Plasma and Fusion Research: Regular Articles

Volume 14, 3401085 (2019)

Characteristics of an ECR Sheet-Shaped Plasma Formed by a
Combination of Permanent Magnets and Field Coils∗)
Arnold Rey B. GINES and Motoi WADA
Graduate School of Science and Engineering, Doshisha University, Kyotanabe, Kyoto 610-0394, Japan(Received 10 January 2019 / Accepted 21 March 2019)
An ECR sheet plasma device employing a 2.45 GHz microwave source and combination of permanent magnets and field coils is being developed. The combined field realized a linear magnetic field that sustained arectilinear confinement of the plasma. The ECR condition eﬀectively lowered the plasma ignition power to below 500 W and minimum plasma sustaining power to 94 W while also increasing the pressure range of low power
ignition. Low ignition power is observed while plasma density is highest at 0.7 - 0.3 Pa pressure range. Stable
plasma operation at input microwave power of up to 3 kW was achieved without overheating in the quartz window
region with the aid of a gas feed mechanism on the quartz window.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: magnetized plasma, electron cyclotron resonance, sheet-shaped plasma, argon plasma, microwave
plasma
DOI: 10.1585/pfr.14.3401085

1. Introduction
A weakly ionized plasma opens new ways to synthesize materials and modify surface structures [1]. Sheet
plasmas have the advantage of producing thin films andfunctional surfaces by generating localized high-density
and temperature gradient regions suitable for specific reactions. A stream of high energy electrons from aplasma cathode eﬃciently excites and/or ionizes atomic
and molecular species which are confined in a linear magnetic field [2, 3]. The so called Uramoto-type sheet plasma
device employs one of the simplest design to realize asheet-shaped plasma by combining a linear magnetic field
produced by electromagnets and a pair of dipole permanent magnets [4]. Because of the performance to create athin plasma, the configuration was utilized among several
research groups [5–7].
To realize a sheet-shaped plasma profile, a linear magnetic field is arranged such that electrons produced in theionizing region are guided along the field line of force.
Among diﬀerent modes of excitation, microwave-based
electron cyclotron resonance (ECR) enables excitation atlower gas pressure and lower input power while also eliminating high temperature cathodes that introduce impurities
into the plasma, thereby, providing greater degrees of freedom on plasma processing parameters. A simple configuration for plasma production by ECR condition and rectangular magnetic confinement has been designed. Yoshida
and Kajinishi [8] employed two coils fed by current of
up to 210 A to produce a field equivalent to 1300 G alongauthor’s e-mail: euq3301@mail4.doshisha.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

the discharge chamber and injected a 2.45 GHz microwave
power through a quartz glass window. Large current tomaintain the magnetic field for long operations is not ideal.
Morishita et al. [9] employed a set of permanent magnets
to produce cusp field near the cathode. While a uniform
sheet plasma in the length of 20 cm is produced, the field
profile is limited by the fix flux density created by the permanent magnets. Noguera and Ramos [10], and Ramos, etal. [11] employed a sheet plasma device with the magnetic
field structure similar to the original Uramoto-type sheet
to form TiN films for hard coatings. Impurity emissions
from the plasma cathode often restrict usage of this type ofdevice for any plasma process that forbids introduction of
trace impurities.
We investigate the production of sheet-shaped plasma
sustained in a moderate intensity magnetic field using a2.45 GHz microwave source. A localized ECR condition isrealized through combining the linear magnetic field produced by coils and a pair of permanent magnets. The discharge conditions satisfying ECR and that with the field
intensity for sub-ECR conditions are reported.

2. Device Set-Up
Figure 1 illustrates the device set-up with the definitions of coordinates. A 290 mm-long stainless steel sixway cross chamber serves as the plasma container. A
2.45 GHz microwave power source, which has a maximum
power output of 3 kW, delivers the electromagnetic radiation to the chamber through a tapered copper waveguideand a 110 mm by 7 mm quartz glass window. The systemis evacuated by a 355 /s turbo molecular pump down to10−5 Pa base pressure.
Three sets of identical pan-cake coils that are electri-

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3401085 (2019)

Fig. 2 Magnetic flux density profile along the downstream central axis inside the chamber. The coil current is 15 A.

