J. Phys. Chem. B 1998, 102, 3959-3966
3959
Chemical Modification of Polystyrene Surfaces by Low-Energy Polyatomic Ion Beams
Earl T. Ada, Oleg Kornienko, and Luke Hanley*
Department of Chemistry, m/c 111, UniVersity of Illinois at Chicago, Chicago, Illinois 60607-7061
ReceiVed: December 30, 1997; In Final Form: March 18, 1998
The chemical modification of polystyrene surfaces by low-energy (10-100 eV) SF5+, C3F5+, and SO3+ ions
was studied by X-ray photoelectron spectroscopy and two-laser ion trap mass spectrometry. The mechanism
of fluorination was found to be dissimilar for SF5+ and C3F5+ ions in this energy range at fluences of 10141016 ions/cm2. SF5+ was found to induce fluorination of the polymer surface by grafting reactive F atoms
upon dissociation at impact. SFn fragments were not found to be grafted or implanted into the polymer.
Sulfur was detected on the polymer surface only at incident energies above 50 eV and was found to be
sulfidic in nature. In contrast, C3F5+ ions induced grafting of both reactive F atoms and molecular CmFn
fragments from the dissociation of the incident projectile. Larger proportions of highly fluorinated sites and
thicker fluorocarbon layers were found for C3F5+ at all energies and fluences. A variety of aliphatic and
aromatic fluorine bonding environments were detected on both SF5+ and C3F5+ modified polystyrene surfaces.
I. Introduction
Interest in low-energy ion beam-surface interactions is driven
by two general applications: (1) the development of analytical
techniques such as low-energy ion scattering spectroscopy,
secondary ion mass spectrometry, and surface-induced dissociation mass spectrometry1-7 and (2) materials processing.1,8,9 Ion
beam processing of materials involves the modification of
surfaces by direct exposure to energetic ions. Ion beams have
been used to graft specific functional groups to polymer surfaces
for biomaterials, adhesion, and printing applications.10-12 Ion
beams have also been used to synthesize or deposit thin films
on metal and semiconductor surfaces with chemical and physical
properties different from the bulk substrate.1,14 Finally, ion
beams have been used to etch features on surfaces for pattern
generation.9
Another important motivation for ion beam-surface experiments is to model plasma-surface interactions. Plasma-based
processing of materials has gained widespread technological
prominence due to its several advantages over other methods
of surface treatment. However, the extreme complexity of the
plasma environment makes it difficult to gain a detailed
understanding of the fundamental plasma-surface interaction,
especially for plasmas of organic molecules.14 Therefore,
several studies have used ion beams of specific mass, kinetic
energy, and fluence incident on well-defined surfaces to model
the energetic species in plasmas that lead to fluorination,13,15,16
nitridation,17,18 hydroxylation, amination,11 and reactive ion
etching.12,16,19 These and other model systems revealed some
of the mechanistic details in the reaction processes, particularly
the importance of energetic ion bombardment of the surface
during plasma treatment.20 For example, specific chemical
functional groups can be grafted onto the surface of polystyrene
by bombardment with OH+ and NH+ ions at specific kinetic
energies (<100 eV) and fluences.11 Radicals also occur in
plasmas, typically at much higher concentrations than ions. Ion
beams can be used to model radical-surface interactions since
ions are often neutralized to radicals near surfaces. While the
* Corresponding author. E-mail: [email protected].
kinetic energy distributions of radicals are often lower than those
of ions in plasmas, this can be at least partially compensated
for by adjusting the kinetic energy distribution in the ion beam.
This lower kinetic energy distribution leads to a shallower
interaction depth with the surface. It is generally believed that
ions in the plasma are responsible for anisotropic etching while
radicals cause isotropic etching.20
Fluorination is one of the best studied of the ion beam induced
processes. This is due to the technological importance of surface
fluorination and the relatively large fluorine chemical shifts in
the core levels detected by X-ray photoelectron spectroscopy.
Both -CF- and -CF2- fluorine bonding environments were
produced in ion induced fluorination of polymer surfaces by 1
keV F+, CF3+, and non-mass-selected CFn+ ions.13 This
fluorination was independent of the type of unsaturated sites in
the target polymer. At this high incident kinetic energy, the
molecular nature of the fluorine-containing ion is unimportant,
and the molecular ion acts only as a convenient vehicle for
delivering reactive F atoms. In contrast, at low incident energies
of 2-100 eV, the molecular identity of the ion (CF+ vs CF2+),
the incident energy, and the reactivity of the target surface (Si
vs SiO2) determine whether the reaction pathway is etching or
fluorocarbon deposition.12 While plasma fluorination of several
polymers including polystyrene have been demonstrated in the
literature, most of the mechanistic discussions center around
the predominant radical species in the plasma as the fluorinating
agent.21-24 Much of this work neglects the contribution of
reactive molecular ions to surface fluorination. However, the
energetic SFn+ and CmFn+ ions that exist in SF6, CF4, C2F6,
and C3F6 plasmas probably contribute to the etching and
deposition processes of the overall plasma-surface chemistry.
