HMDSO/O2 atmospheric pressure plasma chemistry
leading to SiO2 film synthesis
R. Reuter, K. Rügner, D. Ellerweg, T. de los Arcos, A. von Keudell, J. Benedikt
Research Department Plasmas with Complex Interactions, Ruhr-University Bochum
http://reaktiveplasmen.rub.de/
Abstract:
In the past years, a particular type of atmospheric pressure plasma emerged: non equilibrium
microplasmas which operate at low power (< 50 W) and allow the treatment of or deposition on
thermolabile substrates[1,2]. However, the high collision rates and slow transport limit the flux of reactive
species to the substrate and prevent ion bombardment, compromising hence the film quality.
Additionally, the knowledge of plasma composition and growth precursors is only limited due to
difficulties connected with microplasma diagnostics. More understanding and optimisation of deposition
process are needed to improve properties of grown film.
We report the deposition of SiOxCy coatings by means of two different microplasma jets. A coaxial
geometry and a parallel plate geometry are compared. Both systems are driven by RF voltage and
operate in argon or helium gas with small addition of reactive gases (Hexamethyldisiloxane (HMDSO)
and/or oxygen). A novel experimental setup based on a rotating substrate is introduced giving the chance
to analyse the film growth more in detail. The film composition is measured by FTIR and XPS as
functions of gas composition.
Keywords: Microplasma, atmospheric pressure, HMDSO, SiO2
The deposition of SiOx layers at atmospheric
pressure by means of a microplasma jet from
HMDSO was carried out as described in the
following experiments.
The microplasma sources used in this study are
shown in figure 1 and figure 2.
The planar jet shown in figure 1 has two 1 mm
thick electrodes with a length of 10 mm. They
have a distance of 1 mm so that a discharge
channel of 10 mm length and 1 mm² cross
section area is formed. This jet operating at
13.56 MHZ rf-frequency and a voltage of 230
VRMS is driven with helium as plasma forming
gas with small admixtures of HMDSO and/or
oxygen. 0.01–0.1 sccm (sccm denotes cubic
centimetre per minute at STP) HMDSO and/or
0–20 sccm oxygen are admixed into the main
gas flow of 5 slm He (slm denotes cubic
centimetre per minute at STP). The advantages
of this jet are its good optical access, for
example for optical diagnostics, and its robust
and very simple design. A disadvantage is the
fact that the electrodes of this jet are getting
coated with a SiOx layer, when HMDSO is
admixed into the plasma forming gas. This
effect leads to a change of the electrical
properties and to a short source lifetime. This jet
has already been used for many diagnostic
applications[3].
The other microplasma source is a jet in coaxial
design[1,4]. It is shown in figure 2. It consists of a
stainless steel capillary tube with inner and
outer diameters of 0.2 mm and 0.5 mm inserted
into a ceramic tube with an inner diameter of 1
mm leading to a gap of 250 µm between those
two tubes. This ceramic tube has an outer
diameter of 1.5 mm and is surrounded by an
aluminium strip. This strip and the stainless
steel capillary serve as electrodes. This jet is
also driven with a rf-frequency of 13.56 MHz
and a voltage of 230 VRMS. The inner flow
through the capillary is formed by 160 sccm
argon with admixtures of 0–1 sccm HMDSO.
Between the capillary and the ceramic tube a
flow of 3 slm of argon forms the outer flow.
experimental conditions and to avoid impurities
for example because of oxygen or nitrogen
diffusing into the plasma.
The experiments consist of two parts:
In the first part the substrate has been moved in
front of the jet at a distance of 4 mm with a
constant velocity of 0.5 mm/min. Moving the
substrate is helpful for the later analysis of the
film. To deposit the films on the substrate the
microplasma jet with the planar geometry was
used. We call this process Linear Drive Process.
Main goal of this study is to understand the
correlation between film properties and gas
composition.
Ar + HMDSO
Ar
1mm
13.56 MHz
Figure 2. Coaxial Jet [1,4]
Figure 1.
Top: Planar Jet
Bottom: Linear Drive Process: Substrate is moved in front of the
planar jet with a velocity of 0.5 mm/mim. [5]
All experiments were performed in a vacuum
chamber that was pumped down before the
deposition process and afterwards filled with
helium or argon to ensure controlled
The second part of this study deals with a
rotating substrate. Here, a substrate was
mounted on a rotating disc. In this configuration
two or even more jets could treat the surface of
the substrate. In this way we could create a kind
of
alternating
treatment
process.
As
microplasma sources serve the planar jet as well
as the coaxial jet. We call this process Rotating
Disc Process. An exemplary view of the rotating
disc setup is given in figure 3. The disc was
rotated with a frequency of 5 Hz. This
frequency is so high that not more than one
monolayer was deposited in one rotation.