Fig. 1 Cross-section of the sheet plasma device using permanent
magnets and field coils to produce a line cusp field. The
inset shows details of the quartz window region.

cally connected in series produce a linear magnetic fieldwith weak magnetic mirrors inside the vacuum chamber.
The field strength produced by the coils can be adjusted
by controlling the current applied on the coils from a DC
source which can provide up to 15 A current. Figure 2
shows the measured field contribution of the coils along they-axis inside the chamber starting from the plasma-facingside of the quartz glass. At 15 A, the field is maximum(430 G) at the quarts window. To realize resonance condition satisfying ECR at B = 875 G, the field is increased byinstalling Nd-Fe magnets in the region around the quartz
microwave window. These permanent magnets generate
cusp field along the y-axis with a maximum flux densityof 480 G at the quartz window. When combined with thefield coils (at 15 A), the total field generates the intensity
corresponding to an ECR region just several millimetersfrom the quartz window along the axis of the linear magnetic field. A Hall probe coupled to a Lakeshore Model460 3-channel gauss meter measured the field along the yaxis. The magnetic flux density profiles of the total fieldfrom the permanent magnets and the field coils along thechamber axis is also shown in Fig. 2.
Figure 3 a) shows the initial microwave inlet flange
design. During plasma production, localized heating wasobserved in the quartz glass region despite fitting a watercooling system on the outer face of the flange. A substantial heating was observed at non-ECR conditions. Whenthe microwave power exceeds 1.5 kW, the microwave inlet region considerably heated up to eventually burn thehigh temperature O-ring seal and damaged the quartz glass.
To resolve this overheating problem, a gas inlet feature isincorporated into the flange design as shown as shown in
Fig. 3 b). The gas inlet structure supplies neutral gas in theFig. 3 Microwave inlet flange designs. a) Simple flange withquartz glass window, and b) similar flange as [a)] withgas introduction inlet.

vicinity of quartz window surface to enhance recombination for cooling down the temperature and density of thelocal plasma in touch with the window surface realizing
the condition similar to a gas divertor [12]. This configuration can sustain plasma excited with up to 3 kW microwave
power without damaging the seals and the quartz window
for several minutes. The cooling mechanism enables fullrange of possible microwave power available during thisinvestigation.
A Langmuir probe obtains I - V characteristics of theplasma. The cylindrical probe consists of a 0.6 mm diameter tungsten wire enveloped in an alumina tube that leaves a
2 mm exposed end tip of the wire. A grounded copper tube
electrostatically shields the probe signal line from ambient noise. A larger alumina tube shields the copper tube
and its contents from plasma irradiation and the probe signal noise from electromagnetic radiation in the chamber.
A Yokogawa model GS610 source measure unit digitally
records the I - V curve.

3. Results and Discussion
3.1 Ignition and extinction power

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In Fig. 4, the ignition and extinction powers as func-

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Volume 14, 3401085 (2019)

Fig. 4 Ignition (Ig) and extinction (Ex) powers at varying Ar
pressure from diﬀerent field geometries, namely, fieldcoils (FC), permanent magnets (PM), and combination(PM+FC).

