Manufacturability of highly doped Aluminum Nitridefilms
Sergey Mishin
Yury Oshmyansky
Advanced Modular Systems, IncGoleta, CA/USAsmishin@amssb.com
yoshmyansky@amssb.com

Abstract—There have been several investigations [1], [2],[3], that demonstrated benefits of adding dopants such as(Sc) or combination of other materials, like Zr/Mg forexample, to the aluminum nitride (AlN) films in order toincrease coupling coefficient (kt^2) of the Bulk Acoustic
Wave (BAW) devices. For concentrations below 10% atomic
Sc, it is possible to use a single composite target with astandard magnetron design [4]. Most R&D systems that
performed initial investigations on AlScN films with highconcentration of Sc dopant, used two separate targets with
two separate magnetrons: one with pure Al and one with pureSc with different applying power to compensate for the large
difference in sputtering rates of the two materials and get
stoichiometric composition. Unfortunately, depositing fromtwo different targets is only viable for low volume R&D
experiments. The system described in this article uses
standard dual conical magnetron with AC deposition source.
Targets are cut into multiple segments as shown in Figure 1[5].
Figure 1. Multiple material targets

Based on simple geometry of target’s surface, deposited film
composition is proportional to the surface of specific piecesof target material. Unfortunately, Al is eroded at much higher
rate than Sc at the same potential and same magnetic field.
Over the target life, concentration of Sc increases in thedeposited films. In order to maintain same Sc composition
over the entire target life, it is necessary to vary magneticfield locally over the surface of the Al and Sc pieces toprovide same erosion rate of Al vs. Sc at the same target
potential. Adjusting magnetic field for each segment of both
Al and Sc allows for constant deposited film composition
over the entire target life solves this problem.
I. INTRODUCTION
Most 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 or even
impossible. Also, they are very expensive. Binary Al/Sctargets are notoriously difficult to manufacture at Sc
compositions above 10%. Theoretical maximum Sc
concentration in aluminum with stable FCC crystal structure isabout 22%, see Al-Sc phase diagram on figure 2 and table 1
and 2. However, the crystallographic texture should beremained relatively stable, retaining the preferred orientation
ratios for good uniformity and stoichiometry during
sputtering. Orientation ratios, compared to grain size, are themore critical parameter for sputtered film properties.
The AlSc phase diagram below shows the Al-Al3Sc
eutectic reaction at very high temperature [6]. This processcauses hot cracking of composite material, which wereobserved as black spots even below 10% of scandium
composition in aluminum target. Higher Sc composition in Al
target produces even worse problems.

Table 1. Al-Sc Lattice parameter data

Figure 3. Al-Er-Mg ternary compound phase diagram

Table 2. Al-Sc Crystal Structure data

Figure 2. Calculated Al-Sc phase diagram

On other hand, sputtering from multiple targets fromdifferent magnetrons is not practical for high volume
production due to low deposition rate and poor film thickness
uniformity.
In this investigation we used multiple piece targets toproduce highly piezoelectric films with Al, Sc, Mg and Ermaterials.

II.

EQUIPMENTIn this investigation we used Advanced Modular Systems
cluster tool with AlN deposition chambers and ion beamtrimming module (shown in Figure 4).

Composite Targets with three different materials, such as
ternary compounds Al-Er-Mg or Al-Zr-Mg, are even moredifficult to make. For example, Al-Er-Mg alloy may show
multiple intermediate phases [7], [8], which results unstableproperties of the deposited film.

Figure 4. AMSystems cluster tool

AlN deposition uses a dual magnetron system withpositive plasma column and with AC power applied between
targets. Frequency of AC power is 40 kHz and power mayvary from 3 to 10 kW. It is a reactive deposition process indeep poison mode using targets composed of Al and Sc pieces(tiles), see figure 5.

Figure 5. Multiple material targets

High purity research grade 99.9999% argon and nitrogenprocess gasses we used for all depositions.
Substrate rotation is using to compensate variation ofscattering for different materials and composition nonuniformity across the substrate.
The trimming module uses DC focused ion source withargon process gas to improve thickness/uniformity ofdeposited 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 andcomposition control, because it allows avoiding of spending
too much effort on controlling thickness uniformity during
deposition.

III.

