WDS'14 Proceedings of Contributed Papers — Physics, 371–377, 2014.
ISBN 978-80-7378-276-4 © MATFYZPRESS
Sputtering of Glass Dust Grain by Ions and Electrons
M. Vyšinka, J. Vaverka, J. Pavlů, Z. Němeček, and J. Šafránková
Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech
Republic.
Abstract. Glass (SiO2 ) is sputtered not only by ion impact, but the sputtering
yield can be enhanced by electron impact as well. In the case of small silicate
dust grains in the space environment, such effects can influence the lifetime of the
grains. Moreover, there is a suggestion that electric field could further enhance
the sputtering yield from charged non-conducting grains. To reveal these effects,
we performed experiments with a single dust grain levitated in an electrodynamic
quadrupole trap. As a representative of silicate-type space dust, we used spherical
SiO2 grains with diameters around 1 micron. The grain in the trap was bombarded
by 2-keV Ar+ ions and 1-keV electrons. The sputtering yield of the ion-only
bombardment (i.e., at a high surface field) was 1.4 SiO2 /ion, in the case of a
combined ion and electron bombardment, the yield was 1.5 SiO2 /ion, i.e., slightly
higher.
Introduction
Sputtering by a beam of energetic particles is an important process of destructing dust
grains in space. A typical source of such particles is the solar wind that consists of ions with
energies in the keV range and electrons in the eV range. Other source can be shock waves,
supernova explosions, and others. A dust grain is a small spatially limited object, often with
an irregular shape. It means that there is no preferred impact angle for a particular grain.
Sputtering is strongly angular-dependent with the maximum sputtering yield at around 70◦ –
80◦ and can be up to 7 times higher compared to the normal incidence. As a results, the
grain is sputtered by different rates at different areas of the surface. Papers dealing with the
sputtering and the lifetime of the grain [e.g., Flower et al., 1996] usually assume a spherical
grain and calculate the sputtering yield over the sphere using values for a given incident angle
and an angular dependence of the yield. Such approach neglects that the real grain need not
necessarily preserve a spherical shape during the sputtering leading to change in sputtering
yield due to angular dependency and some experimental confirmations of the sputtering rate
will be appreciated.
Dust grain sputtering experiments are usually based on sputtering of dust grain beds using
a mass spectroscopy for detection of sputtered particles [Meyer et al., 2011]. Such experimental
arrangement is intricate due to a possible recaption of sputtered atoms by other grains and a
precise estimation of the sputtering yield is quite difficult. The first study of spherical object
sputtering was performed by Wehner, [1959]. Pavlů et al. [2007] studied sputtering of a single
gold grain with a diameter of 1 micron. The authors sputtered the grain at a high surface
potential and found that the sputtering yield was enhanced by a factor of 4 in comparison
to the normal incidence on a flat sample. They analyzed that a factor of 2 is given by the
geometry and the rest was attributed to a field enhancement caused by the high surface electric
field during the sputtering. They speculated that for an insulating grain, the field factor should
be much larger.
To test check their deductions, we performed a similar experiment as Pavlů et al. [2007]
but with a glass grain as a more space-related material instead of the originally investigated
gold. Our experiment was based on sputtering of a single glass grain by Ar+ ions at a high
surface potential (≈ 1 keV, the potential was limited by field ion emission) and at a low surface
potential (≈ 10 eV, the potential was kept by an 1-keV electron beam). The mass of the grain
was measured every ≈ 3 hours of bombardment. The experiment was similar to that published
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VYŠINKA ET AL.: SPUTTERING OF GLASS DUST GRAIN
Figure 1. Mass measurement. Secular frequency (and Q/m) jumps occur due to the capture or
the release of a single or a few electrons (left). The corresponding charge of the grain in numbers
of electrons is shown in the right side. The data was measured after 31 hours of bombardment
and the obtained grain mass was m = (8.45 ± 0.15) · 10−16 kg.
earlier [Vyšinka et al., 2013] but we developed a new technique for determination of the amount
of impinging ions that improved the sputtering yield determination.