We address in this paper the interaction of fluorinated
molecular ions with polystyrene (PS) by studying mass- and
energy-selected SF5+ and C3F5+ ions incident on PS thin films.
We also briefly examine the sulfidization of PS by comparing
SF5+ and SO3+ modification. We use X-ray photoelectron
spectroscopy and two-laser ion trap mass spectrometry25-27 to
determine the PS surface chemistry induced by these various
molecular ions. We emphasize the molecular character of the
S1089-5647(98)00595-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/22/1998
3960 J. Phys. Chem. B, Vol. 102, No. 20, 1998
projectile ion and the kinetic energies at which this character
directly controls the surface chemistry.
II. Experimental Details
The UHV system used for ion bombardment and analysis of
the PS films was described previously.7 Briefly, it consists of
a differentially pumped ion source attached to a target and
analysis chamber that is maintained at a base pressure of 2 ×
10-10 Torr by a 500 L/s turbomolecular pump. The ions were
formed in an electron impact source at 80 eV electron energy,
extracted, and accelerated to ∼1000 eV, mass-selected by a
Wien filter, bent 3°, decelerated, refocused, and transported by
a series of dc lenses to the target at normal incidence and at the
appropriate kinetic energy. Ions of SF5+, C3F5+, and SO3+ were
formed from SF6, C3F6, and SO3 precursor gases, respectively.
The techniques for handling SO3 gas reported by Roberts and
co-workers28 were followed here. The target chamber pressure
increased to about 1 × 10-8 Torr during ion exposure. Typical
ion currents for the three ions studied were 10-15 nA/cm2 with
energy distributions of 2-3 eV fwhm and a beam width of ∼1
cm. The incident energies were varied from 10 to 100 eV and
total ion fluences from 1014 to 1016 ions/cm2. The species purity
of the mass-selected SF5+ and SO3+ ion beams was estimated
to be 99%. For the C3F5+ ion beam, however, a purity of ∼95%
was estimated due to incomplete separation of this ion from
the C2F4+ and C3F6+ ions also produced in the electron impact
source. A small electron current from the filaments of a sputter
ion gun was delivered to the target surface during ion exposure
to compensate for ion beam induced charging.11 Control
exposures found negligible contributions to PS modification
from the background gas and the neutralizing electron beam.
Target materials consisted of PS films prepared by either spincoating onto 3 in. Si(100) wafers11 or by dip-coating stainless
steel plates from a 0.3% PS solution in CH2Cl2. Survey X-ray
photoelectron spectra were taken to ascertain the absence of
oxygen, solvents, and other surface contaminants (see below).
Survey spectra also monitored the uniformity of the polymer
film by noting the absence of substrate signals. PS films
prepared by this spin-coating method were previously estimated
to be 15-20 nm thick.11
The PS films were analyzed in situ before and after ion beam
exposure by X-ray photoelectron spectroscopy (XPS). Survey
and core-level spectra were taken using a hemispherical electron
energy analyzer operated at constant analyzer energy mode and
an achromatic Mg KR X-ray source (Fisons/VG CLAM2
system). A photoelectron takeoff angle of 45° from the surface
normal and a 50 eV analyzer pass energy were used. To correct
for surface charging during XPS, the -CH- C(1s) main line
was assigned to 285.0 eV for the untreated PS. This C(1s) peak
was used as a binding energy reference for all other XPS
features. No attempts were made to deconvolute the -CHC(1s) signal into the contributions from the alkyl backbone and
the aromatic ring carbon atoms. C(1s), F(1s), O(1s), S(2s),
S(2p), and Si(2p) spectra were measured. Quantitative estimations using peak areas were made after corrections for background (Shirley type subtracted) and photoelectric cross sections.29 Peak fitting was performed using the data analysis
software of the XPS system (Fisons/VG VGX900) with
component peaks having 70:30 Gaussian/Lorentzian character
of variable widths. Typical XPS scan times were 30 min. The
thickness, dF, of the fluorinated film was calculated from the
equation30
ln[(IF/IP) + 1] ) dF/(λF cos θ)
Ada et al.
where IF is the C(1s) intensity of the fluorinated carbons (sum
of -CF-, -CFCFn-, -CF2-, and -CF3- peak areas), IP is
the C(1s) intensity of the unreacted polymer, λF is the mean
free path for C(1s) photoelectrons taken to be ∼10 Å,13 and θ
is the photoelectron takeoff angle measured from the surface
normal. This equation assumes the fluorinated film to be a
homogeneous overlayer. Since the films prepared here probably
have a gradient in fluorine concentration with depth, the
thickness values are only approximate. Control studies were
made to determine the X-ray induced changes to the polymer
thin films. No significant X-ray damage to the films was
observed within the typical XPS acquisition time. Additionally,
a sample of p-fluoro-PS thin film coated onto Si(100) wafer
was analyzed for comparison.
