjusted until theinterferometer signal in the phase sensitive detector/lock-in
amplifier is 0 degrees. If the wafer is removed and replacedwith no changes to the instrument, the same result is obtained.
Once this phase condition is established, the c-MUT wafer isremoved and a wafer bearing AlN of unknown c-axis
orientation is substituted. The phase and amplitude of theresulting AC signal are again observed. If the phase is 0o, thenit is Type CP material. If the phase is 180o, it is Type CN.
Since the AIM is calibrated to read surface peak
displacement in nanometers, the piezoelectric film
displacement can be measured. The ratio of measured peak
displacement to peak driving voltage yields the piezoelectriccoefficient d33 [13].
III. REVERSED C-AXIS ALN FILM GROWTH &EVALUATION

release step tended to destroy the aluminum seed film andfloat off the R-FBAR. To solve this problem, our approach
was to pattern the AlN/Mo/Al seed /stack, then apply a sealing
layer of protective material around the edges of the aluminumseed wasPOR Standard
AlN deposition.
Phase Shift
Δφ = 0 degrees

Aluminum seed layer
AlN deposition.
Phase Shift –Δφ = 157 degrees

We grow Type CN or CP AlN thin films, evaluate them forthe sign and strength of the piezoelectric constant, and thenmeasure the material and piezoelectric material properties.
We deposit standard (POR) AlN by reactively sputtering an
aluminum target in an argon/nitrogen atmosphere. In the AMS
Inc. AlN sputtering tool, one loads a wafer into a firstchamber which is evacuated to remove atmosphericcontaminants, then through a load-lock door into thedeposition chamber.
Process gases are admitted, thesputtering is initiated by applying RF power to create an
ionized atmosphere to deposit the AlN film, and a laserinterferometer monitors the thickness.
A. UNRELEASED TYPE CN ALN FILMS
In the first technique to fabricate Type CN AlN, a flash of
metallic aluminum was deposited over the bottom electrodeby sputtering the AMS aluminum target in pure Argon for a
few seconds. The nitrogen flow was then turned on, whichswitched the plasma to a specified argon/nitrogen ratio at acontrolled pressure and mixture. AlN grows on the freshaluminum layer as high coupling constant Type CN material.
This is illustrated by Fig. 3, which presents AIM interferometer differential phase measurements made on AlN films: a)
Type CP grown on Molybdenum, and b) Type CN grown on
an aluminum metal flash over Molybdenum without breaking
vacuum. In both cases the Mo was on glass (PSG) oversilicon, unreleased, so a low Q transducer response was seen.
From the measured displacement & driving voltage, thepiezoelectric coefficient & sign were determined.
Fig. 4 illustrates the measured d33(CN) for the unreleased
Type CN AlN films, expressed as an electro-acoustic coupling
constant, kt2, relative to the measured d33(CP) value, which is ~6 pm/volt [13].
As the thickness of the aluminum flash layer increases, thecoupling coefficient increases. Figure 4 also illustrates themeasured phase shift for the Type CN AlN, another indicatorof the degree to which the c-axis was reversed. From this andthe known growth condition of an aluminum flash, one
concludes that Type CN material is aluminum face first type.
These results illustrate that 1) Type CN AlN can be grown asa transducer, and 2) the AIM measurement technique works.
It was found that released R-FBAR resonator structures
could be built with aluminum seed layers, but the final HF

Fig. 3 – Phase shift: a) left panel-Type CP AlN grown by
the standard POR process, and b) right panel-Type CN AlN
grown over an aluminum metal flash without breaking
vacuum.

Fig. 4 –Type CN AlN is grown over an aluminum metal
flash. AlN film is not released. Coupling constant (top panel)and relative phase shift (bottom panel), vs. aluminum
thickness, are plotted.
deposited and patterned, and the FBAR was successfully
released in 10:1 hydrofluoric acid, producing an R-FBAR.

Another approach to growing R-FBARS was to change theprocess gas proportions to provide a seed or a reduced surface
on which to grow the Type CN material. This approach wasmuch more successful. We now report on R-FBARS made on
molybdenum electrodes, fully released from the silicon
substrate, & exhibiting high coupling/Q resonators.
It was noticed that one could grow Type CN AlN on tungsten
electrode material with the same type of process. We suspect,
but have not proven, that this process will produce Type CN
AlN on many other metals, such as ruthenium, vanadium,
tantalum, titanium, copper, gold, silver, etc., because the seedmetal electrode growing surface may be reduced and seeded.

The effect is dependent on the type of AlN, the E-field in the
AlN layer, and on a field dependent piezoelectric constant.
The frequency is altered by changes in the stiffened velocity
of sound due to changes in the piezoelectric constant (major),and by piezoelectrically induced changes in the thickness of
the AlN (minor). Since the effect is dependent on the electricfield
Wafer 060, released Type CN AlN.
Molybdenum bottom electrode
Standard Type CN AlN deposition process.
50 Ohm Smith Chart

AlN parameters:
thickness = 1.200 um given.
kt^2=5.81%
Area: 30 kum^2 (given by mask layout)
Moly parameters:
Top thickness = 0.3077um given
Bottom thickness = 0.4393 um given
velocity = 6320 m/s
alpha = 4 dB/cm @1GHz..

