S8-1

Ultra-sensitive measurement of sheath electric fieldby laser-Stark spectroscopy of argon Rydberg states
アルゴンリュドベリ状態のレーザーシュタルク分光による超高感度シース電界計測
Koichi Sasaki
佐々木 浩一Department of Electrical Engineering and Computer Science,
Nagoya University, Nagoya 464-8603
名古屋大学大学院工学研究科電子情報システム専攻〒 464-8603 名古屋市千種区不老町

By applying laser-induced ﬂuorescence-dip spectroscopy to Ar, we have developed a technique
for sensitive measurements of electric ﬁelds in reactive plasmas. A detection limit of 3 V/cm has
been achieved, which is the most sensitive detection limit in the world. We have measured electricﬁelds in the sheath regions of Ar and Ar/SF6 plasmas produced in an inductively coupled plasma
source. In this paper, we show the distributions of sheath electric ﬁelds in electronegative Ar/SF6plasmas, together with the comparison with theoretical calculations based on a ﬂuid model.

rf Power Source
13.56MHzcw

1. Introduction
The measurement of electric ﬁeld is important for optimizing plasma processing of materials since the irradiation of positive ions accelerated in the sheath electric ﬁeld plays essentialroles. In addition, the detailed structure of thesheath electric ﬁeld, especially in the case of electronegative plasmas, is of interest from the view
points of basic plasma physics and basic discharge
physics. We have developed a technique for sensitive measurements of electric ﬁelds in various
processing plasmas [1-3]. The technique is basedon laser-induced ﬂuorescence-dip (LIF-dip) spectroscopy, which was demonstrated by Czarnetzki and coworkers in several years ago [4]. Czarnetzki and coworkers employed H atoms as theprobe particle. We have applied LIF-dip to Ar,
by which we can measure electric ﬁelds in variousplasmas. In this paper, we demonstrate the measurements of sheath electric ﬁelds in electronegative Ar/SF6 plasmas produced in an inductivelycoupled plasma source. The experimental results
are compared with theoretical calculations basedon a ﬂuid model.

ﬁeld from the measurement of the energy levelstructure of high Rydberg states. In LIF-dip, theenergy level structure is detected as a dip spectrum of LIF intensity by employing a two-step
excitation scheme using two tunable lasers. Wedetected a high Rydberg state with a principal
quantum number of n = 58, and we achieved asensitive detection limit of 3 V/cm by measuring
the magnitude of Stark shift of an Rydberg statewith n = 40.

2. Principles
The principles of LIF-dip of Ar have been reported previously. It is a sort of laser Stark
spectroscopy. Since the energy level structure ofhigh Rydberg states is a function of the electric
ﬁeld, we can determine the strength of electric

3. Experimental setup
The experiment was carried out in an
inductively-coupled plasma source shown in
Fig. 1. A movable planar electrode was inserted
in plasmas produced by an internal ICP antenna
activated by an rf power supply at 13.56 MHz.

Gas inlet

OPO
Laser

PMTLensa linewidth of 0.25 cm-1

Monochromator

Polarization

Dye
Laser

Gas exhausta linewidth of 0.2 cm-1

Fig. 1. Experimental apparatus.

3(a) SF : 4%20

α=15.8106210ne=9 x 108 cm-30-10110

Electric potential (V)Electric field (V/cm)

10-20310(b) SF6: 8%
α=18.5102

ne=8 x 108 cm-3

20
100-10110Electric potential (V)Electric field (V/cm)-20
310(c) SF6: 10%α=29.0210ne=4 x 108 cm-320100-10

110Electric potential (V)4. Distributions of electric field in electronegative plasmas
To demonstrate the sensitive measurement, weshow the distributions of electric ﬁeld in Fig. 2.
The horizontal axis is the distance z from theelectrode surface. These distributions were observed in electronegative Ar/SF6 plasmas. The
degree of electronegativity is deﬁned by the ratio of the negative ion density to the electrondensity (α = n− /ne ), which was controlled bychanging the partial pressure of SF6 . The valueof α was measured by probe-assisted laser photodetachment using a XeCl excimer laser as thelight source. We measured the electron density
using a surface-wave probe, since the measurement of electron density was diﬃcult by a Langmuir probe in an electronegative plasmas. The
degree of electronegativity and the electron density in each experimental condition are indicatedin the ﬁgure.
A ﬂuid model for calculating the distributionof electric ﬁeld in an electronegative plasma has
been reported by Kono [5]. We calculated the distribution of the sheath electric ﬁeld in the experimental conditions by using the ﬂuid model. The
theoretical electric ﬁeld is also shown in Fig. 2
by solid curves, together with the distributions ofelectric potential (dashed curves) which was obtained by integrating the theoretical electric ﬁeld.
The most important feature of the electric ﬁeld
observed in the electronegative plasma is the distribution with the two-stage structure, typicallyshown in Fig. 2(c). The almost constant electricﬁeld at z ≥ 3 mm may be due to the detectionof quasi-random microﬁeld, and in not includedin the theoretical calculation. The jump in theelectric ﬁeld strength at z  2.8 mm is explainedby the reﬂection of negative ions with low temperature. This jump was reproduced by the theoretical calculation. The slope of the electric ﬁeld
in the range of 0.6 ≤ z ≤ 2.8 mm is consistent
between the experimental and theoretical results.
However, the jump in the electric ﬁeld strength atz  0.6 mm, which may be due to the reﬂectionof electrons, was not reproduced theoretically.

References[1] K. Takizawa, K. Sasaki, and K. Kadota,
Jpn. J. Appl. Phys. 41, L1285 (2002).
[2] K. Takizawa, K. Sasaki, and A. Kono, Appl.
Phys. Lett. 84, 185 (2004).
[3] K. Takizawa and K. Sasaki, J. Plasma Fusion Res. 80, 3 (2004).
[4] U. Czarnetzki, D. Luggenhölscher, and H.
F. Döbele, Phys. Rev. Lett. 81, 4592
(1998).
[5] A. Kono, J. Phys. D: Appl. Phys. 32, 1357
(1999).

Electric field (V/cm)

The electrode was connected to a dc power supply, and was biased at −20 V with respect tothe grounded chamber wall. The plasma potential was estimated to be +23 V. The distribution of the electric ﬁeld in the sheath in frontof the planar electrode was measured by LIF-dip
spectroscopy. We used a dye laser and an optical parametric oscillator (OPO) for measuring
the structure of Ar high Rydberg state by thetwo-step excitation scheme. The LIF intensity
was detected using a photomultiplier tube via amonochromator.

-2001 2 3 4 5 6 7 8Distance from electrode (mm)Fig. 2. Distributions of sheath electric ﬁeld observedin Ar/SF6 ICP plasmas at various percentages of SF6 .
The voltage between the plasma and electrode potentials was 43 V. The degree of electronegativity deﬁned
by α = n− /ne was examined by laser photodetachment and the electron density ne was measured using
a surface-wave probe. The solid and dashed curves
represent theoretical electric ﬁeld and potential calculated by a ﬂuid model at each experimental condition.