Modification of an NF3 film by sub-excitation electrons

Article abstract of DOI:10.1016/S0009-2614(01)00517-6

By applying the IRAS technique (infrared-absorption-reflection spectroscopy) and electron stimulated desorption (ESD) of negative ions we demonstrate that a 10 monolayer (ML) film of NF₃ is degraded in the course of low-energy electron irradiation in the range 0-5 eV which is far below the electronic excitation of NF₃. Degradation is accompanied by the desorption of F^- fragment ions from the film and formation of NF₂ radicals and N₂F₄ molecules in the film. The energy dependence of the degradation cross-section follows that for resonant (dissociative) electron attachment in the low-energy region (≈0-5eV) and increases above 6 eV. We therefore identify (dissociative) electron capture at low energy as the only initial reaction responsible for the chemical changes in the NF₃ film.

Full text of DOI:10.1016/S0009-2614(01)00517-6

29 June 2001
Chemical Physics Letters 341 ,2001) 401±406
www.elsevier.nl/locate/cplett
Modi®cation of an NF3 ®lm by sub-excitation electrons
Petra Tegeder, Eugen Illenberger *
Institut fur Chemie, Physikalische und Theoretische Chemie, Freie Universitat Berlin, Takustrasse 3, D-14195 Berlin, Germany
Received 23 February 2001; in ®nal form 24 April 2001
Abstract
By applying the IRAS technique ,infrared-absorption-re¯ection spectroscopy) and electron stimulated desorption
,ESD) of negative ions we demonstrate that a 10 monolayer ,ML) ®lm of NF3 is degraded in the course of low-energy
electron irradiation in the range 0±5 eV which is far below the electronic excitation of NF3. Degradation is accompanied
by the desorption of FÀ fragment ions from the ®lm and formation of NF2 radicals and N2F4 molecules in the ®lm. The
energy dependence of the degradation cross-section follows that for resonant ,dissociative) electron attachment in the
low-energy region ꢀꢁ 0±5 eV† and increases above 6 eV. We therefore identify ,dissociative) electron capture at low
energy as the only initial reaction responsible for the chemical changes in the NF3 ®lm. Ó 2001 Elsevier Science B.V.
All rights reserved.
1. Introduction
siderably below the threshold for electronic exci-
tation in NF3ꢀꢁ 7 eV [1]) .
In this contribution we study the reactions in-
duced by a beam of energy selected electrons in-
teracting with a 10 monolayer ,ML) ®lm of NF3
deposited on a cryogenically cooled ,25 K) Au
substrate under UHV conditions. The reactions
are monitored by infrared-absorption-re¯ection-
spectroscopy ,IRAS) and electron stimulated de-
sorption ,ESD) of negative ions, both recorded as
a function of the primary electron energy. By
comparing with data from the gas phase it can be
concluded that in the energy range 0±5 eV reso-
nant dissociative electron capture is responsible for
the degradation of the NF3 ®lm, the desorption of
FÀ ions and the formation of new species in the
®lm. The relevant energy range ,<5 eV) is con-
The electron impact behaviour of gas phase
NF3 ,single molecules and clusters) has attracted
considerable interest in the last years as this com-
pound is used as feed gas in various plasma etching
processes [2±5] and also as a ¯uoride source in
excimer gas laser media [6,7]. Owing to the com-
paratively high dissociative electron attachment
,DA) cross-section it is also used to generate FÀ in
the course of ion±molecule reactions [8,9]. The DA
cross-section in NF3 generating FÀ is 2:2Â10À16
cm2 and thus more than two orders of magnitude
larger than that in CF4 ,1:6 Â 10À18 cm2 [11,12]).
In the gas phase, the DA products FÀ2 and NF2À are
also observed [10,13], however, with cross-sections
about two orders of magnitude below that of FÀ.
The present study is a continuation of our
previous investigation of electron stimulated de-
sorption of FÀ from condensed NF3 [14] where it
was shown that FÀ desorption is driven by DA to
* Corresponding author. Fax: +49-30-8386612.
E-mail address: iln@chemie.fu-berlin.de ,E. Illenberger).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 , 0 1 ) 0 0 5 1 7 - 6

