Spatial distributions of non-molecular species in N2-H2 plasmas measured by a
quartz oscillator
A. Suzuki1, and S. Asahina2
1
Research Institute of Instrumentation Frontier (RIIF),
National Institute of Advanced Industrial Science and Technology (AIST), Japan,
Tsukuba Central 2, Umezono 1-1-1 Tsukuba, Ibaraki 305-8568, Japan, [email protected]
2
Shimane Institute for Industrial Technology,
Shimokou-cho 388-3, Hamada-shi, Shimane 697-0006 Japan, [email protected]
Abstract: Non-molecular species in N2-H2 plasmas were investigated using a quartz
oscillator without a filter which would prevent non-molecular species reaching the quartz
oscillator. Comparison to results obtained by other plasma diagnostic techniques indicated
the kinds of non-molecular species that were detected by the quartz oscillator. Finally,
spatial distributions measured by the quartz oscillator were found to be those of
non-molecular species with relatively high reactivity in the plasma.
Keywords: Plasma diagnostics, quartz oscillator, non-molecular species, spatial
distribution, viscosity measurement
1. Introduction
Non-molecular species in plasmas are usually highly
chemical reactive, and are therefore key species in
plasma processes. They have been investigated using
various plasma diagnostic techniques because of their
importance. However, these techniques are not simple to
apply to plasma processes. In particular, there are few
techniques that can be used to measure chemical species
near plasma.
The plasma diagnostic techniques for non-molecular
species can be categorized into three types: optical, probe,
and mass spectroscopic measurements. However, with
the exception of optical measurement, conventional
techniques to detect non-molecular species in plasmas
have difficulty in detecting them near plasma. For
example, although probe measurement such as Langmuir
probe and Faraday’s cup can measure total amounts of
ions by applying bias voltages to the detector, they
cannot perform measurement near plasma because the
wire or cup detector affects the plasma and likely
produces plasma between these probes for these
measurement and other parts in the plasma chamber. For
mass spectroscopic measurement, the sampling tube also
induces problems.
The measurements mentioned above are not spatially
selective because they have difficulty in detecting
different spatial positions by changing their positions.
Therefore, they cannot obtain spatial distributions in a
plasma chamber. With regard to optical measurement,
optical emission and laser-induced fluorescence
techniques can obtain spatial distributions by changing
the position of the detector and probe laser. However,
only emissive non-molecular species can be detected by
these optical measurements.
A novel plasma diagnostic quartz oscillator
measurement method can obtain information on gas
species by measuring the viscosity and molecular weight
of the measured gas species in plasmas. By measuring
the changes in viscosity and molecular weight of the gas,
information on composition changes of stable gas
molecules can be obtained with some filters that prevent
reactive non-molecular species, such as atoms, ions,
radicals, and electrons, from the Q-sensor [1, 2].
Moreover, information on decomposition of source gas
can also obtained by this method [3]. These results
indicated that the quartz oscillator measurement is useful
to obtain information on gas compositions in plasmas.
Other advantages of this measurement are the small
size of the quartz oscillator, which is about 1x4 mm, and
is applicable to various places because it does not affect
plasmas, as well as the simple pressure measurement.
These advantages make it easy to derive spatial
distributions of information on gas species in plasmas by
simply changing the position of the quartz oscillator.
Spatial distributions give useful information to
understand the kinetic mechanisms of gas species in
plasmas. First, the kinds of species produced by plasmas
can be identified by the location where the change in
quartz oscillator output occurred. H2O evaporation from
plasma electrodes and CO desorption from the inner wall
of the chamber were identified by these spatial
distributions measured by the quartz oscillator [4]. In
addition, from the shape of the spatial distribution, which
reflects the reactivity of the chemical species, the kinds
of chemical species will be identified. As outlined above,
spatial distributions are another type of useful
information to understand the kinetic reactions in
plasmas.
In this study, we focused on the non-molecular species
in N2-H2 plasmas, which are used for plasma nitriding.
Non-molecular species in N2-H2 plasmas are not clear,
but we performed measurements with a quartz oscillator
without any filters to allow all chemical species to reach
and be detected by the quartz oscillator. Information on
non-molecular species in N2-H2 plasmas would be
important to understand the nitriding mechanism. Thus,
the spatial distribution is also helpful to assign the kinds
of non-molecular species in N2-H2 plasmas.
2. Experimental
Figure 1 shows the experimental setup used in this
study, which consisted of a vacuum chamber with
electrodes for radio frequency (13.56 MHz) plasma, a
quartz sensor (Q-sensor), a quartz oscillator, and a
diaphragm gauge. The chamber was connected to a gas
supply and vacuum pumps. In addition, apparatus for
plasma diagnostics, such as quadrupole mass
spectrometer (QMS) for gas analysis and a miniature
spectrometer with a photodetector (PD) for optical
emission spectroscopy.