Figure 6. Sputtering Rate vs. Magnetron Magnetic Field

It is clear, that for Al-Sc deposition, for example, a constantmagnetic field across of the magnetron will produce lower
scandium composition film on the wafer than on the target if
standard AlN magnetron is using. Changing the local
magnetic field on the surface of tiles/segments with different
sputtered materials results higher or lower concentration ofdesire dopant and a uniform target race track. We found that if
we produce the same erosion of the Al and Sc targets, we get
the same composition of Sc on the wafers as on the target.
Based on Sputtering rate vs Magnetic field, we calculated
compositions of different dopant’s concentration in AlN film
depends on concentration of the same dopant in the target.
For Al-Sc deposition, increasing magnetic field on thesurface of the aluminum pieces, increases Sc composition on
the deposited film on the substrate, but it produces opposite
result for Al-Mg deposition, as shown in figure 7.

FILM COMPOSITION CONTROLThe system uses standard dual conical magnetron with AC
deposition source. Targets are cut into multiple pieces/tiles asshown in Figure 5. Films containing various concentrations
Scandium (Sc) and Erbium (Er) in AlSc(x)Er(y)N films,Scandium and Magnesium in AlSc(x)Mg(y)N films or
Scandium in Al(x)Sc(y)N films have been demonstrated using
different number of Sc and/or Er, or Sc and Mg pieces
compare to the number of the Al pieces. As sputtering area ofspecific material increases, composition of this material indeposited film also becomes higher.
But not only pieces/tiles sputtering area effects on film
composition. Sputtering rate of specific material has a verysignificant effect on film composition. Typically, for example,concentration of Sc in the AlScN film is about 66% of what it
is in the deposition target with either composite targets ortargets made out of pieces. It is much more obvious on thetargets made out pieces of Al and Sc, that erosion of Al pieceis significantly higher than the erosion of the adjacent Sc
piece, if the same magnetic field is used. Sputtering rate fordifferent material vs. magnetron magnetic field is shown
below on figure 6.

Figure 7. Film/Target Ratio of Composition of Sc and Mg asa function of the magnetic field at the Al pieces

Some mismatch between calculated and actual film/target
composition ratio versus magnetic field could be considered as
an error for different scattering shape for different type of
dopant (heavier atoms, normally at given target voltage, havebroader scattering and some material is losing due todeposition on shields. Lighter atoms, at given target voltage,

have, usually, more narrow scattering and more material
reaches a substrate). Also, we didn’t consider smooth variationof magnetic field from tile/piece to tile/piece. Since magnetic
field has continuous (smooth) function, and we can’t produce
it as a step function.

Figure 9. Film stress of AlScN deposition as a function oftarget life

Film’s compositions were measured by Rutherford
Backscattering Spectrometry. Rutherford Backscattering
Spectrometry spectra are acquired at a backscattering angle of160° and an appropriate grazing angle (with the sample
oriented perpendicular to the incident ion beam). The
schematic diagram below shows the scattering geometry in a
typical RBS experiments.
Figure 8. Scattering Geometry of
Backscattering Spectrometry Experiments

anRutherford
Figure 10. Film uniformity of AlScN deposition as a functionof target life

Uniformity of sputtering rate changes over the target life.
Fortunately Sc or Mg concentration across wafer and over thetarget life doesn’t change appreciably. This allows for arobust production process that can easily be fixed by ion beamtrimming. Figures 9, 10 and 11 below, show AlScN deposited
film properties over the target life, including thickness
uniformity with parallel trimming process, with adjusted local
magnetic field for Al and Sc tiles/pieces.

Figure 11. Film properties of AlScN deposition as a functionof target lifeIV.

SUMMARY

Using custom and local adjusted magnetic field andthickness trimming, we were able to demonstrate production
worthy highly doped AlN deposition process.

REFERENCES[1] Morito Akiyama, “Influence of temperature and scandium concentration
on piezoelectic response of scandium aluminum nitride alloy thinfilms”, Applied Physics Letters, Oct. 2009, Vol.95, Iss. 16, pp. 21072107-3[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”, MEMS
2013, Taipei, Taiwan, January 20 – 24, 2013
Sergey Mishin, et.al.. “Production worthy system for piezoelectric
multi-material depositions”, IEEE 2014, USA, 2014

[5][6]

M. Asta and V. Ozolins: Structural, Vibrational and Thermodynamic
Properties of Al-Sc Alloys and Intermetallic Compounds, Phys. Rev. B64 (2001) 094104-1-14[7] Jing Hu, et.al “Experimental Phase Diagram of the Al-Er-Zr Ternary
System” , Journal of Phase Equilibria and Diffusion, Vol.32, No 5,2011[8] J.L. Murray “Al-Sc System” , Journal of Phase Equilibria andDiffusion, Vol.19, No , 1998