Experimental techniques
The experimental set-up is similar to that used by Pavlů et al. [2007]. It is based on an
electrodynamic quadrupole (Paul trap) placed in a UHV vacuum chamber. A single dust grain
levitates in the trap and its charge-to-mass ratio (Q/m) can be calculated from the secular
frequency of grain oscillations. To obtain this frequency, light of a red laser diode is scattered
off the oscillating grain. The scattered light is focused by a lens system to a position-sensitive
detector that provides coordinate signals for determining the secular frequency. The grain can
be charged or discharged by electron (KIMABLL PHYSICS EMG-14) and ion (COLUTRON
G-2-D) guns with energies in the range of 0.1–10 keV. More details about the experimental
set-up can be found in the previous papers [Čermák et al., 1995, Žilavý et al., 1998, Pavlů et
al., 2004, Němeček et al., 2011].
Mass measurement
The mass of the grain can be determined from changes of the charge-to-mass ratio caused
by a change of the grain charge by a single or a few electrons (the method was firstly used
for dust grains by Žilavý et al., 1999). The mass can be calculated according to the following
Equation:
e
m= Q,
(1)
∆m
Q
where e is electron charge, ∆ m
is the change of the charge-to-mass ratio of the grain
corresponding to the elementary charge change. The grain was discharged to about hundred
electrons to obtain a higher resolution, thus better precision. Such an example of measurement
is illustrated in Figure 1.
Estimation of number of impinging ions
When the grain undergoes a short shot from the ion gun we can measure a change of
the Q/m ratio (∆Q/m). If the grain mass is known (from mass measurement) and under the
assumption that the mass change can be neglected, the grain charge change can be estimated
(∆Q). If we assume that all impinging ions are captured on the grain surface we can calculate the
number of impinging ions during the shot. The total ion current from the ion gun is measured
by a Faraday cup, the signal is amplified by an electronic system and read in volts. If we take a
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VYŠINKA ET AL.: SPUTTERING OF GLASS DUST GRAIN
Figure 2. Measurements of impinging ions. Left — changes of the charge-to-mass ratio (top,
black dots) caused by the ion shots (blue line). One particular ion shot is magnified (bottom)
to show the ion signal area used to determination of the number of impinging ions. Right —
the charge-to-mass ratio changes as a function of the time integral of the ion signal showing a
relationship between the ion signal and the number of impinging ions.
time integral of the signal (the area under the chart in Figure 2, left) we can plot the dependence
of ∆Q/m on this ion signal area (Figure 2, right). The dependence is linear and we can obtain
the coefficient from a linear regression. Such procedure allows us to estimate the total number
of impinging ions from the ion signal measured by the Faraday cup. This type of measurements
was done prior to each sputtering session, thus we could account for the changes of both the
ion-gun beam intensity and the grain cross-section.
The total number of impinging ions (N) during each sputtering session is given by the
Equation 2,
B · Ia · m
N=
(2)
e
Q
where B is the slope of the dependence of ∆ m
vs. the integrated ion signal that converts the
Faraday cup signal to incoming charge (see Figure 2), Ia is the time integral of the ion signal for
the sputtering session (the total ion signal area), m is the grain mass and e is the elementary
charge.
When the grain is charged to high surface potentials, the number of impinging ions (Ni ) is
reduced according to Equation 3:
e · φmax
(3)
Ni = N0 · 1 −
E0
where N0 is the number of impinging ions according to Equation 2, e is the elementary charge,
φmax is the equilibrium surface potential of the grain, and E0 is the energy of impinging ions.
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VYŠINKA ET AL.: SPUTTERING OF GLASS DUST GRAIN
Sputtering at low surface potentials
To study the sputtering, we used the SiO2 grain with an initial mass m = 1.190 · 10−15
kg and with diameter D = 1.01 µm. The grain mass during the sputtering by 2-keV Ar+ ions
and 1-keV electrons is depicted in Figure 3 as a temporal evolution (a) and as a dependence
on the number of impinged ions (b). The surface potential was ≈ 10 V during the sputtering.