S(2,2)

B. – RELEASED TYPE CN ALN RESONATORS

Wafer 060 measured:
fs =1.586 GHz, Rs = 0.88 Ohms
fp =1.625 GHz, Rp = 1756 Ohms
Xo ~ 50 Ohms .

freq (100.0MHz to 8.000GHz)

IV. R-FBAR RESULTS
A. COUPLING CONSTANT, MATERIAL PROPERTIES

2

2E3

0
1E3

Magnitude Impedance Z(Z = R +jX )-2

Measured.

-4| S11| vs Freq., SE-6

Rreflection Coefficient S11
50 Ohm System-8dB(S(2,2))mag(Z(2,2))

Released R-FBAR resonators were fabricated in the Avago
FBAR process on molybdenum electrodes over PSG glass andsilicon. The typical stack is shown in Table 1.

-10

1E2

-12

-14

-16
-18
1E1

-20

-22
-24

-26

-28
1
-30
0.0

Stack
Top Molybdenum

Thickness
3077 A

AlN- Type CN
Bottom Molybdenum

12,000A
4393 ATABLE 1- R- FBAR STACK.
Individual die with 30 kmicron2 area, reactive impedance ~50 Ohms, are probed on-wafer with a Cascade Technologies
microprobe connected to an Agilent Technologies E5071B
network analyzer. This combination is calibrated to a set ofstandards. The reflection coefficient S11 (f) is measured over afrequency range 0.1 to 8.5 GHz.
To reduce the data, a one dimensional Mason model [14-16]
simulator, implemented in Agilent Advanced Design System(ADS) software, is used to calculate the expected reflection
coefficient vs frequency. The mechanical and piezoelectric
AlN material constants in the model are varied to fit thecalculated to the measured reflection coefficient data.
Fig. 5 shows the broad band refection coefficient result for atypical R-FBAR. One sees a high Q resonance at 1600 MHz,and the first overtone resonance at ~ 6GHz.
For the R-FBAR of Fig. 5, the coupling constant kt2=5.81%,
and the parallel resonance Rp = 1756 Ω. For comparison to
the typical FBAR made from Type CP AlN, the coupling
coefficient kt2 = 6.3%, Rp ~ 2500 Ω.
B. VOLTAGE SHIFT OF RESONANT FREQUENCY
When a DC or low frequency AC voltage is applied across the
AlN layer in a high Q FBAR resonator, a small frequency
shift occurs in both the series and parallel resonant
frequencies. This change is of the order 10 to 100 part per
million, which can only be observed in a high Q resonator.
The shift reverses if the opposite sign of voltage is applied.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

freq, GHz

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

freq, GHz

Fig. 5-Illustrates the broad band reflection coefficient for a
Type CN R-FBAR. Note the high Q, high kt2 resonances, quite
comparable to those obtained for a Type CP FBAR.
(V/to) in the AlN, thinner AlN results in a larger “VoltCo”,
which is defined as the change in frequency per volt applied.
VoltCo is measured by applying both a DC voltage and an
RF signal, via a RF/DC blocking component, to the top
electrode of the resonator & measuring the frequency shift.
Typical values for R-FBARS are: VoltCoCN =-30 to -40
kHz/Volt, and for FBARS: VoltCoCP = +40 kHz/Volt.
The coupling constant was varied by changing the AlN
deposition conditions. Fig. 6 illustrates the data, and a linear
relation of VoltCo vs coupling constant kt,. The effect can besummarized as a voltage dependent piezoelectric constante33(V):
e33(V) = e330 [1 + α* (V/t0)](6)
in which e330 is the piezoelectric constant in Coulomb/m2,
α expresses the electric field dependence, t0 is the AlN film
thickness, and the stiffened sound velocity is:(7)
v0D = v0E *(1 + kt2 )1/2 .
From these relations and a standard definition of kt2, thechange in parallel resonance frequency Δfp per volt is:
Δfp /ΔV ~ fp kt2 α/t0 = VoltCo(8)
For the 1.2 micron thick AlN samples measured here:
αCN = −547 pm/V
(9)αCP = +476 pm/V
(10).

C. TEMPERATURE COEFFICIENT OF FREQUENCY
The temperature coefficient of frequency was measured for
R-FBARS over the range 20 to 120 degree C, and found to be-33 ppm/degree C, slightly greater than that for Type CP
films.
V. APPLICATIONS –A. RBAR
The effect of a Type CN AlN film is only apparent in thepresence of a Type CP film, for example in a side by side

The piezoelectric excitation, product of driving electric field
and the signed e33 coefficient are indicated schematically foreach resonator. The FBAR has a ½ wave length thickness
piezoelectric excitation, and by driving it from the outside
electrodes, the two halves of the structure (each withimpedance ½ Xo) are connected in series, resulting in an input
impedance of Xo. The RBAR is also a half wave length
piezoelectric excitation due to one layer having a reversed caxis sense, but by driving it from the center electrode, one getsa parallel connection of the two resonators, resulting in an
input impedance of ¼ Xo. This is a factor of 4 reduction in
impedance for a given die area.
RBARS were built in both configurations, with 1.2 um of
AlN in each resonator. Fig. 8 shows the real part of themeasured resonator impedance for the RBAR at 1.2 GHz, 36
kum2 area, Xo ~ 22 Ω, followed by a weak FBAR response at
double the frequency, 2.4 GHz, due to the imbalance incoupling constant between the top & bottom resonators.
5E2

