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Applied Surface Science 252 (2006) 6429–6432
Quantitative fundamental SIMS studies using 18O implant standards
Peter Williams *, Richard C. Sobers Jr., Klaus Franzreb, Jan Lorincık 1
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA
Received 12 September 2005; accepted 15 February 2006
Available online 15 May 2006
Abstract
The use of dilute ‘minor-isotope’ 18O implant reference standards for quantification of surface oxygen levels during steady-state SIMS depth
profiling is demonstrated. Some results of two types of quantitative fundamental SIMS studies with oxygen (16O) primary ion bombardment and/or
oxygen flooding (O2 gas with natural isotopic abundance) are presented: (1) Determination of elemental useful ion yields, UY(X�), and sample
sputter yields, Y, as a function of the oxygen fraction cO measured in the total flux emitted from the sputtered surface. Examples include new results
for positive secondary ion emission of several elements (X = B, C, O, Al, Si, Cu, Ga, Ge, Cs) from variably oxidized SiC or Ge surfaces. (2) The
dependence of exponential decay lengths l(Au�) in sputter depth profiles of gold overlayers on silicon on the amount of oxygen present at the
sputtered silicon surface. The latter study elucidates the (element-specific) effects of oxygen-induced surface segregation artifacts for sputter depth
profiling through metal overlayers into silicon substrates.
# 2006 Published by Elsevier B.V.
Keywords: Useful ion yield; Sputter yield; Segregation; Oxygen; Implant standard
1. Introduction
Secondary ion formation and segregation processes are
surface phenomena that are strongly affected by the presence of
oxygen in the top one to two monolayers at the sputtered
surface. Accurate quantification of surface oxygen on this very
narrow depth scale (i.e. within the sputtered particle escape
depth) is a key requirement for efforts to understand the
influence of oxygen on secondary ion emission. Recently we
introduced a powerful new method for quantifying in situ
surface oxygen levels measured in the total emitted flux during
SIMS sputter depth profiling using 18O implant reference
standards [1,2]. In the absence of preferential sputtering effects,
such minor-isotope implants can be used for SIMS quantifica-
tion of 16O levels within the sputtered atom escape depth (�1–
2 atom layers) in steady-state depth profile measurements
under oxygen (16O) primary ion bombardment or oxygen gas
flooding. This method allows quantitative investigation of
variation of sputter yields (Y, defined as the ratio of the total
* Corresponding author. Tel.: +1 480 965 4107; fax: +1 480 965 2747.
E-mail address: pw@asu.edu (P. Williams).1 Present address: Institute of Radioengineering and Electronics, Academy of
Sciences of the Czech Republic, Chaberska 57, 182 51 Praha 8, Czech
Republic.
0169-4332/$ – see front matter # 2006 Published by Elsevier B.V.
doi:10.1016/j.apsusc.2006.02.091
emitted flux of a sample material versus the primary ion flux)
and useful ion yields (UY(X�), defined as the ratio of the
detected ion flux of an element X versus the total emitted
elemental flux) with surface oxygen content [1,2], and similarly
allows investigation of impurity profile distortions arising from
ion beam mixing and oxygen-induced segregation [3,4], again
as a function of surface oxygen levels. The goal of the present
report is to demonstrate the general utility of 18O implant
reference standards for fundamental SIMS studies.
2. Experimental
The results reported here were obtained on a Cameca IMS
3f SIMS instrument using standard depth profiling conditions
at maximum transmission without and with oxygen flooding
(O2 gas with natural isotopic abundance) [1,2]. For the useful
ion yield and sputter yield studies, a Ge and a SiC wafer were
pre-implanted with 18O (dose 1.4 � 1016 cm�2 at 60 keV)
and isotopes of several other elements (11B, 27Al, 63Cu, 69Ga,133Cs, etc.). Positive ion sputter profiles were acquired using
either 8 keV 40Ar+ or 4 keV per atom 16O2+ (ion source
potential +12.5 kV) bombardment at �398 with respect to the
sample normal, in some cases together with O2 flooding from
the gas phase. The location of the 18O implant peak was
determined to be at �120 nm in Ge and �109 nm in SiC,
P. Williams et al. / Applied Surface Science 252 (2006) 6429–64326430
Fig. 2. Oxygen-dependent useful ion yields UYof Ge+ for Ge (rhomb symbols),
and of Si+ and C+ for SiC (circles and triangles, respectively) for 8 keVAr+ (full
symbols) and for 4 keV per atom 16O2+ (open symbols) bombardment (at�398)
without and with O2 gas flooding. For comparison with UY(Si+) from SiC,
corresponding ion yield data of Si+ from Si are included (data taken from [1],
small full square symbols, dotted line).
with 18O peak concentrations calculated from the implant
dose to be �2.1% in Ge and �1.9% in SiC. The amount of
oxygen in the total flux emitted from the sputtered surface in
steady state was obtained from the intensity ratio of 16O+
versus the background-subtracted 18O+ implant peak signal
[1,2]. For the study of elemental decay lengths of gold
impurities in Si substrates after sputtering through Au metal
overlayers, the amount of oxygen in the total flux emitted
from the sputtered Si surface was quantified in a similar way,
but in this case using an external 18O in Si implant depth-
profiled immediately after the Au overlayer sample under
identical conditions as a secondary standard.
