Reducing impact of stress on coupling coefficient in
30% Scandium Aluminum Nitride films
Sergey Mishin

Yury Oshmyansky

Advanced Modular Systems, Inc.
Goleta, CA USA
smishin@amssb.com

Advanced Modular Systems, Inc.
Goleta, CA USA
yoshmyansky@amssb.com

I. INTRODUCTION
Adding Scandium (Sc) to the aluminum nitride (AlN) is
widely used to significantly increase coupling coefficient in
Piezo-MEMS devices [1], [2] and [3]. Stress impact on thecoupling coefficient in 30% Sc films is a major issue [4], [5].
Stress variation across wafer and over the target life is a major
hurdle to keeping coupling coefficient in the range acceptable
for most RF filter applications. We have developed a processthat uses high voltage surface treatment in conjunction withoptimized AlN barrier layer to make coupling coefficient lesssensitive to stress in the film.
II. METHODS/RESULTS
In this investigation we used Advanced Modular Systems
cluster tool with AlN and AlScN deposition chambers and ion
beam trimming module, see Figure 1.
The deposition chambers use standard dual conicalmagnetron with AC deposition source and high-density plasma
configuration. Frequency of AC power is 40 kHz and power
may vary from 3 to 9 kW. Wafer rotation was used tocompensate edge to edge non-uniformity. Additional DC power
supply was using for center to edge thickness uniformity
improvement. Al target bases with inserted Sc pieces were used
to adjust AlScN film composition.
Figure 1.

The trimming module uses DC focused ion source with up to5kV acceleration voltage. This module was used for substratesurface clean up before deposition, as well as for improvementof thickness non-uniformity after AlScN deposition.
Effect of bottom Mo electrode pre-clean with high voltage
bombardment and thickness of AlN seed layer in situ before
AlSc(30%)N deposition was investigated.
Initial investigations on AlN films frequently used XRD(Rocking Curve) measurements to compare quality of the film.
For AlN films there is a good correlation with films having
XRD<1.5 degree on 1um thick films having good coupling
coefficient.
Unfortunately, XRD does not have a good correlation to thecoupling coefficient for the 30%Sc films. As can be seen on
Figure 2, Rocking curve for all data points is about 1.4-degree,
disregard of variation of coupling coefficient.
Figure 2.

HBAR devices on the other hand have good correlation tothe FBAR resonators.
A standard HBAR devices for 3,200 MHz frequency, with
1700A of electrode thickness and 6000A of AlSc(30%)N piezo
thickness were manufactured.
We use two ground pads and one signal pad. The groundpads are 430um x 430um. The signal pad is 98um x 167um andthe pitch for the structure is 43um. We use 350um pitch probes
though. The manufactured HBAR is shown on Figure 3.
Measurements were done on Agilent Network Analyzed
E5071B with Cascade RF probe station.

Figure 3

Treatment of Mo surface with high energy Ion beam hastremendous impact on Coupling coefficient of high dopant
AlScN film. Without Ion Beam treatment, Coupling coefficient
dropped down by almost 30%.
Also, it is important to have this surface treatment done rightbefore AlN barrier deposition. Figure 4 shows correlation
between coupling coefficient and the time wafer sits in the loadlock at 1E-6 Torr vacuum. During delay time between Ion beamtreatment and AlN deposition, surface covered by monolayer ofoxygen and water which dramatically reduce coupling
coefficient of devices.
Figure 4.

As can be seen from the Figure 5, deposition that uses no barrier
layer has lower coupling coefficient . Very thin barrier layer
improves coupling coefficient, but as this layer gets thicker, it,most likely, develops both, crystal orientation and surface
roughness, that have detrimental impact on the orientation of
the 30%Sc films. Additional, thicker AlN seed-layer requires
thinner AlScN film in order to keep the same frequency. This
results in reduction of coupling coefficient.
Result of coupling coefficient on HBAR devices for
combination of optimized AlN seed layer with Ion Millbombardment and surface treatment vs piezo layer film stressfor optimized (Advanced) and non-optimized (STD) processes
is showed on Figure 6.
Figure. 6

AlN has a week correlation between stress and couplingcoefficient. But as Sc is added to the film, more tensile film has
much larger correlation to the stress. Ion mill bombardment of
the Mo surface, improves surface roughness, making film much
more compatible with AlScN deposition.
III. CONCLUSIONSAs can be seen from Figure 4, the best results are obtained
when wafer is immediately loaded into the AlN barrier
deposition chamber. Effect of Ion Beam treatment completely
disappears after about 10…15 min placing a substrate in highvacuum about 1E-6 Torr.
AlN barrier layer provides separation between electrode andthe AlScN film and also is used as seed-layer for AlScN film.
Figure 5.

Using high voltage bombardment of the electrode material
in conjunction with appropriate barrier layer before the AlScN
deposition dramatically reduces the impact of stress variationon the coupling coefficient and provides very tight coupling
coefficient distribution with effective kT2 is more than 23%.
Keywords—Scandium, aluminum nitride, coupling coefficient,film stress
References
[1]

[2]

[3]

[4]

[5]

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