The Q-sensor was fixed near the plasma electrodes
where the top of the Q-sensor holder was fixed 70 mm
from the edge of the plasma electrodes. The spatial
distributions of the results measured by the Q-sensor
were determined by sliding the Q-sensor in the holder.
Here, Z is defined as distance between the head of the
Q-sensor and the edges of the plasma electrodes as
shown in Fig. 1. Z can be varied 0-130 mm in the present
configuration.
Two QMS were used to detect various chemical
species in plasmas and to compare with the results
obtained with the Q-sensor. QMS1 was for electrically
neutral chemical species measured by electron ionization,
and QMS2 was for ions measured without any ionization.
The top of the sampling tube for QMS1 was located 70
mm from the edge of the plasma electrodes and 300 mm
from the ionization region for mass analysis. A sampling
orifice for QMS2 was located just below the center of the
anode. Optical emissions from plasma were detected
Q-sensor
with PD on the miniature spectrometer from the center of
the view window near the plasma without any focusing.
The details of the quartz sensor are shown in Figure 2.
The size of the quartz oscillator was 1 x 4 mm, as shown
in Fig. 2(a), and installed into the Q-sensor together with
a thermocouple and a probe for current measurement as
shown in Fig. 2(b). There was no cover on the quartz
oscillator in the present study, and therefore all chemical
species in plasma could reach and be detected by the
Q-sensor. DC bias was applied to the Q-sensor holder to
investigate the bias voltage dependence of Q-sensor
measurement.
Flow rates of N2 and H2 and total pressure were 12.5,
37.5 sccm, and 300 Pa, respectively. The ratio of flow
rates of each gas, by which maximum change in
composition of stable gas molecules was anticipated, was
selected.
Under the plasma conditions mentioned above, the
Q-sensor measured N2-H2 plasmas with changing Z
together with gas analysis using the QMS and optical
emission spectroscopy using PD.
3. Results and discussion
Temporal evolutions in total pressure, Q-sensor output,
and the pressure and temperature normalized Q-sensor
output (NQO) are presented in Figure 3 with the
discharge on or off. The rf power and Z were fixed to 100
W and 20 mm, respectively.
With discharge, the pressure and Q-sensor output
decreased. During the discharge, pressure increased and
saturated, while the Q-sensor output continued to
decrease. This continuous decrease in Q-sensor output
was attributed to the increase in temperature at the
Q-sensor.
D-gauge
DC
PD
Z
QMS2
70 mm
Anode
Cathode with
earth shield
QMS1 Gas inlet Pump
13.56
MHz
Fig. 1 Experimental setup used in this study
Fig. 2 Outlook of a quartz oscillator (a) and the head of
the Q-sensor (b) which includes the quartz
oscillator
Q-sensor output change
(arb. unit)
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
0
50
100
150
200
250
Rf power (W)
Fig. 4 Rf dependencies of NQO change
NQO was derived to exclude influences of changes in
pressure and temperature using the pressure and
temperature dependence of the Q-sensor output as
reported previously [1]. Gas composition can be
observed by NQO because it only depends on viscosity
and molecular weight of the measured gas in plasmas.
An increase in NQO means decreases in viscosity and
molecular weight of the measured gas, which is the same
qualitative change seen in the results measured by the
Q-sensor with a filter that prevented non-molecular
species from reaching the quartz oscillator [5]. However,
the increase in NQO measured without a filter in this
study was about twice that with the filter, probably
because all chemical species in the plasmas were
detected by the Q-sensor in Fig. 3.
The results measured with the filter explained above
included information on stable gas molecules such as
N2 + 3H2 2NH3.
(1)
Comparing both results suggested that information on
non-molecular species in plasma was included in Fig. 3,
although the kinds of non-molecular species could not be
assigned. This is because only a single output that
depends on viscosity and molecular weight of the
measured gas can be obtained by Q-sensor measurement.
Quantitative evaluation of NQO change does not
directly correlate to amount of non-molecular species;
however, they should be evaluated as relative density. To
confirm this, rf power dependence of NQO change
observed in Fig. 3 was investigated and plotted in Figure
4. Flow rates, pressure, and Z were identical to the results
of Fig. 3. The rf power varied within 10-220 W.
As shown in Fig. 4, NQO change increased with rf
power, suggesting that the observed NQO change seen in
Fig. 3 was attributable to non-molecular species because
they normally increase with rf power. Therefore, it was
concluded that relative amount of non-molecular species
in plasmas can be determined by Q-sensor measurement
without a filter. This is a useful plasma diagnostic
method to detect reactive non-molecular species, which
play important roles in the plasma nitriding process. In
particular, relative amounts of non-molecular species can
be compared when rf power is varied because initial gas
composition is constant and relative change is correlated
with the density change in plasmas.
