나노 응용을 위한 고휘도 집속 이온빔
High-brightness focused ion beam utilizing local sheath plasma for nano applications
Yeong-Shin Park, Yoon-Jae Kim,
Yuna Lee,YoungHwa An,
Man-Jin Park, Kyoung-Jae Chung
and Y. S. Hwang
NUPLEX, Dept. of Nuclear, Seoul National University,
Gwanak 599, Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
2010 한국원자력학회 워크숍- 이온원 개발 및 그 이용 현황
October 20, 2010
Ramada hotel, Jeju, Korea
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Contents
1. Introduction
Focused ion beam (FIB) and its applications
Requirement of high brightness ion source for FIB
2. High brightness plasma ion source with local sheath plasma
Previous works
Principle of local sheath plasma on beam current enhancement
Advantageous features of the local sheath plasma
Brightness comparison with other ion sources
3. Focused ion beam utilizing plasma ion source
Plasma ion source is utilized in conventional focused ion beam system
Images are acquired from focused ion beam from plasma
4. Summary
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Applications of Focused Ion Beam system
▶ FEI Expida 1285 for 300mm wafer
Semiconductors1. Cross Sectioning
2. De-layering
3. TEM specimen preparation
4. Device modification
1 2
3 4
FIB-FESEM
Cross Beam sys.
Dual Beam sys.
Hybrid sys.
Flip-Chips▪ Mounting Chip
Lithography▪ Next Generation Lithography
Micro-sculpture▪ Direct Machining
Applications of FIB-FESEM
SIMS
5
FE-SEM
FIB
▶ Carl Zeiss Leo 1540XB
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High Performance Focused Ion Beam
Performance of Focused Ion Beam (FIB)
High performance FIB is characterized
by high resolution and high processing-yield.
Goal in high performance FIB development
to make nano-scale ion beam (5~100 nm in diameter)
with relatively high current (1~100 pA) for its smallness.
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Structure of Focused Ion Beam system
Focused Ion Beam (FIB)
[1] Reyntjens S and Puers R 2001 J. Micromech. Microeng. 11 287
[1] Major Components in FIB
1. Ion Source
: supplying high performance ion beam
2. Column (Electrostatic Lens)
: transporting, focusing and deflecting
ion beam
3. High resolution Stage
: locating and moving a target material
precisely
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Role of Ion Source in FIB
High Brightness Ion Source is Essential to make high performance FIB.
1. Focused ion beam diameter (d) is determined by virtual diameter(dv) of ion source.
Small virtual diameter is defined as low emittance (small and less diverged) in terms of
ion beam engineering.
2. Beam current from ion source rules the processing yield of FIB.
3. High brightness ion beam means a low emittance and high current ion beam.
Brightness is a physical parameter considering beam current as well as emittance.
2 2 2 2 1 / 2( )
cv sdd M d d
Brightness =
Ion Beam Current
Emittance
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Ion Source Candidates for FIB
Plasma Ion Source (PIS)
1. A promising candidate for advanced FIB. PIS is expected to replace LMIS and overcome the
limitations.
2. Advantages: (a) various ion species (H+, He+, Ar+, Xe+, O+), (b) long life time, (c) favorable to
make multi-beam and (e) suitable for high yield FIB.
3. Challenging issues: increasing brightness of ion beam
Liquid Metal Ion Source (LMIS)
1. Most popular ion source used in FIB because of its high
brightness as well as extremely small virtual diameter.
2. drawbacks: (a) limited ion species (Ga+, In+), (b) short life time,
(c) damage of target by Ga+ ion bombardment, (d) undesired
alloy formation by Ga+ ion, (e) impossible to make multi-beam
and (f) wide ion energy distribution.
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How to increase Brightness of Plasma Ion Source
Extract ions through small aperture from plasma
Small aperture and low ion temerature are preferable to reduce ion beam emittance, ε.
(r0: radius of meniscus, Ti: ion temperature, A: atomic mass number) [1]
Ion extraction is more challenged as the aperture is comparable to or smaller than
sheath thickness. In this sense, plasma density is needed to be higher in order to make
thinner sheath.