A few ion modified PS films were analyzed ex situ by twolaser ion trap mass spectrometry (L2ITMS). The L2ITMS
instrument is discussed in detail elsewhere,26,27 and only a brief
description will be given here. Samples were transferred from
the ion modification/XPS apparatus to the L2ITMS instrument
with a maximum air exposure time of 30 min. The samples
were mounted at the end of a rotatable stainless steel probe in
the L2ITMS. Desorption of the surface species was achieved
by 1064 or 355 nm pulses of a Nd:YAG laser (9 ns pulse
length). The desorbed neutrals entered into the ion trap mass
spectrometer and were ionized by a second Nd:YAG laser.
Either 266 or 118 nm laser pulses of several nanoseconds
duration were used for photoionization. Nonselective singlephoton ionization with 118 nm radiation was shown to have
several advantages over the multiphoton scheme using 266 nm
radiation, including less photoinduced fragmentation of the
desorbed species.27,31,32 The smaller probability for photofragmentation simplifies the interpretation of the mass spectra and
minimizes erroneous assignment of photofragments as surface
species. However, while fluctuations in photoionization efficiencies are expected to be less at 118 nm than at 266 nm,
they may still vary considerably: this indicates that XPS should
be used for quantification of surface species rather than L2ITMS.
The positive ions formed by laser desorption/photoionization
were ejected from the trap sequentially by their mass/charge
(m/z) ratios and detected by a dynode and channeltron.
Alternatively, a narrow m/z range packet of ions was isolated
in the trap by the stored waveform inverse Fourier transform
(SWIFT) method while all other ions were ejected. These
selected ions were then translationally excited by further SWIFT
pulses so that they underwent collision-induced dissociation
(CID) with the ∼5 × 10-4 Torr of He buffer gas that is always
inside the trap. CID was used for the structural analysis of
individual mass ions.
III. Results and Discussion
A. Spectroscopic Analysis of Model Compounds. Figure
1 shows the F(1s) and C(1s) core spectra for the reference
sample of p-fluoropolystyrene (p-fluoro-PS). The F(1s) peak
is centered at 688.2 eV and has a fwhm of 1.7 eV and a π-π*
shake-up feature at ∼695 eV. Similar shake-up features have
been reported for various para-substituted polystyrenes33 and
other aromatic polymers.34 The F(1s) π-π* shake-up feature
is indicative of attachment of the fluorine atom to either an
aromatic ring or an unsaturated carbon atom.33 This shake-up
feature will be used in the discussion of the structure and
bonding in the modified polystyrene (PS) films below. The
X-ray satellite due to Mg KR3,4 occurs at ∼9 eV lower binding
energy (BE) of the main line.
The C(1s) spectrum of p-fluoro-PS is also shown in Figure
1. The C(1s) data (points) are fitted to two component peaks
Chemical Modification of Polystyrene Surfaces
Figure 1. F(1s) and C(1s) X-ray photoelectron (XP) spectra of
p-fluoropolystyrene reference sample coated on Si(100) wafer.
(solid lines) for the -CH- group at 285.0 eV (1.6 eV fwhm)
and the -CF- group at 287.2 eV (1.6 eV). The C(1s) peak
also displays a π-π* shake-up feature as a broad (2.2 eV)
asymmetric peak centered at ∼292 eV: this C(1s) shake-up is
also characteristic of an aromatic polymer.
For the succeeding discussions of the XPS C(1s) peak fitted
envelope, the following assignments are used: CHn at 285.0
eV (1.6 eV fwhm), -CCFn- at 286.5 eV (2.0 eV), -CF- at
287.2 eV (1.6 eV), -CFCFn- at 289.5 eV (1.6 eV), -CF2- at
291.2 eV (1.6 eV), π-π* shake-up at 292 eV (2.2 eV), and
-CF3- at 293.2 eV (1.6 eV). These peak assignments are
consistent with previously reported values in the literature.22,35
Two-laser ion trap mass spectrometry (L2ITMS) is used here
as a complementary surface analysis technique to provide a more
detailed view of the chemical structure of the polymer
surface.25-27 Figure 2 is a series of L2ITM spectra from a test
mixture of 1:1 PS and p-fluoro-PS deposited as a thin film onto
a stainless steel probe. This test mixture was chosen as a model
for the expected presence of fluorinated and unfluorinated
styrene residues resulting from the bombardment of PS by SF5+
and C3F5+ ions. The top spectrum is a single MS of the mixture
desorbed by 355 nm photons and subsequently ionized by 266
nm photons. The dominant peaks at m/z 104 and 122 are
assigned to styrene and p-fluorostyrene monomers, respectively.
