is, B
Applied Surface Science 255 (2008) 970–972
pth
the
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O2
ativ
signal from the most characteristic ions was measured all over the layer.
With 500 eV O2+, real molecular depth-profiles were also obtained in both the positive and the
Contents lists availab
Applied Surfa
els1. Introduction
Organic depth profiling with time-of-flight secondary ion mass
spectrometry (ToF-SIMS) has become amajor challenge, especially
with the use of cluster ion beams which prove successful for depth
profiling a variety of polymers [1–6]. In a recent paper [7], we have
shown that a 75 nm polycarbonate (PC) layer could be depth
profiled by using 200 eV Cs ions as sputtering beam: the most
specific PC fragments signals were surprisingly stable all over the
profile. A strong negative ionisation effect due to Cs implantation
in the polymer was inferred to explain the data. The erosion rate in
this previous study was about 1 nm/min with a relatively low Cs
current (3.6 nA).
In order to draw more general conclusions on organic depth
profiling with low energy ions, we applied the low energy Cs
sputtering on two other polymers: polymethylmethacrylate
(PMMA) and polyethylene terephthalate (PET). Oxygen ions
(O2+) were also used to analyse PMMA films. The general idea in
using low energy reactive ions for sputtering polymer layers is that
the sputtering ion energy should be low enough to avoid complete
degradation of the polymer under irradiation, while the positive or
negative ionisation of fragments is significantly enhanced. This
study primarily aimed at proving the feasibility of low energy ions
depth profiling on several thin polymer films and bulk polymers
and did not focus on sputtering yield measurements.
2. Experimental
PMMA pellets (Scientific Polymer Products Inc., Ontario, NY)
were dissolved in dichloromethane (5.0 g/L) and spuncoated (at
1500 rpm) onto silicon substrates, previously cleaned in
isopropanol and acetone in an ultrasonic bath. The PMMA thin
film’s thickness was not measured. Bulk films of PMMA
(Goodfellow Ltd., UK; 1 mm thick) and PET (1 mm thick) were
also studied.
Depth profiles were acquired on a ToF-SIMS IV instrument
(ION-TOF GmbH, Mu¨nster, Germany) using a 15 keV Ga+ gun for
analysis and a 200 eV Cs+ gun or 500 eV O2+ gun for sputtering.
The incident angle for all beams was 458. Rasters for analysis
and sputtering were respectively 100 mm  100 mm and
300 mm  300 mm. The Ga ion current was about 1.5 pA in
every experiment. Cesium and oxygen currents were respec-
tively 5 nA and 18 nA. A fast destruction of the polymer by the
Ga beam was avoided by limiting the Ga fluence: this was
achieved by using a non-interlaced mode. This mode allows
eroding the surface during several seconds (typically 10 s)
before scanning the gallium area during one analysis cycle,
lasting 1.64 s. Flood gun was used to compensate the charging
negative modes. Once again, the main characteristic fragments of PET or PMMA remain detectable with
stable yields all over the profile.
 2008 Elsevier B.V. All rights reserved.
* Corresponding author.
E-mail address: Laurent.houssiau@fundp.ac.be (L. Houssiau).
0169-4332/$ – see front matter  2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2008.05.027Molecular depth profiling of polymers w
L. Houssiau *, B. Douhard, N. Mine
University of Namur (FUNDP), Physics Department, LISE Laboratory, 61 rue de Bruxelle
A R T I C L E I N F O
Article history:
Available online 8 May 2008
Keywords:
Cs sputtering
Oxygen sputtering
Organic depth profile
PMMA
PET
A B S T R A C T
The need for a molecular de
in the SIMS community in
ion beam depth profiling.
very low energy (down t
surprisingly, wewere able
fluence.
