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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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