CID is performed by first isolating the m/z 122 ion (middle
spectrum) and then by fragmenting this ion by collision-induced
dissociation (CID) with the background He in the ion trap. The
prominent peak in the CID spectrum (bottom, Figure 2) occurs
at m/z 96: it results from the formation of the monofluorobenzyl
cation by removal of the ethylene side chain from the pfluorostyrene monomer via a H atom rearrangement followed
by loss of C2H2. This result complements the XPS data in that
it indicates the fluorine atom is attached as an aromatic ring
substituent and confirms the assigned structure for the peak m/z
122 as the p-fluorostyrene monomer desorbed from the deposited test mixture.
Secondary ion mass spectrometry (SIMS) is currently the
most popular mass spectrometric method used to complement
XPS for polymer surface analysis.36 The positive ion SIM
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3961
Figure 2. Two-laser ion trap mass (L2ITM) spectra of a 1:1 mixture
of polystyrene and p-fluoropolystyrene.
spectrum of PS was previously measured and compared with
the spectra of plasma polymerized benzene, perfluorobenzene,
and perfluorotoluene to rationalize the structures of the latter.35
The SIM spectra of both PS and plasma polymerized benzene
were typical of polymers with alkylbenzene-type structures,
showing peaks at m/z 39, 51, 77, 91, and 104 and an extended
series of clusters ions.38 Our L2ITM spectra of pure PS (not
shown) are essentially identical to the published SIM spectra,
except that we do not observe any clusters beyond the dimer
(not shown). Two major advantages of L2ITMS over SIMS
are the ability to control the ionization step and the reduction
in fragmentation. However, SIMS tends to sample the upper
layers of a polymer surface while our L2ITMS desorbs the entire
polymer film. SIMS has also been used to determine molecular
weight distributions of polymers at interfaces. We do not use
L2ITMS to determine molecular weight changes of the polymer
surface, although these are likely to result from ion bombardment, because the laser desorption step in L2ITMS is accomplished via depolymerization. However, there are other
desorption schemes in L2ITMS that preserve the molecular
weight distribution of the polymer.25
B. Fluorination of Polystyrene by SF5+ and C3F5+. The
top of Figure 3 displays the F(1s) spectra of polystyrene (PS)
before and after exposure to 10 eV C3F5+ ions at fluences of 1
× 1015 and 3 × 1015 ions/cm2. The F(1s) peak is centered at
689 eV (2.1 eV fwhm). Different fluorine bonding environments cannot be resolved via the F(1s) peak shifts by our
spectrometer. However, the 0.8 eV shift in the peak centroid
and the larger width of the F(1s) signal compared to that for
p-fluoro-PS (Figure 1) attest to the presence of various F
bonding environments. Clark et al. reported a similar F(1s)
centroid shift of ∼1 eV in highly fluorinated polymers from
perfluorobenzene plasmas compared to less fluorinated polymers
from monofluorobenzene plasmas.37 The absence of a lowintensity π-π* shake-up in the F(1s) spectra of Figure 3 (cf.
Figure 1) indicates that fluorine atom attachment directly or
immediately adjacent to the aromatic functionality does not
constitute the dominant bonding environment. Rather, the
absence of the π-π* shake-up in the F(1s) spectra indicates
3962 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Ada et al.
Figure 3. F(1s) and C(1s) XP spectra of polystyrene films before and
after exposure to 10 eV C3F5+ ions at 1 × 1015 and 3 × 1015 ions/cm2
fluence.
Figure 4. F(1s) and C(1s) XP spectra of polystyrene films before and
after exposure to 10 eV SF5+ ions at 0.3 × 1016 and 1.0 × 1016 ions/
cm2 fluence.
that most of the covalently bound F atoms are sufficiently distant
from the aromatic group that the wave functions of the F atoms
do not overlap with those of the aromatic system.33 A broad,
low-intensity peak at 32 eV (not shown) is also observed to
appear after exposure to C3F5+ ions: this 32 eV peak is due to
the F(2s) photoelectron line typical of fluorocarbons.13,39
The bottom of Figure 3 displays the C(1s) spectra of PS
before and after exposure to 10 eV C3F5+ ions. Analysis of
the C(1s) line shape after ion exposure reveal the presence of
-CCFn-, -CF-, -CFCFn-, and -CF2- groups. The fluorinated film thickness estimated from the C(1s) component
peak areas is ∼2 Å after an ion exposure of 3 × 1015 ions/cm2.
Also, the CF2/CF area ratio at this fluence is greater than two,
indicating the presence of highly fluorinated sites on the surface.