402
P. Tegeder, E. Illenberger / Chemical Physics Letters 341 ,2001) 401±406
NF3 molecules at or near the surface within a
resonant low-energy feature ,0.5±4 eV).
action pathways involving the associated elec-
tronically excited neutrals can be excluded. To our
knowledge this is the ®rst example of a reaction in
condensed material initiated by electrons appre-
ciable below the threshold for electronic excita-
tion. In that case, only one single possible process,
namely ,resonant) dissociative electron attachment
via the electronic ground state of the negative ion
is responsible for the observed reaction.
Reactions in condensed material initiated by
low-energy electrons have been observed before
and we mention here electron-induced synthesis of
ozone in a dioxygen matrix [15], activation of
adsorbed cyclopropane [16], bond selective disso-
ciation of alkanethiol-based self-assembled mono-
layers [17] and DNA strand breaks by low-energy
,3±20 eV) electrons [18]. In the last two studies the
explicit energy dependence of the corresponding
reactions has been studied resulting in a resonant-
like behaviour peaking around 10 eV. In particular
the ®ndings on DNA strand breaks present a
fundamental challenge to the traditional notion
that genotoxic damage by secondary electrons can
only occur at energies above the onset of ionization
[18]. While all the above-mentioned reactions are
induced by sub-ionization electrons it is also clear
that in each case electronic excitation was involved,
either electronically excited neutral states or elec-
tronically excited resonances.
2. Experimental
The present experiments were carried out in a
UHV apparatus ,Fig. 1) consisting of a trochoidal
electron monochromator ,TEM), a cryogenic-
cooled monocrystalline Au substrate mounted on
a manipulator and a commercial quadrupole mass
spectrometer with an ion extraction system [12].
The substrate can be cooled down to approxi-
mately 25 K by means of a closed cycle He re-
frigerator and resistively heated up to several 100
K. The molecules are condensed on the metallic
substrate by exposing it to a volumetrically cali-
In the present contribution we study reactions
induced by sub-excitation electrons where any re-
Fig. 1. Scheme of the experimental setup to study electron initiated reactions in an NF3 ®lm by means of the IRAS technique and
electron stimulated desorption of fragment anions.

P. Tegeder, E. Illenberger / Chemical Physics Letters 341 ,2001) 401±406
403
brated gas quantity e€using from a capillary lo-
cated 0.7 cm from the gold crystal.
The TEM operates with a weak homogeneous
magnetic ®eld ,ꢁ 30 Gauss) which aligns the
electrons thus preventing spreading of the beam.
The instrument has thereby proven to be particu-
larly suited to study electron-induced processes in
the low-energy domain. The base pressure in the
chamber is in the 10À10 mbar range.
After electron bombardment, the target can be
rotated into a position where an infrared beam is
transmitted through the adsorbate and re¯ected at
a grazing angle ,85°) for applying the IRAS tech-
nique which has recently been established in our
laboratory [19]. As a consequence of the dielectric
properties of the metallic substrate only vibra-
tional modes having a component of the dynamic
dipole moment perpendicular to the surface can be
excited and only at grazing angles of incidence one
can expect appreciable IRAS intensities [20].
The electron energy is calibrated by the onset of
the electron injection curve into the substrate ,0
eV, vacuum level). In the course of the desorption
experiments the energy resolution of the electron
beam was 0.2 eV ,fwhm) at a current of 40 nA
while the IRAS experiments were performed at a
much higher electron intensity ,ꢁ 0:8 lA) and
lower energy resolution ,ꢁ 0:5 eV).
Fig. 2. IRAS signature of the m3 ,asymmetric stretch) and m1
,symmetric stretch) vibration from a 10 ML NF3 ®lm before
and after electron irradiation at an energy of 1.5 eV. The or-
dinate is in units of milliabsorbance ,mAbs).
,908 cmÀ1 [21]) but corresponds to that obtained
from Raman spectroscopy ,930 cmÀ1) recorded on
condensed NF3 at 62 K [22]. It should be noted
that we do not observe a measurable degradation
when the IR spectrum of the NF3 ®lm is recorded
without electron exposure but for times exceeding
those of electron irradiation. At an electron ex-
posure above 35±40 mC degradation reaches sat-
uration. The m3 intensity is then dropped to about
20% of the initial value. The residual intensity is
due either to an incomplete overlap between the
surface area covered by the electron beam and that
by the IR beam or to the fact that electron-induced
degradation is not operative through the entire 10
ML ®lm. While electrons are transmitted through
the entire ®lm ,as recorded by the electron injec-
tion curve) it is not obvious whether DA reactions
are restricted to molecules at or near the surface.
While the lowest DA channel generating FÀ, viz,
3. Results and discussion
Fig. 2 shows the IRAS spectrum of a 10 ML
®lm of NF3 before and after exposition to the
electron beam for di€erent times ,i.e., exposed
charge in mC) at an energy of 1.5 eV. Clearly
visible is the m1 ,symmetric stretch) and m3 ,asym-
metric stretch) vibration at 1030 and 931 cmÀ1
,
respectively. While the m3 vibration has compo-
nents of the dynamical dipole moment perpendic-
ular to the C3m axis of the molecule, that of the m1
vibration is oriented parallel to it. From the facts
that the method is only sensitive to dipole mo-
ments oriented perpendicular to the substrate and
that both modes are active in IRAS and we con-
clude that there is no preferential orientation of
the NF3 molecule in the ®lm. The m3 mode is
shifted with respect to the gas phase value
eÀ ‡ NF3 ! NF2 ‡ FÀ
ꢀ1†
is exothermic by 0.76 eV [13], electron attachment
is only operative for energies above ꢁ 0:4 eV due
to the disposition of the relevant neutral and an-
ionic potential energy surface with the corre-
sponding Franck±Condon Factors [14]. It is hence
a question of the kinetic energy of the electrons
within the ®lm. It is likely that electron transport
through the NF3 ®lm occurs via an electronic

