Improving crystallinity of high concentration
Scandium Aluminum Nitride films deposited on
200mm wafers
Sergey Mishin

Yury Oshmyansky

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

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

Abstract— There have been several investigations [1], [2], [3], [4]that demonstrated benefits of adding Scandium (Sc) to thealuminum nitride (AlN) films in order to increase coupling
coefficient (kt^2) of the Bulk Acoustic Wave (BAW) devices. For
concentrations below 5% atomic Sc, crystallinity of the AlScN
films is very good. There is only single 002 orientation peak thatcan be seen. For higher concentrations deposited on wafers withless than 150mm diameter, there is no significant problem either.
Unfortunately, when working with 200mm wafers and Sc
concentrations higher than 5%, area outside 150mm diameter
develops different crystal orientations of AlScN and abnormal
peaks. This leads to degradation of Rocking curves measurement
and lower coupling coefficient (kt^2) at the edge of the wafer.
Since the edge of the wafer on 200mm wafers has about 40% ofthe dies, this leads to very poor device yields on 200mm wafers.
We have developed and tested up to 20at% of Sc in AlN film
process that involves depositing a thin layer of AlScN and in-situ
focus Ion Beam trimming process to smooth the surface, followedby the rest of the deposition. This process has excellent XRD in
both the center and the edge of the 200mm wafer with strong(002) peak.
Keywords—Scandium; Aluminum Scandium Nitride; AlN;
aluminum nitride; crystallinity

I. INTRODUCTION

At low Sc concentrations, it is easy to deposit highquality films. As Sc concentration is moving higher andhigher in a quest to obtain the maximum possible coupling
coefficient, sporadic nucleation of different crystal
orientations have been observed. Figure 2, 3 and 4 showsdegradation of crystallinity on the edge of 200mm wafers ofAlScN film
Figure 2. ScAlN Omega 18.02 (1.803 degree)

In the last three decades, many applications havebeen developed based on MEMS structures that employ either
a free-standing membrane or a cantilever structure extending
over a cavity.
Examples of such structures include Film
Bulk Acoustic Resonator (FBAR) filters, Piezoelectric MicroMachined Ultrasound Transducers (PMUT) and energy
harvesting devices. Initially, aluminum nitride (AlN) has been
most commonly used for piezoelectric because it is easy tomanufacture in high volume. Recently, inorder to get higher
coupling coefficient, Sc doped aluminum nitride started
making its way into the high-volume manufacturing
applications.
Figure 1. Kt2 vs Sc concentration in AlN-doped film.
Figure 3. 2Theta-Omega Scan PEAK 1 (35.53deg)

Both AlN and AlScN depositions use a dual magnetronwith AC power applied between targets. Frequency of ACpower is 40 kHz and power may vary from 3 to 10 kW. It is areactive deposition process in deep poisoned mode.
We used two target rings made out of pieces/tiles of Al and
Sc to adjust film composition. Preferred embodiment of suchtarget is shown in Figure 6 below:
Figure 6. Multiple material targets

Figure 4. Sporadic nucleation on the surface of AlScN films
by SEM, 2um scale.

We have come up with a very simple process toeliminate this problem. Depositing a thin layer of the AlScN
followed by a short ion mill treatment, prepares the surface
that allows for a deposition of AlScN that has excellent XRD
and no sporadic nucleation.
II. EQUIPMENT
In this experiment, we used Advanced Modular Systems
cluster tool with AlN and AlScN deposition chambers and ion
beam trimming module (shown in Figure 5).
Figure 5. AMSystems cluster tool

Based on simple geometry of target’s surface, deposited
film composition is proportional to the surface of specificpieces of target material. As sputtering area of Sc increases,composition of Sc in deposited film also becomes higher.
Substrate rotation is using to compensate variation of
scattering for different materials and composition nonuniformity across the substrate.
The trimming module uses DC focused ion source withargon process gas to improve thickness/uniformity of either
AlN or AlScN films. It is also used to smooth the surfaces.
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 amuch wider process window for stress control because it
allows avoiding of spending too much effort on controlling
thickness uniformity during deposition.

III. SURFACE TREATMENT WITH IN-SITU ION MILL
All tests were done for two different composition of doped
AlN film with 9% and 20% of Sc content. We depositedfilms between 100nm and 150nm followed by in-situ ion
milling to remove minimum of 30nm. This simple surface
treatment dramatically improved the crystal orientation of
the final AlScN deposition.
Figure 7 and 8 show XRD measurement of ScAlN
folm on the center and edge of the 200mm wafer for 20% Sc in
AlScN film.
Figure 7. 2Theta-Omega measurement of ScAlN on thecenter and the edge of the 200mm wafer.
Above: AlScN 2Theta Scan 30-40, Edge
Figure 9 shows surface of both AlN (left) and AlScN (right)
films by SEM clean from any sporadic nucleation. AlN film
surface is shown for reference.
Figure 9. SEM surface of both AlN (left) and AlScN (right)
films

Above: ScAlN #28 Center (1.302)

Figure 10 below shows AlScN film cross-section with goodcrystallinity.

Above: ScAlN #28 Edge (1.334)
Figure 8. 2Theta measurement of the ScAlN (002)

IV.

SUMMARY

We have been able to demonstrate a simple and effective
method of depositing 20% atomic Sc AlScN films withexcellent crystal orientation and no sporadic crystal growthon 200mm wafers.
Above: AlScN 2Theta Scan 30-40, Center

REFERENCES[1][2]

Morito Akiyama, “Influence of temperature and scandium
concentration on piezoelectic response of scandium aluminum
nitride alloy thin films”, Applied Physics Letters, Oct. 2009,Vol.95, Iss. 16, pp. 2107-2107-3
Pau Muralt, et.al.. “Electromechanical properties of AlScN thinfilms evaluated at 2.5 GHz film bulk acoustic resonators”, Applied
Physics Letters 99, 2011

[3]

[4]

Keiichi Umeda1, et.al.. “PIEZOELECTRIC PROPERTIES OF
ScAlN THIN FILMS FOR PIEZO-MEMS DEVICES”, MEMS
2013, Taipei, Taiwan, January 20 – 24, 2013
Qi Wang et al., “Scandium doped aluminum nitride based
piezoelectric micromachined ultrasound transducers,” in Proc. ofSolid-State Sensors, Actuators and Microsystems Work Shop,
Hilton Head, South Carolina, 2016, pp. 436–439