22B01
Molecular Dynamics Analysis of Ion Incident Angle Dependence ofSi etching with Cl, Br, HBr beams and Neutral Radicals
分子動力学法を用いたSiエッチング表面反応解析：
中性ラジカル付加とイオンビーム入射角度依存性
Nobuya Nakazaki, Hirotaka Tsuda, Yoshinori Takao, Koji Eriguchi and Kouichi Ono
中崎暢也, 津田博隆，鷹尾祥典，江利口浩二，斧 高一
Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyoto University,
Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8115, Japan
京都大学大学院工学研究科航空宇宙工学専攻 〒606-8115 京都市左京区吉田本町
Classical molecular dynamics (MD) simulation has been performed for Si etching with Cl, Br, HBr beams
and neutral radicals. The ion incident energy and incident angle dependence of etching fundamentals (etchyield, surface coverage, stoichiometry, and structure) and the effect of adding neutral radicals for thesefundamentals are analyzed to gain a better understanding of the etching mechanisms for feature sidewalls.

1. Introduction
Profile anomalies and surface roughness are
critical issues to be resolved in the plasma etchingof nanometer-scale microelectronic devices,which in turn requires a better understanding of
the effects of the ion incident energy and incident
angle on surface reaction kinetics. In addition, theexistence of neutral radicals, which attach to thesurface during plasma etching processes
significantly, affects the etching characteristics.
This paper presents a classical moleculardynamics (MD) simulation of Si etching byenergetic halogen beams and low-energy neutral
radicals, where the incident energies and anglesof the halogen beams were varied. We used an
improved Stillinger-Weber interatomic potential
model for Si/halogen interactions [1–3]. We will
discuss the differences between etchingfundamentals with and without radicals, such as
etch yield and stoichiometry, surface coverageand stoichiometry, and surface structure as afunction of ion incident energy and angle.
(a)PlasmaIon : Cl+, Br+, HBr+
Radical : Cl, Br, H and Br
Ei = 20-300 eV
θiPeriodic
Boundary

(100)

(001)

(b)
Case 1Case 2(Beam Only)(Beam + Radical)

Ion
Ion
Impinging Impinging
0.2 ps
0.7 ps

Si1ML= 72 atoms

3.258 nm
(010)
Fixed
bottom layer

SystemCooling

0.7 ps
4.0 ps0.3 ps0.3 psSystemCooling

SystemCooling

Radicals
AddingFig.1. (a) A schematic view and (b) scheme of ourMD simulation for etching.

2. ModelFigure 1 shows a schematic view and scheme of
our MD simulation for etching. A substrate (targetSi atoms) is placed in the simulation cell, initiallyhaving a diamond lattice structure whose top
surface corresponds to the (100) plane. The Si(100)surface concerned is a square 32.58 Å wide, wherea monolayer (ML) contains 72 Si atoms. The
simulation cell initially contains target atoms of 20
ML in a depth of 26 Å. Target atoms in thebottom layer are fixed during the simulation, while
periodical boundaries are imposed in the horizontal
direction. Using this simulation cell, we simulated
two cases: Si etching by beam only (Case 1) and bybeam with radicals (Case 2).
Energetic Cl+, Br+, and HBr+ ions are injected
toward the surface from randomly selectedhorizontal locations above the target. For Case 2, 10
radical atoms of the same element as the ion beam
are introduced from randomly selected horizontal
locations onto surface before every ion incidence.
The initial kinetic energy of a radical atom is 1 eV.
The ion incident energy concerned is in the range Ei= 20–150 eV, and the incident angle is in the range
θi = 0–80˚, where θi = 0˚ corresponds to the normal
incidence onto the surface. Note that impinging
particles are assumed to be charge-neutral with hightranslational energies. After each injection of anenergetic ion, we let the system evolve for 0.7 pswith the total energy being constant, and thenartificially cool the system down to the initial
temperature (300K) of target atoms for 0.3 ps. For
Case 2, after each addition of radicals, we let thesystem evolve for 4.0 ps and cool for 0.2 ps. In
addition, we sometimes add a layer of Si atoms, tomaintain the number of target atoms above ~20