The XPS data indicate that 10 eV C3F5+ ions are grafting
molecular fragments onto the PS surface rather than merely
donating individual F atoms. The structure for the molecular
ion has been reported to be the perfluoroallyl cation, CF2dCFCF2+.40 Addition of CmFn species to the PS surface is reasonable
at 10 eV impact energy since minimal ion fragmentation due
to the 1.5 eV average energy per atom of C3F5+. Furthermore,
a 10 eV projectile ion can only interact with the top few
angstroms of the surface, limiting the depth of fluorination into
the PS surface. Some degree of resputtering of the fluoropolymer is also possible due to either the direct impact of the incident
ion or the formation of volatile CF4 species. The C(1s) π-π*
shake-up feature also decreased in intensity at increasing ion
fluence with the appearance of the -CF2- peak. The decrease
in π-π* shake-up intensity generally indicates the ion induced
damage to the phenyl ring of PS11 and the formation of saturated
C bonding environments on the surface. This was also reported
in the plasma fluorination of polystyrene.22 The appearance of
a -CF3- peak at 293.2 eV at even higher ion fluences (g5 ×
1015 ions/cm2) prevented accurate quantification of the π-π*
shake-up due to peak overlap. Fitting this region of the C(1s)
envelope at these high ion fluences was therefore accomplished
by assuming only -CF2- and -CF3- components and neglecting the contributions of the π-π* shake-up altogether. An
additional error of e5% of the reported component peak areas
is estimated by this approximation.
Figure 4 presents the F(1s) and C(1s) spectra for PS before
and after exposure to 10 eV SF5+ ions at fluences of 3 × 1015
and 1 × 1016 ions/cm2. Comparison of the F(1s) peak areas
indicates that significantly less fluorine is incorporated into the
PS surface by 10 eV SF5+ ions than by C3F5+ ions at similar
energy and fluences. The F(1s) peak is centered at 688.3 eV
(2.1 eV), which is closer to the value found for p-fluoro-PS
than that observed for C3F5+. Similar to the 10 eV C3F5+ treated
PS, no evident π-π* shake-up satellite is found in the F(1s)
spectra of SF5+ treated PS, even at the high fluence of 1 ×
1016 ions/cm2. The absence of the shake-up feature suggests
that fluorine substitution onto the phenyl ring of PS does not
dominate here. Examination of the C(1s) spectra in Figure 4
also shows differences between the two ions. The decrease in
the intensity of the C(1s) π-π* shake-up indicates that, within
the fluorinated film thickness, many of the phenyl rings are
destroyed by the SF5+ ion impact. The CF2/CF area ratio is
less than unity for the SF5+ modified PS, compared with the
greater than two ratio observed for the C3F5+ modified PS. This
lower CF2/CF ratio is expected for SF5+ since, unlike the C3F5+
ion, any -CF- and -CF2- species must form from the reaction
of PS carbon atoms with the fluorine from the SF5+ ion. The
observation of a smaller -CFCFn- peak here, compared with
Figure 3, also lends support to this mechanism. However, our
use of achromatic X-ray radiation makes deconvolution of the
C(1s) peaks to obtain -CFCFn- peak areas somewhat tentative.
The L2ITM spectrum of a 10 eV C3F5+ treated PS film is
shown in Figure 5 (top), obtained using 1064 nm desorption
and 118 nm ionization. The peak assignments for this spectrum
indicate that more species exist on the surface than are
indentified by XPS: the most likely assignment for m/z 50 is
CF2+, m/z 52 is CH2F2+, m/z 56 is C4H8+, m/z 81 is C2F3+, m/z
91 is C7H7+, m/z 104 is C8H8+, m/z 122 is C8H7F+, m/z 131 is
C3F5+, m/z 142 is C8H6F2+, m/z 191 is C9H6F4+, and m/z 219
is C4F9+. The 118 nm photoionization tends to minimize
photofragmentation, indicating that many of these ions correspond to intact surface species. These results indicate that
Chemical Modification of Polystyrene Surfaces
Figure 5. L2ITM spectra of polystyrene film exposed to 10 eV C3F5+
ions at 5 × 1015 ions/cm2 fluence.
the modified surface contains intact unfluorinated styrene (m/z
104), styrene fragments (m/z 50, 52, 56, and 91), fluorinated
substituted styrene (m/z 122, 142, and 191), and deposited CmFn
species (m/z 50, 52, 81, 131, and 219). CID is applied to the
C9H6F4+ ion to determine its structure by isolating and then
fragmenting a packet of ions of m/z 191 ( 3, as shown in the
bottom of Figure 5. The major peak in this CID experiment is
due to m/z 91, which is assigned as either C6H5-CH2+ or its
resonant structure, the cyclic tropyllium ion, C7H7+. The m/z
91 most easily forms by the loss of neutral C2F4 from m/z 191,
indicating that C2F4 remains as an intact species on the m/z 91
fragment. Thus, both the L2ITMS and the XPS data for PS
modified by 10 eV C3F5+ ions indicate the grafting of intact
CmFn fragments from the incident C3F5+ ion onto PS. The
observation of m/z 219 for C4F9+ also indicates the presence of
high mass fluoropolymers arising from the direct polymerization
of projectile ions on the surface: similar species were formed
by gas-phase polymerization in C3F6 plasmas.24
At higher incident energies, greater fluorination is achieved
with both SF5+ and C3F5+ ions. Figure 6 shows the C(1s) and
F(1s) XP spectra for films exposed to either 50 eV SF5+ (top)
or C3F5+ (bottom) ions at fluences of 5 × 1015 ions/cm2. The
fluorinated film thicknesses are estimated to be ∼4 Å for the
SF5+ treated film and ∼13 Å for the C3F5+ treated film. These
fluorinated thicknesses are larger than observed with 10 eV ions
and are expected since the penetration depth of the fluorinating
species into the polymer surface should be deeper at 50 eV.
