Improving frequency Control of Temperature
Compensated Surface Acoustic Wave Devices
Sergey Mishin1, Michael Gutkin1, Alexander Bizyukov2 , Vladimir Sleptsov3
1

Advanced Modular Systems, Inc, Goleta, CA/USA, smishin@amssb.com
2Karazin Kharkiv National University, Kharkiv, Ukraine
3Moscow State Aviation Technological University Moscow, Russia
Email: smishin@amssb.com; mgutkin@amssb.com

Abstract—In this paper, we demonstrated improvement offrequency uniformity and stability for Temperature
Compensated Surface Acoustic Wave (SAW) devices. SiO2 hasbeen used to obtain low Temperature Coefficient (TempCo) inSAW devices for more than three decades [1]. One of the bigissues is that SAW devices have to be processed at temperatures
below 300C. When low temperature SiO2 is exposed to theambient environment, it interacts with ambient humidity [2], [3],[5], [6].
Such interactions can change frequency of the SAW
devices and can make frequency trimming with a focused Ion
Beam [7], [8] extremely challenging. UV and steam treatment of
SiO2 improved trimming rate stability on the first trimming [4]and [9], but was not sufficient to provide tight frequency control
required for SAW devices after the second trimming. Usingsilicon nitride (Si3N4) capping layer on top of SiO2 showed some
improvement in frequency control after trimming process. Most
improvement was obtained using aluminum nitride (AlN)
capping layer on top of SiO2 followed by two trimming steps.
Keywords-SAW, Temperature, coefficient, trimming. frequencyI.

INTRODUCTION

Controlling frequency of temperature compensated devices
is critical in obtaining high yielding production process. The
use of SiO2 films has been recently used to improve behaviorof SAW filters over the required temperature range.
Unfortunately, the diffusion of the moisture into SiO2 film
during exposure of the wafers to the ambient environment
causes the reaction of H20 with strained Si-O bonds andformation of silanol (Si-OH). This has a great effect on thelocal intrinsic SiO2 film stress and leads to the reduction ofcompressive residual stress and progressive increase of thetensile residual film stress.
With above mentioned effect, it is impossible to obtainyield above 90% without use of frequency trimming. In thispaper we address the issues with obtaining best results througha judicious use of both deposition and trimming techniques.
II.

EQUIPMENTIn this research, we used Advanced Modular Systems’(AMSystems) three chamber cluster tool pictured in Figure 1
for deposition of all layers as well as frequency trimming.

SiO2 was deposited using RF diode mode from SiO2
target. We used 12” target at 1kW of RF power and frequency
13.56 MHz. Only Ar process gas had been added during SiO2deposition.
Si3N4 film was deposited by dual AC magnetron fromhighly doped - highly conductive Si target. 40kHz AC power at
1kW had been applied between two Si targets. Ar and N2 wereadded as process gases during deposition. Deposition was donein poisoned nitride mode to get a stable and stoichiometric
Si3N4 film.
Standard process for AlN deposition was used with dual
AC magnetron. AC power applied between targets in areactive deposition mode.
Standard Trimming process was performed on thedeposited films. Process is established by removing film
thickness of the top layer, based on frequency maps using anadjustable focused DC ion source and scanning wafer
mechanism. Trimming process was used to get high yield ofdevices from wafer.
Figure 1: AMSystems Cluster ToolIII.

METHODS FOR IMPROVING TEMPCO CONTROLFor several decades SAW device manufacturers attempted
to utilize SiO2. Unfortunately, low temperature oxide
depositions necessary for the SAW applications, combinedwith a need for low cost equipment, limited options available
to the SAW manufacturers. Over decades many methods havebeen tried in order to reduce variation of frequency due to theinherent oxide interactions with ambient moisture. In thispaper we have compared few methods of protection andsurface modification of the silicon dioxide films in order toprevent the physical adsorption of water vapor molecules on
the surface of silicon dioxide film and reaction between Si-O
bonds with H2O molecules with Si-OH formation.
To prevent physical adsorption of water vapor on Silicon
dioxide film we used two different cap layers: AlN and Si3N4
films. Both films have good passivation properties, stable andhave high grain density. 500A of Si3N4 or AlN films were
deposited on top of SiO2 films without vacuum break. 500A
was chosen to make sure that after trimming process at least
200A of the film is still remaining.