404
P. Tegeder, E. Illenberger / Chemical Physics Letters 341 ,2001) 401±406
conduction band like in rare gas ®lms [23] or in
non-polar insulating liquids [24].
The degradation of the NF3 ®lm is studied by
monitoring the m3 signal for di€erent electron beam
exposure times and at di€erent electron energies.
This is shown in Fig. 3 for an electron energy of
1.5 eV. The ordinate corresponds to the change of
absorbance ,D) in units of milliabsorbance ,mAU).
The time dependence of D can be approximated
[25] by an exponential saturation function of the
form
Fig. 4. Relative degradation cross-section as a function of the
electron energy extracted from the ®tting procedure ,Fig. 3).
Note that each of the four data points are obtained from a
series of independent measurements according to Fig. 3.
D ˆ D ‰1 À expꢀÀrQ=eA†Š;
ꢀ2†
1
with Q the charge quantity corresponding to a
particular electron beam exposure time, r the de-
gradation parameter, A the interaction area be-
tween the electron beam and the molecular ®lm, e
to the electron scattering and electron capture
properties of the target molecule as can be seen by
comparing with gas phase studies [10,13] and
studies on electron stimulated desorption of anions
from an NF3 ®lm [14].
Fig. 5a shows the FÀ yield from gas phase NF3
obtained in a previous study from our laboratory
the elementary charge and D the asymptotic
1
change of the absorbance. Note that both D and
D
1
are negative numbers. For one single mono-
layer the degradation parameter directly corre-
sponds to the degradation cross-section.
An evaluation of data sets shown in Fig. 3 ob-
tained at di€erent electron energies yields the en-
ergy dependence of the ,relative) degradation
cross-section. Fig. 4 indicates that this degradation
cross-section peaks at low energy ,near 4 eV) and
increases again at energies above 6 eV. Although
Fig. 4 contains only four data points it must be
emphasized that each of these data points is ob-
tained from a series of independent measurements
,see Fig. 3). This behaviour can directly be related
Fig. 5. Comparison of FÀ formation from ,a) gas phase NF3
,reproduced from Ref. [11]), and ,b) from an NF3 ®lm at a
thickness ,10 ML) also used for the IRAS studies. The de-
sorption feature is narrower than the gas phase ion yield due to
the kinetic energy ®lter for desorption ,see the text).
Fig. 3. Change of absorbance ,D) as a function of electron ir-
radiation time ,corresponding to a charge Q in mC) at an
electron energy of 1.5 eV. The dotted curve corresponds to a ®t
using the saturation function ,Eq. ,2)).