Furthermore, greater projectile dissociation at impact as well
as polymer bond scission is expected to result in a larger number
of reactive F atoms or CmFn + ions from the projectile and
reactive sites on the polymer surface. These are manifested by
the higher amounts of -CF-, -CF2-, -CFCFn-, and -CF3bonding environments for films exposed to both ions, compared
with the 10 eV data. At 50 eV incident energy, both ions
produce fluorinated films with a CF2/CF area ratio less than
unity. At this higher energy, only smaller amounts of the intact
molecular ion can survive the collision with the polymer surface.
In contrast, the low impact energy of the 10 eV C3F5+ ion allows
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3963
Figure 6. F(1s) and C(1s) XP spectra of polystyrene films exposed to
50 eV SF5+ ions (top) and 50 eV C3F5+ ions (bottom) at 5 × 1015
ions/cm2 fluence.
it to survive the collision intact and subsequently graft onto the
polymer surface. The difference in the fluoropolymer amount
with respect to the hydrocarbon fraction between the SF5+ and
the C3F5+ treated PS films at 50 eV indicates that the SF5+ ion
continues to be a mere vehicle for fluorine atoms which are the
active fluorinating agents. In contrast, the C3F5+ ion can add
intact CmFn fragments onto the surface, in addition to fluorine
atoms. Since the number of available fluorine atoms is the same
for both incident ions, this also indicates that not all of the
available fluorine atoms from SF5+ get incorporated into the
fluoropolymer film. Rather, some volatile SFn species may be
emitted into the vacuum. Similar trends are observed for the
two ions even at 100 eV incident energies. Overall, the thicker
fluorinated films obtained from C3F5+ treatment support the
argument for grafting of CmFn+ onto the PS surface even at
these higher energies.
The top of Figure 7 shows the L2ITM spectra for PS exposed
to 50 eV SF5+ at a fluence of 5 × 1015 ions/cm2. The 266 nm
multiphoton ionization is employed here, as is used in Figure 2
(cf. 118 nm ionization used in Figure 5). Multiphoton ionization
is used here to selectively ionize aromatic species, minimizing
ionization of nonaromatics such as SFn. The nonfluorinated
PS monomer peak at m/z 104 dominates the spectrum. Additionally, the prominent m/z 121 and 142 peaks correspond to
singly and doubly fluorinated PS monomers. The m/z 121 peak
is offset by only one H atom off from the dominant peak of
p-fluoro-PS (Figure 2). Comparison of the mass spectrum of
SF5+ modified PS with the p-fluoro-PS MS indicates enhanced
m/z 50, 53, and 78 peaks in the former. These are common
benzene fragmentation peaks, indicating that 50 eV SF5+ induces
bond cleavage in the styrene monomer.
The bottom of Figure 7 displays the CID spectrum of the
m/z 142 doubly fluorinated ion. Comparison of Figure 7 with
the CID spectrum of p-fluoro-PS (bottom of Figure 2) indicates
a diversity of fluorination sites on the styrene monomer that in
turn generates several different structures of the C8H6F2+ ion
at m/z 142. The predominant feature in CID of the pfluorostyrene monomer ion at m/z 122 is cleavage of the
ethylene side chain via hydrogen rearrangement and loss of C2H2
3964 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Ada et al.
+
Figure 7. L2ITM spectra of polystyrene film exposed to 50 eV SF5
ions at 5 × 1015 ions/cm2 fluence.
C6H5F+
to form
at m/z 96. Additionally, the absence of a strong
m/z 102 or 103 peak indicates no loss of the fluorine atom from
the ring (Figure 2), by either F or HF loss. In contrast, the
CID spectrum of the m/z 142 ion from SF5+ modified PS
(bottom, Figure 7) shows a different fragmentation pattern.
Application of the side chain fragmentation mechanism of
p-fluoro-PS to the m/z 142 ion explains the presence of the m/z
77, 97, and 114 ions by fluorinating both, one, or none of the
side chain carbons, respectively (assuming that either acetylenic
or ethylenic neutrals can be lost). The presence of m/z 121 is
due to a H2F loss from m/z 142, indicating fluorination of the
side chain. The presence of m/z 104 indicates loss of F2: this
ion and m/z 77 both result from F atoms on adjacent carbons.