tions of gas pressure in the three diﬀerent magnetic field
geometries are shown. The microwave power was slowly
increased until a stable discharge is initiated to record theignition microwave power at the corresponding Ar pressure condition. When only the magnetic field from the fieldcoils (FC) are present, ignition occurs at 330 W in the highpressure range at 15 A current on FC equivalent to a maximum field of 430 G at the quartz window. The required
power remains constant down to 3 Pa pressure before it
starts to increase exponentially up to 1200 W at 1.3 Pa.
When the Nd-Fe permanent magnets (PM) are installed in
the microwave inlet region, they provide a maximum magnetic field flux of 480 G at the quartz glass surface. With
only the PM inducing the magnetic field, the minimum microwave power required to commence a discharge is about
480 W and the power remains constant down to 1.5 Pa. A
dip in the ignition power plot occurs at 0.7 - 0.3 Pa only toincrease again up to 900 W at 150 mPa. When the fields
from the permanent magnets and the coils are combinedto achieve ECR condition near the inlet of the quartz microwave inlet region, ignition occurs around 190 W at highpressure and slowly increases only up to 430 W even further down to pressures less than 5 mPa. At the lowest observable pressure, when the plasma glow does not propagate across the chamber and is limited at the surface of thequartz glass, the plot is terminated. The extinction power,or the minimum power required to sustain the plasma is
240 W with coils, 188 W with magnets and only 90 W at
ECR. The extinction power remains constant for each of
the conditions above and is represented by the three horizontal lines in Fig. 4.
The dependence of the ignition and extinction powers
to the strength of the combined fields below 875 G is illustrated in Fig. 5. The DC source of the coils was controlled
to set the maximum field less than 875 G. At 4 A, the total field at the vacuum side of the quartz window surface

Fig. 5 Ignition and extinction powers at varying Ar pressure relative to field strength.

is 600 G. The ignition power profile follows a similar behavior as when only the permanent magnets are present.
This trend continues up to 12 A (820 G max.) with improved ignition towards the lower pressure range. A dip in
the 0.3 - 0.7 Pa range is also present with a significant increase towards lower pressure. Close to the ECR condition
at B = 860 G, there is a considerable improvement in ignition towards lower pressure down to 30 mPa at only 670 W.
The extinction power remains constant at 188 W in all of
the field intensities below ECR and is also shown in Fig. 5.
From the low pressure region in sub-ECR condition
involving permanent magnets, a decrease in the ignition
power as the pressure is increased accounts for the increasein the particle’s mean free path inside the chamber as more
gas is introduced into the system. As such, an increase incollision frequency promotes more eﬃcient ionization until a minimum ignition power is attained at 0.4 - 0.7 Pa. At
pressure greater than at the minimum breakdown, the ignition power is higher but remains invariable. At this point,
the electron temperature decreases due to further increasein collision frequency.
Popov et al. [13] observed in their 2.45 GHz microwave plasma device that a reduction in the minimum
ignition power occurs when the magnetic flux density is set
to be greater than 430 G to which they attribute to the transition from an underdense plasma mode below 430 G tooverdense plasma mode at 430 - 440 G. It is argued that thetransition arise from the abrupt increase of the microwave
power absorption in the resonance on the second gyrotron
harmonic, ω = 2ωce [14, 15]. The observed transition tolower power with respect to the flux density corresponds
to the same range of minimum power occurrence observed
from B > 430 G. The dip starting from B > 430 G manifests this transition towards the minimum power at 188 W
in ECR.

3.2 Pressure dependence of plasma density

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Langmuir probe measurement obtained at the center

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Fig. 6 Ion saturation current (Isati ) at varying pressure along thevacuum chamber’s central axis.

of the chamber at x = 0, y = 0, around 17.5 cm from thequartz window surface along the plasma downstream, provides the ion saturation current, Isati , of the Ar plasma observed at −75 V probe bias. Figure 6 shows the plot of
Isati relative to Ar gas pressure at fixed 1 kW microwave
power. The current is highest at 0.4 - 0.9 Pa pressure range
and decreases beyond this range. An estimate of the ionization mean free path, λmfp , reveals the behavior of Isati
relative to gas pressure. The mean free path is the inverseof the product of cross section, σ, and number density, n,or λmfp = 1/σn. At 0.5 Pa gas pressure, n for argon isaround 1.33 × 1014 cm−3 while an approximate value of theionization cross section σ = 2.5 × 10−16 cm2 from AMDIS
database [16] for a reaction [Ar + e → Ar+ +2e] is used.
From here, λmfp is around 30 cm and is comparable to thedevice dimension. The total distance from the quartz glass
surface to the opposite chamber wall is around 30.5 cm. At
lower pressure, particles diﬀuse towards the chamber wall
while recombination is favored at higher pressure.