The NQO change seen in Fig. 3 was confirmed to be
due to non-molecular species by comparison with the
results obtained by gas analysis using QMS1. Figure 5
shows relative signal intensity from NH3 plotted against
NQO change. NH3 is produced in N2-H2 plasma, and is a
representative probe for the degree of change in gas
molecules in N2-H2 plasma. Figure 5 indicated no clear
correlation between NQO change and NH3 produced;
therefore, the NQO change seen in Fig. 5 was not
correlated with the changes in gas molecules in N2-H2
plasma. This also suggests that the NQO change in Fig. 3
was attributable to the change in non-molecular species
in N2-H2 plasma.
2.5E-6
m/e=17 (arb. units)
Fig. 3 Temporal changes of total pressure (□),
Q-sensor output (△), and pressure and
temperature normalized Q-sensor (NQO)
output (○) with and without discharge.
2.0E-6
1.5E-6
1.0E-6
5.0E-7
0.0E+0
-0.1
0
0.1
0.2
0.3
0.4
0.5
NQO change (arb. units)
Fig. 5 Relative mass signal intensity from NH3
(m/e=17) plotted against NQO change.
NQO change (arb. units)
0.50
0.40
0.30
0.20
30000
Optical emission intensity
(arb. units)
The spatial distribution of NQO change measured with
the Q-sensor without a filter is shown as a function of Z
in Figure 6. Z was varied within 20-130 mm. When Z<20
mm, plasma was introduced into the Q-sensor because
the spacing between the cathode and anode was 20 mm,
and therefore the outputs from the Q-sensor fluctuated
and the measurement became unstable.
As expected, the NQO change by plasmas decreased
with Z, which is reasonable because non-molecular
species in plasma must decrease with distance from the
plasma electrodes, i.e., plasma. Figure 6 shows curve
fitting of our previous results measured by the Q-sensor
with a filter that prevents non-molecular species reaching
the quartz oscillator in the Q-sensor. One is the result of
change in stable gas molecules in N2-H2 plasma, the
other is in NH3 plasma [3]. They are indicated by solid
and broken lines, respectively. The former spatial
distribution results from the chemical reaction of (1)
mentioned previously, which occurs mainly at the
surface of the plasma electrodes. The latter results from
NH3 decomposition, which are wider spatial distributions
because various gas reactions relate in plasma
The spatial distributions of NQO change according to
chemical species with high reactivity should be similar to
those by reaction (1) for NH3 produced at the plasma
electrodes because highly reactive chemical species
immediately react near the plasma electrodes and
disappear with distance from the electrodes. Therefore,
the similarity of the present results in Fig. 6 and those on
stable gas molecules induced by reaction (1) may be
explained as resulting from non-molecular species with
high reactivity.
To identify the kinds on non-molecular species
induced by NQO changes in Figs. 3 and 6, the results
25000
20000
15000
10000
5000
0
-0.1
0
0.1
0.2
0.3
0.4
0.5
NQO change (arb. units)
Fig.7 Relative optical emission intensity of N2*+ (○) and
H (●) plotted against NQO change measured
without filter.
obtained by Q-sensor measurement were compared with
those obtained by conventional plasma diagnostic
methods, such as optical emissive species, ions, and
electrons. Figure 7 presents relative optical emission
intensity as a function of NQO change. Among the
various conventional plasma diagnostic methods, relative
optical emission intensities from highly energetically
excited N2 and H by optical emission spectroscopy
mostly correlated with those by Q-sensor measurement.
Thereby, this may indicate that non-molecular species
measured by the Q-sensor without a filter in the present
study might come from these emissive species. These
emissive species may have higher reactivity than those of
electrically neutral radicals, judging from the shape of
the spatial distribution in Fig. 6.
4. Summary
Information on non-molecular species was observed
by the Q-sensor measurement without filter in N2-H2
plasmas. The spatial distribution obtained by the
measurement was attributable to non-molecular species
with higher reactivity than electrically neutral radicals. In
conclusion, it was shown that the Q-sensor measurement
without a filter is useful to detect non-molecular species
in plasma, and the spatial distribution obtained is helpful
to identify the reactivity of the non-molecular species.
0.10
0.00
-0.10
0
20
40
60
80
100
120
140
Distance from electrode edges; Z (mm)
Fig. 6 Spatial distribution of NQO change. Lines
are fitting curves to our previous results
measured by the Q-sensor with a filter for
N2-H2 plasma (solid line), and for NH3
plasma (broken line).
References
[1] A. Suzuki and H. Nonaka, Rev. Sci. Instrum., 80
(2009): 095109.
[2] A. Suzuki and H. Nonaka, Jpn. J. Appl. Phys., 50
(2011): 01AA03.
[3] A. Suzuki and H. Nonaka, Vacuum, 84 (2010): 1389.
[4] A. Suzuki and H. Nonaka, Vacuum, 84 (2009): 554.
[5] A. Suzuki and S. Asahina, Jpn. J. Appl. Phys., 51
(2012): 01AA03.