Magnetron Deposition of AlN and ScAlN for Massproduction for BAW Devices and MEMS
Sergey Mishin
Yury Oshmyansky
Advanced Modular Systems, IncGoleta, CA/USA
smishin@amssb.com
yoshmyansky@amssb.com

Abstract— Bulk acoustic wave (usually referred to as BAW, FBARand SMR) filter devices used in cell phones have used piezoelectric
AlN for almost two decades.
In the last few years new MEMS devices such as piezoelectric
MEMS oscillators (also known as pMEMS), sensors andmicrophones started using AlN in high volume products.
While it is no longer a great challenge to make limited numbers ofwafers employing reactively sputtered piezoelectric AlN films
suitable for BAW applications, there are several barriers to making
them in a high volume.
Tight control of crystallinity, thickness and thickness uniformity,
deposition rate, coupling coefficient and stress through target
lifetimes and across wafer are major barriers to high volume
production. Independent control of AlN thickness and stressuniformity across wafer is critical for FBAR devices that require
high coupling coefficient with tight distribution.
Novel tool innovations have been successfully implemented toaddress these issues. Data from production tests show that in-situ
laser interferometry thickness monitoring significantly reduces
wafer-to-wafer thickness variations due target rate roll-off.
Use of a unique local enhancement of the magnetron magnetic
field and substrate rotation mechanism to compensate for non-radial
uniformity profiles eliminates changes induced by variations intarget material as well as system asymmetry.
In-situ ion beam milling dramatically improves thickness
uniformity.
ScAlN deposition is used in applications requiring much higher
coupling coefficient than AlN can produce. Although, similar to AlN
deposition, ScAlN process requires much more care to produce
uniform films with good surface roughness and no surface defects
sometimes called “crystallites”. Significant modifications tomagnetic fields and ion mill trimming are used to create highconcentration ScAlN films usable in filter and MEMS applications.

The effect of sputter gas composition and substrate temperature is
widely published [1,2,3]. The distance between cathodes andsubstrate during deposition was also reported to be important to thequality of the AlScN [4].

Keywords—Scandium; Aluminum Scandium Nitride; AlN;
aluminum nitride; crystallinity; stressII.

I. INTRODUCTION
In the past, either AC dual magnetron, (pulsed) DC or RF single
magnetron sputtering were used to deposit AlN ﬁlms withacceptable piezoelectric response. The addition of Sc to form AlScN
increases coupling coefﬁcient as shown in Figure 1 below.

Figure 1. Coupling Coefficient vs Sc content in AlScN film

We have found that using AC plasma with two magnetrons, weonly need to control Sc composition on each target and the angle ofthe sputtering surface vs. substrate to get the best quality films.
Even though many different configurations can produce acceptablefilms on a few wafers, it is very difficult to produce films withuniform Sc concentration, coupling coefficient and stress acrosswafer over the entire target life. With increasing Sc concentration,these problems get worse. Crystallite formation, non-uniform XRD
across wafer, Surface roughness, large Sc concentration variation
across wafer and over the target life, stress variation across wafer
and thickness variation across wafer and over the target life are themost common problems.

EQUIPMENTIn this work, we used Advanced Modular Systems cluster toolwith AlN, AlScN and Mo deposition chambers, and ion beamtrimming module (shown in Figure 2). Both AlN and AlScN
depositions use a dual magnetron with AC power applied between
targets. Frequency of AC power is 40 kHz and power may vary from
3 to 7 kW. High purity 99.9999% argon and nitrogen process gasses

we used for all depositions. The gas flow mixture had between 0%
and 40% of argon. No wafer heating was used in this system.

Figure 4. Stress control and Sc concentration in AlScN film

Figure 2. AMS Cluster tool with AlN, AlScN deposition and Ion
Beam Trimming Modules

We used two targets made out of aluminum bases and Sc pieces toadjust film composition.
The trimming module uses DC focused ion source with argon
process gas to improve thickness/uniformity of either AlN or AlScN
films. Film thickness trimming/tuning is processing based on ion
beam scanning across a wafer with power variation based on film
thickness map. Use of the trimming process opens up a much wider
process window for stress and Sc concentration control because it
allows avoiding of spending too much effort on controlling
thickness uniformity during deposition.
Simple resonators with three layer’s Bragg reflector were
manufactured during investigation for piezoelectric coupling
coefficient measurements.

III.

Since coupling coefficient is very sensitive to Sc composition in
AlN film, in this work we focus on Sc concentration in AlN film
across a substrate and thru target life.
For dual target configuration, concentration of Sc across wafer canbe easy controlled by adjusting concentration of Sc on the inner andouter targets, since most material from inner target is deposited on
the wafer center and most material from outer target is deposited on
the edge of a wafer, until the concentration across a wafer is uniform[5]. Multi-piece targets produced results that are very similar to thecomposite targets with the same concentration of Sc. Figure 5 shows
a comparison of kt^2 between composite targets and multipiecetargets.
Figure 5. Coupling coefficient across wafer as a function ofscandium concentration in two targets in the beginning of the targetlife.

COMPOSITION CONTROL

Above mentioned configuration provides good crystallinity, stressand thickness across a substrate thru whole target life, see figures 3and 4 below.
Figure 3. XRD data: Omega across a substrate and 2Theta at thebeginning and end of target life

Rotating wafer during deposition assures the same Sc
concentration at the same distance from the center of the wafer.
But in order to have production worthy system, it must deliver thesame Sc concentration over the entire target life.
Unfortunately, the same design that gives the best AlN deposition
is not adequate for the AlScN deposition.
One of the problem is accuracy and repeatability of measurement
tools. We picked composition of the inner and outer targets to giveus uniform Sc concentration across wafer. We ran 7% and 20% Sc
depositions for the entire target life. Results are shown in the Figure
6 below.
Figure 6. Sc concentration in film for 7% and 20% configuration
during target life in standard AlN configuration

As can be seen, at lower Sc concentration, because of the noise in
the concentration measurement, which can be as high as 10% of
measurement value, it is not obvious that the concentration increases
over the target life, but at 20%Sc it is clear that concentration rises
over the target life.
Second problem is scattering of ad-atoms during sputtering
process. Al and Sc ad-atoms have different scattering angles underidentical electric and magnetic fields due to difference in mass.
Depending on the angle of the target surface vs. substrate surface,different amount of Sc and aluminum will arrive at different
locations on a wafer.
Over the target life, erosion pattern of target, called “racetrack”,
produces different angles from which material is sputtered. By
adjusting magnetic field, we were able to keep the angle of material
coming from the target and the area of the racetrack balanced insuch a way as to keep Sc concentration uniform and consistent over
the target life. The maximum concentration of Sc on the wafer
comes from the surface that is parallel to the wafer. The minimum
amount comes from the area that is almost perpendicular to thewafer. Figure 7 below demonstrates different concentrations ofscandium over the target life from different magnetic field
configurations. By adjustment of magnetic field and target profile
we were able to control the same Sc concentration during target life.
Figure 7 Impact of magnetic field and target racetrack for Sc
concentration over target life.

IV.

SUMMARY

We demonstrated production worthy AlScN deposition system.
Thickness and thickness uniformity over the target life are
maintained using in-situ laser interferometer, wafer rotation andthickness trimming using ion beam module.
Average stress and stress distribution across a substrate arecontrolled by process gas flow and secondary magnetic field.
Sc concentration across wafer over the entire target life ismaintained by using two independent magnetrons with multi-pieces(pure Al and pure Sc) targets and magnetic fields oriented in such a
way as to compensate with the slope of the target material andmagnetic field keeping area of the “racetrack” in balance to getstable Sc concentration.

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