Practical issues with ion beam milling in acoustic wave resonator
technologies
S. Mishin*, Y. Oshmyansky** and F. Bi****AMSystems Inc., 354 Fairview Av, Goleta, CA, USA, ph. 805-692-0912, s.mishin.ams@cox.net
Avago Technologies, San Jose, CA, ph. 805-889-4889, yury.oshmyansky@avagotech.com***
Avago Technologies, Fort Collins, CO, ph.970-288-9139, frank.bi@avagotech.com**ABSTRACT
Innovative techniques were proven effective inresolving common issues with thickness adjustment by ion
beam milling (also known as trimming) [1]. Two-step ion
milling offers a highly effective technique for overcoming
issues associated with surface oxide in Al, Co, Ni, Fe alloy,Ru and other film’s trimming. Films with slow etchingoxides were trimmed to within less than 5A from desired
thickness and less than 10A total range across the substrate.
Uniformity of aluminum nitride (AlN) and silicon dioxide(SiO2) films were improved to less than 8A (total range)
using two-step milling. Trimming processes were used toallow using one wafer from a batch to provide
compensation feedback in the Bulk Acoustic Wave (BAW)
devices [2,3]. Combining ion mill with deposition in thesame tool has been proven practical in production and R&Denvironments. In a standard aluminum nitride deposition
with in-situ trimming, thickness uniformity of less than
0.1% was demonstrated.
Keywords: trimming, BAW, milling, thickness, uniformity

1INTRODUCTION

In the recent years, the size of the cellular phone wasreduced dramatically in part due to the propagation of
extremely small and high performance filters. These filters
are frequently using either BAW or SAW (SurfaceAcoustic Wave) technologies. As the performance of thesefilters has continued to improve, it became desirable to beable to control thicknesses of the films used in thesetechnologies to below 0.1% uniformity. A number ofdifferent deposition systems are used in making filter incommercially viable environment. Majority of thesemachines provide film uniformities that are an order ofmagnitude worse than acceptable. It is impractical toimprove film thickness uniformity as deposited below
0.5%, but relatively easy to improve the thickness
uniformity after the deposition. In the last decade several
companies used ion beam based machines to improve
uniformity of the films. Beam sizes varied from submillimeter to tens of millimeters.
Eventually, mostcommercial machines chose the beam size between 5 and10mm. Much smaller size tended to reduce the speed ofprocessing, larger size reduced the accuracy of thetrimming. Ion beam milling techniques that have been usedeffectively to improve uniformity in making of optics andtuning of the quartz crystals were successfully adapted tothe BAW and SAW to meet frequency control
requirements. After the trimming tools becamecommercially available, other industries started adapting
trimming equipment to control thickness in the applications
like disc drive magnetic heads.
Initial results enabled commercial use of trimming
machines, but as the requirements got tighter and thetrimming started being used for different applications thefollowing issues became critical:(i) Patterned product wafers don’t always behave as
well as test wafers. Periodically, product wafers can sit
around before ion mill trimming for anywhere between few
minutes to several days. They are also sometimes exposed
to photo-resist or chemical and plasma treatments in thecourse of processing. This frequently leads to thesignificantly different etch rates of the surface layercompared to the bulk material.
(ii) Another manufacturing issue is that depositedmaterials can have variable surface properties (such asindex of refraction, for example) that thickness
measurement tools don’t catch, leading to significantly
different device performance at final test.
(iii) Most ion beam trimming systems have an etch rateuniformity and repeatability limitations. Etch rateuniformity, for the ion mill trimming tool used in thisinvestigation, is about 1-3% across wafer. Wafer-to-wafer
repeatability is about 1-3%. Over the life of the chamberclean cycle etch rate can change as much as 10%. Because,
typical trimming is between 100A and 500A, unlessfrequent etch rate tests are performed, it would not be
uncommon to have 30A to 50A variation in final thickness
from the lowest to the highest thickness in a batch ofwafers. For the acoustic resonator application <20A
variation of film thickness is desired.
(iv) Uniformity maps are not always smooth andcontinuous. Sometimes, because either deposition
equipment limitations or measurement equipment accuracy,
thickness profiles have large anomalies that cause problems
for trimming. Most common problem is a large thickness
gradient across small portions of the substrate. Other times,
measuring equipment algorithms have problems withspecific films or film thicknesses.
(v) Some film characteristics change over time and thefinal product has electrical characteristics that can’t be fixedat the point the devices are measured.

