Method of controlling coupling coefficient of Aluminum
Scandium Nitride deposition in high volume production.
Sergey Mishin1, Michael Gutkin1, Alexander Bizyukov2 , Vladimir Sleptsov3
1

Advanced Modular Systems, Inc, Goleta, CA/USA, smishin@amssb.com
2Karazin Kharkiv National University, Kharkiv, Ukraine
3
Moscow State Aviation Technological University Moscow, Russia
Email: smishin@amssb.com; mgutkin@amssb.com

Abstract— In this paper, we present our studies of theinfluence of the stress on Aluminum Nitride containing
various concentrations Scandium (Sc). Coupling coefficient(kt2) was measured across the wafer and wafer to wafer as afunction of stress and Sc content of the film. Previous studies
demonstrate a considerable increase in kt2 as a function of Sc
content of the film [1], [2], [4], [5]. Unfortunately, when
deposited on 200mm wafers we observed that coupling
coefficient varies significantly more than that of a standard
Aluminum Nitride (AlN). Both stress and concentration of Sc
must be controlled across the wafer to achieve uniform
coupling coefficient acceptable for production of Bulk
Acoustic Resonator (BAW) devices [3], [6], [7]. We wereable to control coupling coefficient across the wafer andwafer-to-wafer by adjusting magnetic fields in dual magnetron
configuration as well as adjusting concentration of Sc in ourtwo sputtering targets.
Keywords-component; Aluminum Nitride; Scandium; coupling
coefficient; stressI.

INTRODUCTION

Most PVD deposition processes require only average stressmeasurement on a wafer to provide reasonable device
performance. It is not uncommon to have 300MPa to 400MPa
variation of stress across wafer on AlN deposition withoutanyone realizing it. Actually stress has a significant impact oncoupling coefficient of AlN film. Local Stress variation acrossa wafer produces variation of piezoelectric coupling
coefficient, and, as the result, dramatically reduces device
yield. Similar problem can be addressed to the stress variationof AlScN film. Figure 1 shows effect of stress on AlN andAlScN films.
Figure 1: Variation of coupling coefficient as a function ofstress for AlN and AlScN films

Tight control of coupling coefficient is essential for
producing high quality devices. Using dual target magnetron
configuration allows very tight control of stress and couplingcoefficient across a wafer.
In this investigation we used different magnetic fields oneach magnetron in conjunction with different concentrations ofSc on each target to produce the tightest control on both stressand coupling coefficient.
II.

EQUIPMENTIn this investigation we used Advanced Modular Systems
cluster tool with AlN and AlScN deposition chambers and ion
beam trimming module (shown in Figure 2).
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 poison mode using two
aluminum or composite targets and, argon and nitrogen process
gasses.

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 is aprocess based on ion beam scanning across a wafer with power
variation based on film thickness map. Utilization of thetrimming module opens up a much wider process window forstress control because it allows to avoid 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

It is clear that a standard 1:1 ratio of magnetic fieldthat produces the best results for AlN process is unacceptablefor AlScN process. We found that the ratio of inner/outer of1.0/0.8 is the best for stress control.

III.

STRESS CONTROL

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 effect on the local stress across a wafer.

Placing an additional magnetic field with an adjustablestrength behind a wafer as shown in Fig.4 creates a variableunbalance effect of the either inner or outer magnetron (and, as
the result, increases or reduces ion bombardment) and provides
an additional fine control of the film stress uniformity across awafer (center to edge) over the target life. This magnetic field
can be varied by either an electro-magnet with adjustableelectrical current or a permanent magnet with a motor that canadjust the distance between the magnet and the wafer.

Figure 4. Magnetron with the enhanced magnetic field

AlN Film stress depends on magnetic field on the surface ofthe target (7). AlScN film stress also depends on magnetic fieldon target surface. Since center of the wafer receives more
deposited material from the inner target and wafer edge
receives more deposition from the outer target, variation of theratio of the strength of magnetic field between inner and outertargets (magnetrons) allows controlling concentric stressuniformity. We found that in AMS system stress variation has
almost perfectly radial distribution that varies with the ratio ofmagnetic field between inner and outer magnetrons. Figure 3
shows change in stress across a wafer as a function of the ratioof the inner to outer magnetron magnetic fields.
Figure 3: Stress variation across wafer as a function of the ratio
of inner to outer target magnetic fields
IV.

COMPOSITION CONTROL

Another advantage of the dual magnetron is its capability touse targets (inner and outer) with different ratio of Sc/Al. Thismethod provides an additional control to obtain a uniform
coupling coefficient across a wafer.
We started our study by using the same composition on bothtargets of about 6.1 atomic % Sc with the rest being Al.
Analysis showed that deposited on the wafer film contained
about 4.2% Sc. In order to achieve desired improvement ofcoupling coefficient we changed to 8 atomic % Sc on the innertarget and varied Sc concentration on the outer targets. Figure
5 shows variation of coupling coefficient across a wafer as theinner target Sc concentration is 8% and outer target Sc
concentration is varied between 0 and 8%.
Figure 5: Coupling coefficient across wafer as a function of Sc
concentration in two targets

V.

SUMMARY

Using dual magnetron design is highly advantageous incontrolling stress and coupling coefficient in AlScN films. By
adjusting magnetic field and concentration on target, it is fairly
easy to obtain highly controlled coupling coefficient and stressin a high volume production environment.
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By using the same Sc concentration of inner and outertargets we get slightly higher concentration of scandium on theedge of the wafer. The optimum concentration profile isobtained with concentration of scandium of about 8% on theinner target and 4% on the outer target. It should be noted thatmost cost efficient method would be to use 8% Sc for innertarget and pure outer aluminum target.
The Coupling
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Figure 6 shows excellent control of both local stress andcoupling coefficient across a wafer using optimized
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