Production Issues in Using Silicon Dioxide Films for
Temperature Compensated Bulk and Surface
Acoustic Wave Devices
Sergey Mishin
Michael Gutkin
Advanced Modular Systems, Inc.
Goleta, CA/USA
smishin@amssb.com
mgutkin@amssb.com

Abstract— In this paper, production oriented aspects ofusing Silicon Dioxide (SiO2) films in manufacturing ofTemperature Compensated Bulk Acoustic Wave (BAW)/
Film Bulk Acoustic Resonator (FBAR) [1], [2] and Surface
Acoustic Wave (SAW) devices have been presented. SiO2
has been used to obtain low Temperature Coefficient(TempCo) in acoustic wave devices for more than threedecades [3]. One of the big issues is that depending on themethod of deposition and the amount of times SiO2 isexposed to the ambient environment [4], it can significantly
alter temperature compensating properties of the film as wellas etch rate in a thickness trimming process with focused Ion
Beam. Plasma Enhanced Chemical Vapor Deposition(PECVD), RF diode and RF magnetron depositions with insitu thickness trimming and capping layers were tested on thetemperature compensated FBAR and SAW structures.
Repeatability of the results was tested with different amountof time before processing steps. Best results were obtainedusing RF diode sputtered SiO2 with in-situ trimming process[5] and in-situ aluminum nitride (AlN) sputtered cappinglayer.
I.

INTRODUCTION

Tight control of TempCo over the entire wafer as well as
wafer to wafer is critical in manufacturing of the temperaturecompensated devices. This can be accomplished by:a) Controlling the thickness of deposited SiO2 film within
0.1% thickness uniformity,b) Tightly controlling TempCo of the deposited SiO2 film.

Both uniformity of the TempCo and trimming (etch) rateof SiO2, as well as frequency shift, can be significantly
impacted by a lengthy exposure to the ambient moisture.
Controlling the deposition process and sealing the SiO2 filmwith sputtered aluminum nitride (AlN) moisture barrier film isa practical way to reduce TempCo variation.
In this investigation we used ion mill based trimming tool
to control thickness and thickness uniformity of the SiO2. It
is very important to limit the time between deposition andtrimming of the SiO2 film in order to control trimming rate.
II.

EQUIPMENTIn this research we used Advanced Modular System's(AMSystems) three chamber cluster tool pictured in fig. 1 forthe deposition and trimming of all films except PECVD
deposited SiO2.
This system had one module with AlN deposition chamberthat uses reactive deposition process with dual magnetronwith AC power applied between two aluminum targets andAr/N2 process gasses. Second module is a trimming chamberwith a focused DC ion source with argon gas used for
improvement of SiO2 thickness uniformity. Third module
was configured as either RF diode or RF magnetron with a3KW RF power supply and argon/oxygen sputtering gases.
High purity SiO2 target was used on both sources.
PECVD reactor was used with RF power at 2KW andSiH4/Ar/O2 gases at 350C.
All etch rate tests were performed on 4” Si substrates andall electrical tests were performed on SAW type of devices on4” lithium-tantalite substrate or BAW type devices on Sisubstrate.

Figure 3. SiO2 Thickness uniformity after trimming process

Fig. 3 shows dramatic improvement of SiO2 film uniformity
across a wafer after trimming process. Good uniformity allows
to reduce and even to eliminate noise signal duringexperiments.
Figure 1. AMSystems cluster toolIII.

TRIMMING FBAR AND SAW DEVICES

We have found that trimming rate of SiO2 reflects film
properties and can vary depending on the time the wafer is
exposed in atmosphere after the deposition and before it istrimmed. Fig. 4 shows variation in the trimming Etch rates
depending on the method of deposition and time period
between deposition and trimming processes.

Thickness variation of SiO2 is a large source of afrequency variation for both FBAR and SAW devices.
Trimming process is used to control thickness variation ofSiO2 over a wafer and from wafer-to-wafer. Fig. 2 shows
typical improvement in thickness uniformity accomplished bya two-step trimming process.

Figure 4. Trimming rates as a function of deposition conditions and timeof atmosphere exposure

Figure 2. Wafer-to-wafer thickness repeatability for two step trimming
process

If trimming rate is well known for the given film,
uniformity and target thickness can be controlled very
precisely.

It can be seen that SiO2 sputtered film is somewhat lesssensitive to the exposure to the ambient moisture, but all films
have somewhat variable etch rates if exposed to the ambient
for a long time.
At the same time, dramatic variation of the magnetic field
during magnetron sputtering of SiO2, has little effect on theetch rate and the etch rate stability over lengthy exposure to
the ambient moisture, see fig. 5.

Figure 5. Etch rates as a function of magnetic field during magnetron
sputtering of SiO2 and ambient exposure time

Figure 7. Cross wafer TempCo variation as a function of deposition
method and capping process

The most consistent trimming rate is obtained by trimming
wafers in-situ right after the deposition without vacuum break.
In this case, trimming etch rate is extremely stable from waferto-wafer and within wafer.
We also observed changes in frequency of SAW devices
during long exposure to the ambient humidity. It is moreevident after trimming process reduces variation across awafer. Fig. 6 shows relative frequency and frequency variation
across a wafer after SiO2 deposition, after trimming process
and after 15 and 30 days exposure to atmosphere with elevatedhumidity.

Figure 8. TempCo variation for different SiO2 preparation

There is an additional option to stabilize properties of
SiO2 films. We found that annealing wafers in a steam
environment at ambient (atmospheric) pressure for about
thirty minutes and then baking them for about 1 hour at 250C
stabilizes etch rate in the trimming module.
Treated wafers have also shown almost no change in either
frequency or etch rate in trimming after a month. Fig. 9 shows
the impact of exposure to high humidity at the elevatedtemperature and the following bake out on the TempCo ofSiO2Figure 6. Average Frequency and Cross wafer Frequency variation as afunction of time exposure to atmosphereIV.

METHODS FOR IMPROVING TEMPCO CONTROL

There is a large variation in contribution of SiO2 toTempCo due to the method of deposition and the consequent
treatment. Fig. 7 and 8 show the impact of the deposition
method and the use of the capping layer on TempCo and its
variation across a wafer.

Figure 9. Average TempCo and TempCo uniformity across a wafer as afunction of humidity and baking treatments after the deposition

It is likely that at low temperatures, that is required for
SAW processing, SiO2 has a lot of strained Si-O bonds. Thesebonds are likely to interact with moisture and may form
silanols (Si-OH bonds). The newly formed bonds tend to be
somewhat unstable leading to variation in all parameters, such
as TempCo, frequency and trimming etch rate. Forcing thesebonds to react with the excess of moisture at elevatedtemperature will cause the maximum reaction as well as
moisture absorption. The following anneal at 250C drives off
the unstable or un-reacted moisture or hydrogen leaving astable matrix of Si-O bonds. Resulting material stays stable
for long periods of time and provides stable trimming rates
and uniform and repeatable TemCo, as well as frequency andfrequency uniformity. Fig. 10 shows frequency and frequency
variation across a wafer after humidity/bake treatment overtime.

Figure 10. Frequency and frequency variation across a wafer afterhumidity/bake treatment over a time periodV.

SUMMARY

When using SiO2 for temperature compensation, the mostfavorable approach is to deposit a capping layer, such as thin
AlN film, to prevent interactions of Si-O bonds in the filmwith the ambient moisture. If such process is not practical,
then using steam treatment at elevated temperature followed
by a bake can stabilize strained Si-O bonds and make themless sensitive to being exposed to the ambient moisture.
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