Thickness Control by Ion Beam Milling in Acoustic Resonator Devices
Sergey Mishin

Yury Oshmyansky and Frank Bi

AMSystems, Inc.
Goleta, CA, USAs.mishin.ams@cox.net

Avago Technologies
Fort Collins, CO,USA

Abstract—In this paper, practical aspects of production
worthy methods for film uniformity adjustment (trimming)
used in manufacturing of Film Bulk Acoustic Resonator(FBAR) filters [1], [2] have been presented. Two-step
trimming in conjunction with thickness “smoothing”
technique control total thickness range to within less than 8A
on product wafers with variable surface film etch rates evenwith difficult to measure film thickness. Trimming processes
were used to allow using one wafer from a batch to provide
compensation feedback in the FBAR devices. Combining ion
mill with deposition in the same tool produces <0.1%
uniformity in the deposited films.

I.

INTRODUCTION

As performance FBAR filters have continued to improve,it became desirable to be able to control thicknesses of thefilms used in these technologies to well below 0.1%
uniformity. A number of different deposition systems areused in making filter in commercially viable environment.
Majority of these machines provide film uniformities that are
an order of magnitude worse than acceptable. It is impracticalto improve film thickness uniformity as deposited below
0.5%, but relatively easy to improve the thickness uniformity
after the deposition. The range of use of FBAR technology has
been extended greatly by developing production worthy
trimming methodology. In the last decade a new techniqueusing ion beam based machines to improve uniformity of thefilms was adapted in making BAW (bulk acoustic wave)
devices [3]. Ion beam milling techniques that have been usedeffectively to improve uniformity in making of optics andtuning of the quartz crystals were successfully adapted to theFBAR, BAW and SAW (surface acoustic wave) devices tomeet frequency control requirements. Use of trimming forthese technologies presented a new set of issues that have tobe addressed:(i) Ion source has to be optimized: beam size of the ion
source must be small enough to allow uniformity correction offilms that have large thickness gradient, but large enough toprovide high wafer throughput. Due to the small beam size, redeposition of the removed film on the wafers surface canresult in degraded electrical performance of the FBAR filters.
Etch rate cross-wafer uniformity and repeatability over time
can result in 30A to 50A variation in thickness from thelowest to the highest thickness in a batch of wafers. For theacoustic resonator application <20A variation of film
thickness is desired.

profiles have large anomalies that cause problems fortrimming.
(iii) FBAR product wafers are usually exposed to photoresist or chemical and plasma treatments in the course ofprocessing. This frequently leads to the significantly different
etch rates of the surface layer compared to the bulk material aswell as erroneous thickness measurements that lead tosignificantly different device performance at final test.
(iv) Film characteristics change over time due to thechanges in the deposition tool conditions. This also leads tovariation of device performance at the final test.
In this paper, an extremely practical approach to trimmingis described. A standard production tool was used to trim
variety of materials. Different techniques are described forproduction and development needs of thickness trimming
technology.

II.

EQUIPMENTIn this investigation we used Advanced Modular Systems
cluster tool with three modules: two aluminum nitride PVD
deposition modules and ion beam trimming module shown in
Figure1.

Figure 1: AMSystems cluster tool

PVD deposition uses a dual conical magnetron with AC
power supply. It is a reactive deposition using aluminumtarget and argon and nitrogen process gasses. Trimming(ii) Uniformity maps are not always smooth and module uses DC source with argon processing gas. Wafer iscontinuous. Sometimes because either deposition equipment moved by linear drive above the source at constant speed. The
limitations or measurement equipment accuracy, thickness source is moved in the direction perpendicular to the wafer
motion by another linear drive. Power of the source isadjusted by the system software based on the wafer thickness
2010 IEEE International Frequency Control Symposiumuniformity map. Ion source has a beam size of about 10mm
FWHM. Figure 2 shows etch pattern from the single scan of
the ion source used in this investigation.
III.

