Improving coupling coefficient distribution on BAW
filters manufactured on 200mm wafers
Sergey Mishin

Yury Oshmyansky

Advanced Modular Systems, Inc
Goleta, CA/USA
smishin@amssb.com

Advanced Modular Systems, Inc
Goleta, CA/USA
yoshmyansky@amssb.com

Abstract— In the past, most BAW and FBAR filters wereproduced on 150mm wafers. Typical yield on 150mm wafers was>90% [1], [2]. In the last few years, most filter manufacturers
started migrating to 200mm wafers in order to reduce the cost ofmanufacturing. Unfortunately, due to variation of coupling
coefficient (kt^2) [3], [4], [5], yields on 200mm wafers weresignificantly lower than on 150mm wafers. Stress in thepiezoelectric material is the largest variable that changes
coupling coefficient [6].
Figure 1. Shows how coupling
coefficient varies with stress for devices that use piezoelectricaluminum nitride (AlN) and molybdenum (Mo) electrodes.
Depending on the exact processing involved in making a filter,
the same AlN deposition process will produce a completely
different stress/kt^2 variation across the wafer. Figure 2
illustrates a difference coupling coefficient variation acrosswafer, using tungsten (W) vs. Mo electrode material whileleaving everything else the same. In order to be able to obtain
the best coupling coefficient with different processing on 200mm
wafer, it is critical to have independent control of film stressacross wafer. In this paper, we will demonstrate a method ofcontrolling coupling coefficient across wafer within less than +/1% (total range) of the target.
Keywords—film stress; coupling coefficient; AlN; aluminum
nitride; stress control

Figure 1. Coupling coefficient as a function of average stressin AlN films.

Figure 2. Coupling coefficient variation across wafer on Mo
vs. W electrodes

I.

INTRODUCTION

As most of the high volume manufacturers of FBAR/BAWfilters are moving to 200mm wafer from 150mm wafer, newchallenges emerged. One of the most critical is significantly
higher variation of kt^2 across 200mm wafer. As can be seenin Figure 3, area of the 200mm wafer that can meet the kt^2specifications is almost the same as on the 150mm wafer. Inorder to meet the same or even tighter specifications of thenew filter products, it is important to be able to producehighly uniform kt^2 across wafer. For different processes itmeans ability to control stress across wafer.
Considering the importance of AlN film sterss and film
thickness uniformity, it is necessary to have independent film
stress and thickness control.

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 andother film properties control, because it allows avoiding ofspending too much effort on controlling thickness uniformity
during the deposition.
Figure 5. Focus Ion Beam ProfileFigure 3. Typical results from 150mm wafer compared to200mm wafer
II.

EQUIPMEMTIn this investigation, we used Advanced Modular
Systems cluster tool with three process modules: AlN
deposition chamber, electrode deposition chamber andfocus ion beam trimming module (shown in Figure 4).

Unlike back-sputter etch module in the traditional cluster
tools, AMS trimming module can be used to both, cleaning
the surface before deposition, as well as improving thickness
uniformity after the deposition. It is very cost effective
solution, because it doesn’t cost any more than RF sputteretch chamber, but does a double duty as a pre-clean chamber
and trimming chamber.
III.

Figure 4. AMSystems cluster tool with three modules(AlN deposition, Mo deposition and pre-clean/trimming)
AlN deposition process uses a high-volume production
dual magnetron system with positive plasma column and withAC power applied between targets (magnetrons). 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 modeusing pure Al targets. High purity research grade 99.9999%
argon and nitrogen process gasses we used for all depositions.
Substrate rotation is used to reduce variation of nonuniformity of deposited film properties across the substrate.
Integrated Laser interferometer is used to control averagedeposition thickness of the AlN films during process.
The trimming module uses DC focused ion beam sourcewith argon process gas to improve thickness/uniformity ofdeposited films. Film thickness trimming/tuning is processing

SYSTEM DESIGN FOR STRESS CONTROL IN
ALUMINUM NITRIDE DEPOSITION

When using a standard rotary magnetron deposition
system for the piezolectric AlN on 200mm wafer, stressvariation across wafer is typically greater than 350MPa. In
most designs, it also produces more tensile films in thecenter of the wafer. This is partially due to the inherent
geometric limitations of the single target system, as well as
the location of the racetracks on the target and the distance
between racetracks and the anode or chamber wall. As aresult, it produces high density plasma with positive
column in the center of the target and low density plasma(due to elimination of positive plasma column) at the edgeof the target. Since more material on the center of thewafer is deposited from the center of the target, and morematerial on the edge of the wafer is deposited from thetarget edge, it produces highly non-uniform center-to-edge
AlN film properties (such as stress and coupling
coefficient). It is very hard to optimize rotary magnetron
system to give both, good thickness uniformity and goodstress variation across wafer.
It is much easier to obtain good stress control andthickness uniformity of deposited film in a system with
two independent magnetrons (targets), that allow fairly
easy geometric manipulation of the target surface, as well

as independent magnetic field control on each target
surface.