In this paper, an extremely practical approach totrimming is described. A standard production tool wasused to trim variety of materials. Different techniques aredescribed for production and development use of thickness
trimming technology.

2450400350

EQUIPMENT300250

etch rate

In this investigation we used Advanced Modular
Systems cluster tool with three modules: two aluminum
nitride PVD deposition modules and ion beam trimming
module [4], shown in Figure 1. PVD deposition uses a dual
conical magnetron with AC power supply. It is a reactive
deposition using aluminum target and argon and nitrogenprocess gasses. Trimming module uses DC source with

200150100500-20-15-10-505101520distance, mm

Figure 2: Etch profile of AMSystems ion mill beam
Electrical characteristics of the trimming source are shownin the Figure 3.
250

Current, m A

200150100

Figure 1: AMSystems cluster tool
argon processing gas [4]. Wafer is moved by linear drive
above the source at constant speed. The source is moved inthe direction perpendicular to the wafer motion by anotherlinear drive. Power of the source is adjusted by the system
software based on the wafer thickness uniformity map.
Beam shape of the source used in this investigation isshown in Figure 2. The size of the beam is about 10mmFWHM.

50Voltage, Kv

000.5

1

1.5

2

2.5

3

3.5

4

Figure 3: V-I characteristics of the DC trimming source

3DATA PROCESSING

Thickness uniformity maps are used to improve film
uniformity by more than twenty times. This performance iseasily achieved on fresh and clean test wafers for both film
uniformity and film target thickness. It is also important tobe able not only to improve cross wafer uniformity but also
to hit the desired target. When thickness is measured on aclean un-patterned monitor wafer it is very easy to getsmooth, continuous uniformity thickness map using a single
pass trimming. Unfortunately, on patterned product wafers
uniformity maps sometimes tend to have a lot of thickness

discontinuities.
Using a software program such as
MatlabTM, data can be “smoothed or filtered” into a profilethat can be easily used for trimming. An example of raw
and “smoothed” data from the same wafer is shown in
Figure 4.

Figure 4: Raw and “smoothed” data

4TWO-STEP TRIMMING

the material. Reflectometers work well on the clean
test/monitor wafers. When product wafers go throughMaterialRate (Ang/min.)Ag
17500
Al6125
Ni5250
NiFe
3500
Au
15750
AlN
1995
Mo
10725
SiO2
4400-5200
W
6825
Cr
3518
Pt
7875
Al2O3
1365-3000
CoNiFe5278
Cu8750Ru
5300-7800
Fe4375FeO
6125
SiC
2650-13650
Si3500
Si3N43000
Note: This table uses ~40mA Ion Beam Current
Table 1: Etch rates of materials on the AMSystems ion
beam trimming tool

Another way to improve the final distribution ofthickness is to trim about 80 to 90% of the total thickness
during the first trim, measure the thickness or anotherthickness dependant parameter again and then finish theremaining material in the second trim. First trimming isdesigned to improve uniformity by factor of 4 to 8 and get
all dice within 50 to 100A of the final target. The second
trimming is done after frequency or thickness is remeasured and gets all dice within <10A of the desired
thickness target. One of the best illustrations of the benefit
from two-step trimming is Aluminum trimming for Surface
Acoustic Wave (SAW) filters. Aluminum forms a surface
oxide that can vary from 5A to 15A, depending on thedeposition technique and the amount of time wafers sit
before trimming [1]. Aluminum oxide etches between 2 to
4 times slower than the bulk aluminum. Target removal foraluminum is typically 200A. When targeting 200A removal
based on the bulk aluminum etch rate, we remove about
160A to 180A during the first trimming. The second
trimming is done within a couple hours targeting 20A to50A and assuming less than 5A aluminum oxide on thesurface. Typical results are within 5A to 10A of the desired
target. This is significantly better than 50A after one trimprocess. Etch rates of different materials trimmed in theAMSystems trimming tool are listed in the Table 1.

patterning process, small amounts of photo-resist can betrapped on the surface. Reflectometers can measure theseareas incorrectly. We split trimming into two steps, 1sttrimming 40A less than the final target thickness, 2ndtrimming 0A to 40A. Because the amount of surface
contamination is variable both wafer-to-wafer and acrosswafer, the one trim process can produce variableimprovement in the thickness uniformity.
Two-step
trimming removes the surface that distorts reflectometer
results and gets thickness close to the desired target.
Second trimming makes fine adjustment on a cleaned
surface. In order to perform second trimming, ion beam has
to remove between 0A and 100A.
Trimming wasaccomplished by moving wafer and source at constant
speed and adjusting the power. Setting power to zero at thethinnest location on the wafer produces precise second
trimming [4].