WAFER PROCESSING

The basic trimming process involves optimizing trimming
module process parameters and getting a good thickness
uniformity map. Several key techniques that produce
excellent results will be discussed in this section.
450400350300etch rate

250200trimming into two passes under the ion source, reducing redeposition in half. Second, we perform first trim to about 80
to 90% of the total required removal. Second trimming step
removes the remaining material, further, reducing redeposition by a factor of 5 to 10. Thus, total re-deposition isreduced by factor of 10 to 20. This technique completelyeliminates Rp degradation problem related to the trimming
process. Using two pass process also improves uniformity oftrimming rate across wafer. When ion source warms up, etchrate increases slightly. In order to keep constant trimming
rate, it is necessary to warm-up source to a steady state
temperature. But even without warm-up two-step trimming
produces only 2-3% variation of etch rate across wafer.
In this investigation, milling was accomplished using
constant speed of wafer and source movement and variable
power. The advantage of varying power over varying speedof wafer movement comes in play when the gradient of thethickness changes very sharply. It is much faster to adjust
power than to mechanically adjust speed of wafer motion [4].
It is even possible to remove close to zero amount of material
at zero power. It is extremely difficult to adjust wafer speed
to accomplish the same result.

150B.
100

500-20-15-10-505

101520distance, mm

Figure 2: Shape of the ion beamA.

Trimming module optimization

The ion source used in the trimming process can havebeam size between sub mm to 20 mm diameter. The smaller
size provides ability to improve uniformity on wafers withlarge gradient of thickness. The larger beam diameterincreases average etch rate. In this investigation we foundthat for 150mm wafers with between 10,000 and 70,000 dice,the beam diameter between 5 and 10mm is most productive.
Because of the re-deposition of the trimmed material on
the ion source, it is important to re-calibrate the source
periodically. We found that performing one calibration per
cassette of wafers is more than sufficient. The calibration
takes care of both changing source condition as well as otherchanges to the system over period of time. Re-deposition ona wafer is a common issue in the small beam trimming
equipment. We found that our Rp (parallel resistance)
degrades as much as 20% after ion standard trimming. The
re-deposited material can be cleaned up with a short sputter
clean in another machine or the trimming process can beadjusted to minimize re-deposition problem. First, we split

Wafer map processing

Basic trimming requires a thickness uniformity map.
Minimum of 25 points are required for a 6” wafer. Using as
many points as practical improves accuracy of trimming.
Trimming tool converts data into a 10,000 point grid thatextrapolates thicknesses between points.
Thickness
uniformity maps can used to improve film uniformity bymore than twenty times. This performance is easily achievedon fresh and clean test wafers for both film uniformity andfilm target thickness.
Unfortunately, on patterned wafers uniformity maps
sometimes tend to have a lot of thickness discontinuities.
Using a software program such as MatlabTM , data can be“smoothed” into a profile that can be easily trimmed.. An
example of raw and “smoothed” data from the same wafer isshown in the Figure 3.

Figure 3: Raw and “smoothed” data2010 IEEE International Frequency Control Symposium

IV.

TWO-STEP TRIMMING

1350

141300121250101200Thickness before trimming

8Thickness after trimming
Uniformity before trimming

1150

6Uniformity after trimming
1100

41050

2

1000

01

2

3

45

6

78Wafer ID

Table 1: Etch rates of materials on the AMSystems
trimming tool
Film uniformity, one sigma, AFilm Thickness, A

Two-step trimming is beneficial not only in reduction ofthe re-deposition but also to the accuracy of the trimming.
By trimming 80 to 90% of the desired thickness in the firststep and the remainder the second step, it is fairly easy tocompensate for most of the systematic errors associated withtrimming hardware. An example of repeatability andaccuracy of trimming is shown in Figure 4.

Pt
7875
Al2O3
1365-3000
CoNiFe
5278
Cu
8750
Ru
5300-7800
Fe4375FeO
6125
SiC
2650-13650
Si
3500
Si3N43000
Note: This table uses ~40mA Ion Beam Current

9 10 11 12 13

Target removal for aluminum is typically 200A. Whentargeting 200A removal based on the bulk aluminum etchrate, we typically remove 160A to 180A during the firsttrimming. The second trimming is done within a couple
hours targeting 20A to 50A and assuming less than 5A
aluminum oxide on the surface. Typical results are within 5A
to 10A of the desired target. This is significantly better than
50A after one trim process.
When product wafers go through patterning process,small amounts of photo-resist can be trapped on the surface.
Reflectometers can measure these areas incorrectly, see
figure 5.

