Energy spectra of sputtered ions: assessment of the instrumental ...

SIMS proceedings paper
Received: 7 October 2011 Revised: 8 December 2011 Accepted: 1 February 2012 Published online in Wiley Online Library: 24 February 2012
(wileyonlinelibrary.com) DOI 10.1002/sia.4908
Energy spectra of sputtered ions: assessment
of the instrumental resolution
Hubert Gnaser*
The energy spectra of Cs + ions sputtered from silicon under 5.5-keV Cs + bombardment were recorded for emission energies
E ≤ 100 eV. The emitted ions were detected in a high-sensitivity double-focusing SIMS (Cameca IMS-4f). The influence of several
instrumental parameters on the energy resolution and the peak position of the measured spectra were investigated to determine
the instrument’s resolution response. Specifically, entrance apertures with four different diameters (410, 280, 50, and 25 mm) were
used in the energy analyzer, whereas the width of its exit slit was varied from 3300 to 15 mm. For the smallest of these values, the
full width at half maximum (FWHM) of the Cs + spectrum amounts to 2.2 eV. A box-type resolution function was found to describe
the energy distributions over a substantial range. Using that function, the Cs + distributions measured for various slit settings can
be reproduced via the convolution of a single original function. The values of the spectral width obtained from this procedure are
essentially identical with the FWHM values of the recorded spectra. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: energy spectra; sputtered ions; Cs + bombardment; energy resolution
Introduction
The bombardment of solids by energetic particles results in the
emission of atoms and molecules from the surface, a process
termed sputtering. [1] The spectral distributions of these sputtered
species in terms of the (kinetic) emission energy and the emission
angle reflect, to a considerable degree, the atomistic processes
occurring during the dissipation of the projectile’s energy in the
solid and in the sputtering event proper. [2] The energy distribution
of the sputtered species can be described quite satisfactorily
by the predictions of the analytical sputtering theory
of Sigmund. [3] This approach [4,5] results in an approximate
energy dependence of the yield Y(E) of sputtered atoms that
scales with the surface binding energy of the released atoms
U, [4] Y(E) / E/(E + U) 3 .
Although the sputtered flux is composed primarily of neutral
species, positively and negatively ionized atoms and molecules
are observed; they form the basis of SIMS. [6] Their number is
determined by the ionization probability P + or P of the positively
or negatively charged ions. Several ionization schemes have been
proposed [7,8] to describe the ionization process of sputtered ions,
but unfortunately, the mechanisms leading to the ionization of
sputtered atoms and molecules are only poorly understood. [9,10]
With the previously given expression for the emission energy
scaling of the total sputtering yield, the energy-dependent yield of
sputtered ions Y + (Y ) could then be given formally by the product
of Y(E) withP + (P ): Y + (E)=Y(E)P + (E) [Y (E)=Y(E)P (E)].
The measured energy spectra of such ions may then provide
(direct) information on the specifics of the ionization processes as
most theoretical concepts predict some kind of energy (velocity)
dependence of the ionization probability of sputtered species. In
fact, this approach has been attempted quite frequently. [2,6,7] It
seems, however, that the influence of the pertinent instrumental
parameters (such as the energy resolution of the specific setup)
has often not been determined with sufficient precision, and
this may thus limit the validity of any conclusions as to detailed
ionization processes.
In an attempt to improve this situation, in the present work the
energy spectra of positive Cs + ions sputtered from a Si sample
under Cs + bombardment were measured for a range of experimental
settings. This choice was largely determined by the fact
that (i) Cs + ions are widely used as projectiles in SIMS analyses
and (ii) sputtered Cs + ions have very high yields. Therefore,
energy spectra of Cs + were recorded for emission energies
E ≤ 100 eV, using 5.5 keV Cs + bombardment. The data will provide
detailed information on the energy resolution as a function of
relevant parameters for a specific double-focusing SIMS instrument
that features electric and magnetic sector fields in series.
