Improving Manufacturability of Bulk Acoustic Wave and Surface Acoustic Wave
Devices
Sergey Mishin 11*Advanced Modular Systems, Inc. Goleta, CA 93117 USA

Corresponding author, E-mail: smishin@amssb.com; Tel.: 805-698-6943

Bulk acoustic wave (BAW/FBAR) or surface acoustic wave (SAW) technologies are widely used in making filters for wirelessapplications [1]. While it is no longer a great challenge to make reactively sputtered piezoelectric aluminum nitride (AlN) films
suitable for BAW applications [3] or to make a standard SAW filter, there are critical devices that require tighter control of film
properties in order to achieve required device performance. Independent control of AlN thickness and stress uniformity acrosswafer is critical for FBAR devices that require high coupling coefficient with tight distribution. Frequency control is big issue in
SAW devices. Magnetic field manipulation was used to control stress uniformity across wafer to less than +/-50MPa. Frequency
control of +/-0.05% was demonstrated on temperature compensated SAW devices. A high volume production cluster tool withboth deposition and trimming performed at the same time was demonstrated.
Keywords: BAW; FBAR; SAW; AlN, Trimming

1.

INTRODUCTION

Many applications utilizing free-standing membranestructures (FBAR filters [2] or cantilever MEMs, forexample) require +/-50MPA stress control not only
wafer-to-wafer but also across wafer. If stress varies toomuch, membranes can develop crack or peel off. In
MEMS devices stress can cause severe distortions indevice performance. In BAW applications that don’t
require stress control for structural reason, it is desirable
to keep stress uniform across wafer in order to maintaintight distribution of coupling coefficient. Figure 1 shows
correlation between stress and coupling coefficient.
AlN (aluminum nitride) and electrode thickness
control of +/-0.1% wafer-to-wafer and across wafer areimportant in order to obtain high yielding wafers. In thispaper we will propose a way to obtain both stress control
and film thickness uniformity.
Independent stresscontrol is obtained by carefully designing sputtering
magnetron with variable magnetic field. Thickness
uniformity is independently controlled by ion mill
trimming module.
SAW filters made today frequently use siliconnitride passivation and temperature compensating silicon
dioxide films. Unfortunately, adding such films leads toSPAWDA 2011©2011 IEEE

degradation of frequency range control. Thickness

Figure 1. Coupling coefficient as a function of stresstrimming ion beam tool is an easy solution for thisproblem.
2.

EQUIPMENTIn this investigation we used Advanced Modular

Systems cluster tool with three modules: AlN deposition1

chamber, electrode deposition chamber and ion
beam trimming module (shown in Fig. 2).

It is much more difficult to control stress across wafer
over the target life. In this investigation we used a dual
conical target magnetron and adjusted the magnetic fieldon each target to obtain best stress uniformity over thetarget life. In a standard single target magnetron, it ismuch harder to optimize magnetic field over the erosion
area without significantly impacting thickness uniformity
across wafer. For this reason, dual target magnetron wasselected. Unfortunately, even with two target magnetron,
target erosion over the target life, causes magnetic fields
on the target surface to change.

This result in thevariation of stress across wafer over the target life isshown in Fig. 3.

Figure 2. AMSystems cluster tool

AlN deposition module uses a high volume
production 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 withDC power supply, electrode target (Mo, W, Pt, etc…)
and argon process gas. The trimming module uses DC
focused ion source with argon process gas to improve

Figure 3. Stress uniformity over target life

thickness/uniformity of either electrode or AlN films.
Use of the trimming module opens up a much wider

Stress variation improves initially, but getsprocess window for stress control because we don’t needsignificantly worse at the end of the target life. Placing

to spend too much effort on controlling thickness

magnetic field with adjustable strength behind a wafer as

uniformity during deposition.

shown in Fig.4 creates a variable unbalance of the either
inner or outer magnetron (and, as result, increases or3.

SYSTEM DESIGN FOR STRESS CONTROL IN
ALUMINUM NITRIDE DEPOSITION

reduces ion bombardment of the deposited AlN film) andprovides a control of film stress uniformity across wafer
over the target life. This magnetic field can be varied by

In a typical FBAR structure electrodes are muchthinner than the AlN film. For this reason, we will focus
on the magnetron design for AlN films.

either an electro-magnet with adjustable electricalcurrent or a permanent magnet with a motor that canadjust the distance between the magnet and the wafer.

It is fairly easy to program deposition
parameters like pressure or substrate bias [4] to control
average wafer stress over the entire target life.