Figure 4: Silicon dioxide repeatability test with twostep trimming.
Two-step trimming is also effective in compensating forthe problems with surface condition and measurement issues.
One of the best illustrations of the benefit from two-step
trimming is Aluminum trimming for SAW filters. Aluminum
forms a surface oxide that can vary from 5A to 15A,
depending on the deposition technique and the amount oftime wafers sit before trimming [3]. Aluminum oxide etches
between 2 to 4 times slower than the bulk aluminum. Etchrates of different materials trimmed in the AMSystems
trimming tool are listed in the Table 1.
MaterialAgAl
NiNiFeAuAlNMo
SiO2
WCrRate (Ang/min.)
17500
6125
5250
3500
15750
1995
10725
4400-5200
6825
3518

Figure 5: The same wafer, film thickness measurement
before and after photolithography process.
Splitting trimming into two steps, 1st trimming 40A lessthan the final target thickness, 2nd trimming 0A to 40A
produces the best results. 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 trimming2010 IEEE International Frequency Control Symposiumremoves the surface that distorts reflectometers results andgets thickness close to the desired target. Second trimming
makes fine adjustment on a cleaned surface. In order toperform second trimming, ion beam has to remove between
0A and 100A. Setting power to zero at the thinnest locationon the wafer produces precise second trimming [4] withminimum loss of material.

days. Based on the measurements collected on this wafer,
SiO2 is trimmed by 0A to 50A to obtain correct TCF on theentire batch.
7060

ONE TOOL DEPOSITION/TRIMMINGVI.

SEND AHEAD WAFER PROCESSING

Deposited films sometimes have different film
characteristics due to the changes in deposition machineconditions. It is only at the final test that these differences
can be detected. It is fairly easy to process one wafer fromthe batch from the deposition step all the way to the final test.
Then adjust deposition based on the data from this “sendahead wafer” and finish the rest of the batch. One suchapplication is silicon dioxide layer used in the bulk acousticresonator technology. This film is used to adjust TCF(temperature coefficient of frequency) of the device. The
amount of temperature compensation per angstrom of SiO2
during a particular deposition is usually consistent within adeposition batch but can vary batch to batch. All wafers in abatch (usually 25 wafers) have SiO2 deposition at the sametime. One wafer is sent-ahead and is finished in couple of50
as deposited40
after the first trim based on
single monitor wafer map

30

after the second trim, basedon individual maps

201023211917151391175301

In a standard FBAR processing, deposition and trimmingare performed in two different systems. When the deposition
tool and trimming tool are combined into a single cluster tool,
it is much more cost effective and saves processing time sincein a cluster tool wafers can be processed simultaneously inboth deposition and trimming modules.
For example, in the FBAR aluminum nitride deposition,
taking a map of the first wafer and using it in a trimming tool
for all of the product wafers improves uniformity by factor of
3 to 5 times. Total processing time for 25 wafer cassette is 12
hours for deposition only and 12.5 hours for deposition andtrimming.
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 andperforming second trim produces less than 5A standard
deviation uniformity required for tight frequency control of
the FBAR devices. Figure 6 demonstrates typical results
obtained in the AMSystems cluster tool containing aluminum
nitride deposition and trimming module on 10,000A films.

U n ifo rm ity, o n e sig m a , AV.

wafer

Figure 6: Deposition and trimming in the same toolVII. 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
necessaryin
SAW/FBAR/BAW
technology
wasdemonstrated.
REFERENCES
[1]

[2]
[3]

[4]

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.
Frank Z. Bi and Bradley P. Barber “Bulk Acoustic wave RFTechnology”, 2008 IEEE
Robert Aigner “Corrective Actions to Meet Extreme Tolerance
Requirements for Thin Films” 2007 Octagon Communication Inc. DBA
MPDigest
Sergey Mishin, “Method and apparatus for film thickness adjustment”,
US Patent Application 200902363

2010 IEEE International Frequency Control Symposium