Experimental
The experiments were performed in a SIMS (Cameca IMS-4f [11] ).
Sputtering was done with Cs + ions produced in a surface-ionization
source. With the source at a potential of +10 kV with respect to
ground and the sample at +4.5 kV, the primary impact energy was
5.5 keV, at an incidence angle of ~40 relative to the surface normal.
The ion current was 20 nA and the beam was raster-scanned across
areas of either (125 mm 125 mm) or (350 mm 350 mm). Sputtered
positive ions were collected from a circular area (centered
within the raster) with a diameter of 8 mm. The instrument
incorporates a double-focusing mass spectrometer consisting of
an electrostatic sector followed by a magnetic sector field, both
with a 90 deflection angle. The electrostatic sector is a spherical
condenser with an energy dispersion D E = 173 mm, whereas the
* Correspondence to: Hubert Gnaser, Department of Physics and Research Center
OPTIMAS, University of Kaiserslautern, Kaiserslautern D-67663, Germany.
E-mail: gnaser@rhrk.uni-kl.de
a Department of Physics and Research Center OPTIMAS, University of Kaiserslautern,
Kaiserslautern D-67663, Germany
b Institute for Surface and Thin-Film Analysis IFOS, Trippstadter Str. 120,
Kaiserslautern D-67663, Germany
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Surf. Interface Anal. 2013, 45, 79–82
Copyright © 2012 John Wiley & Sons, Ltd.

H. Gnaser
80
magnetic sector features a homogeneous magnetic field with
inclined field boundaries. The mass resolution was M/ΔM ~ 300.
The detection of secondary ions was done by a discrete-dynode
electron multiplier, and count rates were limited to

Energy spectra of sputtered ions
A ¼ R e jC e ja 2 (2)
Calculated energy resolution ΔE (eV) FWHM Cs + energy spectrum (eV)
10 2 CA1
CA2
CA3
CA4
10
1
10 10 2 10 3
Exit slit width (µm)
CA1
CA2
CA3
CA4
10
1
10 10 2 10 3
10 2 (a)
Exit slit width (µm)
(b)
The radius of the electrostatic sector is R e = 85 mm, the
constant C e = 3.5, and a is the angle to which the beam is
constrained in the radial direction (a ~ 0.012). [21] With these
numbers, a value of A ~40mm is obtained. Eqn (1) may then be
used to estimate the theoretically predicted values of ΔE for the
different S 1 and S 2 ; Fig. 3(b) shows such an evaluation which
produces a qualitative agreement with the experimental data.
The discrepancies that are found might be due to the presence
of the other optical elements (the magnetic sector and
severa lenses), which follow the energy analyzer and possible
uncertainties associated with the specific values of C e and a.
It was noted in the context of Fig. 1 that for wide exit slits, the
energy spectra exhibit a box-type shape, at least in the lowenergy
and the central parts. This feature is illustrated in Fig. 4,
which depicts the distributions obtained for all four CAs and
S 2 = 1400 mm. In a recent publication, [18] it was proposed that a
box-shaped resolution function R(E) might be an appropriate
description for the bandwidth in electric and magnetic sector-field
instruments such as the one used in the present work. Specifically,
the following normalized resolution function was applied
RE ð Þ ¼ 1
4ΔE 1 þ erf E þ 0:5ΔE
E
pffiffiffi
1 erf
2 s
0:5ΔE
pffiffiffi
2 s
Eqn (3) constitutes the convolution of a rectangular box with a
Gaussian of standard deviation s. [18] Such a box-type function
R(E) is plotted in Fig. 4 (dashed line), using ΔE = 37.6 eV and
s = 1.3 eV. Clearly, the function describes the experimental
spectra quite well in the low-energy and the central parts.