RBAR
Mode

1E2

.

real(Z(2,2))real(Z(1,1))

RBAR Mode –rd
3 HarmonicFig.6-VoltCo vs coupling constant (kt2measured)1/2 exhibits a

FBAR Mode –rd
3 Harmonic

1E1

FBAR
Mode

linear relationship. Coupling kt is varied by growth conditions.

configuration such as a FACT transformer [11], or in astacked resonator format, for example an RBAR or an
RSBAR[17].
An RBAR is stacked resonator with a bottom Type CP
FBAR, and a top Type CN R-FBAR, or vice versa. Fig. 7
illustrates the RBAR stack (left), in comparison to a single
FBAR (top) and DBAR (same c-axis type of AlN, right). The
RBAR is driven from the center electrode, with the top andbottom electrodes grounded, paralleling the two resonators todecrease the area for a given resonator impedance, e.g., 50 Ω.
E-field

Type CPRBAR

GroundE-field

Type CP

C-axisE-field

Type CN

C-axisGroundEfield

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

freq, GHz

Fig. 8 – Illustrates the real part of impedance for an RBAR resonator.
Bottom: Type CN, kt2 bot = 5.5%; Top: Type CP, kt2 top = 6.5%.
The FBAR response is non-zero, due to kt2 imbalance.

Piezoelectric
excitation

E

C-axisVdriveGroundVsignal-e*E

E-field

Top Molybdenum+Type CPType CP C-e*E

1.0

RSBARS are stacked resonators with both AlN types[16].
Figure 9 illustrates two possible RSBAR configurations.

C-axisDBAR

Ground0.5

B. RSBAR

GroundPiezoelectric
excitation
-e*E

0.0

Piezoelectric
excitation
+e*E

FBAR

Vdrive1-

E

Top AlN –Type CP

Caxis

Bottom AlN – CType CN
axis+
Middle Molybdenum,
Floating “growth” surface.

-

Caxis

E

Top AlN –Type CNE

Bottom AlN – CType CP
axis

VsignaBottom Molybdenum

axis
+e*E

RSBAR Configuration “A”

Fig. 7 – Comparison of a standard FBAR (top) andstacked resonators RBAR (left) and DBAR (right) as toelectric field / c-axis orientation & effective terminal
impedance.

RSBAR Configuration “B”

Fig.9 – Schematic diagram of RSBAR configuration.
Devices were built in both configurations, with 0.6 um of
AlN in each resonator. This doubles the frequency, as
compared to that of the standard FBAR with one type AlN

throughout the stack, and thickness 1.2 um. If the comparable
FBAR were made ½ as thick to get the same frequency, the
RSBAR impedance would be twice that of the FBAR.
Fig. 10 shows the real part of the measured RSBAR
resonator impedance. There is a strong RSBAR fundamental
response at 4.5 GHz, and a very weak FBAR response at half
the frequency, 2.1 GHz, due to unbalanced coupling
constants.

VI. DISCUSSION & CONCLUSIONS
We describe a method to characterize the c-axis polarity ofthin film piezoelectric films. An AIM method to differentiate
3E3

1E3

real(Z(2,2))real(Z(1,1))

1E2

RSBAR
Mode

1E1

1

FBAR
Mode
1E-1
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

freq, GHz

Fig.10 – Real part of impedance for an RSBAR resonator.
Config.: Bottom: Type CP, kt2= 6.3%, Top: Type CN AlN, kt2= 5.8%.
Type CP or Type CN AlN films is introduced. For quantitative determination of the piezoelectric constant of an RFBAR resonator, a voltage/frequency shift method is given,
which correlates 100% to the results of the AIM method.
Two methods to deposit Type CN films were usedsuccessfully: 1) aluminum seed layer, 2) surface reduction.
Released AlN R-FBARS were fabricated. RF reflection coefficient measurements were made and a Mason model backfit
to these data, yielded coupling constants kt2 =5.8%, VoltCo =-30 kHz/Volt, and TempCoFrequency =-33 ppm/deg C, comparable to those for FBAR resonators.
One gains an advantage if both types of AlN are present, forexample in FACT transformers [10]. To demonstrate mixed
stacked resonators, two novel devices are presented, an RBAR
low frequency stack, and an RSBAR high frequency stack.
ACKNOWLEDGMENTS
We acknowledge the support of R. Ruby, J. Choy, Y. Oshmyansky, and the engineers of Avago Technologies. C. Fenghelped with the VoltCo measurements. M. Handtmann suggested the RSBAR stack. Particular thanks go to the Fort
Collins Operations R&D Team consisting of M .Bonaventura,
T. Eoff, K. Marincic, D. Wilson, and K. Schumacher.
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