J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)402–405
000–000

Plasma Beam Irradiation into Organic Polymer Surfaces
YAMADA Hideaki and HAMAGUCHI Satoshi
Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Uji, 611-0011, Japan(Received: 9 December 2003 / Accepted: 1 March 2004)

Abstract
Aiming at understanding of interactions between plasmas and carbon-based material surfaces, we have carriedout molecular dynamics (MD) simulations of hydrocarbon plasma injections into a poly (1,4-phenylene) surface using
interatomic potential functions for hydrocarbon systems. Our simulation results show that injected carbon atoms depositon the surface and form a new bonding network. When hydrocarbon beams are injected, on the other hand, we findsurface erosions by hydrogen atoms takes place, which is consistent with experimental observations conducted undersimilar conditions as well as previously reported other MD simulation results for carbon-containing different substratematerials.
Keywords:
plasma processing, plasma-surface interaction, molecular dynamics simulation, low-k material, organic polymer

1. Introduction

polystyrene surfaces was studied [7,8]. However, due to thelack of interatomic potential models for systems including
nitrogen atoms in addition to hydrogen and carbon atoms, few
simulations of plasma-surface interactions concerning
nitrogen atoms have been carried out although nitrogen atoms
play important roles in various applications as describedabove.
To understand physical/chemical mechanisms of surface
reactions during such processes, we have constructed aninteratomic potential model for (H,C,N) systems [9]. We are
now in the process of applying this model to MD simulationsof plasma etching for low-k organic polymer surfaces. In thecase of a hydrogen-nitrogen plasma, the majority of suchincoming clusters include nitrogen atoms. However, thesurface must be also bombarded by some hydrocarbon
clusters, which are recycled from the substrate surface into
the plasma. In the present study, as the first step toward thefull understanding of organic polymer etching by hydrogen/nitrogen plasmas, we examine one of the simplest cases, i.e.,interactions between hydrocarbon beams and hydrocarbon
polymers using Brenner’s potential, which is given in Ref.4.
The results obtained here will be compared with results
obtained under our potential model in the near future.

Carbon-based materials are some of the most promisingand sought-after materials for various applications. For
example, the first walls and diverter plates of magnetic
nuclear fusion devices are often made of carbon-basedmaterials. Such surfaces are directly exposed to fusion
plasmas and can influence the plasma characteristics throughadsorption and desorption of impurities. For industrial
applications, plasmas have been widely employed to processcarbon-based materials in order to fully exploit their potential
advantages, too. Nitrogen ion implantation into carbon
nanotubes and diamond surfaces has been studied in an effort
to have these materials function as semiconductors. Other
examples include organic polymers with low dielectric
constants (low-k), which are considered to be promising
candidates for insulating materials that can reduce RC delay
for future integrated circuits (IC) chips [1-3]. Plasma
processes of such organic polymers to realize microscopicstructures are now also widely studied. In such processes, amixture of hydrogen and nitrogen atoms (or alternativelyammonia) is used as the source gas of plasmas.
Classical molecular dynamics (MD) simulation is a
useful tool to investigate such beam-surface interactions. For
its applications to (H,C) systems (i.e., systems that consistsof H and C atoms only), Brenner proposed an interatomic
potential model [4]. This potential model has been widelyused for studies of, for example, chemical vapor depositions(CVD) of diamond films [4], Young’s moduli of carbon fibers[5], and erosion of hydrogenated amorphous carbon surfaces[6]. With regard to polymer surfaces, ion deposition on

2. Molecular dynamics simulation
As the substrate for our MD simulations presented here,
we select poly (1,4-phenylene), which is a chain of phenylene
rings [Fig. 1]. The polymers are assumed to be arranged in
an almost parallel manner, as shown in Fig. 2. In this figure,white and black spheres represent carbon and hydrogen©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author’s e-mail: yamada-diamond@aist.go.jp1402

Yamada H. et al., Plasma Beam Irradiation into Organic Polymer Surfaces

Fig. 1Fig. 2

Poly (1,4-phenylene), i.e., [C6H4]n. This material is also
called as polyparaphenylene (PPP).