ML in the simulation cell during etching. The etchyield, product stoichiometry, and surface structures
are analyzed by averaging more than 1000 impacts
after the surface and etching characteristics become
stable statistically.

mechanism, as opposed to physical sputtering
represented by single Si desorption. The total yieldin Case 2 for Ei = 20 eV and θi = 60–80˚ is smaller
than that in Case 1 for the same conditions becauseof the effect of the protective Cl layer.
Case 1Case 2(Beam Only)

(a)

(c)

Ei = 20 eV(Beam + Radical)

Ei = 20 eV

θi = 0˚

θi = 70˚

θi = 0˚

θi = 70˚Cl+Si(b)

(d)

1 nm

Ei = 100 eV

Ei = 100 eV

θi = 0˚

θi = 70˚

θi = 0˚

θi = 70˚

Fig.2. Side views of surface reaction layers of Si.
Case 1
3.03.03.03.0Ei = 20 eV

2.5

Y Data1.5

1.01.00.5

0.5010203040506070

80

0 10 20 30 X40
50 60 70 80
Data
2D Graph 1

Column J
Column K
Column LColumn M

3.03.02.02.0Y Data1.5

1.01.00.5

0.51020304050

6070800 10 20 30 X40
50 60 70 80
Data
Column J
Column K
Column L
Column M

20304050

60

0.00.010101010X
X DataData
X Data2020202030303030

X Data405040
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4050
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7070
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0.20.480 0.4

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2D Graph 1

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Column K
Column L
Column M

0.60.80.80.8Ei = 100 eV

0.8
1.0
1.0
1.0
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1.01.00102.52.02.00.00.0000000 10 20 30 X40
50 60 70 800.4
Data

3.03.0Ei = 100 eV

2.5

0.00.0(d)

0.0
0.0
0.0
0.0
0.2
0.20.2

atD
aDDYYYataD
aatataY

Y Data1.5(b)

SiCl
SiCl
SiCl
SiCl

Ei = 20 eV

2.02.01.01.00.00.0Column L
Column LK
ColumnK
Column LLJ
ColumnJ
Column KKI
Column JI
Column J
Column4 IH
Column IH
GColumn3 H
GColumn H
F
Column GF
Column G
Column2 F
E
Column E
ColumnColumn F
E
Column E

2.5

2.02.0Y DataSurface Coverage and Stoichiometry (ML)

Case 2Graph 1(c) (Beam +2DRadical)

Graph 1
(Beam2DOnly)

(a)

1.4
1.6
1.6
1.8
1.6
1.8
2.0
1.8
2.00

10203040506070800 10 20 30 X4050 60 70 802.0
Data

Incident Angle θi (deg)

2D
2D Graph
Graph 1
1Column J
Column K
Column L
Column M

Fig.3. Coverage and stoichiometry of surface reaction 2D Graph 1
layers of Si.
Case 1(a)Graph 1
(Beam2DOnly)

0.40.40

Ei = 20 eV

0.35

0.3

Case 2Graph 1(c) (Beam +2DRadical)
0.40.40

Ei = 20 eV

0.35

0.30.30

0.30

0.25

0.20.20

Y DataY Data0.25

0.20.200.15

0.15

0.10.100.10.100.05

0.05(b)(d)

Y Data0.00

000000Column L
ColumnL
OthersColumnL
ColumnKK
ColumnL
ColumnColumnSi3ClLKLKJJLx
ColumnColumnJ
Column K
Column
LJ
IIJ
KSiClIxColumn
K2 HColumn
IH
ColumnJK
ColumnColumnSiCl4JIHGJIHGHColumnColumnIG
Column IH
ColumnSiCl3GFFFHGFE
ColumnColumnColumnH
ColumnE
Column G
ColumnColumnSiCl EGFFEEColumnColumn2G
ColumnF
ColumnSiCl EEF
ColumnColumn E
20
Si1010201020102010201020X Data
X DataData
X