The temporal evolution of the grain mass is not linear because: (1) a reduction of the grain
cross-section (the grain becomes smaller due to the sputtering thus less ions reach the surface),
and (2) a reduction of the ion flux from the ion gun (given probably by degradation of the ion
gun cathode). Measuring the number of impinging ions as described above, we get a linear
dependence of the grain mass on the ion dose as it is demonstrated in Figure 3b.
During 101 hours of sputtering, 8.34 · 10−16 kg of the grain material (70 % of the initial
mass) was sputtered. The total ion dose was 5.6 · 109 ions and it corresponds to the dose of
9.6 · 1017 ions/cm2 .
The sputtering yield of a SiO2 dust grain as a function of the ion dose (i.e., the number of
the ions that hit the surface) is depicted in Figure 4 left. No dependence of the sputtering yield
on the ion dose (i.e., on the grain diameter) was observed. A mean value of the sputtering yield
was:
amu
SiO2
YSiO2 = (90.2 ± 0.3)
= (1.501 ± 0.004)
,
ion
ion
where amu = 1.66 · 10−27 kg is the atomic mass unit. The value was obtained as a slope of a
linear regression of the mass vs. ion dose dependence (Figure 3b).
Sputtering at high surface potentials
In this case, the initial grain mass was m = 1.280 · 10−15 kg (diameter D = 1.04 µm). The
average charge-to-mass ratio during the sputtering was approx 61 C/kg and it corresponds to
a surface potential of 1.3 kV. The energy of the ion beam was 3 keV, but due to the grain
potential, the ions impinge with a resulting energy of 1.7 keV. The number of impinging ions
was reduced according to Equation 3.
Due to some experimental complications (leading to a higher probability of the grain loss
from the trap during high charge-to-mass ratio changes), there are less data points available
than in the experiment at low surface potentials. During 15.5 hours of sputtering, 6.1 · 10−17 kg
of the grain material (5 % of the initial mass) was sputtered. The total ion dose was 5.2 · 108
ions that corresponds to a dose of 6.1 · 1016 ions/cm2 . The temporal evolution of the grain mass
and its dependence on the number of impinging ions is depicted in Figures 3c and 3d).
The sputtering yield of a SiO2 dust grain as a function of the ion dose (i.e., the number of
ions that hit the surface) is depicted in Figure 4 right. The grain was sputtered by Ar+ ions
with an effective energy of 1.7 keV and the grain surface potential was 1.3 kV. The mean value
of the sputtering yield was:
YSiO2 = (82 ± 38)
amu
SiO2
= (1.36 ± 0.64)
,
ion
ion
The value was obtained again as a slope of a linear regression of the mass vs. ion dose dependence
(Figure 3d).
Discussion
Glass (SiO2 ) is a rather complicated material from the sputtering point of view. It exhibits
preferential sputtering of oxygen by both electron [Fujita et al., 2006] and ion impacts (with
a comparison to neutral beam bombardment) [Mizutani, 1991]. Such preferential sputtering
can lead to changes of the surface composition and to an increase of the surface conductivity
as it was observed by Gonzales de Vicente et al. [2007] after an He+ -ion bombardment of
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VYŠINKA ET AL.: SPUTTERING OF GLASS DUST GRAIN
(a)
(b)
(c)
(d)
Figure 3. Temporal evolution of the grain mass during sputtering by 2-keV Ar+ and 1-keV
electrons (a) and the dependence of the grain mass on the ion dose (b). The sputtering by
3-keV Ar+ beam (surface potential 1.3 kV, hence impinging energy 1.7 keV) as a temporal
evolution of the grain mass (c) and its dependence on the ion dose (d). The lines in the panels
(a) and (c) are intended to guide the eyes, the lines in the panels (b) and (d) are linear fits.
Error bars are given by the errors of the mass measurement data.
Figure 4. The sputtering yield as a function of the ion dose measured on a single SiO2 grain.