Fig. 3A side view of the substrate surface after carbon atom
injections of 2.4 × 1016 atoms/cm2.

Fig. 4A side view of the substrate surface after H2 molecule
injections of 2.4 × 1016 H2 molecules/cm2.

The initial model substrate of PPP used here. The blackand white spheres represent hydrogen and carbon
atoms. The black and white bars represent covalentbonds between H and C atoms and between C and C
atoms.

atoms. Covalent bonds among particles are represented bybars where white (black) bars correspond to carbon-carbon(carbon-hydrogen) covalent bonds. The mass density of thismodel substrate is 1.33 g/cm3, which is consistent with theknown mass density of poly (1,4-phenylene) [10]. The
horizontal cross section of this simulation cell (which isroughly the substrate surface area in our simulation) isapproximately 2.2 × 1.9 nm2. Boundary conditions are thefollowing: in the horizontal directions [x and y directions],
boundary condition is periodic. The bottom layer is fixed. The
typical initial substrate we used for this work has four layersof PPP polymers and each layer consists of 4 chains of 5
phenylene rings per layer. In other words, the initial substrateconsists of 480 C atoms and 320 H atoms. However, we
dynamically adjust the thickness of the substrate by adding
extra polymer layers from the bottom if necessary, so that noatoms would pass through the bottom layer. In oursimulations, we inject C, CH, CH2, CH3, CH4, H and H2 withkinetic energy of 50 eV normally to the substrate surface fromabove. The velocity Verlet method is used for numericalintegration of the equations of motion. The time increment is
set to be 0.5 fsec.

injections whose injection dose is 2.4 × 1016 atoms/cm2. The
thickness of each bar is proportional to the order of covalentbond. As shown in the figure, injected carbon atoms depositon the surface. In addition to the most dominant sp2 type
carbon-carbon bonds, the sp3 type bonds are also observedin this layer. We also observe that very few hydrogen atoms
are contained in this layer.
Figure 4 shows the substrate surface after hydrogenmolecule injections of 2.4 × 1016 H2 molecules/cm2. Unlike
the case of carbon atom injections, atoms are relatively
sparsely arranged in the top part of the substrate and carbon-

3. Results
Figure 3 shows the substrate surface after carbon atom2403

Yamada H. et al., Plasma Beam Irradiation into Organic Polymer Surfaces

carbon bonding networks are not firmly formed. Furthermore
it seems a large layer-like segment of the surface material
can be easily separated from the rest of the substrate.
Figure 5 shows the substrate surface after CH4 cluster
injections of 2.4 × 1016 CH4 molecules/cm2. Similar to thecase of H2 molecule injections, hydrogen atoms of incident
CH4 erode the surface. However, in this case, the separationof a segment is not observed. This is because large momenta
of incident carbon atoms physically break the erodedsubstrate.
As to other hydrocarbon injections, we have observed
that, as the number of hydrogen atoms in the injected cluster
CHx (x = 1–3) increases (decreases), the resultant surface aftersufficient bombardment becomes closer to that of CH4 cluster(C atom) injections. The resultant surface conditions for H
atom injections are similar to those for H2 molecule injections.
Figure 6 shows the sputtering yields for C, H2 and CH4
injections, which are defined as Yα = Nαout – Nαin with Nαout andNαin being the numbers of removed and injected α (= C or H)
atoms per injection. Thus a positive (negative) value of Yα
corresponds to sputtering (deposition) of the species. The
horizontal axis represents injection dose [In our simulations,
100 injections are equivalent to the dose of 2.4 × 1015 /cm2.].
The circle and triangle in the figures represent the sputtering
yields of hydrogen or carbon atoms averaged over each 2.4 ×
1015 /cm2 injections (gray dashed lines) around the indicatedvalue of dose.
Figure 6a shows that, in the case of C atom injections,
H atoms in the top part of the substrate are sputtered (i.e.,removed) in the early stage of injections (filled circles with asolid line, dose ~ 0–7.2 × 1015 /cm2). On the other hand, Fig.
6b shows that almost all injected C atoms are deposited in
the substrate (filled triangles). After almost all hydrogenatoms are removed from the top layer of the substrate, YH
reaches zero around when the injection dose reaches about
7.2–9.6 × 1015 /cm2. This contrasts with the cases where
injected hydrogen and carbon atoms tend to be deposited in
the substrate for H2 and CH4 injections. However, as we saw
in Figs. 4 and 5, due to the erosion by hydrogen atoms,
sputtering yields gradually increase.
For CH4 injections, unlike C or H2 injections, thesputtering yield as a function of the injection dose oscillates,
as shown in Fig. 6. We have observed that, in the case of
CH4 injections, relatively large (but much smaller than thelayer-like segment we observed in the case of H2 injections)clusters such as C19H 12 are sometimes removed from thesurface. When such large clusters are removed from thesurface, sputtering yields drastically increase from its averagevalue. The fluctuation of the sputtering yields seen in Fig. 6
is the results of such large etch products. Sufficient statistical
average over larger number beam injection events will further
reduce such statistical noise in simulation data.

