24B02
Numerical Simulation of O- Ion Trajectory in RF Magnetron Plasma
RFマグネトロンプラズマ中の酸素負イオン軌道解析
Masato Nakabo1, Kazunaga Ono2, Tadashi Morita2, Norikazu Ohshima3,Tatsuo Ishijima4, and Hirotaka Toyoda1,4
中坊 将人1, 小野 一修2，森田 正2，大嶋 則和3，石島 達夫4，豊田 浩孝1,4
1

Department of Electrical Engineering and Computer Science, Nagoya University,Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
2
Tsukuba Institute for Super Materials in Susono Section, ULVAC, Inc. Suyama, Susono 410-1231, Japan
3
NEC Energy Devices, Ltd., Shimokuzawa Chuo-ku Sagamihara, Kanagawa, 252-5298, Japan4
Plasma Nanotechnology Research Center, Nagoya University Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
1,4
名古屋大学工学研究科 〒464-8603 名古屋市千種区不老町C3-1
2
株式会社アルバック 筑波超材料研究所 磁性構造研究室 〒252-5298 裾野市須山 1220-14
3
NEC エナジーデバイス開発本部 〒410-1231 相模原市中央区下九沢 1120
Behavior of high energy nagative oxygen (O-) ions in RF magnetron plasma is investigated by a numericalsimulation in which O- ions are released from the oxide target surface and are accelarated by thetime-varying RF sheath in front of the target. Deflection of O- ions depends not only on a magnetic fieldof the magnetron gun but also on the RF phase at the target surface.

1. Introduction
Magnetron plasmas are one of the most important
tools for sputter deposition of thin films. However,energetic particles from the sputtered target
sometimes induce physical and chemical damages
to the deposited film surface during the sputtering
processes.
For example, magnetic and/or
electrical properties of magnetic recording films ormagnetic tunneling junction for magnetoresistive
random access memory (MRAM) sensitively varywith sputtering conditions, suggesting the damageto the deposited film interface.
Therefore,
measurement of energetic particles in themagnetron plasma is indispensable to improve thedeposition process. So far, we have investigatedbehavior of O- ions in an RF magnetron plasma
with an insulating target materials (MgO),
especially focusing on the O- energy distributionfunction (EDF) and its spatial dependence.[1,2]We have found that O- ions were released from theMgO surface and were accelerated by the sheath
electric field up to a few hundred eV. The
maximum kinetic energy of O- ions varied withradial position of the target due to spatial variationof MgO surface potential caused by the variation of
the sheath thickness. Furthermore, it has been also
pointed out that the O- energy distribution has finestructure due to energy modulation of time-varying
plasma potential. Another interesting feature isthat the O- signal showed drastic change in its
intensity depending on the radial position. This
result suggests that high energy O- ions weredeflected by magnetic field of magnetron gun andwere not detected by a quadrupole mass
spectrometer (QMA) with an energy analyzer. In
this study, O- trajectory is numerically simulatedwith using a model including time-varying sheath
electric field, as well as magnetic field distributionof the magnetron gun.
2. Simulation ModelIn this simulation, spatial profile of magnetic
field strength B is experimentally obtained from thefield measurement of a pair of magnets used in ourprevious experiments.[1,2] Origin of cylindricalcoordinate is set at the center of the magnet forradial position (r) and on the surface of the MgOtarget for axial position (z). The inner (cylindrical)
magnet is 1.7 cm in its radius and the outer(ring-structured) magnet is 2.1 cm and 3.1 cm ininner and outer radii. Parallel magnetic field Br
has the maximum at r=1.7 cm. Figure 1 shows
schematic of the model. In this model, not only
axi-symmetric magnetic field of the magnetron gun
but also time-varying RF sheath in front of theMgO target, spatial variations of MgO surface
potential and the sheath thickness are considered.
Uniform ion density is supposed in the sheath,resulting in linear decease in the electric field withthe distance from the target surface. O- ions are
released at arbitrary RF phase and arbitrary positionof the target and ion trajectories are calculated by
Runge-Kutta method. Here, RF phase angle () of
180o is determined as the minimum spontaneousvoltage of the target surface (0 V). In the sheathregion, both the electric and the magnetic fields aretaken into account. After passing through thesheath, O- ion trajectories are calculated only by themagnetic field. Plasma-sheath boundary moving
with the RF phase is also included in this model.
To ensure the calculation accuracy, number of thecalculation more than 300 steps is guaranteed forthe trajectory calculation inside the sheath region.
3. Calculation Results
Figure 2 shows trajectories of O- ions in thevicinity of the MgO surface (z<1.2 cm). O- ionsare released at r=0, 0.6, 1.2, 1.8 and 2.4 cm with an
RF peak-to-peak voltage of 420, 400, 290, 200 and340 V, respectively. An RF phase in Fig. 2 is =60o,
which corresponds to a spontaneous MgO surface
voltage of 110, 100, 70, 50 and 80 V. At r=0, theO- ion travels along z axis because there is noparallel magnetic field at r=0. At r=0.6 and 2.4cm, however, the O- ions are deflected due to theparallel component of the magnetic field. At
r=1.2 and 1.8 cm, where the parallel magnetic fieldcomponent is much higher, O- ions at z=1.2 cm areshifted about 0.3 mm away from their initial
position due to the deflection. Although theposition shift is less than 1 mm at z=1.2 cm, ionstravel through the plasma to the substrate, typically
positioned at z~10 cm. This means that position
shift becomes a few mm on the substrate.
Figure 3 shows O- flux at the MgO target (z=0)
and the substrate (z=10 cm). When O- flux at theMgO target is assumed to be uniform, O- flux at
substrate is not uniform. Since the calculated Ofluxes are taken into account the RF sheathoscillation, non-uniform O- ions distribution isformed owing to not only magnetic field strength
but also the RF phase variation. To clarify the RFphase dependence, more detailed analysis will bediscussed in the conference.
4. Conclusion
In conclusion, trajectories of O- ions releasedfrom the magnetron target surface were simulated.
Strong ion deflection was observed at a position of
the maximum parallel magnetic field and at low
spontaneous RF voltage phase.
References[1] T. Ishijima, et al., Jpn. J. Appl. Phys., 48 (2009)
116004.
[2] H. Toyoda et al., Appl. Phys. Express, 2 (2009)
126001.