1 / 2
, 00.0016 [ ]
i
n rms
kTr mm mrad
A
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Approaches to extract ion beam through micro-size aperture
Positive Bias Method [2]
Additional Bias Electrode is installed to extract ions more efficiently
Negative Bias Method [1]
[1] K.L. Scott et al., J. Vac. Sci. Technol. B 18 3172 (2000) [2] Y.J. Kim et al, Rev. Sci. Instrum. 77, 03B507 (2006)
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Experimental Setup with Beam Diagnostic System
Additional Electrode (Bias Electrode, BE)
BE is installed In addition to PE, plasma electrode.
Φ 100 μm aperture at 250 μm thick part of BE
BE is insulated from PE by alumina disk.
Small circular area of BE with 3 mm diameter exposes
toward plasma chamber.
Beam Diagnostic system
Faraday cup to measure beam current
Emittance scanner is installed 300 mm away from ion
source and scans diverged beam with large diameter(>20
mm) which is sufficiently larger than slit width (0.1 mm).
Ion energy analyzer is used to estimate axial ion beam
energy spread. When the analyzer is used, ion source are
grounded and the GE (ground electrode) is biased
negatively.
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Introduction of Local Sheath Discharge
Local Sheath Discharge (LSD)
The additional plasma ignites within small sheath in
front of BE with potential difference between
inductively coupled plasma (ICP) and BE. In this
manner, the new plasma is named “local sheath
discharge (LSD).”
Confirmation of LSD
Ion saturation current profiles measured with Langmuir
probe along axis of ion source indicate the existence
of LSD.
Macroscopic observation of LSD in front of biased
electrode immersed in ICP generated in mock-up
device of the ion source.
LSD is similar phenomenon with anode spot, fireball
and so on [1-2].
[1] B. Song et al, J. Phys. D: Appl. Phys. 24, 1789 (1991)
[2] R. Schrittweiser et al, Physica Scripta. T84, 122 (2000)
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Beam extraction from the Local Sheath Plasma
Beam extracted from local sheath plasma
Beam currents increase with bias current rather than
RF power with which density of ambient plasma
increases.
Bias currents indicates the density of the local sheath
plasma.
Ambient plasma rules the bias current
Maximum bias current is proportional to RF power
Bias current, density of local sheath plasma, is limited
by ambient plasma density.
At higher pressure, the local sheath plasma has
higher density, which indicates that the neutral
particles contribute to the sheath plasma density
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Advantageous Features of the Local Sheath Plasma to Ion Sources
Higher pressure is preferable
Beam current increases with pressure at fixed bias
current.
Electron temperature, neutral temperature and ion
temperature is lower at higher pressure.
Low axial ion energy spread
Because of low electron temperature, axial ion energy
spread which is proportional to pre-sheath potential is
small in the case of ion sheath.
There is no pre-sheath in front of electron sheath as
the local sheath plasma generates. For the reason,
only ion temperature affects on ion energy spread.
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Brightness Comparison
Brightness Comparison to RF plasma ion source
LBNL has a record brightness in RF-based plasma ion source [2].
Ion beam is curtailed by 5 μm aperture and the normalized brightness of ion
beam 1.5x103 A/m2SrV.
Brightness of our source utilizing LSD can be estimated as 8x105 A/m2SrV for
5 μm diameter ion beam, which is 100 times higher than conventional RF-base
plasma ion source.
[1] G. D. Alton et al, J. Appl. Phys. 66, 1018 (1989)
Brightness Comparison to LMIS
LMIS brightness is measured in ORNL. [1].
In comparison of normalized brightness with
respect to beam current, plasma ion source
utilizing LSD almost reaches the brightness
of LMIS.
[2] Q. Ji et al, J. Vac. Sci. Technol. B 20, 2717 (2002)
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Installation the Plasma Ion Source on Focused Ion Beam
Plasma ion source Installed on the commercial FIB column.
Vacuum system including mini TMP and an additional rough pump.
Gas injection system including MFC and needle valve is installed.
Base pressure reaches 1.0x10-6 Torr at column region, which is
available to transport ion beam without loss.
Plasma Ion Source FIB Column FIB adopting Plasma Ion Source
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Introduction to Plasma Ion Source adapted FIB (PISFIB)
Plasma ion source
Supply high brightness ion beam.
