Effect of Substrate Material and ElectrodeSurface Preparation on Stress and Piezoelectric
Properties of Aluminum Nitride
Sergey Mishin

Michael Gutkin

Research and Development
Advanced Modular Systems, Inc
Goleta, CA/USA
smishin@amssb.com
Abstract — Intrinsic stress and electromechanicalcoupling coefficient are the most importantparameters of piezoelectric thin films used inmembrane as well as cantilever type MEMS such asenergy harvesters, transducers and others.
In this paper, practical aspects controlling stressand crystallinity in aluminum nitride (AlN) film usedin manufacturing of Bulk Acoustic Wave (BAW)/Film Bulk Acoustic Resonator (FBAR) filters [1],[2], [4] have been presented. Controlling stress bymodifying deposition parameters was compared tocontrolling stress by surface modification. Stress
was found to be an important indicator in predicting
crystallinity of AlN films. Impact of the substratematerial and electrode deposition on stress andcrystal orientation of piezoelectric AlN was studied.
In process applications where control of the surface
layer thickness is critical and surface topography isvariable, use of ion mill to reduce surface roughness
is preferable to CMP. It was generally found that formost amorphous materials such as silicon dioxide orsilicon nitride that are placed under the electrode or
piezoelectric AlN, it is critical to get the best possible
surface smoothness. On the other hand for theelectrode material such as molybdenum, it is more
important to get material with the best crystal
orientation. Polishing of the well oriented electrodematerial did not provide further improvement. It wasalso observed that if AlN is deposited on a smooth orwell oriented layer, orientation of the AlN is
improved when the stress in the film is more tensile.
On the other hand if the tensile stress is caused byresiduals on the surface or surface roughness, tensilestress usually leads to poorly oriented films.

Technical Marketing
Advanced Modular Systems, IncGoleta, CA/USA
mgutkin@amssb.com

I.

INTRODUCION

Early in the development of AlN films, it was observedthat stress could vary for variety of reasons. Sometimes mostcompressive stress in the AlN film produced best crystallinity,
other time tensile stress produced best results. Controlling
stress was particularly important for FBAR applications wheremembranes are susceptible to cracking or delaminating due tohigh stress. Even in BAW technology, high stress can simply
cause yield loss in photo-lithography due to high warpage ofthe wafers. Three major factors must be addressed to control
stress in the AlN films:
Substrate or pre-electrode processingElectrode or pre-AlN processing

Deposition of AlN

We propose some practical guidelines to successful stresscontrol with high electromechanical constant.

II.

EQUIPMENTIn this investigation we used Advanced Modular Systems
cluster tool with three modules: AlN deposition chamber,electrode deposition chamber and ion beam trimming module(shown in Figure 1).
AlN deposition uses a dual magnetron with AC power
applied between targets. It is a reactive deposition usingaluminum target and, argon and nitrogen process gasses.
Electrode deposition chamber is a dual magnetron with DC
power supply, electrode target (Mo, W, Pt, etc…) and argon
process gas. The trimming module uses DC focused ion
sourcewithargon
process
gastoimprovethickness/uniformity of either electrode or AlN films. Use of
the trimming module opens up a much wider process windowfor stress control because we don’t need to spend too much
effort on controlling thickness uniformity during deposition.

Figure 1: AMSystems cluster tool

III.

A.

B.

SUBSTRATE OR PRE-ELECTRODE
PROCESSING

Figure 2: AlN Rocking curves and Stress as a function ofoxide surface roughness

IV.

ELECTRODE OR PRE-ALN
PROCESSING

Surface Cleaning
The surface on which the electrode is deposited canirreparably damage quality of the piezoelectricaluminum nitride. It is very critical to have asmooth and uncontaminated surface before
depositing the electrode material. If substrate was
exposed to the photoresist, it is critical to have thesurface adequately cleaned. Depending on thematerials used, either wet chemical or dry strips canbe effective. If wafers are not adequately cleaned, it
can lead to high tensile stress in the future depositedfilms with very poor crystal orientation and, inworse cases, delamination.