P. Tegeder, E. Illenberger / Chemical Physics Letters 341 ,2001) 401±406
405
[14] and the desorption yield obtained from a 10
monolayer ®lm recorded in the course of the pre-
sent experiments. While degradation observed by
IRAS is operative through the entire molecular
®lm, desorption of ions is probably restricted to
reactions at or near the surface ,vida supra). FÀ
formation exhibits pronounced resonant structures
in the low-energy region, both from gas phase NF3
and as desorption product from condensed mole-
cules. This signal is due to resonant dissociative
electron attachment. In the gas phase the precur-
sor ion NFÀ3 # ,the resonance) decomposes into the
ionic fragments FÀ, F2À and NFÀ2 ,plus the corre-
sponding neutrals) with FÀ by far the dominant
channel. While gas phase NF3 yields a broad and
unresolved signal in the energy range 0.4±5 eV this
feature splits into two energetically separated res-
onances in the desorption yield. As previously
discussed in detail [14] NF3 possesses two ener-
getically overlapping resonances in this energy
range. It is very likely that both resonances are
associated with the electronic ground state of the
anion as electronically excited states of the neutral
are located above 7 eV. While the low-energy
resonance impulsively decomposes into FÀ ‡ NF2
thereby releasing an appreciable amount of kinetic
energy, the second resonance decomposes less di-
rectly yielding a kinetic energy distribution with a
comparably low value of the mean energy. On the
other hand, desorption of an ion from a surface
requires a minimum of translational energy to
overcome the solvation energy. It is in fact this
kinetic energy ®lter which produces the two sepa-
rated features in the desorption yield [14]. While
FÀ is by far the dominant ion in gas phase DA to
NF3, there is no direct information on the neutral
fragments formed. From thermodynamics, how-
ever, it is clear that only above 2.12 eV the multiple
fragmentation channel FÀ ‡ F ‡ NF becomes ac-
cessible while below that energy only the NF2
radical can be formed.
the vibrational excitation functions also exhibit a
low-energy resonance [26] one can conclude that
e€ective vibrational excitation also proceeds via
resonant inelastic scattering, i.e. the transient neg-
ative ion can either dissociate ,formation of FÀ) or
eject the extra electron with some energy loss re-
covering the neutral molecule in a vibrationally
excited state. In a condensed environment, this
energy will immediately dissipate and no chemical
change can occur. It should also be noted that
undissociated neutrals can in fact desorb via short-
lived negative ion resonances: once an ion is
formed at or near the surface, it will ®rst be ac-
celerated towards the surface and after electron
detachment the neutral system may eventually ®nd
itself in the desorption continuum [27]. By per-
forming thermal desorption spectroscopy, how-
ever, we can safely say that the amount of material
in the adsorbate does not signi®cantly change
upon electron exposition and desorption of neutral
molecules can hence be excluded as a major source
of the degradation observed by IRAS.
In contrast to the low-energy domain the in-
crease of the degradation cross-section at higher
energies can no longer be associated with one
single process as at these energies electronic exci-
tation associated with inelastic electron scattering
contributes. The electronically excited states can,
e.g., dissociate into neutral fragments and the in-
elastically scattered electrons can be captured by
further NF3 molecules.
We have also performed an extended IR anal-
ysis of the NF3 ®lm after electron irradiation in
order to get information on new species formed in
the course of degradation. Although these mea-
surements still su€er from a weak signal to noise
level, there is evidence that irradiation at 1.5 eV
creates NF2 radicals and N2F4 molecules. The
latter may be formed by the exothermic associa-
tion of two adjacent NF2 radicals. In contrast to
this exposure of 8 eV electrons gives evidence for
NF and NF2 but not N2F4. This may indicate that
N2F4 which is eventually formed at 8 eV is de-
graded by subsequent electron collisions and that
degradation is not ,or less) operative at low elec-
tron energies.
From the energy dependence we conclude that
the degradation cross-section can directly be re-
lated to resonant electron capture as the only initial
process. This is in fact the only process which can
initiate chemical changes in this energy domain.
Another process operative in this energy range is
vibrational excitation of the neutral molecule. Since
We ®nally note that in a condensed environ-
ment the relaxed NFÀ3 can certainly be formed in