Beam current: several micro-Amperes.
Beam energy: a few tens of kilo-Volts with triode system.
Need to gas injection and mini-TMP.
FIB column
Transport, focus and deflect ion beam.
Two Einzel lens called condense lens (CL, first lens) and
objective lens (OL, second lens) to focus ion beam.
Two aperture is used to curtail unwanted (unfocused) beam.
Deflecting system behind 2nd lens to scan ion beam.
FIB stage
x-y stage is located to place and transfer target sample.
PMT collects the secondary electron from the target by ion
beam impinging.
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Exp. Setup for Ion Beam Current Measurement
Beam current (at upper Faraday cup)
Located below ground electrode of plasma ion source
Have a hole of 0.1 mm diameter at its center to curtail
diverged beam and pass through parallel beam.
Target current (at sample plate on FIB stage)
Current of beam transported and focused via lens are
measured by a plate on FIB stage..
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Ion Beam Current with respect to Extraction Voltage
Beam current with two bumps
Beam current starts to be saturated over 1.2
kV of extraction voltage. However, the current
to extraction voltage shows double bumps
(increase, decrease, increase, decrease)
which is different from the ordinary ion source
having a single bump.
It is because the Faraday cup has a hole at its
center. Therefore, another approaches are
needed to find out optimum extraction voltage.
Target current indicates Opt. voltage
Target current at FIB stage makes us estimate
optimum extraction voltage in which the
maximum target current occurs.
The optimum extraction voltage is about 2 kV
in which the beam current is lowest between
the two dumps.
Specification of extraction
Gas flow rate: 0.4 sccm, RF power: 137 W
Extraction hole diameter: 0.3 mm
Beam energy: 10 kV, Voltage of CL: 5 kV, OL: 2.5 kV
0 1 2 3 4
0.00
0.25
0.50
0.75
Ta
rge
t C
urr
en
t [n
A]
Beam Current
Extraction Voltage [kV]
Be
am
Cu
rre
nt
[A
]
0
100
200
300
Acc. : 10 kV
C.L. : 5 kV
O.L. : 2.5 kV
hole: 0.3 mm, miniTMP, extTMP, 80 W, 0.4 sccm
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Images acquired with Plasma Ion Source adapted FIB: #. 1
Images with PISFIB
Ion beam is scanned on target sample along x-y direction.
Counting secondary electrons emitted from the sample by ion
impinging make images from the electron current signal.
Well focused
poorly focused
Live view
scanning ion beam with 100 kHz
Images are showing real time.
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Images acquired with Plasma Ion Source adapted FIB: #. 2
Capture view
More detailed images are achieved than the live view mode.
Ion beams are scanned more slowly.
A Image is acquired by averaging several scanned signals.
Captured view
live view
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Images acquired with Plasma Ion Source adapted FIB: #. 3
Ion beam interaction with sample
Target samples are damaged as the sample exposes to ion beam in few minutes. Since the damaged part (circle in
right figure) is shown as white point, it is difficult to conclude that there is ion sputtering. However, it is confirmed
that ions are impinging on the sample and the ion beam location.
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Summary and Future work
50 μm
1. Local sheath discharge (LSD) occurs in front of an additional electrode biased
positively compared to ambient plasma potential.
2. LSD is characterized by bias current. The current indicates the LSD density.
3. Since the LSD is characterized as high density, the ion beam extraction is possible
through micro-scale aperture.
4. Brightness of the plasma ion source utilizing LSD is comparable to liquid metal ion
source and surpass the conventional plasma ion source.
5. Install and align the plasma ion source with FIB column and get a image of carbon
sheet with focused plasma ion beam.
6. Start to research the advanced FIB adopting the plasma ion source with LSD
나노 응용을 위한 고휘도 집속 이온빔
High-brightness focused ion beam utilizing local sheath plasma for nano applications
NUPLEX, Dept. of Nuclear, Seoul National University,
Gwanak 599, Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
한국원자력학회 워크숍이온원 개발 및 그 이용 현황
October 20, 2010
Ramada hotel, Jeju, Korea
경청해주셔서 고맙습니다.