Wafer rotation is also used to improve both thickness
uniformity and stress variation across wafer.

The AlN film, deposited mostly from the inner target
produces largest effects on film properties near the centerof the wafer. The AlN film, deposited on the wafer edge,
mostly come from the outer target. Since, the AC power isapplied between two targets (shields do not participated inplasma discharge) it creates symmetrical discharge and thepositive plasma columns that are equal for the discharges.
During the deposition, each target acts alternatively as an
anode or cathode for the half of the cycle. Due to the samelength of plasma discharge and plasma density, AlN film
properties (stress for example) on the center of the wafer
and on the wafer edge are the same. As result, the stressuniformity across the wafer is less than +/- 75 MPa for
200mm wafers, see Figure 6.

Figure 7 below shows a typical configurations (magneticand electrical) used for the 200mm wafers.
Figure 7. Typical configuration used for the 200mmwafers.

Figure 6. Typical AMS AlN film cross wafer stress for
200mm

IV. CONTROLLING COUPLING COEFFICIENT FOR
DIFFERENT PROCESSES
Depending on the processing used in manufacturing
a particular filter technology, it is sometimes required to havea non-uniform stress distribution on a wafer. For example,
when a film, that has perfectly uniform stress distribution ona prime silicon wafer, is deposited on a product wafer, stressis much more compressive in the center than on the edge ofthe wafer. This typically results in lower coupling coefficientin the center of the wafer. There are three potential ways ofaddressing this problem through AlN deposition:
1.
Additional DC power supply, in parallel with AC power
supply, allows to shift reference point for AC power andprecisely control center-to-edge thickness uniformity.

2.
3.

Dis-balancing magnetons magnetic field (inner orouter) with supplemental magnetic fields locatedabove the wafer.
Adjusting magnetic field on the magnetrons topurposely unbalance the stress across the wafer.
Increasing/decreasing the ratio of the deposition
coming from outer vs. inner target.

Since magnetron magnetic field has significant
impact on film stress (see figure 8), it would be reasonable tospend significant amount of effort to control the magnetron
magnetic field.

uniformity the most and will require most trimming tocompensate for thickness non-uniformity.
After making all of the above adjustments to thesystem, we were able to obtain excellent coupling coefficientuniformity over a ten wafer lot of BAW filter 200mm wafers.

Figure 8. AlN film stress depeds on Magnetorn magnetic
field

Figure 10. Impact of the magnetron magnetic field ratio on
stress across wafer.

The easiest way to adjust the stress across wafer is toadjust magnetic field by dis-ballancing magnetic field bychanging the magnetic field strength behing the wafer. Thisrequires either a simple change of the permanent magnets or
adjusting current to the electromagnet located behind thewafer. This methode allows to sputter more compressive or
more tensile stress from inner target to the center of the wafer
and can provide up to 300MPa stress change from the center
to the edge of the wafer. It has a small impact on thickness
uniformity that is easily corrected by the trimming process.

Figure 11. Coupling coefficient across 200mm wafers.

Figure 9. AlN film stress distribution across 200mm waferV.
If a more significant change is required, it may benecessary to change the magnetic field ratio between innerand outer magnetrons. This requires much more extensive
hardware modificaton and process re-carechterization.
Figure 10 shows how adjustment of the magnetic field on two
magnetrons can change stress across wafer.
If the magnetron ratio is already changed to havesignificantly different stress material coming from eachmagnetron on a different part of the wafer, changing theamount of material coming from each target will result ineven bigger change in stress between the center and the edge.
Typically, this kind of change will impact thickness

SUMMARY

We were able to demonstrate highly uniform
coupling coefficient across 200mm BAW/FBAR wafers
using AlN deposition on a dual magnetron deposition
system with integrated pre-clean/trimming module.
Stress and kt^2 uniformity across wafer was achieved
without compromising thickness uniformity or crystal
orientation of the deposited AlN film.
REFERENCES[1][2]

Mishin Sergey, et al. “Sputtering Processes for Bulk AcousticWave Filters”. Semiconductor International, 3/1/2003
Frank Z. Bi and Bradley P. Barber “Bulk Acoustic waveRF Technology”, 2008 IEEE

[3]

[4]
[5]

Sergey Mishin, Brian Sylvia and Daniel R. Marx
“Improving Manufacturability of AlN Deposition Used in
Making Bulk Acoustic Wave Devices”, 2005 IEEEULTRASONICS SYMPOSIUM
K. Hashimoto “RF BULK ACOUSTIC WAVE FILTERSFOR COMMUNICATIONS”, pp.173-195
Robert Aigner “Corrective Actions to Meet Extreme[6]

Tolerance Requirements for Thin Films” 2007 Octagon
Communication Inc. DBA MPDigest
Sergey Mishin, Yury Oshmyansky and Frank Bi“Thickness Control by Ion Beam Milling in Acoustic
Resonator Devices”, 2010 IEEE FREQUENCYCONTROL SYMPOSIUM