3.3

Sheet plasma profile dependence onmagnetic field

A movable stage allows for a 50 mm range of linear motion of the probe tip along the x-axis to determine
the axial profile of the plasma. Several Langmuir probe
scans were performed along the plasma width at 3 mm distance for each scan. Ion saturation currents (Isati ), electron
temperatures (T e ), and electron densities (ne ) were derived
from the I - V curve and are plotted in Fig. 7. Density distribution of the plasma particles and electron temperature
depict the sheet plasma thickness and its behavior in thepresence of varying linear magnetic field. The plasma wasmaintained by 2 kW microwave power at 0.5 Pa while fieldmeasurements were obtained at the center of the chamber along the probe scan direction at x = 0. Starting at
lower field (13 A equivalent to 265 G), the axial profilesof the ion saturation current and electron density behave
similarly and form a gradual gradient peaking at x = 0.
An increase (1 A equivalent to 14 G) in the field strength

Fig. 7 Axial profiles of ion saturation current (Isati ), electron
temperature (T e ) and of electron density (ne ) relative tomagnetic field strength along the x-axis. The indicatedfields were measured at position x = 0.

slightly elevates the gradient while maintaining the same
axial profile. When the current is set at 15 A, the field at
x = 0 is 293 G while an ECR condition is achieved near
the quartz window. The ion saturation and electron density magnitudes form steeper gradient with greater magnitude increase along the center of the plasma sheet achieving electron density ∼1018 m−3 . The electron temperature
axial profile shows high energy electrons populating thecenter of the plasma. The field strength did not aﬀect theelectron temperature largely because the present treatment
to determine the electron temperature is by using the lowtemperature Maxwell component. The ECR condition enhanced the ionization while the increase of magnetic fieldin sub-ECR condition did not improve sheet plasma thickness.
Previously reported sheet plasma thicknesses were defined in terms of ion Larmor radius [17,18] where the sheet
thickness is described as approximately twice the mean ion
Larmor radius. Gyrating ions move relative to guiding centers that lie in the vicinity of the sheet plasma midplane.
Taking the condition at a pressure of 0.5 Pa and magnetic

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Volume 14, 3401085 (2019)
signed and operated. The design includes a gas feed system that cools the quartz window surface allowing plasma
operation at microwave power up 3 kW without damaging the quartz glass microwave inlet structure. The fieldstrength raises the plasma density while ECR condition
significantly enhances plasma production to attain electron density to 1018 m−3 range. The electron temperatureof the bulk plasma remains independent of the magnetic
field strength.

Fig. 8 Images of the sheet plasma illustrating the sheet plasma
when viewed from the top (Top view) and the sheet thickness along the horizontal center of the chamber viewed on
its side (Side view).

flux density B = 293 G, the ion Larmor radius ri is approximately 7 mm when the ion temperature is considered
to be around 0.1 eV. In the meantime, ri can be 38 mm
if we assume ri is determined from ion acoustic velocity for 3 eV electron temperature. When compared to thehalf width half maximum of the plot of the radial profile
of ion saturation current, the plasma thickness is around
34 mm. Figure 8 shows the sheet plasma at the same condition when viewed from the top (Top view) and on its side
(Side view). The second image reveals the plasma thickness based from the observed plasma glow along the midplane. As reference, the circular profile at the center of theimage has a 37 mm diameter. This implies that the plasma
has a thickness less than this value which is comparable tothe estimate of its full width half maximum value at around
34 mm. Clearly, the observed plasma thickness is less than
twice the expected plasma thickness when compared withthe calculated ion Larmor radius.

4. Conclusion
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