SPAWDA 2011©2011 IEEE2

Figure 4. AlN Magnetron with the enhanced magnetic field

Figure 5. AlN thickness uniformity over the target life

Either of these methods allows for an adjustment that canTrimming is also important in improving yield of

be programmed into the target file and provide a

SAW devices.

Some SAW filter require frequency

continuous adjustment to the magnetic field over thecontrol of +/-0.05% in order to achieve yield of >90%.

target life. As result, stress non-uniformity across a wafer

In modern devices silicon nitride is used for passivationis very low and consistent, as shown on Fig.3.

and silicon dioxide films are used for temperature
coefficient compensation.

4.

TRIMMING FOR UNIFORMITY CONTROL

Because 1MHz frequency

shift is produced by 10A to 30A of either film, it iscritical to have film uniformity as good as possible. As

Adjusting stress across wafer does not come without
problems.

The biggest problem is that adjusting

result of the trimming process, device yield increases
dramatically, as shown on Fig. 6.

magnetic field changes film thickness across wafer.
Typical thickness uniformity across wafer is <+/-0.3%.
When stress across wafer is controlled to <+/-50MPa
with the magnetic field control, thickness uniformity
across wafer can be as high as +/-0.5%.
Using trimming of the film thickness with ion beam
[5,6], uniformity of film can be controlled to <+/-0.1%.
Because trimming is done in the cluster tool, it doesn’ttake much extra time. As one wafer is being deposited,
another wafer is being trimmed at the same time. Wafer
maps are stored in the system computer and are updated
throughout the life of the target based on the historical
data. Figure 5 shows results with and without trimming

Figure 6. SAW device yield with and without Trimming Process

throughout the target life.

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Trimming SAW wafer requires a special procedure.

CONCLUSION

Wafers are typically lithium niobate or lithium tantalite
crystal of 200um to 250um thickness. These wafers

Manufacturable solution to producing highly

require special handling to avoid breakage. No clamping

uniform (thickness, stress and coupling coefficient) AlN

or holding clips utilizing tension are allowed.

films

When

SAW wafers get hot they build up high static voltage that

Another issue with SAW wafer is that because they
can’t handle high temperature, silicon dioxide and silicon

FBAR/BAW/MEMS

technologies

wasdemonstrated.

if handled improperly can damage delicate electronics ina transfer mechanism or result in a wafer breakage.

forSame trimming technology was applied to theSAW filter devices with slight hardware and processmodifications and showed great improvement of device
yield.

nitride are usually deposited in a PECVD system at lowtemperature.

Silicon nitride films have high andsomewhat variable hydrogen content.

Silicon dioxide

films have dangling Si-O bonds that interact with
ambient moisture [7]. We have experienced 50% etchrate variation on films deposited at different times on theREFERENCES[1] Mishin S, et al. “Sputtering Processes for Bulk Acoustic
Wave Filters”. Semiconductor International, 3/1/2003

same PECVD system. Films deposited at the same time[2] Frank Z. Bi and Bradley P. Barber “Bulk Acoustic wave
RF Technology”, 2008 IEEE

had less than 2% etch rate variation.

Trimming one[3] Sergey Mishin, Brian Sylvia and Daniel R. Marxwafer from each batch [8] to obtain the etch rate for a“Improving Manufacturability of AlN Deposition Used in

specific batch of wafers deposited at the same time,

Making Bulk Acoustic Wave Devices”, 2005 IEEE

produces excellent results. If tighter distribution isrequired, second trim can be performed as shown in
Fig.7 below.

ULTRASONICS SYMPOSIUM[4] K. Hashimoto “RF BULK ACOUSTIC WAVE FILTERSFOR COMMUNICATIONS”, pp.173-195[5] Robert Aigner “Corrective Actions to Meet ExtremeTolerance Requirements for Thin Films” 2007 OctagonCommunication Inc. DBA MPDigest[6] Sergey Mishin, Yury Oshmyansky and Frank Bi“Thickness Control by Ion Beam Milling in Acoustic
ResonatorDevices”,

2010

IEEE

FREQUENCYCONTROL SYMPOSIUM[7] M.J.Deen, et al. “Silicon Nitride and Silicon Dioxide
Thin Insulating Films”,Vol 97-10, pp.217-231[8] S.Mishin, et al. “Practical issues with ion beam milling inacoustic wave resonator technologies” IC NanoTech
2010

Figure 7. Two step-trimming procedure on SAW wafers

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