Optimizing high concentration 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) iswidely used to significantly increase coupling coefficient in
Piezo-MEMS devices [1], [2], [3] and [4]. Figure 1 shows
relationship between Coupling coefficient and Sc content in
AlN film.
Figure 1. Coupling coefficient vs Sc dopant in AlN film

We have identified critical parameters that makemanufacturing of greater than 30%Sc films possible.
II. METHODS/RESULTS
In this investigation we used Advanced Modular Systems
cluster tool with AlN, Mo and AlScN deposition chambers andion beam trimming module, see Figure 3.
The deposition chambers use standard dual conicalmagnetron with AC/DC configuration for piezoelectric
deposition and DC/DC configuration for Mo deposition, as wellas high-density plasma configuration for all processes.
Frequency of AC power is 40 kHz and power may vary from 3
to 9 kW. Wafer rotation was used to compensate edge to edge
non-uniformity. Additional DC power supply was using forcenter to edge thickness uniformity improvement. Al target
bases with inserted Sc pieces were used to adjust AlScN film
composition.
Figure 3. AMS cluster tool with four process modules.

Optimizing process to keep coupling coefficient high anduniform across wafer without having sporadic “crystallites” isa major challenge, see Figure 2 the AlScN surface with“crystallites”.
Figure 2. SEM of AlSc(30)N film with “crystallite”
formation.

The trimming module uses DC focused ion source with up to
5kV acceleration voltage. This module was used for substratesurface clean up before deposition, as well as for improvementof surface roughness of the AlN barrier layer deposited before
AlScN deposition.

We investigated thickness and stress AlN barrier impact on
the AlSc(30%)N deposition, as well as some deposition
parameters for AlScN deposition itself

Figure 6. Coupling coefficient vs delay time of surface
cleaning/bond activation with Ion Milling

Figure 4. Variations of Coupling Coefficient and XR
Diffraction (rocking curve)

Since rocking curve doesn’t always follow the coupling
coefficient, see figure 4, and in order to investigate mostimportant process parameters, simple HBAR devices weremanufactured to measure kT2.
HBAR devices for 3,200 MHz frequency, with 1700A ofelectrode 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 5.
Actual data for HBARs and FBAR resonators wereconfirmed.

Barrier layer itself is another critical part of getting highquality AlScN film. As can be seen from the Figure 7 below,
deposition, that uses no barrier layer, has lower coupling
coefficient. Very thin barrier layer improves couplingcoefficient. But, as this layer gets thicker, coupling coefficientdrops. This happens due to thicker AlN film has stronger crystal
orientation, but, the higher Sc content in AlN film, the bigger
lattice constant mismatch with pure AlN film. This mechanism
can promote growth of un-desired “crystallites”.
Figure 7. Coupling Coefficient vs AlN barrier film

Figure 5. HBAR structure.
We further investigated the impact of stress in the barrier
layer on the formation of the “crystallites”.
Figure 8. SEM of AlSc(30)N film for compressive AlNbarrier

Measurements were done on Agilent Network Analyzed
E5071B with Cascade RF probe station.
The first parameter is surface cleaning/bond-activation withhigh energy Ion bombardment and delay time between cleaning
and AlN barrier deposition. It is important to have ion beamtreatment done right before AlN barrier layer deposition.
Figure 6 shows correlation between coupling coefficient andthe time wafer sits in the load-lock at 1E-6 Torr vacuum. During
delay time between ion beam treatment and AlN deposition,
surface covered by monolayer of oxygen and water vapor,which dramatically reduce coupling coefficient of devices.

As can be seen in Figure 8, compressive stress in the AlN
barrier film leads to a reduction in the “crystallite” formation inthe AlScN films.
At the same time, tensile stress of AlN barrier leads to“crystallites” formation, see Figure 2.
We also tried depositing thicker barrier layers and trimmingthem with Ion Mill down to 100A thickness. Barrier layers that
had highly compressive stress and were trimmed shown
improvement in both surface roughness of the AlScN films andnumber of the “crystallites”.
Neutral or tensile stress barrier films actually performed
worse after trimming, having rougher AlScN surface, more
crystallites and worse crystal orientation. Figure 9, shows AFM
data of AlScN film, including surface roughness, forcompressive and tensile AlN barrier with Ion mill trimming.

Deposition power has also impact on kT2 of highly dopped
AlN film. As can be seen on Figure 11, there is a range ofdeposition power between 4 to 5.5 kW, where maximum
coupling coefficient can be reached. This is different comparedto pure AlN film, since it can be stable and well oriented withAC power up to 12 kW. This result shows some weakness of
crystals of 30% Sc dopped AlN film
Figure 11. Coupling coefficient vs AC power

Figure 9. AFM of AlSc(30) film and Surface Roughness (Ra)for processes with Compressive (Left) and Tensile (Right) AlN
barrier with Ion Mill Trimming

III. CONCLUSIONS
Using high voltage bombardment of the electrode material
in conjunction optimized barrier layer before the AlScN
deposition improves coupling coefficient and reduces“crystallite” formation in high concentration AlScN films.
Using highly compressive barrier film in conjunction with ion
mill trimming, further improves the quality of the high %Sc
films.
Further optimization of deposition of AlSc(30)N film leads
to stable and repeatable process with high Coupling coefficient.
Regarding AlSc(30)N deposition process itself, parameters,such as AC deposition power and stress have dramatic impact
on device coupling coefficient.
As previously was discussed in [5] AlScN film with moretensile stress provides higher coupling coefficient, see Figure
10, even if rocking curves show about the same values for films
with different stress.
Figure 10. Coupling Coefficient and Rocking curve vsAlSc(30) film stress

Keywords—Scandium, aluminum nitride, coupling coefficient,film stress
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[2]

[3]

[4]

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