303030303030X
Data5040X Data
4050X
X DataData5040405040504050

YD
Y
Y
YYYYD
DDaaaDtaattaaattaaD
atYa0.0 0 10 20 30 40 50 60 70 800.0 0 10 20 30 40 50 60 70 80 0.00 10 20 30 X40
50 60 70 80
0 10 20 30 X40
50 60 70 800.00.0Data
Data
0.00.2
2D Graph 1
2D Graph 1
0.2
0.0Column E
Column E
0.0Column F
Column F
E
=
100
eV
E
=
100
eV 0.20.2
ii0.4
Column G
Column G
0.80.8 Column
0.80.8 Column
0.4
HH
0.4
0.2
0.2
Column I
Column I
0.4
0.7
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Column J
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Column KColumn K
0.6
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0.60.6 Column L
0.60.6 Column L0.6
0.6
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0.1
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1.2
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1.6
0.00.0 0 10 20 30 40 50 60 70 80
0.00.0 0 10 20 30 40 50 60 70 80 1.6
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50
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0102030
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1.6
1.8X Data
Data
1.8
Incident Angle θi (deg)
1.8
1.6
Column EColumn E
1.8
1.8
2.0
Column F
Column F
Column G
Column G
2.02.0Column H
Column H
1.82.0Column I
Column I
2.00.00

Y DataSi Yield and Stoichiometry (per ion)

3. Results and discussion
Figure 2 shows side views of Si(100) surface for
the Si/Cl system, simulated with different ion
incident angles θi = 0˚ and 70˚ and with different
energies Ei = 20 and 100 eV. The results indicatethat the thickness of the surface reaction layer for agiven θi is almost independent of the presence ofradicals. However, at θi = 70˚, the number of Cl
atoms on the surface in the Case 2 is larger than thatin Case 1. This is because most of the radical atoms
attach to the surface owing to their low kinetic
energy, while most of the beam atoms at θi = 70˚
are reflected at the surface due to their high kinetic
energies and large incident angles. At low energy(Ei = 20 eV) and large angle (θi = 70˚), Cl atoms
form a layer like a surface protective film.
Figure 3 shows the surface coverage of Cl atoms
and stoichiometry of surface Si-Cl bonds as afunction of ion incident angle θi. Here, SiClx (x = 1,2, 3, and 4) means that the Si atom has x Si-Clbonds, and the surface coverage is the total numberof adsorbed atoms in the simulation cell. The results
indicate that in both Case 1 and Case 2, thecoverage decreases with increasing θi for a given Ei,
but the coverage at large θi is smaller in Case 2 thanin Case 1. The mechanism for these results aresimilar to those as mentioned above. In addition, atEi = 100 eV and small θi, the number of Si atoms
with two Cl atoms bonded (SiCl2) is increased byadding radicals. This may be explained as follows:
the energetic ion bombardment disarranges the Silattice, and subsequently creates Si-Cl bonds. In
Case 2 (with radicals), disarranged Si atoms canbond with a larger number of Cl, as Cl atoms are
more abundant than Case 1 (beam only).
Figure 4 shows the Si yield and stoichiometry(chemical composition) of etch products SixCly (x ≥1 and y ≥ 0) desorbed, as a function of ion incident
angle θi, simulated for Si/Cl system with different
energies Ei = 20 and 100 eV. Note that the Si yield(per ion) is the total amount of etch products
desorbed from substrate surfaces per ion impact.
The results indicate that the yield of Si atoms
bonded to more than one Cl (i.e. SiCl2 and SiCl3) islarger in Case 2 than that in Case 1. In addition, for
Ei = 100 eV, the total yield becomes larger and theyield of single Si dramatically is decreased byadding radicals. These imply that by adding radicals,
chemical desorption become the significant etchingFig.4. Si etch yield and stoichiometry.
Column J
Column K
Column L

Column J
Column K
Column L

2.0

2D
2D Graph
Graph 1
12DGraph
2DGraph 1
12D
2D Graph
Graph 1
12D Graph 1
References[1] A. Iwakawa et al., Jpn. J. Appl. Phys. 47, 8560
(2008).
[2] T. Nagaoka et al., Jpn. J. Appl. Phys. 105, 023302(2009).
[3] H. Ohta et al., Jpn. J. Appl. Phys. 48, 020225 (2009).

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