Before piezoelectric AlN is deposited, the electrode material
is typically deposited and in most cases patterned. It is
important to keep track of the stress in the electrode films
because it is a good indicator of the problems with thesubstrate preparation. Erratic stress in the electrode usually
results in problems with AlN deposition.

Surface smoothness
After adequate cleaning it is important to makesure that the surface is smooth. Typically, 0.6nm
surface roughness is acceptable. There are two
common methods for the surface smoothing:
Chemical Mechanical Polishing (CMP) and ion
beam milling. CMP can be performed on mostmaterials and with appropriate selection of polishingslurry it is capable of both planarizing andsmoothing the surfaces. One limitation is that it ishard to control the amount of material removed,
especially if there is diverse topography of features
on a wafer. Ion milling can be precise in the amountof material removed, but is only effective on harddielectric materials such as SiN, AlN or SiO2. It
requires too much material removal on metals to beuseful. Figure 2 shows impact of surface roughness
on the crystallinity and stress of AlN deposited on
the PECVD oxide that is polished to different levelof smoothness. Ion beam trimming tool was used toimprove the surface roughness.

Figure 3: AlN properties on differently exposed Ruelectrodes

Some electrode materials such as Ru oxidize in the air if
AlN is not deposited within relatively short period of time.
Significant variations in both stress and rocking curves havebeen observed if Ru electrode is left in the ambient
atmosphere for a long time. Figure 3 shows the impact on
AlN parameter as Ru is left exposed for significant amount oftime.

Even though electrodes such as W and Mo are not easily
oxidized, it was found that removal of 10nm to 20nm of theelectrode material by CMP has beneficial impact on
repeatability of stress as can be seen in Figure 4. Polishing thesurface of the W also produces improvement in rocking curve,
but no improvement was observed for Mo.
Figure 4: Repeatability of AlN stress on polished vs.
unpolished electrodes

substrate interaction with the chamber components andvariable stress results.
Another common cause of stress variation is a plasmasubstrate coupling due to variation of plasma potential andcontact through the electrode structure to the wafer
holding/clamping mechanism. We have observed as much as
700MPa difference in stress due to the conductive contact tothe wafer surface vs. insulated contact.
As the target material gets eroded and a thick insulating
layer builds up on the shields, it is not uncommon to see stresschanging from the beginning of the target life to the end of thetarget life. Usually such changes are repeatable from target totarget and can be built into automated software adjustment on
the deposition system, see Figure 6.
Figure 6: AlN film stress vs. target life

Measuring rocking curve on Mo deposition has been usefulin predicting better crystal orientation in the following AlN
deposition, see Figure 5. Rocking curve measurements had nocorrelation to the rocking curves on AlN or stress repeatabilityfor either W or Ru.
Figure 5: AlN rocking curve depends on Mo electrodepropertyVI.

CONCLUSION

In many cases it is more important to prepare the substrate
and the electrode for the AlN deposition than trying to control
deposition itself. Making sure that both electrode andsubstrate have been treated correctly will go a long way
towards producing excellent AlN films.

REFERENCES[1] Robert Aigner “Corrective Actions to Meet Extreme Tolerance
Requirements for Thin Films” 2007 Octagon Communication Inc. DBA
MPDigest

V. DEPOSITION OF AlN
Stress control in the AlN deposition is typicallyaccomplished by controlling either deposition pressure orsubstrate bias. Higher deposition pressure produces moretensile stress and higher substrate bias more compressivestress [3].
It is important to establish baseline stressrepeatability in the deposition module. High resistivity Si isbest suitable for such application. Wafers with resistivity of>15ohm-cm are acceptable. Lower resistivity can lead to[2] Frank Z. Bi and Bradley P. Barber “Bulk Acoustic wave RFTechnology”, 2008 IEEE
[3]K. Hashimoto “RF BULK ACOUSTIC WAVE FILTERS FORCOMMUNICATIONS”, pp.173-195[4]Sergey Mishin, Dan Marx, Brian Sylvia, V. Lughi, K. L. Turner, and D.
R. Clarke “Sputtered AlN Thin Films on Si and Electrodes for MEMS
Resonators: Relationship between Surface Quality Microstructure and FilmProperties” 2005, IEEE