MASS SPECTROMETRY DIAGNOSTIC DURING RF PLASMA
POLYMERIZATION OF THIOPHENE VAPORS*
B. MITU, V. SATULU, G. DINESCU
National Institute for Lasers, Plasma and Radiation Physics,
Atomistilor 409, PO-Box MG-16, 077125, Măgurele, Romania
E-mail: [email protected]
Received January 24, 2011
This work reports on the mass spectrometry diagnostic of Ar/thiophene RF
plasma generated in parallel-plate discharge configuration at low power levels. The
analysis of mass spectra of neutral species shows that thiophene depletion is reaching
more than 50% for power level above 5 W. At the same time, generation of acetylene,
hydrogen and carbon disulfide molecules in plasma and formation of high mass
oligomers up to C10H8S2+ was evidenced.
Key words: conductive polymers, thiophene monomer, RF plasma polymerization,
mass spectrometry.
1. INTRODUCTION
Plasma polymerization has proven its efficiency in obtaining well adherent
thin films, with conformal coverage, onto various substrates. The process is widely
used for a number of different applications such as barrier layers [1], corrosion
protection [2], biomaterials and hydrophobic textiles [3]. The development of
conductive polymers opens new opportunities to involve plasma polymerization
technique in organic and flexible components in the electronic industry. In this
case, the preservation of conjugated bonds, which are responsible for conduction
properties, is of high importance [4]. Therefore, the investigation of the species
responsible for the deposition and the understanding of the growth mechanisms are
essential. However, only few reports can be find in the literature regarding the
investigation of species assisting the plasma deposition of conductive polymers
[5, 6].
Thiophene (C4H4S) is an example of monomer which can be used for the
synthesis of a conductive polymer. This paper presents the results regarding the
*
Paper presented at the 15th International Conference on Plasma Physics and Applications,
1–4 July 2010, Iasi, Romania.
Rom. Journ. Phys., Vol. 56, Supplement, P. 120–125, Bucharest, 2011
2
Mass spectrometry diagnostic during RF plasma polymerization of thiophene vapors
121
thiophene fragmentation during its plasma polymerization in a RF discharge with
a parallel-plate electrodes configuration. The precursor fragmentation and the
investigation of the neutral and ionic species present in the process, and their
dependence on the applied RF power was monitored by Mass Spectrometry
(MS).
2. EXPERIMENTAL
The experiments have been carried out in a stainless steel vacuum chamber
which was pumped down to 1x10-2 mbar. The capacitively coupled discharge was
generated in RF (13.56 MHz) between two parallel electrodes situated at 8 cm one
from each other. The upper electrode was constructed as a shower to allow the
uniform injection of the precursor, while the lower one, grounded, is used as well
as substrate support [7]. During deposition, the thiophene vapors under controlled
flow of 0.5 g.h-1 were introduced in the discharge mixed with argon carrier gas
(10 sccm) through a control, evaporation and mixing system (CEM, Bronkhorst).
The established pressure inside the reactor was 1.7x10-1 mbar. The applied
RF power was in the range 1-20 W.
The mass spectrometer (EQP 1000, Hiden Analytical) was mounted inside
the reactor, laterally and equidistant in respect to the electrodes. The base pressure
inside the mass spectrometer was 2.5x10-7 mbar, while the pressure during
measurements was reaching 5.5x10-7 mbar. The MS was operated in RGA and
positive ions modes, and the energy distributions of various ions formed in the
Ar/C4H4S discharge were also monitored.
3. RESULTS AND DISCUSSION
A typical MS spectrum of the Ar/C4H4S mixture introduced in the reactor
(10 sccm/0.5 g.h-1 flows ratio) is shown in Figure 1a. It presents the expected
cracking pattern of the thiophene inside the mass spectrometer [8] with the major
peaks corresponding to C4H4S (84 amu), C2H2S (58 amu) and CHS (45 amu), the
Ar – related peaks at 40 and 20 amu and the peaks related to the impurities inherent
to the reactor (N2 – 28 amu and H2O – 18 amu). Once the plasma is ignited, the
mass spectra of the neutral species (see Figure 1b for the case of 20 W RF power)
is modified, both in respect to the relative intensities of the various species, and as
well by the appearance of completely new formed species, as CS2 at 76 amu. They
point out on the plasma induced mechanisms of thiophene bond breaking.
122
B. Mitu, V. Satulu, G. Dinescu
500000
MS - neutrals
Ar flow = 10 sccm
C4H4S flow = 0.5 g/h
600000
Ar
MS - neutrals
Ar flow = 10 sccm
C4H4S flow = 0.5 g/h
a)
500000
C4H4S
400000
C2H2S
CHS
C3 H3
100000
S
100000
C3HS
H2O
Ar/2
C2H2
N2
Intensity (cps)
RF Power = 20 W
H2O
Ar/2
C2H2
N2
Intensity (cps)
b)
400000
RF OFF
H2
Ar
H2
C3H3
600000
3
S
10
20
30
40
50
60
70
80
90
100
10
20
30
40
m/z
CS2
C3HS
0
0
C4H4S
CHS C2H2S
50
60
70
80
90
100
m/z
Fig. 1 – Mass spectra of the neutral species present in Ar/C4H4S mixture, at 1.7x10-1 mbar
a) without plasma; b) with RF plasma ignited at 20 W.
