Characterisation of a He/HMDSO/O2 microplasma jet
by molecular beam mass spectrometry
Dirk Ellerweg, Rüdiger Reuter, Katja Rügner, Teresa de los Arcos, Achim von Keudell and Jan Benedikt
Research Department Plasmas with Complex Interactions, Ruhr-University Bochum, Germany
Abstract: A microscale atmospheric pressure plasma jet is used to deposit thin
organic SiOxCy and inorganic SiOx films on silicon substrates. For this purpose a
small amount (<0.01%) of hexamethyldisiloxane (HMDSO) is admixed to the
helium feed gas (5 slm). It has been observed that the film quality significantly
improves when oxygen is additionally added to the He/HMDSO flow. However,
the HMDSO plasma chemistry at atmospheric pressure leading to SiOx films of
good quality is not well-understood. Here, a molecular beam mass spectrometer
is used to get an insight into the atmospheric pressure plasma chemistry of
HMDSO and HMDSO/O2, respectively. An HMDSO depletion up to 6% can be
measured without O2 addition and several stable reaction products are identified.
This condition leads to organic films. The resulting film quality improves when
O2 is added to the He/HMDSO flow. The measurements revealed that thereby the
HMDSO depletion increases up to 13% and the densities of the main reaction
products increase significantly, too. Additionally, polymerization products larger
than HMDSO appear.
Keywords: microplasma jet, HMDSO, SiOx film deposition, mass spectrometry
1. Introduction
Quartz-like thin films are commonly used for scratch
resistant coating on polymers, permeation barrier in
packaging industry or dielectric in semiconductor
technology. These films are usually deposited by
means of low pressure plasma enhanced chemical
vapor deposition with hexamethyldisiloxane
(HMDSO) as precursor. A drawback for this
deposition technique is that expensive vacuum
systems are needed and implementation in existing
production lines is difficult. This is not the case
when atmospheric pressure plasmas are used.
Microplasma jets are one of several different
concepts [1-4] for atmospheric pressure SiOx thin
film deposition. However, the HMDSO plasma
chemistry at atmospheric pressure is not fully
understood. The flux of radicals to a surface is
crucial to known for the deposition process. A
molecular beam mass spectrometer is capable to
measure these species and can help to get an insight
into the plasma chemistry. The deposited films can
be improved with the so gained knowledge.
2. Experimental Setup
2.1. µ-APPJ
Figure 1. Sketch of the µ-APPJ
The microscale atmospheric pressure plasma jet (µAPPJ) is a capacitive coupled microplasma jet. It
consists of two metal electrodes (length 10mm,
thickness 1mm) separated by a gap of 1mm (cf.
figure 1). Two glass plates confine the plasma
volume of 1x1x10mm³ on both sides. One electrode
is powered by 13.56MHz while the other one is
2.2. Molecular beam mass spectrometer
When a mass spectrometer (MS) is used to analyze
atmospheric pressure plasmas, a differential
pumping system needs to be applied to ensure a
sufficient low pressure at the MS without disturbing
the microplasma. Since the molecular beam mass
spectrometer (MBMS) system used here is described
in detail elsewhere [5,6], only a brief summary is
provided in the following. In contrast to typical
MBMS systems [7], a rotating chopper is installed in
the first pumping stage (cf. figure 2). This rotating
chopper consists of a flat metal disk with four small
embedded skimmers. As long as the embedded
skimmers are not aligned with the other orifices, the
sampled gas is blocked by the metal disk and cannot
penetrate the second stage. The gas can only enter
directly the second stage when one embedded
skimmer is in line of sight with the other orifices.
With this setup low concentrations (down to ppm) of
atmospheric gases can be measured with a signal to
background ratio up to ~14.
3. Results
The MBMS system is used to analyze the
atmospheric pressure plasma chemistry of an
HMDSO plasma. The µ-APPJ is operated with an
applied electrode voltage of 230VRMS and a gas flow
of 5slm He and 0.1sccm HMDSO. This condition
leads to organic SiOxCy coatings [8]. When oxygen
(10sccm) is added to the precursor flow, the
deposition rate doubles and carbon free SiOx films
are deposited.
147 amu (HMDSO)
Plasma off
Plasma on
2.5
MS signal (a.u.)
grounded. The µ-APPJ is operated with a gas flow
of 5slm He and a small admixture of HMDSO or
O2/HMDSO, respectively. During all measurements
the µ-APPJ was placed in a helium atmosphere to
prevent admixture of air.
