22P094-P
Evaluation of electron temperature and electron density in a plasma treatment
device for circuit board by using collisional-radiative model
衝突輻射モデルを用いたプラズマ基板処理装置の電子温度・電子密度評価
Shinya Shiozawa1, Naomichi Ezumi1, Satoshi Hoshino1, Keiji Sawada2, Toshimitu Oda2,
Keita Nakane2 and Kaoru Ono3
塩沢慎也1, 江角直道1, 星野哲志1, 澤田圭司2, 小田敏充2, 中根啓太2, 小野薫3
1

Nagano National College of Technology, 716, Tokuma, Nagano 381-0041,Japan
長野工業高等専門学校 〒381-0041 長野県長野市徳間716
2
Faculty of Engineering, Shinshu University, 4-17-1, Wakasato, Nagano 380-8553, Japan
信州大学工学部 〒380-8553 長野県長野市若里4-17-1
3AIN Co.,Ltd. 374-2, Toyohira, Chino 391-0213, Japan
株式会社アイン 〒391-0213 長野県茅野市豊平374-2
Evaluations of electron temperature and density are important for plasma applications. In this study, we
have observed plasmas in a surface treatment device for circuit boards by optical emission spectroscopy.
Based on a collisional-radiative model, we have attempted to evaluate electron temperature and density
using emission spectrum. As a result, the electron temperature evaluated about 1-2 eV which is close to theresults of probe measurements, Moreover we confirmed discharge power dependence of electron
temperature and density.

1. Introduction
It is important to measure plasma parameterssuch as electron temperature (Te) and electron
density (ne) in plasma applications. So far, wehave measured a plasma surface treatment device
for circuit boards using probe method. In thisstudy, we have measured the plasma using opticalemission spectroscopy (OES) as passive
measurement. Observed results are analyzed
using a collisional-radiative (CR) model forevaluating Te and ne.
2. Experimental Setup
The experimental setup is shown in Fig.1.
Experiments are performed by using a plasma
surface treatment device for circuit boards(Advanced Plasma Systems, PWB-24). The device
has a chamber with the dimension 1585 × 1300 ×1080 mm. 40 kHz AC power voltages are applied to
14 electrodes at regular intervals in a vacuum
chamber. The output power is up to 10 kW. Though
operational neutral gas pressure is typically
100-300 mmTorr for process gases such as He, H2,O2, N2, Ar, etc, we use He for comparing withanalysis model in this study.
A sapphire window is attached to the side port ofthe device for observing visible light emission fromthe plasma. The light through the window isfocused to an optical fiber and the light is guided toa spectrometer (Ocean Optics Inc., HR2000). The
wavelength range is 300-730 nm and the resolution

is about 0.3nm. A partition board is placed on theposition as shown in Fig.1 for defining the observed
volume.

Fig. 1 Experimental Set up

3. Collisional-Radiative model
In order to evaluate Te and ne, we attempt to use
a CR model for He [1-3]. According to CR model,
the temporal variation of the population density ofan excited state p, dn(p)/dt can be described by arate equation as follows,(1)
where C(p,q) is the excitation rate coefficient forelectorn collisions from state p to q and F(q,p) is
the inverse de-excitation rate coefficient. A(p,q) is
the spontaneous transition probability from p to q,and S(p) is the ionization rate coefficient for state p.
Eq.(1) is approximated to 0 for all the states except

the ground state and metastable states. Solving therate equations of each excited states withconsidered photo-excitation from the ground state,n(p) can be described the as follows [3],n( p) = r1( p)n(11 S)ne + r2 ( p)n(21 S)ne + r3 ( p)n(23 S)ne+ r4 ( p)n(11 S)I 31 P + r5 ( p)n(11 S)I 51 P ,€(2)where r1(p), r2(p), r3(p), r4(p), and r5(p) are thepopulation coefficients, each of which is a functionof Te and ne.
4. Results and discussion
Figure 2 shows observed spectra for He plasma
with 8 kW in 250 mTorr. The averaged opticalemission intensities have been obtained by theabsolutely calibrated optical system. This resultshows that typical He emission spectra with Hα line
as contamination described below.
Figure 3 shows the comparison of experimentallyobtained population density and calculated one
using the CR model with lines intensity in Fig.2. In
this result, the calculated population reproduced theobserved one. Most of the population of 31P and
51P are produced by photo excitation from theground states. For other states, the populations areproduced mainly electron impact from the 21S state,
and from 11S state is seems lower.
Figure 4 shows the Te and ne evaluated by CR
model for each discharge power. The Te is about 1.5
eV which is consistent with the temperature
measured by probe method [4]. The ne tend toincreases with discharge power. In the case of 2kW, the ne is higher than other power conditions, Apossible reason is that some error of observed
emission intensities is large because of emission
intensities is low in 2kW.
As mentioned above, Fig.2 shows emission linesof hydrogen especially at 656nm. The hydrogenline considered to be caused by moisture in vacuum
chamber. This contamination would also cause theerror in analysis using the CR model.

Fig.3 Comparison of population distribution between
spectroscopic measurement and CR model analysis.

Fig.4 Evaluated Te and ne using observed spectra basedon CR model for several discharge power.

5. Summary
We have observed plasmas in a surface treatment
device by OES and have attempted to evaluate Te
and ne using CR model. As a result, the Te evaluatedabout 1-2 eV which is consistent with the results ofprobe measurements. And we confirmed discharge
power dependence of Te and ne. Moreover weconfirmed hydrogen lines. It is important result for
accuracy of the evaluation using the CR model.
References[1] T. Fujimoto: J. Quant. Spectrosc. Radiat. Transfer.
21, 439 (1978).
[2] M. Goto: J. Quant. Spectrosc. Radiat. Transfer. 76,331 (2003).
[3] K. Sawada, Y. Yamada, T. Miyachika, N. Ezumi, A.
Iwamae, and M. Goto: Plasma and Fusion Research,5, 001 (2010)[4] K. Yasuhara, K. Imamura, K. Ono, and N. Ezumi:
Extended Abstracts of The 54th Spring Meeting,
JSAP (2007) AP 071108-1, p.178

Fig.2 Observed spectra of He discharge