Polymethylmethacrylat
with 200 eV Cs+ and 500 eV
mode, due to a strong neg
relatively high and stable
journal homepage: www.th very low energy ions
-5000 Namur, Belgium
profiling technique to study organic layers has become a strong incentive
last few years, especially with the recent successes obtained with cluster
his work, we have investigated a thoroughly different approach by using
00 eV) monoatomic or diatomic ions to sputter organic matter. Quite
etain specificmolecular information on various polymers even at very high
MMA) and polyethylene terephthalate (PET) films were depth-profiled
+ ions. With 200 eV Cs ions, the best profiles were obtained in the negative
e ionisation yield enhancement related to Cs retention in the polymer. A
le at ScienceDirect
ce Science
evier .com/locate/apsusc

effect on bulk polymers. The reflectron voltage was set between
20 V and 30 V higher than the surface voltage (measured by the
onset of the secondary ions image). The bias voltage was set to
zero to avoid cesium redeposition in the crater. Depth profiles
were obtained in the two ion polarities. Ion yields are given in
counts/analysis cycle (counts/1.64 s).
3. Results and discussion
3.1. PMMA depth profiling with 200 eV Cs+ ions
The depth profile obtained by sputtering bulk PMMA in the
negative ion mode is shown in Fig. 1. Two categories of ions are
displayed: small fragments, not specific to the polymer (C, 18O,
C2H) and larger fragments directly reflecting the PMMA
chemistry (C3H3O at m/z 55.02, C2H3O2 at m/z 59.01 and
C5H9O2 at m/z 101.04). The Cs signal is also selected in order to
estimate the Cs surface content. A transient regime is first
the sputtering: this effect is described in detail in another
paper [10].
3.2. PMMA depth profiling with 500 eV O2+ ions
The depth profiles obtained by sputtering a spuncoated PMMA
layer on Si with 500 eVO2+ ions, in the negative ionmode, is shown
in Fig. 2. Small ions are displayed (C, C2H), along with PMMA
fingerprint ions (CH3O at m/z 31.02, C3H3O at m/z 55.02 and
C2H3O2 at m/z 59.01). The substrate is identified with the Si ion
signal. The PMMA peaks decrease rapidly during the first seconds
of the profile, revealing a rapid degradation of the polymer.
Nevertheless, the signals stabilize at intensity levels still allowing
molecular identification of the polymers: about 400 counts/cycle
for CH3O, 25 counts/cycle for C3H3O and only 10 counts/cycle for
C2H3O2. The methyl acrylate monomer peak (C5H9O2) remains
detectable but at very low levels (not shown). Negative depth
profiling of PMMAwith 500 eV O2+ ions is then clearly possible but
with much lower intensities than with 250 eV Cs+ ions. In both
L. Houssiau et al. / Applied Surface Science 255 (2008) 970–972 971observed during which all the negative ion intensities rise by
several orders of magnitude. This transient is quite similar to the
one regularly observed on inorganic samples (e.g. from the
semiconductor industry [8]) and is explained by an enhanced
negative ionisation due to Cs implantation. In the case ofmetals or
semiconductors, this is often explained by changes in the work
function [9] but a new model is clearly required to understand
how organic molecules are being ionised by surrounding Cs
atoms. Cs is the most electronegative element and also a very
strong reducing agent (its standard reduction potential is
2.92 V). One can therefore think it easily reduces the organic
fragments by transferring an electron to them, creating negative
ions. Following this short transient, the ion signals reach a
stationary state. As was observed in our previous study on PC [7],
specific fragments of PMMAare detectedwith constant intensities
during the sputtering, proving once again the feasibility of
polymer depth profiling with low energy Cs. In particular, the
C5H9O2 signal remains stable throughout the profile: this is
remarkable as this ion is the methylmethacrylate monomer plus
one hydrogen atom.
Experiments were also carried out in the positive mode (not
shown), but the PMMA fingerprint ions disappeared as soon as
the Cs beam was turned on. This is partly due to the polymer
degradation under irradiation but also to a strong decrease in
the positive ionisation induced by the Cs. However, clusters
made of one or more Cs atoms bound to organic fragments of
the polymer were observed in the spectrum and survivedFig. 1. 200 eV Cs+ negative ion depth profile of bulk PMMA.cases, the energies per atom are similar (250 eV/oxygen atom vs.
200 eV Cs) but no significant negative ionization enhancement
occurs with oxygen.
The Si/PMMA interface is rather poorly defined in the profile.
