22P014-O
Properties of inductively coupled rf Ne/N2 plasmas
誘導性結合型Ne/N2プラズマの特性
Takashi Kimura and Ryotaro Nishimura
木村 高志, 西村 亮太郎
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
名古屋工業大学 〒466-8555 名古屋市昭和区御器所町
The measurements with a floating probe method and optical emission spectroscopy are carried out ininductively coupled rf (13.56 MHz) Ne/N2 discharges in the total pressure range from 5mTorr to 50mTorr,changing the Ne fraction from 50% to 90%. The measured electron density is around 1017m-3 and themeasured electron temperature is in the range of 3-6eV. Both the density and the temperature gradually
increase with the Ne fraction. N2 dissociation rate, which corresponds to the density ratio of N to N2, can be
estimated by actinometry. The dissociation rate reaches more than 10% when the Ne fraction is larger thanabout 80%.

1. Introduction
Inductively coupled plasmas (ICPs) at low
pressures have been widely used in the material
processing, because the high densities and spatiallyhomogeneous profiles of reactive species, such asatoms, free radicals and ions, can be produced andthe ion bombarding energy at the surface can be
independently controlled [1]. The inductively
coupled plasmas are typically operated at pressures
lower than 50-100 mTorr in typical electron density
range of 1016 - 1018 m-3. Such high-density plasmas
containing nitrogen are useful sources of active
species, such as nitrogen atoms, metastable nitrogenmolecules, and nitrogen ions, and have been applied
to semiconductor surface nitridation and etchingof dielectrics. The dissociation degree ofmolecular nitrogen, which should be a significant
parameter for the improvement of the nitridation
processes, has been reported to be low in low
temperature N2 and Ar/ N2 plasmas [2]. However,Akatsuka et al [3] investigated the dissociation
degree of microwave discharge nitrogen plasma
diluted with one of rare gases, and showed themarked increase in the nitrogen dissociation
degree with increasing the mixture ratio of Ne.
In this study, measurements with a floatingprobe method and optical emission spectroscopy
are carried out in inductively coupled rf (13.56MHz) Ne/N2 discharges in the total pressure
range from 5mTorr to 50mTorr, changing the Ne
fraction from 5% to 50%.
2. Experiments
A schematic diagram of the experimental
apparatus is shown in Fig.1. A stainless steel
chamber is cylindrical, 160 mm in inner diameter

MatchingNetworkRF Power
SupplyPlanar CoilQuartz
OpticalEmission
Spectroscope

2R(=160mmφ)
L(=55mm)
20mm
mass-flow
controllers

Floating Probe

Fig 1 Experimental apparatus

and 55 mm in length, with a quartz plate of 10 mmthickness placed at the upper end of the chamber.
Discharges are sustained by an azimuthal electricfield induced by a rf coil current supplied to aplanar three-turn coil from a power source
connected to an L-type capacitive matching
network. The planar coil mounted about 5mm
above the quartz plate is concentric in the center ofthe chamber to maintain the discharge symmetry.
An electrostatic shield can practically eliminate thecapacitive coupling between the coil and the plasma,
resulting in suppression of any rf plasma potentialfluctuation. A planar probe is installed at thechamber center in order to measure the electron
density and the electron temperature［4］. The power
Pabs injected into the plasma, which is estimated bysubtracting the transmitted power without plasma
from that with plasma at the same current, is kept at
170 W in our experiment. The flow rates of Ne andN2 are controlled using two mass-flow controllers,
and the total flow rate corresponding to the sum oftwo flow rates is maintained at 20SCCM.
The atomic nitrogen density in the plasmas is3. Results and Discussions
Figures 2 and 3 show the electron density and its
temperature measured as a function of XNe, whereXNe corresponds to the Ne fraction. As shown inFigs. 2 and 3, the measured electron density does
not strongly depend on XNe below XNe lower than80%, beyond which the increase in the density withXNe is more prominent. On the other hand, theelectron temperature gradually increases withincreasing XNe. The measured electron density isaround 1017 m-3 at any condition, whereas themeasured electron temperature decreases from
4.2eV to 6eV at 5mTorr, from 2.7eV to 4eV at50mTorr. The electron temperature measured by thefloating probe method should correspond to theslope of the high energy region in electron energy
probability function (EEPF). Therefore, thetemperature may be somewhat lower than theeffective electron temperature corresponding to theslope of the bulk region in EEPF.

The dissociation rate (DR), which corresponds tothe density ratio of N to N2, can be estimated byactinometry. The DR estimated as a function of XNe
is shown in Fig. 4. As shown in Fig. 4, the DR
abruptly increases with the increase in XNe at anypressure. However, the dissociation rate does notstrongly depend on the total pressure p, although
the rate coefficient for dissociation of nitrogenmolecules induced by electron impact strongly
depend on p. The fractional rate of the energy loss
due to the dissociation of nitrogen molecules to thetotal energy loss may be a significant parameter inorder to explain the relationship between DR and p.
Asides from the fractional rate of energy loss
described above, the investigation on the influenceof dissociation processes by the collisions between
the metastable Ne and N2 will be required.

p=5mTorr
p=10mTorr
p=25mTorr
p=50mTorr

20DR (%)

estimated from the optical emission intensities at746.8 nm and 750.4 nm (the excited Ar), where asmall amount of Ar (below 3%) is mixed as aactinometer [5].

10ne (×1017 m-3)

10 180p=5mTorr
p=25mTorr
p=50mTorr

5060708090

XNe (%)
Fig.4 DR estimated as a function of XNe.

10 175060708090

X Ne (%)
Fig.2 Electron density measured as a function of XNe

p=5mTorr
p=25mTorr
p=50mTorr

Te (eV)8642050607080
XNe (%)

90Fig.3 Electron temperature measured as a function of XNeReferences[1] M. A. Lieberman and A. J. Lichtenberg: Principlesof Plasma Discharges and Materials Processing(Wiley, NewYork, 2005) 2nd ed., Chap. 11.
[2] T. Kimura and H. Kasugai: J. Appl. Phys. 108 (2010)
033305.
[3] H.Akatsuka, K.Kuwano, A.Nezu and H.Matsuura:
Proc. 7th International Conference on Reactive
Plasmas, Paris, 2010, CTP-063．
[4] M. H. Lee, S. H. Jang, and C. W. Chung: J. Appl.
Phys. 101 (2007) 033305.
[5] T. Czerwiec, F. Greer, and D. B. Graves: J. Phys.
Phys. D 38 (2005) 4278.