S8-3

Highly-Sensitive Monitoring of Electron Densitywith Surface Wave Probes
表面波プローブによる高感度電子密度モニタリング
Keiji NAKAMURA and Hideo SUGAI*
中村圭二，菅井秀郎*
Department of Electrical Engineering, Chubu University,
1200 Matsumoto-cho, Kasugai 487-8501, Japan
中部大学工学部電気システム工学科，〒487-8501 愛知県春日井市松本町1200*
Department of Electrical Engineering and Computer Science, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan*
名古屋大学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This paper reports on monitoring of electron density with surface wave probes. In order to achieve this, theprobe head was modified so as to enhance electrostatic coupling between the probe antenna and plasma.
The modification improved the sensitivity of the probe, and made the density monitoring possible even in
low-density and high-pressure plasmas where electron densities could not be measured with the previouslyused probe. The improvement of the sensitivity extended application range of the probe to actual processconditions. In the present study, density measurements in low-density and high-pressure CVD plasmas anddetection of gas components for endpoint detection in fluorocarbon etching plasmas were examined.

1. Introduction
Electron density is one of essential factors togovern reactive plasmas for materials processing
since a source gas is mainly dissociated by electron
impact process. Therefore development of accurate
simple monitoring techniques of the electron
density is important to control and stabilize theplasma processing.
As a novel tool for the issue, surface wave (SW)probes have been developed[1], and validity of theprobe has been demonstrated for measurements ofelectron densities in various reactive processing
plasmas. However, there are still remain problems,
especially for sensitivity. In the probe, an electron
density is determined from peak frequency ofobtained absorption spectra. The absorption peakintensity decreases with a decrease in electron
density ne as well as a indrease in operatingpressure p. Under conditions for ne>~1010 cm-3 andp>~1 Torr, the decrease in the absorption intensity
becomes serious to make the density measurement
difficult.
To date, the improvement of sensitivity by(a) Standard SW probe(b) Sensitive SW probe

Quartz Tube

Quartz Tube

Coaxial Cable

φ6Coaxial Cable

Fig. 1 Structure of surface wave probeba

removal of glass tube cover has been reported, andthe improved probe has been applied to the plasma
sensor for the discharge pressure up to ~1 Torr [2].
However, in actual plasma process, the glass coveris inevitable in order to avoid metal contamination.
In this report, the improvement of the sensitivityof the glass-covered SW probe is carried out by
modifying the probe head structure. The probe isused for density measurements in actual CVD
plasma sources under low-density and highpressure conditions. Furthermore, the probe is alsoapplied to detection of slight change in gas
component in fluorocarbon etching plasma for endpoint detection.
2. Experimentals
In the present study, two kinds of plasma sources
are used. One is an inductively-coupled plasma(ICP) source. The ICP is produced in a stainlesssteel cylindrical vacuum vessel by supplying 13.56
MHz RF powers through one-turn loop antenna.
Argon is fed into the vessel with a constant flow,
and the discharge pressure is adjusted between 10
and 1000 mTorr by controlling residence time of thedischarge gas. The other is a capacitively-coupledplasma (CCP) of an actual CVD source forformation of low-k films. The CCP is produced bysupplying RF powers between two parallel
electrodes in BCB-containing helium at a dischargepressure of 1 - 4 Torr.
Here two types of SW probes shown in Fig. 1 are3. Experimental Results and Discussions
3.1 Improvement of sensitivity and detection ofslight density variation
At first, a comparison of the sensitivity between
the two types of SW probes was made. In thestandard SW probe, several absorption peaks wereobserved, however the absorption intensities of theall peaks were as low as ~1 dB. However, thesensitive SW probes had peak intensities muchhigher than the standard probe. A pressure
dependence of the absorption intensity was
obtained as shown in Fig. 2. As the pressure
increased with the discharge power almost constant,the absorption intensity decreased for the standard
and the sensitive probes. However, regardless of thepressure, the sensitive probe B had the higher
intensity than the standard probe by a factor of ~10.
Thus the reduction of the distance between theplasma and the probe antenna is useful for
improvement of the sensitivity. The SW probes
allowed us to detect 1% variation of electron
density artificially induced by changing ~1 % thedischarge power. In the sensitive probe, thedetection of the slight density variation waspossible even for high pressure condition of ~500mTorr.
3.2 Density measurements in CVD plasmas
A plasma source for CVD processes is typicallyoperated under low-density (ne<~1010 cm-3) andhigh-pressure (>~1 Torr) conditions. Therefore thestandard SW probe is difficult to be applicable tomeasurements of the electron densities due toinsufficient sensitivity. Here the sensitive probewith the sensitivity improved is applied for thedensity measurements in a CVD plasma source. In10Absorption Intensity (dB)

used: one is a previously-used standard PAP. As
shown in Fig. 1(a), a 50 Ω coaxial cable with a
0.28-mm-diam. and 5-mm-long tiny rod antenna isinserted into a 6-mm-diam. and 1-mm-thick
cylindrical quartz glass tube. One end of the tube isclosed with a semi-spherical shape at the head. Theother is a sensitive SW probe shown in Fig. 1(b).
Basically, the sensitive PAP has the same structure
as the standard PAP, however the shape of the glass
tube at the head is modified. The diameter of theglass tube is reduced at the rod antenna, and the gap
between the rod antenna and the outer surface of theglass tube decreases. This modification of thesensor head enhances the absorption sensitivity ofthe probe due to the stronger antenna-plasma
coupling. In this study, the diameter a of the glass
tube at the probe head is 0.5~1.7 mm.

1Sensitive

0.1

Standard

0.01
1101001000 10000

Pressure (mTorr)

Fig. 2 Pressure dependence of absorption
intensity for standard sensitive SW probe.
the experiment, the discharge pressure is 3 Torr.
Even in such a high pressure condition, absorption
spectra was clearly obtained, and the radial
distribution and the power. dependence of theelectron density was successfully measured.
3.3 Detection of slight change in gas component
Simulating end-point detection in an etchingprocess, a small amount of CO2 gas wereintroduced into the Ar plasma as a etch product, andthe electron density variation induced by the CO2
injection were examined. The SW probe waslocated near the inner surface of the vessel so as torealize an actual situation in an commercial etching
device for mass production. Even for the CO2
fraction as low as ~0.3 %, the sensitive SW probe
was able to detect the density variation. In
comparison with optical emission spectroscopy, theCO2 injection was precisely detected with the SW
probe, suggesting that the SW probe will beapplicable to precise end-point detection.
Acknowledgements
The authors would like to thank N. Toyoda,Nissin Inc. and K. Kinoshita, MIRAI-ASET for
their great support and discussions. Part of thiswork was supported by NEDO. This work was also
supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture,Sports, Science and Technology in Japan.
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Sugai; Jpn. J. Appl. Phys. Vol. 38 (1999) , No.9A(pt.1), pp. 5262- 5266.
[2] K. Nakamura, M. Ohata and H. Sugai; J. Vac. Sci.
Technol. A Vol. 21 (2003) , No. 1, pp. 325- 331