Stress control for highly doped Aluminum Scandium
Nitride films
Sergey Mishin
Yury Oshmyansky
Advanced Modular Systems, Inc
Goleta, CA/USA
smishin@amssb.com
yoshmyansky@amssb.com

Abstract—Recently, highly Sc doped aluminum nitride films havebeen used for variety of applications ranging from microphones to
PMUT sensors. Deposition on different substrate surfaces such as
Pt, Mo, W, Si, SiO2 and SiN (both patterned and un-patterned) leadto requirement to be able to control stress over 1GPa range. In orderto obtain zero stress on different surfaces, equivalent process
condition on Si wafers requires large stress offset. Stress is typicallycontrolled by adjusting deposition pressure and/or substrate bias.
For concentrations above 20%atomic Sc, stress control using
substrate bias results in non-uniform stress across wafer. Stresscontrol using adjustment in deposition pressure can produce goodstress control across a wafer but can only produce stress adjustment
with good thickness uniformity control over a limited stress range.
Furthermore, at Sc concentrations above 30%, sporadic crystal
growth is observed on more tensile depositions. In this presentation,
we demonstrated stress control over 1 GPa range, by using asecondary magnetic field that is easily adjusted from outside of themagnetron and vacuum environment. We were able to obtain stressvariation across 200mm wafer below 100MPa. No sporadic crystal
growth was observed between 20% Sc and 32% Sc range.
Keywords—Scandium; Aluminum Scandium Nitride; AlN;
aluminum nitride; crystallinity; stress

I. INTRODUCTION
In this investigation we used multiple piece targets to produce
highly piezoelectric AlScN films.
When standard magnetic fields, that are used for AlN deposition,
are used for AlScN deposition, stress variation from center to edge ismore than 500MPa. Because piezoelectric response is highly
dependent on stress, see figure 1, this results in unacceptablevariation across wafer.
When RF substrate bias is used to adjust stress, average stressbecomes more compressive at higher substrate bias. Unfortunately,substrate bias has almost no impact on stress uniformity acrosswafer.
When pressure or gas flow is used to control stress. There is alarge impact on average stress and thickness uniformity acrosswafer, but the impact on stress uniformity is typically less than
200MPa at the most.

Figure 1. Coupling Coefficient vs film stress.

II.

EQUIPMENTIn this investigation we used Advanced Modular Systems cluster
tool with AlN and AlScN deposition chambers and ion beamtrimming module.
Figure 2. AMS Cluster tool with AlN, AlScN deposition and Ion
Beam Trimming Modules

Both depositions use a dual magnetron system with positive
plasma column and with AC power applied between targets.
Frequency of AC power is 40 kHz and power may vary from 3 to 10
kW. It is a reactive deposition process in deep poison mode.
Main difference between AlN and AlScN chambers is that
AlScN deposition is using targets composed of Al and Sc pieces
compared to AlN process that uses pure Al targets. Based on simple
geometry of target’s surface, deposited film composition is
proportional to the surface of specific pieces of target material. As
sputtering area of Sc increases, composition of Sc in deposited film
also becomes higher.

Figure 4. AlScN film stress vs secondary magnetic field strength.

Figure 3. Multi pieces AlSc target

Adding magnets underneath the horizontal surface of themagnetron allows us to make stress across the wafer more tensile orcompressive in the center of the wafer depending on the desired
stress profile. Figure 5 shows magnetic field configuration thatallows stress control across wafers.
Figure 5. Magnetic field configuration with outside magnets below
magnetrons
Magnetic field is optimized for each chamber to produce uniform
stress across wafer.
Substrate rotation is used to compensate for the variation ofscattering for different materials and composition non-uniformity
across the substrate.
The trimming module uses DC focused ion source with argon
process gas to improve thickness/uniformity of deposited films. Filmthickness 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 composition control, because it allows avoiding ofspending too much effort on controlling thickness uniformity during
deposition.

III.

FILM STRESS CONTROLThe system uses standard dual conical magnetron with AC
deposition source. Since material from inner target is mostlydeposited on the center of the substrate and material from outertarget is mostly deposited on the edge of the substrate, independent
adjustment and control of target’s properties and magnetronparameters (including magnetic fields) allows an independent
control of film stress on the center of the substrate relative to theedge of the substrate.
For example, targets have a sloped surface. This shape allows us tocontrol angle and energy of the material that is coming off thetargets.
Since magnetron magnetic field also has significant impact on thefilm stress, see figure 4, independent adjusting horizontal andvertical magnets on inner and outer magnetron allows us to dial instress variation on a wafer.

Applying magnetic field with specific polarity on the bottom of
the lower magnetron can increase or decrease stress in the center ofthe wafer.
Another option to control film stress uniformity across a wafer isdisbalancing magnetron magnetic field either on inner target or outertarget, by putting magnets behind a substrate. This created additional
electron traps and, as result, ion clouds either in the center of thesubstrate or on the edge of the substrate. Excessive of ions causehigher bombardment of growth film and, as result, morecompressive film stress in that area of the substrate, see figure 6.

Figure 6. Magnetic field configuration with magnets behind thesubstrate

When stress is made more tensile in the center, it also makes
average stress more tensile. In order to compensate tensile stress, wecan use RF to make average stress more compressive on the wafer
and increase in pressure/gas flow to make average stress moretensile to meet the desired average stress on a wafer target.
Figure 7 shows stress control from more tensile in the center to morecompressive in the center.
Figure 7. Stress control from more tensile (red) in the center (left) tomore compressive (blue) in the center (right).

Figure 9. Stress variation across wafers for high compressive andhigh tensile average stress

Even with high average film stress, AlScN films show highcrystallinity and no sporadic nucleation in film, see figure 10.
Figure 10. 2Theta X-Ray diffraction of AlScN film without any
abnormal peaks

Total stress variation across a wafer can be achieved as low as
100MPa in the wide average stress range, see figure 8 and 9.
Figure 8. Stress variation across two wafers for low average stress

Sometimes is very difficult to obtain desired average film stress,
film stress uniformity across wafer and thickness uniformity acrosswafer at the same time. For this reason, we use trimming process tocorrect thickness uniformity and focus on film stress uniformity withthe magnetic fields control.

IV.

SUMMARY
Using magnetic field adjustment in combination with substratebias, pressure/gas flow control, we were able to demonstrate
excellent stress control from highly tensile to highly compressive
average stress. Thickness uniformity was controlled with trimming.
Having a large number of independently controlled parameters
allows a very precise control of stress and thickness uniformity inAlScN deposition process

REFERENCES[1]

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[2] Pau Muralt, et.al.. “Electromechanical properties of AlScN thin films
evaluated at 2.5 GJz film bulk acoustic resonators”, Applied Physics
Letters 99, 2011

[3] Keiichi Umeda1, et.al. “PIEZOELECTRIC PROPERTIES OF ScAlN

THIN FILMS FOR PIEZO-MEMS DEVICES”, MEMS 2013, Taipei,Taiwan, January 20 – 24, 2013[4] Sergey Mishin, et.al. “ Method of controlling coupling coefficient ofAluminum Scandium Nitride deposition in high volume production .” 2013
IEEE FREQUENCY CONTROL SYMPOSIUM