22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Interaction of atomic fluorine with porous low-k SiOCH films: modeling
A. Palov1, E. Voronina1, D. Lopaev, Yu. Mankelevich1, T. Rakhimova1, S. Zyryanov1, O. Proshina1 and M. Baklanov2
1
Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, RU-119991 Moscow, Russia
2
IMEC vzw, BE-3001 Leuven-Heverlee, Belgium
Abstract: We propose a 3D model of atomic fluorine interaction with porous low-k
SiOCH films. Some possible chemical reactions of fluorine with methyl groups on pore
surfaces and SiO 2 surface were investigated on the level of density functional theory and
implemented in the model. The developed model was finally used to predict the fluorine
distribution inside films after its deposition.
Keywords: organosilicate glasses, plasma etching, fluorine, methyl group, ab initio
1. Introduction
It is well known now fluorine radicals can cause some
damage of organosilicate glasses (OSG) film [1].
Incorporating fluorine atoms in such films leads, for
instance, to replacement of hydrogen atoms in methyl
groups by fluorine ones. Just recently this process was
investigated experimentally and 1D Monte Carlo model to
describe the process was proposed [2]. In current research
we have concentrated on developing the programming
tool having ability to model changes in 3D structure of
films on maps reflecting closely the real OSG films
structure. We applied also ab initio methods to study
possible chemical reactions of fluorine with methyl
groups on the level of the density functional theory to get
confidence in the reaction mechanism proposed [2]. The
mechanism used in 1D model was applied for 3D maps
and changes in OSG film structures can be analyzed now
spatially.
2. Map generation
In order to start modeling of fluorine interaction with
the film surface we need firstly to build a map of porous
films. In order to do this we suggested the OSG films
(further SiOCH films) to consist mainly of SiO 2 matrix
and CH 3 groups distributed on the pore surfaces. The
assumption of CH 3 groups localization on pore surface is
in agreement with conclusions of some other authors [3]
as well. The basic algorithm of building the 3D map can
be described as follows. At the beginning we split the film
volume into cubic cells with a length of 0.3566 nm that
corresponds to the average size of one SiO 2 fragment. At
the second step we produce a porous structure by
subtraction cells from the SiO 2 matrix. Thus we created
randomly distributed pores of the size taken from
experimental data. Finally the CH 3 group were placed
over the pore surface. Note, the pores could be connected
with some neck radii or partially disconnected depending
on the film type. Below Figure 1 presents a typical 3D
map of the porous SiOCH film.
To validate the obtained 3D map we calculated the
dielectric constant and density of the films on its base.
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Comparison of the experimental k-values with computed
ones are listed in Table 1.
Fig. 1. An example of the 3D map of an OSG film. The
SiO 2 fragments are shown in blue while the CH 3 groups
are presented by rose colour.
Table 1. Calculated and measured dielectric constant for two film types.
Experimental data are taken from [4]
OSG film types
Porosity, %
Pore neck/radii, nm
k-value (calc)
k-value (exp)
3
Density (g/cm )
Spin-on
PECVD
51
46
2.7/3.2
1.5/2.0
1.80
1.9
1.79
2.1
0.92
0.96
One can see from Table 1 the perfect agreement of the
dielectric constant calculated for the spin-on family film
and slight disagreement for PECVD one. This fact can be
explained that the presentation of the film matrix by SiO 2
groups only is rather approximate. We concentrate further
on spin-on family films only.
3. Ab initio calculations
1
It was shown previously [2] the reaction mechanism of
fluorine - OSG film interaction includes several stages
depending on the surface group that interacts with F atom.
It is adsorption of fluorine atoms, step-by-step
replacement reaction of hydrogen by fluorine in methyl
groups and etching reactions with SiO 2 . Surface
fluorination includes the first two.
To study fluorination mechanisms, ab initio density
functional theory method was applied. We constructed
different models of SiOCH surfaces and investigated
different ways of fluorine atoms approval of the surface.
Our calculations showed that coming fluorine interacts
intensively with H atoms in methyl groups with formation
of volatile HF molecules (substitution, or replacement
reactions). Fluorine reaching surface Si atoms strongly
interacts with them with formation F adsorption site or SiF bond when breaks one bond in -CH 2 - bridge groups.
Since substitution reactions of fluorine occurs with the
terminated methyl (relatively far from the SiO 2 surface),
they were investigated with a trimethylcyclotrisiloxane
(TMCTS) molecule which can be considered as a Si–O–
Si ring-based model system and was used for studying the
oxygen plasma interactions with OSG materials [5].
Figure 2a - 2b shows two of six stages of replacement
of hydrogen atoms by fluorine ones on the model
molecule including methyl groups. The first subtraction
of hydrogen atom by a fluorine one is presented on Fig.
3a. After that another fluorine atom is landing on the
dangling bond, the next fluorine atom subtracts another
hydrogen atom and so on until all H atoms are subtracted
and replaced.
Fig. 2a. Fluorine atom subtracts the first hydrogen one
(white) from a CH 3 group by establishing a HF molecule.
All ab initio simulation were performed using density
functional theory (DFT) method, implemented in VASP
code (Vienna Ab initio Simulation Package)[6-7] on the
supercomputer Lomonosov [8] .
4. Conclusion
Thus we developed the software tool for building 3D
maps of OSG films. Considering a set of reactions
describing fluorination of the SiO 2 surfaces covered by
methyl groups led to development of software capable to
describe the OSG film fluorination on 3D maps.
Figure 3. Concentration of fluorine and methyl groups
as a function of film depths after 10 min of fluorination.
Figure 3 demonstrates a typical 1D concentration
dependence of fluorine and methyl groups on the film
depth after 10 minutes of OSG film exposure to atomic
fluorine. One can see from this figure how all CH 3 groups
are gradually replaced by CF x ones while many fluorine
atoms cover the available SiO 2 surface. As it was
mentioned above the ab initio analysis confirms the
suggested reaction mechanism of OSG films fluorination
Thus we have developed an approach to describe the
fluorination of OSG films and programming tool which
is based on ab initio data and able to extract spatial detail
of such interaction on the 3D map.
Acknowledgements
A.P., E.V., D.L., Yu. M., S.Z. and T.V.R. thank the
Russian Scientific Fund (RSF) for financial support
(Grant №14-12-01012).
Fig. 2b. Fluorine atom subtracts the last hydrogen one
(white) from a CF 2 H group by establishing one more HF
molecule.
2
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