This is due to ripples at the PMMA surface caused by the
spincoating deposition process which creates high thickness
variations over the analysed area. The interface is reached earlier
(resp. later) in areas where the polymer film is the thinnest (resp.
thickest), producing a slow increase in the Si signal at the same
time as a slow decay of the polymer signal. This effect was
described in detail in our previous study on a PC thin film [7].
The depth profile obtained on the same samplewith 500 eVO2+,
in the positive mode, is presented in Fig. 3. The PMMA layer is
measured by means of the fingerprint peaks C2H3O2+ at m/z 59.01,
C3H3O+ at m/z 55.02 and C4H5O+ at m/z 69.03. The Si interface is
identified by the 30Si+ peak. A drop in the intensities of PMMA
fragments is againmeasured during the first seconds of the profile,
but it is less pronounced than in the negative mode. After this
initial drop, the intensities rise before reaching a relatively high
steady state value. The PMMA remains perfectly identifiable
during this regime, with typically 70 counts/cycle for C2H3O2+, 50
counts/cycle for C4H5O+ or 30 counts/cycle for C3H3O+ and C3H7O+
(not shown).
This profile is interesting because it demonstrates that positive
depth profiling of PMMA is possible with low energy diatomic O2+Fig. 2. 500 eV O2+ negative ion depth profile of PMMA spuncoated on Si.

L. Houssiau et al. / Applied Surface Science 255 (2008) 970–972972ions, with a stable polymer signal and fair yields. This was not the
case with Cs which is only useful for negative depth profiling. The
recovery of the PMMA signals after the initial drop is interpreted
in terms of positive ionisation enhancement due to the oxygen
implantation. Oxygen acts here as an oxidizing agent. This is the
opposite effect to Cs in the negative mode. This profile was
obtained on the same rippled layer as the negative one, which
explains the poor definition of the PMMA/Si interface. In both
polarities, the interface was reached after about a 500 s
sputtering time. Even though the PMMA film thickness is not
known, one can estimate that the erosion rate is not unreason-
ably slow and probably comparable to the one measured on PC
(1 nm/min).
3.3. PET depth profiling with 200 eV Cs+ ions
Following PC and PMMA analysis, bulk PET specimens were
depth profiled with low energy Cs: the negative depth profile is
Fig. 3. 500 eV O2+ positive ion depth profile of PMMA spuncoated on Si.displayed in Fig. 4. Specific PET fragments were selected, namely
C2H3O at m/z 43.02, COOH at m/z 45.00, C2H3O2 at m/z 59.01,
C6H4 at m/z 76.03 and C7H5O2 at m/z 121.03. The C and Cs ion
were also selected.
The variation of the polymer fingerprint ions is rather similar to
the PMMA case, with a transient related to Cs implantation
followed by a stationary regime in which the signals are
surprisingly stable. The strong negative ionization enhancement
due to Cs implantation plays amajor role, boosting the PET specific
signals to values close or above 1000 counts/cycle. The C7H5O2
evolution is particularly outstanding since this ion is a major PET
specific peak, made of a carboxyl group attached to a phenyl ring.
This proves that, at such a low energy, Cs+ ion bombardment
preserves most of the polymer chemical structure, as was already
observed on PMMA and PC.4. Summary
This paper reports on successful organic depth profiling on
PMMA and PET by using simple monatomic (Cs+) or diatomic (O2+)
ions at very low energy. It follows a previous report on PC layers by
the same authors [7]. Two effects prevent the total loss of the
polymer signal under irradiation, namely the ion energy and the
ionization enhancement. The ion energy is low enough to avoid
major modification of the polymer chemistry, such as graphitiza-
tion, so that most of the fingerprint spectrum is maintained. The
ion reactivity improves dramatically the ionization of the polymer
fragments, in the negative mode for Cs+ and in the positive mode
for O2+. Therefore, using very low energy ions appears to be a very
promising way to depth profile organics, in both polarities,
complementary to the cluster ion bombardment.
Acknowledgements
The ToF-SIMS instrument was acquired thanks to the contribu-
tion from the F.R.F.C. N. Mine gratefully acknowledges a grant from
F.R.I.A.
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