In the case of SiO2 films, the final thickness uniformity
and wafer-to-wafer repeatability is mostly limited by theaccuracy of the thickness measurement. Because second
trim removes between 0 and 100A, the final thickness
variation is <10A. Reflectometers are commonly used tomeasure thickness of dielectric films. These machines arefast and can measure up to 100 points per minute. The
drawback is that they don’t measure index of refraction ofAnother common problem encountered in trimming is alarge thickness gradient or simply a large change ofthickness over a small distance.
Figure 5 shows
improvement in uniformity and wafer-to-wafer repeatability
on two-step trimming of SiO2 wafers with a large initial
gradient. It can be seen that both wafer to wafer uniformity
as well as within wafer uniformity are significantly
improved.

1300

12125010Thickness before trimming

8Thickness after trimming
Uniformity before trimming

6Uniformity after trimming
1100

41050

21000

0

100

1

Figure 5: Silicon dioxide repeatability test

23Wafer ID

9 10 11 12 1321819717

615

520

13

4

after the second trim, basedon individual maps

93after the first trim based on
single monitor wafer map

30

112

as deposited

40

7

150

51150

60

31200

70

U n ifo rm ity, o n e sig m a , A

14Film uniformity, one sigma, AFilm Thickness, A

1350

wafer

5

ONE TOOL DEPOSITION/TRIMMING
Figure 6: Deposition and trimming in the same tool

In a standard BAW or SAW processes, deposition andtrimming are performed in two different systems. Whenthe deposition system and trimming system are combined
into a single cluster tool, it is much more cost effective andsaves processing time since in a cluster tool wafers can beprocessed simultaneously in both deposition and trimming
modules.
For example, in a BAW process aluminum nitride
deposition can be continuously adjusted through the target
life. Taking a map of the first wafer and using it in atrimming tool for all of the product wafers improves
uniformity by factor of 3 to 5 times. Because wafer
uniformity is not identical over the life of the target, second
trim is important for the precise thickness control.
Measuring wafers after deposition/trim and performing
second trim produces less than 5A standard deviation
uniformity required for tight frequency control of the BAWdevices. Figure 6 demonstrates typical results obtained in
the AMSystems cluster tool containing aluminum nitride
deposition and trimming module.

6SEND-AHEAD PROCESSING

Sometimes, due to the variations in the criticaldepositions, final product wafers have different
characteristics than desired. It is possible to process one
wafer from a batch at the critical step process, finish it,measure it; adjust the rest of the wafers based on the final
results from the send-ahead wafer. One such application issilicon dioxide layer used in the bulk acoustic resonator
technology. This film is used to adjust temperature
coefficient of frequency of the device. The amount oftemperature compensation per angstrom of SiO2 during aparticular deposition is usually consistent within adeposition batch but can vary over period of time. Allwafers in a batch (usually 25 wafers) have SiO2 depositionat the same time. One wafer is sent-ahead and is finished incouple of days. Based on the measurements collected on
this wafer, SiO2 is trimmed by 0A to 50A. It is important
to trim all layers, including electrodes and piezoelectric
film, for the best results. If all layers on all wafers havemaximum of <20A thickness variation from each other, thistechnique is very effective. If wafers have a lot ofvariations from the send-ahead wafer, this technique ismuch less effective.

7

SUMMARY

A practical approach to addressing production issues ofthickness trimming was demonstrated through data“smoothing”, two-step trimming, send-ahead wafer, anddeposition/trimming cluster tool.
Thickness control
necessary in SAW/BAW technology was demonstrated.

8REFERENCES

1. Robert Aigner “Corrective Actions to Meet Extreme
Tolerance Requirements for Thin Films” 2007 Octagon
Communication Inc. DBA MPDigest
2. Frank Z. Bi and Bradley P. Barber “Bulk Acoustic
wave RF Technology”, 2008 IEEE
3. R. Ruby, P. Bradley, J.D. Larson III, Y. Oshmyansky,"PCS 1900 MHz duplexer using thin film bulk acoustic
resonators (FBARs)," Electronics Letters, vol. 35, no. 10,
pp. 794- 795, 13 May 1999.
4. Sergey Mishin, “Method and apparatus for film
thickness adjustment”, US Patent Application 200902363