3. Results and discussion
3.1. Useful ion yields and sputter yields
Fig. 1 displays sputter yields Y as a function of the oxygen
fraction cO in the total emitted flux for 40Ar+ or 16O2+
bombarded Ge and SiC without and with O2 flood. Sputter yield
data for Si (taken from [1]) are included in Fig. 1. We note that
the measured oxygen flux fraction cO is representative of the
oxygen fraction at the sputtered surface (averaged over the
sputtered particle escape depth) so long as distorting processes
such as preferential sputtering or thermal desorption are
negligible. The sputter yields of SiC are very similar to that of
Si for low oxygen flux fractions, but deviate for cO > 0.4. We
found for SiC (Fig. 1), Si [1] and for Al, Fe, Ni, Cu [5] that the
sample sputter yield is lowered by the addition of more oxygen
at the sputtered surface (this drop of Y for oxygen-covered
surfaces is attributed primarily to a reduction in the number of
atoms of the target within the sputtered particle escape depth).
This trend is not seen for Ge (Fig. 1). It is difficult to fully
oxidize the Ge surface because of its low sticking coefficient for
O2 adsorption and a relatively high sputter yield under O2+
Fig. 1. Oxygen-dependent sputter yields Y for 8 keVAr+ (full symbols) and for
4 keV per atom 16O2+ (open symbols, Y values are given per oxygen atom)
bombardment (at �398) of Ge (rhomb symbols) and SiC (circle symbols)
without and with O2 gas flooding. For comparison with SiC, corresponding
sputter yield data for Si are included (data taken from [1], small square symbols,
two dotted lines). cO denotes the oxygen fraction measured in the total flux
emitted from the sputtered variably oxidized surface.
bombardment [6]. Additional sputter yield and/or relative ion
yield data is available in the literature for partially oxidized Ge
[6–8] and SiC [9].
Fig. 2 shows useful ion yields UYas a function of the oxygen
flux fraction cO for Ge+ as well as C+ and Si+ from partially
oxidized Ge and SiC, respectively. Useful ion yield data of Si+
from oxidized Si (taken from [1]) are included for comparison
in Fig. 2. As might have been expected, the useful ion yields of
the major elements are strongly enhanced by surface oxygen in
all four cases. For a given oxygen level of SiC, we found
UY(C+) to be about two orders of magnitude lower than
UY(Si+), with their values being enhanced by oxidation of the
SiC surface by factors up to �50 and �100, respectively. At
similar oxygen levels, UY(Si+) is nearly the same for partially
or fully oxidized SiC versus Si for cO > 0.3, but is higher by a
factor of�2–3 at low oxygen levels. The latter relatively small
difference might be due to two counter-acting effects, a fairly
small chemical enhancement of Si+ formation in SiC by carbon
neighbors combined with a reduced efficiency in SiC for Si 2p
core excitation that is known as a ‘kinetic process’ for Si+
formation [10]. Note for the case of the sputtered Si surface that
oxygen fractions exceeding that expected for stoichiometric
sputtering of bulk SiO2 (cO > 0.67) have been observed by us
in measurements of the sputtered flux [1,2] and by other groups
in direct measurements of the surface or near-surface
composition [11,12]. This can be understood in terms of the
outermost atomic layer of the sample becoming highly
oxygen-rich (essentially, formation of an adsorbed monolayer
of oxygen) at high oxygen levels; sampling with a technique
that is sensitive only to the outermost one to two layers (SIMS
[1]) or just the outermost layer (LEIS [11]) then is expected to
yield a very high oxygen fraction relative to the value for bulk
SiO2. An oxygen enhancement of UY(Ge+) by a factor of up to
�20 is seen in Fig. 2; note however that full oxidation of the Ge
surface had not been achieved. Over the attainable oxygen
levels the Ge+ ion yield curve parallels that of Si+ and might be
explicable by a similar statistical model [1]. Fig. 3 gives two
P. Williams et al. / Applied Surface Science 252 (2006) 6429–6432 6431
Fig. 3. Oxygen-dependence of useful ion yields UY(X+) of a few trace elements
(X = B, 18O, Al, Cu, Ga, Cs) for 8 keV Ar+ (full symbols) and for 4 keV per
atom 16O2+ (open symbols) bombardment (at �398) of Ge (a) and of SiC (b)
without and with O2 gas flooding. (The label H stands for high-energy (initial
emission energies of 50–170 eV) O+ secondary ions. All other secondary ions
(and the O+ data with label L) were monitored at low initial emission energies
(0–110 eV).)