The main goal of the linear drive process is to
understand how the film composition and the
film properties depend on the composition of
the gases mixed into the plasma. Several films
have been deposited with the planar jet and
different gas compositions. The FTIR spectra of
these films are given in figure 4. Without
addition of oxygen (black curve, pure HMDSO)
a left shift of the SI-O-SI stretching peak at
around 1075 cm-1 wavenumbers is observable.
The other three spectra are recorded from films
that have been deposited with addition of
oxygen. A high oxygen flow leads to an
increasing shoulder at the right hand side of the
Si-O-SI stretching peak at around 1175 cm-1
(green curve, High oxygen flow). For a low
precursor flow carbon-free films can be
achieved indicated by the absence of the SI(CH3)x peak at around 1275 cm-1 (red curve, low
precursor flow). This result was also checked
and confirmed by XPS measurements.
with a rotating substrate were performed. In this
process the HMDSO plasma can be separated
from the oxygen plasma. Now it can be
distinguished between a gas-phase reaction
between oxygen and HMDSO respectively its
dissociation products and surface reaction of
oxygen with HMDSO respectively its
dissociation products on the surface of the
substrate. This has been done in two steps:
Figure 4: FTIR spectra of film deposited in the Linear Drive
Process. Oxygen and HMDSO flows were varied. (Conditions:
Pure HMDSO: 0.1 sccm HMDSO, 0 sccm O2; Low precursor
flow: 0.01 sccm HMDSO, 0.2 sccm O2; High precursor flow:
0.1 sccm HMDSO, 2 sccm O2, High oxygen flow: 0.1 sccm
HMDSO, 10 sccm O2) Adopted from [5]
Figure 3 Rotating disc process. [5]
In the first step it was checked if the deposition
in the rotating disc process differs from the
deposition in the linear drive process. The result
is shown in figure 5a. The FTIR spectra of a
film deposited in the linear drive process is
compared with a spectrum from the rotating disc
process. This indicates that there is no basic
difference between these two processes with
respect to the FTIR spectra.
This experiment has shown that the carbon
content in the film depends on the oxygen and
the precursor flow. To check the role of the
oxygen in this deposition process, experiments
In the second step one jet with HMDSO and the
other one jet with oxygen was used. In this way
the oxygen plasma was separated from the
HMDSO plasma and it can be distinguished
between a gas phase and a surface reaction. The
He + O
2
Jet2
He + HMD
SO
Jet1
He‐Atmosphere @ 1 bar
spectra in figure 5b show no basic difference
between the deposition of a SiOx layer in the
linear drive process with one single HMDSO/O2
plasma and the deposition in the rotating disc
process with two separated plasmas, one with
He/HMDSO and the other one with He/O2. In
other experiments[5] the separation between the
two plasmas was checked and confirmed. This
is a clear evidence that the carbon removal is a
surface reaction.
deposition the coaxial jet has been used. The
rotating disc process now offers the chance to
check if also a surface reaction is reasonable for
the absence of carbon in the films. First
experiments (not shown here) have shown that
in this case a surface reaction is reasonable, too.
By replacing the planar jet driven with He/O2
plasma by the coaxial jet driven only with Ar,
also carbon-free films could be deposited.
We have shown that the amount of oxygen and
HMDSO in the plasma plays an important role
for the film properties and composition. By
introducing the rotating disc process we found a
new diagnostic to analyse the growth
mechanisms of SiOx film deposited from
microplasma jets. With this method we found
the carbon loss being a surface reaction.
This project is supported by DFG within the
framework of the Research Group 1123 and by
the Research Department Plasmas with
Complex Interactions.
[1] J. Benedikt, K. Focke, A. Yanguas-Gil, and
A. von Keudell, Appl. Phys. Lett. 89, 251504
(2006)
[2] J. Schäfer, R. Foest, A. Quade, A. Ohl, and
K-D- Weltmann, J. Phys. D: Appl. Phys. 41,
194010 (2008)
Figure 5: FTIR spectra of film deposited in the Linear Drive
Process. Oxygen and HMDSO flows were varied. (Conditions:
Linear drive: See Figure 4; Rotating disc, pure HMDSO: Jet1:
0.1 sccm HMDSO Jet2: not used; Rotating disc HMDSO + O2:
Jet1: 0.1 sccm HMDSO, Jet2: 2 sccm O2). Adopted from [5].
In the last years it has been shown that carbon
free films can be deposited from HMDSO even
without addition of oxygen[4]. For this
[3] N. Knake, K. Niemi, S. Reuter, V. Schulzvon der Gathen, and Jörg Winter, Appl. Phys.
Lett., 93 131503 (2008)
[4] V. Raballand, J. Benedikt, S. Hofmann, M.
Zimmermann, and A. von Keudell, J. Appl.
Phys. 105, 083304 (2009)
[5] R. Reuter, D. Ellerweg, A. von Keudell, and
J. Benedikt, Appl. Phys. Lett.98, 111502 (2011)