NUKLEONIKA 2015;60(2):327330
doi: 10.1515/nuka-2015-0059

ORIGINAL PAPER

Operation modes of the FALCON
ion source as a part of the AMS cluster
tool

Oleksii Girka,
Alexander Bizyukov,Ivan Bizyukov,
Michael Gutkin,
Sergei Mishin

Abstract. The paper investigates the options to increase the production yield of temperature compensated surface
acoustic wave (SAW) devices with a defined range of operational frequencies. The paper focuses on the preparation of large wafers with SiO2 and AlN/Si3N4 depositions. Stability of the intermediate SiO2 layer is achieved bycombining high power density UV radiation with annealing in high humidity environment. A uniform thicknessof the capping AlN layer is achieved by local high-rate etching with a focused ion beam emitted by the FALCON
ion source. Operation parameters and limitations of the etching process are discussed.
Key words: FALCON • ion source • surface wave • cluster tool

Introduction

O. Girka, A. Bizyukov, I. Bizyukov
V. N. Karazin Kharkiv National University,
4 Svobody Sq., 61022, Kharkiv, Ukraine,
E-mail: oleksiigirka@karazin.ua
M. Gutkin
Micron Surface Technologies,
5033 Dantes View Drive, Calabasas, CA 91301,
United States
S. Mishin
Advanced Modular Systems, Inc,
354 South Fairview Ave., B1, Goleta, CA 93117,
United States
Received: 19 May 2014
Accepted: 5 May 2015

Improvement of frequency uniformity and stability
for surface acoustic wave (SAW) devices [1] is an
important technological task, a solution of which requires advanced methods. Currently, SAW devices arewidely used in mobile communication devices as filters based on converting electrical signals to acousticwaves and vice versa by exploiting the piezoelectric effect. These devices are fabricated by photolithography.
However, the initial stage of manufacturing SAW
devices requires large Si wafers to be covered witha SiO2 intermediate layer and a capping layer consisting of silicon nitride (Si3N4) or aluminum nitride(AlN), see Fig. 1. The thickness of the capping layer
and its bulk properties define the frequency of theSAW device. It is highly desirable to keep the scattering of operational frequencies of SAW devices,
obtained during the manufacturing process, as low
as possible to maximize the frequency uniformity.
Also, the intermediate SiO2 layer should stabilize
the initial properties of the capping layer.
SiO2 has been used to obtain a low temperature
coefficient (TempCo) in SAW devices for more than
three decades [2, 3]. One of the big issues is thatSAW devices have to be processed at temperatures
below 300C. When SiO2 is exposed to air at low
temperature during the manufacturing process, thefrequency of the SAW devices may change [4]. Diffusion of the absorbed water molecules into the SiO2layer causes a reaction of H2O with strained Si-O
bonds and formation of silanol (Si-OH). This has
a big impact on the local intrinsic SiO2 layer stress,
leads to the reduction of compressive residual stress328Fig. 1. A schematic cross section of the wafer with depositions before manufacturing SAW devices with lithography.

and a progressive increase of the tensile residualstress.
Another issue is the choice of the capping layermaterial and its surface processing. Using silicon
nitride (Si3N4) as the capping layer on top of SiO2
showed some improvement in the frequency uniformity after trimming. Up to now, the best results wereobtained using an aluminum nitride (AlN) capping
layer on top of SiO2. While uniform deposition ofthe SiO2 layer (small variation of the layer thickness
over the wafer) is a well-established technologicalprocess, uniform deposition of the capping layer is
not available at the moment and its thickness varies
strongly over the wafer. Therefore, one of the key
operations in the production of SAW devices is thetrimming process [5, 6], which utilizes local high-rate etching combined with a focused ion beam. It
decreases the local thickness of the layer achieving
uniform thickness of the capping layer over the wafer
and, consequently, improves the frequency uniformity.
This paper discusses the options available for improving the frequency uniformity of the SAW devices
and their particular technological implementations.

Experimental setup
We used an Advanced Modular Systems’ (AMSystems) three chamber cluster tool [1] for depositionof all layers as well as for trimming. The cluster tool
allows transport of the processed wafer from one
chamber to another without exposure of its surface
to the air, i.e. all chambers and connecting volumesare maintained at pressures below 10–6 Torr.
In one of the chambers, SiO2 is deposited using
the RF diode mode from 12 inch SiO2 target at 1 kWof RF power and a frequency of 13.56 MHz. Only
Ar process gas was added during SiO2 deposition.
Another chamber was used for deposition of thecapping layer. A Si3N4 layer was deposited by a dualalternative current (AC) magnetron from a highly
doped and highly conductive Si target. 40 kHz AC
power at 1 kW was applied between two Si targets.
Ar and N2 were added as process gases during deposition. Deposition was done in poisoned nitride
mode to get a stable and stoichiometric Si3N4 layer.
The standard process for AlN deposition includes

O. Girka et al.

the application of a dual AC magnetron operated ina reactive deposition mode.
The third chamber had been used for the trimming process, which was performed on the depositedlayers. The process is established by local sputteringof the capping layer using an adjustable focused
direct current (DC) ion source and the scanningwafer mechanism. The trimming process was applied to obtain a higher production yield of devicesfrom a wafer. As it has been mentioned above, twomaterials, namely AlN and Si3N4, were investigatedas cap layers in order to prevent physical adsorptionof water molecules on the silicon dioxide layer. Bothlayers have good passivation properties. They arestable and have a high grain density. Layers of Si3N4or AlN with 500 Å thickness were deposited on topof the SiO2 layers, providing continuous vacuum
conditions during the technological processes. Athickness of the capping layer of 500 Å was chosen
so as to assure that a minimum layer thickness of200 Å remains after the trimming process.

