Production worthy system for piezoelectric multimaterial depositions
Sergey Mishin
Michael Gutkin
Advanced Modular Systems, IncGoleta, CA/USA
smishin@amssb.com
mgutkin@amssb.com

Abstract— In this paper, we describe system for depositing
piezoelectric films with multiple target materials using a standard
production system that is used in high volume production ofAluminum Nitride films. This system uses standard dual conicalmagnetron with AC deposition source [1]. Targets are cut into
multiple pieces/tiles as shown in Figure 1. Films containing various
concentrations Scandium (Sc) and Erbium (Er) in AlSc(x)Er(y)Nfilms have been demonstrated using different number of Sc and Er
pieces compare to the number of the Al pieces. Coupling coefficient(kt2) was measured across wafer and wafer to wafer as a function Sc
and Er content in the target. Previous studies demonstrate aconsiderable increase in kt2 as a function of Sc content of the film [2],[3], [5], [6]. Unfortunately, when higher concentration of Sc or a thirdmaterial such as Er is used, composite targets are too difficult to make.
Multiple target systems can be used to compensate this problem. Bothstress and concentration of Sc and Er must be controlled across wafer
to achieve uniform kt2 acceptable for production of Bulk Acoustic
Resonator (BAW) devices [4], [7]. We were able to control kt2
across wafer and wafer-to-wafer by adjusting magnetic fields in ourmagnetron, rotating wafer during deposition as well as adjusting
concentration of Sc and Er in our two sputtering targets.

Figure 1. Multiple material targets.

Keywords-component; Aluminum Nitride; Scandium; coupling
coefficient; stressI. INTRODUCTIONMost PVD deposition processes that require multiple
materials in a sputtering target use a composite target.
Although it works well for most materials, some composite
targets are extremely difficult to manufacture and are very
expensive.
Al/Sc targets are notoriously difficult tomanufacture at Sc compositions above 10%. Targets with threedifferent materials are even more difficult to make. Sputteringfrom multiple targets is not practical for high volume
production.
In this investigation we used multiple piece targets toproduce highly piezoelectric films with Al, Sc and Er materials.
II. EQUIPMENT
In this experiments, we used Advanced Modular Systems
cluster tool with AlN and AlScErN deposition chambers andion beam trimming module (shown in Figure 2).
Both AlN and AlScErN depositions use a dual magnetronwith AC power applied between targets. Frequency of AC
power is 40 kHz and power may vary from 3 to 10 kW. It is areactive deposition process in deep poison mode. We used twotargets made out of pieces/tiles of Al, Sc and Er to adjust film
composition. High purity 99.9999% argon and nitrogenprocess gasses we used for all depositions.
The trimming module uses DC focused ion source withargon process gas to improve thickness/uniformity of either
AlN or AlScN films. Film thickness trimming/tuning isprocessing based on ion beam scanning across a wafer withpower variation based on film thickness map. Use of thetrimming process opens up a much wider process window forstress 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.
Figure 2: AMSystems cluster tool

Typical stress control techniques involve either substratebias or change in gas pressure. Unfortunately, both of thesemethods are capable of changing average stress on a wafer
without having much impact on the local stress across wafer.
AlN Film stress depends on magnetic field on target (7).
AlScN and AlScErN film stress also depends on magnetic fieldon target surface. We found that our standard configuration ofmagnetic field compensation, that we used for AlScN works forthe multi-piece targets as well for both AlScN and AlScErNfilms.
Figure 4. Magnetron with the enhanced magnetic field

III. STRESS CONTROL
Average film stress and film stress uniformity is very
important, because film strain has a great impact on
piezoelectric properties, see figure 3.

Since wafer center receive more material from the innertarget and wafer edge receive more deposition from the outertarget, variation of the ratio of the strength of magnetic field
between inner and outer targets (magnetrons) allows
controlling concentric stress uniformity.
Figure 5: Stress variation across wafer as a function of the ratio
of inner to outer target magnetic fields

Figure 3: Effect of film stress to piezoelectric coupling
coefficient

We found that in AMS system stress variation has an almost
perfectly radial distribution that varies with the ratio of
magnetic field between inner and outer magnetron. Figure 5
shows change in stress across a wafer as a function of the ratioof the inner to outer magnetron magnetic fields.

It is clear that a standard 1:1 ratio of magnetic field that
produces the best results for AlN process is unacceptable for
AlScN or AlScErN processes. We found that the ration of
inner/outer of 1.0/0.8 is the best for stress control.

IV. COMPOSITION CONTROLOur multi-piece targets in this investigation had 30 pieces in
the inner target and 48 pieces in the outer target. Concentrationof each material on the inner target can be adjusted in 3.3%
increments and on the outer target 2.1%.

same range of Sc concentration results very similar for bothconfigurations.
Figure 8 shows excellent control of both local stress andcoupling coefficient across wafer using optimized combinationof optimum magnetic field ratio and Sc concentration for bothcomposite AlSc and multi-piece AlSc configurations.
Figure 8. Coupling coefficient and local stress on AlScN wafers

We tried Er concentrations 3.3% and 6.6% in combinations
with different Sc concentrations and observed no impact on thecoupling coefficient. For this reason we did not use Er in
further investigations.
Figure 6. Er effect on coupling coefficient

I. SUMMARY
Erbium does not have any effect on kt^2 of the AlScErN
films as compared with AlScN films of the same Sc
concentration. Multi-piece target produces similar results to thecomposite target configuration, but has much more flexibility
for different concentration of Sc and allows much higher Sc
concentrations that a composite targets.
REFERENCES
Using wafer rotation in conjunction with multi-piece targets
produced results that are very similar to the composite targets
with the same concentration of Sc. Figure 7 shows acomparison of kt^2 between composite targets and multipiece targets.
Figure 7. Coupling coefficient across wafer as a function of Sc
concentration in two targets

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