406
P. Tegeder, E. Illenberger / Chemical Physics Letters 341 ,2001) 401±406
[8] M.J. Pellerite, J.I. Brauman, J. Am. Chem. Soc. 105 ,1983)
2672.
the course of electron capture. Density Functional
Theory ,DFT) calculations predict an appreciable
adiabatic electron anity of EA ꢀNF3† ˆ 1:71 eV
and a strong geometry change on going to the
anion ,Cs symmetry with one N±F bond consid-
erably enlarged) [13]. Since so far only calculated
gas phase vibrational frequencies of the anion are
available, we were not successful to unambigu-
ously identify this ion in the molecular ®lm.
[9] J. Langer, S. Matejcik, E. Illenberger, Phys. Chem. Chem.
Phys. 2 ,2000) 1001.
[10] D. Nandi, S.A. Rangwala, S.V.K. Kumar, E. Krishnaku-
mar, Int. J. Mass Spectrom. 205 ,2001) 111.
[11] S.R. Hunter, L.G. Christophorou, J. Chem. Phys. 80
,1984) 6150.
[12] F. Weik, E. Illenberger, J. Chem. Phys. 103 ,1955) 1406.
[13] N. Ruckhaberle, L. Lehmann, S. Matejcik, E. Illenberger,
Y. Bouteiller, V. Periquet, L. Museur, C. Desfranßcois,
J.P. Schermann, J. Phys. Chem. 101 ,1997) 9942.
[14] P. Tegeder, E. Illenberger, Phys. Chem. Chem. Phys. 1
,1999) 5197.
Acknowledgements
[15] S. Lacombe, F. Cemic, K. Jacobi, Phys. Rev. Lett. 79
,1997) 1146.
This work has been supported by the Deutsche
Forschungsgemeinschaft ,Sfb 337, Il 16/15-1), the
Fonds der Chemischen Industrie and the Freie
[16] R. Martel, A. Rochefort, P.H. McBreen, J. Am. Chem.
Soc. 116 ,1994) 5965.
[17] C. Olsen, P.A. Rowntree, J. Chem. Phys. 108 ,1998) 3750.

[18] B. Bodaõ€a, P. Cloutier, D. Hunting, M.A. Huels,
L. Sanche, Science 287 ,2000) 1658.

Universitat Berlin.

[19] P. Tegeder, Doctoral Thesis, Freie Universitat Berlin, 1999.
[20] B.E. Hayden, in: J.T. Yates Jr., Th.E. Madey ,Eds.),
Vibrational Spectroscopy of Molecules at Surfaces, Plenum
Press, New York, 1987.
References
[1] M.B. Robin, Higher Excited States of Polyatomic Mole-
cules, vol. 1, Academic Press, New York, 1974, p. 209.
[2] G. Bruno, P. Capezzuto, G. Cicala, P. Manodoro, J. Vac.
Sci. Technol. 12 ,1994) 690.
[21] T. Shimanouchi, Tables of Molecular Vibrational
Frequencies Consolidated, vol. 1, NSRDS-NBS 39, Wash-
ington, 1972.
[22] M. Gilbert, P. Nectoux, M. Dri€ord, J. Chem. Phys. 68
,1978) 679.
[3] K.E. Greenberg, J.T. Verdeyen, J. Appl. Phys. 57 ,1985)
1596.
[23] L. Sanche, J. Phys. B 23 ,1990) 1597.
[24] W.F. Schmidt, Liquid State Electronics of Insulating
Liquids, CRC Press, Boca Raton, 1997.
ꢀ
ꢀ
[4] J. Perrin, J. Meot, J.M. Siefert, J. Schmitt, Plasma Chem.
Plasma Processing 10 ,1990) 571.
[5] D.M. Manos, D.L. Flamm, Plasma Etching, Academic
Press, Boston, 1989.
[25] C.O. Olsen, P.A. Rowntree, J. Chem. Phys. 108 ,1998)
3750.
[6] P.J. Chantry, in: H.S.W. Massey, E.W. McDaniel,
B. Bederson ,Eds.), Applied Atomic Collision Physics,
vol. 3, Academic Press, New York, 1982.
[26] L. Boesten, Y. Tachibana, Y. Nakano, S. Shinohara,
H. Tanaka, M.A. Dillon, J. Phys. B 29 ,1996) 5475.
[27] J.W. Gadzuk, L.J. Richter, S.A. Buntin, D.S. King,
R.R. Cavanagh, Surf. Sci. 235 ,1990) 317.
[7] M.J. Shaw, J.D.J. Jones, Appl. Phys. 14 ,1977) 393.