나노응용을위한고휘도집속이온빔nuplex.snu.ac.kr/new/archives/upload/pt_pdf89.pdf ·...

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

[email protected]

2010 한국원자력학회 워크숍- 이온원 개발 및 그 이용 현황

October 20, 2010

Ramada hotel, Jeju, Korea

1/21 High-brightness focused ion beam utilizing local sheath plasma for nano applications

PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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.


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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.


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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)


PYS_FIB_KNS-Oct2010

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.


PYS_FIB_KNS-Oct2010

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)


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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.


PYS_FIB_KNS-Oct2010

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)


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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.


PYS_FIB_KNS-Oct2010

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..


PYS_FIB_KNS-Oct2010

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


PYS_FIB_KNS-Oct2010

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.


PYS_FIB_KNS-Oct2010


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


PYS_FIB_KNS-Oct2010


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.


PYS_FIB_KNS-Oct2010

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

[email protected]

한국원자력학회 워크숍이온원 개발 및 그 이용 현황

October 20, 2010

Ramada hotel, Jeju, Korea

경청해주셔서 고맙습니다.

나노응용을위한고휘도집속이온빔nuplex.snu.ac.kr/new/archives/upload/pt_pdf89.pdf ·...

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