23E05
Establishment of Plasma Electron Ablation Method
プラズマ電子アブレーション法の開発	 
Tatsuya Banno
伴野達也
Department of Applied Physics, University of Tokyo
Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
東京大学工学系研究科物理工学専攻 	  〒113-8656	  東京都文京区本郷7-3-1
A new ablation technique is being developed from the investigation of the surface wave plasma. Relatively
low positive bias voltage beyond a certain threshold value applied to small target materials in the plasma
results in a transition to a high electron current flow and in the generation of a steady plume, i.e. plasma
with the much higher density locally. The target materials are eroded and deposited to substrates, as in
the case of Laser ablation method. The dependence of the transition bias voltage on the argon gas pressure shows a minimum as seen in Paschen curve shifted to a lower pressure and lower voltage range.

1. Introduction
In previous studies on the surface wave plasma(SWP), its features relevant to the application to material processing have been revealed [1,2]: the low electron temperature but the high plasma density homogeneously distributed due to the ambipolar diffusion. The discreteness of the coupling mode to thesurface wave allows the independent control of thebias voltage to target materials from the generation of
the SWP: films of FeNi, TiN, AlN were sputter deposited [1] with the negative target bias, whereas
La1-xSrxMnO3 films were prepared with the positive
bias [2].
The latter method seems similar to the pulsed Laser
deposition method [3] as far as erosion of the target
without melting trace, the plume generation, and deposition are concerned, and therefore has been definedas the plasma electron ablation.
In the present study, it is aimed to find appropriate
operation conditions for forming the plume in order tooptimize the rate of ablation of target materials, themechanism of the plume generation, and the role ofthe plume in the ablation.
2. Experimental Procedure
A cylindrical discharge vessel (340mm in innerdiameter, 60cm in height, stainless steel UHVcompatible) has been employed with a top plate
flange having two quartz windows on which a
4-line-slotted waveguide is attached. In this study,
steady-state plasma discharge is maintained withthe microwave power (2.45GHz) ranging from
200W to 350W in Ar gas (1-20 Pa).
A graphite disk (AX60) has been chosen as atarget material to verify whether it emits carbon
without melting. Pellets have been cut out from thedisk and set in three ceramic target holders (type A
with an aperture hole of 5 mmφ and 2 mm indepth, type B with an aperture 3 mmφ and 6 mm
in depth type C with an aperture 3 mmφ and 4mm in depth) so that small area of the pellet is exposed to the SWP. The target holder is placed near
the center axis of the vessel and at 10 cm from thetop plate flange.
Insulation of the electric
feedthroughs and cables to the target has been an
important issue to prevent serious damages from themicrowave power, electrical shortage in the SWP,
and the joule heating of the current. The target bias
voltage ranging up to +100V (10A max) can be applied externally.
3. Results and Discussion
Electron saturation current of 0.2 Acm-2 results
typically when the target is biased positively up to+40 V in SWP in Ar at 1Pa with the MW power of350W, in good accord with the data obtained withelectrostatic probes [1, 2]. But above a certain
value Vtr around +60 V, the electron current density
increased abruptly to the value of 10 Acm-2, and theplume of about 1 cm in length appeared quasi constantly, as shown in Fig.1.
The plume is
not sustained in
the low pressure
range with theexternalbias
voltage only: it
is extinguishedjust as soon as
the MW poweris switched off.
The minimum
values Vtr ofFig.1 A photograph of the plume from a the bias for thegraphite target in the holder type B.
plume to begenerated are plotted in Fig.2 as a function of Ar
pressure for the three types of the target holder A, B,and C. Higher values are needed for Type B in thelower pressure range. The dependence seems toobey the Paschen’s law [4] well known as a universal phenomenological function of Pxd product,
where P is the gas pressure and d is the gap length
between an anode and a cathode. Two Paschen
curves in Fig.2 roughly reduced by a factor 3 along
the ordinate are overlaid with 2 parameters: an effective electrode gap d of 5cm with a secondary
electron emission coefficient γ of 1.2 (green), and d
3cm with γ 1.5 (Blue). The value of d is much
longer than the length of the aperture of the holder

Fig.2 Minimum threshold bias voltage Vtr, to induce theplume in Ar plasma as a function of Ar gas pressure for3 types of target holders. Two Paschen curves are shifted and overlaid.

and seems to be related to the current density,
which depends on various experimental parameters.
Thus the plume is regarded as a DC discharge with
an effective cathode of the SWP, as solution plasmas in pre-ionized fluids.
The temperature of the target remains well below
its melting point even with a maximum heating
power of 150 W (60V x 2.5A), but carbon atoms(ions) are detected in the plasma with optical emission spectroscopy and identified as shown in Fig.3.
Atoms in the surface layers within the electron projectile length of the order of a nanometer may be
excited and ejected, e.g. as in the case of electron
stimulated desorption since the electron energy exceeds some threshold value of 50 eV. A significant line peak appeared at the wavelength467.4nm for C2+ whereas a small hump was seen
for neutral carbon atoms e.g. at 477.2 nm in contrast the low electron temperature plasmas. In thepresent spectra any lines from C+ ions were not
identified yet. The high degree of ionization (exceeding unity) was also reported [5] for the casesof the Laser ablation and of the channel spark interms of the interaction of intense photons and ofelectrons with released atoms. Likewise ejectedcarbon atoms collide with electrons having themaximum energy of 60 eV while they fly or are
confined in the plume. This electron energy value
is greater enough than the second ionization energy of carbon. A simple computation yields anaverage value of the order 104 incident electrons
for a surface carbon atom per one second.
4. Concluding Remarks
The pressure lower than 5 Pa is preferable forthe ablation method taking the mean free path of
the released atoms outside the plume into account,
and also taking the applicable higher electron energy leading to a higher ablation rate into account.
The plume offers the possibility of higher ionization degree of the target atoms and therefore may
influence the deposition process.
ReferencesFig. 3. Optical emission spectra obtained fromthe plasma without target biasing (broken line)
and with the target bias using the target holder
type A (thinner line) and type B (thicker line),
as seen in Fig.2(a) and (b), respectively.

[1] T.Banno, H.Kutsuna and S.Minami: Vacuum 80(2006) 667.
[2] T.Banno: Proc. 10th Intern. Symp. Sputtering andPlasma Processes, Kanazawa, 2009, P.79.
[3] M.Izumi, Y.Konishi, T.Nishihara, S.Hayashi,M.Shinohara, M.Kawasaki, and Y.Tokura: Appl.
Phys. Lett. 73 (1998) 2497..
[4] M. A. Lieberman and A. J. Lichtenberg: Principlesof Plasma Discharges and Materials Processing(Wiley, NewYork, 2005) 2nd ed., Chap. 14, p.546.
[5] Th.Witke, A.Lenk, B.Schultrich, C.Schultheiss:
Surface and Coatings Technol. 74-75 (1995) 580.