Fig. 4. Gold decay lengths l(Au�) in silicon as a function of the oxygen
fraction cO in the total flux emitted from the sputtered Si substrate surface (see
text for details).
examples of useful ion yields UY(X+) of several trace elements
X in partially oxidized Ge (Fig. 3a) and in oxidized SiC
(Fig. 3b). The behavior of UY(B+) as a function of cO is
different for ion emission from Ge versus SiC; additional
UY(B+) data for Si [1] and for Al, Cu [5] are reported
elsewhere.
3.2. Elemental decay lengths in silicon
A sputter profile through a delta-function sub-surface layer
or a step-function overlayer typically exhibits an exponential
decay once the layer material is sufficiently dilute that ion
yields and sputter yields become constant. This is understood to
result from ion-beam mixing effects that disperse the layer
material largely beyond the sputtered atom escape depth,
reducing the efficiency of removal [13,14]. The decay length
l(X�), defined here and in [4] as the sputtered depth (nm/
decade) for which the secondary ion signal of the impurity X
has dropped by a factor of 10 is an indication of the spatial
extent of ion-beam mixing [14]. Williams and Baker [3]
reported that the value of l(Ag+) for a silver delta layer could be
increased by up to a factor of 10 by blowing oxygen onto the
sputtered surface while sputtering with Ar+; however oxyge-
nating the surface by sputtering with O2+ primary ions had
almost no effect on l(Ag+). Hues and Williams [4] showed that
this effect was quite general and seemed to scale with the
relative electronegativities [4] or with the relative heats of oxide
formation [15] of the delta or overlayer element compared to
the silicon matrix. These observations were explained in terms
of chemical segregation or antisegregation of the overlayer/
delta element towards or away from the surface in the steep
oxygen chemical gradient produced by gas flooding. However,
it was later suggested that the overlayer/delta element is
excluded from a stoichiometric oxide layer due to low solid
solubility in the oxide [16], and that Williams and Baker had
seen no such effect because sputtering with O2+ primary ions at
a relatively large angle of incidence did not introduce sufficient
oxygen to produce a stoichiometric oxide layer [17]. The 18O
implant approach allows us to throw new light on these
conflicting explanations.
Fig. 4 shows a plot of exponential decay lengths l(Au�) for
a gold overlayer on a p-type silicon substrate as a function of the
oxygen fraction cO measured in the sputtered flux. Fig. 4
includes SIMS depth profile data for both Au+ and Au�
secondary ions sputtered with 40Ar+ bombardment without and
with O2 flood, and with 16O+ or 16O� beams without oxygen
flood, for a range of impact energies (the numbers next to the
data points give the primary ion energies in units of keV) and
impact angles. Note again that cO can exceed the flux fraction of
0.67 expected for bulk SiO2 [1]. Note also that the 16O� impact
angles and the corresponding sputter yields [2] span the range
from the substoichiometric oxides in [3] to stoichiometric
oxides. Fig. 4 has several interesting features. First, the data
demonstrate a smooth and continuous variation of l(Au�)
P. Williams et al. / Applied Surface Science 252 (2006) 6429–64326432
versus cO, as indicated by the four lines drawn to guide the eye.
No discontinuities were detected near cO = 0.67, as would be
expected for segregation effects due to SiO2 formation. This
observation indicates that exclusion of impurity elements from
a surface oxide layer is not a major cause of the decay length
increase observed here. Additionally, the data demonstrate that
decay lengths increase much more strongly under oxygen
flooding than when oxygen ion beams are used to produce the
same oxygen levels. Both these observations are quite
consistent with the model of segregation in the near-surface
oxygen gradient proposed by Williams and Baker and, at least
for the Au overlayer studied here, do not support a model in
which formation of a stoichiometric SiO2 layer significantly
alters the segregation behavior.
4. Conclusion
The 18O implant technique offers a new and highly
convenient window into the chemistry of sputtered samples,
with a shallow sampling depth (the sputtered atom escape
depth) exactly appropriate for such studies. Quantitative real-
time measurements under sputtering conditions allow new
insights into fundamental issues in sputtered ion formation and
atom relocation during depth profiling. For the first time it is
possible to compare quantitatively the effects of oxygen on ion
yields in different materials, and in this way to disentangle the
effects of oxygen itself and of the matrix elements. Although
the high oxygen fractions measured here (and earlier by LEIS
[11]) are physically quite reasonable in the outermost atom
layer, careful comparisons of 18O SIMS data with LEIS data
will be required to determine the extent to which preferential
sputtering effects cause the oxygen fraction in the sputtered flux
(measured by 18O SIMS) to differ from the surface atom
fraction (which can be measured by LEIS).
Acknowledgement
This work was supported by the National Science
Foundation under Grant No. CHE 0111 654.
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