2.0
0.012
1.5
0.009
1.0
0.006
0.5
0.003
0.000
0.0
HMDSO HMDSO/O2
133 amu
0.025
Plasma off
Plasma on
0.020
Plasma off
Plasma on
0.0030
0.0025
0.0020
3. stage
2. stage
1. stage
HMDSO HMDSO/O2
221 amu
0.0040
0.0035
0.010
pump
photo
diode
pump
0.0010
0.0005
ionizer
sampling
orifice
0.0015
0.005
0.000
pump
Plasma off
Plasma on
0.015
0.015
rotating
skimmer
75 amu
0.018
Figure 2. Sketch of the differential pumping system
HMDSO HMDSO/O2
0.0000
HMDSO HMDSO/O2
Figure 3. Comparison of the MS signals at masses 75, 133,
147, and 221amu under plasma off and plasma on condition
with and without O2 admixture.
Figure 3 shows measurements of four neutral
species: 147amu (corresponds to HMDSO,
(CH3)6Si2O),
75amu
(corresponds
to
trimethylsilanol, (CH3)3SiOH), 133amu (corresponds
to pentamethyldisiloxane, (CH3)5Si2OH), and
221amu (corresponds to octamethyltrisiloxane,
(CH3)8Si3O2). An electron energy of 20eV in the
ionizer was chosen to minimize dissociative
ionization and fragmentation of the molecules.
Please note that the parent ions of these species are
unstable and will release one CH3 group during
ionization. This means that all of these masses are
15amu lower than the masses of the corresponding
parent molecules.
In each case, measurements with plasma off and on
have been done and compared under two conditions:
without and with addition of 10sccm O2. When no
oxygen is admixed to He/HMDSO gas flow, only a
very weak influence of the plasma on the HMDSO
precursor flow can be observed: the HMDSO
consumption is not higher than 6%. Additionally, the
increase of the MS signal of the other masses
(75amu, 133amu, and 221amu) is within the statistic
error. The HMDSO concentration is about 20ppm
and the HMDSO consumption is around 1ppm. The
densities of the reaction products are therefore
around the detection limit of the MBMS system
resulting in a very low signal with large statistic
errors.
When oxygen is admixed to the He/HMDSO flow,
the HMDSO consumption doubles (~13%).
Additionally, the concentrations of the other masses
increase
significantly.
The
production
of
pentamethyldisiloxane (133amu) indicates that the
plasma attacks the Si-C bonds of the HMDSO
molecule, possibly by direct reactions of atomic
oxygen with the methyl group or by excited
molecular oxygen. The increased signal of
trimethylsilanol (75amu) when O2 is admixed shows
an enhanced Si-O bond breaking of the HMDSO
molecule. On the one hand, atomic oxygen can react
with the Si-O bond and break it. On the other hand,
the addition of oxygen leads to changing plasma
parameter like electron density and electron energy
distribution function. Electron impact dissociation of
HMDSO becomes then probably more effective.
This is in agreement with the increase of the
octamethyltrisiloxane
signal
(221amu).
Octamethyltrisiloxane is formed as product of the
reaction of a (CH3)3SiO radical with an HMDSO
molecule.
All of the determined reaction products contain
carbon atoms. Carbon free growth precursors in the
gas phase could not be identified even if oxygen is
admixed. But admixture of oxygen leads to the
deposition of carbon free SiOx films. This fact
indicates that surface reactions and not gas phase
reactions are responsible for carbon removal for
carbon free SiOx film growth. This has been verified
by separated surface treatments with HMDSO and
O2 plasmas [4].
3. Conclusion & Outlook
It has been shown that a carefully designed
molecular beam mass spectrometer is able to analyze
the plasma chemistry at atmospheric pressure. A
He/HMDSO/O2 microplasma jet has been analyzed
to understand the thin SiOx film deposition process
at atmospheric pressure. It was observed that the the
microplasma is a very weak one; when no oxygen is
admixed to the He/HMDSO flow, only an HMDSO
consumption up to 6% can be observed. The plasma
becomes more effective when oxygen is admixed:
the HMDSO consumption increases to 13%.
Additionally, several reaction products can be
detected.
By comparing further MBMS measurements
(variation of the O2 admixture) with measurements
of the deposition rate and the film composition
(XPS), the SiOx film growth can be understood.
Furthermore, measurements of positive ions in the
effluent of the microplasma jet are planned.
Acknowledgment
This project is supported by DFG within the
framework of the Research Group FOR1123 and
with the individual grant KE 1145/1-1 and by the
Research Department 'Plasmas with Complex
Interactions'.
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