As suggested by Wittmaack, [18] the resolution function given in
Eqn (3) could be used to compute energy spectra via a convolution
using R(E). Their comparison with the corresponding experimental
distributions would then constitute a test as to the validity of the
resolution function. This approach was tested for several measured
energy spectra. Figure 5 displays normalized Cs + distributions
(3)
Figure 3. (a) The measured FWHM of the Cs + spectra as a function of the
exit slit width for the four CAs. (b) The energy resolution ΔE calculated
according to Eqn (1), with an aberration of A =40mm.
of the CAs. In addition, optical aberrations may possibly contribute
to the values of the FWHM for small slit sizes.
Theoretically, the relative energy resolution E/ΔE of an
electrostatic sector analyzer such as the one in the present SIMS
instrument depends on the energy dispersion D E , the widths of
the entrance and exits slits, S 1 and S 2 ,theanalyzer’s magnification
M, and some possible contributions due to optical aberration
effects A [6] :
E
ΔE ¼ D E
MS 1 þ S 2 þ A
(1)
Cs + intensity (normalized)
10 -1 1
10 -2
10 -3
S 2
= 1400 μm
CA1
CA2
CA3
CA4
Box-type
In the present case, S 1 is determined by the diameter of the
CAs and M is unity because of the symmetry of the entrance
and exit positions of the sector field. For an electrostatic
sector, the contribution due to aberration in the radial plane is
given by [19,20]
-20 0 20 40 60
Emission energy (eV)
Figure 4. Normalized Cs + energy spectra for the four different CAs and
the exit slit S 2 = 1400 mm (solid symbols). The dashed line is the resolution
function given in Eqn (3).
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Surf. Interface Anal. 2013, 45, 79–82 Copyright © 2012 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/sia

H. Gnaser
Cs + intensity (normalized)
10 -1 1
10 -2
Exp S 2 (μm) Conv
1400
450
190
110
CA4
values of the entrance aperture and the exit slit the FWHM of the
Cs + energy spectra amounts to ~2.2 eV. Generally, the values of
the FWHM are in qualitative agreement with the energy resolution
ΔE calculated for this energy analyzer. The data show that
the different energy spectra can all be reproduced from a single
original spectrum by a convolution procedure that is based on a
box-type shape instrumental resolution function. In future work,
a reversal of that approach (i.e. a deconvolution of measured
spectra) could be attempted, with the objective of determining
the “true” spectra of sputtered ions.
Acknowledgement
The author is grateful to Klaus Wittmaack for useful discussions
related to the subject of this work.
10 -3
obtained for CA4 and S 2 -values of 1400, 450, 190, and 110 mm(solid
symbols with lines). The narrowest energy spectrum recorded in
this study, namely, for S 2 =15mm and CA4 (cf. Fig. 2), was convolved
with the resolution function, Eqn (3), using optimized values of ΔE
and s (37.6/0.8, 10.6/0.6, 5.2/0.6, and 3.0 eV/0.5 eV, respectively).
The resulting spectra are shown in Fig. 5 as open symbols. These
ΔE-values are essentially identical with the FWHM of the respective
spectra (cf. Fig. 3). The graph demonstrates that the experimental
data can be reproduced rather accurately by the convolution procedure.
This agreement corroborates the conjecture that a single type
of resolution function is sufficient to describe the energy spectra
obtained in this instrument, independent of the specific settings
of the entrance and exit slit widths.
Conclusions
-20 0 20 40 60
Emission energy (eV)
Figure 5. Normalized Cs + energy spectra for CA4 and the four different
exit slit widths S 2 of 1400, 450, 190, and 110 mm (solid symbols and lines).
The open symbols are the result of a convolution using the resolution
function of Eqn (3).
The energy spectra of Cs + ions sputtered from silicon under Cs +
bombardment were recorded for different experimental settings
to determine the instrument’s energy resolution. For the smallest
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wileyonlinelibrary.com/journal/sia Copyright © 2012 John Wiley & Sons, Ltd. Surf. Interface Anal. 2013, 45, 79–82