Overall, the CID spectrum of m/z 142 indicates that SF5+
modification can fluorinate any of the carbons on the styrene
monomer and, in some cases, adjacent carbons. These results
are consistent with the absence of a π-π* shake-up feature in
the F(1s) spectra (inset of Figure 6, see above). This complex
surface structure proposed here is in accord with the results of
various fluorinated plasma polymer studies.
Figure 8 plots the percent contribution of various fluorinated
carbon bonding environments to the total C(1s) line shape as a
function of the incident ion energy for SF5+ (top) and C3F5+
(bottom) for a total ion fluence of 5 × 1015 ions/cm2. The
contributions from the -CH- components were not included
in Figure 8 to highlight the differences in the various fluorinated
carbon environments: these can be easily derived from the total
contributions of the fluorinated environments. In general, the
relative amounts of fluorinated carbon environments increased
with the ion energy for both ions. The fluorination of PS
increased much more rapidly with increasing energy of C3F5+
ions than with that of SF5+ ions. For example, from 10 to 100
eV ion energies, the total percentage of surface fluorinated sites
increased from 18% to 71% for C3F5+ ions. In contrast,
fluorinated sites for SF5+ ion exposed samples increased from
14% to 34% in the same energy range. For SF5+ ion
bombardment, any fluorine-bearing carbon on the surface can
only result from the reaction of a polymer C atom with a F
Figure 8. Percentage contribution of fluorinated bonding environments
to the total C(1s) spectrum of polystyrene films as a function of incident
ion energy for SF5+ (top) and C3F5+ (bottom) ions.
atom from the dissociated projectile. In contrast for C3F5+ ions,
the reaction of both the dissociated F atoms and the CmFn+
molecular fragments with the polymer can produce these
fluorinated bonding environments on the surface. As the
collision energy is increased, more projectile dissociation occurs,
generating more reactive F atoms from SF5+ and C3F5+ ions.
This is clearly seen from the steady increase of -CF3environments with ion energy. The projectile penetration depths
are also necessarily increased with ion energy, and therefore,
C atoms deeper in the polymer become available for reaction.
This can also be interpreted from the increasing amounts of
-CCFn- components with increasing ion energy for both ions.
Here again, the difference in the mechanisms of surface -CFformation between the two ions is shown by the larger
proportion of induced -CCFn- environments from C3F5+
ions: direct attachment of CmFn units to the polymer in addition
to F atom attachment. At 10 eV, C3F5+ induces a surface CF2/
CF ratio of ∼2.3, whereas SF5+ gives a ratio of 0.4. This is a
further evidence that CmFn attachment from C3F5+ ions dominates at 10 eV. At increasing ion energies, however, the surface
CF2/CF ratio for the C3F5+ ion decreases to a value below unity
and approaches that for the SF5+ ion at 100 eV.
Figure 9 summarizes the dependence of overall fluorine
incorporation by PS on the ion incident energy and identity.
Values of F/C ratio are estimated in two ways: (1) calculating
the F(1s)/C(1s) peak area ratio and (2) using the C(1s)
component peak fit results. The overall fluorination is similar
for both ions at 10 eV, with F/C ratios of ∼0.2. As the ion
energy increases, there is a larger surface fluorination effect
for C3F5+ ions. At 100 eV, C3F5+ ions are 2-3 times more
effective at fluorination than SF5+, with F/C ratios of ∼0.7 vs
0.3, respectively. As discussed above, the enhanced ability of
C3F5+ to fluorinate PS is due to the deposition of CmFn+
fragments. In contrast, SF5+ species can only donate F atoms
for subsequent reaction with the film. The two methods of
calculating F/C ratios are used to determine the presence of
nonstoichiometric fluorine environments such as trapped F atoms
or SFn fragments or F atoms that reacted with the Si or stainless
steel substrates. The presence of nonstoichiometric F environ-
Chemical Modification of Polystyrene Surfaces
Figure 9. Relative F/C atomic ratio for polystyrene films exposed to
SF5+ and C3F5+ ions plotted as a function of incident ion energy.
ments is indicated by the larger F/C ratios calculated from the
F(1s) and C(1s) core levels compared to those calculated from
the C(1s) components. Reasonable agreement was, however,
found between these two estimates of F/C ratio, within
experimental error.