4. Discussion
In etching experiments of organic low-k polymers,
hydrogen and nitrogen plasmas are usually used. In such
3404

Fig. 5

A side view of the substrate surface after CH4 cluster
injections of 2.4 × 1016 CH4 molecules/cm2.

Fig. 6

Sputtering yields of a) H and b) C atoms as functions ofinjection dose. The sputtering yield is defined as Yα =Nαout – Nαin with N αout and N αin being the numbers ofremoved and injected α = H or C atoms per injection.
Sputtering yields shown here are averaged values overevery 2.4 × 1015 /cm2 of injections around the indicatedvalue of dose.

Yamada H. et al., Plasma Beam Irradiation into Organic Polymer Surfaces

characteristics similar to those observed in polymer etchingby hydrogen/nitrogen plasmas. We shall carry out simulations
under conditions closer to those of actual organic low-k
polymer etching experiments in the near future, using thenewly developed C/H/N potential functions. In thesesimulations, nitrogen atoms and various nitrogen compounds
will be injected into the surface and different roles played bycarbon and nitrogen atoms will be clarified.

experiments, low energy nitrogen atom injections are also
performed [3] and injected nitrogen atoms tend to deposit in
the substrate surface. As we saw in Figs. 3 and 6b, we haveobserved in our MD simulations that injected low energy
carbon atoms tend to deposit in the substrate, which is similar
to the behavior of nitrogen atoms in polymer etchingexperiments by hydrogen/nitrogen plasmas. Furthermore, asobserved in nitrogen radical exposure experiments [1,3], sp3
carbon bonds as well as sp2 bonds are observed. The similarroles played by N and C atoms may be attributed to theirsimilar masses and relatively close bond energies among NN, N-C, C-C as well as N-H and C-H.
On the other hand, it is also known experimentally [3]and numerically [6] that H radicals erode polymers andhydrogenated amorphous carbon surfaces, and therefore
increase their etching rates. This erosion effect by hydrogenatoms is observed also in our simulation, where it is shownthat injected hydrogen atoms break the carbon-carbon boding
networks. Further, injections of hydrogen-rich molecules, suchas CH4, effectively destroy the surface, which is similar toexperimental observation that NH3 and NH4+ usually function
as etchants. Since the injections of C atoms alone do not etchthe surface whereas the CH4 injections seem to effectively
etch the surface, the polymer etching mechanism by CHx orNHx beams must be combined effects of erosion by hydrogenatoms and the destruction by energetic carbon atoms.

Acknowledgment
This work is supported by the Semiconductor
Technology Academic Research Center (STARC).

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5. Summary
In Summary we have performed MD simulations of PPP
etching by hydrocarbon beams. Based on the simulation data,
we have estimated sputtering and deposition yields andexamined surfaces structures during the polymer etchingprocesses. The simulation results also exhibit some4405