Left — sputtering at low surface potentials by 2-keV Ar+ and 1-keV electron beams. Right —
sputtering at high surface potentials (1.3 kV) by 3-keV Ar+ (impinging energy was 1.7 keV).
a glass surface. The increase of the surface conductivity was observed after reaching some
threshold dose (≈ 1016 ions/cm2 ). Such circumstances may explain a broad range of reported
sputtering yields for SiO2 (0.8–2.0 atoms/ion for 2 keV Ar+ ions [Seah et al., 2010]). In the
case of a spherical grain, the sputtering yield should be enhanced by a factor of 1.2–2.0 due to
its angular dependence [Pavlů et al., 2007]. Our results are consistent with these findings.
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VYŠINKA ET AL.: SPUTTERING OF GLASS DUST GRAIN
The charge of the grain influences the number of impinging ions and their effective energy.
When the charge is compensated by beam electrons, these effects can be neglected as the grain
surface potential is around 10 V. When the charge is not compensated by the electron beam, the
surface potential is stabilized by field ion emission (equilibrium between the primary current, the
field ion emission current, and the background current of electrons generated on the electrodes)
and some correction is needed.
The problem of the grain sputtering at high surface potentials is a necessity of discharging
the grain in a broad charge range for the mass measurements. This step increases a risk of losing
the grain from the trap and it makes the experiment time consuming. Long-time experiment
and further data on the sputtering at high surface fields are needed for a stronger conclusion. It
is not clear why the sputtering yield at the 2nd data point in Figure 4 right is so low. Generally,
the ion dose at the sputtering at high surface fields was low and the data point scatter can be
just a result of this.
Pavlů et al. [2007] analyzed a similar experiment on a gold grain at high surface potentials
(without the ion beam compensation by the electron bombardment) and found a sputtering
yield about 2-times larger than expected. They ascribed it to the influence of the surface field
and suggested that the enhancement should be much more pronounced for an insulating grain
but our results do not confirm this hypothesis. The possible reasons can be that the electron
beam used for the ion compensation has a larger effect on the sputtering yield than the electric
field or the preferential sputtering of oxygen leads to a more conductive surface and the grain
loses its insulating properties. Our experimental set-up does not allow us to measure mass
spectra of sputtered particles to confirm the preferential sputtering.
Conclusion
The sputtering yield of an 1-µm SiO2 grain bombarded simultaneously by 2-keV Ar+ ions
and 1-keV electrons that keep the grain surface potential low is YSiO2 = (90.2 ± 0.3) amu
ion =
SiO2
(1.501±0.004) ion . It is consistent with the data published in the literature for SiO2 sputtering
[Seah et al., 2010].
The grain mass evolution shows a linear dose dependence during a prolonged time of the
bombardment. Totally, 8.34×10−16 kg (70 %) of the glass grain was sputtered during 101 hours
of bombardment. No dependence of the sputtering yield on the ion dose was observed during
the 5.6 × 109 ions bombarded the grain surface (9.6 × 1017 ions/cm2 ).
At a high surface potential (1.3 kV), the sputtering yield of the SiO2 grain bombarded by
SiO2
−17 kg
1.7-keV Ar+ ions is YSiO2 = (82 ± 38) amu
ion = (1.36 ± 0.64) ion . Totally, only 6.1 × 10
(5 %) of the glass grain was sputtered during 15.5 hours of bombardment. The total ion dose
was 5.2 · 108 ions and it corresponds to the dose of 6.1 · 1016 ions/cm2 , an order of magnitude
lower than at the low-surface-potential experiment. Reaching a higher ion dose for the highsurface-potential experiment will be needed for a better comparison of the sputtering yield.
A hypothesis about the yield enhancement due to a high surface field of non-conducting materials according to Pavlů et al. [2007] could not be confirmed. At least, the field enhancement
(if any) for the glass grain is smaller than the influence of the electrons.
Acknowledgments. The authors would like to express their appreciation to the Charles University Grant Agency for a support of this work under contract No. 1410213. Prof. DelVı́tek and his group
is greatly acknowledged for never-ending encouragement and endless discussions during the experiment.
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