Figure 3: Cross wafer Frequency variation as a function oftime exposure to atmosphere for different methods of SiO2
treatments and capping processes

For SiO2 surface modification we used two methods:
1.

High power density UV radiation

2.

Annealing of SiO2 film in high humidity environment

High power density UV radiation was done by scanningwafer under excimer laser radiation. XeCl excimer laser withwavelength 308nm and power density greater than 10MW/cm2
per pulse was used for SiO2 surface modification. Surface
densification under this process conditions prevented diffusionof moisture into SiO2 grain bonds.
Annealing of SiO2 film in high humidity was done at thetemperatures between 250…300C in water stem with post
annealing and then, drying. This process allows saturating all
available Si-O bonds and eliminating further changes under
exposure to the ambient.
Figures 2 and 3 show improvements obtained using
different methods commonly used to improve frequency
control in temperature compensated SAW devices.
Figure 2: Average Frequency as a function of time
exposure to atmosphere for different methods of SiO2
treatments and capping processes

The most successful method as we observed is to cap SiO2
with AlN without air exposure and trim it with focused ionbeam. Even though this technique is adequate for a lot ofdevices, there is a further benefit in performing a second trim.
Figure 4 shows the benefits of the two steps trimming vs. one
step trimming.
Figure 4:
Two step trimming on an AlN capped
temperature compensated SAW device

REFERENCES
[1]

[2]

[3]

[4]

[5]

[6]

Depending on the initial frequency distribution, the firsttrim can have 5X improvement. Second trim provides another
4X improvement over the first trim. As the result the totalfrequency improvement can be as big as 20 times.

IV.

[7]

Willingham, C.B., et.al., “Temperature Compensated Surface Wave
Devices”, United States Patent #3965444I. S. Jacobs and C. P. Bean,“Fine particles, thin films and exchange anisotropy,” in Magnetism, vol.
III, G. T. Rado and H. Suhl, Eds. New York: Academic, 1963, pp. 271–350.
H.A.Naseem, et.al., “Stress and bonding characterization of PECVD
silicon dioxide films”,Electrochemical Society, Vol.97-10, pp.217226,4-9 May,1997
Leplan H., “Ressidual stresses in evaporated silicon dioxide thin films:
Correlation with deposition parameters and aging behavior”, Journal ofApplied Physics, pp.962-968, 1995
M.S. Haque, et.al., “Post-deposition processing of low temperature
PECVD silicon dioxide films for enhanced stress stability”, Thin SolidFilms, Volumes 308-309, 1997, pp. 68-73
Leplan H., “Kinetics of residual stress evolution in evaporated silicon
dioxide films exposed to room air”, Journal of Applied Physics,
pp.6926-6931, 1996
Parada E. G., “Aging of photochemical vapor deposited silicon oxidethin films”, Journal of Vacuum Science &Technology, Volume 14,
pp.436-440
Quiang Zou, et.al., “High Coupling Coefficient Temperature
Compensated FBAR Rsonator for Oscillator Apllication with
Wide Pulling Range”, 2010 IEEE International Frequency Control
Symposium, pp. 646-650[8]S. Mishin, “Improving Manufacturability of Bulk Acoustic Wave andSurface Acoustic Wave Devices”, SPAWDA 2011, IEEE[9]S. Mishin, et.al., “Production Issues in Using Silicon Dioxide Films for
Temperature Compensated Bulk and Surface Acoustic Wave Devices”,
2012 IEEE International Frequency Control Symposium

SUMMARY

Capping SiO2 temperature compensated layer with AlN
film and using two steps trimming can provide high yielding
SAW devices for the tough requirements for today’s RF filter
needs.