Results and discussion
For the 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 irradiation was done byscanning the wafer with an excimer laser radiation.
An XeCl excimer laser operating at a wavelength of308 nm and delivering a power density greater than
10 MW/cm2 per pulse was used for SiO2 surfacemodification. Surface densification under theseprocessing conditions prevented diffusion of themoisture into SiO2 grain bonds. The annealing ofSiO2 film in high humidity was done at temperatures between 250 and 300C in water steam, withpost-annealing and later drying. This process allows
saturating all available Si-O bonds and eliminating
further changes under exposure to the ambient.
The most successful method is to cap the SiO2layer with AlN and a subsequent double-step trimming process. The layer is etched using the FALCON
ion source [7], which is essentially an anode layer
thruster with a focused ion beam [8]. It is based on
the design of closed drift thrusters (also known asHall thrusters), which are typically used for space
propulsion due to their compactness, simplicity andvery high efficiency of converting inlet gas molecules
and atoms into ions of the beam [9].
The ion beam current is primarily defined by
the gas type and its flow through the discharge gap.
Figure 2 shows the increase of ion current producedby the FALCON ion source with increasing pressurein the vacuum chamber, which in turn is defined by
the increasing gas flow through the system. One cansee that the most efficient operation of the ion source
is obtained in the pressure range of (1–4) × 10–4 Torr.
Increasing pressure further by increasing the gas flow
leads to a transition of the gas discharge towards themagnetron type resulting in a subsequent deformation

Operation modes of the FALCON ion source as a part of the AMS cluster tool

329

Fig. 2. The dependence of the Ar ion beam current on thevacuum chamber pressure obtained at lower and higher
pumping speed. The accelerating voltage was 5 keV.

of the beam profile, enhanced erosion of cathodes, etc.
Obviously, higher pumping speeds allow higher ion
beam current and higher etching rate to be obtainedin the optimal pressure range.
Another important parameter is the average ion
energy, which defines the sputtering yield for thegiven pair of projectile-target combination and, consequently, the etching rate. By increasing the applied
voltage and, consequently, the average ion energy,
the ion beam current increases, as shown in Fig. 3.
Higher pumping speeds provide higher ion beam
currents at the given acceleration voltage, which isespecially important for lower acceleration voltages.
Both parameters, ion beam current and ion energy, define the etching rate, but at the same time they
define also the total power of the beam and the powertransferred to the processed surface. Therefore, it isimportant to maintain higher etching rates preventing overheating of the processing wafer. While active
cooling is possible, this solution is very expensive;
application of higher pumping speeds and lower ion
energies is preferential for the trimming process.
It has been proven that further benefit is achieved
by applying a second trimming process. This allows
for a further decrease in the variation of the layer
thickness over the wafer and, therefore, for an increase in the production yields of the SAW devices.
Figure 4 shows an example, how the frequency
distribution of the manufactured SAW devices varies depending on the application of the trimming
process. If the layers are deposited without a trim, awide range of frequencies for the final SAW devices
is obtained. Applying a first trim, one can achievesignificant increase of the production yield for SAW
devices from 120 to 250 units with an operation
frequency around 1848 MHz. Moreover, applyinga second trim, one can achieve a further increase of
the yield from 500 to 1100–1200 units. Dependingon the initial frequency distribution, the first trim
can increase the production rate by a factor of 5,
while the second trim provides another improvement
factor of 4 compared to the first trim. As a result,Fig. 3. The total Ar ion beam current as a function of
the accelerating voltage: (a) pumping speed 300 l/s; (b)pumping speed 1200 l/s.

Fig. 4. The effect of the double-step trimming on the production yield of an AlN capped temperature compensated
SAW device with the required frequency range: 1 – theinitially deposited layer before the trimming process; 2 –after the 1st trim; 3 – after the 2nd trim.

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the production yield can be improved by a factor of
20 with double-step trimming.

Summary and conclusions
AMSystems’ three chamber cluster tool with a
FALCON ion source is capable of a high-precisionand high-speed treatment of large wafers. Stable
SiO2 layer can be achieved through high power
density UV radiation and annealing in high humidity environment. A uniform capping layer canbe achieved by a trimming process involving theFALCON focused ion beam. Ion beam current generated by the FALCON ion source is measured for
different pumping rates, providing the possibility forthe selection of the ion beam current and the ion energy and prevent overheating of the processed wafer.
Experimental measures of the operational frequencyof the manufactured SAW devices have shown theefficiency of such a trimming process. As a resultof the double-step trimming a total improvement of
the production yield of AlN based SAW devices bya factor of 20 was achieved.

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