C. Sulfidization of Polystyrene by SF5+ and SO3+. No
sulfur photoelectron peaks are observed from the films exposed
to 10 eV SF5+ even at the highest fluence of 1 × 1016 ions/
cm2. Similarly, sulfur was never detected by XPS on fluoropolymer films formed by the fluorination of PS by SF6
plasmas.23 SF6 plasmas were thought to be sources of F and
SFn radicals, but apparently, SFn radicals do not graft to the
polymer surface. At 10 eV incident energies, the SF5+ ions
react by donating F atoms to the polymer surface in the same
way that SF6 plasmas: by merely acting as convenient sources
of reactive F atoms. At this low incident energy, SF5+ does
not liberate S atoms that can react with the surface since it
requires ∼18 eV to fully dissociate into its constituent atoms.41
Likewise, the 10 eV SF5+ ions do not have enough momentum
to penetrate and implant into the polymer surface. At higher
incident energies, however, beginning at about 50 eV, S(2p)
and S(2s) signals (not shown) are observed whose intensities
increased with total ion fluence. The implications of this
observation are further discussed below for the 50 eV SF5+
exposure.
The observation of sulfur incorporation into the fluoropolymer
film after exposure to SF5+ ions commenced at about 50 eV
incident energy. The amount of sulfur increases with energy
and fluence. Figure 10 shows the S(2p) spectra for the 50 eV
SF5+ exposed PS at 0.5 × 1016 and 1.0 × 1016 ions/cm2 total
fluence (top). Also in Figure 10 is the S(2p) spectra of a PS
film exposed to 50 eV SO3+ ions at 3 × 1015 and 5 × 1015
ions/cm2 total fluence (bottom). A single S(2p) signal centered
at 163.8 eV is observed for the SF5+ exposed film. In
comparison, two S(2p) peaks at 163.8 and 168.6 eV were found
for the SO3+ exposed sample. The peak at 163.8 eV is assigned
to a sulfide functionality, and that at 168.6 eV is assigned to an
SO3 group in accord with the literature values for S(2p) shifts.42
The absence of a S(2p) signal at around 169 eV suggests the
absence of any surface SFn groups on the fluorinated polymer.
This supports the notion that SFn groups are not grafted onto
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3965
Figure 10. S(2p) XP spectra of polystyrene films exposed to 50 eV
SF5+ ions (top) and 50 eV SO3+ ions (bottom) at various fluences.
the PS surface by incident SF5+ ions. The sulfidic nature of
the incorporated sulfur indicates that these reacted on the surface
after complete dissociation of the SF5+ projectile. Any surviving
SFn species (possibly SF4) after projectile impact are apparently
reemitted back into the vacuum.
IV. Conclusions
The chemical modification of polystyrene (PS) thin films by
10-100 eV SF5+, C3F5+, and SO3+ ions was studied using the
complementary techniques of XPS and L2ITMS. Different
mechanisms of fluorination were found to be induced on the
polymer surface by SF5+ and C3F5+ ions. Below 50 eV kinetic
energies, addition of intact molecular and fragment ions of C3F5+
was observed. At these energies, SF5+ caused surface fluorination only by the reaction of liberated F atoms after ion impact
on the surface. The absence of sulfur on the modified surface
at SF5+ incident kinetic energies <50 eV suggested no addition
of the molecular projectile onto the polymer structure. At
kinetic energies of 50-100 eV, higher projectile dissociation
and increased depth penetration of the ion below the polymer
surface resulted in thicker fluorinated films and the appearance
of CF3 bonding environments for both SF5+ and C3F5+ ions.
The fluorinated film thickness obtained from C3F5+ ions at all
energies, however, was consistently larger than that obtained
from SF5+ ions. This further supports the argument for grafting
of CmFn fragments onto the polymer surface. Various fluorine
bonding environments were found by both XPS and L2ITMS
analysis of the modified polymer surface exposed to SF5+ and
C3F5+ ions. These bonding environments included aromatic and
aliphatic sites of fluorine attachments. Both ions were also
found to fragment the styrene monomer on the surface. Sulfide
formation on the surface due to >50 eV SF5+ exposure was
determined by comparison with a 50 eV SO3+ bombarded PS
sample. No high binding energy component peaks were found
in the S(2p) XP spectrum for the SF5+ bombarded films,
indicating the absence of molecular attachment of SFn species
from SF5+ ions onto the PS surface.
Our results have important implications for fluorinated plasma
modification of surfaces. It is clear that the kinetic energy and
chemical structure of the molecular constituents of plasmas will
3966 J. Phys. Chem. B, Vol. 102, No. 20, 1998
contribute to the plasma-surface chemistry. Molecular ions
present in SF6 or C3F6 plasmas will contribute to the surface
fluorination of polymer surfaces. Support is found for the wide
diversity of chemical species formed from plasmas of organofluorine and other molecular compounds. Support is also found
for prediction that plasma precursor gases with low or zero CF/F
ratios will behave as etching plasmas (i.e., SF6) whereas those
with high CF/F ratios will deposit plasma polymers (i.e., C3F6).22
Acknowledgment. We would like to thank Tim Keiderling
for the loan of the Lumonics Nd:YAG laser for the L2ITMS
instrument and Veronica Frydman for providing the p-fluoropolystyrene. This research is supported by the National
Science Foundation (CHE-9632517 and CHE-9457709). L.H.
is supported by a NSF Young Investigator Award (1994-98).
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