In order to monitor closely the dissociation of thiophene monomer by plasma,
the mass signal of the neutrals was analyzed as function of the applied RF power.
The monomer depletion was calculated for the prominent mass signals associated
to thiophene as:
D (%) = (
I OFF − I ON
I OFF
) *100 = (1 −
I ON
I OFF
) *100 ,
(1)
where the ION represents the mass signal associated to the monitored peak at a
given RF power and IOFF is the mass signal of that peak in absence of plasma.
The results are presented in Figure 2 for masses 84, 69, 58 and 45 amu. All
the monitored peaks present a similar trend, with a sharp increase of the monomer
depletion immediately upon plasma ignition (1-3 W) and a much slighter increase
at RF powers above 5 W.
100
Depletion
D = 1 - ION/IOFF
Depletion (%)
80
60
C4H4S
C3HS
40
C2H2S
CHS
20
0
0
5
10
15
20
RF Power (W)
Fig. 2 – Monomer depletion as function of the RF power, monitored via mass signals
corresponding to the important cracking pattern of thiophene.
4
Mass spectrometry diagnostic during RF plasma polymerization of thiophene vapors
123
One must notice that even at such low levels of injected RF power in the
discharge, the thiophene depletion is reaching almost 80% at 20 W. The systematic
lower depletion values corresponding to the peaks of thiophene fragments (C3HS,
C2H2S, CHS) indicate that besides the C4H4S fragmentation inside the mass
spectrometer, an additional dissociation mechanism occurs in the plasma volume
which enhances the mass signal of these fragments.
This phenomenon is even better illustrated in Figure 3 for the case of new
molecules generation, as C2H2, CS2 and H2. We mention that in Figure 3 the
displayed signals related to acetylene (26 amu) and hydrogen (2 amu) count only
for the molecules generation, as the contribution corresponding to thiophene
cracking pattern was extracted from the measured signal. The formation of these
molecules has been also observed in a pulsed RF discharge in thiophene vapors
capacitively coupled with external electrodes [6].
60000
Intensity (cps)
50000
C2H2
40000
CS2
H2
30000
20000
10000
0
0
5
10
15
20
RF Power (W)
Fig. 3 – C2H2, CS2 and H2 molecular production as function of RF power.
The mass spectrometry data of the positive ion species formed in the
Ar/C4H4S plasma generated at 20 W are presented in Figure 4 a) and b) for the
mass range 1–100 amu and 100–200 amu, respectively. Besides the direct
ionization of thiophene molecule, most probably by electron impact, the detection
of various ion fragments from the plasma, as C4H2+, SH2+, CS2+, gives an indication
on the complexity of plasma chemistry which leads to the precursor dissociation,
excitation and ionization. Moreover, the signal associated to the masses higher than
that of thiophene proves that various oligomerization reactions are taking place in
the plasma volume, giving rise to larger ionized fragments as C2H3S2+ (91 amu),
C4H4S2+ (116 amu), and up to C10H8S2+(192 amu).
B. Mitu, V. Satulu, G. Dinescu
8000
SH2
+
C4H2
+
C10H8S2
C9H6S2
+
+
+
+
+
2000
C9H7S
+
C7H5S2
+
C3HS
+
CS2
C8H6S2
+
+
C2H2S
20000
4000
C6H5S2
+
6000
+
C2H2
+
CHS
+
Ar
C2H3S2
+
C3S2
40000
C3H3
+
60000
RF Power = 20 W
+
Intensity (cps)
RF Power = 20 W
+
Intensity (cps)
80000
b)
Y scale zoomed x10
(in respect to a)
MS - positive ions
Ar flow = 10 sccm
C4H4S flow = 0.5 g/h
10000
C5H5S2
+
C8H6S
a)
+
+
5
C7H5S
C4H4S
C4H4S2
MS - positive ions
Ar flow = 10 sccm
C4H4S flow = 0.5 g/h
100000
C3H3S2
+
C6H4S
124
0
0
0
10
20
30
40
50
60
70
80
90
100
100
110
120
130
140
150
160
170
180
190
200
m/z
m/z
Fig. 4 – Mass spectra of the ionic species present in Ar/C4H4S RF plasma ignited at 20 W
a) mass range 1–100 amu; b) mass range 100–200 amu, Y scale zoomed x10 in respect to a).
4. CONCLUSIONS
The neutral and positive ion species present in Ar/thiophene plasma
generated in parallel-plate discharge configuration at low power levels have been
investigated by mass spectrometry diagnostic.
The monomer depletion is increasing with the applied power, reaching
already 80% at 20 W RF power. The thiophene dissociation is accompanied by the
formation of new molecules in the plasma volume, as hydrogen H2, acetylene C2H2
and carbon disulfide CS2.
The mass spectra of the ionic species evidenced the simultaneous
fragmentation and ionization by reaction in plasma. Moreover, the existence of
various oligomeric ionized fragments indicates upon the existence of volume
polymerization reactions.
Acknowledgements. This work has been supported by the Romanian Ministry of Education,
Research, Youth and Sport under the NPRDI II, Partnership contract D11-032/2007 and Human
Resources contract TE-229/2010.
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